On November 5, 1996, a moderate theta auroral event was observed by the Polar spacecraft while traversing the northern polar cap from the pre-midnight to the morning sectors. Polar cap images, continuously taken by the VIS camera, show that the event lasted more than one hour, from about 14:46 UT to 16:29 UT. The prime interval of the theta aurora, however, is approximately from 15:00 to 16:00 UT. During the event, Polar crossed magnetic field lines that mapped to the transpolar arc twice; first from 15:06 to 15:17 UT and second from 15:41 to 15:51 UT. These identifications are accomplished using VIS auroral images, magnetic field mappings with the Tsyganenko 96 model [Tsyganenko, 1996], and simultaneous plasma sheet-like fluxes detected by Hydra and CEPPAD. Parameters used in the field line mapping are the observed values from the Wind spacecraft and ground stations at 15:00 UT: 2.6 nT for IMF By, -2.4 nT for Bz, 4.1 nPa for solar wind dynamic pressure, and -10 for Dst. From our experience, the mapping of the model magnetic field lines from the Polar altitude to the ionosphere is quite insensitive to the input parameters. Plate 1 shows examples of auroral ultraviolet (UV) images from VIS during the crossings, taken at wavelengths of OI 130.4 and 135.6 nm. This event is very similar in the brightness to the transpolar arc observed by DE 1 on October 31, 1981 (cf. Plate 6 of Frank et al. [1986]). The auroral image on the left of Plate 1 clearly indicates a transpolar arc across the morning sector of the polar cap with a second segment almost parallel to the midnight portion of the auroral oval. As illustrated in Plate 1, the ionospheric footprint of the spacecraft shows that Polar crossed the transpolar arc close to local noon. The intensity of emissions from the polar arc is much lower than those from the oval. At later times, this arc evolved further extending from noon to midnight, as demonstrated in the image on the right of Plate 1. The intensity of the transpolar arc decreased. During both crossings, the polar cap was filled with diffuse optical emissions, below the intensity of the transpolar arc. A comparison of both images in Plate 1 shows that the transpolar arc drifted from dawn toward dusk. In summary, beginning at 14:46 UT the transpolar arc brightened and moved duskward across the polar cap until approximately 15:20 UT. After that the theta aurora continued to move duskward but its luminosity diminished until it was barely visible.
Figure 4 shows the IMF components, measured by MFI, in the geocentric solar magnetic (GSM) coordinates from 10:00 to 18:00 UT at a resolution of 46 s. Because the Wind spacecraft was about 100 RE away from the Earth, a significant time was required for the solar wind to travel from the spacecraft to the subsolar magnetopause. A delay time of 26 mins is estimated using the solar wind speed of 390 km/s measured by Wind 3-D Plasma and Energetic Particle Experiment [Lin et al., 1995]. Because the solar wind speed was very steady for the whole day, the delay time remained approximately the same throughout the event. The discussion here uses the corrected universal time (CUT) to which MFI observations are referenced. Therefore, Figure 4 provides IMF information at the subsolar magnetopause at a given time without taking into account the time delay any more. The value of CUT is also the time tag of the VIS auroral images in UT, as shown in the bottom panel. This applys to all the IMF plots presented in the paper. As indicated in the bottom panel of Figure 4, IMF Bz was positive for more than four hours before the occurrence of the theta aurora. It became negative at 14:34 CUT and stayed negative for 45 mins until it turned positive at 15:19 CUT. After that, it stayed positive for about one hour except brief negative excursions at 15:24 and 15:39 CUT. During the prime interval of the theta aurora (15:00-16:00 CUT), IMF Bz was not steadily northward. Rather it was negative for the first 20 mins and positive for the later 40 mins when its absolute value was fluctuating and small. IMF By was positive for about two hours from 14:15 to 16:14 CUT, as shown in the third panel of Figure 4. Before 14:56 CUT, By was steady at about 4 nT, greater than or comparable to |Bz| and then became small. In summary, the IMF pattern for this theta auroral event had a southward turning ~ 12 mins before the first occurrence of the polar arc observed by VIS, followed by a northward turning 45 mins after that, as well as a positive By before and after the turning.
