Introduction

Until recently, it was thought that the dazzling display of light offered by a thunderstorm was confined to the lightning activity in the thunderstorm clouds and between the clouds and the ground. This changed in 1989 with the first scientific documentation of a brilliant optical flash well above a thunderstorm [Franz et al., 1990], following years of intermittent but persistent verbal reports of such high-altitude flashes by pilots and others [Vaughan and Vonnegut, 1989, and references therein]. Publication of the photograph of the flash stoked a great amount of scientific interest in the subject of lightning-associated high-altitude (above cloud) flashes, and within a few years a menagerie of different kinds of flashes had been discovered. The different kinds of flashes were given fanciful names like "sprites" and "elves" to reflect their fleeting and hard-to-catch nature (and to steer clear of names suggesting causative mechanisms which had not yet been pinned down), and the entire category of flashes came to be known as transient luminous events (TLEs). Examples of various kinds of TLEs are shown in Figure 1.

Figure 1: Examples of various transient luminous events: (a) the first recorded sprite (from Figure 1 of Franz et al. [1990]), (b) the first color recording of a sprite (from Figure 1 of Sentman et al. [1995]), (c) the first recorded elve (from Figure 2 of Boeck et al. [1992], (d) one of the first recordings of a jet (from Figure 3 of Wescott et al. [1995]), (e) the first recording of a rare gigantic jet (from Figure 2 of Pasko et al. [2002])

• Sprites are large, brief, and often highly-structured bursts of light occurring high above thunderstorms in response to cloud-to-ground lightning flashes that remove large amounts of charge from the upper portions of a cloud. A single sprite can span altitudes from 40 km up to 80 km (the cloud itself is no higher than 15 km) with a width of several tens of kilometers at its widest. Often, the highest parts of a sprite feature a diffuse, red glow (called a halo) while the lower parts of a sprite feature highly structured streamers of a blue color, although halos can appear without streamers and streamers can appear without halos. Sprites last for several milliseconds to several tens of milliseconds, long enough to be barely perceptible to the human eye. The now-accepted theory of sprite formation was first put forth by the VLF Group [Pasko et al., 1996; Pasko et al., 1997; Pasko et al., 1995].
• Elves are rapidly expanding rings of predominantly red light centered well above a causative cloud-to-ground lightning return stroke. Elves expand radially outward along the lower edge of the ionosphere (80-90 km) with an apparent speed faster than light and can attain diameters up to 300 km across (covering an area similar to the size of West Virginia) on timescales faster than 1 ms. The rapid timescales make elves too quick to see with the naked eye and difficult to catch on standard 30 fps video cameras, but use of specially-designed photometric imaging instruments have shown that elves are easily the most frequently occurring TLE on the planet (a recent 3-year satellite study observed around 600 sprites and over 5000 elves, with a globally averaged elve occurrence rate of over 3 elves/min) [Chen et al., 2008]. Large storms can produce hundreds of elves over the course of a few hours [Newsome and Inan, 2010]. Elves are an example of a natural phenomenon that was predicted to occur before it was actually first observed: the VLF Group first hypothesized the existence of elves in [Inan et al., 1991].
• Jets are associated with upward-directed lightning shooting out of cloud tops and vary greatly in size. Blue in color, the smallest jets are called blue starters and extend only a few kilometers from the cloud top into the clear air above. Jets extend much further into the stratosphere, to altitudes around 40 km. Both starters and jets occur relatively frequently (around the same rate as sprites). Gigantic jets, which reach altitudes as high as 80 km, are on the other hand exceedingly rare. In the same 3-year study cited above where over 5000 elves and around 600 sprites were observed, only 13 gigantic jets were observed, predominantly over oceans [Chen et al., 2008].

