Abstract
[1] We report a superposed epoch analysis of the hemispheric open magnetic flux, maximum nightside auroral intensity, geomagnetic activity, and solar wind and interplanetary magnetic field conditions around the time of substorm onset for three distinct categories of substorms defined by their energetic particle injection signatures. Substorms identified from global auroral imagery are classified into one of three categories based on their energetic particle injection signatures as seen at geosynchronous orbit by the Los Alamos National Laboratory spacecraft. Category 1 events are associated with a “classic” substorm injection, category 2 events show varied activity (i.e., energetic enhancements not following the evolution expected for classic substorms), and category 3 events show no apparent injection activity. The superposed epoch analysis reveals that the three distinct particle injection categories exhibit distinct differences in the level and continuity of magnetospheric driving by the solar wind, such that category 1 events can be described as classic substorm events, category 2 as continuously driven events, and category 3 as weak events. The results of this study suggest that the level and continuity of the dayside solar wind driving of the magnetosphere during substorms have a direct impact on the injection of energetic particles to geosynchronous orbit at substorm onset. These results could have considerable value in empirical predictions of the space weather environment.
Highlights
[2] Substorms are the fundamental global disturbance of the magnetospheric system
In this paper we investigate this finding further by carrying out a superposed epoch analysis of Fpc, auroral intensity, interplanetary magnetic field (IMF), and solar wind conditions, as well as geomagnetic activity, around the time of substorm onset, for the three particle injection categories defined by Boakes et al [2009]
[7] In this study we use a subset of substorms identified by Frey et al [2004] from images of the northern hemispheric auroral oval made by the far ultraviolet (FUV) instrument on board the Imager for Magnetopause‐to‐ Aurora Global Exploration (IMAGE) spacecraft [Mende et al, 2000a, 2000b]
Summary
[2] Substorms are the fundamental global disturbance of the magnetospheric system. The substorm growth phase is usually defined as a period of 30–60 min of southward oriented interplanetary magnetic field (IMF) [McPherron et al, 1973] during which low‐latitude reconnection at the dayside magnetopause opens previously closed terrestrial magnetic field lines to the solar wind. Several studies [e.g., Milan et al, 2003, 2007, 2009a; Coumans et al, 2007; DeJong et al, 2007; Hubert et al, 2008; Boakes et al, 2009; Huang et al, 2009] have employed such methods to show that Fpc typically increases by ∼25%–35% during the substorm growth phase, while the average Fpc at onset is typically between 0.5 and 0.7 GWb (but can be as low as 0.3–0.45 GWb for very weak events and as high as 0.8–1.0 GWb for sawtooth events). In this paper we investigate this finding further by carrying out a superposed epoch analysis of Fpc, auroral intensity, IMF, and solar wind conditions, as well as geomagnetic activity, around the time of substorm onset, for the three particle injection categories defined by Boakes et al [2009]
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