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IMFP effects on the equatorial geomagnetic field and ionosphere-a review
IMFP effects on the equatorial geomagnetic field and ionosphere-a review S. MATSUSHITA High Altitude Observatory of NCAR. Boulder, Colorado 80307, U.S.A. Ahatract-It has recently been suggested by a few scientists that polarity (WFP) has a remarkable influence on geomagnetic and equatorial region, in addition to the effect over the polar region. those equatorial effects are critically examined, and a new result their seasonal variations is presented. After a discussion of possible tions for future investigations are provided.
the interplanetary magnetic field ionospheric phenomena over the All recent reports with regard to of geomagnetic field effects with mechanisms of the effects, sugges-
1. INTRODUCTION
2 Br EFFECTS
Con~m~ng the effects of the interp~ne~ry magnetic field polarity (IMFP) on the geomagnetic field, particularly in high latitudes, numerous papers and review articles (e.g. BURCH, 1974; NISHIDA, 1975; FELDSTEIN,1976) have been published during the past three years. The IMF B is most commonly represented by three ~m~nents (RX, By and Bz) in the Sun-Earth coordinate system, where x, y and z rep resent sunward, eastward and northward, respectively. The z direction is usually taken to be normal to the solar ecliptic plane. Also, to indicate IMF sector structure, the nomenclature TOWARD the Sun (Bx, usually associated with a -By com~nent) and AWAY from the Sun (--By, usually associated with a By component) has often been used. Although the azimuthal westward (-By) or eastward (By) component is probably more relevant for physical interpretations of high-latitude responses, the terms ‘toward’ and ‘away’ are occasionally used in the present report for simplicity and for convenience of discussion on low-latitude responses. As a common and customary practice in the past, the effects caused by Bz and those by the sector structure were discussed separately. Although the IMF effects in the y-z plane (e.g. F~Is-~~s~~ and WILHELM, 1975) or those in all three components are probably more relevant to discuss, the customary sep aration is adopted in the present review for convenience of introducing others’ studies. Discussions of Bz effects are presented in Section 2, and those of sector influence are given in Section 3. A new result of IMFP effects on low-latitude geomagnetic fields, their seasonal dependence and possible mechanism of the effect are also discussed in Section 3. Concluding remarks and necessary future investigations are presented in Section 4.
It has commonly been accepted that the southward IMF (- Bz) often causes either aurora1 oval expansion or trigger action of substorm occurrence, probably due to the so-called field-reconnection process. It is natural, then, that the geomagnetic index named equatorial Dst increases negatively due to the - Bz trigger effect (KANE, 1974). According to MAEZAWA (1976), however, -Bz produces a transpolar current sheet and a Bz increase produces a characteristic current system in the polar cap. It is not clear whether or not these currents flow partly in low latitudes, as for DP2 currents correlating with Bz, which MATSUSHITA and BALSLEY(1972) have previously considered. Good correlations among - Bz, geomagnetic fields at various latitudes, Esq disappearance at Huancayo (geomagnetic -0.6”, 354.3%; dip lat. 1.0’) and E-region E-W electron drifts over Jicamarca (seamagnetic -0.6”, 352.8”E; dip lat. 0.5”) during disturbed periods were presented by MATSUSHITAand BAL~LEY(1972) (see Figs. 12-14 in Balsley’s article in this issue). R~snn;r and CHANDRA (1974) reported that with increases of southward Bz, decreases of the F-region westward drift in the daytime and eastward drift at night are observed at Thumba ~eoma~etic - l.O”, 1467”E; dip lat. -0.7”) India (see top left in Fig. 1). They interpret that this phenomenon is caused by a decrease of the equatorial east-west electrostatic field. RASTOOIand PATEL (1975) reported that large and quick changes of the IMFP from its southward to northward direction are associated with the disappearance of Esq (top right in Fig. 1) or with the reversal of E-region horizontal and F-region vertical drifts (bottom two in Fig. 1). The authors suggest the imposition of magnetospheric dusk-to-dawn electric held
I207
1208
S. MA~USH~TA
Thumbo
6
4
2
1967
0 ElI*
-2
-4
-6
Y
3 July
28 November
1968
1966
at Jxomorco 20 0 -20 100 0 -100 IC field
latitude
900 0" -sv
23
00
01
02
03
04
05
06
07
hr 75” WMT
IZ
13 hr 75’
14
15
16
17
WMT
Fig. I. Variations of the F-region eastward drift speed at Thumba with respect to Bz (top left). The numerical figures near the circles with standard-deviation bars represent numbers of observations for each group (RASWGI and CHANDRA, 1974). Esq disappearance at Huancayo (top right) and large variations of the E-region horizontal and F-region vertical drifts at Jicamarca in the daytime (bottom right) and at night (bottom left) associated with Bz variations (RASTOCIand PATEL. 1975).
in the ionosphere due to the IMFP changes, to explain Esq disappearances and drift reversals. However, it is not clear how the magnetospheric electric fields reach the equatorial ionosphere. Also, those observed results are, in basic, same as the ones reported by MATSUSHITA and BALSLEY (1972).
