A Study of Solar Flare Effects on the Geomagnetic Field Components during Solar Cycles 23 and 24

Round 1

Reviewer 1 Report

Comments on “A study of solar flare effects on the geomagnetic field components during solar cycles 23 and 24” by F O Grodji et al. to Atmosphere journal [2021]

 General:

This study mainly aims at investigating the latitudinal, longitudinal and solar zenith angle dependent variations in geomagnetic crochets in ‘H’ and ‘D’ and ‘Z’ components due to different types of solar flares for the years: 1996-2018 in the sunlit side of the earth during the Solar Cycle (SC)-23/24. The study utilizes INTERMAGNET data across different stations to identify the various correlations. The study finds that solar flares impact the geomagnetic crochets which are found to maximize around equinox than other seasons. It is found that magnetic crochets of the H component were positive at low latitude but negative between the high-latitudes and mid latitudes in both hemispheres. However, unlike H component, the Y and Z components showed that their patterns of variations were not coherent in latitude. The peak amplitude of the solar flare effect (sfe) depends on many parameters such as local time, solar zenith angle, latitude and position of solar flare on the sun and intensity of solar flare. Also these peaks were stronger for the stations around the magnetic equator and very low when the geomagnetic field components were close to their nighttime values. While both cycles showed similar monthly variations, the sfes were higher in solar cycle-23 than in solar cycle-24. The maximum impact of sfes were seen for M and X class flares; however, no significant sfes were noticed for A and B classes. The maximum impact has been noticed when the solar zenith angle is approaching zero and position of active region on Sun. Based on these results, authors have described their results in reference to other similar studies. So, most of these results are based on large data analysis and many of these results are known. However, they are done using basic statistical methods. Some good and new results are also brought-in using the large data and also components data. In this regard, the manuscript is presenting characteristics of different components of magnetic crochets with good data analysis and appropriate discussions were made. Accordingly, I believe it may be suitable for consideration after few revisions.

I would like to suggest the following questions/comments:

1. It is well known that sfes are supposed to have positive and negative crochets of H component over low and mid latitudes. However, why the positive and negative crochets occur in the first place? Please provide reasons for these different variations for low and mid latitudes. Whether it is the E region electron density or primary electric fields or additional currents that are responsible for such variable crochets at low and mid latitudes?
2. It is understandable that positive/negative crochets of H component can be due to sudden variations in the conductivity and also due to vertical drifts. However, how to understand the positive/negative crochets of ‘Y’ and ‘Z’ components of magnetic field?
3. Why sfes were seen to have seasonal dependence with equinox being maximum? Whether it is due to more number of solar flares or any other reason?
4. Recent studies show that there can be both positive and negative crochets of delta H at the magnetic equator and mostly these crochets correlate well with the vertical drifts. However, new studies suggest that delta H and vertical drifts can have opposite direction. Please provide how your results may help us understand them.
5. I highlighted some places where there are issues in English or some other mistakes in the manuscript corrections.
6. I also find that following reference is missing which also talks about the SFEs on the equatorial ionosphere using X-class flare based on INTERMAGNET observations. It is: Sripathi, S., N. Balachandran, B. Veenadhari, R. Singh, and K. Emperumal (2013), Response of the equatorial and low-latitude ionosphere to an intense X-class solar flare (X7/2B) as observed on 09 August 2011, J. Geophys. Res. Space Physics, 118, 2648–2659, doi:10.1002/jgra.50267.
7. Similarly, the following reference is also useful in this study which is missing: Annadurai, N. M. N., Hamid, N. S. A., Yamazaki, Y., & Yoshikawa, A.(2018). Investigation of unusual solar flare effect on the global ionospheric current system. Journal of Geophysical Research: Space Physics, 123, 8599– 8609. https://doi.org/10.1029/2018JA025601

Comments for author File: Comments.zip

Author Response

Dear  reviewer 1.
the corrections in the report are in red color. 
thank you 

Author Response File: Author Response.docx

Reviewer 2 Report

Comments to Manuscript ID: atmosphere-1487312

Title: A study of solar flare effects on the geomagnetic field components during solar cycles 23 and 24

Authors: Oswald Didier Franck GRODJI *, Vafi DOUMBIA *, Paul Amaechi OBIAKARA*, Christine Amory-Mazaudier, Kouassi N'GUESSAN, Kassamba Abdel Aziz DIABY, Tuo ZIE, Kouadio BOKA

Ln 111-113 “Among the 6200 SFs  observed, we selected only 289 that had a clearly visible magnetic impact on the geo-magnetic field components.”

Not any movement on the magnetograms is a Sfe. Authors should check and refer their list against the “official” ones provided by the SRMV (http://www.obsebre.es/en/rapid)

Ln 124-125 “Then we estimated the solar flare effect (sfe) following 124 the approach described in Dmitiev (2008), Ugonabo et al. (2013) Taat et al. (2019).”

