- Open Access
Seasonal evolution of Sq current system at sub-auroral latitude
© The Society of Geomagnetism and Earth, Planetary and Space Sciences (SGEPSS); The Seismological Society of Japan; The Volcanological Society of Japan; The Geodetic Society of Japan; The Japanese Society for Planetary Sciences; TERRAPUB. 2012
- Received: 19 December 2011
- Accepted: 16 April 2012
- Published: 26 November 2012
The quiet-time (ΣKp ≤ 3) daily variations of the geomagnetic field at the Indian Antarctic station, Maitri (Geographic Coord.: 70.75°S, 11.73°E; Geomagnetic Coord.: 66.84°S, 56.29°E) during two consecutive years of a solar minimum are considered in order to investigate the characteristics of the solar quiet (Sq) current system. The present work reports the signatures of the south limb of the Sq current loop of the southern hemisphere over a sub-auroral station. It is observed that the seasonal variation of the Sq current strength over Maitri is strongest during the summer months and weakest during the winter months. In spite of the total darkness during the winter months, an Sq pattern is identified at Maitri. The range of the horizontal field variation in the daily Sq pattern during summer is one order higher than that during winter. An interesting feature regarding the phase of the local time variation in the seasonal pattern is found here. A sharp shift in the time of the peak Sq current to later local times (> 1 hour per month) is observed during January–February and July–August, which may correspond to the transition from the complete presence, or absence, of sunlight to partial sunlight. The differences in the incoming solar UV radiation during such transitions can cause a sudden change in the local ionospheric conductivity pattern, and can also trigger some unusual thermo-tidal activity, that might be responsible for modifying the global Sq pattern.
- Sq current system
- seasonal variation
- sub-auroral latitude
It is well-established that a regular diurnal variation in the geomagnetic field, also referred to as the solar quiet day (Sq) variation, is observed at mid- to low-latitudes on magnetically quiet days (Chapman and Bartels, 1940; Richmond et al., 1976; Matsushita and Campbell, 1987; Campbell, 1997). The dynamo currents flowing in the E-region ionosphere due to atmospheric tidal motion across the geomagnetic field are essentially responsible for the Sq variations, comprising two oval-shaped current loops on the day-side in each hemisphere (Matsushita and Campbell, 1987). The latitudinal extent of this current system is very wideranging from low to high geomagnetic latitudes in both hemispheres.
A extensive study of the Sq current to investigate characteristics such as the solar activity dependence, the seasonal variation of its strength, movement of the Sq foci, tidal dependence, longitudinal variation, etc., have been carried out by many researchers (Yacob and Rao, 1966; Gupta, 1973; Tarpley, 1973; Hibberd, 1985; Patil et al., 1985; Rastogi et al., 1994; Miyahara and Ooishi, 1997; Stening et al., 2007; Yamazaki et al., 2010; Pham Thi Thu et al., 2011). Campbell and Schiffmacher (1985, 1988) have analyzed quiet time geomagnetic field records in the Northern, as well as in the Southern, hemisphere. They showed that the seasonal variation of the Sq current is semiannual in the low to mid latitude region (up to around 50° geomagnetic latitude), and is annual at higher latitudes. The differences in the seasonal responses between mid- to low-, and high-, latitude Sq variations are intriguing. The annual variation at higher latitudes is obvious due to large ionospheric conductivity differences between the summer and winter seasons. At low- to mid-latitudes, the solar zenith angle is a minimum in equinoxes and a maximum in both the solstices, thereby resulting in larger ionospheric conductivities during equinoxes. However, the conductivity differences due to the seasonal movement of the Sun are not enough to account for the observed changes in the seasonal variation of the Sq amplitude at low- to mid-latitudes. In addition, it has been well established that the equatorial electrojet also exhibits a semiannual variation (Chapman and Rajarao, 1965; Campbell and Schiffmacher, 1985, 1988), which cannot be explained merely by ionospheric conductivity. Several justifications have been proposed for the semiannual response of the Sq strength at low latitudes (Tarpley, 1973; Stening, 1991). Tarpley (1973) suggested that the equator-ward movement of either the northern or southern Sq focus at each equinox could cause an equinoctial enhancement of the electrojet. Whereas Stening (1991) explained the semiannual variation near the equator and the Sq focus, in terms of the superposition of a current system generated by various forms of semidiurnal tide.
