Behaviour of the quiet-day geomagnetic variation at Livingston Island and variability of the Sq focus position in the South American-Antarctic Peninsula region
© 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. 2010
Received: 24 March 2009
Accepted: 4 November 2009
Published: 4 March 2010
Characteristics of the regular daily variation, including seasonal and solar cycle variabilites, at the relatively new geomagnetic observatory of Livingston Island (Antarctica) have been studied. Such studies of solar cycle variability were possible due to the current availability of more than 11 years of definitive data. The seasonal behaviour of the quiet-time daily field variations are in agreement with those of earlier studies for a mid-latitude observatory placed at the south of the southern hemisphere current focus. We also found a clear dependence of the Sq amplitude on solar activity, although the Sq amplitude maximum occurs about 2 years later than the sunspot maximum. An analysis of contemporary data for solar cycle 23 was carried out for observatories located in the same longitudinal sector, with the aim of identifying the latitudinal displacements of the current focus that affect the observed Sq variations. This was also determined for solar cycle 20 using data from a different set of observatories. The uncertainties associated with the method employed for determining the focus positions are due to the scarcity of observatory data in the South American-Antarctic Peninsula region, but based on our analysis, we can state with a certain reliability that focus latitudes are higher during the summer and at equinoxes than during the winter. However, it is difficult to establish a correlation between focus latitudes and solar sunspot numbers.
Although the ionospheric current system responsible for the regular daily variation has already been derived from observatory magnetograms (e.g., Chapman and Bartels, 1940; Mayaud, 1965; Matsushita and Maeda, 1965), it is interesting to determine the characteristic seasonal and solar activity dependences of the regular variation at a new observatory location. This regular variation is known as “solar quiet variation”, or simply Sq. The difference between this and the variation termed S can be somewhat subjective on occasion. S usually refers to the variation resulting from an analysis carried out on all days except those classified as disturbed, while the term Sq tends to be assigned to the variation observed when only quiet (or those classified as quiet) days are used. When one deals with the regular variation over a single particular day, the term SR is generally used.
The Livingston Island Observatory (62.67°S, 60.39°W, geomagnetic latitude 52.57°S) is operated by the Ebro Observatory Institute (Spain) and was deployed in December 1996 in the vicinity of the Spanish Antarctic Station, which is situated in the South Shetland Islands, north of the Antarctic Peninsula. The observatory has been operating reliably since its installation. The observatory installations consist of three huts. One hut houses the absolute instrument, the so-called D/I fluxgate theodolite, which enables the Declination and Inclination angles of the vector magnetic field to be manually measured in “absolute” terms (for a discussion of the uncertainties associated with this instrument, see Marsal and Torta, 2007). A second hut houses a variometer of the type δD/δI vector magnetometer (Riddick et al., 1995), which automatically measures the variations of the magnetic field vector once per minute. This instrument consists of two perpendicular pairs of Helmholtz coils, the polarization of which allows Declination and Inclination variations to be measured by means of a proton magnetometer located at their centre (see Marsal et al., 2007 for an assessment of this instrument). The proton magnetometer, in turn, measures the total field intensity, F, when the coils are not polarized. The electronic system controlling this automatic instrument is found in the third hut. The International Association of Geomagnetism and Aeronomy (IAGA) has officially recognised the Livingston Island Observatory and given it the code name LIV. It has served as a base station for the reduction of marine magnetic surveys in the zone (Maldonado et al., 2000; Livermore et al., 2000; Catalán et al., 2006), and its data have already been used in several studies and models (Torta et al., 1999, 2001, 2002; Cain et al., 2003; Gaya-Piqué et al., 2006; Marsal and Torta, 2007; Marsal et al., 2007). Apart from a few data gaps due to power supply interruptions, there are presently more than 11 years of definitive data (observatory data which have been corrected for baseline variations and which have had spikes removed) available, with the particular added value that they coincide with the complete solar cycle 23. Thus, it is now possible to obtain a complete picture of the seasonal and solar cycle evolutions of the amplitude ranges and other relevant characteristics of the Sq field at the location of LIV.
