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Many spacecraft fly within or through a natural and variable particle accelerator powered by the coupling between the magnetosphere and the solar wind: the Earth’s radiation belts. Determining the dominant pathways to plasma energization is a central challenge for radiation belt science and space weather alike. Inward radial transport from an external source was originally thought to be the most important acceleration process occurring in the radiation belts. Yet, when modeling relied on a radial diffusion equation including electron lifetimes, notable discrepancies in model-observation comparisons highlighted a need for improvement. Works by Professor Richard M. Thorne and others showed that energetic (hundreds of keV) electrons interacting with whistler-mode chorus waves could be efficiently accelerated to very high energies. The same principles were soon transposed to understand radiation belt dynamics at Jupiter and Saturn. These results led to a paradigm shift in our understanding of radiation belt acceleration, supported by observations of a growing peak in the radial profile of the phase space density for the most energetic electrons of the Earth’s outer belt. Yet, quantifying the importance of local acceleration at the gyroscale, versus large-scale acceleration associated with radial transport, remains controversial due to various sources of uncertainty. The objective of this review is to provide context to understand the variety of challenges associated with differentiating between the two main radiation belt acceleration processes: radial transport and local acceleration. Challenges range from electron flux measurement analysis to radiation belt modeling based on a three-dimensional Fokker-Planck equation. We also provide recommendations to inform future research on radiation belt radial transport and local acceleration. 

An important question that is being increasingly studied across subdisciplines of Heliophysics is “how do mesoscale phenomena contribute to the global response of the system?” This review paper focuses on this question within two specific but interlinked regions in Near-Earth space: the magnetotail’s transition region to the inner magnetosphere and the ionosphere. There is a concerted effort within the Geospace Environment Modeling (GEM) community to understand the degree to which mesoscale transport in the magnetotail contributes to the global dynamics of magnetic flux transport and dipolarization, particle transport and injections contributing to the storm-time ring current development, and the substorm current wedge. Because the magnetosphere-ionosphere is a tightly coupled system, it is also important to understand how mesoscale transport in the magnetotail impacts auroral precipitation and the global ionospheric system response. Groups within the Coupling, Energetics and Dynamics of Atmospheric Regions Program (CEDAR) community have also been studying how the ionosphere-thermosphere responds to these mesoscale drivers. These specific open questions are part of a larger need to better characterize and quantify mesoscale “messengers” or “conduits” of information—magnetic flux, particle flux, current, and energy—which are key to understanding the global system. After reviewing recent progress and open questions, we suggest datasets that, if developed in the future, will help answer these questions. 

Abstract During the 9 March 2018 event with two consecutive interplanetary shocks compressing the dayside magnetosphere, the azimuthal mode structure and frequency spectrum of ultra low frequency magnetic pulsations are resolved using a cross‐spectral analysis based on high‐fidelity multi‐probe Magnetospheric Multiscale Mission (MMS) magnetometer data. The results based on the MMS 4 and MMS 3 pair of measurements show that shock arrival leads to low mode () magnetic fluctuations in the Pc4‐5 regimes, and smaller spatial scale fluctuations implied by the dominant high mode numbers are observed after both shock signatures hit and passed the magnetosphere. Detailed evolution of the mode structure is also shown for the first shock to reveal the development of high mode structure from a bump‐on‐tail distribution atto a dominant peak atin about 10 min. In addition, an interesting change of sign infrom negative to positive is observed as MMS crosses ∼11 MLT pre‐noon, which is consistent with the picture of wave generation by dayside magnetopause compression and then anti‐sunward propagation. For both shocks, the contribution of higher frequency waves (Pc‐4 range compared with Pc‐5) to the total wave power is found to be negligible before and after the shock impact, but it becomes more significant during the shock impact. 

Abstract The Whole Heliosphere and Planetary Interactions initiative was established to leverage relatively quiet intervals during solar minimum to better understand the interconnectedness of the various domains in the heliosphere. This study provides an expansive mosaic of observations spanning from the Sun, through interplanetary space, to the magnetospheric response and subsequent effects on the ionosphere‐thermosphere‐mesosphere (ITM) system. To accomplish this, a diverse set of observational datasets are utilized from 2019 July 26 to October 16 (i.e., over three Carrington rotations, CR2220, CR2221, and CR2222) with connections of these observations to the more focused studies submitted to this special issue. Particularly, this study focuses on two long‐lived coronal holes and their varying impact in sculpting the heliosphere and driving of the magnetospheric system. As a result, the evolution of coronal holes, impacts on the inner heliosphere solar wind, glimpses at mesoscale solar wind variability, magnetospheric response to these evolving solar wind drivers, and resulting ITM phenomena are captured to reveal the interconnectedness of this system‐of‐systems.