Spectral characteristics of field variations during geomagnetically quiet conditions.

Abstract

Using data from 15 observatories at a wide distribution of latitude locations in North and South America the spectra of geomagnetic field variations having periods from 5 min to 4 hr were examined for six days of 1965 when the Ap index values were 3 or less. At high latitudes, where the maximum amplitudes occurred more poleward on low Ap days than on high Ap days, the ratio of the short to long period amplitudes was higher than at other latitudes. For all spectral samples of the data, a minimum in activity was found near 30 geomagnetic latitude and a dayside enhancement was found at the equator. At all latitudes most amplitudes were larger during the local daytime hours and smallest at the predawn hours. East-west component amplitudes were about the same size as the north-south component amplitudes averaged over the day yet regular diurnal variations were found in the ratio of these two components. On the average there was a 10 to 20 percent reduction of the east-west field component in the region of the ionospheric equatorial and auroral electrojet currents. The vertical component of field was about 40 percent less than the horizontal field. This reduction seemed to be accounted for by an effective crustal conductivity of about 2X10-3mhos/ meter extending to an equivalent depth of about 300km. The general characteristics of the quiet-time spectra seemed to match the projection of the changes in active-day spectra to low Ap values. Empirical relationships between the spectral features and the Ap index are provided to allow some prediction of geomagnetic amplitude levels as a function of period, time, and location on magnetically quiet days.

Seasonal Variations of Sq Current System in Different Longitudinal Sectors and Solar Activities

“…Sq currents also changes with longitude (e.g., Campbell, 1989; Matsushita, 1967), and the geomagnetic field B modulates the ionospheric dynamo and ionospheric conductivity, thus leading to the longitudinal variations of the ionospheric dynamo currents and IHFACs (Fukushima, 1994; Olsen, 1997; Park et al., 2020; Stening, 1971; Le Sager & Huang, 2002; Takeda, 1990; Yamashita & Iyemori, 2002; Yamazaki & Maute, 2017). Matsushita and Maeda (1965) and Campbell and Schiffmacher (1985, 1986, 1988b) analyzed the characteristics of the seasonal variations for Sq currents at different longitude sectors in years of high and low solar activity, and the results showed that the vortex center parameters for internal and external Sq currents were different at different longitudes.…”

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

“…Recognizing this 1ocational problem, Mayaud [1967] in recent years initiated the computation of new K type (and related A type) indices using an improved longitudinal distribution of mid-latitude observatories. Geomagnetic field spectra generally show an increase in amplitude with increasing period [Campbell, 1977]. This spectral characteristic causes the 3-hour range values, from which the K indices are derived, to emphasize the long-period variations in that time sample.…”

Particle diffusion in the geomagnetosphere: Comparison of estimates from measurements of magnetic and electric field fluctuations

“…It is now known that the radial diffusion coefficient for energetic particles in the earth's magnetosphere is not constant as a function of time, as was often assumed in the early days of radiation belt research [Lanzerotti et al, 1970[Lanzerotti et al, , 1978. Rather, third-invariant violation depends upon the level of geomagnetic activity, as would be expected from contemporary knowledge of geomagnetic 'noise' levels [e.g., Surkan and Lanzerotti, 1974;Campbell, 1976Campbell, , 1977. As such, the radial diffusion problem is more complicated than that envisioned a decade and a half ago.…”