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Ground-based magnetometer stations represent a multi-viewpoint and easy-to-access system for sounding Earth’s magnetic field disturbances in the inner magnetosphere. Using Ultra-Low Frequency (ULF) measurements recorded from pairs of meridionally aligned stations, it is possible to determine the Field Line Resonance (FLR) frequencies, which are directly related to the equatorial magnetospheric plasma mass density. Recently, it has been shown by Foldes et al. (J Geophys Res 126(5):e2020JA029008. https://doi.org/10.1029/2020JA029008, 2021) that the Machine Learning (ML) algorithms are valuable tools for detecting FLRs by exploiting the useful information provided by cross-phase Fourier spectra, which are at the heart of the ULF technique for inferring the magnetospheric mass density. The main shortcoming of this approach is that it is not possible to discriminate between active and quiet times in terms of resonances. It is commonly known that detecting FLRs using cross-phase spectra may often be unfeasible due to data gaps, noisy signals, and/or quiescent ULF wave periods. To handle these situations, we implement an ML classification algorithm to identify periods when the resonance frequencies are observable and thus easily estimated. Our algorithm can distinguish samples into three main classes: periods with observed frequency (“Freq" class) from others (“NoFreq"), and, in addition, it can determine whether the considered field line crosses the plasmasphere boundary layer (PBL or plasmapause) at a given time. The results of our method are validated for a particular pair of stations (at L=2.9) along the Equatorial quasi-Meridional Magnetometer Array (EMMA), using a large dataset comprising different geomagnetic conditions. The proposed approach might be combined with a regression algorithm (such as those proposed in Foldes et al. (J Geophys Res 126(5):e2020JA029008. https://doi.org/10.1029/2020JA029008, 2021)) in a two-stage ML pipeline, with the ultimate goal of implementing a completely automated system for the real-time monitoring of the plasmasphere dynamics from ground-based magnetometer stations.

This paper presents how the magnetosphere–plasmasphere–ionosphere system was affected as a whole during the geomagnetic storm peaking on 27 May 2017. The interplanetary conditions, the magnetospheric response in terms of the magnetopause motion, and the ionospheric current flow pattern were investigated using data, respectively, from the WIND spacecraft, from GOES15, GOES13, THEMIS E, THEMIS D and THEMIS A satellites and from the INTERMAGNET magnetometer array. The main objective of the work is to investigate the plasmaspheric dynamics under disturbed conditions and its possible relation to the ionospheric one; to reach this goal, the equatorial plasma mass densities derived from geomagnetic field line resonance observations at the European quasi-Meridional Magnetometer Array (EMMA) and total electron content values obtained through three GPS receivers close to EMMA were jointly considered. Despite the complexity of physical mechanisms behind them, we found a similarity between the ionospheric and plasmaspheric characteristic recovery times. Specifically, the ionospheric characteristic time turned out to be ~ 1.5 days, ~ 2 days and ~ 3.1 days, respectively, at L~ 3, L~ 4 and L~ 5, while the plasmaspheric one, for similar Lvalues, ranged from ~ 1 day to more than 4 days.

In one model, Pi2 pulsations are driven pulse by pulse by fast mode pulses that are launched as periodic bursty bulk flows brake when they approach the Earth. We have examined this model by analyzing data from multiple spacecraft and ground magnetometers for a Pi2 pulsation event. During the event, which started at ∼2226 UT on 8 November 2014, Time History of Events and Macroscale Interactions during Substorms (THEMIS)‐D detected an ∼2‐min‐period plasma bulk flow oscillation in the near‐Earth magnetotail, while THEMIS‐E and Van Allen Probes‐B, both located on the nightside just earthward of the electron plasmapause, detected a Pi2 pulsation consisting of a 10‐mHz oscillation in the azimuthal component of the electric field and a 19‐mHz oscillation in the compressional component of the magnetic field. On the ground, magnetic field oscillations containing both frequencies were observed both on the nightside and on the dayside. The nightside observations indicated that the pulsation had a radially standing structure, which is consistent with plasmaspheric virtual resonances (PVRs) excited in a magnetohydrodynamic simulation assuming an impulsive energy source. Cross‐spectral analysis of the magnetotail flow oscillation and the Pi2 pulsation indicated low coherence between them. These results suggest that the flow oscillation contributed to the Pi2 pulsation as a broadband energy source and that only the spectral components matching the PVR frequencies were detected with well‐defined frequencies. Ionospheric currents connected to the PVRs may be responsible for the appearance of the pulsation on the dayside. Periodic oscillation of earthward plasma flow triggered a Pi2 pulsation event observed in the plasmasphere and on the groundIn the plasmasphere, the event was detected as multifrequency poloidal oscillations consistent with virtual resonancesLow‐latitude ground magnetometers detected the plasmaspheric spectral components both on the nightside and dayside

