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The structure of a connected sequence of magnetospheric substorms
Planet.
Spcrcc SC:., Vol.
24. pp. 269 to 280.
Penmmon
Pms,
1916.
Printed
in Northern
ldand
THE STRUCTURE OF A CONNECTED SEQUENCE MAGNETOSPHERIC SUBSTORMS
OF
D. R. McDIARMID and F. R. HARRIS Her&erg Institute of Astrophysics, National Research Council of Canada, Ottawa KlA OR8, Canada (Received infinal form 10 July 1975) Abstract-The structure of a sequence of four spatially and temporally connected magnetospheric substorms has been determined through the use of a set of mid-latitude magnetometer station data, an auroral zone ma etometer line data set, and the ATS-5 magnetometer record. In two of the substorms, two para$lel westward electrojets were observed to develop during the initial part of the expansion phase. In another, the expansion phase consisted of the development of a westward electrojet westward and equatorward of the pm-existing current system.
INTRODUCTION
1969-1970 tbrougb the work of several groups ‘meridian magnetometer line existed through Fort Churchill. Included in the line were an all-sky camera (Churchill) and a radar (Thompson). The purpose of this paper is to present an analysis of ionospheric and magnetospheric current flow during a sequence of four temporally and spatially connected substorms seen on or near this line between 04:oO and 07 : 00 UT during 30 January 1970. Some correlation with concurrent visual aurora will also be presented. More detailed correlations between radio aurora, visual aurora and ionsopheric current flows will be presented in a subsequent paper in collaboration with A. G. McNamara. For several
months
during the winter
OBSERVATION NETWORK AND DATA ANALYSIS PROCEDURE
The geomagnetic locations of the magnetometers whose records were used in this study are shown in Fig. 1 and both the geographic and corrected geomagnetic (Hakura, 1965) coordinates of the stations are listed in Table 1. The Gillam, Caribou River and Eskimo Point fluxgate magnetometers were operated by F. Harris, and the Winnipeg, The Pas, Lynn Lake and Thompson fluxgate magnetometers were operated by J. K. Walker of the Canadian Department of Energy, Mines and Resources. All other magnetometer records were produced at regular magnetometer stations. We also have magnetometer data from ATS-5 which was situated on a field line which originated in the vicinity of Gillam. All of these records are in analog form. In addition to the magnetometer data, we have used the 35 mm all-sky camera records from Great
Whale and Fort Churchill, and the 48 MHz aurora1 radar record from Thompson. Finally, some data from a meridian-scanning photometer (5577 A) at Fort Churchill were provided by F. Creutzberg. The mid-latitudemagnetometer data were analyzed to determine the field-aligned ‘equivalent’ currents using Kisabeth’s (1972) model. The model consists of current flow down dipole field lines from the magnetosphere to the ionosphere at the eastern limit of the substorm sector, westward current ilow in the ionosphere, upward current flow along dipole field lines at the western limit of substorm sector, and closure via current across the tail. This model accounts for the induced currents by assuming a spherical core of perfect metal some distance, below the surface of the Earth (250 km in our computations).* This approach has been extensively used by Kisabeth (1972), by Kisabeth and Rostoker (1971, 1974), and earlier by Bonnevier et al. (1970). Subsequent to the mid-latitude analysis, the final form of the Churchill line magnetic disturbance profiles were determined using the mid-latitude results to assist in finding the orientation of the effective magnetic disturbance meridian with respect to the geographic meridian. Conclusions were obtained by comparing the observed profiles with the published theoretical profiles of Kisabeth (1972) and of Kisabeth and Rostoker (1971). As described below, the effective magnetic disturbance meridian is that great circle through Churchill which intersects the ionospheric current flow at right angles.
* The work of Nopper and Hermance (1974) indicates that a more realistic and, unfortunately, much more difficult treatment should ultimately be considered. However, we do not believe that such treatment would alter our conclusions in any significant way. 269
D. R. MCDIARMID
270
FIG. 1. THE GEOMAGNETIC LOCATIONS OF THE STATIONS USEDINTHISSTUDY.
The key for the name symbols is included in Table 1. Initial inspection of the data indicated that the electrojets flowed in a direction somewhat north of geomagnetic west (confirmed in later analysis) and our mid-latitude analysis took account of this observation. One would expect this behaviour in the pre-midnight sector for current flow along the oval but in this case, the phenomenon persisted until geomagnetic midnight. Our mid-latitude analysis did not account for the ionospheric portion of the current flow which should not be significant at these latitudes. The cross-tail return current, which represents the deflection of westward tail current through the ionosphere, was that corresponding to the higher L value and conjugacy of current flow was assumed. Thus in the model, which, TABLE 1. THEGEOGRAPHICANDCORREC~DGEOMAGNETIC LOCATIONSOPTHESTATIONSUSEDINTHISSTUDY
Local magnetic midnight was commuted usine Montbrian& (1970) cur&s
T *lTm
64.3
264.0
75.1
320.4
61.1
265.9
72.5
325.3
59.4
265.2
70.7
324.7
58.8
265.9
TO.0
326.0
56.4
265.3
67.9
56.8
259.0
61.3
316.3
55.8
262.2
66.8
321.4
54.0
258.9
64.5
YT.3
49.6
262.9
60.9
324.1
61.2
34.6
69.0
55.3
282.2
68.2
353.8
54.5
246.6
62.5
301.2
64.9
212.2
64.9
260.3
71.3
203.2
69.7
247.0
325.9
45.6
51.0
355.5
50.0
77.9
47.6’
307.3
58.3
.29.8
3.S.l
293.8
38.2
282.6
51.8
352.2
33.0
96.1
43.4
326.9
29.9
3.2
40.0
254.7
49.3
315.7
LB.2
242.5
55.3
299.5
48.5
236.6
53.9
292.6
21.3
2M.O
21.1
266.5
13.6
144.9
4.0
212.9
-
and F. R. HARRIS as will be seen, was applied to a situation where the electrojet flows were primarily in the dusk-midnight quadrant, the substorm eastward cross-tail current does not connect with the eastern field-aligned current. However, we note that we did not account in the model for the fact that near the midnight sector, the field lines are extended tailward (Fairfield, 1968). Nor did we account for the aximuthal bending of field lines tailward across local time sectors (Fairfield, 1968). The analysis procedure was to determine which of a set of field-aligned and cross-tail currents (successive cut and try), taking account of the latitude profile of ionospheric current flow indicated by the magnetometer line, best matched the disturbance fields recorded at Fredericksburg, Dallas, Boulder, Newport and Victoria. These disturbance fields were measured relative to the magnetometer values at 05:22. (All times are UT unless otherwise specified.) In addition to the above sources of error is that of baseline determination. Several sets of relatively quiet days were used and yielded baselines with some scatter. This scatter was significant with respect to the OS:405 disturbance and tolerable with respect to the others (05:55.5, 06:03, 06:12, 06:21, 06:40.5). The best match was in terms of least scatter in the current values resulting from comparison between observed and computed disturbances. The sources of error outlined above plus reading errors lead us to use the results of our analysis in qualitative rather than a quantitative way. Having determined the longitudinal extent and geomagnetic location of the electrojets at six points of time during the sequence of events described below, we then plotted H, D, and 2 protiles for various values of 0 where 0 is the angle between the X magnetic axis and the H-axis. Using the theoretical profiles of Kisabeth (1972) and Kisabeth and Rostoker (1971) for comparison, we determined for each chosen time which 6 seemed to best correspond to a line perpendicular to the current flow, i.e. the orientation of the effective magnetic disturbance meridian. A value of 15’ was chosen. This corresponds to electrojet current flow approximately 9” north of geomagnetic west. As in the mid-latitude analysis, the disturbance fields for the magnetometer chain were determined relative to the magnetometer values at 05 : 22. The meridian chosen above is, of course, not the geomagnetic meridian and thus our H and D components are slightly different from the normally defined ones. Distances along the meridian line relative to Churchill were computed by constructing
271
Magnetospheric substorms
_F-component
Time,
FIG.
