Journal of Atmospheric and Solar-Terrestrial Physics

Solar energetic particle catalogs: Assumptions, uncertainties and validity of reports

Sat, 11/17/2018 - 19:10

Publication date: November 2018

Source: Journal of Atmospheric and Solar-Terrestrial Physics, Volume 180

Author(s): R. Miteva, S.W. Samwel, M.V. Costa-Duarte

Abstract

The aim of this work is to summarize the main underlying assumptions, simplifications and uncertainties while studying solar energetic particles (SEPs). In general, numerous definitions are used for the evaluation of a given SEP parameter and these different methods lead to different outcomes for a given particle event. Several catalogs of SEP events from various instruments are currently available; however, each catalog is specific to the adopted data and analysis. We investigate the differences while comparing several SEP catalogs and outline probable reasons. We focus on SEP statistical studies and quantify the influences of the particle intensity, solar origin location and projection effects. We found that different definitions and criteria used for these parameters change the values of the correlation coefficients between the SEPs and their solar origin.

Editorial Board

Fri, 11/09/2018 - 19:10

Publication date: November 2018

Source: Journal of Atmospheric and Solar-Terrestrial Physics, Volume 179

Author(s):

Seismoionospheric anomalies associated with earthquakes from the analysis of the ionosonde data

Fri, 11/09/2018 - 19:10

Publication date: November 2018

Source: Journal of Atmospheric and Solar-Terrestrial Physics, Volume 179

Author(s): Junaid Ahmed, Munawar Shah, Waqar Ali Zafar, Muhammad Ayaz Amin, Talat Iqbal

Abstract

Previous researchers have reported the earthquakes (EQs) induced ionospheric abnormalities, such as electron density depletion and/or enhancement, which they claimed were related to the impending EQs. In this paper, the peak plasma frequency of the ionosonde data from the Islamabad station was studied related to the July 24, 2015 (M5.1), October 26, 2015 (M7.5) and April 25, 2015 (M7.8) EQs in Pakistan and Nepal, respectively. The baseline of the running 30-day median (commonly used in other studies) was deployed on the foF2 data to detect the abnormality in the long term data before the EQs. A detailed analysis of the foF2 variability in a month before all the EQs of this study is then undertaken, where two out of three EQs showed simultaneous deviation in peak plasma frequency relative to the confidence intervals. There is no percentage deviation in foF2 data for the days associated with M7.8, proved no ionospheric signatures in the peak plasma frequency. We believe that the reason is the distance between the epicenter of M7.8 and the ionosonde station, which was approximately 12° longitudinal extension. On the other hand, the negative deviation beyond the lower confidence interval in critical frequency was detected for the mega thrust of M5.1. Similarly, positive foF2 anomalies were recorded within 10 days before the main shock of M7.5, where there was no high intensity geomagnetic storm. These results suggested that in order to detect reliable seismoionospheric precursor, the ionosonde station must operate within the range of seismic breeding zone.

The mean zonal wind effect on the long-term variation of ultra-fast Kelvin waves in the mesosphere and lower thermosphere and in the upper stratosphere

Fri, 11/09/2018 - 19:10

Publication date: November 2018

Source: Journal of Atmospheric and Solar-Terrestrial Physics, Volume 179

Author(s): Wei-Sheng Chen, C.J. Pan, Uma Das

Abstract

This study investigates 14-year ultra-fast Kelvin wave (UFKW) activity in the mesosphere and lower thermosphere (MLT) and in the upper stratosphere where the mesopause semiannual oscillation (MSAO) and the stratopause semiannual oscillation (SSAO) dominate the two altitude regions, respectively. The wave properties are derived from SABER temperature data for the period from 2003 to 2016 by two-dimensional fast Fourier transform. The investigations focus on the UFKW with zonal wavenumber 1 and periods of 2.5–4.5 days. The spectra, daily variations, and seasonal variations of UFKW are investigated. The wave activity is also compared with the background zonal wind derived from the horizontal wind model 2014 (HWM14) for the MLT region and from European Centre for Medium-Range Weather Forecasts (ECMWF) interim reanalysis for the upper stratosphere. The results are that periods of UFKW are mainly in the range of 2.5–4.5 days, but the dominated periods change from year to year. The UFKW amplitudes increase with altitude, and the largest amplitudes occur at altitudes above 90 km. Above 95 km, the mean zonal wind is westward at all times and the amplitude of UFKW is large most of the time. In the MLT region and the upper stratosphere, the large-amplitude UFKW tend to occur in the westward phase of the MSAO and the SSAO. The seasonal variation of UFKW in the MLT region and the upper stratosphere both show a semiannual variation in which the maximum and the secondary maximum are in August and February, respectively. The correlation analysis shows that the time lags between the 90-km wave variation and lower altitude wave variations do not match the theoretical expectation of the wave upward velocity. Finally, the MSAO and the SSAO act like two filters, which modify the wave amplitude and result in different daily variations in the MLT region and in the upper stratosphere.