This is the weakest point of the paper. The method of Ugonabo et al. (2013) is coarse and has an oversimplification by taking the Sq variation as irrelevant during the Sfe time. Sometimes, especially during long Sfe, Sq variation is higher than that of the Sfe. So, part of the statistics developed using this method could be not enough precise.

Ln 148-149 “The A, B and C class flares are 148 represented by the gray, green and blue color, respectively.”

Ln 157 “Variation of solar flare occurrence during solar cycles 23 and 24. A-class (gray), B-class (Green), C-class (Blue),”

• A, B and C are irrelevant for the study the authors take over.
• Gray color is not in the palette of colors presented in this figure. Do you mean blue?

Ln 172 “We observed a total of 289 SF, 73 of which were of class X and 216 of class M.”

According of what was said in chapter 2.1, the authors considered 6200 Solar Flares and from them only 289 produced Sfe. So, authors observed Sfe, not SF.

Ln 185 “25 October 2013.”

After the point there is a weird dash.

Ln 236 “Figure 5”

This kind of representation seem more appropriate for showing the Sq than the Sfe. In the best cases, Sfe appear as spikes.

Ln 414 “Figure 12”

The number of Sfe per month is very reduced so its statistical representativity is poor. Long error bars indicate that these measures have a high variability and thus the mean value could be not very relevant. So, conclusions on seasonal variability and even that on cycle comparison should be taken with caution.

Ln 427 “C and M-class flares were strongly correlated”

The number of C and M-class flares were strongly correlated

Ln 429-431 “This negative correlation between 429 sunspots and classes A and B is due to the fact that both classes of flare occur more frequently and in great number increase great number during the solar minimum (figure 2) and reduce during solar maximum.”

Small SF correlate with solar cycle. Simply they are lagged respect the solar cycle. This is a clear example why a simple linear correlation is not appropriated to check the relationship between both phenomena.

Ln 436 “100 X class solar flares occur every 125,000 years”

This of course is very extreme, but big flares occur more often:

Sfe: waiting for the big one. Curto, J.J., Castell, J., Del Moral, F.: Journal of Space Weather and Space Climate, 6, A23, 1-8,, DOI: 10.1051/swsc/2016018, 2016.

Ln 439-441 “The reasons for this include the fact that these flares occurred during strong magnetic disturbance when the irregular variation of the geomagnetic field components masked the magnetic crochet”.

This is only one of the reasons for a SF not having a visible magnetic signature. There is abundant literature about this (eg. :

Geoeffectiveness of Solar Flares in magnetic crochet (sfe) Production:  II - Dependence on the detection method. Curto, J.J., Gaya-Piqué, L.R. J. Sol. Atmos. Terr. Phys., 71, 1705-1710. DOI: 10.1016/j.jastp.2007.12.003, 2009

Geomagnetic Solar Flare Effects: a review. Curto, J.J.: Journal of Space Weather and Space Climate, 10, 27. doi: 10.1051/swsc/2020027, 2020.

Ln 441 “Also the flare occurred on a surface of the Sun that did not face the Earth.”

This is not a reason that few SF produce Sfe. If a SF occurred on a surface of the Sun that did not face the Earth, then it was not observed by the satellites and not computed as SF!

Ln 443-444 “we selected only 289 that had a clearly visible magnetic impact on the geomagnetic field components.”

I insist that personal list could have bias and be not representative of true Sfe. So, official lists should be taken as reference.

Ln 444-445 “This represented a percentage of about 5%.”

Most of the 6200 SF considered by the authors have not enough energy to disturb the ionosphere, so it is clear that this figure is not significative.

Ln 445 “about 5%. The 289 SFs were studied taking into consideration”

I suggest transform the stop to full stop (new paragraph). Authors introduce a new subject.

Ln 451 “the effects of solar flares on the Ionosphere-Thermosphere coupling depend on the location of the solar flare on the solar disk”

This is not completely true. See Curto & Gaya, 2009:

Geoeffectiveness of Solar Flares in magnetic crochet (sfe) Production: I - Dependence on their spectral nature and position on the solar disk.  Curto, J.J., Gaya-Piqué, L.R.: J. Sol. Atmos. Terr. Phys., 71, 1695-1704. DOI. 10.1016/j.jastp.2008.06.018, 2009

Ln 454 “limb flare has a smaller effect than a central flare on the ionosphere”

This is due to the EUV absorption in the solar atmosphere.

Ln 466 “Thus, we note that the stations around to magnetic equator during a flare recorded the highest peak of the magnetic crochet.”

This is due to the high conductivity of this region due to the equatorial electroject.

Ln 475 “the magnetic rocket on the H component”

??. I guess authors refer to magnetic crochet

Ln 478-479 “Also, magnetic crochets are not observed in stations synchronously, but separated by a relatively short time (a few seconds).”

Also, magnetic crochets are not observed synchronously (a few seconds), in stations separated by a short distance.

Author Response

Dear  reviewers 2.
the corrections in the report are in red color. 
thank you 

Author Response File: Author Response.docx