Height-integrated conductivity in auroral substorms

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 105, NO. A1, PAGES , JANUARY 1, 2000 Height-integrated conductivity in auroral substorms 2. Modeling J. W. Gjerloev and R. A. Hoffman Laboratory for Extraterrestrial Physics, NASA Goddard Space Flight Center, Greenbelt, Maryland Abstract. Calculations of height-integrated conductivity from 31 individual Dynamics Explorer (DE 2) substorm crossings presented by Gjerloev and Hoffman [this issue] are used to compile empirical models of the height-integrated Pedersen and Hall conductivities (conductances) in a bulge-type auroral substorm. Global auroral images obtained by Dynamics Explorer 1 (DE 1) were used to select substorms displaying a typical bulge-type emission pattern and each individual DE 2 pass was positioned with respecto key features in the observed emission pattern. The conductances were calculated for each DE 2 pass using electron precipitation data and a monoenergeticonductance model. All passes were divided into six different sectors, and average conductance profiles were carefully deduced for each of these sectors. Using a simple boxcar filter, smoothed average sector passes were calculated and from linear interpolation between these, two-dimensional conductance models were compiled. The characteristics of our models are (1) the Hall conductance maximizes in the high-latitude part of the surge at 48 mho with a Hall to Pedersen ratio of 2.4; (2) two channels of enhanced conductance are overlapping in local time near midnight and are fairly separated in latitude; (3) the conductance has a sharp gradient at the highlatitude boundary in the premidnight sector while in the postmidnight sector a broad region of low conductance stretches up to 10 ø invariant latitude poleward of the local peak; and finally, (4) the enhanced conductance region displays a characteristic broadening toward dawn primarily owing to a poleward shift of the high-latitude boundary. 1. Introduction Since the first model of the ionospheric Hall and Pedersen conductances was developed by Wallis and Budzinski [1981], a number of studies have been done with the purpose of producing empirical models of the conductance at highlatitudes. The value of such studies can in general be considered to be threefold: first, to increase the understanding of a fundamental ionospheric electrodynamic parameter; second, to develop self-consistent models of the ionosphere; and third, to provide models needed tbr global MHD simulations. However, unlike other fundamental parameters such as the convection electric field or the field-aligned currents, it is not possible to make direct measurements of the height-integrated conductivity. Instead, it must be calculated from measured parameters such as the electron precipitation spectra. Consequently, results depend heavily upon the quality of the data used for the calculation as well as the calculation method itself and the assumptions inherent in the method. In the case of statistical studies the results will obviously depend on which criteria are used for the selection of the individual data sets (e.g., satellite passes) and the subsequent organizational method, as was pointed out by Gjerloev and Hoffman [this issue] (hereafter referred to as INow at Danish Space Research Institute, Copenhagen, Denmark. Copyright 2000 by the American Geophysical Union. Paper number 1999JA /00/1999JA G&H). These problems result in rather large deviations between the different models and should be kept in mind when comparing results. A number of different approaches have been used in compiling statistical models of conductances. ISIS 2 measurements of the electron precipitation spectra enabled Wallis and Bud inski [1981] to compile models of the Hall and Pedersen conductances for two levels of the Kp index, 0<Kp<3 and 3<Kp<9. Unfortunately, the energetic particle detector only provided measurements of the precipitating electron fluxes at energies of 0.15, 1.27, 9.65, and >22 kev, but power law spectra were used to approximate the fluxes at 33 different energy levels. They included solar photon ionization as well as other background sources such as galactic EUV. Another example of this type of study was done by Spiro et al. [1982], who used electron precipitation data with a 15-s time resolution from the Atmosphere Explorers C and D to produce a binned model (Simons et al. [ 1985], later deduced a more portable Gaussian model based on this binned model). They determined the total precipitating electron energy input for both hemispheres as a function of MLT and found that at all MLT an organization of the data by the AE index resulted in a clear separation of data obtained at different magnetic activity levels. Ahnet al. [1983] used another interesting approach. They determined empirical relationships between the north-south component of the magnetic field (AH) measured at College and Pedersen and Hall conductances deduced from Chatanika radar data. These relationships were used to compile global conductance models on the basis of the distribution of AH. 227

