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Richard B. Horne - One of the best experts on this subject based on the ideXlab platform.
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rapid electron acceleration in low density regions of saturn s radiation belt by whistler mode chorus waves
Geophysical Research Letters, 2019Co-Authors: E E Woodfield, Richard B. Horne, J D Menietti, Sarah A Glauert, T F Averkamp, Yuri ShpritsAbstract:Author(s): Woodfield, EE; Glauert, SA; Menietti, JD; Averkamp, TF; Horne, RB; Shprits, YY | Abstract: Electron acceleration at Saturn due to whistler mode chorus waves has previously been assumed to be ineffective; new data closer to the planet show it can be very rapid (factor of 104 flux increase at 1 MeV in 10 days compared to factor of 2). A full survey of chorus waves at Saturn is combined with an improved plasma density model to show that where the plasma frequency falls below the Gyrofrequency additional strong resonances are observed favoring electron acceleration. This results in strong chorus acceleration between approximately 2.5 R S and 5.5 R S outside which adiabatic transport may dominate. Strong pitch angle dependence results in butterfly pitch angle distributions that flatten over a few days at 100s keV, tens of days at MeV energies which may explain observations of butterfly distributions of MeV electrons near L=3. Including cross terms in the simulations increases the tendency toward butterfly distributions.
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gyroresonant interactions between the radiation belt electrons and whistler mode chorus waves in the radiation environments of earth jupiter and saturn a comparative study
Journal of Geophysical Research, 2012Co-Authors: J D Menietti, Xudong Gu, Richard B. HorneAbstract:In the current study we perform a comparative analysis of the gyroresonant interactions of whistler mode waves with radiation belt electrons in the magnetospheres of Earth, Jupiter, and Saturn. Our primary goal is to evaluate the effect of resonant wave-particle interactions with chorus waves and determine whether chorus waves can produce net acceleration or net loss of radiation belt electrons on the outer planets. The ratio of plasma frequency to Gyrofrequency is a key parameter that determines the efficiency of the pitch angle and energy resonant scattering. We present a comparison of statistical maps of the ratio of plasma frequency to Gyrofrequency for Jupiter, Saturn and Earth in terms of radial distance and latitude. Preliminary maps of the plasma frequency to Gyrofrequency ratio and 2D simulations of pitch angle and energy diffusion using the Versatile Electron Radiation Belt (VERB) indicate that the Kronian plasma environment is not likely to support as efficient gyroresonant interactions with whistler mode chorus waves as in the Terrestrial or Jovian environments. Inefficiency of the local acceleration by whistler mode waves in the Kronian environment raises important questions about the origin of the relativistic electrons in the Saturn's radiation belts. Two-dimensional diffusive simulations of local acceleration and loss to the atmosphere using the VERB code confirm previous suggestions that the acceleration of electrons may be very efficient in the outer radiation belt of Jupiter. However, sensitivity simulations also show that the result of the competition between acceleration and loss in the Jupiter's magnetosphere strongly depends on the currently unknown latitudinal distribution of chorus waves that will be provided by the upcoming Juno mission. If waves extend to high latitudes, it is likely that the loss rates due to whistler mode waves will exceed energization rates.
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global simulation of magnetosonic wave instability in the storm time magnetosphere
Journal of Geophysical Research, 2010Co-Authors: Lunjin Chen, R M Thorne, V K Jordanova, Richard B. HorneAbstract:Coupling between the Rice Convection Model and Ring Current-Atmospheric Interactions Model codes is used to simulate the dynamical evolution of ring current ion phase space density and the thermal electron density distribution for the 22 April 2001 storm. The simulation demonstrates that proton ring distributions (df(perpendicular to)/dv(perpendicular to) > 0) develop over a broad spatial region during the storm main phase, leading to the instability of equatorial magnetosonic waves. Calculations of the convective growth rate of magnetosonic waves for multiples of the proton Gyrofrequency from 2 to 42 are performed globally. We find that the ratio between the perpendicular ring velocity and the equatorial Alfven speed determines the frequency range of unstable magnetosonic waves. Low harmonic waves (omega 20 Omega(H+)) are excited over a broad spatial region of low density outside the morningside plasmasphere
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favored regions for chorus driven electron acceleration to relativistic energies in the earth s outer radiation belt
Geophysical Research Letters, 2003Co-Authors: Nigel P Meredith, Richard B. Horne, R R AndersonAbstract:[1] Pitch angle and energy diffusion rates for scattering by whistler-mode chorus waves are proportional to the wave magnetic field intensity and are strongly dependent on the frequency distribution of the waves and to the ratio between the electron plasma frequency (f(pe)) and the electron Gyrofrequency (f(ce)). Relativistic electrons interact most readily with lower-band chorus (0.1 300 nT). Enhanced waves in these regions could play a major role in electron acceleration to relativistic energies during periods of prolonged substorm activity.
