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Annual and semiannual variations of the geomagnetic field at equatorial locations
J-l hinted
of Atmawbic and in Northern Ireland
TmcsbialPhysics. Vol.43.
No. S/6, pp. 607616,
1981.
0021-9169/81/050607-lOSO2.00/0 Per.gamon Press Ltd.
Annual and semiannual variations of the geomagnetic field at equatorial locations WALLACEH. CAMPBELL U.S. Geological Survey, Denver, CO 80225, U.S.A.
AIra&&-For a year of quiet solar-activity level, geomagnetic records from American hemisphere observatories located between about 0“ and 30” north geomagnetic latitude were used to compare the annual and semiannual variations of the geomagnetic field associated with three separate contributions: (a) the quiet-day midnight level, MDT; (b) the solar-quiet daily variation, Sq; and (c) the quiet-time lunar semidiurnal tidal variation, L(12). Four Fourier spectral constituents (24, 12, 8, 6 h periods) of Sq were individually treated. All three orthogonal elements (H, D and 2) were included in the study. The MDT changes show a dominant semiannual variation having a range of about 7 gammas in H and a dominant annual variation in 2 having a range of over 8 gammas. These changes seem to be a seasonal response to the nightside distortions by magnetospheric currents. There is a slow decrease in MDT amplitudes with increasing latitude. The Sq changes follow the patterns expected from an equatorial ionospheric dynamo electrojet current system. The dominant seasonal variations occur in H having a range of over 21 gammas for the 24 h period and over 12 gammas for the 12 h period spectral components. The higher-order components are relatively smaller in size. The !&J(H) amplitudes decrease rapidly with increasing
latitude. Magnetospheric contributions to the equatorial Sq must be less than a few per cent of the observed magnitude. The L(12) variation shows the ionospheric electrojet features by the dominance of H and the rapid decrease in amplitude with latitude away from the equator. However, the seasonal variation range of over 7 gammas has a maximum in early February and minimum in late June that is not presently explainable by the known ionospheric conductivity and tidal behavior.
1. INTRODUCTION The
purpose of this study is to examine the seasonal behavior of the quiet geomagnetic field at low-latitude locations. The separate contributions of the midnight (MDT), daily variation (Sq), and lunar (I.) parts of the observed fields are compared using data from American hemisphere observatories (Table 1) at low latitudes for the quiet-sun year, 1965. This presentation utilizes the methods of three recent studies (C-BELL, 1980a, 1980b, in press) brought together in a unique fashion to emphasize the equatorial features and to illustrate the differences in the seasonal amplitude and phase changes obtained from a Fourier analysis of annual
and semiannual components in the three orthogonal magnetic-field directions. Early studies of the seasonal changes in the geomagnetic field were reported by CHAPMAN and BARTU~ (1940), VAN W~JK (1953) and MCINTOSH (1959). 2. ANAL.YSls Three slightly different data sets were selected from the geomagnetic records of the 1965 quietsun year. Because the study of midnight values required conditions of quiet magnetosphere, a special-day determination that utilized low Dst (ring current) and AE (aurora1 electrojet) indices
Table 1. Observatory locations Geographic degrees Station
Code
Latitude
E. longitude
Huancayo Tatuoca Paramaribo Honolulu San Juan
HU Pfz
12.05 1.20 5.82 21.32 18.12
284.67 311.48 304.78 202.00 293.85
HO SJ
S s N N N
Geomagnetic Latitude 0.64 9.51 16.93 21.17 29.57
s N N N N
degrees E. longitude 354.27 21.25 14.83 266.99 3.63
608
WALLACEH. CAMPBELL
was employed. The L and Sq changes were investigated at times of quiet ionospheric conditions when the magnetic activity index was low. The days for determining Sq were a subset of those for determining L. The five magnetic observatories from which data were obtained are listed in Table 1. The analysis was arranged in geomagnetic latitude from 0 to 30” as given in that table. In the American hemisphere, within the accuracy of this study, the dip latitudes and geomagnetic latitudes are essentially interchangeable. This study has concentrated on the surface observations of field; the separation of external and internal contributions to the measurements have been omitted. Such separation will not affect the general conclusions sought at this time. A later study by the author will investigate the ionosphericcurrent models and deep earth conductivity values that may be obtained through a source separation. The midnight level of the geomagnetic field (MDT) was taken to be the station’s reported universal time hourly value that occurs during the time of local midnight. For each of the three orthogonal geomagnetic-field components, H, D and Z, in gammas (1 gamma = 1 nanotesla), only the MDT values for 37 days selected from a list representing the quietest magnetospheric conditions (CAMPBELL, 1979) were selected for analysis. Details of the data processing and error estimate are given in CAMPBELL (1980b). Essentially, the procedure for determining the seasonal changes involves the fitting of an observatory’s selected MDT values to a function that contains a constant and linear term, as well as cosine and sine terms of the annual and semiannual Fourier analysis coefficients. If the ionospheric effects are not important in the midnight hours on quiet days, then the annual coefficients a, and b, and the semiannual coefficients a2 and b, of the Fourier terms should show the summer-winter and equinoctial changes in the field about the earth as it is distorted by the stream of particles and fields from the sun and the resulting current systems that arise within the magnetosphere and its tail. Station values of Fourier coefficient amplitudes were plotted versus geomagnetic latitude; straight-line segments were drawn connecting these data points; the values at latitudinal points separated by 2.5” were determined and a smoothing applied to the readings so that a continuous-curve representation of the midnight cosine and sine Fourier terms was obtained. With these terms the MDT amplitude function of combined annual and semiannual variation was recon-
structed and plotted, as a seasonal change at 5” increments, from the equator to 30” North geomagnetic latitude. The three orthogonal-direction field components were treated separately. Figures la, 2a and 3a show the combined annual and semiannual changes in amplitude of the MDT levels. The FAR technique for month-by-month separation of the semidiurnal (12 h) lunar-tidal effect, I-(12), in geomagnetic changes registered by the 2.5 min scaled field values at a station was described by MATSUSHITA and CAMPBELL (1972). The method is essentially a superposed-epoch technique that makes use of the 50.5 min difference in solar and lunar time to separate the diurnal, 12 h (lunar time) changes from the low activity geomagnetic record residuals that have the quiet solartime variations removed. The procedure uses onemonth sets of days when all 8 of the 3 h magnetic activity indices, Kp, were less than or equal to a value of 3+. Moderately disturbed records may be used in the lunar-time superposed-epoch analysis because the geomagnetic activity is essentially initiated in a statistically random fashion and is enhanced with some solar-time modulation; only very large disturbances would completely mask the lunar contribution. For the stations of Table 1 and separated H, D and Z components, the lunar semidiurnal amplitude as well as the phase (hour of first maximum in the variation) have seasonal variations that may be represented by the Fourier cosine and sine coefficients (a, and b, for the annual terms and a2 and b2 for the semiannual terms). When these coefficients are plotted, smoothed, and scaled as a function of latitude in a manner similar to that described in the preceding paragraph and explained in CAMPBELL (198Oa), the lunar variation from 0” to 30” geomagnetic latitude is obtained as represented in Figs. Id, 2d and 3d for the seasonal amplitude changes in the Figs. 4c and 5c for the seasonal phase changes. The method for determining monthly values of Sq for the three components of the geomagnetic field at a station is given in CAMPBELL (1972). In outline, this method may be described this way: for each month the quietest UT days were selected by the condition that for all 8 of the 3 h index intervals, Kp was less than or equal to 2+. This is the lowest value of Kp that would provide a sufficientsize sample of quiet days for this analysis. For each of the 2.5 min data samples of those days, the average values were formed for the H, D and Z components separately and called Sq. The appropriate lunar tidal component described above was
Variations of the geomagnetic field at equatorial locations
ANNUAL
AND SEMIANNUAL
AMPLITUDE
VARlATlON
H COMPONENT L(l2)
MDT
# LA L! I i I JFMAMJJASOND
;
!
