Unexpected ubiquity of chorus emissions in terrestrial magnetotail

Unexpected ubiquity of chorus emissions in terrestrial magnetotail | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Article Unexpected ubiquity of chorus emissions in terrestrial magnetotail Chengming Liu, Boning Zhao, Jinbin Cao, Yangyang Liu This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-8191502/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted You are reading this latest preprint version Abstract Chorus waves are one of the strongest electromagnetic emissions widely occurring in planetary space, and have been documented to play a key role in mediating extreme space climate and driving spectacular auroras. Chorus waves have hitherto been believed to develop typically inside planetary inner magnetospheres, where magnetic dipolar fields dominate and govern their generation and propagation. Here, in contrast to such conventional knowledge, we show that chorus waves are surprisingly ubiquitous in terrestrial neutral sheet where magnetic fields are not dipolar, by surveying years of data from NASA’s Magnetospheric Multiscale spacecraft. We discover that chorus waves in the neutral sheet are mostly falling-tone, unlike inner magnetosphere chorus waves which are predominantly rising-tone, suggesting that global magnetic field topology can mediate the wave pattern. In addition, we find that the falling-tone chorus waves mostly propagate along magnetic field lines, different from obliquely-propagated falling-tone chorus in the inner magnetosphere. We further reveal that local magnetic field inhomogeneity associated with the chorus waves is basically negligible, posing observational constraints for theoretical modeling of chorus waves. These results uncover an unexpected source region for inner-magnetosphere chorus, and provide novel insights into understanding wave-driven space weather and climate. Earth and environmental sciences/Space physics/Aurora Physical sciences/Physics/Plasma physics/Astrophysical plasmas Earth and environmental sciences/Planetary science/Geodynamics Physical sciences/Astronomy and planetary science/Space physics/Magnetospheric physics Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Introduction Wave-particle interaction serves as a fundamental energy transfer mechanism between charged particles and electromagnetic fields, governing particle acceleration and loss processes in the solar system 1–4 . Whistler-mode chorus waves, one of the most intense electromagnetic emissions naturally arising in space, can drive efficient electron acceleration and scattering. These emissions are characterized by their discrete and frequency-varying elements, which have attracted extreme interest since their discovery 5–7 . Chorus emissions have been documented as key drivers of important magnetospheric phenomena including precipitating electrons to produce diffuse and pulsating auroras 8–11 , generating plasmaspheric hiss waves 12,13 and accelerating relativistic electrons in radiation belts 14–16 . They have been extensively studied in terrestrial magnetosphere through in situ observations and numerical simulations for more than 70 years 17–22 . Their global distribution, typical properties and parametric dependence have been demonstrated by spacecraft observations in the near-Earth space 23–25 . Nonlinear wave theories have predicted that magnetic field inhomogeneity plays a decisive role in chorus generation, governing the generation of rising-tone and falling-tone emissions 26–28 . Nonetheless, recent numerical simulations have successfully reproduced chorus emissions in uniform magnetic fields, demonstrating that nonlinear mechanisms keep valid without field inhomogeneity 29,30 . In addition, latest spacecraft observations in mid-tail neutral sheet have found locally-generated chorus waves in regions with negligible magnetic field gradients 31 . Despite these advances, chorus research remains predominantly limited within near-Earth regions dominated by dipolar magnetic fields ( L < 10 R E ), which has led to a biased understanding of chorus generation and evolution in space—largely because of a longstanding belief that chorus generation requires dipolar magnetic field. Resolving the discrepancy between theories, simulations, and observations necessitates comprehensive in situ measurements of chorus generation under various magnetic field conditions, which can provide critical observational constraints for current theoretical frameworks. Here we have conducted, for the first time, a comprehensive investigation of chorus waves in terrestrial magnetotail using state-of-the-art measurements provided by NASA’s Magnetospheric Multiscale (MMS) mission 32 . Despite being primarily designed for magnetic reconnection studies, MMS’s exceptional measurement capabilities have enabled us to search for chorus emissions in the magnetotail region 33 . During its 2017–2020 orbital phase with apogees exceeding 25 R E , MMS eccentric orbits cover terrestrial neutral sheet where magnetic field topology and plasma properties are distinct from those in near-Earth space. Our analysis demonstrates that chorus emissions are surprisingly ubiquitous in the neutral sheet (Fig. 1), with falling-tone emissions dominating, and their characteristic features are significantly different from those in near-Earth region, with larger amplitudes and lower frequency sweep rates. These findings uncover an unexplored regime of chorus waves and provide new observational constraints for theoretical studies of nonlinear wave–particle interactions in collisionless plasmas. Results Observational events We have identified, via singular value decomposition (SVD) analysis 34 , two typical types of chorus (rising-tone and falling-tone) emissions observed by MMS in Earth's neutral sheet ( R E >10), as shown in Fig. 2 . The rising-tone chorus was detected at [-17.9 -10.6 0.5] R E , and the falling-tone chorus was detected at [-20.4 -12.7 1.3] R E . In the two events, magnetic and electric field power spectral density exhibit clear discrete and frequency-varying elements, with frequency sweep rates close to ± 100 Hz/s (positive for the rising-tone, negative for the falling-tone), approximately an order of magnitude lower than the typical values(~ kHz) in the inner magnetosphere. The SVD analysis yields key wave parameters, including ellipticity ≈ 1 (right-handed circular polarization, Fig. 2 c and 2 i), wave normal angles < 30° (quasi-parallel propagation relative to background magnetic field, Fig. 2 d and 2 j), and planarity ≈ 1 (highly coherent wave fronts, Fig. 2 eand 2k). The normalized parallel Poynting flux resolved by the SVD suggests that the rising-tone chorus waves propagate antiparallel to local magnetic field (i.e., away from the equator since local B x < 0; Fig. 2 f), while the falling-tone chorus waves propagate parallel to local magnetic field (i.e., towards the equator since local B x < 0; Fig. 2 l). These results provide compelling evidence for local generation of chorus emissions within the neutral sheet. Global distribution Figure 3 presents the global distribution of chorus emissions in the magnetotail plasma sheet, with occurrence rates normalized by spacecraft dwell time and projected onto X-Y and Z-Y planes. The emissions predominantly occur at X < -15 R E in the X-Y plane (left panel), peaking at X \(\:\approx\:\) -25 R E , where magnetic field lines are highly stretched. This is in contrast with the traditional knowledge that chorus waves are usually related to global or local magnetic dipolar fields. This unexpected distribution may stem from the fact that electron perpendicularly-anisotropic distribution (i.e., pancake distribution), which serve as the energy source for whistler-mode waves, also has large occurrence rates near X \(\:\approx\:\) -25 R E 35 . In addition, the chorus waves preferentially occur in the dusk side, in line with the dawn-dusk asymmetry of electron pancake distribution 35 . In the Z-Y plane (right panel), we find