Dayside response of the magnetosphere to a small shock compression: Van Allen Probes, Magnetospheric MultiScale, and GOES-13.

Observations from Magnetospheric MultiScale (~8 Re) and Van Allen Probes (~5 and 4 Re) show that the initial dayside response to a small interplanetary shock is a double-peaked dawnward electric field, which is distinctly different from the usual bipolar (dawnward and then duskward) signature reported for large shocks. The associated E × B flow is radially inward. The shock compressed the magnetopause to inside 8 Re, as observed by Magnetospheric MultiScale (MMS), with a speed that is comparable to the E × B flow. The magnetopause speed and the E × B speeds were significantly less than the propagation speed of the pulse from MMS to the Van Allen Probes and GOES-13, which is consistent with the MHD fast mode. There were increased fluxes of energetic electrons up to several MeV. Signatures of drift echoes and response to ULF waves also were seen. These observations demonstrate that even very weak shocks can have significant impact on the radiation belts

Large parallel and perpendicular electric fields on electron spatial scales in the terrestrial bow shock

Large parallel ($\leq$ 100 mV/m) and perpendicular ($\leq$ 600 mV/m) electric fields were measured in the Earth's bow shock by the vector electric field experiment on the Polar satellite. These are the first reported direct measurements of parallel electric fields in a collisionless shock. These fields exist on spatial scales comparable to or less than the electron skin depth (a few kilometers) and correspond to magnetic field-aligned potentials of tens of volts and perpendicular potentials up to a kilovolt. The perpendicular fields are amongst the largest ever measured in space, with energy densities of $\epsilon_0 E^2/ n k_b T_e$ of order 10%. The measured parallel electric field implies that the electrons can be demagnetized, which may result in stochastic (rather than coherent) electron heating

Prompt energization of relativistic and highly relativistic electrons during a substorm interval: Van Allen Probes observations

Abstract On 17 March 2013, a large magnetic storm significantly depleted the multi-MeV radiation belt. We present multi-instrument observations from the Van Allen Probes spacecraft Radiation Belt Storm Probe A and Radiation Belt Storm Probe B at ~6 Re in the midnight sector magnetosphere and from ground-based ionospheric sensors during a substorm dipolarization followed by rapid reenergization of multi-MeV electrons. A 50% increase in magnetic field magnitude occurred simultaneously with dramatic increases in 100 keV electron fluxes and a 100 times increase in VLF wave intensity. The 100 keV electrons and intense VLF waves provide a seed population and energy source for subsequent radiation belt enhancements. Highly relativistic (\u3e2 MeV) electron fluxes increased immediately at L* ~ 4.5 and 4.5 MeV flux increased \u3e90 times at L* = 4 over 5 h. Although plasmasphere expansion brings the enhanced radiation belt multi-MeV fluxes inside the plasmasphere several hours postsubstorm, we localize their prompt reenergization during the event to regions outside the plasmasphere. Key Points Substorm dynamics are important for highly relativistic electron energization Cold plasma preconditioning is significant for rapid relativistic energization Relativistic / highly relativistic electron energization can occur in \u3c 5 hrs

Spacecraft charging and ion wake formation in the near-Sun environment

A three-dimensional (3-D), self-consistent code is employed to solve for the static potential structure surrounding a spacecraft in a high photoelectron environment. The numerical solutions show that, under certain conditions, a spacecraft can take on a negative potential in spite of strong photoelectron currents. The negative potential is due to an electrostatic barrier near the surface of the spacecraft that can reflect a large fraction of the photoelectron flux back to the spacecraft. This electrostatic barrier forms if (1) the photoelectron density at the surface of the spacecraft greatly exceeds the ambient plasma density, (2) the spacecraft size is significantly larger than local Debye length of the photoelectrons, and (3) the thermal electron energy is much larger than the characteristic energy of the escaping photoelectrons. All of these conditions are present near the Sun. The numerical solutions also show that the spacecraft's negative potential can be amplified by an ion wake. The negative potential of the ion wake prevents secondary electrons from escaping the part of spacecraft in contact with the wake. These findings may be important for future spacecraft missions that go nearer to the Sun, such as Solar Orbiter and Solar Probe Plus.Comment: 25 pages, 7 figures, accepted for publication in Physics of Plasma

Effect of EMIC waves on relativistic and ultrarelativistic electron populations: Ground-based and Van Allen Probes observations

Abstract We study the effect of electromagnetic ion cyclotron (EMIC) waves on the loss and pitch angle scattering of relativistic and ultrarelativistic electrons during the recovery phase of a moderate geomagnetic storm on 11 October 2012. The EMIC wave activity was observed in situ on the Van Allen Probes and conjugately on the ground across the Canadian Array for Real-time Investigations of Magnetic Activity throughout an extended 18 h interval. However, neither enhanced precipitation of \u3e0.7 MeV electrons nor reductions in Van Allen Probe 90° pitch angle ultrarelativistic electron flux were observed. Computed radiation belt electron pitch angle diffusion rates demonstrate that rapid pitch angle diffusion is confined to low pitch angles and cannot reach 90°. For the first time, from both observational and modeling perspectives, we show evidence of EMIC waves triggering ultrarelativistic (~2-8 MeV) electron loss but which is confined to pitch angles below around 45° and not affecting the core distribution. Key Points EMIC wave activity is not associated with precipitation of MeV electrons EMIC waves do not deplete the ultra-relativistic belt down to 90° EMIC waves cause loss of low pitch angle electrons with energies ~2-8 MeV