We present observational evidence of drift resonance between westward propagating odd mode standing ultralow frequency waves and energetic protons. Compressional ∼13 mHz (Pc4 band) waves and proton flux oscillations at >50 keV were detected at ∼03 hr magnetic local time by the Arase satellite on 15 April 2017. The azimuthal wave number (mnumber) is estimated to be ∼−50 from ground observations, while the theory of drift resonance gives m∼− 49 for odd mode waves and ∼110‐keV protons, providing evidence that the drift resonance indeed took place in this event. We also found a steep earthward gradient of proton phase space density, which can quantitatively explain the wave excitation. The observed waves show typical features of giant pulsations (Pgs), regarding local time, mnumber, and flux oscillations. This study, therefore, has great implications to the field line mode structure and excitation mechanism of Pgs. The Arase satellite of Japan Aerospace Exploration Agency was launched on 20 December 2016 into an elliptical orbit with the apogee of 6 Re. On 15 April 2017, Arase was able to observe a giant pulsation (Pg) that is a kind of ultralow frequency waves with a large amplitude on the ground. The excitation mechanism of Pg has been studied by many authors but is not determined yet. Thanks to the simultaneous observations by ground magnetometers and the Arase satellite, the excitation mechanism of Pg was fully examined in this study. The azimuthal wave structure was estimated from the ground magnetometer stations to confirm whether the observed waves satisfy the resonance condition with energetic (110 keV) protons. Medium‐Energy Particle Experiments‐Ion Mass Analyzer onboard Arase is equipped with as many as 16 detectors of energetic ions and provides three‐dimensional flux data with the sufficient number of particle counts. We found a steep earthward gradient of proton phase space density from the data provided by Medium‐Energy Particle Experiments‐Ion Mass Analyzer and concluded that Pg observed in this event was excited through the wave‐particle interaction driven by the earthward gradient of the phase space density for protons at 110 keV. Pc4 ULF waves and proton flux oscillations were observed by the Arase satellite and ground magnetometersThe azimuthal wave number estimated from ground magnetometers is consistent with the theory of drift resonanceThe steep gradient of proton phase space density is responsible for the wave excitation

Cavity mode oscillations (CMOs) are basic magnetohydrodynamic eigenmodes in the magnetosphere predicted by theory and are expected to occur following the arrival of an interplanetary shock. However, observational studies of shock‐induced CMOs have been sparse. We present a case study of a dayside ultralow‐frequency wave event that exhibited CMO properties. The event occurred immediately following the arrival of an interplanetary shock at 0829 UT on 15 August 2015. The shock was observed in the solar wind by the Time History of Events and Macroscale Interactions during Substorms‐B and ‐C spacecraft, and magnetospheric ultralow‐frequency waves were observed by multiple spacecraft including the Van Allen Probe‐A and Van Allen Probe‐B spacecraft, which were located in the dayside plasmasphere at L∼1.4 and L∼ 2.4, respectively. Both Van Allen Probes spacecraft detected compressional poloidal mode oscillations at ∼13 mHz (fundamental) and ∼26 mHz (second harmonic). At both frequencies, the azimuthal component of the electric field (Eϕ) lagged behind the compressional component of the magnetic field (Bμ) by ∼90°. The frequencies and the Eϕ‐Bμrelative phase are in good agreement with the CMOs generated in a dipole magnetohydrodynamic simulation that incorporates a realistic plasma mass density distribution and ionospheric boundary condition. The oscillations were also detected on the ground by the European quasi‐Meridional Magnetometer Array, which was located near the magnetic field footprints of the Van Allen Probes spacecraft. An interplanetary shock excited magnetospheric ULF oscillationsThe oscillations were observed in the dayside plasmasphere by Van Allen ProbesThe oscillations exhibited properties consistent with cavity mode oscillations

