Umberto Villante

This article presents a concise overview of research developments and advancements in space weather and space climate, with a specific focus on the significant contributions made by members of the Italian Space Weather Community (SWICo). We highlight their achievements in instrument development, observational techniques, and modeling. Furthermore, we introduce a special collection of papers within this journal, entitled “Frontiers in Italian Studies on Space Weather and Space Climate.” This collection features a selection of research articles and presentations from the Second and Third SWICo Congresses, held in Rome in February 2022 and November 2024, respectively, and hosted by the Italian Space Agency.

In a recent paper, Di Ma eo et al. [2024] proposed a sophis cated analysis of the quasiperiodic varia ons 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 spacecra (Wind and ARTEMIS-P1) in the interplanetary medium. They obtained results which, in my opinion, are important also in the context of the scien fic debate regarding the fluctua ons at discrete frequencies observed in the magnetosphere (mostly in range f≈1-5 mHz). Indeed, the occurrence, the origin and the characteris cs of these fluctua ons (which play an important role in the magnetospheric dynamics) have been examined, in the last decades, in a large number of papers, o en with controversial results mainly related to their rela onships with SW fluctua ons at similar frequencies and to the the possible existence (and stability) of sets of favorite magnetospheric frequencies (review by Di Ma eo and Villante, 2025 and papers therein referenced). On the other hand, in recent years, some papers highlighted some cri cal aspects of the data analysis which might have influenced the conclusions of several inves ga ons. In par cular, Di Ma eo and Villante [2017;2018] applied two different methods adopted in the scien fic literature (the Welch method, WM, and the Mul taper method and F-test, MTM) to the same data sets and showed that the WM/MTM agreement in the iden fica on of the wave occurrence and frequency es mate might occur only in ≈50% of cases, both in the SW and in the magnetosphere. In addi on, an analysis conducted by Villante et al. [2022] revealed different characteris cs in the fluctua ons of the SW dynamic pressure (PSW) when the same SW stream was observed by two spacecra at different places in front of the magnetosphere. All these aspects make ambiguous the analysis of the rela onships between the SW and the magnetospheric fluctua ons; as we discuss in this note, the results proposed by Di Ma eo et al. [2024] add other interes ng elements in this context.In their analysis, Di Ma eo et al.[2024] examined the characteris cs of PDS with periods ranging from a few minutes to a few hours (radial length scale of tens to several thousands of megameters). In par cular, they conducted a robust es mate of the spectral proper es of the NSW fluctua ons (they also examined the interplanetary magne c 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 spacecra , obtaining interes ng results. Namely, -79 out of 158 events of NSW fluctua ons iden fied by Wind occurred in the same frequency range (within +/-0.3 mHz) of the corresponding events detected by P1 (P1 iden fied 166 events). In prac ce, for the same SW parcels, comparable frequencies were es mated at the spacecra posi ons for about half of events; meanwhile, the frequency to be a ributed to the other half of the events that will impact the magnetosphere is uncertain. In this context it is interes ng to remind that Viall et al. [2009], who examined the frequencies of PDSs and dayside magnetospheric oscilla ons in the f=0.5-5.0 mHz range using 11 years of Wind andGOES observa ons (1995-2005), reported that in ≈54% of the SW segments with a spectral peak, at least one of the same discrete frequencies was sta s cally significant in the corresponding magnetospheric data segment. Eventually, according to the results of Di Ma eo et al. [2024], this percentage of correspondence between the frequencies of SW and magnetospheric fluctua ons might be related to SW events in which the es mated frequencies of fluctua ons would have been the same at different 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 (≈59%), progressively decreasing and prac cally 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 rela onship between SW and magnetospheric fluctua ons could likely have been inves gated mostly in a frequency range (f≈1-5 mHz) in which the agreement between the frequencies of SW fluctua ons 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 Jan. 1, 2014, Wind iden fied a single fluctua on event (f≈1.8 mHz; Table 1 in Di Ma eo 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 fluctua ons o en differ significantly between the observa ons of the same SW parcel at different places (Villante et al., 2022; Figure 2). All these arguments suggest cau on before assuming a definite iden fica on of the characteris cs of the compressive fluctua ons impinging the magnetosphere when the event is observed by a single spacecra (Di Ma eo and Sivadas, 2022).-Obviously, the global frequency distribu of events at Wind and P1 are not the same (Fig. 5 in Di Ma eo 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 inves ga ons, 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. Interes ngly, in the last 30 years, several papers, proposed the possible existence...