UT
2. MmLATITUDE AND ATS-5 MAGNETCKXUMS. The dashed lines are baselines for the ATS-5 records, which are not plotted beyond 06: 00 because the record was extremely diEicult to read there. Straight lines were used in the Z component plat where the variations were not significant. a new coordinate system in which OUTmetidian was a inczeased after 04: 25. This corre&es with the arrival at the rna~opa~ of a southward interlon@ude line and~~c~l was assigned itscorrected planetary magnetic field (IMF) at approximately geomagnetic latitude. The positions in this coordinthe same time. The field remained southward for ate system of the other stations were determined several hours (Rostoker, private communication). and used to compute tie distance along our meridian Note the positive correlation between the ground from Churchill to the “latitude’ of each of the other station records of the more notable features of the H stations. Ln the profile plots the positive direction component trace. (T&e format of the ATS-5 from Churchi@ is north and the order of the stations magnetometer data obtained by us hides whatever is that of Table 1 with the exception of Thompson whiGh could not her plotted because only its X of these features was seen at the satellite). Both the decrease in the ATS-5 H component and its component was available. general similarity to the Dallas H component record are not unexpected Qmmings and Coleman, 1958; Cummings ef al., 1968) and show that the sequence of events described &low were not The mid-latitude magnetic records fram San Juan, significantly unrepresentative of substorms in Fredericksburg, Dallas, Boulder, Newport and general. We see also that the disturbance in the Victoria are shown in Fig. 2 along with the ATSJ geographic equatorial plane is ba&alIy radially data. The scales have all been equalized and the outward as reported earlier by Cummings er at. dashed line is the baseline for the disturbance seen (1968t under similar circumstances. However, in at AT%-5. The ATS background was taken as the contrast to their typical observation, there was no average of the 25 Jamzuy and 8 February records, concurrent positive H component disturbance seen the only nearby quiet days for which we have good at aurora1 latitudes in the geomagnetic longitude data. Straight lines were used in the Z component sector of the satellite or at any other of the high plot where the variations were not significant. latitude stations in Fig. I with the exception of Prior to about 04~25, all the records show a Port Barrow f-80” west of satellite) which recorded decrease in the X component, the rate of which a disturbance in X of approx, 20 y. However, as
272
D. R. MCD~ARM~and F. R. HARRIS
discussed below, this disagreement was probably the result of masking by another current system. Inspection of Fig. 2 shows the depression in H to have occurred between geomagnetic longitudes 292’E and 352’E but at 3*2’E (San Juan) the amplitude of the depression was significantly attenuated. In other words, the region of depressed H extended from the dusk sector to the midnight sector but is significantly attenuated in the early morning sector-a result similar to the statistical conclusion of Cummings et al. (1968) using ATS-1 data. Further interpretation will be left to the Discussion section. Inspection of the first three all-sky camera pairs of Fig. 3(a) shows southward drifting bands over Great Whale and southwestward drifting bands overChurchill. Theseweredetected from aboutO4:OO at Churchill and earlier at Great Whale. The electric field over Great Whale as deduced from the rate of drift was only several mV/m prior to 04: 16. The intensity at 5577 A of the band near the Churchill zenith at 04:20 was approx. 1.4 kR. For the purpose of permitting the reader to compare our all-sky camera data with those of other systems we note that the f number of the cameramirror system is 1.2 (d&se source), the exposure time was 40 set and the film was Kodak Tri-X (ASA-400). We must, however, point out that operation at Churchill was substandard at the time of interest. First of all, an improper lens was accidentally in use which produced a slightly out-of-focus image. Secondly, the film processing, contrary to our standard practice, was done at the station and resulted in significant fogging. Thus bright sky over Churchill relative to Great Whale in Fig. 3(a) is not real but a result of tihn processing. 2. 04:16-05:23
UT
As noted by Akasofu et al. (1974) in their discussion of this event, an arc near the poleward boundary of the diffuse aurora over Great Whale brightened very slightly at 04: 16. At this time the beginning of a substorm-like disturbance was recorded by the Narassarssuaq magnetometer (see Fig. 4). It was not until 04:26 that a significant brightening of an auroral band occurred over Great Whale although the onset of magnetic activity was recorded several minutes earlier (&:21). The visual aurora did not brighten near the foot of the ATSJ field line (in the vicinity of the lower part of south pointing arm of the all-sky photograph) until about 04:32. As can be seen in Fig. 3(a), the brightening propagated westward from the Great Whale sector. It was accompanied by relatively little magnetic
I
/
I 0400
I
I
I
I
I
0%
I
I
I
I
Oeco
I
I
I
I
0700
I
I 0800
'Time. UT FIG.
4. Tut+ NARSSARSSUAQAND
GREAT
WHALE H
COMPONENTS.
The solid triangles mark the beginning of the O&16 disturbance at each station. activity on the Churchill magnetometer line. Although there was some brightening of the aurora1 bands over Churchill poleward of the brightened band, these bands continued their slow southwestward drift until 05:23 in contrast to the active aurora over Great Whale. When the rate of drift is determined relative to that expected from rotation of an all-sky camera under a fixed oval-aligned band, the electric field (u = E/B) is found to increase from several millivolts/meter before 04:25 to the order of 10 mV/m or a little larger afterwards (prior to 05 :OO). Kelley et al. (1971) have shown that electric fields determined from meridional aurora drift velocities during periods with no expansive phase are equal to or somewhat smaller than the values measured via balloon probes. The brightness of the prominent auroral band south of Churchill at 04: 50 was approx. 25 kR at 5577 A, an order of magnitude brighter than the aurora before 04:26. We note in Fig. 2 that the rate of decrease in H at all the mid-latitude stations and at ATS-5 increased after 04 : 25. What these observations add up to is increased convection in the magnetosphere and concurrent substorm activity between the Greenland and Great Whale sectors. We do not have sufficient data to determine either the longitudinal extent, latitudinal structure or intensity of the 04: 16 current system. We do know that after its initial development near the Greenland sector, its zone of influence spread westward until it was observed at Great Whale after 04: 26. We also know that there is virtually no
Fro.
3. (a,b) SELE~~EDALL-~KYPHOT~ORA~HS (POSITIVE)FROM EITHER GREAT WHALEOR~HIJRCHILL North is upward with west and east to the left and right, respectively.
ASNOTED.
272
FIG. 3 (cunrhed).
273
Magnetospheric substorms ,
.a
4l.I
r
,
I’D
X-component
4”
I
*
Y-component
CH
Gi
TP.
v
2ooy
w. I
05:oo
’
I
4
06:Oi,
I
I
I
07~00.
I 05:oo
I
’
I
Time,
FIG. 5.
MAG~IC
RECORDS The
FROM
If
07~00
iGG& 05:oo
’
osod
0700
UT
THE STATIONS IN THE CHURCHILLMAGNETOMETERLINE.
key for the name symbols appears in Table 1.
evidence of this current recorded on the Churchill magnetometer line. Meanwhile the ATS-5 H record continued to behave in a pre-substorm manner and the electric field over Churchill indicated enhanced magnetospheric convection as expected from the behaviour of the IMF. In this situation, the Great Whale all-sky film was not the best record of behaviour at the foot of the ATS-5 field line (I$ Akasofu et al., 1974). 3. 05:23-OS:58
G4.I
0600
UT
Although there was a minor brightening of the aurora south of Churchill at 05 : 21, the significant brightening began in the geomagnetic southwest at 05 : 23 as shown in Fig. 3(b). Poleward movement toward the Churchill zenith from the same sector began at 05:28+ by which time the magnetic disturbance observed on the Churchill line had increased significantly, as shown in Figs. 5 and 6. In Fig. 3 (b) we see the extent of this movement of the visual aurora at 05:32 and also note that except for the eastern sector, this poleward movement remained stalled between 05: 32 and 05:36. Akasofu et al. (1974) noted that ATS-5 encountered a hot plasma with an intense flux during this event at about
05: 35. At 05 :40 we see the appearance of new aurora in the southeast zenithward of the preexisting aurora. By 05 : 44 the aurora near the southem horizon had faded and the new aurora had brightened. In quantitative terms, the aurora in the south (west side of south arm of the all-sky camera) had a brightness of approx. 20 kR (5577 A) at 05:41 and the more zenithward emission was approx. 10 kR (5577 A). At 05:44, the 10 kR had increased to approx. 90 kR. After 05~45, the visual aurora began to decrease in brightness, both in the background emission and, on average, in the discrete forms until 05: 58. We must point out that brightness comparisons between the Churchill photographs in Figs. 3(a) and (b) cannot be made because, although print exposure times are constant in each figure, they were increased in Fig. 3(b) relative to Fig. 3(a) in order to retain detail in the brighter aurora. Figure 6 shows the Churchill magnetometer line profiles of the current system which developed at 05:23 at several times throughout the event. Some interpretation has been made in producing these plots. The data appear very consistent with the character of the theoretical profiles in Kisabeth
and F. R. HARRIS
D. R. McDr-
274
/ 05~28
/ /
5
;r
.o f
-50
!
