UV ovals and precipitation

Transcription

1 JOURNAL OF GEOPHYSCAL RESEARCH, VOL. 14, NO. A, PAGES , FEBRUARY 1, 1999 Size of the auroral oval: boundaries compared UV ovals and precipitation K. Kauristie, J. Weygand,,3 T.. Pulkkinen, J o S. Murphree, a and P. T. Newell 4 Abstract. The oval boundaries in 44 Viking UV images are compared with three critical boundaries as defined from simultaneous DMSP particle precipitation data. The particle boundaries are the equatorward boundary of the particle oval (often associated with the earthward edge of the main plasma sheet), the boundary between smooth and structured precipitation, and the poleward boundary of the particle oval (close to the open-closed field line separatrix). The UV oval is characterized by the latitude of maximum UV intensity, equatorward boundary, and poleward boundary which are the latitudes corresponding to the half values of the maximum intensity. Differences between the UV and particle boundaries are quantified in various magnetic local time sectors and at different activity levels. The study shows that the poleward boundary of the particle oval is often at _> ø higher latitudes than the most intense UV luminosity. Large differences are typical especially in the midnight and morning sectors. The present results suggest that caution is needed in interpreting the dramatic poleward expansion of the oval in the UV images, or more generally in using UV images to compute changes in the amount of open flux under different states of substorm activity. 1. ntroduction The auroral ovals encircling the magnetic poles are the regions in the ionosphere which receive the main part of the magnetospheric particle precipitation. The precipitating particles excite atmospheric atoms and molecules and thus cause luminosity both in visible and ultraviolet (UV) wavelengths. The exact location of the equatorward oval boundary depends on the energy of the precipitating particles and on the magnetospheric electric and magnetic fields. The poleward boundary of the oval is often taken to separate the closed field lines (field lines connected at both ends to the Earth) from the polar cap covered by open field lines (field lines connected from the Earth to the solar wind). At the boundary of polar cap the precipitation energy flux drops significantly, as the polar rain within the polar cap consists of weak homogeneous fluxes of < 1 kev elec- trons. Geophysical Research, Finnish Meteorological nstitute, Helsinki. 'Now at Physikalisches nstitut, University of Bern, Switzerland. adepartment of Physics and Astronomy, University of Calgary, Calgary, Alberta, Canada. 4Applied Physics Laboratory, Johns Hopkins University, Laurel, Maryland. Copyright 1999 by the American Geophysical Union. Paper number 199JA /99/199JA The oval consists of two different parts: The discrete oval (visual luminosity, e.g., rays and arcs) lies embedded in the continuous, permanent diffuse oval. The diffuse oval is mainly equatorward of the discrete oval, but diffuse precipitation, although often subvisual, appears also poleward of the discrete oval. The connection of the discrete and diffuse ovals to different magnetospheric plasma domains has been under discussion for more than twenty years. Lately, Galperin and Feld- stein [1991] presented plenty of observational evidence which supports the view that the poleward portion of the diffuse oval maps to the plasma sheet boundary layer (PSBL), the discrete oval to the central plasma sheet, and the equatorward part of the diffuse oval to the region between the inner boundary of the plasma sheet and the instantaneous plasmapause. The equatorward boundary of the discrete oval is generally associated with the trapping boundary of energetic (_>3 kev) electrons. During magnetospheric substorms the boundaries of the auroral oval move. During the growth phase, dayside reconnection increases the number of open field lines and consequently the polar cap and oval expand to lower latitudes. Part of the solar wind energy gets stored as magnetic field energy into the lobes of the magnetosphere. During the expansion phase the stored energy dissipates via various processes in the far and near-earth plasma sheet and in the ionosphere. Via nightside reconnection the polar cap contracts as open field lines in the lobe are converted to closed field lines [Hones, 1979; $iscoe and Huang, 195; Moses et al.,

