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Spectra of small-scale sprite structures, downward and upward propagating streamers, glow, and beads, were recorded with a slitless spectrograph at 10,000 frames per second (fps) from aircraft missions in 2009 and 2013. The spectra are dominated by emissions from molecular nitrogen, the first positive band in the red, and in the blue the second positive band plus the first negative band of molecular nitrogen ions. The excitation threshold for the blue emissions is higher than for the red emissions, so the blue/red ratio can, in principle, be used as a proxy for the electron energy leading to the emissions. We extracted for analysis time series of spectra from 11 sprites: 18 time series from downward propagating streamers, 6 from upward propagating streamers, 14 from glow, and 12 from beads. The total number of spectra in the 50 time series is 953. Blue emissions are almost exclusively associated with streamers indicating the more energetic nature of streamers compared with glow and beads. Both downward and upward propagating streamers start and end with low blue emissions indicating time variations in the associated processes. Because the red and blue nitrogen emissions are significantly affected by quenching, which is altitude dependent, and we do not have sufficiently accurate altitudes, the observed spectral blue/red ratios cannot be directly applied to sprite models. ©2020. American Geophysical Union. All Rights Reserved., We gratefully acknowledge the support of the NSF/NCAR High Performance Instrumented Airborne Platform for Environmental Research (HIAPER) project, as well as the pilots and technical staff that made the HIAPER Gulfstream V missions possible. We also acknowledge the contribution by T. Kanmae who did the early analysis of the spectra, and discussions with N. Liu, F. J. Pérez‐Invernón, and A. Malagón‐Romero. The research has been supported in part by National Science Foundation grants 1104441 to the University of Alaska Fairbanks and 1201683 to the US Air Force Academy. A. Luque was supported by the European Research Council (ERC) under the European Union H2020 programme/ERC grant agreement 681257.

This is an open access article under the terms of the Creative Commons Attribution‐NonCommercial‐NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non‐commercial and no modifications or adaptations are made.https://creativecommons.org/licenses/by-nc-nd/4.0/, 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. ©2019. The Authors., Information on how to access the code used to run the simulations as well as the output data analyzed in this study is available in the supporting information. This work was supported by the European Research Council (ERC) under the European Union H2020 program/ERC Grant Agreement 681257. A. Malagon Romero and A. Luque acknowledge financial support from the State Agency for Research of the Spanish MCIU through the "Center of Excellence Severo Ochoa" Award for the Instituto de Astrofisica de Andalucia (SEV-2017-0709). J. Teunissen was supported by postdoctoral fellowship 12Q6117N from Research Foundation-Flanders (FWO). The aircraft observations were sponsored by the Japanese National Broadcasting System, NHK. H. C. Stenbaek-Nielsen and M. G. McHarg were supported by Grants 1104441 and 1201683 from the U.S. National Science Foundation to the University of Alaska Fairbanks and the U.S. Air Force Academy, respectively. The simulation code used for this work is available at this site (https://gitlab.com/MD-CWI-NL/afivo-streamer).The version used for the simulations corresponds to the commit a3e2241fba 15f177b26e79106cef7354cbe09d68. Configuration files as well as the output data are available at this site (https://doi.org/10.5281/zenodo.3478177).

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 respectively

The purpose of this paper is to introduce electrical discharge phenomena known as transient luminous events above thunderstorms to the lightning protection community. Transient luminous events include the upward electrical discharges from thunderstorms known as starters, jets, and gigantic jets, and electrical discharges initiated in the lower ionosphere such as sprites, halos, and elves. We give an overview of these phenomena with a focus on starters, jets, gigantic jets, and sprites, because similar to ordinary lightning, streamers and leaders are basic components of these four types of transient luminous events. We present a few recent observations to illustrate their main properties and briefly review the theories. The research in transient luminous events has not only advanced our understanding of the effects of thunderstorms and lightning in the middle and upper atmosphere, but also improved our knowledge of basic electrical discharge processes critical for sparks and lightning.

International audience; High-speed intensified cameras are commonly used to observe and study the transient luminous events known as sprite halos and sprite streamers occurring in the Earth's upper atmosphere in association with thunderstorm activity. In such observations, the phosphor persistence in the image intensifier, depending on its characteristic decay time, might lead to a significant distortion of the optical signals recorded by those cameras. In this paper, we analyze the observational data obtained using different camera systems to discuss the effects of phosphor persistence on high-speed video observations of sprites, and introduce a deconvolution technique to effectively reduce such effects. The discussed technique could also be used to enhance the high-speed images of other transient optical phenomena in the case when the phosphor persistence has a characteristic decay time that is comparable with the temporal resolution of the cameras required to resolve the phenomena.

[1] The lightning current waveforms observed simultaneously with high-speed video records of a column and a carrot sprite event are incorporated in a plasma fluid model to provide quantitative explanation of these two distinct morphological classes of transient luminous events. We calculate the strength of the lightning-induced electric field at sprite altitudes using a time integral of the ionization frequency ∫0tνi(E/N)dt. For the studied two events, modeling results indicate that these integral values never exceed 18 in the lower ionosphere, which is the minimum value required for the initiation of streamers from single seed electrons according to the Raether-Meek criterion. It is therefore suggested that the presence of electron inhomogeneities is a necessary condition for the initiation of sprite streamers. It is further demonstrated using streamer modeling that a minimum value of the integral ∼10 is necessary to initiate upward negative streamers from inhomogeneities, corresponding to a minimum charge moment change of ∼500 C km under typical nighttime conditions. If the integral values in the entire upper atmosphere are smaller than ∼10, only column sprites can be produced, dominated by downward positive streamers.

