Solar wind control of plasma number density in the near‐Earth plasma sheet

The plasma number density in the plasma sheet depends on the solar wind number density and the north‐south component of interplanetary magnetic field (IMF B z ) with the time lag of several hours. We examined such dependences as functions of ( X , Y ) coordinates in the near‐Earth plasma sheet by fitting observations of plasma sheet and solar wind to an empirical model equation. We explored shortest and longest response time lags to IMF B z by optimizing the correlation coefficient of fit. Analyses were conducted separately for northward and southward IMF dominant conditions. The dependence of plasma sheet number density on solar wind number density is stronger in the near‐tail region ( r > 20 R E ) under the southward IMF dominant condition. The dependence has weak dawn‐dusk asymmetry under the southward IMF dominant condition, whereas it is stronger in the dusk flank under the northward IMF dominant condition. The dependence on IMF B z is globally positive under the northward IMF dominant condition, whereas the dependence is negative in the near‐Earth premidnight region under the southward IMF dominant condition. Both shortest and longest time lags increase from the mid‐tail to the near‐Earth premidnight region under the southward IMF dominant condition. On the other hand, the shortest (longest) time lag increases antisunward (sunward) along flanks under the northward IMF dominant condition. These features can be explained in terms of reconnection and Kelvin‐Helmholtz diffusion mechanisms for entry of magnetosheath plasma into the plasma sheet and electric and magnetic drift transport in the plasma sheet.