2 3 KAURSTE ET AL.' SZE OF THE AURORAL OVAL 199; Lockwood et al., 199]. Thus the size of the polar cap follows the amount of magnetic field energy stored in the tail lobes, and the locations of the poleward and equatorward boundaries of the oval are important parameters when estimating the energy flows in the solar wind-magnetosphere-ionosphere system [Baker et al., 1997]. The oval boundaries can be monitored locally using ground-based and low-altitude satellite observations, while UV imagers onboard high-altitude satellites yield an instantaneous global view on the oval [Graven and Frank, 1991; Elphin tone et al., 199, 1993]. Discriminating he diffuse and discrete ovals from UV images is difficult and usually more sensitive particle instruments yield better estimates of the oval boundary locations. On the other hand, only UV images are useful in the global energy flow studies. Here we compare oval boundary locations defined from particle precipitation data recorded by Defense Meteorological Satellite Program (DMSP) satellites with nearly simultaneous UV images from the Viking satellite. These two data sets do not yield exactly the same boundary locations because the precipitation observed by DMSP satellites causes luminosity in a wide wavelength range and at various altitudes and he Viking UV imager can detect only part of these excitation pro- cesses. Furthermore, differences in the dynamic range and space resolution of the instruments affec the deftnition of the boundaries. n the present study we quantitatively estimate the differences between the particle boundaries and UV oval location in several cases. We do not consider the relative roles of the various observa- tional limitations in the differences but concentrate to resolve where and when UV images yield most reliable information on he oval boundary locationsø Section describes our data set. n section 3 the main results are presented, UV intensity versus magnetic latitude curves are compared with the locations of three particle boundaries defined by the new classification system of DMSP nightside precipitation data [Arewell et al., 1996]. Section 4 discusses the results of the previous section and a few case studies where UV intensity curves are compared with total energy flux recorded by DMSP satellites. Section summarizes our results.. Description of the Data Set The UV imager onboard the Viking satellite was in operation during the period March-November 196. mages were taken above the northern hemisphere at altitudes around - 13 km. The orbital period was 6 min, and the imaging rate was typically one frame per minute. One pixel in the images corresponds - x km area in the ionosphere [Hultqvist, 197]. The Viking imager comprised two CCD cameras with use images from the LBH camera, which, although less sensitive to high energy, recorded fine scale structures more clearly than the O camera [Petsson et al., 199]. The sensitivity of the LBH camera falls off if the average energy of the precipitating electrons is low (below.5- kev), or if the energy flux is too weak (below.5 erg cm-s - for energies _> 4 kev) [Elphinstone et al., 199]. DMSP satellites are Sun synchronou satellites in polar circular orbits at the altitude of - 35 km and with orbital period of - 1 min. They record particles well within the loss cone in the energy range from 3 ev to 3 kev. During the Viking period the tracks of DMSP F7 crossed the oval in the prenoon and premidnight sectors and those of DMSP F6 in the dawn and dusk sectors. This study utilizes the new classification system for nightside precipitation data developed by Newell et al. [1996]. The automated system defines from DMSP recordings the locations of six characteristic particle boundaries in the nightside auroral region. We concentrate on three boundaries which, from low to high latitudes, are e, 4s, and the more poleward of 5e and 5i (hereafter 5pol). Boundary e is the point where the energy of electrons ceases to increase with increasing latitude. Relating low-altitude particle boundaries with different magnetospheric regions is not always straightforward, but e is often associated with the earthward edge of the main (central) plasma sheet [Galperin and Feldstein, 1991]. The 4s is the transition boundary from unstructured to structured precipitation. ts operational definition is based on correlation between con- secutive energy spectra. The definition of unstructured precipitation is naturally limited by the spatial resolution of DMSP recordings (5-1 km). The 5pol marks the poleward boundary of the discrete oval (excluding the diffuse zone observed occasionally poleward of 5e and 5i). For more details about the boundary definitions, see Newell et al. [1996]. At 5pol the precipitating energy flux of either electrons (Se) or ions (5i) typically drops by about an order of magnitude within _< ø in magnetic latitude. We selected from the automated DMSP database for further analysis only those boundary observations which were recorded during the middle parts of Viking imaging periods and in the part of oval which Viking imaged. Particle boundaries were compared with the Viking image closest to the observing time of the 4s boundary. n 91% of the comparisons the time difference between the D MSP and Viking observations is _< 5 min. Keeping the time differences between Viking and DMSP observations this small limits the size of the data set but also reduces the ambiguity due to motion of boundaries between the observations. The UV images were transformed to a two dimen- sional ionospheric plane centered at the magnetic north passbands (O camera) and (LBH camera) [Anger et al., 197]. n this study we pole at the altitude of 1 km. DMSP observations were mapped to the same plane and the boundary lo-

