Observed connections between apparent polar cap features and the instantaneous diffuse auroral oval

Observed connections between apparent polar cap features and the instantaneous diffuse auroral oval

Pfanef. Space SCI. Vol. 29, No. I I, pp. 1143-l 149. 1981 Printed in Great Britain. OBSERVED 00324%33/81/l I 1143M$02.00/0 @ 1981 Pergamon Press Ltd...

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Pfanef. Space SCI. Vol. 29, No. I I, pp. 1143-l 149. 1981 Printed in Great Britain.

OBSERVED

00324%33/81/l I 1143M$02.00/0 @ 1981 Pergamon Press Ltd.

CONNECTIONS BETWEEN APPARENT FEATURES AND THE INSTANTANEOUS DIFFUSE AURORAL OVAL J. S. MURPHREE

CAP

and L. L. COGGER

The University of Calgary, Physics Department, (Received

POLAR

9 March

Calgary, Alberta, Canada, T2N IN4 1981)

Abstract--Images of the instantaneous nightside aurora1 distribution reveal that at times the orientation of aurora1 oval arcs changes to become characteristic of polar cap arcs. These connecting arcs all terminate in the diffuse aurora in the midnight sector, and their separation from the equatorward boundary of the diffuse aurora generally increases away from the midnight termination. The occurrence of these features requires a northward interplanetary magnetic field (positive B,) as well as low magnetic activity. The existence of connecting arcs and the observation that they are at times the poleward boundary of weak diffuse emission indicate that the poleward boundary of aurora1 emissions can be significantly modified during non-substorm periods. Such a distortion implies that there can be a modification of the standard convection pattern in the magnetosphere during periods of positive B, to produce expanded regions of sunward convection in the high latitude ionosphere. INTRODUCTION

Aurora1 researchers have generally assumed that there are at least two topologically distinct regions of emission: polar cap and the instantaneous aurora] oval. Polar cap features are identified visually by their predominant sun-aligned character (Davis, 1963) or spectroscopically by their 6300 A-rich emissions (Akasofu and Yasuhara, 1973). In practice the sharp distinction between polar cap and oval auroras is not clear, particularly for dayside fan arcs, late morning auroras and some continuous midnight arcs (Meng and Akasofu, 1976). In this report we shall discuss satellite observations of aurora1 arcs which appear to be in the polar cap over a range of local times, but are also connected to the instantaneous aurora] oval in the m’idnight sector. SELECTION

CRITERIA

The distinction between polar cap and oval auroras may seem obvious because of the general definition that any feature poleward of the instantaneous oval is in the polar ,cap. However, defining this somewhat arbitrary boundary is at times difficult optically because of unusual aurora1 arc orientations with respect to the midnight aurora1 oval. Examples of such features are shown in Fig. 1 where the Aurora1 Scanning Photometer data from the ISIS-2 satellite are transformed onto a Corrected Geomagnetic Latitude/Magnetic Local Time grid (Murphree and Anger, 1980). All data

are displayed with an intensity grey scale of 0% 4 kR. Intensities greater than 4 kR appear as white. The data are corrected for path length but not for ground scattering. Most of the examples shown in Fig. I are clearly not arcs which simply deviate slightly from being oval aligned; rather, they display significant changes in latitudinal separation from the equatorial boundary of the diffuse aurora. It is this type of aurora that is the object of the present investigation. From over 400 transformed passes examined, only 17 passes contained this feature at a 3914 A intensity 2 0.5 kR in the midnight sector. A criterion which could have been used to preselect possible passes is the sign of the north/south component (B,) of the interplanetary magnetic field (IMF). Davis (1963) showed that polar cap features do not occur during times of high magnetic activity, and more recently Berkey et al. (1976) have shown a near perfect correlation between positive B, and the occurrence of sunaligned arcs. All the examples that were analyzed for this report and for which IMF data were available occurred during periods of positive B, (average value = 2.3 y). OBSERVATIONS

In Fig. 1 are shown 6 passes which display a monotonically increasing separation of an aurora1 arc system from the equatorward boundary of the diffuse aurora. These connections range from a 1143