In addition to the three examples observed on 13 September 1967 (Ap = 29), 3 July 1968 (Ap = 16, one of 5 disturbed days in the month) and 28 November 1968 (Ap = 7) shown in Fig. 1, RASTOGI and PATEL (1975) presented three more examples on 23 April 1967(Ap = 21, one of 5 disturbed days in the month),
IMFP
28 February 1968 (Ap = 30, one of 5 disturbed days in the month) and 6 March 1969 (Ap = 14). Most of these days were geomagnetically disturbed. Caution is necessary, accordingly, in judging whether these equatorial ionospheric variations are directly produced by the IMFP changes or whether they are simply due to common storm effects. For example, a dynamo action by the mid- and low-latitude neutral wind caused by the temperature increase due to electric field, current and Joule heating in the polar cap and the aurora1 region is plausible (MATSUSHITA,1975a). Equatorward thermospheric motions in response to aurora1 region storm activity, as discussed by RICHMOND and MATSUSHITA (1975, see Figs. 3 and 4 therein) may produce eastward electric currents in mid-latitudes and a westward electrostatic field in low latitudes, and hence be responsible for certain ionospheric variations mentioned above. In fact, REDDY(1974) reported equatorward winds above 120 km altitude following the onset of a geomagnetic disturbance, based on the incoherent scatter radar observations of ionization drifts at St. Santin (geomagnetic 46.7’, 82.9”E; dip lat. 46.7”), France. Although further data examinations are needed, the equatorial ionospheric variations introduced in this section seem to be common ionospheric disturbances caused by geomagnetic storms or magnetospheric substorms probably triggered by -Bz, instead of direct production caused by the IMFP changes. 3. SEaOR EFFECKj High latitudes
Concerning the IMFP Bx and By components or sector structures, their effects on the polar cap geomagnetic fields, sometimes indicated as the ‘Mansurov-Svalgaard Effects’, have fairly well been established. Namely, in the northern hemisphere, the toward (or away) IMF sector structure correlates with positive (or negative) deviations of the geomagnetic downward component Z at stations near 85” invariant latitude (such as Thule) and with negative (or positive) deviations of the horizontal component H at stations near 80” invariant latitude (such as Godhavn). In the southern hemisphere, the same story holds for Z but the sign reverses for H. The early analyses were done by visual judgement of the geomagnetic variations (e.g. SVALGAARD, 1972), but CAMPBELL and MATSUSHITA(1973) and CAMPBELL (1976) examined the relation objectively by an automated computer technique and concluded that a fairly good correlation can be obtained mainly in local summer. To explain this correlation, schematic circular electric currents around the polar cap were postulated
effects
1209
by BERTHELIER (1972) and SVALGAARD (1973), taking into consideration a theoretical suggestion given by JDRGENSENet al. (1972), while a shift of magnetospheric convection was suggested by HEPPNER(1972). To facilitate discussion of the physical mechanism involved in the interaction, however, more detailed studies of IMFP effects were required. FRIIS-CHRISTENSEN and WILHJELM(1975) examined polar cap currents for different directions of IMF in the yz plane. MATSUSHITA et al. (1973) investigated IMF effects and polar cap currents during quiet periods, and VOLLAND (1975a, b) presented a theoretical model. As for the quiet day variations in May-June 1965 shown in Fig. 2(a), the most reasonable interpretation for high-latitude variations is a circular electric current belt with about 3 x IO4 amp total current at 80”-85” latitude (MATSUSHITAand RICHMOND,1977). The current direction is clockwise (counterclockwise) corresponding to the westward (eastward) IMF, when it is viewed down over the pole at each polar cap. In other words, the current direction of the same IMF azimuthal component is reversed at the northern and the southern polar caps, when the circular currents in the two polar caps are viewed from the equatorial plane. MATSUSHITA and RICHMOND (1977) suggest that the formation of these circular electric currents can be explained by electric fields associated with the oppositely-moving plasma convections in the northem and the southern polar magnetospheric regions due to magnetic stresses of the IMF azimuthal component on the magnetic field lines of the polar magnetosphere to which they are connected (ATKINSON, 1975). High-latitude effects are briefly included here to contrast with low-latitude effects which are presented in the next subsection, Middle and low latitudes
As seen in Fig. 2(a), IMF sector or By effects on mid- and low-latitudes during quiet periods are shifts of the daily mean values of the northward (X) geomagnetic variation fields. An obvious representation of this effect can be shown by the shift of Sq equivalent current focus, as illustrated in Fig. 2(b). Symmetric day-to-day variability of the Sy foci between northern and southern hemispheres reported by SCHLAPP (1976) needs to be re-examined from the point of view of IMFP effects. AFANAS’YEVA (1973) reported variations of the northward geomagnetic field in mid-latitudes for the individual event of sector reversal during two consecutive days. Although she uses both standard notation Sy and her own Sq* (defined as modified form of Sq), she apparently discussed well-known disturbances associated with sector reversals or sector boundaries.
1210
S. MATSUSHITA
15 \:.
20
-a
--
-
30 35
_I
,,
z
i
x0
,./I
I1
w
,, 30
1
.,I
60
/
III ?g%
30
00
I
30
1 MI
I
I
II
,
2:4
/
y,
‘,
O”
M
I,(,_
4o
Fig. 2(a). The daily mean values of the northward (X), eastward (Y) and downward (Z) geomagnetic variation fields, measured from the average values for all quiet days and averaged separately for toward and away polarities during May and June of 1965, are plotted with respect to geomagnetic latitude (MATSUSHITA et al.. 1973). No special increase of X at the equator (hence, no ionospheric origin) is observed. TOWARD 00
AWAY 00
iI
32
MAY 81JUNE 1965 Fig. 2(b). Sqequivalent overhead current systems with respect to geomagnetic coordinates in the northern hemisphere corresponding to toward (left) and away (right) polarities for May and June of 1965. The contour interval of currents is IO4 amp, and solid and dotted curves indicate counterclockwise and clockwise currents, respectively (MATSUSHITA, 197Sb).
%
IMFP effects ---Day-time field Mean daily field -*-Night-time field
1234567
1234567
Fig. 3(a). Average responses of the geomagnetic H component at Alibag caused by away (toward) sector for seven consecutive days as shown in lower (upper) two panels, where left diagrams are for low solar activity periods and right diagrams are for high activity (BHARGAVAand RANGAR,~JAN, 1975). Using the mean hourly values of northward horizontal geomagnetic variation field H at Alibag (geomagnetic 9.5”, 143.6”E; dip lat. 12.9”) for 1929-1972, BHARGAVAand RANGARAJAN(1975) examined IMFP effects on the field (see Fig. 3). Their results agree -----a-
Day-time field Mean dally field Night-time field
Fig. 3(b). Average responses of H at Alibag for the sector structure changes from away to toward (toward to away) during five consecutive days each, as shown in lower (upper) two panels during low and high solar activity periods in the same way as Fig. 3(a) (BHARGAVAand XANGAWAN, 1975).
