Solar Radar

The possibility of probing the solar corona, solar priminences, and coronal mass ejections (CMEs) from the ground using large radar systems will be examined. Solar radar would utilize direct reflection (i.e. soundings) from the solar plasma along with coherent scatter from Langmuir waves in coronal arcs and CMEs. The value of active sounding to space weather is that it could provide unambiguous information about the range, bearing, and speed of the target. Such information could be crucial for initial-value and assimilative models providing operational space-weather forecasts.

Technical challenges associated with the solar-radar problem are significant but not insurmountable, and most of the design choices are clearcut. For solar studies, the radar wavelength must be longer than the plasma Debye length.  This places a premium on low radar frequencies which overrides the penalty of increased sky and solar noise. However, the radar frequency should not fall below the maximum usable frequency (MUF) since that would invite radar clutter from sky waves. The ideal frequency is therefore between 40--50 MHz. The crucial performance metric is the transmitter power-aperture product which sets the flux that can be delivered to the Sun.  In order to optimize this flux, the antenna for transmission should be a steerable aperture or filled array with about a 1-degree half-power beamwidth.  Steerability is necessary to keep the radar beam trained on the Sun, facilitating long incoherent integration times. The receive array meanwhile must be large enough that most of the noise it receives comes from the solar disk itself and not from the galactic background. However, we have to consider that the main source of noise in solar radar experiments will be type III radio bursts. The noise temperature at VHF frequencies from solar radio bursts can be several orders of magnitude greater than that of the quiet sun, and system performance will depend on discriminating solar echoes from radio bursts. Adaptive beamforming will ultimately be critical for operational solar-radar space-weather applications. It is in this way that a large, modular receiving arrays become important. All things considered, a facility comparable in size and power to the existing NSF Geospace Facilties but operating in the VHF band and possessing spaced-receiver capabilities should be able to detect solar echoes.

A number of attempts have been made already to detect solar echoes. The historical record is mixed and inconsistent, and so the plausibility of the concept remains somewhat mysterious. Recent and ongoing attempts to receive solar echoes at The Jicamarca Radio Observatory near Lima, Peru, will be discussed.