Several critical aspects may influence the analysis of the relationship between the solar wind (SW) and magnetospheric fluctuations, for example, the characteristics and frequency of SW fluctuations that are expected to impinge the magnetosphere may not be the same when they are observed by spacecraft located at different places in front of the magnetosphere; similarly, the choice of analytical methods adopted for the spectral analysis might influence the frequency estimate (as well as the wave identification itself) both in the SW and magnetosphere. Focusing our attention on these aspects, we present an analysis of SW compressional fluctuations (f ≈ 1–5 mHz), following two interplanetary shocks observed by two interplanetary spacecraft, regarded as two different situations in terms of spacecraft separation and distance from the magnetosphere. Our results show that some differences in the characteristics of SW fluctuations emerge when the same stream is observed at different places and confirm the critical role of analytical methods in determining fluctuation characteristics. We compared aspects of SW fluctuations with those of magnetospheric fluctuations following the sudden impulses due to the impact of interplanetary shocks. For this scope, we examined observations by two satellites at geostationary orbit and at several ground-based stations. We found that the magnetospheric fluctuations were related to compressional SW fluctuations approximately at the same frequencies, with no evidence for wave activity of internal origin or directly driven by the shock impact.

Identifying the nature and source of ultra‐low frequencies (ULF) waves (f ⪅ 4 mHz) at discrete frequencies in the Earth's magnetosphere is a complex task. The challenge comes from the simultaneous occurrence of externally and internally generated waves, and the ability to robustly identify such perturbations. Using a recently developed robust spectral analysis procedure, we study an interval that exhibited in magnetic field measurements at geosynchronous orbit and in‐ground magnetic observatories both internally supported and externally generated ULF waves. The event occurred on 9 November 2002 during the interaction of the magnetosphere with two interplanetary shocks that were followed by a train of 90 min solar wind periodic density structures. Using the Wang‐Sheeley‐Arge model, we mapped the source of this solar wind stream to an active region and a mid‐latitude coronal hole just prior to crossing the Heliospheric current sheet. In both the solar wind density and magnetospheric field fluctuations, we separated broad power increases from enhancements at specific frequencies. For the waves at discrete frequencies, we used the combination of satellite and ground magnetometer observations to identify differences in frequency, polarization, and observed magnetospheric locations. The magnetospheric response was characterized by: (a) forced breathing by periodic solar wind dynamic pressure variations below ≈1 mHz, (b) a combination of directly driven oscillations and wave modes triggered by additional mechanisms (e.g., shock and interplanetary magnetic field discontinuity impact, and substorm activity) between ≈1 and 4 mHz, and (c) largely triggered modes above ≈4 mHz.

Identifying the nature and source of Ultra Low Frequencies (ULF) waves (f ≤ 4 mHz) at discrete frequencies in the Earth’s magnetosphere is a complex task. The challenge comes from the simultaneous occurrence of externally and internally generated waves, and the ability to robustly identify such perturbations. Using a recently developed robust spectral analysis procedure, we study an interval that exhibited in magnetic field measurements at geosynchronous orbit and in ground magnetic observatories both internally supported and externally generated ULF waves. The event occurred on November 9, 2002 during the interaction of the magnetosphere with two interplanetary shocks that were followed by a train of 90 min solar wind periodic density structures. Using the Wang-Sheeley-Arge model, we mapped the source of this solar wind stream to an active region and a mid-latitude coronal hole just prior to crossing the Heliospheric current sheet. In both the solar wind density and magnetospheric field fluctuations, we separated broad power increases from enhancements at specific frequencies. For the waves at discrete frequencies, we used the combination of satellite and ground magnetometer observations to identify differences in frequency, polarization, and observed magnetospheric locations. The magnetospheric response was characterized by: (i) forced breathing by periodic solar wind dynamic pressure variations below ≈ 1 mHz; (ii) a combination of directly driven oscillations and wave modes triggered by additional mechanisms (e.g., shock and interplanetary magnetic field discontinuity impact, and substorm activity) between ≈ 1 and ≈ 4 mHz; and (iii) largely triggered modes above ≈ 4 mHz.

This paper addresses the investigation of the interaction of the solar wind energy with the Earth’s magnetosphere, by studying its correlation with the disturbance storm time (Dst) index, a proxy of the geomagnetic activity. Some relevant parameters of the solar wind (the bulk speed and the z-component of the interplanetary magnetic field) are explored in the energy−Dst space. It results that (i) the solar wind energy and the geomagnetic activity are strictly related, with the coronal mass ejections representing the most energetic and geoeffective driver; (ii) the slow solar wind has negligible effects on Earth regardless of its energy content, whereas high-speed streams may induce severe geomagnetic storming depending on the advected energy; and (iii) while at low and mid energies, geomagnetic disturbances are induced provided the magnetic reconnection between the interplanetary and terrestrial magnetic fields occurs, high-energy solar wind plasma can impact Earth even without reconnecting with the geomagnetic field at the dayside magnetopause. The most significant result in the framework of space weather science resides in the observational evidence that the Earth’s magnetosphere has a maximum response to the energetic content of the solar wind, which leads to the derivation of an empirical law allowing the proper forecast of the upper limit of the intensity of any geomagnetic disturbance on the basis of the solar wind energy derived in situ at the Lagrangian point L1.

