Kazue Takahashi

All past spacecraft observations of fundamental poloidal Alfvén (P1) waves attributed to ion drift resonance have been made outside the plasmasphere. In this study, we present the first spacecraft observations of a plasmaspheric P1 wave driven by that resonance. The wave was observed on 6 March 2014 by the Van Allen Probes A and B spacecraft as they moved from = 3 to = 5 in the prenoon sector. The wave had a frequency of 4 mHz, and its harmonic order is determined from the phase difference between the magnetic and electric field components and a theoretical model of the frequency. The wave was accompanied by proton flux oscillations in the energy range 70–100 keV, which exhibited energy‐dependent amplitude and phase. This flux behavior is explained by proton drift resonance with the P1 wave, which is inferred to have propagated westward with an azimuthal wave number, , of ∼. The wave was detected on the ground near the magnetic footprints of the spacecraft as magnetic pulsations in the east‐west component, resembling giant pulsations. The proton phase space density evaluated with the first two adiabatic invariants fixed at the values specified at the center of the wave event exhibits a negative radial gradient in the region of the P1 wave, supporting the theory of drift resonance excitation of P1 waves. Pitch angle scattering of ring current ions by electromagnetic ion cyclotron waves may contribute to the formation of the phase space density gradient.

Pi2 oscillations are known to be associated with cavity mode oscillations (CMOs) in the inner magnetosphere at low latitudes. CMOs can also be excited by interplanetary shocks on the dayside. We present results from a global linear MHD wave model in spherical coordinates that allows us to analyze these waves at equatorial latitudes. We see that the model reproduces some of the principal CMO harmonics. The standing wave cavity modes are set up due to a dense plasmasphere. The CMOs associated with interplanetary shocks are compared with ground observations from the EMMA array at mid‐latitudes. In accordance with observations of Pi2 pulsations, we also see a small phase difference between compressional mode Alfvén waves at midnight at low altitudes and on the noon ground, both at equatorial latitudes. This supports the finding that upon being excited in the nightside inner magnetosphere, these waves travel quickly around to the dayside. We compare our results to observations of the night‐to‐day time delay of ground Pi2 pulsations.

Fundamental poloidal (P1) waves are a class of magnetospheric standing Alfvén waves characterized by field line oscillations in the meridional plane. P1 waves are infrequently excited in the ring current region and are believed to be the source of giant pulsations (Pgs) observed on the ground. Pgs are mostly detected on the morning side with a sharp cutoff near midnight. Motivated by this local time dependence, we examine a longitudinally extended P1 wave event observed on 24 March 2013 by Van Allen Probes (RBSP)‐A and RBSP‐B (midnight sector), Engineering Test Satellite (ETS)‐VIII (morning side), and Time History of Events and Macroscale Interactions during Substorms (THEMIS)‐D (dawn sector). At each spacecraft, the magnetic field perturbations exhibit a strong radial component and a finite compressional component consistent with previously reported P1 waves. A fortuitous conjunction of RBSP‐A and RBSP‐B allows us, for the first time, to determine the magnitude of the azimuthal wave number at midnight to be , if we assume the waves propagate westward. This value implies a strong ionospheric attenuation of the waves, explaining why few Pg events have been reported near midnight relative to other local time sectors. Ion flux data from RBSP and THEMIS indicate that the P1 waves were excited in a longitudinally extended region of a negative radial gradient of the ion phase space density, which is the condition for an instability to excite P1 waves through drift resonance.

Collisionless shocks are fundamental accelerators of energetic particles, yet the observations of nonlinear foreshock structures, which are essential in acceleration processes, differ significantly between interplanetary (IP) shocks and planetary bow shocks. We present a direct comparison of two high-Mach-number, quasi-parallel shocks: an IP shock observed by Solar Orbiter and the Earth’s bow shock measured by the Magnetospheric Multiscale mission during the 2024–2025 “string-of-pearls” campaign. We show that foreshock compressive structures (FCSs) initiate upstream of both shocks at similar normalized distances (≲50 ion inertial lengths, d _i ) when the suprathermal (>10 keV) ion density exceeds ∼1% of the background. However, the IP shock lacks the fully evolved, high-amplitude short large-amplitude magnetic structures characteristic of the terrestrial foreshock. We demonstrate that the “growth zone” capable of sustaining these structures is spatially limited (∼135 d _i ), which, due to the high speed of the propagating IP shock, corresponds to a brief observational window of <10 s. Beyond this observational constraint, we suggest an additional physical mechanism that can inhibit foreshock maturity at IP shocks. The lack of global curvature prevents the lateral supply (“cross talk”) of energetic ions from different shock regions. These findings suggest that while the fundamental physics of FCS initiation is unified across collisionless shocks, the achievement of full nonlinearity can be regulated by the unique shock geometry and upstream properties while ultimately remaining observationally challenging to identify.

