Geomagnetic storms are disturbances of the Earth’s magnetic field caused by the interaction between the solar wind and the Earth’s magnetosphere. Indeed, the solar wind carries part of the solar magnetic field into interplanetary space. This field is carried into interplanetary space by the solar wind and is the interplanetary magnetic field (IMF). It acts as a trigger for a magnetic storm. If the IMF is directed towards the south, in the opposite direction to the Earth’s magnetic field, there is a reconnection between the lines of the IMF and those of the Earth’s magnetic field [1] . This reconnection causes an opening of the Earth’s magnetosphere and thus exposes it to the influence of the solar wind. A geomagnetic storm then occurs. The effects of geomagnetic storms induce ionospheric disturbances called ionospheric storms [2] , characterized by fluctuations in the electron density of the ionosphere, which can disrupt radio and satellite communications, radio wave propagation, ground-based electrical networks, GPS navigation systems [3] . Ionospheric storms are classified into positive and negative storms. Thus, if the electron density of the ionosphere increases during a geomagnetic storm, compared to its density during calm periods, it results in ionospheric disturbances called positive ionospheric storms or positive storms [4] - [6] . On the contrary, if during a geomagnetic storm, the electron density of the ionosphere decreases compared to the density during calm periods, it results in ionospheric disturbances called negative ionospheric storms or negative storms [4] [7] [8] . It is well known that negative ionospheric storms at low and mid-latitudes result mainly from changes in thermospheric composition that make the thermosphere richer in molecular concentration [N2] and poorer in atomic concentration [O] so that chemical recombination becomes faster than normal [9] [10] . As for positive storms at low and mid-latitudes, they result from the combination of the following phenomena during geomagnetic storms: 1) the rapid slowdown of recombination processes and plasma diffusion by the mechanical effects of neutral winds towards the equator [11] - [13] and 2) the rapid strengthening of the equatorial ionospheric fountain by rapidly penetrating electric fields (PPEF) enhanced towards the east [14] - [17] . Ionospheric storms are the responses of the ionosphere to geomagnetic storms.

As for positive storms at low and mid-latitudes, they result from the combination of the following phenomena during geomagnetic storms: 1) the rapid decrease of recombination processes and plasma diffusion by the mechanical effects of neutral winds towards the equator [11] [12] and 2) the rapid strengthening of the equatorial ionospheric fountain by rapidly penetrating electric fields (PPEF) enhanced towards the east [14] - [17] . Ionospheric storms are the responses of the ionosphere to geomagnetic storms.

In the past, several ionospheric parameters have been used to study the responses of the ionosphere to geomagnetic storms. These include the peak electron density (Nmax) [18] - [20] , the critical frequency of the F2 layer (foF2) [21] - [25] , and the Total Electron Content of the ionosphere (TEC) [26] - [31] . These studies have allowed us to understand, among other things, that the responses of the ionosphere to geomagnetic storms depend on the phases of the geomagnetic storm, solar activity, seasons, the geomagnetic position of the station, and times of day. However, important aspects of the responses of the ionosphere still remain to be elucidated, particularly in the sector of the West African equatorial ionosphere, where studies on the responses of the ionosphere to geomagnetic storms are weak. In this manuscript, we use the ionospheric parameter foF2 to study the responses of the ionosphere to geomagnetic storms at a West African equatorial station (Korhogo: Lat. 9.3˚N; Long. 354.6˚E; dip. 0.6˚S). This study contributes to the understanding of the responses of the equatorial ionosphere to weak, moderate, and intense geomagnetic storms. This manuscript consists of two parts. The first part presents the tools and methods used to conduct the study, and the second part presents the results of the study and the discussion.

