Sharp Decreases of Solar Metric Radio Storm Emission

Abstract

We discuss a little-known variety of sharp decreases of long-duration meter-wavelength noise storms and type IV bursts. A survey of the IZMIRAN and AIP radio observations shows that a decrease or nearly complete disappearance of the continuum and bursts developing over tens of minutes without a subsequent recovery of the radio flux occasionally occurs. The decrease is usually preceded by a short-duration (several tens of minutes) enhancement of the radio emission. In these events, the onset of the flux decrease drifts from high to low frequencies with a rate of −(0.05-0.35) MHz s⁻¹, comparable to the drift rates of noise-storm onsets and of chains of type I bursts. White-light coronagraph observations, as well as the characteristics of the accompanying microwave and soft X-ray emissions, provide evidence that such radio decreases appear to be associated with coronal mass ejections (CMEs) and post-CME phenomena. Yohkoh/SXT images show radio flux decrease events which are accompanied by significant rearrangements of coronal structures. We suggest that the radio flux variations are caused by CME interactions with pre-existing coronal arcade structures which are sources of noise storms and energetic electron acceleration. The fact that the noise-storm decreases develop with delays of several tens of minutes relative to the associated microwave burst peak, when the corresponding CME front is located at heights of several R_⊙, however, is not explained.