Latitude and local time variations of topside magnetic field aligned ion drifts at solar minimum

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 116,, doi:10.1029/2011ja016715, 2011 Latitude and local time variations of topside magnetic field aligned ion drifts at solar minimum A. G. Burrell, 1 R. A. Heelis, 1 and R. A. Stoneback 1 Received 1 April 2011; revised 1 August 2011; accepted 29 August 2011; published 11 November 2011. [1] The movement of ions along terrestrial magnetic field lines frequently causes the redistribution of ionization between northern and southern hemispheres. This behavior is known as interhemispheric transport and is an important source of coupling between the ion and neutral gases in the upper atmosphere. The Communications/Navigation Outage Forecast System (C/NOFS) satellite and the Coupled Ion Neutral Dynamics Investigation (CINDI) provide an opportunity to directly measure ion velocities and ion densities in the topside ionosphere, facilitating the study of the field aligned ion motions near the equator. Using data from 2008 and 2009, the field aligned ion velocities shows the presence of and variations in the interhemispheric transport during this extreme solar minimum. Solar local time and corrected magnetic latitude variations in field aligned plasma transport at equinox and solstice are examined for a fixed longitude region and the consistency or the observed trends are compared to the expected behavior of F region neutral winds. Citation: Burrell, A. G., R. A. Heelis, and R. A. Stoneback (2011), Latitude and local time variations of topside magnetic field aligned ion drifts at solar minimum, J. Geophys. Res., 116,, doi:10.1029/2011ja016715. 1. Introduction [2] Above the ion density peak, the topside ionospheric plasma is highly magnetized and mobile along the magnetic field. This allows field aligned plasma drifts to occur readily as a result of plasma pressure gradient, gravitational, or collisional forces that align with the magnetic field. The existence and behavior of field aligned plasma transport has been inferred from observations of ion composition [West et al., 1997; West and Heelis, 1996], ion temperature [Venkatraman and Heelis, 1999; Chao et al.,2004;su et al., 2004], and electron density [Tulasi Ram et al., 2009; Chen et al., 2009] in the topside ionosphere. These studies have described the solar cycle, temporal, and spatial variations of field aligned plasma transport that are consistent with the measured parameters, but had not been directly observed in the plasma. Case studies that relate measurements of the equatorial field aligned ion drifts to previously inferred characteristics have been presented by Greenspan et al. [1994], Venkatraman and Heelis [2000] and Chao et al. [2004]. [3] These studies have outlined the typical behavior of field aligned plasma drifts and identified the causes for such movements. At dawn the onset of ionization increases the plasma pressure at the feet of the flux tubes, causing equatorward field aligned motions opposed by gravity and ionneutral collisions. Later in the day the forces that cause plasma motion along the magnetic field lines are typically 1 William B. Hanson Center for Space Sciences, University of Texas at Dallas, Richardson, Texas, USA. Copyright 2011 by the American Geophysical Union. 0148 0227/11/2011JA016715 balanced, leaving the plasma in a quasi steady diffusive equilibrium condition. At sunset photoionization ceases, the atmosphere cools, and the ions move towards the poles along the magnetic field as a result of chemical losses below the ion density peak. Seasonal and diurnal variations are imposed on these motions by the meridional and zonal neutral winds, which serve to raise and lower the F peak height and change the plasma loss rate. The strongest characteristic that results from the influence of the neutral wind on the field aligned drift is the so called interhemispheric plasma transport, which generally moves plasma from the summer hemisphere to the winter hemisphere. [4] Assuming that the ion and electron populations are ideal gases and that any field aligned temperature gradients are negligible, the field aligned drifts (v k ) are given by equation (1). v k ¼ u k g k k BðT i þ T e Þ r k n ð1þ in nm i in [5] In this equation u k is the field aligned component of the neutral wind, g k is the field aligned component of the gravitational acceleration, n in is the ion neutral collision frequency, k B is the Boltzmann constant, T e and T i are the electron and ion temperatures, m i is the ion mass, and n is the total ion or electron number density. When the field aligned component of the plasma pressure gradient is not balanced by the fieldaligned contributions from collisions with neutral particles and gravity, plasma motions along the magnetic field will be seen. Processes that affect the field aligned plasma pressure gradient include ion production and loss, the neutral winds below the ion density peak, and meridional E B plasma drifts. 1of11

