Extremely low ionospheric peak altitudes in the polar hole region

Transcription

1 Radio Science, Volume 36, Number 2, Pages , March-April 2001 Extremely low ionospheric peak altitudes in the polar hole region Robert F. Benson and Joseph M. Grebowsky Laboratory for Extraterrestrial Physics, NASA Goddard Space Flight Center, Greenbelt, Maryland Abstract. Vertical electron density N e profiles, deduced from newly available ISIS 2 digital ionospheric topside sounder data, are used to investigate the altitude structure of localized ionization depletions within the winter, nighttime polar cap ionosphere during solar minimum. The region investigated, called the polar hole-auroral cavity region, is located just after midnight magnetic local time near the poleward side of the auroral oval. Earlier investigations had at times found very low N e values in this region (down to 200 cm -3 near 300 km). In the present study, latitudinal N e depletions were found, but such low N e values ( 100 cm -3) were only found near the ISIS 2 altitude of 1400 km. The peak ionospheric concentration below ISIS 2 remained fairly constant (-105 cm -3) across the Ne-depleted region, but the altitude of the peak dropped dramatically to the vicinity of 100 km. The observations, in combination with other data, indicate that the absence of an F layer peak may be a frequent occurrence at high latitudes and stress the importance of a knowledge of the vertical N e distribution in high-latitude investigations of horizontal N e structures. 1. Introduction The magnetic polar cap, the region poleward of the auroral oval in each hemisphere, in winter is subject to prolonged periods of darkness and has lower ionospheric electron densities N e than in the oval. It has N e structures known as tongues, patches, blobs, troughs, cavities, and holes. Ignoring the complex time-dependent structures, the average N e in the polar cap ionosphere decreases during the winter night from noon to midnight as the plasma convects antisunward, chemically decaying in the absence of photoionization. If convection is slow, deep N e depressions can develop in the antisolar region just poleward of the oval. We here focus on this polar cap region where broad, deep Ne depressions (horizontal dimensions km) had been detected previously near 300 km. These ionization depressions were designated as holes by Brinton et al. [1978], in their investigation based on Atmospheric Explorer C (AE-C) winter nighttime ion composition and electron temperature T e data. They sometimes found extremely low O + ion densities that were several orders of magnitude less than typical polar cap ion densities (reaching as low as 200 cm -3 at 310 km). The deepest ones were This paper is not subjecto U.S. copyright. Published in 2001 by the American Geophysical Union. Paper number 1999RS observed in the southern hemisphere during solar minimum conditions. In addition to their statistical investigation of ion composition and T e data they presented (1) case studies involving these data and simultaneous low-energy electron flux measurements from four satellite passes and (2) arguments based on plausible plasma convection speeds and recombination rates to conclude that the polar holes they observed resulted from long plasma drift times across the polar cap. Hoegy and Grebowsky [1991] used AE-C and Dynamics Explorer 2 (DE 2) Langmuir probe data to statistically investigate the dependence of the ionization hole region N e on solar activity, universal time (UT), magnetic activity, season, and the hemisphere of observation. They showed that the hole region is not typically characterized by very deep N e depres- sions. Strong dependencies on solar activity (as described by the F10.7 cm solar flux) and UT were found but with only a weak dependency on magnetic activity (described by the Kp index). The hole region N e increased with increasing F10.7 and was, on average, about a factor of 10 larger in the Northern Hemisphere than in the Southern Hemisphere. Doe et al. [1993], using Sondre Stromfjord incoherent scatter radar scans during overpasses of the HILAT and Polar BEAR satellites (which occasionally provided coincident UV images and total electron content and magnetometer measurements),

