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The roles of the north-south component of the interplanetary magnetic field on large-scale auroral dynamics observed by the DMSP satellite
Planet.
Space
Scl.,
Vol. 23, pp. 1349 to 1354.
Pcrgamon
Press,
1975.
Printed
in Northern
Ireland
THE ROLES OF THE NORTH-SOUTH COMPONENT OF THE INTERPLANETARY MAGNETIC FIELD ON LARGESCALE AURORAL DYNAMICS OBSERVED BY THE DMSP SATELLITE Geophysical
S, -1, AKASOPIJ Institute, University of Alaska, Fairbanks,
Abstract-An extensive study of DMSP photographs field data suggests that the quantity detined by S=
s
Alaska 99’701, U.S.A.
and the simultaneous
interplanetary
magnetic
;(@,--crddr
has a fundamental importance in substorm processes, where @u and @N denote the production rate of merged (or open) field lines along the dayside X-line and of reconnected (or closed) fieId lines along the nightside X-line, respectively; d = 0 is measured from the time when the 23, component begins to decrease after a prolonged period of a large positive B, value. It is shown, first of all, that substorms occur so long as S > 0, regardless of the sign of the B, component and its changes (namely, the southward and northward turnings) and of its time derivative as well. Secondly, the intensity of substorms is proportional to Sa. By introducing the quantity S, the recent confusion of the problem of the roles of the north-south component of the interplanetary magnetic field on substorm processes can be removed. Since S is equal to the amount of the open magnetic fluxes at a time reckoned from I = 0, it is proportional to (A, - A,), where A, denotes the minimum polar cap area (namely, the area bounded by the minimum aurora1 oval) and A1 the polar cap area at an arbitrary time t. Therefore, substorms can occur whenever the aurora1 oval is larger than its minimum size. Further, an intense substorm tends to occur along a large oval. The quantity S can also be considered as an excess flux, and thus the substorm can be considered as a process by which the magnetosphere tends to remove sporadically the excess energy associated with S.
the last several years, one of the most important subjects on magnetospheric physics is the roles of the north-south component B, of the interplanetary magnetic field on magnetospheric processes, in particular on magnetospheric substorm processes. Since substorm energy is generated by the solar wind-magnetosphere dynamo and since the north-south component of the interpIanetary magnetic field controls the efhciency of the dynamo, there is no doubt that the B, component plays an important role. The point in question is thus what are the quantities which control directly the occurrence of substorms. Such a study requires a continuous monitoring of both a large-scale aurora1 activity over the entire polar region and the inte~lanetary magnetic field. AIthough we are far from having such an ideal set of data, we have now DMSP photographs (which can be considered to be “snap shots” taken about every 100 minutes) and the corresponding interplanetary magnetic field data for the months of During
October and December 1972 and January 1973; the interplanetary data are provided by the HEOS and IMP-I satellites. Figure 1 shows eight DMSP photographs which are chosen to show how the expansive phase of the aurora1 substorm might appear if DMSP photographs could be taken a few minutes apart. Let tDn be the production rate of merged (or open) field lines along the dayside X-line. This quantity is aiso equal to the potential drop along the X-Line and also to the potential drop across the dawn-dusk meridian in the polar cap. It is given (Sonnerup, 1974) by
where BI = the intensity of the interplanetary magnetic field, Bo = the intensity of the geomagnetic field just inside the magnetopause, 0 = the
1349 1
S.-I. AKASOFU
1350
and high Kp values (& > 2-) and showed that there is no significant differencein the structure of the magnetotail between the two conditions. Akasofu, Hones, Bame, Asbridge and Lui (1973) showed that the plasma sheet was present at a geocentric distance of 18 Rn after more than two days of an extremely quiet period. Under such a condition, it is expected to have a s = ‘(@, - aa,) dt steady state, and both tDn and @‘Nhave the same J0 minimum value. The aurora1 oval has the minimum where the time t = 0 is chosen at the instant when size, and its noon and midnight location is 80’~82’ the interplanetary magnetic field begins to decrease and 70°-‘72’ in invariant Iatitudes, respectively. after having a large northward component (B, 2 Further, the oval also becomes dim (often below the +5~) for an extended period, say 6-12 hours. threshold of the DMSP detector), and there is little Figure 2 illustrates SchematicaIly time variations indication of substorm activity. As we shall see on 24-25 and 27 of @n, DN, (@n - @n) and S = j5, (an - @J.J dt later, the aurora1 condition December 1972 corresponds to this situation. when the north-south component of the interplanetary magnetic field B, varies from a Iarge The fact that such a condition can occur suggests positive value to a negative value for about 2 hr and strongly that there is a minimum magnetic energy in then back to a large positive value again. Note that the magnetotail which is not available as substorm an extended period of a large positive B, value is the energy. That is to say, the magnetic energy in the magnetotail during an extended period of a large initial condition. In constructing Fig. 2, it is assumed that both @n positive B, value cannot be dissipated as substorm and SD, have a finite value even when the B, com- energy. This may also be inferred from the fact that ponent has a large positive value. This is because the the aurora1 oval does not shrink to a point either magnetotail (a product of the solar wind-magnetoduring a very quiet period or most intense substorms. Now, let us suppose that 3, begins to decrease sphere dynamo) appears to be a permanent feature, As 3, begins to and thus there must always be a finite amount of after such a quiet condition. open Aux even when B, has a large positive value for decrease, @n begins to increase immediately. Let us an extended period. Meng and Anderson (1974) suppose afso that B, reaches a minimum value in examined extensively magnetotail data at the lunar Iess than 1 hr after the southward turning and distance (60 Rx) for both low Kp values (Kp < 1 -I-) maintains a steady state value for about 2 hr. an& between Br and B,, V, = AIfven wave speed and L = the length of the merging (X) line. Let us define here also the production rate Qz, of reconnected (or closed) field lines along the nightside X-line. Consider then the quantity S defined by:
Interplanetary
Production
component
north-south
rote of
open . ..._ --~
Production field
rote
of
closed
in%?9
-l2hr-
FIG.
2
%‘POi-BETRXL
CHANGB &,
OF -
THfi & iDx,
COhWONENT
AND
s
=
f;
AND (#,
-
THE a,)
RESULTINQ
df.
