The Sun has a magnetic field which results from the differential rotational speeds of its radiative zone and its convective zone. Solar activity is known as the manifestations of the solar magnetic field (Ouattara 2009). This phenomenon occurs according to a fairly regular cycle of around eleven (11) years (Ouattara et al., 2015). In the vicinity of the Earth, certain solar events disturb the terrestrial environment (magnetosphere and ionosphere). These disturbances can give rise to short-term variation in Earth’s magnetic field called, geomagnetic activity. The geoeffectiveness of a solar event is known as its ability  to  generate disturbances on the Earth's magnetosphere and within this structure. In order to identify the solar sources responsible for the various geomagnetic disturbances observed in the vicinity of the Earth, Legrand and Simon (1989) and Simon and Legrand (1989) proposed to divide geomagnetic activity into four classes. Thus, we note (1) quiet activity due to slow solar wind coming from solar heliosheet, (2) recurrent activity due to solar high wind stream coming from coronal holes, (3) shock activity due to Coronal Mass Ejections (CMEs) and (4) fluctuating activity, consequences of fluctuating solar heliosheet. Zerbo et al. (2012) extends the previous study by breaking down the fluctuating activity class. In this study, we are interested in magnetic clouds activities. Indeed, previous studies have shown that around 80% of magnetic clouds are geoeffective (Zhang et al., 2004; Echer et al., 2005).

As a result, most of them are responsible for the most intense geomagnetic storms (Echer et al., 2008a, b). Magnetic clouds are a specialized subset of interplanetary coronal mass ejections (ICME) comprising between one-third (Gosling, 1990) and one-half (Cane et al., 1997) of all ICME events. In terms of composition, these clouds are regions formed by a high magnetic field, low temperature of protons, low amounts of protons and a slowly rotating magnetic field (Burlaga et al., 1981). Geomagnetic storms caused by magnetic clouds are referred to as magnetic cloud activity. Magnetic cloud activity days correspond to the days when magnetic clouds disturb the geomagnetic field and thus cause geomagnetic activity.

The aim of this article is to make a statistical study of magnetic clouds activity days occurrence taking into account the duration of  their  effects  and  the  phases  of solar cycles 11 to 24. These days are identified using the pixel diagram (Ouattara and Mazaudier, 2009). It is important to note that the pixel diagram represents geomagnetic data as a function of solar activity for each Bartels rotation (27 days). They report on the geoeffectiveness of solar events in general and the activities of magnetic clouds in particular.

Determination of magnetic cloud activity days

For this study, we used the values of the index aa and the dates of the SSCs to construct the pixel diagram plots of the year corresponding to this study. This diagram was used to select magnetic cloud activity days. The values of the aa index and the data of the SSCs are available on the ISGI website. The values of the solar index Rz were used to determine the solar phases. They are available on the OMNIWEB website. The different types of magnetic clouds activity are identified from 152-pixel diagrams (representing pixel diagrams from 1868 to 2019). Figure 1 is an example of a pixel diagram (pixel diagram of the year 2000).

Magnetic cloud activity days correspond to SSC days for which Aa indices are between 20 and 40 nT over one, two or three days (Zerbo et al., 2012). Figure 1 show examples of days of magnetic cloud activity of 1 day duration (January 27 activity); 2 days duration (activity from June 23-24) and 3 days duration (activity from May 1-3).

Determination of the different phases of the solar cycle

To determine the solar phases, we used the values of the solar index Rz and adopt the method used by Ouattara  (2009),  Nanéma and Ouattara (2013), Guibula et al. (2017), and Sandwidi et al. (2020). According to this method, the different phases of solar cycle are defined in Table 1.

For small solar cycles, that is, those whose maximum number of spots is less than 100 (case of solar cycles 12, 13, 14 and 16), the phase maximum is obtained for number of spots greater than 0.8 times the maximum spot number (Rz > 0.8 × Rz max).

Referring to Table 2, between 1868 and 2019 there are 37 years belonging to the phase minimum, 28 years belonging to the descending phase, 43 years belonging to the phase maximum and 43 years belonging to the descending phase. During the same period, we recorded: 1227 magnetic cloud activities, an average of 8 magnetic cloud activities per year.

This result is in agreement Wu et al. (2003) who finds approximately nine (9) magnetic clouds per year. Figure 2 shows total number of any type of magnetic clouds recorded per solar phase. We note 450 and 134 magnetic cloud activities respectively at the solar maximum and at the solar minimum. This observation is in agreement with Wu et al. (2006) for whom, at the  solar minimum, very few magnetic clouds are detected; on the other hand, they are much more numerous around the solar maximum. Indeed, according to Turc (2014), magnetic clouds are transient phenomena and at the solar minimum, many of them do not reach the vicinity of the Earth. This could explain their low number at the solar minimum.

Furthermore, Figure 2 shows that magnetic cloud activity days occurrence during solar phases ascending and descending is not the same (233 and 284 respectively in the ascending and descending solar phases). This observation shows that magnetic clouds activity does not follow the sunspot cycle. This result is in agreement with Wu et al. (2003) who find that the frequency of magnetic clouds occurrence does not follow the sunspot cycle or the frequency of CMEs occurrence.

Figure 3 shows the level of activity of magnetic clouds as a function of solar cycles. According to this figure, we note at solar maximum 122 and 51 magnetic cloud activities during cycles 20 and 14 respectively. This makes cycle 20 the cycle in which magnetic cloud activity was most active and cycle 14 the least active in terms of magnetic cloud activity. Analysis of the same figure (Figure 3)  shows  that  the  last  eight  solar  cycles alone record 70.33% of the total magnetic clouds recorded since solar cycle 11. However, there is a continuous decrease in the level of magnetic cloud activity for the last five solar cycles (solar cycle 20 to cycle 24).

We make a statistical study of magnetic clouds activity days’ occurrence taking into account the duration of their effects and the phases of solar cycles 11 to 24. This study statistical study is a contribution in the understanding of Sun-Earth relations. The results show that: (1) the frequency of magnetic cloud ejections depends on the type of magnetic clouds and the solar cycle phase. Different types of magnetic clouds do not have the same occurrences.

However, regardless of the type of magnetic clouds, the phase maximum is the phase of the solar cycle during which the most  magnetic  cloud  ejections  are  recorded.

(2) Whatever the solar cycle, the number of magnetic clouds whose effect lasts 1 day is greater than that whose effect lasts 2 days, which in turn is greater than that whose effect lasts 3 days. (3) For all types of magnetic clouds, cycles 20, 21, 22 and 23 were the four most active cycles in magnetic cloud ejections. Thus, Cycles 14, 12, 13 and 16 are the least active cycles in magnetic cloud ejections for the last fourteen solar cycles studied.

The authors have not declared any conflict of interests

The authors thank ISGI and OMNIWEB for making the data used in this article available.