IMF Sector Effect on Geomagnetic Field at MID and Low Latitudes during Solar Cycle 23

Geomagnetic data of Tianjing during solar cycle 23 have been analyzed by eigen vector method. The Interplanetary Magnetic Field (IMF) sector effect on the H component of geomagnetic field at mid and low latitudes has been studied. The results show that the sector effect magnitude is from 3 to 11nT at mid and low latitudes in contrast to tens nT at high latitudes. The IMF sector effect reached maximum (about 11nT) during solar activity maximum, and reached minimum (about 3nT) during solar activity minimum. The largest value of the sector effect during solar activity maximum occurred in spring, summer and autumn seasons. However, the largest sector effect occurred one year later in the winter season. In 1996 and 2007, when the IMF is away from (or toward) the Sun, the sector effect increase (or decrease) in H component at daytime and decrease (or increase) at night in summer, while the sector effect increase (or decrease) in H component at daytime during the other seasons. The sector effect causes opposite changes during solar maximum, however, it will have less effect during summer. For high solar activity years, the IMF sector effect on daily mean value variation of the geomagnetic field is the same (or opposite) with the decreasing (or increasing) phase of solar cycle. In the summer, the average increment minimum value arises, and the average increment is irregular. About 60% of the estimations are consistent with satellite observations in summer, spring and autumn months, but about 55% in winter months.