I have attempted a correlation of low amplitude radio bursts at 3 cm with transient features on Hc~ films.
The 130 ft antenna at Owens Valley Radio Observatory has a HPBW of about 3 rain at 3 cm, enabling us to track a single active region which was followed at the same time in He with the 13 cm photoheliograph at Caltech and the 25 cm photoheliographs at Big Bear Solar Observatory.
The original intention of the experiment, as proposed to me by H. Zirin, was to derive statistics of the correlation between He and centimeter bursts for events that were too small to be identified on the usual whole-Sun RF records and He patrol movies.
Unfortunately, over the six days in 1970-71 that the experiment was performed, malfunction of the antenna and cloud cover reduced the reliable simulteneous data to a few hours on July [14][15] 1971.
During this time only 3 well-defined He 'events' occurred, so no statistics can be derived. However, a temperature can be calculated usefully for the one event which resulted in a 3-cm grf burst of" at least 1.2 sfu from McMath Plage 425 A, producing a plage brightening directly behind the P-spot from about 17:15 UT to about 17:50 UT. The flux measured here at 3-cm is a lower limit. The small burst quite possibly occurred away from the center of the narrow beam which was tracking the maximum of the slowly varying component in that active region.
This grf burst correlates well with an event measured in Ottawa at 2800 MHz with a whole-Sun antenna. It began at 17: 10, peaked at 17:30 at 2 sfu, and lasted 60 rain.
The maximum extent of the brightening in He occurred at about 17:30 UT, about 3 rain after the maximum of the 3-cm burst at 17:27:37 UT. At that time it covered about 0.05 sq. rain of arc.
The radio optical depth of the burst, v, may be estimated, where z = k~L and 0.1N 2
k~-v2Te
Taking Ne,-~ 101~ cm -3 and Allen (1962).