TAIL LOBE AND OPEN FIELD LINE REGION ENTRIES AT MID
TO HIGH LATITUDES
J. F. Fennell1, J. B. Blake1, J. L. Roeder1,
R. Sheldon2, and H. E. Spence2
1) Space Sciences Dept., The Aerospace Corporation, Los Angeles, CA, 90009
2) Center for Space Physics, Boston University, Boston,
MA, 02215
ABSTRACT
New data from the POLAR and HEO (high earth orbit) 95-034 satellites are used in a preliminary study to estimate the position of the open/closed (OC) field line boundary using energetic particle and plasma observations. In this study, the OC boundary positions were compared in several different coordinate systems and the GSM and eccentric dipole coordinates were found to be equally effective in organizing the HEO 95-034 data. No coordinate system appeared to organize the POLAR data best. The HEO 95-034 and POLAR data were examined for interplanetary field and plasma dynamic pressure control of the dayside OC boundary. The HEO 95-034 data showed a clear response to solar wind dynamic pressure with the OC boundary being at lower latitude with increasing pressure. The HEO 95-034 data also showed some response to the IMF BZ, hinting that the OC boundary would move to lower latitude with increasingly negative BZ but be unchanged for positive BZ. The POLAR data showed no dependence on either the solar wind dynamic pressure or BZ in this initial study. The data were relatively sparse; about a month of observations for each satellite.
Introduction
This work was motivated, in part, by recent studies of tail lobe entries using low latitude, near geosynchronous altitude data (Korth et al., 1994; Thomsen et al., 1994; Fennell et al., 1995 and 1996; Moldwin et al., 1994 and 1995). At such low latitudes the entries into the nightside tail lobes were found to be a relatively rare occurrence that was usually associated with significant magnetic disturbance levels (Korth et al., 1994; Fennell et al., 1996; Moldwin et al., 1995) or increased solar wind pressure (Fennell et al., 1995). It was also found that the CRRES satellite tail lobe entries occurred above 10° magnetic latitude near geosynchronous orbit (Fennell et al., 1996). On the dayside the transition from closed to open field lines represented either tail lobe entries near dusk and dawn or magnetopause crossings (Fennell et al., 1996; Moldwin et al., 1995; Russell et al., 1976). The magnetopause crossings were deleted from the tail lobe entry studies. A recent case study (Fennell et al., 1995) showed that some tail lobe entries along the dawn and dusk flanks of the magnetosphere occurred at the same time the magnetopause was below geosynchronous altitudes near local noon. A recent GOES-2 study at geosynchronous found that energetic particle flux dropouts could occur at magnetic latitudes as low as 5° (Kopànyi and Korth, 1995), however plasma data were not used to confirm that these were tail lobe entries.
Because of the limited latitudinal coverage of these recent near geosynchronous boundary studies, it was decided to take advantage of newer observations by spacecraft with higher latitude coverage at medium to high altitudes. Earlier high altitude and high latitude studies using the HEOS-2 and Prognoz-7 satellite data (see discussions in Hughes, 1995 and references therein) clearly defined the appearance of the open/closed (hereafter called OC) field line boundary in the plasma, energetic particle, and magnetic field data. We make use of these previous results to guide us in estimating the position of the OC boundary in the POLAR and HEO 95-034 data.
Instrumentation
The data used in the present study were taken by the High Earth Orbit (HEO) satellite 95-034 (hereafter designated HEO 95-034) and the POLAR satellite. HEO 95-034 has an apogee of ~7.2 RE, ~63° inclination, and 12 hour orbital period. POLAR has a ~9 RE apogee, ~86° inclination, and ~18 hour orbital period. HEO 95-034 crosses into the open field line regions in a significant fraction of its orbits and POLAR crosses into the open field line regions every orbit. Satellite HEO 95-034 carries both plasma and energetic particle measurements. The plasma instrument measures ions from ~50 eV to 25 keV/q and electrons from ~70 eV to 30 keV/q. The energetic particle system measures electrons with energies > 130 keV and protons with energies > 80 keV in several channels. A complete plasma spectrum is obtained every ~ 15 seconds and the energetic particle fluxes are sampled every second. For the present study, the HEO 95-034 summary data were used. These have been averaged to provide approximately one minute resolution.