2 228 GJERLOEV AND HOFFMAN: HEIGHT-INTEGRATED CONDUCTIVITY, 2, MODEL Recently, Ahn et al. [1998] has attempted to improve these models using the same technique. Fuller-Rowell and Evans [1987] used years of electron precipitation data obtained by the TIROS, NOAA 6, and NOAA 7 satellites to compile binned models of the Hall and Petersen conductances. In order to organize individual passes they introduced an activity index which was an estimate of the total energy deposited into a single hemisphere, which Foster et al. [1986] found could be related to Kp. In an immense study, Hardy et al. [1987] used million spectra of the electron precipitation data gathered by the polar orbiting satellites DMSP F2, DMSP F4, and P78-1 to compile global maps of the conductance for seven levels of Kp ranging from 0 to > 6-. They found consistently lower values than those in the work by Wallis and Budzinski [1981] and explained the discrepancy by the limited amount of spectral data available from the ISIS 2 instrument and the fact that Wallis and Budzinski [1981] used incorrect collision frequencies [Vickrey et al., 1981; Spiro et al., 1982]. Global auroral images obtained at two different wavelengths enable one to deduce the conductance directly from the images [Rees et al., 1988] and consequently compile global models with a time resolution which equals the imager exposure time. However, since such models are not statistical they will not be reviewed here. Finally, the model by Robinson and Vondrak [1994] should be mentioned, although it perhaps should be referred to as a collection of features rather than an actual model, since it was compiled from a number of different studies. However, it is interesting to see that their model indicates a surge located premidnight and a characteristic channel of enhanced conductance at lower latitude stretching from dusk to dawn. In this paper we present a model of the height-integrated Hall and Pedersen conductivities in the night time sector during auroral substorms. The data used to compile the model were presented by G&H. They used electron precipitation data and the monoenergeticonductance model by Reiff [1984] to calculate the Hall and Petersen conductances for 31 DE 2 substorm crossings occurring during the expansion phase and early recovery phase. These passes were carefully selected and organized in location by the use of global auroral images obtained by DE 1. Our model represents a fifth paper in a series consisting of an overview study of the electrodynamic parameters by Fujii et al. [1994], a field-aligned current study by Hoffman et al. [1994a], an ionospheri convection paper by Hoffman et al. [1994b] and finally a conductance calculation paper by G&H. This paper consists of four sections: in section 2 we compile individual data points from all passes within the specific sector. This was done in two steps: 1. The latitudinal width of each pass was normalized by the use of the boundary and central plasma sheet (BPS and CPS) [Winningham et al., 1975] regions of electron precipitation and stretched to match the average sector latitudinal width, 2. Each pass within a sector was positioned by placing the high-latitude BPS boundary at the average high-latitude BPS boundary for the specific sector. When using the terms BPS and CPS, it needs to be pointed out that we do not intend to describe the topology of the magnetosphere but simply use the terms as a description of the precipitation type. All data points from all 31 passes are shown as dots in Figure 1 after the above explained normalization and positioning. These data plots were used to deduce average passes for each sector by dividing 60o-80 ø ILAT into 100 bins and to calculate a simple average value for each bin. The resulting average passes are also shown in Figure I without any smoothing applied. Owing to the limited number of passes in each sector the average pass is highly sensitive to unusual values in individual passes. An example of this is the inverted V event in east of bulge at 74.5 ø ILAT, and naturally this is a problem in the middle surge, which only includes two passes. The number of points exceeding the maximum of the y scale is shown in each panel as (n?) where n is a very small number compared to the total number of displayed data points in the sector, which ranged from 632 to Note that the y scale of each panel is identical and the Hall scale is double the Pedersen scale. The striking spread in the data shown in Figure I simply illustrates the high variability of one of the fundamental electrodynamic parameters in the high-latitude ionosphere. Consequently, any