Steven R. Spangler
Department of Physics and Astronomy, University of Iowa
Modern radio telescopes are extremely sensitive to plasma on the line of sight from a radio source to an antenna. Plasmas in the corona and solar wind produce measurable changes in the radio wave amplitude and phase, and the phase difference between wave fields of opposite circular polarization. Such measurements can be made of radio waves from spacecraft transmitters and extragalactic radio sources, using existing radio telescopes (such as the Very Large Array) and spacecraft tracking antennas. Data have been taken at frequencies from about 80 MHz to 8000 MHz. Lower frequencies probe plasma at greater heliocentric distances. Analysis of these data yields information on the plasma density, density fluctuations, and plasma flow speeds in the corona and solar wind, and on the magnetic field in the solar corona. I will discuss the ways in which these observations can contribute to topics of interest to this symposium. Specific examples are the magnetic and density structure of CMEs, the apparent absence of MHD fast mode waves in the corona, and measurements of, or upper limits to, electrical currents in the corona. Finally, I summarize plans for future instruments that will provide new capability in this field.