Found 26 entries in the Bibliography.
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Abstract In evaluating the effects of fast magnetosonic (MS) waves on magnetospheric particles, their magnetic spectra are often obtained from satellite observations, while electric field components are usually derived under the cold plasma approximation. However, such an approximation has not been verified with in situ observations yet. In this paper, we report the electromagnetic characteristic for MS waves in various plasma environments with observations of the Van Allen Probe A. It is found that a considerable number of observed MS waves consist of dominated electrostatic components, which also suggest the importance of inspecting the estimation algorithm for the electric field components. Moreover, the comparison between results from statistical and theoretical analysis shows that electromagnetic characteristics of the observed MS waves can be well predicted by cold plasma theory. Our result indicates the validation of cold plasma approximation to estimate the electric field components of MS waves from their magnetic counterparts in the inner magnetosphere.
Published by: Journal of Geophysical Research: Space Physics Published on: 08/2021
YEAR: 2021   DOI: https://doi.org/10.1029/2021JA029759
Abstract We report a simultaneous observation of two band electromagnetic ion cyclotron (EMIC) waves and toroidal Alfvén waves by the Van Allen Probe mission. Through wave frequency analyses, the mass density ρ is found to be locally peaked at the magnetic equator. Perpendicular fluxes of ions (< 100 eV) increase simultaneously with the appearances of EMIC waves, indicating a heating of these ions by EMIC waves. In addition, the measured ion distributions also support the equatorial peak formation, which accords with the result of the frequency analyses. The formation of local mass density peaks at the equator should be due to enhancements of equatorial ion concentrations, which are triggered by EMIC waves’ perpendicular heating on low energy ions.
Published by: Earth and Planetary Physics Published on: 03/2021
YEAR: 2021   DOI: https://doi.org/10.26464/epp2021008
Abstract In this paper, analytical approximation is used to solve the wave equations near the plasmapause boundary layer in order to examine the validity of ray tracing approach for fast magnetosonic (MS) wave propagations, and then analytical solutions for electromagnetic fields of MS waves through the plasmapause boundary layer are provided for the first time. Both theoretical calculations from the analytical expressions and observations of Van Allen Probes have indicated that electric fields of MS waves decrease rapidly but magnetic fields increase rapidly when propagating across the plasmapause boundary layer from the outside. Considering effects of width of the plasmapause and wave frequency, parameter analysis has shown that when the width of the plasmapause boundary layer is narrow in comparison with the wavelength of MS waves, a significant part of waves will be reflected. In these circumstances, the WKB approximation and then ray tracing method might become invalid, and analytical approach provided in this paper could be utilized to solve the wave equation.
Published by: Journal of Geophysical Research: Space Physics Published on: 02/2021
YEAR: 2021   DOI: https://doi.org/10.1029/2020JA028330
Van Allen Probes (VAPs) and multiple ground-based stations simultaneously observed prompt emergences and disappearances of electromagnetic ion cyclotron (EMIC) waves driven by the sequentially enhanced solar wind dynamic pressure in the dayside inner magnetosphere on 6 November 2015. The measured hot protons (> 60 keV) display enhancements of perpendicular temperature during compressions, which provides sufficient temperature anisotropies for the EMIC wave generation so that the calculated linear growth rate also agrees well with the observed wave spectrum. There are bidirectionally propagating EMIC waves observed by VAPs at off equator regions (MLAT from ∼ 13° to ∼ 18°), which indicates local wave excitation under the compressions’ impact. The quick responses of waves and particle distributions to the compressions and decompressions at multiple points in the dayside suggest that the external pressure pulses can be a direct driver for the inner magnetospheric wave evolution and energetic particle dynamics.