The descent of the Stardust Sample Return Capsule on 15 January 2006 was observed with an intensified highframe-rate slitless spectrograph, which was operated in staring mode. Spectra were recorded at 300 frames per second from 09:57:47 to 09:57:57 UTC (Coordinated Universal Time), around the time of peak deceleration of the capsule. At this time, the spectra contained continuum emission from the hot surface and appeared to be featureless otherwise, with the exception of the telluric absorption caused by molecular oxygen in the atmosphere between the capsule and the observer. By averaging all spectra together, however, it was possible to bring outweak emission lines of oxygen and potassium over the time frame 09:57:51–54 UTC and sodium emission during 09:57:48–51 UTC. The spectral sequence was analyzed for signs of variation in the total emission from a possible wobbling of the capsule at periods higher than the spin rate. No such variationwas observed above the detection threshold of 5 amplitude in the line of sight.

Observations of naturally enhanced ion acoustic lines (NEIALs) taken with the Poker Flat Incoherent Scatter Radar (PFISR) using a mode with very high time resolution are presented. The auroral event took place over Poker Flat, Alaska on 8 February 2007 at 09:35 UT (~22:00 MLT), and the radar data are complemented by common-volume high-resolution auroral imaging. The NEIALs occurred during only one of the standard 15-s integration periods. The raw data of this time show very intermittent NEIALs which occur only during a few very short time intervals (≤1 s) within the 15-s period. The time sampling of the raw data, ~19 ms on average, allows study of the time development of the NEIALs, though there are indications that even finer time resolution would be of interest. The analysis is based on the assumption that the NEIAL returns are the result of Bragg scattering from ion-acoustic waves that have been enhanced significantly above thermal levels. The spectra of the raw data indicate that although the up- and down-shifted shoulders can both become enhanced at the same time, (within 19 ms), they are most often enhanced individually. The overall power in the up-and down-shifted shoulders is approximately equal throughout the event, with the exception of one time, when very large up-shifted power was observed with no corresponding down-shifted power. This indicates that during the 480 μs pulse, the strongly enhanced ion-acoustic waves were only traveling downward and not upward. The exact time that the NEIALs occurred was when the radar beam was on the boundary of a fast-moving (~10 km/s), bright auroral structure, as seen in the high resolution auroral imaging of the magnetic zenith. When viewed with high time resolution, the occurrence of NEIALs is associated with rapid changes in auroral luminosity within the radar field of view due to fast-moving auroral fine structures.

Previous observations have shown that pulsating aurora sometimes occurs with patches of a vertical extent that is thinner than would be expected for aurora caused by collisional processes, implying that local ionospheric processes are important in causing the narrow luminosity enhancement. However, Poker Flat Incoherent Scatter Radar (PFISR) data from four pulsating aurora events, during the Rocket Observations of Pulsating Aurora (ROPA) mission in January and February 2007, show that the electron density profile associated with the pulsating patches had a thickness of ∼ 15 – 25 km in all four cases and that, therefore, these are not examples of such thin enhancements. A numerical model of the associated volume emission rates for the night of the ROPA launch supports this conclusion. In the process of modeling the volume emission rates, the PFISR data are inverted to calculate the associated electron energy distribution for comparison with in situ electron measurements from ROPA and the REIMEI satellite. The modeled distribution shows a diffuse plasma sheet population which gradually decreases in energy over the course of the event, resulting in ∼ 6 – 8 keV precipitation by the end of the PFISR data interval, in agreement with the ROPA/REIMEI measurements.

We present results from a coordinated camera and radar study of the auroral ionosphere conducted during March of 2006 from Poker Flat, Alaska. The campaign was conducted to coincide with engineering tests of the first quarter installation of the Poker Flat Incoherent Scatter Radar (PFISR). On 31 March 2006, a moderately intense auroral arc, (~10 kR at 557.7 nm), was located in the local magnetic zenith at Poker Flat. During this event the radar observed 7 distinct periods of abnormally large backscattered power from the F-region. These were only observed in the field-aligned radar beam, and radar spectra from these seven times show naturally enhanced ion-acoustic lines (NEIALs), the first observed with PFISR. These times corresponded to (a) when the polar cap boundary of the auroral oval passed through the magnetic zenith, and (b) when small-scale filamentary dark structures were visible in the magnetic zenith. The presence of both (a) and (b) was necessary for their occurrence. Soft electron precipitation occurs near the magnetic zenith during these same times. The electron density in the vicinity where NEIALs have been observed by previous studies is roughly between 5 and 30×1010 m−3. Broad-band extremely low frequency (BBELF) wave activity is observed in situ by satellites and sounding rockets to occur with similar morphology, during active auroral conditions, associated with the poleward edge of the aurora and soft electron precipitation. The observations presented here suggest further investigation of the idea that NEIALs and BBELF wave activity are differently-observed aspects of the same wave phenomenon. If a connection between NEIALs and BBELF can be established with more data, this could provide a link between in situ measurements of downward current regions (DCRs) and dynamic aurora, and ground-based observations of dark auroral structures and NEIALs. Identification of in situ processes, namely wave activity, in ground-based signatures could have many implications. One specific example of interest is identifying and following the temporal and spatial evolution of regions of potential ion outflow over large spatial and temporal scales using ground-based optical observations.

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.

