scholarly journals The Impulsive Nature of Lightning Initiation

2021 ◽  
Author(s):  
Christopher Sterpka ◽  
Joseph Dwyer ◽  
Ningyu Liu ◽  
Brian Hare ◽  
Olaf Scholten
Keyword(s):  
The Netherlands ◽  
Time Resolution ◽  
Low Frequency ◽  
Propagation Speed ◽  
Timing Accuracy ◽  
Lightning Initiation ◽  
Image Detail

<p>We report results from imaging the initiation region of lightning via 3D interferometric beamforming on data collected by the Netherlands-based core of the Low Frequency Array of Antennas (LOFAR). LOFAR achieves 1 nanosecond timing accuracy and meter-scale spatial precision in lightning imaging on pulses observed in the 30-80 MHz band via the 38 Dutch-based stations. This project complements and enhances the previous work of the LOFAR lightning group of Groningen [Hare, B.M., et al., Nature 568, 360363 (2019)], and [Scholten, O., et al., ESSOAr 10503153] in order to improve image detail in regions with weak sources. This project incorporates beamforming techniques to improve upon previously employed methods with the result of improving both spatial and time resolution of lightning sources. In doing so, we have located and imaged the first non-impulsive sources in lightning flashes. These sources are believed to be caused by a streamer-cascade-like initiation event leading to the formation of the first leader in two separate lightning flashes. The initiation starts from essentially background and within a tens of microseconds ramps up a few orders of magnitude before the first impulsive sources connected with lightning leaders are observed. The events are likely an analog of fast breakdown in narrow bipolar events, and here we report their ramp-up rate, propagation speed, and trajectories.</p>

scholarly journals Trapped Planetary (Rossby) Waves Observed in the Indian Ocean by Satellite Borne Altimeters

2017 ◽  
Author(s):  
Yair De-Leon ◽  
Nathan Paldor
Keyword(s):  
Indian Ocean ◽  
Rossby Waves ◽  
Low Frequency ◽  
Propagation Speed ◽  
Wave Theory ◽  
Planetary Waves ◽  
Sea Surface ◽  
Trapped Wave ◽  
The Indian Ocean ◽  
Frequency Variations

Abstract. Using 20 years of accurately calibrated, high resolution, observations of Sea Surface Height Anomalies (SSHA) by satellite ‎borne altimeters we show that in the Indian Ocean south of the Australian coast the low frequency variations of SSHA are ‎dominated by westward propagating, trapped, i.e. non-harmonic, planetary waves. Our results demonstrate that the ‎meridional-dependent amplitudes of the SSHA are large only within a few degrees of latitude next to the South-Australian ‎coast while farther in the ocean they are uniformly small. This meridional variation of the SSHA signal is typical of the ‎amplitude structure in the trapped wave theory. The westward propagation speed of the SSHA signals is analyzed by ‎employing three different methods of estimation. Each one of these methods yields speed estimates that can vary widely ‎between adjacent latitudes but the combination of at least two of the three methods yields much smoother variation. The ‎estimates obtained in this manner show that the observed phase speeds at different latitudes exceed the phase speeds of ‎harmonic Rossby (Planetary) waves by 140 % to 200 %. In contrast, the theory of trapped Rossby (Planetary) waves in a ‎domain bounded by a wall on its equatorward side yields phase speeds that approximate more closely the observed phase ‎speeds.‎

Defining sources from mobile gas measurements using their typical “fingerprint” 

2021 ◽  
Author(s):  
Daniëlle van Dinther ◽  
Sarah de Bie ◽  
Ilona Velzeboer ◽  
Pim van den Bulk ◽  
Arnoud Frumau ◽  
...  
Keyword(s):  
The Netherlands ◽  
Time Resolution ◽  
Methane Emission ◽  
Gaussian Model ◽  
Google Earth ◽  
Single Shot ◽  
Emission Level ◽  
Greenhouse Gasses ◽  
Mobile Measurements ◽  
Different Sources