A large day-to-day variability of Sq currents (strength and shape) has been observed (Mayaud, 1965; Butcher and Brown, 1981; Chen et al., 2007), and is mainly attributed to changes in the tidal wind, solar radiation, geomagnetic activity, ionospheric conditions, etc. (Torta et al., 1997). Using a method of natural orthogonal components, Chen et al. (2007) analyzed the day-to-day variability of the Sq variations. They found positive and negative correlations for the stations on the same north- or south-side of the Sq current system focus and for the stations on different sides of the Sq focus, respectively. In addition, they reported noticeably enhanced Sq values on some disturbed days, but on other disturbed days they found dramatically reduced Sq variations, suggesting the dominant effects of the disturbance dynamo process.
Thus, an essential understanding of the Sq current system has been acquired through an extensive analysis of low- to mid-latitude station data, covering both sides (equatorward and poleward) of the Sq current system focus in both hemispheres. Although very few studies have been carried out at the edge of the Sq current loop near higher magnetic latitudes (Campbell and Schiffmacher, 1985, 1988; Carafella et al., 1998; Detrick and Lanzerotti, 2001; Sillanpaa et al., 2004). At higher magnetic latitudes, the magnetic variations are affected by a wide range of different phenomena, and magnetospheric processes that may completely dominate the Sq current pattern. The electric fields and currents in the polar region are essentially driven by field-aligned currents caused by solar wind-magnetosphere-ionosphere interactions; resulting in two-cell convection patterns and enhanced ionospheric conductivity in both polar regions (Nagata and Kokubun, 1962). Sillanpaa et al. (2004) used an IMAGE magnetometer array to obtain the average signatures of the high-latitude quiet-time magnetic variations. Figure 7 from their paper shows that the observations for Corrected Geomagnetic (CGM) latitudes less than 65° exhibit the signatures of an Sq current system, while latitudes higher than 65° CGM latitude show different signatures. On the contrary, Detrick and Lanzerotti (2001) reported Sq current variations beyond an 80° geomagnetic latitude, which is well beyond this boundary. The boundary of the auroral oval is highly variable with respect to geomagnetic activity, and in the polar-cap region significant magnetic perturbations are always present even on geomagnetically quiet days (Nagata and Kokubun, 1962). This plays a crucial role in determining the higher latitude extent of the Sq current system and, hence, could account for the differences in the results.
The Indian Institute of Geomagnetism has been operating a digital fluxgate magnetometer at the Indian Antarctica station, Maitri (Geographic coordinates: 70.75°S, 11.73°E; Geomagnetic coordinates: 66.84°S, 56.29°E), since the year 2003. The CGM coordinates of this location are (63.11°S, 53.59°E). The analysis of geomagnetic data at Maitri has proved that the station occupies a sub-auroral position during quiet times and comes under the influence of Auroral Electrojet (AE) and Field Aligned Currents (FAC) with increasing geomagnetic disturbances (Hanchinal et al., 1996). Thus, being a sub-auroral location, Maitri is very sensitive to electromagnetic changes in the geo-environment.
In this paper, we investigate the magnetic field recordings at Maitri and attempt to delineate the characteristics of quiet-time magnetic field variations at the sub-auroral location. The magnetic field variations in this paper are interpreted mainly in terms of the atmospheric current systems and not in terms of ground-induced currents. Section 2 describes the data selected for the present study. Results including the diurnal patterns and seasonal variations of its amplitudes and patterns are discussed in Section 3. Section 4 discusses the results, and Section 5 presents the conclusions of this study.
For the present study, we use magnetic field variations in all three components recorded by the digital fluxgate magnetometer with a one-minute sampling interval. The accuracy of the magnetometer is 0.1 nT. As discussed in the previous section, the location of Maitri station is very sensitive to geomagnetic disturbances, and thus it is very important to select the quiet days with extra care. The variation of the equatorward edge of the auroral oval with the geomagnetic activity can also affect the shape and amplitude of the Sq variation over Maitri. Therefore, for the selection of days suitable for studying the Sq system, it is not sufficient to look for times of low Kp, but, in addition, the activity of the auroral electrojet (AE) index has to be low (Xu, 1989).
