Statistics of Fluctuations in the Solar Soft X-ray Emission

X-ray emission from the Sun Ñuctuates as a result of occasional Ñare events. We have calculated the power spectra of the solar soft X-ray variations using the photometric data of the GOES 6 satellite. The data cover the period 1991 September to 1994 April, about 32 months in total, and we have worked with 10 minute averages. We Ðnd that the total power spectral densities (PSDs) have three distinct components ; a Ñat or slightly decreasing low-frequency section, a 1/f-like moderate decline mediumfrequency section, and steep decline high-frequency parts. The break frequencies separating three parts are Hz and 10~3.8 Hz , respectively. Such downward breaks are expected from the shotfbreak^ 10~4.7 noise (Ñarelike) character of solar X-ray emission, and we interpret the lower break frequency as indicating an upper limit on Ñare timescales. These break frequencies do not vary appreciably with activity level. This suggests the existence of a universal mechanism for triggering Ñares in the solar corona. Moreover, the power-law index (where we assume PSDP f ~b, f being frequency) of each part does not vary appreciably with the level of activity either ; its average is b ^ 0.45, 0.95, and 1.5, respectively. The overall shape of the PSD is quite similar to those of other astrophysical objects such as black hole candidate stars and active galactic nuclei, albeit on a vastly di†erent scale. Subject headings : Sun: activity È Sun: Ñares È Sun: X-rays, gamma rays