Diagnostic study of geomagnetic storm-induced ionospheric changes over VLF signal propagation paths in mid-latitude D-region

Nwankwo, Victor U. J.; Chakrabarti, Sandip K.; Denig, William; Ogunmudimu, Olugbenga; Ajakaiye, Muyiwa P.; Anekwe, Paul I.

doi:https://doi.org/10.5194/angeo-2021-42

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https://doi.org/10.5194/angeo-2021-42

Preprints

06 Aug 2021

Diagnostic study of geomagnetic storm-induced ionospheric changes over VLF signal propagation paths in mid-latitude D-region

Victor U. J. Nwankwo¹, Sandip K. Chakrabarti², William Denig³, Olugbenga Ogunmudimu⁴, Muyiwa P. Ajakaiye¹, and Paul I. Anekwe¹ Victor U. J. Nwankwo et al.

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¹Space, Atmospheric and Radiowave Propagation Laboratory, Department of Physics, Anchor University, Lagos, Nigeria
²Indian Centre for Space Physics, Kolkata-700084, India
³St. Joseph College of Maine, Standish, ME 04084, U.S.A
⁴Department of Electrical Engineering, Manchester Metropolitan University, Manchester, UK

Received: 24 Jul 2021 – Discussion started: 06 Aug 2021

Abstract. We performed a diagnostic study of geomagnetic storm-induced disturbances that are coupled to the lower ionosphere in mid-latitude D-region using propagation characteristics of VLF radio signals. We characterised the diurnal VLF amplitude (from two propagation paths) into five metrics, namely the mean amplitude before sunrise (MBSR), midday amplitude peak (MDP), mean amplitude after sunset (MASS), sunrise terminator (SRT) and sunset terminator (SST). We analysed and monitored the trend in variations of signal metrics for up to 20 storms, to understand deviations in the signal that are attributable to the storms; five storms (and their effects on the signals) were studied in detail, followed by statistical analysis that included 15 other events. Considering the quietient pre-day level following the storm our results showed that the MDP exhibited characteristic dipping in about 67 % and 80 % of the events for GQD-A118 and DHO-A118 propagation paths, respectively. The MBSR showed respective dipping of about 77 % and 60 %, while the MASS dipped by 58 % and 67 %. Conversely, the SRT and SST showed respective dipping of 25 % and 33 %, and 42 % and 47 %. Of the two propagation paths used in this study, the dipping of the amplitude of DHO-A118 propagation path signal is larger (as also observed in previous study). To understand the state of the ionosphere over the signal propagation paths and how it affects the VLF responses, we further analysed virtual heights (h'E, h'F1 and h'F2) and critical frequencies (foE, foF1, and foF2) of the E and F regions (from ionosonde stations near the transmitters). The results of this analysis showed a significant increase and/or fluctuations of the foF2, foF1, h'F2, h'F, h'Es and h'E near both transmitters during the geomagnetic storms. The largest increase in heights of the regions (h'F2, h'F, h'Es and h'E) occured over Juluisruh station (around the DHO transmitter) in Germany, suggesting a strong storm responses over the region leading to the large dipping of the DHO-A118 propagation path signal.

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Victor U. J. Nwankwo et al.

Interactive discussion

Status: closed

RC1:
'Comment on angeo-2021-42', Sergey Sokolov, 07 Sep 2021

The preprint of the article seems to me interesting and quite worthy for its publication.

The authors continued their studies of the effects of geomagnetic storms in the mid-latitude D region, begun in their previous works, for example, in [Nwankwo V. U. J., Chakrabarti S. K. and Ogunmodimu O. Probing geomagnetic storm-driven magnetosphere-ionosphere dynamics in D-region via propagation characteristics of very low frequency radio signals, J. Atmos. Sol-Terr. Phys., 145, 154-169, 2016]. They used VLF data from mid-latitude paths obtained during storms of different intensities and obtained detailed and interesting statistics on the occurrence of VLF signal amplitude anomalies along these paths.

This information itself is very valuable and complements the results of studies of VLF propagation during periods of magnetic storms and substorms carried out over the past decades.

In my opinion, at the end of the article, the authors should say at least a few words about what, in their opinion, are the reasons for the occurrence (or absence) of these anomalies. If these reasons are the precipitation of energetic magnetospheric electrons during and after storms, then in the future, satellite data on such precipitations could be drawn into the data obtained by the authors, of course, if such data are available. In addition, for the periods of storms considered by the authors, it would be possible to analyze the data of riometric measurements, as well as VLF observations on other paths.

