Characteristics of high-latitude vertical plasma flow from the Defense Meteorological Satellite Program

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1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 111,, doi: /2005ja011553, 2006 Characteristics of high-latitude vertical plasma flow from the Defense Meteorological Satellite Program W. R. Coley, 1 R. A. Heelis, 1 and M. R. Hairston 1 Received 29 November 2005; revised 26 June 2006; accepted 28 July 2006; published 15 November [1] We have examined characteristics of the vertical O + flux in the topside high-latitude ionosphere from measurements of the vertical ion drift and ion number density made by the Defense Meteorological Satellite Program (DMSP) F13 spacecraft from June 1996 to January 1997, June 1998 to January 1999, and June 2001 to January In the polar cap the vertical ion flux is, on average, downward at all locations. However, in the auroral zone the ion flux is highly structured, and a net upward flux is produced primarily by spatially and temporally confined events containing upward fluxes in excess of 10 9 cm 2 s 1. These dominant high-flux events tend to be produced more commonly by high densities rather than by high velocities. The range of vertical velocity and number density changes with season and solar cycle with greater variability in the vertical velocity occurring during winter and at lower levels of solar activity. There is also evidence that the vertical fluxes are a function of the interplanetary magnetic field, with upward fluxes in the auroral zones for negative B z and upward fluxes in the polar cap for positive B z. Citation: Coley, W. R., R. A. Heelis, and M. R. Hairston (2006), Characteristics of high-latitude vertical plasma flow from the Defense Meteorological Satellite Program, J. Geophys. Res., 111,, doi: /2005ja Review and Introduction [2] There is evidence that a substantial amount of O + escapes the Earth s ionosphere into the magnetosphere. Even for solar minimum conditions and low levels of geomagnetic activity, data from Dynamics Explorer 1 (DE1) indicate that the O + outflow is in excess of s 1 [Yau et al., 1988]. These outflows are a function of season and solar cycle, reaching rates of over s 1 [Lennartsson et al., 2004]. Indeed, there are indications that at least part of the time the dominant source of plasma for the plasma sheet and the near- Earth portion of the magnetosphere may be the ionosphere [e.g., Moore and Delcourt, 1995; Greenspan and Hamilton, 2002; Cully et al., 2003]. Over the past two decades the appreciation of the role of low-energy (50 ev) ions in the description of magnetospheric plasma has increased significantly. Chappell et al. [1987] showed that with the inclusion of this so-called core plasma, the ionosphere is capable of supplying the observed ion densities throughout most of the magnetosphere. [3] There have been numerous investigations involving spacecraft observations, radar observations, and simulations directed toward understanding the possible mechanisms that drive the upward/outward ionospheric plasma flow observed in the high-latitude F region above 500 km altitude [cf. Horowitz and Moore, 1997; Liu et al., 2001]. These flows are seen to be primarily upward in the cusp and auroral regions and downward over the polar cap [e.g., Loranc et 1 William B. Hanson Center for Space Sciences, University of Texas at Dallas, Richardson, Texas, USA. Copyright 2006 by the American Geophysical Union /06/2005JA al., 1991; Heelis et al., 1992]. Loranc et al. [1991] also found a positive correlation between the magnitude of the outflow and geomagnetic activity (Kp index). Heelis et al. [1984] observed upward ion fluxes exceeding cm 2 s 1 in the auroral zone near 900 km. At higher altitudes, suprathermal ion conics, bowls, rings, and beams are frequently present on active-auroral field lines [Klumpar et al., 1984]. Lockwood et al. [1985] reported observations of low-energy upwelling ions whose source was the dayside cusp and the auroral zone/polar cap boundary. They inferred that upward fieldaligned fluxes on the order of 10 9 cm 2 s 1 should be observed in the ionosphere below the spacecraft. These ions showed evidence of passing through a transverse heating region at the km altitude level. Indeed, as altitude increases, the plasma outflow over the high-latitude region undergoes several characteristic transitions [Ganguli, 1996]: from chemical to diffusion dominance at km, from subsonic to supersonic flow at around km, from collisional to collisionless at km, and from O + to H + dominance at ,000 km. [4] The major proposed mechanisms for driving these upflows include (1) frictional heating caused by convection that causes plasma expansion and outflow [e.g., Heelis et al., 1993]; (2) ionospheric electron temperature enhancement with the consequent increased upward ambipolar electric field [e.g., Whitteker, 1977]; (3) convection sheardriven ion instabilities that can induce heating [Ganguli et al., 1994]; and (4) ring current ion precipitation [Yeh and Foster, 1990]. In addition, high-altitude polar wind and ion outflow can possibly be driven by effects involving hot polar rain electrons, soft electron precipitation, topside ion heating, and photoelectrons. [Barakat and Schunk, 1984; 1of12

