In this study, the seasonal variations of the EEJ longitudinal profiles were examined based on CHAMP satellite magnetic measurements from 2001 to 2010. 7537 satellite noon-time passes across the magnetic dip-equator were analyzed. The EEJ strength was estimated from the latitudinal profiles of its magnetic signatures, with dense coverage of all longitude sectors. Based on these results, the EEJ longitudinal variation was revisited. On the average, the EEJ exhibits the wave-four longitudinal pattern with four maxima located, respectively, around − 170° E, − 80° E, − 10° E and 100° E longitudes. This confirms the results obtained in previous studies (Alken and Maus 2007; Doumbia et al. 2007; Doumbia and Grodji 2016; Doumouya and Cohen 2004; Jadhav et al. 2002; Lühr et al. 2004). However, a detailed analysis of the monthly averages yielded the classification of the longitudinal profiles in two types. Profiles with three main maxima located, respectively, around − 150° E, 0° E and 120° E, were observed in November, December, January and February. In addition, a secondary maximum observed near − 90° E started in November, December and slightly in January, to finally establish the wave-four patterns from March to October. It is to be noticed that the period of wave-four patterns includes the months of equinoxes (E) and June solstice (J) of Lloyd seasons. According to our observations, this period was divided into a period of transition (March, April, May and October) and a period of well-established wave-four structure (June, July, August and September). The period of transition includes two phases. The first phase consists of transition from three to four maxima in March, April and May, and the second phase consists of a short transition from four to three maxima during October. In summary, the patterns the EEJ longitudinal variation have been divided into three groups of 4 months each: (i) the group of three maxima, (ii) the group of transition and (iii) the group of well-established wave-four pattern. While the first group coincides with December solstice (D) of the Lloyd seasons, the second group spans partially on the equinox (March, April and October) and June solstice (May), the third group covers partially June solstice (June, July and August) and equinox (September).
The locations of the three main maxima of the EEJ longitudinal profiles identified from West to East, respectively, as L1, L3 and L5, have been found to clearly oscillate around average positions in longitude. Thus, L1 and L5 move from the east sides to the west side of, respectively, − 160° E and 120° E meridians, while L3 moves from the west side to the east side of − 10° E meridian. During the transition phase, L1 stabilizes at − 160° E and L5 moves westward across 120° E meridian from March to May and eastward in October. In the transition phase, L2 almost stabilizes at − 80° E meridian, moving very little to east during the rest of time. Another secondary maximum (L4) was also observed near 70° E, but only in January and February.
The results above clearly demonstrate the dependence of the EEJ longitudinal structures on season and confirm the finding of previous studies by Alken and Maus (2007), Doumbia et al (2007) and Doumbia and Grodji (2016). Indeed, those studies have shown that the EEJ longitudinal profiles with three maxima were observed during the December solstice, while the profiles with four maxima were observed during equinoxes and the June solstice of Lloyd seasons. However, results are slightly different for the transition phases and well-established wave-four structures, which are instead inter-seasonal. In addition, it is the first time that the motions of various maxima of the EEJ longitudinal structures have been clearly highlighted. The original features of the present work can be summarized as:
The full CHAMP satellite 10-year magnetic data base was used, which statistically better supports the kind of detailed analysis conducted in this manuscript.
Previous works (Alken and Maus 2007; Doumbia et al. 2007; Doumbia and Grodji 2016; Doumouya and Cohen 2004; Jadhav et al. 2002; Lühr et al. 2004) have only made broad remarks on the EEJ longitudinal variations and attributed different morphologies to December solstice (three maxima) and four maxima for the other seasons. In the present study, these features of the EEJ longitude profiles are captured more finely on a monthly basis.
The present study yielded for the first time a special classification of the EEJ longitude profiles in three main categories as shown in this manuscript. In addition, we have shown how transitions are made from one structure to the other.
Our classification shows that most of the features of the EEJ longitude profiles are inter-seasonal, instead of coinciding with a single particular season.
The motions in longitudes of different maxima of the EEJ longitude profiles in course of the year were examined the first time.
These new features open the way to better perspectives in the analysis of the physical processes that govern the EEJ longitudinal variation, especially the roles of thermospheric winds and their seasonal behaviors in this longitudinal variation (England et al. 2006; Immel et al. 2006; Lühr et al. 2008).
The structures and seasonal dependence of the EEJ longitudinal variation have been considered to be linked with the wave structures of the thermospheric winds (Doumbia et al. 2007; Doumbia and Grodji 2016; Immel et al. 2006; Kil et al. 2007; Lühr et al. 2008; Lühr and Maus 2006). In the ionosphere, winds and electric fields are known to be modulated by the tidal excitations that propagate upward from lower atmospheric layers. Doumbia et al. (2007) simulated such tidal excitations for migrating tides diurnal and semi-diurnal components based on the National Center for Atmospheric Research Thermosphere–Ionosphere Electrodynamics General Circulation Model (NCAR TIEGCM). Furthermore, the wave structures of the thermospheric winds described in many studies (Häusler et al. 2007; Häusler and Lühr 2009; Immel et al. 2006), have been found to exhibit similar longitudinal variations. In a companion paper, the influence of thermospheric winds will be examined with combined migrating and non-migrating tidal excitations, for a better understanding of the background physical processes involved in the transition between the various EEJ longitudinal patterns.