1211
with the variations presented in Fig 2(a). However, they made no separation of quiet and disturbed periods and of different seasons, which are extremely important for the identification of IMFP effects on mid- and low-latitude fields, as discussed later. In addition, the sector polarity data judged visually by SVALGAARD(1972) may not always be reliable, as the authors notice. MISHIN (1977) assumed three dimensional electric current systems caused by By, and suggested midand low-latitude geomagnetic variations produced by the equivalent currents, as shown in Fig. 4. Unfortunately, however, the suggested model has a very weak physical basis. Instead of the field-aligned currents shown in the top panel of Fig. 4, it may be more useful to discuss IMFP effects in terms of magnetospheric plasma convections (MATSUSHITAand RICHMOND, 1977) discussed briefly in the preceding section. This is one reason why some discussion of high-latitude behavior is necessary for the present report on the equatorial region. To examine seasonal variations and solar activity dependences of IMFP effects on the daily mean level of the H field in mid- and low-latitudes during quiet periods shown in Fig. 2(a), H variations at several stations during 1964-1973 were examined for three seasons: D months (November-February); E months (March, April, September and October); and J months (May through August). As listed in Table 1, selected stations were seven each in Afro-India (Area l), Austro-Pacific (Area 2) and N-S America (Area 3) corresponding three zones investigated by MATSUSHITA (1967) for his Sq studies. Since monthly tables of hourly H values at each station conveniently provide daily mean values, those values on five international quiet days in each month were examined. Sector assignments were provided by J. M. Wilcox, based on satellite observations. HA - fir at each station was computed for each season in each year, where g is an average of daily mean values of H on quiet days and A and T denote away and toward sectors. Those g” - Hr values were plotted with respect to dip latitudes: some examples are shown in Fig. 5. Although solar activity dependence was not clear, a remarkable seasonal variation was obtained: deviations during D months were reversed from those during J months (see Fig. 5). It may be safely assumed that R” - fir > 0 indicates the same behavior as in Fig. 2(a), while HA - Hr < 0 indicates a reversed deviation. Accordingly, the deviation of the daily mean level is positive (or negative) for toward (or away) sectors during D months. Corresponding to the IMF toward (away) sector structure, the Q-current foci in both northern
S. MATSUSHITA
1212
x ,‘(b)
_--
I
ill
l
lZh_ 8”
J,;-P/
(~058)
cost
24”LT
J,-P;Ccos8)
60° 30”
/’
/
s,n t
J,--P;(cos8)sm
2t
LT
60°
4
60”
30”
300
60” 90”
O0
8
120”
- 30” L
I. 3
I
I 9
15
I
I
I
21
LT
-60”
/ 0
I
12 LT -Ax
---
J
24 Ay
Fig. 4. Three-dimensional electric current systems in the magnetosphere and ionosphere due to By or a northward interplanetary electric field (top three diagrams) and corresponding two-dimensional equivalent currents in mid- and low-latitudes (middle three). Combination of those three equivalent currents is shown at bottom left, and geomagnetic variations to be produced by this current system are presented at bottom right (MISHIN, 1977). The contour interval of equivalent currents is not given but seems to be approximately 5 x IO3 amp based on the amplitude of geomagnetic variations.
and southern hemispheres move equatorward (poleward) by about 4” in June solstitial months (MATSUSHITA, 1975b) and move poleward (equatorward) by a similar amount in December solstitial months. HA - Hr during E months showed irregular variations, such as positive or negative deviations as well as near 0, indicating predominant J or D month influences, as well as cancellation by the two opposed effects. The effects during J and D months may be caused by the magnetospheric expansion or contraction due to the IMF sector structures. That is, the toward (away) sector in June solstitial months tends to yield a northward (southward) IMF component over the
low-latitude magnetopause in the Earth’s frame of reference, and in December solstitial months tends to yield a southward (northward) IMF component. Now, since the northward (southward) IMF causes the magnetospheric expansion (contraction), the northward geomagnetic component measured at the Earth’s surface decreases (increases), and hence the foci of the Sq equivalent current systems move equatorward (poleward). Here, original IMF Bz is assumed near zero during quiet periods. It must be emphasized that these effects are produced by magnetospheric deformations due to the reconnection process of IMF with the magnetospheric fields, instead of direct field influence which acts in
1213
IMFP effects Table 1. Geomagnetic stations in dip latitudes
Trivandrum Addis Ababa
-00.3” -00.5
Moca Nairobi
-08.9 - 14.3
Kakioka Honolulu
30.1” 22.0
Guam
06.5
Apia Port Moresby
Gnangara Toolangi
-
H,-H,(Quiet
Fredericksburg Tucson San Juan
54.4 40.3 32.1
Fuquene
18.2
Huancayo
01.0
La Quiaca
-07.5
Trelew
- 22.4
- 16.3 - 17.3
- 24.0 -34.3 -46.8
Luanda Tananarive Hermanus
-
Area 3
Area 2
Area 1
-48.9 -51.2
days)
HA-H, J months Nov. 1964-
Y
III
Feb. 1965
I
I
I
I
Moy-Aug.
I
I
I
I
0 months
I ’ ’ ’ ’ ’ I ’ ’
““““”
-1 1.
I .
1971
3 30.1”
i
Ll
-60’
I
-400
I
I.1
20”
I’1
0
I 20”
I
I 40’
I1
600
‘/ \.
.& *i
DIP latitude
Fig. 5. Differencesbetween averaged daily mean values on away-sector days H, and those on toward-sector days HT are plotted with respect to dip latitudes. Triangles, circles and squares represent stations in Areas 1, 2 and 3 listed in Table 1, respectively. Open (or solid) symbols are for June (or December) solstitial months, and top (or bottom) half is for low (or moderate) solar-activity periods. There is no particular difference among three geographical areas. Probable error value at each point is satisfactorily small; for example, Kakioka (circle near 30”) presents 9.0 _+6.0, -5.0 + 2.3, 5.8 + 2.5, and -5.7 k 3.3 y, respectively at each panel (downward).
