Substorm time constants

Abstract

We show that during magnetospheric substorms there is a clear tendency for the westward electrojet current to follow a characteristic curve that can be matched with an exponential equation. The substorm recovery phase has a characteristic timescale, or exponential time constant, that does not vary substantially from one substorm to the next. There is a slight trend toward shorter substorms as the peak magnitude of the electrojet current increases. This behavior can be explained by a simple model of the coupling between the currents and electric fields in the magnetotail and ionosphere. We derive an equation for the evolution of the substorm currents, including those in the magnetotail, using fundamental physical principles. There is good agreement between the measured and predicted time constants. This model is also consistent with other reported measurements of plasma flows and magnetic perturbations in the magnetotail.

A superposed epoch investigation of the relation between magnetospheric solar wind driving and substorm dynamics with geosynchronous particle injection signatures

“…The recovery of category 2 AL and AU towards control line values after onset occurs more gradually over a longer time period (∼3 h) than for category 1 events, similar to F pc , I, B z , and ". The peak in the category 1 AL and AU at +26 min is consistent with the range of 25 to 34 min (peak times for substorms with peak AL magnitudes between 600 and 1600 nT and between 400 and 600 nT, respectively) found by Weimer [1994]. In contrast, the category 2 AU and AL magnitudes peak considerably later but may be consistent with the finding that the lower the magnitude of the substorm bay, the longer it takes for the peak to be reached.…”

“…At substorm onset the disruption of the cross-tail current and formation of the substorm electrojet cause a rapid increase in the strength of the AL index and subsequent recovery. Weimer [1994] carried out a superposed epoch analysis of the AL and AU indices for 55 substorm events, finding the peak in activity was reached between +25 and +34 min after the time of substorm onset, with the peak in intensity, and subsequent decay, increasing for more intense substorms.…”

Rayed Southern Lights Observed at Belgrano Station: A Study.

“…Our results, agree with the ones obtained by Weimer (1992Weimer ( , 1994, who used sixteen randomly chosen substorms (withAE peak, less than 600 nT) selected by visual inspection, and considered only the AE signature to define each isolated substorm.…”

“…As a theoretical model, we used the electrical circuit of the coupled magnetosphere-ionosphere system, proposed by Weimer (1992Weimer ( , 1994. In this model, the electric field has the same shape as the electrojet current intensity, and it delineates the current in an electric circuit constituting of an inductor, a capacitor and a resistor connected in parallel.…”

“…The development of the electric field in the plasma sheet, after a disruption of the current sheet flow has been elucidated using an electrical circuit model of the coupled magnetosphere-ionosphere system by Weimer (1992Weimer ( , 1994. Foster et al (1971) adjusted the time corresponding to auroral breakup and the beginning of the expansion phase employing the sharp changes registered in magnetograms corresponding to an auroral zone midnight station and a low-latitude pre-midnight station.…”