Solar Radar Observations
Radar observations of the sun have been made extensively at decameter and low meter wavelengths (Eshleman et al., 1960, and James, 1966). Their interpretation by specular reflection on high density structures with “corner reflector” shape is unlikely from the echo spectral broadening and range depth. Gordon’s (1973) interpretation of the scattering by a 4 wave interaction between radar and coronal Langmoir waves requires a level of 10−2nKT (thermal energy density) of the Langmuir waves. A radar experiment in microwaves with the 300 m dish in Arecibo*) is described, which was able to test this hypothesis. It was based on the idea of scattering radar waves on Langmuir waves by the much more efficient 3 wave interaction. The echo at the beat frequency of the radar (2380 MHz) and the Langmuir wave (170–270 MHz) is then to be expected at 2600 MHz. The results, however, show the absence of echos, from which an upper limit of 6.10−4nKT for the level of Langmuir waves is derived. First results will soon be published (Benz and Fitze, 1979).
Here I report from an other microwave radar experiment in Arecibo which was receiving at the transmitted frequency, similar as the decametric radar observations. We have probed a coronal streamer, a coronal hole, and an active region (emitting noise storm radio bursts).
No echo has yet been detected. Previously, radar observations have been suggested to be possible only at low frequencies, since the optical depth of the plasma layer increases rapidly with frequency, absorbing any echo mirrored at this plasma. However, James’ result clearly shows that the reflection occurs well above the plasma layer.
I propose that decametric and low metric radar echos are produced by interactions of the radar wave with ion acoustic waves. Such low frequency electrostatic waves have been suggested by Benz and Wentzel (1979) to be present in type I radio burst sources due to a current-driven instability, which dissipates free magnetic energy. The requirement of this model on magnetic flux into the corona, 1013 gauss cm2s−1, fits very well the observations. The absence of an echo from Langmuir waves is then due to their low level, and the difficulty to detect echos at high frequency due to the high collisional damping of ion acoustic (and radar) waves at lower heights.
KeywordsCoronal Hole Radio Burst Plasma Layer Radar Observation Langmuir Wave
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