I
I
1964
I
I
I
I
I
1970
I
Ii
/
2”
t
1
I,I,,IIII 1964-65
-51
J
1970-71
Fig. 6. HA - fir values at three stations in AustroJapanese area for June (left) and December (right) solstitial months during about 10 years starting from 1964, which were available at WDC-A. Thick horizontal lines show zero level and thin horizontal ones indicate average values during the period: they are 3.3 + 0.9 y for J months and -1.3 + 0.9 y for D months at Kakioka; 2.5 + 0.7 y for J and -5.1 + 2.3 y for D at Port Moresby; and 2.4 + 0.8 y for J and - 1.8 + 1.0 y for D at Toolangi.
S. MATSUSHITA
1214
an opposite way. In other words, the direct effect of the northward IMF causes a small increase of H (a few gammas) at the Earth’s surface, but a trigger action due to reconnection process produces a large decrease of H (several gammas). As shown in Fig. 6, HA - HT in J (or D) months is sometimes negative (or positive). Possible causes are that, in addition to observational difficulties and predominantly one type of sector: (1) five quiet days examined are not equally quiet, and (2) more critically, IMF has large lBz/ component, overcoming the produced N-S IMF component due to the seasonal inclination of the Earth’s rotation axis with respect to the Sun-Earth line. As conducted by FRIIS-CHRISTENSEN and WILHJELM (1975) for high latitudes, studies of the data, only when Bz is small, will clarify the results.
are discussed. Also, it is emphasized that sector effects in the polar region are mainly caused by polar ionospheric currents, while sector effects in low latitudes are by changed magnetospheric configurations, and hence there is probably no noticeable change of the low-latitude ionosphere during quiet periods. However, some effects from high latitudes due to the sector need to be examined in the future. One of the most important remaining problems is that of Bx and By or sector effects on mid- and lowlatitude fields during disturbed periods. Some preliminary results are available: same relations as quiet periods seem to hold when Dsr component is properly subtracted. However, extensive studies are necessary to establish At
high
the behavior latitudes
well.
where
sector
obvious,
the sense of the geomagnetic
viations
on individual
effects
are
daily-mean
days agrees about
most de-
70% of the
time with the sense indicated
4. CONCLUDING REMARKS As given in the preface of the present special issue of the Fifth ISEA (International Symposium on Equatorial Aeronomy), ‘magnetospheric coupling to lower levels’ and ‘high and low latitude interactions’, as well as ‘strato- and meso-spheric coupling to higher levels in the atmosphere’ were emphasized in the symposium program, and are likely to receive strong emphasis at future equatorial symposia. In the present review, -Bz effects on magnetospheric coupling and importance of high and low latitude interactions, primarily during disturbed periods,
by average values for the appropriate sector (CAMPBELL and MATSUHI~A, 1973). The percentage agreement at low latitudes for the H field is satisfactory compared to the high latitude percentage. However, since the amount of the effect is small (see Fig. 2a), careful and extensive studies are required for quiet and particularly disturbed periods.
Acknowledgement-The author is very grateful to Drs. H. RISHBETHand A. D. RICHMONDfor their helpful discussions. The National Center for Atmospheric Research is sponsored by the National Science Foundation.
REFERENCES
AFANAS’YEVA V. I.
1973
ATKINSONG. BERTHELIERA. BHARGAVAB. N. and RANGARAJAN G. K. BURCH J. L. CAMPBELLW. H. CAMPBELLW. H. and MATSUSHITA S. FELDSTEINYA. I. FRIIS-CHRISTENSEN E. and WILHJELMJ. HEPPNERJ. P. JORGENSEN T. S., FRIIS-CHRISTENSEN E. and WILH~ELMJ. KANE R. P. MAEZAWAK. MATSUHITA S.
1975 1972
MA~U~HITA S. MATSUSHITA S. MATSUSHITA S. and BAL~LEYB. B. MATSUSHITA S. and RICHMONDA. D.
1975 1974 1976 1973 1976 1975 1972 1972
Geomag. Aeronomy 13, 165. J. geophys. Res. 80, 32. C. R. Acad. Sci. !%r. B, 275, 841. Planet. Space Sci. 23, 929. Rev. Geophys. Space Phys. 12, 363. J. geophys. Res. 81, 4731. J. geophys. Res. 78, 2079. Space Sci. Rev. 18, 771. J. aeoohvs. Res. 80. 1248. J. geobh&. Res. 77; 4877. J. geophys. Res. 77, 1976.
1974 1976 1967
J. geophys. Res. 79, 64. J. geophys. Res. 81, 2289. Physics of Geomagnetic Phenomena
1975a 1975b 1972 1977
(Edited by S. MATSUSHITA and W. H. CAMPBELL), p. 301. Academic Press, N.Y. Phys. Earth Planet. Interiors 10. 299.
J. geophys. Res. 80, 4751. Planet. Space Sci. 20, 1259. Planet. Space Sci. 25 (in press)
IMFP effects MATXBHITA S., TARPLEYJ. D. and CAMPBELLW. H. MISHINV. M. NISHIDAA. RAST~GIR. G. and CHANDRAH. RASTOGIR. G. and PATEL V. L. REDDY C. A. RICHKIND A. D. and MATXJSHITAS. SCHLAPPD. M. SVALGAARDL. SVALGAARDL. VOLLANDH. VOLLANDH.
