substorms

Beginner's guide to Substorms

Early studies [Akasofu, 1964] suggested the concept of auroral substorm by studying auroral observations - ASI, mag?
McPherron [1972], McPherron et al., [1973], Horning et al., [1974] deduced the large scale current pattern associated with auroral displays - SCW [Pytte et al., 1976].
Qualitative picture of SCW remained the same since then.

McPherron [1972], McPherron et al., [1973] noted correlation between magnetometer obs on ground and geosynchronous orbit (ATS-1 satellite).
McPherron et al., [1973] suggested tail-current short circuited into the ionosphere.
Mid-lat mag. field singatures of SCW. H-comp is positive and symmetric about the central meridian of the wedge. D-comp is antisymmetric about central meridian.
Similar variations in the D-comp of geosynchronous spacecraft.

Magnetic reconnection [Dungey, 1961] and Viscous drag [Axford and Hines, 1961] are two important modes of energy transfer in to the magnetosphere.
Magnetosphere transports and processes this energy through different modes of response, sporadically leading to reconfiguration of nightside magnetosphere and flow bursts in the tail.
Psuedobreakups, magnetospheric substorms, steady magnetospheric convection intervals, sawtooth events and geomagnetic storms.
These modes share some common properties. However, they occur on different temporal and spatial scales and have several distinct characteristics.
SCW is observed during each of these modes. However, there are differences in location, extent and intensity of SCW.

Near Earth Neutral Line (NENL)/ Outside In Model : Substorms triggered by tail reconnection, generating BBFs which cause current disruption.
Near Earth Current Disruption (NECD)/Inside Out Model : Current disruption occurs first, sending refractive waves tailward driving reconnection.
THEMIS observations show NENL model is probably the correct one [Angelopoulos, 2008]

Phases - growth, onset/expansion, recovery.
Growth - increase in open magnetic flux after dayside reconnection. Thin and stretched magnetotail.
Onset/Expansion - Reconnection in the tail, fast flows earthward and tailward, dipolarization. Formation and expansion of SCW

Begins as a localized brightening near the equatorward boundary of auroral prec ~ 23 MLT. Maps to near-Earth transition region.
Brightening expands azimuthally, exhibiting folds and beads during breakup. Then expands poleward within few minutes.
The poleward expansion transforms into a WTS.
SCW links dynamical changes in the near-Earth transition region with the ionosphere.

Flow burst transports magnetic flux, changing magnetic configuration in a limited sector from stretched to dipolar.
Magnetic shear is generated as regions outside remain stretched (towards dawn and dusk).

2-loop SCW was postulated as early as 1964 and simulated in 1999.
I think observations were presented in 2013.
NOTE : Can't be observed in ground magnetomters - Fukushima theorem.

Well defined feature of the auroral substorm.
Bulge of discrete aurora that represents westward and poleward expansion.
In a simplistic view, it is a visual manifestation of upward current at the duskward edge of SCW.

Substorms are complicated!
Can someone talk about pi2 pulsations, auroral streamers, poleward boundary intensifications, flow bursts etc.