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Observations of ionospheric electron content at medium latitude geomagnetically conjugate stations
Planet. Space
Sci. 1972,
Vol. 20, pp. 2045 to 2050.
Pergamon Press. Printed in Northern Ireland
OBSERVATIONS OF IONOSPHERIC ELECTRON CONTENT AT MEDIUM LATITUDE GEOMAGNETICALLY CONJUGATE STATIONS Ionosphere
K. C. YEH Radio Laboratory, Department of Electrical Engineering, University at Urbana-Champaign Urbana, Illinois 61801, U.S.A.
of Illinois
(Received in final form 17 May 1972) Abstract-Electron content data recorded at Cold Bay, Alaska and Invercargill, New Zealand have been used to study the geomagnetic conjugate effects. The correlation on day-to-day variations at these two stations is found to be negative at extreme magnetic quiet and it then increases to positive values with magnetic activities. When disturbed magnetically, the correlation is +0.73. Evidence is presented to show that, on the first day following the sudden commencement, the afternoon rise and the subsequent pre-sunset fall in electron content is controlled geomagnetically. 1. INTRODUCTION Many geophysical phenomena in the upper atmosphere are known to have geomagnetic conjugate effects. The earliest example of this, recognized in the last century, is the tendency of great auroras to occur simultaneously in the northern and southern aurora1 zones. However, only after the IGY years were more concentrated observations made. Since then conjugate effects have been reported to occur on events related to aurora1 radio absorption, precipitation of electrons in the aurora1 zone, polar magnetic substorms, VLF emissions, occurrence of whistlers and magnetic micropulsations (For a review, see Wescott, 1966 and Special Issue on Conjugate Point Symposium, 1968). At temperate latitudes and in the ionosphere, conjugate work has been carried out on scintillation of radio signals (Yeh ef al., 1968) and conjugate heating (Carlson, 1968; Kwei and Nisbet, 1968; Evans, 1968). Several studies on the behavior off82 at conjugate points have also been made. At low latitudes, Matsushita (1968) and Cummack (1967) have found high positive correlation of variations of f,F2 from the monthly mean. Similar studies at temperature latitudes have uncovered very weak correlations between Campbell Island and a chain of three Alaskan stations (Hooper, 1969). However, Hooper did not separate his data according to geomagnetic activities. As it turns out, the dependence on geomagnetic activities is very important since Ben’kova et al. (1968) have found in-phase variations during geomagnetic disturbances and anti-phase variations during weak activities between Kerguelen fz2 values and those recorded along a chain of stations at Arkhangel’sk Province. 2. NATURE
OF THE EXPERIMENT
In this study we wish to report electron content observations made at Cold Bay, Alaska (55.2”N, 162.75”W) and Invercargill, New Zealand (46.42%, 168.32”E). The electron content values were obtained by monitoring the rotation of wave polarization produced through the Faraday effect on VHF signals (136 MHz) transmitted by the geostationary satellite (Titheridge, 1966; Klobuchar and Whitney, 1966; see also the review by Garriott ef al., 1970). At Invercargill, recording was made on signals transmitted by Symcom III from December 1968 through March 1969 and by ATS I from April 1969 onward. The corresponding subionosphere point was approximately 42”S, 164”E for Symcom III and 42”S, 172’E for ATS I. At Cold Bay, ATS I signals were used for the entire period and its subionospheric point was located at 49”N, 162”W approximately. Computations of 300 km-to-300 km geomagnetic conjugacy indicate that the Invercargill subionospheric point 2045
K. C. YEH
2046
(42”S, 164”E) is mapped along the geomagnetic field into a point at 53”N, 169”W in the northern hemisphere. This point is 4” higher in latitude and 7” larger in west longitude than the Cold Bay subionospheric point. The difference is not expected to be important for most conjugate studies, except perhaps during magnetic storms as discussed in Sections 4 and 5. Except for a few interruptions due to technical difficulties, nearly continuous data have been collected and used in this study for the period December 25, 1968 through September 9, 1969. The average electron content at these two stations shows the seasonal behavior distinct for its own hemisphere and does not appear to be correlated in any way. This is not unexpected since f$2 studies of, for example, Matsushita (1968) show poor correlation on the monthly median values even for low latitude conjugate stations. Our attention is therefore turned to conjugate effects on the variations from the average. 3. DAY-TO-DAY CHANGE IN CONTENT