CHANGES
OF
t&j,
@N,
FIG. I. EIGHT DMSP PHOTOGRAPHS WHICH ARE CHOSEN TO SHOW HOW THE EXPANSIVE PHASE OF THE ALIRORAL SUBSTORM MIGHT APPEAR IF THE PHOTOGRAPHS WERE TAKEN A FEW MINUTES APART. (Note
that they are not chronologically
shown.)
1350
0000
04:oo
0a:oo 28
12:oo
October
16:OO
20:oo
24:OO
1972
UT FIG. 3. SOME OF THE DMSP PHOTOGRAPHS WHICH WERE TAKEN IN THE LATER HALF OF 28 OCTOBER 1972: THE INTERPLANETARY MAGNETIC FIELD DATA (HEOS), B,Bx~~,BY>~,BZP; THE AE INDEX.
639
07:23 UT
13:2RUT
640
OS:05
643
UT
641
10:46UT
14:lOUT 24
December
1972
6:OO
24 December
25 December
1972
lgi’2
UT FIG. 4. SOME
OF THE DMSP PHOTOGRAPHS WHICH WERE TAKEN ON 24-25 DECEMBER 1972: INTERPLANETARY MAGNETIC FIELD DATA (HEOS), B, BXM, By,, Bznr;THE AEINDEX.
THE
IOr B
,”
0
?c
aI
O/B -5-
c" g
5~&u
5_BYM
SC
/ v
&EOl-5-6InQr250-l!G
a
1;
I$. $1
I1
b
18:OO 2000
26
I 24:OO
December
1 0400
”
3 (Dl 4
G u3I
a, CD1
ZI_
I
I 11 12:oo
OS00
27
1972
1
’
1
16:OO
December
I
2000
I 11 24:oO
1972
UT FIG. 5. SOME OF THE
DMSP
PHOTOGRAPHS TAKEN ON 26-21 DECEMBER 1972: THE INTERPLANETARY MAGNETIC FIELD DATA (HEOS); THE AE INDEX.
INUVIK
1l:OO
-B
5-IO+
II:05
ALL-SKY
Photographs
II. IO
I I.08
II:25
II:17
ZM
A_
-5LI
0o:oo
t
I
I
I I
I
04:oo
o&o
I I
I
I2:OO
I
16:OO
t
I
2o:oo
I
1
24:OO
UT Interplanetary
magnetic
field
(HE0.S)
NOAAIERL COLLEGE,
College
magnetogram
4 November
1972
(a) FIGS. 6(a,b). Two AND 5 NOVEMBER
EXAMPLES OF SVBSTORMS ALONG CONTRACTED OVALS, OBSERVED AT INVVIK ON 4 THE INTERPLANETARY MAGNETIC FIELD (HEOS) AND THE COLLEGE MAGNETIC RECORDS.
1972;
INUVIK
IO:35 +-!s---
ALL-SKY
IO:40
10x38
IO:36
Photoqraphs
045
IO:50
QM
0 -5t 0000
I
t 04:oo
!
)_( # 08~00
s
w
L IZOO UT . magnetic
Interplanetary NOAAIERL COLLEGE,
College
I
field
magnetogram
5 November FIG.
6(b)
1972’
I 16:OO
3
(HEOSI
1 2o:OO
1
I 24200
0772
1702
2 January
UT
1973
Cd FGS.
7(a,b).
Two
EXAMPLES
OF
SMALL
AURORAL JANUARY
OVAL
1973).
(DMSP
PHOTOGRAPHS
TAKEN
ON
2
0 7:74
2026
2 January
UT ,
.
.
1973
\
~~~~~~~~~~~
\
\
\
\
FIG. 7(b)
*-‘
-al
/
/
I
IO:40 FIG. 8. THE DISTRIBUTION OF AURORAS JUST PRIOR TO AND AFTER THE ONSET MOST INTENSE SUBSTORMS TO BE OBSERVED DURING THE LAST TWO DECADES CHAPMAN, 1962).
OF ONE OF THE (AKASOFU AND
1351
Interplanetary magnetic field and aurora1 dynamics This immediate response of On to the B, change can be inferred from the fact that the equatorward motion of the cusp or of the midday aurora follows closely B, changes. On the other hand, it is not at present accurately known how rapidly the midnight portion of the oval responds to the initial B, change. This depends on how quickly the information on a change of Qn can reach the nightside X-line and the production rate Q’N begins to respond to it. The minimum delay time T,,, can be estimated by: 7,
=
4(4
- 4) _
4.
min
>
@D where @n = (400 km/set) X (5~) x (15 Rx), and BP denotes the magnetic field intensity in the polar ionosphere, and A, and A, are the area of the minimum oval and an expanded oval, respectively (A,, and A, are assumed to be bounded by the latitude circles of 70” and 6S”, respectively). Because of this delay, the quantity (Qn - @n) should increase initially. Note that this quantity is directly proportional to the rate of change of the polar cap area, namely the area bounded by the aurora1 oval. Thus, when (@n - DN) is positive, the oval expands. However, O’Nbegins to increase about 40 min-1 hr after t = 0, and eventually a new steady state @n = QN will be reached. Then, the quantity (an - a,) becomes null, and the aurora1 oval ceases to expand. Suppose then the B, begins to increase after maintaining a large negative value for about 2 hr. The quantity @n responds immediately to the B, change and begins to decrease. However, again, the corresponding CD N variation will be delayed for about 40 min -1 hr, until the new information on B, can reach the nightside X-line. During this period, the quantity (@n - QN) becomes negative. If a large B, value is maintained for a prolonged period again, both QD and QN reach the same minimum value. Meanwhile, the quantity S = js (@n - aN) dt increases until (@n - QN) becomes null, and then begins to decrease. Note that the area of the polar cap is proportional to the quantity S. Thus, the aurora1 oval expands until S reaches the maximum value and then begins to contract poleward. Eventually, S will become null after a prolonged period of a large B, value (that is, when both QD and @‘Nreach the same minimum value). It should be noted that the quantity S is equal to the amount of open magnetic fluxes at a time reckoned from t = 0 (after a prolonged period of a large positive B, value). S = B,(A1 - A,,).
