the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 25 Nov 2025
High-latitude MSTIDs over the EISCAT-3D site: Solar activity and seasonal dependency
Abstract. This work involves an investigation of high-latitude medium scale traveling ionospheric disturbances (MSTIDs) over the newly established EISCAT-3D radar site. We have used the ground backscatter data from an HF radar located at Hankasalmi, Finland (~62.3° N, ~26.61° E geographic coordinates), which is a part of the SuperDARN (Super Dual Auroral Radar Network). Data from solar maximum (2001 & 2014) and minimum (1996 & 2009) years from solar cycles 23 and 24 have been used to investigate the characteristics, seasonal variation, and possible generating sources of high-latitude daytime MSTIDs. Irrespective of the seasons and solar activity conditions, a dominant fraction of MSTIDs propagate equatorward with velocity in the range of 50 – 150 m s-1 and period in the range of 30 – 60 minutes. Their occurrence shows seasonal and solar activity dependency. They normally occur during winter and equinoctial months. During solar maximum conditions, the occurrence was comparatively higher (~72 %) than during solar minimum years (below 50 %). Furthermore, the MSTIDs’ occurrence showed a dependence on IMF Bz, being generally higher during intervals of prolonged northward or southward IMF Bz, and lower during small or fluctuating IMF Bz conditions. Our results indicate that MSTIDs occurrence showed seasonal variation as well as dependence on the solar forcing over the EISCAT-3D location. Therefore, this statistical study will help in providing comprehensive insight about the MSTIDs which will be effective in scheduling future experimental runs of EISCAT-3D to explore their 3-dimensional structures.
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Status: open (until 06 Jan 2026)
- RC1: 'Comment on egusphere-2025-5769', Anonymous Referee #1, 17 Dec 2025 reply
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RC2: 'Comment on egusphere-2025-5769', Anonymous Referee #2, 19 Dec 2025
reply
Summary:
This manuscript presents a statistical investigation of high-latitude daytime medium-scale traveling ionospheric disturbances (MSTIDs) using ground backscatter observations from the SuperDARN Hankasalmi radar, with a particular focus on the region overlapping the newly established EISCAT_3D site. By comparing solar maximum and minimum years from solar cycles 23 and 24, the authors examine seasonal variability, solar activity dependence, and possible external drivers of MSTIDs. The study is generally well structured and carefully executed. However, several aspects of the interpretation require clarification, particularly concerning observational biases related to ground backscatter, the physical interpretation of the IMF Bz dependence, and the limitations of the analysis methods. For these reasons, this reviewer recommends major revision before being published in Annales Geophysicae.
Major comments:
- A key result of the study is the seasonal variation in MSTID occurrence, with higher occurrence during winter and equinoctial months. At the same time, the authors clearly show that ground backscatter (GBS) itself exhibits a strong seasonal dependence. Although the introduction of the “relative occurrence” metric is a reasonable attempt to mitigate this effect, it remains unclear to what extent this approach fully separates true MSTID climatology from observational bias. In particular, it would be helpful if the authors could discuss more explicitly whether the reduced MSTID occurrence during summer reflects a genuine physical reduction in MSTID activity or is primarily a consequence of reduced detectability due to lower GBS occurrence. A clearer discussion of the statistical robustness and limitations of the relative occurrence metric would strengthen the confidence in the seasonal conclusions.
- The dependence of MSTID occurrence on IMF Bz orientation is another important and interesting result of the paper. The finding that MSTIDs are more frequent during prolonged northward or southward IMF Bz conditions is intriguing and consistent with recent studies suggesting a role for reconnection-driven processes. However, the manuscript would benefit from a clearer definition of what constitutes “prolonged” or “steady” IMF Bz, including the thresholds and time intervals used for classification. Furthermore, the physical interpretation deserves more careful treatment, as northward and southward IMF Bz correspond to fundamentally different coupling mechanisms (e.g., lobe reconnection versus dayside reconnection and enhanced Joule heating). While it is plausible that both scenarios can lead to MSTID generation, the manuscript should more explicitly acknowledge these differences and avoid implying a single dominant mechanism without sufficient evidence
- The comparison between MULMEM and FFT-derived periods is a valuable component of the study and highlights important methodological limitations. The discussion of cases where MULMEM appears to overestimate the MSTID period is informative, but it would be beneficial to generalize this finding. In particular, the authors could clarify under what conditions MULMEM is most likely to mischaracterize MSTID parameters (for example, weak signals, overlapping wave modes, or automatic time-series selection). Providing a more quantitative assessment of how frequently such discrepancies occur would further enhance the usefulness of this comparison for future studies.