2 Figure 1. Contour plot of the number of data points per accumulation bin (2.5 magnetic latitude (MLAT) 1 hour SLT) for two months of summer condition passes in the Northern and Southern Hemispheres from the Defense Meteorological Satellite Program (DMSP) F13 spacecraft on a magnetic latitude and local time polar dial. Note that the band of data for the Southern Hemisphere is wider than that of the Northern Hemisphere, owing to the greater offset of the Southern Hemisphere s magnetic pole from the geographic pole. From Coley et al. [2003]. Wilson et al., 1988; Tam et al., 1995; Seo et al., 1997]. A good overview of possible processes is provided by André and Yau [1997]. [5] The work of Loranc et al. [1991], Horowitz and Moore [1997], Yau and André [1997], and others indicates that the topside ionosphere actively supplies thermal ions to intermediate altitudes. There they may be given escape energy by various mechanisms and transformed into beams and conics. Wu et al. [2002] have recently developed a dynamic fluid kinetic simulation of this process. Regardless of the physical mechanisms involved in moving plasma between the high-latitude topside F region and the inner magnetosphere only limited spatial and temporal relationships between the large-scale ion outflow features observed at low and high altitudes on the same field lines have been established [e.g., Zeng et al., 2001]. Nor has a good empirical model been generated for the overall outflow (inflow) of plasma from (to) the auroral zone and polar cap as a function of geomagnetic and solar activity. In this paper we address the latter problem through the examination of the seasonal, solar cycle, and interplanetary magnetic field variations of vertical ion flow data from the Defense Meteorological Satellite Program (DMSP) series of spacecraft, flying in the topside F region at approximately 840 km altitude. 2. Spacecraft and Data Set [6] The database for this study comes from the special sensor-ions, electrons, and scintillation (SSIES) package aboard the Defense Meteorological Satellite Program (DMSP) F13 satellite [Heelis and Hairston, 1990; Hairston and Heelis, 1993]. This instrument package provides density, temperature, and flow data for the thermal plasma in the upper ionosphere. Of particular interest are the measurements taken by the ion drift meter (DM), retarding potential analyzer (RPA) and the total ion trap (SM) [Heelis and Hanson, 1998]. The DM measures the vertical and horizontal (cross track) components of the plasma drift in the range of ±2800 m s 1 (corresponding to a maximum directed kinetic energy of 0.75 ev for an O + ion), the SM measures total ion density, and the RPA provides ion density, ion temperature, ram ion velocity, and ion composition. Since RPA measurements are not always available in the auroral zone it was decided to use only the more generally available vertical ion velocity measurements from the drift meter and the ion density measurements from the total ion trap in computing ion fluxes. The data set we are using contains measurements taken at a 4 s cadence. [7] The F13 spacecraft was launched in March 1995 and continues to function to the present time. The longitude sampled changes 26 per orbit giving excellent longitudinal coverage at the two local times sampled by the spacecraft which is in a polar Sun-synchronous circular (840 km altitude) orbit. Figure 1 illustrates the highlatitude coverage with a contour plot of the distribution of data points from two months of summer condition data from the DMSP F13 spacecraft for both Northern and Southern Hemispheres. Note that the nightside auroral region is poorly sampled, particularly in the Northern Hemisphere and that the dayside cusp region is sometimes only partly sampled. This implies that the vertical flow associated with 2of12

3 Figure 2. Scatterplot of vertical plasma velocity versus plasma density in the polar cap for summer, winter, and equinox conditions during periods of low solar activity ( , F ), medium solar activity ( , F ), and high solar activity ( , F ). The curved lines are 10 8,10 9, and cm 2 s 1 flux contours. 3of12

4 Figure 3. Scatterplot of vertical plasma velocity versus plasma density in the auroral zone (65 75 MLAT) for summer, winter, and equinox conditions during periods of low solar activity ( , F ), medium solar activity ( , F ), and high solar activity ( , F ). The curved lines are 10 8,10 9, and cm 2 s 1 flux contours. 4of12