comparison between individual events (e.g., satellite passes) and models has to be made with utmost caution, even if the time and position of the event with respecto the substorm is determined. However, despite this spread a number of interesting features are maintained in each sector and between sectors. The width of the enhanced conductance region increases considerably from the west of bulge to the east of bulge primarily as a result of a characteristic poleward shift of the high-latitude boundary (also see G&H, Figure 5). The latitudinal profile becomes increasingly asymmetric toward midnight and dawn due to the change in the precipitation pattern. From the surge to the east of bulge the conductance in the high-latitude part decreases, evident of a softening of the BPS-type precipitation, while on the other hand the low-latitude part which the CPS produces is fairly constant. the model; section 3 is a discussion including a comparison with previously published models; and finally in section 4 we It should be noted that the technique for obtaining an average pass in a sector unavoidably decreases the gradients summarize and draw our conclusions. in the conductanceseen in individual passes, in spite of our data selection and sorting method. However, the method does 2. Modeling maintain primary characteristics of the profiles such as the rather sharp gradients at the high-latitude boundaries of the The 31 passes used in this study consisted of 24 expansion west of bulge through surge sectors. phase and 7 early recovery phase passes. Owing to the limited number of passes and the nonuniform local time distribution of the passes (e.g., we have nearly no passes near Since we organized the data by global images rather than magnetic local time, the position of the average passes from each sector can only be related to their position with respect midnight), we chose not to fit a two-dimensional spherical to the generic aurora. Nevertheless, we reintroduce a function to all passes. Instead, we divided the substorm into six sectors as described by Fujii et al. [1994](also see G&H) and carefully calculated a sector average pass using all generalized magnetic local time in order to create a model and defined the magnetic local time at the center of each sector to be (from dusk to dawn): 1840, 1940, 2100, 2240, 0120 and

3 GJERLOEV AND HOFFMAN: HEIGHT-INTEGRATED CONDUCTIVITY, 2, MODEL Pedersen... i,.,,, i...! West of Bulge (t2) (to) ''' I... ".... o ;...:.._; ,-r-- - I,,,, '"":i' ' 'i- :'c.i.i- ". '"'"' 70- r' '" Surge Horn "'..".-", '. ;. (t7) ' '...,u..,,-'"', '"; ' : ';' I I /I :':. '%: ' "'* t ::'. " ' '" '" ,: -.':';,:..'.,.:.,v..,, I' '"'' ' o Surge (to) Hall I... Average I... 6O 4O t [ 0.,.; r Middle Surge.. I ; I I I lo o... ":' '-': : I : ', : :...,...,., : ', I '. ' ' ' 30 i.,.ii... :: (to),.,' (t )...,. 6o 0 e' ' '... '"=' ' ":' ' T ',t;)' ' '... '...,'.'"-?:.':,'.:,?-.-?.: o -..:? ::r'"':-.'?;,%<:--',:...:;.,:'i.,;.'; ; :':.:.':i(;.:::.::. (..".' ;""-".""... :', ". :'":'.' ".,...,.~.:05... ' '...';.'.'."2.'.'... ' 0,.;, '''' '"'"""" "'-"" '"'' ""' ',.,'-.... "'.' :... " '- '''" ' -,.:',"-; 'b,:4,"; MLT. While the position of the passes in the substorm By the use of a simple sliding boxcar filter a smoothed coordinate system shows a weak MLT dependence average pass was calculated for each sector and between the postmidnight, the premidnight passes have no MLT sectors linear interpolation was used to create a uniform dependency, as pointed out in Figure 2 of G&H. MLT-ILAT grid consisting of 100 by 100 points covering Consequently, the above chosen local times are more or less MLT and ø ILAT. The resulting models of arbitrary. However, a combination of the average MLT for all the Pedersen conductance, Hall conductance and the Hall to passes within a specific sector and the generic aurora defined Pedersen ratio are shown in Plate 1 as polar contour plots in by Fujii et al. [1994] can provide a rough estimate of the the generalized magnetic local time-invariant latitude MLT position of the sectors. We extended the local time coordinate system. The enhanced conductance region is coverage to the entire nighttime sector by adding a pass primarily confined between 61 ø and 75 ø ILAT much like identical to the west of bulge average pass at 18 MLT and previous models. The position of the low-latitude boundary is another pass identical to the east of bulge average pass at fairly constant from MLT to dawn while the poleward 0600 MLT. boundary displays the characteristic high-latitude kink

4 ,.....,, : GJERLOEV AND HOFFMAN' HEIGHT-INTEGRATED CONDUCTIVITY, 2, MODEL 40 a0] o o,,, o 2o,, o,,,.,,. o { Plate 1. Polar plots showing the Pedersen conductance, the Hall conductance and the Hall to Pedersen ratio as function of invariant latitude and a generalized magnetic local time derived from the average passeshown in Figure 1.

