Magnetic quiet daily variation: variations in the magnetic declination on abnormal quiet days

Abstract

For stations on the poleward side of the Sq focus the minimum in H on quiet days occurs near 1 100 LT. In fact, on about 80% orquiet days, the minimum occurs between 0830 and 1330 h local time (LT). Such days were termed 'normal quiet days' (NQD) by Brown and Williams (1969) whereas thosedays where the minimum occurs outside this time range were termed 'abnormal quiet days' (AQD). It has been shown that when AQD occur, the amplitude of the normal H variation is significantly reduced and the daily minimum is formed by a magnetospheric substorm event. The reduction in the H variation is caused by a current which flows in the ionosphere, in a west-east dircction on both sides of the focus, and therefore causes an increase in the amplitude of H at stations on the equatorward side of the focus (see Fig. 1). The strength of this current has been found to be dependent on the polarity of the interplanetary magnetic field (IMF), it being larger on days when the IMF is away from the sun (A-days) that when it is toward the sun (T-days). This current also affects the position of the Sq focus, causing it to move poleward on AQD, the effect being greatest on A-days