[Paper Review] First High-speed Video Camera Observations of a Lightning Flash Associated with a Downward Terrestrial Gamma-ray Flash

Rasha Abbasi, Marcelo M. F. Saba+150 more|arXiv (Cornell University)|May 10, 2022

Lightning and Electromagnetic Phenomena4 citations

TL;DR

This study presents the first high-speed video observations of a lightning flash associated with a downward terrestrial gamma-ray flash (TGF), capturing a fast, bright downward negative leader propagating below the cloud base that produced a 154 kA return stroke and a unique TGF burst. The data reveal that the TGF occurred during leader propagation, not at ground contact, and suggest downward TGFs may be a variant of the same physical mechanism as upward TGFs.

ABSTRACT

In this paper, we present the first high-speed video observation of a cloud-to-ground lightning flash and its associated downward-directed Terrestrial Gamma-ray Flash (TGF). The optical emission of the event was observed by a high-speed video camera running at 40,000 frames per second in conjunction with the Telescope Array Surface Detector, Lightning Mapping Array, interferometer, electric-field fast antenna, and the National Lightning Detection Network. The cloud-to-ground flash associated with the observed TGF was formed by a fast downward leader followed by a very intense return stroke peak current of -154 kA. The TGF occurred while the downward leader was below cloud base, and even when it was halfway in its propagation to ground. The suite of gamma-ray and lightning instruments, timing resolution, and source proximity offer us detailed information and therefore a unique look at the TGF phenomena.

Motivation & Objective

• To investigate the optical and electrical characteristics of a lightning flash associated with a downward terrestrial gamma-ray flash (TGF) using high-speed video and electromagnetic instrumentation.
• To determine the precise timing and spatial evolution of the lightning leader and return stroke relative to the TGF burst.
• To examine whether downward-directed TGFs are a variant of the same physical mechanism responsible for upward-directed TGFs.
• To analyze the energy deposit and duration of the TGF in relation to the lightning discharge process.
• To establish a correlation between optical emissions (in 337.0 nm, 391.2 nm, and 777.4 nm bands) and TGF production using synchronized photometers and high-speed video.

Proposed method

• Simultaneous deployment of a high-speed video camera (40,000 fps) to capture optical dynamics of the lightning flash.
• Use of the Telescope Array Surface Detector (TASD) to detect and time the TGF with high precision.
• Integration of lightning detection systems: the Lightning Mapping Array (LMA), Fast Antenna (FA), and INTerFerometer (INTF) for 3D mapping of discharge processes.
• Deployment of photometers at the TASD site to record optical emissions at 337.0 nm (2PN₂), 391.2 nm (1N₂⁺), and 777.4 nm (OI) with high timing resolution.
• Correlation of high-speed video frames with TGF detection and electromagnetic data to establish temporal and spatial relationships.
• Use of PCC 3.6 Phantom software to analyze and reconstruct the optical data from the high-speed camera.

Experimental results

Research questions

• RQ1 What is the precise optical and electrical evolution of the lightning flash that produced the downward TGF?
• RQ2 At what stage of the lightning discharge process—leader propagation or return stroke—does the TGF occur?
• RQ3 How do the energy deposit and duration of the TGF compare to those of upward-directed TGFs?
• RQ4 Is there a measurable correlation between specific optical emissions (e.g., 337.0 nm, 391.2 nm, 777.4 nm) and TGF production?
• RQ5 Can downward-directed TGFs be explained by the same physical mechanism as upward-directed TGFs?

Key findings

• The TGF burst occurred during the propagation of a fast, bright downward negative leader, not at the time of the return stroke.
• The leader was propagating below the cloud base when the TGF was detected, with the last trigger occurring when it was approximately halfway to the ground.
• The TGF exhibited unique features in both energy deposit and duration, distinguishing it from previously observed TGFs.
• The high peak current return stroke associated with the flash reached -154 kA, indicating a strong electrical discharge.
• The storm producing this TGF was uncommon and accounted for a significant fraction of all ground TGFs detected over a ten-year period at the TASD site.
• The synchronized photometer data at 337.0 nm, 391.2 nm, and 777.4 nm will enable future analysis of optical signatures linked to TGF initiation and help verify if downward and upward TGFs stem from the same mechanism.