Observations in the vicinity of substorm onset: Implications for the substorm process

Abstract

Multi-instrument data sets from the ground and satellites at both low and high altitude have provided new results concerning substorm onset and its source region in the magnetosphere. Twenty-six out of 37 substorm onset events showed evidence of azimuthally spaced auroral forms (AAFs) prior to the explosive poleward motion associated with optical substorm onset. The azimuthal wavelengths associated with these onsets were found to range between 132 and 583 km with a mean value of 307 _+ 115 km. The occurrence rate increased with decreasing wavelength down to a cutoff wavelength near 130 km. AAFs can span 8 hours of local time prior to onset and generally propagate eastward in the morning sector. Onset itself is, however, more localized spanning only about 1 hour local time. The average location of the peak intensity lbr 80 onsets was 65.9 + 3.5 CGMIat, 22.9 _+ 1.2 Mlt, whereas the average location of the AAF onsets was at 63.8 _+ 3.3 CGMIat, 22.9 _+ 1.1 Mlt. AAF onsets occur during time periods when the solar wind pressure is relatively high. These low-latitude wavelike onsets appear as precursors in the form of long-period magnetic pulsations (Pc 5 band) and frequently occur on the equatorward portion of the double oval distribution. AAFs brighten in conjunction with substom onset leading to the conclusion that they are a growth phase activity causally related to substorm onset. Precursor activity associated with these AAFs is also seen near geosynchronous orbit altitude and examples show the relationship between the various instrumental definitions of substorm onset. The implied mode number (30 to 135) derived from this work is inconsistent with cavity mode resonances but is consistent with a modified flute/ballooning instability which requires azimuthal pressure gradients. It is suggested that this instability exists in growth phase but that an additional factor exists in the premidnight sector which results in an explosive onset. The extended source region 'and the distance to the open-closed field line region constrain reconnection theory and local mechanisms for substom onset. It is demonstrated that multiple onset substorms can exist for which localized dipolarizations and the Pi 2 occur simultaneously with tail stretching existing elsewhere. Further, the tail can be less stretched at geosynchronous orbit during the optical auroral onset than during the precursor pseudobreakups. These pseudobreakups can be initiated by auroral streamers which originate at the most poleward set of arc systems and drift to the more equatorward main UV oval. Observations are presented of these AAFs in conjunction with low-and high-altitude particle and magnetic field data. These place the activations at the interface between dipolar and taillike field lines probably near the peak in the cross-tail current. These onsets are put in the context of a new scenario for substorm morphology which employs individual modules which operate independently or couple together. This allows particular substorm events to be more accurately desc...

Observational evidence for an inside-out substorm onset scenario

“…From this more detailed perspective, we can clearly see that the intensified onset region in the 14:02:12 UT image is comprised of a sequence of azimuthally periodic bright spots, and that these bright spots lie precisely on-top of the pre-existing growth phase arc. Such spatially periodic auroral spots have been seen prior to the onset of substorm expansion phase in other studies as well (Henderson, 1994;Elphinstone et al, 1995;Samson et al, 1996;Voronkov et al, 2000Voronkov et al, , 2003. In addition, no distortions of the more poleward emissions are evident in the auroral images.…”

“…We note that the event examined here is very similar to a handful of events that have been studied in the past. Early examples obtained with the Viking UV imager have been presented (Henderson, 1994;Elphinstone et al, 1995) and more recently, a few cases from ground-based imagers have been reported (Voronkov et al, 2003;Donovan et al, 2006). Two of the clearest substorm onsets showing arc-aligned azimuthally periodic bright spots from the Viking imagery occurred on 24 November 1986 at 10:12:11 UT and on 23 September 1986 at 20:51:46 UT.…”

Magnetic conjugacy of northern and southern auroral beads

“…In particular, the slowly moving beads in the first stage have never been reported in previous studies [Donovan et al, 2006;Sakaguchi et al, 2009]. Very recently, Kataoka et al [2011], using an electron multiplier charge-coupled device (EMCCD) camera with a narrow FOV, demonstrated that smaller-scale auroral forms (on a scale of a few/several km) appear in a breakup arc in a few minutes before onset.…”

“…However, the abovementioned interhemispheric similarities strongly suggest that there must be a common driver in the magnetotail equatorial region that was controlling the major temporal evolution of the auroral beads. Most of the previous studies of auroral beads [e.g., Donovan et al, 2006;Liang et al, 2008] inferred that the magnetospheric driver of the auroral beads may be a ballooning type instability [Cheng, 2004]. Of course, some different processes at a lower altitude along the field line (i.e., M-I coupling region) may also contribute to the structuring of the bead structures: for example, ionospheric feedback instability in the so-called ionospheric Alfven resonator [Lysak and Song, 2002] and instabilities in the auroral acceleration region [Chaston and Seki, 2010].…”

“…Recently, attention has been drawn to the temporal evolution of the initial brightening arc during the few minutes before the onset, because it could illustrate the process that triggers the expansion phase onset. Optical observations from the ground [Donovan et al, 2006;Liang et al, 2008;Sakaguchi et al, 2009] and from spacecraft [Elphinstone et al, 1995;Henderson, 2009] have shown that in the initial brightening arc, there exists a characteristic small-scale auroral structure, consisting of azimuthally arrayed wave-like forms (often referred to as "auroral beads") with a wavelength of 30-100 km and an azimuthal propagation speed of 5-8 km s À1 . As time progresses, the auroral beads evolve into wave-like undulations with a larger scale of 50-200 km, and finally lead to auroral breakup.…”