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.
Key Points
Beads and glows in sprites at distant points of a channel decay exponentially at the same rate
Beads and glows result from an attachment instability in air
Sprites are strongly coupled systems
Diameter-speed relation of sprite streamers Kanmae, T; Stenbaek-Nielsen, H C; McHarg, M G ...
Journal of physics. D, Applied physics,
07/2012, Letnik:
45, Številka:
27
Journal Article
Recenzirano
Propagation and splitting of sprite streamers has been observed at high temporal and spatial resolution using two intensified high-speed CMOS cameras recording at 10 000 and 16 000 frames per second. ...Concurrent video recordings from a remote site provided data for triangulation allowing us to determine accurate altitude scales for the sprites. Diameters and speeds of the sprite streamers were measured from the high-speed images, and the diameters were scaled to the reduced diameters based on the triangulated locations. The sprite streamers with larger reduced diameter move faster than those with smaller diameter; the relation between the reduced diameter and speed is roughly linear. The reduced diameters at 65-70 km altitude are larger than streamer diameters measured at ground pressure in laboratory discharges indicating a deviation from the similarity law possibly due to the effects of the photoionization and an expansion of the streamer head along its propagation over a long distance. The reduced diameter and speed of the sprite streamers agree well with the diameter-velocity relation proposed by Naidis (2009 Phys. Rev. E 79 057401), and the peak electric field of the sprite streamers is estimated to be approximately 3-5 times the breakdown threshold field.
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.
Streamer tip splitting in sprites McHarg, M. G.; Stenbaek-Nielsen, H. C.; Kanmae, T. ...
Journal of Geophysical Research: Space Physics,
August 2010, Letnik:
115, Številka:
A8
Journal Article
Recenzirano
Odprti dostop
Five sprite events were observed above mesoscale thunderstorms in the midwest of the United States on 23 June 2007 with images recorded at 10,000 frames per second. We observe 75 streamer tips that ...propagate with no splitting and 42 streamer tips that split at some point while in the field of view. In general, streamer tips that propagate and do not split are smaller in width, and less bright. The median streamer tip radius for propagating streamer tips is 193 m, while the median value for splitting streamer tips is 389 m. Streamer tips that split become wider and brighter shortly before they are observed to split. Our observations compare favorably with theoretical models based on viscous fingering of the streamer heads leading to streamer tip splitting.
Altitude-time development of sprites McHarg, Matthew G.; Haaland, Ryan K.; Moudry, Dana ...
Journal of Geophysical Research - Space Physics,
November 2002, Letnik:
107, Številka:
A11
Journal Article
Recenzirano
Odprti dostop
Data from sprites using a 16‐channel, multi‐anode photometer (MAP) have been recorded at 0.1 ms resolution. The majority of the sprites have onsets at an altitude of 70–75 km and subsequently ...propagate both upward and downward from this initial altitude. The statistical lifetime of the emissions measured by the blue‐sensitive MAP is approximately 1.3 ms. The velocities of propagation are between 107 and 108m s−1, with larger velocities being measured at higher altitudes. The larger propagation velocities of the upward and downward sprite luminosity channels are consistent with the characteristics of highly over‐voltage streamers in the mesosphere.
The Cluster-mission has enabled a study of the near-Earth magnetospheric lobes throughout the waning part of solar cycle 23. During the first seven years of the mission the satellites crossed this ...region of space regularly from about July to October. We have obtained new and more accurate plasma densities in this region based on spacecraft potential measurements from the EFW-instrument. The plasma density measurements are found by converting the potential measurements using a functional relationship between these two parameters. Our observations have shown that throughout this period a full two thirds of the measurements were contained in the range 0.007–0.092 cm−3 irrespective of solar wind conditions or geomagnetic activity. In fact, the most probable density encountered was 0.047 cm−3, staying roughly constant throughout the entire observation period. The plasma population in this region seems to reflect an equilibrium situation in which the density is independent of the solar wind condition or geomagnetic activity. However, the high density tail of the population (ne>0.2 cm−3) seemed to decrease with the waning solar cycle. This points to a source region influenced by the diminishing solar UV/EUV-intensity. Noting that the quiet time polar wind has just such a development and that it is magnetically coupled to the lobes, it seems likely to assume that this is a prominent source for the lobe plasma.
High-Speed Imaging of Sprite Streamers McHarg, M. G.; Stenbaek-Nielsen, H. C.; Kanmae, T. ...
