Space of Quiet Fields

Astronomical measurements of light polarization have shown that our Milky Way Galaxy can exhibit magnetic fields of nearly 0.1 gamma. Similar magnetic fields have been detected at places in the intergalactic space. Our Sun's field is thought to extend to a heliospheric boundary at nearly 100 times the Sun-Earth distance.

FIGURE 3.15 ► The boundary of the magnetospheric field pattern can extent from about 6 to 25 Re toward the Sun, depending on the compression by the solar wind. Downwind, away from the Sun, the direction of the magnetospheric tail boundary can stretch far past the moon's orbit (60 Re). Shaded regions represent the inner and outer radiation (Van Allen) belts where charged particles accumulate.

FIGURE 3.15 ► The boundary of the magnetospheric field pattern can extent from about 6 to 25 Re toward the Sun, depending on the compression by the solar wind. Downwind, away from the Sun, the direction of the magnetospheric tail boundary can stretch far past the moon's orbit (60 Re). Shaded regions represent the inner and outer radiation (Van Allen) belts where charged particles accumulate.

In the space around the Earth, out to a distance of several Earth radii (1 Earth radius, Re, — 6371 km or 3959 miles), the main field has approximately the form of the eccentric axis dipole. However, the Earth's field becomes distorted beyond that distance, primarily because of a strong and varying assault of charged particles and fields from the Sun, called the solar wind. This solar wind bounds the entire region of space dominated by the Earth's main field and forces the magnetospheric outer boundary into an extended tear-drop shape. On the day side, in times of extreme quiet, the sunward boundary of the magnetosphere can extend to 25 Re\ but, on average, that stand-off position is approximately 11 or 12 Re (Figure 3.15). During major blasts of the solar wind, the sunward boundary can be compressed to 6 Re. The main field distortion at such times at low latitudes can reach 40 gammas.

The solar wind further restricts the full magnetospheric envelope on the night side. A long tail of the magnetosphere is blown outward, two or more times the Moon's orbital distance at 60 Re. This constant deformation of the magnetosphere is detectable at the magnetic observatories located about the world.

In the yearly path of the Earth about the Sun, which defines the ecliptic plane, the tilt of the Earth's axis gives us our seasonal climate changes. The magnetospheric tail is always extended toward the downwind, antisolar direction. Thus, from our viewpoint on Earth, the tail appears to shift seasonally north and south of the geomagnetic equator, opposite to the apparent seasonal

Sun position. This shift, verified by satellite measurements, can be detected at the Earth-surface magnetic observatories as an apparent seasonal change in the night-time field level of about 10 gammas at mid-latitudes.

The route for the arrival of the many charged particles that the Earth encounters in space is determined by the Earth's magnetospheric field. That field also arranges the many special current patterns that attend bursts of solar-terrestrial activity that we will examine in Chapter 4. In addition, the Earth is bombarded by very high-energy particles, called galactic cosmic rays, that travel throughout our Milky Way galaxy. Curiously, during strong solar winds, the cosmic rays are swept away from the Earth by that wind; scientists detect a decrease in arriving cosmic rays at such times (Forbush effect).

Principally at the two distances of approximately 1.2 Re to 4.0 Re and 4.5 to 6.0 Re, a great number of solar-terrestrial charged particles organized by the magnetospheric field gather to form two donut-like girdle patterns about the Earth, called the inner and outer radiation (Van Allen) belts (Figure 3.15). Some of these particles drift to much lower altitudes, particularly where the Earth's main field is weak, toward the South America-South Atlantic Ocean region (Figure 2.21). Man-made satellites are usually routed to avoid the potential damage by the concentration of belt particles in that region (Figure 2.20).

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