MLT dependence in the relationship between plasmapause, solar wind, and geomagnetic activity based on CRRES: 1990-1991

Using the database of CRRES in situ observations of the plasmapause crossings, we develop linear and more complex plasmapause models parametrized by (a) solar wind parameters V (solar wind velocity), BV (where B is the magnitude of the interplanetary magnetic field (IMF)), and d(mp)/dt (which combines different physical mechanisms which run magnetospheric activity), and (b) geomagnetic indices Dst, Ap, and AE. The complex models are built by including a first harmonic in magnetic local time (MLT). Our method based on the cross-correlation analyses provides not only the plasmapause shape for different levels of geomagnetic activity but additionally yields the information of the delays in the MLT response of the plasmapause. All models based on both solar wind parameters and geomagnetic indices indicate the maximal plasmapause extension in the postdusk side at high geomagnetic activity. The decrease in the convection electric field places the bulge toward midnight. These results are compared and discussed in regard to past works. Our study shows that the time delays in the plasmapause response are a function of MLT and suggests that the plasmapause is formed by the mechanism of interchange instability motion. We observed that any change quickly propagates across dawn to noon, and then at lower rate toward midnight. The results further indicate that the instability may propagate much faster during solar maximum than around solar minimum. This study contributes to the determination of the MLT dependence of the plasmapause and to constrain physical mechanism by which the plasmapause is formed.