X-rays from the Sun were measured by instruments on early rocket flights and their association with solar flares was recognized immediately. NOAA has flown solar x-ray monitors on its Geostationary Operational Environmental Satellites (GOES) since 1975 as part of its Space Environment Monitor. The solar x-ray flux has been measured in two bandpasses by these instruments: 0.5 to 4.0 Å and 1.0 to 8.0 Å. The x-ray flux is given on a logarithmic scale with A and B levels as typical background levels depending upon the phase of the cycle, and C, M, and X levels indicating increasing levels of flaring activity. The number of M-class and X-class flares seen in the 1.0 – 8.0 Å band tends to follow the sunspot number as shown in Figure 15. The two measures are well correlated (r = 0.948, r2 = 0.900) but there is a tendency to have more flares on the declining phase of a sunspot cycle (the correlation is maximized for a 2-month lag). In spite of this correlation, significant flares can, and have, occurred at all phases of the sunspot cycle. X-class flares have occurred during the few months surrounding sunspot cycle minimum for all of the cycles observed thus far (Figure 15).
Coronal mass ejections (CMEs) are often associated with flares but can also occur in the absence of a flare. CMEs were discovered in the early 1970s from spacecraft observations from OSO 7 (Tousey, 1973) and from Skylab (MacQueen et al., 1974). Routine CME observations began with the Solar Maximum Mission and continue with SOHO. The frequency of occurrence of CME’s is also correlated with sunspot number (Webb and Howard, 1994).
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