Anomalous DS-variation in equatorial latitudes during geomagnetic storms

Planet. Space Sci. 1974, Vol. 22, pp. 991 to I001, Pergamon Press. Printed in Northern Ireland

ANOMALOUS

DS-VARIATION IN EQUATORIAL DURING GEOMAGNETIC STORMS

LATITUDES

A. GRAFE

Zentralinstitut fiir solar-terrestrische Physik der Akademie der Wissenschaften der DDR, DDR--1199 Berlin-Adlershof, Rudower Chaussee 5 (Received 8 November 1973)

Abstract--For several storms the asymmetry of the magnetic disturbance values at equatorial latitudes has been investigated. Asymmetrieswere found for which the maximum and minimum depressions of the horizontal intensity occur at midnight and at noon, respectively, (anomalous DS-variation); other asymmetries, where the maximum and minimum depressions were observed in the morning and in the evening, respectively (inverse DS-variation). The DS-variation at equatorial latitudes was discussed in connection with the polar magnetic substorm and the Dst-variation. INTRODUCTION It is generally accepted that during geomagnetic storms the equatorial ring-current field has an asymmetric distribution depending on the local time. In this case the strongest ring-current effects in the horizontal component occur in the evening and the weakest early in the morning (Akasofu and Chapman, 1964; Cummings, 1966). This asymmetry is said to be caused by the inflow of protons having an energy of 10 keV into the evening sector (Cahill 1966; Frank 1970). According to Frank (1970) the protons flow within the energy range from 31 to 49 keV in the evening sector have an intensity ten times higher than that occurring in the noon sector. On the other hand the ring current was always supposed to be symmetrical during periods without geomagnetic disturbances. However, there are also indications that the ring current shows an asymmetric structure even on undisturbed days. According to Bhargava and Yacob (1971), a maximum of the ring current occurs on undisturbed days by 1900 LT and a minimum by 0800 LT. Akasofu and Chapman (1964) derived the asymmetry of the ring current from the fact that DS-variations at low latitudes occur also in cases where substorm variations at auroral latitudes are absent. This is probably true. However, the question arises, which effects cause the DS-variations at low latitudes in the presence of substorms. In principle the following sources are possible. 1. The ionospheric return currents of the polar electrojets. 2. The asymmetric ring current. 3. The current flowing in the neutral sheet of the magnetospheric tail. 4. The magnetopause current caused by the compressions of the magnetosphere. To separate these contributory factors is very difficult even with the use of satellite data. Crooker and Siscoe (t971) and Crooker (1972) speak of a two-component model of the asymmetry of the disturbance field. The ionospheric return current of the electrojets is one of the components, the magnetospheric currents being the other one. We believe (Grafe, 1972) that the asymmetry of the ring current in effect is reflected by the trend of the DS-variation at low latitudes and that the short-periodic portions of the DS-variation result from the ionospheric return currents of the polar electrojets. The ATS-1 measurements, as they were described by Coleman and Cummings (1971), have clarified the situation insofar as it was shown that during the main phase the magnetopause current is effective in addition to the tail current and that only in the recovery phase does the ring current 991

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exist alone. However, until now it has been far from clear how these individual disturbance fields influence the development at the Earth's surface. This paper is intended to show that at tow latitudes DS-variations can occur winch differ from the known standard shape of the DS-variation of the horizontal component occurring at low latitudes during geomagnetic storms and showing a minimum in the evening and a maximum in the morning. Here these former variations shall be called anomalous DS-variations. EVENT INVESTIGATIONS