i
1D-component
/ 1
*Z-component
1
‘W-component
H -100
50; .f: 2 4 3 5: 2 0) -took s
I
-1000
Line
-500
distance
0
from
Fort
Churchill,
500
0
-500
-1000
km Line distance
from
Fort
Churchill,
500
km
FIG. 6. SELECTED ‘LATITUDE' PROFILES (CHURCHILL LINE) OF THE H, D ANLI Z COMPONENTS FOR THE 05: 23 SUBSTORM. Distance is positive poleward
and Rostoker (1971) and in Kisabeth (1972) for two parallel westward electrojets (see, e.g. Fig. 3(c) in the first reference) and our profiles are plotted accordingly. In Fig. 6 we see the development of two parallel electrojets, one near Gillam and the weaker one near Churchill. The southern electrojet moved somewhat equatorward after the initial onset. It would be interesting to conclude that the poleward electrojet reached a maximum at OS:44 when the visual aurora was most intense in the Churchill vicinity but the data (as opposed to our interpretive plotting) do not unequivocally support such a conclusion. Although the H component profiles do not unmistakably indicate a double electrojet
from
Churchill.
system prior to 05:40, the nature of the D and Z component profiles clarify the situation. The data for the 05 :28 and 05 :40 Z component profiles clearly show the two maxima and two minima which are characteristic of a parallel electrojet system (Kisabeth and Rostoker, 1971). The Ed-latitude analysis discussed below indicates that the central meridian of the combined current system was close to the magnetometer line at 05:405. Now if the central meridian of the low latitude electrojet was west of the magnetometer line, one would expect a D component profile not dissimilar to what was observed at 05:40 and at most other times. In other words, these profiles
Magnetospherlc substorms also indicate a double current system. The large negative minimum of the 05:32 D component profile remains an enigma. In contrast to the observations of Kisabeth and Rostoker (1971), both electrojets developed simultaneously during the expansive phase. Although the discrete visual aurora dimmed after 05 :45 and gradually became confined to the southern sky (Churchill), both electrojets continued to flow until OS:58 and, in fact, the poleward electrojet reached a maximum (stronger than the equatorward electrojet) around 05 : 50. In this regard our observations are not inconsistent with the northern border current intensifications reported by Kisabeth and Rostoker (1971). An analysis of the mid-latitude disturbance field of the 05:23 current system was performed on the 05:405 data. Two parallel electrojets, the equatorward twice as intense as the other (see Fig. 6), were assumed in the model. The analysis procedure was that described above and it was found that the ionospheric portion of the electrojet flowed west between geomagnetic longitudes 360’ and 295’ approx. Analysis of the 05 : 55.5 data indicated an eastward shift of the end points by approx. 10’. Both analyses indicated a total current flow of approximately 1O5A. A comparison between our observed aurora1 zone profiles and the theoretical profiles of Kisabeth and Rostoker (1971) or of Kisabeth (1972) indicates a somewhat higher value than lo5 A. To summarize, at 05: 23, a current system of 50°60° longitudinal extent developed west of or extended out of the 04: 16 current system. Since we do not know the latitudinal structure of the 04:16 current system, we cannot say when the double electrojet developed. This extension was accompanied by substorm behaviour of the visual aurora over Churchill. 4. 05:58-06:13
UT
As stated above, by 05:58 the visual aurora was south of Churchill. At this time the band close to the horizon in the west to southwest brightened (see Fig. 3(b)). This brightening reached its peak at 05 : 59 and by 06 : 01 the brightness had diminished to a level comparable to the more zenithward aurora. Thereafter until 06: 12 the visual aurora became more and more confined to the southern horizon. In Fig. 7 we see the magnetic consequences of the disturbance. Almost immediately the equatorward peak in the H component increased and by 06:03 had broadened southward. The dramatic change occurred at The Pas and, as can be seen in Fig. 8,
275
this behavionr persisted until 06: 12. Analysis of the mid-latitude data for 06:03 and 06:12 clarified these profiles. Since most of the change occurred in a confined latitude region, it appears to be the result of the addition of a new current system to the one existing before 05 : 58 which remained unchanged. To investigate this possibility, we subtracted the mid-latitude disturbance values just prior to 05:58 from those for 06: 03 and 06: 12. The subsequent midlatitude analysis showed the new disturbance to result from a current system well to the west of our magnetometer line and a similar treatment on the profile data showed the system to be equatorward of the preexisting current flows. Evidence of this current system is contained in Fig. 2; the H component disturbance is much greater at Newport and Victoria than at the more easterly stations. Specifically, at 06:03 the ionospheric portion of current flow (westward) was between geomagnetic longitudes of 310’ and 270’ approximately. The situation at 06:12 was unchanged except for the amplitude of the current flow which, from the mid-latitude analysis, increased to 3(106) A from 2(106) A at 06:03. In other words, the total current flow at 06:03 was 3(105) A and at 06:12 was 4(105) A when all current systems are included. In summary, at 05: 58 a new and ultimately stronger current system was added to that previously in existence. The ionospheric portion of the new current system was equatorward and westward (overlapping) of the other system. In addition, a positive bay was observed at College indicating an eastward electrojet in the dusk sector. At 06:02, ATSJ encountered an even hotter plasma than it did at 05:35 (Akasofu et al., 1974). 5. After 06:13 UT As stated above, the visual aurora was confined to the sky well south of Churchill by 06:12. At 06:13 a band extending from the south to the southwest brightened (see Fig. 3(b)) and by 06:15 zenithward movement from the southwest was apparent. By 06 : 20 aurora was again present in the zenith and the discrete forms were accompanied by an overall diffuse emission with the result that most of the stars were masked out in the all-sky camera record. The poleward boundary of the discrete forms moved northward to about Baker Lake by 06:26 and remained there until 06: 59 when it began to move equatorward. New plasma was observed by ATS-5 at about 06: 17 (Akasofu et al., 1974). The magnetic profiles of this event are shown in Fig. 8. First of all, the new current system of 05:58
D. R. MCDIW
276 200 T U
I 05:58
OS00
. >:
c
r
and F. R. HARRIS
*cd
.I--;--.
--.
%
.
/ ,
A---’ \.
,-
---.
--__,
_./’
1
x 3
E
-lOO* H-component 1 D-comDonent . Z-component
g N
.> 06~03
0606
I
+
-500
-1000
Line
distance
500
0
from Fort Churchill,
km
Line
distance
from Fort
Churchill,
km
FIG. 7. SELECTED ‘LATITUDE’ PROFILES (CHURCHILL LINE) OFH, D AND Z COMPONENTS FORTHE 05 : 58 SUBSTORM. Distance is positive poleward from Churchill. Note the change in the scale of the ordinate axis from the previous figure. which was seen primarily at The Pas appears to decay before the 06:13 current system began to grow. This new current system was similar to the 05 : 23 system in both position and latitude structure. Again we see two parallel westward electrojets which simultaneously increase in strength but this time the two are more equal in intensity. Also this time there is clear indication of a parallel electrojet system in both the Hand Z component profiles. The parallel electrojet system produced a maximum of the profile peak intensities at about 06:21 and several minutes thereafter became much less confined in meridian cross-section in what was probably a merging of the two electrojets. This is seen in the 06:33 profile. Note that current flow existed well north of Churchill at this time. Following 06:33 there was an increase in intensity of the current flows, particularly in the equatorward electrojet. This was followed by a decrease which showed most clearly in the attenuation of the H component disturbance of the southern electrojet but also showed in the decrease of the latitudinal extent of the influence of the northern electrojet. This is seen in the 06:44 profile. Thereafter as the
substorm decayed the profiles became more complicated and will not be discussed here. The changes in total current flow after 06:33 indicated by the profiles are not large and would require a detailed model analysis to quantify. The result of the mid-latitude analysis for 06:12 (06 : 03 plot) is shown in Fig. 9. The result for 06 : 21, the end of the first rapid intensification of the 06: 13 event, was a parallel electrojet system of total current flow around 4(105) A extending from 350“ to 240’ geomagnetic longitude as shown in Fig. 9 wherein the Earth’s rotation has been accounted for. Examination of Fig. 2 shows that after 06:21 the total current flow continued to increase but the western end of the electrojet moved eastward until about 06: 30 after which the eastern end of the electrojet then also moved eastward such that at 06:405 the total current flow was of the order of 6-8(105) A and ionospheric currents flowed between approx. 5’ and 290’ geomagnetic longitudes. An attempt was made to take account in these analyses of the existence of an eastward electrojet in the vicinity of College. The Hcomponent disturbance at College reached a maximum at 06:25 before diminishing
Magnetospheric substorms
277
06:44
Line distance from Fort
FIG. 8. SELECTED
Churchill,
Linedistancefrom
km
Fort
‘LATITUDE' PROFILES (CHURCHILL LINE)• SUBSTORM.
