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Title: The Re-Acceleration of Galactic Electrons at the Heliospheric Termination Shock

Abstract

Observations by the Voyager spacecraft in the outer heliosphere presented several challenges for the paradigm of diffusive shock acceleration (DSA) at the solar wind termination shock (TS). In this study, the viability of DSA as a re-acceleration mechanism for galactic electrons is investigated using a comprehensive cosmic-ray modulation model. The results demonstrate that the efficiency of DSA depends strongly on the shape of the electron spectra incident at the TS, which in turn depends on the features of the local interstellar spectrum. Modulation processes such as drifts therefore also influence the re-acceleration process. It is found that re-accelerated electrons make appreciable contributions to intensities in the heliosphere and that increases caused by DSA at the TS are comparable to intensity enhancements observed by Voyager 1 ahead of the TS crossing. The modeling results are interpreted as support for DSA as a re-acceleration mechanism for galactic electrons at the TS.

Authors:
; ;  [1]
  1. Centre for Space Research, North-West University, 2520 Potchefstroom (South Africa)
Publication Date:
OSTI Identifier:
22663829
Resource Type:
Journal Article
Resource Relation:
Journal Name: Astrophysical Journal; Journal Volume: 836; 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; COMPARATIVE EVALUATIONS; COSMIC RADIATION; EFFICIENCY; ELECTRON SPECTRA; ELECTRONS; HELIOSPHERE; MODULATION; SIMULATION; SOLAR WIND; SUN; VOYAGER SPACE PROBES