Table 1. Binning Criteria for Season, Magnetic Latitude, and Solar Local Time Season Magnetic Latitude Solar Local Time Region Starting Date Ending Date Region Range Region Range December Solstice 14 November 2008 17 February 2009 Northern 20 to 5 Morning 07:30 10:30 Equinox 18 February 2009 21 April 2009 Equatorial ±5 Day 10:30 15:00 21 August 2009 20 October 2009 Southern 5 to 20 Dusk 17:30 21:30 June Solstice 22 April 2009 20 August 2009 Night 23:00 03:00 [6] The goal of this paper is to explore the relative roles of the physical drivers causing field aligned plasma transport throughout the day during the most recent deep and extended solar minimum. A systematic description of the equatorial field aligned drifts using direct measurements has not previously been undertaken for these solar conditions and will be a valuable asset to the validation of ionospheric modeling efforts. 2. Data [7] In this paper the influence of various physical drivers on the field aligned ion drift is explored at different solar local times and corrected magnetic latitudes using data from the CINDI instruments onboard the C/NOFS satellite, along with an empirical model of F region neutral winds. C/NOFS is an Air Force mission whose goal is to locate and predict areas of ionospheric scintillation [de la Beaujardière et al., 2004]. The satellite was launched into a low Earth, equatorial orbit on April 16, 2008. The orbit has an inclination of 13 and it takes the orbital plane about 3 months to precess 24 hours in local time. The orbital period is about 97 min, with perigee near 400 km and apogee near 860 km. The ion velocity meter (IVM) used in this study is part of CINDI, a NASA (National Aeronautics and Space Administration) Mission of Opportunity selected to be a part of the C/NOFS payload. IVM measures the ion velocity, temperature, composition and density using a retarding potential analyzer (RPA) and an ion drift meter (IDM) [Heelis and Hanson, 1998]. 2.1. Data Processing [8] The IVM data is provided in pre processed files at a 0.5 second resolution. The data files are subsequently postprocessed to remove geophysically suspect data and data where the instrument performance was compromised. Altitudes above 550 km are removed from consideration because the low ionized oxygen density at these heights stressed the instrumental limits at the low solar activity levels experienced during 2008 and 2009. The data are filtered for outliers chronologically, smoothed using a running median, and resampled to a 20 second cadence along the satellite track. [9] This study considers three seasons of four months each. A single equinox season is used to improve the seasonal data coverage and assumes that both the March and September equinoxes behave similarly. The December solstice is shorter than the June solstice due to the operational profile of the C/NOFS satellite. Nevertheless, the 2008 data are preferred because the increasing F 10.7 during the 2009 December solstice season would distinguish it from the lower levels seen during the other seasons. During the year presented in this paper the National Oceanic and Atmospheric Administration (NOAA) reported a maximum F 10.7 index of 76 and a median index of 69, confirming the low solar activity levels. A low level of geomagnetic activity is assured for this study by limiting the 3 hour k P index to a maximum of 3, as reported by the National Geophysical Data center. [10] The entire filtered data set was searched for points that lay within the desired altitude, longitude, corrected magnetic latitude and solar local time range for a given season. A single dependent variable was chosen (corrected magnetic latitude or solar local time) and the data were then smoothed using a running median boxcar with the scatter represented by error bars that extend to the upper and lower quartiles (the medians of the measurements that lie above and below the median of the entire sample). The scatter shows the uncertainty in the data medians introduced by measurement error and spatiotemporal variations in the controlled dependent variables. A minimum number of 50 smoothed data points within each boxcar was required to prevent spurious variations in the median. When considering the solar local time dependence of the field aligned drift, a 4 hour wide boxcar was used to perform a running median and was advanced at an 8 minute cadence. When considering the corrected magnetic latitude dependence of the field aligned drift, a 4 boxcar was used to perform a running median and was advanced at a 0.22 cadence. The bins used to consider the latitude and local time variations in the data as well as the seasonal ranges are shown in table 1. The number of smoothed field aligned ion drifts that were used to compute each running median for the solar local time and corrected magnetic latitude regions are shown in Figures 1 and 2, respectively. [11] Because the field aligned drift depends on longitude, the local time and magnetic latitude variations were only considered between 140 and 250 geographic longitude. This geographic region, shown in Figure 3, was chosen because it is centered about the intersection of the geomagnetic and geographic equators. This selection minimizes the geographic differences between the solar zenith angles at the northern and southern extents of the magnetic field while providing the most complete coverage of the northern and southern geomagnetic hemispheres. The positive magnetic declination that characterizes this longitude region does introduce a local time offset in sunrise and sunset at northern and southern latitudes, which is illustrated in Figure 3 for sunrise over a magnetic field line with a 10 declination. 2.2. Analysis [12] The physical drivers that cause field aligned plasma transport have previously been identified as the fieldaligned force components from direct collisions with neutral particles, gravity, and plasma pressure gradients, where plasma pressure gradients can be modified by changes in ion 2of11