Electron content is known to have large day-to-day fluctuations. A change of 20 per cent is fairly common and on occasion it may be as large as 50 per cent or more. In general, the daytime day-to-day changes are larger than the corresponding night-time changes. At temperate latitudes these daytime changes are correlated geographically in a region which is 800 km in the north-south direction and 2000 km in the east-west direction, but not over a distance 9000 km in the east-west direction (Rao et al., 1970). It is therefore of interest to investigate whether such changes are correlated at conjugate locations. To this end, each daily electron content curve is averaged over a 4-5 hr interval centered about the maximum of the monthly curve. This is done to remove temporal fluctuations created by, for example, traveling disturbances. Then day-to-day variations of this averaged content are computed for both Cold Bay data and Invercargill data and the correlation coefficients are calculated between these two sets of data under different geomagnetic conditions. When the conditions are geomagnetically disturbed, the scatter plot of day-to-day variations at Cold Bay vs. those at Invercargill is shown in Fig. 1. As seen from Fig. 1, these variations are well correlated with a correlation coefficient O-73 and the 95 per cent confidence interval 0-53-0-85. Similar scatter plots have also been made for different ranges of magnetic activities and the corresponding correlation coefficients and confidence intervals have been computed. These plots can be found in the original report (Yeh et al., 1970) and will not be presented here, but the results are summarized in Fig. 2. Figure 2 shows that the day-to-day electron content variations are negatively correlated during extreme magnetic quiet (i.e. daily sum of k, index less than 9). This negative correlation suggests that an increase in electron content is correlated with a decrease in electron content at the conjugate point. Unfortunately due to lack of these extreme quiet days (only 10 days’ data are used here) the confidence interval is rather large. With increased magnetic activities, the correlation coefficient as shown in Fig. 2 also increases; it passes through zero to some positive value. On magnetically disturbed days, the variation is highly correlated at conjugate stations. This suggests that the storm behavior studied by Papagiannis et al. (197 1) has an approximate symmetry with respect to the two hemispheres. A rough indication of this hemispheric symmetry has been found for the February 11,1969 storm which is discussed in the next section. 4. CONTEST
BEHAVIOR DURING A MAGNETIC STORM
A magnetic storm occurred in February 10, 1969 with a sudden commencement at 2024 UT. It ended around 3 UT, February 12. Before the storm the Cold Bay content
ELECTRON
CONTENT
AT CONJUGATE
STATIONS
2047
AN, at Cold Bayin 10’7electro”slm* 0
2
i
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I
0
I
I 3
ot Invercargitt
* in
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-2.8m
4
i
FIG.~. C~RREUTI~NOPDAY-TO-DAYVARIATIONSINELE~~ONCONTENTAT COLDBAY, ALASKA THOSE AT INVERCARGILL,NEW ZEALAND DURING MAGNETICALLY DISTURBEDDAYS. THE aORRELATlON COEFFICENTIS 0,733 AND THE 95 PER CENT CONFIDEN~JEINTERVAL IS o-53-0*85.
wrm
r
1.0
0.8 -
0.6 -
0.4 Q 5 g
0.2-
k g u
0.0
c>
I
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3 14 Ranges ofMagnetic
c Activity
z G _I -0.2 ii
-
1
-0.4 -
-0.6 -
-0.8 FIG.~. CORRELA~ONOFDAY-TO-DAYVARIATIONSINELECTRONCONTENTATCOLD BAY, ALASKA WITHTHOSEATINVERCARGILL,NEWZEALANDASAFUNCTIONOFMAGNETICA~ITY. RANGES ~RRESPoNDSTODAYSwHEN&,(SUM)< 9; RANGE 2,9 Q &(SUM) < 17; RANGE 3,&(SuM)> 17; RANGE 4, MAGNEnCALLY DISTURBED.
K. C. YEH
2048
Ro. 3. THE ELECTRONCONTENTBEHAVIORDURING THEMAGNETICSTORMOF FEBRUARY10-l 1, 1969. (a)COLD BAY, ALASKA AND INVERCARGILL,NEW ZEALAND. (b) URBANA, ILLINOIS.
