Astronomy and Astrophysics – Astronomy
Scientific paper
Dec 2002
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002agufmsh52a0444c&link_type=abstract
American Geophysical Union, Fall Meeting 2002, abstract #SH52A-0444
Astronomy and Astrophysics
Astronomy
2169 Sources Of The Solar Wind, 6949 Radar Astronomy, 6964 Radio Wave Propagation, 7509 Corona
Scientific paper
Radar echoes from the Sun were first detected in 1959 at 25 MHz and an extensive set of measurements was made at 38 MHz between 1960 and 1969. The results were unexpected and could not be explained at the time. Interest in the technique waned and radar astronomy evolved to the use of higher frequencies so it became impossible to repeat the measurements. The early observations can be explained in the light of our present understanding of the corona. New radar observations, with correlative optical, UV, and soft X-ray observations, would be very useful in probing the corona near the origin of the solar wind. Radar measures the range to the reflection point and the plasma velocity at the reflection point. Reflection occurs where the dielectric constant goes to zero, which is polarization dependent. Thus dual polarization observations provide estimates of the electron density, magnetic field, and velocity at the reflection point. Solar echoes can be observed at frequencies between 18 MHz and 100 MHz, corresponding to reflection heights between (roughly) 1.8 Rs and 1.15 Rs. It may be possible to operate up to 200 MHz and probe to the edge of the transition region. Here we will review the early observations; explain their basic features; outline existing and potential opportunities for new observations; and speculate on the future development of the technique.
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