The Dynamic Global Core Plasma Model (DGCPM) is an empirical dynamical model of the plasmasphere which, despite its simple mathematical form, or perhaps because of its simple mathematical form, has enjoyed wide use in the space physics modeling community. In this paper we present some recent observations from the European quasi‐Meridional Magnetometer Array (EMMA) and compare these with the DGCPM. The observations suggest more rapid daytime refilling and loss than what is described in the DGCPM. We then modify the DGCPM by changing the values of some of its parameters, leaving the functional form intact. The modified DGCPM agrees much better with the EMMA observations. The modification resulted in an order‐of‐magnitude faster daytime refilling and nighttime loss. These results are also consistent with previous observations of daytime refilling. The Dynamic Global Core Plasma Model requires modification to agree with ground‐based magnetic observations in the inner magnetosphereWe have proposed specific modifications to DGCPM to bring it into better agreement with observationsThe excellent agreement between the modified DGCPM and the EMMA observations support their use for plasmasphere sounding

The investigation of heavy ions dynamics and properties in the Earth's magnetosphere is still an important field of research as they play an important role in several space weather aspects. We present a statistical survey of the average ion mass in the dayside magnetosphere made comparing plasma mass density with electron number density measurements and focusing on both spatial and geomagnetic activity dependence. Field line resonance frequency observations across the European quasi‐Meridional Magnetometer Array, are used to infer the equatorial plasma mass density in the range of magnetic L‐shells 1.6–6.2. The electron number density is derived from local electric field measurements made on Van Allen Probes using the Neural‐network‐based Upper‐hybrid Resonance Determination algorithm. The analysis is conducted separately for the plasmasphere and the plasmatrough during favorable periods for which both the plasma parameters are observed simultaneously. We found that throughout the plasmasphere the average ion mass is ≃1 amu for a wide range of geomagnetic activity conditions, suggesting that the plasma mainly consist of hydrogen ions, without regard to the level of geomagnetic activity. Conversely, the plasmatrough is characterized by a variable composition, highlighting a heavy ion mass loading that increases with increasing levels of geomagnetic disturbance. During the most disturbed conditions, the average radial structure shows a broad maximum around 3–4 Earth radii, probably correlated with the accumulation of oxygen ions near the plasmapause. Those ions are mostly observed in the post‐dawn and pre‐dusk longitudinal sectors. The space surrounding the Earth is permeated by plasma whose composition could affect the space weather conditions. The near Earth region, the plasmasphere, roughly co‐rotates with our planet, while the external part, the plasmatrough, streams toward the Sun direction. The solar wind conditions determine the level of the geomagnetic activity and eventually the radial position of the layer separating the plasmasphere from the plasmatrough. We investigate the average ion mass in the dayside region between 1.6 and 6 Earth Radii, using ground based magnetic field observations recorded at the European quasi‐Meridional Magnetometer Array and plasma waves measurements made on Van Allen Probes spacecrafts. The analysis shows that the plasmasphere is practically insensitive to the level of geomagnetic activity being composed predominantly by hydrogen ions. Contrarily, the contribution of oxygen ions in the plasmatrough increases with increasing geomagnetic activity and maximizes between 3 and 4 Earth radii and in the dawn and dusk longitudinal sectors. The plasmasphere mainly consists of hydrogen ions for a wide range of geomagnetic activity conditionsThe contribution of heavy ions in the plasmatrough increases as the geomagnetic activity increases, predominantly close to the plasmapauseThe highest ion mass loading in the plasmatrough is observed in the longitudinal sectors 7–9 and 15–17 MLT The plasmasphere mainly consists of hydrogen ions for a wide range of geomagnetic activity conditions The contribution of heavy ions in the plasmatrough increases as the geomagnetic activity increases, predominantly close to the plasmapause The highest ion mass loading in the plasmatrough is observed in the longitudinal sectors 7–9 and 15–17 MLT