had the typical winter season behavior and the Invercargill content had the typical summer season behavior as shown in Fig. 3(a). The sudden commencement started in the local morning for both stations as indicated by an arrow. Following the storm both stations showed the dramatic afternoon increase of the kind observed by Papagiannis et al. (1971) and Klobuchar et al. (1971). The electron content on the day following the storm was depressed both at Cold Bay and Invercargill. On the third day, the daytime content recovered to normal values at both stations. For comparison, the content behavior at Urbana (40.069”N, 88.225”W) for the same storm was also plotted as Fig. 3(b). The sudden commencement occurred in the local afternoon. The increase near sunset so apparent on Cold Bay and Invercargill data was nonexistent on Urbana data. But following the sunset, the electron content did not decay as rapidly to normal night-time value. On the second day the Urbana content was depressed, even though it was not as much as that at Cold Bay or at Invercargill. The electron content curve was back to normal on the third day. 5. IXsctrssIoN The behavior of the ionosphere is controlled by many processes. When geomagne~c~ly disturbed, its behavior generally becomes even more complex. Suggested processes responsible for the disturbed behavior are E x B drift (Martyn, 1953; Maeda and Sato, 1959; Papagiannis et al., 1971); atmospheric and ionospheric heating (Yonezawa, 1963); compression and inflation of the magnetosphere (Bauer and Krishnamurty, 1968); composition change in the neutral atmosphere (Chandra and Herman, 1969) and changes in meridional winds (Jones and Rishbeth, 1971). Numerical computations have shown that of the two processes the neutral wind effect is more important than the electric field effect in an undisturbed ionosphere while the converse is true when it is geomagnetically disturbed (Riister, 1969, 1971).
ELECTRON
CONTENT
AT CONJUGATE
STATIONS
2049
This study has shown that during days of extreme magnetic quiet, the day-to-day variations in electron content at two geomagnetic conjugate stations are correlated negatively. The correlation coefficient then increases to a positive value for geomagnetically disturbed days. One of the outstanding features connected with magnetic storms is the occurrence of the pre-sunset sharp rise in electron content studied by Papagiannis et al. (1971). Such rises have been observed at both Cold Bay and Invercargill for the February 10-11, 1969 storm. As is usually the case, the rise is followed by a very steep fall before the local sunset. Such falls can be seen in Fig. 3(b), although the rnverc~~~ fall is not as dramatic as Cold Bay fall. The steepness in the fall may serve as an excellent time reference to discern whether the phenomenon is solar controlled or geomagneticaily controlled. The subionospheric points of Cold Bay and Invercargill differ in longitudes by 34” which corresponds to a difference of 2 hr 16 min in local time. By inspection of Fig. 3b, the presunset steep fall following the sudden commencement is not brought to better agreement if the Invercargill curve is advanced by 2 hr 16 min relative to the Cold Bay curve. Now as mentioned in Section 2 the computed conjugate of Invercargill subionospheric point is to the west of Cold Bay subionospheric point by approximately 7’ which corresponds to 30 min. If the Invercargill curve of Fig. 3(b) is advanced by 30 min relative to the Cold Bay curve, the steep falls at both stations become more nearly coincident. Therefore, it suggests that the pre-sunset enhancement on the first day following the sudden commencement occurs nearly simul~neously at geomagnetic conjugate points and is controlled geomagnetically. Because the megnetic fields are skewed, this enhancement and the subsequent fall occurs earlier at Invercargill (approximately 14:30 LT) than at Cold Bay (approximately 16:30 LT) by about 2 hr in local time. Papagiannis et al. (1971) have suggested that during magnetic storms the rotational electric field is partially cancelled by the electric field related to magnetospheric convection motions at the dusk sector. As a result the ionization corotation with the Earth is hindered, causing a pre-sunset rise in electron content. Our observation of geomagnetic control is consistent with their model. However, their model also predicted that the pre-sunset rise is more pronounced at higher L values than at lower L values, contrary to our experimental results shown in Fig. 3 where Urbana (L = 2.7) values show little effect while both Cold Bay and Invergargill (L = 2-3) values show very large effect. This discrepancy may be reconciled if the electric fieId related to convection motions was not fully developed at the time of Urbana sunset as was the case at the time of Cold Bay