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Alfvén waves play an important role in the stability, heating, and transport of magnetized plasmas. They are found to be ubiquitous in the solar wind, mainly propagating outward from the Sun, especially in high-speed streams emanating from coronal holes. When high-speed streams impinge on the Earth’s magnetosphere, the impact of Alfvénic fluctuations can cause magnetic reconnection between the intermittent southward IMF and the Geomagnetic field, leading to energy injection from solar wind into the Earth’s magnetosphere. In this work, we tested a rotation procedure from theHeliocentric Earth Ecliptic (HEE) to the Mean-Field Aligned (MFA) reference frame, identified by means of the Empirical Mode Decomposition (EMD), of both solar wind velocity and interplanetary magnetic field at 1 AU. Our aim is to check the reliability of the method and its limitations in identifying Alfvénic fluctuations through the spectral analysis of time series in the MFA reference frame. With this procedure, we studied the fluctuations in the main-field-aligned direction and those in the orthogonal plane to the main field. To highlight the peculiarities of each case of study and be able to better identify Alfvén waves when applying this procedure to real data, we reproduced the magnetic and velocity fields of a typical corotating high-speed stream. We tested the procedure in several cases, by adding the presence of Alfvén waves and noise. We performed the spectral analysis of the MFA component of both magnetic and velocity fields to define the power related to the two main directions: the one aligned to the ambient magnetic field and the one orthogonal to it. The efficiency of the procedure and the result’s reliability are supported by Monte Carlo tests. The method is as well applied to a real case represented by a selected corotating solar wind stream. The results are also compared with those obtained by using the Elsässer variables to analyze the Alfvénicity of fluctuations via the cross-helicity, which is related to the degree of correlation between the solar wind velocity and the magnetic field fluctuations.

In the frame of the geomagnetic depth sounding (GDS) using Single Station Induction Arrows (SSIA), data from an island and nearby coastal sites can provide useful information regarding the surrounding crust structure. In this regard, ultra low frequency (ULF, 1 mHz–5 Hz) waves, originated by solar wind-magnetosphere interactions, represent a natural probing source of the ground from sea-crust surface down to the mantle due to their skin depth in the salt water (1-D infinite half space) of the order of ~104 m for a frequency of ~1 mHz and a conductivity of ~3 S/m. The INGV geomagnetic observatory installed at Lampedusa island (geographic coordinates 35°31′N; 12°32′E, altitude 33 m a.s.l.), and operating since 2007, offers a great opportunity for probing the Earth’s crust in the frame of 1-D models. In this work, geomagnetic field variations collected from 2017 to 2021 are examined by using spectral approaches, providing evidence of the influence of sea salt water with the SSIA approach in relation to different geomagnetic activity levels, seasons and tidal modes. We also study a possible relationship with the sea level variations driven by astronomical tides. A possible link between SSIA variations with water flow modifications, probably linked to the induced electric field of sea salt water flows embedded into the main Earth’s magnetic field, is shown.

At present, the geomagnetic observatory at Lampedusa (south of Sicily — Italy, geographic coordinates 35°31′N; 12°32′E, altitude 33 m a.s.l. - provisional IAGA code: LMP) is the southernmost point of observation in European territory, and since 2007 it contributes at filling the spatial observational gap in the whole south Mediterranean and North African areas. A signature of very low electromagnetic noise is expected at LMP, since it is located in the inner part of a wild park with limited access, far away from the urbanized areas of the island. LMP lies in the middle of the Mediterranean Sea, while the other two Italian observatories (Castello Tesino – CTS and Duronia - DUR, in North and central Italy, respectively) are located in the continental territory.Comparisons among the three observatories, in both time and frequency domains, allow to magnetically characterize the Italian territory. Both 1-minute and 1-second data for the years 2017-2020 are analyzed under a statistical approach and also single event analysis is performed. Superposed Epoch Analysis (SEA) of geomagnetic data from the three observatories returns individual responses to external triggers during geomagnetic storms as well as SSC and SI events, indicating that in correspondence to impulsive inputs a peculiar feature arises at LMP, probably as contribution of electric currents in the surrounding sea salt water. Magnetic responses in the Ultra-Low-Frequency (ULF, 1 mHz–5 Hz) range from spectral, local Signal-to-Noise Ratio (SNR) analyses under different local time are computed, showing that the signal emerges mainly during morning hours, as expected for upstream waves related ULF source waves: in particular, the distinct narrow band characteristic of SNR at LMP indicates that the ULF signals are here mainly uncontaminated by local Field Line Resonance (FLR) as at DUR and CTS, while lower noise levels estimated at LMP suggest a smaller anthropogenic contamination in this frequency range. Moreover, for the first time at such low latitudes in the Mediterranean region, we find evidence of FLR events on Duronia–Lampedusa intermediate field line with the application of the gradient method, a consolidated technique that provides estimates of the ULF standing wave frequencies.. Results from data retrieved by geomagnetic observatories, whose long time series of data are of primary importance, demonstrate a unique contribution in characterizing the magnetospheric response to external events.