Figure 1. Number of 20 second resolution ion velocity measurements used to obtain each median field aligned drift value as a function of solar local time. production and loss, low altitude neutral winds, and the meridional E B drift. The most rapid changes in ion production occur at sunrise, when photoionization is initiated. Conversely, changes due to chemical loss processes are strongest after sunset when there are a large number of ions present after the cessation of photoionization. Fieldaligned plasma drifts from net ion production or loss are caused by unequal production or loss at the northern and southern feet of the magnetic flux tubes, which occurs when there are differences in the solar zenith angles, differences in local twilight, or differences in atmospheric composition at the conjugate flux tube feet. Solar zenith angle differences can be caused by the planetary tilt at solstice seasons and from an offset between the geographic and geomagnetic equators. Differences in the onset of twilight occur when the solar terminator and magnetic field lines are misaligned, as illustrated in Figure 3. Differences in the ion chemistry are due to hemispheric asymmetries in the neutral and ion particle densities and scale heights, specifically those of O +, O, and N 2. The production and loss rates depend on these particle populations because O + is the dominant ion species in the altitudes under consideration and the recombination rate for this ion at the flux tube feet is proportional to the N 2 density. The O to N 2 ratio largely controls the density of the ion peak (N m F 2 ) while the winds more strongly influence the height of the ion density peak (h m F 2 )[Rishbeth et al., 2000]. In this analysis the geographic region of interest lies over the Pacific ocean, within 20 of the geographic equator, and during a period of low solar activity; thus changes due to asymmetric neutral densities are small Figure 2. Number of 20 second resolution ion velocity measurements used to obtain each median field aligned drift value as a function of corrected magnetic latitude. Figure 3. Selected geographic latitudes and longitudes with magnetic declination contours and mean sunrise solar terminators over a 10 magnetic field line for each season. 3of11

Figure 4. Behavior of the field aligned drift, meridional drift, and low altitude neutral wind between 5 and 20 corrected magnetic latitude as a function of solar local time and season. compared to the changes in the h m F 2 and N m F 2 induced by meridional winds and differences in the solar zenith angle as witnessed by the lack of a seasonal h m F 2 anomaly [Torr and Torr, 1973; Rishbeth, 1998]. Nevertheless, seasonal changes in the O to N 2 ratio contribute in concert with larger drivers. [13] The neutral winds in the lower thermosphere, at altitudes below the h m F 2, have a large collisional influence on the ion motion. This allows the neutral wind to raise or lower h m F 2, producing hemispheric asymmetries in the plasma density and resulting in field aligned motions. In order to provide a measure of the effect of this neutral wind on the field aligned drifts in the topside ionosphere, the meridional and zonal neutral winds at 250 km (where the northern and southern end of the flux tubes threading the observation points lie) are estimated using the Horizontal Wind Model (HWM07 [Drob et al., 2008]). HWM07 was chosen to provide neutral wind estimates due to the lack of available measurements. This solution is not ideal, as the model does not incorporate observations taken under such low solar activity levels. However, the horizontal winds provided by HWM07 are adequate, since only the guidance from the climatological behavior of the neutral wind is desired for this study. [14] For the year under consideration, HWM07 was run at a 30 minute universal time cadence at the magnetic flux tube feet. The field aligned component of the neutral wind was computed from the modeled meridional and zonal horizontal winds using magnetic coordinates provided by the International Geomagnetic Reference Model (IGRF [Maus et al., 2009]). The locations of the magnetic flux tube feet were found independently for each solar local time and magnetic latitude bin by tracing down the dipole field lines that intersect the mean apex height of each bin at 150, 195, and 240 longitude. These three longitudes are at the edges and