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The UV auroral distribution: Its impulsive nature
Adv. Space Res. Vol. 10, No. 6, pp. (6)167—(6)177, 1990 Printed in Great Britain. All rights reserved
0273—1177/90 $0.00 + .50 Copyright © 1989 COSPAR
THE UV AURORAL DISTRIBUTION: ITS IMPULSIVE NATURE L. L. Cogger and J. S. Murphree Department of Physics, The University of Calgary, Calgary, Alberta 72N 1N4, Canada
ABSTRACT
The UV Imager on the Swedish Viking satellite obtained near—instantaneous (1 second exposure) spatial distributions of the entire Northern Hemisphere aurora. The short exposure time coupled with a repetition rate as rapid as three images per minute permitted scientists to observe rapid variations of the aurora in space and time. Numerous examples of impulsive changes were recorded during the 1986 emission, and many of these were of a sufficient scale that they can be expected to affect the high latitude thermosphere. Such events are not uncommon and thus present a difficult challenge for global thermospheric modelers who desire realistic auroral input parameters for their models. One important aspect is the contribution of auroral energy input not only on the nightside due to substorm effects, but impulsive events on the dayside as well. The purpose of this paper is to acquaint scientists with the dramatic variations observed by presenting selected examples of the image data. INTRODUCTION Although a great amount of knowledge of the Earth’s atmosphere, magnetosphere and the solar wind has been acquired since the development of scientific satellites, there remains much to learn about the processes that couple energy from one region to another. These processes are not only complex in nature, but they vary dramatically with time and position. One natural indicator of magnetospheric processes is the global distribution of optical aurora; the upper atmosphere serves as a screen which provides indirect, but valuable, information about the incoming energy flux that would be impossible to acquire with in situ measurements either on the ground or in space, and for this reason considerable effort has been expended on obtaining global auroral images from satellites. Ideally, such images should be instantaneous views of the aurora, at a number of pertinent wavelengths covering both the dayside and nightside of the earth with temporal and spatial resolution consistent with the scale size of the relevant magnetospheric processes. Aeronomers could utilize these images in the computation of auroral—driven effects in the ionosphere and thermosphere. Such an ideal instrument does not exist at present due to obvious technical difficulties; however, considerable progress has been achieved since the first auroral imager was flown on the ISIS 2 satellite in 1971 /1/. A number of imaging instruments have been designed for low—altitude polar orbiting satellites: ISIS 2, the DMSP series /2/, HILAT /3/, Polar Bear. These acquire images over an auroral region only once per orbit because they use the orbital motion to provide one spatial dimension and either the satellite spinning motion or a moving mirror for the second dimension. Usually only partial views of the auroral distribution are possible due to low satellite altitude, but the spatial resolution can be very good. Other imagers have been designed for high—latitude satellites: KYOKKO /4/, DE 1 /5/, Viking /6, 7/. These can acquire continuous sequences of images of the entire auroral distribution (single hemisphere) over ‘periods’ of one or more hours. The DE instrument utilizes narrow—field photometers and a combination of the spinning satellite and a scanning mirror to obtain images of the earth at a repetition rate of one every 6 to 12 minutes. The KYOKKO instrument viewed the entire field by means of an image tube, and use a rotating mirror to compensate for the satellite spin. The Viking instrument also viewed the entire field, but used an intensifier CCD array detector to both provide the digital image and to compensate for the satellite spin. The latter two instruments could obtain images at rates up to the spin period, 2 minutes for KYOKKO and 20 seconds for Viking. A common feature of many instruments Is the capability of
L. L. Cogger and J. S. Murphree
(6)168
The purpose of this paper is not to answer questions related to the cause of these events, but to describe the characteristics that would likely affect the global dynamics of the thermosphere. AURORAL VARIATIONS
It is well known that the auroral distribution exhibits major variations in morphology and intensity due to the coupling of the earth’s magnetosphere to the sun by means of the solar wind. Investigations of the aurora from ground stations has led to the development of statistical ovals that are representative of various magnetic conditions, e.g. Feldsteiri and Starkov /8/. While such representations are useful for modelling purposes, it is recognized that the actual auroral distribution at any time can be very different and can change dramatically over short time scales. The examples that follow have been selected to illustrate some of the kinds of variations that occur over time scales from days to minutes and which can have significant implications for thermospheric modelling. The first example illustrates the dramatic dependence of the auroral distribution on the general level of magnetic activity as depicted by the Kp index. Because this is a three—hour index, it is most useful for time scales from hours to days. During the period July 24 to 26, 1986, the Kp index ranged from a low of 0+ to a high of 6, as shown in Fig. 1. Representative images of the aurora are shown in Fig. 2 for 6 of the orbits during that period. All the images shown in this paper were taken with the LBH camera and are presented as a false—colour picture. Latitudes and times shown on the images are in eccentric dipole latitude/magnetic local time coordinates. In Fig. 2 the terminator lies along a line from the top left to bottom right in each image and for the cases of weaker auroral emissions the dayglow obscures those emissions. For the quietest periods, represented by the images for orbits 840 (860724/143159 UT) and 846 (860725/165215 UT), the auroral oval was weak throughout and essentially below the threshold values chosen for Fig. 2. Orbits 843 (860725/035936 UT) and 849 (860726/054157 UT) correspond to moderate magnetic disturbance, and the images for orbits 842 (860724/233558 UT) and 847 (860725/205405 UT) were taken during very disturbed periods. In the latter cases, the optical distribution was much brighter and thicker and dominated by discrete features. It is clear that the instantaneous oval can reflect the gross magnetic activity as described by the Kp index although as will be shown below the rapidity with which large scale optical features change may not be adequately represented in the normal indices used to describe auroral activity.
