Comparison between the ring current energy injection and decay under southward and northward IMF Bz conditions during geomagnetic storms

The geomagnetic storm is one of the most important geomagnetic disturbance phenomena in the geospace. Many studies assume that input of solar wind energy into the Earth's ring current only occurs when IMF Bz in GSM coordinates is southward; the ring current energy injection and decay under northward IMF Bz are not well understood and still need further investigation. In this paper, by using the large amount of data from the year 1964 to 2010, we use the empirical phase space analysis method to study the ring current energy injection and decay under northward IMF Bz and compare the results with those under southward IMF Bz condition. We have found that the largest injection Q under northward IMF Bz condition is only 7% of the largest injection under southward IMF Bz, implying that there is a very limit energy injected into the ring current region when IMF Bz is northward. The decay time tau decreases as VBz increases and shows a good linear trend for southward IMF Bz; while for the northward IMF Bz, there is not a clear relation between tau varies and VBz. Having taken tau as a function of injection Q instead of VBz, we have obtained the empirical relation of tau with Q for northward and southward IMF Bz conditions: the two categories are further combined together and the empirical relation tau=e ((2.6+0.039-Q)) is derived. Further, the pressure-corrected Dst formula is derived for both southward and northward IMF Bz conditions, where the coefficients b and c are 6.9/10.4 and 10.0/15.2 when IMF Bz is southward and northward respectively. The statistical results on between the different geomagnetic indices (Dst, Kp, AE) and IMF Bz are also obtained.