5 .. _ GJERLOEV AND HOFFMAN' HEIGHT-INTEGRATED CONDUCTIVITY, 2, MODEL 231 associated with the surge, which is also seen in global auroral images. The other kink near midnight is usually not seen in auroral ultraviolet images owing to the low energy flux of the precipitating electrons. The latitudinal width increases significantly from dusk to dawn, primarily due to the poleward shift of the high-latitude precipitation boundary. Peak values of EM-48 mho and EM/Ep-2.4 are found in the high-latitude part of the surge at MLT. It is interesting to note that while this maximum has a well-defined westward, poleward and equatorward boundary it appears to be stretched far eastward across the middle surge and into the bulge at MLT, thereby forming a high conductance channel within the enhanced conductance region. This channel which is produced by the BPS precipitation overlaps in MLT with a similar low-latitude channel produced by the CPS precipitation. This second channel covering a larger area stretches from MLT to dawn across three sectors with maximum values of E -22 mho and EH/Ep-2.1 located in the bulg e at -01:30 MLT. The Pealersen conductance distributions display similar features although the Pealersen conductance in general is less than the Hall conductance. Actually, E. exceeds or equals E, throughouthe substorm region, which is clearly illustrated in the third panel. Highest ratios are found in the high-latitude part of the surge produced by energetic BPS-type precipitation and in the low-latitude postmidnight region produced by typical CPS-type precipitation (see G&H for typical surge and east of surge passes). Postmidnight both the Pealersen and Hall conductance plots show large areas of lower conductance (-5 mho) extending to high latitudes above the high-latitude conductance channel. This is produced by the weak BPS-type precipitation in the bulge and east of bulge sectors. 3. Comparisons With Previous Models As mentioned in the introduction calculations of the conductance on which models are based are subject to a number of possible uncertainties, and in addition the compilation of our model itself further reduce structures in the modeled parameters. A direct comparison between some previously published models [e.g., Wallis and Budzinski, 1981; Spiro et al., 1982; Fuller-Rowell and Evans, 1987; Hardy et al., 1987] and the model presented in this study is further compromised by the fact that none of the above studies are directly substorm related. These models typically express the conductance as a function of invariant latitude and magnetic local time and an activity index such a: AE or Kp. Obviously Kp, being a 3-hour index, is not well suited for substorm studies. Even a disturbed AE does not necessarily mean substorm conditions, and sorting data by AE does not differentiate between substorm phases. In all previous models MLT has been used to organize individual satellite Hall Conductance for AE>600 nt Contour Interval: 4.0 mho Maximum: 34.3 mho Figure 2. Hall conductance model after Spiro et al. [1982] for broad region of low conductances, E -4 mhos, poleward of the postmidnight maximum, but at a later local time than our large region of low conductance. The model also shows a secondary peak at dusk at the same location as the Spiro et al. peak. It may be possible that our data selection process could passes and as pointed out by G&H this introduces a data smearing, especially in the premidnight sector. Further, we believe that our selection of satellite passes which fit a specific substorm type minimizes the possibility of mixing different geophysical conditions more effectively than a have missed a region of enhanced conductance at such an early local time. 2. To perform further comparisons, we list in Table 1 separately for the premidnight and postmidnight regions the locations of conductance maxima, their Pedersen and Hall magnetic activity index such as AE or Kp. The model values and the ratios for all the models available. Our presented in this study principally only applies to bulge-type premidnight maxima for the Hall conductance is larger than auroral substorms, although that is a common type. any other values by 33% to 150% and the Pedersen 1. The most appropriate substor models for comparison conductance by 20% to 65%. Note, in general, that the with our model are those