3 _ KAURSTE ET AL.: SZE OF THE AURORAL OVAL 33 i i! i i 1 MLT -."' 6ML*T... i...,,,,,",,, ",,,..-..,,,,,,,,,, / :.-" 7... '... ", '- '. ' : ' ' " GO""...' ' ' ' ' 1 ' : ' ' : "" ' '"' " ' : : ' ' / - : : : : - : : : ':' e: '")"' ', / :. :,,,,,'..-' / '.,,i,,. ' x.,-. X Figure 1. Distribution of the data points used in this study. Poleward, middle, and equatorward boundaries are marked with circles, dots, and crosses, respectively. The symbols of particle observations (5pol, 4s, and e) are larger than those of UV observations. cations were transformed to eccentric dipole latitude (MLAT) and magnetic local time (MLT). For each particle boundary observation, UV intensity versus MLAT curve was defined at the same MLT, and from this curve the MLAT of the intensity maximum and the equatorward and poleward half values were defined. These locations are hereafter called maximum, poleward and equatorward boundary of the UV oval. As this study concentrates to the nightside oval boundaries, the minor effects of dayglow background were not eliminated from the UV images. The data selection procedure covered the entire operation period of the Viking LBH-imager, 44 Viking images were selected for the analysis. Figure shows the distribution of the particle and UV boundary observations. The common coverage of UV images and particle observations is clearly best in the premidnight sector, while only a few comparisons can be made in the morning sector. The D MSP data set consists of 45 observations of both e and 4s boundaries and of 4 observations of 5pol. The total observations of the UV equatorward, maximum, and poleward boundaries are, 111, and 93. f we consider boundaries are 6. ø and 7.1ø), the maximum of UV intensity is located - ø poleward of the unstructuredstructured (4s) boundary (average MLATs 6.1 ø and 66.ø). The scatter in the UV boundary observations is somewhat larger (standard deviation cr = 4 ø) than that in the particle observations (c = 3-3.5ø). Note that the definitions of the equatorward and poleward boundaries of UV oval do not have similar physical interpretation (i.e., definition based on spectral char- acteristics) as the particle boundaries considered here. Locating the boundaries at half the UV intensity maximum value implies scattering due to the variable shapes of the intensity curves, but on the other hand is more objective and quantitative than selection by eye. n the example shown in Plate the UV intensity curves are very peaked and consequently the UV oval is narrowø The UV poleward boundary is close to the poleward boundary of the particle oval and approximately at the location which one would select by eye, while the equatorward UV boundary is at a higher latitude than the particle data (and eye) suggest. Locating UV boundaries according to 1/e values would give more consistent only the mean values (merging all MLT sectors), the estimates in this case. The results shown in the next particle oval seems to be at higher latitudes (average sections were defined using the half value criterion be- MLAT of e and 5pol are 64. ø and 71.5 ø) than the UV cause it was not so often hampered by the background oval (average MLAT of the equatorward and poleward scatter than the 1/e criterion. However, general appli-

4 34 KAURSTE ET AL.: SZE OF THE AURORAL OVAL cability of the results was checked with a reference data set consisting of the cases favorable for the 1/e criterion. 3. Differences Between the Particle and UV Boundaries ': z loo Z 5O ß -- Boundary Boundary 1 UV intensity along the MLT of 5pol in4_ MLAT UV intensity along the MLT of 4s in4_9... o 7 ' ;5 ' MLAT 15 1 UV intensity along the MLT of e in4_5 Histograms depicting the MLAT differences between the particle and UV boundaries are shown in Figures, 3, and 4. Each figure shows a particle boundary and its relation to the three UV boundaries. n the histograms the MLAT differences are grouped to 1 ø bins (e.g., the column at 5 ø shows the number of events between 4.5 ø and 5.5ø). The differences are positive when the particle boundary is poleward of the UV boundary. The columns at 4-15 ø include all the points outside the range from-14.5 ø to 14.5 ø. The encircled numbers in Figures, 3, and 4 show the total number of events in the colums to the left and right of the zero column. These numbers lead us to conclude that the UV oval and the particle oval are always at least partially colocated: The equatorward edge of the particle oval (Figure ) is always equatorward of the poleward UV boundary and the poleward edge of the particle oval (Figure 4) is poleward of the equatorward UV boundary. The data set of UV boundaries based on the 1/e criterion shows similar behavior. The boundary between unstructured and structured precipitation (Figure 3) is equatorward of the UV maximum boundary (cf. the example