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.

Earth's inner magnetosphere is host to a population of highly variable, highly dynamic, and highly energetic particles known as the Van Allen radiation belts (Li & Hudson, 2019;Van Allen et al., 1958, 1959. Of particular interest is the outer radiation belt population that typically occupies radial distances greater than 3-4 R E and is host to extremely energetic MeV electrons. During geomagnetic storms, this population undergoes dramatic enhancements as well as rapid flux dropouts (e.g., Baker et al., 2004;Murphy et al., 2018;Turner et al., 2012). The MeV electron component of the outer radiation belt can cause problematic satellite

During the interval 2012 March 7-11 the geospace experienced a barrage of intense space weather phenomena including the second largest geomagnetic storm of solar cycle 24 so far. Significant ultra-low-frequency wave enhancements and relativistic-electron dropouts in the radiation belts, as well as strong energetic-electron injection events in the magnetosphere were observed. These phenomena were ultimately associated with two ultra-fast (>2000 km s −1 ) coronal mass ejections (CMEs), linked to two X-class flares launched on early 2012 March 7. Given that both powerful events originated from solar active region NOAA 11429 and their onsets were separated by less than an hour, the analysis of the two events and the determination of solar causes and geospace effects are rather challenging. Using satellite data from a flotilla of solar, heliospheric and magnetospheric missions a synergistic Sun-to-Earth study of diverse observational solar, interplanetary and magnetospheric data sets was performed. It was found that only the second CME was Earth-directed. Using a novel method, we estimated its near-Sun magnetic field at 13 R e to be in the range [0.01, 0.16] G. Steep radial fall-offs of the near-Sun CME magnetic field are required to match the magnetic fields of the corresponding interplanetary CME (ICME) at 1 AU. Perturbed upstream solar-wind conditions, as resulting from the shock associated with the Earth-directed CME, offer a decent description of its kinematics. The magnetospheric compression caused by the arrival at 1 AU of the shock associated with the ICME was a key factor for radiation-belt dynamics.

Energy coupling between the solar wind and the Earth's magnetosphere can affect the electron population in the outer radiation belt. However, the precise role of different internal and external mechanisms that leads to changes of the relativistic electron population is not entirely known. This paper describes how ultralow frequency (ULF) wave activity during the passage of Alfvénic solar wind streams contributes to the global recovery of the relativistic electron population in the outer radiation belt. To investigate the contribution of the ULF waves, we searched the Van Allen Probes data for a period in which we can clearly distinguish the enhancement of electron fluxes from the background. We found that the global recovery that started on 22 September 2014, which coincides with the corotating interaction region preceding a high-speed stream and the occurrence of persistent substorm activity, provides an excellent scenario to explore the contribution of ULF waves. To support our analyses, we employed ground-and space-based observational data and global magnetohydrodynamic simulations and calculated the ULF wave radial diffusion coefficients employing an empirical model. Observations show a gradual increase of electron fluxes in the outer radiation belt and a concomitant enhancement of ULF activity that spreads from higher to lower L-shells. Magnetohydrodynamic simulation results agree with observed ULF wave activity in the magnetotail, which leads to both fast and Alfvén modes in the magnetospheric nightside sector. The observations agree with the empirical model and are confirmed by phase space density calculations for this global recovery period.