that the emissions are concentrated near the equatorial region (-2 R E < Z < 5 R E ), with an apparent north-south asymmetry, showing larger occurrence rates in the northern hemisphere. This north-south asymmetry is also consistent with the global distribution of perpendicularly-anisotropic electrons in the tail. Note that previous studies have established that chorus emissions in the inner magnetosphere originate near the magnetic equator, similar to the distribution of magnetotail chorus waves. Wave property We further examine the statistical distribution of frequency sweep rate, normal angle and magnetic wave amplitude of the rising-tone (Fig. 4 a- 4 c) and falling-tone (Fig. 4 d- 4 f) chorus waves. Rising-tone chorus waves generally propagate quasi-parallel to the background magnetic field, with a mean normal angle of 11.25°. Their amplitudes range from 50 to 250 pT, with a mean value of 145.0 pT. The central frequency is below 0.3 f ce , with a mean frequency of 0.17 f ce . These features are comparable to those of the near-Earth chorus, which are also characterized by parallel propagation, large amplitude and frequency in low-frequency band. Falling-tone chorus waves, however, show similar properties to the rising-tone chorus in terms of normal angle, amplitude and frequency range, making them distinct from those in the inner magnetosphere. In particular, their normal angles remain largely below 30°, with a mean normal angle of 13.37°, different from the highly-oblique propagation of the near-Earth falling-tone chorus 36 . Such difference may result from the magnetic field topology in the tail, where magnetic field lines are much longer and more stretched than in the near-Earth region, hence posing distinct effects on the propagation of falling-tone chorus waves. In addition, the mean amplitude of falling-tone chorus is 158.3 pT, with peaks exceeding 400 pT, stronger than the rising-tone waves. This contrasts with the near-Earth observations, where falling-tone chorus are typically weaker than their rising counterparts 36 . The mean central frequency of falling-tone chorus is 0.14 f ce , close to that of rising-tone chorus. Although the falling-tone chorus hosts similar properties to the rising-tone, their proportions are quite different. The proportion of falling-tone chorus waves reaches 90%, which differs strikingly from the near-Earth chorus statistics, where falling-tone chorus accounts for only ~ 16% of chorus events. 37 This striking difference may arise from background magnetic field inhomogeneity, which has been suggested by simulation to play a decisive role in setting the chirping direction 30 . Unlike the dipolar magnetic field in the near-Earth region, the field topology of magnetotail may be disturbed as anti-dipolar type, leading to the dominance of falling-tone chorus therein. Parametric dependence The properties of magnetotail chorus differ markedly from those observed in the near-Earth region, especially for falling-tone chorus, indicating that background plasma and magnetic field conditions play a crucial role in determining the wave properties. We examine the key plasma parameters during the chorus intervals (Fig. 5 ). In agreement with earlier studies 38 – 40 , chorus waves are concentrated near the magnetic equator (i.e., neutral sheet in the magnetotail), where the B x is very small ( = 1.81 nT; Fig. 5 a). The average B x is positive, consistent with the northern-southern asymmetry shown in Fig. 3 b. The electron density during the chorus wave interval is low ( = 0.32 cm − 3 ; Fig. 5 b). Note that the electron population in the neutral sheet is mainly hot electrons, different from the electron population in the inner magnetosphere where cold electrons dominate over hot electrons. The two-component (hot and cold) electron population is a key assumption widely used in previous simulations and theoretical studies. The one-component electron population in the neutral sheet thus provides a new background for further chorus research. Nonlinear theory predicted that magnetic field gradient governs the chorus generation and propagation, while the magnetic field gradient is generally weak in the neutral sheet. We have calculated the gradient term in the wave second-order resonance equation, with the aid of MMS four-point satellite measurements, yielding a mean value of -0.01 (Fig. 5 c). Therefore, chorus can indeed develop in quasi-homogeneous magnetic fields, consistent with recent simulation predictions 29 – 31 . We find that during the chorus interval, electron temperature is typically close to several keV with a mean of 1471 eV (Fig. 5 d), similar to previous findings in the near-Earth region 41 . Most chorus events exhibit electron perpendicular temperature anisotropy A e exceeding 1, fulfilling the criterion for local whistler instability. Nonetheless, A e is not large for most chorus events, typically between 1.0 and 1.2, with a mean of 1.07 (Fig. 5 f), indicating that in the neutral sheet, weak anisotropy is sufficient to excite whistler-mode waves 42 , unlike in the inner magnetosphere where higher anisotropy is required 43 . Another difference between the magnetotail chorus and the inner magnetosphere chorus is plasma-to-cyclotron frequency ratio f pe / f ce . The typical value of f pe / f ce ratio is below 5 in the inner magnetosphere, while it typically exceeds 10 in the magnetotail, with a mean of 30.85 (Fig. 5 e). Overall, these results suggest that chorus emissions in the neutral sheet develop in a different plasma regime compared with those in the inner magnetosphere. We investigate the correlation between chorus properties and local plasma parameters. In order to reliably quantify the chorus properties, such as frequency sweep rate and element duration, we have further selected 20 events which show unambiguous, well-defined wave elements. Figure 6 displays a correlation heatmap between the wave properties and the environmental parameters for these 20 events. The wave normal angle and magnetic field amplitude show no clear correlation with local plasma parameters. The central frequency of the chorus waves increases with magnetic field strength (correlation coefficient, cc ≈ 0.52), consistent with wave generation by cyclotron resonance with thermal electrons. The wave central frequency is negatively correlated (cc ≈ -0.51) with temperature anisotropy, which is not expected from linear cyclotron resonance, where larger temperature anisotropy should yield higher frequency waves. Therefore, the temperature anisotropy may also affect wave nonlinear growth, which is indeed glimpsed from the negative correlation (cc ≈ -0.43) between frequency sweep rate and temperature anisotropy. Interestingly, we find that the frequency sweep rate is not clearly correlated with local magnetic field gradient, indicating that the observed chorus waves are mainly detected near the source region, where wave generation and propagation are controlled by nonlinear processes. The sweep rate is positively correlated (cc ≈ 0.43) with electron plasma-to-cyclotron ratio, consistent with prediction by nonlinear wave theories. We also find a positive correlation (cc ≈ 0.52) between wave element duration and electron plasma-to-cyclotron ratio. Nevertheless, we find no correlation between the frequency sweep rate and the wave amplitude, different from previous theoretical prediction. Discussion Chorus emissions have been extensively characterized in the near-planet regions ( L ≤ 10) and have been believed to be closely related to magnetic dipolar field. Our statistical analysis reveals unexpected ubiquity of chorus waves in the mid-tail (L > 15), where the effect of planetary dipolar fields is negligible. These mid-tail chorus emissions exhibit properties strikingly different from their near-Earth counterparts, due to contrasting magnetic field properties (e.g., |B| ~ 10 nT in the mid tail vs. ~ 100 nT in the near-Earth region) and plasma conditions (plasma β ~ 1 in the mid tail vs. ~0.01 in the near-Earth region). This