center of the geographic region under consideration. Running HWM07 for this range of times and locations allows the spatiotemporal variations within the area under consideration to be specified by error bars. As was the case with the CINDI data, the central variations and scatter in the neutral wind predictions for each season in the solar local time and corrected magnetic latitude regions were represented by the median and upper and lower quartiles. Over 1000 points went into the computation of each central variation in the low altitude neutral winds shown in the subsequent figures. The neutral winds in the upper thermosphere are not considered in this study since it is assumed that their influence on the ion drifts will be small due to the low collision rate between the ion and neutral populations at these heights. 4of11

Figure 5. Behavior of the field aligned drift, meridional drift, and low altitude neutral wind between 20 and 5 corrected magnetic latitude as a function of solar local time and season. [15] The meridional E B drift moves plasma perpendicular to the magnetic field and between flux tubes. The resulting change in the flux tube volume and field aligned plasma pressure gradient produces field aligned motions that, combined with the meridional E B drift, are called the fountain effect. Although the fountain effect does not cause interhemispheric transport, it will enhance or diminish any existing field aligned drifts. Upward meridional E B drifts diminish field aligned drifts towards the equator from both hemispheres, while downward meridional E B drifts enhance such motions. Thus, if there is a net ion drift across the flux tube apex, the drifts in one hemisphere will be enhanced while those in the opposing hemisphere will be lessened. The meridional E B drifts are typically directed upwards during the day and downward at night, with a possible period of pre reversal enhancement near sunset [Scherliess and Fejer, 1999]. In the region used in this study, the meridional E B drift will depend on season and its typical behavior will be modulated by the solar activity level. During periods of low solar activity, such as the one under consideration, the pre reversal enhancement is greatly diminished. During the 2008 June solstice, which experienced similar solar activity levels, it has even been shown to be undetectable [Pfaff et al., 2010; R. A. Stoneback et al., Observations of the vertical ion drift in the equatorial ionosphere during the solar minimum period of 2009, submitted to Journal of Geophysical Research, 2011]. 3. Results and Discussion [16] This investigation examines the magnetic latitude, solar local time, and seasonal dependence of field aligned plasma transport. Recall that the results shown here are all confined to the longitude region between 140 and 250 where the magnetic equator is on average centered about the geographic equator. Figures 4 10 show the seasonal medians of v k, v mer, and u k with respect to solar local time and corrected magnetic latitude. In these figures, the fieldaligned drifts and winds are defined as positive when they flow toward magnetic north, while the meridional drifts are defined to be positive when they are directed upwards. These directions (north, south, up, and down) are denoted by their first letter on the righthand axis of each drift and wind plot. The neutral winds are shown at the northern and southern flux tube feet separately so that the relative influence of each hemisphere may be examined. Note that the scale for the field aligned drifts, meridional drifts, and fieldaligned neutral winds are self consistent and identical across seasons. 5of11

Figure 6. Behavior of the field aligned drift, meridional drift, and low altitude neutral wind between 5 and 5 corrected magnetic latitude as a function of solar local time and season. 3.1. Solar Local Time Dependence [17] The solar local time dependence of the drifts and winds are shown for three latitude regions. Figures 4, 5, and 6 display the northern magnetic latitudes near 10, the southern magnetic latitudes near 10, and the equatorial magnetic latitudes respectively. Figures 4a, 5a, and 6a show the variation in the field aligned ion drift, Figures 4b, 5b, and 6b show the variations in the meridional E B drift, and Figures 4c, 4d, 5c, 5d, 6c, and 6d show the strength of the neutral wind below the F peak as a function of local time at the northern and southern magnetic flux tube feet respectively. Recall that Figure 1 revealed a drop in the number of quality observations near 06:00 during the December solstice at northern and southern magnetic latitudes, and so gaps are seen in the December solstice ion drifts in Figures 4 and 5. [18] Figure 4 shows the local time variations of the ion drifts in the northern hemisphere and the corresponding neutral winds for each season. At these latitudes, the positive northbound flows are headed downward and away from the magnetic equator while the negative southbound flows are headed upwards towards the magnetic equator. In Figure 4a it can be seen immediately that flows are predominately northbound during the December solstice (winter) and southbound during the June solstice (summer). During equinox the fieldaligned drifts are generally weak except around sunrise. Inspection of the effective neutral wind in the lower thermosphere indicates that the field aligned drifts are generally consistent with the cumulative effects of this driver in each hemisphere and more closely follow the variations seen at the northern flux tube feet than those seen at the southern flux tube feet. Significant differences between these two variables in specific local time intervals, however, indicate the presence of other factors affecting the field aligned drifts. [19] In the post dawn sector, weak positive drifts away from the equator in the December solstice and strong negative drifts toward the equator in the June solstice are suggested by inspection of the low altitude neutral wind. Instead, weak negative drifts toward the equator and strong negative drifts toward the equator are observed respectively at these times. The field aligned drifts directed toward the equator at all seasons are evidently produced by the postsunrise production of ionization, which exerts a significant influence on the plasma drifts at this time. The field aligned drifts induced by ionization production are strongest during the June solstice when the positive magnetic declination places the northern feet of the flux tubes in daylight much sooner than those in the south, as illustrated in Figure 3. The southbound field aligned drifts weaken through equinox to reach a minimum in the December solstice when the solar terminator and the magnetic meridian are more closely 6of11

be positive and upward during the June solstice and negative and downward during equinox and the December solstice, contrary to meridional drifts observed at higher solar activity levels [Scherliess and Fejer, 1999]. As previously discussed, the upward meridional plasma transport results in fieldaligned motions away from the equator, in this case such motions would be northward and in opposition to the fieldaligned drifts induced by ion production. The downward Figure 7. Behavior of the field aligned drift, meridional drift, and low altitude neutral wind between 07:30 and 10:30 solar local time as a function of corrected magnetic latitude and season. aligned, lessening the hemispheric asymmetries in the ionization production since the difference between onset times at the northern and southern flux tube feet has decreased. During the December solstice the prevailing neutral wind also opposes the expansive field aligned drifts in the northern magnetic latitudes shown here. This decline in the magnitude of the southbound field aligned drifts as the seasons progress away from the June solstice is mitigated by the influence of the meridional E B drift, which is shown in Figure 4b to Figure 8. Behavior of the field aligned drift, meridional drift, and low altitude neutral wind between 10:30 and 15:00 solar local time as a function of corrected magnetic latitude and season. 7of11

raising and lowering the h m F 2, the density distribution of the neutral population below this altitude affect the influence of the neutral wind on the field aligned ion drifts. During solstices, the sub solar point is nearly collocated with the location of the flux tube feet in the summer hemisphere, leading to more rapid ion production and the subsequent depletion of the neutral density. Thus, the neutral winds at the northern flux tube feet should exert a stronger influence Figure 9. Behavior of the field aligned drift, meridional drift, and low altitude neutral wind between 17:30 and 21:30 solar local time as a function of corrected magnetic latitude and season. meridional E B drifts seen in equinox and the December solstice would result in negative field aligned drifts in the northern hemisphere, which would help to maintain strong southbound field aligned drifts after dawn. [20] Throughout the day at all seasons the field aligned drifts are primarily driven by the low altitude neutral winds [Heelis and Hanson, 1980], although the strength of their influence is tied to ionization processes and the neutral composition. Since the neutral winds affect ion motions by Figure 10. Behavior of the field aligned drift, meridional drift, and low altitude neutral wind between 23:00 and 03:00 solar local time as a function of corrected magnetic latitude and season. 8of11

