Interactions between the solar wind and the Earth's magnetosphere manifest many important space weather phenomena. In this paper, magnetosphere‐ionosphere drivers of intense dB/dt produced during geomagnetic storms that occurred on 9 March 2012 and 17 March 2015 are analyzed. A multi‐instrument approach combining Time History of Events and Macroscale Interactions during Substorms (THEMIS) mission space‐borne and ground‐based observations was adopted to examine the magnetosphere‐ionosphere signatures associated with the dB/dt extremes during each storm. To complement the THEMIS measurements, ground‐based magnetometer recordings and All‐Sky Imager observations, equivalent ionospheric currents derived from magnetometer chains across North America and Greenland, and geosynchronous observations from the Los Alamos National Laboratory Synchronous Orbit Particle Analyzer are also examined. Our results show that the most extreme dB/dt variations are associated with marked perturbations in the THEMIS magnetospheric measurements, poleward expanding discrete aurora passing over the magnetometer sites (seen by the ground‐based THEMIS All‐Sky Imagers), intense Pc5 waves, rapid injection of energetic particles, and intense auroral westward currents. Substorms are considered as the major driver with a possible contribution from magnetospheric waves. The findings of this study strongly suggest that the localization of extreme dB/dt variations is most likely related to the mapping of magnetosphere currents to local ionospheric structures.

Ultralow frequency (ULF) waves play a fundamental role in the dynamics of the inner magnetosphere and outer radiation belt during geomagnetic storms. Broadband ULF wave power can transport energetic electrons via radial diffusion, and discrete ULF wave power can energize electrons through a resonant interaction. Using observations from the Magnetospheric Multiscale mission, we characterize the evolution of ULF waves during a high-speed solar wind stream (HSS) and moderate geomagnetic storm while there is an enhancement of the outer radiation belt. The Automated Flare Inference of Oscillations code is used to distinguish discrete ULF wave power from broadband wave power during the HSS. During periods of discrete wave power and utilizing the close separation of the Magnetospheric Multiscale spacecraft, we estimate the toroidal mode ULF azimuthal wave number throughout the geomagnetic storm. We concentrate on the toroidal mode as the HSS compresses the dayside magnetosphere resulting in an asymmetric magnetic field topology where toroidal mode waves can interact with energetic electrons. Analysis of the mode structure and wave numbers demonstrates that the generation of the observed ULF waves is a combination of externally driven waves, via the Kelvin-Helmholtz instability, and internally driven waves, via unstable ion distributions. Further analysis of the periods and toroidal azimuthal wave numbers suggests that these waves can couple with the core electron radiation belt population via the drift resonance during the storm. The azimuthal wave number and structure of ULF wave power (broadband or discrete) have important implications for the inner magnetospheric and radiation belt dynamics.

Magnetopause shadowing and wave‐particle interactions are recognized as the two primary mechanisms for losses of electrons from the outer radiation belt. We investigate these mechanisms, using satellite observations both in interplanetary space and within the magnetosphere and particle drift modeling. Two interplanetary shocks/sheaths impinged upon the magnetopause causing a relativistic electron flux dropout. The magnetic cloud (MC) and interplanetary structure sunward of the MC had primarily northward magnetic field, perhaps leading to a concomitant lack of substorm activity and a 10 daylong quiescent period. The arrival of two shocks caused an unusual electron flux dropout. Test‐particle simulations have shown ∼ 2 to 5 MeV energy, equatorially mirroring electrons with initial values of L≥5.5 can be lost to the magnetosheath via magnetopause shadowing alone. For electron losses at lower L‐shells, coherent chorus wave‐driven pitch angle scattering and ULF wave‐driven radial transport have been shown to be viable mechanisms.

Our previous studies have produced phenomenological models for turbulence in solar wind plasmas on large-(inertial) magnetohydrodynamic scales, based on observations by the Voyager, Ulysses, and THEMIS missions. Here we consider turbulence in the Earth’s magnetosheath, where timescales are often far shorter than those in the heliosheath, using observations from the currently operating Magnetospheric Multiscale (MMS) mission on much smaller kinetic scales. We employ a standard statistical analysis to obtain energy density spectra for the magnetic field strength and the ion speed at high time resolution. We find a clear breakpoint of the magnetic spectrum exponent from −0.8 to −5/2 near the ion gyrofrequency of 0.25 Hz. In fact, just behind the bow shock and near the magnetopause, the availability of the highest-resolution magnetic field observations enables us also to identify the expected spectral exponent of about −3, which is further followed by steeper spectra with the slopes from −7/2 to −11/2 (−16/3) in the kinetic regime above 20 Hz, possibly resulting from the kinetic Alfvén waves. Because the resolution of the ion plasma parameters is somewhat lower than that for the magnetic field, spectra for the ion velocity can only be resolved near the onset of kinetic scales. On the other hand, deep inside the magnetosheath, where only low-resolution data are available and we are still in the magnetohydrodynamic scale range, we recover the well-known −5/3 Kolmogorov’s spectrum. The obtained results on kinetic scales may be useful for better understanding the physical mechanisms governing turbulence.