Churchill,
FTHEH,
D
km
AND
Z
COMPONENTSFORTHE
06:13
Distance is positive poleward from Churchill. Note the change in the scale of the ordinate axis from the previous figure. rapidly. The College and Port Barrow disturbances are consistent with the existence of an eastward electrojet significantly weaker than the westward electrojet system and equatorward of it when they overlapped. Figure 2 also indicates that after 06:40-45, the total electrojet current flow peaked and began to decrease and the western end of the current system moved westward again. Figure 9, which shows the results of our midlatitude analyses, serves as a resume of the event sequence described above. We see the initial substorm of undetermined longitudinal extent and latitudinal structure, its expansion westward after 05: 23, the appearance of the new current system in the west after 05 : 58 followed by its disappearance after 06: 13 with the development of the current
system of the final substorm in the sequence. The 04: 16 current system is only indicated in the plot for which we have some information concerning it. We included the current values resulting from the analyses presented in the text above for the purpose of showing the extent of their variation throughout the event. As noted above, comparison of our profiles with those of Kisabeth and Rostoker (1971) or of Kisabeth (1972) indicates somewhat larger values than those produced by our mid-latitude analysis. DISCUSSION
AND CONCLUSIONS
We will consider first of all the current system responsible for the pre-OS:23 depression in the The mid-latitude and ATS-5 H components.
278
D. R. MCDIARMIDand F. R. HARRls
The solid lines are the westward electrojets which were analyzed, the alternate dash-dot line is the 04: 16 westward electrojet which could not be resolved, and the dashed lines are the unresolved eastward electrojets. The 04:16 current system is only indicated in the plot for which we have some iaformation concerning it. The hatched area in the 06:405 plot is used to indicate the latitudinal spreading of the two electrojets and the black dot at the top of each diagram denotes geomagnetic noon. behaviour of the latter was the same as has been commonly observed in the pre-midnight sector prior to the onset of a substorm expansion phase (Cummings et al., 1968). The source of the disturbance is believed to be a partial ring current in the evening sector. We have noted above the general correlation between the ATS-5 I;T component variation and those of the mid-latitude ground stations as well as the fine structure correlations between the latter. Since mid-latitude stations are particularly sensitive to field-aligned currents, the data suggest these were flowing even prior to the iirst expansion phase onset at 04: 16. Two models have been proposed to account for the pre-substorm expansive phase magnetic disturbances and both contain field-aligned currents. One model consists of a partial ring current in the pre-midnight sector with closure through fieldaligned currents and eastward current in the ionosphere (Cummings et al., 1969; their Fig. 8). The other model was recently proposed by Rostoker (1974) to explain both the ATS W component and Triad field-aligned current observations (e.g. Zmuda and Armstrong, 1974). His presubstorm current co~~mtion is shown in Fig. 3 of his paper and consists of a pre-midnight eastward electrojet,
a post-midnight westward electrojet and outward field-aligned current at the junction of these electrojets (Harang discontinuity). This field-aligned current couples to the ring current and consequently increases the ring current in the evening sector as observed by the ATS satellites. Closure to the electrojets again is assumed to occur diffusely on the dayside. As we will discuss later, Rostoker proposes that substorms consist of steplike westward motions of the Harang discontinuity. We can put limits on the position of the upward field-aiigned current in the midnight sector after 04: 16, The large depression in the ATS-5 H component requires the field-aligned current to be east of the satellite. The behaviour of the Dcomponent at the satellite is consistent with this conclusion if we assume conjugacy of current flow and note the position of the satellite south of the tail sheet as shown in Fig. 9 of Cummings et aZ. (1968). On the other hand, it could not have been very far east of the satellite since the depression in H was significantly attenuated at San Juan and the western end of the 04: 16 disturbance was near the Great Whale sector. In order to compare the models described above with our 04:0@-05 23 observations, we had to
279
Magnetospheric substorms include the 04:16 current system into them. This was difficult to attempt because of the unknown extent of the 04:16 current system and the relative error of the observed data before 05 : 23. However, an intensification occurred at the western end of the 04: 16 system beghming at 04: 50. The Fredericksburg, San Juan and St. John’s (not shown) magnetograms provided a good indication of the longitudinal extent of the intens~cation and the comparison between the calculated and observed mid-latitude disturbances for the intensification indicated that both models described the situation reasonably well. A comparison for the 04:00-04:45 period was inconclusive. At first glance, both of these modeis predict the observation of eastward current flow over the Churchill magnetometer line. This was not observed. However, the westward electrojet in the Great Whale sector produced Hand Z disturbances which tended to cancel those of an overhead eastward electrojet. The opposite is true of the D component and, in fact, there was evidence of combined eastward and westward current flows in the D component records after 04 : 30. Turning now to the substorm sequence, we saw in both the 05 :23 and 06: 13 substorms the simultaneous development of two parallel eiectrojets. Parallel electrojets have been observed before (Kisabeth and Rostoker, 1971) but their development was not simultaneous. Rather the equatorward electrojet developed first and the poleward one developed 15-20 mm later and underwent periodic fluctuations in amplitude. Kisabeth and Rostoker fmd this behaviour to be common so the substorm sequence described herein must be relatively uncommon. Wiens and Rostoker (1973) have further concluded that the poleward electrojet which develops 15-20 min after the beginning of the expansive phase is a current system which, obviously poieward of the other, is also displaced westward. They then see substorm sequences as a series of impulsive westward movements of the westward end of the electrojet system. As mentioned above, Rostoker (1974) feels this may correspond to the same movement of the Harang discontinuity. A sibilant modification of this picture is required to make it compatible with our substorm sequence. If it is assumed that the 0.5: 58 disturbance, which developed both equatorward and westward of the existing current flows, is the beginning of a new substorm sequence, then the next development in the sequence (06: 13) was the appearance of a new current system poleward of the 05:59 system
but which extended beyond it in both the east and the west. The change in the Churchill line profiles and the nature of the visual and radio aurora in that sector establish the significant enhancement of current flow in that sector after 06: 13. One is then left with the problem of relating such behaviour to movements of the Harang discontinuity. If, on the other hand, the OS:58 disturbance is part of the 04: 16 sequence, we must allow westward and e~aato$~ard developments into the sequence. are indebted to J. K. Walker for providing us with his magnetograms and also a covy of the ATSJ data. For the latter data we are ulttmately indebted to T. L. Skillman, now retired from Acknowledgements-We
NASA Goddard Space Flight Center. We would also like to thank F. Creutzberg for giving us the reduced photometer data. One of the authors (DRM) is indebted to G. Rostoker for advice on reading magnetograms. The standard observatory magnetograms were supplied by World Data Center A and by the Earth Physics Branch of the Canadian Department of Energy, Mines and Resources. REFERENCES Akasofu. S.-I., DeForest, S. and McIlwain, C. (1974). Aurora1 displays near the ‘foot’ of the field line of the ATS-5 satellite. Planet. Suace Sci. 22.25. Bonnevier, B., Bostrom, R. and Rostokeri G. (1970). A three-dimensional model current system for polar magnetic substorms. J. geo@s. Res. 75,107. Cummings, W. D., Barfleld, J. N. and Coleman, P. J., Jr. (1968). Magnetospheric substorms observed at the synchronous orbit. J. geophys. Res. 73,6687. Cummings, W. D. and Coleman, P. J., Jr. (1968). Simultaneous magnetic field variations at the earth’s surface and at synchronous, equatorial distance. Part 1. Bay-associated events. Radio SC&.3,758. Fairfield, D. A. (1968). Average magnetic field configuration of the outer magnetosphere. f. geophys. Res. 73,7329. Hakura, Y. (1965). Tables and maps of geomagnetic coordinates corrected by higher order spherical harmonic trems. Rep. Zonosph. Space Res. Japan. 19,121. Kelley, M. C., Starr, J. A. and Mozer, F. S. (1971). Relationshio between ma~netos~heric electric fields and the motion of auroril forms. f. geophys. Res. 76, 5269. Kisabeth, J. L. (1972). The dynamical development of the polar electrojets. Ph.D. dissertation. University of Alberta, Edmonton. Kisabeth. J. L. and Rostoker, G. (1971). DeveIopment of the polar electrojet during polar magnetic substorms. J. geophys. Res. 76,6815. Kisabeth, J. L. and Rostoker, G. (1974). The expansive phase of magnetospheric substorms. 1. Development of the aurora1 electrojets and aurora1 arc configuration during a substorm. J. geophys. Res. 79, 972. Montbriand, L. E. (1970). A simple method for determining the local time of corrected geomagnetic midnight. J. geophys. Res, 75,5634.
280
D. R. MCDIARMID and F. R. HARRIS
Nopper, R. W., Jr. and Hermance, J. F. (1974). Phase relations between polar magnetic substorm fields at the surface of a finite conducting earth. J. geophys. Res. 79,4799. Rostoker, G. (1974). Current flow in the magnetosphere during magnetospheric substorms. J. geophys. Res. 79,1994.