Citation Formats

Prinsloo, P. L., Potgieter, M. S., and Strauss, R. D., E-mail: marius.potgieter@nwu.ac.za. The Re-Acceleration of Galactic Electrons at the Heliospheric Termination Shock. United States: N. p., 2017. Web. doi:10.3847/1538-4357/836/1/100.
Prinsloo, P. L., Potgieter, M. S., & Strauss, R. D., E-mail: marius.potgieter@nwu.ac.za. The Re-Acceleration of Galactic Electrons at the Heliospheric Termination Shock. United States. doi:10.3847/1538-4357/836/1/100.
Prinsloo, P. L., Potgieter, M. S., and Strauss, R. D., E-mail: marius.potgieter@nwu.ac.za. Fri . "The Re-Acceleration of Galactic Electrons at the Heliospheric Termination Shock". United States. doi:10.3847/1538-4357/836/1/100.
@article{osti_22663829,
title = {The Re-Acceleration of Galactic Electrons at the Heliospheric Termination Shock},
author = {Prinsloo, P. L. and Potgieter, M. S. and Strauss, R. D., E-mail: marius.potgieter@nwu.ac.za},
abstractNote = {Observations by the Voyager spacecraft in the outer heliosphere presented several challenges for the paradigm of diffusive shock acceleration (DSA) at the solar wind termination shock (TS). In this study, the viability of DSA as a re-acceleration mechanism for galactic electrons is investigated using a comprehensive cosmic-ray modulation model. The results demonstrate that the efficiency of DSA depends strongly on the shape of the electron spectra incident at the TS, which in turn depends on the features of the local interstellar spectrum. Modulation processes such as drifts therefore also influence the re-acceleration process. It is found that re-accelerated electrons make appreciable contributions to intensities in the heliosphere and that increases caused by DSA at the TS are comparable to intensity enhancements observed by Voyager 1 ahead of the TS crossing. The modeling results are interpreted as support for DSA as a re-acceleration mechanism for galactic electrons at the TS.},
doi = {10.3847/1538-4357/836/1/100},
journal = {Astrophysical Journal},
number = 1,
volume = 836,
place = {United States},
year = {Fri Feb 10 00:00:00 EST 2017},
month = {Fri Feb 10 00:00:00 EST 2017}
}
  • Observations by the plasma and magnetic field instruments on board the Voyager 2 spacecraft suggest that the termination shock is weak with a compression ratio of {approx}2. However, this is contrary to the observations of accelerated particle spectra at the termination shock, where standard diffusive shock acceleration theory predicts a compression ratio closer to {approx}2.9. Using our focused transport model, we investigate pickup proton acceleration at a stationary spherical termination shock with a moderately strong compression ratio of 2.8 to include both the subshock and precursor. We show that for the particle energies observed by the Voyager 2 Low Energymore » Charged Particle (LECP) instrument, pickup protons will have effective length scales of diffusion that are larger than the combined subshock and precursor termination shock structure observed. As a result, the particles will experience a total effective termination shock compression ratio that is larger than values inferred by the plasma and magnetic field instruments for the subshock and similar to the value predicted by diffusive shock acceleration theory. Furthermore, using a stochastically varying magnetic field angle, we are able to qualitatively reproduce the multiple power-law structure observed for the LECP spectra downstream of the termination shock.« less
  • Pickup ions (PUIs) are created from a number of neutral sources both inside and outside the heiosphere. The combination of recent observational and theoretical work has completed the catalog of at least the major sources of Heliospheric PUIs. These PUIs are the seed population for anomalous cosmic rays (ACRs), which are accelerated to high energies at the termination shock (TS). Recently, Voyager 1 (V1) encountered strong intensity enhancements in the energy range where pickup ions are energized into ACRs. These observations may indicate that V1 has already passed beyond the TS into the bath of accelerated pickup ions in themore » inner heliosheath. However, opposing this conclusion, V1 magnetic field and higher energy particle observations appear very typical. Here, we advance the concept that V1 was in a special region in the heliosphere where the magnetic field is highly distorted due to solar wind shearing and footpoint motions at the Sun. These Favored Acceleration Locations at the Termination Shock (FALTS) efficiently inject pickup ions into diffusive shock acceleration due to slightly increased radial field components in the FALTS field regions. Such regions provide much more direct field-line connection to the TS than along strongly wound Parker field lines found outside of FALTS. Inside the TS, FALTS naturally enhance fluxes of accelerated pickup ions. Thus, FALTS may be responsible for the V1 observations of energetic particle enhancements. Further, FALTS provide a conduit for an inward electron heat flux from hot electrons beyond the TS, thereby heating the plasma and increasing the electron impact ionization rate. This additional ionization causes mass loading that locally weakens the TS and draws it in toward the Sun. Thus, FALTS can cause both energetic particle intensity enhancements and a weaker, locally drawn in TS; these effects may help explain the otherwise contradictory V1 observations.« less
  • It is discussed how a time-dependent focused transport model, using a time series of shock obliquities at the termination shock based on Voyager 1 observations to model magnetic field-line random walk, can reproduce observational features of energetic ions at the termination shock and in the heliosheath which is beyond the scope of standard cosmic-ray transport models.
  • A shock in a collisionless plasma should be treated as a place where the superthermal particles are continuously produced. The Rankine-Hugoniot conditions modified so as to describe the loss of the energy flux during the production process are presented. The model of the interaction between the two colliding shocks in the collisionless magnetized plasma is used to describe the behavior of the heliospheric termination shock. Both forward-reverse and reverse-reverse shock pairs are considered. In consequence of these interactions the termination shock is in a constant in and out motion with velocities equal to 100-200 km/s. The dependence of the interactionmore » parameters on the production efficiency of the superthermal population is discussed. 15 refs., 6 figs.« less
  • The Los Alamos hybrid simulation code is used to examine heating and the partition of dissipation energy at the perpendicular heliospheric termination shock in the presence of pickup ions. The simulations are one-dimensional in space but three-dimensional in field and velocity components, and are carried out for a range of values of pickup ion relative density. Results from the simulations show that because the solar wind ions are relatively cold upstream, the temperature of these ions is raised by a relatively larger factor than the temperature of the pickup ions. An analytic model for energy partition is developed on themore » basis of the Rankine-Hugoniot relations and a polytropic energy equation. The polytropic index {gamma} used in the Rankine-Hugoniot relations is varied to improve agreement between the model and the simulations concerning the fraction of downstream heating in the pickup ions as well as the compression ratio at the shock. When the pickup ion density is less than 20%, the polytropic index is about 5/3, whereas for pickup ion densities greater than 20%, the polytropic index tends toward 2.2, suggesting a fundamental change in the character of the shock, as seen in the simulations, when the pickup ion density is large. The model and the simulations both indicate for the upstream parameters chosen for Voyager 2 conditions that the pickup ion density is about 25% and the pickup ions gain the larger share (approximately 90%) of the downstream thermal pressure, consistent with Voyager 2 observations near the shock.« less