suggests that chorus generation is controlled by fundamental plasma processes capable of operating across diverse plasma environments, largely independent of local magnetic field geometry. In other words, chorus waves can develop anywhere in space, suggesting that they should be investigated in various regions with different magnetic fields and particle profiles to fully understand how the wave generation and propagation are affected by local physical conditions. Chorus waves in the mid-tail plasma sheet show several important features distinct from those in the near-Earth region. First, chorus waves in the mid tail are primarily falling-tone, different from the near-Earth chorus waves which are mainly rising-tone. Such difference may provide the first observational evidence for the mediation of chorus sweep direction by local magnetic field inhomogeneity, as predicted by recent simulation 31 . Second, the falling-tone chorus waves in the mid tail propagate mostly along magnetic field lines, in contrast with the oblique propagation of falling-tone in the inner magnetosphere. The quasi-parallel propagation of falling-tone chorus waves can also help to explain their dominance in the mid tail since they would experience much weaker Landau damping compared with the obliquely-propagated falling-tone chorus waves in the inner magnetosphere. Such difference in the propagation direction between mid-tail and inner-magnetosphere chorus waves may arise from three facts: 1) the magnetic field lines in the mid-tail are more stretched than in the inner magnetosphere and exhibit weaker gradients, allowing for more field-aligned outward propagation of chorus waves from the source region; 2) the observed falling-tone chorus waves occur mainly near the neutral sheet which is likely close to the wave source region; 3) the mid-tail chorus waves are associated with much lower anisotropy of electron temperature, indicating lower threshold for triggering whistler instability due to higher plasma beta in the mid-tail. Another important difference is the wave sweep rate, which is the hallmark of chorus waves. The mid-tail chorus waves have sweep rates close to ~ 100 Hz/s, approximately an order of magnitude smaller than those (~ 1 kHz/s) of the near-Earth chorus waves. The wave sweep rate, based on the second-order resonance equation, can be controlled by magnetic field inhomogeneity or wave magnetic field amplitude. The magnetic field inhomogeneity and the observed wave magnetic field amplitude are both weaker in the mid tail than in the near-Earth region, hence yielding smaller wave sweep rates. Note that the two controlling factors typically dominate at different stages during the wave evolution: wave magnetic field amplitude is crucial during the wave growth stage, and magnetic field inhomogeneity plays more important role during the wave outward propagation from the source region. Therefore, one may not find a clear correlation between the wave sweep rate and these two factors from observations, since the stage of wave evolution usually cannot be directly determined. In short summary, the present statistical analysis reveals that chorus emissions are ubiquitous in Earth’s magnetotail plasma sheet, and exhibit distinct characteristics compared to those observed in the inner magnetosphere. Using multi-point measurements from MMS, we provide direct quantification of local magnetic field gradients during chorus wave activity, demonstrating that these emissions can be excited in regions characterized by weak field gradients. These results suggest that chorus generation is governed by a fundamental plasma process independent of local magnetic geometry. These findings open a new avenue for studying chorus waves in space and highlight the need to investigate these emissions in regions with non-dipolar magnetic fields, particularly during geomagnetically active periods. These will provide novel insights into understanding wave-particle interactions widely observed throughout the heliosphere. Methods Availability of MMS measurement MMS mission was designed to measure the potential electron-scale physical processes in the Earth’s magnetotail, which have made it possible to measure the statistical properties of chorus emissions in the neutral sheet. In this study, the Fluxgate Magnetometer 44 (FGM) provides background magnetic field measurements at 128 Hz, which are used to determine local electron cyclotron frequencies ( f ce ) for normalizing observed wave frequencies ( f / f ce ). The Search-Coil Magnetometer 45 (SCM) provides high-resolution three-axis measurements of magnetic field fluctuations from 1 Hz to 6 kHz, and the Electric Field Double Probes 46 , 47 (EDP) capture electric field waveforms from DC to 100 kHz in three dimensions. All data are presented in Geocentric Solar Magnetospheric (GSM) coordinates. To estimate the magnetic field gradient, FGM data from the other spacecraft are linearly interpolated in time to align with the MMS1 timestamps. Leveraging high-cadence, four-point measurements from the MMS, the gradient of the magnetic field can be derived under the assumption that the background magnetic field varies linearly across the spacecraft tetrahedron. Given the FGM measurement accuracy of approximately 0.1 nT, the uncertainty in the gradient estimation may become non-negligible when the magnitude of ∇ B ₀ exceeds 0.1 nT divided by the spacecraft separation distance. Event selection The analysis utilizes MMS observations from summer seasons between 2017 and 2020, when the spacecraft traversed the magnetotail. We focus on whistler-mode chorus waves within the characteristic frequency range of 0.1–0.8 f ce (where f ce is the electron gyrofrequency). Magnetic field measurements are obtained from the Fluxgate Magnetometer (FGM), which provides high-cadence data at 128 Hz in burst mode—sufficient to resolve electron gyration frequencies in the magnetotail under typical conditions. All burst-mode data are divided into 30-second intervals, corresponding to the FGM’s positional sampling. We computed wavelet spectrograms based on the Fast Fourier Transform (FFT) algorithm, implementing a Morlet wavelet transform for time-frequency analysis. To distinguish chorus emissions from background noise, we implement a power spectral density (PSD) threshold of 10 − 4 nT 2 /Hz, consistent with established noise-floor levels in prior studies. Chorus events are defined as frequency-varying emissions detected in the spectrograms. For each 30-second window, the 3 seconds before and after the maximum PSD intensity will be identified as a chorus event by visually inspecting. Declarations Author contributions B.N.Z. prepared the manuscript and performed the data processing and analysis. C.M.L. conceived the research and contributed to the manuscript writing. J.B.C. oversaw the research and gave suggestions. Y.Y.L. and J.X.Z. gave suggestions on the manuscript. All authors reviewed the manuscript. Acknowledgements We greatly appreciate the MMS Science Data Center for providing the data and IRFU-MATLAB for providing the analysis codes for this study. This research is supported by NSFC (No. 42522409), Beijing Natural Science Foundation (No.1252024), the Fundamental Research Funds for the Central Universities, and the young talent supporting project of the China Association for Science and Technology. Data Availability Statement The data used in the present study is collected by the NASA’s MMS mission and publicly available at https://lasp.colorado.edu/mms/sdc/public/about/browse-wrapper Code availability All the data plots in this study were generated with the IRFU-MATLAB software applied to the publicly available MMS database. The IRFU-MATLAB software is available for downloading from https://github.com/irfu/irfu-matlab References Kitamura N et al (2018) Direct measurements of two-way wave-particle energy transfer in a collisionless space plasma. Science 361:1000–1003 Gershman DJ et al (2017) Wave-particle energy exchange directly observed in a kinetic Alfvén-branch wave. Nat Commun 8:14719 Allison HJ, Shprits YY, Zhelavskaya IS, Wang D, Smirnov AG (2021) Gyroresonant wave-particle interactions with chorus waves during extreme depletions of plasma density in the Van Allen radiation belts. 