of Plate 1). Thus the most intense UV auroras can usually be associated with structured particle precipitation. The distributions of the MLAT differences are quite broad. However, the equatorward boundary of the particle oval is often quite close to, or slightly poleward of the UV equatorward boundary; the distribution is highly peaked around ø and 1 ø. Furthermore, the difference between unstructured-structured precipitation boundary and UV equatorward boundary is peaked around 5 ø, indicating a relatively constant width of the unstructured particle oval. Peaked distributions Plate 1. A comparison between Viking and DMSP observations. The top panel shows a UV image from the Viking orbit 4 (April , 133:4 UT). n the UV oval the colors at the red end of the spectrum show the most intense emission while the blue end shows the weak luminosity. The thin white lines mark the eccentric dipole latitudes (MLAT) and the magnetic local time (MLT, noon is at the top of the figure, midnight is at the bottom). The color-coded straight line shows the closest D MSP F7 track. White color marks the track from the e boundary to 4s and purple from 4s to 5pol. Three lower panels show the UV intensity versus MLAT curves at the MLTs of 5pol, 4s, and e. The vertical line in each plot mark the location of the particle boundary. The horizontal solid and dashed lines mark the levels of the half and 1/e values of the maximum intensity.

5 KAURSTE ET AL.: SZE OF THE AURORAL OVAL e6 4 E z 6 Equatorward boundary of particle oval (e) UV poleward. boundary total=33 UV maximum - total=3 UV equatorward bound The results of Figure 5 are interesting from the viewpoint of polar cap boundary location. f we assume the most poleward boundary observation to be the best estimate for polar cap boundary the histogram in the right panel of bottom row, which peaks at zero, suggests that Viking and DMSP observations do about equally well. The corresponding histogram based on the 1/e boundaries peaks also at zero, but it shows more negative difference values than the one in Figure 5. Binning according to MLT revealed that the negative differences were observed especially in the dusk sector where the dayglow effect is more pronounced that in the pre- and postmidnight sectors. As our definitions of the poleward boundary of UV oval are artificial without a physical meaning, we have examined the UV maximum boundary as well. n more than half of the comparisons the poleward boundary of the particle oval appeared to be Unstructured-structured particle boundary (4s),,,.UV poleward boundary total= ) Figure. Histograms showing the distribution of the MLAT differences between the equatorward boundary of the particle oval (e) and (from top to bottom) UV poleward, maximum and equatorward boundary. Positive differences mean that e was observed poleward of the considered UV boundary. The encircled numbers show the total number of differences in the colums to the left and right of the zero column (including differences from -.5 ø to.5ø). were not achieved when the UV equatorward boundaries were defined according to the 1/e criterion. One cause for the fiat distributions in Figures -4 is that all MLT sectors have been merged together. This is especially true at the poleward boundaries. The location of UV poleward boundary dependstrongly on MLT and the particle poleward boundary probably has! to the same property. n Figure 5 UV maximum and pole- ward boundaries are compared with the particle pole- Figure 3. Histograms showing the distribution of the ward boundary after binning the data points into three MLAT differences between the unstructured-structured MLT groups. Evening, midnight, and morning sec- precipitation boundary (4s) and (from top to bottom) tors were defined as MLT<.,.<_MLT<_4., and UV poleward, maximum, and equatorward boundary. MLT>4., respectively. Note that in Figure 5 the lat- Positive differences mean that 4s was observed poleward itude division of the histograms 4 times courser than in the previous figures. of the considered UV boundary. For more details, see the caption of Figure. 6 > 4 E z UV maximum UV equatorward 6 boundary total=39,,,,

6 36 KAURSTE ET AL.' SZE OF THE AURORAL OVAL Poleward boundary of particle oval (5pol), UV poleward boundary than 5pol. Temporal evolution between the DMSP and Viking observations may explain the deviations, although the time differences between the D MSP and Viking observations were not exceptionally long (_ 5 min) in these events. Alternativ explanations are prob- total=9 5pol vs UV max 5pol vs UV poleward boundary Evening Evening UV maximum ,o,a1:34 : R boundary UV equatorward >6 o 4 E z Midnigh ::. Midnig :::. ::.,,, o Uorning Morning Figure 4. Histogram showing the distribution of the MLAT differences between the poleward boundary of the particle oval (5pol) and (from top to bottom) UV poleward, maximum, and equatorward boundary. Positive differences mean that 5pol was observed poleward of the considered UV boundary. For more details, see the caption of Figure. 