On the latitude of the Sq(H) focus at sunspot minimum

Abstract

The latitude of the S 4 ( H ) focus along the 0" longitude meridian in the northern hemisphere has been determined for all the quiet days, as determined from the aa indices, for the sunspot minimum years 1963-64-65. It is shown that: (a) most of the large variability of the focus latitude is due to the effect of a superposed northward magnetic field that is present on AQDs and which tends to move the apparent focus latitude poleward in the northern hemisphere, and (b) a smaller equatorward motion is caused by the negative AQD events that occur in the 0830-1330 LT range. When these two classes of days are removed from the data set, the focus latitude is found to be completely contained within the range 36"-48" for the months MarchOctober with an average value of 41.5T2.3, whilst in winter the range is larger with an average value of 36.7k3.4. However, since the magnitude of the superposed northward field is longitude-dependent, it may be present even on days not classed as AQDs. It is shown that much of the variability in the focus latitude of the normal days along the 0" longitude meridian is caused by variations in the amplitude of the superposed northward field.

The effect of disturbance on the day-to-day variability of Sq

“…This shows that the M group, although nominally classed as quiet, still contains a considerable amount of residual disturbance. The reduction in variance agrees qualitatively with the findings of Butcher (1982) for variability of the latitude of the S, focus. Bearing this in mind, the small changes in correlation coefficient between the Q and M days for a given distance of separation suggest that if one could remove all disturbance and thus find the spatial coherence of S, it would turn out to be not much different from that shown in Fig.…”

“…They showed that, for stations on the 0" meridian, it was possible to identify the bay-like events by examination of the H magnetograms at Meanook, 7% hr west of this meridian. Butcher (1982) showed that, when days on which these baylike events occurred were removed from the analysis, the day-to-day variability of the latitude of the S, focus was reduced, and the seasonal variation of the latitude of the focus became more in accordance with theory.…”

“…Days contaminated by bay-like disturbance fall into ( I ) above. Such days were removed by a method similar to that of Butcher (1982), who used the magnetogram at Meanook to identify disturbance effects superimposed on the noon-time S, minimum at Hartland. In our case, in order to detect disturbance in both northern and southern hemispheres, we chose eight such 'selection stations' for each meridian chain, symmetrically arranged so that there were two stations in each of the quadrants formed by the geographical equator and the chain meridian great circle.…”

Day-to-day variability of geomagnetic hourly amplitudes at low latitudes

“…The studies of the variability of the current centre and focus of the worldwide part of Sq (W Sq ) include those of Chapman & Bartels (1940), Hasegawa (1960), Matsushita (1960, 1967), Hutton (1967b), Schlapp (1976), Malin & Gupta (1977), Butcher & Brown (1980, 1981b), Butcher (1982b), Campbell (1989) and references therein, and Onwumechili et al . (1996).…”

Global scale of the day-to-day variability of Sq

“…It is much less effective in reducing contributions from ionospheric non-Sq currents. Such currents include at least those associated with sub-storms and bays (Butcher & Brown 1981;Butcher 1982) and with DP2 (Nishida, 1978), which are known to extend at times into lower latitudes, so that there is justification for regarding some, if not all, of the fluctuations of H and AH in Fig. 3 as non-Sq effects rather than as part of the day-to-day variability of Sq (see also Mayaud 1978).…”