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whistler absorption and electron heating near the plasmapause
Journal of Geophysical Research, 1996Co-Authors: R M Thorne, Richard B. HorneAbstract:Using the HOTRAY code, we demonstrate that lightning-generated whistlers which enter the magnetosphere over a broad range of latitudes (Δλ ≈ 5°) just inside the plasmapause are strongly focused by the steep plasma density gradient into a narrow range of L shells near the equatorial region. The wave normal angle also remains closely aligned (±20°) with the magnetic field direction along the entire ray path. Under such conditions, Landau resonance is relatively unimportant, and the wave amplitude is controlled by cyclotron resonant interactions with energetic electrons. All waves with frequencies comparable to or larger than one third of the equatorial electron Gyrofrequency can be strongly absorbed by resonant electrons, leading to electron heating perpendicular to the ambient magnetic field at energies above 100 eV. Consequently, in the presence of this strongly focused source of wave energy, the electron distribution should evolve toward a marginally stable anisotropic equilibrium distribution with T⊥>T‖. In order to simulate this perpendicular heating, we allow the anisotropy of the electron distribution to evolve so that damping is minimized at a frequency of 5 kHz, corresponding to the peak in the power spectrum of spherics above the ionosphere. When the plasmapause is located at Lp = 4.5, whistlers above 4 kHz experience more than 20 dB attenuation owing mainly to cyclotron resonance with 0.1 to 1 keV electrons near the equator. It is unlikely that these waves would be detectable on the ground. This attenuation will produce an upper cutoff in the whistler frequency considerably below one half the equatorial electron Gyrofrequency for waves that are guided along the plasmapause. In contrast, lower-frequency whistlers (ƒ ≈ 1–3 kHz) should be amplified by the anisotropic electron population; such waves are able to propagate to the conjugate ionosphere and thus be detected on the ground. This energy transfer between whistlers and cyclotron resonant electrons is relatively unimportant when Lp ≤ 3.0, but it should become significant for Lp ≥ 4.5.
R M Thorne - One of the best experts on this subject based on the ideXlab platform.
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statistical distribution of emic wave spectra observations from van allen probes
Geophysical Research Letters, 2016Co-Authors: Xiaojia Zhang, Vassillis Angelopoulos, Jacob Bortnik, W. Li, W S Kurth, C A Kletzing, R M Thorne, G B HospodarskyAbstract:It has been known that electromagnetic ion cyclotron (EMIC) waves can precipitate ultrarelativistic electrons through cyclotron resonant scattering. However, the overall effectiveness of this mechanism has yet to be quantified, because it is difficult to obtain the global distribution of EMIC waves that usually exhibit limited spatial presence. We construct a statistical distribution of EMIC wave frequency spectra and their intensities based on Van Allen Probes measurements from September 2012 to December 2015. Our results show that as the ratio of plasma frequency over electron Gyrofrequency increases, EMIC wave power becomes progressively dominated by the helium band. There is a pronounced dawn-dusk asymmetry in the wave amplitude and the frequency spectrum. The frequency spectrum does not follow the commonly used single-peak Gaussian function. Incorporating these realistic EMIC wave frequency spectra into radiation belt models is expected to improve the quantification of EMIC wave scattering effects in ultrarelativistic electron dynamics.