P.S. I noticed one typo in the text of the preprint. In line 60, instead of "Kleimenov et al ..." you need "Kleimenova et al ..."

Citation: https://doi.org/10.5194/angeo-2021-42-RC1
- AC1: 'Reply on RC1', Victor U J Nwankwo, 06 Oct 2021
  
  We (authors) thank the reviewer (Referee #1) for accepting and making time to review our manuscript. Your effort and expertise are highly appreciated. It is also encouraging that you identified with relevance of this work in the field.
  In my understanding, the two important suggestions/concerns to address in the paper are (i) to at least infer or confirm the cause of the observed anomalies in the VLF characteristics (e.g., energetic magnetospheric electron precipitation) and (ii) to support the claim with relevant data in the future. You are also of the opinion that we consider the inclusion of riometer measurements in the investigation.
  We have probed further into the dynamics of some of the storms investigated in this work as it affects VLF propagation (in a separate study) by including the ancillary data/information of the timing, classification and location of associated solar flares, coronal mass ejections (CMEs), solar particle events (SPEs), and the timings for the sudden storm commencements (SSCs). With the results at our disposal, I believe we now have the basis to speculate on the cause of the observed anomaly. The details will be updated/included in the revised version of this manuscript (depending on the Editor’s recommendation). Authors have also noted and are now discussing the possibility of including available riometer measurements in future work in order to support the findings.
  We have taken note of the typo and will correct accordingly.
  Thank you very much for your valuable comments.
  
  Citation: https://doi.org/10.5194/angeo-2021-42-AC1
RC2:
'Comment on angeo-2021-42', Anonymous Referee #2, 01 Oct 2021

Report for:

Diagnostic study of geomagnetic storm-induced ionospheric changes over VLF signal propagation paths in mid-latitude D-region by Nwankwo et al.

This paper presents VLF signal analysis over two propagation paths associated with 15-20 geomagnetic storms in the mid-latitude region from September 2011 to October 2012. The authors characterized VLF signal disturbances according to the five metrics/parameters defined at different times in the diurnal variation. Based on analysis they found dipping in five VLF parameters (ranging from 25% to 80% of the analyzed cases) during the storms compared to the respective pre-storm values. Further, the authors added virtual heights and critical frequencies of the E- and F-regions from ionosonde stations nearby the VLF transmitters.

The paper is interesting, however, based on my observation, I recommend major revision with the following modifications.

1) The propagation disturbances of the VLF/LF waves have been extensively studied for several decades showing that the signals are strongly affected by the geomagnetic storms at high and mid-latitudes. However, the previous studies are not properly referred to in the text

and so the results presented by the authors are not properly evaluated with reference to the previous studies. The authors are recommended to state clearly what results are newly added to our knowledge about the VLF propagation disturbances and D-region ionosphere.

2) Authors are advised to mention the significance of the five metrics or how they are connected to the ionospheric variation/properties. What do these five metrics tell us about the D-region ionosphere? This has to be discussed clearly.

3) Authors combined VLF observations of D-region ionosphere with ionosonde observations of E and F regions ionosphere. It will be meaningful to compare the D-region parameters (like electron density, or D-region reference height) deduced from VLF observations with the ionosonde parameters. This is the major concern for the paper.

4) Statistical results should be summarised effectively with one/two figures. Repetition of the same kind of figures confuses the goal of the paper.

5) "The MDP signal appears to be more responsive (about 68% for combined analysis shown in figs 7 and 9) to geomagnetic perturbations than other signal metrics"

A more detailed discussion is needed. For example, how does geomagnetic storm dominates over daytime solar ionization in determining VLF amplitudes?

6) "A rise in SRT and SST amplitude under geomagnetic storm conditions"; what does this mean in connection to ionosphere during the geomagnetic storms? An explanation is needed.

7) What could be the physical reason for "strong storm responses" on DHO path compared to the responses on GQD path, though both the GQD and DHO are almost at the same latitude (GQD is slightly higher). Ionosonde results may also be checked with satellite electron precipitation data in this regard.

8) Figure 2: Mention the name of the transmitters in Caption.

