A critical condition in magnetotail pressure for leading to a substorm expansion onset: Geotail's observations

Variations in the magnetotail total pressure are studied to search for the critical value leading to an onset of the major auroral breakup. Thirty‐seven sudden decreases in the total pressure in conjunction with the onsets of auroral breakups were identified to occur. These pressure events are termed as the magnetotail deflation in this study. Plotting the total pressure as a function of the Dst index, it is found that the data points obtained just prior to the deflation events are located near the upper limit of the entire data points. Confirming Nakai and Kamide [2003a] , this implies that a magnetotail deflation is preferably triggered when the magnitude of the total pressure becomes greater than a threshold value, which is determined depending on the Dst index. The minimum values of the total pressure after the deflations are also examined. It is found that the difference in the total pressure between before and after deflations depends negatively on the Dst index. From these results it is estimated that the radius of the mid‐distant magnetotail reduces, on average, from 20 to 14 R E for Dst = 0 during the magnetotail deflation.