Weins, R. G. and Rostoker, G. (1973). Ground based magnetic signatures of the phases of magnetospheric substorms-a reconciliation. Ebs Trans. Am. Geophys. Un. 54,412. Zmuda, A. J. and Armstrong, J. C. (1974). The diurnal flow pattern of field-aligned currents. J. geophys. Res. 79,4611.
Spcrcc SC:., Vol.
24. pp. 269 to 280.
Penmmon
Pms,
1916.
Printed
in Northern
ldand
THE STRUCTURE OF A CONNECTED SEQUENCE MAGNETOSPHERIC SUBSTORMS
OF
D. R. McDIARMID and F. R. HARRIS Her&erg Institute of Astrophysics, National Research Council of Canada, Ottawa KlA OR8, Canada (Received infinal form 10 July 1975) Abstract-The structure of a sequence of four spatially and temporally connected magnetospheric substorms has been determined through the use of a set of mid-latitude magnetometer station data, an auroral zone ma etometer line data set, and the ATS-5 magnetometer record. In two of the substorms, two para$lel westward electrojets were observed to develop during the initial part of the expansion phase. In another, the expansion phase consisted of the development of a westward electrojet westward and equatorward of the pm-existing current system.
INTRODUCTION
1969-1970 tbrougb the work of several groups ‘meridian magnetometer line existed through Fort Churchill. Included in the line were an all-sky camera (Churchill) and a radar (Thompson). The purpose of this paper is to present an analysis of ionospheric and magnetospheric current flow during a sequence of four temporally and spatially connected substorms seen on or near this line between 04:oO and 07 : 00 UT during 30 January 1970. Some correlation with concurrent visual aurora will also be presented. More detailed correlations between radio aurora, visual aurora and ionsopheric current flows will be presented in a subsequent paper in collaboration with A. G. McNamara. For several
months
during the winter
OBSERVATION NETWORK AND DATA ANALYSIS PROCEDURE
The geomagnetic locations of the magnetometers whose records were used in this study are shown in Fig. 1 and both the geographic and corrected geomagnetic (Hakura, 1965) coordinates of the stations are listed in Table 1. The Gillam, Caribou River and Eskimo Point fluxgate magnetometers were operated by F. Harris, and the Winnipeg, The Pas, Lynn Lake and Thompson fluxgate magnetometers were operated by J. K. Walker of the Canadian Department of Energy, Mines and Resources. All other magnetometer records were produced at regular magnetometer stations. We also have magnetometer data from ATS-5 which was situated on a field line which originated in the vicinity of Gillam. All of these records are in analog form. In addition to the magnetometer data, we have used the 35 mm all-sky camera records from Great
Whale and Fort Churchill, and the 48 MHz aurora1 radar record from Thompson. Finally, some data from a meridian-scanning photometer (5577 A) at Fort Churchill were provided by F. Creutzberg. The mid-latitudemagnetometer data were analyzed to determine the field-aligned ‘equivalent’ currents using Kisabeth’s (1972) model. The model consists of current flow down dipole field lines from the magnetosphere to the ionosphere at the eastern limit of the substorm sector, westward current ilow in the ionosphere, upward current flow along dipole field lines at the western limit of substorm sector, and closure via current across the tail. This model accounts for the induced currents by assuming a spherical core of perfect metal some distance, below the surface of the Earth (250 km in our computations).* This approach has been extensively used by Kisabeth (1972), by Kisabeth and Rostoker (1971, 1974), and earlier by Bonnevier et al. (1970). Subsequent to the mid-latitude analysis, the final form of the Churchill line magnetic disturbance profiles were determined using the mid-latitude results to assist in finding the orientation of the effective magnetic disturbance meridian with respect to the geographic meridian. Conclusions were obtained by comparing the observed profiles with the published theoretical profiles of Kisabeth (1972) and of Kisabeth and Rostoker (1971). As described below, the effective magnetic disturbance meridian is that great circle through Churchill which intersects the ionospheric current flow at right angles.
* The work of Nopper and Hermance (1974) indicates that a more realistic and, unfortunately, much more difficult treatment should ultimately be considered. However, we do not believe that such treatment would alter our conclusions in any significant way. 269
D. R. MCDIARMID
270
FIG. 1. THE GEOMAGNETIC LOCATIONS OF THE STATIONS USEDINTHISSTUDY.
The key for the name symbols is included in Table 1. Initial inspection of the data indicated that the electrojets flowed in a direction somewhat north of geomagnetic west (confirmed in later analysis) and our mid-latitude analysis took account of this observation. One would expect this behaviour in the pre-midnight sector for current flow along the oval but in this case, the phenomenon persisted until geomagnetic midnight. Our mid-latitude analysis did not account for the ionospheric portion of the current flow which should not be significant at these latitudes. The cross-tail return current, which represents the deflection of westward tail current through the ionosphere, was that corresponding to the higher L value and conjugacy of current flow was assumed. Thus in the model, which, TABLE 1. THEGEOGRAPHICANDCORREC~DGEOMAGNETIC LOCATIONSOPTHESTATIONSUSEDINTHISSTUDY
Local magnetic midnight was commuted usine Montbrian& (1970) cur&s
T *lTm
64.3
264.0
75.1
320.4
61.1
265.9
72.5
325.3
59.4
265.2
70.7
324.7
58.8
265.9
TO.0
326.0
56.4
265.3
67.9
56.8
259.0
61.3
316.3
55.8
262.2
66.8
321.4
54.0
258.9
64.5
YT.3
49.6
262.9
60.9
324.1
61.2
34.6
69.0
55.3
282.2
68.2
353.8
54.5
246.6
62.5
301.2
64.9
212.2
64.9
260.3
71.3
203.2
69.7
247.0
325.9
45.6
51.0
355.5
50.0
77.9
47.6’
307.3
58.3
.29.8
3.S.l
293.8
38.2
282.6
51.8
352.2
33.0
96.1
43.4
326.9
29.9
3.2
40.0
254.7
49.3
315.7
LB.2
242.5
55.3
299.5
48.5
236.6
53.9
292.6
21.3
2M.O
21.1
266.5
13.6
144.9
4.0
212.9
-
and F. R. HARRIS as will be seen, was applied to a situation where the electrojet flows were primarily in the dusk-midnight quadrant, the substorm eastward cross-tail current does not connect with the eastern field-aligned current. However, we note that we did not account in the model for the fact that near the midnight sector, the field lines are extended tailward (Fairfield, 1968). Nor did we account for the aximuthal bending of field lines tailward across local time sectors (Fairfield, 1968). The analysis procedure was to determine which of a set of field-aligned and cross-tail currents (successive cut and try), taking account of the latitude profile of ionospheric current flow indicated by the magnetometer line, best matched the disturbance fields recorded at Fredericksburg, Dallas, Boulder, Newport and Victoria. These disturbance fields were measured relative to the magnetometer values at 05:22. (All times are UT unless otherwise specified.) In addition to the above sources of error is that of baseline determination. Several sets of relatively quiet days were used and yielded baselines with some scatter. This scatter was significant with respect to the OS:405 disturbance and tolerable with respect to the others (05:55.5, 06:03, 06:12, 06:21, 06:40.5). The best match was in terms of least scatter in the current values resulting from comparison between observed and computed disturbances. The sources of error outlined above plus reading errors lead us to use the results of our analysis in qualitative rather than a quantitative way. Having determined the longitudinal extent and geomagnetic location of the electrojets at six points of time during the sequence of events described below, we then plotted H, D, and 2 protiles for various values of 0 where 0 is the angle between the X magnetic axis and the H-axis. Using the theoretical profiles of Kisabeth (1972) and Kisabeth and Rostoker (1971) for comparison, we determined for each chosen time which 6 seemed to best correspond to a line perpendicular to the current flow, i.e. the orientation of the effective magnetic disturbance meridian. A value of 15’ was chosen. This corresponds to electrojet current flow approximately 9” north of geomagnetic west. As in the mid-latitude analysis, the disturbance fields for the magnetometer chain were determined relative to the magnetometer values at 05 : 22. The meridian chosen above is, of course, not the geomagnetic meridian and thus our H and D components are slightly different from the normally defined ones. Distances along the meridian line relative to Churchill were computed by constructing
271
Magnetospheric substorms
_F-component
Time,
FIG.
