M. K. Hudson

Acceleration of relativistic electrons via drift‐resonant interaction with toroidal‐mode Pc‐5 ULF oscillations

There has been increasing evidence that Pc-5 ULF oscillations play a fundamental role in the dynamics of outer zone electrons. In this work we examine the adiabatic response of electrons to toroidal-mode Pc-5 field line resonances using a simplified magnetic field model. We find that electrons can be adiabatically accelerated through a drift-resonant interaction with the waves, and present expressions describing the resonance condition and half-width for resonant interaction. The presence of magnetospheric convection electric fields is seen to increase the rate of resonant energization, and allow bulk acceleration of radiation belt electrons. Conditions leading to the greatest rate of acceleration in the proposed mechanism, a nonaxisymmetric magnetic field, superimposed toroidal oscillations, and strong convection electric fields, are likely to prevail during storms associated with high solar wind speeds.

Simulation of the prompt energization and transport of radiation belt particles during the March 24, 1991 SSC

The authors model the rapid ([approximately]1 min) formation of a new electron radiation belt at L [approx equal] 2.5 that resulted from the Storm Sudden Commencement (SSC) of March 24, 1991 as observed by the CRRES satellite. Guided by the observed electric and magnetic fields, the authors represent the time-dependent magnetospheric electric field during the SSC by an asymmetric bipolar pulse that is associated with the compression and relaxation of the Earths magnetic field. The authors follow the electrons using a relativistic guiding center code. The test-particle simulations show that electrons with energies of a few MeV at L > 6 were energized up to 40 MeV and transported to L [approx equal] 2.5 during a fraction of their drift period. The energization process conserves the first adiabatic invariant and is enhanced due to resonance of the electron drift motion with the time-varying electric field. Their simulation results, with an initial W[sup [minus]8] energy flux spectra, reproduce the observed electron drift echoes and show that the interplanetary shock impacted the magnetosphere between 1500 and 1800 MLT. 121 refs., 5 figs.

Experimental evidence on the role of the large spatial scale electric field in creating the ring current

This paper presents the first simultaneous in situ measurements of the large-scale convection electric field and the ring current induced magnetic field perturbations in the equatorial plane of the inner magnetosphere and compares them to the evolution of major geomagnetic storms as characterized by Dst. The measurements were obtained from the University of California, Berkeley double-probe electric field experiment and the Air Force Geophysics Laboratory fluxgate magnetometer on the CRRES spacecraft. This spacecraft had an apogee near geosynchronous orbit and a perigee near 300 km altitude. We focus on the major geomagnetic storm on March 24, 1991, for which the maximum negative excursion of Dst was about −300 nT. During the main phase of the storm, the large-scale electric field repeatedly penetrated earthward, maximizing between L = 2 and L = 4 with magnitudes of 6 mV/m. These magnitudes were larger than quiet time values of the electric field by a factor of 60 or more. Electric potential drops across the dusk region from L = 2 to L = 4 ranged up to 50–70 kV in concert with increases in Kp up to 9 and dDst/dt (an indicator of the net ring current injection rate) which ranged up to −50 nT/hr. These electric fields lasted for time periods of the order of an hour or more and were capable of injecting ring current ions from L = 8 to L = 2.4 and energizing particles from initial plasma sheet energies of 1–5 keV up to 300 keV through conservation of the first adiabatic invariant. The data obtained during the recovery phase of this storm provide the first direct experimental evidence in the equatorial plane that the electric field is systematically diminished or shielded earthward of the inner edge of the ring current during this phase of the geomagnetic storm. Also observed during the 2-week recovery phase were episodic enhancements in the electric field which coincided and were colocated with enhancements of in situ ring current intensity and which also coincided with decreases in Dst. These enhancements in the electric field and in the ring current magnetic field perturbation occurred at progressively larger radial positions as the recovery phase continued. Evidence for regions of reversed convection near midnight during the recovery phase is provided. An unexpected and important feature of this data set, during both main and recovery phases, near 1800–2100 MLT, is that electric fields are often much stronger earthward of L = 4 or L = 5 than at positions more distant than L = 6. This suggests important features of the interaction between the hot ring current plasma and the large-scale electric field in the inner magnetosphere are not yet understood.

A long-lived relativistic electron storage ring embedded in Earth's outer Van Allen belt.

Van Allen Variation The two rings of relativistic particles called Van Allen Belts that encircle Earth were discovered during the space age, and are known to pose risks to satellites in geostationary orbit. NASA launched twin spacecraft, the Van Allen Probes, on 30 August 2012 to measure and characterize Earths radiation belt regions. Baker et al. (p. 186, published online 28 February) have shown that a third, unexpected and temporary, radiation belt formed on 2 September 2012 to disappear 4 weeks later in response to changes in the solar wind. NASA’s Van Allen Probes revealed an additional, dynamic belt of relativistic particles surrounding Earth. Since their discovery more than 50 years ago, Earth’s Van Allen radiation belts have been considered to consist of two distinct zones of trapped, highly energetic charged particles. The outer zone is composed predominantly of megaelectron volt (MeV) electrons that wax and wane in intensity on time scales ranging from hours to days, depending primarily on external forcing by the solar wind. The spatially separated inner zone is composed of commingled high-energy electrons and very energetic positive ions (mostly protons), the latter being stable in intensity levels over years to decades. In situ energy-specific and temporally resolved spacecraft observations reveal an isolated third ring, or torus, of high-energy (>2 MeV) electrons that formed on 2 September 2012 and persisted largely unchanged in the geocentric radial range of 3.0 to ~3.5 Earth radii for more than 4 weeks before being disrupted (and virtually annihilated) by a powerful interplanetary shock wave passage.