Thus, the amount of S can be monitored if one can observe continuously the area of the aurora1 oval. 2. THE
CONDITION S > 0 AS A NECESSARY CONDITION
An extensive study of DMSP photographs and the simultaneous interplanetary magnetic field data indicates thzt substorms can occur when the aurora1 oval is larger than its minimum size. Their occurrence does not depend on the sign of the B, component and its changes (namely, the southward and northward turnings) and of its time derivative as well. Referring to Fig. 2, it can also be concluded that substorms can occur so long as the quantity S is positive. No substorms were observed when the oval had approximately the minimum size (namely, during prolonged periods of a large positive B, component) and then when S N 0. In order to substantiate the above claims, let us group changes of the interplanetary magnetic field as follows: (a) The B, component is positive, but is decreasing. (b) The B, component becomes negative (namely, the southward turning). (cc) The B, component few hours.
remains
negative
for a
(d) The B, component (either positive or negative) is increasing. (e) The B, component becomes positive (namely, the northward turning). (f) The B, component is positive for a few hours. It is not difficult at all to find DMSP photographs which show intense aurora1 activity for the above six conditions. Akasofu et al. (1974) have already demonstrated that aurora1 substorms were observed under all the six conditions. Since substorms under the conditions (b) and (cc>have been extensively documented (McPherron et al., 1974), it is instructive to see a few examples of substorms under the conditions (a), (d), (e) and (f). Figure 3 shows several DMSP photographs which were taken in the later half of 28 October 1972. The interplanetary magnetic field data on that day show that the B, component turned northward at about 1O:lO UT and remained positive until about 19:30 UT. Some of the DMSP photographs taken during this period (orbits 275, 276, 277 and 278) show clearly intense substorm activity. The substorms observed during orbits 275,276,277 and 278 occurred under the conditions (a) or (f). All of them showed typical substorm features and show no qualitative difference from substorms observed
1352
S.-I. AKAsOFU
during orbits 279 and 280 when the B, component was negative. Figure 4 shows a series of DMSP photographs which were taken on 24-25 December 1972. They provide one of the most illuminating examples in studying the roles of the interplanetary magnetic field on substorm processes, although simifar situations were aIso observed at a number of occasions during the period under study. At about 08:OOUT on 24 December, the B, component became positive, and a prolonged period of the northward directed field began. It can be seen that auroras were quite active a least until about 14: 10 UT (orbits 640,641,642 and 643), in spite of large positive B, values during that period (the condition f). The substorm observed during orbit 642 occurred under the condition (a). One of the most interesting features of aurora1 activity during this period is that the size of the aurora1 oval was gradually decreasing after each substorm. The oval at orbit 643 was considerably smaller than that at orbit 640. The oval became even smaller and quite dim at orbit 645. During the next two orbits (646 and 647), the oval contracted even further polewards. The fact that the oval was very small during orbit 647 suggests that a small decrease of the B, component after a prolonged period of a large positive value contributes only a small amount to the quantity S. However, during orbit 648, a typical substorm feature was observed (the condition a). The substorm observed during orbit 649 occurred under the condition (f). The B, component was positive throughout the day of 25 December 1972, except for a short period between 19:45 and 21:30 UT. The aurora1 oval was very dim during orbits 650 and 651. Although it is not shown here, the 3, component had small positive values between 04 : 00 and 13 : 00 UT on that day and a dim contracted oval was seen around 12 : 00-14: 00 UT, together with weak substorm activity; its midnight latitude was about 70’. It is likely that S had a small positive value during that period. A prolonged period of Iarge positive B, values occurred also on 27 December. Figure 5 shows DMSP photographs, the interplanetary magnetic data and the AE index between 18: 00 UT, 26 December and 24:00 UT, 27 December. A dim small oval was observed during orbits 674 and 67.5. Then, a weak substorm activity was observed during orbits 676 and 677, although the AE index had no typical substorm features. The DMSP photograph at orbit 679 shows a bright arc in the afternoon part of the oval; the B, component was negative for
about 1 hr before that time. After orbit 679, the oval became too dim to study in detail in DMSP photographs during the rest of the day. However, auroras were seen over Sachs Harbour (one of the Alaska meridian chain stations, at invariant lat. ~74~) at least until 12:OO UT. It should be emphasized that our conclusion differs significantly from that put forward by McPherron, Russell and Aubry (1974) who claimed that the southward turning of the B, component would lead to a series of processes (perhaps such as an increase of the magnetic flux in the magnetotail and thinning of the plasma sheet) and eventually to the expansive phase of the substorm. Our observation indicates that so long as S is positive, a completely opposite series of processes (namely, the northward turning of the B, component, a decrease of the magnetic flux in the magnetotail and expansion of the plasma sheet) leads to the expansive phase of the substorm. Therefore, it is very doubtful that their proposed series of processes has any importance in substorm processes. As Akasofu (1974) pointed out, their proposed growth phase signatures must simply be effects of the southward turning. They considered only the first substorm after the southward turning. Further, although it is not essential, one of the reasons for the present confusion on the roles of the interplanetary magnetic field on substorm processes is that the size of the oval is controlled by the .B, component and thus that a “typical” substorm cannot be observed unless the 3, component turns southward, so that the oval expands to the latitude of the aurora1 zone in the midnight sector where the AEmagnetic stations are located. It is notpossibleto monitor properly substorm activity along a contracted oval which occurs when the BZ component has a positive value. Figures 6(a, b) show a typical substo~ activity observed over Inuvik (one of the Alaska meridian chain stations, inv. lat. ~70”) during a prolonged period of a positive 3, value (4 and 5 November 1972). Several important substorm features can easily be recognized in these photographs; Montbriand (1962) showed that substorms along a contracted oval are best characterized by westward traveling surges. The figures show also the corresponding magnetograms from College (inv. lat. 64.7’) which show little indication of substorms. Thus, the AEindex cannot monitor such substorms. 3. THE QUANTITY S BEING PROPORTIONAL TO THE INTENSITY OF SUBSTORMS