UT
2. MmLATITUDE AND ATS-5 MAGNETCKXUMS. The dashed lines are baselines for the ATS-5 records, which are not plotted beyond 06: 00 because the record was extremely diEicult to read there. Straight lines were used in the Z component plat where the variations were not significant. a new coordinate system in which OUTmetidian was a inczeased after 04: 25. This corre&es with the arrival at the rna~opa~ of a southward interlon@ude line and~~c~l was assigned itscorrected planetary magnetic field (IMF) at approximately geomagnetic latitude. The positions in this coordinthe same time. The field remained southward for ate system of the other stations were determined several hours (Rostoker, private communication). and used to compute tie distance along our meridian Note the positive correlation between the ground from Churchill to the “latitude’ of each of the other station records of the more notable features of the H stations. Ln the profile plots the positive direction component trace. (T&e format of the ATS-5 from Churchi@ is north and the order of the stations magnetometer data obtained by us hides whatever is that of Table 1 with the exception of Thompson whiGh could not her plotted because only its X of these features was seen at the satellite). Both the decrease in the ATS-5 H component and its component was available. general similarity to the Dallas H component record are not unexpected Qmmings and Coleman, 1958; Cummings ef al., 1968) and show that the sequence of events described &low were not The mid-latitude magnetic records fram San Juan, significantly unrepresentative of substorms in Fredericksburg, Dallas, Boulder, Newport and general. We see also that the disturbance in the Victoria are shown in Fig. 2 along with the ATSJ geographic equatorial plane is ba&alIy radially data. The scales have all been equalized and the outward as reported earlier by Cummings er at. dashed line is the baseline for the disturbance seen (1968t under similar circumstances. However, in at AT%-5. The ATS background was taken as the contrast to their typical observation, there was no average of the 25 Jamzuy and 8 February records, concurrent positive H component disturbance seen the only nearby quiet days for which we have good at aurora1 latitudes in the geomagnetic longitude data. Straight lines were used in the Z component sector of the satellite or at any other of the high plot where the variations were not significant. latitude stations in Fig. I with the exception of Prior to about 04~25, all the records show a Port Barrow f-80” west of satellite) which recorded decrease in the X component, the rate of which a disturbance in X of approx, 20 y. However, as
272
D. R. MCD~ARM~and F. R. HARRIS
discussed below, this disagreement was probably the result of masking by another current system. Inspection of Fig. 2 shows the depression in H to have occurred between geomagnetic longitudes 292’E and 352’E but at 3*2’E (San Juan) the amplitude of the depression was significantly attenuated. In other words, the region of depressed H extended from the dusk sector to the midnight sector but is significantly attenuated in the early morning sector-a result similar to the statistical conclusion of Cummings et al. (1968) using ATS-1 data. Further interpretation will be left to the Discussion section. Inspection of the first three all-sky camera pairs of Fig. 3(a) shows southward drifting bands over Great Whale and southwestward drifting bands overChurchill. Theseweredetected from aboutO4:OO at Churchill and earlier at Great Whale. The electric field over Great Whale as deduced from the rate of drift was only several mV/m prior to 04: 16. The intensity at 5577 A of the band near the Churchill zenith at 04:20 was approx. 1.4 kR. For the purpose of permitting the reader to compare our all-sky camera data with those of other systems we note that the f number of the cameramirror system is 1.2 (d&se source), the exposure time was 40 set and the film was Kodak Tri-X (ASA-400). We must, however, point out that operation at Churchill was substandard at the time of interest. First of all, an improper lens was accidentally in use which produced a slightly out-of-focus image. Secondly, the film processing, contrary to our standard practice, was done at the station and resulted in significant fogging. Thus bright sky over Churchill relative to Great Whale in Fig. 3(a) is not real but a result of tihn processing. 2. 04:16-05:23
UT
As noted by Akasofu et al. (1974) in their discussion of this event, an arc near the poleward boundary of the diffuse aurora over Great Whale brightened very slightly at 04: 16. At this time the beginning of a substorm-like disturbance was recorded by the Narassarssuaq magnetometer (see Fig. 4). It was not until 04:26 that a significant brightening of an auroral band occurred over Great Whale although the onset of magnetic activity was recorded several minutes earlier (&:21). The visual aurora did not brighten near the foot of the ATSJ field line (in the vicinity of the lower part of south pointing arm of the all-sky photograph) until about 04:32. As can be seen in Fig. 3(a), the brightening propagated westward from the Great Whale sector. It was accompanied by relatively little magnetic
I
/
I 0400
I
I
I
I
I
0%
I
I
I
I
Oeco
I
I
I
I
0700
I
I 0800
'Time. UT FIG.
4. Tut+ NARSSARSSUAQAND
GREAT
WHALE H
COMPONENTS.
The solid triangles mark the beginning of the O&16 disturbance at each station. activity on the Churchill magnetometer line. Although there was some brightening of the aurora1 bands over Churchill poleward of the brightened band, these bands continued their slow southwestward drift until 05:23 in contrast to the active aurora over Great Whale. When the rate of drift is determined relative to that expected from rotation of an all-sky camera under a fixed oval-aligned band, the electric field (u = E/B) is found to increase from several millivolts/meter before 04:25 to the order of 10 mV/m or a little larger afterwards (prior to 05 :OO). Kelley et al. (1971) have shown that electric fields determined from meridional aurora drift velocities during periods with no expansive phase are equal to or somewhat smaller than the values measured via balloon probes. The brightness of the prominent auroral band south of Churchill at 04: 50 was approx. 25 kR at 5577 A, an order of magnitude brighter than the aurora before 04:26. We note in Fig. 2 that the rate of decrease in H at all the mid-latitude stations and at ATS-5 increased after 04 : 25. What these observations add up to is increased convection in the magnetosphere and concurrent substorm activity between the Greenland and Great Whale sectors. We do not have sufficient data to determine either the longitudinal extent, latitudinal structure or intensity of the 04: 16 current system. We do know that after its initial development near the Greenland sector, its zone of influence spread westward until it was observed at Great Whale after 04: 26. We also know that there is virtually no
Fro.
3. (a,b) SELE~~EDALL-~KYPHOT~ORA~HS (POSITIVE)FROM EITHER GREAT WHALEOR~HIJRCHILL North is upward with west and east to the left and right, respectively.
ASNOTED.
272
FIG. 3 (cunrhed).
273
Magnetospheric substorms ,
.a
4l.I
r
,
I’D
X-component
4”
I
*
Y-component
CH
Gi
TP.
v
2ooy
w. I
05:oo
’
I
4
06:Oi,
I
I
I
07~00.
I 05:oo
I
’
I
Time,
FIG. 5.
MAG~IC
RECORDS The
FROM
If
07~00
iGG& 05:oo
’
osod
0700
UT
THE STATIONS IN THE CHURCHILLMAGNETOMETERLINE.
key for the name symbols appears in Table 1.
evidence of this current recorded on the Churchill magnetometer line. Meanwhile the ATS-5 H record continued to behave in a pre-substorm manner and the electric field over Churchill indicated enhanced magnetospheric convection as expected from the behaviour of the IMF. In this situation, the Great Whale all-sky film was not the best record of behaviour at the foot of the ATS-5 field line (I$ Akasofu et al., 1974). 3. 05:23-OS:58
G4.I
0600
UT
Although there was a minor brightening of the aurora south of Churchill at 05 : 21, the significant brightening began in the geomagnetic southwest at 05 : 23 as shown in Fig. 3(b). Poleward movement toward the Churchill zenith from the same sector began at 05:28+ by which time the magnetic disturbance observed on the Churchill line had increased significantly, as shown in Figs. 5 and 6. In Fig. 3 (b) we see the extent of this movement of the visual aurora at 05:32 and also note that except for the eastern sector, this poleward movement remained stalled between 05: 32 and 05:36. Akasofu et al. (1974) noted that ATS-5 encountered a hot plasma with an intense flux during this event at about
05: 35. At 05 :40 we see the appearance of new aurora in the southeast zenithward of the preexisting aurora. By 05 : 44 the aurora near the southem horizon had faded and the new aurora had brightened. In quantitative terms, the aurora in the south (west side of south arm of the all-sky camera) had a brightness of approx. 20 kR (5577 A) at 05:41 and the more zenithward emission was approx. 10 kR (5577 A). At 05:44, the 10 kR had increased to approx. 90 kR. After 05~45, the visual aurora began to decrease in brightness, both in the background emission and, on average, in the discrete forms until 05: 58. We must point out that brightness comparisons between the Churchill photographs in Figs. 3(a) and (b) cannot be made because, although print exposure times are constant in each figure, they were increased in Fig. 3(b) relative to Fig. 3(a) in order to retain detail in the brighter aurora. Figure 6 shows the Churchill magnetometer line profiles of the current system which developed at 05:23 at several times throughout the event. Some interpretation has been made in producing these plots. The data appear very consistent with the character of the theoretical profiles in Kisabeth
and F. R. HARRIS
D. R. McDr-
274
/ 05~28
/ /
5
;r
.o f
-50
!
