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Sprite Streamer Initiation from Natural Mesospheric Structures

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Sprite Streamer Initiation from Natural Mesospheric Structures ARTICLE Received 12 Sep 2014 | Accepted 19 May 2015 | Published 29 Jun 2015 DOI: 10.1038/ncomms8540 Sprite streamer initiation from natural mesospheric structures Ningyu Liu1, Joseph R. Dwyer1,w, Hans C. Stenbaek-Nielsen2 & Matthew G. McHarg3 Sprites are large, luminous electrical discharges in the upper atmosphere caused by intense cloud-to-ground lightning flashes, manifesting an impulsive coupling mechanism between lower and upper atmospheric regions. Their dynamics are governed by filamentary streamer discharges whose propagation properties have been well studied by past work. However, how they are initiated is still under active debate. It has recently been concluded that ionospheric/ mesospheric inhomogeneities are required for their initiation, but it is an open question as to what the sources of those inhomogeneities are. Here we present numerical simulation results to demonstrate that naturally-existing, small-scale mesospheric structures such as those created by gravity waves via instability and breaking are viable sources. The proposed theory is supported by a recent, unique high-speed observation from aircraft flying at 14-km altitude. The theory naturally explains many aspects of observed sprite streamer initiation and has important implications for future observational work. 1 Department of Physics and Space Sciences, Florida Institute of Technology, Melbourne, Florida 32901, USA. 2 Geophysical Institute, University of Alaska, Fairbanks, Alaska 99775, USA. 3 Department of Physics, United States Air Force Academy, Colorado Springs, Colorado 80840, USA. w Present address: Department of Physics, University of New Hampshire, Durham, New Hampshire 03824, USA. Correspondence and requests for materials should be addressed to N.L. (email: nliu@fit.edu). NATURE COMMUNICATIONS | 6:7540 | DOI: 10.1038/ncomms8540 | www.nature.com/naturecommunications 1 & 2015 Macmillan Publishers Limited. All rights reserved. ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms8540 ntense cloud-to-ground (CG) lightning flashes are capable of of the long-delayed sprites, because observed sprites are rarely generating a brief, strong quasi-electrostatic (QE) field in the accompanied by meteors. It is possible that some delayed sprite Imesosphere/lower ionosphere to trigger electrical discharges streamers are initiated by the remnants of previous, dim known as sprites1–4. The observation of sprites was first reported streamers that are not bright enough to be detected by the in scientific literature in the early 1990s5. Subsequent work high-speed imaging system. According to simulations based on indicated that they appear above thunderstorms within a few to electrical engineering circuit models33, another possible hundreds of milliseconds from the parent lightning and may span mechanism to initiate long-delayed sprites is the moderately an altitude range of 40–90 km (refs 6,7). The luminosity of a strong continuing current of the parent CG, which can generate sprite persists only for a few to tens of milliseconds, but its an electric field greater than Ek from 55- to 67-km altitude after a electrical and chemical effects last much longer7. It has been more than 20 ms delay. In this paper, we will consider the found that filamentary streamer discharges are the building possibility of initiation of the long-delayed sprite streamers from blocks of sprites8–10. A streamer is a highly nonlinear ionization small-scale mesospheric structures such as those created by wave that can propagate self-consistently in an electric field below gravity waves. the air breakdown threshold field by carrying a strongly enhanced It is known that thunderstorms also generate gravity waves field region at its wave front. The propagation characteristics of a with a full range of spatial scales including short-period, single sprite streamer, such as acceleration, expansion and small-scale gravity waves with a wavelength of a few tens of brightening, are relatively well understood10–18. However, how kilometres34–40. The generated gravity waves propagate upwards sprite streamers are initiated, a critical question to understand the and spread radially outwards. They reach the mesosphere within responses of the upper atmosphere to sudden, strong QE field tens of minutes and extend laterally hundreds of kilometres or impacts, is still under active debate14,19–26. more above the convection core of thunderstorms. The small- In particular, recent observation-based studies indicate that scale gravity waves have been observed to create band structures sprites can be initiated even if the lightning electric field is only with a typical wavelength range of 20–30 km at the airglow layer 27,28 about a few tenths of the breakdown threshold field Ek . Those around 80–100-km altitude. Smaller, ripple-like structures on the studies also show that about half of the observed sprites appear order of 5–20 km can then be produced via instability or after a more than 10 ms delay from the causative CGs27, and they breaking, and