The POLAR satellite is a fully instrumented NASA science satellite (see Russell, 1995). For the present study, the summary data from the CEPPAD Imaging Proton and Imaging Electron Spectrometers (IPS and IES) were used to get boundary determinations. IPS measures protons with energies from ~18 keV to 1.5 MeV. IES measures electrons with energies from ~20 keV to ~450 keV. These spectrometers make simultaneous measurements with a temporal resolution of < 0.2 sec. IPS and IES each have nine detector ìpixelsî in a plane covering the full 180° relative to the POLAR satellite spin axis (see Blake et al., 1995 for details). The satellite rotation is used to obtain complete coverage of the 4¹ sphere. The summary data, used in this study, provide 96 second averages of the IPS and IES measurements.
Observations
The time periods of the data used in this preliminary study were July 28 - August 27, 1995 for HEO 95-034 and March 2 - May 1, 1995 for POLAR. Only northern hemisphere crossings that occurred on the upward and downward legs of the orbit to/from apogee were used. The one minute and 96 sec. data averages were deemed sufficient because of the low satellite velocity at the > 3 RE altitude of the OC boundary crossings. Only those crossings that showed a clean transition from the radiation belts/plasma sheet to the open field line regions (and the reverse) were selected. Recontacts with possible closed field line regions were ignored. Here we focused on the “clean” OC boundary crossings. For simplicity, we take the OC boundary to be consistent with the low latitude edge of the cusp near noon, the high latitude edge of the boundary layer away from noon on the dayside, and the high latitude edge of the plasma sheet boundary layer in the pre-dusk through midnight to pre noon regions. As noted below, small errors in this identification are negligible in terms of the spatial position because of the slow motions of the satellites. Examples of the HEO 95-034 and Polar data, with the OC boundary positions, selected for this study, are shown below in Figures 1 and 2, respectively.
Figure 1 shows a typical HEO 95-034 survey plot from July 28, 1995. The top two panels contain spectrograms of the plasma electron (panel (a)) and proton energy flux (panel (b)); proton energy increasing downward. Panels (c) and (d) contain line plots of the energetic proton and energetic electron fluxes, respectively. The boundaries, marked with the vertical lines near 1250 and 1930 UT, were the estimated open/closed field line transitions for this orbit of HEO 95-034. Note that these selected OC boundaries fall significantly poleward of what would normally be identified as the energetic electron trapping boundary. This was typical for the HEO 95-034 data. Note also that the OC boundary, determined from the plasma data, agreed well with the point at which the energetic proton ( > 80 keV) fluxes fell to background levels in Figure 1. For the more than sixty orbits of HEO 95-034 data used in this study, the position of the OC boundary, based on the plasma data, is essentially indistinguishable from that based on the energetic protons. While there may be small timing differences, the differences in position coordinates is negligible.
Figure 2 shows a typical POLAR CEPPAD-IPS survey plot taken on March 22, 1995. It shows stacked energy-time spectrograms for five different IPS view directions relative to POLARís spin axis. The ion flux boundaries marked T1, T3, and T4 were used as estimates of the OC boundaries for this orbit. Similar plots for CEPPAD-IES (not shown) were used to help in the boundary determinations. The excellent agreement between the HEO 95-034 energetic proton and plasma determinations of the OC boundary gives us confidence in using the CEPPAD >18 keV IPS and >25 keV IES data to estimate the positions of the OC boundary crossings for POLAR. (In the future we plan include the HYDRA plasma data. This could not be done in time for this report. There will be small timing differences between the OC boundary determinations using the CEPPAD energetic particle data and those obtained with the HYDRA plasma data. But, as with the HEO 95-034 data, the differences in position coordinates should be negligible.) The region near T2 in Figure 2 (~ 1025 - 1235 UT) was ignored. It marks a possible recontact of the plasma sheet or a high latitude acceleration event not necessarily associated with the OC boundary. Similarly, the complete region between ~1500 - 1700 UT in Figure 1 was also ignored and not considered in this study. Such high latitude events, which occur a significant distance from the obvious transition from trapped radiation to open/closed field lines, will be the topic of a future more exhaustive study.