i
1D-component
/ 1
*Z-component
1
‘W-component
H -100
50; .f: 2 4 3 5: 2 0) -took s
I
-1000
Line
-500
distance
0
from
Fort
Churchill,
500
0
-500
-1000
km Line distance
from
Fort
Churchill,
500
km
FIG. 6. SELECTED ‘LATITUDE' PROFILES (CHURCHILL LINE) OF THE H, D ANLI Z COMPONENTS FOR THE 05: 23 SUBSTORM. Distance is positive poleward
and Rostoker (1971) and in Kisabeth (1972) for two parallel westward electrojets (see, e.g. Fig. 3(c) in the first reference) and our profiles are plotted accordingly. In Fig. 6 we see the development of two parallel electrojets, one near Gillam and the weaker one near Churchill. The southern electrojet moved somewhat equatorward after the initial onset. It would be interesting to conclude that the poleward electrojet reached a maximum at OS:44 when the visual aurora was most intense in the Churchill vicinity but the data (as opposed to our interpretive plotting) do not unequivocally support such a conclusion. Although the H component profiles do not unmistakably indicate a double electrojet
from
Churchill.
system prior to 05:40, the nature of the D and Z component profiles clarify the situation. The data for the 05 :28 and 05 :40 Z component profiles clearly show the two maxima and two minima which are characteristic of a parallel electrojet system (Kisabeth and Rostoker, 1971). The Ed-latitude analysis discussed below indicates that the central meridian of the combined current system was close to the magnetometer line at 05:405. Now if the central meridian of the low latitude electrojet was west of the magnetometer line, one would expect a D component profile not dissimilar to what was observed at 05:40 and at most other times. In other words, these profiles
Magnetospherlc substorms also indicate a double current system. The large negative minimum of the 05:32 D component profile remains an enigma. In contrast to the observations of Kisabeth and Rostoker (1971), both electrojets developed simultaneously during the expansive phase. Although the discrete visual aurora dimmed after 05 :45 and gradually became confined to the southern sky (Churchill), both electrojets continued to flow until OS:58 and, in fact, the poleward electrojet reached a maximum (stronger than the equatorward electrojet) around 05 : 50. In this regard our observations are not inconsistent with the northern border current intensifications reported by Kisabeth and Rostoker (1971). An analysis of the mid-latitude disturbance field of the 05:23 current system was performed on the 05:405 data. Two parallel electrojets, the equatorward twice as intense as the other (see Fig. 6), were assumed in the model. The analysis procedure was that described above and it was found that the ionospheric portion of the electrojet flowed west between geomagnetic longitudes 360’ and 295’ approx. Analysis of the 05 : 55.5 data indicated an eastward shift of the end points by approx. 10’. Both analyses indicated a total current flow of approximately 1O5A. A comparison between our observed aurora1 zone profiles and the theoretical profiles of Kisabeth and Rostoker (1971) or of Kisabeth (1972) indicates a somewhat higher value than lo5 A. To summarize, at 05: 23, a current system of 50°60° longitudinal extent developed west of or extended out of the 04: 16 current system. Since we do not know the latitudinal structure of the 04:16 current system, we cannot say when the double electrojet developed. This extension was accompanied by substorm behaviour of the visual aurora over Churchill. 4. 05:58-06:13
UT
As stated above, by 05:58 the visual aurora was south of Churchill. At this time the band close to the horizon in the west to southwest brightened (see Fig. 3(b)). This brightening reached its peak at 05 : 59 and by 06 : 01 the brightness had diminished to a level comparable to the more zenithward aurora. Thereafter until 06: 12 the visual aurora became more and more confined to the southern horizon. In Fig. 7 we see the magnetic consequences of the disturbance. Almost immediately the equatorward peak in the H component increased and by 06:03 had broadened southward. The dramatic change occurred at The Pas and, as can be seen in Fig. 8,
275
this behavionr persisted until 06: 12. Analysis of the mid-latitude data for 06:03 and 06:12 clarified these profiles. Since most of the change occurred in a confined latitude region, it appears to be the result of the addition of a new current system to the one existing before 05 : 58 which remained unchanged. To investigate this possibility, we subtracted the mid-latitude disturbance values just prior to 05:58 from those for 06: 03 and 06: 12. The subsequent midlatitude analysis showed the new disturbance to result from a current system well to the west of our magnetometer line and a similar treatment on the profile data showed the system to be equatorward of the preexisting current flows. Evidence of this current system is contained in Fig. 2; the H component disturbance is much greater at Newport and Victoria than at the more easterly stations. Specifically, at 06:03 the ionospheric portion of current flow (westward) was between geomagnetic longitudes of 310’ and 270’ approximately. The situation at 06:12 was unchanged except for the amplitude of the current flow which, from the mid-latitude analysis, increased to 3(106) A from 2(106) A at 06:03. In other words, the total current flow at 06:03 was 3(105) A and at 06:12 was 4(105) A when all current systems are included. In summary, at 05: 58 a new and ultimately stronger current system was added to that previously in existence. The ionospheric portion of the new current system was equatorward and westward (overlapping) of the other system. In addition, a positive bay was observed at College indicating an eastward electrojet in the dusk sector. At 06:02, ATSJ encountered an even hotter plasma than it did at 05:35 (Akasofu et al., 1974). 5. After 06:13 UT As stated above, the visual aurora was confined to the sky well south of Churchill by 06:12. At 06:13 a band extending from the south to the southwest brightened (see Fig. 3(b)) and by 06:15 zenithward movement from the southwest was apparent. By 06 : 20 aurora was again present in the zenith and the discrete forms were accompanied by an overall diffuse emission with the result that most of the stars were masked out in the all-sky camera record. The poleward boundary of the discrete forms moved northward to about Baker Lake by 06:26 and remained there until 06: 59 when it began to move equatorward. New plasma was observed by ATS-5 at about 06: 17 (Akasofu et al., 1974). The magnetic profiles of this event are shown in Fig. 8. First of all, the new current system of 05:58
D. R. MCDIW
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FIG. 7. SELECTED ‘LATITUDE’ PROFILES (CHURCHILL LINE) OFH, D AND Z COMPONENTS FORTHE 05 : 58 SUBSTORM. Distance is positive poleward from Churchill. Note the change in the scale of the ordinate axis from the previous figure. which was seen primarily at The Pas appears to decay before the 06:13 current system began to grow. This new current system was similar to the 05 : 23 system in both position and latitude structure. Again we see two parallel westward electrojets which simultaneously increase in strength but this time the two are more equal in intensity. Also this time there is clear indication of a parallel electrojet system in both the Hand Z component profiles. The parallel electrojet system produced a maximum of the profile peak intensities at about 06:21 and several minutes thereafter became much less confined in meridian cross-section in what was probably a merging of the two electrojets. This is seen in the 06:33 profile. Note that current flow existed well north of Churchill at this time. Following 06:33 there was an increase in intensity of the current flows, particularly in the equatorward electrojet. This was followed by a decrease which showed most clearly in the attenuation of the H component disturbance of the southern electrojet but also showed in the decrease of the latitudinal extent of the influence of the northern electrojet. This is seen in the 06:44 profile. Thereafter as the
substorm decayed the profiles became more complicated and will not be discussed here. The changes in total current flow after 06:33 indicated by the profiles are not large and would require a detailed model analysis to quantify. The result of the mid-latitude analysis for 06:12 (06 : 03 plot) is shown in Fig. 9. The result for 06 : 21, the end of the first rapid intensification of the 06: 13 event, was a parallel electrojet system of total current flow around 4(105) A extending from 350“ to 240’ geomagnetic longitude as shown in Fig. 9 wherein the Earth’s rotation has been accounted for. Examination of Fig. 2 shows that after 06:21 the total current flow continued to increase but the western end of the electrojet moved eastward until about 06: 30 after which the eastern end of the electrojet then also moved eastward such that at 06:405 the total current flow was of the order of 6-8(105) A and ionospheric currents flowed between approx. 5’ and 290’ geomagnetic longitudes. An attempt was made to take account in these analyses of the existence of an eastward electrojet in the vicinity of College. The Hcomponent disturbance at College reached a maximum at 06:25 before diminishing
Magnetospheric substorms
277
06:44
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FIG. 8. SELECTED
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06:13
Distance is positive poleward from Churchill. Note the change in the scale of the ordinate axis from the previous figure. rapidly. The College and Port Barrow disturbances are consistent with the existence of an eastward electrojet significantly weaker than the westward electrojet system and equatorward of it when they overlapped. Figure 2 also indicates that after 06:40-45, the total electrojet current flow peaked and began to decrease and the western end of the current system moved westward again. Figure 9, which shows the results of our midlatitude analyses, serves as a resume of the event sequence described above. We see the initial substorm of undetermined longitudinal extent and latitudinal structure, its expansion westward after 05: 23, the appearance of the new current system in the west after 05 : 58 followed by its disappearance after 06: 13 with the development of the current
system of the final substorm in the sequence. The 04: 16 current system is only indicated in the plot for which we have some information concerning it. We included the current values resulting from the analyses presented in the text above for the purpose of showing the extent of their variation throughout the event. As noted above, comparison of our profiles with those of Kisabeth and Rostoker (1971) or of Kisabeth (1972) indicates somewhat larger values than those produced by our mid-latitude analysis. DISCUSSION
AND CONCLUSIONS
We will consider first of all the current system responsible for the pre-OS:23 depression in the The mid-latitude and ATS-5 H components.