ORBIT t 840
842 843
846 847
849
~JULY (date) Figure 1: The variation of the Kp index during July 24—26, 1986. in Fig. 2 were taken at the times marked by an X.
The images shown
On a shorter time scale than 3 hours, the isolated auroral substorm is a feature that is associated with an impulsive injection of energetic particles into the upper atmosphere. These are easily identified in auroral images. The sequence in Fig. 3 for 1 April, 1986 consists of 9 selected images from the period 1831 to 1914 UT. The substorm developed from a bright intensification near 23 MLT around 185055 UT into an expanding region of precipitation. A helpful description of this event was prepared by Rostoker et al. /9/. Fig 4 depicts the change in brightness and area as a function of time using images separated by about two minutes. The brightness is shown as the average signal level in data numbers (DN)
Figure 2: A sequence of images for the period July 24—26, 1986 selected to illustrate the range of auroral activity present. The dates and UT are shown in the bottom of each image; the latitudes are given in eccentric dipole magnetic coordinates; and the times on the image are magnetic local time. Red corresponds to the highest signal levels.
Figure 3~ A sequence of nine images showing the variation of the nightside aurora during a period of 45 minutes on 1 April 1986. The circles to the east and west of the substorm indicate the locations of Dixon Island and Kiruna respectively.
The
UV Auroral
Distribution
(6)171
signal level is somewhat energy—dependent due to absorption by 0~. The effect is not severe for primary electron energies in the range of 1 to several keV /12/, but for larger energies serious error would be introduced. There is also a small contribution to the LBH signal from other species, and their energy dependence may be somewhat different. With these cautions, the reader may wish to multiply signal levels given in the diagrams in DN by the factor .11 to obtain a rough estimate of the energy flux. The initial peak in average intensity prior to 184115 UT was due to the polar arc connection seen in the images from 183149 UT to 183951 UT. Such intensifications are quite common /13/ and themselves represent a non trivial source of energy input during quiet times. D~ring the period of expansion, area and 2 per mm. The product of the the area increased at a rate approximately 0.1indicator x 10 km of the variation in the total energy average signal level ofcan be used as an deposition rate, i.e. power. This product, plotted in Fig. 5, shows that the rate of energy input increased from onset to at least the.end of the satellite pass at 1915 UT. Other examples of isolated substorms in the Viking data base are consistent with the development for orbit 214 and provide evidence that although these substorms can be considered to be impulsive events, the precipitated energy does not increase in a step—function manner that would be convenient for the theoretical prediction of thermospheric effects. SUBSTORM GROWTH. ORBIT 214
184115
1852~
190345
111 (~amssUT) Figure 4: The area and average signal level variation for the substorm development shown in Fig. 3 SUBSTORM GROWTH, ORBIT 214 a
183~~0
184115
185220
190345
1915g~
IDE ll’lamss UT)
Figure 5: The product of the two plots given in Fig. 4. The values here represent the variation in the rate of energy deposition into the substorm bulge. There are other interesting aspects of this sequence that are relevant to thermospheric studies. The location of the substorm onset was east of the Kiruna magnetometer station and west of the Dixon Island station. This likely explains why the 11—station AE index shown in Fig. 6 did not increase sharply until about 1900 UT (about 12 minutes after the onset was observed from the Viking satellite) when the substorm expanded sufficiently for the ionospheric currents to be measured by the stations. The conclusion from this is that the AE index may not always be a suitable reference for timing the start of isolated substorms.
(6)172
L. L. Cogger and J. S. Murphree
indication of the time required for the high latitude convection to response to the IMP. the arc is an indicator of the poleward edge of the plasmasheet, then the area of open magnetic field lines expanded significantly with the disappearance of the arc.