sunset. Acknowledgements-The Squadron Commanderand his personnel of the U.S. Air Force Base at Cold Bay, Alaska assisted in setting up and maintaining the Cold Bay station. The Cold Bay data were collected and reduced under the supervision of B. J. Flaherty. Dr. J. E. Titheridge, the University of Auckland, New Zealand supplied the Invercargill data for this conjugate study. I-I. Nomani did some prorating and and his comments data processing. I would like to thank Mr. John A. Klobuchar for his en~u~gement on the t&t draft. The research was sponsored by the Air Force Cambridge Research Laboratories, Air Force Systems Command, under Contract F19628-70-C-001, but the work does not necessarily reflect endorsement by the sponsor. The Urbana data were collected under the NASA Grant NGR 14-005-002. REFERFlNCES B. V. (1968). Behavior of the topside ionosphere during a great magnetic storm. P&et. Space Sci. 16,653-663. BEN'KOVA, N. P., BUK~N,G. V., DAVOUST, K., KERBLAY, J. S. and TAIEB,C. (1968). Synoptic Observations of thef,F2 layer of the ionosphere in the magnetically conjugate regions Kerguelen-Arkhangel’sk Province. Geomugn. Aeron. (English Translation) 8, 342-345. See also (1969) Les observations synoptiques de la couche F2 days les regions magnetiquement conjuguees de Kerguelen et Arkangelsk. Annls Gdophys. 25,67-71. BALER, S. J. and KRXS~A~THY,
2050
K. C. YEH
CA~I.~N, H. C., JR. (1968). Most recent studies of low latitude effects due to conjugate location heating. Rodi Sci. 3, 668. CHANDRA, S. and HERMAN,J. R. (1969). F-region ionization and heating during magnetic storms. Planer. Space Sci. 17, 841. CUMMACK,C. H. (1967). The wnjugacy of F-region perturbations. J. atnws. terr. Phys. 29,811-818. EVANS, J. V. (1968). Sunrise behavior of the F-layer at midlatitudes. J. geophys. Res. 73,3489. GARRIOIT,0. K.. DA ROSA. A. V. and Ross, W. J. (1970). Electron content obtained from Faraday rotation and phase path length variations. J. atmos. terr. Phys. 32,705-727. HOOPER,A. W. (1969). The correlation of the critical frequencyf,F2 between a pair of mid-latitude magnetically conjugate points. J. atmosph. terr. Phys. 31, 1373. JONES,K. L. and RLSHBETH,H. (1971). The origin of storm increases of mid-latitude F-layer electron concentration. J. atmos. terr. Phys. 33,391401. KLOBUCMR, J. A. and WHITNEY, H. E. (1966). Middle-latitude ionospheric total electron content: Summer 1965. Radio Sci. 1,1149-1154. KUBUCHAR, J. A., MENDILLO,M., SMITH,F. L., III, FRITZ, R. B., DA ROSA,A. V., DAVIS, M. J.. YUEN, P. C. ROEL~F~,T. H., YEH, K. C. and FLAHERTY,B. J. (1971). Ionospheric storm of March 8,197O. J.geophys Res. 76,6202-6207. Kwer, M. W. and NISBET,J. S. (1968). Presunrise heating of the ionosphere at Arecibo due to conjugate uoint nhotoelectrons. Radio Sci. 3, 674-679. MXEDA.~K. I. and SATO, K. (1959). The F region during magnetic storms. Proc. IRE 47,232-239. MARTYN, D. F. (1953). Electric currents in the ionosphere III Ionization drift due to winds and electric fields. Phil. Trans. R. Sot. A246, 306-20. MATSUSHITA,S. (1968). Ionospheric F2 behavior at conjugate places in low latitudes. Radio Sci. 3, 658. PAPAGIANNIS,M. D., MENDILU), M. and KL~BUCHAR,J. A. (1971). Simultaneous storm-time increase of the ionospheric total electron content and the geomagnetic field in the dusk sector. Planet. Space Sci. 19,503-l 1. RAO, N. N., Ym, K. C. and YOUAKIM,M. Y. (1970). Ionospheric electron content at temperature latitudes during the increasing phase of the solar cycle. Aust. J. Phys. 23,37-43. Rtisrsa, R. (1969). Theoretical treatment of the dynamical behavior of the F-region during geomagnetic bay disturbances. J. atmos. terr. Phys. 31,765-80. ROsrrn, R. (1971). The relative effects of electric fields and atmospheric composition changes on the electron concentration in the mid-latitude F-layer, J. atrnos. terr. Phys. 33,275-280. Special Issue on conjugate point symposium (1968) Radio Sci. 3 (New Series), 7, 645-773. TITHERIDGE, J. E. (1966). Continuous records of the total electron content of the ionosphere. J. atmos. terr. Phys. 28, 1130-l 150. Wescorr, E. M. (1966). Magnetoconjugate phenomena. Space Sci. Rev. 5,507. YEH, K. C., SIMONICH,D., MANDSLEY,J. and PRFDDEY,G. F. (1968). Scintillation observations at medium latitude geomagnetically conjugate stations. Radio Sci. 3, 690. YEH, K. C., FLAHERTY,B. J., 0x0, H. R. and NOMANI,H. (1970). Investigations of ionospheric total electron wntent behavior at conjugate points and during a solar eclipse, Tech. Report No. 41, Ionosphere Radio Laboratory, Department of Electrical Engineering, University of Illinois. YONEZAWA, T. (1963). The characteristic behavior of the F2 layer during severe magnetic storms. Proc. Intern. Conf. Zonosphere, pp. 128-133. The Institute of Physics and the Physical Society, London.
Sci. 1972,
Vol. 20, pp. 2045 to 2050.
