Geotail was at (-23.1, 6.1, -1.4) Re GSM at 2100 UT on Dec. 15, 1996. It observed a particle energization starting at 2054 UT, accompanied by positive-then-negative Bz. The Bz variations are typical of a plasmoid like structure. It looks like reconnection took place very close to Earth at this time, but it is unclear at what distance.
Electric and magnetic field data at 3s minute resolution are shown in three figures. In the same figures data from the EPIC instrument's ICS head, looking slightly off the spin plane (ecliptic) towards south are also shown. The ICS head data provide information on "all ions" (channels E3, E4) "protons" (Channel P2) and Helium (He2).
Figure 1 shows a 6 hour overview of the event. There are some spikes on the plots that require removal mentally. The E- and B- fields have not had the background subtracted.
Figure 2 shows a 2 hour overview of the event around the time of interest. Same as above with respect to backgrounds.
Figure 3 shows a 20 minute overview of the event around the time of interest. It is evident that the time of onset of the anisotropy increase is at 20:54:06. At that time both the gradient anisotropy (duskward fluxes) and the tailward anisotropy increase. The gradient anisotropy was there before- hand, but intensified at the time of interest. The same might be said for the tailward anisotropy evident in the He2 channel. Electrons (ed1, >28 keV) are isotropic and indicate closed field line topology.
In Figure 3 Bz has been offset by the expected value during this period (Fairfield, private communication) and the E-field has been offset to account for a <1. mV/m uncertainty that depends on density and temperature. The offset was computed from the earlier, adjacent quiet period.
It is evident that the perpendicular component of the flow velocity computed from the E-field matches the impression from the anisotropy, regarding the plasma flow. The plasma started to flow gradually at 20:54:20 UT. The flow velocity perpendiccular to the instantaneous field retains its sign and magnitude through a sign-change of Bz and a corresponding sign change of Ey, as is typical of plasmoids (Cattell et al. 1984). We may be observing a generation of a plasmoid, but never cutting the field line that reconnects because we are far inside the plasma sheet and in addition we never cross field lines that reconnect. This ought to be contrasted with the energatic particle beam observation within the plasma sheet interepreted as separatrix observation inside the plasma sheet, as reported by Angelopoulos et al., 1995 GRL.
The fact that the gradient anisotropy is present prohibits determination of the time delays experienced by different particle species and energies along the field line. In addition we do not see an electron beam. We do not have an abrupt increase of the fluxes, either, indicating we never crossed the separatrix as mentioned above. The intensification is simply due to convection in this case.
Note the significant presence of Oxygen at the time of onset at Figure 4 which was taken from the APL Web database. The other EPIC figures from the same data base have been copied for convenience here and are: Figure 5
