Daily variation of the geomagnetic field in equatorial region and sector boundary passage

Planet 3p.w

Sci., Vol. 25. pp. 1183 to 1185. Pqpmoa

Press, 19T’. Printed in Northern lrehd

RESEARCH

NOTES

DAILY VARIATION OF THE GEOMAGNETIC FIELD IN EQUATORIAL REGION AND SECTOR BOUNDARY PASSAGE (Received in final form 26 July 1977) Abdmct-The

mean daily range in horizontal intensity at low latitudes shows a signi8cant departure on the day of a sector boundary passage in relation to its magnitude on adjacent days with a measure of dependence on phase of the solar activity. It is shown that this arises because of a substantial difference. in the nature of the response to sector boundary passage, between the instantaneous maximum field and minimum field. From the fact that the responses at three stations spanning the latitudes near dip equator to that near the focus of Sq currents are almost identical, it is suggested that the cause of the observed feature is primarily disturbance and is essentially non-ionospheric. Differences in the nature of responses between pre-1957 and post-1957 periods reported earlier in the planetary indices or low latitude disturbance indices are shown to be true tor the daily range, maximum and minimum fields at low latitudes. Studies on the range of daily variation at low latitudes have been quite extensive. Analysis of data from observatories in the vicinity of dip equator have shown that the ranges there and at stations outside the in&tenor. of equatorial electrojet are not well-correlated (Osborne 1966, 1968: Onbuehi er al.. 1967, and others). Kane (197I) showed -&at, for quiet days; the correlation between the ranges at Trivandrum and Alibag were consistently low and only for disturbed days the correlation improved. Yacob and Sen (1974) have studied the variations of maximum and minimum daily values of H at low latitudes in relation to the daily range and showed that quiet day variation is predominantly a day-time phenomenon. With the discovery of the sector stmcture of IMF, several interesting relations between geomagnetic activity and IMF direction in the ecliptic plane have been detected. Sector boundary passage was shown to be associated with increased geomagnetic activity (Wilcox, 1968). Polar cap magnetograms depict signatures of the IMF polarity (Svalgaard, 1968; Mansurov, 1969) and these have been used as diagnostic tools for inferring the polarity of IMF for each day. Using the subjective list of inferred polarity given by Svalgaard (1972), Bhargava and Rangarajan (1975) showed that the response of the low latitude field differed signitiamtly depending on the nature of the polarity, the type of boundary and the phase of the solar activity. Following comments that the subjective list of inferred polarity was biased, Svalgaard (1975, 1976a) compiled an atlas of A/C index using satellite observations, polar cap magnetograms and occasionally polar cap boundary magnetograms. Rangarajan (1977) showed that even with this list, the response in geomagnetic activitv at low latitudes depend signiflcantlv on the type of boundary. Recently, M&urshi~et al. (1973) and Matsushita (1975) found that the level of the mid-latitude field on quiet days changes depending on the direction of IMF and suggested equatorward (poleward) shift of the Sq current focus corresponding to toward (away) polarity of IMF. Matsushita (1975) indicated this feature could be used for inferring the polarity of IMF. In this communication, we analyse the range of daily variation in H recorded at the Indian magnetic observatories at Sabhawala (Dipole lat. 20.8ON), Alibag (Dipole 7

lat. 9.5’N) and Trivandrum (Dipole lat. 1.1’S) as a function of the solar magnetic sector boundary passage at Earth’s orbit. The three stations are such that the latitude of the 8rst is near the focus of Sq current system, the second is outside the influence of the electrojet while being away from the location of Sq focus and the third is under the immediate. influence of the equatorial electrojet. To understand the nature of the response in daily range, maximum and minimum values of the daily field have also been analysed. As varying geomagnetic response to the sector stmcture for the periods 1947-1956 and post-1957 was indicated by Svalgaard (1976b) and Rangarajan (1977). ranges at kibaghave been studied separately for two blocks, 1947-1956 and 1964-1974. The daily range used in the present study is defined as the difference between the instantaneous maximum and minimum of the daily horixontal field in a Greenwich day. Ranges derived from Trivandrum and Alibag records for the period 1959-1974 have been used in a superposed epoch analysis with sector boundary dates given by Svalgaard (1975, 1976a) as the key day. The results for the years, 1959-60 and 1967-70, near maximum of solar cycle were combined to get response representative of behaviour during conditions of high solar activity and the rest as that during low solar activity. DAILY