The geomagnetic observatories managed by INGV (Istituto Nazionale di Geofisica e Vulcanologia), both in Italy and Antarctica, send data to a server and the data are collected and stored in a MySQL database. The database has been operating for almost two decades and it is implemented on a local server which serves also as a web portal for the data display and distribution. By analyzing the data of all the INGV geomagnetic observatories at middle and polar latitudes, i.e. the values of the H, D and Z components, the F module and the K indices, the algorithm aims to distinguish the activity of the Earth's magnetic field in the following categories: “Quiet Period”, “Local disturbance” and “Magnetic Activity”, possibly distinguishing, within the latter, the level and the kind of event (sudden impulse, sudden ionospheric and magnetic disturbance driven by solar flare, magnetic storm or substorm). A preliminary automatic procedure allows to detect possible instrumental failure from a comparison between the vector components and the total field intensity in each observatory. A second level of check allows to discriminate local or regional against global features with the final goal to reject local noise, possibly of anthropic nature, eventually present in a single observatory through a majority logic based procedure. After these first filtering steps an automatic software procedure provides an empirical estimation of the current Magnetic Activity (nowcasting) organized according to the above three possible categories. The embedded algorithm in the procedure operates on the geomagnetic field element (H, D, Z and F) and the local K indices of all observatories. The operations that the algorithm performs are aimed to identify the impulsive components in the signal, which are caused by external events. The quiet field component is removed from the signal, leaving the impulsive components present in the signal almost unaltered. If in the residual field is present a significant activity (with respect to an appropriate threshold) a procedure is performed that distinguishes between an isolated impulse or a cluster of impulses, by using time windows of different sizes. The whole procedure allows us to generate two different geomagnetic activity indices, one at low and the other at high latitude. In a final step we compute a cross correlated geomagnetic index comparing processed data at low and high latitude to retrieve a large spatial scale index.

In this work, we analyze the geomagnetic field measurements collected from 2017 to 2020 at the Italian observatories of Lampedusa and Duronia (an island and inland site, respectively) for investigating a possible signature of the tidal sea water level changes on the local magnetic variations. We obtain the following main results: (a) evidence of the geomagnetic power spectral peaks at the solar and lunar tidal main frequencies at both sites is found; (b) by using a robust fit procedure, we find that the geomagnetic field variations at Lampedusa are strongly influenced by the lunar tidal variations in the sea level, while at Duronia, the main effects on the geomagnetic field variations are associated with diurnal solar ionospheric tides; (c) a single-station induction arrows (SSIAs) investigation reveals different behaviors between Lampedusa and Duronia. Specifically, Lampedusa shows that the induction arrows in different frequency ranges point toward different directions with different amplitudes, probably related to the surrounding regions with different water depths, while Duronia shows a persistent coast effect, with the induction arrows pointing toward the Adriatic sea; and (d) a Superposed Epoch Analysis reveals, only for Lampedusa, a close relationship between SSIAs with a frequency of >2 mHz (

