Local time dependence of the dominant frequency of Pi2 pulsations at mid- and low-latitudes

We carried out a statistical analysis of Pi2 pulsations using the geomagnetic field data obtained at three ground stations. A local time dependence of the dominant frequency of Pi2 was found on the nightside. The frequency of mid-latitude Pi2 pulsations is lower on the dusk side than that on the dawn side. This tendency is attributed to the shape of the plasmasphere which bulges out to the dusk side. It was confirmed that the Pi2 frequency depends also on the geomagnetic activity measured with Kp index. During the disturbed periods, Pi2 pulsations have higher frequency than that in the quiet periods. This dependence is interpreted to be caused by the size of the plasmapause which is smaller under the disturbed conditions than that under the quiet conditions. The dominant frequency of Pi2 pulsations at lower latitudes has a peak in post-midnight, and a Kp dependence similar to that at mid-latitudes is also observed. However, the result for low-latitude Pi2’s is different from that for mid-latitude Pi2. We consider that the dominant mechanism of mid-latitude Pi2 is the plasmaspheric surface wave. In order to examine the idea that the surface wave on the plasmapause is the dominant mechanism of Pi2 pulsations at mid-latitudes, we estimated the resonance frequency of the surface wave on the plasmapause using a plasmaspheric model which includes the effect of the plasmaspheric bulge. The estimated frequency of the surface wave is higher on the dawn side than that on the dusk side, which is essentially consistent with the observational results. The predicted frequency under quiet conditions (Kp ≤ 3) is nearly equal to the observed Pi2 frequency at mid-latitudes. These results suggest that the dominant frequency of Pi2 pulsations at mid-latitudes depends on the structure of the plasmapause.