4 _ ø at higher latitudes than the most intense UV luminosity, in % of the cases the difference was even more than 6 ø. The differences between 5pol and the UV maximum are large in the midnight and morning sectors. n the evening sector the UV maximum is typically within ø from the poleward edge of the particle oval, while in the midnight and morning sectors the typical value is between ø and 6øø Similar trend can be seen in the comparisons between the poleward particle and poleward UV boundary (right panel of Figure 5)' Number of differences > ø is larger both in the midnight and morning sectors than in the evening sector (This is true also when using the 1/e criterion.) Associating the polar cap boundary with the poleward boundary of the particle oval is not always a correct assumption. The middle panel of Figure 4 shows five cases where the UV maximum is at higher latitudes 16 All 1 4 ' :": -:" : All Figure 5. Histograms showing the distribution of the MLAT differences between the particle poleward boundary and the UV (left) maximum and (right) poleward boundary. The three uppermost panels deal with the differences binned into evening (MLT<.), midnight (._ MLT_ 4.), and morning (MLT>4.) sector observations. Note that the MLAT division is here coarser than in Figures -4' The zero column includes differences from - ø to ø. The bottom panel shows the two uppermost panels of Figure 4 as a refer- ence.

7 ß... KAURSTE ET AL.' SZE OF THE AURORAL OVAL 37 6 e vs UV max AE<5 nt 5pol vs UV max AE<5 T tionary conditions (lower panel). n the subgroup with geostationary activity the differences are larger than in the quiet time group. This suggests that geostationary activity is often accompanied by intense UV auroras located clearly equatorward of the open-closed field line separatrix (here assumed to be in the vicinity of 5pol) Discussion 4.1. Diffuse or Discrete Oval? >6. 4 z AE>5 nt AE>5 nt -'9-' The definition of the particle oval used in this study does not cover entirely the diffuse oval. The diffuse zone poleward of the discrete oval is totally excluded because it is poleward of 5pol. Ground-based observations suggesthat especially the dusk sector of the main diffuse oval (equatorward of the discrete oval) is caused by energetic ion precipitation [Fukunishi, 1975; Samson, 1994] which is partly beyond the reach of DMSP instruments. These instruments viewing toward the zenith may occasionally underestimate particle fluxes especially at low latitudes where nearly trapped, energetic particles can cause a significant part of the diffuse precipitation. n the low-altitude precipitation model by Galperin Figure 6. Histograms showing the distribution of the MLAT differences between the UV maximum boundary and (left) equatorward and (right) poleward boundary of the particle oval. The differences have been and Feldstein ([1991], see also references therein) the binned into two groups according magnetic activity, entire diffuse oval is equatorward of the e boundary. (top) AE <5 nt and (bottom) AE >5 nt. The They used the electron energy dispersion with increaszero column includes differences from -1.5 ø to 1.5 ø. 5pol vs UV max lems in the mapping of the UV images or in the classification system of DMSP data, which may fail in some cases (e.g., due to unfavorable viewing angle or exceptional distribution of particle precipitation). Figure 6 compares the location of the UV maximum with the poleward and equatorward boundaries of the particle oval at two different magnetic activity levels, i.e. when the auroral electrojet (AE) index is below and above 5 nt. A shift of the differences toward more negative values with increasing AE is visible in the histograms in the left panel, which indicates that the distance of the UV maximum from the particle equatorward boundary (e) increases with increasing activ- c 4 ity. On the other hand, the distance between the UV maximum and the particle poleward boundary often decreases as the level of activity increases. Consequently, most intense UV auroras shift closer to the polar cap boundary with increasing AE activity. o Figure 7 examines a possible connection between o intense UV auroras and geostationary activity. The geostationary activity was characterized using electron Figure?. Are geostationary electron flux injections flux recordings by the Los Alamos National Labora- associated with intense UV auroras? Upper panel shows the distribution of differences between the maximum tory (LANL) instruments. The histogramshow differ- UV boundary and poleward particle boundary observed ences between the poleward particle boundary and the at times when the LANL instruments recorded rapid UV maximum for two subgroups, one for events when flux variations. Lower panel shows the distribution of LANL instruments observed abrupt flux variations (up- a reference group of differences observe during quiet per panel of Figure 7) and the other for quiet geosta- geostationary conditions.