On 3 November 2021, an interplanetary coronal mass ejection impacted the Earth’s magnetosphere leading to a relevant geomagnetic storm (Kp = 8-), the most intense event that occurred so far during the rising phase of solar cycle 25. This work presents the state of the solar wind before and during the geomagnetic storm, as well as the response of the plasmasphere–ionosphere–thermosphere system in the European sector. To investigate the longitudinal differences, the ionosphere–thermosphere response of the American sector was also analyzed. The plasmasphere dynamics was investigated through field line resonances detected at the European quasi-Meridional Magnetometer Array, while the ionosphere was investigated through the combined use of ionospheric parameters (mainly the critical frequency of the F2 layer, foF2) from ionosondes and Total Electron Content (TEC) obtained from Global Navigation Satellite System receivers at four locations in the European sector, and at three locations in the American one. An original method was used to retrieve aeronomic parameters from observed electron concentration in the ionospheric F region. During the analyzed interval, the plasmasphere, originally in a state of saturation, was eroded up to two Earth’s radii, and only partially recovered after the main phase of the storm. The possible formation of a drainage plume is also observed. We observed variations in the ionospheric parameters with negative and positive phase and reported longitudinal and latitudinal dependence of storm features in the European sector. The relative behavior between foF2 and TEC data is also discussed in order to speculate about the possible role of the topside ionosphere and plasmasphere response at the investigated European site. The American sector analysis revealed negative storm signatures in electron concentration at the F2 region. Neutral composition and temperature changes are shown to be the main reason for the observed decrease of electron concentration in the American sector.

The points where the horizontal component of the geomagnetic field vanishes are located in polar areas, far away from the geomagnetic (analytic) poles and the poles of rotation of the Earth and, differently from the geomagnetic poles, can be found experimentally with a magnetic survey to determine where the field is vertical. The experimental determination of the area where the total field is perfectly vertical, commonly known as dip pole, is not simple, due to the remoteness and harsh climatic conditions; another difficulty is related to the short term geomagnetic field variations, due to the interaction with the external solar wind, which causes the magnetospheric dynamics, particularly evident at high latitude, and as a consequence a displacement of the dip pole. Actually, the study of the dip pole displacements over short time scales can be an important tool for monitoring the magnetospheric dynamics at high latitude. In this study we present the updated location of the the dip poles, using data from the Swarm ESA’s constellation of satellites along their almost polar orbits. We also analyse the spatial shift of these areas during different seasons and interplanetary magnetic field orientations.

We use low frequency geomagnetic field measurements at two Antarctic stations to statistically investigate the longitudinal location of the polar cusp. The two stations are both located in the polar cap at a geomagnetic latitude close to the cusp latitude; they are separated by one hour in magnetic local time. At each station the Pc5 power maximizes when the station approaches the cusp, i.e. around magnetic local noon. The comparison between the Pc5 power at the two stations allows to determine the longitudinal location of the cusp. Our analysis is conducted considering separately different orientation of the interplanetary magnetic field. The results, which indicate longitudinal shifts of the polar cusp depending on the selected conditions, are discussed in relation to previous studies of the polar cusp location based on polar magnetospheric satellite data.

This paper is based on the statistical analysis of the diurnal variation as observed at six polar geomagnetic observatories, three in the Northern and three in the Southern hemisphere. Data are for 2006, a year of low geomagnetic activity. We compared the Italian observatory Mario Zucchelli Station (TNB; corrected geomagnetic latitude: 80.0°S), the French–Italian observatory Dome C (DMC; 88.9°S), the French observatory Dumont D’Urville (DRV; 80.4°S) and the three Canadian observatories, Resolute Bay (RES; 83.0°N), Cambridge Bay (CBB; 77.0°N) and Alert (ALE, 87.2°N). The aim of this work was to highlight analogies and differences in daily variation as observed at the different observatories during low geomagnetic activity year, also considering Interplanetary Magnetic Field conditions and geomagnetic indices.