On the basis of an extensive examination of DMSP photographs, substorms along a contracted
1353
Interplanetary magnetic field and aurora1 dynamics oval are, in general, much less intense than those along a small oval. Typical substorms along contracted ovals are shown in Figs. 7(a, b). Like the substorms shown in Figs. 6(a, b), westward traveling surges are the most characteristic feature. The intensity of substorms may be defined on the basis of the area covered by bright auroras as a measure (this definition is similar to that of the intensity of solar flares). Indeed, the difference of the intensity of substorms in Fig. 1 and those in Figs. 6(a, b) is considerable. From such an observation, it is not diEcult to infer that substorms tend to be, in general, intense when S is large. Since Sis proportional to (A, - A,). the intensity of substorms is greater when the size of the oval is larger. Indeed, an intense substorm tends to occur along an expanded oval. One of the most intense substorms during the last two decades occurred during the historic storm of 11 February 1958 along one of the most expanded ovals ever documented accurately (Akasofu and Chapman, 1962). Figure 8 shows how extensive the expansive feature of this particular substorm was. 4. THE
SIGNIFICANCE
OF THE
(6) Thus, the southward turning of the B, component and the subsequent chain of processes, proposed by McPherron et al. (1973), do not have any significance in causing the expansive phase. Substorms can occur frequently after a northward turning so long as S > 0. (7) The ma~etosphere is always in a state of “growth phase” except for prolonged periods of a large positive B, value. (8) The substorm can be considered as a process by which the magnetosphere tends to remove the excess energy associated with S. Since the total energy E in the magnetotail is given (Gonzalez and Mozer, 1974) by:
where dl = the noon-midnight polar cap, dz = the dawn-dusk polar cap, Vi = the solar wind radius of the magnetotail, P = magnetotail and the energy d(d, = dJ is given by:
dimension dimension speed, R, the length available
of the of the = the of the E$ for
FINDINGS
By introducing the quantity S, therecentconfusion on the problem of the roles of the interplanetary magnetic field on substorm processes can be removed. (1) The ma~etotail and the aurora1 oval appear to be present even during a prolonged period of a large positive & value. (2) Thus, there appears to be a finite amount of the open magnttic fluxes (= A&,) even during such a period. (3) Substorms are absent during such a period. (4) Thus, the magnetic energy associated with the flux A,&, is not available as substorm energy. {5) Substorms can occur so long as there are additional magnetic fluxes 5’ in the magnetotail, regardless of the sign of the B, component and its changes (namely, the southward and northward turnings) and of its time derivative as well. McPherron et al. (1973) chose only a small fraction of substorms which happen to occur after southward turnings of the B, component. In fact, Caan, McPherron and Russell (1973) noted that their proposed growth phase features can be seen only for the first substorm after the southward turning. As Akasofu (1974) pointed out, their proposed growth phase features must simply be the southward turning effects.
(9) The sporadic nature of substorms indicates that this removal process is not a steady process. Akasofu (1975) suggested a certain analogy of this process with cyclogenesis. (10) The removal may be accomplished by ford ming a new neutral Iine, deep in the magnetotai1, along which the rate of reconnection @, is explosively enhanced. (11) Recent studies show conchrsively that the thinning of the plasma sheet occurs within a few minutes of the onset time of the expansive phase, but not prior to the onset (Hones, Akasofu and Perreault, 1975). Thus, it is not fruitful to look for instabilities which are associated with a thinned plasma sheet. It is suggested that the formation of the neutral line is a cause of the thinning, rather than a result. If the ion-tearing mode instability is importal~t in the neutral line formation (Schinder, 1974) and if it requires the removai of the magnetic field component normal to the neutral sheet, it cannot be the thinning of the plasma sheet which causes the removal. Acknowledgements-I would like to thank Dr. Joseph Kan and Dr. Y. Kamide for their valuable discussions during the preparation of this paper. The HEOS data were supplied by Dr. P. C. Hedgecock, through the WDC-A, Boulder, Colorado, U.S.A. The work reported here was supported in part by a grant from the National
1354
S.-f. Amom
Science Fo~datjon, Atmospheric Sciences Section, GA-36873 and also by the United States Air Force contract FI9628-720301 with the Geophysical Institute of the University of Alaska. REFERENCES Akasofu, S.-I. (1974). The aurora and the magnetosphere: the Chapman memorial lecture. PIamt. space Sci. 22, 885. Akasofu, S.-I. (1975). The solar wind-magnetosphere dynamo and the magnetospheric substorm. Planet. Spuce A’&. 23, 817. Akasofu, S.-I. and Chapman, S. (1962). Large-scale aurora1 motions and polar magnetic dist~ban~s-III, The aurora and magnetic storm of 11 February, 1958. $. atms. terr. Phys. 24, 785. Akasofu, S.-I., Hones, E. W., Jr., Bame, S. J., Asbridge, J. R. and Lui, A. T. Y. (1973). Magnetotail and boundary layer plasmas at a geocentric distance of -18Rn: Vela 5 and 6 observations. J. geophys. Res. 78, 7257. Akasofu, S.-I., Perreault, P. D., Yasuhara, F. and Meng, C.-I. (1973). Aurora1 substorms and the interplanetary magnetic field. J. geophys. Res. 78, 7490.
Caan, M. N., McPherron, R. L. and Russell, C. T. (1973). Solar wind and substorm-related changes in the lobes of the geomagnetic tail. J. geuphys. Res. 78,8087. Gonzalez, W. D. and Mozer, F. S. (1974). A quantitative model for the potential resulting from reconnection with an arbitrary interplanetary magnetic field. J. geophys. Res. 79, 4186. Hones, E. W., Jr., Akasofu, S.-I. and Perreault, P. (1975). Associations of IMF polarity occnrrence on March 6, 1970. Submitted to J. geophys. Res. McPherron, R. L., Russeli, C. T. and Aubry, M. P. (1973). Satellite studies of magnetospheric substorms on August 15, 1968: a. Phenomenonological model for substorms. J. geophys. Res. 78, 3131. Meng, C.-I. and Anderson, K. A. (1974). Magnetic field co~g~ation in the magnetotail near 60R~. J. geophys, Res. 79, 5143. Montbriand, L. E. (1969)). Morphology of aurora1 hydrogen emissions during aurora1 substorms. Ph.D. Thesis, University of Saskatchawan. Schindler, K. (1974). A theory of the substorm mechanism. J. geophys. Res. 79, 2803. Sonnerup, B. U. 0. (1974). Magnetopause reconnection rate, J. geophys. Res. 79, 1546.