- While the manuscript places the results in the context of previous high-latitude MSTID studies, the definition of “high-latitude MSTIDs” adopted here could be discussed more explicitly. Because the present study focuses on daytime events detected via ground backscatter at specific radar ranges, it is not immediately clear whether the same population of MSTIDs is being sampled as in optical or incoherent scatter radar studies. A short clarification of how the present observations relate to, or differ from, earlier high-latitude MSTID climatologies would help to better position the contribution of this work.
Minor comments:
- The abstract and conclusions repeatedly emphasize the relevance of the results for future EISCAT_3D operations. While this is an important motivation, some repetition could be reduced to improve conciseness
- Line 112: How do the authors select the offsets between the three cells, i.e., plus-minus 2 in beam and 4 in range. If you change these offsets, would you have different results?
- Figure 2: Meaning of black rectangles should also be described in the caption
- Line 117-118: The authors describe that the derived parameters are used for statistics if they are similar between different cell sets. What is the procedure of this data selection?
- Figure 4d: What is the unit of the relative MSTID occurrence?
- Figure 6: Unit (m/s) should be added to the labels in the radial axis
- Line 210: This reviewer recommends the authors describe how the significant test was applied to the results in more detail
- The criteria used to classify IMF Bz conditions (e.g., thresholds and time windows) would be clearer if summarized in a table or an appendix. In particular, how do the authors evaluate the fluctuation level of IMF Bz?
- There appears to be a typographical error in the discussion section where “MMF Bz” is used instead of “IMF Bz”
- Although the overall quality of the English is good, several long sentences in the Discussion section could be shortened to improve readability
Citation: https://doi.org/10.5194/egusphere-2025-5769-RC2
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- 1
Review of High-latitude MSTIDs over the EISCAT-3D site: Solar activity and seasonal dependency.
By Rathi et al.
The paper presents a study of MSTIDs using observations from the SuperDARN radar at Hankasalmi, Finland. It examines data from four separate years, two of which were solar maximum years, while the other two were solar minimum years. The data were analyzed using a technique called “multichannel maximum entropy method (MULMEM)”, which determines wave characteristics such as period and velocity. Statistics of the MSTID observations were accumulated and compared against various parameters such as season and level of solar activity. The findings of the study largely agree with similar previous studies. There are no major flaws in the paper, but there are several points that should be addressed. The paper may be suitable for publication after revision.
Around line 105 the time window for analysis is described as being 160 minutes, while later points are presented for every half-hour window. Does this mean that the 160 minute window is applied to overlapping data segments that are centered 30 minutes apart? The paper should describe how the window was applied. (The parenthetical reference to the Grocott paper is not enough.)
Around line 113, it is stated that the MULMEM method is used to detect the presence of MSTIDs, however there isn’t any description of the technique or what constitutes a detection. While the technique is described elsewhere, this paper should provide a bit of discussion of at least how MULMEM detects waves and its performance at doing to.
How does the MULMEM technique perform when multiple waves from different directions are present? If multiple waves are detected were they counted as separate events in the occurrence statistics?
The paper should give uncertainty estimates for the parameters determined by MULMEM, and should describe how the estimates were performed.
Around line 125: “The number of such cases are very less and in most of these cases MSTIDs were observed at different times.” The grammar of this sentence is very less than it should be.
Also at line 125, the sentence “The cases where the parameters showed high variation with larger standard deviations and/or scattered values with time...” is too imprecise for scientific writing. “high variation” should be defined. The comparison word “larger” requires something to compare to. It should read something like “with standard deviations larger than XXX…”
At line 130 the paper says that there were cases where MULMEM failed, though it doesn’t say how that failure was determined. To determine that it failed, it must have been known that a TID was present as determined by some other means. Somewhere in the paper, the overall procedure should be discussed. Was a set of intervals selected by visual inspection or was the technique applied to all intervals within the years of interest?
At line138 the paper says that table 1 gives a comparison between the number of MSTIDs detected by MULMEM and the number observed in the RT plots. The text should explain how the number observed in the RT plots was determined.
Around line 154 the paper says “the higher the ionospheric density the greater will be the level of GBS”. What is meant by the “level of GBS”. Does that mean higher SNR? Larger spatial extent?
At line 177 insert “been” in the sentence “Therefore, there are chances that MSTIDs seasonal variation might have influenced by the GBS seasonal variation”
Around line 210 the paper should provide more details of the significance test.
Around line 228 the term “prolonged northward” is used, which is imprecise. The paper should define what is meant by “prolonged”, “fluctuating”, and “near zero”
At line 295: the sentence “To extensively explore this aspect a separate study during different IMF Bz conditions is needed.” should be reworded.