Active Plasma Experiment North Star was launched from Poker Flat Research Range, Alaska, on 22 January 1999 at 13:57:03 UT, with two explosive-type generators that produced an artificial aluminum plasma jet. The purpose of this experiment was to study the interaction of the artificial plasma jet with the ambient plasma. The first release occurred at 363 km, and a ∼90% reduction of the geomagnetic field was observed on three separate daughter payloads. The diamagnetic signatures suggest that the plasma cloud was highly localized (i.e., cloud dimensions ∼Al + gyroradius) traveling with a velocity of roughly 25 km/s perpendicular to the geomagnetic field. A hybrid code simulation provided an estimate of the plasma distribution and a qualitative description of the evolution of the plasma cloud. The simulation showed that the plasma cloud polarized and E x B drifted while transferring momentum to the ambient plasma via an Alfvenie disturbance. The model results are in good qualitative agreement with data from the plasma diagnostics payload.

Extraterrestrial organic matter may have been chemically altered into forms more ameanable for prebiotic chemistry in the wake of a meteor after ablation. We measured the rate of cooling of the plasma in the meteor wake from the intensity decay just behind a meteoroid by freezing its motion in high frame-rate 1000 frames/s video images, with an intensified camera that has a short phosphor decay time. Though the resulting cooling rate was found to be lower than theoretically predicted, our calculations indicated that there would have been insufficient collisions to break apart large organic compounds before most reactive radicals and electrons were lost from the air plasma. Organic molecules delivered from space to the early Earth via meteors might therefore have survived in a chemically altered form. In addition, we discovered that relatively small meteoroids generated far-ultraviolet emission that is absorbed in the immediate environment of the meteoroid, which may chemically alter the atmosphere over a much larger region than previously recognized.

Historically, the process of transient luminous event (TLE) detection has required an alert human observer on a low light level television (LLTV) monitor, either in real time or playback. The 1999 Sprites Balloon Campaign payloads had all-sky upward looking photometers not sensitive to events below the balloons. The photometer data was examined at the time of National Lightning Detection Network (NLDN) lightening strokes to find TLEs that were missed visually. 3602 events were analyzed in 4.1 h of storm time. Threshold current moments of ∼50 kA km for the positive cloud to ground (+CG) TLEs and ∼−5 kA km for negative cloud to ground (−CG) TLEs are found.

A bright, estimated magnitude −3, Leonid meteor was observed from the University of Alaska Fairbanks Poker Flat Research Range north of Fairbanks, Alaska, at 10:42 UT on 18 November 2001. Images of the meteor were recorded at 1000 frames per second. The millisecond time resolution allows spatial structures in the ablation cloud to be recorded without “smearing” due to the ∼72 km/s Leonid entry velocity. The meteor was first observed at an altitude of about 123 km. As it descended deeper into the atmosphere it brightened and a tail developed. In addition, a bright structure developed in front of the meteor, much resembling a shock, reaching its maximum brightness at an altitude of 104 km. The spatial scale-size of the shock-like structure is large, several 100 m. Observation of such structure has, to our knowledge, not been reported before. We do not have an explanation for its cause. If its formation is similar to the shock observed around re-entry vehicles it would indicate the size of the meteor to be ∼100 m, which is clearly not realistic. The meteor will create ionization, but at 105 km the ion mean-free path is only ∼1 m and it is difficult to see how ionospheric plasma instabilities driven by the ionization caused by the meteor can result in ∼100 m structures. It is more likely that the structure is associated with photo-chemical processes driven by the intense UV radiation from the hot meteor. A simple computer simulation appears to be able to reproduce the observations. However, the process cannot just be photo ionization of the ambient molecular oxygen or nitrogen as the required total photon flux is too large to be energetically possible.

Elves, halos and sprites were observed during August 1999 with a 1 ms high speed imager. The higher time resolution compared to conventional television cameras (17 or 20 ms ) allowed excellent images of the three phenomena temporally separate from each other to be obtained. Analysis of images of elves and halos indicates that the causal lightning-generated electromagnetic pulse and quasi-electro static fields are homogeneous and any small-scale (sub- 10 km ) structure, if visible, is most likely due to a structured atmosphere. Observations of sprites initiated to the side of a halo, without a halo, and from beads left over from a previous sprite, respectively, all suggest sub-pixel ( km ) background structures in atmospheric pressure or composition as being the dominant factors in determining the sprite “seed” location, or site of sprite initiation.

The active geophysical rocket experiment “North Star” was carried out in the auroral ionosphere on January 22, 1999, at the Poker Flat Research Range (Alaska, USA) using the American research rocket Black Brant XII with explosive plasma generators on board. Separable modules with scientific equipment were located at distances of from 170 to 1595 m from the plasma source. The experiment continued the series of the Russian–American joint experiments started by the “Fluxus” experiment in 1997. Two injections of aluminum plasma across the magnetic field were conducted in the “North Star” experiment. They were different, since in the first injection a neutral gas cloud was formed in order to increase the plasma ionization due to the interaction of neutrals of the jet and cloud. The first and second injections were conducted at heights of 360 and 280 km, respectively. The measurements have shown that the charged particle density was two orders of magnitude higher in the experiment with the gas release. The magnetic field in the first injection was completely expelled by the dense plasma of the jet. The displacement of the magnetic field in the second injection was negligible. The plasma jet velocity in both injections decreased gradually due to its interaction with the geomagnetic field. One of the most interesting results of the experiment was the conservation of high plasma density during the propagation of the divergent jet to considerable distances. This fact can be explained by the action of the critical ionization velocity mechanism.

