Investigation of external triggering of substorms with Polar ultraviolet imager observations

Abstract

The triggering mechanism(s) for the substorm expansion phase onset is one of the outstanding issues in magnetospheric physics. Previous studies have shown that an impingement of the high solar wind dynamic pressure may lead to the expansion phase onset of a substorm. These previous studies typically used a negative magnetic bay as the main proxy for a substorm, but we now know that some magnetic bays are associated not with substorms but with the enhancement of convection. Therefore it is reasonable to cast doubts on the compression trigger mechanism. In this study we use classical substorm onsets, “auroral breakups,” which are the most reliable substorm onset indicator when identified with global auroral images, to reinvestigate this issue. We examine 43 interplanetary shock events that occurred between 1996 and 1999 with simultaneous global auroral images from the Polar ultraviolet imager images and the auroral electrojet indices. It is found, indeed, that ∼52% of the shocks produce magnetic bays (AL < −100 nT). While most of the shocks enhance auroral luminosities, only 4 events (∼9%) appear to have an auroral breakup preceded by an SSC/SI by 20 min (two events by 10 min). These results strongly indicate interplanetary shocks can produce negative magnetic bays but not auroral breakups (thus christened “compression bays”). An examination of precisely timed interplanetary magnetic field (IMF) during 11 substorms that occurred near the shock‐induced SSCs/SIs indicates that northward turnings of IMF occurred more than 50% of time. Given such a low probability, it is concluded that shock compression is not likely to trigger substorms but enhances magnetospheric currents and auroral particle precipitation. In line with many previous study results, on the other hand, the northward turnings of IMF can be a plausible substorm triggering mechanism.

Geomagnetic and ionospheric response to the interplanetary shock on January 24, 2012

“…The initial brightening is known to immediately propagate to the midnight sector, occupying most of the oval in ~20 min. In contrast to this auroral transient, a sudden brightening of auroral patches with lifetimes of 5-6 min in the midday sub-auroral zone detached from the oval is observed within 1 min of the SC (Liou et al 2003). This auroral transient is associated with the precipitation of preexisting trapped central plasma sheet particles caused by magnetospheric compression.…”

“…However, whether all substorms are triggered is still a controversial issue, because not all magnetic bays are associated with auroral substorms. Although ~50% of the IP shocks do produce negative magnetic bays at nightside, only less than 10% of the SC causes an auroral breakup (Liou et al 2003). Therefore, it is likely that the observed magnetic bay at PBK is not associated with the auroral breakup but with enhanced convection due to a sudden compression and should be classified as a "compression bay.…”

“…Solar wind changes are believed to externally trigger a substorm (e.g., McPherron et al 1986;Liou et al 2003), though it remains a controversial issue whether the substorm triggering is caused by the sudden IMF changes that occur simultaneously with a pressure increase, or if the compression itself has a potential to trigger a substorm. In addition, the magnetospheric compression and inward (anti-sunward) displacement of magnetospheric plasma is to be accompanied by an induction electric field, E λ ~ ∂B z /∂t.…”

“…An observed response to an IP shock or discontinuity may be rather complicated due to the nearly simultaneous excitation of several transient processes. The IP shock is known to stimulate a global compression of the magnetosphere (Araki 1994), with excitation of transient field-aligned current systems (Fujita et al 2005), generation of transient ULF waves in a wide frequency band from Pc1 (Parkhomov 1990) to Pc5-6 (Yumoto et al 1994), triggering of VLF wave activity and enhancement of energetic electron precipitation (Gail et al 1990), brightening of the dayside aurora ("shock aurora") at auroral (Spann et al 1998;Vorobiev et al 2008) and sub-auroral (Liou et al 2003) latitudes, sudden vertical Open Access *Correspondence: space.soliton@gmail.com 2 Geophysical Center, Moscow, Russia Full list of author information is available at the end of the article shifting of the ionospheric plasma (Yumoto et al 1989), prompt injection and energization up to MeV energies of magnetospheric particles (Blake et al 1992), and triggering of a nightside "compression" magnetic bay or even a substorm (Lyons 1996;Liou et al 2003). In addition, a SC pulse serves as an indicator of the onset of a more intense solar wind driving and buffeting the magnetosphere.…”

Ionospheric Response to Solar Wind Pressure Pulses Under Northward IMF Conditions

“…The initial brightening is localized and the region of brightening often extends quickly antisunward to cover a much wider local-time sector of the oval. The effect of magnetospheric compression is not limited to the dayside auroral oval; compression of the magnetosphere by shocks can also enhance auroral activity in the nighttime sector Boudouridis et al 2003;Liou et al 2003), widen the auroral zone width, and reduce the polar cap size (Boudouridis et al 2003(Boudouridis et al , 2004. Furthermore, Liou et al (2006) demonstrated that global auroral responses to negative pressure pulses (magnetospheric decompression) are opposite to that of positive pressure pulses (i.e., decompression of the magnetosphere quenches the global development of auroras).…”