Superflare
Since superflares can occur on stars apparently equivalent in every way to the Sun, it is natural to ask if they can occur on the Sun itself. An estimate based on the original Kepler photometric studies suggested a frequency on solar-type stars (early G-type and rotation period more than 10 days) of once every 800 years for an energy of 10 erg and every 5000 years at 10 erg. One-minute sampling provided statistics for less energetic flares and gave a frequency of one flare of energy 10 erg every 5–600 years for a star rotating as slowly as the Sun; this would be rated as X100 on the solar flare scale. This is based on a straightforward comparison of the number of stars studied with the number of flares observed. An extrapolation of the empirical statistics for solar flares to an energy of 10 erg suggests a frequency of one in 10,000 years.
Astrophysical X-ray source
Solar flares usually follow the solar cycle. CORONAS-F was launched on July 31, 2001 to coincide with the 23rd solar cycle maximum.
The solar flare of October 29, 2003 apparently showed a significant degree of linear polarization (> 70% in channels E2 = 40-60 keV and E3 = 60-100 keV, but only about 50% in E1 = 20-40 keV) in hard X-rays, but other observations have generally only set upper limits.
Coronal loops form the basic structure of the lower corona and transition region of the Sun. These highly structured and elegant loops are a direct consequence of the twisted solar magnetic flux within the solar body. The population of coronal loops can be directly linked with the solar cycle, it is for this reason coronal loops are often found with sunspots at their footpoints. Coronal loops populate both active and quiet regions of the solar surface. The Yohkoh Soft X-ray Telescope (SXT) observed X-rays in the 0.25-4.0 keV range, resolving solar features to 2.5 arc seconds with a temporal resolution of 0.5–2 seconds. SXT was sensitive to plasma in the 2-4 MK temperature range, making it an ideal observational platform to compare with data collected from TRACE coronal loops radiating in the EUV wavelengths.