substorm: The pseudobreakup phase

Abstract. We study the isolated substorm that occurred after a long quiet period, which showed all of the substorm sig-natures except for the first half hour of the expansion phase, which could be characterized as a pseudobreakup sequence, rather than a full-scale substorm onset. During the consid-ered event, the substorm’s instability leads to a current dis-ruption, which starts at the near-Earth plasma sheet and then propagates tailward. Based on auroral observations, the anal-ysis of geosynchronous plasma injections, and the plasma sheet observations at ∼15 RE at the meridian of auroral sub-storm development we show that (1) before and probably during “pseudobreakup phase”, the plasma sheet stayed cold and dense, (2) during the pseudobreakup phase, particle in-jections at 6.6 RE were only seen in unusually low energy components, and (3) the electron precipitation into the iono