5 Table 1. Number of Measurements Used in Figures 2 and 3 Season/Region Low ( ) Solar Activity Level Medium ( ) High ( ) Summer auroral zone Equinox auroral zone Winter auroral zone Summer polar cap Equinox polar cap Winter polar cap either of these two dynamic regions (e.g., nightside flux variations during substorms) will not be fully reflected in the results in this paper. 3. Observations [8] The first set of observations presented here covers three intervals of data taken by the F13 spacecraft from June 1996 to January 1997, June 1998 to January 1999, and June 2001 to January These are three periods of low, medium, and high solar activity with mean daily F10.7 indices of 74, 131, and 195, respectively. Note that these are average values and there is generally a wide range of activity over the course of a 27-day solar rotation period. F13 has a dawn-dusk orbital plane, crossing the equator near 0600 and 1800 SLT. In all cases O + is the dominant high-latitude ion at the altitudes sampled. [9] Some major features of vertical fluxes have been described by Coley et al. [2003] for northern and southern high-latitude regions during summer solstice (June July 1998 and December 1998 January 1999, respectively) when a two-cell convection pattern could be observed. The average hourly Z component of the IMF (B z ) over these periods of two-cell convection was 1.9 nt with values ranging from 13.9 to 8.5 nt. The auroral zones are characterized by upward fluxes that extend in local time across the dayside and into the nightside. In the prenoon and postnoon periods the convection reversal boundary rather cleanly separates regions of upward flux in the auroral zone and regions of downward flux in the polar cap. Fluxes in the auroral zone show more spatial structure and can reach cm 2 s 1 in individual passes, but those in the polar cap are more uniformly distributed and rarely exceed 10 8 cm 2 s 1. [10] The mechanisms known to produce upward ion fluxes are by nature both spatially and temporally confined [André and Yau, 1997]. Thus an average upward or downward flux may not represent the instantaneous physical situation observed. Coley et al. [2003] show that the polar cap is indeed characterized on the average by only downward fluxes and that at the DMSP altitude 75% of the events lie in the range cm 2 s 1. By contrast the auroral zone contains both upward and downward fluxes. They emphasize the significance of upward fluxes in the auroral zone in excess of 10 9 cm 2 s 1. While such fluxes are seen in the DMSP data only about 5% of the time, the complete absence of downward fluxes of the same magnitude means that this small number of events control the average properties of the auroral zone flux. It is important to note that the average flux distribution described earlier characterizes the entire auroral zone with an average upward flux. This implies that these large upward flux events are probably uniformly distributed in space in the auroral zone. In the absence of any spatial organization of these fluxes it was concluded that they are temporally variable events. The incidence and characteristics of these large flux events are addressed in more detail below using the larger data set of this study. It should also be noted that Coley et al. [2003] demonstrate instrumental error can lead to baseline uncertainties in the computed vertical flux on the order of 10 7 cm 2 s Seasonal/Solar Cycle Variations [11] One question that arises is whether high ion densities or high ion velocities are more important in the production of the observed vertical fluxes. Figures 2 and 3 present scatterplots of vertical ion drift versus ion density for different seasons (columns) and different levels of solar activity (rows) in both the polar cap and auroral zones, respectively. The data used again come from two-cell passes with >40 kv cross-cap potential and data from both hemispheres are combined in these plots. The curved lines are contours of constant flux at the 10 8 cm 2 s 1,10 9 cm 2 s 1, and cm 2 s 1 levels. As in the treatment by Coley et al. [2003], the measurements are located with respect to the convection reversal. The polar cap is defined as that region poleward of the convection reversal (75 MLAT in our normalized coordinates) and here the auroral zone is arbitrarily defined as the 10 MLAT region equatorward of the polar cap (65 75 MLAT). Table 1 presents the total number of data points for each of the panels in Figures 2 and 3. When we examine a single region for a given level of solar activity, Figure 2 clearly shows the seasonal (daylight/darkness) variation of the average ion density with a relatively narrow range of densities observed during summer and proceeding to a greater range of densities extending to lower values in the equinox and winter polar caps. In a similar fashion, when ion densities are compared for a given season for different levels of solar activity the range of ion densities extends to higher values with increasing F10.7. Similar patterns are seen in the auroral zone data of Figure 3. [12] Further examination of Figure 2 reveals several important features of the vertical ion velocity in the polar cap. The point distribution is characterized by a disk-shaped envelope with a center of mass that lies where the vertical ion drift is slightly negative. This is consistent with the previously reported average downward flux in this region of the topside ionosphere. The shape of the envelope is also revealing. Note for example that the highest downward vertical fluxes appear in the regions of largest ion density and smallest vertical ion drift. The largest upward fluxes on the other hand are present when the ion number density is lower and the upward ion drift is higher. [13] In the auroral zone Figure 3 shows that the distribution of vertical ion drift and ion number density has a similar disk shape. In this case the center of mass of the distribution lies where the vertical ion drift is slightly positive. Again downward vertical ion fluxes in excess of 10 9 cm 2 s 1 occur in general only for large number densities and the smallest velocity, while upward fluxes of comparable magnitude may be due to larger upward ion velocities. In the summer and equinox plots during high 5of12