We have selected days with ΣKp ≤ 3 for the years 2009 and 2010. As mentioned above, the criteria of a low ΣKp only is not sufficient, and, therefore, we inspected each day individually to confirm that at least the local daytime is free of disturbance. The AE indices on these days were verified to have lower values (daily average <60 nT). Since the solar minima of solar cycle 23 took place around 2009–10, it was possible to acquire a good number of quiet days with ΣKp ≤ 3. During the year 2009, we obtained 3 days in each month, except for the months of April and November, in which only 2 days in each month satisfied the ΣKp ≤ 3 condition. In the year 2010, seven months had 3 days in each month; three months had 2 days in each month and two months had only one day in each month. On average, we obtained more than 5 days per month in two years. The averages of the magnetic field variations on these days were used to obtain the typical patterns of quiet-time magnetic field variations at Maitri.
3.1 Diurnal pattern
The signatures of the H- and D-variations shown in Fig. 1 are of a typical kind due to a clockwise horizontal current flowing in the southern hemisphere, which is a result of the global ionospheric dynamo. The latitudinal location of Maitri is well beyond the focus of the Sq current system, and, hence, experiences the westward limb of the Sq current loop during daytime, peaking near local noon; and negative-positive bipolar variations in the D-component are due to the meridional currents flowing in the morning and afternoon sectors. Normally, the ground magnetic field variations seen in the Z-component are contaminated by the induced subsurface currents. Since we do not intend to study the induced current effects here, we will not discuss this component.
3.2 Seasonal variation
3.2.1 Range of Sq variation
3.2.2 Local time of Sq peak
Therefore, depending upon the rate of the time shift of the Sq peak, one can classify the months of the year into the following three categories: (1) Months of highest gradients, viz. January–February and July–August; (2) Months of moderate gradients, viz. February–May and August–November; and (3) Months of lowest gradients, viz. May–June–July and November–December–January.
Likewise, we also record the timings of maximum and minimum D, which are shown in Figs. 4(b) and 4(d). It can be assumed that the cross-over of D-variations from morning negative to afternoon positive variations takes place approximately at the midway between the two extrema occurring in the morning and afternoon hours. Therefore, for the sake of convenience, we refer to the midpoint of the local times of the minimum and maximum D-variation as the cross-over point, which indirectly represents the peak of the Sq variation (shown in Fig. 4(c)). Figure 4(e) shows the time interval between the minimum and maximum D-deviation. One can observe from Fig. 4(b), that the time of maximum D sharply increases by more than one hour from January to February. Similarly, the time of minimum D (Fig. 4(d)), and the midpoint of the local times of the two extrema in the D-variations (Fig. 4(c)) also display sharp changes (>1 hr) between these two months. Then the LT of maximum D gradually moves towards lower local times (going nearer to noon) until the month of July. From July to August, once again a sudden shift (of ~1 hr) towards evening times (~16 LT) is observed (Fig. 4(b)). In Fig. 4(b), the LT of minimum D moves gradually towards local noon, but during July and August there is an abrupt change in the LT of minimum D; and again moves gradually towards earlier LT’s. These movements of maximum and minimum D-variations towards noon between March to June could be attributed to the shrinking of the Sq current system due to a decreasing amount of daylight time with the gradual arrival of winter. The mid LT of minimum and maximum D-variation shown in Fig. 4(c) also shows the evidence of abrupt changes between January–February, and July–August.
In general, to some extent, variations in Figs. 4(b), (c) and (d), can be classified into similar categories of months during the year as listed above for the time of maximum depression in the H-variation.
Figure 4(e) shows the time range between minimum and maximum D-variation, that can provide a nearly proportionate estimate of the time duration of the Sq variations. It shows that the time interval does not change abruptly between January–February, or July–August, rather it is almost the same during these months. The time interval changes from a longer to a shorter duration from summer to winter, which is obvious due to the day-night differences during summer and winter.
The seasonal variability of the range of the Sq variation over a sub-auroral station, described in this paper, indicates an annual variation-peaking during summer and a weakening during the winter months. Campbell (1997) has summarized the daily magnetic field variations due to the Sq current system at various latitudes for different months of the year in both hemispheres. His figure 2.24 indicates a summer peak and a winter minima at higher latitudes, while the variation is of a semiannual type at low to middle latitudes, peaking at equinoxes and a minimum during solstices. Thus, the present results are in agreement with earlier high-latitude observations.
