Sounding of sporadic E layers from CSES radio occultation and comparing with ionosonde measurements

Gan, Chengkun; Hu, Jiayu; Luo, Xiaomin; Xiong, Chao; Gu, Shengfeng

doi:https://doi.org/10.5194/angeo-2022-1

Preprints

https://doi.org/10.5194/angeo-2022-1

Preprints

11 Jan 2022

Status: a revised version of this preprint is currently under review for the journal ANGEO.

Sounding of sporadic E layers from CSES radio occultation and comparing with ionosonde measurements

Chengkun Gan¹, Jiayu Hu², Xiaomin Luo³, Chao Xiong^4,5, and Shengfeng Gu¹ Chengkun Gan et al.

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¹GNSS Research Center, Wuhan University, Wuhan 430079, China
²School of Geodesy and Geomatics, Wuhan University, Wuhan 430079, China
³School of Geography and Information Engineering, China University of Geosciences (Wuhan), Wuhan 430074, China
⁴Department of Space Physics, School of Electronic Information, Wuhan University, Wuhan 430072, China
⁵Hubei Luojia Laboratory, Wuhan 430079, China

Received: 06 Jan 2022 – Discussion started: 11 Jan 2022

Abstract. GNSS radio occultation (RO) plays an important role in ionospheric electron density inversion and sounding of sporadic E layers. As the China's first electromagnetic satellite, China Seismo Electromagnetic Satellite (CSES) has collected the RO data from both GPS and BDS-2 satellites since March 2018. In this study, we extracted the carrier to noise density ratio (CNR) data of CSES and calculated the standard deviation of normalized CNR. A new criterion is developed to determine the Es events, that is when the mean value of the absolute value of the difference between the normalized CNR is greater than 3 times of the standard deviation. The statistics show that sporadic E layers have strong seasonal variations with highest occurrence rates in summer season at middle latitudes. It is also found that the occurrence height of Es is mainly located at 90–110 km, and the period of local time 15:00–18:00 is the high incidence period of Es. In addition, the geometric altitudes of a sporadic E layer detected in CSES radio occultation profiles and the virtual heights of a sporadic E layer obtained by the Wuhan Zuo Ling Tai (ZLT) ionosonde show four different space-time matching criterions. Our results reveal that there is a good agreement between both parameters which is reflected in the significant correlation.

Chengkun Gan et al.

Status: final response (author comments only)

RC1:
'Comment on angeo-2022-1', Anonymous Referee #1, 08 Feb 2022

General comments:

The paper deals with radio occultation measurement of sporadic layers. It presents Es occurence rate depending on seasons (Spring, Summer, Autumn) and heights (70 km to 120 km), and global distribution of Es during different seasons. Included is a chapter which compares radio occultation with ionosonde measurement. The paper is well and clearly written. I have following suggestions and questions to the authors:

1. Figure 3 and corresponding text deals with histograms of Es occurence depending on height. The plots show number of Es observations in each height bin (resolution 1 km). My question is: is it possible to present the data as percentage of Es observation / total of measurement rather than absolute numbers? The authors discuss decreased number of Es observations in summer due to lack of data. I think that using relative number rather than absolute number can help with this issue.

2. Figure 7 shows electron density profiles by CSES and ZLT ionosonde. Could you please explain if the Es can be seen in the ionosonde derived profile (I cannot tell if the peak is Es or E layer) and if yes, give details about the electron density computation? Regarding this, I strongly suggest that you show ionogram which shows Es situation, not only computed electron density profiles.

3. The authors claim that the virtual height of Es can be influenced by the ionization of the ionosphere below Es. Can you estimate by how much can the h'Es theoretically differ from real height of Es for your situtation?

4. Could you please provide brief information about the ionosonde used and software which computes electron density profile?

5. Figure 8 shows comparison of radio occultation Es heights vs. ionosonde derived heights. It shows a line y=x. In first two panels I had an impression that it is a regression line. I suggest that you show the regression line and corresponding statistical coefficients describing the regression line.