Space
Scl.,
Vol. 23, pp. 1349 to 1354.
Pcrgamon
Press,
1975.
Printed
in Northern
Ireland
THE ROLES OF THE NORTH-SOUTH COMPONENT OF THE INTERPLANETARY MAGNETIC FIELD ON LARGESCALE AURORAL DYNAMICS OBSERVED BY THE DMSP SATELLITE Geophysical
S, -1, AKASOPIJ Institute, University of Alaska, Fairbanks,
Abstract-An extensive study of DMSP photographs field data suggests that the quantity detined by S=
s
Alaska 99’701, U.S.A.
and the simultaneous
interplanetary
magnetic
;(@,--crddr
has a fundamental importance in substorm processes, where @u and @N denote the production rate of merged (or open) field lines along the dayside X-line and of reconnected (or closed) fieId lines along the nightside X-line, respectively; d = 0 is measured from the time when the 23, component begins to decrease after a prolonged period of a large positive B, value. It is shown, first of all, that substorms occur so long as S > 0, regardless of the sign of the B, component and its changes (namely, the southward and northward turnings) and of its time derivative as well. Secondly, the intensity of substorms is proportional to Sa. By introducing the quantity S, the recent confusion of the problem of the roles of the north-south component of the interplanetary magnetic field on substorm processes can be removed. Since S is equal to the amount of the open magnetic fluxes at a time reckoned from I = 0, it is proportional to (A, - A,), where A, denotes the minimum polar cap area (namely, the area bounded by the minimum aurora1 oval) and A1 the polar cap area at an arbitrary time t. Therefore, substorms can occur whenever the aurora1 oval is larger than its minimum size. Further, an intense substorm tends to occur along a large oval. The quantity S can also be considered as an excess flux, and thus the substorm can be considered as a process by which the magnetosphere tends to remove sporadically the excess energy associated with S.
the last several years, one of the most important subjects on magnetospheric physics is the roles of the north-south component B, of the interplanetary magnetic field on magnetospheric processes, in particular on magnetospheric substorm processes. Since substorm energy is generated by the solar wind-magnetosphere dynamo and since the north-south component of the interpIanetary magnetic field controls the efhciency of the dynamo, there is no doubt that the B, component plays an important role. The point in question is thus what are the quantities which control directly the occurrence of substorms. Such a study requires a continuous monitoring of both a large-scale aurora1 activity over the entire polar region and the inte~lanetary magnetic field. AIthough we are far from having such an ideal set of data, we have now DMSP photographs (which can be considered to be “snap shots” taken about every 100 minutes) and the corresponding interplanetary magnetic field data for the months of During
October and December 1972 and January 1973; the interplanetary data are provided by the HEOS and IMP-I satellites. Figure 1 shows eight DMSP photographs which are chosen to show how the expansive phase of the aurora1 substorm might appear if DMSP photographs could be taken a few minutes apart. Let tDn be the production rate of merged (or open) field lines along the dayside X-line. This quantity is aiso equal to the potential drop along the X-Line and also to the potential drop across the dawn-dusk meridian in the polar cap. It is given (Sonnerup, 1974) by
where BI = the intensity of the interplanetary magnetic field, Bo = the intensity of the geomagnetic field just inside the magnetopause, 0 = the
1349 1
S.-I. AKASOFU
1350
and high Kp values (& > 2-) and showed that there is no significant differencein the structure of the magnetotail between the two conditions. Akasofu, Hones, Bame, Asbridge and Lui (1973) showed that the plasma sheet was present at a geocentric distance of 18 Rn after more than two days of an extremely quiet period. Under such a condition, it is expected to have a s = ‘(@, - aa,) dt steady state, and both tDn and @‘Nhave the same J0 minimum value. The aurora1 oval has the minimum where the time t = 0 is chosen at the instant when size, and its noon and midnight location is 80’~82’ the interplanetary magnetic field begins to decrease and 70°-‘72’ in invariant Iatitudes, respectively. after having a large northward component (B, 2 Further, the oval also becomes dim (often below the +5~) for an extended period, say 6-12 hours. threshold of the DMSP detector), and there is little Figure 2 illustrates SchematicaIly time variations indication of substorm activity. As we shall see on 24-25 and 27 of @n, DN, (@n - @n) and S = j5, (an - @J.J dt later, the aurora1 condition December 1972 corresponds to this situation. when the north-south component of the interplanetary magnetic field B, varies from a Iarge The fact that such a condition can occur suggests positive value to a negative value for about 2 hr and strongly that there is a minimum magnetic energy in then back to a large positive value again. Note that the magnetotail which is not available as substorm an extended period of a large positive B, value is the energy. That is to say, the magnetic energy in the magnetotail during an extended period of a large initial condition. In constructing Fig. 2, it is assumed that both @n positive B, value cannot be dissipated as substorm and SD, have a finite value even when the B, com- energy. This may also be inferred from the fact that ponent has a large positive value. This is because the the aurora1 oval does not shrink to a point either magnetotail (a product of the solar wind-magnetoduring a very quiet period or most intense substorms. Now, let us suppose that 3, begins to decrease sphere dynamo) appears to be a permanent feature, As 3, begins to and thus there must always be a finite amount of after such a quiet condition. open Aux even when B, has a large positive value for decrease, @n begins to increase immediately. Let us an extended period. Meng and Anderson (1974) suppose afso that B, reaches a minimum value in examined extensively magnetotail data at the lunar Iess than 1 hr after the southward turning and distance (60 Rx) for both low Kp values (Kp < 1 -I-) maintains a steady state value for about 2 hr. an& between Br and B,, V, = AIfven wave speed and L = the length of the merging (X) line. Let us define here also the production rate Qz, of reconnected (or closed) field lines along the nightside X-line. Consider then the quantity S defined by:
Interplanetary
Production
component
north-south
rote of
open . ..._ --~
Production field
rote
of
closed
in%?9
-l2hr-
FIG.
2
%‘POi-BETRXL
CHANGB &,
OF -
THfi & iDx,
COhWONENT
AND
s
=
f;
AND (#,
-
THE a,)
RESULTINQ
df.