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J. S. MURPHREE and L. L. COGGER

smooth separation (Figs. lb and d) to an almost perpendicular connection (Figs. la, c, e and f). The connections can occur at any M.L.T. in the midnight sector and unconnected polar cap features may also be present (Figs. lc and f). The arcs, which in all cases are imbedded in the diffuse aurora1 oval in the midnight sector, would normally be classified as polar cap features because of their orientation (Davis, 1963). Clearly however, these features reflect a modification of the poleward boundary of the aurora1 oval so that in these cases it cannot be considered to be circular (Holzworth and Meng, 1975). In all cases the arc connecting to the diffuse aurora has an intensity that is the same as or greater than that of neighboring oval auroras. It is clear from the examples in Fig. 1 that significant structure may occur along an arc (Figs. la and d) and that even if an individual arc terminates, other arc segments may exist as a continuation of the feature into the polar cap (Figs. lc and f). Of the 17 examples found, 2 displayed a somewhat different morphology, see Fig. 2. Here the aurora1 arc, rather than monotonically increasing its separation from the equatorward boundary of the aurora1 oval, changes its direction as it leaves the oval. This type of connection is similar to that reported by Meng and Akasofu (1976), although their interpretation that the arcs are on open field lines is inconsistent with the arc being continuous into the diffuse aurora. The pass shown in Fig. 2a (3914 A) occurred on 720116 at 0135 U.T. and is unusual in that it also contains the rare trough region aurora called patches (Moshupi et al., 1977). Both this and the 721012 pass at 0444 U.T. shown in Fig. 2b (6300 A) display significant discrete arc systems in the aurora1 oval whereas the examples in Fig. 1 do not. All observations of connections are shown schematically in Fig. 3. The lines are drawn from where the arc intersects the aurora1 oval to where the arc ends (i.e. to where its intensity drops below 0.5 kR). It is evident from Fig. 3 that there is no clear separation in time of the features based on B, as might be expected (Lassen, 1979). This may be due to the difficulty in relating instantaneous observations to an hourly averaged index. The same problem arises in the case of the B, component of the IMF as shown at the top of Fig. 3. Here the maximum separation of the connecting arc system from the equatorward boundary of the diffuse aurora has been divided by the local time extent over which the separation occurred and plotted against B, (in gammas) to show the mean

rate of separation as a function of B,. The large scatter is expected as noted above, but there is a trend indicating that as B, becomes more positive the orientation change of the feature with respect to the oval becomes greater. Thus the features extend farther into the “classical” polar cap with increasing B,. Similar observations have been reported by Lassen (1979) based on a statistical analysis of the distribution of quiet time aurora1 arcs: for positive B, the distribution of aurora1 arcs is widely spread over the high latitude ionosphere. In many cases the connecting arc systems reported here are in fact the poleward boundary of weak diffuse emission (Fig. lb shows this best for the intensity grey scale chosen for that figure) and sufficiently sensitive measurements are needed to see this effect. An example of this is shown in Fig. 4 for a low value of B, ( = 0.4 y). The 3914 A data for this pass (740213 at 0457 U.T.) is shown with two intensity grey scales: 0.5-4 kR on the left and l-5 kR on the right. Only with the more sensitive image can the diffuse emissions be seen, In this example the connecting arc is not very well formed, but there is clearly an increasing separation of the poleward extent of the diffuse emission toward earlier M.L.T. (2400 M.L.T. is vertically down from the cross representing the pole). Thus the morphology of the poleward extent of aurora1 emissions can be misinterpreted if low intensities are not included in the analysis. DISCUSSION The occurrence frequency of aurora in the polar cap is significantly less than in the oval and this in fact is the definition of the polar cap boundary from the viewpoint of the statistical aurora1 oval (Feldstein, 1963). Ismail et al. (1977) found that the occurrence frequency for sun-aligned arcs is about 6%. The connecting arc systems discussed in this report are somewhat rarer (about 4% occurrence probability) and thus they represent an unusual aurora1 distribution and hence an atypical particle precipitation pattern. However, the observation that the poleward boundary of the aurora1 oval may be significantly distorted during non-substorm times is important in its magnetospheric implications. The diffuse aurora has been identified as the optical signature of the plasmasheet (Lui et al., 1977) and thus these observations imply that the poleward extent of the plasmasheet in the midnight sector is not necessarily a smooth surface at its boundary with the high latitude lobe. Figure 5 is

observations

Fro. I.