The observation regarding the seasonal variation of the time of the Sq peak over Maitri (Fig. 4) is quite puzzling. Nevertheless, these observations are interesting and have not been reported earlier. The sudden shift of the daily local time of the strongest Sq current over Maitri, between Jan–Feb and July–August, has been checked carefully. This feature is found to be quite persistent in both years, and, hence, it is important to look for the cause of such a rapid phase shift.
One can see from these plots that during November, December and January, Maitri experiences 24-hour sunlight, whereas during May, June and July, it experiences darkness throughout the day. Thus, for a period of about three months centered at mid-December, Maitri is continuously in sunlight, indicating that the ionosphere over Maitri is illuminated by solar UV throughout the day. In contrast, during a period of about three months centered at mid-June, Maitri is in total darkness, indicating that the ionosphere over Maitri does not receive any solar UV radiation. The electrical conductivity due to the ionization of the upper atmosphere caused by solar UV rays is important in producing the ionospheric electric currents and, subsequently, the ground magnetic signatures. At Maitri, the differences in the ionospheric conductivity during summer and winter are large. Therefore, one would expect a maximum variation in the geomagnetic field during the Antarctic summer. Nevertheless, during winter, with a complete lack of solar radiation, it is still possible to have some ionospheric conductivity, although much lower than that during summer. Wang et al. (2005) found that the ionospheric conductivity in the auroral region strongly depends on the solar zenith angle (solar irradiance) during daytime, but during nighttime the effects of precipitating electrons play a dominant role in maintaining the ionospheric conductivity. Due to different types of energetic-particle ionization during geomagnetic activity and significantly different conditions of solar illumination over Maitri (polar type), the ionospheric conductivities at Maitri are much different from that at low to middle latitudes. The extreme conditions of sunlight during summer and winter over the polar to sub-auroral region cause concentrations of many chemically active species (that are crucial in determining the ozone distribution) to be considerably different from those typical of mid latitudes. In addition, large latitudinal temperature gradients (due to differences in sunlight and ozone distribution) can cause large-scale dynamical circulation of the upper and middle atmosphere, which could be responsible for the ionospheric conductivities during a winter of total darkness (Roederer et al., 1982). Also to some extent, the conductivity of the conjugate ionosphere of summer can be mapped to the winter ionosphere.
Although the Sq variations in the geomagnetic field are due to the global dynamo currents induced by neutral winds associated with the tides in the lower thermosphere, they can also be modulated by local conductivities. The abrupt changes in the local conductivity patterns can lead to abrupt modifications of the Sq current system as well. During January, the Sun is in the sky all the time (24 hours), and then the local sunrise and sunset start appearing in the month of February. In the same way, in the month of July, Maitri does not experience local sunrise and sunsets, but in the month of August it begins to emerge. This could be responsible for our observation of a sudden time shift in the Sq peak during January–February and July–August (Fig. 4). During January, there was continuous solar illumination causing no drastic day-night conductivity differences. Similarly, during July, there was no distinctive termination boundary of ionospheric conductivity. However, during the subsequent months, viz. February and August, sharp boundaries of day-night start appearing. Thus, this could lead to a transformation from a situation of continuous ionization, or total absence of ionization, due to the solar radiation, to some kind of generation (sunlight time) and termination (absence of sunlight) of the ionization. Since the Sq current system is of a global nature, somehow local variations have to be coupled with the global pattern. However, in doing so, this may result in a sudden shift in the pattern. Therefore, a sudden delay of about 1–1.5 hr in the local time (LT) of the Sq peak in the months of February and August (Fig. 4(a)), could appear with the emergence of the day-night terminator (DNT). If a maximum shift of the Sq peak is seen due to high gradients in the ionization and temperature across the terminator line when DNT appears (January–February and July–August), then the disappearance of DNT in the months of May and November should also result in a sudden morning-side shift of the Sq peak. But, we do not observe such a sharp change during April–May and October– November, although we do see a time shift of ~30 minutes during these pairs of months. This may indicate that the maximum shift of the Sq peak in LT is seen only when the system departs from prolonged extreme local conditions of ionospheric conductivities. Migration from locally-modified ionospheric conductivities over a prolonged time to the appearance of DNT (which normally occurs at other locations under the Sq system) leads to a sudden shift of the Sq peak towards latter local times. Whereas migration from DNT occurrence to extreme local ionospheric conditions (disappearance of DNT) does not result in the rapid movement of the Sq peak. The quasi-steady state of the local Sq system (forming a part of the global Sq current system) in the southern hemisphere is established at a faster rate with the occurrence of DNT and is destroyed at a slower rate when the DNT disappears. While local ionospheric conditions, when DNT is absent, seem to achieve a quasi-steady state slowly. Therefore, when local conditions are overtaken by the appearance of DNT, Maitri experiences a sudden shift in the LT of the Sq peak, but does not observe a sharp change when DNT disappears and local conditions dominate. We envisage that the system remains in the quasi-steady state even after the DNT is withdrawn and, hence, there is no sudden change in the Sq pattern.
