Shui Wang

STEERB: A three‐dimensional code for storm‐time evolution of electron radiation belt

[1]xa0We present a three-dimensional code (storm-time evolution of electron radiation belt, STEERB) for the storm-time evolution of electron radiation belt following the preceding work. STEERB code is based on the solution of three-dimensional modified Fokker-Planck equation, which covers the inner and outer radiation belts with incorporation of the Coulomb collisions, radial diffusion due to magnetic and electric field perturbations, and local pitch angle, energy, and cross-pitch angle-energy diffusion due to various wave-particle interactions. It is implemented by a split operator technique, in conjunction with the recently developed hybrid finite difference method for local wave-particle interaction, and the fully implicit finite difference method for radial diffusion as well as Coulomb collisions. The resulting numerical model is robust, efficient, and easily parallelizable. Some of the dominant characteristics of electron radiation belt during both quiet and active periods can be well reproduced by STEERB code. Test simulations are performed to evaluate the respective roles of radial diffusion, cyclotron resonant interaction with chorus and plume waves in the global radiation belt dynamics, the sensitivity of numerical results to the uncertainty of wave spectrum, and the importance of cross-pitch angle-energy diffusion in the three-dimensional simulations.

CRRES observation and STEERB simulation of the 9 October 1990 electron radiation belt dropout event

We examine and simulate the electron radiation belt dropout event on 9 October 1990. CRRES observations show that significant depletions of electron fluxes occurred at energies similar to 0.1- 1.0 MeV beyond 6 R-E and at energies >similar to 0.4 MeV within 6 R-E. The three- dimensional kinetic radiation belt model STEERB is used to simulate this dropout event, taking into account the magnetopause shadowing, adiabatic transport, radial diffusion, and plume and chorus wave- particle interactions. Our results show that STEERB code can basically reproduce the observed depletion of similar to 0.1-1.0 MeV electron fluxes throughout the outer radiation belt, suggesting that the competition and combination of all these physical mechanisms can well explain this electron radiation belt dropout event. Citation: Su, Z., F. Xiao, H. Zheng, and S. Wang (2011), CRRES observation and STEERB simulation of the 9 October 1990 electron radiation belt dropout event, Geophys. Res. Lett., 38, L06106, doi: 10.1029/2011GL046873.

Evolution of electron pitch angle distribution due to interactions with whistler mode chorus following substorm injections

[1]xa0During substorms, pitch angle distribution (PAD) of freshly injected plasma sheet electrons can develop into the typical pancake distribution from the nearly isotropic distribution, which has previously been suggested as the result of resonant scattering driven by electrostatic cyclotron harmonic (ECH) and whistler mode waves in the equatorial regions. In this paper, using the two-dimensional bounce-averaged Fokker-Planck equation, we present a quantitative analysis of the electrons PAD evolution at L = 6 due to interactions with upper (ω/∣Ωe∣ > 0.5) and lower (ω/∣Ωe∣ < 0.5) band chorus based on the observed wave characteristics during substorms. It is found that the upper band chorus can efficiently scatter the electrons with energies 0.1–2 keV into loss cone and drive the electrons with energies above 2 keV toward loss cone. The lower band chorus can only cause precipitation loss of the electrons with energies above 1 keV by resonant scattering. The PADs of electrons with energies above 0.1 keV can develop into the pancake-shaped distributions due to the combined resonant scattering by upper and lower band chorus, and the pancake index PI, defined as the flux ratio between 90° and 70°, can reach 6 after 5 hours since substorm injections. The PI variation timescale and amplitude are consistent with previous statistical results. The numerical results suggest that the resonant scattering of electrons by the whistler mode chorus waves can be a substantial mechanism responsible for the formation of a pancake distribution outside L = 6. Besides, the rapid precipitation of electrons (∼keV) driven by resonant scattering of chorus waves may also be responsible for the diffuse aurora.

