Simulated magnetopause losses and Van Allen Probe flux dropouts

Abstract

Three radiation belt flux dropout events seen by the Relativistic Electron Proton Telescope soon after launch of the Van Allen Probes in 2012 (Baker et al., 2013a) have been simulated using the Lyon-Fedder-Mobarry MHD code coupled to the Rice Convection Model, driven by measured upstream solar wind parameters. MHD results show inward motion of the magnetopause for each event, along with enhanced ULF wave power affecting radial transport. Test particle simulations of electron response on 8 October, prior to the strong flux enhancement on 9 October, provide evidence for loss due to magnetopause shadowing, both in energy and pitch angle dependence. Severe plasmapause erosion occurred during~14 h of strongly southward interplanetary magnetic field B z beginning 8 October coincident with the inner boundary of outer zone depletion.

Magnetohydrodynamic modeling of three Van Allen Probes storms in 2012 and 2013

“…Magnetopause location is calculated from the maximal gradient of the number density sampled along the Sun-Earth line. As shown by Hudson et al (2014a), LFM-RCM predicts the magnetopause location well with respect to a widely used empirical model of Shue et al (1998). Panel (b) is mean ULF wave power between L = 4 and L mp − 1, where L mp is the magnetopause location.…”

“…The third component along the spin axis is measured by two spherical sensors separated by 15 m. The instrument works nominally in a survey mode of 32 samples per second but can use two burst memories with sampling rate of 512 or 16 000 samples per second. An accurate measurement of the electric field is particularly important in determining the effects of particle acceleration (Foster et al, 2015), the losses to both the atmosphere and magnetopause (Hudson et al, 2014a), and also microphysical phenomena like double layers (Mozer et al, 2014). Thanks to the orbit design of the two spacecraft, we receive a current picture of the radiation belts with at least 4.5 h resolution (half orbital period), and are able to separate temporal from spatial variations on a shorter timescale using the spatial separation of the two spacecraft.…”

“…A dip in solar wind pressure and an extended period ∼ 14 h of southward interplanetary magnetic field (IMF) B z was correlated with a sudden increase in the relativistic electron population deep inside geosynchronous orbit. Hudson et al (2014a) analyzed the 8 October 2012 event in detail using MHD simulations combined with a 3-D particle-tracing technique (Kress et al, 2007) to study the electron loss mechanism due to magnetopause shadowing. The energy and pitch angle dependence of the losses analyzed reveal a clear signature of magnetopause loss at higher L on 8 October observed by the Van Allen Probes ECT instrument.…”

“…This behavior was interrupted by prompt loss of the outer zone on 17 March following the arrival of another strong CME shock, which produced a D st of −132 nT and subsequent local acceleration comparable to the 8-9 October 2012 storm (Li et al, 2014a). The role of ULF waves during this and the preceding 8-9 October 2012 storm event, which were examined globally in Hudson et al (2014a, b), is examined in more detail here, including further analysis of the magnetosonic impulse which accompanies magnetosonic compression.…”

“…Inward motion of the magnetopause causes loss on the timescale of magnetopause compression Hudson et al, 2014a) evident in loss of electrons at high L values. The Van Allen Probes measure electron and ion populations across a broad range of energies with complementary electric and magnetic field measurements designed to answer fundamental questions about the balance between source and loss processes affecting the most energetic (multi-MeV) outer-zone electrons in particular (Mauk et al, 2013).…”