3.6 Flares and Coronal Mass Ejections

Carrington (1859) and Hodgson (1859) reported the first observations of a solar flare from white-light observations on September 1, 1859. While observing the Sun projected onto viewing screen Carrington noticed a brightening that lasted for about 5 minutes. Hodgson also noted a nearly simultaneous geomagnetic disturbance. Since that time flares have been observed in H-alpha from many ground-based observatories and characterizations of flares from these observations have been made (cf. Benz, 2008).

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 15View Image. 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 15View Image).

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Figure 15: Monthly M- and X-class flares vs. International Sunspot Number for the period of March 1976 to January 2010. These two quantities are correlated at the 94.8% level but show significant scatter when the sunspot number is high (greater than 100).
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Figure 16: Monthly X-class flares and International Sunspot Number. X-class flares can occur at any phase of the sunspot cycle – including cycle minimum.

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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