In the Wind

In the solar wind, the bursts of high-energy charged particles and fields from the especially active solar regions are superposed on a constant stream of the solar plasma (Figure 4.3) that moves outward at about 300 to 350 km/sec (190 to 220 miles/sec). The matter exploding from the Sun (called coronal mass ejection, CME; see Plate 2) travels outward faster to cause a shock wave as it encounters the slower, more constant plasma stream. The solar plasma describes an immense spiral pattern, determined by the outward radial speed of the ejected particles and the rotation of the Sun's surface (Figure 4.4). Such motion can be compared to the spiral we see in the water path from a rotating lawn sprinkler. Although each particle of water is shot out radially, the sprinkler head has moved a little before the next particle leaves. The pattern of outflowing water or solar particles forms what is called an Archimedes spiral.

FIGURE 4.4 ► The Sun's dipole-like field at quiet times is illustrated by this hairy ball. Low-latitude fields are mostly closed on the solar surface; high-latitude fields are mostly open. The rectangular surface represents the ecliptic plane (defined by the Earth's annual path about the Sun), slightly tilted with respect to the Sun's equator. At differing solar longitudes, either the Sun's northern or southern high-latitude fields dominate the ecliptic plane, creating the toward or away field patterns carried to the Earth by the solar wind radial outflow of particles (see Section 3.5.4, p. 94). The solar rotation produces an Archimedes spiral pattern of particle flow in the ecliptic plane.

FIGURE 4.4 ► The Sun's dipole-like field at quiet times is illustrated by this hairy ball. Low-latitude fields are mostly closed on the solar surface; high-latitude fields are mostly open. The rectangular surface represents the ecliptic plane (defined by the Earth's annual path about the Sun), slightly tilted with respect to the Sun's equator. At differing solar longitudes, either the Sun's northern or southern high-latitude fields dominate the ecliptic plane, creating the toward or away field patterns carried to the Earth by the solar wind radial outflow of particles (see Section 3.5.4, p. 94). The solar rotation produces an Archimedes spiral pattern of particle flow in the ecliptic plane.

The outward flow of solar plasma, in both quiet and active times, carries along a magnetic field that is determined near the Sun itself. This locked-in solar-wind magnetic field is described in terms of a vector (an arrow with a magnitude and a direction) that can be resolved into three perpendicular components. The toward-away (from the Sun) component defines what are called Toward and Away Sectors (Figure 4.4), which affect the Earth's polar region field (Section 3.5.4, p. 94). The most important direction for the wind interaction with the Earth is the north-south component of wind field. Its interaction with the Earth's main field, which has the opposite direction, determines the severity of a resulting magnetic storm. In this chapter, we explore some of the magnetic effects of the solar-wind bursts of particles and fields when they arrive at the Earth to cause major disturbances.

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