A statistical analysis of low frequency geomagnetic fluctuations at the two Antarctic stations Mario Zucchelli Station (geographic coordinates: 74.7°S, 164.1°E; corrected geomagnetic coordinates: 80.0°S, 306.8°E) and Dumont D’Urville (geographic coordinates: 66.7°S, 140.0°E; corrected geomagnetic coordinates: 80.4°S, 236.0°E) is shown. The analysis focuses on power spectra, coherence and phase difference between the stations, which are both located in the polar cap, with a 5-h magnetic local time displacement along a geomagnetic parallel; in this situation, the phase difference between geomagnetic fluctuations indicates the direction of their azimuthal propagation. Coherent fluctuations have been found to occur preferably when both stations are on the same side (dawnward or duskward) with respect to the polar cusp; moreover, around local magnetic midnight, they occur essentially during open magnetospheric conditions. The phase difference for coherent fluctuations indicates a propagation direction away from local geomagnetic noon and midnight. Also the analysis of three individual pulsation events, occurring at different times during the day, is shown; they are characterized at the two stations by simultaneous, coherent fluctuations, whose phase difference finds correspondence with the statistical behaviour.

We analyzed a number of forward shocks detected by Wind in the interplanetary medium to determine the orientation and speed of the shock fronts. Assuming a planar shock geometry and a constant propagation speed (both con9rmed by a comparison with available Geotail observations), we determined when the shock would hit the bow shock (BS) and evaluated the propagation time of associated disturbances between the BS and ground. We found delay times were ∼ 5 min: for almost radially propagating structures, this delay time would imply magnetosheath speeds of shock associated disturbances of ∼ 1 – 1 4 of the external shock speed. c

On Jan. 10–11, 1997 a wide magnetic cloud reached the Earth triggering intense geomagnetic activity. Observations performed at low and very high latitude show that the same features appear simultaneously in correspondence to different changes in the solar wind conditions. In particular, highly polarized modes are simultaneously observed at the same discrete frequencies after the passage of the high density solar wind region following the cloud. SI's and ULF waves polarization are also examined in a wide latitudinal and longitudinal extent.

During the 1986-87 austral summer a geomagnetic observatory was installed at Terra Nova Bay. During the firstyears both geomagnetic field time variation monitoring and absolute measurements were carried out only duringsummer. Since 1991 variometer measurements are automatically performed throughout the year, while absolutemeasurements are still performed only during summer. In spite of this, interesting observations were obtainedduring the life (quite long for Antarctica) of the geomagnetic observatory. In particular, this paper brieflypresents some of the most important results: studies on secular variation, daily variation (and its dependencefrom solar cycle and seasons) and geomagnetic higher frequency variations, such as geomagnetic pulsations.

During the 1986-87 austral summer a geomagnetic observatory was installed at the Italian Antarctic Base Mario Zucchelli Station. In the first three years continuous time variation monitoring and absolute measurements of the geomagnetic field were carried out only during summer expeditions. Starting 1991 an automatic acquisition system, operating through all the year, was put in operation. We present here some peculiarities of the daily variation as observed for fourteen years (1987-2000). The availability of a long series of data has allowed the definition of seasonal, as well as solar cycle effects, on short time variations as observed at a cusp-cap observatory. In particular, contrary to mid latitude behaviour, a clear dependence of the daily variation amplitude on the global geomagnetic K index was well defined.

A network of three absolute magnetometer stations and the geomagnetic observatory of LAquila (42°23N, 13°19E) monitors possible seismo- or tectonomagnetic effects in Central Italy, using LAquila Observatory as a reference for differentiation. A system of two VLF search coil wide-band antennas, working in two different frequency bands, at the LAquila Observatory, monitors possible electromagnetic effects related to seismic events occurring in Central Italy. Absolute magnetic field observations and VLF signals have been collected for several years. In particular the tectono-magnetic network started its operations in 1989. In this paper we report on the time variation of above mentioned data for the most recent years 2002 and 2003, also in connection with older measurements time series; we also report on seismic activity recorded in this area by the national seismic network. In the above mentioned time interval, no strong earthquake activity was recorded, and at the same time no clear evidence for magnetic or electromagnetic signals related to seismic events was found.

