Dst Index

Many of the currents flowing in the auroral electrojet close in a loop through the conducting lower-latitude region of the ionosphere; others return to the magnetosphere as field-aligned currents. The current-carrying region of the ionosphere has its conductivity significantly enhanced both by the auroral process and by the ionization attending the day-side solar illumination. Thus, some of the night-hour auroral zone storm-time ionospheric currents become gradually redirected toward the more conducting day side of the Earth while spreading toward the lower latitudes. Extending to the equator, the storm-time currents are then concentrated east-west by the narrow band of high conductivity that follows the day-side equatorial dip-latitude position (see Figure 4.8). As a result, magnetometers register a large, storm-time, eastward electrojet current at the day-side dip equator.

All magnetic fields decrease in intensity with increasing distance from the source current to the measurement position. Therefore, at the low and equatorial latitudes, nearby ionospheric currents often have a proportionally greater effect at the Earth's surface at daytime than the stronger, but more distant,

July 4.1974 July 5,1974

FIGURE 4.17 ► The AE (auroral electrojet) index is the measure (in gammas) of the range of field between the highest (upper envelope, AU) and the lowest (lower envelope, AL) values determined from an overplot of the horizontal field (change from its quiet-time level) for hourly values measured at a group of auroral zone magnetic observatories. In this example, for 4 and 5 July, 1974, the fact that there were nine contributing observatories is indicated in a bracket. Illustration from World Data Center A, NGDC/NOAA.

July 4.1974 July 5,1974

FIGURE 4.17 ► The AE (auroral electrojet) index is the measure (in gammas) of the range of field between the highest (upper envelope, AU) and the lowest (lower envelope, AL) values determined from an overplot of the horizontal field (change from its quiet-time level) for hourly values measured at a group of auroral zone magnetic observatories. In this example, for 4 and 5 July, 1974, the fact that there were nine contributing observatories is indicated in a bracket. Illustration from World Data Center A, NGDC/NOAA.

magnetospheric currents. Cross-tail magnetospheric currents dominate the night-side measurements. A global collection of four to six magnetic records from low-latitude stations are averaged to form an hourly Disturbance Storm-Time Index, Dst (Figure 4.18). The expected quiet-day values of Sq have first been removed so that the zero index levels occur at quiet times.

0 0

Post a comment