of Spiro et al. [1982] for high AE conductance maxima are at higher latitudes than for the index and Ahn et al. [1983] for the maximum epoch of a postmidnight maxima. The Hall conductances vary AE>600 nt. substorm when AL(70)=-583 nt and AU(70)= 175 nt. In Figure 2 we have plotted the Spiro et al. [1982] Hall conductance data for AE > 600 nt which shows one local maximum of 34 mhos very early in MLT ( MLT) at 72 ø with the conductance ratio of 2.3, and another premidnight maximum of the same intensity at MLT, but at an even higher latitude of 74 ø. The dusk maximum is well to the west of the typical location of surges, whereas the 2100 to 2200 MLT maximum agrees in local time with our surge as well as with our conductance ratio, but at a latitude about 5 ø higher. The postmidnight maximum coincides well with our conductance location and value, though with a higher conductance ratio. The Ahn et al. [1983] model in Figure 3 shows peak values of the Hall conductance of 37 mhos at MLT at a latitude of 67 ø, which is rather late for a surge location, with another maximum postmidnight of 25 mhos, coinciding with our lower-latitude conductance channel. This model also shows a

6 232 GJERLOEV AND HOFFMAN: HEIGHT-INTEGRATED CONDUCTIVITY, 2, MODEL SIX MERIDIAN CHAINS SIX MERIDIAN CHAINS 1200UT MARCH 19, UT MARCH 19, 1978 HALL CONDUCTIVITY PEDERSEN CONDUCTIVITY 12 MLT IS MLT 50 ø. L--8-0ø;, 0o CONTOUR INTERVAL: 4.0 MHO (b) CONTOUR INTERVAL: 2.0 MHO (c) MAX: 36.8 MHO MAX: 12.2 MHO MIN: 2.2 MHO MIN: 1.4 MHO Figure 3. Conductance distributions by Ahn et al. [1983] for AE=758 nt. considerably more than the Pedersen conductances. We attribute our larger values to the selection and organization methods of the data which prevent data from different geophysical conditions and locations within a substorm from being mixed and to the modeling technique of normalizing the width of the passes in a sector and adjusting the high- latitude boundaries. These procedures minimize smearing of the data and help maintain the gradients. On the other hand, the conductance values and latitudes of the maxima from the various rrlodels agree well in the postmidnight region, although the local times of the maxima are hardly consistent, though the majority agree with our broad maximum around 0130 MLT. One must remember that our sector locations were somewhat arbitrarily assigned, and we have only two sectors in the postmidnight hours. However, we cannot explain the Wallis and Budzinski [1981] and Hardy et al. [1987] maxima as late as 0500 and MLT, respectively. This maximum may be due to the conductivity produced by substorm recovery phase electron precipitation in the dawn region [Ostgaard et al., 1999] that would be included in these models but not in ours. 3. Possibly, the most significant difference between our model and all others is the two separated high conductance channels overlapping in MLT. Since both channels in our model cover several sectors with good latitudinal alignment and clearly appear in the data from the individual passes, the existence of these channels during a substorm must be considered well documented. Note that the fairly deep minimum between the conductance channels from 2200 MLT to 0200 MLT is only 1 ø to 2 ø wide, so it could be easily smeared out in statistical studies. 4. Further, the conductance profiles are highly structured in the high-latitude part of the premidnight region with a characteristic sharp decrease at the high-latitude boundary associated with the BPS cutoff. Owing to the smoothing and averaging this structure is not reproduced well in the model Table 1. Magnitude and Location of the Maximum Hall and Pedersen Conductance in Six Different Models. Activit 7 Zti Zr Wallis and Budzinski [ 1981 ] Kp> Spiro et al. [ 1982] AE> Ahn et al. [1983] AE= Fuller-Rowell and Evans [1987] Level= Har(v et al. [1987] Kp> Premidnight Postmidnight ILAT MLT Z,H Z T ILAT MLT 68 ø ø 5 74 ø ø ø ø 3-65 ø ø 1-1: ø 4-4:30 Ahn et al. [ 1998] Kp= ø 1-2 Gjerloev and Hoffman [ 1999] Substorm ø ø 1:30 Position is in invariant latitude and magnetic local time. A third local maximum near 1800 MLT is present in the Spiro et al. [1982] model at 72ø/ MLT; in the Ahn et al. [1983] model at 73ø/1800 MLT and in the Hardy et al. [1987] model at 67ø/1900 MLT. Notice that Hardy et al. [1987] uses corrected geomagnetic latitude.

