ALBERT

Description: We investigate the plasma sheet pressure variations in the near Earth magnetotail (Radius distance, R , from 7.5 R E to 12 R E and Magnetic Local Time, MLT, from 18:00 to 06:00) during substorm growth phase with Time History of Events and Macroscale Interactions during Substorms (THEMIS) observations. It is found that, during the substorm growth phase, about 39.4% (76/193) of the selected events display a phenomenon of equatorial plasma pressure ( P eq ) decrease. The occurrence rates of P eq decrease cases are higher in the dawn (04:00 to 06:00) and dusk (18:00 to 20:00) flanks (〉 50%) than in the midnight region (20:00 to 04:00, 〈 40%). The mean values of the maximum percentages of P eq decrease during the substorm growth phases are larger in the dawn and dusk flanks (~ - 20%) than in the midnight region (~ 〉 - 16%). The mean value of P eq increase percentages at the end of substorm growth phase is the highest (~ 40%) in the pre-midnight MLT bin (22:00 to 00:00) and is almost unchanged in the dawn and dusk flanks. Further investigations show that 13.0% of the events have more than 10% of P eq decrease at the end of substorm growth phase comparing to the value before the growth phase, and ~ 28.0% of the events have small changes (〈 10%), and ~ 59.0% events have a 10% increase. This study also reveals the importance of electron pressure ( P e ) in the variation of P eq in the substorm growth phase. The P e variations often account for more than 50% of the P eq changes, and the ratios of P e to ion pressure often display large variations (~ 50%). Among the investigated events, during the growth phase, an enhanced equatorial plasma convection flow is observed, which diverges in the midnight tail region and propagates azimuthally towards the dayside magnetosphere with velocity of ~ 20 km/s. It is proposed that the P eq decreases in the near Earth plasma sheet during the substorm growth phase may be due to the transport of closed magnetic flux towards the dayside magnetosphere driven by dayside magnetopause reconnection. Both solar wind and ionospheric conductivity effects may influence the distributions of occurrence rates for P eq decrease events and the P eq increase percentages in the investigated region.