3.2 Soft X-rays from flares
Quite often, the first visible signature of a flare appears in soft X-rays with energies up to some tens of
keV. It is thermal flare plasma radiation that signalizes the sudden heating of coronal plasma to
temperatures of some 107 K. This radiation is due to the bremsstrahlung continuum and to a multitude of
lines of heavily stripped ions, e.g., the line complex around 0.186 nm from helium-like iron ions.
The time profiles of soft X-ray bursts are often very similar to those of simultaneous radio
microwaves. Since the onset of regular observations from space in 1968, the intensity of soft
X-rays has been used for flare classification. It is based on measurements (using calibrated
satellite-carried instruments) of the soft X-ray emission in the 0.1 to 0.8 nm band as published in
real-time by NOAA (
http://www.sec.noaa.gov/today.html). For example, the big X-ray
flare on October 28, 2003 was classified as X 17.2, corresponding to the measured power of
1.72 mWm−2 (see Figure 4). NOAA is presently operating its own Soft X-ray Imager (SXI) as
part of their space weather service (see http://www.sec.noaa.gov/sxi/index.html). More
information on solar soft X-ray emission and flares can be obtained from the YOHOH-webpage at http://solar.physics.montana.edu/sxt/.
). NOAA is presently operating its own Soft X-ray Imager (SXI) as
part of their space weather service (see 