Highlatitude plasma outflow as measured by the ... - Wiley Online Library

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 108, NO. A12, 1441, doi:10.1029/2003JA009890, 2003

High-latitude plasma outflow as measured by the DMSP spacecraft W. R. Coley, R. A. Heelis, and M. R. Hairston William B. Hanson Center for Space Sciences, University of Texas at Dallas, Richardson, Texas, USA Received 6 February 2003; revised 21 July 2003; accepted 13 August 2003; published 19 December 2003.

[1] We have examined the vertical ion flux in the topside high-latitude ionosphere from

measurements of the vertical ion drift and ion number density made by the DMSP F13 satellite. We found that the average vertical flux over the entire high-latitude region near 800 km altitude is quite small. This suggests that most of the vertical ion flux is associated with a large-scale ‘‘breathing’’ of the upper ionosphere. In the polar cap the vertical ion flux is uniformly downward at all locations. However, in the auroral zone the ion flux is highly structured, and a net upward flux is produced only by spatially and temporally confined events containing upward fluxes in excess of 1010 cm 2 s 1 that have INDEX TERMS: 2431 Ionosphere: Ionosphere/magnetosphere no downward counterparts. interactions (2736); 2437 Ionosphere: Ionospheric dynamics; 2407 Ionosphere: Auroral ionosphere (2704); 2475 Ionosphere: Polar cap ionosphere; 2481 Ionosphere: Topside ionosphere; KEYWORDS: magnetosphereionosphere interaction, outflow, upflow, flux, oxygen, joule heating Citation: Coley, W. R., R. A. Heelis, and M. R. Hairston, High-latitude plasma outflow as measured by the DMSP spacecraft, J. Geophys. Res., 108(A12), 1441, doi:10.1029/2003JA009890, 2003.

1. Review and Introduction [2] There are indications that 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]. Over the past 2 decades the role of lowenergy (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. Horwitz and Moore, 1997]. These flows are generally seen to be primarily upward in the cusp and auroral regions and downward over the polar cap [e.g., Loranc et 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] saw upward ion fluxes exceeding 1010 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 109 cm 2 s 1 should be observed in the ionosphere below the spacecraft. These ions showed evidence of passing through a transverse heating Copyright 2003 by the American Geophysical Union. 0148-0227/03/2003JA009890

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region at the 1000 – 2000 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 500 – 800 km, from subsonic to supersonic flow at 1000 – 2000 km, from collisional to collisionless at 1500– 2000 km, and from O+ to H+ dominance at 5000 – 10,000 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 shear-driven ion instabilities that can induce heating [Ganguli et al., 1994], and (4) ring current ion precipitation [Yeh and Foster, 1990]. In addition, for high-altitude polar wind and ion outflow effects involving hot polar rain electrons, soft electron precipitation, topside ion heating, and photoelectrons have also been considered [Barakat and Schunk, 1984; Li et al., 1988; Tam et al., 1995; Seo et al., 1997]. [5] The work of Loranc et al. [1991], Horwitz and Moore [1997], and others indicates that the topside ionosphere actively supplies thermal ions to higher altitudes. There they may be given escape energy by various mechanisms and may be transformed into beams and conics. Regardless of the physical mechanisms involved in moving plasma between the inner magnetosphere and the high-latitude topside F region, an exact spatial and temporal relationship between the large-scale ion outflow features observed at low and high altitudes on the same field lines has not been established, nor has a good empirical model been generated for the outflow (or inflow) of plasma from (or to) the auroral zone and polar cap as a function of geomagnetic and solar activity. This study seeks to begin to address the latter

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COLEY ET AL.: HIGH-LATITUDE PLASMA OUTFLOW FROM DMSP

Figure 1. Contour plots of the number of data points per accumulation bin for 2 months of summer condition passes in the Northern and Southern Hemispheres from the 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 for the Northern Hemisphere because of the greater offset of the Southern Hemisphere’s magnetic pole from the geographic pole. problem through the utilization of ion flow data from the Defense Meteorological Satellite Program (DMSP) series of spacecraft flying in the topside F region at the 800 km altitude level.

