Soft X-ray luminosity and photospheric magnetic field in quiet sun

We use full-disk soft X-ray data from Yohkoh and Kitt Peak daily magnetogra ms to study the coronal irradiance and photospheric magnetic held remote fr om active regions between 1991 November and 1998 December. For every image of our data set we extract three areas 4 degrees x 4 degrees in size center ed at N00 degrees W00 degrees, N50 degrees W00 degrees, and S50 degrees W00 degrees and compute X-ray irradiance and unsigned magnetic flux for each o f these areas. Between 1991 (active Sun) and 1996 (quiet Sun) the X-ray irr adiance at the heliographic center decreased by more than a factor of 7, wh ile the magnetic flux decreased by only a factor of 2. A similar tendency i s observed for our high-latitude samples. Apart from the cycle-related vari ations, all three areas of quiet Sun exhibit significant nonperiodic change s in X-ray irradiance. These variations occur on 9-12 month intervals and c learly correlate with changes in sunspot activity. Similar variations are p resent in the total X-ray irradiance averaged over the solar disk. By contr ast, the magnetic fluxes from the photosphere beneath these same areas show no corresponding variations on this time scale. In our opinion, coronal he ating models based on the reconnection of quiet-Sun magnetic elements (vari ously called chromospheric network, "magnetic carpet," or "salt and pepper" field) can at best account for a minimal contribution to heating the milli on-degree corona observed by the Yohkoh soft X-ray telescope. We conclude t hat the X-ray irradiance in the quiet Sun (at least in the Yohkoh temperatu re range, >2 MK) is primarily associated with the strong magnetic fields of active regions, not with weak photospheric fields. The association, howeve r, is not direct. We interpret the enhanced X-ray irradiance above the quie t Sun not as a result of the coronal "canopy" formed by the active-region m agnetic field above the quiet-Sun areas, but as the large-scale relaxation process in the corona triggered by the evolution of active regions. To furt her support this conclusion, we show examples of active and quiet hemispher es in 1996 with similar weak-held properties but greatly different global X -ray emission and a pronounced change in X-ray irradiance over the entire v isible hemisphere that was associated with the emergence of a single small active region.