Negative Microwave Bursts

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Introduction

The microwave spectrum, observable by radio observatories, has a close relationship with the X-ray spectrum as observed by RHESSI. Typically one sees bursts in both spectral bands, but - rarely - one sees negative bursts in the microwaves. Such bursts were described in the 1950s by the eminent radio astronomer Arthur Covington, who also introduced the much-used F10.7 radio index of solar activity. The negative bursts are in a sense easy to understand - the whole Sun emits microwave radiation from near the base of the corona, and so a relatively cool and absorbing coronal feature - for example, a prominence - may obscure parts of this emission. Indeed, Covington had also used the Moon (via an eclipse observed in Ottawa, in 1946!) as an absorber to show that solar radio emission was concentrated spatially in sunspot regions and to establish the importance of magnetism in solar activity.

The physics of negative microwave bursts may not be simple, and therefore many things may be learned about the behavior of the solar corona by observing it with modern equipment. This includes not only the microwave domain, but also the EUV as currently observed, for example, via the [Solar Dynamics Observatory http://sdo.gsfc.nasa.gov/] among others. We have now begun further studies of these phenomena, as introduced in Ref. [1].

Some examples

Negative microwave bursts often accompany solar flares, but that is not always the case. Sometimes coronal activity can result in sudden absorptions in the absence of an obvious flare or CME; we show examples of non-flare (left) and flare (right) negative bursts in Figure 1. The SDO movies of the latter make the possible utility of microwave absorption quite obvious! In the remarkable case on the left, the dip at long wavelengths at about 01:10 UT corresponds to dimming seen at 304 A, due to He I, which clearly implicates relatively cool material embedded in the otherwise hot corona. The dimming for the flare event may involve this phenomenon, as well as the effects of the CME eruption associated with this very powerful flare.

Figure 1: Left, a microwave negative burst not obviously associatd with a flare; right, that observed during the huge event SOL2011-06-07. The movies in this link illustrate how clouds of material move through the corona, often obscuring sources at lower altitude.

The radio observatories involved in the observations of Figure 1 are in Australia (Lear), Japan (NoRP), and Russia (Voro), and one can clearly see the wavelength dependence (the free-free opacity increases as the square of the wavelength). One of the most exciting prospects here is that the radio data can be calibrated very precisely, and that there are radio images with much higher spatial resolution than heretofore available. Thus a multi-wavelength analysis of radio, white light, EUV, and X-ray signatures will obviously enrich our knowledge of the physics involved in the negative-burst phenomena.

References

[1] "Microwave Negative Bursts as Indications of Reconnection between Eruptive Filaments and Large-Scale Coronal Magnetic Environment"