The study of coronal plasma structures and fluctuations with Faraday rotation measurements

Abstract

view Abstract Citations (39) References (47) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Study of Coronal Plasma Structures and Fluctuations with Faraday Rotation Measurements Sakurai, Takayuki ; Spangler, Steven R. Abstract We report dual-frequency, polarimetric measurements of Faraday rotation of extragalactic radio sources viewed through the solar corona. The observations were made at the Very Large Array in 1990 during solar maximum. Of the nine observed, an excess rotation measure of -12.6 rad m^-2^ was detected for one source (0010 + 005), which was observed at an elongation of about 9 R_sun_. This measurement is in fair agreement with an a priori model rotation measure of -8.6 rad m^-2^ estimated from coronal potential field models and the electron density model of Patzold et al. (1992). Our measurement provides a value for the coronal magnetic field strength at 9 R_sun_, given a knowledge of the magnetic field sector structure, of 12.5 +/- 2.3 microG. Rotation measure measurements of 0010 + 005 were made approximately once per hour over an 11 hr period. During this interval, a slow change of about 1 rad m^-2^ hr^-1^ in rotation measure was detected. Although we are not absolutely certain that this drift is not unremoved ionospheric Faraday rotation, extensive analysis of data from the other sources suggests that this is not the case (Sakurai & Spangler 1994). The very long timescale for this variation argues against the agency of MHD waves, and we suggest occultation of 0010+005 by relatively static plasma structures in the corona. We filtered our rotation measure time series to search for varia- tions on an hourly timescale, such as those reported by Hollweg et al. (1982), which could be attributed to coronal MHD waves. We were unable to detect such fluctuations and can report only an upper limit to the rms variation of 1.6 rad m^-2^. This upper limit is of the same order, but slightly larger than the values typically reported by Hollweg et al. (1982). This upper limit to the rotation measure fluctuations limits the dimensionless wave amplitude δB/B in the corona to be less than 0.7. Using this number, we estimate the MHD wave flux at the coronal base to be less than 1.6 x 10^5^ ergs cm^-2^ s^-1^. This is less than the amount of wave energy flux required by wave-driven models of the solar wind. Finally, we discuss a number of ways in which such observations could be improved in the future. Publication: The Astrophysical Journal Pub Date: October 1994 DOI: 10.1086/174780 Bibcode: 1994ApJ...434..773S Keywords: Extragalactic Radio Sources; Faraday Effect; Magnetohydrodynamic Waves; Plasma Dynamics; Polarization (Waves); Solar Corona; Solar Magnetic Field; Solar Wind; Stellar Models; Astronomical Polarimetry; Very Large Array (Vla); Solar Physics; MAGNETOHYDRODYNAMICS: MHD; SUN: SOLAR WIND; SUN: CORONA; SUN: MAGNETIC FIELDS; TURBULENCE full text sources ADS | data products NED (13)