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12:09:44","extension":"pdf","order_by":6,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":31446,"visible":true,"origin":"","legend":"","description":"","filename":"fig6.pdf","url":"https://assets-eu.researchsquare.com/files/rs-8191502/v1/5e98d00dc6c8219b6dbcc51f.pdf"},{"id":96995573,"identity":"cb505098-d273-45a6-8097-233a66f565b2","added_by":"auto","created_at":"2025-11-28 12:09:44","extension":"json","order_by":7,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":5856,"visible":true,"origin":"","legend":"","description":"","filename":"NCOMMS2595988T.json","url":"https://assets-eu.researchsquare.com/files/rs-8191502/v1/8780b04986fc0531194fb515.json"},{"id":96995572,"identity":"db3642d3-39fd-433e-9321-8dda8fe2db31","added_by":"auto","created_at":"2025-11-28 12:09:44","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":714232,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eA schematic diagram of the distribution of chorus emissions in Earth's neutral sheet.\u003c/strong\u003e Chorus emissions, characterized by their distinct rising and falling tones, have historically been observed almost exclusively in the near-Earth region. There, strong inhomogeneities in the dipolar magnetic field were considered essential for their generation and propagation. However, this study reveals the unexpected ubiquity of these waves in the mid-tail neutral sheet—a region characterized by a long, stretched magnetic field configuration that starkly contrasts with the dipolar topology. \u003cstrong\u003ea\u003c/strong\u003e, Rising-tone chorus. \u003cstrong\u003eb\u003c/strong\u003e, Falling-tone chorus.\u003c/p\u003e","description":"","filename":"fig171.png","url":"https://assets-eu.researchsquare.com/files/rs-8191502/v1/f238859079d39b652636353d.png"},{"id":96995567,"identity":"b9220760-40b8-4c4c-b5b8-2e0d556de53f","added_by":"auto","created_at":"2025-11-28 12:09:44","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":780138,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eRising-tone and falling-tone events observed in magnetotail neutral sheet.\u003c/strong\u003e \u003cstrong\u003ea\u003c/strong\u003e–\u003cstrong\u003ef\u003c/strong\u003e, A rising-tone chorus event.\u003cstrong\u003e a\u003c/strong\u003e, Power spectral density of magnetic field. \u003cstrong\u003eb\u003c/strong\u003e, Power spectral density of electric field. \u003cstrong\u003ec\u003c/strong\u003e, Ellipticity. \u003cstrong\u003ed\u003c/strong\u003e, Wave normal angle. \u003cstrong\u003ee\u003c/strong\u003e, Planarity. \u003cstrong\u003ef\u003c/strong\u003e, Normalized parallel Poynting flux. \u003cstrong\u003eg\u003c/strong\u003e–\u003cstrong\u003el\u003c/strong\u003e, A falling-tone chorus event with the same set of parameters as in\u003cstrong\u003e a\u003c/strong\u003e–\u003cstrong\u003ef\u003c/strong\u003e. The lines in each panel denote 0.1 and 0.5 \u003cem\u003ef\u003c/em\u003e\u003csub\u003ece\u003c/sub\u003e.\u003c/p\u003e","description":"","filename":"fig172.png","url":"https://assets-eu.researchsquare.com/files/rs-8191502/v1/91c59708c3ade66ebe5814d7.png"},{"id":96995569,"identity":"98d2de11-abc1-4399-8f61-9b23673e0bd8","added_by":"auto","created_at":"2025-11-28 12:09:44","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":780138,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eSpatial distribution of chorus emission occurrence rates in the magnetotail.\u003c/strong\u003e \u003cstrong\u003ea\u003c/strong\u003e, Distribution projected onto the X-Y (equatorial) plane. \u003cstrong\u003eb\u003c/strong\u003e, Distribution projected onto the Z-Y (meridional) plane. The occurrence rates are normalized by spacecraft dwell time.\u003c/p\u003e","description":"","filename":"fig173.png","url":"https://assets-eu.researchsquare.com/files/rs-8191502/v1/376737a791402828fb1bce8c.png"},{"id":96995563,"identity":"05f0b51d-801a-48d1-953d-8136686fa74c","added_by":"auto","created_at":"2025-11-28 12:09:44","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":39421,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eStatistical distributions of chorus wave properties. a\u003c/strong\u003e–\u003cstrong\u003ec\u003c/strong\u003e, Occurrence of the wave normal angle, wave amplitude and wave frequency for falling-tone chorus. \u003cstrong\u003ed\u003c/strong\u003e–\u003cstrong\u003ef\u003c/strong\u003e, Corresponding distributions for rising-tone chorus.\u003c/p\u003e","description":"","filename":"fig174.png","url":"https://assets-eu.researchsquare.com/files/rs-8191502/v1/e61666a30068dc60dc97e364.png"},{"id":97137596,"identity":"1c03b657-08cb-4c18-9665-43687f5000eb","added_by":"auto","created_at":"2025-12-01 09:57:58","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":44463,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eStatistical distributions of plasma parameters during chorus emissions. a,\u003c/strong\u003e \u003cem\u003ex\u003c/em\u003e-component of magnetic field. \u003cstrong\u003eb\u003c/strong\u003e, Electron density. \u003cstrong\u003ec\u003c/strong\u003e, Electron temperature. \u003cstrong\u003ed\u003c/strong\u003e, Ratio of electron plasma frequency to electron cyclotron frequency. \u003cstrong\u003ee\u003c/strong\u003e, Electron temperature anisotropy. \u003cstrong\u003ef\u003c/strong\u003e, Magnetic field gradient.\u003c/p\u003e","description":"","filename":"fig175.png","url":"https://assets-eu.researchsquare.com/files/rs-8191502/v1/33b48ea1de31dcab5e57fb06.png"},{"id":97139775,"identity":"330504e3-3751-4bef-b348-555335ec6408","added_by":"auto","created_at":"2025-12-01 10:02:28","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":65651,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eCorrelations analysis of chorus wave parameters. a\u003c/strong\u003e, Correlation heatmap between chorus wave properties (e.g., amplitude, frequency) and local plasma parameters (e.g., density, temperature). \u003cstrong\u003eb\u003c/strong\u003e, Correlation heatmap among the different wave properties.\u003c/p\u003e","description":"","filename":"fig176.png","url":"https://assets-eu.researchsquare.com/files/rs-8191502/v1/c4eccd713f186840fc701f75.png"},{"id":97144831,"identity":"cb8419c6-5b59-4ed3-b8cd-ff7eb36ecc0a","added_by":"auto","created_at":"2025-12-01 10:12:13","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":3158275,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-8191502/v1/3c9389a1-887f-44c8-9659-5bfae573ff94.pdf"}],"financialInterests":"There is \u003cb\u003eNO\u003c/b\u003e Competing Interest.","formattedTitle":"Unexpected ubiquity of chorus emissions in terrestrial magnetotail","fulltext":[{"header":"Introduction","content":"\u003cp\u003eWave-particle interaction serves as a fundamental energy transfer mechanism between charged particles and electromagnetic fields, governing particle acceleration and loss processes in the solar system\u003csup\u003e1\u0026ndash;4\u003c/sup\u003e. Whistler-mode chorus waves, one of the most intense electromagnetic emissions naturally arising in space, can drive efficient electron acceleration and scattering. These emissions are characterized by their discrete and frequency-varying elements, which have attracted extreme interest since their discovery\u003csup\u003e5\u0026ndash;7\u003c/sup\u003e. Chorus emissions have been documented as key drivers of important magnetospheric phenomena including precipitating electrons to produce diffuse and pulsating auroras\u003csup\u003e8\u0026ndash;11\u003c/sup\u003e, generating plasmaspheric hiss waves\u003csup\u003e12,13\u003c/sup\u003e and accelerating relativistic electrons in radiation belts\u003csup\u003e14\u0026ndash;16\u003c/sup\u003e. They have been extensively studied in terrestrial magnetosphere through \u003cem\u003ein situ\u003c/em\u003e observations and numerical simulations for more than 70 years\u003csup\u003e17\u0026ndash;22\u003c/sup\u003e. Their global distribution, typical properties and parametric dependence have been demonstrated by spacecraft observations in the near-Earth space\u003csup\u003e23\u0026ndash;25\u003c/sup\u003e.