Published by: Geophysical Research Letters Published on: 12/2020
YEAR: 2020   DOI: https://doi.org/10.1029/2020GL091479
In this study, the excitation of narrowband fast magnetosonic (MS) waves near the lower hybrid resonance frequency (fLHR) has been investigated with observations from Van Allen Probes mission and linear growth theory. A typical wave event is first examined to show that these waves can be excited through linear instabilities driven by partial shell distributions of protons. Then it is found that these narrowband MS waves from 188 wave events observed by the Van Allen Probe A between January 1, 2013 to December 31, 2015 have central frequencies around 0.7fLHR with a bandwidth of 0.2fLHR. In addition, these waves are observed mainly in the dayside and dusk sectors outside the plasmapause, which is different from previously reported results. Moreover, the simultaneously observed energetic protons during wave activities show that the ratios of the ring speed Vr to the Alfvén speed VA mainly fall into the range of 0.8 < Vr/VA < 1, and this preferred condition for excitations of narrowband MS waves near fLHR is also verified by a parameter analysis of calculating linear wave growth rates combined with wave observations.
Published by: Journal of Geophysical Research: Space Physics Published on: 12/2020
YEAR: 2020   DOI: https://doi.org/10.1029/2020JA028158
Based on observations of Van Allen Probe-A during the period from 19 September 2012 to 28 February 2016, the relations of the fast magnetosonic (MS) wave amplitude Bw with kp index, the wave normal angle (WNA), and the wave normalized frequency (norF) are presented. Then, we establish an analytical regression model for MS wave amplitude as a function of geomagnetic storm activity (presented by kp index), L-shell (L), magnetic local time (MLT), magnetic latitude (λ), and the characteristics of MS wave, that is, wave norF and WNA. From the analytical Bw models, we found MS wave amplitude Bw has a positive relation with the intensity of geomagnetic activities both inside and outside the plasmapause, while the Bw can reach higher values inside the plasmapause than it does outside the plasmapause as the kp index increases. The Bw distribution on the norF demonstrates that most of the wave energies are concentrated on the lower harmonics part, which results from the excitation mechanism of MS waves. In addition, the Bw distribution on the WNA shows that the waves with larger normal angles have higher values of wave amplitude. Our analytic MS wave model agrees with the observed distribution in 3-D space of L, MLT, and λ well with high value of determine coefficient R2. The extended λ dimension will help us to calculate the more accurate bounced averaged diffusion coefficients during particles transit time.
Published by: Journal of Geophysical Research: Space Physics Published on: 10/2020
YEAR: 2020   DOI: https://doi.org/10.1029/2020JA028527
In this letter, we present unique conjugated satellite observations of ionospheric signatures of ring current (RC) ions scattered by fast magnetosonic (MS) waves. In the plasmasphere, the Van Allen Probe in situ observed MS waves. At ionospheric altitudes, the NOAA 16 satellite at the footprint of Van Allen Probe simultaneously observed obvious enhancements of mirroring RC ions, but no obvious variations of precipitating RC ions at subauroral latitudes. Theoretical calculations of pitch angle diffusion coefficients for RC ions confirm that observed MS waves can lead to flux enhancements only for mirroring but not for precipitating RC ions, which is in agreement with the observations of NOAA 16. Our result provides a direct link between in situ inner magnetospheric observations of MS waves and conjugated ionospheric observations of flux enhancements for mirroring RC ions caused by MS waves so as to reveal the ionospheric signature of RC ions scattered by MS waves.
Published by: Geophysical Research Letters Published on: 08/2020
YEAR: 2020   DOI: https://doi.org/10.1029/2020GL089032
In this paper, we present evolutions of the phase space density (PSD) spectra of ring current (RC) ions based on observations made by Van Allen Probe B during a geomagnetic storm on 23–24 August 2016. By analyzing PSD spectra ratios from the initial phase to the main phase of the storm, we find that during the main phase, RC ions with low magnetic moment μ values can penetrate deeper into the magnetosphere than can those with high μ values, and that the μ range of PSD enhancement meets the relationship: S(O+) > S(He+) > S(H+). Based on simultaneously observed ULF waves, theoretical calculation suggests that the radial transport of RC ions into the deep inner magnetosphere is caused by drift-bounce resonance interactions, and the efficiency of these resonance interactions satisfies the relationship: η(O+) > η(He+) > η(H+), leading to the differences in μ range of PSD enhancement for different RC ions. In the recovery phase, the observed decay rates for different RC ions meet the relationship: R(O+) > R(He+) > R(H+), in accordance with previous theoretical calculations, i.e., the charge exchange lifetime of O+ is shorter than those of H+ and He+.
