Correlations between high- and low-latitude Pi 2's as a function of the position relative to the auroral breakup region

*Teiji Uozumi[1] ,Kiyohumi Yumoto [1],Hideaki Kawano [1]
Akimasa Yoshikawa [1],Shinichi Ohtani [2]
John V. Olson [3],Syunichi Akasofu [4],S. I. Solovyev [5]
E. F. Vershinin [6],Kan Liou [2],C.-I. Meng [2]
Graduate School of Sciences, Kyushu University[1]
Applied Physics Laboratory, The Johns Hopkins University[2]
Geophysical Institute, University of Alaska Fairbanks[3]
International Arctic Research Center, University of Alaska Fairbanks[4]
Institute of Cosmophysical Reserch and Aeronomy [5]
Institute of Cosmophysical Reserch & Radiowaves Propagation[6]

In order to investigate the generation and propagation mechanisms of Pi 2s in the magnetosphere, we made a comparative study between magnetometer data observed at high- and low-latitude CPMN stations and the ultraviolet image (UVI) data of aurora obtained by the Polar satellite. Correlation coefficients between low- and high-latitude Pi 2 were calculated. We studied the dependence of the correlation coefficient of Pi 2s on the position relative to the auroral breakup region. It has been found the tendency that as the distance between a high-latitude station and the auroral break up region become larger, correlation coefficients among H component of low-latitude Pi 2 and H, D component of high-latitude Pi 2 tended to become higher.

In order to investigate the generation and propagation mechanisms of Pi 2s in the magnetosphere, we made a comparative study between magnetometer data observed at high- and low-latitude CPMN stations and the ultraviolet image (UVI) data of aurora obtained by the Polar satellite. Correlation coefficients among H component of low-latitude Pi 2 and H, D component of high-latitude Pi 2 were calculated. Where, in this paper, we assumed and used high- and low-latitude Pi 2s as the manifestation of fast and Alfven mode wave propagating in the magnetosphere, respectively. Results were classified by the relative position between each high-latitude station and the auroral breakup region. We then studied the dependence of the correlation coefficient of Pi 2s on the position relative to the auroral breakup region. Twenty-one Pi 2 events occurred in January, 1997 were analyzed. In this paper we have found the following results. When high-latitude stations were separated longitudinally from auroral break up region larger than 1 hour, the average correlation coefficient between H component of low-latitude Pi 2 and H, D component of high-latitude Pi 2 became 0.86 and 0.87, respectively. On the other hand, when high-latitude stations were located in the same longitudinal span of auroral break up region, those coefficients decrease 0.69 and 0.63 respectively. It has been found the tendency that as the distance between a high-latitude station and the auroral break up region become larger, correlation coefficients among H component of low-latitude Pi 2 and H, D component of high- latitude Pi 2 tended to become higher. It means that the coupling between fast and Alfven mode wave were observed clearly as away from auroral breakup region. Those observational results suggested that high-latitude Pi 2's, which occur at a distance from auroral breakup region, are generated via mode conversion from the fast to the Alfven mode. That is, fast mode waves are first generated at the Pi 2 source region, propagate from there in a three dimensional manner, and then excite shear Alfven mode waves via mode conversion. It is suggested that generation of fast mode wave in the magnetosphere at the onset of substorm is the essential process for global Pi 2 occurrence.