Pressure pulse from a lightning stroke

Hill, E. L.; Robb, J. D.

doi:10.1029/jb073i006p01883

Cited by 20 publications

(4 citation statements)

References 16 publications

(8 reference statements)

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“…We model the effects of small-scale air density perturbations and ionization by a preceding streamer as suggested in Babich et al (2015), Eichwald et al (1996), Navarro-González et al (2001, Hill and Robb (1968), Kacem et al (2013), Liu and Zhang (2014), Marode et al (1979), Villagrán-Muniz et al (2003), and Plooster (1970). We choose sinusoidal air density perturbation in the radial direction with the minimum on the axis (r = 0) and the maximum at the outer boundary (r = L r )…”

Section: Air Density Perturbationsmentioning

confidence: 99%

High‐Energy Emissions Induced by Air Density Fluctuations of Discharges

Köhn

Chanrion

Neubert

2018

Geophysical Research Letters

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Bursts of X‐rays and γ‐rays are observed from lightning and laboratory sparks. They are bremsstrahlung from energetic electrons interacting with neutral air molecules, but it is still unclear how the electrons achieve the required energies. It has been proposed that the enhanced electric field of streamers, found in the corona of leader tips, may account for the acceleration; however, their efficiency is questioned because of the relatively low production rate found in simulations. Here we emphasize that streamers usually are simulated with the assumption of homogeneous gas, which may not be the case on the small temporal and spatial scales of discharges. Since the streamer properties strongly depend on the reduced electric field E/n, where n is the neutral number density, fluctuations may potentially have a significant effect. To explore what might be expected if the assumption of homogeneity is relaxed, we conducted simple numerical experiments based on simulations of streamers in a neutral gas with a radial gradient in the neutral density, assumed to be created, for instance, by a previous spark. We also studied the effects of background electron density from previous discharges. We find that X‐radiation and γ‐radiation are enhanced when the on‐axis air density is reduced by more than ∼25%. Pre‐ionization tends to reduce the streamer field and thereby the production rate of high‐energy electrons; however, the reduction is modest. The simulations suggest that fluctuations in the neutral densities, on the temporal and spacial scales of streamers, may be important for electron acceleration and bremsstrahlung radiation.

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Section: Air Density Perturbationsmentioning

confidence: 99%

High‐Energy Emissions Induced by Air Density Fluctuations of Discharges

Köhn

Chanrion

Neubert

2018

Geophysical Research Letters

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“…In a recent issue of the Joui'naI we presented measurements of atmospheric pressure pulses from triggered lightning flashes [Hill and Robb, 1968]. The magnitudes of the overpressures found were considerably smaller than the magnitudes predicted earlier by Zhivlyuk and Mandel'shtam [1961] on the basis of a theoretical model of the spark breakdown channel first developed by Drabkina [1951] and later extended by Braginskii [1958].…”

Section: Introductionmentioning

confidence: 82%

Spectroscopic and thermal temperatures in the return stroke of lightning

Hill¹,

Robb²

1969

J. Geophys. Res.

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Two different models of the temperature and state of ionization in the return stroke of a cloud‐ground lightning stroke are in use in the literature. In the model that has been adopted for interpretation of the spectroscopic data on lightning, the mechanism of capture of free electrons to form negative oxygen ions is disregarded, the ionization being assumed to exist in the form of free electrons and heavy positive ions. In this case the thermalization time of the channel is expected to be short, so that the optical and thermal temperatures are effectively equal. In the model that accepts negative ion formation as a fundamental process, the ionization behind the tip of the return stroke is considered to be rapidly interchangeable between free electrons and heavy atomic and molecular oxygen ions. It is expected on this model that there exists a distinct time lag of the thermal temperature behind the optical temperature.

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“…Flowers [27] argued that experiments had shown that a time of 1000 ps was required for thermal equilibrium to be approached in the air arc. In 1968 Hill and Robb [28] reported: 'At present, the quantitative evidence indicates that in the lightning channel the thermalisation time is of the order of 25-50ps, or even longer; therefore, the thermal temperature never rises above a few thousand degrees.' Orville [29], who carried out the most detailed spectroscopic lightning temperature measurements, was guarded when he said: 'Under the assumption of LTE (local thermodynamic equilibrium), which is probably valid after the first few microseconds in the return-stroke channel, .…”

Section: Discussionmentioning

confidence: 99%

The cause of thunder

Graneau¹

1989

J. Phys. D: Appl. Phys.

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The cause of thunder is one of the oldest riddles of recorded scientific speculation. Three centuries BC Aristotle published the first thunder theory. Many other theories were proposed until at the beginning of the present century a consensus evolved which assumed thunder must begin with a shockwave in air due to the sudden thermal expansion of the plasma in the lightning channel. The only experimental support for this theory came from spectroscopic temperature determinations up to 36000 K. Any one of the assumptions made in equating 'optical' to thermodynamic temperatures can be challenged and some have been disputed. Experiments with short atmospheric arcs of lightning strength revealed average arc pressures in excess of 400 atm and peak pressures approaching 1000 atm. These results demand much higher temperatures than those found by lightning spectroscopy. Furthermore, when the strength of the short arc explosions was plotted against the action integral of the current pulse it followed an electrodynamic law rather than a heating curve. Arc photography then proved conclusively that the plasma did not expand thermally in all directions, but preferentially at right angles to the current, as if driven by organised electrodynamic action. Possible electrodynamic forces which might drive the thunder shockwave are the Lorentz pinch force, the longitudinal Ampere force and the alpha-torque force of the Ampere-Neumann electrodynamics. The pinch force was found to be far too small and in the wrong direction to be the cause of thunder. Longitudinal and alpha-torque forces act in the correct direction but, so far, quantitative agreement has not been achieved. This may have to wait for a complete Ampere MHD.

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Pressure pulse from a lightning stroke

Cited by 20 publications

References 16 publications

High‐Energy Emissions Induced by Air Density Fluctuations of Discharges

High‐Energy Emissions Induced by Air Density Fluctuations of Discharges

Spectroscopic and thermal temperatures in the return stroke of lightning

The cause of thunder

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