The VLF Group and TLEs

The VLF Group has been deeply involved in TLE research since the subject's scientific inception in 1989, making numerous theoretical contributions and observational discoveries. Much of the VLF Group's work has been driven by in-the-field TLE observation campaigns carried out all over the world with novel imaging instrumentation developed and built at Stanford. Locations for these campaigns over the years have included the Rocky Mountain Front Range of Colorado (looking into the US Great Plains), the Magdalena Mountains of New Mexico, Arecibo Observatory in Puerto Rico, Pic du Midi in the French Pyrenees, Japan, and South Africa.

Figure 2: Photos from various TLE observation campaigns over the years: (lower left) Yucca Ridge Field Site near Ft. Collins, Colorado, (middle and lower right) Observatoire Midi-Pyrenees at the top of Pic du Midi in the French Pyrenees, (right side) South African Astronomical Observatory near Sutherland, South Africa, (top) Langmuir Laboratory at the top of South Baldy Peak in the Magdalena Mountains near Socorro, New Mexico

Novel approaches to TLE imaging and imaging instruments developed by the VLF Group include:

• The PIPER Instrument [Marshall et al., 2008], a 64-anode two-dimensional photometric array imager capable of free-running (non-triggered) ground-based recording of aggregate elve activity over active thunderstorms [Newsome and Inan, 2010]. PIPER was the first instrument to document hundreds of elves from a single storm system as well as observe an unusual class of elve (elve doublets, see Figure 3) in large numbers. PIPER has been deployed for TLE observation in New Mexico, Colorado, Puerto Rico, and France.
• High Speed Video with >1000 fps frame rates (sub-millisecond time resolution) have been used to study streamer propagation in sprites from Langmuir Laboratory in New Mexico [Marshall and Inan, 2005; Marshall and Inan, 2006].
• Telescopic Imaging involving a 41 cm aperture f/4.5 Dobsonian telescope operated from Langmuir Laboratory made the first discovery of small bead-like streamer propagation in the highly-structured lower portion of sprites [Gerken et al., 2000; Gerken and Inan, 2003; Gerken and Inan, 2002].
• The WASP (Wide-angle Array for Sprite Photometry), a two-row horizontal photometer array, used in numerous sprite observation campaigns, including the conjugate sprite observation campaign from South Africa [Marshall et al., 2005].
• The Fly's Eye, one of the first photometer arrays developed and deployed to study elves, made significant discoveries about elves' shapes and motions [Inan et al., 1997] production by negative cloud-to-ground-flashes [Barrington-Leigh and Inan, 1999] as well as the discovery of halos as TLEs distinct from sprites [Barrington-Leigh et al., 2001].

Figure 3: Observation of a rare elve doublet over West Texas as seen by the PIPER instrument from Langmuir Laboratory in New Mexico

Acknowledgements

Recent study of TLEs by the VLF Group has been funded by the Office of Naval Research (ONR).

Portions of this material are based upon work supported by the National Science Foundation under Grant No. ATM-0836326.

Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.

Bibliography

Barrington-Leigh, C. P., and U. S. Inan, Elves triggered by positive and negative lightning discharges, Geophys. Res. Lett., 26(6), 683-686, 1999.

Barrington-Leigh, C. P., U. S. Inan, and M. Stanley, Identification of sprites and elves with intensified video and broadband array photometry, J. Geophys. Res., 106(A2), 1741-1750, 2001.

Boeck, W. L., O. H. Vaughan, R. Blakeslee, B. Vonnegut, and M. Brook, Lightning induced brightening in the airglow layer, Geophys. Res. Lett., 19(2), 99-102, 1992.

Chen, A. B., et al., Global distributions and occurrence rates of transient luminous events, J. Geophys. Res., 113, A08306, doi: rm10.1029/2008JA013101, 2008.

Franz, R. C., R. J. Nemzek, and J. R. Winckler, Television image of a large upward electric discharge above a thunderstorm, Science, 249(4964), 48-51, 1990.

