On the nature of abnormal quiet days in Sq(Hs)

Abstract

It is well known that the time of occurrence of the minimum in the horizontal component of the Earth's magnetic field (Hmin) on quiet days at a mid-latitude station on the poleward side of the Sq focus shows considerable variability from day to day. This variability has previously been discussed in terms of the incidence of so-called 'abnormal quiet days' (AQD). arbitrarily defined for the station Hartland as quiet days when Hmin occurred outside an interval of +2% hr centred on the most common time of 1 I30 LT. AQDs have some interesting properties, which have been documented, but their precise nature and cause have not been elucidated. In this paper we report the results of a study of AQDs as identified at Hartland using a chain of Northern hemisphere stations situated approximately along the 0" longitude meridian and extending on both sides of the Sq focus. It is found that there are two effects on AQDs: (i) a northward component field varying in LT is superposed at all latitudes throughout the day, so reducing the amplitude of the normal Sq(H) variation at stations poleward of the focus and increasing it on the equatorward side, (ii) a southward perturbation field, of most probable magnitude 6.0 nT for the period studied, is imposed for about 4 lir at a fixed UT at all latitudes. so constituting an 'AQD event' which can lead to the occurrence of Hmin at an anomalous local time for a station poleward of the focus. It is shown that the AQD event may be of large geographical extent and that it is related to the interplanetary magnetic field. All the main properties of AQD occurrences are explained. and it is suggested that much of the dayto-day variability in the amplitude and phase of the normal Sq(H) variation probably also arises from the occurrence of AQD events at times close to the diurnal turning points.

Abnormal quiet day variations in Indian region along 75° E meridian

“…To find the source of these AQD variations, data sets are compared with IMF parameters B, By, and Bz that are downloaded from ACE satellite. AQD events are also linked to the IMF and are well correlated with negative values of the Bz component (Butcher and Brown 1981). Figure 10a shows an example of Alibag (ABG) Hcomponent with B, By, and Bz on 9 July 2004.…”

“…This is equivalent to an additional west-east current flowing in the northern hemisphere on AQDs, producing a superposed northward field (SPNF) at low and mid-latitudes (Butcher and Brown 1981) as shown in Figs. 2, 3, 4, and 5.…”

“…Most of the bays occur during the main and recovery phases of magnetic storms but as suggested by Ratcliffe (1972), they may also occur during quiet times. The energy associated with bay activity is transferred from the solar wind into the tail of the magnetosphere, i.e., the IMF is directed southward, although the mechanisms of the energy, momentum, and mass transfer are not yet understood (Butcher and Brown 1981). Rostoker (1969) suggests that long period bay could be produced at mid-and low latitudes by asymmetric ring current system.…”

“…Rastogi (1993) has brought out changes in the summer-winter variation pattern in the eastward field based on magnetic field component data for the period 1975. Campbell et al (1993 Butcher and Brown (1981)) i.e., the normal positive excursion was greater on AQDs than on NQDs. This is equivalent to an additional west-east current flowing in the Northern Hemisphere on AQDs producing a superposed northward field (SPNF) at all latitudes (Butcher et al 1993).…”

“…The magnitude of the SPNF is found to be latitude and longitude dependent. Butcher and Brown (1981) and Butcher (1982) studied northern hemispheric stations along the 0°longitude meridian and found that SPNF have a peak at 35°-40°latitude in winter and~55°in summer and in both seasons tending to zero near 20°and 60°N geomagnetic latitudes.…”