Analysis of geomagnetic sudden impulses at low latitudes

Abstract

[1] We present an analysis of the SI characteristics at l % 36°examining 54 events for a 5-year interval (2000)(2001)(2002)(2003)(2004). In the H component, the ground responses are typically greater than expected for changes of the magnetopause current; they are also accompanied by significant changes of the D component. Both these aspects suggest a remarkable, long-lasting contribution of the ionospheric currents on geomagnetic measurements associated with changes of the solar wind pressure. The relative response (i.e., as normalized to the change of the square root of the solar wind pressure) exhibits a local time modulation, with highest values in the afternoon hours for both components. For the H component, the average ratio between the observed and the predicted response is %1.5 in the central part of the day and %1.2 in the dark sector.

On the discrimination between magnetospheric and ionospheric contributions on the ground manifestation of sudden impulses

“…Figure a shows a model SI event as it would appear at ground midlatitudes in the morning hours: as previously said, according to Araki [], the H behavior (solid line, left) can be interpreted in terms of the superposition of a longer time variation, D L (dotted line), and a shorter time variation, D P (dashed line); such D P contribution emerges after removing the D L field from the experimental signal. In Figure a (right), we show a typical behavior of the D component [ Villante and Piersanti , ], which is not considered in the Araki [] representation. Figures b–d summarize our approach, as follows: We estimate the global magnetospheric contribution ( D L m , where m stands for “magnetosphere”) from a comparison between the observations of two spacecraft ( B z , B x in Figure b; magnetospheric observations are expressed in the GSM coordinate system in which the X axis is directed from the Earth to the Sun; the Y axis is defined to be perpendicular to the Earth's magnetic dipole so that the X ‐ Z plane contains the dipole axis and the positive Z axis is chosen to be in the same sense as the northern magnetic pole), ∼4 h apart along geostationary orbit, and the predictions of the TS04 model, assuming transition between two steady states of the magnetospheric field (Figure b).…”

“…Figure b provides an ideal scheme in which the magnetospheric observations would be well interpreted on both components in terms of the cumulative magnetopause and ring current field: according to our approach, in this ideal case D L m = B CF + R . In such representation we explicitly show that the magnetospheric currents often influence also the magnetospheric B x component (and the ground D component as well) [ Villante and Piersanti , ]. Given the representation of the D L m field at geostationary orbit, we evaluate the expected signal of D L m at ground stations ( D L H and D L D , where H stands for H and D component) by means of TS04 model and determined the LT sectors in which the long‐term variation in ground measurements reveals a good agreement with the modeled D L m field. At these stations the residual field ( D P i ) is determined subtracting from ground observations the estimated D L H , i and D L D , i fields (Figure c).…”

“…The primary causes of SIs are interplanetary fast shocks or discontinuities of the incoming solar wind (SW) colliding with the magnetopause. Several works have studied the morphological aspects of SIs at geosynchronous orbit and in the outer magnetosphere [ Patel and Coleman Jr. , ; Kokubun , ; Lee and Lyons , ; Villante and Piersanti , , ]. They revealed a general dependence of the jump of the magnetic field on the increase of the square root of the SW pressure (Δ P 1/2 ).…”

“…In addition, Tsunomura [] showed that at middle and low latitudes the H variations are accompanied by explicit negative variations of the D component (east‐west), interpreted in terms of the influence of ionospheric current systems. As a matter of fact, Villante and Piersanti [, ], analyzing 55 SI events, suggested the concurring influence of the magnetospheric and ionospheric currents on both the H and D components.…”

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.…”