High-speed observation of sprite streamers

This article is distributed under the terms of the Creative Commons Attribution License which permits any use, distribution, and reproduction in any medium, provided the original author(s) and the source are credited.Sprites are optical emissions in the mesosphere mainly at altitudes 50–90 km. They are caused by the sudden re-distribution of charge due to lightning in the troposphere which can produce electric fields in the mesosphere in excess of the local breakdown field. The resulting optical displays can be spectacular and this has led to research into the physics and chemistry involved. Imaging at faster than 5,000 frames per second has revealed streamer discharges to be an important and very dynamic part of sprites, and this paper will review high-speed observations of sprite streamers. Streamers are initiated in the 65–85 km altitude range and observed to propagate both down and up at velocities normally in the 106–5 9 107 m/s range. Sprite streamer heads are small, typically less than a few hundreds of meters, but very bright and appear in images much like stars with signals up to that expected of a magnitude -6 star. Many details of streamer formation have been modeled and successfully compared with observations. Streamers frequently split into multiple sub-streamers. The splitting is very fast. To resolve details will require framing rates higher than the maximum 32,000 fps used so far. Sprite streamers are similar to streamers observed in the laboratory and, although many features appear to obey simple scaling laws, recent work indicates that there are limits to the scaling.Research funding has been provided by the US National Science Foundation grants to the University of Alaska Fairbanks, and the US Air Force Academy, and by DARPA through a grant to the University of Florida

Sprite beads and glows arising from the attachment instability in streamer channels

The complex dynamics of a sprite discharge are not limited to the propagation of streamers. After the passage of a streamer head, the ionized channel established in its wake develops intricate luminous patterns that evolve on timescales from 1 up to 100 ms. To investigate these patterns, conventionally called beads and glows, we present high-speed recordings of their onset and decay; our main observation here is that in many cases distant points within a channel decay at the same rate despite considerable differences in the underlying air density. We then show that the properties of beads and glows, including this synchronized decay, are explained by the tendency of electric current within a streamer channel to converge to an uniform value and by an attachment instability of electric discharges in air. However, we also discuss the uncertainty about the chemical reactions that affect the electron density during the sprite decay.©2016. American Geophysical Union. All Rights Reserved.A.L. was supported by the Spanish Ministry of Economy and Competitiveness (MINECO) under projects ESP2013-48032-C5-5-R and FIS2014-61774-EXP and by the Junta de Andalucia, Proyecto de ExcelenciaFQM-5965. H.C.S.-N, M.G.M., and R.H. have been supported by the U.S. National Science Foundation grants 1104441 and 1201683 to the University of Alaska Fairbanks and to the U.S. Air Force Academy respectivelyPeer Reviewe

On the emergence mechanism of carrot sprites

We investigate the launch of negative upward streamers from sprite glows. This phenomenon is readily observed in high‐speed observations of sprites and underlies the classification of sprites into carrot or column types. First, we describe how an attachment instability leads to a sharply defined region in the upper part of the streamer channel. This region has an enhanced electric field, low conductivity and strongly emits in the first positive system of molecular nitrogen. We identify it as the sprite glow. We then show how, in the most common configuration of a carrot sprite, several upward streamers emerge close to the lower boundary of the glow, where negative charge gets trapped and the lateral electric field is high enough. These streamers cut off the current flowing toward the glow and lead to the optical deactivation of the glow above. Finally, we discuss how our results naturally explain angel sprites

Conjugate Breakup

Observation of auroral breakup sequences with all-sky cameras carried on conjugately flying aircraft indicates that the breakup occurs simultaneously or near-simultaneously in the two hemispheres. Observed differences in the auroral behavior during breakup are compatible with the general finding that auroras on the 256° meridian tend to be brighter and more numerous in the northern hemisphere

Simultaneous observations of mesospheric gravity waves and sprites generated by a midwestern thunderstorm

The present report investigates using simultaneous observations of coincident gravity waves and sprites to establish an upper limit on sprite-associated thermal energy deposition in the mesosphere. The University of Alaska operated a variety of optical imagers and photometers at two ground sites in support of the NASA Sprites99 balloon campaign. One site was atop a US Forest Service lookout tower on Bear Mt. in the Black Hills, in western South Dakota. On the night of 18 August 1999 we obtained from this site simultaneous images of sprites and OH airglow modulated by gravity waves emanating from a very active sprite producing thunderstorm over Nebraska, to the Southeast of Bear Mt. Using 25 s exposures with a bare CCD camera equipped with a red filter, we were able to coincidentally record both short duration (\u3c10 ms) but bright (\u3e3 MR) N2 1PG red emissions from sprites and much weaker (~1 kR), but persistent, OH Meinel nightglow emissions. A time lapse movie created from images revealed short period, complete 360° concentric wave structures emanating radially outward from a central excitation region directly above the storm. During the initial stages of the storm outwardly expanding waves possessed a period of τ≈10 min and wavelength λ≈50 km. Over a 1 h interval the waves gradually changed to longer period τ≈11 min and shorter wavelength λ≈40 km. Over the full 2 h observation time, about two dozen bright sprites generated by the underlying thunderstorm were recorded near the center of the outwardly radiating gravity wave pattern. No distinctive OH brightness signatures uniquely associated with the sprites were detected at the level of 2% of the ambient background brightness, establishing an associated upper limit of approximately ΔT ≤ 0.5 K for a neutral temperature perturbation over the volume of the sprites. The corresponding total thermal energy deposited by the sprite is bounded by these measurements to be less than ~1 GJ. This value is well above the total energy deposited into the medium by the sprite, estimated by several independent methods to be on the order of ~1–10 MJ

Use of wild fish and other aquatic organisms as feed in aquaculture: a review of practices and implications in the Asia-Pacific

The solar-wind magnetosphere interaction primarily occurs at altitudes where the dipole component of Earth’s magnetic field is dominating. The disturbances that are created in this interaction propagate along magnetic field lines and interact with the ionosphere–thermosphere system. At ionospheric altitudes, the Earth’s field deviates significantly from a dipole. North–South asymmetries in the magnetic field imply that the magnetosphere–ionosphere–thermosphere (M–I–T) coupling is different in the two hemispheres. In this paper we review the primary differences in the magnetic field at polar latitudes, and the consequences that these have for the M–I–T coupling. We focus on two interhemispheric differences which are thought to have the strongest effects: 1) A difference in the offset between magnetic and geographic poles in the Northern and Southern Hemispheres, and 2) differences in the magnetic field strength at magnetically conjugate regions. These asymmetries lead to differences in plasma convection, neutral winds, total electron content, ion outflow, ionospheric currents and auroral precipitation