:::°
If
~L~_L_Li 7~.LLJ. T~ ~
-
10
15
20
UT
Figure 6: The provisional AE index for 1 April 1986 obtained from the magnetogram records at 11 stations, and the IMP B component obtained from the IMP 8 satellite measurements on the same day. Four orbits after the previous example of substorm development the satellite was in a position to view another such event near 12 UT on 2 April 1986. This was a very quiet period, with Kp = 1, the IMP magnitude steady at about 5 nT, and B near 0. Images from this pass have been published elsewhere /14/; here the discussion is li~itedto the temporal development. The area covered by the auroral bulge expanded for about 15 mm. then contracted for the next 15 minutes before a second expansion commenced at about 1240 UT. The average intensity increased sharply to a maximum at the beginning of the substorm, and thereafter subsided. From this variation, the inferred energy input rate shown in Fig. 7 increased for the first 10 mm. and then decreased for the remainder of the substorm. Differences in the development of this substorm and the previous example are evident, but the main features appear to be quite similar. The expansion of this type of auroral event takes place both poleward and longitudinally, reaching typical dimensions of 10 deg. in latitude and 80 degrees in longitude within about 15 mm. The energy input rate reaches a maximum somewhat earlier than the area. Short term variations during a disturbed period are illustrated in Fig. 8 using orbit 842 (24 July 1986). It is clear that a major change in the auroral distribution took place between the image at 2301 UT and 2317 UT. The auroral distribution brightened and expanded in the dawn and dusk sectors, with relatively little response in the midnight sector. This contradicts the usual picture of aurora in which the maximum activity occurs near midnight. The abrupt change in the IMP (obtained from the IMP 8 satellite) that occurred at approximately 2310 UT is shown in Fig. 9. The area of the auroral distribution has been measured both before and after the northward turning of B ; changes for three local time sectors are shown in Fig. 10. The midnight region, 23.5 ~o 0.5 MLT, remained fairly uniform in width and brightness until just after 2342 UT when a rapid expansion began. The segment SUBSTORM GROWTH, ORBIT 218 a
__________________________________________
Figure 8: A sequence of auroral images from 24 July 1986 illustrating the dramatic changes in the auroral oval over the period of 2250 to 2336 UT. The impulsive broadening of the oval in the dawn and dusk sectors occurred at about 2310 UT.
Figure 12: An example from 27 September 1986 showing very bright and dynamic dayside aurora.
The
UV Auroral Distribution
(6)175
of the auroral distribution bounded by 18.5 and 19.5 8fl~T began to expand rapidly at about 2310 UT; the maximum width occurred at about 2337 UT and then it began to contract. The dawnside of the distribution expanded and contracted in a similar fashion, but it was not as intense. The image at 233556 UT gives the impression that the dawn side was broader than the dusk, but this appearance is due to the viewing direction. The corresponding plots for relative energy deposition rate are given in Figs. 11. The energy deposition rate was greatest in the 18.5 and 19.5 MLT sector throughout the period. This sequence of events is explainable in terms of the sudden change in the IMP at 2310 UT that caused an immediate perturbation of the earth’s magnetic field giving rise to the auroral enhancements in the 860724 orbit 842
- ~
~
231000
325500
232500
711 )Nimi~s IT)
Figure 9: The components B , B , and B of the IMP for the period covered by the auroral images in Fig. 9. ~he ~MP data were obtained from measurements on the IMP 8 satellite. dawn and dusk sectors, followed by an nagnetotail unloading event some 30 mm. later that caused the disturbance in the midnight sector. Rapid auroral response to IMF impulsive events has been described by Craven et al. /15/ using DE 1 images. It is interesting to note that the directly—driven auroral enhancements continued to expand in area until about 5 mm. before the unloading event commenced, and the two types thereafter coexisted.
ARtA C~ANGtIN 3
RCGIDNS.
ORBIT
842
4.5 mIt - 5.5 mit
23.5 mit
N
-
es
cit
~1t-19.5mit
225t02
230738
TIet
232500 U,~,assUT)
234230
240222
Figure 10: Area of portions of the oval bounded by 15 degrees in longitude for one— hour time sectors in the morning, evening, and at midnight during the period 2250 to 2350 UT on 24 July 1986.