The relative importance of propagating and cavity mode waves remains an important question regarding the generation of Pi2 pulsations detected on the ground. To determine the wave mode, we statistically generate spatial maps of magnetospheric oscillations that are coherent with ground Pi2 pulsations. The magnetospheric observations were made by the two Van Allen Probes spacecraft over a 7‐year period. The amount and quality of the spacecraft data allow us to investigate the mode structure of Pi2 pulsation in ways that were not possible in previous studies. We use the Hcomponent of low‐latitude ground Pi2 pulsations detected in the 22–02 magnetic local time (MLT) sector as the reference signal to generate L‐MLT and meridional maps of the coherence, amplitude, and phase of the magnetospheric electric and magnetic field components defined in magnetic field aligned coordinates. We identify low‐frequency and high‐frequency components in Pi2 power spectra, and we are able to determine the mode structure of the high‐frequency events for the first time. The maps demonstrate that the poloidal components have higher coherence than the toroidal components. For each frequency component, the maps of the poloidal components agree with those of cavity mode oscillations obtained in a numerical simulation using realistic models for the magnetospheric mass density and magnetic field. This result is conclusive evidence of the cavity mode nature of Pi2 pulsations detected in the inner magnetosphere. The magnetospheric phenomena known as Pi2 pulsations are oscillations of the electric and magnetic fields with periods around 1 min. The pulsations typically occur at the onset of nightside disturbances known as substorms, which result in sudden brightening of auroras. Pi2 pulsations propagate globally and are detected on the ground even at noon, serving as a useful monitor of substorms. This is analogous to propagation of seismic waves generated by earthquakes. Similar to the solid Earth, the magnetosphere sustains both propagating waves and normal mode oscillations (cavity modes), with the latter exhibiting discrete frequencies and standing wave structures that depend on the plasma distribution in the magnetosphere. There is debate on whether Pi2 pulsations are propagating waves or cavity modes. We construct spatial maps of the amplitude and phase of magnetospheric Pi2 pulsations using observations made by the Van Allen Probes spacecraft. The maps show theoretically predicted mode structures of cavity modes. Van Allen Probes data are used to generate maps of the amplitude and phase of Pi2 pulsations in the inner magnetosphereA secondary frequency component of the pulsations is treated separately from the main componentThe maps for both frequency components show radial structures of cavity mode resonances Van Allen Probes data are used to generate maps of the amplitude and phase of Pi2 pulsations in the inner magnetosphere A secondary frequency component of the pulsations is treated separately from the main component The maps for both frequency components show radial structures of cavity mode resonances

Previous spacecraft studies showed that stormtime poloidal ultralow‐frequency (ULF) waves in the ring current region have an antisymmetric (second harmonic) mode structure about the magnetic equator. This paper reports Van Allen Probes observations of symmetric ULF waves in the postnoon sector during a moderate geomagnetic storm. The mode structure is determined from the presence of purely compressional magnetic field oscillations at the equator accompanied by strong transverse electric field perturbations. Antisymmetric waves were also detected but only very late in the recovery phase. The symmetric waves were detected outside the plasmasphere at L= 3.0–5.5 and had peak power at 4–10 mHz, lower than the frequency of the local fundamental toroidal standing Alfvén wave. During the wave events, the flux of protons was enhanced at energies below ∼5 keV, which appears to be a prerequisite for the waves. The protons may provide free energies to waves through drift resonance instability or drift compressional instability, which occur in the presence of radial gradients of plasma parameters. During geomagnetic storms, the intensity of the flux of ions is elevated in the magnetosphere including the region inward of geostationary orbits. The ions are capable of exciting a variety of plasma waves including ultralow‐frequency (ULF) waves in the frequency range 1–20 mHz. Stormtime ULF waves with a strong perturbation of the magnetic field magnitude are commonly detected at geostationary orbit and are known to commonly have a standing wave structure along the background magnetic field that is antisymmetric about the magnetic equator. Observations made by the Van Allen Probes provide new evidence that symmetric waves are also excited inward of geostationary orbits. The waves are observed in association with enhancement of the flux of protons at energies below 5 keV. The waves may be excited by the spatial gradient of parameters associated with the ions. Symmetric compressional ULF (4–10 mHz) waves were excited at L= 3.0–5.5 during a geomagnetic stormThe waves were associated with enhancement of the flux of protons at energy <5 keVDrift compressional instability is a possible wave generation mechanism Symmetric compressional ULF (4–10 mHz) waves were excited at L= 3.0–5.5 during a geomagnetic storm The waves were associated with enhancement of the flux of protons at energy <5 keV Drift compressional instability is a possible wave generation mechanism

