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On separating space and time variations of auroral precipitation: Dual DMSP-F6 and -F8 observations
Adv. SpaceRes. Vol. 20, No. 3, pp. 453--456, 1997 © 1997 COSPAR. Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain 0273-I 177/97 $17.00 + 0.00 PII: S0273-1177(97)00710-2
Pergamon
ON SEPARATING SPACE AND TIME VARIATIONS OF AURORAL PRECIPITATION: DUAL DMSP-F6 A N D -F8 OBSERVATIONS A. M. Jorgensen and H. E. Spence Centerfi)r Space Physics and Department of Astronomy, Boston University, 725 CommzmwealthAvenue, Boston, MA 02215, U.S.A.
ABSTRACT After the launch of DMSP-FS, into an almost identical orbit to DMSP-F6, there occurred periods every approximately 11 days when the two satellites were very close to each other, thereby allowing for distinguishing between temporal and spatial effects on measured auroral precipitation. We present observations of one such period, when we were able to resolve space and time through multi-point measurements. In this period, the DMSP satellites observed the same general auroral zone precipitation pattern. However one large-scale feature changed vastly in the two minute time separation between the satellites. We demonstrate how this feature can be identified and mapped using two low altitude satellites together with ground magnetic field and radar data. This example underscores the utility of "clustered" satellite constellations at low earth orbit for auroral observations. © 1997 COSPAR. Published by Elsevier Science Ltd.
INTRODUCTION Approximately every 11 days the DMSP-F6 and -F8 positions almost coincided as -F6 overtook -F8 owing to its slightly smaller orbit. When these close encounters occured in the auroral zone, there was the potential for particularly interesting multipoint measurements that would allow for the separation of temporal effects from spatial effects. One of these events (April 4, 1990) was discussed by Waterman et al (1993) in the context of the two satellites passing through a radar's field of view. They concluded that the auroral precipitation showed little variation during the 0-4 see time delay between DMSP-F6 and -F8. The importance of multi-point measurements has been stressed a great deal recently, and was the main focus of the CLUSTER project in the region 4 to 20 Re. Multi-point measurements can also be important at low altitudes in the auroral zone. In the following we will show that auroral variation on minute timescales can have a significant impact on scientific studies, and if not taken into account, could lead to erroneous conclusions. DATA SET/OBSERVATIONS For this study, we focus on an event that occurred on September 26, 1989. At the time, Kp was 5, and the magnetosphere was in the main phase of a magnetic storm, Dst was -75 nT, and the storm reached a minimum of150 nT. A multiple onset substorm had started an hour earlier at 11 UT. We will be presenting data from the SSJ/4 and the SSIES instruments on DMSP-F6 and -F8, as well as the IDM instrument on -F8 (Rich et al., 1985). The satellites DMPS-F6 and -F8 overflew the auroral zone at approximately 12:30 UT. F6 was at an altitude of approximately 815 km while F8 was at an altitude of approximately 850 km. The satellite tracks were separated by only a few minutes in magnetic local time, and the time delay between them arriving at a given magnetic latitude was close to two minutes. Under these circumstances, one would normally expect the large-scale precipitation signatures from the two satellites to be almost indistinguishable. 453
454
A.M. Jorgensen and H. E. Spence
D M S P - F 8 Ion Drift Meter 2
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Figure I. (a) Spectrograms of precipitating electrons and protons for DMSP-F6 and -F8. Both spacecraft can be seen to enter the electron aurora at 56 degrees latitude, and leave it again at 72 degrees. Between 60 and 63 degrees latitude F6 measured enhanced precipitation, but F8 - which passed 2 minutes later - measured a depletion. (b) The -F8 drift meter data shows two regions of enhanced eastward flow, the more southward one coincident with the region of depleted precipitation. The flow is generally eastward throughout the auroral zone up to the convection reversal boundary just south of the poleward edge of the aurora. The long-dashed lines mark a region that also displays enhanced flows.
