Radial streaming anisotropies of charged particles accelerated at the solar wind termination shock

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Abstract

Recent observations of an increase in energetic particle intensities on the Voyager 1 spacecraft, for several months in late 2002 to early 2003, suggest new phenomena associated with its approach to the termination shock of the solar wind. An important diagnostic used by the experimenters to interpret the event has been the radial anisotropy of the energetic particle distribution. In this Letter we consider the transport of energetic charged particles accelerated at the termination shock, both in the diffusion approximation and by directly integrating test particle trajectories in a turbulent magnetic field. We determine the radial anisotropy in a number of models. In general, we find that the radial anisotropy of the particles is complicated and that it cannot reliably be used to determine the radial velocity of the background plasma. In all cases considered, we find that the average radial anisotropy in the region upstream of the shock is directed toward the shock and is less than or equal to the Compton-Getting anisotropy in the slower wind downstream. We attribute the difference from the Compton-Getting anisotropy to the diffusive anisotropy, which is directed upstream away from the shock and which is part of the acceleration process. We suggest that the small radial anisotropy observed on Voyager 1 at low energies may actually be an indication that Voyager 1 was located upstream of the shock during the entire event. In addition, we find large fluctuating anisotropies along the magnetic field (nearly transverse to the radial direction).

Original languageEnglish (US)
JournalAstrophysical Journal
Volume605
Issue number2 II
DOIs
StatePublished - Apr 20 2004

Fingerprint

solar wind
charged particles
anisotropy
shock
energetic particles
upstream
energetics
Voyager 1 spacecraft
particle intensity
magnetic field
particle
particle trajectories
magnetic fields
radial velocity
indication
spacecraft
trajectory
plasma
approximation
energy

Keywords

  • Acceleration of particles
  • Interplanetary medium
  • Shock waves

ASJC Scopus subject areas

  • Space and Planetary Science

Cite this

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title = "Radial streaming anisotropies of charged particles accelerated at the solar wind termination shock",
abstract = "Recent observations of an increase in energetic particle intensities on the Voyager 1 spacecraft, for several months in late 2002 to early 2003, suggest new phenomena associated with its approach to the termination shock of the solar wind. An important diagnostic used by the experimenters to interpret the event has been the radial anisotropy of the energetic particle distribution. In this Letter we consider the transport of energetic charged particles accelerated at the termination shock, both in the diffusion approximation and by directly integrating test particle trajectories in a turbulent magnetic field. We determine the radial anisotropy in a number of models. In general, we find that the radial anisotropy of the particles is complicated and that it cannot reliably be used to determine the radial velocity of the background plasma. In all cases considered, we find that the average radial anisotropy in the region upstream of the shock is directed toward the shock and is less than or equal to the Compton-Getting anisotropy in the slower wind downstream. We attribute the difference from the Compton-Getting anisotropy to the diffusive anisotropy, which is directed upstream away from the shock and which is part of the acceleration process. We suggest that the small radial anisotropy observed on Voyager 1 at low energies may actually be an indication that Voyager 1 was located upstream of the shock during the entire event. In addition, we find large fluctuating anisotropies along the magnetic field (nearly transverse to the radial direction).",
keywords = "Acceleration of particles, Interplanetary medium, Shock waves",
author = "Jokipii, {J. Randy} and Joe Giacalone",
year = "2004",
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language = "English (US)",
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journal = "Astrophysical Journal",
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T1 - Radial streaming anisotropies of charged particles accelerated at the solar wind termination shock

AU - Jokipii, J. Randy

AU - Giacalone, Joe

PY - 2004/4/20

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N2 - Recent observations of an increase in energetic particle intensities on the Voyager 1 spacecraft, for several months in late 2002 to early 2003, suggest new phenomena associated with its approach to the termination shock of the solar wind. An important diagnostic used by the experimenters to interpret the event has been the radial anisotropy of the energetic particle distribution. In this Letter we consider the transport of energetic charged particles accelerated at the termination shock, both in the diffusion approximation and by directly integrating test particle trajectories in a turbulent magnetic field. We determine the radial anisotropy in a number of models. In general, we find that the radial anisotropy of the particles is complicated and that it cannot reliably be used to determine the radial velocity of the background plasma. In all cases considered, we find that the average radial anisotropy in the region upstream of the shock is directed toward the shock and is less than or equal to the Compton-Getting anisotropy in the slower wind downstream. We attribute the difference from the Compton-Getting anisotropy to the diffusive anisotropy, which is directed upstream away from the shock and which is part of the acceleration process. We suggest that the small radial anisotropy observed on Voyager 1 at low energies may actually be an indication that Voyager 1 was located upstream of the shock during the entire event. In addition, we find large fluctuating anisotropies along the magnetic field (nearly transverse to the radial direction).

AB - Recent observations of an increase in energetic particle intensities on the Voyager 1 spacecraft, for several months in late 2002 to early 2003, suggest new phenomena associated with its approach to the termination shock of the solar wind. An important diagnostic used by the experimenters to interpret the event has been the radial anisotropy of the energetic particle distribution. In this Letter we consider the transport of energetic charged particles accelerated at the termination shock, both in the diffusion approximation and by directly integrating test particle trajectories in a turbulent magnetic field. We determine the radial anisotropy in a number of models. In general, we find that the radial anisotropy of the particles is complicated and that it cannot reliably be used to determine the radial velocity of the background plasma. In all cases considered, we find that the average radial anisotropy in the region upstream of the shock is directed toward the shock and is less than or equal to the Compton-Getting anisotropy in the slower wind downstream. We attribute the difference from the Compton-Getting anisotropy to the diffusive anisotropy, which is directed upstream away from the shock and which is part of the acceleration process. We suggest that the small radial anisotropy observed on Voyager 1 at low energies may actually be an indication that Voyager 1 was located upstream of the shock during the entire event. In addition, we find large fluctuating anisotropies along the magnetic field (nearly transverse to the radial direction).

KW - Acceleration of particles

KW - Interplanetary medium

KW - Shock waves

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