Energetic cosmic-ray secondary electron distribution at thunderstorm altitudes

Fri, 11/09/2018 - 19:10

Publication date: November 2018

Source: Journal of Atmospheric and Solar-Terrestrial Physics, Volume 179

Author(s): N.C. Lindy, E.R. Benton, W.H. Beasley, D.A. Petersen

Abstract

At present, there is no complete theory describing how lightning discharges are initiated in thunderstorms. The greatest deficiency in existing lightning initiation model is the lack of an explanation for how lightning discharges occur in thunderstorm electric fields that are much weaker than the electric fields required for dielectric breakdown (Bazelyan & Raizer, 1998; McGorman and Rust 1998). Recently it has been suggested energetic cosmic ray secondary electrons propagating in strong thunderstorm electric fields could initiate lightning discharges (Gurevich, A. V. et al., 1992; Carlson et al. 2008). However, the distribution and energy spectrum of secondary electrons at thunderstorm altitudes in the atmosphere have been heretofore not well known. In this paper we report on use of the Monte Carlo code CORSIKA 6.790 to simulate the distribution and determine the energy spectrum of secondary electrons at thunderstorm altitudes. Results from these simulations show that the average secondary electron energy is > 10 MeV with ∼10% of secondary electrons having energy >100 MeV in energy at thunderstorm altitudes. This result is consistent with previous studies.

Relative role of the azimuthal Pedersen current component in the substorm global electric circuit

Fri, 11/09/2018 - 19:10

Publication date: November 2018

Source: Journal of Atmospheric and Solar-Terrestrial Physics, Volume 179

Author(s): M.A. Kurikalova, V.M. Mishin, V.V. Mishin, S.B. Lunyushkin, Yu.V. Penskikh

Abstract

In the early, well-known model of the substorm current wedge (SCW) circuit, the azimuthal Pedersen current in the ionosphere closes a pair of the down- and up-ward field-aligned currents connecting the magnetosphere's generator with the ionosphere. In an alternative SCW model, the Pedersen currents flow through the ionosphere in the meridional direction. In this paper, we describe the dynamics of electric currents during the expansion phase of two selected, winter and summer substorms to compare the relative role of the meridional and azimuthal Pedersen currents at the expansion phase peak.

Spatial characteristics of recurrent ionospheric storms at low latitudes during solar minimum

Fri, 11/09/2018 - 19:10

Publication date: November 2018

Source: Journal of Atmospheric and Solar-Terrestrial Physics, Volume 179

Author(s): A. Dmitriev

Abstract

Low-latitude ionospheric disturbances with amplitudes of 20 TECU occurred during moderate recurrent magnetic storms were analyzed in time period from 2007 to 2008. The analysis of these so-called recurrent ionospheric storms (RISs) was based on global ionospheric maps of vertical total electron content derived from the ground based GPS network. It was found that positive RISs were developed predominantly in the noon and postnoon sector and they have very large latitudinal and longitudinal extensions of up to 70 and 160°, respectively. Negative RISs occur mainly in the evening sector and they have ∼1.5 time smaller latitudinal and longitudinal extensions. The different location and spatial scales can be explained by different origins of the ionospheric storms. The large-scale positive ionospheric storms are generated by the mechanism of prompt penetration electric field, operating on the dayside, and by equatorward neutral wind disturbance. The negative storms can be related to the mechanism of disturbance dynamo electric field, operating in the evening sector, and to the changing of the thermospheric neutral composition, operating on sunlight side.