I
!
Ii
LLL.1. I 1.1 JFMAMJJASOND
b 11.1.1
.u
L.I
i
I I 1 I
1 I 1 i-:-1
JFMAMJJASOND
LII_Lu_LuI_LLJ JFMAMJJASOND
Fig. 1. The H-component, quiet-time, anmual and semiannual combined variation of the geomagnetic field amplitudes in 5” increments from the equator (bottom row) to 30” (top row) geomagnetic latitude, displayed for (columns left to right): (a) midnight field level, MDT; (b) quiet daily variation, 24h spectral component, Sq(24); (c) quiet daily variation, 12 h spectral component, Sq(12); and (d) the semidiurnal lunar tidal variation, L(12). Manths of the year are indicated by letters at the bottom of the columns, The distance between 5’ latitude lines measures a 5-gamma (1 gamma = t nT) amplitude scale size. As variations become less than 0.5 gammas in amplitude they grow less significant.
ANNUAL
AND SEMIANNUAL
AMPLITUDE
VARIATION
Z COMPONENT MDT
U-L-L_.IIIL.LLu JFMPMJJASOND
U..J. L_lI_Lu._i JFMAMJJASOND
iLLl _1-i. LIIJ JFMAMJJASOND
I__LLLL_LL_LJ_LIJ JFMAMJJ4SOND
MONTH OF YEAR
Fig. 2. Similar to Fig. 1, except that the Z-component
of field is depicted.
WALLACE
610
H. CAMPBELL
ANNUAL AND SEMIANNUAL
AMPLITUDE
VARIATION
D COMPONENT L (PI
sq m
5944)
llIIlII1IlI~I JFYAMJJASOND
I~lllllllll~l JFYAYJJASOND
MONTH
OF YEAR
Fig. 3. Similar to Fig. 1, except that the D-component
then subtracted and the residual was taken to be the monthly Sq variation for each station. Monthby-month Fourier analysis of these Sq gave amplitude and phase values of the constituent 24, 12, 8 and 6 h components of the quiet field levels. Next, a seasonal Fourier analysis of these components yielded the annual (al and b,) and semiannual (a, and b2) coefficients of the separated amplitude and phase changes. At this point the data processing continued in a fashion similar to that used in the lunar analysis above and included line segment plots of the station coefficients, 5-point smoothing, and reconstruction of the amplitude and phase change variations for 5” latitude increments, Figure 6 shows the annual and semiannual variation of the amplitudes during the year for the 24, 12, 8 and 6 h spectral components of Sq at 0, 15 and 30” latitude locations. The figure shows that the amplitudes generally decrease for higher-order harmonics. The 8 and 6 h seasonal variations are similar in seasonal behavior to that of the 12 h component. Thus, in subsequent discussions I will show only the two lowest order spectral components, Sq(24) and Sq(12). Figures lb, c, 2b, c, 3b, c illustrate the summation of annual and semiannual changes in amplitude for latitudes from 0 to 30”. The latitude and seasonal change in the phase of Sq(24) and Sq(12) are shown in Figs. 4 and 5.
of field is depicted.
3.RESuLTS
3.1. Quiet daily uariation, Sq The best interpretation of Sq-field changes supports a model that pictures the dynamo effect of diurnal and semidiumal tidal winds in the lower ionosphere generated in situ by solar heating (RKHMOND, 1979). Higher harmonics appear in the field records because of the rapid change in ionization near sunrise and sunset. Most models consider the E-region near 100 km, which disappears at night-time, to be the locale of supporting conductivity for the dynamo effect. However, RISHBETH (1971) suggested that the high-altitude F-region winds may produce a dynamo effect at night hours when its fields cannot be shorted out by the conducting E-region. Bishbeth has calculated that under the most favorable conditions such a field would be no larger than 3 gammas. The annual and semiannual variation of the field components for the Sq behave as one would expect for a typical eastward current of the equatorial electrojet system. Both the 24 and 12 h components of H, shown in Figs. lb and c, display two prominent features. There is a large, northward enhancement of field (+H) during the equinoxes at the equator and a steep decrease of amplitude with increasing latitude to about 15 or 20”. The extreme
Variations of the geomagnetic field at equatorial locations
ANNUAL
PHASE
0.
611
VARIATION
15*
30. Ii
H
4
D
2 H H
2 D
Sq(l2) H D \
2 ‘c\
2
LINE LENGTH
SCALE
D 2
D
H
D
H
H
D
f 2
Fig. 4. The annual variations of the phase characteristics of geomagnetic field at 0, 15 and 30” geomagnetic latitude (columns left to right) displayed in three rows representing (top to bottom): (a) the 24 h spectral component of the quiet daily variation, Sq(24); (b) the 12 h spectral component of the quiet daily variation, Sq(12); and (c) the semidiurnal lunar tidal variation, L(12). The magnitude of the annual phase shift is indicated by the length of the vectors (scale, in hours, is given to the lower right of the figure). The time of year for the maximum in annual phase change is indicated by the vector directions (scale, in months, is shown to the upper right of the figure). Separate vectors are shown for the H, D and Z orthogonal field components followed in the analysis. For example, the vector plotted for H of the Sq(24) at 30” indicates that the annual variation for the 24 h Fourier component of the change in Sq shows an annual phase change of about 1.5 h with an increase in June. Note that phase changes of less than 0.5 h in magnitude or those phase changes arising for annual field change amplitudes (Figs. 1,2 and 3) less than 0.5 gammas are of minimal importance.
range of variation in Sq at the equator is over 21 gammas in magnitude for the diurnal component, Sq(24), and over 12 gammas in magnitude for the semidiumal component, Sq(12), of II. Figure 7 shows the solar zenith angle, x, and Chapman function, m, for the E-region ionization at the geographic equator at the equinox and the solstice. In this region, although the day and night are always of equal duration, the sun is vertical (x = 0) twice a year at the equinoxes. The seasonal, solar isolation changes cause the thermally generated wind systems to have a semiannual fluctuation. The overhead E-region ionization, roughly represented by m, shows a semiannual change that supports an increase in eastward electrojet current at the equinoxes. This semi-
annual characteristic of x, of course, decreases with increasing latitude to the Tropic of Cancer at 23”27’ geographic latitude. The horizontal alignment of the earth’s main field at ionospheric altitudes near the equator that promotes the high conductivity supporting the electrojet existence decreases more rapidly with latitude than x. The strong electrojet effect on H shows little annual or semiannual variation in its phase (Figs. 4a, b, 5a and b). Semiannual and annual changes in the Zamplitude for Sq could indicate both location and amplitude changes of the electrojet current. The amplitude changes should be related to the H changes and therefore should appear semiannually. For indicating a seasonal motion of an ionospheric
WALLACE H. CAMPBELL
612
Sq (24)
SEMIANNUAL
PHASE
O0
w
Dy
VARIATION 30.