<p>Mobile measurements of greenhouse gasses are used more often for emission evaluation studies (https://h2020-memo2.eu). Over the last few years, TNO have carried out multiple studies using a van to measure greenhouse gasses mobile (e.g. Hensen et.al., 2018 and Hensen & Scharff, 2001). Evaluation of the campaign data sets, where nitrous oxide (N<sub>2</sub>O) is released as a tracer release and meteorological conditions (windspeed and -direction) are measured, has provided a great quantity of information both on the different sources that are investigated as well as on the evaluation method itself. This study examines a subset of the “random” survey datasets that were obtained while driving in the Netherlands. In general, these are single pass plume measurements that can be used to generate a single shot emission estimate as long as the exact location of the source and local meteorological data are known. In order to automatically indicate different sources, it is assumed that different source types emit different mixtures of trace gasses into the atmosphere, which leave behind a typical ‘’fingerprint’’. A combustion source, for instance, might leak methane (CH<sub>4</sub>) as well as ethane (C<sub>2</sub>H<sub>6</sub>) and produce carbon monoxide (CO) and nitric oxide/nitrogen dioxide (NO/NO<sub>2</sub>). Farms, on the other hand, produce CH<sub>4</sub>, ammonia (NH<sub>3</sub>) and potentially N<sub>2</sub>O, but in principal no C<sub>2</sub>H<sub>6</sub>, CO, NO and NO<sub>2</sub>. For the mobile measurements of greenhouse gasses the Aerodyne TDLAS instrument was used. This instrument measures CH<sub>4</sub>, C<sub>2</sub>H<sub>6</sub>, N<sub>2</sub>O, CO<sub>2</sub>, and CO simultaneously and data is stored at a 1 second time resolution. Since December 2020, the MIRO instrument, which measures CH<sub>4</sub>, N<sub>2</sub>O, CO, NH<sub>3</sub>, Sulphur dioxide (SO<sub>2</sub>), NO and NO<sub>2</sub> on a 1 second time resolution, was added in the van as well. The expected co-emitted species are then used in an algorithm to automatically categorize the mixture of gas in the observed gas plumes into five different source types (farms, traffic, burning, fossil and wastewater treatment plants) and can be viewed per category in Google Earth. Emission levels are subsequently calculated using the TNO Gaussian model that is used in many of our emission studies (e.g. Hensen et.al., 2019) and calibrated versus N<sub>2</sub>O tracer release tests, which can then be compared to emission registration (ER) numbers. In this study, a subset of available datasets will be shown covering a large part of the Netherlands. Different sources were assigned a source category and, if possible, these sources were assigned an emission level. Some of these locations, for instance along major highways, have multiple “hits” in a year. For these sources, an average and standard deviation in the emission level numbers are provided and compared to ER numbers.</p><p><strong>References:</strong></p><p>Hensen, A., Bulk, W.C.M. van den, Dinther, D. van, 2018. Methaan emissiemetingen aan buiten gebruik gestelde olie- en gaswinningsputten. ECN-E—18-032, Petten.</p><p>Hensen, A., Scharff, H., 2001. Methane emission estimates from landfills obtained with dynamic plume measurements. Water, Air and Soil Pollution Kluwer focus1:455-464.</p><p>Hensen, A., Velzeboer, I., Frumau, K.F.A., Bulk, W.C.M. van den, Dinther, D. van, 2019. Methane emission measurements of offshore oil and gas platforms, TNO report 2019 R10895, Petten.</p>

Methods for the assessment of low-frequency noise from mining activities in the Netherlands

2021 ◽  
Vol 263 (1) ◽  
pp. 5175-5186
Author(s):  
Jelle Assink ◽  
Rosan Nusselder ◽  
Kim White ◽  
Olivier den Ouden ◽  
Erik de Graaff ◽  
...  
Keyword(s):  
The Netherlands ◽  
Public Awareness ◽  
Standard Procedure ◽  
Low Frequency ◽  
World Health ◽  
Frequency Noise ◽  
Mining Activities ◽  
Low Frequency Noise ◽  
Research Attention ◽  
Presentation Methods