During February–March–April and August–September– October, Maitri station sees DNT, and behaves normally as any low- to mid-latitude station. In the presence of DNT, there is a steady gradual shift of the Sq peak to earlier local times by 30 minutes at Maitri as we move to the subsequent month during February–March–April and August–September–October (Fig. 4(a)). The magnitude of the shift may vary with the location of the station. In the months of absence of DNT (May–June–July and November–December–January), the Sq peak gradually shifts either towards morning or towards evening, from one month to another. However, this shift is small (~10–15 min/month) as compared with that during the months when DNT is present (~30 min/month). The disappearance of DNT marks the decrease in the local time shift of the Sq peak from 30 min to 15 min towards the earlier local times as seen from May–June and November–December. A prolonged absence of DNT marks the reversal in the trend of the LT shift of the Sq peak.
In the absence of DNT, the local ionospheric conditions can vary slowly (quasi-statically) and will cause a steady LT shift in the Sq peak. This is what we observe during November–December–January and May–June–July. The appearance of DNT causes an adiabatic (sudden) change in the local ionospheric conditions and leads to a sudden large LT shift of the Sq peak. That is why we observe the sudden change in LT of the Sq peak from January to February and July to August. Once the DNT appears, again the local ionospheric conditions change to quasi-statically. Therefore, during February–March–April and August–September–October, we observe steady changes in the LT of the Sq peak. Withdrawal of DNT after its continuous presence does not result in adiabatic changes in the local ionospheric conditions because the Sq system remains in the quasi-steady state even after the disappearance of DNT. This could be the reason that we do not see a sudden change in the Sq current pattern during April–May and October– November.
Furthermore, the differences in the intensity of the impinging solar UV radiation can activate different types of neutral wind patterns driving the ionospheric dynamo currents, thus triggering unusual thermotidal activity. The tidal motion activated by the sudden ‘switching on’ and ‘switching off’ of the solar irradiation could be a potential candidate for the modulation of the dynamo currents and a shifting of the Sq peak in the local time.
It is worth noting the observations by Stening et al. (2007), who investigated the seasonal variation of the Sq focus on quiet days. Their Figure 7 shows a sharp equator-ward shift of the southern Sq focus from January to February and a similar equatorward movement of the southern focus was seen during July–August as well. Therefore, the question arises: “is there any association between this equatorward movement of the Sq focus and the observed shift of the Sq peak to later local times over Maitri?” If the answer is “yes”, then a similar shift in the peak Sq time should be present at lower latitudes too. Therefore, it is important to examine the time of the Sq peak at lower latitudes. However, while carrying out such exercise, one should also keep in mind that a tilted Sq current loop is a commonly-observed feature of the Sq system (Mayaud, 1965), and, hence, a shift in the Sq peak will be less prominent at low- to mid-latitudes compared to that at higher latitudes. Also, Chen et al. (2007) found that the latitudinal shift of the Sq current focus is more important than current intensity variation to the day-to-day variability in the Sq daily range. It is possible that some antisymmetric tidal modes become activated, either due to the differences in the extreme conditions of incoming solar UV radiations at higher latitudes or due to a day-to-day variability in the overall atmospheric conditions, which could be responsible for the latitudinal movement of the Sq focus. This aspect of the diurnal Sq-pattern needs to be examined in future studies.
This paper investigates geomagnetic field variations due to a quiet Sq current system observed by a ground-based magnetometer located at a southern sub-auroral latitude. Although the study of the Sq current has a long history and its climatological features have already been well documented, the high-latitude part of Sq is relatively not well studied, partly because the effect of the magnetospheric current is usually intense there. Also, it is useful to know the quiet-time variation as a baseline at sub-auroral latitudes. We have extracted quiet geomagnetic field variations from the magnetic data, and examined the average local time and seasonal variations.