Measurements of singly ionized oxygen () ions in the inner magnetosphere during geomagnetic storms are important because the ions affect various magnetospheric processes. We apply a magnetoseismic technique to Van Allen Probes data to statistically determine the spatial and temporal development of the region of elevated number density , referred to as the oxygen torus, during geomagnetic storms. This study is motivated by previous studies that reported magnetic local time (MLT) localization of the torus to the morning side. In our study, we first determine the frequencies of the fundamental through third harmonics of toroidal standing Alfvén waves to estimate the mass density and then use the electron density derived from plasma wave spectra to define the average ion mass (=). We obtain from by making an assumption about the number density relative to the number density. All quantities are evaluated in a 15 min moving data window that moves in 5 min steps. By generating ‐MLT maps of for different phases of 102 storms with SYMH minimum lower than nT, we find that a region of high appears on the morning side during the storm main phase and lasts for a few days. is also high on the morning side during the main phase, but this MLT skewing disappears during the recovery phase. Therefore, the MLT asymmetry of the oxygen torus depends on what parameter is considered.

Ultra-low frequency waves, with periods between 1-1000 s, are ubiquitous in the near-Earth plasma environment and play an important role in magnetospheric dynamics and in the transfer of electromagnetic energy from the solar wind to the magnetosphere. A class of those waves, often referred to as Pc3 waves when they are recorded from the ground, with periods between 10 and 45 s, are routinely observed in the dayside magnetosphere. They originate from the ion foreshock, a region of geospace extending upstream of the quasi-parallel portion of Earth’s bow shock. There, the interaction between shock-reflected ions and the incoming solar wind gives rise to a variety of waves, and predominantly fast-magnetosonic waves with a period typically around 30 s. The connection between these waves upstream of the shock and their counterparts observed inside the magnetosphere and on the ground was inferred already early on in space observations due to similar properties, thereby implying the transmission of the waves across near-Earth space, through the shock and the magnetopause. This review provides an overview of foreshock 30-second/Pc3 waves research from the early observations in the 1960s to the present day, covering the entire propagation pathway of these waves, from the foreshock to the ground. We describe the processes at play in the different regions of geospace, and review observational, theoretical and numerical works pertaining to the study of these waves. We conclude this review with unresolved questions and upcoming opportunities in both observations and simulations to further our understanding of these waves.

On the ground, Pi2 magnetic pulsations are detected at low latitudes at all magnetic local times (MLTs), unlike in the inner magnetosphere. To gain insight into the mechanism for the global appearance, we study the MLT dependence of the properties of low‐latitude ground Pi2 pulsations detected at four longitudinally separated stations. The pulsation properties are defined with respect to compressional magnetic field oscillations detected by Van Allen Probes at = 2.5–6.5 within 2 hr of midnight. Up to two peaks between 6.7 and 40 mHz found in the spectrum are selected as possible signatures of the source of ground Pi2 pulsations. For each spectral peak, we compute the coherence of the ground horizontal northward component with , and those events exhibiting high coherence are used in statistical analyses. The radial mode structure of the oscillations indicates they are fundamental or second harmonics of cavity mode oscillations (CMOs). Ground pulsations appear primarily in the component with time delays of less than a few seconds and amplitudes comparable relative to the oscillations in the low‐ region. The observations suggest that, if the dayside ground Pi2 pulsations are driven by ionospheric currents as previously proposed, the current must be coupled to the CMOs, not to the currents flowing on field lines connected to the auroral zone.

The Earth's magnetosphere supports a variety of Magnetohydrodynamic (MHD) normal modes with Ultra Low Frequencies (ULF) including standing Alfvén waves and cavity/waveguide modes. Their amplitudes and frequencies depend in part on the properties of the magnetosphere (size of cavity, wave speed distribution). In this work, we use ∼13 years of Time History of Events and Macroscale Interactions during Substorms satellite magnetic field observations, combined with linearized MHD numerical simulations, to examine the properties of MHD normal modes in the region L > 5 and for frequencies 5 depend on both the magnetopause location and the location of peaks in the radial Alfvén speed profile. Finally, we discuss how these results might be used to better model radiation belt electron dynamics related to ULF waves.

Many previous studies have reported that magnetospheric ultralow frequency waves excited by interplanetary shocks exhibit a strong toroidal component, corresponding to azimuthal displacement of magnetic field lines. However, the toroidal oscillations excited by an interplanetary shock on 27 February 2014 and observed on the dayside by multiple spacecraft were accompanied by a strong poloidal component (radial field line displacement). The frequency of the toroidal oscillations changed with the radial distance of the spacecraft as expected for standing Alfvén waves. We run a 3D linear numerical simulation of this wave event using a model magnetosphere with a realistic radial variation of the Alfvén velocity. The simulated wave fields, when sampled at a radial distance comparable to those of the observations in the real magnetosphere, exhibit polarization similar to the observations. In the simulation, the poloidal component comes from radially standing fast mode waves and the toroidal component results from a field line resonance driven by the fast mode waves. As a consequence, the relative amplitude and phase of the toroidal and poloidal components change with radial distance.

Whitepaper #063 in the Decadal Survey for Solar and Space Physics (Heliophysics) 2024-2033. Main topics: basic research. Additional topics: planetary magnetospheres; ground-based missions/projects; space-based missions/projects.