Published by: Earth and Planetary Physics Published on: 03/2020
YEAR: 2020   DOI: 10.26464/epp2020019
Fast magnetosonic (MS) waves can play an important role in the evolution of the inner magnetosphere. However, there is still not an effective method to quantitatively identify such waves for observations of the Van Allen Probes reasonably. In this paper, we used Van Allen Probes data from 18 September 2012 to 30 September 2014 to find a more comprehensive automatic detection algorithm for fast MS waves through statistical analysis of the major properties, including the planarity, ellipticity, and wave normal angle of whole fluctuations using the singular value decomposition method. According to a control variate method, we find an obvious difference between fast MS waves and other waves in the statistical distribution of their major properties. After eliminating the influence of background noises, by excluding fluctuations at L < 1.8, we set up an automatic detection algorithm applied to fast MS waves, that is, smaller than 0.2 for the absolute value of wave ellipticity, larger than 70\textdegree for the wave normal angle, with frequency range of 2 Hz to 1.5 fLHR (fLHR is the local lower hybrid resonance frequency). Finally, we have checked the planarity to verify availability of this method and tested this completely automatic method on the Van Allen Probes data and found some results consistent with previous studies. Inside the plasmapause, we found that there is a more obviously favorable occurrence of MS waves at dusk sector with increasing magnetic latitudes.
Published by: Journal of Geophysical Research: Space Physics Published on: Apr-05-2021
YEAR: 2019   DOI: 10.1029/2018JA026387
In this letter, detailed evolution process of parallel electromagnetic ion cyclotron waves in the inner magnetosphere has been investigated through quasilinear theory. A new saturation has been found to occur after the usual first saturation. During the interval between these two saturations, the energy transfers from H+ band to He+ band electromagnetic ion cyclotron waves. Moreover, through a best fitting, we obtain new model parameters for the anisotropy-beta inverse relation of hot H+, which identifies the threshold of ion cyclotron instabilities in the inner magnetosphere. In situ observations of the Van Allen Probe mission also verify these new model parameters. Therefore, our results reveal the evolution process and saturation characteristics of parallel electromagnetic ion cyclotron waves in the inner magnetosphere.
Published by: Geophysical Research Letters Published on: 07/2019
YEAR: 2019   DOI: 10.1029/2019GL083630
Low-frequency chorus emissions have recently attracted much attention due to the suggestion that they may play important roles in the dynamics of the Van Allen Belts. However, the mechanism (s) generating these low-frequency chorus emissions have not been well understood. . In this letter, we report an interesting case in which background plasma density lowered the lower cutoff frequency of chorus emissions from above 0.1 f ce (typical ordinary chorus) to 0.02 f ce (extremely low-frequency chorus). Those extremely low-frequency chorus waves were observed in a rather dense plasma, where the number density N e was found to be several times larger than has been associated with observations of ordinary chorus waves. For suprathermal electrons whose free energy is supplied by anisotropic temperatures, linear growth rates (calculated using in-situ plasma parameters measured by the Van Allen Probes) show that whistler mode instability can occur at frequencies below 0.1 f ce when the background plasma density N e increases. Especially when N e reaches 90 cm\textendash3 or more, the lowest unstable frequency can extend to 0.02 f ce or even less, which is consistent with satellite observations. Therefore, our results demonstrate that a dense background plasma could play an essential role in the excitation of extremely low-frequency chorus waves by controlling the wave growth rates.