2 . 278 BENSON AND GREBOWSKY: IONOSPHERIC PEAK ALTITUDES IN POLAR HOLE REGION 12 1t3 6 0 'Avera9e Hole Location M/T N Cavity Figure 1. Average location of high-latitude ionization hole [Brinton et al., 1978] and N e cavities [Doe et al., 1993] in relation to the main trough, a representative plasma drift configuration and a quiet time auroral oval. Adapted from Brinton et al. [1978] and Doe et al. [1993]. identified narrow high-latitude depletions (horizontal dimensions less than 100 km) which they called auroral ionospheric cavities. They were associated with downward directed electric currents. These Northern Hemisphere features, observed during periods of moderate magnetic activity, had N e depletions of 20-70% below surrounding values which, on average, were 7.8 x 104 cm -3. They were observed just poleward of the statistical auroral oval and were colocated in the hole region delineated by Btinton et al. [1978], as illustrated in Figure 1. Hence we call this region the polar hole-auroral cavity region. Crowley et al. [1993] investigated the morphology and evolution of this region above Greenland using a digital ionosonde, a 250-MHz scintillation receiver, and simultaneous in situ measurements by the Defense Meteorological Satellite Program F8 and F9 satellites. This investigation was conducted during quiet magneticonditions (Kp = 1 +) during solar maximum. The in situ satellite measurements detected ion densities as low as 103 cm -3 at 840 km inside an N e hole. The ground-based ionosonde enabled them to determine the electron density altitude profile Ne(h) up to the altitude of the F layer peak and to determine the altitude and maximum N e at the peak (referred to as h max and N e (max), respectively). In one of the two polar holes encountered during their 24-hour experimental run, ionosonde observations indicated that Ne(max) decreased by a factor of 4 within the hole. They also observed an increase in hma x by about 10% within the holes relative to the surrounding values and noted that h max often exceeded 300 km in the holes. In this paper we present new observations concerning the physics of the polar hole-auroral cavity region. The observations are based on newly available ISIS 2 digital ionospheric topside sounder data (available from National Space Science Data Center at which are used to produce topside Ne(h) profiles in this region and to estimate the important ionospheric parameters hmax and Ne(max). The original goal of this study was to use the ISIS 2 observations to determine Ne(h) profiles from the satellite altitude of 1400 km down to h ma x within deep polar cap region holes of the type found by Btinton et al. [1978]. The motivation for obtaining these profiles was to see if the Ne(h) distribution could be a partial explanation for why ion densities as low as observed have not been reported in later in situ satellite investigations. While the ISIS 2 topside sounder is not capable of detecting the narrow cavities found by Doe et al. [1993], because the best latitudinal resolution of the sounder-derived N e profiles is approximately 100 km, it is ideally suited for exploring the broader low-n e regions character- istic of the observations of Btinton et al. [1978]. We found no evidence for N e values near 300 km in the polar hole-auroral cavity region as low as they reported but did observe an interesting Ne(h ) profile behavior in this region. 2. Observations Since the original purpose of this study was to determine the N e(h) profiles associated with the broad, deep polar cap region holes detected near 300 km by Btinton et al. [1978] in the winter, nighttime Southern Hemisphere during solar minimum conditions, a search of the ISIS 2 digital topside sounder database (which mainly covers the period from 1973 through 1984) was made for relevant ionograms. This search (conducted in July 1998) revealed that highlatitude Southern Hemisphere ionograms were only available during the solar maximum near A search was then conducted to locate ISIS 2 digital topside sounder data recorded in the Northern Hemisphere during the solar minimum of 1976 (the time period of the Btinton et al. [1978] study). Since the onboard tape recorder was no longer operational at