7 GJERLOEV AND HOFFMAN' HEIGHT-INTEGRATED CONDUCTIVITY, 2, MODEL 233 Baumjohann et al. [ 1981] : B I m w i +F 40 MHO i,., i,. Opgenoorth et al. [ 1983]... -at tilt... o..m;mmmmmmm m m IIIIIII II I lltl I Inhester et al. [ 1981 ] 30 MHO + 20 MHO Gjerloev and Hoffman [ 1999] +El 50 MHO Figure 4. The conductance distribution in three self-consistent WTS models by Baumjohann et al. [1981], Opgenoorth et al. [1983], and Inhester et al. [1981] and in the bottom panel a linearized section of our model. For comparison, all models are equally scaled and aligned at the head of the surge. The square and the cross denote Hall and Pedersen conductance, respectively. and the sharp high-latitude boundary is somewhat softened, dependency. These discrepancies arise most likely as a result while on the other hand the CPS produces a smoother profile, of the selection and organization of passes as well as the which is well reproduced by the model. However, our high- binning and smoothing methods used in the other papers. latitude cutoffs in the premidnight region are still much Neither AE nor Kp is well suited to differentiate between sharpe[ than any other model, showing a drop by a factor of different geophysical conditions. For example a disturbed AE 10 in less than 2 ø latitude compared to 10 ø or more in other or Kp does not necessarily mean substorm conditions nor can models. The conductance profiles from individual DE 2 they differentiate between different substorm phases. passes as well as the average passes are strongly asymmetric Consequently, using geomagnetic indices to select events will with respecto latitude. Typical of the other models, Hardy et unquestionably result in a mix of data from many different al. [1987] states that in general the profile only has one types of activity. As to the organization of individual events, maximum and varies smoothly with latitude although the binning of the data in an invariant latitude-mlt (or profile is not symmetric, which is in agreemere with our equivalent) coordinate system will necessarily lead to further results. However, Hardy et al. finds that although the smearing. It is well known that the latitudinal position and amplitudes are changed considerably, the shape of the width of the auroral oval not only is dependent upon the latitudinal profiles are fairly identical at dusk, midnight and substorm phase but it is different from substorm substorm. dawn (see their Figures 6 and 7). This is in contrasto our Further, as pointed out by G&H, the magnetic local time latitudinal profiles, which show a distinct local time position of substorm features located from the dusk to the

8 234 GJERLOEV AND HOFFMAN: HEIGHT-INTEGRATED CONDUCTIVITY, 2, MODEL middle surge are completely randomly distributed within a 5- hour interval, while features located east of the middle surge do show a weak MLT dependency. In every one of these three steps we have done everything possible to minimize smearing. Images ensured that all selected passes occurred during similar geophysical conditions and were used to organize individual substorm crossings, which prevented mixing of data from different pans of the substorm. Finally, when compiling the model we adjusted tbr the difference in latitudinal position and width of the auroral oval, using a latitudinal normalization and positioning of each individual pass. 5. Finally, three interesting westward traveling surge studies by Inhester et al. [1981], Opgenoorth et al. [1983], and Baumjohann et al. [1981] produced self-consistent models by the use of a variety of data sources. The ionosphere was divided into 50 km by 50 km cells in which all electrodynamic parameters were assumed constant. The electric field within each cell was determined from a combination of measurements and interpolation. Simple Hall and Pedersen conductance distributions were assumed which in combination with the electric field allowed a calculation of the height-integrated ionosphericurrents. Finally, upward and downward FACs were calculated from divergence of the ionospheric currents thereby ensuring current continuity. Using this scheme, they were able to reproduce the observed ground magnetic perturbations. The assumedistribution for each of the three studies is shown in Figure 4 along with a linearized section of our model. We find E.=48 mho and EH/Z,=2.4 within the surge, while Inhester et al. [1981] uses EH=I 1.2 mho and Z./Z,=4.0, Opgenoorth et al. [1983] uses Z =25 mho and E./Z,=I.0, and finally in better agreement with our results Baumjohann et al. [1981] uses a more complex distribution with peak values of Z =32 mho and E./Z,-2.1. Hence fairly large quantitative differences between the models are present which perhaps can be attributed to the different ionosphericonditions under which these case studies were done. However, it is noteworthy that all models show an -200 km wide highly conductance "channel" within the enhanced conductance region. In the Opgenoonh et al. and Inhester et al. models the eastern by their position relative to key features in the global auroral images obtained from DE 1 and subsequently divided into six sectors. Data from all passes within a sector were normalized to the average sector latitudinal width and shifted in latitude to the average high-latitude boundary. Average sector passes were then calculated and smoothed using a simple boxcar filter. Models of the Hall and Pedersen conductances and Hall to Pedersen ratio were compiled by a linear interpolation between the smoothed average passes from each of the six sectors. On the basis of this work we can make the following conclusions: 1. The enhanced conductance region is in general confined within 61 ø-75ø ILAT. 2. Despite a careful selection and organization of the satellite passes we find large differences between the individual passes in a sector. 