Radiation belt electron dynamics driven by adiabatic transport, radial diffusion, and wave-particle interactions

The adiabatic transport process is introduced into our recently developed three-dimensional physics-based electron radiation belt model (STEERB, Storm-Time Evolution of Electron Radiation Belt) via adopting a time-varying Hilmer-Voigt geomagnetic field. The current STEERB model contains more complete physical processes: adiabatic transport, radial diffusion, and various in situ wave-particle interactions. In particular, the influence of adiabatic transport on storm time radiation belt electron dynamics is investigated by some idealized simulations. It is found that the adiabatic transport alone (without plume hiss and electromagnetic ion cyclotron (EMIC) waves) is unable to reproduce the observed main phase loss of energetic outer radiation belt electron fluxes in the presence of a strong chorus-driven acceleration process. However, these adiabatic and nonadiabatic processes for radiation belt electron dynamics are coupled to each other. The adiabatic transport, together with radial diffusion and cyclotron resonant interactions with chorus, plume hiss, and EMIC waves, contributes significantly to the main phase loss and the recovery phase enhancement of energetic electron fluxes. In the absence of adiabatic transport, the energetic outer radiation belt electron fluxes are found to be overestimated by a factor of 5-30 over all the pitch angles during the main phase and to be underestimated by a factor of 2-5 at larger pitch angles (alpha(e) > 50 degrees) during the recovery phase. These numerical results suggest that the adiabatic transport in a time-varying geomagnetic field model should be incorporated into the future radiation belt models for space weather application.

Quantitative Analysis of CME Deflections in the Corona

In this paper, ten CME events viewed by the STEREO twin spacecraft are analyzed to study the deflections of CMEs during their propagation in the corona. Based on the three-dimensional information of the CMEs derived by the graduated cylindrical shell (GCS) model (Thernisien, Howard, and Vourlidas in Astrophys. J.652, 1305, 2006), it is found that the propagation directions of eight CMEs had changed. By applying the theoretical method proposed by Shen etxa0al. (Solar Phys.269, 389, 2011) to all the CMEs, we found that the deflections are consistent, in strength and direction, with the gradient of the magnetic energy density. There is a positive correlation between the deflection rate and the strength of the magnetic energy density gradient and a weak anti-correlation between the deflection rate and the CME speed. Our results suggest that the deflections of CMEs are mainly controlled by the background magnetic field and can be quantitatively described by the magnetic energy density gradient (MEDG) model.

Nonstorm time dynamics of electron radiation belts observed by the Van Allen Probes

Storm time electron radiation belt dynamics have been widely investigated for many years. Here we present a rarely reported nonstorm time event of electron radiation belt evolution observed by the Van Allen Probes during 21-24 February 2013. Within 2 days, a new belt centering around L=5.8 formed and gradually merged with the original outer belt, with the enhancement of relativistic electron fluxes by a factor of up to 50. Strong chorus waves (with power spectral density up to 10(-4)nT(2)/Hz) occurred in the region L>5. Taking into account the local acceleration driven by these chorus waves, the two-dimensional STEERB can approximately reproduce the observed energy spectrums at the center of the new belt. These results clearly illustrate the complexity of electron radiation belt behaviors and the importance of chorus-driven local acceleration even during the nonstorm times. Key Points A rarely reported nonstorm time event of RB reformation observed by RBSP Formation of a new belt near the outer boundary of the original outer belt Importance of chorus-driven local acceleration: observation and simulation

Bounce-averaged advection and diffusion coefficients for monochromatic electromagnetic ion cyclotron wave: Comparison between test-particle and quasi-linear models