6 Table 2. Vertical Velocity Statistics for Figure 3, Auroral Zone, Where jfluxj >10 9 cm 2 s 1 Season/Direction Low Solar Activity Level ( ) V z, s v, ms 1 ms 1 Median, ms 1 Percent Medium Solar Activity Level ( ) V z, s v, ms 1 ms 1 Median, ms 1 Percent High Solar Activity Level ( ) V z, s v, ms 1 ms 1 Median, ms 1 Summer up Summer down Equinox up Equinox down Winter up Winter down Percent solar activity ( ) there are isolated groupings of unusually high and low vertical drift. Examination of individual orbits reveals that these are due to isolated velocity spikes in the Southern Hemisphere during periods of high geomagnetic activity. Detailed examination of this phenomenon is beyond the scope of this paper but will be investigated later. [14] Examining the data at different levels of solar activity shows that the largest variation in the vertical ion drift occurs at moderate levels of solar activity. For high solar activity vertical ion drifts greater than 600 m s 1 are quite rare and for low levels of solar activity they predominate at the lowest observed ion number densities. By contrast, moderate levels of solar activity display relatively large vertical ion drifts when the ion number density is moderately large ( cm 3 ). We note that if the upward vertical flows are related to ion temperature increases from frictional heating then we might expect the largest drifts to occur when the ion concentration is a minimum as is the case at solar minimum. [15] Comparing different levels of solar activity in Figures 2 and 3 for a given season we find that as solar activity increases the average ion density also increases but the variability of the vertical velocity reaches a maximum at moderate solar activity. In particular, the higher levels of F10.7 show less scatter. A possible explanation for this variability in the auroral zones is the greater relative effect of frictional heating events on the lower background plasma density during the winter (nighttime) and low solar activity conditions. Finally, note that the envelope containing most of the data in a given panel is lozenge shaped. This is consistent with our earlier observation that the vertical fluxes show a great deal of spatial and temporal variability in that the most variation in velocity is seen around the level of the average ion density where most of the measurements are taken. [16] Detailed information on the incidence and velocity structure of the high flux events are presented in Tables 2 and 3. These utilize the data from Figures 2 and 3 where the absolute magnitude of the vertical flux is greater than 10 9 cm 2 s 1. In the leftmost column of Table 2 auroral zone data are broken down by season and the direction of the vertical velocity (upward/downward). The data are then broken out by solar activity level. Under each solar activity level heading are four entries: the mean vertical velocity (V z ), the standard deviation of the vertical velocity (s v ), the median vertical velocity, and the percentage of the total number of observations in this season/solar activity range (see Table 1) in which the absolute magnitude of the vertical flux is greater than 10 9 cm 2 s 1 (i.e., the occurrence probability). Looking at the data it can be seen that, as is also evident from the figures, the velocities of high flux events are generally higher in the low period of the solar cycle. However, the occurrence probabilities of the high fluxes are largest when the solar cycle is at its peak. This seems to indicate that it is the elevated ion densities that are primarily responsible for the elevated fluxes. The largest absolute and relative difference in occurrence probability between large upward and downward flows is seen in the summer (medium solar activity) when the probability of a large upward flux is over six times that of a downward one. When the polar cap data are examined in Table 3 it is seen that the mean and median vertical velocities during high flux events follow the same pattern with the largest velocities at the low part of the solar cycle and the largest occurrence probabilities at the high part. Indeed, in the medium and high parts of the solar cycle the probabilities of large fluxes being observed in the polar cap are generally larger than in the auroral zone with large Table 3. Vertical Velocity Statistics for Figure 2, Polar Cap, Where jfluxj >10 9 cm 2 s 1 Season/Direction Low Solar Activity Level ( ) V z, s v, ms 1 ms 1 Median, ms 1 Percent Medium Solar Activity Level ( ) V z, s v, ms 1 ms 1 Median, ms 1 Percent High Solar Activity Level ( ) V z, s v, ms 1 ms 1 Median, ms 1 Summer up Summer down Equinox up Equinox down Winter up Winter down of12 Percent