In this study, the critical frequency of the F2 layer of the ionosphere (foF2) was used to characterize the responses of the ionosphere to geomagnetic storms. The data of this parameter come from the recordings of the Korhogo ionosonde (Lat. 9.3˚N; Long. 354.6˚E; dip. 0.6˚S). This station is an equatorial station that is under the influence of the equatorial electrojet [32] . Figure 1 presents the position of this station. The foF2 data of the station are available on the GIRGEA website: https://www.girgea.org/recherches/logiciels/

The Dst index was used to identify classes of geomagnetic storms. Data from this index are available on the website whose link is: https://wdc.kugi.kyoto-u.ac.jp/dstae/index.html

The Rz index was used to identify the solar phases of the years of the study using the method described by [33] . The data for this parameter are available on the website: https://omniweb.gsfc.nasa.gov/form/dx1.html

The dates of the five (05) calm days of the month were used to identify periods without geomagnetic storms. These dates are available on the site: https://wdc.kugi.kyoto-u.ac.jp/dstae/index.html

The diurnal profiles of foF2 during geomagnetic storm periods correspond to the daily hourly variations of the foF2 averages during storm periods. Similarly, the diurnal profiles of foF2 during storm-free periods (foF2_q) correspond to the daily hourly variations of the foF2 average during the five calmest days of the month.

A magnetic storm has an impact on a foF2 profile if this profile does not have the same characteristics as that of the calm period (period without storms). Otherwise, the storm has no effect on the profile. The types of foF2 profiles and their characteristics are those described by [34] .

According to these authors, there are five types of foF2 profiles in the equatorial region: 1) the “Noon bite out” or “B” type, characterized by a double peak (one occurs in the morning and the other in the evening) and an ionization trough around local noon; 2) the “Dome” or “D” type, characterized by a single ionization maximum during the day; 3) the “Plateau” or “P” type characterized by a constant trend of ionization during the day; 4) the “Morning peak” or “M” type characterized by a single ionization peak in the morning followed by a decreasing trend of foF2 during the day; 5) the “Reversed” or “R” type characterized by the presence of a single ionization peak in the evening.

An ionospheric storm is characterized by relation (1), which has been used by authors such as [35] - [37] .

$\Delta \text{foF}2=\frac{\text{foF}{2}_{\_\text{S}}-\text{foF}{2}_{\_\text{q}}}{\text{foF}{2}_{\_\text{q}}}\times 100\%$(1)

ΔfoF2 is the relative deviation of foF2 in disturbed periods compared to the calm period. foF2__S is the hourly average of foF2 in storm periods and foF2__q is the hourly average of foF2 of the five calm days of the month. A geomagnetic storm generates a positive storm (Resp. negative) if ΔfoF2 ≥ 25% (Resp. ΔfoF2 ≤ −25%) for at least three hours [18] .

Based on the minimum Dst values, geomagnetic storms are classified as follows: Weak storm (−50 nT < Dst ≤ −30 nT), moderate storm (−100 nT < Dst ≤ −50 nT), intense storm (−200 nT < Dst ≤ −100 nT), severe storm (−350 nT < Dst ≤ −200 nT) and extreme storm (Dst ≤ −30 nT) [38] [39] . Based on this classification and taking into account the years of the present study, the occurrence of geomagnetic storms is summarized in Table 1 .

In this work, the responses of the ionosphere to weak, moderate, and intense geomagnetic storms were studied. The ionospheric parameter used is the critical frequency of the F2 layer (foF2), whose data come from the Korhogo station (Lat. 9.3˚N; Long. 354.6˚E; dip. 0.6˚S). The years of study are chosen according to the solar phases: 1993 (descending solar phase), 1995 (solar minimum), 1998 (ascending solar phase), 2001 (solar maximum). The results show that weak storms have no significant effect on all types of diurnal profiles of foF2. However, moderate and intense storms disturb the “Noon Bite Out” type profiles but do not disturb the other types of profiles. The analysis of the evolution of ΔfoF2 shows that all classes of magnetic storms generate positive storms whose peaks are more marked in the night-morning sector. Moderate and intense storms sometimes generate negative storms in the night-morning sector. Positive storms are more significant during intense storms, but negative storms are more marked during moderate storms. These results, which account for the responses of the equatorial ionosphere to a set of geomagnetic storms, open the way to future work on the impact of particular storms on the ionosphere of the same attitude sector.

The authors express their gratitude to the teams of GIRGEA, World Data Center for Geomagnetism, Kyoto, and Omniweb.