U. Villante
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- Dipartimento Scienze Fisiche e Chimiche, Università and Area di Ricerca in Astrogeofisica, L’Aquila, Italy
Introduction
In a recent paper, Di Matteo et al. (2024) 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 Di Matteo and Villante (2024) 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, Di Matteo and Villante (2017) and Di Matteo and Villante (2018) 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 Villante et al. (2022) 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 Di Matteo et al. (2024) add other interesting elements in this context.
The comparison between PSDs and magnetospheric fluctuations at discrete frequencies
In their analysis, Di Matteo et al. (2024) examined the characteristics of PDS with 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 NSW fluctuations (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 SW streams (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 Viall et al. (2009), 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 Di Matteo et al. (2024), 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 Di Matteo et al., 2024) 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 (Villante et al., 2022; 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 (Di Matteo and Sivadas, 2022).
- Obviously, the global frequency distributions of events at Wind and P1 are not the same (Figure 5 in Di Matteo et al., 2024). 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., Samson et al., 1991; Ruohoniemi et al., 1991; Samson et al., 1992; Walker et al., 1992; Francia and Villante, 1997; Villante et al., 2001). 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 (Kepko et al., 2002; Kepko and Spence, 2003; Villante et al., 2007; Viall et al., 2009; Villante et al., 2013, 2016), the results of Di Matteo et al. (2024) 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 Di Matteo et al. (2024) 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 Di Matteo and Villante (2024) 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 (Di Matteo and Villante, 2017, Di Matteo and Villante, 2018) 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 Di Matteo and Villante (2024), 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.
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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References
Di Matteo, S., Katsavrias, C., Kepko, L., and Viall, N. M. (2024). Azimuthal size scales of solar wind periodic density Structures. Astrophys. J. 969 (67), 67. doi:10.3847/1538-4357/ad479e
Di Matteo, S., and Sivadas, N. (2022). Solar-wind/magnetosphere coupling: understand uncertainties in upstream conditions. Front. Astron. Space Sci. 9. doi:10.3389/fspas.2022.1060072
Di Matteo, S., and Villante, U. (2017). The identification of solar wind waves at discrete frequencies and the role of the spectral analysis techniques: the Identification of Solar Wind Waves. J. Geophys. Res. 122 (5), 4905–4920. doi:10.1002/2017JA023936
Di Matteo, S., and Villante, U. (2018). The identification of waves at discrete frequencies at the geostationary orbit: the role of the data analysis techniques and the comparison with solar wind observations. J. Geophys. Res. 123, 1953–1968. doi:10.1002/2017JA024922
Di Matteo, S., and Villante, U. (2024). Simultaneous occurrence of magnetospheric fluctuations at different discrete frequencies (f ≈ 1 – 5 mHz): a review, sub. Space Sci. Rev. C.
Francia, P., and Villante, U. (1997). Some evidence of ground power enhancements at frequencies of global magnetospheric modes at low latitude. Ann. Geophys. 15 (17), 17–23. doi:10.1007/s00585-997-0017-2
Kepko, L., and Spence, H. E. (2003). Observations of discrete, global magnetospheric oscillations directly driven by solar wind density variations. J. Geophys. Res. 108 (A6), 1257. doi:10.1029/2002JA009676
Kepko, L., Spence, H. E., and Singer, H. J. (2002). ULF waves in the solar wind as direct drivers of magnetospheric pulsations: ULF waves in solar wind. Geophys. Res. Lett. 29 (8), 39–1394. doi:10.1029/2001GL014405
Ruohoniemi, J. M., Greenwald, R. A., Baker, K. B., and Samson, J. C. (1991). HF radar observations of Pc 5 field line resonances in the midnight/early morning MLT sector. J. Geophys. Res. 96 (A9), 15697–15710. doi:10.1029/91JA00795
Samson, J. C., Greenwald, R. A., Ruohoniemi, J. M., Hughes, T. J., and Wallis, D. D. (1991). Magnetometer and radar observations of magnetohydrodynamic cavity modes in the Earth’s magnetosphere. Can. J. Phys. 69 (8-9), 929–937. doi:10.1139/p91-147
Samson, J. C., Harrold, B. G., Ruohoniemi, J. M., and Greenwald, R. A. (1992). Field line resonances associated with MHD waveguides in the magnetosphere. Geophys. Res. Lett. 19 (5), 441–444. doi:10.1029/92GL00116
Viall, N. M., Kepko, L., and Spence, H. E. (2009). Relative occurrence rates and connection of discrete frequency oscillations in the solar wind density and dayside magnetosphere. J. Geophys. Res. 114 (A1). doi:10.1029/2008JA013334
Villante, U., Del Corpo, A., and Francia, P. (2013). Geomagnetic and solar wind fluctuations at discrete frequencies: a case study. J. Geophys. Res. 118 (1), 218–231. doi:10.1029/2012JA017971
Villante, U., Di Matteo, S., and Piersanti, M. (2016). On the transmission of waves at discrete frequencies from the solar wind to the magnetosphere and ground: a case study. J. Geophys. Res. 121 (1), 380–396. doi:10.1002/2015JA021628
Villante, U., Francia, P., and Lepidi, S. (2001). Pc5 geomagnetic field fluctuations at discrete frequencies at a low latitude station. Ann. Geophys. 19 (3), 321–325. doi:10.5194/angeo-19-321-2001
Villante, U., Francia, P., Vellante, M., di Giuseppe, P., Nubile, A., and Piersanti, M. (2007). Long-period oscillations at discrete frequencies: a comparative analysis of ground, magnetospheric, and interplanetary observations. J. Geophys. Res. 112 (A4), 04210. doi:10.1029/2006JA011896
Villante, U., Recchiuti, D., and Di Matteo, S. (2022). The transmission of ULF waves from the solar wind to the magnetosphere: an analysis of some critical aspects. Front. Astron. Space Sci. 9, 835539. doi:10.3389/fspas.2022.835539
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.
Edited by:
Nithin Sivadas, National Aeronautics and Space Administration, United StatesReviewed by:
Qusai Al Shidi, West Virginia University, United StatesCopyright © 2025 Villante. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
*Correspondence: U. Villante, dW1iZXJ0by52aWxsYW50ZUBhcXVpbGEuaW5mbi5pdA==