Dear Dr. Hansen, dear co-authors,

It is my pleasure to say, that the manuscript you submitted to Annales Geophysicae is interesting and brings some interesting/new results of the continuing work on the topic. I have also read the comments of both reviewers. However, referees had some substantive comments/suggestions to the present version of the manuscript, and this is a reason why I suggest a moderate revision. Please, consider carefully and discuss in the revised version of the manuscript the comments of both referees. Me and one of the referees will review the manuscript again.
If you are prepared to undertake the improvements required, please submit the revised manuscript as well as a list of changes or a rebuttal against each point, which is being raised when you submit the revised manuscript.

Kindest regards
Cordially

D. Buresova

Dear Dr. Hansen, dear co-authors,

I have evaluated the most recently received reviews of your article and would like to inform you that the current state of the article requires minor revision. Please consider the comments of the referees (see below) and make the required corrections/additions to the text. Please send us the revised article as soon as possible so that I can conclude the review process.

Kindest regards

Cordially
D. Buresova

# Referee comments
The authors properly answered all reviewers’ questions.
There are only minor revisions to be done before being acceptable for publication.
Line 97: change ‘evets’ by ‘events’
Line 164: ‘marked as black dots’ I think must be ‘marked with dots in the same line color’
Figure 4: the labels ‘e’ and ‘d’ must be changed. (d) is the map

#Referee comments
Revision of "Investigation of the October effect in VLF signals" by Hansen et al.

The topic is very interesting and within the scope of the Annales Geophysicae. The authors have reported some observation data based on the VLF signals shows an abrupt decrease of the amplitude of the detected signal around October, which is known as October effect. In their investigations, they showed large year-to-year variability from 2006 to 2022.They also studied the latitudinal and longitudinal dependency. Some results were expected as the latitudinal dependency, in which the start time of the October effect is early in high latitudes. On the other hand, the longitudinal dependency seems to be something new. According to the author, there is nothing conclusive regarding de temporal variability. So, although I recognize the importance of this study, I am not confident to recommend the publication as it is. So, I have some important aspects that were not clear, that must be addressed before the manuscript is published.

1. In the methodology, the authors were not criterios on the possible effects of the magnetic storm in the ionosphere and, consequently, in the propagation of the VLF wave in this medium under these particular conditions. For example, they stated "During geomagnetic 100 storms the VLF amplitude may be decreased for a couple of days...", however, during a geomagnetic storm two different events occur in the ionosphere: dirtuebed dynamo and prompt penetration electric field. Although those effects are predominant in the F region. but due to the connection between the E and F region though the magnetic field lines, it can reflect the lower part of the ionosphere and the consequences must be different. I am also concerned about how the solar x-ray burst could interfere in the results if they occur around noon? I have a suggestion, how could be the result affected if the authors removed the disturbed period from the data? Maybe, removing those days, the results must be more convincing.

2. Discussion is very superficial. I understand that there is no consensus about the likely mechanism to explain the October effect. Physically, we believe that it could be the result of the modification in the atmosphere since the ionospheric condition remains stable during those periods. The paper by Macotela et al. (2021 and references therein) explores those possibilities that can be used by the authors to improve the discussion.

In summary, the manuscript presents interesting results, but a more criterios method for processing the data and an improvement of the scientific discussion are really necessary to support its publication.

#Referee comments
This well-written paper presents an interesting analysis of the "October effect" in Very Low Frequency (VLF) signals and proposes an in-depth investigation of its latitude and longitude dependencies. While the study has considerable merit, some areas would benefit from further development to strengthen its scientific foundation and methodological clarity before this paper can be considered suitable for publication.

General Comments:

1. The paper describes the "October effect" as a sharp decrease in VLF signal amplitude but lacks a clear physical explanation for this phenomenon. This absence of a more in-depth explanation, supported by previous studies, limits understanding the physics underlying this effect. Further exploration of the mechanisms contributing to the October effect is needed.

2. Using derivatives to identify the "October effect" may be highly sensitive to noise and short-term data variations, increasing the risk of false positives or negatives in detection, especially given the use of a predefined threshold. Does the methodology incorporate any form of supervised classification, or how do the authors address this potential sensitivity to noise?


3. The authors adopt a series of criteria to identify the October effect, including a threshold of 0.005 dB/day². Given the challenges mentioned in the text, this raises questions about the reproducibility of the methodology. Since VLF measurements can be influenced by various external factors (e.g., solar flares, geomagnetic storms) that may cause amplitude fluctuations, it is unclear how this specific threshold was determined. Additionally, would this threshold remain valid for studying the October effect during periods of intense geomagnetic storms, or should data from such periods be excluded? Clarification on these points would strengthen the reproducibility of the detection method.


Minor Comments:

Line 86: Change “doesn’t” to “does not” for a more formal tone.

Figure 1 caption: Correct “propagations paths” to “propagation paths.”

Line 97: Correct the typo “evets” to “events.”

Line 135: Change “show” to “shown”.