2. Spacecraft and Data Set [6] The database for this study comes from the special sensor ions, electrons, and scintillation (SSIES) package aboard the 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 total ion trap (SM). The DM measures the vertical and horizontal (cross track) components of the plasma velocity in the range of ±3000 m s 1 (corresponding to a maximum 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. A limitation in computing the full vector ion velocity is imposed at high latitudes because of the fact that the RPA requires 4 s to complete a measurement cycle. This corresponds to a distance of 30 km of spacecraft travel. In the auroral regions, ionospheric conditions often change significantly over this time and distance scale, making it impossible to compute the RPA-derived geophysical parameters (i.e., the ram component of ion velocity). It was therefore decided to use only the more generally available cross-track velocity measurements from the drift meter and the ion density measurements from the total ion trap in computing ion fluxes. [7] The F13 spacecraft was launched in March 1995 and continues to function to the present time. Since the duty cycle of the instruments approaches 100%, a substantial database is available for study consisting of 14 polar passes in each hemisphere per day (5000 passes yr 1). The longitude sampled changes 26° per orbit, giving

excellent longitudinal coverage at the two local times sampled by the spacecraft. The DMSP spacecraft are in polar Sun-synchronous circular orbits (840 km altitude, 98° inclination), so they always make their measurements at the same solar local times (SLT) when crossing the equator. Most of the DMSP data taken in recent years were obtained from satellites in the 0900 –2100 and 0600 – 1800 SLT (i.e., F13) orbital planes. However, at high latitudes the 98° orbital inclination combined with the offset of the magnetic pole from the geographic pole allows the spacecraft to traverse a large range of local times over the course of a day, thus giving coverage of most of the auroral zone and polar cap. Figure 1 illustrates the high-latitude coverage with contour plots of the distribution of data points from 2 months of summer condition data from the DMSP F13 spacecraft for both the Northern and Southern Hemispheres.

3. Observations [8] The observations presented here were taken by the F13 spacecraft from June 1998 to January 1999, a period with a mean daily F10.7 index of 130.8. F13 has a 0600– 1800 SLT orbital plane. We first desire to examine the seasonal variations in the plasma outflow from the topside ionosphere. This is necessary in order to distinguish such seasonal variations in the auroral zone and polar cap from variations produced by changes in magnetic activity. As noted in other studies [e.g., Coley and Heelis, 1998], the larger offset of the magnetic pole from the geographic pole in the Southern Hemisphere can be expected to lead to possibly significant differences in ionospheric behavior. Therefore data from the Northern and Southern Hemispheres are initially examined separately. [9] Figure 2 shows data taken from a polar pass of F13 through the high-latitude regions in the Northern Hemisphere. The inclination of the satellite is such that this pass cuts almost radially through the auroral zones and across the magnetic pole. In this case the horizontal ion drift, desig-


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Figure 2. Plasma parameters for 1650 –1718 UT from the DMSP spacecraft F13 on date 99015 (15 January 1999) in the Northern Hemisphere. (a) Ion and electron temperatures from the retarding potential analyzer (RPA) and Langmuir probe, respectively. (b) Vertical ion flux. (c) Horizontal cross drift and (d) vertical components of the ion velocity from the RPA and drift meter (DM). (e) Ion density from the RPA showing the total ion density (solid line) and the H+ density (dotted line). nated Vy (sunward flow is positive), provides a signature of the large-scale convection pattern, and the vertical ion drift, designated Vz, is dominated by the field-aligned flow. The northern winter hemisphere is characterized by upward fluxes of O+ in the auroral zones near 0600 and 1800 LT near 800 km altitude. At highest latitudes in the polar cap

the O+ fluxes in the winter hemisphere are predominantly downward at 800 km altitude. As discussed in section 2, the ram component of the plasma velocity is often not available at high latitudes, and therefore vertical plasma fluxes are used instead of field-aligned fluxes. Figure 3 shows data from the same DMSP pass taken in the summer hemisphere.

Figure 3. Same as Figure 2 for 1742 – 1803 UT in the Southern Hemisphere.