\u003c/p\u003e\n\u003cp\u003eNonlinear wave theories have predicted that magnetic field inhomogeneity plays a decisive role in chorus generation, governing the generation of rising-tone and falling-tone emissions\u003csup\u003e26\u0026ndash;28\u003c/sup\u003e. Nonetheless, recent numerical simulations have successfully reproduced chorus emissions in uniform magnetic fields, demonstrating that nonlinear mechanisms keep valid without field inhomogeneity\u003csup\u003e29,30\u003c/sup\u003e. In addition, latest spacecraft observations in mid-tail neutral sheet have found locally-generated chorus waves in regions with negligible magnetic field gradients\u003csup\u003e31\u003c/sup\u003e. Despite these advances, chorus research remains predominantly limited within near-Earth regions dominated by dipolar magnetic fields (\u003cem\u003eL\u003c/em\u003e \u0026lt; 10 \u003cem\u003eR\u003c/em\u003e\u003csub\u003eE\u003c/sub\u003e), which has led to a biased understanding of chorus generation and evolution in space\u0026mdash;largely because of a longstanding belief that chorus generation requires dipolar magnetic field. Resolving the discrepancy between theories, simulations, and observations necessitates comprehensive \u003cem\u003ein situ\u003c/em\u003e measurements of chorus generation under various magnetic field conditions, which can provide critical observational constraints for current theoretical frameworks.\u003c/p\u003e\n\u003cp\u003eHere we have conducted, for the first time, a comprehensive investigation of chorus waves in terrestrial magnetotail using state-of-the-art measurements provided by NASA\u0026rsquo;s Magnetospheric Multiscale (MMS) mission\u003csup\u003e32\u003c/sup\u003e. Despite being primarily designed for magnetic reconnection studies, MMS\u0026rsquo;s exceptional measurement capabilities have enabled us to search for chorus emissions in the magnetotail region\u003csup\u003e33\u003c/sup\u003e. During its 2017\u0026ndash;2020 orbital phase with apogees exceeding 25 \u003cem\u003eR\u003c/em\u003e\u003csub\u003eE\u003c/sub\u003e, MMS eccentric orbits cover terrestrial neutral sheet where magnetic field topology and plasma properties are distinct from those in near-Earth space. Our analysis demonstrates that chorus emissions are surprisingly ubiquitous in the neutral sheet (Fig. 1), with falling-tone emissions dominating, and their characteristic features are significantly different from those in near-Earth region, with larger amplitudes and lower frequency sweep rates. These findings uncover an unexplored regime of chorus waves and provide new observational constraints for theoretical studies of nonlinear wave\u0026ndash;particle interactions in collisionless plasmas.\u003c/p\u003e"},{"header":"Results","content":"\u003cdiv id=\"Sec2\" class=\"Section2\"\u003e\u003ch2\u003eObservational events\u003c/h2\u003e\u003cp\u003eWe have identified, via singular value decomposition (SVD) analysis\u003csup\u003e\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e\u003c/sup\u003e, two typical types of chorus (rising-tone and falling-tone) emissions observed by MMS in Earth's neutral sheet (\u003cem\u003eR\u003c/em\u003e\u003csub\u003eE\u003c/sub\u003e \u0026gt;10), as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e. The rising-tone chorus was detected at [-17.9 -10.6 0.5] \u003cem\u003eR\u003c/em\u003e\u003csub\u003eE\u003c/sub\u003e, and the falling-tone chorus was detected at [-20.4 -12.7 1.3] \u003cem\u003eR\u003c/em\u003e\u003csub\u003eE\u003c/sub\u003e. In the two events, magnetic and electric field power spectral density exhibit clear discrete and frequency-varying elements, with frequency sweep rates close to \u0026plusmn;\u0026thinsp;100 Hz/s (positive for the rising-tone, negative for the falling-tone), approximately an order of magnitude lower than the typical values(~\u0026thinsp;kHz) in the inner magnetosphere. The SVD analysis yields key wave parameters, including ellipticity\u0026thinsp;\u0026asymp;\u0026thinsp;1 (right-handed circular polarization, Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ec and \u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ei), wave normal angles\u0026thinsp;\u0026lt;\u0026thinsp;30\u0026deg; (quasi-parallel propagation relative to background magnetic field, Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ed and \u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ej), and planarity\u0026thinsp;\u0026asymp;\u0026thinsp;1 (highly coherent wave fronts, Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eeand 2k). The normalized parallel Poynting flux resolved by the SVD suggests that the rising-tone chorus waves propagate antiparallel to local magnetic field (i.e., away from the equator since local \u003cem\u003eB\u003c/em\u003e\u003csub\u003ex\u003c/sub\u003e \u0026lt; 0; Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ef), while the falling-tone chorus waves propagate parallel to local magnetic field (i.e., towards the equator since local \u003cem\u003eB\u003c/em\u003e\u003csub\u003ex\u003c/sub\u003e \u0026lt; 0; Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003el). These results provide compelling evidence for local generation of chorus emissions within the neutral sheet.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e\u003ch2\u003eGlobal distribution\u003c/h2\u003e\u003cp\u003eFigure \u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e presents the global distribution of chorus emissions in the magnetotail plasma sheet, with occurrence rates normalized by spacecraft dwell time and projected onto \u003cem\u003eX-Y\u003c/em\u003e and \u003cem\u003eZ-Y\u003c/em\u003e planes. The emissions predominantly occur at \u003cem\u003eX\u003c/em\u003e \u0026lt; -15 \u003cem\u003eR\u003c/em\u003e\u003csub\u003eE\u003c/sub\u003e in the \u003cem\u003eX-Y\u003c/em\u003e plane (left panel), peaking at \u003cem\u003eX\u003c/em\u003e\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\approx\\:\\)\u003c/span\u003e\u003c/span\u003e-25 \u003cem\u003eR\u003c/em\u003e\u003csub\u003eE\u003c/sub\u003e, where magnetic field lines are highly stretched. This is in contrast with the traditional knowledge that chorus waves are usually related to global or local magnetic dipolar fields. This unexpected distribution may stem from the fact that electron perpendicularly-anisotropic distribution (i.e., pancake distribution), which serve as the energy source for whistler-mode waves, also has large occurrence rates near \u003cem\u003eX\u003c/em\u003e\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\approx\\:\\)\u003c/span\u003e\u003c/span\u003e-25 \u003cem\u003eR\u003c/em\u003e\u003csub\u003eE\u003c/sub\u003e\u003csup\u003e35\u003c/sup\u003e. In addition, the chorus waves preferentially occur in the dusk side, in line with the dawn-dusk asymmetry of electron pancake distribution\u003csup\u003e\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e\u003c/sup\u003e. In the \u003cem\u003eZ-Y\u003c/em\u003e plane (right panel), we find that the emissions are concentrated near the equatorial region (-2 \u003cem\u003eR\u003c/em\u003e\u003csub\u003eE\u003c/sub\u003e \u0026lt; \u003cem\u003eZ\u003c/em\u003e \u0026lt; 5 \u003cem\u003eR\u003c/em\u003e\u003csub\u003eE\u003c/sub\u003e), with an apparent north-south asymmetry, showing larger occurrence rates in the northern hemisphere. This north-south asymmetry is also consistent with the global distribution of perpendicularly-anisotropic electrons in the tail. Note that previous studies have established that chorus emissions in the inner magnetosphere originate near the magnetic equator, similar to the distribution of magnetotail chorus waves.