(6)176
L. L. Cogger and J. S. Murphree
ENERGY CHANGE
IN 3 REGIONS.
a
4.5 mIt
Ui
s~ a a, 5-—
..~
5.5 mIt
-
X>~
_
23.5 mIt
-
0.5 nIt
18.5 tilt
-
19.5 mIt
I
a
225000
ORBIT 842
230730
232500
234230
240800
Figure 11: The relative variation in the energy deposition rate into 3 sectors of the auroral oval during the period 2250 to 2350 UT on 24 July 1986. followed by a return to lower intensities. By 024607 UT the disruption reached its maximum extent, approximately 500 km along the 14 MLT meridian. Subsequently the curvature of the auroral distribution relaxes so that after 0250 UT the afternoon sector appeared as normal. In fact after this tine the maximum optical activity shifted to the morning sector (see Fig. 12.). Such clear modifications of the auroral distribution represent significant energy input outside of the normal statistical oval often used by modelers. DISCUSSION Significant advances in our understanding of thermospheric dynamics have come about through a combination of observational programs and the development of increasingly realistic numerical simulations of the global thermosphere. One of the main challenges over the past ten years has been to evaluate the high latitude sources of momentum and heating resulting from magneto— spheric processes. A comprehensive review of the developments in thermospheric modelling has been given by Rees et al. /16/. Improvements in empirical neutral models /17/, ionospheric models, e.g. /18, 19/, polar electric field models /20, 21/, and precipitation models, e.g. /22/ have been incorporated into self—consistent simulations. Major consequences of the magnetospheric—induced high latitude thermospheric/ionospheric processes have been identified and better understood through the use of these models, e.g. /16, 23/. Advances in observational techniques have permitted the measurement of high latitude neutral winds, temperature and composition that have served as tests for the simulation models, e.g. /24/. Refinements such as UT dependence and the effects of variation of the IMP are being incorporated. As the models become more inclusive, and the observations more comprehensive, the need to account for the temporal variation of the magnetospheric source becomes apparent. A recent study by Mazaudier et al. /25/ has been successful in calculations of the main properties of the thermospheric wind from magnetic storms using the AZ index as a means to estimate the Joule heating that was assumed to drive the winds. As our physical understanding of the fundamental processes develops, it will be necessary to monitor with increased resolution, the actual auroral distribution. It is no surprise that the instantaneous geographic distribution of aurora rarely, if ever, conforms to a statistical model. Moreover, we have shown in this paper that with improvements in temporal resolution, the dynamic nature of aurora appears ever more complex. Large changes in the global energy deposition take place in short intervals of time; the location of the auroral precipitation can rapidly move in latitude, longitude, and even switch from one tine sector to another. We have shown evidence that both directly—driven and loading—unloading types of aurora can occur simultaneously. CONCLUSION The employment of auroral imagers from space has led to major advances in the understanding of the magnetosphere. It is clear that global images of the instantaneous auroral distribution can also provide major support to the study of the thermosphere. Sequences of
.
The LIV Auroral Distribution
(6)177
ACKNOWLEDGEMENTS We wish to acknowledge the assistance of R. Elphinstone and J. Smith in the preparation of the figures. The IMP data were provided by R. Lepping. This research is supported by grants from the Natural Sciences and Engineering Council of Canada. REFERENCES
1.
C.D. Anger, T. Fancott, J. McNally, and 11.5. Kerr, ~ppl. Opt.
12, 1753, (1973)
2.
E.H. Rogers, D.F. Nelson, and R.C. Savage, Science, 183, 951, (1974)
3.
C.—I. Meng and R.E. Huffman, Geophys. Res. Lett.
4.
K. Hirao and T. Itoh,
5.
L.A. Frank, J.D. Craven, K.L. Ackerson, M.R. English, R.H. Eather, and R.L. Carovillano,
11, 315, (1984)
Solar Terr. Env. Res. in Japan, 2, 148, (1978)
Space Sci.Inst. 5, 369, (1981) 6.
C.D. Anger, S.K. Babey, A.L. Broadfoot, R.G. Brown, L.L. Cogger, R. Gattinger, J.W. Haslett, R.A. King, D.J. McEwen, J.S. Murphree, E.H. Richardson, W.R. Sandel, K. Smith, and A. Vallance Jones, Geophys Res. Lett. 14, 387, (1987)
7.
R.A. King, A.L. Broadfoot, B.R. Sandel, and A. Vallance Jones, App. (1988)
B.
Y.I. Feldstein and G.V. Starkov, Planet. Space Sci. 15, 209, (1967)
9.
C. Rostoker, A. Vallance Jones, R.L. Gattinger, C.D. Anger and J.S. Murphree, Res Lett., 14, 399, (1987)
opt.,
27, 2048,
Geophys
10.
A. Vallance Jones, R.L. Gattinger, F. Creutzberg, R.A. King, P. Pikryl, L.L. Cogger, D.J. McEwen, F.R. Harris, C.D. Anger, J.S. Murphree, and R.A. Koehier, Geophys. Res. Lett., 14, 391, (1987)
11.