Flight 3 of the Sprites99 balloon campaign flew from 00:39:32 to 11:12:00 UTC on 21 August 1999, launched from Ottumwa, Iowa. Three axis electric and magnetic field and ground-based low light level TV observations from three sites were made of more than 68 transient luminous events (TLEs) in the middle atmosphere above thunderstorms in South Dakota and Kansas. At least four TLEs were recorded by two or more stations, including sprites at 0955:36, 0541:58 and 0546:10 UTC and a sprite halo at 0746:35. An event triggered on-board memory, sampling 10 quantities at a rate of 50 kHz per channel for 160 ms per event, recorded bursts for some of the sprites and elves. At the range of the balloon from the lightning and TLE (∼400 km), the vertical electric field perturbation is roughly simulataneous with the light emission. The presence of a radial component of some magnetic pulses suggests that signal propagation was not entirely in the TM mode.

Momentum transfer between an ionized gas cloud moving relative to an ambient magnetized plasma is a general problem in space plasma physics. Obvious examples include the Io–Jupiter interaction, comets, and coronal mass ejections. Active plasma experiments have demonstrated that momentum transfer rates associated with Alfven wave propagation are poorly understood. Barium injection experiments from the Combined Release and Radiation Effects Satellite (CRRES) have shown that dense ionized clouds are capable of E×B drifting over large distances perpendicular to the magnetic field. The CRRES “skidding” distances were much larger than predicted by magnetohydrodynamic theory and it has been proposed that parallel electric fields were a key component in the skidding phenomenon. A two-fluid code was used to demonstrate the role of parallel electric fields in reducing momentum transfer between two distinct plasma populations. In this study, a dense plasma was initialized moving relative to an ambient plasma and per...

Initial results from the Active Plasma EXperiment North Star experiment are presented. The North Star active experiment included two separate plasma jet injections, both perpendicular to the Earth's magnetic field. The objective of the experiment is to investigate the interaction of high-speed (7–40 km/s) plasma jets with the ionospheric plasma and the coupling to the magnetosphere and lower ionosphere. The plasma jets were produced using an Explosive Type Generator device. This device is a shaped-charge device that vaporizes porous aluminum inside the generator and forces the vaporized products out of a nozzle, resulting in the production of a high-speed, partially ionized aluminum plasma jet. Instrumentation on three different payloads was used to obtain multi-point observations of the plasma jet properties, optical radiation from the jets, and ionospheric perturbations. Imagery, high-speed photometry, and spectrographic imagery using ground- and space-based sensors were used to monitor the dynamics and spectral content of the plasma jet. This paper describes the experiment and summarizes the initial results from the North Star experiment.

Sprites are spectacular optical emissions in the mesosphere induced by transient lightning electric fields above thunderstorms. Recently, significant efforts have been devoted to the understanding of the inception mechanism of sprite streamers, the origin of different sprite morphologies, and the lightning polarity asymmetry in producing sprites. In this paper, we present a combination of observational and modeling results explaining the physical parameters and processes that are important for the resolution of these outstanding issues. We first emphasize the presence of plasma inhomogeneities in the lower ionosphere as a necessary condition for the initiation of sprite streamers. Then we explain the relation between sprite morphology and the characteristics of the causative lightning discharges using plasma fluid modeling results in comparison with optical and radio observations. Finally, we discuss the critical factors that account for the lightning polarity asymmetry in producing sprites.

Sprites are spectacular optical emissions in the mesosphere induced by transient lightning electric fields above thunderstorms. Although the streamer nature of sprites has been generally accepted, how these filamentary plasmas are initiated remains a subject of active research. Here we present observational and modelling results showing solid evidence of pre-existing plasma irregularities in association with streamer initiation in the D-region ionosphere. The video observations show that before streamer initiation, kilometre-scale spatial structures descend rapidly with the overall diffuse emissions of the sprite halo, but slow down and stop to form the stationary glow in the vicinity of the streamer onset, from where streamers suddenly emerge. The modelling results reproduce the sub-millisecond halo dynamics and demonstrate that the descending halo structures are optical manifestations of the pre-existing plasma irregularities, which might have been produced by thunderstorm or meteor effects on the D-region ionosphere.

Blue jets and blue starters are partially ionized luminous cones of primarily blue light that propagate upward out of the top of thunderstorms at speeds of order 100 km s−1. Blue jets propagate up ∼40 km, but blue starters, which resemble blue jets, terminate abruptly after only a few kilometers of upward travel. Theories on the origin of blue jets have proposed that they are due to either positive or negative streamers or runaway electrons. Quantitative analysis of new multi-instrument observations of a blue starter from an aircraft during the Energetics of Upper Atmospheric Excitation by Lightning, 1998 (EXL98) campaign of July 1998, shows that the ionization accounts for ∼3% of the observed intensity. Quantitative analysis of a remarkable color photograph of a blue jet taken from Reunion Island in the Indian Ocean shows that the minimum optical energy deposition was ∼0.5 MJ. The same photograph shows details of streamers never before seen.

Optical observations of the CRRES plasma injection experiments revealed a highly structured ion cloud. In an attempt to understand these structures, we have developed a three-dimensional hybrid code to study the interaction between an injected plasma cloud and an ambient magnetized plasma. The primary advantage of this code is a seamless interface between a small plasma cloud, treated with fully kinetic particles, and an MHD fluid (ambient plasma). The results of the simulation are in good agreement with the observations. According to the simulation, a very dense ion core was formed as the neutral barium photoionized. The core polarized and moved in the initial injection direction via E × B drift. The simulation also provided insight into a structured secondary neutral cloud created by Ba-Ba+ charge exchange. Specifically, the ion core split into two parts along the magnetic field, creating a nonuniform distribution of charge-exchanged neutrals. This structured ion core also leads to a herringbone structure in the trailing ion cloud. The code can be applied to similar problems including comets, coronal mass ejections, the Io-Jupiter interaction, and other plasma injection experiments.

