Discovery of the Earth's Van Allen radiation belts by instruments flown on Explorer 1 in 1958 was the first major discovery of the Space Age. The dynamic properties of trapped outer zone electrons and the outer boundary of the inner zone proton population, along with source populations, have recently been studied in great detail by instruments on National Aeronautics and Space Administration's Van Allen Probes spacecraft, as well as other data sources like operational spacecraft designed for navigation and terrestrial weather forecasting. The vulnerability of the myriad of spacecraft that is strongly affected by space weather disruptions, as compared to 1958, has motivated the radiation belt community to develop essential improved models for forecasting the space environment we will inhabit in the 21st century and evaluate its impacts on our technological society. In this paper, we provide a review on historical background and recent advances in understanding and modeling acceleration, transport, and loss processes of energetic particles in the Earth's Van Allen radiation belts, followed by outstanding challenges for developing future radiation belt models. The findings on the fundamental physics of the Van Allen radiation belts potentially provide insights into understanding energetic particle dynamics at other magnetized planets in the solar system, exoplanets throughout the universe, as well as in astrophysical and laboratory plasmas. Given the potential Space Weather impact of radiation belt variability on technological systems, these new radiation belt models are expected to play a critical role in our technological society in the future much as meteorological models do today.

In space, huge amounts of energy are released explosively by a mysterious mechanism: magnetic reconnection. Reconnection can abruptly convert energy stored in magnetic fields to energy in charged particles, and power such diverse phenomena as solar and stellar flares, magnetic storms and aurorae in near-Earth space, and major disruptions in magnetically confined fusion devices. It is behind many of the dangerous effects associated with space weather, including damage to satellites, endangering astronauts, and impacting the power grid and pipelines. Understanding reconnection enables us to quantitatively describe and predict these magnetic explosions. Therefore, magnetic reconnection has been at the forefront of scientific interest for many years, and will be for many more. Measuring reconnection is incredibly difficult. However, recently scientists have been able to peek into its machinery. Combining measurements from NASA's Magnetospheric Multiscale mission with supercomputer modeling, scientists have now been able to analyze the inner workings of this elusive mechanism. Even though open questions remain, this new understanding has broad implications. Here, we describe magnetic reconnection, where it plays a role, its impacts on society, and what we now know about it. We point to future research challenges, including implications and the utility of our recently developed knowledge.

In the upper part of the ionosphere at the highest latitudes of the Earth, we often observe isolated chunk of high-density plasma at around 300 km altitude. Those localized regions of enhanced plasma density is called “polar cap patches”. Polar cap patches are known to occur when the interplanetary magnetic field (IMF) is directed southward and the magnetosphere-ionosphere coupling system becomes open to the energy input from the solar wind. Of course, the direction of IMF is an important factor which determines the production of patches. But, we still do not know what controls the diurnal and seasonal variations of patch activity. To answer this question, we have carried out a statistical analysis of the occurrence probability of polar cap patches by using the in situ plasma density data from the low-Earth orbiting satellite Swarm. The statistics clearly show that the diurnal and seasonal variations of patch activity are strongly characterized by the spatial overlap between the high-latitude convection and the high density source plasma in the sunlit region. The statistics also demonstrate the existence of remarkable interhemispheric difference in the patch occurrence distribution, which can be interpreted by the difference in the offset between the geographic and geomagnetic poles between the two hemispheres.

Electromagnetic ion cyclotron (EMIC) waves occur throughout our solar system. The waves have been observed near Earth and are more likely to achieve large amplitudes in the magnetized plasma that exists in a region of space called Earth's dusk-side magnetosphere. Multiple plasma populations exist in this region that can be organized into groups of cold or hot plasmas. Although the hot plasmas can be measured most of the time, the cold plasmas are usually hidden from plasma sensors due to positive spacecraft charging issues; cold plasmas are therefore usually unavailable to help provide a detailed understanding of why dusk-side EMIC waves are generated. The purpose of our study was to investigate measurements made by the NASA Magnetospheric Multiscale satellites to during a time period when the cold plasma species were not hidden and apply these measurements to improve understanding of these dusk-side EMIC waves. The results showed why comprehensive measurements are needed to continue advancing our understanding of EMIC waves as seen by other spacecraft in different regions in Earth's magnetosphere, and how these waves impact other plasma populations.