M.H. Rees and R.G. Roble, Can. J. Phys., 64, 1608,
12.
D.J. Strickland, J.R. Jasperse and J.A. Whalen, J. Geophys. Res., 88, 8051, (1983)
13.
J.S. Murphree, L.L. Cogger, C.D. Anger, D.D. Wallis, and G.G. Shepherd, Geophys. Res. Lett., 14, 403, (1987)
14.
Cogger, L.L., J.S. Murphree and C.D. Anger, Physica Scripta, 37, 432, (1988)
15.
J.D. Craven, L.A. Frank, C.T. Russell, E.J. Smith and R.P Lepping, Solar Wind—Magnetosphere Coupling, edited by Y. Kamide and J.A. Slavin, D. Reidel Publishing Co., Dordecht, 367, (1986)
16.
D. Rees, T.J. Fuller—Rowel]., S. Que gan, R.J. Moffett and G.J. Bailey, Ann. Geophys., 5A, 303, (1987)
17.
A.E. Hedin, J. Geophys. Res., 92, 4649, (1987)
18.
J.J. Sojka and R.W. Schunk, J. Geophys. Res., 90, 5285, (1985)
19.
M.F. Smith, D. Rees and T.J. Fuller—Rowe].]., Planet. Space Sci., 30, 1259, (1982)
20.
J.P. Heppner and N.C. Maynard, J. Geophys. Res., 92, 4467, (1987)
21.
R.A. Heelis, J.K. Lowell and R.W. Spiro, J. Geophys. Rca., 87, 6339, (1982)
22.
T.J. Fuller—Rowell and D.S. Evans, J. Geophys. Res., 7606, (1987)
23.
R.G. Roble and E.C. Ridley, Ann. Geophys. 5A, 369, (1987)
24.
T.L. Killeen, P.B. Hays, N.W. Spencer, and L.E. Wharton, Geophys. Res. Lett. 9, 957, (1982)
(1986)
0273—1177/90 $0.00 + .50 Copyright © 1989 COSPAR
THE UV AURORAL DISTRIBUTION: ITS IMPULSIVE NATURE L. L. Cogger and J. S. Murphree Department of Physics, The University of Calgary, Calgary, Alberta 72N 1N4, Canada
ABSTRACT
The UV Imager on the Swedish Viking satellite obtained near—instantaneous (1 second exposure) spatial distributions of the entire Northern Hemisphere aurora. The short exposure time coupled with a repetition rate as rapid as three images per minute permitted scientists to observe rapid variations of the aurora in space and time. Numerous examples of impulsive changes were recorded during the 1986 emission, and many of these were of a sufficient scale that they can be expected to affect the high latitude thermosphere. Such events are not uncommon and thus present a difficult challenge for global thermospheric modelers who desire realistic auroral input parameters for their models. One important aspect is the contribution of auroral energy input not only on the nightside due to substorm effects, but impulsive events on the dayside as well. The purpose of this paper is to acquaint scientists with the dramatic variations observed by presenting selected examples of the image data. INTRODUCTION Although a great amount of knowledge of the Earth’s atmosphere, magnetosphere and the solar wind has been acquired since the development of scientific satellites, there remains much to learn about the processes that couple energy from one region to another. These processes are not only complex in nature, but they vary dramatically with time and position. One natural indicator of magnetospheric processes is the global distribution of optical aurora; the upper atmosphere serves as a screen which provides indirect, but valuable, information about the incoming energy flux that would be impossible to acquire with in situ measurements either on the ground or in space, and for this reason considerable effort has been expended on obtaining global auroral images from satellites. Ideally, such images should be instantaneous views of the aurora, at a number of pertinent wavelengths covering both the dayside and nightside of the earth with temporal and spatial resolution consistent with the scale size of the relevant magnetospheric processes. Aeronomers could utilize these images in the computation of auroral—driven effects in the ionosphere and thermosphere. Such an ideal instrument does not exist at present due to obvious technical difficulties; however, considerable progress has been achieved since the first auroral imager was flown on the ISIS 2 satellite in 1971 /1/. A number of imaging instruments have been designed for low—altitude polar orbiting satellites: ISIS 2, the DMSP series /2/, HILAT /3/, Polar Bear. These acquire images over an auroral region only once per orbit because they use the orbital motion to provide one spatial dimension and either the satellite spinning motion or a moving mirror for the second dimension. Usually only partial views of the auroral distribution are possible due to low satellite altitude, but the spatial resolution can be very good. Other imagers have been designed for high—latitude satellites: KYOKKO /4/, DE 1 /5/, Viking /6, 7/. These can acquire continuous sequences of images of the entire auroral distribution (single hemisphere) over ‘periods’ of one or more hours. The DE instrument utilizes narrow—field photometers and a combination of the spinning satellite and a scanning mirror to obtain images of the earth at a repetition rate of one every 6 to 12 minutes. The KYOKKO instrument viewed the entire field by means of an image tube, and use a rotating mirror to compensate for the satellite spin. The Viking instrument also viewed the entire field, but used an intensifier CCD array detector to both provide the digital image and to compensate for the satellite spin. The latter two instruments could obtain images at rates up to the spin period, 2 minutes for KYOKKO and 20 seconds for Viking. A common feature of many instruments Is the capability of