The positions of the OC boundary determined from the HEO 95-034 and POLAR data are shown in Figure 3, panels (a) through (e) and (aa) through (ee), respectively. Panels (a) and (aa) show the magnetic latitude of the OC boundary versus local time for HEO 95-034 and POLAR, respectively. The eccentric dipole (ECD) field model was used to calculate the magnetic latitude and local time for this preliminary study. Note that both data sets show the expected decrease in the OC boundary latitude away from noon towards midnight. Near noon, the two data sets show the OC boundary to be in the range of 60° - 75° magnetic latitude. These same data are plotted as the radial distance to the OC boundary in the YZ plane (RYZ) versus magnetic local time (MLT), using three different coordinate systems: SM, eccentric dipole, and GSM, for HEO 95-034 (panel (b)) and POLAR (panel (bb)). The HEO 95-034 data was pre-selected for |RX| < 2.2 RE. The HEO 95-034 points clearly separated into two groups, with those in the SM and ECD coordinates being nearly identical, while in GSM coordinates the values were at a systematically lower radial distance. The POLAR data (panel (bb)) did not show such a systematic difference in the RYZ boundary for the different coordinates. Some of the POLAR points showed a dramatic difference in the RYZ for different coordinates, while others showed almost no difference for the three coordinate systems. (Note, the POLAR points were not specifically constrained to small RX although |RX| was generally small for the local noon data and relatively large (|RX| > 2.2 RE) near local midnight.)
The solar wind velocity (VSW) and density (NP) from the WIND satellite were used to compute the solar wind pressure (PSW). (See Russell (1995) for descriptions of the instruments producing these data.) The appropriately delayed PSW values were obtained based on the XGSM position of the WIND spacecraft and VSW. Panels (c) and (cc) show the dependence of the OC-boundary ECD latitude on PSW. The HEO 95-034 data (panel (c)) shows a hint of PSW control of the latitude of the near noon OC boundary. The POLAR data (panel (cc)), taken near local noon, do not show PSW control of the OC-boundary latitude.
Panel (d) shows the relationship between the ECD latitude of the OC boundary with IMF BZ for HEO 95-034. The linear regressions for BZ < 0 and BZ > 0 appear to show some degree of latitudinal dependence of the OC boundary for BZ < 0. These data have large scatter and the apparent BZ < 0 dependence may not be significant. The POLAR data taken near local noon (panel (dd)) did not show a BZ dependence of the OC boundary position.
Finally, the latitude of the OC boundary was plotted versus IMF BY for the HEO 95-034 and POLAR data in panels (e) and (ee), respectively. Linear regressions on these data (not shown) did not indicate any significant BY dependence for either data set.
Summary
Both the HEO 95-034 and POLAR preliminary OC boundary positions show a clear local time dependence, with OC boundary latitude decreasing away from local noon. The MLT dependencies are very similar for the two data sets with a decrease of about 30° in latitude from noon to midnight. This is as expected and is consistent with low-altitude polar orbiting satellite results. The HEO 95-034 data also show an indication of solar wind pressure (PSW) control of the OC boundary on the dayside magnetosphere and possible IMF BZ control for BZ < 0. The POLAR data did not show as clear an IMF influence on the position of the OC boundary as did the HEO 5-034 data. There are two possible reasons for this. First, the POLAR data was taken near spring equinox while the HEO 95-034 data was taken one month prior to autumnal equinox. The difference in effective geomagnetic latitude of the solar wind flow may be important. Second, the HEO 95-034 is in a 12 hour orbit and thus visits the same longitudes day after day whereas POLAR, with its ~18 hour orbit, repeats the same longitude regions every ~3 days. Thus, HEO has more effective coverage at a single dipole longitude than does POLAR and this may also be important. These results are very preliminary and represent a relatively small data set at this point. In the future we will increase the data coverage to all local times; include, as much as possible, data taken from identical time periods; and include separation of the data by magnetic activity levels and ranges of the IMF parameters. On the night side, we need to compare substorm phase with the OC boundary position. This will require a significance increase in the data set. These data are being taken now.
Acknowledgements
The authors acknowledge the WIND SWE and MFI teams for allowing us to use their key parameter data sets to specify the interplanetary conditions for this study. We especially wish to acknowledge the support of K. Ogilvie and R. Lepping. The authors also wish to acknowledge the efforts of G. Boyd and M. Redding of The Aerospace Corporation and K. Hirsch of Boston University for helping to make the reduced HEO 95-034 and POLAR CEPPAD data available. Work at Aerospace and Boston University was supported by NASA Contract NAS5-30368. Work at Aerospace was also supported in part by the Aerospace Sponsored Research program.
Figures:
Figure 1. HEO 95-034 data for July 28, 1995 showing examples of OC boundary selections for this study.
Figure 2. POLAR CEPPAD-IPS data for March 22, 1996 showing examples of OC boundary selections
Figure 3. (Large image) OC boundary positions for HEO 95-034 and POLAR
Figure 3. (Small image) OC boundary positions for HEO 95-034 and POLAR
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