To understand Sq behaviour, it is necessary to examine the sources from which such variations originate: the current systems flowing in the so-called ionospheric dynamo region and the induced telluric currents in the Earth’s upper mantle. These currents, in turn, generate additional magnetic field variations that are almost in phase with the primary variations. The morphology of the atmospheric tides gives the ionospheric currents a whorl configuration, with two vortices, one in each hemisphere, and foci at mid-geomagnetic latitudes that occur about 1 h before local noon. The variability of the current intensity and the complicated morphology of the corresponding whorl and its latitudinal or local time displacements with respect to a given observatory location give rise to different patterns of the daily variation of the geomagnetic elements at that specific observatory (Mayaud, 1965). Thus, it is also important to analyse contemporaneous patterns at neighbouring observatories, with the aim of identifying the shape and displacements of the current focus.
Geomagnetic observatories in the South American-Antarctic Peninsula region are rather scarce in comparison with those other regions of the world, such as Europe, North America or Eastern Asia. In those latter, more densely covered areas, it is possible to perform regional harmonic analyses that provide a motion picture representation of the current system, thereby allowing the study of continuous variations with either local or universal time (Haines and Torta, 1994; Torta et al., 1997; Gaya-Piqué et al., 2008; Stening, 2008). However, under regular conditions, the ionospheric current system is generally assumed to remain approximately constant in form over a given day, fixed with respect to the Sun, which is equivalent to assuming that the variations only depend on latitude and local time. For this reason, it is possible to carry out an estimation of the position of the current system focus using data available from only a certain group of observatories distributed in latitude over a narrow sector of the Earth (Stening et al., 2005, 2007).
With all this in mind, the aim of the study reported here was to investigate the variations of Sq at LIV using the data corresponding to quiet days taken from the whole time span of data. We also examined the latitudes of the focus of the southern hemisphere ionospheric current system in the respective sector using the available observatory data. The results of our investigation are discussed relative to those from earlier studies.
The values for the Sq variations were obtained by subtracting from the observatory hourly means, a trend determined by the two local midnight levels on each side of the quiet day. The local midnight level was computed as a four-hourly mean centred on local midnight. The study was carried out on a monthly basis, based on monthly averages using the five International Quiet Days of each month. This selection was chosen for simplicity and could certainly be improved, as it is well-known (Campbell, 1979; Torta et al., 1997; Janzhura and Troshichev, 2008) that special attention has to be paid to the selection of the representative quiet days for the definition of the genuine Sq variation. Furthermore, the magnetic quiescence is defined for astronomical days (0–24 UT), which do not necessarily coincide with the LT days at the given observatories, which are located between 43° and 65° west of the Greenwich meridian. However, stricter selections frequently give rise to months with very few representative days left, or even none at all (Torta et al., 1997). In addition to the scarcity of observatories, observatory data are not complete, and some important gaps are detected from time to time, so those better selections would have precluded obtaining representative results for the whole seasonal and solar cycle spans.
3. Variations at LIV
H amplitudes range from around 10 nT in winter to 30 nT in summer at the solar minimum, and from 15 nT to 70 nT at the solar maximum. D amplitudes range from around 2 arcmin in winter to 8 arc-min in summer at the solar minimum, and from 3 arc-min in winter to 15 arc-min in summer at the solar maximum. Z ranges from around 5 to 25 nT, and from 7 to 65 nT, respectively.