According to the World Health Organisation, noise ranks among the environmental stressors with the highest impact on public health. The contribution of low-frequency noise (LFN) is not well-understood and deserves more research attention from several perspectives, that include epidemiology, acoustics as well as civil engineering. The attribution of symptoms to LFN is increasing due to a growth in public awareness and the expansion of industrial installations and traffic. In the Netherlands, numbers of LFN-related complaints are rising. Several of those have been attributed to the mining industry. However, an effective methodology for the assessment of such complaints is not yet available. In this presentation, methods are defined to assess LFN from mining activities in the Netherlands, focusing on the extraction, processing, transportation and storage of gas, salt and geothermal heat. Through a literature review and interviews with domain experts, methodologies have been derived with regards to 1) the prediction of LFN generation at the source, 2) observational techniques and 3) potential impacts on health. A broad low-frequency band is considered, spanning from the often-discarded infrasonic frequencies to up to 200 Hz. Based on this study's results, recommendations are given for establishing a standard procedure to assess LFN produced by mining activities.

Propagation properties of foreshock cavitons and spontaneous hot flow anomalies: Statistical results from a global hybrid-Vlasov simulation

2021 ◽  
Author(s):  
Vertti Tarvus ◽  
Lucile Turc ◽  
Markus Battarbee ◽  
Jonas Suni ◽  
Xóchitl Blanco-Cano ◽  
...  
Keyword(s):  
Magnetic Field ◽  
Solar Wind ◽  
Rest Frame ◽  
Low Frequency ◽  
Propagation Speed ◽  
Bow Shock ◽  
Linear Interaction ◽  
Frame Motion ◽  
Suprathermal Ions ◽  
Propagation Properties

<p>Foreshock cavitons are transient structures forming in Earth's foreshock as a result of non-linear interaction of ultra-low frequency waves. Cavitons are characterised by simultaneous density and magnetic field depressions with sizes of the order of 1 Earth radius. These transients are advected by the solar wind towards the bow shock, where they may accumulate shock-reflected suprathermal ions and become spontaneous hot flow anomalies (SHFAs), which are characterised by an enhanced temperature and a perturbed bulk flow inside them.<br>    Both spacecraft measurements and hybrid simulations have shown that while cavitons and SHFAs are carried towards the bow shock by the solar wind, their motion in the solar wind rest frame is directed upstream. In this work, we have made a statistical analysis of the propagation properties of cavitons and SHFAs using Vlasiator, a hybrid-Vlasov simulation model. In agreement with previous studies, we find the transients propagating upstream in the solar wind rest frame. Our results show that the solar wind rest frame motion of cavitons is aligned with the direction of the interplanetary magnetic field, while the motion of SHFAs deviates from this direction. We find that SHFAs have a faster solar wind rest frame propagation speed than cavitons, which is due to an increase in the sound speed near the bow shock, affecting the speed of the waves in the foreshock.</p>

LOFAR4SW – Space Weather Science and Operations with LOFAR

2021 ◽  
Author(s):  
Hanna Rothkaehl ◽  
Barbara Matyjasiak ◽  
Carla Baldovin ◽  
Mario Bisi ◽  
David Barnes ◽  
...  
Keyword(s):  
The Netherlands ◽  
Radio Telescope ◽  
Space Weather ◽  
Low Frequency ◽  
Point Of View ◽  
Current Status ◽  
Civic Society ◽  
Horizon 2020 ◽  
Final Design ◽  
Areas Of Interest

<p>Space Weather (SW) research is a very important topic from the scientific, operational and civic society point of view. Knowledge of interactions in the Sun-Earth system, the physics behind observed SW phenomena, and its direct impact on modern technologies were and will be key areas of interest.  The LOFAR for Space Weather (LOFAR4SW) project aim is to prepare a novel tool which can bring new capabilities into this domain. The project is realised in the frame of a Horizon 2020 INFRADEV call.  The base for the project is the Low Frequency Array (LOFAR) - the worlds largest low frequency radio telescope, with a dense core near Exloo in The Netherlands and many stations distributed both in the Netherlands and Europe wide with baselines up to 2000 km.  The final design of LOFAR4SW will provide a full conceptual and technical description of the LOFAR upgrade, to enable simultaneous operation as a radio telescope for astronomical research as well as an infrastructure working for Space Weather studies.  In this work we present the current status of the project, including examples of the capabilities of LOFAR4SW and the project timeline as we plan for the Critical Design Review later in 2021.</p>