7 Figure 4. Ion density (N i ) and vertical drift (V z ) values binned and plotted versus ion flux for (a) auroral summer conditions, (b) auroral winter conditions, (c) polar cap summer conditions, and (d) polar cap winter conditions for The densities and drift values are normalized to the value of the lowest flux bin. Upward and downward values are plotted separately, and the mean densities in the lowest upward and downward bins are indicated on each panel. downward fluxes observed in the winter polar cap at solar maximum 16.5 percent of the time. These large downward polar fluxes are indicative of unobserved upward flow elsewhere, possibly the nightside auroral zone. [17] The relative importance of ion density and ion velocity changes as a function of the vertical flux can be seen in Figure 4. It presents the ion density and vertical ion drift values binned and plotted versus ion flux for auroral summer conditions, auroral winter conditions, polar cap summer conditions, and polar cap winter conditions for the moderate solar activity conditions of The densities and drift values are normalized to the value of the 7of12

8 Figure 5. Plots of maximum vertical flux versus the z component of the IMF in the morningside auroral zone, eveningside auroral zone, and the polar cap for both the Northern and Southern Hemispheres during a period of low solar activity (F ). lowest flux bin and upward and downward values are plotted separately. The normalization number densities N i0 (up) and N i0 (down) are shown in each panel. In summer auroral zone larger ion fluxes are due mostly to increases in the vertical velocity, while in the winter auroral zone as the total flux increases the average ion density increases more rapidly than the average upward velocity. It is worth noting that there is less difference between the velocity and density 8of12

9 Figure 6. Plots of maximum vertical flux versus the z component of the IMF in the morningside auroral zone, eveningside auroral zone, and the polar cap for both the Northern and Southern Hemispheres during a period of medium solar activity (F ). values than in the summer case. One possible interpretation of this pattern is that in the summer the local times sampled are always in daylight and there is relatively little variation in the already elevated plasma densities while in the winter there exist significant variations in both density and velocity. The larger summer polar fluxes follow the same pattern as the summer auroral zone. The winter polar region downward fluxes are controlled by the ion number density while the changes in the upward flux are driven equally by changes in the number density and upward velocity. [18] The behavior of the upward vertical ion velocity and the vertical ion fluxes appears to be consistent with expansion of the plasma from an input heat source. We see that upward fluxes vary by a factor of 30 or more in the auroral zone and polar cap during the winter and the summer. In the summer the number density changes by about a factor of 2, 9of12

10 Figure 7. Plots of maximum vertical flux versus the z component of the IMF in the morningside auroral zone, eveningside auroral zone, and the polar cap for both the Northern and Southern Hemispheres during a period of high solar activity (F ). indicating a wide range of heat input that changes the vertical velocity by more than a factor of 10. During the winter the same change in the vertical flux is accompanied by approximately equal changes in the velocity and the number density. This is consistent with observations in the topside ionosphere where in the summer the satellite is located closer to the peak density than in the winter. The downward fluxes represent a relaxation of previous upward flux events, probably upward fluxes in the auroral zone. In the winter the vertical velocity is almost constant and the flux is determined by the number density that flows through the surface at the spacecraft altitude. In the summer a reduction in the downward ion flux is accompanied by reductions in the velocity and the number density as the scale height of the plasma in the topside returns to nominal values in the absence of heating. 10 of 12