We present simultaneous measurements of thermospheric winds, auroral emissions, and ionospheric currents over Alaska, obtained from four separate instruments. Thermospheric (F region) wind maps were recorded by an all-sky imaging Fabry-Perot spectrometer located at Poker Flat and observing at λ630.0 nm. Auroral images at λ557.7 nm were obtained from the low-resolution visible imager on board the Polar satellite. White-light all-sky auroral images were recorded by ground-based all-sky cameras located in Alaska at Poker Flat (65° 07′N, 212° 34′E) and at Kaktovik (70° 06′N, 217° 24′E). Finally, the east-west component of the ionospheric F region plasma convection was inferred using the Alaskan meridian chain of magnetometers. Montage images of these four data sets are presented, projected onto a geographic map of the Alaskan region. We examine a 10-hour period during the Alaskan local nighttime of February 10, 1997. These montages illustrate a close relationship between spatial structures occurring in the aurora, in the ionospheric plasma convection, and in the F region wind field. Latitudinal shear of the geomagnetic zonal wind, often observed in the premidnight time sector, was seen to be associated with both the equatorward and poleward boundaries of the discrete aurora. We focus particularly on a period commencing just after 0900 UT, when a strong shear in the zonal wind was observed to sweep southward across Alaska. After magnetic midnight the wind field was dominated by the emergence of the “cross-polar jet” from the polar cap. This overwhelmed any wind features associated with local auroral processes.

Sprite halos were recently identified as an impulsive but spatially diffuse phenomenon that sometimes occurs just prior to, but distinct from, sprites. The lack of discernible spatial structure and the temporal development sequence in halos differs markedly from the highly structured bodies and tendrils and the complex development sequences of sprites. However, both phenomena are thought to result from an electric field due to charge moment changes usually associated with large positive cloud-to-ground (CG) lightning but also following negative CG flashes. Three-dimensional triangulations of sprites and sprite halos were made between stations in South Dakota and Wyoming in August 1999 during the NASA Sprites99 balloon campaign. Halos were found to have a Gaussian 1/e diameter of ∼66 km and 1/e thickness of ∼4 km. Comparison with the location of the underlying lightning strokes, as recorded by the National Lightning Detection Network (NLDN), confirms that the horizontal position of sprites may be laterally offset by as much as 50 km from the underlying parent lightning discharge, as has been previously reported. The point of maximum apparent brightness for sprite halos occurs at an altitude of ∼78 km, similar to that of sprites. However, unlike sprites, this point tends to be centered directly above the underlying parent lightning discharge, 4.6 ± 2.7 km mean distance from the center of the halo to the NLDN location. This difference in spatial location relative to the underlying lightning suggests that the electrical breakdown associated with discrete sprites may require a random ionizing event such as a micrometeor. In contrast, sprite halos do not appear to require such a random component.

Images of sprites have been recorded at 1 ms resolution revealing several new sprite properties. Sprites appear to occur in a highly structured mesosphere, and we suggest that the cause of some of this structure is the sprite activity itself. Evidence is seen in events where a subsequent nearby sprite appears to re-activate the volume of a previous sprite, in sprites where tendrils and branches develop away from the normally observed vertical direction, and in beads that lasts much longer that the parent sprite. Sprites can be large with horizontal widths of more than 40 km and can extend from the clouds up to the lower ionosphere thus affecting a large volume of the atmosphere. The total horizontal area of sprites during one storm over Nebraska was a significant fraction of the area covered by the associated thunderstorm raising the possibility of larger scale measurable mesospheric effects.

Optical observations of the combined release and radiation effects satellite (CRRES) plasma injection experiments have revealed signatures of the coupling processes between the injected and ambient plasmas. The elongation of the ion cloud along the geomagnetic field indicated the presence of parallel electric fields. A dense core of ions polarized and E×B drifted along the satellite trajectory for 6–10 s. In an effort to understand the momentum coupling between the two plasma populations, a three-dimensional hybrid code is used. The simulation, which neglected electron inertia, showed a very efficient transfer of momentum via Alfven waves and the skidding ion core was stopped within 2 s. To reconcile the difference between the observed and simulated skidding times, we propose that the cloud must have been decoupled from the ambient plasma via parallel electric fields. The parallel fields may be associated with inertial Alfven waves propagating in filamentary current layers at the edges of the ion cloud, o...

The Observations of Electric-field Distributions in the Ionospheric Plasma—A Unique Strategy C (OEDIPUS-C) rocket payload was launched from Poker Flat, Alaska, into an evening aurora at 0638 UT, on November 7, 1995. The payload included a tethered HF transmitter-receiver pair which acted as a topside sounder. The bistatic (two-point) configuration allowed an in situ calibration of the radiated power. The conditions in the magnetosphere and ionosphere during the experiment were monitored by a ground-based network of instruments and by instruments on the GOES 7 satellite in a geosynchronous orbit. In this paper we present results of the data analysis of topside ionograms that were obtained during the down-leg flight of OEDIPUS-C (OC). The relatively low altitudes through which OC carried out topside sounding make the resulting ionograms a novel data set. Ionospheric reflections of the 10-W transmissions were detected at payload heights between 780 and 160 km on the down leg. Near apogee at 824 km, extremely low electron densities (∼100 cm−3) were observed. The monotonic rise in electron density at the payload from apogee to reentry clearly showed that there was no ionospheric F layer peak. The topside-sounding echoes came from all heights between the payload and the E layer peak around 100 km altitude. Strong X-mode ionospheric reflections plus strong O-mode ground reflections were observed. OC thus has provided a close-hand view of a thick, highly structured, auroral E layer sounded at small ranges. The RF signal was efficiently guided along the magnetic field aligned density depletions that were located at the equatorward edges of auroral arcs. Large pulse-to-pulse variations in the amplitude of the ionospheric reflection are not explained by ducting in the geometric-optics sense.

