Agitation in the Fields

► Storm Fingerprints

The most outstanding features in a collection of geomagnetic records are the storms. Field strengths during large storms can reach several hundred gammas at mid-latitude stations and above 1000 gammas at the auroral zones (recall Figure 1.13). The disturbances span a range of periods from seconds to many hours, with larger amplitudes generally associated with the longer periods (Figure 4.8).

Magnetic observatories around the world record the amplitude and direction of disturbance fields that are superposed on the internal main field of the Earth. On the records, at the onset of a geomagnetic storm, there often occurs a sudden spike-like change in field strength shown by the northward (H) component. This spike is thought to represent the shock from the arrival of the solar-wind disturbance at the day side of the magnetospheric boundary. Then there typically follows a slower, H-component increase (called the initial phase; Figure 4.9) that has been identified with the compression of the Earth's Sun-facing side of the main field. The magnetospheric stand-off position (Figure 4.5) can be estimated from the size of this positive field increase

FIGURE 4.8 ► Example of the spectral amplitudes of the 30- and 60-minute-period components of the horizontal variation of the geomagnetic field on a magnetic storm day of high daily activity index (Ap = 149, see Section 4.1.5, p. 116). In these displays, the geomagnetic latitude locations are all north of the equator in the American continents; constant latitude lines indicate the location of the observatories. The amplitude scale of the particular spectral component (30 minutes or 60 minutes) is in proportion to the peak value (in gammas) indicated on each display. Note the principal auroral zone activity for this event near 65 to 75° latitude and the daytime enhancement at the magnetic equator (local time there is about 5 hours earlier than Universal Time).

FIGURE 4.8 ► Example of the spectral amplitudes of the 30- and 60-minute-period components of the horizontal variation of the geomagnetic field on a magnetic storm day of high daily activity index (Ap = 149, see Section 4.1.5, p. 116). In these displays, the geomagnetic latitude locations are all north of the equator in the American continents; constant latitude lines indicate the location of the observatories. The amplitude scale of the particular spectral component (30 minutes or 60 minutes) is in proportion to the peak value (in gammas) indicated on each display. Note the principal auroral zone activity for this event near 65 to 75° latitude and the daytime enhancement at the magnetic equator (local time there is about 5 hours earlier than Universal Time).

at the Earth's surface. After this compression, a main phase and recovery phase of the storm follow (Figure 4.9) as a summation of the many different current systems that have been newly created.

The arriving solar-wind disturbance modifies the currents on the outer magnetospheric boundary, producing field changes that are detectable at the Earth's surface. During the storm development, a special circulation forms in the magnetospheric tail, with a considerably intense internal sheet current. This tail current is roughly perpendicular to the Earth's eccentric dipole-field axis (see Figure 4.5), and forms the north and south tail lobes. The growth and movement of this tail current is a primary contributor to the observed surface fields at low-latitude locations near the midnight hours during a storm (Figure 4.10). Of course, the magnetotail current position changes seasonally to a downwind location, north or south of the magnetic equator, in response to the solar wind direction.

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