8 3 KAURSTE ET AL.: SZE OF THE AURORAL OVAL ing latitude as their definition of the diffuse precipitation. Diffuse and discrete auroras defined this way cannot be associated directly with the structured and unstructured precipitation defined here. The location of e should rather be considered as the low-latitude limit of the region of where discrete auroras could exist if they were present. However, in DMSP recordings structured precipitation only occasionally starts from this low-latitude boundary as 4s is typically poleward of e. The UV oval (as defined with the half value criterion) and particle oval have on average similar widths ( 7ø), and UV equatorward boundary tends to be equatorward of e (bottom panel of Plate 1). Comparisons with simultaneous D MSP data reveal that the UV oval is partly in the unstructured and partly in the structured precipitation region. Particles causing significant UV luminosity originate both from the main plasma sheet but also closer to the Earth, from the region where curvature and gradient drifts start to dominate. These drifts cause the energy dispersion effect which DMSP instruments observe equatorward of the e boundary. The UV maximum is usually within in the region of structured particle precipitation. Defining the locations of the oval equatorward boundary and the boundary between diffuse and discrete precipitation always depends on the observing instrument. Furthermore, selecting the relevant parameters for the definitions is difficult as the exact mechanisms caus- ferent activity levels suggesthat there may be differing diffuse and discrete precipitation are not yet known ences in the expansion rates. The particle oval and po- [Lyons, 1997]. Unique definitions, however, are essential lar cap are known to expand equatorward during the when the connection of the different low-altitude pregrowth phase and with increasing activity [Feldstein cipitation regimes with various magnetospheric plasma and $tarkov, 1967; Hardy and Gussenhoven, 195; Spiro domains are made [Galperin and Feldstein, 1991; Samet al., 19; Kauristie, 1995]. On the other hand, during son, 1994]. A concensus at the ionospheric end could be expansion phase intense UV auroras often shift polefound by systematic comparisons between ground-based ward with the expanding auroral bulge. DMSP elecand low-altitude satellite observations. With such deftric field recordings (as derived from the ion drift data) initions, it would be easier to establish the large-scale could provide additional reference information about linkages to the magnetospheric plasma domains. the polar cap dynamics. As the potential drop across 4.. Searching for the Polar Cap Boundary the polar cap follows changes of polar cap size [Lockwood et al., 199], estimates of the potential drop de- The histograms in Figures 4-7 show that UV images rived from DMSP recordings could be compared with are not always reliable when estimating the location the polar cap size variations derived from UV images. of the polar cap boundary: 5pol is often more that ø poleward of the UV maximum location, and the real open-closed field line separatrix is still poleward of 5pol (which corresponds to the equatorward boundary of PSBL). A rough estimate of the error in the open flux area is achieved if the polar cap is assumed to be circular (radius 15 ø in colatitude) and the error in boundary location about ø. Then the error in the area would be 7 %. ncreasing the sensitivity of the UV imagers (e.g., onboard Polar satellite [Tort et al., 1995])may provide better estimates of the polar cap boundary location, but the reliability will still vary as a function of magnetic local time and geomagnetic activity. Furthermore, finding the best criterion for defining the UV oval