Zhenpeng Su

Modeling of outer radiation belt electrons by multidimensional diffusion process

[1] In this study, we develop a new code which uses a hybrid finite difference (HFD) method to solve a 2-D bounce-averaged energy-pitch angle diffusion equation in the outer radiation belts. We implement the numerical algorithm by adopting a split operator technique, an implicit scheme for dealing with diagonal diffusion coefficients, and an alternative direction implicit scheme for off-diagonal (or cross) diffusion coefficients. We show that our code HFD can successfully overcome the unstable problem when the large and rapidly varying cross diffusion coefficients are included. Particularly, we examine whether resonant interactions with obliquely propagated whistler mode chorus and hiss waves result in a net acceleration or loss of relativistic electrons in the outer radiation belt L = 4.5. Numerical simulations show that chorus waves can yield significant acceleration of relativistic electrons, while hiss waves are primarily responsible for scattering equatorially mirroring electrons toward the loss cone.

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.

Wave-driven butterfly distribution of Van Allen belt relativistic electrons

Van Allen radiation belts consist of relativistic electrons trapped by Earths magnetic field. Trapped electrons often drift azimuthally around Earth and display a butterfly pitch angle distribution of a minimum at 90° further out than geostationary orbit. This is usually attributed to drift shell splitting resulting from day–night asymmetry in Earths magnetic field. However, direct observation of a butterfly distribution well inside of geostationary orbit and the origin of this phenomenon have not been provided so far. Here we report high-resolution observation that a unusual butterfly pitch angle distribution of relativistic electrons occurred within 5 Earth radii during the 28 June 2013 geomagnetic storm. Simulation results show that combined acceleration by chorus and magnetosonic waves can successfully explain the electron flux evolution both in the energy and butterfly pitch angle distribution. The current provides a great support for the mechanism of wave-driven butterfly distribution of relativistic electrons.

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.

Chorus acceleration of radiation belt relativistic electrons during March 2013 geomagnetic storm

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.

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 10 h, 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.

Quantifying the relative contributions of substorm injections and chorus waves to the rapid outward extension of electron radiation belt

We study the rapid outward extension of the electron radiation belt on a timescale of several hours during three events observed by Radiation Belt Storm Probes and Time History of Events and Macroscale Interactions during Substorms satellites and particularly quantify the contributions of substorm injections and chorus waves to the electron flux enhancement near the outer boundary of radiation belt. A comprehensive analysis including both observations and simulations is performed for the first event on 26 May 2013. The outer boundary of electron radiation belt moved from L = 5.5 to L > 6.07 over about 6 h, with up to 4 orders of magnitude enhancement in the 30 keV to 5 MeV electron fluxes at L = 6. The observations show that the substorm injection can cause 100% and 20% of the total subrelativistic (similar to 0.1 MeV) and relativistic (2-5 MeV) electron flux enhancements within a few minutes. The data-driven simulation supports that the strong chorus waves can yield 60%-80% of the total energetic (0.2-5.0 MeV) electron flux enhancement within about 6 h. Some simple analyses are further given for the other two events on 2 and 29 June 2013, in which the contributions of substorm injections and chorus waves are shown to be qualitatively comparable to those for the first event. These results clearly illustrate the respective importance of substorm injections and chorus waves for the evolution of radiation belt electrons at different energies on a relatively short timescale.