This event occured between Jan. 18, 2001, 20UT and Jan. 19, 2001, 02UT. The conjunction was not on the same MLT meridian but rather GT and Cluster were on opposite sides of the substorm meridian. Figure: IonoProj.gif shows the footpath of the two satellites (Geotail is the small line along the poleard boundary in the cap, and Cluster are one trace intersecting the northern oval). The snapshop is taken at 23:20 UT. Midnight is down and noon is up.
Figure: equatorialproj.ps shows the equatorial projections of the Cluster satellites and of Geotail for the time indicated. Cluster equatorial footpoints are displaced for clarity.
Figure: CLtetraplotB.pdf shows the relative position of the 4 spacecraft in GSE coordinates and superimposed the magnetic field vector (2000nT/Re). SC1 moves first through the region, then come SC3, then SC2, last is SC4. Time differences are about: SC1=0min, SC2=4min, SC3=3min, SC4=5.5min
Figure: AE.gif shows the AE index for the event. A small substorm onset took place around 2300 UT.
Figure: Crete.gif shows Pi2 onsets from Crete. Onset precursor was at 22:42 and main onset at 22:52UT. From the Y deviation (Westward) we can infer that the onset meridian was East of Crete, i.e., near Cluster and at least 1.5 hours away from the MLT of Geotail.
Figure: geotail_poynting_01Jan18_2000.ps is an overview of the tail conditions at (near) midnight. There are two surprizes: First, significant flows are seen throughout the period, i.e. even prior to onset. Second the (most) rapid bursty flows (>1000km/s) (expected at onset) and dipolarization occur at at least 20minutes after the main onset. The latter is more easy to explain, given the Pi2 behavior at Crete: Since the onset meridiant was >1.5 MLT away from Geotail, it takes time for the flows to expand laterally and engulf Geotail. The former requires images (IMAGE) and will be explained away later.
The first page of the above figure shows the Poynting flux computed from E and B measurements at Geotail. The second page of the above figure shows the power spectrum of that Poynting flux locally (bottom) and mapped at the ionosphere at 100km.
Figure: CL1234_Spar.epsi shows the parallel Poynting flux at Cluster sc 1,2,3,4. The Poynting flux is on average towards the ionosphere (i.e., negative in the southern pass, at ~2100 UT and positive in the northern pass, at 23UT). Since the substorm hit at 2252UT Cluster must have been going right through the onset location at the onset time. Very lucky!! This explains the huge spike at SC3 and (somewhat smaller at) SC1. The detailed analysis of these spikes, their temporal and spatial variations and their association with IMAGE images will be the topic of the AGU poster.
Figure: GTCL1234.epsi shows the Poynting flux spectrum combined at Geotail and Cluster. It is evident that, just as the POLAR-Geotail conjunction presented in our previous JGR paper ( paper ) (in press), there is plenty more power at Geotail to account for the POLAR fluctuations than you would want to admit. This corroborates that the earlier results are not just symptomatic of that event, but rather the norm. This puzzle and possible explanations are discussed in our previous paper.
OK, more to come next week... Vassilis, 11/16/2001