GLOBAL

of connecting aurora1 arcs

IMAGESFOR~ PASSES ILLIJSTRATINGAURORAI.ARCSYSTEMSCDNNECTINGTOTHE DlFFUSEAURORAINTHEMIDNlGHTSECTOR.

All data have been transformed to the Corrected Geomagnetic Latitude/Magnetic Local Time coordinate grid. The orientation for all passes in the top row is as shown in the left hand frame with midnight extending along the right edge of each frame. In the bottom row the centre M.L.T. extending vertically through the middle of each frame is 0300, 2400 and 2400 M.L.T. from left to right. The intensity grey scale is 0.5-4.0 kR for all passes. (Left to right) (a) 740218 at 0418 U.T.. 5577 .&; (b) 740220 at 0545 U.T., 3914& (c) 751129 at 0522 U.T., 3914A; (d) 730204 at IO28 U.T., 5577 A: (e) 751210 at 0057 U.T., 3914 i%i;(f) 740218 at 0045 U.T., 3914 A.

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J. S. MURPHREEand L. L. C~GGER

1146

FIG.~.COORDINATETRANSFORMS ~0nTwo

PASSESDISPLAYING ~NUSUALARCSYSTEMSCONNECT1NGTOTHENIGHTSIDEDiFFUSEAURORAI,OVAL.

Left (a) 720116 at 0135 U.T. for 3914 A (1.5-10 kR grey scale); Right (b) 721012 at 0444 U.T. for 6300 A (0.4-3 kR grey scale).

.‘

i

FIG. 3. TOP:THE MEAN RATE OF SEPARATION BETWEEN EACH CONVECTING ~ORWAR~BOUNDARYOFTHEDIF~USEAURORAASAFUNCTIONOF

This plot indicates the relationship BOTTOM:

SCNEMATIC

DIAGRAM

ARC AND THE EQUA-

B,. between the orientation of the connecting arcs and B;.

SHOWING ALLOBSERVED AURORAL ~~~MKDNIGHTSECTOR.

ARC SYSTEM CONNECTIONSIN

Arc systems have been drawn from the point where they intersect the diffuse aurora! oval to where their intensity drops below 0.S kR (3914 A).

Observations

of connecting

amoral arcs

1147

FIG. 4. COORDINATETRANSFORMATION OF THE 3914 8, DATA FOR THE 740213 PASS AT 0457 U.T. The 24 M.L.T. meridian is vertically down the centre of each frame from the cross marking the pole. Left: 0.5-4 kR grey scale; Right: 1-5 kR grey scale.

i4 (a)

-DISCRETE KJ DIFFUSE

i4

ii4

(b)

(c)

FIG. 5. SCHEMATIC DRAWING ILLUSTRATING THE MAIN DISTORTIONS WHICH OCCUR WARDBOUNDARYOFAURORALEMISSIONSWHEN B, ISPOSITIVE.

IN THE POLE-

Observations

of connecting aurora1 arcs

a schema of the main variations in aurora1 morphology observed when B, is positive based on the instantaneous images described in this report. Figures 5(a) and (b) show that the distortion can be observed mainly as a dawn/dusk asymmetry in the poleward boundary of the diffuse emissions. These distributions correspond to cases where the connecting arcs form the poleward boundary of the diffuse aurora1 oval. The configuration in Fig. 5(a) where the asymmetry occurs on the evening side may occur predominantly when B, is positive (Lassen, 1979) while that shown in Fig. 5(b) may occur when B, is negative although our dataset is too small to confirm this. In Fig. S(c) the distortion in the poleward boundary reflects cases where the connecting arc system simply extends some distance into the polar cap. The observation of such distortions in the poleward boundary of emissions suggests that modifications may be necessary to the standard two cell convection pattern which normally is used to describe the electric field measurements in the high latitude ionosphere (Cauffman and Gurnett, 1972). Indeed many workers (Heppner, 1977; Burke et al., 1979; Heelis and Hanson, 1980) have found that the nightside convection pattern is highly variable when magnetic activity is low (B, positive). While it is not possible to devise a unique convection pattern based only on an instantaneous view of the aurora1 distribution, it seems clear that the expanded regions of diffuse aurora and the associated connected arcs imply an expanded region of sunward convection. This is in agreement with Lassen (1979) who proposed a three cell convection pattern to explain the distribution of quiet-time aurora1 arcs (Lassen and Danielsen, 1978) when B, was positive. The result is predominantly sunward flow in the high latitude regions. CONCLUSION

