Application of Fengyun 3C GNSS occultation sounder for assessing global ionospheric response to magnetic storm event

Abstract. The rapid advancement of global navigation satellite system (GNSS) occultation technology in recent years has made it one of the most advanced space detection technologies of the 21st century. GNSS radio occultation has many advantages, including all-weather operation, global coverage, high vertical resolution, high precision, long-term stability, and self-calibration. Data products from GNSS occultation sounding can greatly enhance ionospheric observations and contribute to space weather monitoring, forecasting, modeling, and research. In this study, GNSS occultation sounder (GNOS) results from a radio occultation sounding payload aboard the Fengyun 3-C (FY3-C) satellite were compared with ground-based ionosonde observations. Correlation coefficients for peak electron density (NmF2) derived from GNOS Global Position System (GPS) and Beidou navigation system (BDS) products with ionosonde data were higher than 0.9, and standard deviations were less than 20 %. Global ionospheric effects of the strong magnetic storm event in March 2015 were analyzed using GNOS results supported by ionosonde observations. The magnetic storm caused a significant disturbance in NmF2 and hmF2 levels. Suppressed daytime and nighttime NmF2 levels indicated mainly negative storm conditions. In the zone of geomagnetic inclination between 40–80 °, average NmF2 during the geomagnetic storm showed the same basic trends in GNOS measurements, and in observations from 17 ground-based ionosonde stations, and confirmed the negative effect of the event on the ionosphere. The analysis demonstrates the reliability of the GNSS radio occultation sounding instrument GNOS aboard the FY3-C satellite, and confirms the utility of ionosphere products from GNOS for statistical and event-specific ionospheric physical analyses. Future FY3 series satellites, and increasing numbers of Beidou navigation satellites, will provide increasing GNOS occultation data on the ionosphere, which will contribute to ionosphere research and forecasting applications.

The primary findings are that GNOS provides reliable ionospheric NmF2, and that GNOS measurements can be used to observe the average trend of ionospheric NmF2 and hmF2 associated with a geomagnetic storm at mid to high latitudes.The validation of NmF2 is fine, however I have concerns with the results based on averaging of the occultation-derived NmF2, and the characterization of the ionosphere in localized regions based on this averaging.The concerns along with other specific comments are itemized below.
The ionospheric response to the March 2015 storm has been extensively studied using (e.g.Astafyeva et al. 2015, Nava et al., 2016, etc.. a quick Google search reveals many).Discussion and references to these previous studies should be added, as well as consistencies or inconsistencies between GNOS RO observations and pervious results.Also add discussion on Habarulema et al., 2016, "Long-term analysis between radio occultation and ionosonde peak electron density and height during geomagnetic storms", which is directly related to the analysis attempted in your study.Also, GNOS occultation measurements are a valuable contribution to existing RO constellations.It would be worthwhile to add some discussion highlighting the uniqueness of GNOS RO measurements in terms of existing RO capabilities, and the specific RO studies that the high elevation, sun-synchronous FY3-C orbit may allow for.

Specific Comments:
1. P2 L30-37 Please specify receiver and antenna models.3. P3 L33: Equation 1 does not eliminate differential code biases due to receiver and satellite hardware, as implied in the text.Please discuss the techniques applied to account for these biases.2: Direct inversion of the TEC is usually sufficient for obtaining F region ionospheric densities.Is there a reason for using the bending angle inversion? 5. Equation 2: Please provide the method used for obtaining bending angle from the excess phase.Also specify how bending angles above satellite altitude are accounted for (since you are integrating to infinity).

Equation
6. Please indicate whether ionograms were scaled manually or "auto-scaled".Ionospheric parameters derived from manually scaled ionograms are generally more reliable.
7. State the maximum tangent point -ionosonde separation distance used in NmF2 validations.8. Since occultation hmf2 is being used for analysis in this study, it should be validated as well.It shouldn't be too difficult to compare occultation and ionosonde hmf2, similar to the Nmf2 validation.9.The Discussion in Section 3.2 seems to imply that the variations in NmF2 are a geographical effect (e.g.Line 28: "large increases in the South Atlantic region"), however these are observations over a 10 hour period, and thus temporal effects would be large, particularly during a geomagnetic storm.I'm not convinced that a few occultation events over a 10 hour period are sufficient to characterize the ionospheric behavior in a particular geographical region.From Figures 7a-b, the most I would conclude is that equatorial/low latitude NmF2 increases during daytime, and decreases at night.I have the same concerns for the geographical trends are also discussed on Page 7, Lines 17-23.10. Figure 8 averages NmF2/hmF2 from mid-latitude, trough, and auroral regions.Since the ionospheric structure can vary significantly over these regions, please comment on the potential effects of this averaging, and whether the trends shown in Figure 8 would change if only mid-latitudes or auroral regions were considered.
11. On a related note, please comment on the occurrence magnetospheric substorm activity, which can result in significantly enhanced ionization in the nighttime auroral region.I would strongly suggest analyzing mid-latitude and auroral regions separately, instead of a broad region covering 40 to 80 degrees inclination.
12. The standard deviation for each averaged HmF2 and NmF2 value should be shown in Figures 8 and 12, perhaps as error bars.
13. P6 L8: NmF2 is maximum on March 16 according to Figure 8, as opposed to March 17 as stated in the text.
14. P6 L32: Is there an explanation for the few stations that observed a sustained daytime NmF2 enhancement on March 18? 15.Integrating NmF2 over all local times in the top panel of Figure 12 seems meaningless.
16. P7 L5-6: GNOS observations at 40-80 magnetic inclination extend into the auroral region, well poleward of the northernmost ionosonde station (Moscow) in Figure 9, and thus the averaged ionosonde NmF2 wouldn't include significant auroral region effects.This may help explain discrepancies in Figure 12.For completeness, consider including ionosonde measurements from stations north of Moscow, of which there are several.17.P7 L6-7: "..indicating significant differences still exist between the two measurement techniques."Please specify the differences this statement is referring to.
18. P7 L13: Spherical homogeneity of ionospheric density is also a very large assumption at high latitudes (trough, auroral, polar cap regions).
19. P8 L7: Instead of "GNOS data", specify that GNOS NmF2 values are reliable, since this was the only parameter validated in the manuscript.

Technical Corrections:
P1 L16: I suggest "space detection" be replaced with "space-based remote sensing" or something along those lines.P1 L26-28: Unless I'm misinterpreting the intended meaning, this sentence should read something like: "In the zone of 40-80° magnetic inclination, average NmF2 observed by GNOS and 17 ground-based ionosondes showed the same basic trends during the geomagnetic storm.

2. Figure 2 :
Individual hardware components shown in the figure should be labelled.
Figure 1 axis labels are difficult to read.