1973 1977 1975 1974 1975 1974 1975 1976 1972 1973 1975a 1975b
1215 Radio Sci. 8, 963. Space Sci. Rev. 19 (in press). Space Sci. Rev. 17, 353. J. atmos. terr. Phys. 36, 377. Proc. Indian Acad. Sci. 82, A(4), 12 I J. atmos. terr. Phys. 36. 1561. J. geophys. Res. 80, 2839. J. atmos. terr. Phys. 38, 573. J. geophys. Res. 77, 4027. J. geophys. Res. 78, 2064. J. geophys. Res. 80, 23 I 1. Annls Giophys. 31, 159.
the interplanetary magnetic field ionospheric phenomena over the All recent reports with regard to of geomagnetic field effects with mechanisms of the effects, sugges-
1. INTRODUCTION
2 Br EFFECTS
Con~m~ng the effects of the interp~ne~ry magnetic field polarity (IMFP) on the geomagnetic field, particularly in high latitudes, numerous papers and review articles (e.g. BURCH, 1974; NISHIDA, 1975; FELDSTEIN,1976) have been published during the past three years. The IMF B is most commonly represented by three ~m~nents (RX, By and Bz) in the Sun-Earth coordinate system, where x, y and z rep resent sunward, eastward and northward, respectively. The z direction is usually taken to be normal to the solar ecliptic plane. Also, to indicate IMF sector structure, the nomenclature TOWARD the Sun (Bx, usually associated with a -By com~nent) and AWAY from the Sun (--By, usually associated with a By component) has often been used. Although the azimuthal westward (-By) or eastward (By) component is probably more relevant for physical interpretations of high-latitude responses, the terms ‘toward’ and ‘away’ are occasionally used in the present report for simplicity and for convenience of discussion on low-latitude responses. As a common and customary practice in the past, the effects caused by Bz and those by the sector structure were discussed separately. Although the IMF effects in the y-z plane (e.g. F~Is-~~s~~ and WILHELM, 1975) or those in all three components are probably more relevant to discuss, the customary sep aration is adopted in the present review for convenience of introducing others’ studies. Discussions of Bz effects are presented in Section 2, and those of sector influence are given in Section 3. A new result of IMFP effects on low-latitude geomagnetic fields, their seasonal dependence and possible mechanism of the effect are also discussed in Section 3. Concluding remarks and necessary future investigations are presented in Section 4.
It has commonly been accepted that the southward IMF (- Bz) often causes either aurora1 oval expansion or trigger action of substorm occurrence, probably due to the so-called field-reconnection process. It is natural, then, that the geomagnetic index named equatorial Dst increases negatively due to the - Bz trigger effect (KANE, 1974). According to MAEZAWA (1976), however, -Bz produces a transpolar current sheet and a Bz increase produces a characteristic current system in the polar cap. It is not clear whether or not these currents flow partly in low latitudes, as for DP2 currents correlating with Bz, which MATSUSHITA and BALSLEY(1972) have previously considered. Good correlations among - Bz, geomagnetic fields at various latitudes, Esq disappearance at Huancayo (geomagnetic -0.6”, 354.3%; dip lat. 1.0’) and E-region E-W electron drifts over Jicamarca (seamagnetic -0.6”, 352.8”E; dip lat. 0.5”) during disturbed periods were presented by MATSUSHITAand BAL~LEY(1972) (see Figs. 12-14 in Balsley’s article in this issue). R~snn;r and CHANDRA (1974) reported that with increases of southward Bz, decreases of the F-region westward drift in the daytime and eastward drift at night are observed at Thumba ~eoma~etic - l.O”, 1467”E; dip lat. -0.7”) India (see top left in Fig. 1). They interpret that this phenomenon is caused by a decrease of the equatorial east-west electrostatic field. RASTOOIand PATEL (1975) reported that large and quick changes of the IMFP from its southward to northward direction are associated with the disappearance of Esq (top right in Fig. 1) or with the reversal of E-region horizontal and F-region vertical drifts (bottom two in Fig. 1). The authors suggest the imposition of magnetospheric dusk-to-dawn electric held
I207
1208
S. MA~USH~TA
Thumbo
6
4
2
1967
0 ElI*
-2
-4
-6
Y
3 July
28 November
1968
1966
at Jxomorco 20 0 -20 100 0 -100 IC field
latitude
900 0" -sv
23
00
01
02
03
04
05
06
07
hr 75” WMT
IZ
13 hr 75’
14
15
16
17
WMT
Fig. I. Variations of the F-region eastward drift speed at Thumba with respect to Bz (top left). The numerical figures near the circles with standard-deviation bars represent numbers of observations for each group (RASWGI and CHANDRA, 1974). Esq disappearance at Huancayo (top right) and large variations of the E-region horizontal and F-region vertical drifts at Jicamarca in the daytime (bottom right) and at night (bottom left) associated with Bz variations (RASTOCIand PATEL. 1975).
in the ionosphere due to the IMFP changes, to explain Esq disappearances and drift reversals. However, it is not clear how the magnetospheric electric fields reach the equatorial ionosphere. Also, those observed results are, in basic, same as the ones reported by MATSUSHITA and BALSLEY (1972).