Citation: https://doi.org/10.5194/angeo-2021-42-RC2
- AC2: 'Reply on RC2', Victor U J Nwankwo, 06 Oct 2021
  
  We (authors) thank the reviewer (Referee #2) for accepting and making time to review our manuscript. Your effort and expertise are highly appreciated. We are happy that this effort is appreciated and hope to take advantage of your suggestions to ensure a better version of work.
  We have taken note of your observations and depending on the Editor’s recommendations, we will address them in a revised version accordingly. However, I will provide preliminary short responses to them in the meantime.
  1) This manuscript is a revised work of a paper we submitted in May 2016 (due to extended delay in getting the required data). While it is true that “VLF/LF waves have been extensively studied for several decades”, we consulted and duly cited the works at our disposal at the time (e.g., see lines 56-73). This work is also built on our previous effort (e.g., Nwankwo et al. 2016) in which we cited many other supporting works. However, your observation is noted, and we will include relevant prior work in the revised version.
  2) Some of the factors on which our characterized metrics are based include i) the diurnal signature and ii) the propagation characteristics of VLF narrowband measurements. We will include the significance accordingly. Some authors reported the overall depression of the diurnal signal with respect to a baseline but these metrics allowed us to study both the storm effects and the local time-variant signal responses.
  3) We have already addressed this issue in a separate work in which we combined simultaneously observed VLF variations with TEC data from multiple GNSS/GPS stations (around the transmitter and receiver) to probe geomagnetic storm effects as they propagate down to the lower ionosphere from the magnetosphere. Although there is a revised version of this work, you may look up the idea here: https://www.essoar.org/doi/10.1002/essoar.10504067.1. Appropriate connection between the two will be done in the revised version.
  4) There is an important observation/finding associated with the statistical analysis done here. We have statistically analysed the metrics for (i) 1-day (mean value) before, during and after the storms (figure 7) and (ii) 2-day (mean value) before, during and after the storms (figure 9). Interestingly, the percentage dip of the MBSR and MASS increased significantly in the 2-day mean signals before the events (when compared with the 1-day mean value). It will be challenging to summarise the statistics in one/two figures because of the need to show results of the two propagation paths (GQD-A118 and DHO-A118). Also, the plots need be large enough for readers to see and compare. However, we will look into ways of better summarizing the results.
  5) We will work on this important suggestion and revert accordingly. However, we speculate that the responses are related to positive storm effect which affects, albeit small, the attenuation of the VLF radio waves (Fagundes et al. 2016).
  6) The SRT and SST indirectly relate to ionospheric responses at sunrise and sunset. Our findings show that storms-induced disturbances do not have significant impact on such responses, and since the sunrise and sunset signatures relate to mode conversion in the VLF propagation path this might imply that the D-region density is not a significant contributor to this effect. This and more detailed explanation will be provided in the revised manuscript.
  7) This is a very good scientific question! We observed a trend associated with the DHO-A118 region in our TEC analysis (not included here), which may (to a good extent) address this important question. This will be updated in the revised manuscript. We will also check with satellite electron precipitation data as suggested, and perhaps perform Ovation-Prime auroral model runs for the intervals of interest – see https://www.ngdc.noaa.gov/stp/ovation_prime/data/
  8) The name of the transmitters will be mentioned in the caption as suggested.
  Thank you very much for your valuable comments.
  
  Citation: https://doi.org/10.5194/angeo-2021-42-AC2

Peer review completion

AR: Author's response | RR: Referee report | ED: Editor decision

ED: Publish subject to revisions (further review by editor and referees) (13 Oct 2021) by Keisuke Hosokawa

AR by Victor U J Nwankwo on behalf of the Authors (19 Feb 2022) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (07 Mar 2022) by Keisuke Hosokawa

RR by Sergey Sokolov (28 Mar 2022)

RR by Anonymous Referee #2 (07 Apr 2022)

ED: Publish as is (13 Apr 2022) by Keisuke Hosokawa

Interactive discussion

Status: closed

RC1:
'Comment on angeo-2021-42', Sergey Sokolov, 07 Sep 2021

The preprint of the article seems to me interesting and quite worthy for its publication.

The authors continued their studies of the effects of geomagnetic storms in the mid-latitude D region, begun in their previous works, for example, in [Nwankwo V. U. J., Chakrabarti S. K. and Ogunmodimu O. Probing geomagnetic storm-driven magnetosphere-ionosphere dynamics in D-region via propagation characteristics of very low frequency radio signals, J. Atmos. Sol-Terr. Phys., 145, 154-169, 2016]. They used VLF data from mid-latitude paths obtained during storms of different intensities and obtained detailed and interesting statistics on the occurrence of VLF signal amplitude anomalies along these paths.

This information itself is very valuable and complements the results of studies of VLF propagation during periods of magnetic storms and substorms carried out over the past decades.

