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Title: SEP acceleration in CME driven shocks using a hybrid code

Abstract

We perform hybrid simulations of a super-Alfvénic quasi-parallel shock, driven by a coronal mass ejection (CME), propagating in the outer coronal/solar wind at distances of between 3 to 6 solar radii. The hybrid treatment of the problem enables the study of the shock propagation on the ion timescale, preserving ion kinetics and allowing for a self-consistent treatment of the shock propagation and particle acceleration. The CME plasma drags the embedded magnetic field lines stretching from the sun, and propagates out into interplanetary space at a greater velocity than the in situ solar wind, driving the shock, and producing very energetic particles. Our results show that electromagnetic Alfvén waves are generated at the shock front. The waves propagate upstream of the shock and are produced by the counter-streaming ions of the solar wind plasma being reflected at the shock. A significant fraction of the particles are accelerated in two distinct phases: first, particles drift from the shock and are accelerated in the upstream region, and second, particles arriving at the shock get trapped and are accelerated at the shock front. A fraction of the particles diffused back to the shock, which is consistent with the Fermi acceleration mechanism.

Authors:
; ;  [1]; ;  [2]
  1. GoLP/Instituto de Plasmas e Fusão Nuclear, Instituto Superior Técnico, 1049-001 Lisboa (Portugal)
  2. Rutherford Appleton Laboratory, Chilton, Didcot OX11 0QX (United Kingdom)
Publication Date:
OSTI Identifier:
22365245
Resource Type:
Journal Article
Resource Relation:
Journal Name: Astrophysical Journal; Journal Volume: 792; 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; ALFVEN WAVES; COMPUTERIZED SIMULATION; EMISSION; INTERPLANETARY SPACE; IONS; MAGNETIC FIELDS; MASS; PLASMA; SHOCK WAVES; SOLAR CORONA; SOLAR WIND; SUN; TRAPPING; VELOCITY