3 BENSON AND GREBOWSKY: IONOSPHERIC PEAK ALTITUDES IN POLAR HOLE REGION ' ' I ' ' I ' ' I ' ' I * I * ' I * ' I ' ' I ' ' I ø ' I ' t 300 o ' $0 f' '...5 0,, I,, I,, I,, I,, I,, I,, I,, I,, I,, I,, YEAR Figure 2. Time intervals of ISIS 2, AE-C, and DE 2 operation relative to the Fi0.7 solar activity. The data time intervals indicated by the numbers 1-5 correspond to the investigations of Brinton et al. [1978], the present ISIS 2 study, Doe et al. [1993], Crowley et al. [1993], and Hoegy and Grebowsky [1991], respectively. this time, the data coverage was restricted to realtime recording over ground stations. The Canadian station at Resolute Bay (265øE, 75øN) was selected as the best candidate for this initial study. Digital data corresponding to six satellite passes in December 1976 over this station were found that contained topside ionograms recorded with magnetic local times between 0000 and 0300 MLT and at invariant lati- tudes between 72 ø and 79 ø, i.e., within the hole region delineated by Brinton et al. [1978](Figure 1). The time period for these data, and for the data of the earlier investigations discussed in section 1, is indicated on a plot of solar activity in Figure 2. All of the ISIS 2 topside ionograms from these passes were inspected for evidence of extremely low N e values in the F region below the satellite altitude of 1400 km. Though N e values as low as 200 cm -3, as observed by Brinton et al. [1978] (at 310 km), were encountered at the satellite altitude on all six of the passes, none of these passes revealed such low values in the remote regions below the satellite. On one of these passes (beginning at 0813 UT on day 355) a conversion of the ionospheric reflection traces to Ne(h) profiles, using the inversion algorithm developed by Jackson [1969], indicated a depletion of the N e peak values about 5 ø invariant latitude poleward of the average hole location region in Figure 1 and near an altitude of 300 km. The N e decrease was slightly more than an order of magnitude (down to - 3 x 103 cm -3) but still an order of magnitude above the low values observed by Brinton et al. [1978] at these altitudes in the hole region. Many of the reflection traces on the ionograms on this pass, however, were very difficult to interpret (mainly because of scatter returns commonly referred to as spread F), and it often required many trace-scaling attempts before the Jackson inversion program would converge on a solution. Even though none of the ISIS 2 passes indicated an Ne(max) in the cm -3 range, three (which appeared to contain mainly ionograms with good ionospheric reflection traces in the region of interest) were selected for inversion into N e (h) profiles. Three ISIS 2 ionograms from one of the passes are reproduced in Figure 3. Each shows local plasma resonances (at the top) and remote ionospheric reflection traces embedded in spread F. The resonance just beyond 0.1 MHz on each ionogram occurs at the electron plasma frequency, indicating that N e only 3 slightly exceeds 124 cm- at the satellite altitude of 1400 km. The Ne(h) profiles deduced from these ionograms are shown in Figure 4. By combining

4 BENSON AND GREBOWSKY: IONOSPHERIC PEAK ALTITUDES IN POLAR HOLE REGION [ ' ' looo ß ',',.i':.:,!. '5' 617. ' ':' i..t'.,; '..,.: z 1500 z i 3000 Isis i... i i... i t i i i t i t,... i i i i t TIME AFTER FRAME SYNC (sec) O, , FREQUENCY (MHz) Figure 3. Resolute Bay ISIS 2 ionograms recorded (top) poleward of (1858:39 UT), (middle) within (1902:11 UT), and (bottom) equatorward of (1904:19 UT) a polar holencounter on day 354 of Each ionogram is a mix of fixed (0.48 MHz) and swept ( MHz) frequency operation.