Pergamon Press. Printed in Northern Ireland
OBSERVATIONS OF IONOSPHERIC ELECTRON CONTENT AT MEDIUM LATITUDE GEOMAGNETICALLY CONJUGATE STATIONS Ionosphere
K. C. YEH Radio Laboratory, Department of Electrical Engineering, University at Urbana-Champaign Urbana, Illinois 61801, U.S.A.
of Illinois
(Received in final form 17 May 1972) Abstract-Electron content data recorded at Cold Bay, Alaska and Invercargill, New Zealand have been used to study the geomagnetic conjugate effects. The correlation on day-to-day variations at these two stations is found to be negative at extreme magnetic quiet and it then increases to positive values with magnetic activities. When disturbed magnetically, the correlation is +0.73. Evidence is presented to show that, on the first day following the sudden commencement, the afternoon rise and the subsequent pre-sunset fall in electron content is controlled geomagnetically. 1. INTRODUCTION Many geophysical phenomena in the upper atmosphere are known to have geomagnetic conjugate effects. The earliest example of this, recognized in the last century, is the tendency of great auroras to occur simultaneously in the northern and southern aurora1 zones. However, only after the IGY years were more concentrated observations made. Since then conjugate effects have been reported to occur on events related to aurora1 radio absorption, precipitation of electrons in the aurora1 zone, polar magnetic substorms, VLF emissions, occurrence of whistlers and magnetic micropulsations (For a review, see Wescott, 1966 and Special Issue on Conjugate Point Symposium, 1968). At temperate latitudes and in the ionosphere, conjugate work has been carried out on scintillation of radio signals (Yeh ef al., 1968) and conjugate heating (Carlson, 1968; Kwei and Nisbet, 1968; Evans, 1968). Several studies on the behavior off82 at conjugate points have also been made. At low latitudes, Matsushita (1968) and Cummack (1967) have found high positive correlation of variations of f,F2 from the monthly mean. Similar studies at temperature latitudes have uncovered very weak correlations between Campbell Island and a chain of three Alaskan stations (Hooper, 1969). However, Hooper did not separate his data according to geomagnetic activities. As it turns out, the dependence on geomagnetic activities is very important since Ben’kova et al. (1968) have found in-phase variations during geomagnetic disturbances and anti-phase variations during weak activities between Kerguelen fz2 values and those recorded along a chain of stations at Arkhangel’sk Province. 2. NATURE
OF THE EXPERIMENT
In this study we wish to report electron content observations made at Cold Bay, Alaska (55.2”N, 162.75”W) and Invercargill, New Zealand (46.42%, 168.32”E). The electron content values were obtained by monitoring the rotation of wave polarization produced through the Faraday effect on VHF signals (136 MHz) transmitted by the geostationary satellite (Titheridge, 1966; Klobuchar and Whitney, 1966; see also the review by Garriott ef al., 1970). At Invercargill, recording was made on signals transmitted by Symcom III from December 1968 through March 1969 and by ATS I from April 1969 onward. The corresponding subionosphere point was approximately 42”S, 164”E for Symcom III and 42”S, 172’E for ATS I. At Cold Bay, ATS I signals were used for the entire period and its subionospheric point was located at 49”N, 162”W approximately. Computations of 300 km-to-300 km geomagnetic conjugacy indicate that the Invercargill subionospheric point 2045
K. C. YEH
2046
(42”S, 164”E) is mapped along the geomagnetic field into a point at 53”N, 169”W in the northern hemisphere. This point is 4” higher in latitude and 7” larger in west longitude than the Cold Bay subionospheric point. The difference is not expected to be important for most conjugate studies, except perhaps during magnetic storms as discussed in Sections 4 and 5. Except for a few interruptions due to technical difficulties, nearly continuous data have been collected and used in this study for the period December 25, 1968 through September 9, 1969. The average electron content at these two stations shows the seasonal behavior distinct for its own hemisphere and does not appear to be correlated in any way. This is not unexpected since f$2 studies of, for example, Matsushita (1968) show poor correlation on the monthly median values even for low latitude conjugate stations. Our attention is therefore turned to conjugate effects on the variations from the average. 3. DAY-TO-DAY CHANGE IN CONTENT