Published by: Earth and Planetary Physics Published on: 02/2019
YEAR: 2019   DOI: 10.26464/epp2019001
Electromagnetic ion cyclotron waves and fast magnetosonic waves are found to be simultaneously modulated by background plasma density: both kinds of waves were observed in high plasma density regions but vanished in low density regions. Theoretical analysis based on Snell\textquoterights law and linear growth theory have been utilized to investigate the physical mechanisms driving such modulation. It is suggested that the modulation of fast magnetosonic waves might be due to trapping by plasma density structures, which results from a conservation of the parameter Q during their propagation. Here Q = nrsinψ, with n the refractive index, r the radial distance, and ψ the wave azimuthal angle. As for electromagnetic ion cyclotron waves, the modulation might be owed to the ion composition difference between different plasma density regions. Our results indicate the alternative mechanism for simultaneous appearance of electromagnetic ion cyclotron waves and fast magnetosonic waves (rather than wave excitations of both two wave emissions), which might take combined effects on the evolution of radiation belt electrons.
Published by: Journal of Geophysical Research: Space Physics Published on: 02/2019
YEAR: 2019   DOI: 10.1029/2018JA026149
With observations of Van Allen Probe A, in this letter we display a typical event where banded whistler waves shifted up their frequencies with frequency bands broadening as a response to the enhancement of solar wind dynamic pressure. Meanwhile, the anisotropy of electrons with energies about several tens of keV was observed to increase. Through the comparison of the calculated wave growth rates and observed wave spectral intensity, we suggest that those banded whistler waves observed with frequencies shifted up and frequency bands broadening could be locally excited by these hot electrons with increased anisotropy. The current study provides a great in situ evidence for the influence on frequencies of banded whistler waves by the enhancement of solar wind dynamic pressures, which reveals the important role of solar wind dynamic pressures playing in the frequency properties of banded whistler waves.
Published by: Geophysical Research Letters Published on: Mar-08-2020
YEAR: 2018   DOI: 10.1029/2018GL078849
In this letter, we present unique conjugated satellite observations of MeV relativistic electron precipitation caused by electromagnetic ion cyclotron (EMIC) waves. On the outer boundary of the plasmasphere, the Van Allen probe observed EMIC waves. At ionospheric altitudes, the NOAA 16 satellite at the footprint of Van Allen probe simultaneously detected obvious flux enhancements for precipitating >MeV radiation belt electrons, but not for precipitating
Published by: Geophysical Research Letters Published on: 11/2018
YEAR: 2018   DOI: 10.1029/2018GL080481
Using the Van Allen Probe A observations, we obtained the global distribution of proton rings and calculated the linear wave growth rate of fast magnetosonic (MS) waves in the region L ~ 3-6. Our statistical and calculated results demonstrate that MS waves can be locally excited on the dayside outside the plasmapause, as well as in the dusk sector inside the plasmapause. The frequency range of unstable MS waves is strongly modulated by the ratio of the proton ring velocity (Vr) to the local Alfv\ en speed (VA). High harmonic MS waves (ω>20ΩH+) can be excited outside the plasmapause where Vr/VA<1 while low harmonic MS waves (ω<10ΩH+) with frequencies less than ~30 Hz are found to be excited both outside and inside the plasmapause where 1 Published by: Geophysical Research Letters Published on: 09/2018 YEAR: 2018   DOI: 10.1029/2018GL079999
Published by: Geophysical Research Letters Published on: 09/2018
YEAR: 2018   DOI: 10.1029/2018GL079999
Fast magnetosonic (MS) waves play an important role in the dynamics of the inner magnetosphere. Theoretical prediction and simulation have demonstrated that MS waves can heat cold ions. However, direct observational evidence of cold ion heating by MS waves has so far remained elusive. In this paper, we show a typical event of cold ion heating by magnetosonic waves in a density cavity of the plasmasphere with observations of the Van Allen Probe mission on 22 August 2013. During enhancements of the MS wave intensity in the density cavity, the fluxes of trapped H+ and He+ ions with energies of 10\textendash100 eV were observed to increase, implying that cold plasmaspheric ions were heated through high-order resonances with the MS waves. Based on simultaneous observations of ring current protons, we have calculated local linear growth rates, which demonstrate that magnetosonic waves can be locally generated in the density cavity. Our results provide a direct observational proof of the energy coupling process between the ring current and plasmasphere; that is, through exciting MS waves, the free energy stored in the ring current protons with ring distributions is released. In the density cavity of the plasmasphere, both cold H+ and He+ ions are heated by MS waves. As a result, the energy of the ring current can be transferred into the plasmasphere