5 _ BENSON AND GREBOWSKY: IONOSPHERIC PEAK ALTITUDES IN POLAR HOLE REGION 281 sequential Ne(h) profiles obtained along the pass an N e contour plot was created and is presented in the top panel of the left column of Figure 5. The middle and bottom panels in the left column of Figure 5 present the corresponding Ne(max ) and hma x values, respectively. Note that the former remains practically unchanged whereas the latter decreases dramatically in the polar hole-auroral cavity region around 1902 UT I I ooo '".},, The results from the other two ISIS 2 passes 500 processed to produce N e (h) profiles are shown in the center and right columns of Figure 5. Both show similar behavior in the polar hole-auroral cavity regions, but the hma x values are not as depressed as in 0,,,,,,I,,,,,I,,, ll the pass discussed above. The pass shown in the center column reveals the lowest overall N e values of N.(cm) the three passes (at all altitudes) as the satellite crosses the polar cap and enters the polar hole- Figure 4. Electron density profiles deduced from the auroral cavity region (near the low hmax values just ionograms in Figure 3, using the inversion algorithm develprior to 1018 UT) from the high-latitude side. The oped by Jackson [1969], corresponding to ISIS 2 locations erratic behavior of hmax between approximately 1018 poleward of (solid curve corr,esponding to 1858:39 UT), within (dotted curve corresponding to 1902:11 UT), and and 1021:30 UT is due to interference on the ionoequatorward of (dashed curve corresponding to 1904:19 grams which prevented reliable scaling of the high- UT) the polar hole (1976, day 354). frequency portions of the ionogram traces. The low hma x values after UT correspond to the equatorward moving satellite entry into the auroral oval- frequency sweep range from 0.1 to 10 MHz (see main trough [Muldrew, 1965] region. The interpreta- Figure 3), the latitudinal separation of adjacent N e tion of the ionograms in this region was also profiles obtained from the polar-orbiting ISIS 2 compromised by interference in addition to weak sounder was, approximately, either 100 or 300 km. multiple traces with spread F. The ionospheric reflec- During the other two passes represented in Figure 5 tion traces on the last ionogram of this sequence, (center and right columns) the sounder was operawhere the calculated hma x value was below 100 km, tional on every ionogram. These ionograms were also were particularly weak. The results for the third pass separated by 14 s, so the latitudinal separation beinvestigated, presented in the right column of Figure tween all profiles obtained from these two passes was 5, again reveal two regions of reduced h max values: approximately 100 km. one on the dayside near the cusp region (0737 UT) The deduced hmax in the lower three panels of and one within the average polar hole-auroral cavity Figure 5 can differ from the true altitude of the region (as identified in Figure 1) at 0747 UT. In each ionosphere peak for two reasons. First, the reflection case, it is notable that negligible changes in N e (max) traces on the ionograms may not be of sufficient accompanied the reduced hma x values. clarity (because of an inappropriate antenna orienta- The N e latitudinal resolution is different in the tion, interference, or spread F) to yield accurate N e three passes presented in Figure 5. The data in the information all the way down to the true peak. Under left column were collected when the sounder was in a these conditions the deduced h ma x value can be mode of operation that produced two active sounding higher than the altitude of the ionization peak. As ionograms followed by two passive ionograms where discussed above, this factor was the likely cause of the only the sounder-receiver was in operation. In the top erratic behavior of h ma x in the center column of left panel of Figure 5, the satellite positions are Figure 5. Second, the analysis program assumes verindicated only for the active soundings which pro- tical propagation in a horizontally stratified ionoduced the data leading to the N e profiles (see the sphere. At high latitudes, radio propagation guided open circles at the top of Figure 5). Since the along the magnetic field direction often dominates ionograms were spaced 14 s apart, corresponding to a over vertical propagation. When reflection traces due