Van Allen Probes observations of direct wave-particle interactions

Abstract Quasiperiodic increases, or bursts, of 17-26 keV electron fluxes in conjunction with chorus wave bursts were observed following a plasma injection on 13 January 2013. The pitch angle distributions changed during the burst events, evolving from sinN(α) to distributions that formed maxima at α = 75-80°, while fluxes at 90° and \u3c60° remained nearly unchanged. The observations occurred outside of the plasmasphere in the postmidnight region and were observed by both Van Allen Probes. Density, cyclotron frequency, and pitch angle of the peak flux were used to estimate resonant electron energy. The result of ∼15-35 keV is consistent with the energies of the electrons showing the flux enhancements and corresponds to electrons in and above the steep flux gradient that signals the presence of an Alfvén boundary in the plasma. The cause of the quasiperiodic nature (on the order of a few minutes) of the bursts is not understood at this time

Excitation of nightside magnetosonic waves observedby Van Allen Probes

Abstract During the recovery phase of the geomagnetic storm on 30-31 March 2013, Van Allen Probe A detected enhanced magnetosonic (MS) waves in a broad range of L = 1.8-4.7 and magnetic local time (MLT) = 17-22 h, with a frequency range ∼10-100 Hz. In the meanwhile, distinct proton ring distributions with peaks at energies of ∼10 keV, were also observed in L = 3.2-4.6 and L = 5.0-5.6. Using a subtracted bi-Maxwellian distribution to model the observed proton ring distribution, we perform three-dimensional ray tracing to investigate the instability, propagation, and spatial distribution of MS waves. Numerical results show that nightside MS waves are produced by proton ring distribution and grow rapidly from the source location L = 5.6 to the location L = 5.0 but remain nearly stable at locations L \u3c 5.0. Moreover, waves launched toward lower L shells with different initial azimuthal angles propagate across different MLT regions with divergent paths at first, then gradually turn back toward higher L shells and propagate across different MLT regions with convergent paths. The current results further reveal that MS waves are generated by a ring distribution of ∼10 keV proton and proton ring in one region can contribute to the MS wave power in another region. Key Points: Correlated Van Allen Probe data of MS wave and proton ringGrowth rates are peaked at the harmonics of the proton gyrofrequencyMS waves propagate inward divergently and outward convergently

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

Abstract The dual-spacecraft Van Allen Probes mission has provided a new window into mega electron volt (MeV) particle dynamics in the Earth\u27s 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. Key Points Clear observations to higher energy than ever before Precise detection of where and how acceleration takes place Provides new eyes on megaelectron Volt

Interactions of energetic electrons with ULF waves triggered by interplanetary shock: Van Allen Probes observations in the magnetotail

Abstract We present in situ observations of a shock-induced substorm-like event on 13 April 2013 observed by the newly launched Van Allen twin probes. Substorm-like electron injections with energy of 30-500 keV were observed in the region from L∼5.2 to 5.5 immediately after the shock arrival (followed by energetic electron drift echoes). Meanwhile, the electron flux was clearly and strongly varying on the ULF wave time scale. It is found that both toroidal and poloidal mode ULF waves with a period of 150 s emerged following the magnetotail magnetic field reconfiguration after the interplanetary (IP) shock passage. The poloidal mode is more intense than the toroidal mode. The 90 phase shift between the poloidal mode Br and Ea suggests the standing poloidal waves in the Northern Hemisphere. Furthermore, the energetic electron flux modulations indicate that the azimuthal wave number is ∼14. Direct evidence of drift resonance between the injected electrons and the excited poloidal ULF wave has been obtained. The resonant energy is estimated to be between 150 keV and 230 keV. Two possible scenaria on ULF wave triggering are discussed: vortex-like flow structure-driven field line resonance and ULF wave growth through drift resonance. It is found that the IP shock may trigger intense ULF wave and energetic electron behavior at L∼3 to 6 on the nightside, while the time profile of the wave is different from dayside cases

New Insights into Dissipation in the Electron Layer During Magnetic Reconnection

Detailed comparisons are reported between laboratory observations of electron-scale dissipation layers near a reconnecting X-line and direct two-dimensional full-particle simulations. Many experimental features of the electron layers, such as insensitivity to the ion mass, are reproduced by the simulations; the layer thickness, however, is about 3-5 times larger than the predictions. Consequently, the leading candidate 2D mechanism based on collisionless electron nongyrotropic pressure is insufficient to explain the observed reconnection rates. These results suggest that, in addition to the residual collisions, 3D effects play an important role in electron-scale dissipation during fast reconnection.Comment: 17 pages, 4 figure