Heating events in the quiet solar corona: multiwavelength correlations

Coronal, transition region and chromospheric lines and centimeter radio emission of the quiet Sun have been simultaneously observed by SoHO and the VLA. The corona above the magnetic network has a higher pressure and is more variable than above the interior of supergranular cells. The Fourier transform in time is found to have steeper spectra in the corona and upper chromosphere than in the transition region. The temporal sequence of brightenings has been determined by cross-correlations of identical picture elements in different emissions. The method allows to study statistically the faintest fluctuations in the corona and relate them to the layers below. The cross-correlations yield that (i) the first emissions to peak in time are O V and He I originating in the transition region and the upper chromosphere, respectively. (ii) The coronal line of Fe XII lags by about 5 minutes and Fe IX/X peaks a further half a minute later in the average, latest of all emissions. The interpretation of these lags follows readily from analogy with regular flares in active regions, where O V and He I correlate closely with hard X-rays emitted by beam electrons impinging on the chromosphere. The coronal iron lines are then emitted by the evaporating plasma expanding into the corona and cooling by conducting part of the energy to increase the emission in Fe IX. (iii) The radio emission peaks before the coronal emission measure, similar to the Neupert effect in flares, but shows considerable variation relative to O V. It is proposed that there are two emission processes at work radiating both thermal emission and non-thermal gyrosynchrotron emission at various fluxes. These statistical results show that the coronal heating events follow the properties of regular solar flares and thus may be interpreted as microflares or nanoflares.