Separating Space and Time in the Auroral
455
In the present case, however, the precipitation signatures showed some major differences. Figure la shows energytime spectrograms for precipitating electrons and protons for both satellites. Both satellites entered the auroral zone (as marked by electron precipitation) at approximately 55 degrees magnetic latitude, and left it again at 72 degrees magnetic latitude. F6 measured strongly enhanced precipitation between 60 and 63 degrees magnetic latitude. Two minutes later F8 cut through those same latitudes, and measured a significantly reduced precipitation compared to the rest of the auroral zone precipitation. F8 is also equiped with an ion drift meter (IDM), which is capable of measuring vertical ion drifts as well as cross-track ion drifts (approximately east-west). Figure lb shows the F8IDM data for approximately the same time interval. On this graph we have marked with dotted lines from left to fight: the southern edge of the aurora; the southern edge of the anomalous precipitation region; the northern edge of the anomalous precipitation region; and the northern edge of the aurora. Note that the convection reversal boundary and the northern edge of the aurora do not coincide. The region between the two likely maps to the low latitude boundary layer° DISCUSSION It is interesting to note how well the eastward flow region coincides with the region of enhanced/decreased precipitation. In fact this leads us to conclude that the two are electrodynamically coupled. From the IDM data we have identified and marked another flow region (flow channel) with dashed lines, and on the spectrogram one can in fact see weak signatures of it. From these and ancillary data, we conclude that in both flow channels, alternating auroral bright patches and dark regions are flowing east at the velocity measured by the DMSP-F8 IDM, which
12:30:00 UT, September 26, 1989
Figure 2. Here we have plotted the flow and precipitation boundaries (identified in Figure 1) encountered by the DMSP satellites onto a satellite projection map. The solid boundaries are encountered by F6, whereas the dashed ones are encountered by F8. For F6 the boundaries are (from bottom to top): equatorward edge of aurora, equatorward edge of enhanced precipitation, poleward edge of enhanced precipitation, poleward edge of aurora. For F8 the first three boundaries are the same (except that it is a depletion), the next two lines mark the poleward enhanced flow region marked as two dashed lines on Figure 1b, and the last boundary is the poleward edge of the aurora.
456
A.M. Jorgensen m~d H. E. Spence
amounts to approximately 1 km/s. Bright aurora is caused by strong precipitation, whereas dark regions are the result of weakened precipitation. Inspection of the difference between the -F6 and -F8 spectrograms leads to the above conclusion. We have been able to relate this difference in precipitation signatures to other measurements, both ground based and space based. Figure 2 presents a sketch of the different data sources, and the identified DMSP boundaries. It should be noted that these features are well south of the convection reversal boundary, suggesting that they map to the inner magnetosphere. In a separate paper we will show that all these data sources are consistent with auroral fib band features (See e.g. Akasofu et al. (1964) or Rostoker et al. (1981)) observed both on the ground by magnetometer, riometer and radar; at low altitude by the two DMSP satellites, and at near geostationary orbit by GOES-6, GOES-7 and SCATHA. In that paper we conclude that the auroral signature of O-bands map to extremely localized and intense field rotations near geostationary orbit. Let us for a moment investigate another important consequence of the dual measurement. If only the -F6 pass is used, one would identify the boundaries of the auroral zone to be at approximately 56 and 72 degrees latitude. With the -F8 data only, the boundaries could mistakenly be identified at 63 and 72 degrees - a possible discrepancy of 8 degrees on the equatorward boundary location. This discrepancy could potentially cause severe problems for models which make use of such information as input parameters. With both satellite measurements we are able to separate out the space and time variations, and even infer longitudinal auroral structure. It is important to stress that this interesting event was identified initially by virtue of the unexpectedly large differences in auroral precipitation morphology witnessed by the two DMSP satellites. Each satellite measured quite typical auroral zone precipitation that would not necessarily have stood out as especially unusual. However, when combined these surprisingly different time series yield a much more comprehensive view of a dynamic auroral event. In the end, the dual DMSP data set revealed an event that is now allowing us to better understand auroral f2-bands and their mapping to the inner magnetosphere. CONCLUSION We have demonstrated the value of two-point measurements of the (well-known) dynamics of auroral precipitation on minute timescales. Single satellite measurements will always suffer from the space/time ambiguity. This event emphasizes the importance of separating the two, and the possibility of reaching erroneous scientific conclusions by interpreting a single pass or a single point measurement without knowledge of the time evolution of the phenomenon. ACKNOWLEDGMENTS We wish to thank Drs. T. J. Hughes and D McDiarmid for providing the ancillary ground data not shown, as well as invaluable discussion and interpretation. This work was supported by NASA grant NAGW-3953. REFERENCES Akasofu, S.-I., D. S. Kimball. The dynamics of the aurora - I Instabilities of the aurora, J. Geophys. Res., 26,205211, 1964. Rich, F. J., D. A. Hardy, M. S. Gussenhoven, Enhanced ionosphere-magnetosphere data from the DMSP satellites, Eos, 66, 513, 1985. Rostoker, G., K. S. Apps. Current flow in auroral forms responsible for Ps 6 magnetic disturbances, J. Geophys., 49, 163-168, 1981. Watermann, J., O. De la Beaujardiere, H. E. Spence, Space-time structure of the morning aurora inferred from coincident DMSP-F6, -F8 and Sondrestrom incoherent scatter radar observations, J.Atmosp. Terr. Phys. 55, 14, 1729-1739, 1993.
