Fig. 3: Further evidence for magnetic reconnection in association with the plasma cavity.

a, The magnetic fields in LMN coordinates. b–d, The V_L (b), V_M (c), and V_N (d) components of different ions measured by STATIC. e, The ion temperatures. f, The ion densities. g, The plasma beta, magnetic pressure (P_B) and thermal pressure (P_th). The dashed lines are positioned as in Fig. 2. The L, M, and N directions in MSO coordinates as well as the ratios of the maximum to intermediate and intermediate to minimum variances are given in b. The ion jet and heating accompanying the magnetic strength dip clearly indicate magnetic reconnection. The reconnection jet is along the −L direction, with the outflow plasma in the Southern Hemisphere primarily moving in the −Z MSO direction, indicating outward movement. The densities of \(\mathrm{O}_{2}^{+}\) and other heavy ions decreased by nearly two orders of magnitude within the reconnection exhaust, implying extensive plasma ejection into space and resulting in an ionospheric plasma cavity. The surrounding regions exhibit a low plasma beta. The reconnection site, at a lower altitude, has a stronger crustal field, further reducing the plasma beta and promoting magnetic reconnection.