In addition to the three examples observed on 13 September 1967 (Ap = 29), 3 July 1968 (Ap = 16, one of 5 disturbed days in the month) and 28 November 1968 (Ap = 7) shown in Fig. 1, RASTOGI and PATEL (1975) presented three more examples on 23 April 1967(Ap = 21, one of 5 disturbed days in the month),
IMFP
28 February 1968 (Ap = 30, one of 5 disturbed days in the month) and 6 March 1969 (Ap = 14). Most of these days were geomagnetically disturbed. Caution is necessary, accordingly, in judging whether these equatorial ionospheric variations are directly produced by the IMFP changes or whether they are simply due to common storm effects. For example, a dynamo action by the mid- and low-latitude neutral wind caused by the temperature increase due to electric field, current and Joule heating in the polar cap and the aurora1 region is plausible (MATSUSHITA,1975a). Equatorward thermospheric motions in response to aurora1 region storm activity, as discussed by RICHMOND and MATSUSHITA (1975, see Figs. 3 and 4 therein) may produce eastward electric currents in mid-latitudes and a westward electrostatic field in low latitudes, and hence be responsible for certain ionospheric variations mentioned above. In fact, REDDY(1974) reported equatorward winds above 120 km altitude following the onset of a geomagnetic disturbance, based on the incoherent scatter radar observations of ionization drifts at St. Santin (geomagnetic 46.7’, 82.9”E; dip lat. 46.7”), France. Although further data examinations are needed, the equatorial ionospheric variations introduced in this section seem to be common ionospheric disturbances caused by geomagnetic storms or magnetospheric substorms probably triggered by -Bz, instead of direct production caused by the IMFP changes. 3. SEaOR EFFECKj High latitudes
Concerning the IMFP Bx and By components or sector structures, their effects on the polar cap geomagnetic fields, sometimes indicated as the ‘Mansurov-Svalgaard Effects’, have fairly well been established. Namely, in the northern hemisphere, the toward (or away) IMF sector structure correlates with positive (or negative) deviations of the geomagnetic downward component Z at stations near 85” invariant latitude (such as Thule) and with negative (or positive) deviations of the horizontal component H at stations near 80” invariant latitude (such as Godhavn). In the southern hemisphere, the same story holds for Z but the sign reverses for H. The early analyses were done by visual judgement of the geomagnetic variations (e.g. SVALGAARD, 1972), but CAMPBELL and MATSUSHITA(1973) and CAMPBELL (1976) examined the relation objectively by an automated computer technique and concluded that a fairly good correlation can be obtained mainly in local summer. To explain this correlation, schematic circular electric currents around the polar cap were postulated
effects
1209
by BERTHELIER (1972) and SVALGAARD (1973), taking into consideration a theoretical suggestion given by JDRGENSENet al. (1972), while a shift of magnetospheric convection was suggested by HEPPNER(1972). To facilitate discussion of the physical mechanism involved in the interaction, however, more detailed studies of IMFP effects were required. FRIIS-CHRISTENSEN and WILHJELM(1975) examined polar cap currents for different directions of IMF in the yz plane. MATSUSHITA et al. (1973) investigated IMF effects and polar cap currents during quiet periods, and VOLLAND (1975a, b) presented a theoretical model. As for the quiet day variations in May-June 1965 shown in Fig. 2(a), the most reasonable interpretation for high-latitude variations is a circular electric current belt with about 3 x IO4 amp total current at 80”-85” latitude (MATSUSHITAand RICHMOND,1977). The current direction is clockwise (counterclockwise) corresponding to the westward (eastward) IMF, when it is viewed down over the pole at each polar cap. In other words, the current direction of the same IMF azimuthal component is reversed at the northern and the southern polar caps, when the circular currents in the two polar caps are viewed from the equatorial plane. MATSUSHITA and RICHMOND (1977) suggest that the formation of these circular electric currents can be explained by electric fields associated with the oppositely-moving plasma convections in the northem and the southern polar magnetospheric regions due to magnetic stresses of the IMF azimuthal component on the magnetic field lines of the polar magnetosphere to which they are connected (ATKINSON, 1975). High-latitude effects are briefly included here to contrast with low-latitude effects which are presented in the next subsection, Middle and low latitudes
As seen in Fig. 2(a), IMF sector or By effects on mid- and low-latitudes during quiet periods are shifts of the daily mean values of the northward (X) geomagnetic variation fields. An obvious representation of this effect can be shown by the shift of Sq equivalent current focus, as illustrated in Fig. 2(b). Symmetric day-to-day variability of the Sy foci between northern and southern hemispheres reported by SCHLAPP (1976) needs to be re-examined from the point of view of IMFP effects. AFANAS’YEVA (1973) reported variations of the northward geomagnetic field in mid-latitudes for the individual event of sector reversal during two consecutive days. Although she uses both standard notation Sy and her own Sq* (defined as modified form of Sq), she apparently discussed well-known disturbances associated with sector reversals or sector boundaries.
1210
S. MATSUSHITA
15 \:.
20
-a
--
-
30 35
_I
,,
z
i
x0
,./I
I1
w
,, 30
1
.,I
60
/
III ?g%
30
00
I
30
1 MI
I
I
II
,
2:4
/
y,
‘,
O”
M
I,(,_
4o
Fig. 2(a). The daily mean values of the northward (X), eastward (Y) and downward (Z) geomagnetic variation fields, measured from the average values for all quiet days and averaged separately for toward and away polarities during May and June of 1965, are plotted with respect to geomagnetic latitude (MATSUSHITA et al.. 1973). No special increase of X at the equator (hence, no ionospheric origin) is observed. TOWARD 00
AWAY 00
iI
32
MAY 81JUNE 1965 Fig. 2(b). Sqequivalent overhead current systems with respect to geomagnetic coordinates in the northern hemisphere corresponding to toward (left) and away (right) polarities for May and June of 1965. The contour interval of currents is IO4 amp, and solid and dotted curves indicate counterclockwise and clockwise currents, respectively (MATSUSHITA, 197Sb).
%
IMFP effects ---Day-time field Mean daily field -*-Night-time field
1234567
1234567
Fig. 3(a). Average responses of the geomagnetic H component at Alibag caused by away (toward) sector for seven consecutive days as shown in lower (upper) two panels, where left diagrams are for low solar activity periods and right diagrams are for high activity (BHARGAVAand RANGAR,~JAN, 1975). Using the mean hourly values of northward horizontal geomagnetic variation field H at Alibag (geomagnetic 9.5”, 143.6”E; dip lat. 12.9”) for 1929-1972, BHARGAVAand RANGARAJAN(1975) examined IMFP effects on the field (see Fig. 3). Their results agree -----a-
Day-time field Mean dally field Night-time field
Fig. 3(b). Average responses of H at Alibag for the sector structure changes from away to toward (toward to away) during five consecutive days each, as shown in lower (upper) two panels during low and high solar activity periods in the same way as Fig. 3(a) (BHARGAVAand XANGAWAN, 1975).