Electrons and positive ions were measured by the CEPPAD and Hydra detectors during the northern polar cap traversal by Polar. Plate 2 presents CEPPAD and Hydra electron and ion spectra in an energy-time spectrogram format from 08:00 to 20:00 UT, covering energies from 10 eV to 500 keV for electrons and 10 eV to 2 MeV for ions. Average omnidirectional fluxes of Hydra DDEIS electrons and ions with energy from 10 eV to 20 keV are shown in the second and the fourth panel, respectively. The color bar to the right of the Hydra spectrograms shows particle counts that are proportional to the differential energy fluxes with one count corresponding to 1.88 × 105 eV/(cm2·s·sr·eV). During the time interval covered in Plate 2, Hydra observed plasmas in the nightside auroral oval (08:10-09:15 UT), the nightside polar cap (09:20-15:06 UT), regions associated with the transpolar arc (15:06-15:17 and 15:41-15:51 UT), the dayside polar cap (15:17-15:41 and 15:51-18:50 UT), and the cusp (18:50-20:00 UT). The equatorward region of nightside auroral oval (08:10-08:25 UT) consists of hot, intense electrons with energies across the entire measured spectrum and ions with energies above 10 keV. This region is generally associated with the plasma sheet and ring current [e.g., Winningham et al., 1975]. The poleward part of the auroral oval (08:25-09:15 UT) consists of electrons and ions with relatively lower energies. The peak electron fluxes at ~ 1 keV originate in the distant plasma sheet, and have been designated phenomenologically as "boundary plasma sheet" (BPS) fluxes by Winningham et al. [1975]. This should be distinguished from the similarly sounding term "plasma sheet boundary layer" (PSBL) [DeCoster and Frank, 1979] used to describe the boundary layer between the lobes of the magnetotail and the core of the plasma sheet. It is characterized by field-aligned fluxes of ions emerging from a reconnection line in the magnetotail [Schindler and Birn, 1987]. In the ionosphere PSBL fluxes are detected in a latitudinally narrow strip at the poleward edge of BPS auroral precipitation [Zeleyni et al., 1990; Burke et al., 1994]. Since the observed plasma energies and energy fluxes in this region are more like those encountered in the poleward part of the auroral oval, we hereafter refer to them as BPS. In the polar cap fluxes of electrons are less intense, have average energies < 100 eV and are referred to as polar rain [Winningham and Heikkila, 1974]; the fluxes of ions are nearly imperceptible. Electron fluxes in the nightside polar cap are less intense than those in the dayside polar cap, perhaps due to the difference in the IMF Bz orientation. During the dayside polar cap traversal, Bz shown in the bottom panel of Figure 4, is mainly negative and the polar rain is more intense. The cusp spectrum shows much colder magnetosheath electrons and the typical energy-latitude dispersion of the ions, due to the velocity filter effect for southward IMF conditions [e.g., Reiff et al., 1977; Onsager et al., 1995]. The two regions within the polar cap that are associated with the transpolar arc show enhanced electrons in the energy range ~ 10-10,000 eV and ions with energies from 80 eV to above 10 keV. In these regions, the peak energy flux of the electrons is 1.5 × 107 eV/(cm2·s·sr·eV), more than 10 times higher than the neighboring polar rain fluxes, and the maximum energy flux of ions is 3.4 × 105 eV/(cm2·s·sr·eV). The average energies of the electrons and ions, indicated by dashed lines on the spectrograms, are 80 eV and 1 keV, respectively.
