Statistical study of the substorm onset: its dependence on solar wind parameters and solar illumination

Abstract

Abstract. Based on 1829 well-defined substorm onsets in the Northern Hemisphere, observed during a 2-year period by the FUV Imager on board the IMAGE spacecraft, a statistical study is performed. From the combination of solar wind parameter observations by ACE and magnetic field observations by the low altitude satellite CHAMP, the location of auroral breakups in response to solar illumination and solar coupling parameters are studied. Furthermore, the correspondence of the onset location with prominent large-scale field-aligned currents and electrojets are investigated. Solar illumination and the related ionospheric conductivity have significant effects on the most probable substorm onset latitude and local time. In sunlight, substorm onsets tend to occur 1 h earlier in local time and 1.5 • more poleward than in darkness. The solar wind input, represented by the merging electric field, integrated over 1 h prior to the substorm, correlates well with the latitude of the breakup. Most poleward latitudes of the onsets are found to range around 73 • magnetic latitude during very quiet times. Field-aligned and Hall currents observed concurrently with the onset are consistent with the signature of a westward travelling surge evolving out of the Harang discontinuity. The observations suggest that the ionospheric conductivity has an influence on the location of the precipitating energetic electron which causes the auroral break-up signature.

Interhemispheric comparison of average substorm onset locations: evidence for deviation from conjugacy

“…Finally, a substorm onset is only accepted if at least 30 min had passed after the previous onset (Frey et al, 2004). Figure 1 in Wang et al (2005b) shows two auroral images from IMAGE FUV-WIC in the frame of magnetic latitude (MLat) and local time (MLT) coordinates. The coordinate frame used to order FUV observations is the Apex coordinate system described by Richmond (1995).…”

“…In a previous study Wang et al (2005b) have performed a superposed epoch analysis of the substorm onset time with respect to various solar wind parameters (which had been time-shifted to the dayside magnetopause). That study was based on two years (2002)(2003) of substorm onset events in the Northern Hemisphere.…”

“…The solar wind data have been propagated from the ACE satellite to the magnetopause. The solar wind parameters at the magnetopause are averaged over a one hour period before the onset (for justification see Wang et al, 2005b).…”

“…Liou et al (2001) found that the substorm onsets tended to start at earlier local times in northern hemispheric summer than in winter, and at intermediate local times in spring. Our previous study of northern hemispheric events disclosed that substorm onsets in sunlight tend to occur 1 h earlier in local time and 1.5 • MLat more poleward than in darkness, suggesting that the ionospheric conductivity has an influence on the location of substorm onset (Wang et al, 2005b). Newell et al (1996) found that an increased ionospheric conductivity decreases the likelihood of establishing energetic electron precipitation (which is needed to excite discrete auroras).…”

“…Østgaard et al, 2006). They proposed that longitudinal asymmetry was primarily controlled by IMF clock angle defined in GSM coordinates, which is regarded as a follow on to the Northern Hemisphere study by Wang et al (2005b). In this work we will investigate the statistical properties of substorm onset locations in both hemispheres, including their dependence on solar wind input and solar illumination.…”