A three-dimensional hybrid code has been developed to study the interaction between small dense plasma clouds and an ambient plasma. The primary advantage of this code is a seamless interface between kinetic particles (plasma cloud) and an MHD fluid (ambient plasma). This interface provides momentum coupling between two distinct ion populations. As a preliminary test of our code, we have simulated the first 3 minutes of the December 1984 AMPTE artificial comet. The results show good agreement with observations of the magnetic field distribution as well as the lateral motion of the comet head. Examples of other applications for this code include comets, coronal mass ejections, Io's plasma torus, and plasma injection experiments.

We present high speed photometric measurements of the auroral intensity of flickering aurora. These measurements reveal the existence of intensity modulations in the discrete aurora above the Nyquist frequency of standard 30 frame per second TV cameras. The intensity fluctuations observed are primarily below 80 Hz, although frequencies up to 180 Hz have been detected. Changes in the spectral characteristics from essentially band-limited to broadband intensity fluctuations and then to narrowband fluctuations are seen in individual discrete arcs within a few minutes. The center frequency of the observed fluctuations also change during this period. Data obtained from a standard narrow field TV camera observing the same arc show that only the low ( ≈ 10 Hertz) narrowband modulations would be considered standard flickering aurora.

Optical observations conjugate to the FAST satellite show good agreement between the widths of auroral structures observed optically and those inferred from the measured electron energy flux. The implication is that these structures are imposed by processes at or above the -4000 km altitude of FAST. A variety of widths down to about 2 km were observed, but there were no examples of finer scale structures. A pre-breakup weak discrete arc at the poleward edge of the diffuse aurora showed electron produced optical structures located on either side of upward going ion beams. The optical emission in the equatorward part of the diffuse aurora was caused almost exclusively by precipitating ions. The optical observations were made over northern Alaska between Jan 31 and Feb 16, 1997, from a jet aircraft carrying an all-sky and three narrow-field TV cameras.

Sprites are spectacular optical emissions in the mesosphere induced by transient lightning electric fields above thunderstorms. These luminous gas discharges often exhibit a brief descending high-altitude diffuse glow in the shape of a pancake with diameters up to ∼80 km near ∼75 km altitude, referred to as a sprite halo, that sometimes is accompanied by development of fine-structured filaments with diameters up to several hundred meters in the altitude range of ∼40 to ∼90 km, commonly referred to as sprite streamers. Since the first video documentation [Franz et al., Science, 249, 48, 1990], sprites have attracted extensive research interest in the last two decades, primarily due to their potential as natural resources for the study of streamer physics, their potential impact on the chemistry in the upper atmosphere, and their ability to perturb the subionospheric radio signals [Qin J., PhD Dissertation, Penn State, 2013, and references therein]. However, up to date, how these filamentary plasmas are initiated in the lower ionosphere remains a subject of active research and existing theories differ fundamentally from each other [Luque and Ebert, Nature Geoscience, 2, 757, 2009; Qin et al., JGR, 116, A06305, 2011]. To better understand the initiation mechanism of sprite streamers, in the present work, a plasma fluid model developed previously by Qin et al. [JGR, 118, 2623, 2013] is used to investigate the initiation of sprite streamers during the development of a sprite halo. High-speed video observations of sprites are also studied in a comparison manner. The reported results suggest that the presence of plasma inhomogeneities in the lower ionosphere is a necessary condition to initiation sprite streamers, in agreement with the recently proposed sprite theory of Qin et al. [2011]. The polarization of the plasma inhomogeneities produces strong space charge field in a localized region, which greatly facilitates the initiation of sprite streamers. The presented results once again reiterate the idea that the strength of the lightning induced electric field produced by a relatively large charge moment change is a necessary but not sufficient factor for initiation of sprite streamers [Lang et al., JGR, 116, A10306, 2011] and the presence of inhomogeneities is an important factor, especially for low charge moment changes.

Auroral arcs frequently exhibit bands of enhanced intensity along the bottom edge. These bands have sharp upper borders and are referred to as enhanced lower borders or auroral hems. They are thought to be related to thin layers of enhanced luminosity observed both in auroral arcs and in pulsating auroras. These anomalous distributions of intensity as a function of altitude are referred to collectively as enhanced aurora (EA) and are of interest because they suggest a more complex process for producing auroral ionization and luminosity than the collisional degradation of precipitating electrons. Spectra of eight auroral hems were obtained with an imaging spectrograph and showed consistently that within the hem, there is an enhanced ratio of N2 1st positive emissions (red) to N2+ 1st negative emissions (blue). The enhancement in the red/blue ratio ranged from 10% to 100%. There was a generally smaller and less consistent enhancement in the ratio of the O2+ 1st negative band at 5275 A to the N2+ 1st negative. Observations of the far red spectra of several hems showed no enhancement in the ratio of N2 1st positive to N2+ Meinel emissions. There were no simultaneous measurements of the Meinel and the N2+ 1st negatives. It is argued that the enhanced spectral ratios are indicative of a population of suprathermal electrons with a sharp energy cutoff near the ionization potential of N2 and that the electrons are energized in situ through wave-particle interactions or possibly by dc electric fields. It is also noted that some but not all of the hems are type b red lower borders.

