Correlated interball/ground-based observations of isolated substorm: The pseudobreakup phase

We study the isolated substorm that occurred after a long quiet period, whi ch showed all of the substorm signatures except for the first half hour of the expansion phase, which could be characterized as a pseudobreakup sequen ce, rather than a full-scale substorm onset. During the considered event, t he substorm's instability leads to a current disruption, which starts at th e near-Earth plasma sheet and then propagates tailward. Based on auroral ob servations, the analysis of geosynchronous plasma injections, and the plasm a sheet observations at similar to 15 R-E at the meridian of auroral substo rm development we show that (1) before and probably during "pseudobreakup p hase", the plasma sheet stayed cold and dense, (2) during the pseudobreakup phase, particle injections at 6.6 R-E were only seen in unusually low ener gy components, and (3) the electron precipitation into the ionosphere was v ery soft. We conclude that the basic difference between pseudobreakups and "real" substorm activations was found in the low energy of all manifestatio ns. We suggest that high density and low electron temperature in the plasma sheet are the reasons for low energization in the magnetic reconnection op erated on closed field lines in the plasma sheet, as well as the weak field -aligned acceleration, as predicted by the Knight's relationship. The low H all conductivity could then be the reason for the weal, ground magnetic eff ects observed. This explanation suggests that the role of the ionospheric c onductivity is "passive" as the plasma sheet, rather than the ionosphere, c ontrols the development of the magneto spheric instability.