6 282 BENSON AND GREBOWSKY: IONOSPHERIC PEAK ALTITUDES IN POLAR HOLE REGION,'... I... i... i... i... i...,!.... i... i... i... i... i... i... i... ;i... i... i... i,, i,... i... I... i i... I... i... i... I... o ( mo) firlfilxvfi )d. ISN3(] NOI:LL03'13 (u l) 'Xt )d. ISN3(] NOU.L03'13 do '.LlV

7 BENSON AND GREBOWSKY: IONOSPHERIC PEAK ALTITUDES IN POLAR HOLE REGION 283 to such propagation are evaluated using the vertical propagation assumption, the longer slant ranges lead calculation was expanded to explore the polar cap ion composition dependence on variations in magnetic to calculated reflection layer altitudes that are too latitude, local time, altitude, and UT. Sojka et al. low. In the three polar hole regions presented in obtained O + densities as low as 200 cm-3 in the hole Figure 5 the dip angles (not shown) are large (87 ø, region at 300 km, in agreement with the observations 85 ø, and 83 ø for the left, center, and right columns, of Btinton et al. [1978]. Their predicted Ne(h ) within respectively), and the error introduced by such possi- the polar hole had no F 2 region peak; rather, ble field-aligned propagation is small (2, 5, and 9 km, Ne(max ) (of approximately 2 x 103 cm -3) was respectively). The error could be more significant, located at the lower boundary of their model near 150 however, for the lowest-altitude point in the lower panel of the center column of Figure 5, where the dip km. An upgraded version of this global time-dependent ionospheric model was used later in a climatoangle is 76 ø. If field-aligned propagation was involved logical sense by Sojka et al. [1991] to show that it was in the traces used to deduce hma x in this case, the calculated value shown could be too low by nearly 40 km. in agreement with the global AE-C and DE 2 observations of Hoegy and Grebowsky [1991] obtained over one solar cycle. The agreement included the conclusion that the average hole N e was about a factor of Discussion Several different interpretations have been given to larger in the Northern Hemisphere than in the Southern Hemisphere. Sojka et al. [1991] also supported the conclusion of Btinton et al. [1978] that the polar hole explain observations of polar ionospheric depletions. is formed in regions of weak convection. There is Btinton et al. [1978] attributed their AE-C polar holes to long plasma drift times across a dark polar cap while ionization chemical decay progressed. High convincing evidence that the smaller-scale auroral cavities observed in the same region, on the other hand, are due to field-aligned current evacuation plasma drift speeds, which can significantly increase [Doe et al., 1993]. the O + chemical loss rate through dissociative recombination, can also produce deep troughs. Btinton et al. [1978] ruled this mechanism out on the basis of Even though the three ISIS 2 passes used to produce the Ne(h) contours of Figure 5 passed through the polar hole-auroral cavity region of Figion composition and the low plasma temperatures ure 1, there is no evidence that they encountered N e measured in the holes. Similar conclusions about hole formation were reached by Sojka et al. [1981b], who theoretically modeled the high-latitude ionosphere under conditions of low magnetic activity at solar minimum. These conditions were selected to compare their results with the data of Btinton et al. [1978]. Sojka et al. [1981b] were able to show that the polar hole was a natural consequence of competing highlatitude chemical and dynamical processes. Their calculated hole, however, was smaller and in a slightly different location than the average hole location found by Btinton et al. [1978]. These differences may have been due to differences between the modeled and observed conditions since Sojka et al. [1981b] modeled the Northern Hemisphere whereas the ob- servations of Btinton et al. [1978] corresponded to the Southern Hemisphere. The model of Sojka et al. [1981b](for low magnetic activity during solar minimum) produced an hmax value within