Introduction
In a recent paper, proposed a sophisticated analysis of the quasi-periodic variations of the solar wind (SW) number density (NSW), referred to as Periodic Density Structures (PDSs; f ≈ 0.45–4.65 mHz), which were observed by two spacecraft (Wind and ARTEMIS-P1) in the interplanetary medium. They obtained results which, in my opinion, are important also in the context of the scientific debate regarding the fluctuations at discrete frequencies observed in the magnetosphere (mostly in range f ≈ 1–5 mHz). Indeed, the occurrence, the origin and the characteristics of these fluctuations (which play an important role in the magnetospheric dynamics) have been examined, in the last decades, in a large number of papers, often with controversial results mainly related to their relationships with SW fluctuations at similar frequencies and to the possible existence (and stability) of sets of favorite magnetospheric frequencies [review by and papers therein referenced]. On the other hand, in recent years, some papers highlighted some critical aspects of the data analysis which might have influenced the conclusions of several investigations. In particular, and applied two different methods adopted in the scientific literature (the Welch method, WM, and the Multitaper method and F-test, MTM) to the same data sets and showed that the WM/MTM agreement in the identification of the wave occurrence and frequency estimate might occur only in ≈50% of cases, both in the SW and in the magnetosphere. In addition, an analysis conducted by revealed different characteristics in the fluctuations of the SW dynamic pressure (PSW) when the same SW stream was observed by two spacecraft at different places in front of the magnetosphere. All these aspects make ambiguous the analysis of the relationships between the SW and the magnetospheric fluctuations; as we discuss in this note, the results proposed by add other interesting elements in this context.
The comparison between PSDs and magnetospheric fluctuations at discrete frequencies
In their analysis,
examined the characteristics of
PDSwith periods ranging from a few minutes to a few hours (radial length scale of tens to several thousands of megameters). In particular, they conducted a robust estimate of the spectral properties of the
NSWfluctuations (they also examined the interplanetary magnetic field’s intensity, not considered in the present note) in the frequency range
f≈ 0.45–4.65 mHz, that were associated with 68
PDSs observed by Wind and ARTEMIS-P1 in front of the magnetosphere over 9 years (2012–2020) and occurring during intervals of high density (maximum values above 15 cm
−3), slow
SWstreams (below 450 km/s); they also determined the level of coherence between the events observed by the two spacecraft, obtaining interesting results. Namely,
- 79 out of 158 events of NSW fluctuations identified by Wind occurred in the same frequency range (within ±0.3 mHz) of the corresponding events detected by P1 (P1 identified 166 events). In practice, for the same SW parcels, comparable frequencies were estimated at the spacecraft positions for about half of events; meanwhile, the frequency to be attributed to the other half of the events that will impact the magnetosphere is uncertain. In this context it is interesting to remind that , who examined the frequencies of PDSs and dayside magnetospheric oscillations in the f = 0.5–5.0 mHz range using 11 years of Wind and GOES observations (1995–2005), reported that in ≈54% of the SW segments with a spectral peak, at least one of the same discrete frequencies was statistically significant in the corresponding magnetospheric data segment. Eventually, according to the results of , this percentage of correspondence between the frequencies of SW and magnetospheric fluctuations might be related to SW events in which the estimated frequencies of fluctuations would have been the same at different places in front of the magnetosphere.
- Considering only the events characterized by high level of coherence (43) between Wind and P1, the percentage of agreement is higher below f ≈ 1 mHz (≈59%), progressively decreasing and practically vanishing at higher frequencies. Reinforcing previous arguments, these conclusions are important in this context in that the frequencies below ≈1 mHz have been rarely explored in the magnetosphere; consequently, the analysis of the relationship between SW and magnetospheric fluctuations could likely have been investigated mostly in a frequency range (f ≈ 1–5 mHz) in which the agreement between the frequencies of SW fluctuations observed at two different places might be poor.
