Occurrence of AE and Dst geomagnetic index levels and the selection of the quietest days in a year

Abstract

Three groups of indices were studied as indicators for quiet day geomagnetic field level determinations: one was the AE index, and the other two were the positive and negative value groups of the Dst index. A comparison of the behaviors of these three indices for 17 years showed their solar cycle changes and the differences in distribution of index levels between the times of high and low sunspot numbers. Two selections of the quietest days were made to provide at least 19 and 30 chosen days per year. These selections required the correspondence of equivalent fractional portions of the distribution of days in which all hourly values of indices were below specified levels. Lists of these days are given for the years 1958-1974. A comparison of these lists with the 5 days per month selection derived from the geomagnetic activity index Kp shows that for the same total number of quiet days per year only about one third to one half of the low-Kp days would be quiet by AE and Dst index standards.

Sq and EEJ—A Review on the Daily Variation of the Geomagnetic Field Caused by Ionospheric Dynamo Currents

“…Disturbance winds can persist for more than a day after a geomagnetic storm (Huang et al 2005a). Other studies employed AE and Dst indices in order to define more strictly quiet days (Campbell 1979;Torta et al 1997). At high latitudes (above 60°magnetic latitudes), the geomagnetic field is almost always disturbed and a visual inspection of the data is recommended for the selection of quiet days.…”

“…The actual level of geomagnetic activity for IQDs varies with activity of the Sun. Consequently, IQDs during active solar periods are not as quiet as IQDs during quiet solar periods (Campbell 1979). An alternative way of defining quiet days is to introduce Kp criteria in selecting the data.…”

The quiet geomagnetic field at geosynchronous orbit and its dependence on solar wind dynamic pressure

“…The Dst index, currently calculated every hour, is a measure of the equatorial ring current. This index is enhanced during intense magnetic activity that decreases the surface magnetic field [Campbell, 1979] …”

“…We also selected ground-based geomagnetic indices, AE and Dst (rather than Kp), which we feel improves the description of the magnetic activity at the spacecraft and hence the quiet field. The Kp index on some occasions may be contaminated by effects of ionospheric currents not directly related to magnetospheric magnetic activity; see, for example, Rostoker [1972] and Campbell [1979]. Furthermore, Kp is only available every 3 hours.…”

Effects of substorms on the stormtime ring current index <i>Dst</i>

“…It is useful to note here that Campbell (1979) has investigated the relationships among the parameters Dst, AE and ap (where ap is the linear counterpart of Kp). He found a linear correlation coecient for AE and ap of 0.89, which in recent time would be attributed to the fact that high levels of directly driven activity are normally (but not always) accompanied by strong substorm expansive phases.…”