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Title: Energetic Particles of keV–MeV Energies Observed near Reconnecting Current Sheets at 1 au

Abstract

We provide evidence for particle acceleration up to ∼5 MeV at reconnecting current sheets in the solar wind based on both case studies and a statistical analysis of the energetic ion and electron flux data from the five Advanced Composition Explorer Electron, Proton, and Alpha Monitor (EPAM) detectors. The case study of a typical reconnection exhaust event reveals (i) a small-scale peak of the energetic ion flux observed in the vicinity of the reconnection exhaust and (ii) a long-timescale atypical energetic particle event (AEPE) encompassing the reconnection exhaust. AEPEs associated with reconnecting strong current sheets last for many hours, even days, as confirmed by statistical studies. The case study shows that time-intensity profiles of the ion flux may vary significantly from one EPAM detector to another partially because of the local topology of magnetic fields, but mainly because of the impact of upstream magnetospheric events; therefore, the occurrence of particle acceleration can be hidden. The finding of significant particle energization within a time interval of ±30 hr around reconnection exhausts is supported by a superposed epoch analysis of 126 reconnection exhaust events. We suggest that energetic particles initially accelerated via prolonged magnetic reconnection are trapped and reaccelerated in small- ormore » medium-scale magnetic islands surrounding the reconnecting current sheet, as predicted by the transport theory of Zank et al. Other mechanisms of initial particle acceleration can contribute also.« less

Authors:
 [1];  [2]
  1. Heliophysical Laboratory, Pushkov Institute of Terrestrial Magnetism, Ionosphere and Radio Wave Propagation of the Russian Academy of Sciences (IZMIRAN), Moscow (Russian Federation)
  2. Center for Space Plasma and Aeronomic Research (CSPAR), University of Alabama in Huntsville, Huntsville, AL 35805 (United States)
Publication Date:
OSTI Identifier:
22663463
Resource Type:
Journal Article
Resource Relation:
Journal Name: Astrophysical Journal; Journal Volume: 843; Journal Issue: 1; Other Information: Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
79 ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; ACCELERATION; ELECTRONS; HELIOSPHERE; KEV RANGE; MAGNETIC FIELDS; MAGNETIC ISLANDS; MAGNETIC RECONNECTION; MEV RANGE 01-10; PROTONS; SOLAR PARTICLES; SOLAR WIND; SUN; TAIL IONS; TRANSPORT THEORY; TRAPPING; TURBULENCE