Electron content is known to have large day-to-day fluctuations. A change of 20 per cent is fairly common and on occasion it may be as large as 50 per cent or more. In general, the daytime day-to-day changes are larger than the corresponding night-time changes. At temperate latitudes these daytime changes are correlated geographically in a region which is 800 km in the north-south direction and 2000 km in the east-west direction, but not over a distance 9000 km in the east-west direction (Rao et al., 1970). It is therefore of interest to investigate whether such changes are correlated at conjugate locations. To this end, each daily electron content curve is averaged over a 4-5 hr interval centered about the maximum of the monthly curve. This is done to remove temporal fluctuations created by, for example, traveling disturbances. Then day-to-day variations of this averaged content are computed for both Cold Bay data and Invercargill data and the correlation coefficients are calculated between these two sets of data under different geomagnetic conditions. When the conditions are geomagnetically disturbed, the scatter plot of day-to-day variations at Cold Bay vs. those at Invercargill is shown in Fig. 1. As seen from Fig. 1, these variations are well correlated with a correlation coefficient O-73 and the 95 per cent confidence interval 0-53-0-85. Similar scatter plots have also been made for different ranges of magnetic activities and the corresponding correlation coefficients and confidence intervals have been computed. These plots can be found in the original report (Yeh et al., 1970) and will not be presented here, but the results are summarized in Fig. 2. Figure 2 shows that the day-to-day electron content variations are negatively correlated during extreme magnetic quiet (i.e. daily sum of k, index less than 9). This negative correlation suggests that an increase in electron content is correlated with a decrease in electron content at the conjugate point. Unfortunately due to lack of these extreme quiet days (only 10 days’ data are used here) the confidence interval is rather large. With increased magnetic activities, the correlation coefficient as shown in Fig. 2 also increases; it passes through zero to some positive value. On magnetically disturbed days, the variation is highly correlated at conjugate stations. This suggests that the storm behavior studied by Papagiannis et al. (197 1) has an approximate symmetry with respect to the two hemispheres. A rough indication of this hemispheric symmetry has been found for the February 11,1969 storm which is discussed in the next section. 4. CONTEST
BEHAVIOR DURING A MAGNETIC STORM
A magnetic storm occurred in February 10, 1969 with a sudden commencement at 2024 UT. It ended around 3 UT, February 12. Before the storm the Cold Bay content
ELECTRON
CONTENT
AT CONJUGATE
STATIONS
2047
AN, at Cold Bayin 10’7electro”slm* 0
2
i
0 0
t I -
a
l
a
l
a )
l
I
I
-3
I -2
I
l
I -1..
l
l.O .O c
l
O-3.1
0
.‘*a,_
I I
I 2 AN,
I
0
I
I 3
ot Invercargitt
* in
10’7electrons/m2
-2.8m
4
i
FIG.~. C~RREUTI~NOPDAY-TO-DAYVARIATIONSINELE~~ONCONTENTAT COLDBAY, ALASKA THOSE AT INVERCARGILL,NEW ZEALAND DURING MAGNETICALLY DISTURBEDDAYS. THE aORRELATlON COEFFICENTIS 0,733 AND THE 95 PER CENT CONFIDEN~JEINTERVAL IS o-53-0*85.
wrm
r
1.0
0.8 -
0.6 -
0.4 Q 5 g
0.2-
k g u
0.0
c>
I
C) 2
3 14 Ranges ofMagnetic
c Activity
z G _I -0.2 ii
-
1
-0.4 -
-0.6 -
-0.8 FIG.~. CORRELA~ONOFDAY-TO-DAYVARIATIONSINELECTRONCONTENTATCOLD BAY, ALASKA WITHTHOSEATINVERCARGILL,NEWZEALANDASAFUNCTIONOFMAGNETICA~ITY. RANGES ~RRESPoNDSTODAYSwHEN&,(SUM)< 9; RANGE 2,9 Q &(SUM) < 17; RANGE 3,&(SuM)> 17; RANGE 4, MAGNEnCALLY DISTURBED.
K. C. YEH
2048
Ro. 3. THE ELECTRONCONTENTBEHAVIORDURING THEMAGNETICSTORMOF FEBRUARY10-l 1, 1969. (a)COLD BAY, ALASKA AND INVERCARGILL,NEW ZEALAND. (b) URBANA, ILLINOIS.