For these investigations the reduced magnetograms of the horizontal intensity recorded during the period from January 1967 to December 1971, were available. These had been derived and drawn (and published in part) in Report UAG of the World Data Center A (see for instance Figs. 2(a) and (b) published by Akasofu and Kawasaki, 1970). In these magnetograms, So is eliminated. Table 1 shows the stations at low latitude and at the latitude of the northern auroral zone from where these magnetograms usually were available. The distribution of these stations showed minor variations from one case to another. However, as a rule magnetograms from seven stations of the northern auroral zone and five stations of low latitudes were available for each event, so that a sufficient distribution of stations along a geomagnetic circle of latitude was ensured. Instantaneous disturbance values differing from the undisturbed level were recorded, and the disturbance values obtained at low latitudes were converted into the geomagnetic equator. These disturbance values were represented for the equatorial latitude as a function of the geographic local time and, in the case of the disturbance values of the northern auroral zone, as a function of the magnetic local time. Figure 1 shows four examples of the normal DS-variation at equatorial latitudes. The largest depression of the horizontal component occurs at about 1800 LT, which is in agreement with the findings of Crooker and Siscoe (1971), and the smallest depression between 0600 and 0800 LT, as was found also for undisturbed days (Bhargava and Yakob, 1971). Figure 2 shows several examples of anomalous DS-variations at equatorial latitudes. Here the essential fact is that the minimum depression has been shifted towards noon and that the maximum depression occurs at local midnight. TABLE 1

Observatory Honolulu Kakioka Tangerang Taschkent M'Bour San Juan College Cape Wellen Tixie Bay Dixon Island Cheljuskin Sodankylii Abisko Leirvogur Great Whale River Meanook

Geomagnetic lat (deg) 21.1 26.0 17.6 32.4 21.3 29.9 64.6 6 I. 8 60.5 63-0 65.9 63-8 66.0 70.2 66.8 61.8

Geomagnetic long (deg) 266.5 206-1 175.4 143.7 55.0 3.2 256-5 237.0 191.0 161-4 177-5 120.0 115.0 71-0 347.2 301-0

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EXAMPLES OF ANOMALY DS-VARIATION NEAR THE GEOMAGNETIC EQUATOR DURING THE STORM MAIN PHASE.

In what follows, the DS-variations occurring at equatorial latitudes are investigated in connection with the variations at the latitudes of the northern auroral zone. A m o n g the great number of the storms evaluated, five cases are discussed here, the Dst-variations of which are shown by Fig. 3. a. 14 December 1970, 0600-1500 UT For this period Fig. 4 shows the variations of the disturbance component at equatorial latitude and at the latitude of the northern auroral zone as functions of the local time for intervals of half an hour. At about 0600 U T the main phase begins. The m a x i m u m substorm activity is reached with a range of 1300 7 at 0730 UT. However, this maximum does not coincide at all with that of the asymmetry found at the Equator. The maximum asymmetry at the Equator is reached only at 0800 U T when the DS-range in the northern auroral

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zone was only just 1000 y. This is the same phenomenon as described already by Kawasaki and Akasofu (1971) and Crooker (1972), i.e. that the maximum of asymmetry at the Equator occurs only after that of the substorm activity. Moreover, it can be observed that a normal DS-variation occurs at the Equator if substorms are present. At 0900 UT the substorm activity has visibly died out. The maximum of the main phase is reached, and the asymmetry at the Equator is surprisingly low. Consequently it appears that in cases of low substorm activity also an asymmetry at the Equator does not always exist, as was found by Akasofu and Chapman (1964). After 1000 UT the asymmetry at the equator again increases and a considerable shift of the minimum depression towards local noon appears. Furthermore a clear formation of anomalous DS-variation takes place. In this case the occurrence of an anomalous DS-variation can be correlated with the beginning of the recovery phase. b. 9 November 1969, 1000-1800 UT