Published by: Journal of Geophysical Research: Space Physics Published on: 02/2018
YEAR: 2018   DOI: 10.1002/2017JA024919
A typical case of electromagnetic ion cyclotron (EMIC) emissions with both He+ band and O+ band waves was observed by Van Allen Probe A on 14 July 2014. These emissions occurred in the morning sector on the equator inside the plasmasphere, in which region O+ band EMIC waves prefer to appear. Through property analysis of these emissions, it is found that the He+ band EMIC waves are linearly polarized and propagating quasi-parallelly along the background magnetic field, while the O+ band ones are of linear and left-hand polarization and propagating obliquely with respect to the background magnetic field. Using the in situ observations of plasma environment and particle data, excitation of these O+ band EMIC waves has been investigated with the linear growth theory. The calculated linear growth rate shows that these O+ band EMIC waves can be locally excited by ring current protons with ring velocity distributions. The comparison of the observed wave spectral intensity and the calculated growth rate suggests that the density of H+ rings providing the free energy for the instability has decreased after the wave grows. Therefore, this paper provides a direct observational evidence to the excitation mechanism of O+ band EMIC waves: ring current protons with ring distributions provide the free energy supporting the instability in the presence of rich O+ in the plasmasphere.
Published by: Geophysical Research Letters Published on: 02/2018
YEAR: 2018   DOI: 10.1002/grl.v45.310.1002/2018GL077109
During 04:45:00\textendash08:15:00 UT on 13 September in 2015, a case of Electromagnetic ion cyclotron (EMIC) waves covering wide L shells (L = 3.6\textendash9.4), observed by the Magnotospheric Multiscale 1 (MMS1) are reported. During the same time interval, EMIC waves observed by Van Allen Probes A (VAP-A) only occurred just outside the plasmapause. As the Van Allen Probes moved outside into a more tenuous plasma region, no intense waves were observed. Combined observations of MMS1 and VAP-A suggest that in the terrestrial magnetosphere, an appropriately dense background plasma would make contributions to the growth of EMIC waves in lower L shells, while the ion anisotropy, driven by magnetospheric compression, might play an important role in the excitation of EMIC waves in higher L shells. These EMIC waves are observed over wide L shells after three continuous magnetic storms, which suggests that these waves might obtain their free energy from those energetic ions injected during storm times. These EMIC waves should be included in radiation belt modeling, especially during continuous magnetic storms. Moreover, two-band structures separated in frequencies by local He2+ gyrofrequencies were observed in large L shells (L > ~6), implying sufficiently rich solar wind origin He2+ likely in the outer ring current. It is suggested that multiband-structured EMIC waves can be used to trace the coupling between solar wind and the magnetosphere.
Published by: Journal of Geophysical Research: Space Physics Published on: 07/2017
YEAR: 2017   DOI: 10.1002/2017JA023982
We report in situ observations by the Van Allen Probe mission that magnetosonic (MS) waves are clearly relevant to appear relevant to the background plasma number density. As the satellite moved across dense and tenuous plasma alternatively, MS waves occurred only in lower density region. As the observed protons with \textquoteleftring\textquoteright distributions provide free energy, local linear growth rates are calculated and show that magnetosonic waves can be locally excited in tenuous plasma. With variations of the background plasma density, the temporal variations of local wave growth rates calculated with the observed proton ring distributions, show a remarkable agreement with those of the observed wave amplitude. Therefore, the paper provides a direct proof that background plasma densities can modulate the amplitudes of magnetosonic waves through controlling the wave growth rates.
