The Atmosphere-Space Interactions Monitor (ASIM) is an instrument suite on the International Space Station (ISS) for measurements of lightning, Transient Luminous Events (TLEs) and Terrestrial Gamma-ray Flashes (TGFs). Developed in the framework of the European Space Agency (ESA), it was launched April 2, 2018 on the SpaceX CRS-14 flight to the ISS. ASIM was mounted on an external platform of ESA’s Columbus module eleven days later and is planned to take measurements during minimum 3 years. The instruments are an x- and gamma-ray monitor measuring photons from 15 keV to 20 MeV, and an array of three photometers and two cameras measuring in bands at: 180–250 nm, 337 nm and 777.4 nm. Additional objectives that can be addressed with the instruments relate to space physics like aurorae and meteors, and to Earth observation such as dust- and aerosol effects on cloud electrification. The paper describes the scientific objectives of the ASIM mission, the instruments, the mission architecture and the international collaboration supported by the ASIM Science Data Centre. ASIM is the first space mission with a comprehensive suite of instruments designed to measure TLEs and TGFs. Two companion papers describe the instruments in more detail (Ostgaard et al. in Space Sci. Rev., 2019; Chanrion et al. in Space Sci. Rev., 2019).

/pdf/the-asim-mission-on-the-international-space-station-30a43our20.pdf

The ASIM Mission on the International Space Station

Radio emissions continue to provide insight into the production of terrestrial gamma ray flashes (TGFs) by thunderstorms, including the critical question of the conditions under which they are generated. We have identified several TGF-associated lightning radio emissions in which the altitudes of in-cloud lightning leader pulses that precede and follow the TGF can be measured. We combine these with high absolute timing accuracy TGF observations from the Fermi satellite to determine the development of the lightning channel before, during, and after the TGF production. All of these TGFs were produced several milliseconds after the leader had initiated and when the leaders reached 1–2 km in length. After the TGFs, the leaders all continued to ascend for several more kilometers with no dramatic change in their characteristics, although they all exhibited high average velocities of 0.8–1.0 × 106 m/s. Implications in the context of TGF models are discussed. These results paint the first clear picture of the lightning processes that occur before, during, and after TGF production.

/pdf/lightning-leader-altitude-progression-in-terrestrial-gamma-4kmyyfygvh.pdf

Lightning Leader Altitude Progression in Terrestrial Gamma-Ray Flashes†

We report on a terrestrial gamma ray flash (TGF) that occurred on 15 August 2014 coincident with an altitude-triggered lightning at the International Center for Lightning Research and Testing (ICLRT) in North Central Florida. The TGF was observed by a ground-level network of gamma ray, close electric field, distant magnetic field, Lightning Mapping Array (LMA), optical, and radar measurements. Simultaneous gamma ray and LMA data indicate that the upward positive leader of the triggered lightning flash induced relativistic runaway electron avalanches when the leader tip was at about 3.5 km altitude, resulting in the observed TGF. Channel luminosity and electric field data show that there was an initial continuous current (ICC) pulse in the lightning channel to ground during the time of the TGF. Modeling of the observed ICC pulse electric fields measured at close range (100–200 m) indicates that the ICC pulse current had both a slow and fast component (full widths at half maximum of 235 μs and 59 μs) and that the fast component was more or less coincident with the TGF, suggesting a physical association between the relativistic runaway electron avalanches and the ICC pulse observed at ground. Our ICC pulse model reproduces moderately well the measured close electric fields at the ICLRT as well as three independent magnetic field measurements made about 250 km away. Radar and LMA data suggest that there was negative charge near the region in which the TGF was initiated.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1002/2015JD024426

Ground-level observation of a terrestrial gamma ray flash initiated by a triggered lightning

We investigated National Lightning Detection Network reports and lightning radio waveforms in a 44 day observation period to analyze the in-cloud (IC) events producing currents above 200 kA. The results show that there are two distinct classes of IC lightning events with very high peak currents: the well-known narrow bipolar events, and a previously unreported type that we call energetic in-cloud pulses (EIPs). Their temporal and spatial context shows that EIPs are generated from existing negative polarity leaders that are propagating usually upward but sometimes downward. The nearly identical characteristics of EIPs and some previously reported terrestrial gamma ray flashes (TGFs) indicate a likely connection between the two, which further suggests the possibility of downward directed TGFs. These very high peak current IC events also suggest the association of EIPs with ionospheric perturbations and optical emissions known as elves.

/pdf/insights-into-high-peak-current-in-cloud-lightning-events-50ttj4jv19.pdf

Insights into high peak current in-cloud lightning events during thunderstorms

Beijing Lightning Network (BLNET) and the observation on preliminary breakdown processes

Many details of how thunderstorms generate terrestrial gamma ray flashes (TGFs) and other forms of high-energy radiation remain uncertain, including the basic question of where they are produced. We exploit the association of distinct low-frequency radio emissions with generation of terrestrial gamma ray flashes (TGFs) to directly measure for the first time the TGF source altitude. Analysis of two events reveals source altitudes of 11.8 ± 0.4 km and 11.9 ± 0.9 km. This places the source region in the interior of the thunderstorm between the two main charge layers and implies an intrinsic TGF brightness of approximately 10 18 runaway electrons. The electric current in this nontraditional lightning process is found to be strong enough to drive nonlinear effects in the ionosphere, and in one case is comparable to the highest peak current lightning processes on the planet.

/pdf/the-source-altitude-electric-current-and-intrinsic-4q62sdbccn.pdf

The source altitude, electric current, and intrinsic brightness of terrestrial gamma ray flashes

We report on the development of an easily deployable LF near-field interferometric-time of arrival (TOA) 3-D Lightning Mapping Array applied to imaging of entire lightning flashes. An interferometric cross-correlation technique is applied in our system to compute windowed two-sensor time differences with submicrosecond time resolution before TOA is used for source location. Compared to previously reported LF lightning location systems, our system captures many more LF sources. This is due mainly to the improved mapping of continuous lightning processes by using this type of hybrid interferometry/TOA processing method. We show with five station measurements that the array detects and maps different lightning processes, such as stepped and dart leaders, during both in-cloud and cloud-to-ground flashes. Lightning images mapped by our LF system are remarkably similar to those created by VHF mapping systems, which may suggest some special links between LF and VHF emission during lightning processes.

/pdf/a-low-frequency-near-field-interferometric-toa-3-d-lightning-6mo9rm0wrj.pdf

Rahulkumar Solanki

Papers

The source altitude, electric current, and intrinsic brightness of terrestrial gamma ray flashes

A low-frequency near-field interferometric-TOA 3-D Lightning Mapping Array