Citation Formats

Khabarova, Olga V., and Zank, Gary P. Energetic Particles of keV–MeV Energies Observed near Reconnecting Current Sheets at 1 au. United States: N. p., 2017. Web. doi:10.3847/1538-4357/AA7686.
Khabarova, Olga V., & Zank, Gary P. Energetic Particles of keV–MeV Energies Observed near Reconnecting Current Sheets at 1 au. United States. doi:10.3847/1538-4357/AA7686.
Khabarova, Olga V., and Zank, Gary P. Sat . "Energetic Particles of keV–MeV Energies Observed near Reconnecting Current Sheets at 1 au". United States. doi:10.3847/1538-4357/AA7686.
@article{osti_22663463,
title = {Energetic Particles of keV–MeV Energies Observed near Reconnecting Current Sheets at 1 au},
author = {Khabarova, Olga V. and Zank, Gary P.},
abstractNote = {We provide evidence for particle acceleration up to ∼5 MeV at reconnecting current sheets in the solar wind based on both case studies and a statistical analysis of the energetic ion and electron flux data from the five Advanced Composition Explorer Electron, Proton, and Alpha Monitor (EPAM) detectors. The case study of a typical reconnection exhaust event reveals (i) a small-scale peak of the energetic ion flux observed in the vicinity of the reconnection exhaust and (ii) a long-timescale atypical energetic particle event (AEPE) encompassing the reconnection exhaust. AEPEs associated with reconnecting strong current sheets last for many hours, even days, as confirmed by statistical studies. The case study shows that time-intensity profiles of the ion flux may vary significantly from one EPAM detector to another partially because of the local topology of magnetic fields, but mainly because of the impact of upstream magnetospheric events; therefore, the occurrence of particle acceleration can be hidden. The finding of significant particle energization within a time interval of ±30 hr around reconnection exhausts is supported by a superposed epoch analysis of 126 reconnection exhaust events. We suggest that energetic particles initially accelerated via prolonged magnetic reconnection are trapped and reaccelerated in small- or medium-scale magnetic islands surrounding the reconnecting current sheet, as predicted by the transport theory of Zank et al. Other mechanisms of initial particle acceleration can contribute also.},
doi = {10.3847/1538-4357/AA7686},
journal = {Astrophysical Journal},
number = 1,
volume = 843,
place = {United States},
year = {Sat Jul 01 00:00:00 EDT 2017},
month = {Sat Jul 01 00:00:00 EDT 2017}
}
  • The problem of current instability leading to anomalous resistivity in current sheets in which reconnection is occurring is considered. The cause of reconnection is not considered. It is simply assumed that because of external forces, oppositely directed magnetic fields are pressed together on a time scale much longer than the time scale for the development of current instability. Three regimes are delineated. (1) Current instability does not occur because the sheet reaches a thickness for steady state reconnection before the condition for current instability can be satisfied. (2) Current instability occurs when the sheet has contracted past the steady statemore » thickness of the diffusion region with anomalous resistivity but before it reaches the thickness of the diffusion region in the presence of normal or inertial resistivity. This leads to a pulsating regime of reconnection in which the reconnection is rapid when anomalous resistivity becomes fully developed, the diffusion region expands so that the conditions for current instability are no longer satisfied, the anomalous resistivity decays, reconnection becomes slow, the diffusion region contracts, and the whole process repeats. (3) Current instability occurs before any part of the sheet contracts to the thickness of the diffusion region, which occupies only part of the sheet. In this case, steady state reconnection occurs with a diffusion region whose thickness is determined by anomalous resistivity. Possible applications to reconnection in the vicinity of the polar cusp, the magnetospheric tail, and solar flares are considered briefly.« less
  • A design is gfiven of a sinusoidal mechanism with an optical system of subtraction. The results of measurements are tabulated and correlated with previous data. (R.V.J.)
  • Proton and electron acceleration in a fragmenting periodic current sheet (CS) is investigated, based on the forced magnetic reconnection scenario. The aim is to understand the role of CS fragmentation in high-energy beam generation in solar flares. We combine magnetohydrodynamics and test-particle models to consider particle trajectories consistent with a time-dependent reconnection model. It is shown that accelerated particles in such a model form two distinct populations. Protons and electrons moving in open magnetic field have energy spectra that are a combination of the initial Maxwellian distribution and a power-law high-energy (E>20 keV) part. The second population contains particles movingmore » in a closed magnetic field around O-points. These particles move predominantly along the guiding field and their energies fall within quite a narrow range between {approx}1 MeV and {approx}10 MeV. It is also found that particles moving in an open magnetic field have a considerably wider pitch-angle distribution.« less
  • We perform resistive magnetohydrodynamic simulations to study the internal structure of current sheets that form during solar eruptions. The simulations start with a vertical current sheet in mechanical and thermal equilibrium that separates two regions of the magnetic field with opposite polarity which are line-tied at the lower boundary representing the photosphere. Reconnection commences gradually due to an initially imposed perturbation, but becomes faster when plasmoids form and produce small-scale structures inside the current sheet. These structures include magnetic islands or plasma blobs flowing in both directions along the sheet, and X-points between pairs of adjacent islands. Among these X-points,more » a principal one exists at which the reconnection rate reaches maximum. A fluid stagnation point (S-point) in the sheet appeared where the reconnection outflow bifurcates. The S-point and the principal X-point (PX-point) are not co-located in space though they are very close to one another. Their relative positions alternate as reconnection progresses and determine the direction of motion of individual magnetic islands. Newly formed islands move upward if the S-point is located above the PX-point, and downward if the S-point is below the PX-point. Merging of magnetic islands was observed occasionally between islands moving in the same direction. Reconnected plasma flow was observed to move faster than blobs nearby.« less