In my opinion, at the end of the article, the authors should say at least a few words about what, in their opinion, are the reasons for the occurrence (or absence) of these anomalies. If these reasons are the precipitation of energetic magnetospheric electrons during and after storms, then in the future, satellite data on such precipitations could be drawn into the data obtained by the authors, of course, if such data are available. In addition, for the periods of storms considered by the authors, it would be possible to analyze the data of riometric measurements, as well as VLF observations on other paths.

P.S. I noticed one typo in the text of the preprint. In line 60, instead of "Kleimenov et al ..." you need "Kleimenova et al ..."

Citation: https://doi.org/10.5194/angeo-2021-42-RC1
- AC1: 'Reply on RC1', Victor U J Nwankwo, 06 Oct 2021
  
  We (authors) thank the reviewer (Referee #1) for accepting and making time to review our manuscript. Your effort and expertise are highly appreciated. It is also encouraging that you identified with relevance of this work in the field.
  In my understanding, the two important suggestions/concerns to address in the paper are (i) to at least infer or confirm the cause of the observed anomalies in the VLF characteristics (e.g., energetic magnetospheric electron precipitation) and (ii) to support the claim with relevant data in the future. You are also of the opinion that we consider the inclusion of riometer measurements in the investigation.
  We have probed further into the dynamics of some of the storms investigated in this work as it affects VLF propagation (in a separate study) by including the ancillary data/information of the timing, classification and location of associated solar flares, coronal mass ejections (CMEs), solar particle events (SPEs), and the timings for the sudden storm commencements (SSCs). With the results at our disposal, I believe we now have the basis to speculate on the cause of the observed anomaly. The details will be updated/included in the revised version of this manuscript (depending on the Editor’s recommendation). Authors have also noted and are now discussing the possibility of including available riometer measurements in future work in order to support the findings.
  We have taken note of the typo and will correct accordingly.
  Thank you very much for your valuable comments.
  
  Citation: https://doi.org/10.5194/angeo-2021-42-AC1
RC2:
'Comment on angeo-2021-42', Anonymous Referee #2, 01 Oct 2021

Report for:

Diagnostic study of geomagnetic storm-induced ionospheric changes over VLF signal propagation paths in mid-latitude D-region by Nwankwo et al.

This paper presents VLF signal analysis over two propagation paths associated with 15-20 geomagnetic storms in the mid-latitude region from September 2011 to October 2012. The authors characterized VLF signal disturbances according to the five metrics/parameters defined at different times in the diurnal variation. Based on analysis they found dipping in five VLF parameters (ranging from 25% to 80% of the analyzed cases) during the storms compared to the respective pre-storm values. Further, the authors added virtual heights and critical frequencies of the E- and F-regions from ionosonde stations nearby the VLF transmitters.

The paper is interesting, however, based on my observation, I recommend major revision with the following modifications.

1) The propagation disturbances of the VLF/LF waves have been extensively studied for several decades showing that the signals are strongly affected by the geomagnetic storms at high and mid-latitudes. However, the previous studies are not properly referred to in the text

and so the results presented by the authors are not properly evaluated with reference to the previous studies. The authors are recommended to state clearly what results are newly added to our knowledge about the VLF propagation disturbances and D-region ionosphere.

2) Authors are advised to mention the significance of the five metrics or how they are connected to the ionospheric variation/properties. What do these five metrics tell us about the D-region ionosphere? This has to be discussed clearly.

3) Authors combined VLF observations of D-region ionosphere with ionosonde observations of E and F regions ionosphere. It will be meaningful to compare the D-region parameters (like electron density, or D-region reference height) deduced from VLF observations with the ionosonde parameters. This is the major concern for the paper.

4) Statistical results should be summarised effectively with one/two figures. Repetition of the same kind of figures confuses the goal of the paper.

5) "The MDP signal appears to be more responsive (about 68% for combined analysis shown in figs 7 and 9) to geomagnetic perturbations than other signal metrics"

A more detailed discussion is needed. For example, how does geomagnetic storm dominates over daytime solar ionization in determining VLF amplitudes?

6) "A rise in SRT and SST amplitude under geomagnetic storm conditions"; what does this mean in connection to ionosphere during the geomagnetic storms? An explanation is needed.

7) What could be the physical reason for "strong storm responses" on DHO path compared to the responses on GQD path, though both the GQD and DHO are almost at the same latitude (GQD is slightly higher). Ionosonde results may also be checked with satellite electron precipitation data in this regard.