Synthesis of Vortex Rossby Waves. Part II: Vortex Motion and Waves in the Outer Waveguide

2015 ◽  
Vol 72 (10) ◽  
pp. 3958-3974 ◽  
Author(s):  
Israel Gonzalez ◽  
Amaryllis Cotto ◽  
Hugh E. Willoughby
Keyword(s):  
Nonlinear Model ◽  
Rossby Waves ◽  
Critical Radius ◽  
Vortex Motion ◽  
Low Frequency ◽  
Wave Interaction ◽  
Linear Acceleration ◽  
Propagation Speed ◽  
Large Radius ◽  
Subsequent Work

Abstract Beta, the meridional gradient of planetary vorticity, causes tropical cyclones to propagate poleward and westward at approximately 2 m s−1. In a previous shallow-water linear model, the simulated vortex accelerated without limit, ostensibly because beta forced a free linear mode. In the analogous nonlinear model, wave–wave interaction limited the propagation speed. Subsequent work based upon the asymmetric balance (AB) approximation was unable to replicate the linear result. The present barotropic nondivergent model replicates the linear beta gyres as a streamfunction dipole with a uniform southeasterly ventilation flow across the vortex. The simulated storm accelerates to unphysical, but finite, speeds that are limited by vorticity filamentation. In the analogous nonlinear model, nonlinearly forced wavenumber-1 gyres have opposite phase to the linear gyres so that their ventilation flow counteracts advection by the linear gyres to limit the overall vortex speed to approximately 3 m s−1. A bounded mean vortex with zero circulation at large radius must contain an outer annulus of anticyclonic vorticity to satisfy the circulation theorem. The resulting positive mean vorticity gradient constitutes an outer waveguide that supports downstream-propagating, very-low-frequency vortex Rossby waves. It is confined between an inner critical radius where the waves are absorbed and an outer turning point where they are reflected. Vorticity filamentation at the critical radius limits the beta-drift acceleration. The original unlimited linear acceleration stemmed from too-weak dissipation caused by second-order diffusion applied to velocity components instead of vorticity. Fourth-order diffusion and no outer waveguide in the Rankine-like vortex of the AB simulations plausibly explain the different results.

Investigating Thunderstorm HF/VHF Radio Bursts with Weak Lower Frequency Radiation

2020 ◽  
Author(s):  
Ningyu Liu ◽  
Joseph Dwyer
Keyword(s):  
High Frequency ◽  
Large Scale ◽  
Strong Field ◽  
Large Fraction ◽  
Low Frequency ◽  
Very High Frequency ◽  
Lightning Initiation ◽  
Study Support ◽  
Volcanic Lightning ◽  
Frequency Radiation

<p>While the spectrum of lightning electromagnetic radiation is known to peak around 5-10 kHz in the very low frequency (VLF) range, intense high frequency/very high frequency (HF/VHF) radiation can be produced by various lightning related processes. In fact, thunderstorm narrow bipolar events (NBEs), which are capable of initiating lightning, are the most powerful HF/VHF sources in nature on Earth. But even for NBEs, the spectral intensity in HF/VHF is still many orders of magnitude weaker than that of lower frequencies (Liu et al., JGR, 124, https://doi.org/10.1029/2019JD030439, 2019). HF/VHF bursts with weak VLF signals, however, can also be produced by thunderstorms. These bursts may be related to the thunderstorm precursor events noted by Rison et al. (Nat. Commun., 7, 10721, 2016) and are also found to precede a large fraction of lightning initiation (Lyu et al., JGR, 124, 2994, 2019). They are also known as continual radio frequency (CRF) radiation associated with volcanic lightning (Behnke et. al., JGR, 123, 4157, 2018).</p><p> </p><p>In this talk, we report a theoretical and modeling study to investigate a physical mechanism for production of those HF/VHF bursts. The study is built on the theory developed recently concerning the radio emissions from an ensemble of streamers (Liu et al., 2019). We find an ensemble of streamer discharges that develop in random directions can produce HF/VHF radiation with intensity comparable to those all developing in a single direction, but the VLF intensity is many orders of magnitude weaker. The results of our study support the conclusions of Behnke et. al (2018) that CRF is produced in the absence of large-scale electric field, it results in insignificant charge transfer, and it is caused by streamers. In the context of the HF/VHF bursts preceding lightning initiation (Lyu et. al, 2019), our results imply that highly localized strong field regions exist in thunderstorms and streamers take place in those regions, which somehow precondition the medium for lightning initiation.</p>