CHANGES
OF
t&j,
@N,
FIG. I. EIGHT DMSP PHOTOGRAPHS WHICH ARE CHOSEN TO SHOW HOW THE EXPANSIVE PHASE OF THE ALIRORAL SUBSTORM MIGHT APPEAR IF THE PHOTOGRAPHS WERE TAKEN A FEW MINUTES APART. (Note
that they are not chronologically
shown.)
1350
0000
04:oo
0a:oo 28
12:oo
October
16:OO
20:oo
24:OO
1972
UT FIG. 3. SOME OF THE DMSP PHOTOGRAPHS WHICH WERE TAKEN IN THE LATER HALF OF 28 OCTOBER 1972: THE INTERPLANETARY MAGNETIC FIELD DATA (HEOS), B,Bx~~,BY>~,BZP; THE AE INDEX.
639
07:23 UT
13:2RUT
640
OS:05
643
UT
641
10:46UT
14:lOUT 24
December
1972
6:OO
24 December
25 December
1972
lgi’2
UT FIG. 4. SOME
OF THE DMSP PHOTOGRAPHS WHICH WERE TAKEN ON 24-25 DECEMBER 1972: INTERPLANETARY MAGNETIC FIELD DATA (HEOS), B, BXM, By,, Bznr;THE AEINDEX.
THE
IOr B
,”
0
?c
aI
O/B -5-
c" g
5~&u
5_BYM
SC
/ v
&EOl-5-6InQr250-l!G
a
1;
I$. $1
I1
b
18:OO 2000
26
I 24:OO
December
1 0400
”
3 (Dl 4
G u3I
a, CD1
ZI_
I
I 11 12:oo
OS00
27
1972
1
’
1
16:OO
December
I
2000
I 11 24:oO
1972
UT FIG. 5. SOME OF THE
DMSP
PHOTOGRAPHS TAKEN ON 26-21 DECEMBER 1972: THE INTERPLANETARY MAGNETIC FIELD DATA (HEOS); THE AE INDEX.
INUVIK
1l:OO
-B
5-IO+
II:05
ALL-SKY
Photographs
II. IO
I I.08
II:25
II:17
ZM
A_
-5LI
0o:oo
t
I
I
I I
I
04:oo
o&o
I I
I
I2:OO
I
16:OO
t
I
2o:oo
I
1
24:OO
UT Interplanetary
magnetic
field
(HE0.S)
NOAAIERL COLLEGE,
College
magnetogram
4 November
1972
(a) FIGS. 6(a,b). Two AND 5 NOVEMBER
EXAMPLES OF SVBSTORMS ALONG CONTRACTED OVALS, OBSERVED AT INVVIK ON 4 THE INTERPLANETARY MAGNETIC FIELD (HEOS) AND THE COLLEGE MAGNETIC RECORDS.
1972;
INUVIK
IO:35 +-!s---
ALL-SKY
IO:40
10x38
IO:36
Photoqraphs
045
IO:50
QM
0 -5t 0000
I
t 04:oo
!
)_( # 08~00
s
w
L IZOO UT . magnetic
Interplanetary NOAAIERL COLLEGE,
College
I
field
magnetogram
5 November FIG.
6(b)
1972’
I 16:OO
3
(HEOSI
1 2o:OO
1
I 24200
0772
1702
2 January
UT
1973
Cd FGS.
7(a,b).
Two
EXAMPLES
OF
SMALL
AURORAL JANUARY
OVAL
1973).
(DMSP
PHOTOGRAPHS
TAKEN
ON
2
0 7:74
2026
2 January
UT ,
.
.
1973
\
~~~~~~~~~~~
\
\
\
\
FIG. 7(b)
*-‘
-al
/
/
I
IO:40 FIG. 8. THE DISTRIBUTION OF AURORAS JUST PRIOR TO AND AFTER THE ONSET MOST INTENSE SUBSTORMS TO BE OBSERVED DURING THE LAST TWO DECADES CHAPMAN, 1962).
OF ONE OF THE (AKASOFU AND
1351
Interplanetary magnetic field and aurora1 dynamics This immediate response of On to the B, change can be inferred from the fact that the equatorward motion of the cusp or of the midday aurora follows closely B, changes. On the other hand, it is not at present accurately known how rapidly the midnight portion of the oval responds to the initial B, change. This depends on how quickly the information on a change of Qn can reach the nightside X-line and the production rate Q’N begins to respond to it. The minimum delay time T,,, can be estimated by: 7,
=
4(4
- 4) _
4.
min
>
@D where @n = (400 km/set) X (5~) x (15 Rx), and BP denotes the magnetic field intensity in the polar ionosphere, and A, and A, are the area of the minimum oval and an expanded oval, respectively (A,, and A, are assumed to be bounded by the latitude circles of 70” and 6S”, respectively). Because of this delay, the quantity (Qn - @n) should increase initially. Note that this quantity is directly proportional to the rate of change of the polar cap area, namely the area bounded by the aurora1 oval. Thus, when (@n - DN) is positive, the oval expands. However, O’Nbegins to increase about 40 min-1 hr after t = 0, and eventually a new steady state @n = QN will be reached. Then, the quantity (an - a,) becomes null, and the aurora1 oval ceases to expand. Suppose then the B, begins to increase after maintaining a large negative value for about 2 hr. The quantity @n responds immediately to the B, change and begins to decrease. However, again, the corresponding CD N variation will be delayed for about 40 min -1 hr, until the new information on B, can reach the nightside X-line. During this period, the quantity (@n - QN) becomes negative. If a large B, value is maintained for a prolonged period again, both QD and QN reach the same minimum value. Meanwhile, the quantity S = js (@n - aN) dt increases until (@n - QN) becomes null, and then begins to decrease. Note that the area of the polar cap is proportional to the quantity S. Thus, the aurora1 oval expands until S reaches the maximum value and then begins to contract poleward. Eventually, S will become null after a prolonged period of a large B, value (that is, when both QD and @‘Nreach the same minimum value). It should be noted that the quantity S is equal to the amount of open magnetic fluxes at a time reckoned from t = 0 (after a prolonged period of a large positive B, value). S = B,(A1 - A,,).