j
“6
/ D
Ii
Ii D
H D
sq(12)
d_
2 / D
r 2
2
LINE LENGTII SCALE 0
2
I
3
HOURS
LO21
D n 3 2
I\
n
2
2-D n
D
Fig. 5. Similar to Fig. 4, except that the semiannual variation of the phase characteristic is depicted. electrojet current, the vertical components of the field amplitudes observed at the earth’s surface are considerably more sensitive than are the horizontal components of the field amplitudes. Thus, small location changes for the electrojet should show an annual variation in Z. We observe both large annual and semiannual terms in Fig. 2b. This may indicate that for the eastward electrojet current, the increasing negative value of Z in the December solstice is caused by a current displacement southward toward the summer hemisphere, just as might be expected from the solar control of the ionization maximum. The large semiannual terms follow the equinoctial enhancement of electrojet intensity. The range of Z variation is about 6 gammas at the equator. The interesting feature of the seasonal D variation is the dominant annual change for the diurnal component of about 11 gammas near 30” latitude. The Sq current patterns show foci close to this latitude (MATSUSHITA, 1967). The seasonal variation in the foci strongly varies the north-south component of the horizontal ionospheric current system (TARPLEY, 1973). Although the current is strongest close to the equator, the amplitude ratio of D to H is largest close to the foci (CAMPBELL, 1977).
SARABHAI and NAIR (1969) proposed a ring current source for the Sq variations at the earth’s surface. BHARGAVA (1972a) analyzed a 45 y data sample at Alibag and showed that the annual and semiannual variations varied independently with local time and that there was significant components of these variations even at local midnight hours. This result indicated that some nonionospheric effects were likely appearing in the annual and semiannual variations. BHARGAVA (1972b) also showed that only the night-time annual components of field at low latitude stations varied significantly with geomagnetic activity level. He concluded that the probable cause for the annual variation at night is the modulation of the magnetospheric ring current. CURME (1976) found that there was an annual change in the base level associated with Sq. OLSON (1970) studied the quiet-time daily, Sqtype variations of field at the earth’s surface that are due to the changing magnetospheric form as seen at the earth’s surface. He also computed the seasonal variation in the north and vertical components for the daily mean of this field change due to the magnetosphere. His seasonal equatorial variations of H (about 1 gamma in range) showed semiannual maxima at the equinoxes of less than a
613
Variations of the geomagnetic field at equatorial locations
Sq ANNUAL AND SEMIANNUAL AMPLITUDE VARIATKINS
JFYAWJJASONO
-
24 hr
--_---_
12 hr 6h#
_________
6h, 111
LI 11I1I1II JFYAYJJASONO
111 I 11 JFYAYJJASONO
I
I
Ill
II
MONTHOFYEAR
Fig. 6. The quiet annual and semiannual variation for the geomagnetic amplitudes of Sq at (bottom to top rows) 0, 15, 30” geomagnetic latitude steps displayed for (left to right columns) the three orthogonal field components H, D and Z. The 24, 12, 8 and 6 h Fourier spectral components of the variations are marked with the special line codes shown. Months of the year are indicated by the letters at the bottom of the columns. An amplitude scale in gammas (1 gamma = 1 n7’) is shown. As variations become less than 0.5 gammas in amplitude, they grow less significant.
few per cent of the change shown in Fig. 6. At 30”
latitude he found an annual periodicity (about 2 gammas range) with a summer maximum that does not correspond to my observations. Olson’s report of a Z-component annual change at the equator (about 1 gamma in range) also does not match my results. At 30” latitude his Z prediction of amplitude change (about 1 gamma range) may correspond to Fig. 6. In light of the above shortcomings, it would appear that the magnetospheric model contributions to Sq observations near the equator are quite small, 3.2. Midnight field level, MDT In the early days of satellite exploration of the earth’s magnetosphere, it was found that the dipole field shape was severely distorted into a long taillike form away from the sun and that a current sheet existed on the dark side of the extended magnetosphere. WILLUMS and MEAD (1965) and KENDALL et al. (1969) found that the shape of the field lines in the noon-midnight meridian plane of the nightside magnetosphere could be adequately
modeled by the addition of a magnetotail current sheet. SIGNORA et al. (1971) reported that the satellite, midnight-meridian observations of the magnetospheric field clearly indicated the existence of a ‘disk-shaped’ quiet-time ring current present at even the lowest values of Kp. CAMPBELL (1973) showed that the observed field levels near midnight at geomagnetic latitudes lower than 40” varied during periods of high geomagnetic activity in a consistent fashion to indicate a current source at an antisolar position within the nightside magnetosphere. In a somewhat similar study MALIN and ISIKARA (1976) analyzed the annual variation of monthly mean field levels at midnight for a distribution of world stations. At latitudes below 60” they found a global variation of the night-time surface field that could be best explained by the annual change in location of the mean latitude for the magnetospheric ring current that moved north in December and south in June. Their results seem to be dominated by the active periods. At quiet times, on the nightside, the earth’s field in space may be represented by the superposition
614
WAILME H. CAMPBELL
EQUATORIAL
SOLAR
IONIZATION
30
0 0
2
4
6
6 LOCAL
IO
14
12
MERIDIAN
16
16
20
22
24
TIME
Fig. 7. The daily variation, in local meridian time, for the solar zenith angle, ,Y,(solid lines and scale to left) and the Chapman ionization function, &Jos x), (dashed lines and scale to right) for the solstice and equinox at the equator.
of magnetospheric ring and tail currents upon the dipole field. The fields of such currents are seen at the earth’s surface as a decrease in H at low latitudes. With the seasonal change in solar direction, these current contributions change location so that an equatorial measurement of the field, AB, that must be added to the dipole component would be in the direction indicated by Fig. 8a. H is positively directed northward and Z is positively directed into the earth. Thus, we may see that the expected seasonal variation of this magnetospheric distortion on the nightside of the earth would produce equatorial variations in H,,, and Z,,, as shown in Fig. 8b. H has a semiannual variation with minima at the equinoxes; Z has an annual variation with a maximum near the June solstice. The amplitudes of these seasonal variations in MDT should change slowly with latitude from the equator to 30” because of the scale and geometry of the magnetospheric contribution. Note that H,,, is out of phase with the expected value of Sq(H). MDT values of H and Z shown in Figs. la and 2a seem to be consistent with the behavior expected of the
H,,, and Z,,, displayed in Fig. 8. The F-region, night-level dynamo-current variations (&WBETH, 1971) would be too small to be considered as a source of these changes. An estimate of the external components, H, and Z,, of the seasonal variation in quiet-time, midnight field levels may be obtained from the H = 7.0 gammas and Z = 8.0 gammas (seasonal MDT in Figs. 1 and 2) near the equator using the induction factors 2/3 and 2.0 (respectively) as was done in an earlier paper (CAMPEML,L,1973). This gives H. = 4.7 gammas and Z, = 16.0 gammas. Assuming these values to be equal to H,,, and Z,,, respectively at the when solstice the magnetospheric-distortioncurrent axis is at an angle of 32.5” to the equatorial plane, the geometric relationships give a magnitude of about 30 gammas for the field contribution from the distorted magnetosphere. SUGIURA et al. (1971) show observed values of AB (the magnitude of the field in space minus the main reference field) to be 40 gammas at low Kp index times. My selection of MDT data days provided for quieter magnetospheric conditions than the low Kp selection would
Variations of the geomagnetic field at equatorial locations
-.
