Simultaneous VLA and UVCS/SOHO Observations

of the Solar Corona

Steven R. Spangler (University of Iowa), Mari Paz Miralles, Steven R. Cranmer, and John C.

Raymond (Harvard-Smithsonian CFA)

Knowledge of the coronal magnetic field is crucial for assessing theories of coronal heating and origin of the solar wind. One of the few ways of measuring the magnetic field in the corona is via Faraday rotation of linearly polarized radio signals which have propagated through the corona and been received on Earth. An example of such a source of linearly polarized waves is a radio galaxy or quasar.

Coronal Faraday Rotation

Faraday rotation observations need additional information to be interpreted. On August 16, 2003, we made observations of the radio galaxy 3C228 (viewed through the corona at heliocentric distances of 7-6.2 solar radii) with the VLA. Simultaneous observations of that part of the corona were made with the UVCS instrument on SOHO. The VLA observations measured linear polarization at 1465 and 1665 MHz.

The corona on August 16,

2003

Radio line of sight during session

UVCS slit positions

Radio source 3C228 Image at left is from reference observations. Three source components were bright enough in polarized emission for a p.a. measurement in a single 10 minute scan.

A

B

C

Measured Coronal Faraday Rotation

20 degrees = 8.3 rad/m2

Solid symbols: 21cm; open 18cm

B A C

UVCS Measurements

UVCS observations yielded line profiles of Lyman Alpha and OVI, and polarized brightness in white light

The UVCS observations yield (preliminary) measurements of the hydrogen and OVI temperatures (perpendicular and parallel), solar wind outflow speed, and plasma density in the streamers and coronal holes.

Why was such a large Rotation Measure event observed?

Point of closest approach of the line of sight was on the neutral line of the coronal magnetic field: this is the condition for maximum rotation measure

Thanks to Bill Coles, UCSD, for providing and maintaining the programs to plot these trajectories.

Work in the future will consist of modeling these observations to yield the strength and functional form of the

coronal magnetic field at heliocentric distances of 5 – 10 solar radii. The rotation measure observations presented

here are consistent with earlier results regarding the strength and form of the coronal field (Sakurai and Spangler 1994;

Mancuso and Spangler 2000).

Thanks

This work was supported at the University of Iowa by grants ATM99-86887 and ATM03-54782 from the Division of Atmospheric Sciences of the National

Science Foundation. The Very Large Array is operated by the National Radio Astronomy Observatory, which is also supported by NSF. This work was supported at the Harvard-Smithsonian Center for Astrophysics by

grants NNG04GE84G and NAG5-11913 from the National Aeronautics and Space Administration, by the

Agenzia Spaziale Italiana, and by the Swiss contribution to the ESA PRODEX program.