No abstract

Abstract. At 1130 UT on November 28, 1995, two spacecraft, Interball-Tail and Geotail, were in a favorable position to study the plasma sheet activity and an auroral breakup observed on the ground near the spacecraft ionospheric footpoints. Both spacecraft were near the neutral sheet, and they were nearly aligned along the magnetic meridian. During the auroral breakup observed at the equatorward half of the auroral oval (also registered as an AKR burst at Interball) both spacecraft

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-

Abstract. We use five years of Geotail and AMPTE/IRM measurements in the near-Earth magnetotail to compute angles between vectors of fast earthward plasma flow and the local magnetic field. In the low-fi parts of the magnetotail, fast flows were found to be nearly field-aligned with an average angle of --•20 ø. In the high-fi plasma sheet the average angle was larger than 45 ø. The existence of a substantial convective component in the plasma sheet confirms the importance of the bursty bulk flow phenomenon for the magnetotail convection process.

On 23 March 2009 between 6:00 and 6:40 UT, three Time History of Events and Macroscale Interactions during Substorms probes (P3, P4, and P5) were at about −11.5 Earth radii (RE) and two (P1 and P2) were at −14 RE downtail. The inner probes (P3–P5) started to observe oscillatory flow braking with plasma sheet dipolarization due to flux pileup at about 6:04 UT. After 6:16 UT the flux pileup region (FPR) expanded tailward as the outer probes (P1 and P2) moved closer to the neutral sheet and began to observe oscillatory braking also. During the FPR tailward expansion, the flow oscillation period increased from about 3.5 min at P3–P5 to about 6.2 min at P1 and P2. Meanwhile, as observed by the all‐sky camera at Rankin Inlet, auroral activity gradually moved northward indicating that the characteristics of oscillatory flows at the tailward retreating FPR may be crucial for understanding the magnetosphere‐ionosphere coupling.