The physics of lightning

It is now well established that both thunderclouds and lightning routinely emit x-rays and gamma-rays. These emissions appear over wide timescales, ranging from sub-microsecond bursts of x-rays associated with lightning leaders, to sub-millisecond bursts of gamma-rays seen in space called terrestrial gamma-ray flashes, to minute long glows from thunderclouds seen on the ground and in or near the cloud by aircraft and balloons. In particular, terrestrial gamma-ray flashes (TGFs), which are thought to be emitted by thunderclouds, are so bright that they sometimes saturate detectors on spacecraft hundreds of kilometers away. These TGFs also generate energetic secondary electrons and positrons that are detected by spacecraft in the inner magnetosphere. It is generally believed that these x-ray and gamma-ray emissions are generated, via bremsstrahlung, by energetic runaway electrons that are accelerated by electric fields in the atmosphere. In this paper, we review this newly emerging field of High-Energy Atmospheric Physics, including the production of runaway electrons, the production and propagation of energetic radiation, and the effects of both on atmospheric electrodynamics.

https://link.springer.com/content/pdf/10.1007%2Fs11214-012-9894-0.pdf

High-Energy Atmospheric Physics: Terrestrial Gamma-Ray Flashes and Related Phenomena

Observational analyses have shown the width of the tropical belt increasing in recent decades as the world has warmed. This expansion is important because it is associated with shifts in large-scale atmospheric circulation and major climate zones. Although recent studies have attributed tropical expansion in the Southern Hemisphere to ozone depletion, the drivers of Northern Hemisphere expansion are not well known and the expansion has not so far been reproduced by climate models. Here we use a climate model with detailed aerosol physics to show that increases in heterogeneous warming agents--including black carbon aerosols and tropospheric ozone--are noticeably better than greenhouse gases at driving expansion, and can account for the observed summertime maximum in tropical expansion. Mechanistically, atmospheric heating from black carbon and tropospheric ozone has occurred at the mid-latitudes, generating a poleward shift of the tropospheric jet, thereby relocating the main division between tropical and temperate air masses. Although we still underestimate tropical expansion, the true aerosol forcing is poorly known and could also be underestimated. Thus, although the insensitivity of models needs further investigation, black carbon and tropospheric ozone, both of which are strongly influenced by human activities, are the most likely causes of observed Northern Hemisphere tropical expansion.

/pdf/recent-northern-hemisphere-tropical-expansion-primarily-3g1pnqc6qn.pdf

Recent Northern Hemisphere tropical expansion primarily driven by black carbon and tropospheric ozone

In the present paper, the experimental set-up of Institut Fresnel used to measure the scattered fields of different elongated objects is precisely described. Since the special issue on 'Testing inversion algorithms against experimental data', the modifications of this system, outlined here, have mostly been done to improve the synchronization of the apparatuses and the precision of our measurements. Due to a large number of requests from the inverse problem community, it has been decided to add new measurements to the Institut Fresnel's database. All the new targets presented here are two-dimensional inhomogeneous ones. They are made of different dielectrics or are mixing metal and dielectric parts. Both TE and TM polarizations are measured for each target, from 2 to 10 GHz and even 18 GHz for the most complex target. In the first part of this paper the set-up is described precisely. The second part is devoted to the presentation of the targets. Finally, some TE and TM comparisons of measurements and direct problem simulations are shown to accredit our experimental method and to give an idea of the accuracy of these measurements.

Free space experimental scattering database continuation : experimental set-up and measurement precision : Testing inversion algorithms against experimental data: Inhomogeneous targets

Evidence from Greenland ice cores shows that year-to-year temperature variability was probably higher in some past cold periods, but there is considerable interest in determining whether global warming is increasing climate variability at present. This interest is motivated by an understanding that increased variability and resulting extreme weather conditions may be more difficult for society to adapt to than altered mean conditions. So far, however, in spite of suggestions of increased variability, there is considerable uncertainty as to whether it is occurring. Here we show that although fluctuations in annual temperature have indeed shown substantial geographical variation over the past few decades, the time-evolving standard deviation of globally averaged temperature anomalies has been stable. A feature of the changes has been a tendency for many regions of low variability to experience increases, which might contribute to the perception of increased climate volatility. The normalization of temperature anomalies creates the impression of larger relative overall increases, but our use of absolute values, which we argue is a more appropriate approach, reveals little change. Regionally, greater year-to-year changes recently occurred in much of North America and Europe. Many climate models predict that total variability will ultimately decrease under high greenhouse gas concentrations, possibly associated with reductions in sea-ice cover. Our findings contradict the view that a warming world will automatically be one of more overall climatic variation.

No increase in global temperature variability despite changing regional patterns

[1] Lightning observations in the very high frequency band and measurements of ultra low frequency magnetic fields are analyzed to investigate the charge transfer and in-cloud structure of eight positive cloud-to-ground (+CG) strokes in a mesoscale convective system. Although no high altitude images were recorded, these strokes contained large charge moment changes (1500–3200 C·km) capable of producing nighttime sprites. Even though the convective region of the storm was where the flashes originated and where the CG strokes could occur, the charge transferred to ground was mainly from the stratiform region. The post-stroke long continuing currents were connected to highly branched negative leader extension into the stratiform region. While the storm dissipated, the altitude of negative leader propagation in the stratiform area dropped gradually from 8 to 5 km, indicating that in some and perhaps all of these strokes, it was the upper positive charge in the stratiform region that was transferred.