The present study identifies the signatures of the south limb of the solar quiet current loop of the southern hemisphere over the Indian Antarctic station, Maitri (Corrected Geo. Mag. Lat. = 63.11°S), on all the magnetically quiet days (ΣKp ≤ 3) throughout the years 2009 and 2010.
Marked seasonal changes are observed in the H-, and D-components. Because the amount of solar UV radiation received by the ionosphere is highest during summer, the amplitude of the Sq variations is large during summer (a few tens of nT), and weak during winter (a few nTs), exhibiting an annual variation. In spite of the total darkness during winter months, an Sq pattern is identified at Maitri.
The obtained climatological variations are generally in agreement with previous studies, therefore confirming the annual variation of Sq at sub-auroral region.
A discernible change in the phase of the local time variation through the change in time of the peak of the Sq current is evident during particular months. We believe that this may be due to a sudden change in the ionospheric conductivity pattern and/or trigger unusual thermo-tidal activity, due to the transitions from the complete presence, or absence, of sunlight to partial sunlight, and vice versa.
Accordingly, the present paper highlights the role of the extreme conditions of impinging solar UV radiation in the sub-auroral region, which can trigger particular thermo-tidal activity, and can modify the global Sq current pattern. A better picture of the seasonal evolution of the Sq pattern could be obtained using a larger database. This will be a part of our future studies.
We would like to thank Prof. R. Rajaram for useful comments and discussion.
- Butcher, E. C. and G. M. Brown, On the nature of abnormal quiet days in Sq(H), Geophys. J. R. Astron. Soc., 64, 527–537, 1981.View ArticleGoogle Scholar
- Campbell, W. H., Introduction to Geomagnetic Fields, Cambridge University Press, 1997.Google Scholar
- Campbell, W. H. and E. R. Schiffmacher, Quiet ionospheric currents of the northern hemisphere derived from quiet field records, J. Geophys. Res., 90, 6475–6486, 1985.View ArticleGoogle Scholar
- Campbell, W. H. and E. R. Schiffmacher, Quiet ionospheric currents of the southern hemisphere derived from quiet field records, J. Geophys. Res., 93 (A2), 933–944, 1988.View ArticleGoogle Scholar
- Carafella, L., A. Meloni, and P. Palangio, Solar cycle 22 control on daily geomagnetic variation at Terra Nova Bay (Antarctica), Ann. Geofis., 41, 805–811, 1998.Google Scholar
- Chapman, S. and J. Bartels, The Solar Daily Variation on Quiet Days, Sq Geomagnetism, Vol. 1 Oxford University Press, 1940.Google Scholar
- Chapman, S. and K. O. Rajarao, The H and Z variation along and near equatorial electrojet in India, Africa and the Pacific, J. Atmos. Terr. Phys., 27, 559–581, 1965.View ArticleGoogle Scholar
- Chen, G. X., W. Y. Yao, A. M. Du, Y. Y. Wu, B. Chen, and X. C. Liu, Statistical characteristics of the day to day variability in the geomagnetic Sq field, J. Geophys. Res., 112, A06320, doi:10.1029/2006JA012059, 2007.Google Scholar
- Detrick, D. L. and L. J. Lanzerotti, Geomagnetic quiet-time (Sq) variations at high latitudes, Geophys. Res. Lett., 28, 2581–2584, 2001.View ArticleGoogle Scholar
- Gupta, J. C., Movement of the Sq foci in 1958, Pure Appl. Geophys., 110, 2076–2084, 1973.View ArticleGoogle Scholar
- Hanchinal, A., K. Jeeva, A. Dhar, and G. Rajaram, Response of Geomagnetic Field at Indian Antarctic Station Maitri to Increasing Electromag netic Disturbance in Geospace, DOD Scientific Publication No. 10, 47–57, 1996.Google Scholar
- Hibberd, F. H., The geomagnetic Sq variation—annual, semiannual and solar cycle variations and ring current effects, J. Atmos. Terr. Phys., 47, 341–352, 1985.View ArticleGoogle Scholar