In contrast to the above case, here a storm is involved, which distinguishes itself by a Ds~-field of a relatively low intensity. Nevertheless, the asymmetry at equatorial latitude is rather large as is shown by Fig. 5, 1200 UT. It is even larger than that which occurred during the main phase on 8 January 1967, which can be observed in Fig. 8. Also in this case it can be seen very clearly that the asymmetry at equatorial latitudes increases only after the substorm has already died out. At 1000 UT and 1430 UT, when heavy substorms occurred at College, the asymmetry was quite low. With the previously described storm we had an example of low asymmetry at equatorial latitudes for a low substorm activity and, in the present case, we have examples of low asymmetry for a high substorm activity. In this case the end of the main phase and the beginning of the recovery phase cannot readily be determined; however, by 1300 UT at the latest, we must expect the occurrence of an anomalous DS-variation. A well-defined noon maximum and a distinct midnight minimum appear in the curves shown. From 1700 UT, when the Ds~-variation has become negligible, an inverse DS-variation, which has a curve minimum in the morning and a maximum in the afternoon, is obtained. At this time substorm disturbances are absent. c. 14 January 1967, 0000-0400 UT This period is shown by Fig. 6. In this case we had to deal with a more regular Dstvariation. The period from 0000--0400 UT falls within the main phase of the storm. In

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this case large asymmetries occurred at equatorial latitudes, although the substorm activity is high (see 0130 UT). Indeed, the curve minimum for the DS-variation at equatorial latitudes cannot be clearly defined, but with decreasing substorm activity a shift of the curve maximum towards noon can be observed. By 0300 UT at the latest we have an anomalous DS-variation, the curve minimum likewise occurring near local midnight. The phenomenon is similar to that described by Crooker and Siscoe (1971), according to which with decreasing substorm activity the local time of maximum depression in the horizontal component is

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increasingly shifted towards midnight. Here it must be emphasized that in this case an anomalous DS-variation occurs during the main phase of the storm. Moreover at 0400 UT we again have an example of low asymmetry at equatorial latitudes during the maximum of the main phase for low substorm activity. d. 14 January 1967, 0800-1200 UT According to Fig. 3 this period fell within the recovery phase of the storm recorded on 14 January. The DS-variatidns of this period are shown by Fig. 7. During this recovery phase the substorm activity had again increased especially at Leirvogur and Great Whale River; normal DS-variations occur at equatorial latitudes. The distribution of disturbance values at equatorial latitudes by 0900 UT is most typical. At this moment the curve maximum occurs at 0800 LT and the minimum at 1800 LT. After 1000 UT the substorm activity in effect has died out. Nevertheless a perceptible asymmetry exists at equatorial latitudes which is, however, inverse to the usually observed asymmetry. Here we have an example of an inverse DS-variation, which is similar to that of case b. The situation is not, as frequently stated, that during the recovery phase there are symmetrical conditions and

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hence only a symmetrical ring current exists. After all the DS-range during the recovery phase amounts to 50 7 in this case. e. 8 January 1967, 0100-0600 U T

As compared to the previous cases, Fig. 8 shows relatively low asymmetries at equatorial latitudes during the main phase. The intensity and the shape of the asymmetry hardly varied during this period. Only at 0300 UT are the conditions almost symmetrical, although a substorm occurs. Here nearly exclusively anomalous DS-variations at equatorial latitudes are encountered. The strongest depression in the horizontal component occurs near local midnight. The largest deviation from this anomalous DS-variation occurs at 0330 UT when the asymmetry at the latitudes of the northern auroral zone also reaches its maximum value. Thus we have a typical case where during the main phase only an anomalous DS-variation occurs. SUMMARY AND CONCLUSIONS

The results of these investigations provide a rather intricate picture. However, we should always remember that in the magnetic field observations of the Earth's surface we have to deal with the effects of at least four different sources. I. The ionospheric return currents of the polar electrojets. 2. The asymmetric ring current. 8

