One of the most important issues in space physics is to identify the dominant processes that transfer energy from the solar wind to energetic particle populations in Earth's inner magnetosphere. Ultra-low-frequency (ULF) waves are an important consideration as they propagate electromagnetic energy over vast distances with little dissipation and interact with charged particles via drift resonance and drift-bounce resonance. ULF waves also take part in magnetosphereionosphere coupling and thus play an essential role in regulating energy flow throughout the entire system. This review summarizes recent advances in the characterization of ULF Pc3-5 waves in different regions of the magnetosphere, including ion and electron acceleration associated with these waves.

Although the Earth's Van Allen radiation belts were discovered over 50 years ago, the dominant processes responsible for relativistic electron acceleration, transport and loss remain poorly understood. Here we show evidence for the action of coherent acceleration due to resonance with ultra-low frequency waves on a planetary scale. Data from the CRRES probe, and from the recently launched multi-satellite NASA Van Allen Probes mission, with supporting modelling, collectively show coherent ultra-low frequency interactions which high energy resolution data reveals are far more common than either previously thought or observed. The observed modulations and energy-dependent spatial structure indicate a mode of action analogous to a geophysical synchrotron; this new mode of response represents a significant shift in known Van Allen radiation belt dynamics and structure. These periodic collisionless betatron acceleration processes also have applications in understanding the dynamics of, and periodic electromagnetic emissions from, distant plasma-astrophysical systems.

Ultralow frequency (ULF) electromagnetic waves in Earth's magnetosphere can accelerate charged particles via a process called drift resonance. In the conventional drift resonance theory, a default assumption is that the wave growth rate is time independent, positive, and extremely small. However, this is not the case for ULF waves in the real magnetosphere. The ULF waves must have experienced an earlier growth stage when their energy was taken from external and/or internal sources, and as time proceeds the waves have to be damped with a negative growth rate. Therefore, a more generalized theory on particle behavior during different stages of ULF wave evolution is required. In this paper, we introduce a time‐dependent imaginary wave frequency to accommodate the growth and damping of the waves in the drift resonance theory, so that the wave‐particle interactions during the entire wave lifespan can be studied. We then predict from the generalized theory particle signatures during different stages of the wave evolution, which are consistent with observations from Van Allen Probes. The more generalized theory, therefore, provides new insights into ULF wave evolution and wave‐particle interactions in the magnetosphere.

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[1] The adiabatic drift-resonant interaction between relativistic, equatorially mirroring electrons and narrowband, Pc 5 ultra low frequency (ULF) waves in the magnetosphere is investigated using a time-dependent magnetohydrodynamic (MHD) wave model. Attention is focused on the effect of a ULF wave packet with finite duration on the equatorially mirroring, relativistic electron phase space density (PSD) profile. It is demonstrated that a burst of narrow band ULF waves can give rise to the growth of strong localized peaks in PSD with L-shell by nondiffusive radial transport. This contrasts with the diffusive ''external source acceleration mechanism'' described by Green and Kivelson (2004), a radial transport mechanism often attributed to ULF waves, which cannot produce peaks in PSD that increase with time. On the basis of this paradigm, observations of locally growing PSD peaks are usually attributed to very low frequency (VLF) wave acceleration by resonant interactions with lower-band chorus (e.g., Horne et al., 2005). However, we show that in situations where large amplitude, narrow bandwidth ULF waves are also observed, these time-limited coherent ULF waves can also generate growing PSD peaks and under such circumstances may offer an alternative explanation.

[1] On the basis of magnetohydrodynamic simulation results for northward interplanetary magnetic field (IMF) and significant dipole tilt, we describe internal reconnection processes that occur earthward of the magnetopause subsequent to magnetopause reconnection. We discuss the associated ionospheric signatures and show that the internal reconnection occurs not only between a summer lobe and a winter lobe field line but also between a summer lobe field line and a closed field line. The latter internal reconnection drives a pair of convection cells circulating outside the polar cap in the winter ionosphere. In this paper, we refer to these convection cells as ''reciprocal cells'' and the corresponding reconnection as reciprocal cell reconnection. The reciprocal cells are coupled to the so-called lobe cells that are driven by magnetopause reconnection between an IMF line and a summer lobe field line (lobe cell reconnection); these lobe cells circulate inside the polar cap in the summer ionosphere. The reciprocal cell reconnection converts an overdraped lobe field line to a relaxed lobe field line, while the lobe cell reconnection converts a relaxed lobe field line to an overdraped lobe field line. Thus the reciprocal cell reconnection reciprocates with the lobe cell reconnection through the exchange of magnetic flux.

Abstract.A model is presented that describes the excitation of density perturbations and parallel electric fields by standing shear Alfvdn waves on dipole fields in Earth's magnetosphere. The model includes the effects of electron inertia and gyro-kinetic dispersion, accounting for field-aligned variations of the electron and ion temperatures and the ambient plasma density. In a model dipole magnetosphere, it is found that dispersion and nonlinearity determine the depth, spatial structure, and temporal evolution of large-amplitude density fluctuations near to the polar ionospheres. The characteristics of magnetospheric density cavities and their relationship to auroral luminosity, or field-aligned currents, is discussed in the context of recent satellite and ground based observations.

ULF waves can accelerate/decelerate the charged particles including the ring current ions via drift‐bounce resonance, which play an important role in the dynamics of ring current during storm times. This study compares the different behaviors of oxygen ions (10.5–35.1 keV) and protons (0.3–12.3 keV) which simultaneously interact with Pc5 ULF waves observed by Cluster on 3 June 2003. The ULF waves are identified as the fundamental mode oscillations. Both oxygen ions and protons show periodic energy dispersion and pitch angle dispersion signatures, which satisfy the drift‐bounce resonance condition of N=2. The different behaviors of oxygen ions and protons include (1) the resonant energy of oxygen ions is higher than that of protons due to mass difference; (2) the phase space density (PSD) of oxygen ions show relative variations (3.6–6.3) that are much larger than that of protons (<0.4), which indicates a more efficient energy exchange between oxygen ions and ULF waves; (3) the PSD spectra show that oxygen ions are accelerated, while protons are decelerated, which depend on the radial gradient of their PSD; (4) the pitch angle distributions (PADs) of the oxygen ions and protons show negative slope and bidirectional field‐aligned features, respectively, which is related to the preexisting state of ion PADs before the interaction with the ULF waves. In addition, the resonant ions with peak fluxes tracing back to the magnetic equator are always collocated with the accelerating (westward) electric field, which indicate that the ions are mainly accelerated near the magnetic equator and the electric field intensity of ULF waves peaks there.