Using the Cluster cusp crossings data,dependence of the cusp location at the mid-altitude on the geomagnetic dipole tilt during northward IMF is studied.The results show that the cusp center moves 0.051°Invariant Latitude(ILAT)upon the increase of 1°in the dipole tilt angle at the average altitude of 5.8 RE(Earth radius).According to the present results obtained at the altitude of the Cluster orbit and previous results obtained at other altitudes of other satellite orbits,it is found that the higher the altitude in the cusp region is,the bigger the dependence of cusp location on the dipole tilt angle will be.If the altitude increases by 1 RE in the cusp region,the dependence will increase by 0.012°ILAT upon the increase of 1°in the dipole tilt angle.Some possible physical mechanisms are discussed and it shows that the cusp location will be more sensitive to the solar wind dynamic pressure if the altitude is high.
We have studied 172 field-aligned currents （FACs） cases observed by the ClusterlI satellites when they crossed the plasma sheet boundary layer （PSBL） in the magnetotail from July to October 2001. We mainly analyzed the relationship between the characteristic of FACs at the PSBL in magnetotail and the Kp index. The main results indicated the followings： 1） In the different geomagnetic activity levels, the relative occurrence of FACs in PSBL increased monotonically with geomagnetic activity. 2） The density of FACs in PSBL increased monotonically with Kp index. In the storm main phase, the density of FACs increased dramatically, the maximum FACs approximately equaled 19.05 nA m-2 while Kp equaled 5.3） The variation of FACs density in PSBL was consistent with the variation of the Kp index. However, when AE〈800 nT, FACs density in PSBL increased with increasing AE, and when AE〉800 nT, it decreased with increasing AE. Therefore, our results suggested that the FACs density in PSBL had a closer correlation with Kp index.
The ionospheric responses to two strong storms on 17-19 August 2003 and 22-23 January 2004 are studied,using the data from Irkutsk(52.5°N,104°E) and Hainan(19.5°N,109°E) ionospheric stations.The analysis of variations in relative deviations of the critical frequency △f_0F_2 revealed that at middle latitudes(Irkutsk) negative disturbances were observed in the summer ionosphere; positive and negative ones,in the winter ionosphere during the main and recovery phases respectively.At low latitudes(Hainan),the disturbances were positive in all the cases considered. Mechanisms of the disturbances were analyzed with the aid of empirical models of the neutral atmosphere NRLMSISE-00 and thermospheric wind HWM07.The main factors determining △f_0F_2 variations at middle latitudes during the storms were demonstrated to be the disturbed equatorward thermospheric wind transporting the disturbed atmospheric composition,the increase in the atomic oxygen concentration,and the passage of internal gravity waves.At low latitudes,the effects associated with neutral composition variations are less significant than those of the thermospheric wind and electric fields.
E B RomanovaG A ZherebtsovK G RatovskyN M PolekhJ K ShiX WangG J Wang
To interpret density holes in the solar wind,which are nonlinear structures observed by Cluster and Double Star,we propose an electrostatic ion fluid model.We derive the Sagdeev potential from the magnetohydrodynamic(MHD)equations and study the characteristics of nonlinear structures in our model.The results show that density depletions(or holes)can develop from linear ion acoustic waves or ion cyclotron waves in space plasmas when parameters such as Mach number,initial electric field and ratio of ion to electron temperature satisfy certain conditions.In our model,the relative density depletion(or density holes)is from 0 to 1, and the time duration of density holes is from 2 s to more than 98 s.These are in good agreement with the observations by Cluster and Double Star in the solar wind.Our model also shows that the density holes should be accompanied by bipolar electric field solitary structures,which have also been observed by Cluster in the solar wind.
QURESHI M N SSHI JianKuiCHENG ZhengWeiLIU ZhengXing