Energetic electron and ion measurements from CEPPAD IES and IPS during the northern polar cap pass by Polar do not exhibit exactly the same patterns as those observed by Hydra. As shown in the third panel of Plate 2, CEPPAD IPS observed energetic ions with energy above 10 keV in the nightside auroral oval (08:00-09:15 UT), transpolar arc (15:06-15:17 and 15:41-15:51 UT), and a portion of the cusp (19:45-19:52 UT) as previously identified by Hydra. The boundary between the two distinct regions in the nightside auroral oval is very obvious from the CEPPAD and Hydra electron measurements, indicated in the top two panels of Plate 2. Polar encountered this sharp boundary at about 08:25 UT. The equatorward region of nightside auroral oval (08:00-08:25 UT) contains hot, intense fluxes of electrons and ions across the entire energy spectra, as shown in the first and the third panel, respectively. The intense ion fluxes at the highest energies show the signature of an energy-latitude dispersion. Similar to the Hydra observations, this region is associated with the plasma sheet and ring current. In the poleward part of the auroral oval (08:25-09:15 UT), identified as BPS in the Hydra data, the energies of intense ion fluxes drop significantly to 50 keV and below. Only electrons with energies below 30 keV are present, at low flux levels. Such low fluxes of electrons intermittently exist in the polar cap and the transpolar arc. Nonetheless, in the regions associated with the transpolar arc, ions with energies below 50 keV are present. However, their fluxes are much lower than those in the poleward part of the nightside auroral oval. Furthermore, the correspondence of ions in these two regions is similar to that measured by Hydra. In general, the energetic plasmas observed by CEPPAD in the regions associated with the transpolar arc and the poleward part of the nightside auroral oval are consistent with being the high energy limits of the Hydra results presented above.
Ion composition data were obtained from TIMAS along the Polar trajectory. During the time interval covered in Plate 2 (08:00-20:00 UT), in the poleward part of the nightside auroral oval and regions associated with the transpolar arc, TIMAS detected energetic ions with energies and angular distributions consistent with the Hydra observations. The ion composition is primarily H+, with fluxes of He+, He++, and O+ close to the TIMAS detection threshold.
Plate 3 illustrates the flux difference (or net flux) of downward (0° - 20° pitch angles) and upward (160° - 180° pitch angles) flowing plasmas for the interval 12:30 to 18:30 UT, in the unit of the Poisson error of the mean measurements, indicated by the color scale to the right. Since the net flux is meaningless in regions where the observed omnidirectional flux is below the detection threshold or the absolute value of net flux is within the error bar of the measurements, it is displayed in gray and black, respectively. In the nightside polar cap region (12:30-15:06 UT), the electrons with energies above 50 eV had a net downward flow along the magnetic field line (indicated by yellow) and electrons with energies below 30 eV moved upward (indicated by blue). In the dayside polar cap region after ~ 17:00 UT, electrons moved upward and outflows of low energy ions were also present (17:07-18:00 UT). The reason for the difference may be due to a change in the IMF Bz polarity. During the time interval from 16:46 to 17:50 CUT, Bz was negative and electrons and ions may escape from the ionosphere more effectively. In the two transpolar arc crossings, low energy electrons around 40 eV and high energy ions had a net flow along the magnetic field lines, shown in yellow and red. Low energy ions had a net flow along the magnetic field lines away from the ionosphere, shown in blue. This suggests that the electrons and energetic ions came from a distant tail source and the low energy ions from the ionosphere. This is consistent with the source of plasmas in the transpolar arc being in the distant plasma sheet [e.g., Peterson and Shelley, 1984; Frank et al., 1986].