RANGEIN

AND OWE

THE ELECTROJET

REGRON

In Fig. 1 are shown the changes in daily range at Trivandrum and Alibag for four days on either side of the sector boundary separately for low and high solar activity conditions. The primary feature noticeable is that irrespective of the phase of solar activity the response, shown as departures from that four days prior to the boundary passage, at Trivandrum is almost exactly the same as that at Alibag for both types of sector boundaries, indicating a complete absence of equatorial enhancement in the response of daily range as a function of sector boundary passage. This result suggests that apart from the known enhancement in the daily range from Alibag to Trivandrum by a factor of about 2.5 (Sastri and Jayakar, 1970), the equatorial ionospheric conductivity above the dip equator is not sensibly modified by the passage of an IMF

1183

Research notes

1184 Low SOLAR ACTlVllV

HIGH SOLAR ACTlVlTV

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F1o.1. &.WONSE OF RANGE OF DAILY VARIATION AT TR~VANDRIJM AND ALIBAG TO PASSAGE OF SECTOR BOUNDARY.

(1973) showed that the departure of the mean daily X component, on a day with a defined polarity, from tbe mean derived from all days showed a smooth latitudinal profile with largest deviation of about 5 nT near the dipole equator. The results in Fig. 1 showing very little difference in response of range between Alibag and Trivandrum, separated by about lo” in latitude is in conformity with the profile shown by Matsushita et al. (1973). Some additional features seen in Fig. 1 are: 1. Difference in the magnitude of the response dependent on the nature of the boundary appears to be confined to conditions of low solar activity. 2. Response during conditions of low solar activity is higher in magnitude especially in association with +/ type boundary, a feature shown to be present in the disturbance index K at low latitudes by Rangarajan (1977), and earlier by Bhargava and Rangarajan (1975). sector boundary. Matsushita et al.

DAD.,Y RANGE NEAR FOCUS OF Sq CURRENT

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F1G.2. &sPONSE OF RANGE OF DAILY VARIATION AT SABELWALA AND ALIBAG TO PASSAGE OF SECXOR BOUNDARY.

ALIBAG

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SYSTEM

An association between movement of the Sq focus and polarity of &fF has been suggested by Matsushita et al. (1973) and Matsushita (1975), which would imply a corresponding difference in the magnitude of the daily range at low latitude stations dependant on polarity. To ascertain the effect of sector boundary passage on the daily range at a station near the focus of the Sq current system, we have analysed, by superposed epoch method, the range at Sabhawala (Din 45.3%) as a function of sector

boundary passage for the period 19641974 as the station commenced functioning only from 1964. For comparison, data of Alibag for the same period is also analysed. In Fig. 2 are shown the response at both the stations separately for the two types of boundary. It is immediately apparent that the responses are practically identical between the two stations for either type of boundary. If the response in range were linked with quiet day variation, some difference between Alibag and Sabhawala, associated with the movement of focus ascribed to IMF polarity would have been noticed in view of their separation in latitude. Also at Alibag, the nature of the response should have shown difIerence between (+/-) and (-/+) type of boundary passage, a lower value of range prior to the (-/+) boundary a sharp increase across the boundary followed by a higher level and vice versa for the +/boundary. Absence of these two features clearly indicates that the change in the daily ranges with passage of a sector boundary is associated only with a component of disturbance originated by the passage of a sector boundary. The comparable magnitudes of the responses at Sabhawala, Alibag and Trivandrum indicates that the source of the disturbance is definitely non-ionospheric. Even for the shifts in Sq focus associated with IMF direction, Matsushita (1975) stressed that the primary cause is not necessarily ionospheric electric current changes. Kane (1972) indicated that the daily range has both a positive and a negative component unrelated to each

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FELDANDMINIMUMFIELD RE~P~NSEOFMAXIMUM ATAUBAGTOPASSAGEOFSE~RB~UNDARY.

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1185

Research notes (- I+ Boundary)

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FIG.