had the typical winter season behavior and the Invercargill content had the typical summer season behavior as shown in Fig. 3(a). The sudden commencement started in the local morning for both stations as indicated by an arrow. Following the storm both stations showed the dramatic afternoon increase of the kind observed by Papagiannis et al. (1971) and Klobuchar et al. (1971). The electron content on the day following the storm was depressed both at Cold Bay and Invercargill. On the third day, the daytime content recovered to normal values at both stations. For comparison, the content behavior at Urbana (40.069”N, 88.225”W) for the same storm was also plotted as Fig. 3(b). The sudden commencement occurred in the local afternoon. The increase near sunset so apparent on Cold Bay and Invercargill data was nonexistent on Urbana data. But following the sunset, the electron content did not decay as rapidly to normal night-time value. On the second day the Urbana content was depressed, even though it was not as much as that at Cold Bay or at Invercargill. The electron content curve was back to normal on the third day. 5. IXsctrssIoN The behavior of the ionosphere is controlled by many processes. When geomagne~c~ly disturbed, its behavior generally becomes even more complex. Suggested processes responsible for the disturbed behavior are E x B drift (Martyn, 1953; Maeda and Sato, 1959; Papagiannis et al., 1971); atmospheric and ionospheric heating (Yonezawa, 1963); compression and inflation of the magnetosphere (Bauer and Krishnamurty, 1968); composition change in the neutral atmosphere (Chandra and Herman, 1969) and changes in meridional winds (Jones and Rishbeth, 1971). Numerical computations have shown that of the two processes the neutral wind effect is more important than the electric field effect in an undisturbed ionosphere while the converse is true when it is geomagnetically disturbed (Riister, 1969, 1971).
ELECTRON
CONTENT
AT CONJUGATE
STATIONS
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This study has shown that during days of extreme magnetic quiet, the day-to-day variations in electron content at two geomagnetic conjugate stations are correlated negatively. The correlation coefficient then increases to a positive value for geomagnetically disturbed days. One of the outstanding features connected with magnetic storms is the occurrence of the pre-sunset sharp rise in electron content studied by Papagiannis et al. (1971). Such rises have been observed at both Cold Bay and Invercargill for the February 10-11, 1969 storm. As is usually the case, the rise is followed by a very steep fall before the local sunset. Such falls can be seen in Fig. 3(b), although the rnverc~~~ fall is not as dramatic as Cold Bay fall. The steepness in the fall may serve as an excellent time reference to discern whether the phenomenon is solar controlled or geomagneticaily controlled. The subionospheric points of Cold Bay and Invercargill differ in longitudes by 34” which corresponds to a difference of 2 hr 16 min in local time. By inspection of Fig. 3b, the presunset steep fall following the sudden commencement is not brought to better agreement if the Invercargill curve is advanced by 2 hr 16 min relative to the Cold Bay curve. Now as mentioned in Section 2 the computed conjugate of Invercargill subionospheric point is to the west of Cold Bay subionospheric point by approximately 7’ which corresponds to 30 min. If the Invercargill curve of Fig. 3(b) is advanced by 30 min relative to the Cold Bay curve, the steep falls at both stations become more nearly coincident. Therefore, it suggests that the pre-sunset enhancement on the first day following the sudden commencement occurs nearly simul~neously at geomagnetic conjugate points and is controlled geomagnetically. Because the megnetic fields are skewed, this enhancement and the subsequent fall occurs earlier at Invercargill (approximately 14:30 LT) than at Cold Bay (approximately 16:30 LT) by about 2 hr in local time. Papagiannis et al. (1971) have suggested that during magnetic storms the rotational electric field is partially cancelled by the electric field related to magnetospheric convection motions at the dusk sector. As a result the ionization corotation with the Earth is hindered, causing a pre-sunset rise in electron content. Our observation of geomagnetic control is consistent with their model. However, their model also predicted that the pre-sunset rise is more pronounced at higher L values than at lower L values, contrary to our experimental results shown in Fig. 3 where Urbana (L = 2.7) values show little effect while both Cold Bay and Invergargill (L = 2-3) values show very large effect. This discrepancy may be reconciled if the electric fieId related to convection motions was not fully developed at the time of Urbana sunset as was the case