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3. The neutral-sheet current. 4. The magnetopause current. Consequently the DS-variation observed at the Earth's surface at equatorial latitudes will be quite different. The following results of observation are summarized here. 1. The largest asymmetry found at the Equator frequently occurs onlyafter themaximum ofsubstorm activity (case (a), case b). The time shift between the maximum asymmetry at equatorial latitudes and the substorm maximum can however be different. In case a this time shift amounts to approx 30 rain; in case b to almost 2 hr. 2. As a rule normal DS-variations occur when substorms are present (cases a, e and d). However, it is also possible that the equatorial asymmetry is very low in the presence of heavy substorms (case b). However, the formation of anomalous DS-variations is favoured when the substorm activity decreases (case c, case el. 3. According to Akasofu and Chapman (1964) an equatorial asymmetry also exists independently of whether or not substorms occur. In the present investigations, however, cases were also encountered for which, even during the main phase, no asymmetry at equatorial latitudes was observed irrespective of the presence (cases a and b) or absence (cases a and c) of substorms. 4. If the equatorial asymmetry is discussed in connection with Ds~-variation, then the following picture results: high equatorial asymmetries occur simultaneously with large Dst-variations (case a); the asymmetry at equatorial latitudes can also be relatively low during a high main phase of Dst-variation (case e); conversely it is also possible that during a low Dst-variation a high equatorial asymmetry is observed (case b).

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5. An anomalous DS-variation was found to occur during the main phase (cases b and e) and during the recovery phase (case a). 6. The occurrence of an inverse DS-variation certainly will be limited to the recovery phase. In each case the existence of an equatorial asymmetry during the recovery phase must also be expected (cases b and d). The first result of the above summary fully confirms the results on the relationship between the AE-index and the equatorial asymmetry, as obtained by Kawasaki and Akasofu (1971), Crooker and Siscoe (1971), and Crooker (1972). Usually the maximum of equatorial asymmetry occurs later than that of the substorm activity. Therefore it is not immediately obvious how, on the basis of this observation, it can be concluded as was done by Crooker and McPherron (1972), that the asymmetry at equatorial latitudes indicates that the partial ring current is connected with the eastward directed electrojet over field-aligned currents. This conclusion would only be justified if a coincidence between the substorm maximum occurring in the evening and the maximum of ring-current asymmetry were observed. However, from measurements of the stormtime disturbance field by means of the ATS-1 satellite, which were described by Coleman and Cummings (1971), we know that during the main phase also the neutral-sheet current and the magnetopause current are active in addition to the ring current. But we do not know how the individual disturbance fields influence the equatorial asymmetry at the Earth's surface. We are of the

1000

A GRAFE

same opinion as Crooker and Siscoe (1971), that the shift of the maximum depression o! the horizontal component at equatorial latitudes towards noon during the occurrence ~;f a substorm indicates at least a two-component model of the disturbance-field asymraecry. Certainly ionospheric return ctlrrents of the polar electrojets play no unessential rc,lc ill this connection. The asymmetry component, which is not attributed to these return current~; (called NBG by Crooker and Siscoe, 1971), then has a maximum depression occurring near 2300 LT. If this asymmetry phenomenon is attributed to an asymmetric ring current, then a maximum of the ring current should be expected by midnight and it could be said that the asymmetry phenomena at equatorial latitudes, which are called anomalous DS-variations in this paper, are effects of an asymmetric ring current. We remember that anomalous DS-variations were found also during the main phase. But there is still another possible variant. In those cases where the anomalous DS-variation is observed the influence of the neutral-sheet current might exceed that of the ring current. An indication that, with increasing geomagnetic activity, the maximum of magnetic field depression is shifted from the evening meridian to the midnight meridian is provided by the results of the magnetic field observations with OGO3 and OGO5, which were described by Sugiura and Poros (t973). According to this paper for kp ,- 0-1, l_ 4, at the evening meridian, stronger field depressions occur than at the midnight meridian. For/,-p --- 2-3 the highest negative tield values were measured at the midnight meridian. According to Coleman and Cummings (1971) during the recovery phase both the neutralsheet current and the magnetopause current have already died out. Only the ring current is still present. Therefore we have to interpret the inverse DS-variations at equatorial latitudes, which are found during the recovery phase, as effects of an asymmetric ring current. Coleman and Cummings (1971) believe that with the beginning of the recovery phase the ring current moves towards the Earth. It is clear that in this case a symmetrical ring current cannot be expected to occur during the recovery phase. Moreover, the OGO3 and OGO5 field measurements carried out by Sugiura and Poros (1973) for kp == 2-3 show stronger field depressions for the morning meridian than for the evening meridian. It is difficult to explain the fact that cases also occur where no equatorial asymmetry exists during the main phase. An accidental cancellation of the asymmetry due to superposition of the individual effects is hardly imaginable, because the sources show approximately the same asymmetric behaviour. Therefore it can only be supposed that the ring current is symmetrical during this period. However, we are also forced to assume certain variations of the propagation conditions for the ionospheric return currents of the polar electrojets; for the occurrence of heavy substorms in connection with a very low equatorial asymmetry is also possible. Likewise a high Ds:activity does not necessarily result in a marked equatorial asymmetry and vice versa. The Ds:variation is a measure of the intensity of the storm and, as is known, also of the ring-current activity. Perhaps it is more justified to consider the equatorial asymmetry to be a consequence of the tail current. However, there are still many unsolved problems in this connection which can be clarified only by means of satellite measurements. One should take care not to attribute the observed asymmetry of the magnetic field disturbance values at equatorial latitudes merely to an asymmetric ring current. As long as there is no clear interpretation of the equatorial asymmetry observed at the Earth's surface, conceptions of a common three-dimensional current system between the eastward electrojet and the asymmetric ring current (see Zaitzev, 1971; Kamide and Fukushima, 1972) can only be considered with scepticism.