Thus, the amount of S can be monitored if one can observe continuously the area of the aurora1 oval. 2. THE
CONDITION S > 0 AS A NECESSARY CONDITION
An extensive study of DMSP photographs and the simultaneous interplanetary magnetic field data indicates thzt substorms can occur when the aurora1 oval is larger than its minimum size. Their occurrence does not depend on the sign of the B, component and its changes (namely, the southward and northward turnings) and of its time derivative as well. Referring to Fig. 2, it can also be concluded that substorms can occur so long as the quantity S is positive. No substorms were observed when the oval had approximately the minimum size (namely, during prolonged periods of a large positive B, component) and then when S N 0. In order to substantiate the above claims, let us group changes of the interplanetary magnetic field as follows: (a) The B, component is positive, but is decreasing. (b) The B, component becomes negative (namely, the southward turning). (cc) The B, component few hours.
remains
negative
for a
(d) The B, component (either positive or negative) is increasing. (e) The B, component becomes positive (namely, the northward turning). (f) The B, component is positive for a few hours. It is not difficult at all to find DMSP photographs which show intense aurora1 activity for the above six conditions. Akasofu et al. (1974) have already demonstrated that aurora1 substorms were observed under all the six conditions. Since substorms under the conditions (b) and (cc>have been extensively documented (McPherron et al., 1974), it is instructive to see a few examples of substorms under the conditions (a), (d), (e) and (f). Figure 3 shows several DMSP photographs which were taken in the later half of 28 October 1972. The interplanetary magnetic field data on that day show that the B, component turned northward at about 1O:lO UT and remained positive until about 19:30 UT. Some of the DMSP photographs taken during this period (orbits 275, 276, 277 and 278) show clearly intense substorm activity. The substorms observed during orbits 275,276,277 and 278 occurred under the conditions (a) or (f). All of them showed typical substorm features and show no qualitative difference from substorms observed
1352
S.-I. AKAsOFU
during orbits 279 and 280 when the B, component was negative. Figure 4 shows a series of DMSP photographs which were taken on 24-25 December 1972. They provide one of the most illuminating examples in studying the roles of the interplanetary magnetic field on substorm processes, although simifar situations were aIso observed at a number of occasions during the period under study. At about 08:OOUT on 24 December, the B, component became positive, and a prolonged period of the northward directed field began. It can be seen that auroras were quite active a least until about 14: 10 UT (orbits 640,641,642 and 643), in spite of large positive B, values during that period (the condition f). The substorm observed during orbit 642 occurred under the condition (a). One of the most interesting features of aurora1 activity during this period is that the size of the aurora1 oval was gradually decreasing after each substorm. The oval at orbit 643 was considerably smaller than that at orbit 640. The oval became even smaller and quite dim at orbit 645. During the next two orbits (646 and 647), the oval contracted even further polewards. The fact that the oval was very small during orbit 647 suggests that a small decrease of the B, component after a prolonged period of a large positive value contributes only a small amount to the quantity S. However, during orbit 648, a typical substorm feature was observed (the condition a). The substorm observed during orbit 649 occurred under the condition (f). The B, component was positive throughout the day of 25 December 1972, except for a short period between 19:45 and 21:30 UT. The aurora1 oval was very dim during orbits 650 and 651. Although it is not shown here, the 3, component had small positive values between 04 : 00 and 13 : 00 UT on that day and a dim contracted oval was seen around 12 : 00-14: 00 UT, together with weak substorm activity; its midnight latitude was about 70’. It is likely that S had a small positive value during that period. A prolonged period of Iarge positive B, values occurred also on 27 December. Figure 5 shows DMSP photographs, the interplanetary magnetic data and the AE index between 18: 00 UT, 26 December and 24:00 UT, 27 December. A dim small oval was observed during orbits 674 and 67.5. Then, a weak substorm activity was observed during orbits 676 and 677, although the AE index had no typical substorm features. The DMSP photograph at orbit 679 shows a bright arc in the afternoon part of the oval; the B, component was negative for
about 1 hr before that time. After orbit 679, the oval became too dim to study in detail in DMSP photographs during the rest of the day. However, auroras were seen over Sachs Harbour (one of the Alaska meridian chain stations, at invariant lat. ~74~) at least until 12:OO UT. It should be emphasized that our conclusion differs significantly from that put forward by McPherron, Russell and Aubry (1974) who claimed that the southward turning of the B, component would lead to a series of processes (perhaps such as an increase of the magnetic flux in the magnetotail and thinning of the plasma sheet) and eventually to the expansive phase of the substorm. Our observation indicates that so long as S is positive, a completely opposite series of processes (namely, the northward turning of the B, component, a decrease of the magnetic flux in the magnetotail and expansion of the plasma sheet) leads to the expansive phase of the substorm. Therefore, it is very doubtful that their proposed series of processes has any importance in substorm processes. As Akasofu (1974) pointed out, their proposed growth phase signatures must simply be effects of the southward turning. They considered only the first substorm after the southward turning. Further, although it is not essential, one of the reasons for the present confusion on the roles of the interplanetary magnetic field on substorm processes is that the size of the oval is controlled by the .B, component and thus that a “typical” substorm cannot be observed unless the 3, component turns southward, so that the oval expands to the latitude of the aurora1 zone in the midnight sector where the AEmagnetic stations are located. It is notpossibleto monitor properly substorm activity along a contracted oval which occurs when the BZ component has a positive value. Figures 6(a, b) show a typical substo~ activity observed over Inuvik (one of the Alaska meridian chain stations, inv. lat. ~70”) during a prolonged period of a positive 3, value (4 and 5 November 1972). Several important substorm features can easily be recognized in these photographs; Montbriand (1962) showed that substorms along a contracted oval are best characterized by westward traveling surges. The figures show also the corresponding magnetograms from College (inv. lat. 64.7’) which show little indication of substorms. Thus, the AEindex cannot monitor such substorms. 3. THE QUANTITY S BEING PROPORTIONAL TO THE INTENSITY OF SUBSTORMS