Interferometric Observations of the Active Regions in Radio Domain Before and After the Total Solar Eclipse on 21 August 2017

2020 ◽  
Author(s):  
Bartosz Dabrowski ◽  
Paweł Flisek ◽  
Christian Vocks ◽  
Diana Morosan ◽  
Peijin Zhang ◽  
...  
Keyword(s):  
The Netherlands ◽  
Solar Eclipse ◽  
Low Frequency ◽  
Active Regions ◽  
Total Solar Eclipse ◽  
Frequency Resolution ◽  
Radio Bursts ◽  
Type Iii ◽  
Dynamic Spectra ◽  
Before And After

<p>We hereby present the interferometric LOFAR observations made before and after the total solar eclipse on 21 August 2017, during which the type III radio bursts have been detected.</p><p>The LOw-Frequency ARray (LOFAR) is a large radio interferometer operating in the frequency range of 10–240 MHz, designed and constructed by ASTRON (the Netherlands Institute for Radio Astronomy). The LOFAR telescope is an array of stations distributed throughout the Netherlands and other parts of Europe. Currently the system consist of 52 LOFAR stations located in Europe. Apart from the high time and frequency resolution of the dynamic spectra, LOFAR allows also a 2D imaging of the radio sources and tracking of their positions through the solar corona.</p><p>In this work we present a preliminary analysis of the dynamic spectra of type III radio bursts with radio images.</p>

scholarly journals Coal Seam Roof: Lithology and Influence on the Enrichment of Coalbed Methane

2019 ◽  
Vol 23 (4) ◽  
pp. 359-364
Author(s):  
Yunlan He ◽  
Xikai Wang ◽  
Hongjie Sun ◽  
Zhenguo Xing ◽  
Shan Chong ◽  
...  
Keyword(s):  
Coal Seam ◽  
Coalbed Methane ◽  
Low Frequency ◽  
Propagation Speed ◽  
Wave Impedance ◽  
P Wave ◽  
Borehole Data ◽  
Seismic Frequencies ◽  
Argillaceous Siltstone ◽  
Gas Bearing

To identify the lithology of coal seam roof and explore the influence of these roofs on the enrichment of coalbed methane, low-frequency rock petrophysics experiments, seismic analyses and gas-bearing trend analyses were performed. The results show that the sound wave propagation speed in rock at seismic frequencies was lower than that at ultrasound frequencies. Additionally, the P-wave velocities of gritstone, fine sandstone, argillaceous siltstone and mudstone were 1,651 m/s, 2,840 m/s, 3,191 m/s and 4,214 m/s, respectively. The surface properties of the coal seam roofs were extracted through 3D seismic wave impedance inversion. The theoretical P-wave impedance was calculated after the tested P-wave velocity was determined. By matching the theoretical P-wave impedance of the four types of rocks with that of the coal seam roofs, we identified the lithology of the roofs. By analyzing known borehole data, we found that the identified lithology was consistent with that revealed by the data. By comparing and analyzing the coal seam roof lithology and the gas-bearing trends in the study area, we discovered that the coal seam roof lithology was related to the enrichment of coalbed methane. In the study area, areas with high gas contents mainly coincided with roof zones composed of mudstone and argillaceous siltstone, and those with low gas contents were mainly associated with fine sandstone roof areas. Thus, highly compact areas of coal seam roof are favorable for the formation and preservation of coalbed methane. 

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