Citation Formats

Gargaté, L., Fonseca, R. A., Silva, L. O., Bamford, R. A., and Bingham, R., E-mail: Ruth.Bamford@stfc.ac.uk. SEP acceleration in CME driven shocks using a hybrid code. United States: N. p., 2014. Web. doi:10.1088/0004-637X/792/1/9.
Gargaté, L., Fonseca, R. A., Silva, L. O., Bamford, R. A., & Bingham, R., E-mail: Ruth.Bamford@stfc.ac.uk. SEP acceleration in CME driven shocks using a hybrid code. United States. doi:10.1088/0004-637X/792/1/9.
Gargaté, L., Fonseca, R. A., Silva, L. O., Bamford, R. A., and Bingham, R., E-mail: Ruth.Bamford@stfc.ac.uk. Mon . "SEP acceleration in CME driven shocks using a hybrid code". United States. doi:10.1088/0004-637X/792/1/9.
@article{osti_22365245,
title = {SEP acceleration in CME driven shocks using a hybrid code},
author = {Gargaté, L. and Fonseca, R. A. and Silva, L. O. and Bamford, R. A. and Bingham, R., E-mail: Ruth.Bamford@stfc.ac.uk},
abstractNote = {We perform hybrid simulations of a super-Alfvénic quasi-parallel shock, driven by a coronal mass ejection (CME), propagating in the outer coronal/solar wind at distances of between 3 to 6 solar radii. The hybrid treatment of the problem enables the study of the shock propagation on the ion timescale, preserving ion kinetics and allowing for a self-consistent treatment of the shock propagation and particle acceleration. The CME plasma drags the embedded magnetic field lines stretching from the sun, and propagates out into interplanetary space at a greater velocity than the in situ solar wind, driving the shock, and producing very energetic particles. Our results show that electromagnetic Alfvén waves are generated at the shock front. The waves propagate upstream of the shock and are produced by the counter-streaming ions of the solar wind plasma being reflected at the shock. A significant fraction of the particles are accelerated in two distinct phases: first, particles drift from the shock and are accelerated in the upstream region, and second, particles arriving at the shock get trapped and are accelerated at the shock front. A fraction of the particles diffused back to the shock, which is consistent with the Fermi acceleration mechanism.},
doi = {10.1088/0004-637X/792/1/9},
journal = {Astrophysical Journal},
number = 1,
volume = 792,
place = {United States},
year = {Mon Sep 01 00:00:00 EDT 2014},
month = {Mon Sep 01 00:00:00 EDT 2014}
}
  • Our code of solar energetic particle (SEP) acceleration and transport developed in Arizona is combined with the realistic CME simulations of Michigan, using the solar wind and magnetic field data of the Michigan CME-simulation as input to the SEP code. We suggest that, in addition to the acceleration at the shock significant acceleration may also occur in the sheet behind the shock, where magnetic field lines are compressed as they are bent around the expanding cloud. We consider field aligned motion and cast the proper Fokker-Planck equation into a non-inertial comoving frame, that follows field lines as they evolve. Illustrativemore » simulation results are presented.« less
  • We compare the behavior of heavy ion spectra during an Energetic Storm Particle (ESP) event that exhibited clear evidence of wave excitation with that observed during an intense, large gradual Solar Energetic Particle (SEP) event in which the associated <0.2 MeV/nucleon ions are delayed >12 hr. We interpret that the ESP event is an example of the first-order Fermi acceleration process where enhancements in the magnetic field power spectral densities around local ion cyclotron frequency {nu}{sub pc} indicate the presence of Alfven waves excited by accelerated protons streaming away from the in-situ interplanetary shock. The softening or unfolding of themore » CNO energy spectrum below {approx}200 keV/nucleon and the systematic organization of the Fe and O spectral roll-overs with the E/q ratio during the ESP event are likely due to M/Q-dependent trapping and scattering of the heavy ions by the proton-excited waves. Based on striking similarities in the spectral behavior observed upstream of both, the ESP and the SEP event, we suggest that coupling between proton-generated Alfven waves and energetic ions is also operating at the distant CME shock during the large, gradual SEP event, thereby providing us with a new, powerful tool to remotely probe the roles of shock geometries and wave-particle interactions at near-Sun CME-driven shocks.« less
  • Observations relating the characteristics of electrons seen near Earth (solar energetic particles [SEPs]) and those producing flare radiation show that in certain (prompt) events the origin of both populations appears to be the flare site, which shows strong correlation between the number and spectral index of SEP and hard X-ray radiating electrons, but in others (delayed), which are associated with fast coronal mass ejections (CMEs), this relation is complex and SEPs tend to be harder. Prompt event spectral relation disagrees with that expected in thick or thin target models. We show that using a more accurate treatment of the transportmore » of the accelerated electrons to the footpoints and to Earth can account for this discrepancy. Our results are consistent with those found by Chen and Petrosian for two flares using nonparametric inversion methods, according to which we have weak diffusion conditions, and trapping mediated by magnetic field convergence. The weaker correlations and harder spectra of delayed events can come about by reacceleration of electrons in the CME shock environment. We describe under what conditions such a hardening can be achieved. Using this (acceleration at the flare and reacceleration in the CME) scenario, we show that we can describe the similar dichotomy that exists between the so-called impulsive, highly enriched ({sup 3}He and heavy ions), and softer SEP events and stronger, more gradual SEP events with near-normal ionic abundances and harder spectra. These methods can be used to distinguish the acceleration mechanisms and to constrain their characteristics.« less
  • Particle spectra at a CME-driven shock often exhibit a power law to certain energies, then roll over exponentially beyond. However, there are cases where a spectrum evolves to another power law above a certain energy (e.g. the Oct. 29th, 2003 event). Here we introduce an effective 'loss term' into the particle transport equation and study the consequent particle spectra behavior at a CME-driven shock. The loss term represents the effect of particle leaking out from a finite shock and is related to the turbulence power at and near the shock. We show that the shape of particle spectra are tightlymore » related to the form of upstream turbulence. Under certain circumstances, broken power-law spectrum can be obtained. The physical meaning of the 'loss term' and its relationship to the upstream turbulence is discussed.« less
  • The authors have made a direct comparison between computer simulations of a plane, parallel, collisionless shock including particle acceleration to energies typical of those of diffuse ions observed at the earth bow shock. Despite the fact that the one-dimensional hybrid and Monte Carlo techniques employ entirely different algorithms, they give surprisingly close agreement in the overall shapes of the complete distribution functions for protons as well as heavier ions. Both methods show that energetic ions emerge smoothly from the background thermal plasma with approximately the same relative injection rate and that the fraction of the incoming plasma`s energy flux thatmore » is converted into downstream enthalpy flux of the accelerated population (i.e., the acceleration efficiency) is similar in the two cases. The fraction of the downstream proton distribution made up of superthermal particles is quite large, with at least 10% of the energy flux going into protons with energies above 10 keV. In addition, an upstream precursor, produced by backstreaming energetic particles, is present in both shocks, although the Monte Carlo precursor is considerable longer than that produced in the hybrid shock. These results offer convincing evidence that, at least in these ways, the two simulations are consistent in their description of parallel shock structure and particle acceleration, and they lay the groundwork for development of shocks models employing a combination of both methods. 30 refs., 6 figs.« less