First seasonal and annual variations of atmospheric electric field at a subtropical station in Islamabad, Pakistan

Fri, 11/09/2018 - 19:10

Publication date: November 2018

Source: Journal of Atmospheric and Solar-Terrestrial Physics, Volume 179

Author(s): S.F. Gurmani, N. Ahmad, J. Tacza, T. Iqbal

Abstract

The variation of the atmospheric electric field from the ground is analyzed by using an electric field mill sensor for Islamabad, Pakistan. The first early results of atmospheric electric field are presented in this paper. These results are based on fair-weather days for the diurnal, seasonal, and annual variations in Islamabad from 2015 to 2017. The variation in the values of the atmospheric electric field is studied extensively during the monsoon, the pre-monsoon and the winter season. Minimum values are observed during the monsoon season, and maximum values are observed during winter and spring. The measured values of the atmospheric electric field are also compared with the universal standard values of the Carnegie curve. The trend of the diurnal variation shows a sharp single oscillation in fair-weather with a maximum at ∼ 4:00 UT (universal time), which could be due to the influence of local effects such as aerosol.

Variation of the TEC at a dip equatorial station, Trivandrum and a mid latitude station, Hanle during the descending phase of the solar cycle 24 (2014–2016)

Fri, 11/09/2018 - 19:10

Publication date: November 2018

Source: Journal of Atmospheric and Solar-Terrestrial Physics, Volume 179

Author(s): P.R. Shreedevi, R.K. Choudhary, Sneha Yadav, SmithaV. Thampi, A. Ajesh

Abstract

Morphological variations in the Total Electron Content (TEC) of the ionosphere at a dip equatorial station, Trivandrum (8.47°N, 76.91° E) and a mid latitude station, Hanle (32.78° N, 78.95° E) during the period 2014–2016 is studied using the GPS TEC measurements from the InSWIM (Indian Network for Space Weather Impact Monitoring) network of stations. The variation of the TEC at Hanle, which is located in the same longitude sector as that of Trivandrum, is presented here for the first time. At Hanle, the TEC peaks at local noontime while its peak appears with a delay (1600 LT) at Trivandrum. The minima in the TEC at both stations appear during the early morning hours. Nighttime enhancements in the TEC are seen at Trivandrum during the equinoctial months while at Hanle they appear in the summer months. The TEC at both the stations exhibit semi-annual anomaly and equinoctial asymmetry. However, the winter anomaly is seen only at Trivandrum. The TEC at both Trivandrum and Hanle is seen to display strong solar activity dependence with the average TEC decreasing as solar activity decreases. We surmise that the observed diurnal/seasonal/annual variations in the TEC at the dip equatorial station, Trivandrum is controlled by the processes induced by electrodynamics while the TEC variations at the mid latitude station, Hanle is mostly affected by the neutral dynamical processes. The morphological comparison between the TEC at Trivandrum derived from the IRI 2012 model and GPS TEC measurements shows only a qualitative agreement. The IRI 2012 model is found to overly underestimate the daytime as well as nighttime TEC at the dip equatorial station, Trivandrum while it is well in agreement with the TEC measurements at the mid latitude station, Hanle.

A mathematical model of quasistationary electric field penetration from ground to the ionosphere with inclined magnetic field

Fri, 11/09/2018 - 19:10

Publication date: November 2018

Source: Journal of Atmospheric and Solar-Terrestrial Physics, Volume 179

Author(s): V.V. Denisenko, S.A. Nesterov, M.Y. Boudjada, H. Lammer

Abstract

A quasi stationary model of the electric fields and currents in the conductor that includes the Earth's atmosphere and ionosphere is developed. The role of the inclination of the magnetic field is considered. A mathematical simulation has shown that the resulting electric field in the ionosphere decreases. The penetration of the electric field and current from the ground to the ionosphere can be a physical process which creates observed ionospheric precursors of earthquakes only if the electric field strength and conductivity of the air near ground as well as the size of the pre-earthquake perturbations zone can simultaneously reach extreme possible values. Additional experiments are necessary to verify such a possibility. Other models are analyzed and compared to the main outcomes of our investigations.

Analysis of the differential emission measure distributions for solar flares observed by RESIK

Fri, 11/09/2018 - 19:10

Publication date: November 2018

Source: Journal of Atmospheric and Solar-Terrestrial Physics, Volume 179

Author(s): A. Kepa, B. Sylwester, J. Sylwester, M. Gryciuk, M. Siarkowski

Abstract

The Polish X-ray spectrometer RESIK observed the spectra in four wavelength bands from 3.3 Å to 6.1 Å. This spectral range contains many emission lines of H- and He-like ions for Si, S, Ar and K formed in the high temperature of solar coronal plasma. Analysis of measured spectra gives a possibility to study the differential emission measure distributions (DEM) in the temperature range between 1 MK and 30 MK. We present the analysis of DEM distributions of the multi-peaked C9.8 flare observed by RESIK on 9 January 2003 adopting the model of elementary flare profile (EFP). The model allows to distinguish the individual flare components based on the observed light curves in selected spectral bands.

