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UNDERSTANDING THE GEOMETRY OF ASTROPHYSICAL MAGNETIC FIELDS

Abstract

Faraday rotation measurements have provided an invaluable technique for probing the properties of astrophysical magnetized plasmas. Unfortunately, typical observations provide information only about the density-weighted average of the magnetic field component parallel to the line of sight. As a result, the magnetic field geometry along the line of sight, and in many cases even the location of the rotating material, is poorly constrained. Frequently, interpretations of Faraday rotation observations are dependent upon underlying models of the magnetic field being probed (e.g., uniform, turbulent, equipartition). However, we show that at sufficiently low frequencies, specifically below roughly 13(RM/1 rad m{sup -2}){sup 1/4}(B/1 G){sup 1/2} MHz, the character of Faraday rotation changes, entering what we term the 'super-adiabatic regime' in which the rotation measure (RM) is proportional to the integrated absolute value of the line-of-sight component of the field. As a consequence, comparing RMs at high frequencies with those in this new regime provides direct information about the geometry of the magnetic field along the line of sight. Furthermore, the frequency defining the transition to this new regime, {nu}{sub SA}, depends directly upon the local electron density and magnetic field strength where the magnetic field is perpendicular to the line of sight, allowing  More>>
Authors:
Broderick, Avery E; [1]  Blandford, Roger D., E-mail: aeb@cita.utoronto.c [2] 
  1. Canadian Institute for Theoretical Astrophysics, 60 St. George St., Toronto, ON M5S 3H8 (Canada)
  2. Kavli Institute for Particle Astrophysics and Cosmology, 2575 Sand Hill Rd., Menlo Park, CA 94309 (United States)
Publication Date:
Aug 01, 2010
Product Type:
Journal Article
Resource Relation:
Journal Name: Astrophysical Journal; Journal Volume: 718; Journal Issue: 2; Other Information: DOI: 10.1088/0004-637X/718/2/1085
Subject:
79 ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; ASTROPHYSICS; BLACK HOLES; ELECTRON DENSITY; EMISSION; FARADAY EFFECT; GALAXY NUCLEI; MAGNETIC FIELDS; MILKY WAY; PLASMA; POLARIZATION; ROTATION; TURBULENCE; GALAXIES; MOTION; PHYSICS
OSTI ID:
21455083
Country of Origin:
United States
Language:
English
Other Identifying Numbers:
Journal ID: ISSN 0004-637X; ASJOAB; TRN: US10Q7297049545
Availability:
Available from http://dx.doi.org/10.1088/0004-637X/718/2/1085
Submitting Site:
INIS
Size:
page(s) 1085-1099
Announcement Date:
Jul 16, 2011

Citation Formats

Broderick, Avery E, and Blandford, Roger D., E-mail: aeb@cita.utoronto.c. UNDERSTANDING THE GEOMETRY OF ASTROPHYSICAL MAGNETIC FIELDS. United States: N. p., 2010. Web. doi:10.1088/0004-637X/718/2/1085.
Broderick, Avery E, & Blandford, Roger D., E-mail: aeb@cita.utoronto.c. UNDERSTANDING THE GEOMETRY OF ASTROPHYSICAL MAGNETIC FIELDS. United States. doi:10.1088/0004-637X/718/2/1085.
Broderick, Avery E, and Blandford, Roger D., E-mail: aeb@cita.utoronto.c. 2010. "UNDERSTANDING THE GEOMETRY OF ASTROPHYSICAL MAGNETIC FIELDS." United States. doi:10.1088/0004-637X/718/2/1085. https://www.osti.gov/servlets/purl/10.1088/0004-637X/718/2/1085.
@misc{etde_21455083,
title = {UNDERSTANDING THE GEOMETRY OF ASTROPHYSICAL MAGNETIC FIELDS}
author = {Broderick, Avery E, and Blandford, Roger D., E-mail: aeb@cita.utoronto.c}
abstractNote = {Faraday rotation measurements have provided an invaluable technique for probing the properties of astrophysical magnetized plasmas. Unfortunately, typical observations provide information only about the density-weighted average of the magnetic field component parallel to the line of sight. As a result, the magnetic field geometry along the line of sight, and in many cases even the location of the rotating material, is poorly constrained. Frequently, interpretations of Faraday rotation observations are dependent upon underlying models of the magnetic field being probed (e.g., uniform, turbulent, equipartition). However, we show that at sufficiently low frequencies, specifically below roughly 13(RM/1 rad m{sup -2}){sup 1/4}(B/1 G){sup 1/2} MHz, the character of Faraday rotation changes, entering what we term the 'super-adiabatic regime' in which the rotation measure (RM) is proportional to the integrated absolute value of the line-of-sight component of the field. As a consequence, comparing RMs at high frequencies with those in this new regime provides direct information about the geometry of the magnetic field along the line of sight. Furthermore, the frequency defining the transition to this new regime, {nu}{sub SA}, depends directly upon the local electron density and magnetic field strength where the magnetic field is perpendicular to the line of sight, allowing the unambiguous distinction between Faraday rotation within and in front of the emission region. Typical values of {nu}{sub SA} range from 10 kHz (below the ionospheric cutoff, but above the heliospheric cutoff) to 10 GHz, depending upon the details of the Faraday rotating environment. In particular, for resolved active galactic nuclei, including the black holes at the center of the Milky Way (Sgr A*) and M81, {nu}{sub SA} ranges from roughly 10 MHz to 10 GHz, and thus can be probed via existing and up-coming ground-based radio observatories.}
doi = {10.1088/0004-637X/718/2/1085}
journal = {Astrophysical Journal}
issue = {2}
volume = {718}
place = {United States}
year = {2010}
month = {Aug}
}