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e\n\u003ch3\u003eWave property\u003c/h3\u003e\n\u003cp\u003eWe further examine the statistical distribution of frequency sweep rate, normal angle and magnetic wave amplitude of the rising-tone (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ea-\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ec) and falling-tone (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ed-\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ef) chorus waves. Rising-tone chorus waves generally propagate quasi-parallel to the background magnetic field, with a mean normal angle of 11.25\u0026deg;. Their amplitudes range from 50 to 250 pT, with a mean value of 145.0 pT. The central frequency is below 0.3 \u003cem\u003ef\u003c/em\u003e\u003csub\u003ece\u003c/sub\u003e, with a mean frequency of 0.17 \u003cem\u003ef\u003c/em\u003e\u003csub\u003ece\u003c/sub\u003e. These features are comparable to those of the near-Earth chorus, which are also characterized by parallel propagation, large amplitude and frequency in low-frequency band. Falling-tone chorus waves, however, show similar properties to the rising-tone chorus in terms of normal angle, amplitude and frequency range, making them distinct from those in the inner magnetosphere. In particular, their normal angles remain largely below 30\u0026deg;, with a mean normal angle of 13.37\u0026deg;, different from the highly-oblique propagation of the near-Earth falling-tone chorus\u003csup\u003e\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e\u003c/sup\u003e. Such difference may result from the magnetic field topology in the tail, where magnetic field lines are much longer and more stretched than in the near-Earth region, hence posing distinct effects on the propagation of falling-tone chorus waves.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eIn addition, the mean amplitude of falling-tone chorus is 158.3 pT, with peaks exceeding 400 pT, stronger than the rising-tone waves. This contrasts with the near-Earth observations, where falling-tone chorus are typically weaker than their rising counterparts\u003csup\u003e\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e\u003c/sup\u003e. The mean central frequency of falling-tone chorus is 0.14 \u003cem\u003ef\u003c/em\u003e\u003csub\u003ece\u003c/sub\u003e, close to that of rising-tone chorus. Although the falling-tone chorus hosts similar properties to the rising-tone, their proportions are quite different. The proportion of falling-tone chorus waves reaches 90%, which differs strikingly from the near-Earth chorus statistics, where falling-tone chorus accounts for only\u0026thinsp;~\u0026thinsp;16% of chorus events.\u003csup\u003e\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e\u003c/sup\u003e This striking difference may arise from background magnetic field inhomogeneity, which has been suggested by simulation to play a decisive role in setting the chirping direction\u003csup\u003e\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e\u003c/sup\u003e. Unlike the dipolar magnetic field in the near-Earth region, the field topology of magnetotail may be disturbed as anti-dipolar type, leading to the dominance of falling-tone chorus therein.\u003c/p\u003e\n\u003ch3\u003eParametric dependence\u003c/h3\u003e\n\u003cp\u003eThe properties of magnetotail chorus differ markedly from those observed in the near-Earth region, especially for falling-tone chorus, indicating that background plasma and magnetic field conditions play a crucial role in determining the wave properties. We examine the key plasma parameters during the chorus intervals (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e). In agreement with earlier studies\u003csup\u003e\u003cspan additionalcitationids=\"CR39\" citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e40\u003c/span\u003e\u003c/sup\u003e, chorus waves are concentrated near the magnetic equator (i.e., neutral sheet in the magnetotail), where the \u003cem\u003eB\u003c/em\u003e\u003csub\u003e\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003e is very small (\u0026lt;\u0026thinsp;\u003cem\u003eB\u003c/em\u003e\u003csub\u003ex\u003c/sub\u003e\u0026thinsp;\u0026gt;\u0026thinsp;=\u0026thinsp;1.81 nT; Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ea). The average \u003cem\u003eB\u003c/em\u003e\u003csub\u003ex\u003c/sub\u003e is positive, consistent with the northern-southern asymmetry shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eb. The electron density during the chorus wave interval is low (\u0026lt;\u0026thinsp;\u003cem\u003eN\u003c/em\u003e\u003csub\u003ee\u003c/sub\u003e\u0026thinsp;\u0026gt;\u0026thinsp;=\u0026thinsp;0.32 cm\u003csup\u003e\u0026minus;\u0026thinsp;3\u003c/sup\u003e; Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eb). Note that the electron population in the neutral sheet is mainly hot electrons, different from the electron population in the inner magnetosphere where cold electrons dominate over hot electrons. The two-component (hot and cold) electron population is a key assumption widely used in previous simulations and theoretical studies. The one-component electron population in the neutral sheet thus provides a new background for further chorus research. Nonlinear theory predicted that magnetic field gradient governs the chorus generation and propagation, while the magnetic field gradient is generally weak in the neutral sheet. We have calculated the gradient term in the wave second-order resonance equation, with the aid of MMS four-point satellite measurements, yielding a mean value of -0.01 (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ec). Therefore, chorus can indeed develop in quasi-homogeneous magnetic fields, consistent with recent simulation predictions\u003csup\u003e\u003cspan additionalcitationids=\"CR30\" citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eWe find that during the chorus interval, electron temperature is typically close to several keV with a mean of 1471 eV (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ed), similar to previous findings in the near-Earth region\u003csup\u003e\u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e\u003c/sup\u003e. Most chorus events exhibit electron perpendicular temperature anisotropy \u003cem\u003eA\u003c/em\u003e\u003csub\u003ee\u003c/sub\u003e exceeding 1, fulfilling the criterion for local whistler instability. Nonetheless, \u003cem\u003eA\u003c/em\u003e\u003csub\u003ee\u003c/sub\u003e is not large for most chorus events, typically between 1.0 and 1.2, with a mean of 1.07 (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ef), indicating that in the neutral sheet, weak anisotropy is sufficient to excite whistler-mode waves\u003csup\u003e\u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e\u003c/sup\u003e, unlike in the inner magnetosphere where higher anisotropy is required\u003csup\u003e\u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e43\u003c/span\u003e\u003c/sup\u003e. Another difference between the magnetotail chorus and the inner magnetosphere chorus is plasma-to-cyclotron frequency ratio \u003cem\u003ef\u003c/em\u003e\u003csub\u003epe\u003c/sub\u003e/\u003cem\u003ef\u003c/em\u003e\u003csub\u003ece\u003c/sub\u003e. The typical value of \u003cem\u003ef\u003c/em\u003e\u003csub\u003epe\u003c/sub\u003e/\u003cem\u003ef\u003c/em\u003e\u003csub\u003ece\u003c/sub\u003e ratio is below 5 in the inner magnetosphere, while it typically exceeds 10 in the magnetotail, with a mean of 30.85 (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ee). Overall, these results suggest that chorus emissions in the neutral sheet develop in a different plasma regime compared with those in the inner magnetosphere.