Published by: Geophysical Research Letters Published on: 07/2017
YEAR: 2017   DOI: 10.1002/2017GL074681
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+ cutoff 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
Published by: Journal of Geophysical Research: Space Physics Published on: 07/2016
YEAR: 2016   DOI: 10.1002/2016JA022573
We present Van Allen Probes observations of modulations in the flux of very energetic electrons up to a few MeV and protons between 1200 - 1400 UT on February 19th, 2014. During this event the spacecraft were in the dayside magnetosphere at L*≈5.5. The modulations extended across a wide range of particle energies, from 79.80 keV to 2.85 MeV for electrons and from 82.85 keV to 636.18 keV for protons. The fluxes of π/2 pitch angle particles were observed to attain maximum values simultaneously with the ULF compressional magnetic field component reaching a minimum. We use peak-to-valley ratios to quantify the strength of the modulation effect, finding that the modulation is larger at higher energies than at lower energies. It is shown that the compressional wave modulation of the particle distribution is due to the mirror effect, which can trap relativistic electrons efficiently for energies up to 2.85 MeV, and trap protons up to ≈600 keV. Larger peak-to-valley ratios at higher energies also attributed to the mirror effect. Finally, we suggest that protons with energies higher than 636.18 keV can not be trapped by the compressional ULF wave efficiently due to the finite Larmor radius effect.
Published by: Journal of Geophysical Research: Space Physics Published on: 05/2016
YEAR: 2016   DOI: 10.1002/2016JA022706
On 23 November 2012, a sudden dropout of the relativistic electron flux was observed after an interplanetary shock arrival. The dropout peaks at \~1MeV and more than 80\% of the electrons disappeared from the drift shell. Van Allen twin Probes observed a sharp electron flux dropout with clear energy dispersion signals. The repeating flux dropout and recovery signatures, or \textquotedblleftdropout echoes\textquotedblright, constitute a new phenomenon referred to as a \textquotedblleftdrifting electron dropout\textquotedblright with a limited initial spatial range. The azimuthal range of the dropout is estimated to be on the duskside, from \~1300 to 0100 LT. We conclude that the shock-induced electron dropout is not caused by the magnetopause shadowing. The dropout and consequent echoes suggest that the radial migration of relativistic electrons is induced by the strong dusk-dawn asymmetric interplanetary shock compression on the magnetosphere
Published by: Geophysical Research Letters Published on: 05/2016
YEAR: 2016   DOI: 10.1002/2016GL069140
By examining the compression-induced changes in the electron phase space density and pitch angle distribution observed by two satellites of Van Allen Probes (RBSP-A/B), we find that the relativistic electrons (>2MeV) outside the heart of outer radiation belt (L*>= 5) undergo multiple losses during a storm sudden commencement (SSC). The relativistic electron loss mainly occurs in the field-aligned direction (pitch angle α< 30\textdegree or >150\textdegree), and the flux decay of the field-aligned electrons is independent of the spatial location variations of the two satellites. However, the relativistic electrons in the pitch angle range of 30\textdegree-150\textdegree increase (decrease) with the decreasing (increasing) geocentric distance (|ΔL|< 0.25) of the RBSP-B (RBSP-A) location, and the electron fluxes in the quasi-perpendicular direction display energy-dispersive oscillations in the Pc5 period range (2 - 10min). The relativistic electron loss is confirmed by the decrease of electron phase space density at high-L shell after the magnetospheric compressions, and their loss is associated with the intense plasmaspheric hiss, electromagnetic ion cyclotron (EMIC) waves, relativistic electron precipitation (observed by POES/NOAA satellites at 850km) and magnetic field fluctuations in the Pc5 band. The intense EMIC waves and whistler-mode hiss jointly cause the rapidly pitch angle scattering loss of the relativistic electrons within 10 hours. Moreover, the Pc5 ULF waves also lead to the slowly outward radial diffusion of the relativistic electrons in the high-L region with a negative electron phase space density gradient.