boundaries is not straightforward as it varies from case to case. When the magnetotail is very strecthed even a difference of one degree in the ionosphere can correspond to distances of several Earth radii at the magnetospheric equator [Pulkkinen et al., 1995]. Thus the 5pol boundary and the most intense UV auroras usually map to different regions of the magnetosphere. However, during substorm recovery phases the UV oval occasionally shows a doubly-peaked latitude profile, called double oval [Elphinstonet al., 1995b]. The data set analyzed here included four UV images showing partial double- oval configuration. n all these cases the most intense UV-auroras were located in the additional oval at high latitudes. Simultaneous 5pol observation was available for one case where 5pol was.7 ø poleward of the UV maximum location. The 5pol being just at the poleward edge of the high-latitude oval is consistent with the conclusion of Elphinstonet al. [1995a], who suggested that the additional oval is a signature of reconnection in the far tail (X#s m _ -RE). The time evolution of the auroral oval during substorms can be continuously monitored using UV images [Craven and Frank, 1991]. The orbital period of the DMSP satellites is too long for monitoring the particle oval similarly. Thus it is difficult to confirm that the particle and UV boundaries move together. Our results that compare the oval boundaries at dif Comparisons With Total Energy Flux Curves ntense auroras are generally associated with high energy flux of precipitating electrons. This can be found also when comparing D MSP total flux recordings with UV intensity curves from Viking. Seven premidnight oval crossings of DMSP F7 took place approximately along constant MLT and thus are suitable for comparison with the simultaneous UV intensity versus MLAT curves. Figure shows one example and Figure 9 the result of a superposed epoch analysis including all the seven curves. The superposed curves show how the av-

9 ._ KAURSTE ET AL.' SZE OF THE AURORAL OVAL 39 April : Total electron flux by DMSP F7 113,,,,,,,,,,, u) e,,4s,, 5pol , i MLAT 5 -,,... Viking orbit 4, intensity along.1 MLT, r ak -- eql,15 - ß 1-5 A1 pol i J i MLAT Figure. (top) Total energy flux of electrons recorded by DMSP F7 compared with (bottom) UV intensity curve by Viking. The particle data were recorded during an oval crossing in premidnight sector (at.1 MLT). erage values of flux and UV intensity vary around the MLAT of the UV maximum. The total energy flux curves are generally more structured that the UV intensity curves (Figure ). When the UV image is not recorded directly above the oval the mapping procedure smooths somewhat the intensity curves. This is because auroras occurring over an altitude range get mapped onto different locations in a plane at a constant altitude. As UV images were selected near Viking apogee, smearing due to oblique viewing angle should not be significant. The example of Figure shows features typical also for the other events: At the equatorward boundary of the oval the electron flux increase is more gradual than at the poleward boundary. The region of gradual energy flux increase is equatorward of the e boundary and 5pol marks the abrupt flux drop. The UV-intensity curves tend to be more peaked than the total flux curves. 113 Total electron flux by DMSP F7: Superposed epoch analysis 1,_ 11 l := 1 TM ß 11o 19 - i i i i -4 - Q 4 MLAT-UVmaxlat! UV intensity by Viking: Superposed epoch analysis i i i MLAT-UVmaxlat Figure 9. Superposed epoch analysis of seven (top) total electron energy flux and (bottom) UV intensity curves. The data were recordeduring oval crossings around. MLT. The origin of the horizontal axis in both panels corresponds to the MLAT of the UV maximum intensity.