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TO DAILY IMUM

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OFDAILYVARIATIONAT OF SECTOR BOUNDARY. (a) AND (b)

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(d)

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Parameters of daily variation (1947-1956) Commenting on the contamination of the inference of polarity of IMP by !!iq signature at polar cap stations during calm periods, Svalgaard (1976b) suggested that the distinct difference in response of geomagnetic activity between pre-1957 and post-1957 was not due to any systematic bias, but is a real phenomenon, not in violation of any physical laws. Analysis of K indices (Rangarajan, 1977) has also shown that such a feature can be prominantly observed even in low latitude disturbance index. The daily range, maximum and minimum fields at Alibag for the period 1947-1956 were studied as a function of sector boundary passage and the responses of the different parameters are shown in Fig. 4. While the change in the daily range in the vicinity of a ( + / -) sector boundary (Fig. 4b) has the same form as that for the 1964-1974 epoch (Fig. 2) the response to a (-I+) boundary passage has drastically changed (Fig. 4a). The response is neither significantly large nor systematic. For both types of boundaries the maximum and miniium fields show distinct differences between the two epochs. Also for the 1947-1956 period, responses of the maximum and minimum fields are strongly dependent on the nature of the boundary as is the daily range. These results for the parameters of the daily variation at low latitudes are in agreement with the earlier findings of Rangarajan (1977) wherin three hourly indices of irregular geomagnetic activity were considered for analysis. Acknowledgement-The author is grateful to Prof. B. N. Bhargava, Director, Indian Institute of Geomagnetism for useful suggestions. G. K. Rangarajan Indian Znstimte of Geomagneiism, Bombay 400 005, India.

OF THE FIELD.

other. While the negative component is exclusively of magnetospheric origin, the positive component could be contributed by both magnetospheric and ionospheric sources. Early analyses of the relation between AH, the daily range and H,,,, or Hmin the maximum or minimum values during the day) have indicated different magnitudes and sign of correlations. To understand the behaviour of the daily range in vicinity of an IMF sector boundary, we have used the two parameters instantaneous maximum (H_) and minimum (H,& in a superposed epoch analysis results of which are graphed in Fig. 3. The data Pertain to the period 1964 1974 for Alibag. A striking ditIerence in the nature of response centred on the key date is at once apparent. While the H,,,, field has a large positive deviation on the day, the Hmin value depicts largest negative value leading to a significantly larger response of the range on the day of the passage of the boundary, as observed. As the H,, is generally observed near local noon hours and the Hmin field during night or pre-dawn hours, it is inferred that the nature of the response for the daytime and night-time fields to the passage of a sector boundary differ significantly. Data of the field averaged over 3 h centred on local noon and local midnight at Alibag have again shown behaviour similar to that depicted in Fig. 3 but with a reduced magnitude.

REFERENCES

Bhargava, B. N. and Rangarajan, G. K. (1975). Planet. Space Sci. 23, 929. Kane, R. P. (1971). J. atmos. ten. Plays. 33, 319. Kane, R. P. (1972). J. ammos. terr. Phys. 34, 1105. Mansurov, S. M. (1969). Geomagn. Aeron. 9, 622. Matsushita, S. (1975). J. geophys. Res. SO, 4751. Matsushita, S., Tarpley, J. D. and Campbell, W. H. (1973). Radio Sci. 8,963. Ogbuehi, P. O., Onwumechilli, A. and Ifedelli, S. 0. (1976). 1. atmos. ten. Phys. 29, 149. Osborne, D. G. (1966). J. afmos. ten. Phys. 24445. Osborne, D. G. (1968). .Z. atmos. terr. Phys. Jo, 1429. Rangarajan, G. K. (1977). Planet. Space Sci. In press. Sastri, N. S. and Javakar, R. W. (1970). Indian J. Me& Ge&hys. 21, 279.Svalgaard, L. (1968). Geophys. Papers R-6, Danish Meteorol. Institute. Svalgaard, L. (1972). Geophys. Papers, R-29, Danish Meteorol. Institute. Svalgaard, L. (1975). Stanford University Institute for Plasma Res. No. 629. Svalgaard, L. (1976a). Stanford University Institute for Plasma Res. No. 648. Svalgaard, L. (1976b). Stanford University Institute for Plasma Res. No. 649. Yacob, A. and Sen, A. K. (1974). PAGEOPH 112,464.