I have read the manuscript "Seasonal features of geomagnetic activity: evidence for solar activity dependence?". The authors present an extended analysis of the semi-annual variation occurrence in various solar wind parameters, geomagnetic indices and the occurrence rates of storms with various magnitudes, substorms and HILDCAAs. Nevertheless, there are points in the manuscript that need further clarification and, moreover, there are certain aspects of the statistical analysis which need further testing. Therefore my suggestion is major revision.

Even though all my comments are included in the attached pdf, I'm pointing out some important comments below.

1) Even though it is not adequately explained, the reader understands that the authors use the monthly mean of the occurrence rate  of substorms, HILDCAAs, etc. to perform statistics. If indeed the authors are using monthly mean of the occurrence, it could introduce several artifacts in the results due to very low values. For example, in figure 1, the occurrence of HILDCAAs or super storms take only a couple of values (0, 1, 2). It would be helpful to provide the same results using the total occurrence rate per month instead of the mean. Another option would be to normalize the monthly occurrence rates with respect to the maximum occurrence for the whole dataset.

 2) The significance level in the Lomb periodogram, as a statistical metric, is much affected by the strongest periodicity (e.g. 11 years). This could result to artifacts when discussing much lower periodicities which statistically be weaker and probably showed below this confidence level. One way to overcome this feature is to filter the time-series in the desired period range (either way the 11 years periodicity is well known and of no importance for the present work). Another way is to limit the Lomb periodogram in the desired range (for example 3 - 24 months).

3) The authors should further discuss the reason why the occurrence of substorms exhibits the semi-annual variation, while the AE index, which is a proxy for substorm activity, does not.

4) The authors should discuss the discrepancies between odd/even and strong/weak cycles after they have clearly stated what a strong/weak cycle is.

Finally, I think that the questionmark in the title of the manuscript is contradicting. If the conclusions of this work are indeed correct, then there is a dependence in Solar activity.

Revision of:

Title: Seasonal features of geomagnetic activity: evidence for solar activity dependence?

Author(s): Adriane Marques de Souza Franco et al.

MS No.: angeo-2021-27

MS type: Regular paper

Summarization:

The work intends to study the Earth’s geomagnetic seasonal features produced by solar influences. Concerning the main geomagnetic activities, the authors consider the Sun-Earth electrodynamics coupling modulation captured by (1) geomagnetic indices: the low-latitude equatorial effects (Dst), planetary effects ap, auroral effects AE, and Akasouf Epsilon parameter, and (2) the solar influences defined by some significant parameters: the solar flux (the F10.7), Interplanetary magnetic field Magnitude Bo, the South-North oriented component (supposed to be in GSM) Bs, the solar wind speed Vsw, the D500 (percentage of the days with the Vsw peak equal or higher than 500 km/s). Although several periods have been obtained by signal analysis from the Lomb-Scargle periodograms, the investigation focus is short cycles, less or equal to an annual variation.

The data interval involves the solar cycles 20 to 24.

General comments:

The work is interesting. The text is well written using clear ideas. The contents are well enchained. The statistical technique is simple. Nevertheless, the current work needs some improvements. Beyond the simple statistical interpretation, some physical discussions are expected to be included.

Suggestions (major remarks):

In the introduction, the authors can describe the supposed sources of the Earth Geomagnetic activity modulations more clearly.

On the one hand: the solar cycle activity (11year), the solar rotation (27 days), the solar activity features (in general lines): the electromagnetic radiation, the corpuscular radiation, plasmas emission phenomena, the heliospheric current region occurrence. On the other hand: The Earth’s translational movement (solstices), the inter-hemispheric symmetry (equinoxes), and the effect of the Coordination reference systems (GSE x GSM, for instance)

Discussion problems not adequately addressed:

Line 115: “...Thus the AE index shows an annual variation...”. There are several geomagnetic stations located only in the North Hemisphere to calculate AE (http://wdc.kugi.kyoto-u.ac.jp/aedir/ae2/AETABLE1.html). The asymmetric pole exposition to the Sun during the Earth translation could contribute to this annual variation.

Line 121: “...VBs exhibits a semi-annual variation...”. It was not declared at work; however, the GSM seems the choice (from the OMNI web service option), which creates a (statistical) artefact. The Bs calculated is affected by a diary cycle (magnetic dipole attitude spinning around the rotation axis) and by a 6-month interval, i.e., this latter concerns the attitude of the Earth rotation-axis during the translation.

Line 136: “...HILDCAAs, on the other hand, exhibit a ~4.1-year periodicity...”. Could it be related to the 11-year solar activity cycle (concerning the ascending and descending phases)?

Line 151: “...On the other hand, the ~1-year periodicity in Vsw/D500 can be a source of the annual variation in the AE index...”. Beyond the Line 121 remark, could the Earth’s slightly elliptical orbit also contribute to it?

Lines 229-270: The Conclusion section will be affected by the earlier remarks. Please, pay attention to other parts of the text. Some written contents in Results Section will require an update of interpretation or discussion.

Minor remarks:

Line 86-87: Please, justify the solar cycles to be grouped into the even and odd ones.

Figure 3: The bottom panel presents colour (VBs and D500) confusingly.

Line 146: write “...in the southern hemisphere...”