Spatial, temporal and source study of black carbon in the atmospheric aerosols over different altitude regions in Southern India

Fri, 11/09/2018 - 19:10

Publication date: November 2018

Source: Journal of Atmospheric and Solar-Terrestrial Physics, Volume 179

Author(s): B. Vijay Bhaskar, R.M. Rajeshkumar, K. Muthuchelian, S. Ramachandran

Abstract

Ambient equivalent black carbon (EBC) aerosol mass concentration is regularly observed over a period of 2 years (June 2014 to May 2016) at Kodaikanal, a high-altitude and Madurai, a semi-arid location to study its spatial and temporal characteristics and its sources. The diurnal variation of EBC had two peaks, early morning sharp peak that occurred between 06:00 and 09:00 h (LT) almost an hour after the local sunrise and the evening broad peak that occurred between 19:00 to 22:00 h (LT), while in Kodaikanal late afternoon peaks showed maximum contribution. The difference in the sunshine hours with respect to seasons also caused an effect on the EBC concentration. The mean seasonal values of EBC over Kodaikanal are 2.58 ± 0.15 μg m−3, 1.98 ± 0.19 μg m−3, 1.01 ± 0.19 μg m−3 and 2.48 ± 0.15 μg m−3 and over Madurai 8.97 ± 2.34 μg m−3, 4.69 ± 2.14 μg m−3, 6.70 ± 3.83 μg m−3 and 3.34 ± 2.53 μg m−3 during winter, summer, monsoon and post-monsoon seasons, respectively. The effects of meteorological parameters on EBC at the two stations are investigated and it is found that wind speed and rainfall plays a major role. Source identification is done by calculating absorption Angstrom exponent (α). The absorption Angstrom exponent values ranged from 0.99 to 1.15 with the average value of 1.05 for Madurai and a range from 0.54 to 1.92 with an average value of 1.43 for Kodaikanal, indicating that the major sources of EBC on the study site are from biomass burning and fossil fuel burning sources.

Circulation characteristics of EP and CP ENSO and their impacts on precipitation in South China

Fri, 11/09/2018 - 19:10

Publication date: November 2018

Source: Journal of Atmospheric and Solar-Terrestrial Physics, Volume 179

Author(s): Jiangnan Li, Dazhen Huang, Fangzhou Li, Zhiping Wen

Abstract

EP and CP events have different effects on precipitation in South China (SC). In EP El Niño years, the winter precipitation increases and the summer precipitation is distributed in the form of a tri-pole; specifically, it increases in the northeastern and southwestern parts of SC and decreases in the central and northwestern parts. In EP La Niña years, the winter precipitation decreases in the most part of SC and the summer precipitation increases in the central part of SC and decreases in the eastern and southwestern parts. In CP El Niño years, the summer precipitation increases and the winter precipitation increases in the northeastern part of SC and decreases in the southwestern part. In CP La Niña years, the winter precipitation generally decreases and the summer precipitation decreases in the western and southeastern parts of SC but increases in the central and northeastern parts.

The results of both numerical simulations and diagnostic analyses show that in EP El Niño winters, there is a northerly wind on the lower troposphere of SC conducive to the southward movement of cold air to SC, converging in the south and diverging in the north, in agreement with the distribution of the precipitation anomalies. In the lower troposphere of SC in summer, an abnormal southwester is dominant, facilitating the movement of warm and moist air in the southwest toward SC, which is conducive to an increase in precipitation there. In EP La Niña winter, there is a southwester anomaly in the lower troposphere of SC, which is adverse to the southward movement of cold air from the north to SC, with the divergence field diverging in the south and converging in the north, in agreement with the distribution of precipitation anomalies in this region. In EP La Niña summer, there is a northeaster anomaly in SC that is in the water vapor convergence area, which is conducive to precipitation in SC. In CP El Niño summer. There are northerly winds over SC and an anomalous cyclone over the western Pacific and South China Sea, which was beneficial to precipitation.

