The Atmosphere‐Space Interactions Monitor (ASIM) was launched to the International Space Station on 2 April 2018. The ASIM payload consists of two main instruments, the Modular X‐ray and Gamma‐ray Sensor (MXGS) for imaging and spectral analysis of Terrestrial Gamma‐ray Flashes (TGFs) and the Modular Multi‐spectral Imaging Array for detection, imaging, and spectral analysis of Transient Luminous Events and lightning. ASIM is the first space mission designed for simultaneous observations of Transient Luminous Events, TGFs, and optical lightning. During the first 10 months of operation (2 June 2018 to 1 April 2019) the MXGS has observed 217 TGFs. In this paper we report several unprecedented measurements and new scientific results obtained by ASIM during this period: (1) simultaneous TGF observations by Fermi Gamma‐ray Burst Monitor and ASIM MXGS revealing the very good detection capability of ASIM MXGS and showing substructures in the TGF, (2) TGFs and Elves produced during the same lightning flash and even simultaneously have been observed, (3) first imaging of TGFs giving a unique source location, (4) strong statistical support for TGFs being produced during the upward propagation of a leader just before a large current pulse heats up the channel and emits a strong optical pulse, and (5) the t50 duration of TGFs observed from space is shorter than previously reported.

A steady-state, analytical model of energetic particle acceleration in radio-jet shear flows due to cosmic-ray viscosity is explored, including particle scattering both into and out of the shear flow acceleration region. This involves solving a mixed Dirichlet–Von Neumann boundary value problem at the edge of the jet. The spectrum of the accelerated particles is harder than the free-escape case from the edge of the jet. The flow velocity   u  = u(r) e z is along the axis of jet (the z-axis).   u is independent of distance z along the jet axis, and u(r) is a monotonically decreasing function of cylindrical radius r from the jet axis. The scattering time where p is the particle momentum in the fluid frame in the shear flow region 0 < r < r 2, and outside the jet (r > r 2). Green’s functions are obtained for monoenergetic injection of particles with momentum p = p 0 at radius r = r 1 (0 < r 1 < r 2). The Green’s function and Green’s formula are used to determine solutions for a general spectrum of particles at . Solutions are obtained corresponding to a monoenergetic spectrum at infinity. We discuss the implications of these results for the acceleration of ultra-high-energy cosmic-rays in active galactic nucleus jet sources. Leaky box models of particle acceleration in shear flows, including synchrotron losses and particle escape, are used to describe the momentum spectrum of accelerated particles. The use of the relativistic telegrapher transport equation model is discussed.

In the spring of 2017 an ER‐2 aircraft campaign was undertaken over continental United States to observe energetic radiation from thunderstorms and lightning. The payload consisted of a suite of instruments designed to detect optical signals, electric fields, and gamma rays from lightning. Starting from Georgia, USA, 16 flights were performed, for a total of about 70 flight hours at a cruise altitude of 20 km. Of these, 45 flight hours were over thunderstorm regions. An analysis of two gamma ray glow events that were observed over Colorado at 21:47 UT on 8 May 2017 is presented. We explore the charge structure of the cloud system, as well as possible mechanisms that can produce the gamma ray glows. The thundercloud system we passed during the gamma ray glow observation had strong convection in the core of the cloud system. Electric field measurements combined with radar and radio measurements suggest an inverted charge structure, with an upper negative charge layer and a lower positive charge layer. Based on modeling results, we were not able to unambiguously determine the production mechanism. Possible mechanisms are either an enhancement of cosmic background locally (above or below 20 km) by an electric field below the local threshold or an enhancement of the cosmic background inside the cloud but then with normal polarity and an electric field well above the Relativistic Runaway Electron Avalanche threshold.

A steady-state, analytical model for the acceleration of energetic charged particles owing to cosmic ray viscosity and fluid shear in relativistic jets is explored. The model extends the work of Webb et al. to alternative forms of the mean scattering time τ (r, p). The flow velocity profile u  = u(r) e z of the jet is independent of distance z along the axis of the jet. u(r) is a monotonic decreasing function of cylindrical radius r about the jet axis. The scattering time is a power-law function of the particle momentum p as measured in the fluid frame. The solutions are eigenfunction expansions involving J 0 Bessel functions and power-law functions of p. The solutions are used to discuss particle acceleration in shear flows in jets, and to determine if high-energy cosmic rays (i.e., with kinetic energies T ∼ EeV) can be accelerated to these energies in candidate AGN jet sources. Green’s function solutions involving J m Bessel functions and more general boundary conditions at the outer edge of the jet are described. We use a time-dependent model to assess the effects of cosmic ray inertia in limiting the upper particle momentum p max(t) due to cosmic ray viscosity and from second-order Fermi acceleration due to Alfvén waves. The model describes the competition between energy gains due to momentum space diffusion and energy losses of the particles due to synchrotron losses or inverse Compton losses.

In its first 2 years of operation, the ground-based Terrestrial gamma ray flash and Energetic Thunderstorm Rooftop Array (TETRA)-II array of gamma ray detectors has recorded 22 bursts of gamma rays of millisecond-scale duration associated with lightning. In this study, we present the TETRA-II observations detected at the three TETRA-II ground-level sites in Louisiana, Puerto Rico, and Panama together with the simultaneous radio frequency signals from the lightning data sets VAISALA Global Lightning Dataset, VAISALA National Lightning Detection Network, Earth Networks Total Lightning Network, and World Wide Lightning Location Network. The relative timing between the gamma ray events and the lightning activity is a key parameter for understanding the production mechanism(s) of the bursts. The gamma ray time profiles and their correlation with radio sferics suggest that the gamma ray events are initiated by lightning leader activity and are produced near the last stage of lightning leader channel development prior to the lightning return stroke.