Small substorms: Solar wind input and magnetotail dynamics

Abstract

We investigated properties of 43 small magnetospheric substorms. Their general signatures were found to be consistent with the so-called contracted oval or northern Bz substorms. Small but clear pressure changes in the tail corresponding to growth and expansion phases detected in about a half of cases testify that these substorms follow the same loading-unloading scheme as the larger ones. However, rate of the solar wind energy accumulation in the magnetosphere was low due to azimuthal IMF orientation with dominating IMF By and small fluctuating IMF B,.. Plasma sheet signatures could be very strong and likely were localized in their cross-tail size. Negative bays in auroral X magnetograms were of order of 100-300 nT, with maxima at Bear Island station (71øgeomagnetic latitude) and in few cases were delayed after magnetotail onsets by tens of minutes. Small substorms probably differ from their la, rger counterparts in a, way that .coherency of the magnetotail reconfiguration in the inner and middle-tail regions and across the tail is lost in smaller substorms. 21,109 21,110 PETRUKOVICH ET AL.' SMALL SUBSTORMS 2. Instrumentation and Data Set Selection and coordinates of ground stations are in standard geographic frame. We compiled our collection of events after the analysis of winter (October-February) periods of 1996-1997 and 1997-1998, when almost continuous magnetotail coverage by the Interball-Tail (IT), Geotail (GT) was available. We selected substorms with isolated smallamplitude negative bays (within-400 nT at the first stage of the selection process) in the X magnetograins of the International Monitor for Auroral Geomagnetic Effects (IMAGE) meridional chain of magnetometers [Viljane,, and Hfi'kkinen, 1997], which spans a wide range of geomagnetic latitudes. In some cases, magnetograms from the Russian stations Amderma and Dikson (to the east of IMAGE) were used. Only substorms that commenced within approximately 3 hours of magnetic local time (MLT) from the IMAGE magnetic midnight were considered. Also, all cases that were associated with short intervals of pronounced southward IMF B, (Bz <• -2 liT) were excluded in order to limit our data set to cases wit. h a low level of the magnetospheric convection. In order to cross-check selection criteria we looked at Polar ('a,p (PC) index [T•'oshtche•' •'t al., 1988], which should be less than unity when the magnetospheric convection is at the low level [Tro.shzchev et al., 1999]. Our collection of events turned out to be quite consistent in this sense' All selected substorms occurred while PC was less than unity, while the majority of rejected cases was associated with higher PC.It should be noted that events with amplitudes of ground negative bays smaller than -100 nT are not. covered well in our data set. Such a, ctivitv is more simi-

Different magnetospheric modes: solar wind driving and coupling efficiency

“…This finding is in agreement with the level of fluctuations in the solar wind (the standard deviation of the solar wind parameters) being much lower for the SMCs than for any other event type (Partamies et al, 2009). Without obvious particle injections at the onset, the isolated substorms may seem like what was described as small substorms during a contracted auroral oval by Petrukovich et al (2000) and Lui et al (1976). The average AL peak of −200 nT for our substorm events is in the range of small substorms by Petrukovich et al (2000).…”

“…Without obvious particle injections at the onset, the isolated substorms may seem like what was described as small substorms during a contracted auroral oval by Petrukovich et al (2000) and Lui et al (1976). The average AL peak of −200 nT for our substorm events is in the range of small substorms by Petrukovich et al (2000). However, the other criteria for small substorms are not fulfilled by our event group: instead of the PC index staying below unity, we found peak values of 1.5, and instead of the substorms occurring at and north of Bear Island station (71.45 • MLAT) the mean latitude of the onset latitude for our data set is 66.5 • MLAT.…”

X lines in the magnetotail for southward and northward IMF conditions

“…Substorms can occur for southward as well as northward IMF [Akasofu et al, 1973;Lui et al, 1976;Lee et al, 2010]. However, substorms are much weaker for northward than southward IMF conditions [Petrukovich et al, 2000;Kullen and Karlsson, 2004;Pulkkinen et al, 2007;Peng et al, 2013]. Recent simulation result showed that the Earth's magnetosphere system may be more strongly driven during southward IMF [Tsurutani and Gonzalez, 1995;Lu et al, 2013].…”

On the conditions preceding sudden magnetotail magnetic flux unloading

“…The role of the magnetotail field as the main reservoir of energy dissipated during substorms have been questioned by Pulkkinen et al [2006]. Several examples of substorm onsets, not preceded by a pressure or magnetic flux increase in the magnetotail, have been presented [Petrukovich et al, 2000;Yahnin et al, 1994Yahnin et al, , 2001DeJong et al, 2008]. This has also been statistically confirmed by Dmitrieva et al [2004] who noticed that ∼1/3 out of 145 isolated substorm events in the midtail studied did not show any total pressure growth before unloading, whereas a classical growth phase was observed only in the other 1/3 of events (the remaining 1/3 showed a mixed behavior).…”