Peak flux density spectra of large solar radio bursts and proton emission from flares

We have reexamined the relationship between “U‐shaped” peak flux density microwave spectra and solar proton events for ∼200 large ( Sp (≥2 GHz) ≥ 800 solar flux units (sfu)) microwave bursts (1965–1979). The radio spectra fell into two basic classes: U‐shaped, with two maxima (≥ 800 sfu) in the range from 200 MHz to ≥ 10 GHz (59% of all events), and cutoff spectra, with a maximum ≥800 sfu at f ≥ 2 GHz and Sp (200 MHz) < 100 sfu (18%). Nine percent of the events had “intermediate” spectra with a maximum ≥800 sfu at f ≥ 2 GHz and 100 sfu ≤ Sp (200 MHz) < 800 sfu. We were unable to classify 15% of the events because of incomplete data. The associations of the three classes of spectra with type II (and /or type IV) meter wavelength bursts and > 10‐MeV proton events of any size (≥0.01 protons cm −2 s −1 sr −1 ) are as follows: U‐shaped: type II/IV (90% of large microwave bursts with U‐shaped spectra are associated with type II/IV events), protons (77%); intermediate: type II/IV (78%), protons (73%); and cutoff: type II/IV (22%), protons (33%). These statistics affirm various lines of evidence linking coronal shock waves and interplanetary proton events. They also suggest that the meter wavelength branch of the U‐shaped spectrum may be attributable to second‐phase (versus flash phase) accelerated electrons. We have examined this latter supposition and find that it cannot be true in general. In our sample a type II event was in progress at the time of the peak of the low‐frequency branch for only about half of the bursts with U‐shaped spectrum (U bursts). For these events we cannot rule out a possible contribution to the peak 200‐MHz flux from either the second harmonic of the type II burst or from flare continuum of the type FC II, provided that the starting frequency of the fundamental type II burst is ≳100 MHz. The low‐frequency branch of the U burst appears to be more closely related to impulsive phase type III emission. We note that the small sample of U bursts that lacked type II/IV association is also poorly associated with proton events. We conclude that the observed association between U bursts and proton events probably results from the big flare syndrome rather than a close physical link between these two phenomena. If the current National Oceanic and Atmospheric Administration prediction threshold of J (> 10 MeV) ≥ 10 protons cm −2 s −1 sr −1 had been in effect during the period covered by our data base (1965–1979), the U burst “yes or no” forecast tool would have had a false alarm rate of 50–70% and would have failed to provide warning for 40–50% of the significant prompt proton events attributable to disk flares. We note that several (eight of 46) of the prompt proton events with J (>10 MeV) > 10 protons cm −2 s −1 sr −1 observed from 1965 to 1979 originated in flares that had relatively weak (≤300 sfu) burst emission at 200 MHz.