during the December solstice and the neutral winds at the southern flux tube feet will be more influential during the June solstice. [21] After sunset it is expected that the field aligned plasma drift would be poleward and positive, since the plasma is lost to recombination at lower altitudes. The lowaltitude neutral winds reinforce this motion during the December solstice but typically oppose it during the June solstice, as is reflected in the observations shown in Figure 4 near 20:00 solar local time. This situation reverses at southern magnetic latitudes, where the seasons are reversed. [22] Figure 5 shows the solar local time variations of the ion drifts in the southern hemisphere and the corresponding neutral winds for each season. Keep in mind that at these magnetic latitudes the drifts toward the magnetic equator are now northbound and positive. As in the northern magnetic latitudes, the drift variations in solar local time show a prevailing flow from the summer to the winter hemisphere induced by the low altitude neutral winds, especially those at the southern flux tube feet. [23] Across local sunrise and into the morning sector the field aligned drifts are weak, but still show the strongest field aligned drifts towards the equator during the December solstice, when the winds and the ion production induced flows act together. At equinox the positive drift expected from the filling of the flux tubes is still present, though it has disappeared by the time of the opposing June solstice. As there is little difference between the flux tube foot locations in the northern and southern magnetic latitude regions, as evidenced by the nearly identical winds predicted by HWM07 in Figures 4c, 4d, 5c, and 5d, the weak influence of ion production at these seasons is likely due to the changes in the meridional E B drift. Figure 5b shows that strong upward drifts are seen much earlier than they were at northern latitudes. These upward meridional drifts lead to southbound field aligned drifts in the southern hemisphere and would oppose the northbound drifts caused by ion production. The asymmetry of the meridional E B drifts seen between the northern and southern magnetic latitude regions is a result of longitudinal variations and coverage biases within the geographic region used in this study. To reconcile this with the expected conjugate symmetry, a hemispheric average of the E B drifts is used to assess the influence on the field aligned drifts studied here. [24] During the daytime the largest field aligned drifts in the southern hemisphere are again directed towards the winter hemisphere during the solstices. After sunset the field aligned drifts during the December solstice remain northbound, even at these southern latitudes, despite a southbound drift that would be induced by ion loss. This is likely due in part to the magnetic declination that places the northern foot of the flux tubes in darkness much before the southern foot and therefore produces a hemispheric asymmetry in the plasma loss rate. [25] Figure 6 shows the solar local time variations of the ion drifts near the magnetic equator and the corresponding neutral winds for each season. The field aligned drifts seen here very closely represent the average of the behaviors previously seen at higher magnetic latitudes (±10 ), even though the continuity of these previous drifts and the associated plasma flux would exist at an equatorial apex height about 250 km above the equatorial altitudes considered here. Interhemispheric drift exists at all seasons during the daytime and is consistent with the hemispheric asymmetries in the neutral winds at the flux tube feet. Near sunrise and across the early sunlit hours the upward flows resulting from ionization production in each hemisphere approximately balance at the equator. However, at sunset during the December solstice a large summer to winter (positive) drift is present. This flow is consistent with the neutral wind and the hemispheric asymmetry in the plasma loss rate associated with the magnetic declination observed at higher latitudes. 3.2. Corrected Magnetic Latitude Dependence [26] A more detailed description of the field aligned plasma drifts produced by the hemispheric asymmetries in the neutral wind and ion production and loss processes can be obtained by examining the latitude profile of the fieldaligned drifts and winds for the four solar local time sectors outlined in Table 1. Figure 7a shows the magnetic latitude variations in the plasma drifts in the morning between 07:30 and 10:30 for the three seasonal periods discussed previously. Figure 7b shows the variations of the meridional E B drifts and Figure 7c shows the separate contributions of the neutral wind parallel to the magnetic field at the northern and southern flux tube feet for each season over the same solar local time period. It has already been seen in Figures 4 6 that the large upward flows associated with flux tube filling have diminished at the onset of this local time period, making a latitudinal examination during these hours possible. [27] At this time Figure 7b shows that the meridional E B drift is consistently directed upward at all seasons. Note that the latitude profile of the E B drift should show the expected conjugate behavior. Departures from this behavior (seen primarily during the December solstice) are due in part to longitude variations, seasonal differences in the sampling, and uncertainties in the orientation of the magnetic field with respect to the sensors. For the purposes of this study the drifts at any given magnetic latitude can be represented by the average value across the northern and southern hemispheres. [28] The effect of the upward meridional E B drift on the field aligned drift can be seen in the asymmetric latitude profile shown in Figure 7a. At northern magnetic latitudes the strongest positive field