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

Abstract

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.

Local time dependence of the frequency of Pi2 waves simultaneously observed at 5 low-latitude stations

“…L = 1.2 ∼ 3.4), Kosaka et al (2002) found that the frequencies of Pi2 pulsations have local time dependence, that is, they are lower on the dusk side than those on the dawn side. Two stations they used were located in the southern hemisphere and the other one is in the northern hemisphere.…”

“…2. The start time of the 5-minute data set is generally taken at somewhere between the start time of 10-minute data set and the onset of the Pi2, which is nearly the same with that used by Kosaka et al (2002). The start time of the trailing-5-minute data set is defined as the time at 2 wavelengths after the onset, which is shown by the coarse dash line in upper panel of the Fig.…”

“…As for defining the onset and duration of Pi2s, different criteria have been used by different authors (Takahashi et al, 1995;Cheng et al, 2000;Kosaka et al, 2002). In this paper, based on the length of the data, we adopted three types of data sets which are called "10-minute data set", "5-minute data set" and "trailing-5-minute data set", respectively.…”

“…This verified the suggestions that the surface waves could be excited in that area. By a model calculation, Kosaka et al (2002) showed that the plasmaspheric bulge on the dusk side causes the LT dependence of the surface wave frequency. However, it was not confirmed in their study that whether "each" Pi2 event has the LT dependence or not, because the events were selected independently at each station, and there could be a bias in the event selection which could cause the apparent LT dependence.…”

Pi2 pulsations in a small and strongly asymmetric plasmasphere

“…However, the same frequency was observed at both stations, THJ and KAK, and the observed frequency was lower than the expected frequency of the surface wave on the plasmapause (24 mHz $ ffiffi ffi 2 p times the FLR frequency at 0100 MLT) [Chen and Hasegawa, 1974]. A statistical study of surface mode Pi2 pulsations at midlatitudes and low latitudes showed also that the frequency $11 mHz is lower than the statistical result at 0100 MLT at Kp = 4 [Kosaka et al, 2002]. Thus the surface wave model is not appropriate for our Pi2 event.…”