Tectonic events, like earthquakes and volcanic eruptions, may generate variations in the local geomagnetic field intensity. To detect possible effects related to seismic activity in central Italy, the INGV (Istituto Nazionale di Geofisica e Vulcanologia) installed a network of four magnetometers in 1989, in a region extending from 41° to 43° latitude N and 12° to 15° longitude E. The study area is characterized by active faulting and seismicity, and by historical destructive earthquakes. The total geomagnetic field intensity was synchronously sampled at the magnetometer sites since July 1989 and averaged on a daily basis. These values were then differentiated with re- spect to the data recorded at the permanent nearby National Geomagnetic Observatory of L'Aquila (42°23lN, 13°19lE). The aim of our work was to detect possible significant magnetic field variations of tectonic origin. Da- ta have been regularly recorded for about 15 years with some interruptions due to instrumental temporary fail- ures. This data set represents a long series of recordings that is valuable to characterize the local response of each station to the secular variation in a tectonically active area. In this paper we applied a new signal processing on data by means of a wavelet analysis to the differentiated geomagnetic records for the period 2000-2005. In an ongoing paper the same technique is extended back to the remaining ten years of data (1989-1999).

[1] We report a statistical analysis of low-frequency magnetic variations (magnetic pulsations, 0.8–7 mHz) at South Pole (74°S corrected geomagnetic latitude) and Terra Nova Bay (80°S) during 1996. The results show that at South Pole (at cusp latitudes) the pulsation power exhibits two maxima during the day, one in the local premidnight and the other in the morning. The first maximum disappears when the analysis is restricted to northward interplanetary magnetic field conditions (Bz > 1 nT), suggesting that it might be associated to substorm phenomena. During closed magnetospheric conditions, when the cusp is expected to be located poleward with respect to the station, the spectral and polarization characteristics of pulsations between 1 and 3 mHz suggest that resonant oscillations of the outermost closed field lines commonly occur at South Pole in the local morning. At Terra Nova Bay, in the polar cap, the pulsation power is much lower and its diurnal variation is characterized by a single maximum, which occurs around local noon, when the station approaches the cusp. The corresponding polarization pattern indicates that Terra Nova Bay is always located poleward with respect to resonant field lines.

Two-dimensional (2-D) statistical distributions of spectral power and coherence of polar geomagnetic variations with quasi-periods about 10 min are analyzed using data from magnetometer arrays in Antarctica. Examination of the 2-D patterns of spectral power and coherence shows the occurrence of significant variations in geomagnetic power levels but with low spatial coherence near the cusp projection and in the auroral region. At the same time, low-amplitude pulsations, which we coin Pi(cap)3 pulsations, are very coherent throughout the polar cap. The region occupied by coherent Pi(cap)3 pulsations is shifted toward local MLT night from the geomagnetic pole and is decoupled from the regions of auroral and cusp ULF activity. The spectral power varies with time at polar latitudes in a manner different from that at auroral latitudes. Diurnal variations of power at different stations at the same geomagnetic latitude exhibit different behavior depending on the station's position relative to geomagnetic and geographic poles. This asymmetry is shown to be partly attributed to the variations of the ionospheric conductance. The primary source of polar pulsations is probably related to intermittent magnetosheath turbulence and tail lobe oscillations, though a particular propagation mechanism has not as yet been identified.

