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G. Kopp, G.M. Lawrence, G. Rottman, T. Woods (CU LASP)
We report on the first definitive observation of a solar flare in total solar irradiance (TSI) and on TSI observations of several other flares during the active Oct./Nov. time period. Solar flares are most prominent in EUV or X-ray wavelengths, since they release significant energy in these spectral regions where the Sun itself has a relatively low background. Despite their high energies, flares are minuscule compared to the entire energy output of the Sun, and thus cause very little change in TSI. Indeed, in 25 years of space-based irradiance monitoring prior to October 2003, no previous solar flare had been measured in TSI.
The Oct. 28, 2003 X17 flare was measured by the Total Irradiance Monitor (TIM) on NASA's SORCE. The TIM measures power across the entire solar spectrum, integrating X-ray to far infrared wavelengths. TIM data show a sudden increase of almost 270 parts per million slightly preceding the flare's soft X-ray peak at 11:10 UT. The TSI signature is similar to hard X-ray emissions in that it roughly coincides with the maximum rate of change of the soft X-ray emission.
The TSI measurement provides the spectrally integrated flare energy. We estimate the Oct. 28 flare had total energy exceeding 4.6e25 Joules. Preliminary estimates of the flare energy at wavelengths shorter than 200 nm, based on solar EUV measurements from other SORCE instruments and from an instrument on NASA's TIMED mission, only account for 23% of this energy, meaning the majority of the flare's energy was at longer wavelengths. This, combined with the timing of the flare in TSI, may indicate that portions of the visible and UV included in TSI also respond to the initiation phase of the flare.
We appreciate the support of NASA for this work.
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Bulletin of the American Astronomical Society, 36 #2
© YEAR. The American Astronomical Soceity.