278
D. R. MCDIARMIDand F. R. HARRls
The solid lines are the westward electrojets which were analyzed, the alternate dash-dot line is the 04: 16 westward electrojet which could not be resolved, and the dashed lines are the unresolved eastward electrojets. The 04:16 current system is only indicated in the plot for which we have some iaformation concerning it. The hatched area in the 06:405 plot is used to indicate the latitudinal spreading of the two electrojets and the black dot at the top of each diagram denotes geomagnetic noon. behaviour of the latter was the same as has been commonly observed in the pre-midnight sector prior to the onset of a substorm expansion phase (Cummings et al., 1968). The source of the disturbance is believed to be a partial ring current in the evening sector. We have noted above the general correlation between the ATS-5 I;T component variation and those of the mid-latitude ground stations as well as the fine structure correlations between the latter. Since mid-latitude stations are particularly sensitive to field-aligned currents, the data suggest these were flowing even prior to the iirst expansion phase onset at 04: 16. Two models have been proposed to account for the pre-substorm expansive phase magnetic disturbances and both contain field-aligned currents. One model consists of a partial ring current in the pre-midnight sector with closure through fieldaligned currents and eastward current in the ionosphere (Cummings et al., 1969; their Fig. 8). The other model was recently proposed by Rostoker (1974) to explain both the ATS W component and Triad field-aligned current observations (e.g. Zmuda and Armstrong, 1974). His presubstorm current co~~mtion is shown in Fig. 3 of his paper and consists of a pre-midnight eastward electrojet,
a post-midnight westward electrojet and outward field-aligned current at the junction of these electrojets (Harang discontinuity). This field-aligned current couples to the ring current and consequently increases the ring current in the evening sector as observed by the ATS satellites. Closure to the electrojets again is assumed to occur diffusely on the dayside. As we will discuss later, Rostoker proposes that substorms consist of steplike westward motions of the Harang discontinuity. We can put limits on the position of the upward field-aiigned current in the midnight sector after 04: 16, The large depression in the ATS-5 H component requires the field-aligned current to be east of the satellite. The behaviour of the Dcomponent at the satellite is consistent with this conclusion if we assume conjugacy of current flow and note the position of the satellite south of the tail sheet as shown in Fig. 9 of Cummings et aZ. (1968). On the other hand, it could not have been very far east of the satellite since the depression in H was significantly attenuated at San Juan and the western end of the 04: 16 disturbance was near the Great Whale sector. In order to compare the models described above with our 04:0@-05 23 observations, we had to
279
Magnetospheric substorms include the 04:16 current system into them. This was difficult to attempt because of the unknown extent of the 04:16 current system and the relative error of the observed data before 05 : 23. However, an intensification occurred at the western end of the 04: 16 system beghming at 04: 50. The Fredericksburg, San Juan and St. John’s (not shown) magnetograms provided a good indication of the longitudinal extent of the intens~cation and the comparison between the calculated and observed mid-latitude disturbances for the intensification indicated that both models described the situation reasonably well. A comparison for the 04:00-04:45 period was inconclusive. At first glance, both of these modeis predict the observation of eastward current flow over the Churchill magnetometer line. This was not observed. However, the westward electrojet in the Great Whale sector produced Hand Z disturbances which tended to cancel those of an overhead eastward electrojet. The opposite is true of the D component and, in fact, there was evidence of combined eastward and westward current flows in the D component records after 04 : 30. Turning now to the substorm sequence, we saw in both the 05 :23 and 06: 13 substorms the simultaneous development of two parallel eiectrojets. Parallel electrojets have been observed before (Kisabeth and Rostoker, 1971) but their development was not simultaneous. Rather the equatorward electrojet developed first and the poleward one developed 15-20 mm later and underwent periodic fluctuations in amplitude. Kisabeth and Rostoker fmd this behaviour to be common so the substorm sequence described herein must be relatively uncommon. Wiens and Rostoker (1973) have further concluded that the poleward electrojet which develops 15-20 min after the beginning of the expansive phase is a current system which, obviously poieward of the other, is also displaced westward. They then see substorm sequences as a series of impulsive westward movements of the westward end of the electrojet system. As mentioned above, Rostoker (1974) feels this may correspond to the same movement of the Harang discontinuity. A sibilant modification of this picture is required to make it compatible with our substorm sequence. If it is assumed that the 0.5: 58 disturbance, which developed both equatorward and westward of the existing current flows, is the beginning of a new substorm sequence, then the next development in the sequence (06: 13) was the appearance of a new current system poleward of the 05:59 system
but which extended beyond it in both the east and the west. The change in the Churchill line profiles and the nature of the visual and radio aurora in that sector establish the significant enhancement of current flow in that sector after 06: 13. One is then left with the problem of relating such behaviour to movements of the Harang discontinuity. If, on the other hand, the OS:58 disturbance is part of the 04: 16 sequence, we must allow westward and e~aato$~ard developments into the sequence. are indebted to J. K. Walker for providing us with his magnetograms and also a covy of the ATSJ data. For the latter data we are ulttmately indebted to T. L. Skillman, now retired from Acknowledgements-We
NASA Goddard Space Flight Center. We would also like to thank F. Creutzberg for giving us the reduced photometer data. One of the authors (DRM) is indebted to G. Rostoker for advice on reading magnetograms. The standard observatory magnetograms were supplied by World Data Center A and by the Earth Physics Branch of the Canadian Department of Energy, Mines and Resources. REFERENCES Akasofu. S.-I., DeForest, S. and McIlwain, C. (1974). Aurora1 displays near the ‘foot’ of the field line of the ATS-5 satellite. Planet. Suace Sci. 22.25. Bonnevier, B., Bostrom, R. and Rostokeri G. (1970). A three-dimensional model current system for polar magnetic substorms. J. geo@s. Res. 75,107. Cummings, W. D., Barfleld, J. N. and Coleman, P. J., Jr. (1968). Magnetospheric substorms observed at the synchronous orbit. J. geophys. Res. 73,6687. Cummings, W. D. and Coleman, P. J., Jr. (1968). Simultaneous magnetic field variations at the earth’s surface and at synchronous, equatorial distance. Part 1. Bay-associated events. Radio SC&.3,758. Fairfield, D. A. (1968). Average magnetic field configuration of the outer magnetosphere. f. geophys. Res. 73,7329. Hakura, Y. (1965). Tables and maps of geomagnetic coordinates corrected by higher order spherical harmonic trems. Rep. Zonosph. Space Res. Japan. 19,121. Kelley, M. C., Starr, J. A. and Mozer, F. S. (1971). Relationshio between ma~netos~heric electric fields and the motion of auroril forms. f. geophys. Res. 76, 5269. Kisabeth, J. L. (1972). The dynamical development of the polar electrojets. Ph.D. dissertation. University of Alberta, Edmonton. Kisabeth. J. L. and Rostoker, G. (1971). DeveIopment of the polar electrojet during polar magnetic substorms. J. geophys. Res. 76,6815. Kisabeth, J. L. and Rostoker, G. (1974). The expansive phase of magnetospheric substorms. 1. Development of the aurora1 electrojets and aurora1 arc configuration during a substorm. J. geophys. Res. 79, 972. Montbriand, L. E. (1970). A simple method for determining the local time of corrected geomagnetic midnight. J. geophys. Res, 75,5634.
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Nopper, R. W., Jr. and Hermance, J. F. (1974). Phase relations between polar magnetic substorm fields at the surface of a finite conducting earth. J. geophys. Res. 79,4799. Rostoker, G. (1974). Current flow in the magnetosphere during magnetospheric substorms. J. geophys. Res. 79,1994.
Weins, R. G. and Rostoker, G. (1973). Ground based magnetic signatures of the phases of magnetospheric substorms-a reconciliation. Ebs Trans. Am. Geophys. Un. 54,412. Zmuda, A. J. and Armstrong, J. C. (1974). The diurnal flow pattern of field-aligned currents. J. geophys. Res. 79,4611.