the hole that was well below 300 km. (Considerably higher values have been observed corresponding to low magnetic activity during solar maximum [Crowley et al., 1993].) In a later work [Sojka et al., 1981a], the full model depletions of the type reported in the coordinated satellite-ground-based observing campaigns of Doe et al. [1993] and Crowley et al. [1993], which also obtained N e (h) profiles through the region of interest. In each of these studies, large decreases in Ne(max) were observed. In the ISIS 2 data, extreme decreases were observed in hmax, but they were accompanied by only negligible changes in Ne(max) in the polar hole-auroral cavity region. These observed decreases in hma x are consistent with the predictions of Sojka et al. [1981a], and they would lead to "depletions" at a constant altitude of 310 km (appropriate to the AE-C altitude in the Btinton et al. [1978] study) of as much as 85% (in the case of Figure 4). Allowing for the factor of 10 difference found between the Northern (present ISIS 2 study) and Southern [Btinton et al., 1978] Hemispheres found by Hoegy and Grebowsky [1991], these results at 310 km are fairly comparable. The predictions of Sojka et al. [1981a], however, yield much smaller Ne (max) values than were observed by ISIS 2. Unfortunately, there are no ISIS 2 softparticle spectrometer data available at these times that could have been used to check for possible

8 284 BENSON AND GREBOWSKY: IONOSPHERIC PEAK ALTITUDES IN POLAR HOLE REGION electron precipitation to explain the observed lowaltitude ionization. Electric-Field Distributions in the Ionospheric Plasma: A Unique Strategy (OEDIPUS-C) bistatic rocket radio sounder, and Zhu et al. [2000] modeled substorm conditions that yielded similar hma x values. In the former, the profile was uniquely determined by a combination of in situ and remote measurements based on ionospheric soundings, with the receiver and transmitter separated by 1 km, as the payload descended from its apogee of 824 km. (It is important to recall, as stated in section 2, that the h ma x values determined from topside sounders may be higher than the true altitude of the ionization peak of the Ne(h ) profile.) In the latter, a combined global substorm electrodynamic and ionospheric model is used to produce Ne(h) profiles showing an ionospheric lowering caused by a downward E x B drift. 4. Summary The present results, combined with the results of other studies, suggesthat high-latitude regions with a single ionospheric N e peak altitude down in the E region may be common. Such profiles may be characteristic features of polar holes at solar minimum and, possibly, the cusp and auroral regions. These distinctive altitude changes in the N e (h) distribution stress that high-latitude ionospheric structure should be viewed as changes in both vertical as well as High-latitude N e distributions with very low altitude peaks have been observed previously. Bates and Hunsucker [1974, Figure 10] observed E layer peaks near 100 km with no evidence of an F layer during intense auroral precipitation events with the Chatanika incoherent scatter radar. Jelly and Petrie [1969, Figure 5] presented an Alouette-1 topside N e profile over Alaska showing a monotonic increase in N e from 1000 km to a maximum at 175 km. The ISIS 2 horizontal plasma distributions. The results from this experimenters prepared four archived data volumes containing the results of coordinated ionospheric and magnetospheric observations [Klumpar, 1980; Murphree, 1980; Shepherd, 1980; Burrows et al., 1981]. A first targeted inspection of the topside N e (h) structures through the polar cap-auroral cavity region with the digital ISIS 2 topside sounder data invites additional research using this growing digital database. search of these volumes revealed 89 N e high-latitude orbit plane contours, similar to those presented in the top panels of Figure 5. Most of them corresponded to the Northern Hemisphere during 1971 and Acknowledgments. We are grateful to G. M. Burgess and W. B. Schar for assistance in the ISIS 2 data analysis and to the two referees for their helpful comments. Thus they were recorded several years before