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 (External Linkhttp://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 mWm2 (see Figure 4View Image). NOAA is presently operating its own Soft X-ray Imager (SXI) as part of their space weather service (see External Linkhttp://www.sec.noaa.gov/sxi/index.html). More information on presently ongoing observations of solar soft X-ray emission and flares can be found on the websites for the RHESSI mission (External Linkhttp://hesperia.gsfc.nasa.gov/hessi/), the Hinode mission (External Linkhttp://hinode.nao.ac.jp/index_e.shtml), and not to forget the excellent presentation in Wikipedia (External Linkhttp://en.wikipedia.org/wiki/Solar_X-ray_astronomy). UpdateJump To The Next Update Information
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