L. L. Cogger and J. S. Murphree
(6)168
The purpose of this paper is not to answer questions related to the cause of these events, but to describe the characteristics that would likely affect the global dynamics of the thermosphere. AURORAL VARIATIONS
It is well known that the auroral distribution exhibits major variations in morphology and intensity due to the coupling of the earth’s magnetosphere to the sun by means of the solar wind. Investigations of the aurora from ground stations has led to the development of statistical ovals that are representative of various magnetic conditions, e.g. Feldsteiri and Starkov /8/. While such representations are useful for modelling purposes, it is recognized that the actual auroral distribution at any time can be very different and can change dramatically over short time scales. The examples that follow have been selected to illustrate some of the kinds of variations that occur over time scales from days to minutes and which can have significant implications for thermospheric modelling. The first example illustrates the dramatic dependence of the auroral distribution on the general level of magnetic activity as depicted by the Kp index. Because this is a three—hour index, it is most useful for time scales from hours to days. During the period July 24 to 26, 1986, the Kp index ranged from a low of 0+ to a high of 6, as shown in Fig. 1. Representative images of the aurora are shown in Fig. 2 for 6 of the orbits during that period. All the images shown in this paper were taken with the LBH camera and are presented as a false—colour picture. Latitudes and times shown on the images are in eccentric dipole latitude/magnetic local time coordinates. In Fig. 2 the terminator lies along a line from the top left to bottom right in each image and for the cases of weaker auroral emissions the dayglow obscures those emissions. For the quietest periods, represented by the images for orbits 840 (860724/143159 UT) and 846 (860725/165215 UT), the auroral oval was weak throughout and essentially below the threshold values chosen for Fig. 2. Orbits 843 (860725/035936 UT) and 849 (860726/054157 UT) correspond to moderate magnetic disturbance, and the images for orbits 842 (860724/233558 UT) and 847 (860725/205405 UT) were taken during very disturbed periods. In the latter cases, the optical distribution was much brighter and thicker and dominated by discrete features. It is clear that the instantaneous oval can reflect the gross magnetic activity as described by the Kp index although as will be shown below the rapidity with which large scale optical features change may not be adequately represented in the normal indices used to describe auroral activity.
ORBIT t 840
842 843
846 847
849
~JULY (date) Figure 1: The variation of the Kp index during July 24—26, 1986. in Fig. 2 were taken at the times marked by an X.
The images shown
On a shorter time scale than 3 hours, the isolated auroral substorm is a feature that is associated with an impulsive injection of energetic particles into the upper atmosphere. These are easily identified in auroral images. The sequence in Fig. 3 for 1 April, 1986 consists of 9 selected images from the period 1831 to 1914 UT. The substorm developed from a bright intensification near 23 MLT around 185055 UT into an expanding region of precipitation. A helpful description of this event was prepared by Rostoker et al. /9/. Fig 4 depicts the change in brightness and area as a function of time using images separated by about two minutes. The brightness is shown as the average signal level in data numbers (DN)
Figure 2: A sequence of images for the period July 24—26, 1986 selected to illustrate the range of auroral activity present. The dates and UT are shown in the bottom of each image; the latitudes are given in eccentric dipole magnetic coordinates; and the times on the image are magnetic local time. Red corresponds to the highest signal levels.
Figure 3~ A sequence of nine images showing the variation of the nightside aurora during a period of 45 minutes on 1 April 1986. The circles to the east and west of the substorm indicate the locations of Dixon Island and Kiruna respectively.