/pdf/charge-transfer-and-in-cloud-structure-of-large-charge-3ue45ebcxr.pdf

Charge transfer and in‐cloud structure of large‐charge‐moment positive lightning strokes in a mesoscale convective system

To characterize lightning processes that produce terrestrial gamma ray flashes (TGFs), we have analyzed broadband (<1 Hz to 30 kHz) lightning magnetic fields for TGFs detected by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) satellite in 2004-2009. The majority (96%) of 56 TGF-associated lightning signals contain single or multiple VLF impulses superposed on a slow pulse that reflects a process raising considerable negative charge within 2-6 ms. Some TGF lightning emissions also contain VLF signals that precede any appreciable slow pulse and that we term precursor sferics. The analyses of 9 TGFs related to lightning discharges with location uncertainty <100 km consistently indicate that TGFs are temporally linked to the early portion of the slow process and associated VLF impulses, and not to precursor sferics. The nearly universal presence of a slow pulse suggests that the slow process plays an important role in gamma ray production. In all cases the slow process raises negative charge with a typical mean current moment of +30 kA km. The resulting charge moment change ranges from small values below +10 C km to a maximum of +200 C km, with an average of +64 C km. The current moment waveform extracted from TGF sferics with single or multiple VLF impulses also shows that the slow process initiates shortly before the major TGF-associated fast discharge. These features are generally consistent with the TGF-lightning sequence reported by Lu et al. (2010), suggesting that the majority of RHESSI TGFs are produced during the upward negative leader progression prevalent in normal polarity intracloud flashes.

/pdf/characteristics-of-broadband-lightning-emissions-associated-2n91yueumh.pdf

Characteristics of broadband lightning emissions associated with terrestrial gamma ray flashes

[1] Significant temporal variability of the nighttime D region is well known, but its inaccessibility means that the time scales, magnitudes, and sources for that variability are not well understood. We probed the ionospheric D region by measuring the high‐power broadband very low frequency (VLF) signals launched by lightning and propagating in the Earth‐ionosphere waveguide. We analyzed broadband sferic data of July and August 2005 recorded by our sensors located near Duke University by comparing measured sferic spectra to model results and extracted the height of an assumed exponential electron density profile for each measurement. The measured nighttime D region electron density profile heights showedlarge temporal variations ofseveral kilometers onsomenights andrelatively stable behaviors on others. The measured hourly average heights in 260 h ranged between 82.0 and 87.2 km, with a mean value of 84.9 km and a standard deviation of 1.1 km. The maximum variation in the 5 h period was around just above 4.0 km and the maximum variation in the 1 h period was around 1.3 km, with sharper gradients observed over shorter time periods. We also observed spatial variability as large as 2.0 km over 5° latitudes on some nights and no spatial variability on others. On some nights, the temporal variability exhibited a close correlation with the occurrence rate of lightning discharges under the probed region. This suggests that the direct energy coupling between lightning discharges and lower ionosphere can be a significant source of the D region variability. However, on other nights, the measured height temporal variability showed weak to no correlation with local lightning, displaced lightning (as would be expected for lightning‐induced electron precipitation), or geomagnetic activities. These measurements suggest that nighttime D region variability may be driven by many sources.

/pdf/midlatitude-nighttime-d-region-ionosphere-variability-on-47hlw0cmjk.pdf

Midlatitude nighttime D region ionosphere variability on hourly to monthly time scales

Gigantic jets emerge from the top of thunderstorms and extend all the way to the ionosphere at altitudes of 90 km. Simultaneous video images and magnetic field measurements of a gigantic jet demonstrate an electric charge transfer between the thunderstorm and the ionosphere that is comparable to that observed in cloud-to-ground lightning.

/pdf/quantification-of-the-troposphere-to-ionosphere-charge-1ez4i1j6u1.pdf

Quantification of the troposphere-to-ionosphere charge transfer in a gigantic jet

Reverse-time migration (RTM) has shown its advantages over other conventional migration algorithms for ground-penetrating radar (GPR) imaging. RTM is preferred to be implemented in the computationally attractive 2-D domain, whereas a real measurement can only be conducted in a 3-D domain. Thus, we propose an asymptotic 3-D-to-2-D data conversion filter in the frequency domain for preprocessing of the recorded data for 2-D RTM. The accuracy of the data conversion filter is verified by two numerical tests on a homogeneous and a layered model. Then, we evaluate the effectiveness of the data conversion filter on the imaging result of 2-D RTM, which is applied to simulated multioffset GPR data from a buried pipe model. With the filter, subsurface image by the 2-D RTM matches better with the 3-D RTM result especially in the aspect of phase congruency. Therefore, we conclude that this data conversion filter is necessary for 2-D RTM. We also conducted a laboratory experiment on a volcanic ash pit using a multiinput–multioutput GPR system, which is adopted on the Chang-E 5 lunar exploration lander and works in a stationary mode. The 3-D-to-2-D data conversion filter is applied to the measured multioffset GPR data before the 2-D RTM. The imaging results demonstrate that three marble slabs buried at different depths up to 2 m are clearly imaged.

Feng Han

Papers

Charge transfer and in‐cloud structure of large‐charge‐moment positive lightning strokes in a mesoscale convective system

Characteristics of broadband lightning emissions associated with terrestrial gamma ray flashes

Midlatitude nighttime D region ionosphere variability on hourly to monthly time scales

Quantification of the troposphere-to-ionosphere charge transfer in a gigantic jet

Two-Dimensional Reverse-Time Migration Applied to GPR With a 3-D-to-2-D Data Conversion