- EQUINOXES
615
-
JFMAMJJASOND MONTH
Fig. 8. (a) (top) Heavy lines are the locations of the equivalent magnetospheric currents (out of page) at the December solstice (DS), equinoxes (EQ) and June solstice (JS) with respect to the earth (N = north pole) when the sun is to the left and the vectors represent the geomagnetic field (AB) directions at the equator due to the model magnetospheric currents in the three seasonal locations; (b) (bottom) the annual and semiannual variation of the horizontal (H,,,) and vertical (Z,,,) geomagnetic field components obtained from the seasonal change in the model magnetospheric current position.
provide (CAMPBELL, 1980b) so the two values seem to be consistent. If the nightside current system responsible for the seasonal change in MDT is as close to the earth as has been indicated by SUGIURA et al. (1971) and CAMPBELL(1973) then one must assume that there would be a significant diurnal change in field at the equatorial stations due to the earth’s rotation within a distorted magnetosphere. The mean I-I component diurnal variation for Sq(24) at the equator is about 100 gammas in size (CAMPBELL,in press). As an upper limit, (for no distortion on the dayside) the magnetospheric contribution to Sq(24) in I-I would be about 3.5 gammas (one-half of the annual change). Baselines for Sq drawn from night levels of field may be uncertain by this amount. The annual change in MDT(D) is smaller in amplitude than MDT(H) and MDT(Z). This D variation in h4DT seems to be opposite to that of Sq and the form of the variation is modified rapidly with increasing latitude. Such behavior may be due to a seasonal adjustment of east-west distortion of the magnetosphere and, in small part, to a night-
time contribution dynamo current.
by F~WFJETH’S (1971) F-region
3.3. Lunar tidal field, L(12) The semidiurnal lunar variation shows principally an annual change in H (Fig. Id) having a range in variation of over 7 gammas with a maximum in early February and a minimum in early July. This strong seasonal change that disappears at latitudes above 10” suggests behavior typical of an ionospheric current source. The lunar variation is principally a daytime phenomenon. The annual change would seem to indicate increased eastward electrojet lunar-tidal current contributions during the southern hemisphere summer and fall. Such behavior has been reported for equatorial stations a number of years ago (ONWUMJXHELI and ALJZXANDER, 1959; MATSUSHITA and CAMPBEU, 1972); the phenomenon is, as yet, unexplained. Only the large afternoon counter-electrojet events show a similar annual occurrence maximum in JanuaryFebruary (MAYAUD, 1977). There is no seasonal correspondence between the I.,( 12) and the Sq (12).
WALLACE H.
616
Figures Id, 2d and 3d show the amplitude changes; whereas, Figs. 4c and 5c illustrate the seasonal phase variations. Only the H annual variation seems to be significant. MALIN et al. (1975) have proposed an F-region location of the L(12) current because of differences in Sq and L field effects. However, such explanation is inconsistent with the present results because the equinoctial F-region ionization values at equatorial stations far exceed both the December and June solstitial ionization values (RAJARAM and RASTOGI, 1977). The annual and semiannual changes of L(12) differ appreciably from Sq(12). This difference may indicate that the short period harmonics of Sq arise, in part, from the Fourier analysis representation of the sudden conductivity changes at sunrise and sunset times (Fig. 7), whereas the L(12), obtained by a superposed epoch technique, is largely representative of a semidiurnal, tidal wind system. 4. CONCLUSIONS
equatorial annual and semiannual midnight field level (MDT) variations of the northward and vertical components at quiet periods seem to repThe
CAMPBELL
resent the expected seasonal night-time magnetospheric distortions. The seasonal equatorial region solar quiet daily variations in Sq follow closely the annual and semiannual patterns expected to be caused by ionospheric conductivity and heating variations that give rise to a dynamo current at Eregion heights. The difference in the seasonal changes in MDT and Sq would imply that the base line levels for the ionospheric source variations in H and Z would best be taken as the night-time field levels adjusted for a local-time magnetospheric change of a few per cent. The lunar seasonal variations show characteristics of dayside ionospheric electrojet origin; however, the cause of the annual, early-February enhancement of the lunar tidal field is unknown and the explanation should be sought in the atmospheric wind behavior. Acknowkdgemen&--I am grateful to ALANH. SHAPLEY, J. VIRGINIA LINCOLN,JOSEPH H. ALLEN, and WILLIAM PAULISHAK,of the World Data Center A in Boulder, Colorado, for providing the data and facilities for some of the analysis. I thank Dr S. MATSUSHITAfor discussions regarding Sq and L(12) interpretation. The research was supported, in part, by the U.S. Office of Naval Research.
REFERENCE8
BHARGAVAB. N. BHARGAVAB. N. CAMPBELLW. H. CAMPBELL W. H. CMBELL W. H. CMEU W. H. C~BELL W. H. CAMF’BELL W. H. CHAPMANS. and BARTEU J. CURRE R. G. KENDALLP. C., WINDLE D. W., AKASOFUS.-I. and CHAPMAN S. MALIN S. R. C., CECERE A. and PALUMBOA. m S. R. C. and ISIKARAA. M. MA~USHITA
S.
MATSUSHITAS. and CAMPBELL W. H. MAYAUDP. N. MCINTOSHD. H. OLSONW. P. ONWIJMECHILLI C. A. and ALEXANDER N. S. RAJARAMG. and RASTOGIR. G. RICHMONDA. D. RISHEiETHH. SARABHAIV. and NAIR K. N. SUGIURAM., LEDLEYB. G., SKILLMANT. L. and HEPPNERJ. P. TARPLEYJ. D. VAN WUK A. M. WILLIAMSD. J. and MEAD G. D.
1972a 1972b 1972 1973 1977 1979 1980a 1981 1940 1976 1969
Planet. Space Sci. 20, 423. Annls Gkoohvs. 28. 357. Planet. Sp&e’Sci. i0, 61. J. atmos. terr. Phys. 35, 1127. J. ahnos. terr. Phys. 39, 1217. J. geophys. Res. 84, 875. .7. Geomag. Geoelecr. 32, 105. (in preparation). Geomagnetism. Clarendon, Oxford. J. geophys. Res. 81, 2935. Geophys. J. R. astr. Sot. 17, 185.
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Geophys. J. R. ask Sot. 41, 115. Geophys. J. R. astr. Sot. 47, 445. Physics of Geomagnetic Phenomena, Academic Press, New York. J. atmos. Ierr. Phys. 34, 1187. J. atmos. terr. Phys. 39, 1055. Phil. Trans. R. Sot. A251, 525. Planet Space Sci. 18, 1471. J. ahos. terr. Phys. 16, 115. J. atmos. terr. Phys. 39, 1175. J. Geomag. Geoelect. 31, 287. Planet. Space Sci. 19, 263. Nature 223, 603. J. geophys. Res. 76, 7552.