3. The average conductance profiles have distinct local time dependencies and are strongly asymmetric in latitude at all nighttime local times. 4. Two high conductance channels are present within the enhanced conductance region, fairly separated in latitude but overlapping in MLT from 2200 hours to 0200 hours. The high-latitude channel is produced by typical energetic, structured BPS-type precipitation maximizing in the surge at Z.=48 mho with ZH/Z,-2.4. The low-latitude channel is produced by typical diffuse CPS-type precipitation maximizing in the bulge at E.=22 mho and E./Z, The enhanced conductance region displays a characteristic broadening toward dawn, primarily as a result of a poleward shift of the high-latitude precipitation boundary. The high-latitude boundary is well defined in the surge where the conductance decreases by a factor of 10 in -2 ø ILAT. Postmidnight a broad region with E.-5 mho stretches up to 10 ø ILAT poleward of the local peak. 6. The Hall to Pedersen conductance ratio is equal to or larger than I throughout the substorm. It minimizes in the west of bulge at E./E,-1.0 and, as expected, it maximizes in the high-latitude part of the surge sector at./e,-2.4. However, it should be noted that the ratio is relatively high throughouthe postmidnight CPS region with typical ratios of E,?Z Significant differences between our model and boundary, unfortunately, is defined by the simulation domain and consequently we cannot determine whether these also previously published models can be explained by the unique exhibit the premidnight enhanced conductance channel seen data selection and organization method used in this study as in our model. In contrasthe Baumjohann et al. model has a well as the method used for compiling the model itself. region of very small conductances east of the surge itself. 8. In comparing with previously published self-consistent The western boundary of the "channel" is less sharp in our WTS models we find that the maximum Hall conductance in model which is due to the inclusion of the surge horn and our model is -50%-450% larger, while on the other hand the possibly the linear interpolation between the average passes spatial distribution of the enhanced conductance region is in that we used. We find fairly high conductances south and southwest of the surge, a feature not seen in any of the other good agreement. As pointed out in the introduction this model represents a models. first step toward a self-consistent model of the electrodynamics during bulge-type auroral substorms 4. Summary and Conclusion including conductances, electric fields, ionospheric Hall and Pedersen currents and field-aligned currents (FACs). In doing We have compiled models of the ionospheric Hall and Pedersen conductances in the high-latitude nighttime sector during bulge type substorms. The models are based upon calculated conductance profiles from 31 individual DE 2 so it is essential to maintain gradients in the conductance and electric field models since the gradients determine the location and magnitude of the FACs and consequently the validity of the self-consistent model itself. Therefore it is of substorm crossings presented by G&H which used fundamental importance that virtually everything be done to measurements of the electron precipitation and a minimize smearing of characteristic features during each step monoenergetic conductance model by Reiff [1984]. toward the self-consistent electrodynamic model of the Following Fujii et al. [1994], these 31 passes were organized ionospheric substorm.

9 GJERLOEV AND HOFFMAN: HEIGHT-INTEGRATED CONDUCTIVITY, 2, MODEL 235 Acknowledgements. The first author would like to thank J.A.Slavin for giving me the opportunity to do part of my Ph.D. at the Laboratory for Extraterrestrial Physics at NASA's Goddard Space Flight Center and the Department of Automation at the Technical University of Denmark for their flexibility and support. The work was done under a grant from the Technical University of Denmark and USRA contract NAS The authors appreciate the very helpful suggestions and comments made by the two referees. Janet G. Luhmann thanks Barbara A. Emery and Phillip C. Anderson for their assistance in evaluating this paper. References Ahn, B.-H., R. M. Robinson, Y. Kamide, and S.-I.Akasofu, Electric conductivities, electric fields and auroral particle energy injection rate in the auroral ionosphere and their empirical relation to the horizontal magnetic disturbances, Planet. Space. Sci., 31, 641, Ahn., B.-H., A.D. Richmond, Y. Kamide, H. 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