An analysis of sudden impulses at geosynchronous orbit

Abstract

An analysis of sudden impulses (SI) at geosynchronous orbit (2000–2004) confirms a general dependence of the SI amplitude on the variation of the square root of the solar wind pressure, together with an explicit LT dependence, with greater responses at satellites located closer to noon meridian. In the dayside hemisphere the magnetospheric response, which mostly influences the Bz component, is well consistent with the magnetic field jump expected for changes of the magnetopause current alone, driven by changes of the solar wind pressure. In the dark hemisphere, where the changes of the Bx component are often relevant, the competing contributions of several current systems (from the magnetopause, cross‐tail current, ring current, Birkeland current) determine a large variety of responses that cannot be interpreted in a statistical sense. Depending on the solar wind conditions, different situations emerge for nightside events. We present a case in which a remarkable magnetospheric compression determined field variations which can be interpreted in terms of a strongly dominant contribution of the magnetopause current even in the midnight sector, while in other cases the observed features are consistent with the predictions of the global current system. We also speculated that additional elements (such as the geocentric distance of the hinging point, the separation point between closed and open field lines in the geomagnetic tail) might play a crucial role in determining the aspects of the magnetospheric response. The correspondence between model predictions and observations persists even in cases of moderate Southward orientations of the IMF.

Sudden Impulses in the Magnetosphere and at Ground

“…different magnetopause currents, Tsyganenko, 2002aTsyganenko, , 2002b. As a matter of facts, the observed average values reveal in the central part of the day a close correspondence with the predicted responses, a feature confirmed by an analysis of individual events (Villante and Piersanti, 2008). It suggests that, in this region, the field jumps are basically determined by the changes of the magnetopause current alone.…”

“…Kokubun (1983) evaluated the role of the magnetopause and tail currents and concluded that the geosynchronous responses were ≈30% smaller than expected. Figure 2b (after Villante and Piersanti, 2008), compares the 3-hr averages of R z with the theoretical profiles evaluated assuming that the magnetic field change is determined by the transition between two steady states of the magnetosphere under different SW pressure conditions (i.e. different magnetopause currents, Tsyganenko, 2002aTsyganenko, , 2002b.…”

Identification of the different magnetic field contributions during a geomagnetic storm in magnetospheric and ground observations

“…During a GS, this baseline shows large amplitude variations, and it could be used as a measure of the local magnetospheric current activity by a comparison between B z0 (t) and the TS04 magnetospheric field model (Tsyganenko and Sitnov, 2005). In fact, since the TS04 model is modular, it can be used to evaluate the magnetospheric current that best fits the B z0 (t) variations (Villante and Piersanti, 2008;Piersanti and Villante, 2016). Moreover, during an SSQ period, the same baseline could be used to efficiently calibrate the IGRF model.…”

Sudden impulses at geosynchronous orbit and at ground

“…Basically, we focus our attention on the change of the magnetic field between approximately two steady states on opposite sides of the event and compare such change with the value predicted by theoretical models for transition between two steady state representations of the magnetospheric and ground field under different SW conditions (i. e. before and after the SW pressure change): as we discuss in the following, the comparison between low latitude and geosynchronous observations, as well as with the model predictions, represents an useful tool for a better understanding of basic aspects of the SI manifestation and of the role of the competing magnetospheric/ionospheric current systems. In our previous analysis the magnetic field response at geostationary orbit (Villante and Piersanti, 2008, hereafter referred as paper 1) and at ground (Villante and Piersanti, 2009; paper 2) was compared with the predictions obtained by the magnetic field representation proposed by Tsyganenko (2002a, b; such model is usually referred as T01 in the scientific literature). In the present paper, the same set of events, as observed both at geosynchronous orbit and at ground, is compared with the predictions of a more recent model (Tsyganenko and Sitnov, 2005;T04) which is expected to provide a more confident representation of the magnetospheric field and allows separate analysis of the contributions of the competing current systems.…”