aligned drifts or the weakest negative drifts are seen, while at southern magnetic latitudes the field aligned drifts exhibit either the strongest negative or weakest positive magnitudes. The field aligned drifts are not always directed poleward in each hemisphere due to the diffusive influences also in play. [29] At equinox hemispheric asymmetries in the winds and ion production and loss are minimized. Thus the poleward field aligned drifts in each hemisphere, associated with an upward E B drift, dominate the latitude dependence. The northern displacement of the crossing point for the field aligned drift alludes to stronger southbound flows near the equator, attributable to the inequalities in the low altitude neutral wind predicted in Figure 7c. [30] Figure 7c reveals inequalities in the neutral wind magnitudes during the solstices as well, but in each case the winds at the northern and southern flux tube feet are directed towards the winter hemisphere (northbound during the 9of11

December solstice and southbound during the June solstice). The prevailing summer to winter drifts seen at the solstices have the poleward drifts induced by the meridional E B drift motion superimposed upon them in each hemisphere. Figure 7a clearly shows a great magnitude difference between the solstice drifts, however. With little observable difference in the seasonal averages of the meridional E B drift at this local time, it is likely that the neutral wind induced the field aligned drifts and would therefore be larger than predicted by HWM07. [31] Figure 8 shows the solar local time distribution of the ion drifts and neutral winds in the daytime solar local time sector between 10:30 and 15:00 for the three seasonal periods previously discussed. A comparison with Figure 7 reveals larger field aligned drifts away from the equator than are seen during the solstices even though hemispheric asymmetries in the low altitude neutral winds have decreased. This is consistent with the decline in the upward E B drift seen in Figure 8b in this local time sector. The increase in hemispheric asymmetry is not seen at equinox and in all other aspects the field aligned ion drifts are similar to those observed earlier. This indicates that daytime ion production and loss processes, moderated by the lowaltitude neutral winds, continue to operate in the same way. [32] Figure 9 shows the latitude variation in the plasma drifts and neutral winds observed in the dusk local time sector, between 17:30 and 21:30. This local time region is characterized by the onset of net ion loss and a change in the direction of the winds, making them generally weaker than seen earlier. At sunset the alignment of the solar terminator and the magnetic meridian is the reverse of that at sunrise, thus during the December solstice the magnetic meridian and the terminator are misaligned to the maximum extent in this longitude region and the northern feet of the flux tubes are at much larger solar zenith angles than the southern feet. The resulting differences in plasma loss maximize the fieldaligned plasma pressure gradient. At northern latitudes, where the field aligned diffusion due to plasma loss and the prevailing neutral wind act in concert, the largest fieldaligned drifts are seen. [33] At equinox the difference in solar zenith angles at the flux tube feet also encourages a northbound drift that is reinforced by a weak meridional wind. During the June solstice the misalignment of the magnetic meridian and the terminator is small, but serves to augment the effect of negative field aligned drifts caused by ion loss in the south while diminishing the positive field aligned drifts that result from the cumulative ion loss in the north. Nevertheless, a latitude gradient in the field aligned drifts is formed and is augmented by the southbound neutral wind, which has a greater magnitude at the southern flux tube feet. [34] Figure 10 shows the latitude distribution of the ion drifts and neutral winds at night between 23:00 and 03:00 solar local time. During this period the neutral wind driver has diminished from that seen earlier and the downward drift induced by net ion loss continue. The meridional E B drift also plays a role at this local time, although its magnitude and direction, shown in Figure 10b, vary significantly with season. [35] During the December solstice, contrary to expectation formed by observations at higher solar activity levels, a strong upward E B drift is seen. Thus field aligned drifts away from the equator in each hemisphere are reinforced by similar drifts induced by chemical losses. These drifts are moderated by a prevailing neutral wind that creates a summer to winter flow toward the northern hemisphere. At the equinox the meridional E B drift is weakly downward, inducing flows towards the equator in the north and south and opposing the drifts induced by chemical loss. The influence of the neutral wind is also small at this time, leading to the smallest field aligned drifts seen at all latitudes. During the June solstice the meridional E B drift at this local time is close to zero. Thus the field aligned drifts reflect a combination of neutral wind induced southbound, summer to winter drifts and poleward drifts, positive in the north and negative in the south, induced by chemical losses. 