In the study of relativistic jets one of the key open questions is their interaction with the environment. Herewe study the initial evolution of both electron-proton (e p --+ ) and electron-positron (e ± ) relativistic jets, focusing on their lateral interaction with ambient plasma. We follow the evolution of toroidal magnetic fields generated by both the kinetic Kelvin-Helmholtz and Mushroom instabilities. For an e p --+ jet, the induced magnetic field collimates the jet and electrons are perpendicularly accelerated. As the instabilities saturate and subsequently weaken, the magnetic polarity switches from clockwise to counterclockwise in the middle of the jet. For an e ± jet, we find strong mixing of electrons and positrons with the ambient plasma, resulting in the creation of a bow shock. The merging of current filaments generates density inhomogeneities thatinitiate a forward shock. Strong jetambient plasma mixing prevents a full development of the jet (on the scale studied), revealing evidence for both jet collimation and particle acceleration in the forming bow shock. Differences in the magnetic field structure generated by e p --+ and e ± jets may contribute to the polarization properties of the observed emission in AGN jets and gamma-ray bursts.

We here propose a model to capture the complexity of the streamer corona adjacent to leader stepping and relate it to the production of energetic electrons serving as a source of X-rays and γ-rays, manifesting in terrestrial gamma-ray flashes (TGFs). During its stepping, the leader tip is accompanied by a corona consisting of multitudinous streamers perturbing the air in its vicinity and leaving residual charge behind. We explore the relative importance of air perturbations and preionization on the production of energetic run-away electrons by 2.5D cylindrical Monte Carlo particle simulations of streamers in ambient fields of 16 kV cm −1 and 50 kV cm −1 at ground pressure. We explore preionization levels between 10 10 m −3 and 10 13 m −3 , channel widths between 0.5 and 1.5 times the original streamer widths and air perturbation levels between 0% and 50% of ambient air. We observe that streamers in preionized and perturbed air accelerate more efficiently than in non-ionized and uniform air with air perturbation dominating the streamer acceleration. We find that in unperturbed air preionization levels of 10 11 m −3 are sufficient to explain run-away electron rates measured in conjunction with terrestrial gamma-ray flashes.

Nuclear astrophysics is a field at the intersection of nuclear physics and astrophysics, which seeks to understand the nuclear engines of astronomical objects and the origin of the chemical elements. This white paper summarizes progress and status of the field, the new open questions that have emerged, and the tremendous scientific opportunities that have opened up with major advances in capabilities across an ever growing number of disciplines and subfields that need to be integrated. We take a holistic view of the field discussing the unique challenges and opportunities in nuclear astrophysics in regards to science, diversity, education, and the interdisciplinarity and breadth of the field. Clearly nuclear astrophysics is a dynamic field with a bright future that is entering a new era of discovery opportunities.

Terrestrial gamma ray flashes (TGFs) are beams of high-energy photons associated to lightning. These photons are the bremsstrahlung of energetic electrons whose origin is currently explained by two mechanisms: energizing electrons in weak, but large-scale thundercloud fields or the acceleration of low-energy electrons in strong, but localized fields of lightning leaders. Contemporary measurements by the Atmosphere-Space Interactions Monitor suggest that the production of TGFs is related to the leader step and associated streamer coronae when upward moving intracloud lightning illuminates. Based on these observations, we apply a particle-in-cell Monte Carlo code tracing electrons in the superposed electric field of two encountering streamer coronae and modeling the subsequent photon emission. We also perform a parameter study by solving the deterministic equations of motion for one electron. We find that this mechanism can explain the occurrence of TGFs with photons energies of several MeV lasting for tens to hundreds of μs, in agreement with observations. Plain Language Summary For more than two decades, it has been known that thunderstorms emit high-energy X-rays and rays, the so-called terrestrial gamma ray flashes (TGFs) lasting for tens to hundreds of μs, which are the bremsstrahlung ("braking radiation") of energetic electrons and are the most energetic natural phenomena on Earth. Within the last years, two theories have been crystallized out to explain the origin of energetic electrons: the acceleration and multiplication of energetic electrons as remnants of cosmic rays in the large-scale electric fields of thunderclouds or the acceleration of thermal electrons in high electric fields in the vicinity of the tips of lightning leaders. Contemporary measurements of the Atmosphere-Space Interactions Monitor (ASIM) show that TGFs are produced at the onset of the main optical lightning pulse, indicating that the electron acceleration is related to the upward pointing lighting leader tip. We have performed computational simulations of the electron acceleration in the superposed electric field of two encountering streamer coronae, a compilation of small plasma channels with high-field tips, arising in the proximity of the lightning leader tip and the upper charge layer. We find that this scenario can explain the occurrence of TGFs with energies and durations compatible with previous and contemporary measurements.

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