Storm Explanations

In the early years of the first man-made satellites, it was believed that the large negative field seen during a magnetic storm at low-latitude observatories was simply a feature of the particle radiation belts circling the Earth (see Figure 3.15). A gigantic Saturn-like ring current was thought to grow with the arrival of solar wind ions and decay as the charged particles disappeared upon recombination. Subsequent direct measurement by satellites found this ring-current model to be a general representation of an ensemble of source

Sudden I Initial | Main I Commencement ^ Phase J Phase]4- Recovery Phase

Sudden I Initial | Main I Commencement ^ Phase J Phase]4- Recovery Phase

Universal Time Dec. 19,1980 Dec. 20,1980

FIGURE 4.9 ► Example of a typical geomagnetic storm variation for the northward (H) component of fields recorded a four low-latitude stations (listed at left), spaced in longitude around the Earth. The contribution of the quiet-day field variation has been removed from each record. The storm field pattern is so reproducible that special names (indicated at the top of the figure) have been assigned to each part.

Universal Time Dec. 19,1980 Dec. 20,1980

FIGURE 4.9 ► Example of a typical geomagnetic storm variation for the northward (H) component of fields recorded a four low-latitude stations (listed at left), spaced in longitude around the Earth. The contribution of the quiet-day field variation has been removed from each record. The storm field pattern is so reproducible that special names (indicated at the top of the figure) have been assigned to each part.

current patterns. In the region of approximately 3 to 8 Re, many currents were found that stayed briefly in part of the ring region, but dumped their particles as field-aligned currents (see B and C in Figure 4.5) into and away from the auroral latitude ionosphere of both the northern and southern auroral zones. Field-aligned current (Figure 4.11) is another principal contributor to the storm fields sensed by magnetometers from the polar regions to the middle latitudes.

Closing currents within the conducting ionosphere form a westward auroral electrojet current (current F of Figure 4.5) that dominates the magnetic fields in that region. Because of the ionospheric conductivity, a part of these electrojet currents is led away from the auroral zone into the day-side lower latitude ionosphere, creating a major contribution to the storm-time fields at low and equatorial latitudes.

For convenience in representation, all the magnetic fields of the auroral electrojet current and the field-aligned currents measured at observatories in the auroral and polar regions can be represented as contours of current flowing parallel to the Earth's surface in the ionosphere. Such displays are called equivalent storm currents (Figure 4.12). A concentration of these currents

FIGURE 4.10 ► During a magnetic storm, strong cross-tail currents flow (see D in Figure 4.5). These currents produce a disturbed field with a specific direction at low-latitude observatories on the midnight side of the Earth. Because of the solar wind flow, the magnetotail extends away from the Earth into a downstream direction that shifts seasonally (June, southward; December, northward) with the antisolar location. The cross-tail currents move similarly. An offset of the Earth's eccentric dipole axis, away from the spin axis, modifies this seasonal shift of tail current depending on the Earth's magnetic dipole location during the midnight field observation.

FIGURE 4.10 ► During a magnetic storm, strong cross-tail currents flow (see D in Figure 4.5). These currents produce a disturbed field with a specific direction at low-latitude observatories on the midnight side of the Earth. Because of the solar wind flow, the magnetotail extends away from the Earth into a downstream direction that shifts seasonally (June, southward; December, northward) with the antisolar location. The cross-tail currents move similarly. An offset of the Earth's eccentric dipole axis, away from the spin axis, modifies this seasonal shift of tail current depending on the Earth's magnetic dipole location during the midnight field observation.

typically occurs in the post-midnight to pre-dawn hours and is called an auroral electrojet.

In the auroral region, during the geomagnetic storm, a number of related processes occur that can be detected simultaneously. Instruments on high-altitude balloons, measuring the radiation from the individual bombarding electrons as they encounter atmosphere molecules, count the electrons that are arriving. As the air molecules are split apart, the number of ions increase in the ionosphere. That ionization causes the region to become more conducting and opaque to the reception of constant cosmic noise emitted by our galaxy in radiowave frequencies. The northward magnetic field at the surface decreases, attending the growth of a westward auroral electrojet current in the ionosphere. The magnetic disturbance is always accompanied by a sharp increase in the field pulsations with periods of seconds. Figure 4.13 illustrates the simultaneous occurrence of all these phenomena.

FIGURE 4.11 ► Here we see a typical pattern for some field-aligned currents at the polar region auroral oval during geomagnetic storms. These currents travel into the ionosphere (+, positive) and away from the ionosphere (—, negative). Although much of the auroral region is affected during the hours of major disturbance, only a few of these currents might be seen at any one time. Local time is indicated around the circumference.

FIGURE 4.11 ► Here we see a typical pattern for some field-aligned currents at the polar region auroral oval during geomagnetic storms. These currents travel into the ionosphere (+, positive) and away from the ionosphere (—, negative). Although much of the auroral region is affected during the hours of major disturbance, only a few of these currents might be seen at any one time. Local time is indicated around the circumference.

Because the Earth's electrical conductivity increases quite rapidly with depth below its surface, that high-conductivity property shields an observatory on one side of the Earth from sensing field variations that occur on the other side of the Earth. Therefore, fields from the partial ring currents, field-aligned currents, and ionospheric currents observed at the Earth's surface are mostly those whose current sources are near the same Earth side as the observatory itself.

Both the Northern and Southern Hemispheres of the Earth receive the field changes. The variations in the two fields would be completely symmetrical were it not for the offset of the eccentric magnetic dipole axis with respect to the spin axis of the Earth and the summer-winter differences that distort the magnetosphere and change the conductivity of the ionosphere. During a magnetic storm, the surface magnetic fields all around the Earth are disturbed. At high and auroral latitude locations that are identified as lying at the opposite Earth-bound feet of a main field line (called conjugate positions) the field disturbance changes are quite similar in appearance because of the symmetrical way that the particles and fields from the Sun enter the Earth's environment. Figure 4.14 shows how magnetic records appear at conjugate auroral latitude stations during a disturbed period near equinox.

FIGURE 4.12 ► The equivalent ionospheric current contours (higher current represented by closer contour lines) for fields of a single storm at Northern Hemisphere observatories are plotted in geomagnetic latitude and geomagnetic time (local time adjusted for the geomagnetic longitude) coordinates. Figure adapted from Akasofu and Chapman.

FIGURE 4.12 ► The equivalent ionospheric current contours (higher current represented by closer contour lines) for fields of a single storm at Northern Hemisphere observatories are plotted in geomagnetic latitude and geomagnetic time (local time adjusted for the geomagnetic longitude) coordinates. Figure adapted from Akasofu and Chapman.

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