1973 1953 1965
J. atmos. terr. Phys. 35, 1063. J. geophys. Res. 58, 418. J. geophys. Res. 70, 3019.
Reference is also made to the following unpublished material: 1980b CAMPBELLW. H.
J. geophys. Res. 85,6557.
p.
301.
of Atmawbic and in Northern Ireland
TmcsbialPhysics. Vol.43.
No. S/6, pp. 607616,
1981.
0021-9169/81/050607-lOSO2.00/0 Per.gamon Press Ltd.
Annual and semiannual variations of the geomagnetic field at equatorial locations WALLACEH. CAMPBELL U.S. Geological Survey, Denver, CO 80225, U.S.A.
AIra&&-For a year of quiet solar-activity level, geomagnetic records from American hemisphere observatories located between about 0“ and 30” north geomagnetic latitude were used to compare the annual and semiannual variations of the geomagnetic field associated with three separate contributions: (a) the quiet-day midnight level, MDT; (b) the solar-quiet daily variation, Sq; and (c) the quiet-time lunar semidiurnal tidal variation, L(12). Four Fourier spectral constituents (24, 12, 8, 6 h periods) of Sq were individually treated. All three orthogonal elements (H, D and 2) were included in the study. The MDT changes show a dominant semiannual variation having a range of about 7 gammas in H and a dominant annual variation in 2 having a range of over 8 gammas. These changes seem to be a seasonal response to the nightside distortions by magnetospheric currents. There is a slow decrease in MDT amplitudes with increasing latitude. The Sq changes follow the patterns expected from an equatorial ionospheric dynamo electrojet current system. The dominant seasonal variations occur in H having a range of over 21 gammas for the 24 h period and over 12 gammas for the 12 h period spectral components. The higher-order components are relatively smaller in size. The !&J(H) amplitudes decrease rapidly with increasing
latitude. Magnetospheric contributions to the equatorial Sq must be less than a few per cent of the observed magnitude. The L(12) variation shows the ionospheric electrojet features by the dominance of H and the rapid decrease in amplitude with latitude away from the equator. However, the seasonal variation range of over 7 gammas has a maximum in early February and minimum in late June that is not presently explainable by the known ionospheric conductivity and tidal behavior.
1. INTRODUCTION The
purpose of this study is to examine the seasonal behavior of the quiet geomagnetic field at low-latitude locations. The separate contributions of the midnight (MDT), daily variation (Sq), and lunar (I.) parts of the observed fields are compared using data from American hemisphere observatories (Table 1) at low latitudes for the quiet-sun year, 1965. This presentation utilizes the methods of three recent studies (C-BELL, 1980a, 1980b, in press) brought together in a unique fashion to emphasize the equatorial features and to illustrate the differences in the seasonal amplitude and phase changes obtained from a Fourier analysis of annual
and semiannual components in the three orthogonal magnetic-field directions. Early studies of the seasonal changes in the geomagnetic field were reported by CHAPMAN and BARTU~ (1940), VAN W~JK (1953) and MCINTOSH (1959). 2. ANAL.YSls Three slightly different data sets were selected from the geomagnetic records of the 1965 quietsun year. Because the study of midnight values required conditions of quiet magnetosphere, a special-day determination that utilized low Dst (ring current) and AE (aurora1 electrojet) indices
Table 1. Observatory locations Geographic degrees Station
Code
Latitude
E. longitude
Huancayo Tatuoca Paramaribo Honolulu San Juan
HU Pfz
12.05 1.20 5.82 21.32 18.12
284.67 311.48 304.78 202.00 293.85
HO SJ
S s N N N
Geomagnetic Latitude 0.64 9.51 16.93 21.17 29.57
s N N N N
degrees E. longitude 354.27 21.25 14.83 266.99 3.63
608
WALLACEH. CAMPBELL
was employed. The L and Sq changes were investigated at times of quiet ionospheric conditions when the magnetic activity index was low. The days for determining Sq were a subset of those for determining L. The five magnetic observatories from which data were obtained are listed in Table 1. The analysis was arranged in geomagnetic latitude from 0 to 30” as given in that table. In the American hemisphere, within the accuracy of this study, the dip latitudes and geomagnetic latitudes are essentially interchangeable. This study has concentrated on the surface observations of field; the separation of external and internal contributions to the measurements have been omitted. Such separation will not affect the general conclusions sought at this time. A later study by the author will investigate the ionosphericcurrent models and deep earth conductivity values that may be obtained through a source separation. The midnight level of the geomagnetic field (MDT) was taken to be the station’s reported universal time hourly value that occurs during the time of local midnight. For each of the three orthogonal geomagnetic-field components, H, D and Z, in gammas (1 gamma = 1 nanotesla), only the MDT values for 37 days selected from a list representing the quietest magnetospheric conditions (CAMPBELL, 1979) were selected for analysis. Details of the data processing and error estimate are given in CAMPBELL (1980b). Essentially, the procedure for determining the seasonal changes involves the fitting of an observatory’s selected MDT values to a function that contains a constant and linear term, as well as cosine and sine terms of the annual and semiannual Fourier analysis coefficients. If the ionospheric effects are not important in the midnight hours on quiet days, then the annual coefficients a, and b, and the semiannual coefficients a2 and b, of the Fourier terms should show the summer-winter and equinoctial changes in the field about the earth as it is distorted by the stream of particles and fields from the sun and the resulting current systems that arise within the magnetosphere and its tail. Station values of Fourier coefficient amplitudes were plotted versus geomagnetic latitude; straight-line segments were drawn connecting these data points; the values at latitudinal points separated by 2.5” were determined and a smoothing applied to the readings so that a continuous-curve representation of the midnight cosine and sine Fourier terms was obtained. With these terms the MDT amplitude function of combined annual and semiannual variation was recon-
structed and plotted, as a seasonal change at 5” increments, from the equator to 30” North geomagnetic latitude. The three orthogonal-direction field components were treated separately. Figures la, 2a and 3a show the combined annual and semiannual changes in amplitude of the MDT levels. The FAR technique for month-by-month separation of the semidiurnal (12 h) lunar-tidal effect, I-(12), in geomagnetic changes registered by the 2.5 min scaled field values at a station was described by MATSUSHITA and CAMPBELL (1972). The method is essentially a superposed-epoch technique that makes use of the 50.5 min difference in solar and lunar time to separate the diurnal, 12 h (lunar time) changes from the low activity geomagnetic record residuals that have the quiet solartime variations removed. The procedure uses onemonth sets of days when all 8 of the 3 h magnetic activity indices, Kp, were less than or equal to a value of 3+. Moderately disturbed records may be used in the lunar-time superposed-epoch analysis because the geomagnetic activity is essentially initiated in a statistically random fashion and is enhanced with some solar-time modulation; only very large disturbances would completely mask the lunar contribution. For the stations of Table 1 and separated H, D and Z components, the lunar semidiurnal amplitude as well as the phase (hour of first maximum in the variation) have seasonal variations that may be represented by the Fourier cosine and sine coefficients (a, and b, for the annual terms and a2 and b2 for the semiannual terms). When these coefficients are plotted, smoothed, and scaled as a function of latitude in a manner similar to that described in the preceding paragraph and explained in CAMPBELL (198Oa), the lunar variation from 0” to 30” geomagnetic latitude is obtained as represented in Figs. Id, 2d and 3d for the seasonal amplitude changes in the Figs. 4c and 5c for the seasonal phase changes. The method for determining monthly values of Sq for the three components of the geomagnetic field at a station is given in CAMPBELL (1972). In outline, this method may be described this way: for each month the quietest UT days were selected by the condition that for all 8 of the 3 h index intervals, Kp was less than or equal to 2+. This is the lowest value of Kp that would provide a sufficientsize sample of quiet days for this analysis. For each of the 2.5 min data samples of those days, the average values were formed for the H, D and Z components separately and called Sq. The appropriate lunar tidal component described above was
Variations of the geomagnetic field at equatorial locations
ANNUAL
AND SEMIANNUAL
AMPLITUDE
VARlATlON
H COMPONENT L(l2)
MDT
# LA L! I i I JFMAMJJASOND
;
!
