Fast low‐level light pulses from the night sky observed with the SKYFLASH program

Abstract

This paper presents further discussion of and new data on fast subvisual increases in the luminosity of the night sky described in our previous papers. A detailed technical description of the simple telescopic photometers used in the project SKYFLASH and their mode of operation including the detection of polarized Rayleigh‐scattered flashes is provided. Distant lightning storms account for many of the events, and the complex relations between short and long luminous pulses with and without sferics are shown by examples from a new computerized data system, supplemented by two low‐light‐level TV cameras. Of particular interest are the previously observed “long” events having a slow rise and fall, 20‐ms duration, and showing small polarization and no coincident sferic. A group of such events on September 22–23 during the invasion of U.S. coasts by Hurricane Hugo, is discussed in detail. The recently observed “plume” cloud‐top‐to‐stratosphere lightning event is suggested as a possible source type for these flashes. An alternative source may be exploding meteors, recently identified during SKYFLASH observations by low‐light‐level television techniques as the origin of some sky‐wide flash events described herein.

Observations of upper atmospheric optical flashes recorded from an aircraft

“…Television observations from the ground (Franz et al, 1990;Winckler et al, 1993) and from the space shuttle (Boeck et al, 1990;Vaughan et al, 1992;Boeck et al, 1993) confirmed the existence of a previously reported (Boys, 1926; Malan, 1937;Vonnegut, 1980;Vaughan and Vonnegut, 1982; Vaughan and Vonnegut, 1989), but apparenfiy rare, form of upper atmospheric optical emission. Farrell and Desch (1992) calculated that electromagnetic emissions from such longduration (10-20 ms) events would maximize in power at frequencies below ~100 Hz, if they were electrical discharges.…”

On the physics of high altitude lightning

“…The parameter X determines the delay time between the signals arriving at z from the discharge current and from its ground reflection. In addition, the HAL seems to be correlated with lightning pulses with duration ,,-20 ms, called "long events" [Winckler et al, 1993]. For longer delays (broken curve) the reflected field appears as a second peak in Etot, following the decay of the field due to the discharge current.…”

“…A number of authors [Boeket al, 1992;Franz et al, 1990;Winckler et al, 1993;Vaugham and Vonnegut, 1989;Vaugham et al, 1992;Lyons, 1994] reported the presence of optical flashes at high altitudes above thunderstorm clouds. Sentman and Wescott [1993] describe the phenomenon, often called high altitude lightning (HAL), as a luminous column stretched between 30 and 80 kin, with peak luminosity in the vicinity of 60 kin.…”

Electric fields of initial breakdown in positive ground flash

“…Sprites, luminous glows occurring at altitude ranging from 50 to 90 km [Winckler et al, 1993;Sentman and Wescott, 1993;Lyons, 1994], are coincident with large-amplitude positive cloud-to-ground lightning flashes that exhibit extremely large peak return-stroke current and long horizontal progression of in-cloud discharges in the mesoscale convective storm systems The questions above as well as an interest in the nature of the initial breakdown process of positive ground flashes motivated us to investigate the electric field pulses which appear at the beginning of the positive stepped leader. In this paper, the investigated results are presented.…”