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.

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 addition to having the right surface temperature, a planet needs an atmosphere to keep surface liquid water stable. Although many planets have been found that may lie in the right temperature range, the existence of an atmosphere is not guaranteed. In particular, for planets that are kept warm by being close to dim stars, there are a number of ways that the star may remove a planetary atmosphere. These atmospheric escape processes depend on the behavior of the star as well as the nature of the planet, including the presence of a planetary magnetic field. Under certain conditions, a magnetic field can protect a planet's atmosphere from the loss due to the direct impact of the stellar wind, but it may actually enhance total atmospheric loss by connecting to the highly variable magnetic field of the stellar wind. These enhancements happen especially for planets close to dim stars. We review the complete range of atmospheric loss processes driven by interaction between a planet and a star to aid in the identification of planets that are both the correct temperature for liquid water and that have a chance of maintaining an atmosphere over long periods of time.

Electrons can become trapped in the near-Earth space environment and stay trapped for days to years, damaging sensitive electronics on space assets. The flux of relativistic electrons varies over time due to source and loss processes, some of which are due to interactions with electromagnetic waves. When the wave amplitudes are small, the effect of the waves on electrons can be modeled as a diffusion equation, with the diffusion coefficients proportional to the wave power. The theory currently used to compute diffusion coefficients was developed in the 1960s using a coordinate system based on all three components of the wave propagation direction. The theory was adapted to a coordinate system that uses the wave frequency and wave normal angle, and has been used for 50 years in a variety of heliophysics models. It is shown for the first time here that the transformation of the coordinate system is inappropriate when the precise distribution of wave k-vectors is available, and the correct transformation is derived. The newly computed diffusion coefficients can be orders of magnitude different than the old coefficients, potentially implying large differences in the predicted time scales over which the source and loss processes will occur for some situations.