8.1 The solar wind

The solar wind consists of outward flowing plasma whose radial velocity is roughly constant with distance causing its density to fall as $r ~ r-2$ . By the time it reaches the Earth (at $1 AU$ ) the density has dropped to $ne ~ 3-10 cm -3$ . During solar minimum the overall structure is the clearest, and high and low heliospheric latitudes exhibit different characteristic speeds giving rise to the terms “fast” and “slow” solar wind. The fast solar wind at high latitudes is very steady at $-1 700 -800 km s$ with very little variation over a solar cycle (Phillips et al., 1995 ). At low latitudes the wind is sometimes fast and sometimes slow ( $300-500 km s-1$ ).

The magnetic field embedded in the solar wind appears very weak ( $B ~ 5× 10-5 G$ at $1 AU$ ) when its energy density is compared to either the plasma’s pressure or kinetic energy density. The field is therefore readily deflected by the wind and generally lies parallel to the flow. In his early model Parker (1958 ) predicted that the outward solar wind would “open” the magnetic field into a split magnetic monopole: field lines directed outward in one hemisphere and inward in the other, with magnitude decreasing with distance as $r-2$ . Between the hemispheres the field reverses sign in a layer called the heliospheric current sheet. Magnetic observations made within the ecliptic, from the Earth for example, show reversals in the field direction as the heliospheric current sheet sweeps past, rotating with the Sun. These transitions are known as sector boundaries, as they divide sectors of inward and outward directed magnetic field. The current sheet is deformed to accommodate the complex field structure it separates, so there can be more than two sector boundary crossings per solar rotation.

A simultaneous measurement of the magnetic field within an entire hemisphere is not possible, since measurements are made at single points by spacecraft. Taking a typical measured value $Br -~ 3 × 10-5 G$ as the representative of the entire hemisphere gives a net flux $Popen = 2p(1 AU)2Br ~ 4 × 1022 Mx$ . Because they are anchored to a rotating Sun, the field lines in each hemisphere sweep backward in a pattern called the Parker spiral. In situ measurements of the magnetic field vectors by various spacecraft confirm the average field to be oriented in good agreement with the Parker spiral: $-~ 45o$ from radial in the ecliptic plane at $1 AU$ .

The connection of an interplanetary field line to the Sun can be experimentally inferred by the presence of a uni-directional high-energy electron population called “strahl” (Feldman et al., 1975 ). Such measurements generally corroborate the picture that almost all field lines in the solar wind connect back to the Sun at one end and are open to interplanetary space at the other¹⁸ . Notable exceptions are transient events known as magnetic clouds which appear to be ropes of closed field lines (both ends anchored to the Sun) whose apices are entrained in the solar wind (Burlaga et al., 1981 ) discussed further below. A more serious challenge to the simple Parker spiral picture came from the Ulysses spacecraft which flew to very high heliospheric latitudes and observed populations of electrons and ions typically associated with low latitude activity. These observations suggest either that the electrons are capable of diffusing across field lines much more readily than expected or that heliospheric field lines are not confined to latitudinal cones as they would be in a Parker spiral (Fisk, 1996 ). The open field lines which compose the solar wind must all have footpoints located somewhere on the solar photosphere. X-ray images of the solar corona revealed extensive dark regions, generally near the poles, dubbed coronal holes. It was quickly understood that coronal holes probably corresponded with the open field lines from which the out-flowing solar wind originated. The plasma density on these field lines would be lower than on the closed active region field lines, causing them to appear darker in soft X-rays. While they are typically confined to high latitudes, the boundaries of coronal holes can be complex near solar maximum, sometimes even crossing the equator.