The
UV Auroral
Distribution
(6)171
signal level is somewhat energy—dependent due to absorption by 0~. The effect is not severe for primary electron energies in the range of 1 to several keV /12/, but for larger energies serious error would be introduced. There is also a small contribution to the LBH signal from other species, and their energy dependence may be somewhat different. With these cautions, the reader may wish to multiply signal levels given in the diagrams in DN by the factor .11 to obtain a rough estimate of the energy flux. The initial peak in average intensity prior to 184115 UT was due to the polar arc connection seen in the images from 183149 UT to 183951 UT. Such intensifications are quite common /13/ and themselves represent a non trivial source of energy input during quiet times. D~ring the period of expansion, area and 2 per mm. The product of the the area increased at a rate approximately 0.1indicator x 10 km of the variation in the total energy average signal level ofcan be used as an deposition rate, i.e. power. This product, plotted in Fig. 5, shows that the rate of energy input increased from onset to at least the.end of the satellite pass at 1915 UT. Other examples of isolated substorms in the Viking data base are consistent with the development for orbit 214 and provide evidence that although these substorms can be considered to be impulsive events, the precipitated energy does not increase in a step—function manner that would be convenient for the theoretical prediction of thermospheric effects. SUBSTORM GROWTH. ORBIT 214
184115
1852~
190345
111 (~amssUT) Figure 4: The area and average signal level variation for the substorm development shown in Fig. 3 SUBSTORM GROWTH, ORBIT 214 a
183~~0
184115
185220
190345
1915g~
IDE ll’lamss UT)
Figure 5: The product of the two plots given in Fig. 4. The values here represent the variation in the rate of energy deposition into the substorm bulge. There are other interesting aspects of this sequence that are relevant to thermospheric studies. The location of the substorm onset was east of the Kiruna magnetometer station and west of the Dixon Island station. This likely explains why the 11—station AE index shown in Fig. 6 did not increase sharply until about 1900 UT (about 12 minutes after the onset was observed from the Viking satellite) when the substorm expanded sufficiently for the ionospheric currents to be measured by the stations. The conclusion from this is that the AE index may not always be a suitable reference for timing the start of isolated substorms.
(6)172
L. L. Cogger and J. S. Murphree
indication of the time required for the high latitude convection to response to the IMP. the arc is an indicator of the poleward edge of the plasmasheet, then the area of open magnetic field lines expanded significantly with the disappearance of the arc.
:::°
If
~L~_L_Li 7~.LLJ. T~ ~
-
10
15
20
UT
Figure 6: The provisional AE index for 1 April 1986 obtained from the magnetogram records at 11 stations, and the IMP B component obtained from the IMP 8 satellite measurements on the same day. Four orbits after the previous example of substorm development the satellite was in a position to view another such event near 12 UT on 2 April 1986. This was a very quiet period, with Kp = 1, the IMP magnitude steady at about 5 nT, and B near 0. Images from this pass have been published elsewhere /14/; here the discussion is li~itedto the temporal development. The area covered by the auroral bulge expanded for about 15 mm. then contracted for the next 15 minutes before a second expansion commenced at about 1240 UT. The average intensity increased sharply to a maximum at the beginning of the substorm, and thereafter subsided. From this variation, the inferred energy input rate shown in Fig. 7 increased for the first 10 mm. and then decreased for the remainder of the substorm. Differences in the development of this substorm and the previous example are evident, but the main features appear to be quite similar. The expansion of this type of auroral event takes place both poleward and longitudinally, reaching typical dimensions of 10 deg. in latitude and 80 degrees in longitude within about 15 mm. The energy input rate reaches a maximum somewhat earlier than the area. Short term variations during a disturbed period are illustrated in Fig. 8 using orbit 842 (24 July 1986). It is clear that a major change in the auroral distribution took place between the image at 2301 UT and 2317 UT. The auroral distribution brightened and expanded in the dawn and dusk sectors, with relatively little response in the midnight sector. This contradicts the usual picture of aurora in which the maximum activity occurs near midnight. The abrupt change in the IMP (obtained from the IMP 8 satellite) that occurred at approximately 2310 UT is shown in Fig. 9. The area of the auroral distribution has been measured both before and after the northward turning of B ; changes for three local time sectors are shown in Fig. 10. The midnight region, 23.5 ~o 0.5 MLT, remained fairly uniform in width and brightness until just after 2342 UT when a rapid expansion began. The segment SUBSTORM GROWTH, ORBIT 218 a
__________________________________________
Figure 8: A sequence of auroral images from 24 July 1986 illustrating the dramatic changes in the auroral oval over the period of 2250 to 2336 UT. The impulsive broadening of the oval in the dawn and dusk sectors occurred at about 2310 UT.
Figure 12: An example from 27 September 1986 showing very bright and dynamic dayside aurora.
