Some aspects of the geomagnetic response to solar wind pressure variations: a case study at low and middle latitudes

Abstract

Abstract. We examined geomagnetic field observations at low and middle latitudes in the Northern Hemisphere during a 50-min interval (12 May 1999), characterized by a complex behaviour of the solar wind dynamic pressure. For the entire interval, the aspects of the geomagnetic response can be organized into four groups of events which show common characteristics for the H and D components, respectively. The correspondence between the magnetospheric field and the ground components reveals different aspects of the geomagnetic response in different magnetic local time (MLT) sectors. For the H component, the correspondence is highly significant in the dusk and night sectors; in the dawn and prenoon sectors it shows a dramatic change across a separation line that extends approximately between (6 MLT, 35 • ) and (13 MLT, 60 • ). For the D component, the correspondence has significant values in the dawn and prenoon regions. We propose a new approach to the experimental data analysis which reveals that, at each station, the magnetospheric field has a close correspondence with the geomagnetic field projection along an axis (M1) that progressively rotates from north/south (night events) to east/west orientation (dawn events). When projected along M1, the geomagnetic signals can be interpreted in terms of a one-dimensional pattern that mostly reflects the field behaviour observed at geostationary orbit. Several features appear more evident in this perspective, and the global geomagnetic response to the SW pressure variations appears much clearer than in other representations. In particular, the MLT dependence of the geomagnetic response is much smaller than that one estimated by previous investigations. A clear latitudinal dependence emerges in the dusk sector. The occurrence of low frequency waves at ∼2.8 mHz can be interpreted in terms of global magnetospheric modes driven by the SW pulse. This event occurred in the recovery phase after the day the SW almost disappeared (11 May 1999): in this sense our results suggest a rapid recovery of almost typical magnetospheric conditionsCorrespondence to: U. Villante (umberto.villante@aquila.infn.it) soon after a huge expansion. Overshoot amplitudes, greater than in other cases, are consistent with a significant reduction of the ring current.

An analysis of sudden impulses at geosynchronous orbit

“…Indeed, we found a case in which an extreme 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. On the other hand, an explicit correspondence between SI observations in the nightside hemisphere and external SW pressure changes was already remarked by Villante and Di Giuseppe [2004], who examined the geomagnetic field observations at low and middle latitudes during a time interval characterized by a significant SW pressure change and identified in the dusk and night sectors a correlation between interplanetary and ground measurements much better than in other sectors. In other cases the observed features are better consistent with the predictions of the global current system and the external conditions (and, in particular, the IMF orientation) may occasionally drive changes in the tail configuration and dynamics (for example in terms of the position of the hinging point and on the thickness of the plasma sheet) which significantly influence the magnetospheric response.…”

ULF fluctuations of the geomagnetic field and ionospheric sounding measurements at low latitudes during the first CAWSES campaign

“…On the other hand, in agreement with the present results, the low-latitude extension of the DP fields has been reported in previous investigations (Kikuchi and Araki, 1985;Tsunomura, 1998;Villante et al, 2005a): in the present case, the occurrence of the initial negative pulse might lead to speculate, for the period of interest, a latitudinal and MLT extension of the afternoon vortex larger than the morning one, a situation predicted by Araki (1994) as a consequence of the different morning and afternoon conductivity. As to the amplitude of the geomagnetic response, the comparison of different stations suggests a steady-state response H (as evaluated 5 min after the peak response) decreasing with decreasing latitude as ∼1 nT/ • : as previously underlined, this behaviour is odd with respect to predictions for the DL field alone and also confirms a significant contribution of the DP field on the longer term variation (Villante and Di Giuseppe, 2004;Villante et al, 2005a).…”

Sudden impulses at geosynchronous orbit and at ground

“…In ground manifestations the preliminary impulse is exclusively due to current systems of polar origin, DP PI , whereas the main impulse is due to the combined effect of DL and DP MI (Sastri et al, 2001). It is worth noting, in addition, that such representation is usually referred to the behaviour of the H component alone: by contrast, Villante and Di Giuseppe (2004) revealed, for a case study, that the trace of the magnetospheric field in the noon quadrant found a close correspondence at low latitudes in the projection of the geomagnetic field along an axis that progressively rotated from the H (in the nightside) to the D component (dawn sector); it suggests that the effects of the magnetopause current do not influence the H component alone. The basic paradigm proposed by Araki (1994) has been successfully applied to a number of SI/SSC events; in the meanwhile, more complex manifestations have been reported in several cases (for example, Araki et al, 1997;Sastri et al, 2001;Moiseyev et al, 2002).…”

“…On the other hand, since early investigations, it is well known that different waveforms of the H component are detected at different stations (Matsushita, 1962;Nishida and Jacobs, 1962;Villante and Di Giuseppe, 2004). Araki (1994) summarized the principal characteristics of the SI/SSC manifestations, as they appear at different latitudes and LT. Basically, at auroral latitudes, the H waveform typically consists of two successive pulses with opposite sense (''preliminary impulse'', PI, and ''main impulse'', MI, with typical duration of E1-2 min and E5-10 min, respectively).…”