A microwave study of coronal ejecta

Using Nobeyama 17 GHz data, we have studied the radio properties of 19 coro nal jets identified in Yohkoh soft X-ray imaging telescope (SXT) X-ray obse rvations. The radio data provide information on the physical conditions in the jets, which complements the data from the X-ray surveys. Microwave emis sion was associated with the majority of the X-ray jets in our sample. The radio emission typically came from the base or the base and lower part of t he jets. We detected radio emission from almost all jets that showed flarel ike activity at their bases. The jets that were not associated with radio e mission did not show any significant increase in X-ray emission at their ba ses. The strongest radio emission came from two of the largest jets in our sample. Our data show a general correlation between the X-ray jet fluxes an d the associated radio fluxes. The 17 GHz time profiles were gradual and un polarized, implying that the emission was thermal. In a two-sided-loop jet (1992 July 22 event) and one anemone-type jet (1993 February 9 event), the observed microwave fluxes from the lower part of the jets were well above t he fluxes calculated from the computed physical parameters of the soft X-ra y-emitting material on the basis of thermal free-free emission. We interpre t the large discrepancies in terms of the presence of lower temperature mat erial, which cannot be detected by the SXT (the SXT is most sensitive to ho t plasma above 2 x 10(6) K), but which produces strong microwave free-free emission. This is the first time that such material has been observed in tw o-sided-loop-type jets. We also observed motion of a jet-associated microwa ve source with a velocity of 55 km s(-1). The microwave motion occurred aft er the appearance of the X-ray jet. There is clear evidence that the microw ave emission of that source was associated with the jet and not with the as sociated small hare.