Gerken, E. A., and U. S. Inan, A survey of streamer and diffuse glow dynamics observed in sprites using telescopic imagery, J. Geophys. Res., 107(A11), doi: rm10.1029/2002JA009248, 2002.

Gerken, E. A., and U. S. Inan, Observations of decameter-scale morphologies in sprites, J. Atmos. Sol.-Terr. Phys., 65(5), 567-572, doi: rm10.1016/S1364-6826(02)00333-4, 2003.

Gerken, E. A., U. S. Inan, and C. P. Barrington-Leigh, Telescopic imaging of sprites, Geophys. Res. Lett., 27(17), 2637-2640, 2000.

Inan, U. S., T. F. Bell, and J. V. Rodriguez, Heating and ionization of the lower ionosphere by lightning, Geophys. Res. Lett., 18(4), 705-708, 1991.

Inan, U. S., C. Barrington-Leigh, S. Hansen, V. S. Glukhov, T. F. Bell, and R. Rairden, Rapid lateral expansion of optical luminosity in lightning-induced ionospheric flashes referred to as 'elves', Geophys. Res. Lett., 24(5), 583-586, 1997.

Marshall, R., R. Newsome, and U. Inan, Fast photometric imaging using orthogonal linear arrays, IEEE Trans. Geosci. Remote Sens., 46(11), 3885-3893, doi: rm10.1109/TGRS.2008.2000824, 2008.

Marshall, R. A., and U. S. Inan, High-speed telescopic imaging of sprites, Geophys. Res. Lett., 32, L05804, doi: rm10.1029/2004GL021988, 2005.

Marshall, R. A., and U. S. Inan, High-speed measurements of small-scale features in sprites: Sizes and lifetimes, Radio Sci., 41, RS6S43, doi: rm10.1029/2005RS003353, 2006.

Marshall, R. A., U. S. Inan, T. Neubert, A. Hughes, G. Satori, J. Bor, A. Collier, and T. H. Allin, Optical observations geomagnetically conjugate to sprite-producing lightning discharges, Ann. Geophys., 23, 2231-2237, doi: rm10.5194/angeo-23-2231-2005, 2005.

Newsome, R. T., and U. S. Inan, Free-running ground-based photometric array imaging of transient luminous events, J. Geophys. Res., A00E41, doi: rm10.1029/2009JA014834, 2010.

Pasko, V. P., U. S. Inan, Y. N. Taranenko, and T. F. Bell, Heating, ionization and upward discharges in the mesosphere due to intense quasi-electrostatic thundercloud fields, Geophys. Res. Lett., 22(4), 365-368, 1995.

Pasko, V. P., U. S. Inan, and T. F. Bell, Blue jets produced by quasi-electrostatic pre-discharge thundercloud fields, Geophys. Res. Lett., 23(3), 301-304, 1996.

Pasko, V. P., U. S. Inan, T. F. Bell, and Y. N. Taranenko, Sprites produced by quasi-electrostatic heating and ionization in the lower ionosphere, J. Geophys. Res., 102(A3), 4529-4561, 1997.

Pasko, V. P., M. A. Stanley, J. D. Mathews, U. S. Inan, and T. G. Wood, Electrical discharge from a thundercloud top to the lower ionosphere, Nature, 416, 152-154, 2002.

Sentman, D. D., E. M. Wescott, D. L. Osborne, D. L. Hampton, and M. J. Heavner, Preliminary results from the sprites94 aircraft campaign: 1. red sprites, Geophys. Res. Lett., 22(10), 1205-1208, 1995.

Vaughan, O. H., and B. Vonnegut, Recent observations of lightning discharges from the top of a thundercloud into the clear air above, J. Geophys. Res., 94(D11), 13,179-12,182, 1989.

Wescott, E. M., D. Sentman, D. Osborne, D. Hampton, and M. Heavner, Preliminary results from the sprites94 aircraft campaign: 2. blue jets, Geophys. Res. Lett., 22(10), 1209-1212, 1995.