The relationship between apparently different types of aurora can only be evaluated when sufficiently sensitive and large scale observations are made. Meridian scans (either ground based or satellite) through most of these features would not reveal the connection of these features to the diffuse aurora1 oval and would incorrectly describe some of these features as polar cap arcs completely distinct (i.e. separate source and acceleration mechanism) from oval auroras. These features represent significant distortions in the poleward boundary of aurora1 emissions either as dawn/dusk asymmetry in the latitude of the emis-

1149

sions or as an extension of the connecting arc into the polar cap. Such distortions are always associated with positive B, and imply expanded regions of sunward convection into the high latitude ionosphere. Acknowledgements-The 63OOA data for Fig. 2 were provided by Dr. G. G. Shepherd. This work was supported by the National Research Council of Canada (now NSERC) grants A-7 and A-6762. REFERENCES

Akasofu, S.-I. and Yasuhara, F. (1973). Red auroras in the morning sector. J. geophys. Res. 78, 3027. Berkey, F. T., Cogger, L. L., Ismail, S. and Kamide, Y. (1976). Evidence for a correlation between sun-aligned arcs and the Interplanetary Magnetic Field direction. Geophys. Res. Lett. 3, 145. Burke, W. J., Kelley, M. C., Sagalyn, R. C., Smiddy, M. and Lai, S. T. (1979). Polar cap electric field structures with a northward Interplanetary Magnetic Field. Geophys. Res. Lett. 6, 21.

Cauffman, D. P. and Gurnett, D. A. (1972). Satellite measurements of high latitude convection electric fields. Space Sci. Rev. 13, 369. Davis, T. N. (1963). Negative correlation between polarcap visual aurora and magnetic activity. J. geophys. Res. 58, 4447.

Feldstein, Ya. I. (1963). Some problems concerning the morphology of auroras and magnetic disturbances at high latitudes. Geomagn. and Aeronomy 3, 183. Hellis, R. A. and Hanson, W. B, (1980). High-latitude ion convection in the nighttime F region. J. geophys. Res. 85, 1995.

Heppner, J. P. (1977). Empirical models of high-latitude electric fields. J. Geophys. Res. 82, 1115. Holzworth, R. H. and Meng, C.-I. (1975). Mathematical representation of the aurora1 oval. Geophys. Res. Left. 2, 377.

Ismail, S., Wallis, D. D. and Cogger, L. L. (1977). Characteristics of polar cap sun-aligned arcs. J. geophys. Res. 82, 4741. Lassen, K. and Danielsen, C. (1978). Quiet time pattern of aurora1 arcs for different directions of the Interplanetary Magnetic Field in the Y-Z plane. J. geophys. Res. 83, 5277. Lassen, K. (1979). The quiet-time pattern of aurora1 arcs as a consequence of magnetospheric convection. Geophys. Res. Lett. 6, 777.

Lui, A. T. Y., Venkatesan,

D., Anger, C. D., Akasofu, S.-I., Heikkila, W. J.,Winningham, J. D. and Burrows, J. R. (1977). Simultaneous observations of particle precipitations and aurora1 emissions by the ISIS-2 satellite in the 19-24 MLT sector. J. geophys. Res. 82, 2210. Meng, C.-I. and Akasofu, S.-I. (1976). The relation between the polar cap aurora1 arc and the aurora1 oval arc. J. geophys. Res. 81, 4004. Moshupi, M. C., Cogger, L. L., Wallis, D. D., Murphree, J. S. and Anger, C. D. (1977). Aurora1 patches in the vicinity of the plasmapause. Geophys. Res. Lett. 4, 37. Murphree, J. S. and Anger, C. D. (1980). An observation of the instantaneous optical aurora1 distribution. Can, J. Phys. 58,214.