- In extreme cases, moreover, Wind and P1 provided, for the same PDS, very different results: for example, for a parcel observed on 1 January 2014, Wind identified a single fluctuation event (f ≈ 1.8 mHz; Table 1 in ) while four peaks emerged in the power spectra at P1 (f ≈ 0.7, ≈1.5, ≈2.2, ≈3.7 mHz). It confirms that the aspects of the SW fluctuations often differ significantly between the observations of the same SW parcel at different places (; Figure 2). All these arguments suggest caution before assuming a definite identification of the characteristics of the compressive fluctuations impinging the magnetosphere when the event is observed by a single spacecraft ().
- Obviously, the global frequency distributions of events at Wind and P1 are not the same (Figure 5 in ). Nevertheless, in both cases, they manifest the highest occurrence at f ≈ 0.5–0.8 mHz (a frequency range rarely explored in magnetospheric investigations, as previously remarked), with some evidence for enhancements around f ≈ 1.9 mHz, and, less explicit, around f ≈ 2.7–2.9 mHz and f ≈ 3.2–3.8 mHz. Interestingly, in the last 30 years, several papers, proposed the possible existence of frequencies more common than others for magnetospheric fluctuations, in particular f1 ≈ 1.3, f2 ≈ 1.9 (most common), f3 ≈ 2.6–2.7, and f4 ≈ 3.2–3.4 mHz (e. g., ; ; ; ; ; ). In this sense, corroborating the conclusions of several analysis in favor of magnetospheric fluctuations directly driven by compressional SW modes approximately at the same frequencies (; ; ; ; , ), the results of might confirm, at least in a statistical sense, that several magnetospheric fluctuations at discrete frequencies might be associated with the interaction of PDSs with the magnetosphere.
Conclusion
As discussed in the previous paragraph, several results obtained for PDS by might find correspondence in those obtained, over many years, in the analysis of the magnetospheric fluctuations at discrete frequencies (such as the enhancements of the event occurrence at given discrete frequencies, more evident around f ≈ 1.9 mHz … ; review by and papers therein referenced). On the other hand, other aspects such as the general ≈50% of (dis)agreement between the frequencies of fluctuations observed by the two spacecraft (, ) as well as the strong disagreement in the fluctuations content occasionally obtained when the same SW parcel is observed at different places confirm that, as underlined by , further investigations of the relationship between SW and magnetospheric fluctuations should pay careful attention to several critical aspects which may strongly influence the results of the data analysis (i.e., the method of data processing which may be critical for the identification of events; the unambiguous identification of the characteristics of the SW fluctuations impinging the magnetosphere); in addition, it is useful to remind that the magnetospheric response is expected to be different in different regions (and intermixed with concurring local generation processes) and strongly influenced, at least in terms of the frequency of fluctuations, by the daily, seasonal and solar cycle variation of the local resonant frequency.
Statements
Author contributions
UV: Writing–original draft.
Funding
The author declares that no financial support was received for the research, authorship, and/or publication of this article.
Acknowledgments
Thanks to M. Piersanti (University of L’Aquila), this research activity was carried out within the Ministry of University and Research, MUR, with the project PRIN2022 No. 2022ZBBBRY “Characterization of the Lithosphere-Ionosphere coupling during seismic phenomena”, CUP E53D23004560006.
Conflict of interest
The author declares that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
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Summary
Keywords
solar wind, magnetosphere, fluctuations, discrete frequencies, methods of analysis
Citation
Villante U (2025) Comment to the paper “Azimuthal size scales of solar wind periodic density structures” by Di Matteo et al. (2024). Front. Astron. Space Sci. 12:1565669. doi: 10.3389/fspas.2025.1565669
Received
23 January 2025
Accepted
19 February 2025
Published
21 March 2025
Volume
12 - 2025
Edited by
Nithin Sivadas, National Aeronautics and Space Administration, United States
Reviewed by
Qusai Al Shidi, West Virginia University, United States
Updates
Copyright
© 2025 Villante.
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*Correspondence: U. Villante, umberto.villante@aquila.infn.it
Disclaimer
All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article or claim that may be made by its manufacturer is not guaranteed or endorsed by the publisher.