next up previous
Next: Blue Jets and Blue Up: Phenomenology Previous: Phenomenology

Sprites

Anecdotal reports of flashes above thunderstorms from ground based observers and airplane pilots date back to the late 19th century (Vaughan and Vonnegut(1989), and references therein). Early considerations of thunderstorm effects on the middle atmosphere included breakdown ionization (Wilson(1925)). The first recorded observation of non-lightning optical emissions associated with thunderstorm activity was made on July 6, 1989 with an intensified low light level camera being calibrated for a sounding rocket campaign (Winckler(1995),Franz et al.(1990)). This initial evidence led to several large campaigns to characterize sprites, many of these were centered around the Yucca Ridge Field Station in Colorado (Lyons(1995),Lyons(1996)) and aircraft flights over the central plains of the United States (Sentman et al.(1995b),Wescott et al.(1995)).

The brightest features of sprites (the `body') occur at ~70 km altitude. Often bright `branches' diverge upward from the `body' of the sprite. Further, there is usually also `hair' topping the sprite, with no presently resolvable structure, and `tendrils' extend to regions below the `body' of the sprite. The upper and lower extremes in altitude have been triangulated to extend above 95 km and below 40 km (Wescott et al.(1998b),Sentman et al.(1995b)). The horizontal extent of a sprite event can be as large as 50 km, but there is a great deal of both vertical and horizontal structure in sprites (Moudry et al.(1998)). Recently, a telescope/imaging system recorded filament-like structures only several tens of meters wide (the resolution of the imaging system) in the `body' of sprites (). In general, as the resolving capabilities of sprite observing instrumentation has increased, further structure has been found indicating sprites are highly structured on a fine scale rather than simply a diffuse glow.

Intensified imaging systems used for sprite, jet, and elves observations have primarily been standard television video rate systems (providing 17 ms resolution from interlaced fields). One of the first optical observations of sprites was made in 1993 using photometers (with 100 us resolution) in a comparative study of optical and Very Low Frequency (VLF) electric field observations (Winckler et al.(1996)). In 1995, observations of sprites and elves using a 4 channel 15us resolution photometer were made by (Fukunishi et al.(1996)). The photometers were occasionally run with a red-pass filter (passing light with wavelength greater 650 nm). One channel in this study was operated with a wide field of view, while three channels were positioned in a vertical array. This study determined that sprites generally begin at ~70 km and then propagate both upward and downward. Sprites were also observed to start with predominantly blue emissions (lasting less than 100 us) followed by a much longer red emission from sprites (Takahashi et al.(1995)). Photometers filtered for specific N2 groups operated at 1.3 ms time resolution were used to make measurements in 1995-1998 (Armstrong et al.(1998)) and are discussed further in the energetics section. In 1996, a blue filtered imager made observations in conjunction with a 50 us resolution blue filtered photometer (Suszcynsky et al.(1998)). The photometer observations and high speed imager observations (with 1 ms resolution) (Stanley et al.(1999)) show that sprites develop and evolve at sub-millisecond time scales. All the spectral observations discussed later in this paper are made using video imaging systems (17 ms resolution) and therefore average over much of the temporal evolution of sprites.


next up previous
Next: Blue Jets and Blue Up: Phenomenology Previous: Phenomenology
Matt Heavner 2002-02-13