Pergamon
ON SEPARATING SPACE AND TIME VARIATIONS OF AURORAL PRECIPITATION: DUAL DMSP-F6 A N D -F8 OBSERVATIONS A. M. Jorgensen and H. E. Spence Centerfi)r Space Physics and Department of Astronomy, Boston University, 725 CommzmwealthAvenue, Boston, MA 02215, U.S.A.
ABSTRACT After the launch of DMSP-FS, into an almost identical orbit to DMSP-F6, there occurred periods every approximately 11 days when the two satellites were very close to each other, thereby allowing for distinguishing between temporal and spatial effects on measured auroral precipitation. We present observations of one such period, when we were able to resolve space and time through multi-point measurements. In this period, the DMSP satellites observed the same general auroral zone precipitation pattern. However one large-scale feature changed vastly in the two minute time separation between the satellites. We demonstrate how this feature can be identified and mapped using two low altitude satellites together with ground magnetic field and radar data. This example underscores the utility of "clustered" satellite constellations at low earth orbit for auroral observations. © 1997 COSPAR. Published by Elsevier Science Ltd.
INTRODUCTION Approximately every 11 days the DMSP-F6 and -F8 positions almost coincided as -F6 overtook -F8 owing to its slightly smaller orbit. When these close encounters occured in the auroral zone, there was the potential for particularly interesting multipoint measurements that would allow for the separation of temporal effects from spatial effects. One of these events (April 4, 1990) was discussed by Waterman et al (1993) in the context of the two satellites passing through a radar's field of view. They concluded that the auroral precipitation showed little variation during the 0-4 see time delay between DMSP-F6 and -F8. The importance of multi-point measurements has been stressed a great deal recently, and was the main focus of the CLUSTER project in the region 4 to 20 Re. Multi-point measurements can also be important at low altitudes in the auroral zone. In the following we will show that auroral variation on minute timescales can have a significant impact on scientific studies, and if not taken into account, could lead to erroneous conclusions. DATA SET/OBSERVATIONS For this study, we focus on an event that occurred on September 26, 1989. At the time, Kp was 5, and the magnetosphere was in the main phase of a magnetic storm, Dst was -75 nT, and the storm reached a minimum of150 nT. A multiple onset substorm had started an hour earlier at 11 UT. We will be presenting data from the SSJ/4 and the SSIES instruments on DMSP-F6 and -F8, as well as the IDM instrument on -F8 (Rich et al., 1985). The satellites DMPS-F6 and -F8 overflew the auroral zone at approximately 12:30 UT. F6 was at an altitude of approximately 815 km while F8 was at an altitude of approximately 850 km. The satellite tracks were separated by only a few minutes in magnetic local time, and the time delay between them arriving at a given magnetic latitude was close to two minutes. Under these circumstances, one would normally expect the large-scale precipitation signatures from the two satellites to be almost indistinguishable. 453
454
A.M. Jorgensen and H. E. Spence
D M S P - F 8 Ion Drift Meter 2
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12:24
12:26
12:28
12:30 12:32 12:34 UT (Hours) S e p t e m b e r 26, 1989
12:36
12:38
Figure I. (a) Spectrograms of precipitating electrons and protons for DMSP-F6 and -F8. Both spacecraft can be seen to enter the electron aurora at 56 degrees latitude, and leave it again at 72 degrees. Between 60 and 63 degrees latitude F6 measured enhanced precipitation, but F8 - which passed 2 minutes later - measured a depletion. (b) The -F8 drift meter data shows two regions of enhanced eastward flow, the more southward one coincident with the region of depleted precipitation. The flow is generally eastward throughout the auroral zone up to the convection reversal boundary just south of the poleward edge of the aurora. The long-dashed lines mark a region that also displays enhanced flows.