I
!
Ii
LLL.1. I 1.1 JFMAMJJASOND
b 11.1.1
.u
L.I
i
I I 1 I
1 I 1 i-:-1
JFMAMJJASOND
LII_Lu_LuI_LLJ JFMAMJJASOND
Fig. 1. The H-component, quiet-time, anmual and semiannual combined variation of the geomagnetic field amplitudes in 5” increments from the equator (bottom row) to 30” (top row) geomagnetic latitude, displayed for (columns left to right): (a) midnight field level, MDT; (b) quiet daily variation, 24h spectral component, Sq(24); (c) quiet daily variation, 12 h spectral component, Sq(12); and (d) the semidiurnal lunar tidal variation, L(12). Manths of the year are indicated by letters at the bottom of the columns, The distance between 5’ latitude lines measures a 5-gamma (1 gamma = t nT) amplitude scale size. As variations become less than 0.5 gammas in amplitude they grow less significant.
ANNUAL
AND SEMIANNUAL
AMPLITUDE
VARIATION
Z COMPONENT MDT
U-L-L_.IIIL.LLu JFMPMJJASOND
U..J. L_lI_Lu._i JFMAMJJASOND
iLLl _1-i. LIIJ JFMAMJJASOND
I__LLLL_LL_LJ_LIJ JFMAMJJ4SOND
MONTH OF YEAR
Fig. 2. Similar to Fig. 1, except that the Z-component
of field is depicted.
WALLACE
610
H. CAMPBELL
ANNUAL AND SEMIANNUAL
AMPLITUDE
VARIATION
D COMPONENT L (PI
sq m
5944)
llIIlII1IlI~I JFYAMJJASOND
I~lllllllll~l JFYAYJJASOND
MONTH
OF YEAR
Fig. 3. Similar to Fig. 1, except that the D-component
then subtracted and the residual was taken to be the monthly Sq variation for each station. Monthby-month Fourier analysis of these Sq gave amplitude and phase values of the constituent 24, 12, 8 and 6 h components of the quiet field levels. Next, a seasonal Fourier analysis of these components yielded the annual (al and b,) and semiannual (a, and b2) coefficients of the separated amplitude and phase changes. At this point the data processing continued in a fashion similar to that used in the lunar analysis above and included line segment plots of the station coefficients, 5-point smoothing, and reconstruction of the amplitude and phase change variations for 5” latitude increments, Figure 6 shows the annual and semiannual variation of the amplitudes during the year for the 24, 12, 8 and 6 h spectral components of Sq at 0, 15 and 30” latitude locations. The figure shows that the amplitudes generally decrease for higher-order harmonics. The 8 and 6 h seasonal variations are similar in seasonal behavior to that of the 12 h component. Thus, in subsequent discussions I will show only the two lowest order spectral components, Sq(24) and Sq(12). Figures lb, c, 2b, c, 3b, c illustrate the summation of annual and semiannual changes in amplitude for latitudes from 0 to 30”. The latitude and seasonal change in the phase of Sq(24) and Sq(12) are shown in Figs. 4 and 5.
of field is depicted.
3.RESuLTS
3.1. Quiet daily uariation, Sq The best interpretation of Sq-field changes supports a model that pictures the dynamo effect of diurnal and semidiumal tidal winds in the lower ionosphere generated in situ by solar heating (RKHMOND, 1979). Higher harmonics appear in the field records because of the rapid change in ionization near sunrise and sunset. Most models consider the E-region near 100 km, which disappears at night-time, to be the locale of supporting conductivity for the dynamo effect. However, RISHBETH (1971) suggested that the high-altitude F-region winds may produce a dynamo effect at night hours when its fields cannot be shorted out by the conducting E-region. Bishbeth has calculated that under the most favorable conditions such a field would be no larger than 3 gammas. The annual and semiannual variation of the field components for the Sq behave as one would expect for a typical eastward current of the equatorial electrojet system. Both the 24 and 12 h components of H, shown in Figs. lb and c, display two prominent features. There is a large, northward enhancement of field (+H) during the equinoxes at the equator and a steep decrease of amplitude with increasing latitude to about 15 or 20”. The extreme
Variations of the geomagnetic field at equatorial locations
ANNUAL
PHASE
0.
611
VARIATION
15*
30. Ii
H
4
D
2 H H
2 D
Sq(l2) H D \
2 ‘c\
2
LINE LENGTH
SCALE
D 2
D
H
D
H
H
D
f 2
Fig. 4. The annual variations of the phase characteristics of geomagnetic field at 0, 15 and 30” geomagnetic latitude (columns left to right) displayed in three rows representing (top to bottom): (a) the 24 h spectral component of the quiet daily variation, Sq(24); (b) the 12 h spectral component of the quiet daily variation, Sq(12); and (c) the semidiurnal lunar tidal variation, L(12). The magnitude of the annual phase shift is indicated by the length of the vectors (scale, in hours, is given to the lower right of the figure). The time of year for the maximum in annual phase change is indicated by the vector directions (scale, in months, is shown to the upper right of the figure). Separate vectors are shown for the H, D and Z orthogonal field components followed in the analysis. For example, the vector plotted for H of the Sq(24) at 30” indicates that the annual variation for the 24 h Fourier component of the change in Sq shows an annual phase change of about 1.5 h with an increase in June. Note that phase changes of less than 0.5 h in magnitude or those phase changes arising for annual field change amplitudes (Figs. 1,2 and 3) less than 0.5 gammas are of minimal importance.
range of variation in Sq at the equator is over 21 gammas in magnitude for the diurnal component, Sq(24), and over 12 gammas in magnitude for the semidiumal component, Sq(12), of II. Figure 7 shows the solar zenith angle, x, and Chapman function, m, for the E-region ionization at the geographic equator at the equinox and the solstice. In this region, although the day and night are always of equal duration, the sun is vertical (x = 0) twice a year at the equinoxes. The seasonal, solar isolation changes cause the thermally generated wind systems to have a semiannual fluctuation. The overhead E-region ionization, roughly represented by m, shows a semiannual change that supports an increase in eastward electrojet current at the equinoxes. This semi-
annual characteristic of x, of course, decreases with increasing latitude to the Tropic of Cancer at 23”27’ geographic latitude. The horizontal alignment of the earth’s main field at ionospheric altitudes near the equator that promotes the high conductivity supporting the electrojet existence decreases more rapidly with latitude than x. The strong electrojet effect on H shows little annual or semiannual variation in its phase (Figs. 4a, b, 5a and b). Semiannual and annual changes in the Zamplitude for Sq could indicate both location and amplitude changes of the electrojet current. The amplitude changes should be related to the H changes and therefore should appear semiannually. For indicating a seasonal motion of an ionospheric
WALLACE H. CAMPBELL
612
Sq (24)
SEMIANNUAL
PHASE
O0
w
Dy
VARIATION 30.