10 33 KAURSTE ET AL.' SZE OF THE AURORAL OVAL The most equatorward boundary of the DMSP precipitation categorization system is the zero-energy convection boundary (le, i) [Newell et al., 1996]. As zeroenergy particles do not curvature or gradient drift, the Earthward edge of their drift trajectories is controlled purely by the E x B drift. Thus, in first-order approximation (assuming steady state convection) the equatorward cutoffs of ion and electron precipitation should coincide. Figures and 9 suggest that using the zeroenergy convection boundary instead of e as the equatorward particle boundary would yield particle ovals wider than the UV ovals. n DMSP data ion and elec- 1 MLT '"'" t ' b,mlat,,",,-" 'YO, /v.q uvo, ',,,,', :,", o6:,,,: : ', 1 po tron zero-energy cutoffs coincide frequently, but not always, which is the reason why l e,/-boundary was not Maximum UV intensity used in this study. DMSP particle precipitation data: The superposed curves of UV intensity and total en- equatorward de/dmlat>, of gradual the particle increase oval of defined total ener l¾ in th,s flux, study. ergy flux maximize at the same latitude. Thus the particle data and UV images locate the ionospheric region re- "'a de/dmlat Unstructured not precipitation, > anymore high total energy flux, ceiving the most intense precipitation on average to the same latitudes. However, the polar cap boundary shows Structured de/dmlat not precipitation, > anymore high total energy flux, results consistent with those discussed above: The pole- Figure 1. Schematic illustration summarizing the avward edge of the UV curve decreases more rapidly than erage locations of the particle and UV oval based on that of the particle flux curve. More exactly, the par- DMSP and Viking observations. The poleward and ticle flux does not drop by an order of magnitude (cf. equatorward boundaries of the UV oval are defined to the definition of poleward oval boundary by Newell et be at the poleward and equatorward half values of the maximum UV intensity. al. [1996]) within the MLAT difference where the UV intensity drops to its half value. to structured precipitation. Analysis of relative differ- 5. Summary and Conclusions ences (Figures 4 and 5) reveals that in more than half Oval boundaries located from UV images of Viking satellite have been compared with simultaneous particle precipitation data from DMSP satellites. The parof the comparisons the poleward boundary of the particle oval was >_ ø poleward of the most intense UV luminosity. Large differences were observed especially ticle data have been characterized with three critical in the midnight and morning sectors. boundaries: The boundary where the average electron The location of UV maximum with respect to the energy stops to increase with increasing latitude (equa- poleward and equatorward boundaries of the particle torward boundary of the oval), the boundary where unstructured precipitation changes to structured, and the boundary where the particle fluxes drop down abruptly oval has been further studied by binning the latitude differences according to the AE index and LANL electron flux data. The UV maximum is usually closer (poleward boundary of the discrete oval). The UV oval to the poleward boundary of the particle oval during has been located by the maximum UV intensity so that the poleward and equatorward boundaries of the oval are defined to be the poleward and equatorward half higher AE activity. On the other hand, rapid variations of the geostationary fluxes are often accompanied by intense UV auroras at latitudes clearly equatorward values of the maximum intensity. UV oval boundaries of the poleward particle boundary. as defined according to the 1/e value of the maximum intensity have been used as a reference data set. n general, the two data sets show similar behavior, but they also demonstrate that the distribution of UV intensity hardly provides any easy and unique way to define the The real polar cap boundary may be at higher latitudes than the poleward boundary of the UV oval because reconnection and other processes in the far tail (but in closed field lines) do not always cause auroras. An exception is the double oval configuration where the oval boundary locations. Consequently, our conclusions high-latitude oval is close to the poleward boundary of are mainly based on the comparisons between the UV maximum location and the particle boundaries. The analysis includes 44 Viking images and concentrates on nightside oval boundaries. Figure 1 summarizes schematically the average lothe particle oval and thus yields a fairly good approximation for the location of the open-closed field line separatrix. Our results support the conclusion that the motions of the UV oval boundaries tend to overestimate the substorm-time changes in the polar cap area [E1- cations of the oval boundaries. n all cases in this study phinston et al., 1993]. The auroral bulge visible in UV the UV oval and the particle oval were partially colocated. The most intense UV luminosity was usually recorded poleward of the transition from unstructured images during the expansion phase develops within the particle oval. Reconnection converting open field lines to closed field lines may take place at the head of the UV oval

11 KAURSTET AL.: SZE OF THE AURORAL OVAL 331 bulge, but the bulge is not intruding to the region of open field lines as dramatically as it seems in the expansion phase UV images. Despite their limitations the global auroral images are fundamental observables when estimating the energy flows in the solar wind-magnetosphere-ionosphere system. As a first step toward more quantitative analysis, we used here relative intensity units for defining the UV oval boundaries. A next step toward more rigorous analysis could be comparisons between particle data and absolute UV intensity values recorded e.g. by the Polar satellite. Oval boundaries defined according to the absolute values could behave more systematically than the boundaries used here, especially if the various altitude and particlenergy spectrum effects in the UV intensity [Jones, 1974] are taken into account in the 199. Elphinstone, R.D., J.S. Murphree, D.J. Hearn, W. Heikkila, M.G. Henderson, L.L. Cogget, and. Sandahl, The au- K. 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