they are often found to superimpose on the band are initiated around 70-km altitude, lower than that of prompt structure. The instability and breaking of the gravity waves can sprites27–29. The long-continuing current of the parent lightning further result in even smaller turbulent structures. The can maintain a moderate electric field at the sprite initiation disturbances associated with the gravity waves typically modify altitude, and sometimes sprite initiation occurs during a 5–20 ms neutral density and temperature by a few per cent. However, rise of the current27. However, there is no indication of sudden because the coefficients of the governing basic discharge processes crossing of a certain threshold condition at the moment of sprite of electrical discharges in the atmosphere are a very sensitive initiation. Another important observational finding is that some function of the ratio of electric field and neutral density, a few per streamers of prompt sprites are initiated from visible structures in cent perturbation in neutral density can result in a few tens of per preceding halos, while the streamers of delayed sprites normally cent change in those coefficients10,41. emerge out of the dark background30. Halos are relatively Here we present numerical simulation results to show that homogeneous glows that are also driven by the QE field of CGs. small-scale mesospheric structures can initiate sprite streamers They typically appear within a few milliseconds of the CGs, under the influence of the lightning electric field inferred from and are centred on 78–80-km altitude with a horizontal extent measurements. Our focus is to investigate the initiation of the of tens of kilometres and a vertical thickness of several long-delayed sprites, but a short discussion on prompt sprite kilometres. A recent study31 reported detection of a initiation is also given. The proposed theory is supported by high- mesospheric irregularity with a volume of 3–19 km3 at 80–85- speed images of a long-delayed sprite event recently recorded at km altitude that was illuminated by a lightning flash causing a aircraft altitude. The combined results from the models and nearby sprite event. observations indicate that the optical intensity ensemble of halos, The initiation of sprite streamers at sub-breakdown conditions streamers and sprites spans a much larger dynamic range than was addressed by a few recent modelling studies21,22,25. It has that of the current imaging system. Therefore, many small, dim been found that streamers can be initiated from thin, dense glows caused by lightning likely elude the detection of current column ionization patches with a plasma density more than three imaging instruments. orders of magnitude, higher than the ambient ionospheric electron density21,22 or from hundreds of metre wide spherical patches with a density about two orders of magnitude higher than Results the ambient value25. Modelling studies have also shown that Model formulation. The simulations are conducted by using a plasma inhomogeneities with a density of two orders of cylindrically symmetric plasma discharge model that accounts for magnitude higher than the ambient are also required for sprite all the processes important for electrical discharges in the upper streamer initiation at overbreakdown conditions19,20,23. The atmosphere. A detailed description of the model can be found in candidate sources of the inhomogeneities include the the Methods section. The observational finding that long-delayed 27,28 ionospheric effects of thunderstorm/lightning activities, meteor sprites are initiated at a few tenths of Ek is generally trails and gravity waves21,22, but no study has unambiguously consistent with the results from a recent halo modelling study41. demonstrated the feasibility of streamer initiation from the This modelling study shows that the front of a typical halo as an inhomogeneities created by any of those processes. A more recent ionization wave descends to 70- to 75-km altitude after a few modelling study has rigorously shown that the dynamic aspects of milliseconds, and the peak electric field becomes smaller than Ek. the luminous halo structures leading to streamer initiation can be At such a field, ionization growth takes place on a timescale of a reproduced by subjecting a dense (108 m À 3) plasma column of few to tens of milliseconds, resulting in the formation of a sharp, 12-km long and 2-km wide that might be created by meteor trails almost stationary halo front. Now, if mesospheric perturbations to a typical lightning field26. However, the lifetime of electron with a spatial scale on the order of the halo front thickness are density is very short (B1 s) at 70-km altitude32. It is reasonable to present in the high-field region of the halo, there will be sufficient expect that meteor trails do not play a major role in the initiation time for them to grow through field enhancement, electron 2 NATURE COMMUNICATIONS | 6:7540 | DOI: 10.1038/ncomms8540 | www.nature.com/naturecommunications & 2015 Macmillan Publishers Limited. All rights reserved. NATURE COMMUNICATIONS | DOI: 10.1038/ncomms8540 ARTICLE density increase, further field enhancement and so on.
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