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earthward flows in mid-tail plasma sheet
2016Co-Authors: J. Liang, R M Thorne, C. M. Cully, E. F. Donovan, V AngelopoulosAbstract:Abstract. In this study we perform a statistical survey of the extremely-low-frequency wave activities associated with fast earthward flows in the mid-tail central plasma sheet (CPS) based upon THEMIS measurements. We reveal clear trends of increasing wave intensity with flow enhancement over a broad frequency range, from below fLH (lower-hybrid resonant frequency) to above fce (electron Gyrofrequency). We mainly investigate two electromagnetic wave modes, the lower-hybrid waves at frequencies below fLH, and the whistler-mode waves in the frequency range fLH <f <fce. The waves at f <fLH dramatically intensify during fast flow intervals, and tend to contain strong electromagnetic compo-nents in the high-plasma-beta CPS region, consistent with the theoretical expectation of the lower-hybrid drift insta-bility in the center region of the tail current sheet. UL
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statistical results describing the bandwidth and coherence coefficient of whistler mode waves using themis waveform data
Journal of Geophysical Research, 2014Co-Authors: J Bortnik, R M Thorne, Xinliang Gao, V Angelopoulos, Xin Tao, S WangAbstract:The bandwidths and coherence coefficients of lower band whistler mode waves are analyzed using Time History of Events and Macroscale Interactions during Substorms (THEMIS) waveform data for rising tones, falling tones, and hiss-like emissions separately. We also evaluate their dependences on the spatial location, electron density, the ratio of plasma frequency to local electron Gyrofrequency (fpe/fce), and the wave amplitude. Our results show that the bandwidth normalized by the local electron Gyrofrequency (fce) of rising and falling tones is very narrow (~0.01 fce), smaller than that of the hiss-like emissions (~0.025 fce). Meanwhile, the normalized bandwidth of discrete emissions gradually decreases with increasing wave amplitude, whereas that of hiss-like emissions increases slowly. The coherence coefficient of rising and falling tones is extremely large (~1), while the coherence coefficient of hiss-like emissions is smaller but is still larger than 0.5. For all categories of whistler mode waves, the normalized bandwidth increases at larger L shells. Furthermore, the normalized bandwidth is positively correlated with local fpe/fce but is inversely correlated with the electron density. Interactions between radiation belt electrons and whistler mode waves have been widely described by quasi-linear diffusion theory. Our results suggest that although quasi-linear theory is not entirely applicable for modeling electron interactions with rising and falling tones due to their narrow bandwidth and high coherence coefficient, it is suitable to treat wave-particle interactions between electrons and low-amplitude hiss-like emissions. Moreover, the correlations between the normalized bandwidth of chorus waves (especially the discrete emissions) and other parameters may provide insights for the generation mechanism of chorus waves.
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global simulation of magnetosonic wave instability in the storm time magnetosphere
Journal of Geophysical Research, 2010Co-Authors: Lunjin Chen, R M Thorne, V K Jordanova, Richard B. HorneAbstract:Coupling between the Rice Convection Model and Ring Current-Atmospheric Interactions Model codes is used to simulate the dynamical evolution of ring current ion phase space density and the thermal electron density distribution for the 22 April 2001 storm. The simulation demonstrates that proton ring distributions (df(perpendicular to)/dv(perpendicular to) > 0) develop over a broad spatial region during the storm main phase, leading to the instability of equatorial magnetosonic waves. Calculations of the convective growth rate of magnetosonic waves for multiples of the proton Gyrofrequency from 2 to 42 are performed globally. We find that the ratio between the perpendicular ring velocity and the equatorial Alfven speed determines the frequency range of unstable magnetosonic waves. Low harmonic waves (omega 20 Omega(H+)) are excited over a broad spatial region of low density outside the morningside plasmasphere
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relativistic electron pitch angle scattering by electromagnetic ion cyclotron waves during geomagnetic storms
Journal of Geophysical Research, 2003Co-Authors: Danny Summers, R M ThorneAbstract:During magnetic storms, relativistic electrons execute nearly circular orbits about the Earth and traverse a spatially confined zone within the duskside plasmapause where electromagnetic ion cyclotron (EMIC) waves are preferentially excited. We examine the mechanism of electron pitch-angle diffusion by gyroresonant interaction with EMIC waves as a cause of relativistic electron precipitation loss from the outer radiation belt. Detailed calculations are carried out of electron cyclotron resonant pitch-angle diffusion coefficients Dααfor EMIC waves in a multi-ion (H+, He+, O+) plasma. A simple functional form for Dαα is used, based on quasi-linear theory that is valid for parallel-propagating, small-amplitude electromagnetic waves of general spectral density. For typical observed EMIC wave amplitudes (l-10nT), the rates of resonant pitch-angle diffusion are close to the limit of "strong" diffusion, leading to intense electron precipitation. In order for gyroresonance to take place, electrons must possess a minimum kinetic energy Emin which depends on the value of the ratio (electron plasma frequency/ electron Gyrofrequency); Emin also depends on the properties of the EMIC wave spectrum and the ion composition. Geophysically interesting scattering, with Emin comparable to 1 MeV, can only occur in regions where (electron plasma frequency/electron Gyrofrequency) ≥ 10, which typically occurs within the duskside plasmapause. Under such conditions, electrons with energy ≥ 1 MeV can be removed from the outer radiation belt by EMIC wave scattering during a magnetic storm over a time-scale of several hours to a day.
Herbert O Funsten - One of the best experts on this subject based on the ideXlab platform.