- Matsushita, S. and W. H. Campbell, Solar quiet and Lunar Daily variation fields, Physics of Geomagnetic Phenomena, Vol. 1, Academic Press, 1967.Google Scholar
- Mayaud, P. N., Analyse morphologique de la variabilité jour-à-jour de la variation journalière “règulière” SR du champ magnètique terrestre, II.—Le système de courants CM (Règions non-polaires), Ann. Geo phys., 21, 514–544, http://www.ann-geophys.net/21/514/1965/, 1965.Google Scholar
- Miyahara, S. and M. Ooishi, Variation of Sq induced by atmospheric tides simulated by a middle atmosphere general circulation model, J. Geomag. Geoelectr., 49, 77–87, 1997.View ArticleGoogle Scholar
- Nagata, T. and S. Kokubun, An additional geomagnetic daily variation field (Spq-field) in the polar region on geomagnetically quiet day, Rep. Ionosph. Space Res. Jpn., 16, 256–274, 1962.Google Scholar
- Patil, A., B. R. Arora, and R. G. Rastogi, Daily variations of the geomagnetic field near the focus of Sq current system in Indian longitudes, Proc. Ind. Acad. Sci., (Earth Planet. Sci.), 92, 239–245, 1983.Google Scholar
- Patil, A., B. R. Arora, and R. G. Rastogi, Seasonal variations in the intensity of Sq current system and its focus latitude over the Indian region, Indian J. Radio Space Phys., 14, 131–135, 1985.Google Scholar
- Pham Thi Thu, H., C. Amory-Mazaudier, and M. Le Huy, Sq field char acteristics at Phu Thuy, Vietnam, during solar cycle 23: comparisons with Sq field in other longitude sectors, Ann. Geophys., 29, 1–17, doi:10.5194/angeo-29-1-2011, 2011.View ArticleGoogle Scholar
- Rastogi, R. G., S. Alex, and A. Patil, Seasonal variations of geomagnetic D, H and Z fields at low latitudes, J. Geomag. Geoelectr., 46, 115–126, 1994.View ArticleGoogle Scholar
- Richmond, A. D., S. Matuhita, and J. D. Tarpley, On the production mechanism of electric currents and fields in the ionosphere, J. Geophys. Res., 81, 547–555, 1976.View ArticleGoogle Scholar
- Roederer, J., J. Evans, R. Helliwell, L. Lanzerotti, G. Reid, and M. Walt, Study of the Upper Atmosphere and Near Earth Space in Polar Regions, National Academy Press, Washington, D.C., 1982.Google Scholar
- Sillanpaa, I., H. Luhr, A. Viljanen, and P. Ritter, Quiet-time magnetic variations at high latitude observatories, Earth Planets Space, 56, 47–65, 2004.View ArticleGoogle Scholar
- Stening, R. J., Variability of the equatorial electrojet: Its relation to the Sq current system and semidiurnal tides, Geophys. Res. Lett., 18, 1979–1982, 1991.View ArticleGoogle Scholar
- Stening, R. J., T. Reztsova, and L. H. Minh, Variation of Sq focus latitudes in the Australian/Pacific region during a quiet sun year, J. Atmos. Sol-Terr. Phys., 69 (6), 734–740, 2007.View ArticleGoogle Scholar
- Tarpley, J. D., Seasonal movement of the Sq current foci and related effects in the equatorial electrojet, J. Atmos. Terr. Phys., 35, 1063–1071, 1973.View ArticleGoogle Scholar
- Torta, J. M., J. J. Curto, and P. Bencze, Behavior of the quiet day ionospheric current system in the European region, J. Geophys. Res., 102, 2483–2494, 1997.View ArticleGoogle Scholar
- Wang, H., H. Luhr, and S.-Y. Ma, Solar zenith angle and merging electric field control of field-aligned currents: A statistical study of the southern hemisphere, J. Geophys. Res., 110, A03306, doi: 10.10292004JA010530, 2005.Google Scholar
- Xu, W.-Y., Polar region Sq, in Quiet Daily Geomagnetic Fields, edited by W. H. Campbell, pp. 317–393, Birkhäuser Verlag, Basel, 1989.Google Scholar
- Yacob, A. and D. R. K. Rao, Solar cycle and annual variations of Sq(H) at Alibag, J. Atmos. Terr. Phys., 28, 351–360, 1966.View ArticleGoogle Scholar
- Yamazaki, Y., K. Yumoto, T. Uozumi, S. Abe, M. G. Cardinal, D. Mc-Namara, R. Marshall, B. M. Shevtsov, and S. I. Solovyev, Reexamination of the Sq EEJ relationship based on extended magnetometer networks in the east Asian region, J. Geophys. Res., 115, A09319, doi:10.1029/2010JA015339, 2010.Google Scholar