The velocity space distributions of electrons observed by Hydra in several different regions along the Polar trajectory are shown in Figure 5 at a resolution of ~ 13 s. In the poleward part of nightside auroral oval, the distributions of electrons with energies below ~ 300 eV reveal a counter-streaming-beam structure, as shown in Figure 5a. These electrons have a net flow along the field line. Energetic electron fluxes, however, are more isotropic. The electron beams have the parallel temperatures greater than their perpendicular temperatures. The electron distributions in Figures 5c and 5d, associated with the transpolar arc, show similar patterns. The counter streaming beams, however, are relatively colder and have lower energies; below ~ 250 eV for the first transpolar arc crossing and below ~ 100 eV for the second crossing. Also the temperature of these beams is more isotropic. Nevertheless, the feature of counter-streaming-beam electrons suggests that the transpolar arc is on closed magnetic field lines. In the polar cap which is usually on open field lines, the polar rain electrons have very low and more isotropic distribution functions, as illustrated in Figures 5b and 5e. In the nightside polar cap, there are electron outflows at low energy presumably from the ionosphere, whereas, in the dayside polar cap, electron outflows occur at all energies and more profoundly at very low energies. These features of electron distributions in the transpolar arc and polar cap agree with the results shown in Plate 3. The electron distributions in the cusp regions, shown in Figure 5f, demonstrate a pattern similar to those in the dayside polar cap, but they are much colder and have much higher fluxes. In summary, the electron distributions in the two encounters with the transpolar arc are similar to those detected in the poleward part of the nightside auroral oval. They are very different from the distributions found in the adjacent polar cap and in the cusp.
During this theta auroral event (14:46-16:00 UT), high-latitude plasma convection in the ionosphere was continuously monitored by SuperDARN radars. As indicated in the bottom panel of Figure 4, IMF Bz measured by MFI was negative in the early part of this period and became more or less irregular in the latter part. This change of IMF caused ionospheric convection to evolve from steady to turbulent states, as observed by the SuperDARN network. Figure 6 shows the calculated cross polar cap potential drop estimated from the SuperDARN observations from 13:31 to 15:59 UT at a 2-min time resolution. The potential drop was about 30 kV and exhibited a very irregular pattern before 14:43 UT. It increased gradually after that reaching a maximum value of 67 kV at 15:15 UT. After this, decreased back to ~ 30 kV near 16:00 UT. As illustrated in Figure 6, variations of the cross polar cap potential well correlated with IMF Bz, shifted in time by approximately 35 mins due to the solar wind propagation and the ionosphere response.
By using larger time intervals, reasonable polar cap convection patterns can be derived from the measurements of SuperDARN network for this event. The results for a time interval of 6 mins duration are presented in Figure 7. Figure 7a shows the high-latitude ionospheric convection from 15:12 to 15:18 UT while Polar traversed the region associated with the transpolar arc for the first time. The dayside data are best represented by the typical skewed two-cell pattern for negative IMF Bz and positive By [e.g., Heppner and Maynard, 1987]. After taking into account the time required to drive the plasma convection by the merging at low latitudes for southward IMF, the system was evolving to the southward IMF configuration from a positive Bz state. The observed flow patterns on the nightside exhibit remnants of a previous configuration, consistent with IMF measurements shown in Figure 4. As illustrated in the left image of Plate 1, the ionospheric footprint of the Polar spacecraft, traced by using the Tsyganenko 96 magnetic field model, is very close to the magnetic pole and poleward of the region covered by SuperDARN. This implies that the transpolar arc region for this interval has antisunward flows. However, the theta auroral arc shown in Plate 1 is narrower than the grid scale for SuperDARN observations. Thus the flow pattern derived from SuperDARN may not apply to the auroral arc region. During the second transpolar arc crossing by Polar, the derived convection pattern, from 15:42 to 15:48 UT shown in Figure 7b has only one large cell. Any existing morning cell is not derivable from the SuperDARN data. This dominance of the afternoon cell can happen when IMF Bz is near zero or slightly positive and By is positive (cf. Figure 5c of Crooker [1979]). As shown in the bottom two panels of Figure 4, from 15:12 to 15:19 CUT, IMF Bz was slightly positive and By was also positive; after 15:19 CUT both By and Bz were positive and approximately equal. Therefore, the observed flow pattern during the second theta auroral arc crossing reflects a transition between two different IMF configurations. Comparing the convection pattern in Figure 7b with the auroral image on the right of Plate 1, we see that the transpolar arc region encountered by Polar during this time interval is not covered by SuperDARN radars. In summary, the result of the SuperDARN observations is that dayside polar cap convection was mainly antisunward during the first theta auroral arc crossing, and it was a one-cell dominant pattern during the second crossing, consistent with IMF variations. The plasma convection associated with the transpolar arc could not be resolved in the SuperDARN measurements.