8) Figure 2: Mention the name of the transmitters in Caption.

Citation: https://doi.org/10.5194/angeo-2021-42-RC2
- AC2: 'Reply on RC2', Victor U J Nwankwo, 06 Oct 2021
  
  We (authors) thank the reviewer (Referee #2) for accepting and making time to review our manuscript. Your effort and expertise are highly appreciated. We are happy that this effort is appreciated and hope to take advantage of your suggestions to ensure a better version of work.
  We have taken note of your observations and depending on the Editor’s recommendations, we will address them in a revised version accordingly. However, I will provide preliminary short responses to them in the meantime.
  1) This manuscript is a revised work of a paper we submitted in May 2016 (due to extended delay in getting the required data). While it is true that “VLF/LF waves have been extensively studied for several decades”, we consulted and duly cited the works at our disposal at the time (e.g., see lines 56-73). This work is also built on our previous effort (e.g., Nwankwo et al. 2016) in which we cited many other supporting works. However, your observation is noted, and we will include relevant prior work in the revised version.
  2) Some of the factors on which our characterized metrics are based include i) the diurnal signature and ii) the propagation characteristics of VLF narrowband measurements. We will include the significance accordingly. Some authors reported the overall depression of the diurnal signal with respect to a baseline but these metrics allowed us to study both the storm effects and the local time-variant signal responses.
  3) We have already addressed this issue in a separate work in which we combined simultaneously observed VLF variations with TEC data from multiple GNSS/GPS stations (around the transmitter and receiver) to probe geomagnetic storm effects as they propagate down to the lower ionosphere from the magnetosphere. Although there is a revised version of this work, you may look up the idea here: https://www.essoar.org/doi/10.1002/essoar.10504067.1. Appropriate connection between the two will be done in the revised version.
  4) There is an important observation/finding associated with the statistical analysis done here. We have statistically analysed the metrics for (i) 1-day (mean value) before, during and after the storms (figure 7) and (ii) 2-day (mean value) before, during and after the storms (figure 9). Interestingly, the percentage dip of the MBSR and MASS increased significantly in the 2-day mean signals before the events (when compared with the 1-day mean value). It will be challenging to summarise the statistics in one/two figures because of the need to show results of the two propagation paths (GQD-A118 and DHO-A118). Also, the plots need be large enough for readers to see and compare. However, we will look into ways of better summarizing the results.
  5) We will work on this important suggestion and revert accordingly. However, we speculate that the responses are related to positive storm effect which affects, albeit small, the attenuation of the VLF radio waves (Fagundes et al. 2016).
  6) The SRT and SST indirectly relate to ionospheric responses at sunrise and sunset. Our findings show that storms-induced disturbances do not have significant impact on such responses, and since the sunrise and sunset signatures relate to mode conversion in the VLF propagation path this might imply that the D-region density is not a significant contributor to this effect. This and more detailed explanation will be provided in the revised manuscript.
  7) This is a very good scientific question! We observed a trend associated with the DHO-A118 region in our TEC analysis (not included here), which may (to a good extent) address this important question. This will be updated in the revised manuscript. We will also check with satellite electron precipitation data as suggested, and perhaps perform Ovation-Prime auroral model runs for the intervals of interest – see https://www.ngdc.noaa.gov/stp/ovation_prime/data/
  8) The name of the transmitters will be mentioned in the caption as suggested.
  Thank you very much for your valuable comments.
  
  Citation: https://doi.org/10.5194/angeo-2021-42-AC2

Peer review completion

AR: Author's response | RR: Referee report | ED: Editor decision

ED: Publish subject to revisions (further review by editor and referees) (13 Oct 2021) by Keisuke Hosokawa

AR by Victor U J Nwankwo on behalf of the Authors (19 Feb 2022) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (07 Mar 2022) by Keisuke Hosokawa

RR by Sergey Sokolov (28 Mar 2022)

RR by Anonymous Referee #2 (07 Apr 2022)

ED: Publish as is (13 Apr 2022) by Keisuke Hosokawa

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Victor U. J. Nwankwo et al.

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We characterised the diurnal VLF amplitude into five metrics – mean amplitude before sunrise (MBSR), midday amplitude peak (MDP), mean amplitude after sunset (MASS), sunrise terminator (SRT) and sunset terminator (SST). We analysed their trend in variations of the signal during geomagnetic storms and found that the signal exhibit significant dipping due to the storms. Responses of high frequency radio pulses also showed evidence of markedly disturbed ionosphere over the signal paths.


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