Video images of the CRRES Gl, G9, and G1lA barium releases made at 400–500 km altitudes over the Caribbean in July 1991 have been analyzed to identify morphological features formed in the very early phases of the releases with the aim of associating specific physical processes with these features. Three features are presented: the observation that the inner edge of the ion cloud is located some distance from the release point along the orbital track (“skidding”); the formation of an elongated striation at the inner edge of the ion cloud and an ion depleted region behind the striation; and the formation of a flat neutral barium disk perpendicular to the magnetic field B that is centered some tens of kilometers from the release point. We are linking these morphological features with effects associated with the polarization of the initially dense ion cloud created by solar UV illumination. A simple physical model of the release and the polarization process is presented. Model predictions are both qualitatively and quantitatively in good agreement with the observations. The skidding distance appears to be controlled by the background ionospheric density and the size of the release; the structure in the early ion cloud is due to electric fields primarily setup and maintained by the polarization of the densest part of the ion cloud; and the neutral disk is formed by charge exchange between counterstreaming ions and neutrals outside the polarized core. For the three releases analyzed, no additional internal plasma processes nor processes coupling the release to the ionosphere appear to be critical for the formation of the observed features in the initial ion cloud.

The Sounding of the Cleft Ion Fountain Energization Region (SCIFER) experiment was conducted to investigate the ionospheric origin of the Cleft Ion Fountain (CIF). In the previous decade several high altitude spacecraft studies concluded that the CIF is the principal source of mass for the magnetosphere, especially O+. Yet the ionospheric cleft in the altitude range between 1000 km and 2000 km had not been explored since the ISIS spacecraft experiments in the 1970s. SCIFER was designed to fill that gap with instrumentation that provided continuous spatial/temporal resolution two orders of magnitude better than that achieved by previous orbiting spacecraft.

Instruments on the Auroral Turbulence rocket detected several intervals of weak electromagnetic oscillations at frequencies of 6–13 Hz in a strongly flickering auroral arc. These oscillations have amplitudes of up to δB ∼ 3 nT and δE ∼ 4 mV/m and have downward field-aligned Poynting fluxes of up to ∼10−5 W/m². Fluctuations in the parallel electron flux at about 9 Hz were observed in association with the strongest of these oscillations. Simultaneous ground-based optical data show that the arc was flickering at frequencies of 8–15 Hz. The observed frequencies would match the oxygen cyclotron frequency at ∼4500 km altitude. In one wave/particle event the apparent lag of the waves behind the modulated electrons implies a modulation source altitude of 2500–5000 km. We interpret these waves as electromagnetic ion cyclotron waves originating in the auroral acceleration region.

[1] Charge rearrangement by sprites is analyzed for a mesoscale convective system (MCS) situated in north Texas and east New Mexico on 15 July 2010. During the thunderstorm, electric field data were recorded by the Langmuir Electric Field Array (LEFA), while magnetic field data were recorded by the charge-moment network near Duke University. A high speed (12500 fps) video system operated at Langmuir Laboratory recorded telescopic images of the sprites. Data from the National Lightning Detection Network (NLDN) show that each sprite was preceded by a series of cloud discharges and cloud-to-ground discharges. The triggering event preceding the sprite was typically a positive cloud-to-ground (+CG) stroke. For one out of the 10 sprites that were recorded, there was a positive hump in the electric field a few milliseconds after the +CG return stroke. The size and shape of the hump roughly matched the light intensity emitted from the sprite. The electric field hump is fit by a sprite current that originates in the ionosphere and propagates downward, producing the same effect as a downward moving positive current. The integral under the current hump was 23.9 C when the velocity of the current pulse was between 0.25 c and 0.55 c. The large sprite current was followed by impulsive electromagnetic radiation which has not been previously reported and could be a recoil effect similar to what is called a “K-change” when it is observed in a lightning flash.

[1] The Cascades2 auroral sounding rocket provides a case study for comparing multipoint in situ ionospheric observations of a nightside auroral poleward boundary intensification with ground-based optical observations of the same event. Cascades2 was launched northward from Poker Flat Alaska on 20 March 2009 at 11:04 UT. The 13 min flight reached an apogee of 564 km over the northern coast of Alaska. The experiment included a five-payload array of in situ instrumentation, ground cameras at three different points under the trajectory, multiple ground magnetometers, the Poker Flat Incoherent Scatter Radar (PFISR) radar, and the Time History of Events and Macroscale Interactions during Substorms (THEMIS) spacecraft in the magnetotail. The rays of the poleward boundary intensification (PBI) curtain have along-arc motions of 8.5 km/s and along-arc spacings of 16 km. Modulated maximum energy envelopes and energy fluxes of the associated electron precipitation correspond to this spatial structure of the visible rays. The electron precipitation is additionally modulated at a higher frequency, and velocity dispersion analysis of these 8 Hz signatures implies Alfvenic wave-particle acceleration of an ambient ionospheric electron source occurring a few hundred km above the observation point. These observations parameterize the curtain of Alfvenic activity above the PBI event, both in the dispersive ionosphere and in the magnetotail reconnection region. The along-arc variations in brightness correspond to variations in precipitating electron energy flux interpreted as an along-arc modulation of the maximum energy of the Alfvenic wave-particle acceleration process; this is a new interpretation of the formation of rayed structures in auroral curtains. We consider the various possible magnetospheric and ionospheric drivers for the control of the observed along-arc structuring and motions.