The
UV Auroral Distribution
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of the auroral distribution bounded by 18.5 and 19.5 8fl~T began to expand rapidly at about 2310 UT; the maximum width occurred at about 2337 UT and then it began to contract. The dawnside of the distribution expanded and contracted in a similar fashion, but it was not as intense. The image at 233556 UT gives the impression that the dawn side was broader than the dusk, but this appearance is due to the viewing direction. The corresponding plots for relative energy deposition rate are given in Figs. 11. The energy deposition rate was greatest in the 18.5 and 19.5 MLT sector throughout the period. This sequence of events is explainable in terms of the sudden change in the IMP at 2310 UT that caused an immediate perturbation of the earth’s magnetic field giving rise to the auroral enhancements in the 860724 orbit 842
- ~
~
231000
325500
232500
711 )Nimi~s IT)
Figure 9: The components B , B , and B of the IMP for the period covered by the auroral images in Fig. 9. ~he ~MP data were obtained from measurements on the IMP 8 satellite. dawn and dusk sectors, followed by an nagnetotail unloading event some 30 mm. later that caused the disturbance in the midnight sector. Rapid auroral response to IMF impulsive events has been described by Craven et al. /15/ using DE 1 images. It is interesting to note that the directly—driven auroral enhancements continued to expand in area until about 5 mm. before the unloading event commenced, and the two types thereafter coexisted.
ARtA C~ANGtIN 3
RCGIDNS.
ORBIT
842
4.5 mIt - 5.5 mit
23.5 mit
N
-
es
cit
~1t-19.5mit
225t02
230738
TIet
232500 U,~,assUT)
234230
240222
Figure 10: Area of portions of the oval bounded by 15 degrees in longitude for one— hour time sectors in the morning, evening, and at midnight during the period 2250 to 2350 UT on 24 July 1986.
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L. L. Cogger and J. S. Murphree
ENERGY CHANGE
IN 3 REGIONS.
a
4.5 mIt
Ui
s~ a a, 5-—
..~
5.5 mIt
-
X>~
_
23.5 mIt
-
0.5 nIt
18.5 tilt
-
19.5 mIt
I
a
225000
ORBIT 842
230730
232500
234230
240800
Figure 11: The relative variation in the energy deposition rate into 3 sectors of the auroral oval during the period 2250 to 2350 UT on 24 July 1986. followed by a return to lower intensities. By 024607 UT the disruption reached its maximum extent, approximately 500 km along the 14 MLT meridian. Subsequently the curvature of the auroral distribution relaxes so that after 0250 UT the afternoon sector appeared as normal. In fact after this tine the maximum optical activity shifted to the morning sector (see Fig. 12.). Such clear modifications of the auroral distribution represent significant energy input outside of the normal statistical oval often used by modelers. DISCUSSION Significant advances in our understanding of thermospheric dynamics have come about through a combination of observational programs and the development of increasingly realistic numerical simulations of the global thermosphere. One of the main challenges over the past ten years has been to evaluate the high latitude sources of momentum and heating resulting from magneto— spheric processes. A comprehensive review of the developments in thermospheric modelling has been given by Rees et al. /16/. Improvements in empirical neutral models /17/, ionospheric models, e.g. /18, 19/, polar electric field models /20, 21/, and precipitation models, e.g. /22/ have been incorporated into self—consistent simulations. Major consequences of the magnetospheric—induced high latitude thermospheric/ionospheric processes have been identified and better understood through the use of these models, e.g. /16, 23/. Advances in observational techniques have permitted the measurement of high latitude neutral winds, temperature and composition that have served as tests for the simulation models, e.g. /24/. Refinements such as UT dependence and the effects of variation of the IMP are being incorporated. As the models become more inclusive, and the observations more comprehensive, the need to account for the temporal variation of the magnetospheric source becomes apparent. A recent study by Mazaudier et al. /25/ has been successful in calculations of the main properties of the thermospheric wind from magnetic storms using the AZ index as a means to estimate the Joule heating that was assumed to drive the winds. As our physical understanding of the fundamental processes develops, it will be necessary to monitor with increased resolution, the actual auroral distribution. It is no surprise that the instantaneous geographic distribution of aurora rarely, if ever, conforms to a statistical model. Moreover, we have shown in this paper that with improvements in temporal resolution, the dynamic nature of aurora appears ever more complex. Large changes in the global energy deposition take place in short intervals of time; the location of the auroral precipitation can rapidly move in latitude, longitude, and even switch from one tine sector to another. We have shown evidence that both directly—driven and loading—unloading types of aurora can occur simultaneously. CONCLUSION The employment of auroral imagers from space has led to major advances in the understanding of the magnetosphere. It is clear that global images of the instantaneous auroral distribution can also provide major support to the study of the thermosphere. Sequences of
.
The LIV Auroral Distribution
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ACKNOWLEDGEMENTS We wish to acknowledge the assistance of R. Elphinstone and J. Smith in the preparation of the figures. The IMP data were provided by R. Lepping. This research is supported by grants from the Natural Sciences and Engineering Council of Canada. REFERENCES
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(1986)