Disturbances in Sight On the

The Sun's surface is constantly boiling out particles from small and large eruptions, especially in the regions near the sunspots (Figure 2.27) and at places where the solar magnetic field provides a guiding outward path. Solar disturbances (Plate 1) cause most of the large transient changes in the Earth's magnetic field.

We know that the Sun has an activity cycle, just under 11 years on average, that corresponds to the sunspot number, R. The spots are somewhat cooler regions (about 3600° F, or 2000° C) on the visible solar surface (photosphere), which is about 10,500°F (5800°C). The R index is related to the area size of all the visible spots. The solar activity cycle begins with the first spots at about 35° north and south latitudes. The number of spots and their size increase as their locations move equatorward toward about 10° to 15° solar latitudes (Figure 4.1). The cycle ends with just a few small spots near the Sun's equator. The most recent sunspot maximum (number 23) occurred in

FIGURE 4.1 ► E. W. Maunder's original 1922 plot of the central locations for sunspots from 1874 until 1913 (larger letters for years and latitudes have been added). Although the Sun's southern hemisphere has more spots in the years depicted here, the hemisphere dominance changes in other years. Note the limitation of sunspots to latitudes below about 40° and the overlap of the ending of one sunspot cycle with the beginning of another.

FIGURE 4.1 ► E. W. Maunder's original 1922 plot of the central locations for sunspots from 1874 until 1913 (larger letters for years and latitudes have been added). Although the Sun's southern hemisphere has more spots in the years depicted here, the hemisphere dominance changes in other years. Note the limitation of sunspots to latitudes below about 40° and the overlap of the ending of one sunspot cycle with the beginning of another.

the year 2000. Irregularities in a smooth growth and decay of an R cycle often repeat in another cycle when similar solar latitude regions contribute sunspots.

Sunspots provide evidence of a slowly rotating solar surface (once every 27 days near 20° solar latitudes) in the same direction as the Earth's yearly path about the Sun. The general persistence of active areas for several or more rotations of the Sun provides some monthly predictability of solar activity. Those who track solar disturbances and the Sun's mass ejection of energetic particles identify (Figures 4.2):

1. Plages, which are uniformly bright areas on the solar surface.

2. Solar flares, usually near sunspots, which are brilliant outbursts of particles and radiation.

3. Spectacular prominences, which are grand glowing arches of solar gases seen on the solar limb (called filaments when viewed on the solar disk).

4. Large coronal holes of singular magnetic field polarity, where concentrated streams of particles, corotating with the solar surface, are observed leaving the Sun.

FIGURE 4.2 ► The active Sun seen through a filter that emphasizes plages, prominences, filaments, and arches of solar gases. Photo from SEC/NOAA.

5. The solar corona, which surrounds the solar surface with a luminous glow of the outward-streaming particles (Figure 4.3).

The Sun displays unique magnetic field patterns that often provide a guiding path for the gas of high-energy charged particles (called plasma) which leaves the Sun as a solar wind usually taking 2 to 3 days to reach the Earth. The Sun also has both an overall magnetic dipole field (Figure 4.4) and very strong fields associated with the spots and disturbance areas. The Sun's dipole field switches its north and south poles every 11-year cycle. That alternation is also found in changes of the special fields within sunspot and coronal hole regions.

FIGURE 4.3 ► Image of an eclipse of the Sun by the Moon, showing the glowing corona of outward-streaming particles. The coronal energy delivered to the solar wind represents approximately one-millionth of the total radiation from the Sun. Photo by the High Altitude Observatory of NCAR.

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