An important aspect of the interaction between the solar wind (SW) and the Earth’s magnetosphere concerns the possible relationship between SW and magnetospheric fluctuations under different SW conditions. In recent investigations (Di Matteo and Villante, 2017,2018) we revealed the critical role of the analytical methods and the spectral analysis techniques in the identification of fluctuations between ≈1-5 mHz in the SW parameters as well as in the magnetospheric field measurements at the geostationary orbit and developed a new approach, based on the joint use of the Welch and the Multitaper methods, for a more robust identification of these oscillations in both regions. Here, we extend the analysis to ground measurements, analyzing 22 years of magnetic field measurements along the H and D components at low latitude (L’Aquila, Italy, λ≈36.3°, L≈1.6). We found that, in general, the much steeper spectrum of the geomagnetic fluctuations with respect to the ones estimated in the SW parameters and magnetospheric field, might deeply influence the identification of real events. We then examined, for the entire period, consecutive two hours intervals through the day during low geomagnetic activity conditions (Dst>-50), and, for each interval, we carefully evaluated the characteristics of the background spectrum. As a matter of fact, in the ≈1-5 mHz frequency range the spectral indices of both components typically range between -3.5 and -2 with a steeper spectrum in the night sector when the fluctuations power is lower. Simulations of red noise representations, with spectral indices similar to the observed ones, combined with the Sq variation show a systematic reduction of the rate of identification of real events up to ≈2 mHz.Ref.Di Matteo, S., and U. Villante, J. Geophys.Res. Space Physics, 122, 4905–4920, doi:10.1002/2017JA023936.Di Matteo, S., and U. Villante, Journal of Geophysical Research: Space Physics, 123, doi.org/10.1002/2017JA024922.

Following a recent investigation in which we examined the performance of two methods (the Welch method and the multitaper windowing and F test) on the identification of fluctuations of the solar wind dynamic pressure in streams following interplanetary shocks, we extend our analysis to the magnetospheric field fluctuations detected at the geostationary orbit after the occurrence of the related sudden impulses. First, we show that a contamination of the experimental results, dramatic in statistical investigations, might come from the rotation of measurements in the field‐aligned coordinates when the field direction is determined, point by point, by the running averages: this procedure creates spurious events at frequencies related to the length of the running average window. These aspects do not appear in a fixed coordinate system, as determined by evaluating the average field vector for the entire interval. In this system, we examined 124 magnetospheric structures following sudden impulses: the Welch method/multitaper windowing and F test agreement in the wave identification and frequency estimate was achieved for ≈50% of events, and some evidence for higher percentages of events occurs at f ≈ 1.5–1.7, f ≈ 2.2–2.4, ≈3.9–4.2, and, more explicitly, at f ≈ 4.2–4.7 mHz. Oscillation modes with the same characteristics at different local times were rarely observed. An analysis of case events reveals that fluctuations of the solar wind pressure may drive magnetospheric fluctuations at the same frequencies; in addition, the manifestation of modes, not appearing in the solar wind, might be related to the magnetospheric compression or to other processes triggered by the shock arrival.

The occurrence of waves at discrete frequencies in the solar wind (SW) parameters has been reported in the scientific literature with some controversial results, mostly concerning the existence (and stability) of favored sets of frequencies. On the other hand, the experimental results might be influenced by the analytical methods adopted for the spectral analysis. We focused attention on the fluctuations of the SW dynamic pressure (P_SW) occurring in the leading edges of streams following interplanetary shocks and compared the results of the Welch method (WM) with those of the multitaper method (MTM). The results of a simulation analysis demonstrate that the identification of the wave occurrence and the frequency estimate might be strongly influenced by the signal characteristics and analytical methods, especially in the presence of multicomponent signals. In SW streams, P_SW oscillations are routinely detected in the entire range f ≈ 1.2–5.0 mHz; nevertheless, the WM/MTM agreement in the identification and frequency estimate occurs in ≈50% of events and different sets of favored frequencies would be proposed for the same set of events by the WM and MTM analysis. The histogram of the frequency distribution of the events identified by both methods suggests more relevant percentages between f ≈ 1.7–1.9, f ≈ 2.7–3.4, and f ≈ 3.9–4.4 (with a most relevant peak at f ≈ 4.2 mHz). Extremely severe thresholds select a small number (14) of remarkable events, with a one‐to‐one correspondence between WM and MTM: interestingly, these events reveal a tendency for a favored occurrence in bins centered at f ≈ 2.9 and at f ≈ 4.2 mHz.