j
“6
/ D
Ii
Ii D
H D
sq(12)
d_
2 / D
r 2
2
LINE LENGTII SCALE 0
2
I
3
HOURS
LO21
D n 3 2
I\
n
2
2-D n
D
Fig. 5. Similar to Fig. 4, except that the semiannual variation of the phase characteristic is depicted. electrojet current, the vertical components of the field amplitudes observed at the earth’s surface are considerably more sensitive than are the horizontal components of the field amplitudes. Thus, small location changes for the electrojet should show an annual variation in Z. We observe both large annual and semiannual terms in Fig. 2b. This may indicate that for the eastward electrojet current, the increasing negative value of Z in the December solstice is caused by a current displacement southward toward the summer hemisphere, just as might be expected from the solar control of the ionization maximum. The large semiannual terms follow the equinoctial enhancement of electrojet intensity. The range of Z variation is about 6 gammas at the equator. The interesting feature of the seasonal D variation is the dominant annual change for the diurnal component of about 11 gammas near 30” latitude. The Sq current patterns show foci close to this latitude (MATSUSHITA, 1967). The seasonal variation in the foci strongly varies the north-south component of the horizontal ionospheric current system (TARPLEY, 1973). Although the current is strongest close to the equator, the amplitude ratio of D to H is largest close to the foci (CAMPBELL, 1977).
SARABHAI and NAIR (1969) proposed a ring current source for the Sq variations at the earth’s surface. BHARGAVA (1972a) analyzed a 45 y data sample at Alibag and showed that the annual and semiannual variations varied independently with local time and that there was significant components of these variations even at local midnight hours. This result indicated that some nonionospheric effects were likely appearing in the annual and semiannual variations. BHARGAVA (1972b) also showed that only the night-time annual components of field at low latitude stations varied significantly with geomagnetic activity level. He concluded that the probable cause for the annual variation at night is the modulation of the magnetospheric ring current. CURME (1976) found that there was an annual change in the base level associated with Sq. OLSON (1970) studied the quiet-time daily, Sqtype variations of field at the earth’s surface that are due to the changing magnetospheric form as seen at the earth’s surface. He also computed the seasonal variation in the north and vertical components for the daily mean of this field change due to the magnetosphere. His seasonal equatorial variations of H (about 1 gamma in range) showed semiannual maxima at the equinoxes of less than a
613
Variations of the geomagnetic field at equatorial locations
Sq ANNUAL AND SEMIANNUAL AMPLITUDE VARIATKINS
JFYAWJJASONO
-
24 hr
--_---_
12 hr 6h#
_________
6h, 111
LI 11I1I1II JFYAYJJASONO
111 I 11 JFYAYJJASONO
I
I
Ill
II
MONTHOFYEAR
Fig. 6. The quiet annual and semiannual variation for the geomagnetic amplitudes of Sq at (bottom to top rows) 0, 15, 30” geomagnetic latitude steps displayed for (left to right columns) the three orthogonal field components H, D and Z. The 24, 12, 8 and 6 h Fourier spectral components of the variations are marked with the special line codes shown. Months of the year are indicated by the letters at the bottom of the columns. An amplitude scale in gammas (1 gamma = 1 n7’) is shown. As variations become less than 0.5 gammas in amplitude, they grow less significant.
few per cent of the change shown in Fig. 6. At 30”
latitude he found an annual periodicity (about 2 gammas range) with a summer maximum that does not correspond to my observations. Olson’s report of a Z-component annual change at the equator (about 1 gamma in range) also does not match my results. At 30” latitude his Z prediction of amplitude change (about 1 gamma range) may correspond to Fig. 6. In light of the above shortcomings, it would appear that the magnetospheric model contributions to Sq observations near the equator are quite small, 3.2. Midnight field level, MDT In the early days of satellite exploration of the earth’s magnetosphere, it was found that the dipole field shape was severely distorted into a long taillike form away from the sun and that a current sheet existed on the dark side of the extended magnetosphere. WILLUMS and MEAD (1965) and KENDALL et al. (1969) found that the shape of the field lines in the noon-midnight meridian plane of the nightside magnetosphere could be adequately
modeled by the addition of a magnetotail current sheet. SIGNORA et al. (1971) reported that the satellite, midnight-meridian observations of the magnetospheric field clearly indicated the existence of a ‘disk-shaped’ quiet-time ring current present at even the lowest values of Kp. CAMPBELL (1973) showed that the observed field levels near midnight at geomagnetic latitudes lower than 40” varied during periods of high geomagnetic activity in a consistent fashion to indicate a current source at an antisolar position within the nightside magnetosphere. In a somewhat similar study MALIN and ISIKARA (1976) analyzed the annual variation of monthly mean field levels at midnight for a distribution of world stations. At latitudes below 60” they found a global variation of the night-time surface field that could be best explained by the annual change in location of the mean latitude for the magnetospheric ring current that moved north in December and south in June. Their results seem to be dominated by the active periods. At quiet times, on the nightside, the earth’s field in space may be represented by the superposition
614
WAILME H. CAMPBELL
EQUATORIAL
SOLAR
IONIZATION
30
0 0
2
4
6
6 LOCAL
IO
14
12
MERIDIAN
16
16
20
22
24
TIME
Fig. 7. The daily variation, in local meridian time, for the solar zenith angle, ,Y,(solid lines and scale to left) and the Chapman ionization function, &Jos x), (dashed lines and scale to right) for the solstice and equinox at the equator.
of magnetospheric ring and tail currents upon the dipole field. The fields of such currents are seen at the earth’s surface as a decrease in H at low latitudes. With the seasonal change in solar direction, these current contributions change location so that an equatorial measurement of the field, AB, that must be added to the dipole component would be in the direction indicated by Fig. 8a. H is positively directed northward and Z is positively directed into the earth. Thus, we may see that the expected seasonal variation of this magnetospheric distortion on the nightside of the earth would produce equatorial variations in H,,, and Z,,, as shown in Fig. 8b. H has a semiannual variation with minima at the equinoxes; Z has an annual variation with a maximum near the June solstice. The amplitudes of these seasonal variations in MDT should change slowly with latitude from the equator to 30” because of the scale and geometry of the magnetospheric contribution. Note that H,,, is out of phase with the expected value of Sq(H). MDT values of H and Z shown in Figs. la and 2a seem to be consistent with the behavior expected of the
H,,, and Z,,, displayed in Fig. 8. The F-region, night-level dynamo-current variations (&WBETH, 1971) would be too small to be considered as a source of these changes. An estimate of the external components, H, and Z,, of the seasonal variation in quiet-time, midnight field levels may be obtained from the H = 7.0 gammas and Z = 8.0 gammas (seasonal MDT in Figs. 1 and 2) near the equator using the induction factors 2/3 and 2.0 (respectively) as was done in an earlier paper (CAMPEML,L,1973). This gives H. = 4.7 gammas and Z, = 16.0 gammas. Assuming these values to be equal to H,,, and Z,,, respectively at the when solstice the magnetospheric-distortioncurrent axis is at an angle of 32.5” to the equatorial plane, the geometric relationships give a magnitude of about 30 gammas for the field contribution from the distorted magnetosphere. SUGIURA et al. (1971) show observed values of AB (the magnitude of the field in space minus the main reference field) to be 40 gammas at low Kp index times. My selection of MDT data days provided for quieter magnetospheric conditions than the low Kp selection would
Variations of the geomagnetic field at equatorial locations
-.