Separating Space and Time in the Auroral
455
In the present case, however, the precipitation signatures showed some major differences. Figure la shows energytime spectrograms for precipitating electrons and protons for both satellites. Both satellites entered the auroral zone (as marked by electron precipitation) at approximately 55 degrees magnetic latitude, and left it again at 72 degrees magnetic latitude. F6 measured strongly enhanced precipitation between 60 and 63 degrees magnetic latitude. Two minutes later F8 cut through those same latitudes, and measured a significantly reduced precipitation compared to the rest of the auroral zone precipitation. F8 is also equiped with an ion drift meter (IDM), which is capable of measuring vertical ion drifts as well as cross-track ion drifts (approximately east-west). Figure lb shows the F8IDM data for approximately the same time interval. On this graph we have marked with dotted lines from left to fight: the southern edge of the aurora; the southern edge of the anomalous precipitation region; the northern edge of the anomalous precipitation region; and the northern edge of the aurora. Note that the convection reversal boundary and the northern edge of the aurora do not coincide. The region between the two likely maps to the low latitude boundary layer° DISCUSSION It is interesting to note how well the eastward flow region coincides with the region of enhanced/decreased precipitation. In fact this leads us to conclude that the two are electrodynamically coupled. From the IDM data we have identified and marked another flow region (flow channel) with dashed lines, and on the spectrogram one can in fact see weak signatures of it. From these and ancillary data, we conclude that in both flow channels, alternating auroral bright patches and dark regions are flowing east at the velocity measured by the DMSP-F8 IDM, which
12:30:00 UT, September 26, 1989
Figure 2. Here we have plotted the flow and precipitation boundaries (identified in Figure 1) encountered by the DMSP satellites onto a satellite projection map. The solid boundaries are encountered by F6, whereas the dashed ones are encountered by F8. For F6 the boundaries are (from bottom to top): equatorward edge of aurora, equatorward edge of enhanced precipitation, poleward edge of enhanced precipitation, poleward edge of aurora. For F8 the first three boundaries are the same (except that it is a depletion), the next two lines mark the poleward enhanced flow region marked as two dashed lines on Figure 1b, and the last boundary is the poleward edge of the aurora.
456
A.M. Jorgensen m~d H. E. Spence
amounts to approximately 1 km/s. Bright aurora is caused by strong precipitation, whereas dark regions are the result of weakened precipitation. Inspection of the difference between the -F6 and -F8 spectrograms leads to the above conclusion. We have been able to relate this difference in precipitation signatures to other measurements, both ground based and space based. Figure 2 presents a sketch of the different data sources, and the identified DMSP boundaries. It should be noted that these features are well south of the convection reversal boundary, suggesting that they map to the inner magnetosphere. In a separate paper we will show that all these data sources are consistent with auroral fib band features (See e.g. Akasofu et al. (1964) or Rostoker et al. (1981)) observed both on the ground by magnetometer, riometer and radar; at low altitude by the two DMSP satellites, and at near geostationary orbit by GOES-6, GOES-7 and SCATHA. In that paper we conclude that the auroral signature of O-bands map to extremely localized and intense field rotations near geostationary orbit. Let us for a moment investigate another important consequence of the dual measurement. If only the -F6 pass is used, one would identify the boundaries of the auroral zone to be at approximately 56 and 72 degrees latitude. With the -F8 data only, the boundaries could mistakenly be identified at 63 and 72 degrees - a possible discrepancy of 8 degrees on the equatorward boundary location. This discrepancy could potentially cause severe problems for models which make use of such information as input parameters. With both satellite measurements we are able to separate out the space and time variations, and even infer longitudinal auroral structure. It is important to stress that this interesting event was identified initially by virtue of the unexpectedly large differences in auroral precipitation morphology witnessed by the two DMSP satellites. Each satellite measured quite typical auroral zone precipitation that would not necessarily have stood out as especially unusual. However, when combined these surprisingly different time series yield a much more comprehensive view of a dynamic auroral event. In the end, the dual DMSP data set revealed an event that is now allowing us to better understand auroral f2-bands and their mapping to the inner magnetosphere. CONCLUSION We have demonstrated the value of two-point measurements of the (well-known) dynamics of auroral precipitation on minute timescales. Single satellite measurements will always suffer from the space/time ambiguity. This event emphasizes the importance of separating the two, and the possibility of reaching erroneous scientific conclusions by interpreting a single pass or a single point measurement without knowledge of the time evolution of the phenomenon. ACKNOWLEDGMENTS We wish to thank Drs. T. J. Hughes and D McDiarmid for providing the ancillary ground data not shown, as well as invaluable discussion and interpretation. This work was supported by NASA grant NAGW-3953. REFERENCES Akasofu, S.-I., D. S. Kimball. The dynamics of the aurora - I Instabilities of the aurora, J. Geophys. Res., 26,205211, 1964. Rich, F. J., D. A. Hardy, M. S. Gussenhoven, Enhanced ionosphere-magnetosphere data from the DMSP satellites, Eos, 66, 513, 1985. Rostoker, G., K. S. Apps. Current flow in auroral forms responsible for Ps 6 magnetic disturbances, J. Geophys., 49, 163-168, 1981. Watermann, J., O. De la Beaujardiere, H. E. Spence, Space-time structure of the morning aurora inferred from coincident DMSP-F6, -F8 and Sondrestrom incoherent scatter radar observations, J.Atmosp. Terr. Phys. 55, 14, 1729-1739, 1993.