On the basis of an extensive examination of DMSP photographs, substorms along a contracted
1353
Interplanetary magnetic field and aurora1 dynamics oval are, in general, much less intense than those along a small oval. Typical substorms along contracted ovals are shown in Figs. 7(a, b). Like the substorms shown in Figs. 6(a, b), westward traveling surges are the most characteristic feature. The intensity of substorms may be defined on the basis of the area covered by bright auroras as a measure (this definition is similar to that of the intensity of solar flares). Indeed, the difference of the intensity of substorms in Fig. 1 and those in Figs. 6(a, b) is considerable. From such an observation, it is not diEcult to infer that substorms tend to be, in general, intense when S is large. Since Sis proportional to (A, - A,). the intensity of substorms is greater when the size of the oval is larger. Indeed, an intense substorm tends to occur along an expanded oval. One of the most intense substorms during the last two decades occurred during the historic storm of 11 February 1958 along one of the most expanded ovals ever documented accurately (Akasofu and Chapman, 1962). Figure 8 shows how extensive the expansive feature of this particular substorm was. 4. THE
SIGNIFICANCE
OF THE
(6) Thus, the southward turning of the B, component and the subsequent chain of processes, proposed by McPherron et al. (1973), do not have any significance in causing the expansive phase. Substorms can occur frequently after a northward turning so long as S > 0. (7) The ma~etosphere is always in a state of “growth phase” except for prolonged periods of a large positive B, value. (8) The substorm can be considered as a process by which the magnetosphere tends to remove the excess energy associated with S. Since the total energy E in the magnetotail is given (Gonzalez and Mozer, 1974) by:
where dl = the noon-midnight polar cap, dz = the dawn-dusk polar cap, Vi = the solar wind radius of the magnetotail, P = magnetotail and the energy d(d, = dJ is given by:
dimension dimension speed, R, the length available
of the of the = the of the E$ for
FINDINGS
By introducing the quantity S, therecentconfusion on the problem of the roles of the interplanetary magnetic field on substorm processes can be removed. (1) The ma~etotail and the aurora1 oval appear to be present even during a prolonged period of a large positive & value. (2) Thus, there appears to be a finite amount of the open magnttic fluxes (= A&,) even during such a period. (3) Substorms are absent during such a period. (4) Thus, the magnetic energy associated with the flux A,&, is not available as substorm energy. {5) Substorms can occur so long as there are additional magnetic fluxes 5’ in the magnetotail, regardless of the sign of the B, component and its changes (namely, the southward and northward turnings) and of its time derivative as well. McPherron et al. (1973) chose only a small fraction of substorms which happen to occur after southward turnings of the B, component. In fact, Caan, McPherron and Russell (1973) noted that their proposed growth phase features can be seen only for the first substorm after the southward turning. As Akasofu (1974) pointed out, their proposed growth phase features must simply be the southward turning effects.
(9) The sporadic nature of substorms indicates that this removal process is not a steady process. Akasofu (1975) suggested a certain analogy of this process with cyclogenesis. (10) The removal may be accomplished by ford ming a new neutral Iine, deep in the magnetotai1, along which the rate of reconnection @, is explosively enhanced. (11) Recent studies show conchrsively that the thinning of the plasma sheet occurs within a few minutes of the onset time of the expansive phase, but not prior to the onset (Hones, Akasofu and Perreault, 1975). Thus, it is not fruitful to look for instabilities which are associated with a thinned plasma sheet. It is suggested that the formation of the neutral line is a cause of the thinning, rather than a result. If the ion-tearing mode instability is importal~t in the neutral line formation (Schinder, 1974) and if it requires the removai of the magnetic field component normal to the neutral sheet, it cannot be the thinning of the plasma sheet which causes the removal. Acknowledgements-I would like to thank Dr. Joseph Kan and Dr. Y. Kamide for their valuable discussions during the preparation of this paper. The HEOS data were supplied by Dr. P. C. Hedgecock, through the WDC-A, Boulder, Colorado, U.S.A. The work reported here was supported in part by a grant from the National
1354
S.-f. Amom
Science Fo~datjon, Atmospheric Sciences Section, GA-36873 and also by the United States Air Force contract FI9628-720301 with the Geophysical Institute of the University of Alaska. REFERENCES Akasofu, S.-I. (1974). The aurora and the magnetosphere: the Chapman memorial lecture. PIamt. space Sci. 22, 885. Akasofu, S.-I. (1975). The solar wind-magnetosphere dynamo and the magnetospheric substorm. Planet. Spuce A’&. 23, 817. Akasofu, S.-I. and Chapman, S. (1962). Large-scale aurora1 motions and polar magnetic dist~ban~s-III, The aurora and magnetic storm of 11 February, 1958. $. atms. terr. Phys. 24, 785. Akasofu, S.-I., Hones, E. W., Jr., Bame, S. J., Asbridge, J. R. and Lui, A. T. Y. (1973). Magnetotail and boundary layer plasmas at a geocentric distance of -18Rn: Vela 5 and 6 observations. J. geophys. Res. 78, 7257. Akasofu, S.-I., Perreault, P. D., Yasuhara, F. and Meng, C.-I. (1973). Aurora1 substorms and the interplanetary magnetic field. J. geophys. Res. 78, 7490.
Caan, M. N., McPherron, R. L. and Russell, C. T. (1973). Solar wind and substorm-related changes in the lobes of the geomagnetic tail. J. geuphys. Res. 78,8087. Gonzalez, W. D. and Mozer, F. S. (1974). A quantitative model for the potential resulting from reconnection with an arbitrary interplanetary magnetic field. J. geophys. Res. 79, 4186. Hones, E. W., Jr., Akasofu, S.-I. and Perreault, P. (1975). Associations of IMF polarity occnrrence on March 6, 1970. Submitted to J. geophys. Res. McPherron, R. L., Russeli, C. T. and Aubry, M. P. (1973). Satellite studies of magnetospheric substorms on August 15, 1968: a. Phenomenonological model for substorms. J. geophys. Res. 78, 3131. Meng, C.-I. and Anderson, K. A. (1974). Magnetic field co~g~ation in the magnetotail near 60R~. J. geophys, Res. 79, 5143. Montbriand, L. E. (1969)). Morphology of aurora1 hydrogen emissions during aurora1 substorms. Ph.D. Thesis, University of Saskatchawan. Schindler, K. (1974). A theory of the substorm mechanism. J. geophys. Res. 79, 2803. Sonnerup, B. U. 0. (1974). Magnetopause reconnection rate, J. geophys. Res. 79, 1546.