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Acknowledgement--I wish to thank Professor S.-I. Akasofu for providing me with reduced magnetograms. REFERENCES AKASOFU,S.-I. and CHAPMAN,S. (1964). On the asymmetric development of magnetic storm fields in low and middle latitudes. Planet. Space Sci. 12, 607-626. AKASOFtJ, S.-I. and KAWASAKI,K. (1970). Geomagnetic disturbances (26 October-5 November 1968) associated with the McMath Plage Region 9740. Report UAG--8 Part II March, pp. 243-252. BHARGAVA,B. N. and YACOB, A. (1971). Solar wind associated component in the low-latitude magnetic daily variation. J. Geomag. Geoelect. 23, 249-253. CAmLL, L. J. (1966). Inflation of the inner magnetosphere during a magnetic storm. J. geophys. Res. 71, 4505-4519. COLEMAN,P. J. and CUMMINGS,W. D. (1971). Stormtime disturbance fields at ATS I. J. geophys. Res. 76, 51-62. CROOKER,N. U. (1972). High-time resolution of the low-latitude asymmetric disturbance in the geomagnetic field. J. geophys. Res. 77, 773-775. CROOKER,N. U. and MCPHERRON,R. L. (1972). On the distinction between the auroral electrojet and partial ring current systems. J. geophys. Res. 77, 6886-6889. CROOKER,N. U. and SISCOE,G. L. (1971). A study of the geomagnetic disturbance field asymmetry. Radio Sci. 6, 495-501. CtJMMINGS, W. D. (1966). Asymmetric ring currents and the low-latitude disturbance daily variation. J. geophys. Res. 71, 4495-4503. FRANK, L. A. (1970). Direct detection of asymmetric increases of extraterrestrial ring current proton intensities in the outer radiation zone. J. geophys. Res. 75, 1263-1268. GRAFE, A. (1972). About the connection between equatorial ring current and polar electrojet. Planet. Space Sci. 20, 183-204. KAM1DE, Y. and FLrKUSHIMA,N. (1972). Positive geomagnetic bays in evening high-latitudes and their possible connection with partial ring current. Rep. Ionosph. Space Res. Japan 26, 79-101. KAWASAKI,K. and AI~ASOFtr,S.-I. (1971). Low-latitude DS-component of geomagnetic storm field. J. geophys. Res. 76, 2396-2405. StJGIU~A, M. and POROS, D. J. (1973). A magnetospheric field model incorporating the OG03 and OG05 magnetic field observations. GSFC X-645-73-34. ZAITZEV, A. N. (1971). The space-time development of magnetospheric storm according to the groundbased data. Lecture XV IUGG General Assembly, Moscow.