the solar minimum conditions corresponding to the data References used in the present study and that of Brinton et al. Bates, H. F., and R. D. Hunsucker, Quiet and disturbed [1978](see Figure 2). Many of the orbit plane conelectron density profiles in the auroral zone ionosphere, tours presented in the ISIS 2 volumes did not inter- Radio Sci., 9, , sect the average hole location shown in Figure 1. Brinton, H. C., J. M. Grebowsky, and L. H. Brace, The Nevertheless, an inspection of all of the N e contours high-latitude winter F region at 300 km: Thermal plasma was made to compare the minimum altitudes of observations from AE-C, J. Geophys. Res., 83, , N e (max) with the present results. Even though none of them had hma x values as low as the 105-km layer seen in Figure 4 and the lower left panel of Figure 5, Burrows, J. R., L. L. Cogger, and H. G. James, Coordinated Ionospheric and Magnetospheric Observations From the 13 passes (or 15%) of the ensemble of contours had ISIS 2 Satellite by the ISIS 2 Experimenters, vol. 4, A. Large hma x -< 155 km (one as low as 120 km) and 22 passes Storms, B. Airglow and Related Measurements, C. VLF Observations, Rep , Natl. (or 25%) recorded hma x -< 185 km. More recently, Space Sci. Data Cent., Greenbelt, Md., June Prikryl et al. [2000] observed an h max = 115 km during Crowley, G., H. C. Carlson, S. Basu, W. F. Denig, J. auroral conditions in Alaska with the Observations of Buchau, and B. W. Reinisch, The dynamic ionospheric polar hole, Radio Sci., 28, , Doe, R. A., M. Mendillo, J. F. Vickrey, L. J. Zanetti, and R. W. Eastes, Observations of nightside auroral cavities, J. Geophys. Res., 98, , Hoegy, W. R., and J. M. Grebowsky, Dependence of polar hole density on magnetic and solar conditions, J. Geophys. Res., 96, , Jackson, J. E., The reduction of topside ionograms to electron-density profiles, Proc. IEEE, 57, , Jelly, D. H., and L. E. Petrie, The high-latitude ionosphere, Proc. IEEE, 57, , Klumpar, D. M., Coordinated Ionospheric and Magnetospheric Observations from the ISIS 2 Satellite by the ISIS 2 Experimenters, vol. 3, High-Latitude Charged Particle, Magnetic Field, and Ionospheric Plasma Observations During Northern Summer, Rep , Natl. Space Sci. Data Cent., Greenbelt, Md., Nov

9 BENSON AND GREBOWSKY: IONOSPHERIC PEAK ALTITUDES IN POLAR HOLE REGION 285 Muldrew, D. B., F-layer ionization troughs deduced from Alouette data, J. Geophys. Res., 70, , Murphree, J. S., Coordinated Ionospheric and Magnetospheric Observations From the ISIS 2 Satellite by the ISIS 2 Experimenters, vol. 1, Optical Auroral Images and Related Direct Measurements, Rep , Natl. Space Sci. Data Cent., Greenbelt, Md., July Prikryl, P., H. G. James, D. J. Knudsen, S.C. Franchuk, H. C. Stenbaek-Nielsen, and D. D. Wallis, OEDIPUS-C topside sounding of a structured auroral E region, J. Geophys. Res., 105, , Shepherd, G. G., Coordinated Ionospheric and Magnetospheric Observations From the ISIS 2 Satellite by the ISIS 2 Experimenters, vol. 2, Auroral Optical Emissions, Magnetic Field Perturbations, and Plasma Characteristics, Measured Simultaneously on the Same Magnetic Field Line, Rep , Natl. Space Sci. Data Cent., Greenbelt, Md., Dec Sojka, J. J., W. J. Raitt, and R. W. Schunk, Theoretical predictions for ion composition in the high-latitude win- ter F-region for solar minimum and low magnetic activity, J. Geophys. Res., 86, , 1981a. Sojka, J. J., W. J. Raitt, and R. W. Schunk, A theoretical study of the high-latitude winter F-region at solar mini- mum for low magnetic activity, J. Geophys. Res., 86, , 1981b. Sojka, J. J., R. W. Schunk, W. R. Hoegy, and J. M. Grebowsky, Model and observation comparison of the universal time and IMF By dependence of the ionospheric polar hole, Adv. Space Res., 11(10), 39-42, Zhu, L., R. W. Schunk, J. J. Sojka, and M. David, Model study of ionospheric dynamics during a substorm, J. Geophys. Res., 105, 15,807-15,822, R. F. Benson and J. M. Grebowsky, Laboratory for Extraterrestrial Physics, NASA Goddard Space Flight Center, Mail Code 692, Greenbelt, MD (u2rfb@lepvax. gsfc.nasa.gov; u5jmg@lepvax.gsfc.nasa.gov) (Received December 2, 1999; revised August 23, 2000; accepted August 29, 2000.)