Comparisons of Polar satellite observations of solitary wave velocities in the plasma sheet boundary and the high altitude cusp to those in the auroral zone

Characteristics of solitary waves observed by Polar in the high altitude cusp, polar cap and plasma sheet boundary are reported and compared to observations in the auroral zone. The study presented herein shows that, at high altitudes, the solitary waves are positive potential structures (electron holes), with scale sizes of the order of 10s of Debye lengths, which usually propagate with velocities of a few thousand km/s. At the plasma sheet boundary, the direction of propagation can be either upward or downward; whereas at the leading edge of high altitude cusp energetic particle injections, it is downward. For these high altitude events, explanations based on ion modes and on electron modes are both examined, and the electron mode interpretation is shown to be more consistent with observations.

Increase in relativistic electron flux in the inner magnetosphere: ULF wave mode structure

Abstract Pc 5 ULF waves are seen concurrently with the rise in radiation belt fluxes associated with CME magnetic cloud events. A 3D global MHD simulation of the 10–11 January, 1997 event has been analyzed for mode structure and shown to contain field line resonance components, both toroidal and poloidal, with peak power on the nightside during southward IMF conditions. A mechanism for inward radial transport and first-invariant conserving acceleration of relativistic electrons is assessed in the context of ULF mode structure analysis, and compared with groundbased and satellite observations.

Simulations of radiation belt formation during storm sudden commencements

MHD fields from a global three-dimensional simulation of the great March 24, 1991, storm sudden commencement (SSC) are used to follow the trajectories of particles in a guiding center test particle simulation of radiation belt formation during this event. Modeling of less intense events during the lifetime of the CRRES satellite, with similar morphology but less radial transport and energization, is also presented. In all cases analyzed, a solar proton event was followed by an SSC, leading to the formation of a new proton belt earthward of solar proton penetration. The effect on particle energization of varying solar wind and model pulse parameters is investigated. Both a seed population of solar protons and the SSC shock-induced compression of the magnetosphere are necessary conditions for the formation of a new proton belt. The outer boundary of the inner zone protons can be affected by an SSC and a newly formed belt can be affected by the ensuing or a subsequent storm, which may occur in rapid succession, as was the case in June and July 1991. The acceleration process is effective for both northward and southward IMF, with more energization and inward radial transport for the southward case for otherwise comparable solar wind parameters, because of the initially more compressed magnetopause in the southward case. The inner boundary and stability of the newly formed belt depends on the magnitude of radial transport at the time of formation and subsequent ring current perturbation of adiabatic trapping.

Simulated magnetopause losses and Van Allen Probe flux dropouts

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 Bz beginning 8 October coincident with the inner boundary of outer zone depletion.

Gradual diffusion and punctuated phase space density enhancements of highly relativistic electrons: Van Allen Probes observations

The dual-spacecraft Van Allen Probes mission has provided a new window into mega electron volt (MeV) particle dynamics in the Earths radiation belts. Observations (up to E ~10 MeV) show clearly the behavior of the outer electron radiation belt at different timescales: months-long periods of gradual inward radial diffusive transport and weak loss being punctuated by dramatic flux changes driven by strong solar wind transient events. We present analysis of multi-MeV electron flux and phase space density (PSD) changes during March 2013 in the context of the first year of Van Allen Probes operation. This March period demonstrates the classic signatures both of inward radial diffusive energization and abrupt localized acceleration deep within the outer Van Allen zone (L ~4.0 ± 0.5). This reveals graphically that both “competing” mechanisms of multi-MeV electron energization are at play in the radiation belts, often acting almost concurrently or at least in rapid succession.

Simulation of proton radiation belt formation during the March 24, 1991 SSC

The rapid formation of a new proton radiation belt at L ≃ 2.5 following the March 24, 1991 Storm Sudden Commencement (SSC) observed at the CRRES satellite is modelled using a relativistic guiding center test particle code. The SSC is modelled by a bipolar electric field and associated compression and relaxation in the magnetic field, superimposed on a dipole magnetic field. The source population consists of both solar and trapped inner zone protons. The simulations show that while both populations contribute to drift echoes in the 20–80 MeV range, primary contribution is from the solar protons. Proton acceleration by the SSC differs from relativistic electron acceleration in that different source populations contribute and nonrelativistic conservation of the first adiabatic invariant leads to greater energization of protons for a given decrease in L. Model drift echoes and flux distribution in L at the time of injection compare well with CRRES observations.