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generation of extremely low frequency chorus in van allen radiation belts
Journal of Geophysical Research, 2017Co-Authors: Fuliang Xiao, Chang Yang, Qinghua Zhou, H E Spence, G D Reeves, Zhenpeng Su, Zhaoguo He, Yihua He, D N Baker, Herbert O FunstenAbstract:Recent studies have shown that chorus can efficiently accelerate the outer radiation belt electrons to relativistic energies. Chorus, previously often observed above 0.1 equatorial electron Gyrofrequency fce, was generated by energetic electrons originating from Earth's plasmasheet. Chorus below 0.1 fce has seldom been reported until the recent data from Van Allen Probes but its origin has not been revealed so far. Because electron resonant energy can approach the relativistic level at extremely low frequency relativistic effects should be considered in the formula for whistler-mode wave growth rate. Here we report high-resolution observations during the 14 October 2014 small storm and firstly demonstrate, using a fully relativistic simulation, that electrons with the high energy tail population and relativistic pitch angle anisotropy can provide free energy sufficient for generating chorus below 0.1 fce. The simulated wave growth displays a very similar pattern to the observations. The current results can be applied to Jupiter, Saturn and other magnetized planets.
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in situ evidence of the modification of the parallel propagation of emic waves by heated he ions
Journal of Geophysical Research, 2016Co-Authors: Zhigang Yuan, Xiongdong Yu, Haimeng Li, John R Wygant, Dedong Wang, Shiyong Huang, Zheng Qiao, Tao Yu, Herbert O FunstenAbstract:With observations of the Van Allen Probe B, we report in situ evidence of the modification of the parallel propagating electromagnetic ion cyclotron (EMIC) waves by heated He+ ions. In the outer boundary of the plasmasphere, accompanied with the He+ ion heating, the frequency bands of H+ and He+ for EMIC waves merged into each other, leading to the disappearance of a usual stop band between the Gyrofrequency of He+ ions (ΩHe+) and the H+ cut-off frequency (ωH+co) in the cold plasma. Moreover, the dispersion relation for EMIC waves theoretically calculated with the observed plasma parameters also demonstrates that EMIC waves can indeed parallel propagate across ΩHe+. Therefore, the paper provides an in situ evidence of the modification of the parallel propagation of EMIC waves by heated He+ ions.
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chorus acceleration of radiation belt relativistic electrons during march 2013 geomagnetic storm
Journal of Geophysical Research, 2014Co-Authors: Fuliang Xiao, G B Hospodarsky, Herbert O Funsten, W S Kurth, C A Kletzing, Chang Yang, Qinghua Zhou, H E Spence, G D Reeves, J B BlakeAbstract:The recent launching of Van Allen probes provides an unprecedent opportunity to investigate variations of the radiation belt relativistic electrons. During the 17-19 March 2013 storm, the Van Allen probes simultaneously detected strong chorus waves and substantial increases in fluxes of relativistic (2 - 4.5 MeV) electrons around L = 4.5. Chorus waves occurred within the lower band 0.1-0.5fce (the electron equatorial Gyrofrequency), with a peak spectral density approximate to 10-4 nT2/Hz. Correspondingly, relativistic electron fluxes increased by a factor of 102-103 during the recovery phase compared to the main phase levels. By means of a Gaussian fit to the observed chorus spectra, the drift and bounce-averaged diffusion coefficients are calculated and then used to solve a 2-D Fokker-Planck diffusion equation. Numerical simulations demonstrate that the lower-band chorus waves indeed produce such huge enhancements in relativistic electron fluxes within 15 h, fitting well with the observation.
Devrie S Intriligator - One of the best experts on this subject based on the ideXlab platform.
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a reexamination of interstellar ion waves previously identified in pioneer 10 magnetic field data
Geophysical Research Letters, 1998Co-Authors: Neil Murphy, E J Smith, Joyce Wolf, Devrie S IntriligatorAbstract:Pioneer 10 magnetic field measurements, believed to include waves associated with the pick-up of interstellar hydrogen atoms, have been further analyzed. Power spectra and waveform data reveal that the signals previously interpreted as waves are an artifact. They resulted from a very low telemetry/sampling rate, associated with the large distance of Pioneer 10 from Earth, which caused aliasing of a sinusoidal signal at the spacecraft spin frequency to a low frequency just above the proton Gyrofrequency. Thus the Pioneer 10 plasma analyzer evidence for interstellar pickup ions must be considered separately from the magnetometer data: The validity of the plasma measurements is not affected by this data artifact.