Convection electric fields along the Polar orbit were measured by EFI which can indicate the plasma flow in and around the transpolar arc for this event. Because the spin axis measurement is not always reliable, and it is desired to keep the reliable data separated from the less accurate data, the coordinate system used for EFI measurements is a non-rotating spacecraft frame. The electric field E in this frame is denoted by (EX-Y, EZ, E56). This reference frame is related to the geocentric solar ecliptic (GSE) coordinate system. The unit vector of EX-Y is in the spin plane and is nearly parallel to the ecliptic plane with the positive sense away from the Sun. The unit vector of EZ is also in the spin plane with the positive component in the +ZGSE direction. E56 is along the spin axis, positive in the sense that completes an orthogonal, right-handed coordinate system. Spin-fit measurements are presented for EX-Y and EZ every 6 s. The plasma drift velocity is given by (E × B)/B2, where the magnetic field B is obtained from the Magnetic Fields Investigation (MFE) [Russell et al. 1995] on-board Polar. The relative electric potential can be obtained by integrating the electric field along the satellite path.
During this event, E56 from EFI measurements was not reliable due to non-correctable DC offsets. Therefore, the flow components in the spin plane, VX-Y and VZ can not be accurately calculated. An assumption that E · B = 0 is made to estimate E56. However, because the magnetic field lay close to the spin plane during the event, errors in the other two components of the electric field are greatly magnified in the calculated E56 value. Nevertheless, by using the estimated E56 to calculate VX-Y and VZ, the trends of these quantities provide useful information. Since the spacecraft orbital velocity lies very close to the spin plane, the integrated electric potential along the satellite path is calculated from EX-Y and EZ.
Figure 8 shows the spin-plane components of the electric field, the relative potential along the spacecraft trajectory, one component of plasma drift velocity in the spin plane, VX-Y and one along the spin axis, V56 from 15:00 to 16:00 UT.
As indicated in the top two panels, the spin-plane components of the electric field were very small for the whole interval, 
2 mV/m.
As shown in the bottom panel of Figure 8, the component of the plasma drift transverse to the spin plane was initially negative and then mostly positive.
Positive value indicates a duskward flow and negative value indicates a dawnward flow.
This result seems to agree with the duskward motion of the transpolar arc observed by VIS.
Although we do not believe the magnitude of calculated VX-Y, its directional trend can be trusted.
The Polar orbital plane was approximately parallel to the 10:00 MLT meridian.
As illustrated in the fourth panel, VX-Y was positive prior to 15:06 UT, from 15:17 to 15:41 UT and after 15:50 UT, indicative of antisunward convection.
In the other two intervals: 15:06-15:17 and 15:41-15:50 UT, which correspond to the intervals of transpolar arc crossings identified by VIS and Hydra data above, it was mainly negative, indicative of sunward convection.
The integrated electric potential shown in the third panel of Figure 8 is relative to the starting point of integration, i.e., the Polar position at 15:00 UT. It is presented in two reference frames: the corotating frame (dotted line) in which the induced potential due to the Earth's rotation has been subtracted for easy comparison to the ionospheric pattern and the inertial frame (solid line) without the subtraction. Both show almost an identical trend. From 15:00 to 16:00 UT, electric potential along the Polar orbit changed slowly indicating the trajectory was close to equipotentials. Initially the positive deflection indicates motion away from the center of the afternoon cell. The potential change in both contacts with the transpolar arc is negative, indicative of a more complex pattern.
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