11 Figure 8. Maximum vertical fluxes versus IMF B z for the eveningside auroral zone (65 75 MLAT) using only passes showing a two-cell convection pattern. The left panel uses raw MLAT values. The right panel normalizes the data so that the convection reversal occurs at 75 MLAT. Both raw data points (colored triangles and squares) and fluxes binned by IMF (large black triangles) with associated variances are shown. The average fractional variance of the vertical flux in a bin is approximately 24% less in the normalized case IMF Variations [19] We next examine the vertical O + fluxes using all available data regardless of convective flow as a function of the Z component (B z ) of the interplanetary magnetic field (IMF) values taken from the National Space Science Data Center (NSSDC) OMNI files. Since we have established that it is the occasional strong vertical fluxes that dominate, particularly in the auroral zones, we will concentrate on the maximum fluxes observed in the regions of interest during a pass. Figures 5, 6, and 7 each present during low, medium, and high solar activity, respectively, plots of the maximum (most positive) vertical flux observed during each orbit of F13 versus the Z component of the IMF (hourly averages from the NSSDC OMNI files) averaged into 1 nt bins in the morningside auroral zone ( MLT), eveningside auroral zone ( MLT), and the polar cap for both the Northern and Southern Hemispheres. There is approximately one year s worth of data in each plot. The error bars indicate the standard deviation of the data within each bin. Since all available passes are being used we cannot normalize the coordinate system using our previous scheme, so the auroral zone/polar cap boundary is arbitrarily defined to be 75 MLAT for our classification purposes. While this definition will result in some inappropriate data classification it will be adequate to show general trends. [20] Looking first at the moderate solar activity conditions of Figure 6 we note the generally negative correlation between maximum vertical flux and B z when B z is negative for all four auroral zone regions. The correlation is somewhat less apparent for the Southern Hemisphere presumably because the greater offset of the magnetic pole leads to a greater range of background ionospheric conditions and thus a greater range of fluxes. Both the northern and southern polar caps show a slight positive correlation with IMF when B z is positive possibly reflecting the greater Joule and particle heating expected from the greater levels of convective structure and particle precipitation in the polar cap that are expected during positive B z. There are also two weaker apparent correlations: a positive correlation in the auroral zone for positive B z and a small negative correlation in the polar cap for negative B z. While the cause is not certain these possibly reflect contamination from the opposite region due to the arbitrary polar cap boundary used. Comparing these results with those from other levels of solar activity we find, unsurprisingly perhaps, that during low F10.7 conditions (Figure 5) the maximum fluxes are generally lower for a given negative value of B z in the auroral zone than during high F10.7 periods (Figure 7). Similarly the maximum fluxes in the polar cap are lower for a given positive value of B z during low F10.7 conditions. [21] In examining the relationship between the O + fluxes and B z a possible improvement that immediately suggests itself is the use of a dynamic coordinate system to ensure that all data are properly classified as auroral zone or polar cap measurements. Figure 8 illustrates this with the two-cell passes used earlier. The eveningside Northern Hemisphere auroral zone maximum vertical fluxes are plotted as a function of B z when the auroral zone/polar cap boundary is placed at the convection reversal (right panel) and when a fixed value of 75 MLAT is used for the convection reversal (left panel). Both the raw vertical fluxes and the data binned by IMF B z are shown. There is less scatter in the data for the normalized case. The relative standard deviation of the data in each bin is 1.10 for the unnormalized case and 0.83 for the normalized case. The utility of this technique is limited in that only cases of twocell convection can be used. In the future we hope to make use of a more general automated procedure, such as that of Sotirelis and Newell [2000] or Andersson et al. [2004] that 11 of 12

12 uses a dynamic coordinate system based on geophysically meaningful auroral zone boundaries. We anticipate that the use of more than a decade s worth of available data from the various DMSP spacecraft will allow the development of a model of plasma upflow from the high-latitude topside ionosphere. 4. Summary [22] Plasma flow data from the DMSP F13 spacecraft was examined during periods of two-cell convection for three levels of solar activity (low: , medium: , and high: ). The use of this data set allowed the determination of the polar cap auroral zone boundary in a dynamical coordinate system tied to the location of the convection reversal. Though upward and downward vertical O + fluxes at the 840 km altitude level were found in the polar cap and auroral zones at all seasons and levels of solar activity the polar cap (defined as poleward of the convection reversal) is characterized by an average downward plasma fluxes in both hemispheres during all seasons while a net upward flux is seen in the auroral zone. The dominant high-flux events (>10 9 cm 2 s 1 ) tend to be produced more commonly by high densities rather than high velocities. There are distinct seasonal and solar cycle changes in the outflow pattern with greatest velocity structure visible during winter and low F10.7. Vertical fluxes tend to maximize at moderate values of plasma density and vertical velocity. [23] When all data, irrespective of the convection pattern, are used a negative correlation is found between the vertical O + flux and B z in the auroral zones for negative B z while a positive correlation is found between the vertical O + flux and positive B z in the polar caps. The use of a dynamic coordinate system to determine geophysical boundaries has the potential to improve our understanding of the relationship between the fluxes and geomagnetic activity. [24] Acknowledgments. This work was supported by NSF grant ATM , NSF grant ATM , NASA grant NAG , and NASA grant NAG The OMNI data used were obtained from the GSFC/SPDF OMNIWeb interface at [25] Zuyin Pu thanks Andrew Yau and Jiannan Tu for their assistance in evaluating this paper. References Andersson, L., W. K. Peterson, and K. M. McBryde (2004), Dynamic coordinates for auroral ion outflow, J. Geophys. Res., 109, A08201, doi: /2004ja André, M., and A. W. 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