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Fine structure of the X-ray and radio emissions of the quiet solar corona
Arnold O. Benz 1,
Säm Krucker 1,
Loren W. Acton 2 and
Tim S. Bastian 3
Received 10 July 1996 / Accepted 27 September 1996
Two deep soft X-ray exposures of a quiet region on the Sun were made with the SXT telescope on board the Yohkoh satellite on 20 Feb 1995. We report on the spatial X-ray fine structure. Regions of enhanced X-ray emission, more than two orders of magnitude fainter than previously reported X-ray bright points, are loosely associated with bipolar regions in the magnetic network. The power spectrum of quiet X-ray images at small spatial scales is similar to that of active regions, but exhibits a kink at a scale of km, possibly connected to the supergranular structure. The spatial X-ray structures in the time averaged image amount to an rms amplitude which is 6% of the mean value. The X-ray structures correlate with contemporaneous radio maps obtained by the VLA at wavelengths of 1.3, 2.0, and 3.6 cm. The amplitude of the brightness variations in the images increases with radio wavelength, i.e., with increasing height. The cross-correlation coefficient with the absolute magnetic field strength, however, generally decreases with height, consistent with the idea of bipolar regions in the network and of the magnetic field deviating from vertical in the upper chromosphere.
The X-ray observations require an enhanced pressure in the corona above the magnetic network, but suggest similar temperatures. Model calculations show that, under a constant temperature, an rms density increase (relative to that in the cell interior) ranging from about 20% in the chromosphere to 60% in the low corona is sufficient to explain the observed standard deviations due to the spatial structures in radio waves and soft X-rays, respectively.
Key words: Sun: corona; chromosphere; transition region Sun: X-rays; radio radiation
Send offprint requests to: A.O. Benz
Online publication: June 30, 1998