4. Variability of the Sq Focus Position
Using the available data from the chain of observatories described in Section 2, we estimated the variability of the Sq focus position with the method suggested by Stening et al. (2005, 2007). The first step was to determine the time t0 when the variation of D passes through zero at each observatory. The variation △H evaluated at time t0 is then plotted against latitude, and the focus position is obtained from a least squares fit; namely, that latitude at which this line crosses zero. The known strong day-to-day variability of the focus position along with some complexities that differ from the standard Sq current behaviour, which converts the monthly patterns into averages of heterogeneous behaviours on many occasions (Torta et al., 1997), dissuaded us to study month-by-month changes. Thus, the analysis was developed on a seasonal basis (taking the variations of H and D from all the available international quiet days for each Lloyd’s season of each year) and for the time span bounded by the availability of LIV data, i.e., 1997–2007, which roughly coincides with solar cycle 23. Lloyd’s seasons (Lloyd, 1861) at the Southern hemisphere are divided into three categories: (1) winter (months of May, June, July and August); (2) equinoxes (March, April, September and October); (3) summer (January, February, November and December).
To further evaluate the consistency of the procedure, we determined from a different set of observatories the position of the focus for solar cycle 20, which has a maximum amplitude of SSN similar to solar cycle 23. As seen in Fig. 2, the observations used for our analysis of solar cycle 20 were from LQA, PIL, TRW and AIA. The results of this new determination are given in Fig. 1. The focus latitudes for each season can be seen to be similar to those obtained for solar cycle 23, with the exception of equinoxes in 1964 and from 1969 onwards, which tend to appear in higher latitudes. This could be due to the lack of data from TRW in those years, which reduced to three the number of observatories available for the determination. The resultant uncertainties in summer are now larger than those obtained for solar cycle 23.
The major characteristics of the quiet-time daily field variations and their associated current functions found in this study for the Livingston Island Observatory are those expected based on the results from earlier studies. The largest amplitudes of Sq occur during the summer while the smallest occur in the winter. For locations south of the southern hemisphere current focus but still away from the auroral region, H decreases from dawn to about noon, when a minimum is achieved; it later increases up to dusk (the overhead vortex in the northern hemisphere circulates anticlockwise while the overhead vortex in the southern hemisphere moves clockwise). The variations of D were also found to consist of the normal type, the “South type” (Mayaud, 1965), i.e., a minimum in the west followed by a maximum in the east. Z variations show a maximum at about noon, given the clockwise sense of the southern hemisphere current circulation. In winter, however, negative Z variations can appear (see Fig. 3). A plausible explanation for the appearance of the latter can be found in a combination of factors affecting the induced Z signals, which at ground level have an opposite sign compared with the external signal. In general, the induced signals for Sq over the oceans are larger than those over the continents (simply due to their relative higher conductivity). In addition, as already mentioned for the case of the anomalous behaviour detected at PST, oceanic motional induction effects, such as those from ocean tides and global ocean circulation (e.g., Kuvshinov, 2008, and references therein) can also contribute to the variations. Irregularities are always more easily detected in the winter, when the Sq variation is particularly weak, so that the superposition of “contaminating” fields can mask the true Sq variation pattern.
The Sq amplitude is also clearly dependent on solar activity, which can be mainly explained by the effect of ionospheric conductivity. The time lag between both signals may be related to that observed globally between SSN and geomagnetic activity (through the aa index). Although the electrical conductivity is mainly controlled by the extreme ultraviolet (EUV) radiation, which is at its maximum in the toroidal part of the solar cycle (Simon and Legrand, 1989), the effects of the FAC are at a maximum in the poloidal part of the solar cycle via the solar wind (Legrand and Simon, 1989). The poloidal or dipolar field roughly coincides with the minimum of the solar cycle and transforms into a toroidal field producing the migration of the sunspot groups from the pole to the equator. Its maximum coin-(which could be estimated in 2 years), which shows up as an cides with the maximum of the solar cycle. These effects ellipse-like shape (Fig. 5). However, the ellipse of Fig. 5 is would distort the apparent Sq signal by displacing the point not perfect, and it changes towards a lower slope at around of maximum variation and producing the observed delay SSN = 70. This does not agree with the result of Olsen (1993), who found a completely linear dependence between the strength of the Sq and the SSN. In a long-term analysis of different observatories distributed worldwide, Torta et al. (2009) also found that the Y-component Sq range varies linearly with the SSN, regardless of the activity level. However, slightly different behaviours of low and moderate activity were found in that study when the authors plotted the Sq range versus the F10.7 solar radio flux, although this was almost inappreciable between moderate and high activity. These earlier results are in contrast to what we found for LIV in Fig. 5.
