Longitudinal dependence of characteristics of low-latitude Pi2 pulsations observed at Kakioka and Hermanus

Abstract

Abstract We statistically investigated longitudinal dependence of characteristics of low-latitude Pi2 pulsations to find the longitudinal structure of the plasmaspheric cavity mode. We used the geomagnetic field data from two ground stations, Kakioka (27.2° geomagnetic latitude, 208.5° geomagnetic longitude) and Hermanus (—33.9° geomagnetic latitude, 82.2° geomagnetic longitude), and auroral image data acquired by the ultraviolet imager onboard the Polar satellite for the period of December 4, 1996 to March 3, 1997. Our findings include the following: (1) Pi2 amplitude is the largest around the magnetic local time of the auroral breakup site and decreases away from it; (2) when a nightside Pi2 pulsation has large amplitude, a dayside Pi2 pulsation can be observed with a similar waveform; (3) Pi2 pulsations generally have no clear phase differences (mean phase difference of 3.3°) between Kakioka and Hermanus, except for some events; and (4) the phase difference is independent on AMLT (difference of magnetic local time between a station and the auroral breakup). These observations suggest that the plasmaspheric cavity mode can be excited globally with a very small value of the azimuthal wave number (m ≈ 0).