Radial transport of radiation belt electrons due to stormtime Pc5 waves

Abstract

Abstract. During geomagnetic storms relativistic electron fluxes in the outer radiation belt exhibit dynamic variability over multiple orders of magnitude. This requires radial transport of electrons across their drift shells and implies violation of their third adiabatic invariant. Radial transport is induced by the interaction of the electron drift motion with electric and magnetic field fluctuations in the ULF frequency range. It was previously shown that solar-wind driven ULF waves have long azimuthal wave lengths and thus can violate the third invariant of trapped electrons in the process of resonant interaction with their gradient-curvature motion. However, the amplitude of solar-wind driven ULF waves rapidly decreases with decreasing L. It is therefore not clear what mechanisms are responsible for fast transport rates observed inside the geosynchronous orbit. In this paper we investigate wether stormtime Pc5 waves can contribute to this process. Stormtime Pc5s have short azimuthal wave lengths and therefore cannot exhibit resonance with the the electron drift motion. However we show that stormtime Pc5s can cause localized random scattering of electron drift motion that violates the third invariant. According to our results electron interaction with stormtime Pc5s can produce rapid radial transport even as low as L 4. Numerical simulations show that electron transport can exhibit large deviations from radial diffusion. The diffusion approximation is not valid for individual storms but only applies to the statistically averaged response of the outer belt to stormtime Pc5 waves.

Association of radiation belt electron enhancements with earthward penetration of Pc5 ULF waves: a case study of intense 2001 magnetic storms

“…During such intense magnetic storms, when the ring current is intensified, storm-time Pc5 waves are observed, closely linked to quasisinusoidal disturbances at Pc5 frequencies and injections of ring current ions in the inner magnetosphere (Barfield and McPherron, 1978). Storm-time Pc5 waves can reach radial distances of up to L = 7.4 (Barfield and Lin, 1983) and break the third adiabatic invariant through drift resonance with the outer radiation belt electrons to produce rapid radial transport even as low as L = 4 (Ukhorskiy et al, 2009).…”

“…Such enhanced Pc5 wave activity penetrating deep in the magnetosphere and its effects on particle motion are linked with radial diffusion (Ukhorskiy et al, 2009;Ozeke et al, 2012Ozeke et al, , 2014. Semi-periodic variations in magnetic and electric fields steering the drift motion of trapped electrons lead to particle diffusion across drift shells from regions of lower magnetic field strength to regions of increased magnetic field strength.…”

“…Radial diffusion requires a population of seed electrons of a few hundred kiloelectron volts which are supplied by substorms and subsequently energized by Pc5 waves. Depending on the general particle distribution, radial diffusion may act to increase electron flux levels in the inner magnetosphere as electrons diffuse earthward, breaking the drift invariant, , while maintaining the gyro invariant, µ, and bounce invariant, J , associated with the trapped particles' motion (Ukhorskiy et al, 2009;Ozeke et al, 2012Ozeke et al, , 2014. Radial diffusion may also act as a loss process as the aggregate of particles drift outward and are lost to the magnetopause (Turner et al, 2012).…”

The Electric and Magnetic Field Instrument Suite and Integrated Science (EMFISIS) on RBSP

“…The global distribution and variability of low frequency Pc4 and Pc5 waves can be monitored by ground-based magnetometers and by satellites (Liu et al 2009), and the observed wave spectral characteristics have been used to evaluate radial diffusion coefficients (Brautigam et al 2005;Perry et al 2005;Ukhorskiy et al 2005;Huang et al 2010) and employed in dynamic modeling of the outer radiation belt (Loto'aniu et al 2006;Ukhorskiy et al 2009;Chu et al 2010). The properties of magnetospheric ULF waves, excited in response to solar wind variability, have also been obtained from global MHD simulations and used to study the dynamic variability of radiation belt electrons (Fei et al 2006;Kress et al 2007).…”

ULF wave activity during the 2003 Halloween superstorm: multipoint observations from CHAMP, Cluster and Geotail missions

“…Low-frequency instabilities of the ring current plasma during periods of intense geomagnetic activity are closely related with waves observed during geomagnetic storms (Ukhorskiy et al, 2009). The drift and bounce motions of energetic particles of the ring current may lead to fluctuations of electric and magnetic fields in the magnetosphere and ionosphere in the case of excess available energy (Baddeley et al, 2002).…”