The electromagnetic ion cyclotron (EMIC) wave has long been suggested to be responsible for the rapid loss of radiation belt relativistic electrons. The test-particle simulations are performed to calculate the bounce-averaged pitch angle advection and diffusion coefficients for parallel-propagating monochromatic EMIC waves. The comparison between test-particle (TP) and quasi-linear (QL) transport coefficients is further made to quantify the influence of nonlinear processes. For typical EMIC waves, four nonlinear physical processes, i.e., the boundary reflection effect, finite perturbation effect, phase bunching and phase trapping, are found to occur sequentially from small to large equatorial pitch angles. The pitch angle averaged finite perturbation effect yields slight differences between the transport coefficients of TP and QL models. The boundary reflection effect and phase bunching produce an average reduction of >80% in the diffusion coefficients but a small change in the corresponding average advection coefficients, tending to lower the loss rate predicted by QL theory. In contrast, the phase trapping causes continuous negative advection toward the loss cone and a minor change in the corresponding diffusion coefficients, tending to increase the loss rate predicted by QL theory. For small amplitude EMIC waves, the transport coefficients grow linearly with the square of wave amplitude. As the amplitude increases, the boundary reflection effect, phase bunching and phase trapping start to occur. Consequently, the TP advection coefficients deviate from the linear growth with the square of wave amplitude, and the TP diffusion coefficients become saturated with the amplitude approaching 1 nT or above. The current results suggest that these nonlinear processes can cause significant deviation of transport coefficients from the prediction of QL theory, which should be taken into account in the future simulations of radiation belt dynamics driven by the EMIC waves.

Ultra-low-frequency wave-driven diffusion of radiation belt relativistic electrons

Van Allen radiation belts are typically two zones of energetic particles encircling the Earth separated by the slot region. How the outer radiation belt electrons are accelerated to relativistic energies remains an unanswered question. Recent studies have presented compelling evidence for the local acceleration by very-low-frequency (VLF) chorus waves. However, there has been a competing theory to the local acceleration, radial diffusion by ultra-low-frequency (ULF) waves, whose importance has not yet been determined definitively. Here we report a unique radiation belt event with intense ULF waves but no detectable VLF chorus waves. Our results demonstrate that the ULF waves moved the inner edge of the outer radiation belt earthward 0.3 Earth radii and enhanced the relativistic electron fluxes by up to one order of magnitude near the slot region within about 10u2009h, providing strong evidence for the radial diffusion of radiation belt relativistic electrons.

Intense duskside lower band chorus waves observed by Van Allen Probes: Generation and potential acceleration effect on radiation belt electrons

Local acceleration driven by whistler mode chorus waves largely accounts for the enhancement of radiation belt relativistic electron fluxes, whose favored region is usually considered to be the plasmatrough with magnetic local time approximately from midnight through dawn to noon. On 2 October 2013, the Van Allen Probes recorded a rarely reported event of intense duskside lower band chorus waves (with power spectral density up to 10(-3)nT(2)/Hz) in the low-latitude region outside of L=5. Such chorus waves are found to be generated by the substorm-injected anisotropic suprathermal electrons and have a potentially strong acceleration effect on the radiation belt energetic electrons. This event study demonstrates the possibility of broader spatial regions with effective electron acceleration by chorus waves than previously expected. For such intense duskside chorus waves, the occurrence probability, the preferential excitation conditions, the time duration, and the accurate contribution to the long-term evolution of radiation belt electron fluxes may need further investigations in future.

Plasmatrough exohiss waves observed by Van Allen Probes: Evidence for leakage from plasmasphere and resonant scattering of radiation belt electrons

Exohiss waves are whistler mode hiss observed in the plasmatrough region. We present a case study of exohiss waves and the corresponding background plasma distributions observed by the Van Allen Probes in the dayside low-latitude region. The analysis of wave Poynting fluxes, suprathermal electron fluxes, and cold electron densities supports the scenario that exohiss leaks from the plasmasphere into the plasmatrough. Quasilinear calculations further reveal that exohiss can potentially cause the resonant scattering loss of radiation belt electrons similar to< MeV on a comparable time scale to that associated with the storm time plasmaspheric hiss. These results clearly illustrate that exohiss may need to be taken into account in future radiation belt models.