at the time of Cold Bay sunset. Acknowledgements-The Squadron Commanderand his personnel of the U.S. Air Force Base at Cold Bay, Alaska assisted in setting up and maintaining the Cold Bay station. The Cold Bay data were collected and reduced under the supervision of B. J. Flaherty. Dr. J. E. Titheridge, the University of Auckland, New Zealand supplied the Invercargill data for this conjugate study. I-I. Nomani did some prorating and and his comments data processing. I would like to thank Mr. John A. Klobuchar for his en~u~gement on the t&t draft. The research was sponsored by the Air Force Cambridge Research Laboratories, Air Force Systems Command, under Contract F19628-70-C-001, but the work does not necessarily reflect endorsement by the sponsor. The Urbana data were collected under the NASA Grant NGR 14-005-002. REFERFlNCES B. V. (1968). Behavior of the topside ionosphere during a great magnetic storm. P&et. Space Sci. 16,653-663. BEN'KOVA, N. P., BUK~N,G. V., DAVOUST, K., KERBLAY, J. S. and TAIEB,C. (1968). Synoptic Observations of thef,F2 layer of the ionosphere in the magnetically conjugate regions Kerguelen-Arkhangel’sk Province. Geomugn. Aeron. (English Translation) 8, 342-345. See also (1969) Les observations synoptiques de la couche F2 days les regions magnetiquement conjuguees de Kerguelen et Arkangelsk. Annls Gdophys. 25,67-71. BALER, S. J. and KRXS~A~THY,
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CA~I.~N, H. C., JR. (1968). Most recent studies of low latitude effects due to conjugate location heating. Rodi Sci. 3, 668. CHANDRA, S. and HERMAN,J. R. (1969). F-region ionization and heating during magnetic storms. Planer. Space Sci. 17, 841. CUMMACK,C. H. (1967). The wnjugacy of F-region perturbations. J. atnws. terr. Phys. 29,811-818. EVANS, J. V. (1968). Sunrise behavior of the F-layer at midlatitudes. J. geophys. Res. 73,3489. GARRIOIT,0. K.. DA ROSA. A. V. and Ross, W. J. (1970). Electron content obtained from Faraday rotation and phase path length variations. J. atmos. terr. Phys. 32,705-727. HOOPER,A. W. (1969). The correlation of the critical frequencyf,F2 between a pair of mid-latitude magnetically conjugate points. J. atmosph. terr. Phys. 31, 1373. JONES,K. L. and RLSHBETH,H. (1971). The origin of storm increases of mid-latitude F-layer electron concentration. J. atmos. terr. Phys. 33,391401. KLOBUCMR, J. A. and WHITNEY, H. E. (1966). Middle-latitude ionospheric total electron content: Summer 1965. Radio Sci. 1,1149-1154. KUBUCHAR, J. A., MENDILLO,M., SMITH,F. L., III, FRITZ, R. B., DA ROSA,A. V., DAVIS, M. J.. YUEN, P. C. ROEL~F~,T. H., YEH, K. C. and FLAHERTY,B. J. (1971). Ionospheric storm of March 8,197O. J.geophys Res. 76,6202-6207. Kwer, M. W. and NISBET,J. S. (1968). Presunrise heating of the ionosphere at Arecibo due to conjugate uoint nhotoelectrons. Radio Sci. 3, 674-679. MXEDA.~K. I. and SATO, K. (1959). The F region during magnetic storms. Proc. IRE 47,232-239. MARTYN, D. F. (1953). Electric currents in the ionosphere III Ionization drift due to winds and electric fields. Phil. Trans. R. Sot. A246, 306-20. MATSUSHITA,S. (1968). Ionospheric F2 behavior at conjugate places in low latitudes. Radio Sci. 3, 658. PAPAGIANNIS,M. D., MENDILU), M. and KL~BUCHAR,J. A. (1971). Simultaneous storm-time increase of the ionospheric total electron content and the geomagnetic field in the dusk sector. Planet. Space Sci. 19,503-l 1. RAO, N. N., Ym, K. C. and YOUAKIM,M. Y. (1970). Ionospheric electron content at temperature latitudes during the increasing phase of the solar cycle. Aust. J. Phys. 23,37-43. Rtisrsa, R. (1969). Theoretical treatment of the dynamical behavior of the F-region during geomagnetic bay disturbances. J. atmos. terr. Phys. 31,765-80. ROsrrn, R. (1971). The relative effects of electric fields and atmospheric composition changes on the electron concentration in the mid-latitude F-layer, J. atrnos. terr. Phys. 33,275-280. Special Issue on conjugate point symposium (1968) Radio Sci. 3 (New Series), 7, 645-773. TITHERIDGE, J. E. (1966). Continuous records of the total electron content of the ionosphere. J. atmos. terr. Phys. 28, 1130-l 150. Wescorr, E. M. (1966). Magnetoconjugate phenomena. Space Sci. Rev. 5,507. YEH, K. C., SIMONICH,D., MANDSLEY,J. and PRFDDEY,G. F. (1968). Scintillation observations at medium latitude geomagnetically conjugate stations. Radio Sci. 3, 690. YEH, K. C., FLAHERTY,B. J., 0x0, H. R. and NOMANI,H. (1970). Investigations of ionospheric total electron wntent behavior at conjugate points and during a solar eclipse, Tech. Report No. 41, Ionosphere Radio Laboratory, Department of Electrical Engineering, University of Illinois. YONEZAWA, T. (1963). The characteristic behavior of the F2 layer during severe magnetic storms. Proc. Intern. Conf. Zonosphere, pp. 128-133. The Institute of Physics and the Physical Society, London.