Small changes:

Please change "we first calculates...", and "then extracts..." in page 3.

Citation: https://doi.org/10.5194/angeo-2022-1-RC1
- AC1: 'Reply on RC1', Shengfeng Gu, 26 Feb 2022
  
  Thank you very much. The detailed point by point reply can be found within the attached document.
  
  Citation: https://doi.org/10.5194/angeo-2022-1-AC1
RC2:
'Comment on angeo-2022-1', Anonymous Referee #2, 11 Feb 2022
The paper deal with the detection of sporadic E (Es) layers on a global scale applying the radio occultation (RO) technique. For their study, the authors use data obtained from the Chinese CSES mission. The authors developed a new algorithm to detect sporadic E signatures from RO profiles. The results show that Es appears mainly at heights between 90-110km and preferably in the summer hemisphere in the local daytime hours. The comparison with co-located ionosonde measurements shows a relatively high correlation between both measurements techniques.

The results more or less confirm what we know about sporadic E layer occurrence from former global studies. The paper does not provide new knowledge on the Es phenomenon. Nevertheless, I support the publication of the manuscript after a careful revision since it introduces the valuable and widely unknown RO data set of the CSES satellite to the community. Please find my detailed comments below.

It would be informative to add some more details about the CSES satellite. At which altitude and inclination is it flying?

Do the RO profiles cover the whole globe? Are the data equally distributed in local time?

Could you add some information to the "Methods" section about the altitude resolution of your RO profiles? What is the signal tracking frequency?

Figures 4-6 are my major point of criticism: Due to the relatively low amount of RO profiles, the plots 4-6 are not very informative and deviate distinctly from existing global Es plots. I recommend increasing the grid size slightly (maybe 10° in longitude) or working with sliding windows of a bigger size.

line 240-243: There is a contradiction between figure 6 and the text. In the text, you write that the high incidence of Es in the local afternoon is related to high solar radiation. In Fig. 4 (summer plot) the values of Es occurrence are of the same magnitude at 3-6 in the morning where there is definitely no sunshine. Could these high early morning values simply be relicts from data availability? Is an effect from the wind possible? Please comment on it.

You show electron density profiles obtained from RO measurements. These profiles are frequently not accurate for smaller-scale ionospheric phenomena since they rely on assumptions like spherical symmetry which is not valid for sporadic E. Could you comment a bit on the assumptions used for calculating the electron density profiles here?

Figure 8: I assume the black line is no regression but x=y line, correct? It is a little bit misleading since there is definitely an offset between both parameters simply because virtual heights are always deviating from geometric ones.

What is the mean offset between both techniques and can this be explained by different altitude systems?

In the lower right plot, there should be 5 couples. I only see 4. Is it convincing enough to calculate a correlation coefficient from 4-5 values only?

Please carefully revise the complete references section. There are many typos and different styles in citing existing literature.

small improvements:

line 185-186. I assume there is a detail missing in this sentence. For me, it is hard to follow your intention.

line 201, 222, and 239: ...results with spring.... "with" is not the correct word here. Please reformulate.
Citation: https://doi.org/10.5194/angeo-2022-1-RC2
- AC2: 'Reply on RC2', Shengfeng Gu, 26 Feb 2022
  
  Thank you very much. The detailed point by point reply can be found within the attached document.
  
  Citation: https://doi.org/10.5194/angeo-2022-1-AC2

Chengkun Gan et al.

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Short summary

A new criterion is developed to determine Es, that is when the mean value of the absolute value of the difference between the normalized CNR is greater than 3 times of the standard deviation. The statistics show that Es have strong seasonal variations with highest occurrence rates in summer season at middle latitudes, mainly locating at 90–110 km, at 15:00–18:00 LT. The comparison of Es altitudes from RO profiles with ionosonde revealed a large correspondence between both measurement techniques.


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