- EQUINOXES
615
-
JFMAMJJASOND MONTH
Fig. 8. (a) (top) Heavy lines are the locations of the equivalent magnetospheric currents (out of page) at the December solstice (DS), equinoxes (EQ) and June solstice (JS) with respect to the earth (N = north pole) when the sun is to the left and the vectors represent the geomagnetic field (AB) directions at the equator due to the model magnetospheric currents in the three seasonal locations; (b) (bottom) the annual and semiannual variation of the horizontal (H,,,) and vertical (Z,,,) geomagnetic field components obtained from the seasonal change in the model magnetospheric current position.
provide (CAMPBELL, 1980b) so the two values seem to be consistent. If the nightside current system responsible for the seasonal change in MDT is as close to the earth as has been indicated by SUGIURA et al. (1971) and CAMPBELL(1973) then one must assume that there would be a significant diurnal change in field at the equatorial stations due to the earth’s rotation within a distorted magnetosphere. The mean I-I component diurnal variation for Sq(24) at the equator is about 100 gammas in size (CAMPBELL,in press). As an upper limit, (for no distortion on the dayside) the magnetospheric contribution to Sq(24) in I-I would be about 3.5 gammas (one-half of the annual change). Baselines for Sq drawn from night levels of field may be uncertain by this amount. The annual change in MDT(D) is smaller in amplitude than MDT(H) and MDT(Z). This D variation in h4DT seems to be opposite to that of Sq and the form of the variation is modified rapidly with increasing latitude. Such behavior may be due to a seasonal adjustment of east-west distortion of the magnetosphere and, in small part, to a night-
time contribution dynamo current.
by F~WFJETH’S (1971) F-region
3.3. Lunar tidal field, L(12) The semidiurnal lunar variation shows principally an annual change in H (Fig. Id) having a range in variation of over 7 gammas with a maximum in early February and a minimum in early July. This strong seasonal change that disappears at latitudes above 10” suggests behavior typical of an ionospheric current source. The lunar variation is principally a daytime phenomenon. The annual change would seem to indicate increased eastward electrojet lunar-tidal current contributions during the southern hemisphere summer and fall. Such behavior has been reported for equatorial stations a number of years ago (ONWUMJXHELI and ALJZXANDER, 1959; MATSUSHITA and CAMPBEU, 1972); the phenomenon is, as yet, unexplained. Only the large afternoon counter-electrojet events show a similar annual occurrence maximum in JanuaryFebruary (MAYAUD, 1977). There is no seasonal correspondence between the I.,( 12) and the Sq (12).
WALLACE H.
616
Figures Id, 2d and 3d show the amplitude changes; whereas, Figs. 4c and 5c illustrate the seasonal phase variations. Only the H annual variation seems to be significant. MALIN et al. (1975) have proposed an F-region location of the L(12) current because of differences in Sq and L field effects. However, such explanation is inconsistent with the present results because the equinoctial F-region ionization values at equatorial stations far exceed both the December and June solstitial ionization values (RAJARAM and RASTOGI, 1977). The annual and semiannual changes of L(12) differ appreciably from Sq(12). This difference may indicate that the short period harmonics of Sq arise, in part, from the Fourier analysis representation of the sudden conductivity changes at sunrise and sunset times (Fig. 7), whereas the L(12), obtained by a superposed epoch technique, is largely representative of a semidiurnal, tidal wind system. 4. CONCLUSIONS
equatorial annual and semiannual midnight field level (MDT) variations of the northward and vertical components at quiet periods seem to repThe
CAMPBELL
resent the expected seasonal night-time magnetospheric distortions. The seasonal equatorial region solar quiet daily variations in Sq follow closely the annual and semiannual patterns expected to be caused by ionospheric conductivity and heating variations that give rise to a dynamo current at Eregion heights. The difference in the seasonal changes in MDT and Sq would imply that the base line levels for the ionospheric source variations in H and Z would best be taken as the night-time field levels adjusted for a local-time magnetospheric change of a few per cent. The lunar seasonal variations show characteristics of dayside ionospheric electrojet origin; however, the cause of the annual, early-February enhancement of the lunar tidal field is unknown and the explanation should be sought in the atmospheric wind behavior. Acknowkdgemen&--I am grateful to ALANH. SHAPLEY, J. VIRGINIA LINCOLN,JOSEPH H. ALLEN, and WILLIAM PAULISHAK,of the World Data Center A in Boulder, Colorado, for providing the data and facilities for some of the analysis. I thank Dr S. MATSUSHITAfor discussions regarding Sq and L(12) interpretation. The research was supported, in part, by the U.S. Office of Naval Research.
REFERENCE8
BHARGAVAB. N. BHARGAVAB. N. CAMPBELLW. H. CAMPBELL W. H. CMBELL W. H. CMEU W. H. C~BELL W. H. CAMF’BELL W. H. CHAPMANS. and BARTEU J. CURRE R. G. KENDALLP. C., WINDLE D. W., AKASOFUS.-I. and CHAPMAN S. MALIN S. R. C., CECERE A. and PALUMBOA. m S. R. C. and ISIKARAA. M. MA~USHITA
S.
MATSUSHITAS. and CAMPBELL W. H. MAYAUDP. N. MCINTOSHD. H. OLSONW. P. ONWIJMECHILLI C. A. and ALEXANDER N. S. RAJARAMG. and RASTOGIR. G. RICHMONDA. D. RISHEiETHH. SARABHAIV. and NAIR K. N. SUGIURAM., LEDLEYB. G., SKILLMANT. L. and HEPPNERJ. P. TARPLEYJ. D. VAN WUK A. M. WILLIAMSD. J. and MEAD G. D.
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Geophys. J. R. ask Sot. 41, 115. Geophys. J. R. astr. Sot. 47, 445. Physics of Geomagnetic Phenomena, Academic Press, New York. J. atmos. Ierr. Phys. 34, 1187. J. atmos. terr. Phys. 39, 1055. Phil. Trans. R. Sot. A251, 525. Planet Space Sci. 18, 1471. J. ahos. terr. Phys. 16, 115. J. atmos. terr. Phys. 39, 1175. J. Geomag. Geoelect. 31, 287. Planet. Space Sci. 19, 263. Nature 223, 603. J. geophys. Res. 76, 7552.
1973 1953 1965
J. atmos. terr. Phys. 35, 1063. J. geophys. Res. 58, 418. J. geophys. Res. 70, 3019.
Reference is also made to the following unpublished material: 1980b CAMPBELLW. H.
J. geophys. Res. 85,6557.
p.
301.