1211
with the variations presented in Fig 2(a). However, they made no separation of quiet and disturbed periods and of different seasons, which are extremely important for the identification of IMFP effects on mid- and low-latitude fields, as discussed later. In addition, the sector polarity data judged visually by SVALGAARD(1972) may not always be reliable, as the authors notice. MISHIN (1977) assumed three dimensional electric current systems caused by By, and suggested midand low-latitude geomagnetic variations produced by the equivalent currents, as shown in Fig. 4. Unfortunately, however, the suggested model has a very weak physical basis. Instead of the field-aligned currents shown in the top panel of Fig. 4, it may be more useful to discuss IMFP effects in terms of magnetospheric plasma convections (MATSUSHITAand RICHMOND, 1977) discussed briefly in the preceding section. This is one reason why some discussion of high-latitude behavior is necessary for the present report on the equatorial region. To examine seasonal variations and solar activity dependences of IMFP effects on the daily mean level of the H field in mid- and low-latitudes during quiet periods shown in Fig. 2(a), H variations at several stations during 1964-1973 were examined for three seasons: D months (November-February); E months (March, April, September and October); and J months (May through August). As listed in Table 1, selected stations were seven each in Afro-India (Area l), Austro-Pacific (Area 2) and N-S America (Area 3) corresponding three zones investigated by MATSUSHITA (1967) for his Sq studies. Since monthly tables of hourly H values at each station conveniently provide daily mean values, those values on five international quiet days in each month were examined. Sector assignments were provided by J. M. Wilcox, based on satellite observations. HA - fir at each station was computed for each season in each year, where g is an average of daily mean values of H on quiet days and A and T denote away and toward sectors. Those g” - Hr values were plotted with respect to dip latitudes: some examples are shown in Fig. 5. Although solar activity dependence was not clear, a remarkable seasonal variation was obtained: deviations during D months were reversed from those during J months (see Fig. 5). It may be safely assumed that R” - fir > 0 indicates the same behavior as in Fig. 2(a), while HA - Hr < 0 indicates a reversed deviation. Accordingly, the deviation of the daily mean level is positive (or negative) for toward (or away) sectors during D months. Corresponding to the IMF toward (away) sector structure, the Q-current foci in both northern
S. MATSUSHITA
1212
x ,‘(b)
_--
I
ill
l
lZh_ 8”
J,;-P/
(~058)
cost
24”LT
J,-P;Ccos8)
60° 30”
/’
/
s,n t
J,--P;(cos8)sm
2t
LT
60°
4
60”
30”
300
60” 90”
O0
8
120”
- 30” L
I. 3
I
I 9
15
I
I
I
21
LT
-60”
/ 0
I
12 LT -Ax
---
J
24 Ay
Fig. 4. Three-dimensional electric current systems in the magnetosphere and ionosphere due to By or a northward interplanetary electric field (top three diagrams) and corresponding two-dimensional equivalent currents in mid- and low-latitudes (middle three). Combination of those three equivalent currents is shown at bottom left, and geomagnetic variations to be produced by this current system are presented at bottom right (MISHIN, 1977). The contour interval of equivalent currents is not given but seems to be approximately 5 x IO3 amp based on the amplitude of geomagnetic variations.
and southern hemispheres move equatorward (poleward) by about 4” in June solstitial months (MATSUSHITA, 1975b) and move poleward (equatorward) by a similar amount in December solstitial months. HA - Hr during E months showed irregular variations, such as positive or negative deviations as well as near 0, indicating predominant J or D month influences, as well as cancellation by the two opposed effects. The effects during J and D months may be caused by the magnetospheric expansion or contraction due to the IMF sector structures. That is, the toward (away) sector in June solstitial months tends to yield a northward (southward) IMF component over the
low-latitude magnetopause in the Earth’s frame of reference, and in December solstitial months tends to yield a southward (northward) IMF component. Now, since the northward (southward) IMF causes the magnetospheric expansion (contraction), the northward geomagnetic component measured at the Earth’s surface decreases (increases), and hence the foci of the Sq equivalent current systems move equatorward (poleward). Here, original IMF Bz is assumed near zero during quiet periods. It must be emphasized that these effects are produced by magnetospheric deformations due to the reconnection process of IMF with the magnetospheric fields, instead of direct field influence which acts in
1213
IMFP effects Table 1. Geomagnetic stations in dip latitudes
Trivandrum Addis Ababa
-00.3” -00.5
Moca Nairobi
-08.9 - 14.3
Kakioka Honolulu
30.1” 22.0
Guam
06.5
Apia Port Moresby
Gnangara Toolangi
-
H,-H,(Quiet
Fredericksburg Tucson San Juan
54.4 40.3 32.1
Fuquene
18.2
Huancayo
01.0
La Quiaca
-07.5
Trelew
- 22.4
- 16.3 - 17.3
- 24.0 -34.3 -46.8
Luanda Tananarive Hermanus
-
Area 3
Area 2
Area 1
-48.9 -51.2
days)
HA-H, J months Nov. 1964-
Y
III
Feb. 1965
I
I
I
I
Moy-Aug.
I
I
I
I
0 months
I ’ ’ ’ ’ ’ I ’ ’
““““”
-1 1.
I .
1971
3 30.1”
i
Ll
-60’
I
-400
I
I.1
20”
I’1
0
I 20”
I
I 40’
I1
600
‘/ \.
.& *i
DIP latitude
Fig. 5. Differencesbetween averaged daily mean values on away-sector days H, and those on toward-sector days HT are plotted with respect to dip latitudes. Triangles, circles and squares represent stations in Areas 1, 2 and 3 listed in Table 1, respectively. Open (or solid) symbols are for June (or December) solstitial months, and top (or bottom) half is for low (or moderate) solar-activity periods. There is no particular difference among three geographical areas. Probable error value at each point is satisfactorily small; for example, Kakioka (circle near 30”) presents 9.0 _+6.0, -5.0 + 2.3, 5.8 + 2.5, and -5.7 k 3.3 y, respectively at each panel (downward).
