Effects of sudden stratospheric warmings on the global ionospheric total electron content using a machine learning analysis

Ma, Guanyi; Hocke, Klemens

doi:https://doi.org/10.5194/egusphere-2024-3177

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https://doi.org/10.5194/egusphere-2024-3177

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28 Oct 2024

| 28 Oct 2024

Status: this preprint is open for discussion and under review for Atmospheric Chemistry and Physics (ACP).

Effects of sudden stratospheric warmings on the global ionospheric total electron content using a machine learning analysis

Guanyi Ma and Klemens Hocke

Abstract. A sudden stratospheric warming is a breakdown of winter stratospheric polar vortex. It has atmospheric effects in both the Northern and Southern hemispheres, leading to disturbances in the whole ionosphere. Previous works with case studies have shown that SSW effect is mainly in low-latitude ionosphere and each SSW event may have a different effect on the ionosphere due to complex dynamics from solar/geomagnetic activities and seasonal changes. However, the SSW induced tidal variability in mid to high-latitude ionosphere is only identified for several events and its behaviour is not well understood. Here we analyze SSWs’ influences on diurnal/semidiurnal variations of global ionosphere with the global maps of total electron content (TEC) from 1998 to 2022. We use machine learning (ML) with neural network to establish the TEC (ML-TEC) model related to the solar/geomagnetic activities and seasonal change from the long-term global TEC data. The TEC variations due to SSWs are extracted by subtracting the ML-TEC from the observed TEC. Comprehensive composite analysis of 18 SSW events shows for the first time a globally SSW-induced enhancement in diurnal/semidiurnal TEC variations. The enhancement is the strongest at equatorial ionospheric anomaly (EIA) crests, moderate in mid-latitude and vague in high-latitude ionosphere. It also exhibits hemispheric asymmetry and longitudinal differences. While the semidiurnal enhancement starts earlier and peaks at ~8 days after SSW onset, the diurnal one starts on the SSW onset day and peaks around 20–30 days after SSW onset. The enhancement of both semidiurnal and diurnal TEC variations lasts to about 50 days after SSW onset. The SSW related E-region dynamo is likely the dominant mechanism which is not strong enough to produce discernible TEC variations in high-latitude ionosphere. ML-TEC does not contain the SSW effect and is thus a valuable reference for the ionospheric state without an SSW.

Received: 13 Oct 2024 – Discussion started: 28 Oct 2024

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RC1: 'Comment on egusphere-2024-3177', Anonymous Referee #1, 15 Dec 2024 reply

Tidal changes in the ionosphere during SSW has been focused for a decade of years. Although it is believed that the changes are global, most of the studies were concerned mainly in the ionospheric variation the low latitudes. Based on the global TEC map data with resolution of 2 hour, and 2.5° latitude *5° longitude, the ionospheric background morphology has been obtained using a neural network algorithm. Further the global distribution of the diurnal and semidiurnal tide components was analyzed using the residual of the TEC data focused on 18 major SSW events in northern winter hemisphere. This study can provide a comprehensive understanding of the global effects on the ionosphere due to SSW. On the whole, the obtained results are clear. But there are still some unclear situations and some discussions about the results are needed further.

Comments:
1, The TEC map data used here are obtained with interpolation algorithm based on about 300 GNSS stations that are very unevenly distributed. Very limited GNSS data can used in the ocean region especially in the southern hemisphere. Usually, the TEC map with such low spatial and temporal resolution are used for revealing the ionospheric background morphology and large ionosphere disturbance, such as ionospheric storm. So it is better to give some analysis about the availability of such TEC data for deriving the tidal components in the study.

2，What is the criteria for the SSW events in Table 1 as MAJOR?
In general, the major SSW event mainly occurs in the winter period of the Northern Hemisphere, which is manifested by the reversal from eastwards to westwards of the zonal wind zonal mean and the increase of the stratospheric temperature in the polar region.
How is the Central date determined, and is it the same time as the SSW onset in Figure 4? The date of the SSW event in 2010 and 2020 is 20100323 and 20200322. Strictly speaking, these two events should be classified as Final Warming, and the background condition of the zonal wind zonal mean during this kind of warming is different with the normal SSW event occurred in winter period.

3， The input layer of the neural network algorithm shown in Figure 1 only takes into account the annual and diurnal variations of the ionosphere. Why doesn't consider the seasonal variation, i.e., the 180-day period variation? The selected SSW events happened in the northern winter period, whether this collection of input layer without seasonal variation component affect the final results?

4，Why the sites of diurnal tidal and semidiurnal tidal component given in figure 3 are different? It may be better to give the diurnal and semidiurnal tidal components during the same event at same site.

5， Figure 6 and Figure 7 give the latitudinal distribution of the diurnal and semidiurnal components at certain meridian line during SSW period, respectively. Why are there no results for the same longitude? It is suggested to provide the latitudinal distribution of the diurnal and semidiurnal component at each meridian lines. In addition, little observational data in the southern hemisphere in the sector of 80 E meridian line, and the map data in this region is basically interpolated, the result in this meridian line is it reliable?
By the way, the vertical line in each figure is not clear.

6， In the discussion section, it is necessary to analyze why the semi-diurnal tides in the mid-latitudes of the Southern Hemisphere are stronger than those in the mid-latitudes of the Northern Hemisphere, and what are the possible mechanisms. In addition, it has been suggested that the semi-diurnal tidal component is probably related to the enhanced semi-lunar tidal (M2) during the SSW, and the discussion about the M2 component enhancement in the Northern and Southern Hemisphere during SSW should be added in the discussion section. The following related papers can be referred.

Goncharenko, L. P., Harvey, V. L., Randall, C. E., Coster, A. J., Zhang, S.-R., Zalizovski, A., et al. (2022). Observations of Pole-to-Pole, Stratosphere-to-Ionosphere Connection. Frontiers in Astronomy and Space Sciences, 8, 768629. https://doi.org/10.3389/fspas.2021.768629
Liu, J., Zhang, D., Goncharenko, L. P., Zhang, S., He, M., Hao, Y., & Xiao, Z. (2021). The latitudinal variation and hemispheric asymmetry of the ionospheric lunitidal signatures in the American sector during major Sudden Stratospheric Warming events. Journal of Geophysical Research: Space Physics. https://doi.org/10.1029/2020ja028859
Jing Liu, Donghe Zhang, Shuji Sun, Yongqiang Hao, Zuo Xiao, Ionospheric Semidiurnal Lunitidal Perturbations During the 2021 Sudden Stratospheric Warming Event: Latitudinal and Inter‐Hemispheric Variations in the American, Asian‐Australian, and African‐European Sectors, Journal of Geophysical Research: Space Physics, 10.1029/2022JA030313, 127, 9, (2022).

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Citation: https://doi.org/10.5194/egusphere-2024-3177-RC1

Guanyi Ma and Klemens Hocke

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Short summary

We analyze the influences of sudden stratospheric warming (SSW) on diurnal/semidiurnal variations of ionosphere with the global total electron content (TEC) from 1998 to 2022. We use machine learning (ML) to establish the TEC (ML-TEC) model related to the solar/geomagnetic activities and seasonal change from the TEC data. Subtracting the ML-TEC from the observed TEC, we find a globally SSW-induced enhancement in diurnal/semidiurnal TEC variations.


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