This is interesting paper, which deals with another possibility how to study long-term trends in foF2 (critical frequency of ionospheric F2 layer, which corresponds to maximum electron density in the ionosphere). The main advantage of approach suggested by authors is global geographical coverage of foF2 trends compared to local or regional trends analyzed until, which were based on ground-based observations. The utilization of the global empirical model the International Reference Ionosphere (IRI) allows for global coverage. On the other hand, the results are proxy of trends and cannot reach accuracy of local trends but the results are the first results providing some information on foF2 trends globally. I recommend publish the paper after moderate revision.

Comments:

Line 46: “geomagnetic equator” should be “geomagnetic equator in some longitudinal ranges” – to be more correct.

Lines 92-93: The 15^th day of a month need not be good approximation of monthly medians. If the 15^th day occurs in the maximum or minimum of the 27-day variation, then it differs significantly from monthly median. Smooth variation in IRI does not mean no variation. However, we can assume that in the case of large number of data as it is the case of long-term trend investigations the effects of maxima and minima of the 27-day variation essentially cancel out. Nevertheless the usage of monthly medians in future work would increase reliability of results.

Lines 106-108: Lastovicka and Buresova (2023) recommended F30 as the best solar proxy followed by Mg II used by authors. However, I understand that authors prepared their paper essentially before the paper by Lastovicka and Buresova has been published and Mg II appears also useful solar proxy.

Line 161: “Stronger” should be “Weaker” according to Fig. 2 – trends in February and June are only about -1%/decade.

Page 8, Table 4: Some MREs, particularly for 00 LT, are too high – e.g. for Townsville α is not small at 00 LT (the second highest), nevertheless the corresponding MREs are very high. Please make a comment on that in the paper with possible explanation.

Line 218: Delete “(highest values above the geomagnetic equator)” – this is unnecessary and incorrect statement.

Line 269: “represented by” should be “derived from” – this is more accurate.

Wording and misprints:

Line 107: “based in recent” should be “based on recent”

Line 235 and throughout the paper: “valley” – the term used usually in literature is “trough”

Lastovicka, J., Buresova, D., 2023. Relationships between foF2 and various solar activity proxies. Space Weather, 21, e2022SW003359. https://doi.org/10.1029/2022SW003359

This paper explores the use of the semi-empirical IRI model for investigating long-term (multi-decadal) trends in the ionosophere - specifically the height and critical frequency (proportional to the electron concentration) of the main F2 layer. The authors show that there is reasonably good agreement between long-term changes predicted by IRI data with the solar cycle variation removed, and a number of ionosonde stations at different geomagnetic latitudes. There is also fair agreement with the WACCM-X high top chemistry-climate model (used in two previously published studies). The conclusion is that the IRI model, which is straightforward to run on a single processor, is a useful substitute for a whole atmosphere chemistry-climate model.

The major issue that was not clear to me is the role of greenhouse gases in these trends (also raised in the comment by David Themens). The authors state at line 247 "The overall negative trends in both, foF2 and hmF2, is in agreement with that expected from increasing greenhouse concentration. Taking into account that IRI model does not include any forcing linked to these gases, the trends  observed can be attributed to the data." What does this second sentence mean? What is the "data" being referred to? If the IRI model is periodically fitted to ionosonde observations, which are affected by greenhouse gas-induced changes, then it must already implicitly incorporate the effect of greenhouse gases. Although you state at line 80: "According to IRI general specifications, we expect it to somehow force variations linked to changes in the geomagnetic field, since it uses the IGRF model to specify geomagnetic poles and equator, but not those variations expected from the increasing greenhouse gases concentration." This is all very unclear.

Since the IRI model is fitted to ionosonde data, it is surely to be expected that there will be good agreement with the ionosonde data shown in Figs. 3 and 4. It seems rather circular, so I don't understand what the comparison really tests.  It would be very helpful to provide a deeper description for the reader of exactly how the IRI model is fitted to ionosonde data e.g. how often the fitting takes place, over how many stations, are satellite measurements also used? 

Minor points and corrections   

line 75: "used to fix solar..."

line 78: "we decided to ..."

line 90: define CCIR maps

line 162: Figure 2 does not contain upper and lower panels

line 169: "generally good agreement"

line 205: "in the NmF2 trend case..."

line 214: "hmF2, the Cnossen (2020)..."

line 241: "...the Cnossen (2020) negative band"

line 254: "...to the hmF2 case."