Inter-hemispheric field-aligned currents (IHFACs) are one of the major current systems causing changes in geomagnetic field around low and mid latitudes. IHFACs flow from the summer hemisphere to the winter hemisphere in the dawn sector and from the winter hemisphere to the summer hemisphere in the noon and dusk sectors (Fukushima, 1994). Van Sabben (1966) first suggested the existence of IHFACs by using the north-south difference of equivalent Sq current. The IHFACs are caused by inter-hemispheric imbalance of the ionospheric solar quiet (Sq) current system at the mid-low latitudes (Van Sabben, 1966; 1969; 1970), due to asymmetry of the north-south Sq current vortices that established a potential difference between the northern and southern hemispheres (Fukushima, 1979; Tomás et al., 2009). Many numerical calculation studies have predicted IHFACs intensity and supported the dependence of IHFACs current polarity changing with local time (Maeda, 1974; Schieldge et al., 1973; Stening, 1977; Takeda, 1982; Van Sabben,1969, 1970).
Fukushima’s IHFACs model is characterized by (1) IHFACs flow from the summer hemisphere to the winter hemisphere in the dawn sector and the opposite current flows in the noon sector, (2) IHFACs current polarity between the noon and dusk sector is in-phase and (3) the absolute intensity of IHFACs is stronger in both dawn and noon sectors than the dusk sector. The current polarity of Fukushima’s IHFACs model has been supported by in-situ satellite observations and ground-based magnetometer data analysis as well as the above-mentioned numerical studies. The first experimental evidence has been provided from Magsat observations (Olsen, 1997). The morphology of IHFACs has been extensively described by many papers, for example, seasonal climatology of IHFACs by Ørsted satellite (Yamashita and Iyemori, 2002), seasonal, longitudinal and local-time IHFACs climatology by CHAMP satellite (Park et al., 2011) and Swarm satellite constellation (Lühr et al., 2015; 2019; Fathy et al., 2019), ground-based local-time and seasonal dependence (Bolaji et al., 2012; Shinbori et al., 2017; Abidin et al., 2019), latitudinal dependence (Owolabi et al., 2018; Park et al., 2020a).
In recent years, unexpected IHFACs characteristics in dusk sector have been reported by the ground-based magnetometer (Shinbori et al., 2017) and satellite observations (Lühr et al., 2015; 2019; Fathy et al., 2019; Park et al., 2011; 2020a; 2020b). The dusk-side current polarity of IHFACs is often inconsistent with the Fukushima’s IHFACs model. These papers show the dusk-side IHFACs flowing southbound irrespective of season, by using the average value of magnetic field obtained from in-situ satellite or ground-based magnetometer observations for each month in 1-hour local time (LT) bin. Although this analysis revealed the major distribution of IHFACs polarity, a little attention has been paid to the day-to-day IHFACs variations. In addition, their analysis periods were almost limited to demonstrate the solar cycle dependence of IHFACs, namely less than solar cycle period (11 years) (e.g. Park et al., 2011; Owolabi et al., 2018; Lühr et al., 2019). Shinbori et al. (2017), only, used long-term (59 years) ground-based magnetometer data for the comparison of IHFACs intensity between higher and lower solar activity periods. They, however, separated the observational data into two groups and discussed the solar cycle dependence of IHFACs, not examined the day-to-day IHFACs variations. Therefore, the dusk-side IHFACs polarity still remains controversial.
In order to conclude the existence of the dusk-side IHFACs predicted Fukushima’s model, it is important to conduct a long-term time-series analysis of IHFACs variations in terms of day-to-day IHFACs variations. In contrast to the in-situ satellite observations which provides the global distribution of IHFACs on the two-dimensional map (longitude-latitude), the ground-based magnetometer data allows us to investigate the day-to-day IHFACs variations. Especially, the equatorial D-component magnetic fields are used in this paper, since they include more essential effects of IHFACs variation than the equatorial H-component magnetic fields which are dominated by the equatorial electrojet effect (Yamazaki and Maute, 2017).
The field-aligned currents (FACs) observed at the high latitude regions are excited by the plasma environment and its dynamics in the magnetosphere. Their extinction strongly reflects the interaction between the solar wind, magnetosphere, and ionosphere. On the other hand, the IHFAC, which we focus on in this study, is a current system flows along the magnetic field line that is excited by reflecting the asymmetry of ionospheric current system between northern and southern hemispheres. At the low and mid-latitudinal region, electromotive forces for generation of ionospheric current are dynamo effect by the atmospheric wind and penetrated electric field from polar to equatorial ionosphere in which involves variety range of spatiotemporal phenomena. Therefore, a close examination of the IHFAC is very important for understanding the magnetosphere-ionosphere coupling system excited by the solar wind, the global atmospheric motion driven by sunlight, and the electromagnetic environment of the global near-earth system resulting from their coupling. However, most of the related studies that have been carried out so far confirm the existence of IHFACs, and even the morphology of their appearance tendency has not yet been established. Since IHFACs are excited along the magnetic field lines at the mid- and low-latitude regions, their development is known to be remarkably appeared in the east-west component of the ground magnetic field data. The purpose of this study is to understand the long-term variation of the IHFAC using the east-west component of the low-latitude geomagnetic field data continuously observed for two solar activity cycles, and to develop the leading edge of systematic IHFAC research.
