Figure 4.1 Intensities of protons near Earth during the most intense series of SPEs in solar cycle 22. Particles with the energies shown in this figure (>30 MeV) can penetrate spacecraft hulls .

As mentioned in Section 2.3, solar flares and CMEs contribute energetic particles to SPEs, although the particles they produce have different time profiles, compositions, fluences, and distributions in space. Compared with CME particles, flare particles generally arrive at Earth more promptly, have higher fluxes of relativistic electrons, are confined to a fairly narrow region of space that is magnetically connected to the flare site (on average, Earth is magnetically connected to flare sites at about 60 degrees west solar longitude), and have smaller total fluence. Compared with flare particles, CME particles generally take longer to arrive at Earth, have fewer relativistic electrons, reach higher energies, are spread over a wide area centered on the central meridian of the Sun, and have greater total fluence. Both CMEs and flares can produce their characteristic kind of SPEs in the absence of the other.2

A CME is an injection of solar material into interplanetary space at speeds often in excess of 1,000 km/s (2,000,000 mph). A typical travel time to Earth is about 48 hours. (The CME arrival in Figure 4.1 was atypically fast.) If a CME moves fast enough (supersonically with respect to the solar wind), a shock wave forms ahead of it that energizes particles as it moves through the interplanetary medium. According to one model of shock acceleration, a shock can produce both a long SPE profile, lasting 1 or 2 days, and a shock spike such as that seen in the first SPE in Figure 4.1.3 The severity of a shock-generated SPE depends on the speed of the parent CME and on its launch site on the Sun; the most intense cases are associated with events that occur near the center of the visible disk, as was the case for the event shown in Figure 4.1.

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