Imaging of sprites at 16,000 frames per second indicates that streamers may be spawned from spatial structure in the preceding sprite halo. The halo structures descend rapidly with the sprite halo, but slow down and stop to form the stationary glow seen in the vicinity of the streamer onset. The streamers emerge suddenly from the halo structure propagating downwards at ∼107 m/s. The velocities during the rapid descend of the halo are similar to those of the streamers. The luminous halo structures last longer than the main halo, which fades prior to streamer formation.

[1] Simultaneous analyses of measured sprite initiation altitudes with predicted initiation altitudes from simulations enable an examination of our understanding of the sprite initiation mechanism and the modeling techniques to simulate this mesospheric electrical phenomenon. In this work, we selected a subset of sprites optically observed from Langmuir Laboratory, NM; locations near Las Vegas, NM, in 2007 and near Portales, NM, in 2008; and a Duke University field station. The sprites were observed by high-speed imaging with time resolutions of at least 1 ms and by low light level imagers. Sprite initiation altitudes were determined by triangulation between Langmuir Laboratory and either Portales or Las Vegas, while star field analysis determined the approximate measured initiation altitudes for Duke observations. These video observations were coordinated with electromagnetic field measurements from Yucca Ridge Field Station and Duke University, respectively. With a 2-D finite difference time domain model, we simulated the lightning-driven electric fields and predict the likely altitude of sprite initiation and compare these findings with the measured initiation altitude of each sprite analyzed. Of 20 discrete sprite events analyzed, both the measured and the simulation-predicted initiation altitudes indicate that long-delayed sprites tend to initiate at lower altitude. The average discrepancy between the measurements and the simulation results is 0.35 km with a standard deviation of 3.6 km. This consistency not only confirms previous results about the relationship between sprite initiation altitude and time delay but also helps to develop confidence in the models to reveal the sprite physics.

[1] The current configuration in the auroral region is known to consist typically of downward and upward field-aligned current (FAC) sheets, connected in the ionosphere by meridional Pedersen currents, while divergence free electrojets (EJs) flow azimuthally as Hall currents. This configuration of the auroral current circuit was introduced by Bostrom (1964) and labeled as “Type 2,” while he suggested also an alternative “Type 1” configuration, with filamentary FACs connected in the ionosphere by azimuthal Pedersen currents. By using an updated version of the recently developed ALADYN technique, we investigated the divergence of the auroral electrojets for a few FAST crossings over the auroral oval in the 20–22 MLT sector, two of which are presented in detail. Although a precise estimate of the electrojet divergence is difficult, because of several error sources, the results suggest that this divergence can be significant over certain latitude ranges, comparable with the FAC density. Direct FAC-EJ coupling appears to contribute to the ionospheric current closure not only during active times, as already known, but also during rather quiet periods. The quiet time FAC-EJ coupling is likely to be achieved in a mixed “Type 1/Type 2” configuration, with the FAC sheet (Type 1) azimuthally connected to the Pedersen component of the EJ (Type 2). This configuration requires a non-zero tangential component of the electric field, and is therefore more likely realized inside or near the Harang region. At the same time, the divergence of the Hall current is presumably negligible, and likewise the ionospheric polarization, consistent with statistical results published recently. During more active intervals and possible reconfigurations of the auroral current circuit, our results suggest that the FAC-EJ coupling could be also achieved by Hall currents. We conclude by exploring a tentative scenario for the integrated evolution of the ionospheric current closure and Cowling mechanism during the substorm cycle. A systematic examination of more experimental evidence is needed to validate this scenario.

[1] The Cascades-2 sounding rocket was launched on 20 March 2009 at 11:04:00 UT from the Poker Flat Research Range in Alaska, and flew across a series of poleward boundary intensifications (PBIs). The rocket initially crosses a diffuse arc, then crosses the equatorward extent of one PBI (a streamer), and finally crosses the initiation of a separate PBI before entering the polar cap. Each of the crossings have fundamentally different in situ electron energy and pitch angle structure, and different ground optics images of visible aurora. It is found that the diffuse arc has a quasi-static acceleration mechanism, and the intensification at the poleward boundary has an Alfvenic acceleration mechanism. The streamer shows characteristics of both types of acceleration. PFISR data provide ionospheric context for the rocket observations. Three THEMIS satellites in close conjunction with the rocket foot point show earthward flows and slight dipolarizations in the magnetotail associated with the in situ observations of PBI activity. An important goal of the Cascades-2 study is to bring together the different observational communities (rocket, ground cameras, ground radar, satellite) with the same case study. The Cascades-2 experiment is the first sounding rocket observation of a PBI sequence, enabling a detailed investigation of the electron signatures and optical aurora associated with various stages of a PBI sequence as it evolves from an Alfvenic to a more quasi-static structure.

In two barium releases made over the Caribbean during the summer of 1991, the ion cloud was observed to last more than 10 hours. Observations were made with a low light level TV imager and a TV spectrograph operated onboard a Russian research vessel conducting ionospheric research and participating in the NASA-CRRES mission. The two clouds were observed near the release region for 15 to 25 minutes during the twilight following the initial release. One of the releases was made during evening twilight and the ion cloud reappeared the following morning, while the other was a morning release and the late-time observations made the next evening. The observations are highly surprising since, generally, it has been assumed that the ion cloud would dissipate within a few hours. However, the long life time appears consistent with current knowledge about dissipative processes. The long lifetime of barium ion clouds may provide for the most sensitive experimental testing of ionospheric convection models, a possibility with clear implications for future barium release experiments.