[1] The analysis of a pulsation event recorded at a gradient installation in central Italy (L = 1.55) and at a higher latitude station (Nagycenk, Hungary, L = 1.88) is presented. The event occurred on 16 August 1993 during the main phase of a geomagnetic storm. Field line resonance signatures have been identified at the gradient installation by using different spectral techniques, comprising standard methods, such as interstation amplitude ratio, phase difference, and single station H/D amplitude ratio, and more advanced amplitude-phase methods. The capacity of determining the local resonant frequency fr by the different techniques is compared; in particular, we find that the H/D amplitude ratio fails to provide in some cases an estimate of fr. A clear early morning decrease of fr both at L = 1.55 and at L = 1.88 has been found, confirming previous experimental and theoretical results. The estimated fr values at L = 1.55 are significantly higher than those usually reported at these latitudes, suggesting peculiar conditions of the ionosphere-plasmasphere system during this event. The local time variation of the toroidal mode ULF pulsation eigenfrequencies has been numerically calculated at both latitudes using the standard empirical neutral gas model MSIS–86 and the empirical neutral wind model HWM90. Further numerical simulation runs involved a particular disturbed neutral gas composition which is appropriate for stormy periods and an improved meridional wind estimated as best approximation to the observed hmF2 values. The calculated eigenfrequencies agree with the experimentally observed values at L = 1.88 but are smaller than the values observed in the early morning hours at L = 1.55. The effect of the E × B vertical drift is suggested to be the cause of the discrepancy. Possible signatures of the 2nd harmonic at L = 1.55, never reported so far at such low latitudes, have been found. We estimated a harmonic ratio f2/f1 ∼ 1.7–1.8 in agreement with theoretical models. A high-frequency resolution analysis at the gradient installation confirms previous suggestions of the existence of a discrete spectrum of field line resonances driven by some cavity/waveguide modes.

In this paper we present a statistical study of the local time behaviour of low frequency (0.7-4.0 mHz) geomagnetic field fluctuation power at a low latitude station (corrected geomagnetic latitude 36.2°N). The analysis was conducted for two contiguous years during the ascending phase of the solar cycle. We found that the power of the horizontal east-west geomagnetic field component shows a diurnal and seasonal variation which can be related to variations of the ionospheric current system (Sq) mainly produced by dynamo-action in the ionospheric E region. The power of the horizontal north-south geomagnetic field component is higher in the local afternoon with respect to the morning; this asymmetry is more evident during time intervals characterized by high solar wind dynamic pressure.

A statistical analysis of the power spectra (0.7–5 mHz) of the geomagnetic field components H and D recorded at Terra Nova Bay (Antarctica) during three austral summers close to the maximum of solar activity reveals power enhancements in the H component at ≈ 3.3, 3.9 and 4.5 mHz, which become more evident during daytime intervals. During intervals characterized by higher solar wind speeds these spectral features more clearly emerge and are also accompanied by other enhancements at lower frequencies (≈ 1.2, 1.9 and 2.7 mHz). The observed frequencies are close to the ones detected both at auroral and low latitudes.

We examine seven periods during which IMP 8 made multiple crossings of Earth's bow shock during times when IMP 7 data were available to monitor external solar wind conditions. The positions of the bow shock encounters are consistent with reference shock shape models normalized to the solar wind conditions. We find that multiple crossings can usually be interpreted as being due to changes in the external solar wind parameters. We also find that inward motion of the shock is accompanied by large magnetosheath densities just before the shock sweeps across the spacecraft. We perform a chi-square minimization analysis using a limited set of Rankine-Hugoniot conditions across the bow shocks in order to determine their speeds and normals; we find that the shock velocities are generally consistent with the postulated inward and outward bow shock motions. Whether the crossings are observed on the dawnside or the duskside, most of the bow shock structures are quasi-perpendicular due to changes in the external field orientation just upstream of the shock. The orientations of the normals are consistent with a model in which effects of changes in external conditions propagate as shock shape deformations which move downstream from the nose to the flanks.

On an hourly time-scale the different roles of the solar wind and interplanetary magnetic field (IMF) parameters on ground micropulsation activity can be better investigated than at longer time-scales. A long-term comparison between ground measurements made at L'Aquila and IMP 8 observations confirms the solar wind speed as the key parameter for the onset of pulsations even at low latitudes, although additional control of the energy transfer from the interplanetary medium to the earth's magnetosphere is clearly exerted by the cone angle. Above about 20 mHz the frequency of pulsations is confirmed to be closely related to the IMF magnitude while, in agreement with model predictions, the IMF magnitude is related to the amplitude of the local fundamental resonant mode. We provide an interesting example in which high resolution measurements simultaneously obtained in the foreshock region and on the ground show that external transversal fluctuations do not penetrate deep into the low latitude magnetosphere.