Monitoring the plasmasphere is an important task to achieve in the Space Weather context. A consolidated technique consists of remotely inferring the equatorial plasma mass density in the inner magnetosphere using Field Line Resonance (FLR) frequencies estimates. FLR frequencies can be obtained via cross‐phase analysis of magnetic signals recorded from pairs of latitude separated stations. In the last years, machine learning (ML) has been successfully applied in Space Weather, but this is the first attempt to estimate FLR frequencies with these techniques. We survey several supervised ML algorithms for identifying FLR frequencies by using measurements of the European quasi‐Meridional Magnetometer Array. Our algorithms take as input the 2‐hour cross‐phase spectra of magnetic signals and return the FLR frequency as output; we evaluate the algorithm performance on four different station pairs from L= 2.4 to L= 5.5. Results show that tree‐based algorithms are robust and accurate models to achieve this goal. Their performance slightly decreases with increasing latitude and tend to deteriorate during nighttime. The estimation error does not seem to depend on the geomagnetic activity, although at high latitudes the error increases during highly disturbed geomagnetic conditions such as the main phase of a storm. Our approach may represent a prominent space weather tool included into an automatic monitoring system of the plasmasphere. This work represents only a preliminary step in this direction; the application of this technique on a more extensive data set and on more pairs of stations is straightforward and necessary to create more robust and accurate models. We investigated the performances of several machine learning algorithms to identify geomagnetic Field Line Resonance (FLR) frequencies from ultralow frequency signals detected at European quasi‐Meridional Magnetometer ArrayTree‐based methods have higher performance than kernel‐based methods and they result a robust technique to estimate FLR frequenciesThe estimation error of our approach shows a dependence on the station latitude, but it is stable on highly variable geomagnetic conditions We investigated the performances of several machine learning algorithms to identify geomagnetic Field Line Resonance (FLR) frequencies from ultralow frequency signals detected at European quasi‐Meridional Magnetometer Array Tree‐based methods have higher performance than kernel‐based methods and they result a robust technique to estimate FLR frequencies The estimation error of our approach shows a dependence on the station latitude, but it is stable on highly variable geomagnetic conditions

Excitation of toroidal mode standing Alfvén waves in the midnight sector of the inner magnetosphere in association with substorms is well documented, but studies are sparse on dayside sources for the waves. This paper reports observation of midnight toroidal waves by the Van Allen Probe B spacecraft during a geomagnetically quiet period on 12–13 May 2013. The spacecraft detected toroidal waves excited at odd harmonics below 30 mHz as it moved within the plasmasphere from ∼2100 magnetic local time to ∼0030 magnetic local time through midnight in the dipole Lrange 4.2–6.1. The frequencies and the relationship between the electric and magnetic field components of the waves are consistent with theoretical toroidal waves for a reflecting ionosphere. At the time of the nightside toroidal waves, compressional waves were observed by geostationary satellites located on the dayside, and the amplitudes of both types of waves varied with the cone angle of the interplanetary magnetic field. The nightside toroidal waves were likely driven by fast mode waves that resulted from transmission of upstream ultralow frequency waves into the magnetosphere. Ground magnetometers located near the footprint of the spacecraft did not detect toroidal waves. Multiharmonic toroidal standing Alfvén waves were detected in the midnight sector of the plasmasphereInterplanetary magnetic field cone angle was small, which suggests that foreshock ULF waves were the energy sourceThe toroidal waves were not detected on the ground at stations located in the midnight sector

A local time dependent empirical model of the equatorial plasma mass density in the inner magnetosphere is derived from field line resonance (FLR) observations at the European quasi‐Meridional Magnetometer Array (EMMA). Models of the plasmasphere, plasmatrough, and plasmapause are derived separately and then combined. The whole model is limited to the local time (LT) sector 06:00‐18:00 and to the range of equatorial distances 2.3RE