The main aim of this paper is to investigate whether the dusk-side IHFACs polarity is completely inconsistent with the current direction predicted by Fukushima’s IHFACs model. The present study puts its focus on analyzing the time-series data analysis of the day-to-day IHFACs variations and using long-term ground-based equatorial magnetometer data from 1998 to 2020 for the examination of the solar cycle dependence in IHFACs. About 22-year long-term analysis enables us to investigate the solar cycle dependence of IHFACs (Fujimoto et al., 2016), which is the secondary purpose of this work. In Sect. 2, we briefly describe observation data, their sources, and data analysis method to determine the variation of IHFACs (∆D). In Sect. 3, we show day-to-day variations of ∆D, climatology of IHFACs polarity and solar cycle dependence of IHFACs. In Sect. 4, we discuss the seasonal dependence of dusk-side IHFACs polarity and solar cycle dependence of periodicity and intensity of IHFACs. We summarize the present study in Sect. 5.
Observation data and Analysis Method
We used long-term MAGnetic Data Acquisition System/Circum-pan Pacific Magnetometer Network Data (MAGDAS/CPMN) magnetometer 1-hour time resolution data at Davao station (Geographical latitude 7˚N, Geographical longitude 124.5˚E, Geomagnetic latitude − 2.22˚N, Geomagnetic longitude 197.9˚E, Dip latitude − 0.24˚) operated by International Center for Space Weather Science and Education (ICSWSE), Kyushu University (Yumoto et al.,1996; 2001; 2006; 2007), from 1998 August-2020 July. The D-component (east-west direction) of magnetic field was analyzed for investigating the characteristics of IHFACs, since the northward (southward) IHFACs induce the westward (eastward) magnetic field variations on the ground. In this paper, “northward (southward) IHFACs” is referred as the current flowing from southern (northward) hemisphere to northern (southward) hemisphere. The westward (eastward) ∆D magnetic field corresponds to the northward (southward) IHFACs.
To derive IHFAC effect from the D-component variation, we first subtracted the base line calculated by using the midnight averaged values from each hour data point. Since the source of the daytime ionospheric wind dynamo is the solar daily radiations, the magnetic effect of this dynamo is generally negligible during the nighttime (Yamazaki and Maute, 2017). Fambitakoye and Mayaud (1976) determined the base level by interpolating linearly between two midnights neighboring the day considered. This derivation manner was adapted in this paper. The residual of D-component variations was defined as ∆D, and ∆D was calculated according to the Eq. (4).
Daily variation of the D component was calculated with respect to the midnight-to-midnight baseline (daily baseline). Midnight data points for the daily baseline were selected as hourly values of previous day 23:00H (Dp23h, the subscript “p” indicates “previous”), previous midnight 00:00H (Dp00h), target day 01:00H (D00h), target day 23:00H (D23h), next midnight 00:00H (Dn00h, the subscript “n” indicates “next”) and next day 01:00H (Dn01h). The baseline was calculated as a linear function which fits for these six data points of adjacent midnights. The linear function is represented as the following Eq. (1) and slope (a) and y-intercept (b) are constants to be determined.
Domain values for x were determined as (-1, 0, 1, 23, 24, 25) according to the function (2) below so that the distance between x values of any adjacent hours become one and the distance between any x values gives the number of hours between those data points.
The linear function constants a and b were calculated using Least Squares Polynomial Fit technique so that the calculated linear function can represent as possible as all data points. The projected data points of the daily base line for any given hour were calculated according to the Eq. (3) below.
Db(h) = ah + b, {h є Z|-1 ≤ h ≤ 25} (3)
D component variation with respect to daily baseline (∆D) is calculated as below. ∆D represent the daily variation of the D component relative to the daily base line. Value of ∆D was calculated as per the equation below for each day.
ΔDh = Dh - Db(h), {h є Z}|0 ≤ h ≤ 23} (4)
F10.7 cm Radio Flux index (F10.7) and Dst index were used to monitor the solar cycle activities and the magnetospheric disturbances, respectively. Daily F10.7 and Dst indexes data were obtained from SPDF OMNIWeb (King and Papitashvili 2005; SPDF-OMNIWeb Service 2020). The analysis periods from 1998 to 2020 have almost covered last two solar cycles SC23 (1996–2008) and SC24 (2008–2019, NASA officially released that SC25 has begun in 2019 December [NASA 2020]).