4. Conclusions [36] The observations show that, at solar minimum, the direction and magnitude of field aligned plasma drifts in the topside ionosphere vary strongly with season, solar local time, and magnetic latitude. In exploring these variations, the local time dependence and latitudinal extent of interhemispheric plasma transport was established and probable physical processes behind its presence or absence were identified. The neutral winds in the bottomside F region, meridional E B drifts, and hemispherical differences in ion production and loss processes all contribute to the observed flows in recognizable ways. [37] The most consistently influential driver for fieldaligned drifts in the topside ionosphere was found to be the component of the bottomside neutral wind that acts along the magnetic meridian. During both solstices the lowaltitude neutral wind blows from the summer hemisphere to winter hemisphere, resulting in summer to winter interhemispheric transport. Equinox marks a transitional period for the low altitude neutral wind, which thus blows towards different hemispheres depending on the local time. During the daytime, when the influence of the other drivers are typically balanced, the topside field aligned ion drifts reproduce the variations seen in the bottomside field aligned neutral wind. [38] The C/NOFS satellite inclination does not allow a study of the latitude variations in the field aligned flows at all longitudes and thus this study was confined to the longitude region between 140 and 250 where the magnetic declination is positive. This declination always places the northern flux tube feet at later local times than the southern flux tube feet and results in asymmetric ionization production and loss. This effect is most noticeable on field aligned plasma flows in the topside ionosphere across the dawn and dusk terminators. Away from the magnetic equator the upward field aligned drift produced by ionization is largest in the summer (during the June solstice at northern latitudes and during the December solstice at southern latitudes). At this time and location the equatorward drift of the ions due to ionization production in the bottomside acts in concert with the summer to winter flow of the low altitude neutral wind. In winter, however, the equatorward drift caused by ion production moves the ions towards the summer hemi- 10 of 11

sphere and against the direction of the neutral wind. In the longitude region where the magnetic declination is positive, the magnetic meridian and the dawn terminator are most strongly misaligned during the June solstice with about 30 separating the magnetic meridian and the dawn terminator and most closely aligned at equinox and during the December solstice, where the separation is about ±10 respectively. In the northern latitudes during the December solstice ion production overcomes the influence of the neutral wind and field aligned drifts remain upward toward the equator. However, during the June solstice at southern latitudes the neutral wind prevails resulting in very small summer to winter drifts away from the equator. Near the magnetic equator large field aligned drifts are not present across the dawn terminator as they were in the northern and southern hemispheres. [39] Large summer to winter field aligned drifts are also seen at northern and equatorial magnetic latitudes during the December solstice after sunset. At these latitudes the magnetic declination places the northern feet of the magnetic flux tubes in darkness much sooner than the southern feet. This causes asymmetries in the ion loss rate that, along with the bottomside neutral wind, are responsible for the large drifts observed. This configuration of the magnetic flux tubes and the terminator during the June solstice reflects that seen during the December solstice at dawn and thus similarly large flows at southern magnetic latitudes are not seen after sunset. [40] While it has been previously shown that the largest hemispheric asymmetries in the field aligned flows were produced by a combination of the low altitude neutral wind and ionization production and loss, it has also been noted that the meridional E B drift can enhance a pre existing hemispherical asymmetry in the field aligned drift that has the same direction. During solar minimum this effect was found to have influenced the field aligned drifts unexpectedly at certain times due to the deviation between the observed meridional E B drifts and those typically expected under higher solar activity levels. [41] Acknowledgments. This work was supported by NASA grants NAS5 01068 and NNX10AT02G. The authors are grateful to all colleagues of the Coupled Ion Neutral Dynamics Investigation (CINDI) team for providing valuable discussions and suggestions. [42] Masaki Fujimoto thanks the reviewers for their assistance in evaluating this paper. References Chao, C., S. Y. Su, and H. 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