Nonthermal radio emission from solar soft X-ray transient brightenings

Dale E. Gary, Michael D. Hartl, Toshifumi Shimizu

Research output: Contribution to journalArticlepeer-review

52 Scopus citations


We compare microwave total power spectral data from the Owens Valley Radio Observatory Solar Array with soft X-ray transient brightenings observed with the Yohkoh soft X-ray telescope. We find that the transient brightenings are clearly detected in microwaves in 12 of 34 events (35%), possibly detected in another 17 of 34 events (50%), and only five of 34 events (15%) had no apparent microwave counterpart. Comparing the radio and soft X-ray characteristics, we find that (1) the soft X-ray peak is delayed relative to the microwave peak in 16 of 20 events, (2) the microwave flux is correlated with the flux seen in soft X-rays, (3) when radio fluence is used instead of radio flux (24 events) the correlation increases substantially, (4) the microwave spectra in the range 1-18 GHz vary greatly from event to event, (5) the microwave spectra often peak in the range 5-10 GHz (13 of 16 events), and (6) the microwave spectra of some events show narrowband spectra with a steep low-frequency slope. We conclude that the emission from at least some events is the result of a nonthermal population of electrons, and that transient brightenings as a whole can therefore be identified as microflares, the lowenergy extension of the general flare energy distribution. Soft X-ray transient brightenings, and therefore microflares, cannot heat the corona.

Original languageEnglish (US)
Pages (from-to)958-968
Number of pages11
JournalAstrophysical Journal
Issue number2 PART I
StatePublished - 1997
Externally publishedYes

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science


  • Sun: X-rays, gamma rays
  • Sun: radio radiation


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