1. A photon is a unit or quantum of electromagnetic radiation and has energy ( E ) = hc/λ where h is Planck's constant and c is the speed of light. For example an x-ray photon with λ = 1 nm has an E = 1240 eV where 1 eV = 1.602 × 10 −19 J is an energy unit. X-ray photons have energies greater than about 100 eV.
2. P. Foukal Solar Astrophysics (Wiley New York 1990).
3. D. F. Cioffi in Physical Processes in Hot Cosmic Plasma W. Brinkmann A. C. Fabian F. Giovannelli Eds. (Kluwer Dordrecht Netherlands 1990) pp. 1–16.
4. Cravens T. E., Adv. Space Res. 26, 1443 (2000).
5. One earlier x-ray observation of comet C/1979 Y1 (Bradfield) by the orbiting Einstein observatory yielded negative results (7). Lisse et al. reported on measurements made with two ROSAT instruments and one Rossi X-ray Timing Explorer (XTE) instrument. These instruments were the high-resolution imager (HRI) with a spectral range (or band pass) of 0.10 to 2.0 keV (i.e. soft x-rays); the wide-field camera with a spectral range in the S1A filter used of 0.09 to 0.206 keV (i.e. EUV emission); and the XTE proportional counter array (PCA) with a spectral range of 2 to 60 keV. The XTE PCA observations were negative (79). EUV emission was also observed by the EUVE satellite (17).