IEEE transactions on plasma science,
2011-Nov., 2011-11-00, 20111101, Letnik:
39, Številka:
11
Journal Article
Recenzirano
The spatial and temporal developments of sprite streamers in the mesosphere are discussed in the context of the data recorded on August 27, 2009. Streamers initiate at approximately 75 km in altitude ...and are seen to propagate first downward and subsequently upward. Streamer splitting is observed as the streamers propagate. The streamer tips are approximately three times brighter than the planet Jupiter.
Recent actions can benefit or disrupt our current actions and the prefrontal cortex (PFC) is thought to play a major role in the regulation of these actions before they occur. The left PFC has been ...associated with overcoming interference from past events in the context of language production and working memory. The right PFC, and especially the right IFG, has been associated with preparatory inhibition processes. But damage to the right PFC has also been associated with impairment in sustaining actions in motor intentional disorders. Moreover, bilateral dorsolateral PFC has been associated with the ability to maintain task-sets, and improve the performance of current actions based on previous experience. However, potential hemispheric asymmetries in anticipatory regulation of action have not yet been delineated. In the present study, patients with left (n=7) vs. right (n=6) PFC damage due to stroke and 14 aged- and education-matched controls performed a picture naming and a verbal Simon task (participants had to say “right” or “left” depending on the color of the picture while ignoring its position). In both tasks, performance depended on the nature of the preceding trial, but in different ways. In the naming task, performance decreased if previous pictures were from the same rather than from different semantic categories (i.e., semantic interference effect). In the Simon task, performance was better for both compatible (i.e., response matching the position of the stimulus) and incompatible trials when preceded by a trial of the same compatibility (i.e. Gratton effect) relative to sequential trials of different compatibility. Left PFC patients were selectively impaired in picture naming; they had an increased semantic interference effect compared to both right PFC patients and aged-matched controls. Conversely, right PFC patients were selectively impaired in the Simon task compared to controls or left PFC patients; they showed no benefit when sequential trials were compatible (cC vs. iC trials) or a decreased Gratton effect. These results provide evidence for a double dissociation between left and right PFC in the anticipatory regulation of action. Our results are in agreement with a preponderant role of the left PFC in overcoming proactive interference from competing memory representations and provide evidence that the right PFC, plays a role in sustaining goal-directed actions consistent with clinical data in right PFC patients with motor intentional disorders.
•We investigate two measures of proactive control in chronic frontal stroke patients.•All patients and controls performed a picture naming and a verbal Simon task.•Left and right PFC patients are differentially impaired in proactive control.•The left PFC helps to overcome proactive interference from memory representations.•The right PFC helps sustain goal-directed actions from one trial to the next.
The retrogenesis model of Alzheimer's disease (AD) posits that white matter (WM) degeneration follows a pattern that is the reverse of myelogenesis. Using diffusion tensor imaging (DTI) to test this ...model, we predicted greater loss of microstructural integrity in late-myelinating WM fiber pathways in AD patients than in healthy older adults, whereas differences in early-myelinating WM fiber pathways were not expected. We compared 16 AD patients and 14 demographically-matched healthy older adults with a whole-brain approach via tract-based spatial statistics (TBSS), and a region of interest (ROI) approach targeting early-myelinating (posterior limb of internal capsule, cerebral peduncles) and late-myelinating (inferior longitudinal fasciculus ILF, superior longitudinal fasciculus SLF) fiber pathways. Permutation-based voxelwise analysis supported the retrogenesis model. There was significantly lower fractional anisotropy (FA) in AD patients compared to healthy older adults in late-myelinating but not early-myelinating pathways. These group differences appeared to be driven by loss of myelin integrity based on our finding of greater radial diffusion in AD than in healthy elderly. ROI analyses were generally in agreement with whole-brain findings, with significantly lower FA and increased radial diffusion in the ILF in the AD group. Consistent with the retrogenesis model, AD patients showed demonstrable changes in late-myelinating WM fiber pathways. Given greater change in the ILF than the SLF, wallerian degeneration secondary to cortical atrophy may also be a contributing mechanism. Knowledge of the pattern of WM microstructural changes in AD and its underlying mechanisms may contribute to earlier detection and intervention in at-risk groups.
We report on the cryogenic characterization of Red Green Blue - High Density (RGB-HD) SiPMs developed at Fondazione Bruno Kessler (FBK) as part of the DarkSide program of dark matter searches with ...liquid argon time projection chambers. A cryogenic setup was used to operate the SiPMs at varying temperatures and a custom data acquisition system and analysis software were used to precisely characterize the primary dark noise, the correlated noise, and the gain of the devices. We demonstrate that FBK RGB-HD SiPMs with low quenching resistance (RGB-HD-LRq) can be operated from 40 K to 300 K with gains in the range 105 to 106 and noise rates at a level of around 1 Hz/mm2.
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