\u003c/p\u003e\u003cp\u003eWe investigate the correlation between chorus properties and local plasma parameters. In order to reliably quantify the chorus properties, such as frequency sweep rate and element duration, we have further selected 20 events which show unambiguous, well-defined wave elements. Figure\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e displays a correlation heatmap between the wave properties and the environmental parameters for these 20 events. The wave normal angle and magnetic field amplitude show no clear correlation with local plasma parameters. The central frequency of the chorus waves increases with magnetic field strength (correlation coefficient, cc\u0026thinsp;\u0026asymp;\u0026thinsp;0.52), consistent with wave generation by cyclotron resonance with thermal electrons. The wave central frequency is negatively correlated (cc \u0026asymp; -0.51) with temperature anisotropy, which is not expected from linear cyclotron resonance, where larger temperature anisotropy should yield higher frequency waves. Therefore, the temperature anisotropy may also affect wave nonlinear growth, which is indeed glimpsed from the negative correlation (cc \u0026asymp; -0.43) between frequency sweep rate and temperature anisotropy. Interestingly, we find that the frequency sweep rate is not clearly correlated with local magnetic field gradient, indicating that the observed chorus waves are mainly detected near the source region, where wave generation and propagation are controlled by nonlinear processes. The sweep rate is positively correlated (cc\u0026thinsp;\u0026asymp;\u0026thinsp;0.43) with electron plasma-to-cyclotron ratio, consistent with prediction by nonlinear wave theories. We also find a positive correlation (cc\u0026thinsp;\u0026asymp;\u0026thinsp;0.52) between wave element duration and electron plasma-to-cyclotron ratio. Nevertheless, we find no correlation between the frequency sweep rate and the wave amplitude, different from previous theoretical prediction.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e"},{"header":"Discussion","content":"\u003cp\u003eChorus emissions have been extensively characterized in the near-planet regions (\u003cem\u003eL\u003c/em\u003e\u0026thinsp;\u0026le;\u0026thinsp;10) and have been believed to be closely related to magnetic dipolar field. Our statistical analysis reveals unexpected ubiquity of chorus waves in the mid-tail (L\u0026thinsp;\u0026gt;\u0026thinsp;15), where the effect of planetary dipolar fields is negligible. These mid-tail chorus emissions exhibit properties strikingly different from their near-Earth counterparts, due to contrasting magnetic field properties (e.g., |B| ~ 10 nT in the mid tail vs. ~ 100 nT in the near-Earth region) and plasma conditions (plasma β\u0026thinsp;~\u0026thinsp;1 in the mid tail vs. ~0.01 in the near-Earth region). This suggests that chorus generation is controlled by fundamental plasma processes capable of operating across diverse plasma environments, largely independent of local magnetic field geometry. In other words, chorus waves can develop anywhere in space, suggesting that they should be investigated in various regions with different magnetic fields and particle profiles to fully understand how the wave generation and propagation are affected by local physical conditions.\u003c/p\u003e\u003cp\u003eChorus waves in the mid-tail plasma sheet show several important features distinct from those in the near-Earth region. First, chorus waves in the mid tail are primarily falling-tone, different from the near-Earth chorus waves which are mainly rising-tone. Such difference may provide the first observational evidence for the mediation of chorus sweep direction by local magnetic field inhomogeneity, as predicted by recent simulation\u003csup\u003e\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e\u003c/sup\u003e. Second, the falling-tone chorus waves in the mid tail propagate mostly along magnetic field lines, in contrast with the oblique propagation of falling-tone in the inner magnetosphere. The quasi-parallel propagation of falling-tone chorus waves can also help to explain their dominance in the mid tail since they would experience much weaker Landau damping compared with the obliquely-propagated falling-tone chorus waves in the inner magnetosphere. Such difference in the propagation direction between mid-tail and inner-magnetosphere chorus waves may arise from three facts: 1) the magnetic field lines in the mid-tail are more stretched than in the inner magnetosphere and exhibit weaker gradients, allowing for more field-aligned outward propagation of chorus waves from the source region; 2) the observed falling-tone chorus waves occur mainly near the neutral sheet which is likely close to the wave source region; 3) the mid-tail chorus waves are associated with much lower anisotropy of electron temperature, indicating lower threshold for triggering whistler instability due to higher plasma beta in the mid-tail.\u003c/p\u003e\u003cp\u003eAnother important difference is the wave sweep rate, which is the hallmark of chorus waves. The mid-tail chorus waves have sweep rates close to ~\u0026thinsp;100 Hz/s, approximately an order of magnitude smaller than those (~\u0026thinsp;1 kHz/s) of the near-Earth chorus waves. The wave sweep rate, based on the second-order resonance equation, can be controlled by magnetic field inhomogeneity or wave magnetic field amplitude. The magnetic field inhomogeneity and the observed wave magnetic field amplitude are both weaker in the mid tail than in the near-Earth region, hence yielding smaller wave sweep rates. Note that the two controlling factors typically dominate at different stages during the wave evolution: wave magnetic field amplitude is crucial during the wave growth stage, and magnetic field inhomogeneity plays more important role during the wave outward propagation from the source region. Therefore, one may not find a clear correlation between the wave sweep rate and these two factors from observations, since the stage of wave evolution usually cannot be directly determined.\u003c/p\u003e\u003cp\u003eIn short summary, the present statistical analysis reveals that chorus emissions are ubiquitous in Earth\u0026rsquo;s magnetotail plasma sheet, and exhibit distinct characteristics compared to those observed in the inner magnetosphere. Using multi-point measurements from MMS, we provide direct quantification of local magnetic field gradients during chorus wave activity, demonstrating that these emissions can be excited in regions characterized by weak field gradients. These results suggest that chorus generation is governed by a fundamental plasma process independent of local magnetic geometry. These findings open a new avenue for studying chorus waves in space and highlight the need to investigate these emissions in regions with non-dipolar magnetic fields, particularly during geomagnetically active periods. These will provide novel insights into understanding wave-particle interactions widely observed throughout the heliosphere.\u003c/p\u003e"},{"header":"Methods","content":"\u003cdiv id=\"Sec8\" class=\"Section2\"\u003e\u003ch2\u003eAvailability of MMS measurement\u003c/h2\u003e\u003cp\u003eMMS mission was designed to measure the potential electron-scale physical processes in the Earth\u0026rsquo;s magnetotail, which have made it possible to measure the statistical properties of chorus emissions in the neutral sheet. In this study, the Fluxgate Magnetometer\u003csup\u003e\u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e44\u003c/span\u003e\u003c/sup\u003e (FGM) provides background magnetic field measurements at 128 Hz, which are used to determine local electron cyclotron frequencies (\u003cem\u003ef\u003c/em\u003e\u003csub\u003ece\u003c/sub\u003e) for normalizing observed wave frequencies (\u003cem\u003ef\u003c/em\u003e/\u003cem\u003ef\u003c/em\u003e\u003csub\u003ece\u003c/sub\u003e). The Search-Coil Magnetometer\u003csup\u003e\u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e45\u003c/span\u003e\u003c/sup\u003e (SCM) provides high-resolution three-axis measurements of magnetic field fluctuations from 1 Hz to 6 kHz, and the Electric Field Double Probes\u003csup\u003e\u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e46\u003c/span\u003e,\u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e47\u003c/span\u003e\u003c/sup\u003e (EDP) capture electric field waveforms from DC to 100 kHz in three dimensions. All data are presented in Geocentric Solar Magnetospheric (GSM) coordinates. To estimate the magnetic field gradient, FGM data from the other spacecraft are linearly interpolated in time to align with the MMS1 timestamps. Leveraging high-cadence, four-point measurements from the MMS, the gradient of the magnetic field can be derived under the assumption that the background magnetic field varies linearly across the spacecraft tetrahedron. Given the FGM measurement accuracy of approximately 0.1 nT, the uncertainty in the gradient estimation may become non-negligible when the magnitude of \u0026nabla;\u003cem\u003eB\u003c/em\u003e₀ exceeds 0.1 nT divided by the spacecraft separation distance.