I
I
1964
I
I
I
I
I
1970
I
Ii
/
2”
t
1
I,I,,IIII 1964-65
-51
J
1970-71
Fig. 6. HA - fir values at three stations in AustroJapanese area for June (left) and December (right) solstitial months during about 10 years starting from 1964, which were available at WDC-A. Thick horizontal lines show zero level and thin horizontal ones indicate average values during the period: they are 3.3 + 0.9 y for J months and -1.3 + 0.9 y for D months at Kakioka; 2.5 + 0.7 y for J and -5.1 + 2.3 y for D at Port Moresby; and 2.4 + 0.8 y for J and - 1.8 + 1.0 y for D at Toolangi.
S. MATSUSHITA
1214
an opposite way. In other words, the direct effect of the northward IMF causes a small increase of H (a few gammas) at the Earth’s surface, but a trigger action due to reconnection process produces a large decrease of H (several gammas). As shown in Fig. 6, HA - HT in J (or D) months is sometimes negative (or positive). Possible causes are that, in addition to observational difficulties and predominantly one type of sector: (1) five quiet days examined are not equally quiet, and (2) more critically, IMF has large lBz/ component, overcoming the produced N-S IMF component due to the seasonal inclination of the Earth’s rotation axis with respect to the Sun-Earth line. As conducted by FRIIS-CHRISTENSEN and WILHJELM (1975) for high latitudes, studies of the data, only when Bz is small, will clarify the results.
are discussed. Also, it is emphasized that sector effects in the polar region are mainly caused by polar ionospheric currents, while sector effects in low latitudes are by changed magnetospheric configurations, and hence there is probably no noticeable change of the low-latitude ionosphere during quiet periods. However, some effects from high latitudes due to the sector need to be examined in the future. One of the most important remaining problems is that of Bx and By or sector effects on mid- and lowlatitude fields during disturbed periods. Some preliminary results are available: same relations as quiet periods seem to hold when Dsr component is properly subtracted. However, extensive studies are necessary to establish At
high
the behavior latitudes
well.
where
sector
obvious,
the sense of the geomagnetic
viations
on individual
effects
are
daily-mean
days agrees about
most de-
70% of the
time with the sense indicated
4. CONCLUDING REMARKS As given in the preface of the present special issue of the Fifth ISEA (International Symposium on Equatorial Aeronomy), ‘magnetospheric coupling to lower levels’ and ‘high and low latitude interactions’, as well as ‘strato- and meso-spheric coupling to higher levels in the atmosphere’ were emphasized in the symposium program, and are likely to receive strong emphasis at future equatorial symposia. In the present review, -Bz effects on magnetospheric coupling and importance of high and low latitude interactions, primarily during disturbed periods,
by average values for the appropriate sector (CAMPBELL and MATSUHI~A, 1973). The percentage agreement at low latitudes for the H field is satisfactory compared to the high latitude percentage. However, since the amount of the effect is small (see Fig. 2a), careful and extensive studies are required for quiet and particularly disturbed periods.
Acknowledgement-The author is very grateful to Drs. H. RISHBETHand A. D. RICHMONDfor their helpful discussions. The National Center for Atmospheric Research is sponsored by the National Science Foundation.
REFERENCES
AFANAS’YEVA V. I.
1973
ATKINSONG. BERTHELIERA. BHARGAVAB. N. and RANGARAJAN G. K. BURCH J. L. CAMPBELLW. H. CAMPBELLW. H. and MATSUSHITA S. FELDSTEINYA. I. FRIIS-CHRISTENSEN E. and WILHJELMJ. HEPPNERJ. P. JORGENSEN T. S., FRIIS-CHRISTENSEN E. and WILH~ELMJ. KANE R. P. MAEZAWAK. MATSUHITA S.
1975 1972
MA~U~HITA S. MATSUSHITA S. MATSUSHITA S. and BAL~LEYB. B. MATSUSHITA S. and RICHMONDA. D.
1975 1974 1976 1973 1976 1975 1972 1972
Geomag. Aeronomy 13, 165. J. geophys. Res. 80, 32. C. R. Acad. Sci. !%r. B, 275, 841. Planet. Space Sci. 23, 929. Rev. Geophys. Space Phys. 12, 363. J. geophys. Res. 81, 4731. J. geophys. Res. 78, 2079. Space Sci. Rev. 18, 771. J. aeoohvs. Res. 80. 1248. J. geobh&. Res. 77; 4877. J. geophys. Res. 77, 1976.
1974 1976 1967
J. geophys. Res. 79, 64. J. geophys. Res. 81, 2289. Physics of Geomagnetic Phenomena
1975a 1975b 1972 1977
(Edited by S. MATSUSHITA and W. H. CAMPBELL), p. 301. Academic Press, N.Y. Phys. Earth Planet. Interiors 10. 299.
J. geophys. Res. 80, 4751. Planet. Space Sci. 20, 1259. Planet. Space Sci. 25 (in press)
IMFP effects MATXBHITA S., TARPLEYJ. D. and CAMPBELLW. H. MISHINV. M. NISHIDAA. RAST~GIR. G. and CHANDRAH. RASTOGIR. G. and PATEL V. L. REDDY C. A. RICHKIND A. D. and MATXJSHITAS. SCHLAPPD. M. SVALGAARDL. SVALGAARDL. VOLLANDH. VOLLANDH.
1973 1977 1975 1974 1975 1974 1975 1976 1972 1973 1975a 1975b
1215 Radio Sci. 8, 963. Space Sci. Rev. 19 (in press). Space Sci. Rev. 17, 353. J. atmos. terr. Phys. 36, 377. Proc. Indian Acad. Sci. 82, A(4), 12 I J. atmos. terr. Phys. 36. 1561. J. geophys. Res. 80, 2839. J. atmos. terr. Phys. 38, 573. J. geophys. Res. 77, 4027. J. geophys. Res. 78, 2064. J. geophys. Res. 80, 23 I 1. Annls Giophys. 31, 159.