[1] Latitudinal and diurnal distributions of spectral power and spatial coherency parameters of the geomagnetic variations in the Pc5-6 (1–6 mHz) frequency range are analyzed using data of magnetometer stations in Antarctica. The available stations give the possibility to form a latitude chain along the geomagnetic meridian 40°E stretching from magnetic latitude 69°S to 86°S. Long-period ULF activity at polar cap latitudes is characterized by lower amplitudes and wider spectra with lower central frequencies as compared with typical auroral Pc5 pulsations. The meridional distribution of average Pc5-6 spectral power is nonmonotonic and has a minimum near 80°. In general, the low-frequency broadband ULF activities in the polar cap and at auroral latitudes seem to be decoupled. This long-period ULF activity in the polar cap could be an image of wave activity in the tail lobes or the manifestation of turbulent component of the ionospheric convection at very high latitudes, but this requires further investigation.

A statistical analysis of geomagnetic sudden impulses has been performed for data acquired at a low-latitude station during northward interplanetary magnetic field conditions. The local time dependence of the ground response, characterized by very low values in the morning with respect to the afternoon and night sectors and by a clear maximum just after local noon, is very similar to that observed at auroral latitudes, suggesting that signatures of auroral ionospheric currents might be observed also at low latitude. The sense of polarization of the sudden impulses shows a clear reversal from counterclockwise to clockwise a few hours after local noon, consistent with the results obtained for low-frequency geomagnetic fluctuations at the same station. In some cases a transient response (overshoot) is clearly observed; its amplitude is greater when the corrected Dst index is more negative, that is, when the ring current intensity is stronger; moreover, it tends to occur within 1-2 hours after the closest interplanetary magnetic field northward turning. These results seem to indicate that at our station overshoots characterize sudden impulses occurring a short time after periods with high magnetospheric activity. Since the local field line is embedded in the plasmasphere, the overshoot might tentatively be interpreted in terms of an overcompression of the plasmasphere which, after periods with high magnetospheric activity, is less dense and more elastic.

A statistical analysis of the polarization pattern of low-frequency geomagnetic field fluctuations (0.7-7.4 mHz) covering the entire 24-h interval was performed at the Antarctic station Terra Nova Bay (80.0°S geomagnetic latitude) throughout 1997 and 1998. The results show that the polarization pattern exhibits a frequency dependence, as can be expected from the frequency dependence of the latitude where the coupling between the magnetospheric compressional mode and the field line resonance takes place. The polarization analysis of single pulsation events shows that wave packets with different polarization sense, depending on frequency, can be simultaneously observed.

Tectonomagnetic field observations from absolute magnetic field level measurements were undertaken in Central Italy in an area extending between latitude 41°N and 43°N and between longitude 13°E and 15°E. Moreover,natural electromagnetic signals from a system of two VLF search coil wide-band antennas were collected at the geomagnetic observatory of L Aquila (42º23'N, 13º19'E). The analysis of these data allowed the investigation of the electromagnetic properties of the study area at different time and spatial lengthscales. Tectonomagnetic field observations were obtained comparing data simultaneously recorded at three magnetometer stations using L'Aquila Observatory as a reference for differentiation. We report on the time evolution of magnetic and electromagnetic indicators related to local and regional seismic activity.

A statistical analysis of the polarization pattern of low-frequency geomagnetic field fluctuations (0.8–3.6 mHz) covering the entire 24-hour interval has been performed at an Antarctic station (Terra Nova Bay) and a low-latitude station (L'Aquila, Italy) during the entire 1995. The results show a complex pattern in which, in agreement with predictions, four polarization reversals occur at high latitude during the local day. A comparison with another Antarctic station, McMurdo, during a shorter interval in 1994 confirms these results. At low latitude the polarization sense in the afternoon reverses with respect to the morning, but the reversal is delayed by a few hours after the expected local noon. In Antarctica the polarization pattern does not show any dependence on frequency and season, while at L'Aquila it is better defined for frequencies below ≈2 mHz and during local summer.