Published by: Journal of Geophysical Research: Space Physics Published on: 11/2015
YEAR: 2015   DOI: 10.1002/2015JA021460
Electromagnetic ion cyclotron (EMIC) waves; outer radiation belt; Outward radial diffusion driven by ULF waves; Plasmaspheric Hiss; relativistic electron loss; Storm sudden commencement; Van Allen Probes
The Van-Allen probes, low-altitude NOAA satellite, MetOp satellite and riometer are used to analyze variations of precipitating energetic electron fluxes and cosmic radio noise absorption (CNA) driven by plasmaspheric hiss with respect to geomagnetic activities. The hiss-driven energetic electron precipitations (at L~4.7-5.3, MLT~8-9) are observed during geomagnetic quiet condition and substorms, respectively. We find that the CNA detected by riometers increased very little in the hiss-driven event during quiet condition on September 06, 2012. The hiss-driven enhancement of riometer was still little during the first substorm on September 30, 2012. However, the absorption detected by the riometer largely increased while the energies of the injected electrons became higher during the second substorm on September 30, 2012. The enhancement of CNA (ΔCNA) observed by the riometer and calculated with precipitating energetic electrons are in agreement during the second substorm, implying that the precipitating energetic electrons increase CNA to an obviously detectable level of the riometer during the second substorm on September 30, 2012. The conclusion is consistent with Rodger et al. (2012), which suggests that the higher level of ΔCNA prefer to occur in the substorms, because substorms may produce more intense energetic electron precipitation associated with electron injection. Furthermore, the combination of the observations and theory calculations also suggests that higher-energy electron (>55 keV) precipitation contribute more to the ΔCNA than the lower-energy electron precipitation. In this paper, the higher-energy electron precipitation is related to lower-frequency hiss.
Published by: Journal of Geophysical Research: Space Physics Published on: 06/2015
YEAR: 2015   DOI: 10.1002/2015JA021113
Utilizing the data from the magnetometer instrument which is a part of the Electric and Magnetic Field Instrument Suite and Integrated Science (EMFISIS) instrument suite onboard the Van Allen Probe A from Sep. 2012 to Apr. 2014, when the apogee of the satellite has passed all the MLT sectors, we obtain the statistical distribution characteristic of EMIC waves in the inner magnetosphere over all local times from L=3 to L=6. Compared with the previous statistical results about EMIC waves, the occurrence rates of EMIC waves distribute relatively uniform in the MLT sectors in lower L-shells. On the other hand, in higher L-shells, there are indeed some peaks of the occurrence rate for the EMIC waves, especially in the noon, dusk and night sectors. EMIC waves appear at lower L-shells in the dawn sector than in other sectors. In the lower L-shells (L<4), the occurrence rates of EMIC waves are significant in the dawn sector. This phenomenon may result from the distribution characteristic of the plasmasphere. The location of the plasmapause is usually lower in the dawn sector than that in other sectors, and the plasmapause is considered to be the favored region for the generation of EMIC waves. In higher L-shells (L>4) the occurrence rates of EMIC waves are most significant in the dusk sector, implying the important role of the plasmapause or plasmaspheric plume in generating EMIC waves. We have also investigated the distribution characteristics of the hydrogen band and the helium band EMIC waves. Surprisingly, in the inner magnetosphere, the hydrogen band EMIC waves occur more frequently than the helium band EMIC waves. Both them have peaks of occurrence rate in noon, dusk and night sectors, and the hydrogen band EMIC waves have more obvious peaks than the helium band EMIC waves in the night sector, while the helium band EMIC waves are more concentrated than the hydrogen band EMIC waves in the dusk sector. Both them occur significantly in the noon sector, which implies the important role of the solar wind dynamic pressure.
Published by: Journal of Geophysical Research: Space Physics Published on: 05/2015
YEAR: 2015   DOI: 10.1002/2015JA021089
Through polarization and spectra analysis of the magnetic field observed by the Van Allen Probe A, we present two typical cases of O+ band EMIC waves in the outer plasmasphere or plasma trough. Although such O+ band EMIC waves are rarely observed, 18 different events of O+ band EMIC waves (16 events in the outer plasmasphere and 2 events in the plasma trough) are found from September 2012 to August 2014 with observations of the Van Allen Probe A. We find that the preferred region for the occurrence of O+ band EMIC waves is in L = 2-5 and MLT = 03-13, 19-20, which is in accordance with the occurrence region of O+ ion torus. Therefore, our result suggests that the O+ ion torus in the outer plasmasphere during geomagnetic activities should play an important role in the generation of EMIC waves in O+ band.
Published by: Geophysical Research Letters Published on: 02/2015
YEAR: 2015   DOI: 10.1002/2015GL063250