As illustrated in Plate 4, the transpolar arc of September 6 occurred in the northern polar cap and was very bright. Auroral images from VIS are available only after 07:12 UT. By this time, the transpolar arc was well developed. Figure 9 shows the IMF measurements from 04:00 to 12:00 UT with a time delay of ~ 19 mins for the solar wind to propagate from the Wind spacecraft to the subsolar magnetopause. The theta aurora pattern lasted more than two hours. IMF Bz for this event, shown in the bottom panel, followed the pattern suggested by Newell and Meng [1995] and our model. IMF Bz turned southward at 06:42 CUT (06:23 UT) after one and a half hour of northward Bz. It then frequently changed sign. IMF By component shown in the third panel of Figure 9 was positive, greater than or comparable to |Bz| for about 20 mins after the first southward turning of the IMF. Then like the Bz component, it changed sign rapidly. The observed transpolar arc moved dawnward after By became steadily negative at 07:24 UT, as illustrated in Plate 4. This direction of the motion is the same as the direction of the IMF By component.
Polar crossed regions that were associated with the transpolar arc from 08:00 to 08:15 and from 09:04 to 09:13 UT, identified from the VIS auroral images and particle measurements of Hydra, CEPPAD and TIMAS. Plate 5 shows the electron and ion measurements from CEPPAD and Hydra from 07:00 to 10:00 UT in an energy-time spectrogram format. During this interval, both CEPPAD and Hydra sampled plasmas in the nightside auroral oval, polar cap and the transpolar arc regions. The two transpolar arc regions traversed by Polar had plasma characteristics similar to those in the poleward part of the nightside auroral oval (~ 07:17-07:27 UT). Ion energies extended from Hydra into the CEPPAD ranges, to about 60 keV in the poleward part of the auroral oval and the transpolar arc regions. The ion fluxes, however, were much lower in the latter region. The electron energies extended to about 3 keV in both regions. Similar to the ions, the fluxes of electrons were relatively low in the transpolar arcs. The overall characteristics of the electrons and ions in the transpolar arcs are almost the same as those observed in the November 5 event shown in Plate 2.
Ion composition along the Polar trajectory is also available from TIMAS for this event. Plate 6 presents from top to bottom panels H+, O+, He+ and He++ fluxes in an energy-time spectrogram format and fluxes of four species, integrated from 16 eV/e to 1.2 keV/e, in an pitch angle-time spectrogram format. As indicated in the top panel, significant H+ fluxes extended from 10 eV to about 20 keV in the poleward part of the nightside auroral oval and in the transpolar arcs, consistent with CEPPAD and Hydra measurements shown in the bottom two panels of Plate 5. There are some low energy H+ adjacent to the transpolar arc regions. These are the ionospheric outflows in the polar cap, identified by their pitch angle distributions. As shown in the second panel, low energy O+ ions, presumably ionospheric origin, appeared in the auroral oval and the transpolar arc regions. He+ and He++ fluxes in these regions are very low as indicated in the third and fourth panels. The pitch angle spectra reveal that significant ion fluxes of all species except He++ are present in the poleward part of the auroral oval, transpolar arc regions and polar cap. In addition, H+ has an isotropic component in the first two regions. The dominant species of the outflows is H+. Plate 6 shows that in the poleward part of auroral oval and transpolar arcs low energy ions of all species flow away from the Earth. The energetic H+ in the two regions is isotropic.
Please send questions, comments, or suggestions about the paper to:
Shen-Wu Chang
Department of Physics and Astronomy, The University of Iowa, Iowa City, IA 52242
Phone:(319)335-3828; Fax:(319)335-1753;
swc@space-theory.physics.uiowa.edu