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The dominant ion near 800 km altitude is again O+. However, the vertical flows in the auroral zones and the polar cap are much smaller that those seen in the winter under the same conditions. It should be recognized that the important parameter to specify is the flux of ionization in the topside ionosphere. The reduced vertical flows seen in the summer are associated with larger ion densities than are seen in the winter hemisphere. Thus the vertical fluxes are comparable in the summer and the winter. Spatial structure in the fluxes is more evident in the winter hemisphere, and in the summer hemisphere the region of vertical flows extends to subauroral latitudes. [10] It is expected that in general, plasma flows at midlatitudes (50° magnetic latitude(MLAT)) are minimal and can be expected to be near zero over a broad range of latitudes. Examination of the vertical drift Vz near 1651 – 1655 UT in Figure 2 reveals an apparent constant downward drift of 100 m s 1. Because of a combination of instrument offsets and spacecraft attitude uncertainty, this offset leads to a substantial nonphysical value of downward ion flux in this region. To correct this problem, it was decided to perform a linear fit to the median vertical drift values in the 50°– 55° MLAT regions on both the morningside and eveningide of the polar pass and to remove this baseline from the vertical velocity data. As shown later, this technique results in a substantial reduction in the baseline values of the averaged vertical plasma flux. In some cases however, it was not possible to do this correction because of the presence of significant (20% of total) concentrations of H+ and He+. The large thermal velocities of these light ions make the drift meter measurements unreliable. In such cases the raw drifts were used in the averaging process. [11] To obtain seasonally averaged fluxes, it was decided to first examine passes that displayed the two-cell convection patterns typical of the southward interplanetary magnetic field (IMF) with a cross-cap potential >40 kV. The use of such passes allows the determination of a high-latitude convection reversal boundary. The requirement of a two-cell pattern necessitates the removal from consideration of some of the most sunward passes (particularly in the Northern Hemisphere) that just skim the auroral zone and for which no clear convection pattern could be determined. The location of the convection reversal in each pass was defined as the boundary of the polar cap, and the data from individual passes were then binned by magnetic local time (MLT) and MLAT into bins of 1 hour and 2.5°, respectively, after normalizing the coordinates such that the polar cap boundary (convection reversals) occurred at 75° MLAT. Figure 4 displays contour plots of the average vertical ion flux over the northern and southern high-latitude regions for winter and summer solstices (June – July 1998 and December 1998 to January 1999) and for the fall equinox period (August – October 1998) using DMSP F13 passes that display two-cell convection patterns with cross-cap potential >40 kV. Upward fluxes are red and downward fluxes are blue. The black dotted contours labeled 0 divide upward and downward fluxes. The flux contour levels are in units of log10(cm 2 s 1). In all cases the dominant ion was O+. The average Bz component of the IMF over these passes was 1.9 nT. By comparison with Figure 1 it can be seen that the most sunward and antisunward contours in each plot are artifacts due to the limited region of data coverage and should be disregarded in the interpretation

of these plots. The heavy green line in the contour plots delimits the regions where there are at least 100 data points in a bin. Generally, the high-latitude and dawn-dusk regions have enough data for reliable averages to be computed. In some cases (i.e., Figure 4b), there is an island of insufficient data in the region near the pole. The average variance of the computed flux in each bin is 2.4 108 cm 2 s 1. [12] The summer hemispheres (Figures 4a and 4b) in the north and the south are characterized by upward and downward fluxes with relatively large-scale distributions compared to those seen at other seasons. The auroral zones are characterized by upward fluxes that extend across the dayside as seen in the Northern Hemisphere and into the nightside as seen in the Southern Hemisphere. In the prenoon and postnoon periods the convection reversal boundary, located at 75° MLAT in the plots, 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 frequently exceed 1010 cm 2 s 1, but in the polar cap they are more uniformly distributed and rarely exceed 108 cm 2 s 1. In the region across local noon the region of upward fluxes clearly extends poleward across the reversal boundary and into the polar cap in northern summer. This attribute is not seen in the southern summer, perhaps because of the limit in spatial coverage. [13] Figures 4c and 4d show the distribution of average fluxes observed during the fall equinox in the Northern Hemisphere and the spring equinox in the Southern Hemisphere, respectively. The latitudinally narrow region of upward flux seen in the summer cusp region is now a much broader feature. We have noted that the heavy green line marks the boundaries of reasonable average specification, and thus the observations show a similar separation of upward and downward flux in the auroral zones and polar cap, respectively. [14] Figures 4e and 4f show the vertical ion flux distribution seen in the winter. In this case the sunlight conditions have a dramatic effect on the observations. On the dayside, where the solar zenith angle is only slightly larger than 90°, the distribution of ion flux again shows regions of upward and downward flux that are clearly delineated by the polar cap boundary. However, on the nightside (seen only in the Southern Hemisphere) the solar zenith angle is very large, and both the auroral zone and polar cap are characterized by spatially confined regions of highly variable ion fluxes. [15] Figure 5 shows the distribution of ion flux in the highlatitude topside ionosphere in the same format as Figure 4. In this case the data represent the situation in which a two-cell convection pattern is present but with weak convective flows contributing to a cross-polar cap potential 40 kV cross-cap potential. See text for details.

zones and polar cap, respectively. However, as the solar illumination decreases, observations at equinox and winter show considerably more structure with regions of upward flux considerably diminished. [17] Figure 6 shows a cut through the northern summer hemisphere data shown in Figure 3a along a track that reaches a maximum MLAT of 84°. While the auroral zones

and polar cap are clearly visible, examination of the lowlatitude data reveals evidence that the downward flux of 107 cm 2 s 1 plotted at these latitudes probably represents the zero flux level as left by the baseline removal procedure described earlier. Indeed, the instrumental uncertainty of 10 m s 1 combined with an ion density of 105 cm 3 could provide an offset 10 times as large in the raw data before

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Figure 5. Same as Figure 4 for passes that have a low (40 kV cross-cap potential (Bz < 0), there is a net high-latitude plasma inflow for summer conditions and an outflow for equinox and winter conditions. [32] (8) For two-cell convection conditions with