Radial distribution of magnetic field in earth magnetotail current sheet

摘要

Knowing the magnetic field distribution in the magnetotail current sheet (CS) is essential for exploring magnetotail dynamics. In this study, using a joint dataset of Cluster/TC-1, the radial profile of the magnetic field in the magnetotail CS with radial distances covering 8 ＜ r ＜ 20R_E under different geomagnetic activity states (i.e., AE ≤ 100 nT for quiet intervals while AE ＞ 100 nT for active times) and solar wind parameters are statistically surveyed. Our new findings demonstrate that, independent of the activity state, the field strength and B_z component (GSM coordinates) start the monotonic increase prominently as r decreases down to ～11.5R_E. which means the dipole field starts to make a significant contribution from there. At least in the surveyed radial range, the B_z component is found to be weaker in the midnight and dusk sectors than that in the dawn sector, displaying a dawn-dusk asymmetry. The occurrence rate of negative B_z in active times also exhibits a similar asymmetric distribution, which implies active dynamics may occur more frequently at midnight and dusk flank. In comparison with that in quiet intervals, several features can be seen in active times: (1) a local B_z minimum between 10.5＜r＜ 12.5R_E is found in the dusk region, (2) the B_z component around the midnight region is generally stronger and experiences larger fluctuations, and (3) a sharp positiveegative-excursion of the B_y component occurs at the dawn/dusk flank regions inside r＜10R_E. The response to solar wind parameters revealed that the B_z component is generally stronger under higher dynamic pressure (P_(dy) ＞ 5 nPa), which may support the dawn-dusk squeezing effect as presented by Miyashita et al. (2010). The CS B_y is generally correlated with the interplanetary magnetic field (IMF) B_y component, and the correlation quality is found to be better with higher penetration coefficient (the ratio of CS B_y to IMF B_y) when IMF B_z is positive. The implications of the present results are discussed.