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interstellar pickup h ions at 8 3 au pioneer 10 plasma and magnetic field analyses
Geophysical Research Letters, 1996Co-Authors: Devrie S Intriligator, G L Siscoe, David W MillerAbstract:Analysis of Pioneer 10 plasma and magnetic field observations at 8.3 AU in 1975 provides new evidence for the presence of interstellar pickup hydrogen (H + ) ions. Use of plasma sensors that look far from the solar wind direction confirms the spherical shell distribution of the pickup ions in velocity space. Phase space density and flux estimates are closely consistent with those from Ulysses measured under similar conditions. Power spectral analyses of the magnetic field data show a distinct signal a little above the proton Gyrofrequency, consistent with the presence of Doppler-shifted ion-cyclotron waves generated by H + pickup ions. These results show that the Pioneer data set has the potential for systematic studies of the global properties of interstellar pickup ions.
G B Hospodarsky - One of the best experts on this subject based on the ideXlab platform.
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statistical distribution of emic wave spectra observations from van allen probes
Geophysical Research Letters, 2016Co-Authors: Xiaojia Zhang, Vassillis Angelopoulos, Jacob Bortnik, W. Li, W S Kurth, G B HospodarskyAbstract:It has been known that electromagnetic ion cyclotron (EMIC) waves can precipitate ultrarelativistic electrons through cyclotron resonant scattering. However, the overall effectiveness of this mechanism has yet to be quantified, because it is difficult to obtain the global distribution of EMIC waves that usually exhibit limited spatial presence. We construct a statistical distribution of EMIC wave frequency spectra and their intensities based on Van Allen Probes measurements from September 2012 to December 2015. Our results show that as the ratio of plasma frequency over electron Gyrofrequency increases, EMIC wave power becomes progressively dominated by the helium band. There is a pronounced dawn-dusk asymmetry in the wave amplitude and the frequency spectrum. The frequency spectrum does not follow the commonly used single-peak Gaussian function. Incorporating these realistic EMIC wave frequency spectra into radiation belt models is expected to improve the quantification of EMIC wave scattering effects in ultrarelativistic electron dynamics.
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statistical distribution of emic wave spectra observations from van allen probes
Geophysical Research Letters, 2016Co-Authors: Xiaojia Zhang, Vassillis Angelopoulos, Jacob Bortnik, W. Li, W S Kurth, C A Kletzing, R M Thorne, G B HospodarskyAbstract:It has been known that electromagnetic ion cyclotron (EMIC) waves can precipitate ultrarelativistic electrons through cyclotron resonant scattering. However, the overall effectiveness of this mechanism has yet to be quantified, because it is difficult to obtain the global distribution of EMIC waves that usually exhibit limited spatial presence. We construct a statistical distribution of EMIC wave frequency spectra and their intensities based on Van Allen Probes measurements from September 2012 to December 2015. Our results show that as the ratio of plasma frequency over electron Gyrofrequency increases, EMIC wave power becomes progressively dominated by the helium band. There is a pronounced dawn-dusk asymmetry in the wave amplitude and the frequency spectrum. The frequency spectrum does not follow the commonly used single-peak Gaussian function. Incorporating these realistic EMIC wave frequency spectra into radiation belt models is expected to improve the quantification of EMIC wave scattering effects in ultrarelativistic electron dynamics.
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chorus acceleration of radiation belt relativistic electrons during march 2013 geomagnetic storm
Journal of Geophysical Research, 2014Co-Authors: Fuliang Xiao, G B Hospodarsky, Herbert O Funsten, W S Kurth, C A Kletzing, Chang Yang, Qinghua Zhou, H E Spence, G D Reeves, J B BlakeAbstract:The recent launching of Van Allen probes provides an unprecedent opportunity to investigate variations of the radiation belt relativistic electrons. During the 17-19 March 2013 storm, the Van Allen probes simultaneously detected strong chorus waves and substantial increases in fluxes of relativistic (2 - 4.5 MeV) electrons around L = 4.5. Chorus waves occurred within the lower band 0.1-0.5fce (the electron equatorial Gyrofrequency), with a peak spectral density approximate to 10-4 nT2/Hz. Correspondingly, relativistic electron fluxes increased by a factor of 102-103 during the recovery phase compared to the main phase levels. By means of a Gaussian fit to the observed chorus spectra, the drift and bounce-averaged diffusion coefficients are calculated and then used to solve a 2-D Fokker-Planck diffusion equation. Numerical simulations demonstrate that the lower-band chorus waves indeed produce such huge enhancements in relativistic electron fluxes within 15 h, fitting well with the observation.