\u003c/p\u003e\u003c/div\u003e\n\u003ch3\u003eEvent selection\u003c/h3\u003e\n\u003cp\u003eThe analysis utilizes MMS observations from summer seasons between 2017 and 2020, when the spacecraft traversed the magnetotail. We focus on whistler-mode chorus waves within the characteristic frequency range of 0.1\u0026ndash;0.8 \u003cem\u003ef\u003c/em\u003e\u003csub\u003ece\u003c/sub\u003e (where \u003cem\u003ef\u003c/em\u003e\u003csub\u003ece\u003c/sub\u003e is the electron gyrofrequency). Magnetic field measurements are obtained from the Fluxgate Magnetometer (FGM), which provides high-cadence data at 128 Hz in burst mode\u0026mdash;sufficient to resolve electron gyration frequencies in the magnetotail under typical conditions. All burst-mode data are divided into 30-second intervals, corresponding to the FGM\u0026rsquo;s positional sampling. We computed wavelet spectrograms based on the Fast Fourier Transform (FFT) algorithm, implementing a Morlet wavelet transform for time-frequency analysis. To distinguish chorus emissions from background noise, we implement a power spectral density (PSD) threshold of 10\u003csup\u003e\u0026minus;\u0026thinsp;4\u003c/sup\u003e nT\u003csup\u003e2\u003c/sup\u003e/Hz, consistent with established noise-floor levels in prior studies. Chorus events are defined as frequency-varying emissions detected in the spectrograms. For each 30-second window, the 3 seconds before and after the maximum PSD intensity will be identified as a chorus event by visually inspecting.\u003c/p\u003e"},{"header":"Declarations","content":"\u003ch2\u003eAuthor contributions\u003c/h2\u003e\u003cp\u003eB.N.Z. prepared the manuscript and performed the data processing and analysis. C.M.L. conceived the research and contributed to the manuscript writing. J.B.C. oversaw the research and gave suggestions. Y.Y.L. and J.X.Z. gave suggestions on the manuscript. All authors reviewed the manuscript.\u003c/p\u003e\u003ch2\u003eAcknowledgements\u003c/h2\u003e\u003cp\u003eWe greatly appreciate the MMS Science Data Center for providing the data and IRFU-MATLAB for providing the analysis codes for this study. This research is supported by NSFC (No. 42522409), Beijing Natural Science Foundation (No.1252024), the Fundamental Research Funds for the Central Universities, and the young talent supporting project of the China Association for Science and Technology.\u003c/p\u003e\u003ch2\u003eData Availability Statement\u003c/h2\u003e\u003cp\u003eThe data used in the present study is collected by the NASA\u0026rsquo;s MMS mission and publicly available at \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://lasp.colorado.edu/mms/sdc/public/about/browse-wrapper\u003c/span\u003e\u003cspan address=\"https://lasp.colorado.edu/mms/sdc/public/about/browse-wrapper\" targettype=\"URL\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/p\u003e\u003ch2\u003eCode availability\u003c/h2\u003e\u003cp\u003eAll the data plots in this study were generated with the IRFU-MATLAB software applied to the publicly available MMS database. The IRFU-MATLAB software is available for downloading from \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://github.com/irfu/irfu-matlab\u003c/span\u003e\u003cspan address=\"https://github.com/irfu/irfu-matlab\" targettype=\"URL\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eKitamura N et al (2018) Direct measurements of two-way wave-particle energy transfer in a collisionless space plasma. Science 361:1000\u0026ndash;1003\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eGershman DJ et al (2017) Wave-particle energy exchange directly observed in a kinetic Alfv\u0026eacute;n-branch wave. 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Space Sci Rev 199:167\u0026ndash;188\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"nature-portfolio","isNatureJournal":true,"hasQc":false,"allowDirectSubmit":false,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"","title":"Nature Portfolio","twitterHandle":"","acdcEnabled":false,"dfaEnabled":false,"editorialSystem":"ejp","reportingPortfolio":"","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"","lastPublishedDoi":"10.21203/rs.3.rs-8191502/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-8191502/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eChorus waves are one of the strongest electromagnetic emissions widely occurring in planetary space, and have been documented to play a key role in mediating extreme space climate and driving spectacular auroras. Chorus waves have hitherto been believed to develop typically inside planetary inner magnetospheres, where magnetic dipolar fields dominate and govern their generation and propagation. Here, in contrast to such conventional knowledge, we show that chorus waves are surprisingly ubiquitous in terrestrial neutral sheet where magnetic fields are not dipolar, by surveying years of data from NASA’s Magnetospheric Multiscale spacecraft. We discover that chorus waves in the neutral sheet are mostly falling-tone, unlike inner magnetosphere chorus waves which are predominantly rising-tone, suggesting that global magnetic field topology can mediate the wave pattern. In addition, we find that the falling-tone chorus waves mostly propagate along magnetic field lines, different from obliquely-propagated falling-tone chorus in the inner magnetosphere. We further reveal that local magnetic field inhomogeneity associated with the chorus waves is basically negligible, posing observational constraints for theoretical modeling of chorus waves. These results uncover an unexpected source region for inner-magnetosphere chorus, and provide novel insights into understanding wave-driven space weather and climate.\u003c/p\u003e","manuscriptTitle":"Unexpected ubiquity of chorus emissions in terrestrial magnetotail","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-11-28 12:09:39","doi":"10.21203/rs.3.rs-8191502/v1","editorialEvents":[],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"nature-communications","isNatureJournal":true,"hasQc":false,"allowDirectSubmit":false,"externalIdentity":"NCOMMS","sideBox":"Learn more about [Nature Communications](http://www.nature.com/ncomms/)","snPcode":"","submissionUrl":"https://mts-ncomms.nature.com/","title":"Nature Communications","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"ejp","reportingPortfolio":"Nature Communications","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"a0b28fde-7ee9-4255-b1b3-14afdc29c59f","owner":[],"postedDate":"November 28th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[{"id":58698942,"name":"Earth and environmental sciences/Space physics/Aurora"},{"id":58698943,"name":"Physical sciences/Physics/Plasma physics/Astrophysical plasmas"},{"id":58698944,"name":"Earth and environmental sciences/Planetary science/Geodynamics"},{"id":58698945,"name":"Physical sciences/Astronomy and planetary science/Space physics/Magnetospheric physics"}],"tags":[],"updatedAt":"2026-04-13T15:21:57+00:00","versionOfRecord":[],"versionCreatedAt":"2025-11-28 12:09:39","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-8191502","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-8191502","identity":"rs-8191502","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}