Apart from the expected regular behaviour, we identified other properties that are characteristic of any continuous near-Earth geomagnetic recording. For example, even on exceptionally quiet days, one can find examples of residual disturbances still causing certain variations, thereby contaminating the average regular variation obtained from such days. This variation is usually negative and occurs in the late evening (Mayaud, 1980). When these short-term (1– 2 h) disturbances of magnetospheric origin occur around— or close to—local midnight, they can shift the local midnight levels from which the baseline of the daily variation is computed.
Our analysis of the Sq focus position in the South American continent for a complete solar cycle has its origins in a study that had been pending since Kane (1990) proposed it in the conclusions of his analysis. Kane’s analysis was limited to PIL and TRW and to the high sunspot year 1958. New data that have become available for high latitudes, such as data from PST and LIV, have finally allowed us to perform such a study. Our analysis confirms that TRW is, in general, very close to the southern Sq focus, although some movements do exist which depend on the season and the level of solar activity. Focus latitudes are higher during the summer and equinoxes than in the winter. Despite the different methods of analysis, this result does not seem to agree with those of Stening et al. (2007) and the other results discussed there, which showed an equator-ward shift in the southern focus in November (they argued that if the northern focus moves poleward, the southern focus moves equatorward, but we only analysed the southern focus). Our results do agree with the CM4 model (see Fig. 7) and with the results reported by Torta et al. (1997) for Europe and with the fact that, taking into account the change in electron density with latitude (it varies according to the solar zenith angle), the southern focus is expected to be located most poleward during the summer solstice. However, an explanation is still needed as to why the focus at the equinoxes presents such a poleward position, with latitudes that are as high as those observed in the summer—or even slightly higher in some years. We also attempted to find out whether a certain correlation exists between focus latitudes and SSN. Shiraki (1973) studied the latitudinal changes due to solar activity in the West Pacific and North America and concluded that the focus is at a higher latitude during solar quiet years than during solar active years. In our case, such a correlation is difficult to establish due to the lack of sufficient data for such a robust determination (such as in 1997 and 2007) or because in years of solar maximum (such as 2003), many of the five quiet days for each month contain a number of disturbed intervals that prevent a precise determination.
In terms of the time of appearance of the focus, in agreement with Torta et al. (1997), we found it to be closest to noon during the equinoxes. In winter, these times are erratic, probably due to the superposition of field-aligned currents resulting from inter-hemispherical asymmetries, as first suggested by van Sabben (1966). This superposition would give rise to the apparent invasions of one hemisphere’s current pattern by that of the opposite hemisphere (Mayaud, 1965).
The results of this study confirm that Sq is a very changeable phenomenon, with a strong day-to-day variation, and that it is superimposed on magnetic disturbances of a magnetospheric origin that affect the determination of the true Sq variation. This can have repercussions for the derivation of the equivalent ionospheric currents in general and in the determination of their focus position in particular. Averaging over quiet days and seasons tends to smooth out the variability, and a seasonal and solar-cycle characteristic behaviour determination can be attempted, although sometimes the averages are of heterogeneous quantities.
Although we have been able to make a fairly consistent estimation of the focus position based on data from a few well-distributed stations, a dense network of geomagnetic observatories would facilitate the task. For this reason, we note the importance of a good coverage of ground-based observatories in the Southern hemisphere and encourage the corresponding agencies to continue with their recording task for the sake of modelling and understanding the global processes related to this branch of geophysics.
The research results presented in this paper rely on the data collected at several geomagnetic observatories, and we thank the agencies that support them and the individuals who maintain them and collect and process their data. For obvious reasons, we are especially grateful to our colleagues of the Ebro Observatory, who have contributed to installing and maintaining the Livingston Island Observatory since 1995. This research has been supported by Spanish project CGL2006-12437-C02-02/ANT of MEC.
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