Automatic identification of Spread F using decision trees

Fri, 11/09/2018 - 19:10

Publication date: November 2018

Source: Journal of Atmospheric and Solar-Terrestrial Physics, Volume 179

Author(s): Ting Lan, Yuannong Zhang, Chunhua Jiang, Guobin Yang, Zhengyu Zhao

Abstract

Spread F is a commonly observed phenomenon on ionograms caused by plasma irregularities or wave-like structures in the ionosphere. In general, Spread F could induce fluctuations of amplitude and phase of radio waves which travel through the ionosphere. Therefore, investigation of Spread F could be used to not only reveal ionospheric electrodynamics process, but also have a significant engineering application. Due to a large amount of ionograms recorded by ionosondes, it is a challenge work to manually identify ionograms with Spread F to study characteristics of Spread F. Thus, much work has been devoted to automatic identification of Spread F. In the present study, a machine learning method related to decision tree was adopted to automatically identify Spread F from ionograms. First, ionograms were processed by image method and projection techniques to provide input parameters for decision tree. The output of the proposed decision tree is whether Spread F is present or not on ionograms. Then, a set of ionograms was used to construct a decision tree. At last, a set of ionograms was adopted to validate the performance of this decision tree. In this study, ionograms recorded at Puer station (Geographic latitude and longitude: 22.7°N, 101.5°E; Geomagnetic latitude: 12.8°N) in the Yunnan province were used. Results indicate that the decision tree performed well in automatic identification of Spread F on ionograms. The accuracy of automatic identification in a set of ionograms with Spread F was reached up to 89%. It inspires us to continually improve the performance of automatic identification of Spread F in the future work.

Kinematics of CMEs and related shocks from LASCO data: Comparative analysis

Fri, 11/09/2018 - 19:10

Publication date: November 2018

Source: Journal of Atmospheric and Solar-Terrestrial Physics, Volume 179

Author(s): V.G. Fainshtein, Ya.I. Egorov, Yu.S. Zagainova

Abstract

We estimated the positions and velocities of the 10 fast limb CMEs with sources within the 30-degree from the limb and 10 fast halo CMEs with sources within the 30-degree from the solar disk center (central CMEs) using the LASCO-C2 and C3 images. The considered velocities, on which the results are based, include projection effects. We compared kinematic characteristics of the CME body with CME-related shocks whose linear projection velocities greater than 1500 km/s. For all the considered events: (1) the distance between the CME body and the related shock ΔR=(Rsh−Rb) increases with time; (2) the shock velocity Vsh is greater than the CME body velocity Vb; (3) the CME body acceleration modulus |ab| and the related shock acceleration modulus |ash| differ. Herewith, in some cases, |ash|>|ab|, and in other cases, on the contrary, |ash|<|ab|; (4) the difference in the velocities ΔV=(Vsh−Vb) grows with time for limb CMEs and decreases for central CMEs; (5) the variations of velocity-time profiles for the CME and shock are established to be equal: the velocities of both coronal structures either increase or decrease simultaneously. The following averaged values characterize the differences in the kinematics of the CME and shock: 〈ΔR〉  = 1.7 R⊙ (R⊙ is the solar radius), 〈ΔV〉  = 250 km/s, 〈Vsh/Vb〉  = 1.2, 〈ab〉  = 65 m/s2, 〈ash〉  = 70 m/s2.

Study of particulate matter of Akure, Nigeria using a sharp-cut inertial filter combined with an impactor-a preliminary study

Fri, 11/09/2018 - 19:10

Publication date: November 2018

Source: Journal of Atmospheric and Solar-Terrestrial Physics, Volume 179

Author(s): F.O. Abulude, M.M. Ndamitso, Y.A. Iyaka, A. Abdulkadir

Abstract

In this study, the particulate matter (PM10) was studied over a six month period (from August 2017 to January 2018) at an urban and a suburban site (Akure, Ondo State, Nigeria). Three different sites were used, namely: Federal University of Technology, Akure (FUTA); the National Commission for Museum and Monuments, Akure, and Science and Education Development Institute (SEDInst), Oba Ile. The monthly PM10 in these locations during the studied period ranged from 60.73 to 287.97 μm3 at FUTA, 96.1–175.63 μm3 for the National Commission for Museum and Monuments, Akure, while it ranged from 92.72 to 204.48 μm3 at SEDInst. The results were compared with national and international standards. It was observed that the PM10 were above the recommended limits. Meteorological parameters (Rainfall, relative humidity (RH), temperature, precipitation, wind speed and direction) were obtained from a meteorological center and analyzed statistically. The essence in the study was to find out their relationship with PM10. The value of R of the PM10 samples had positive correlation with rain (r = 0.561, 0.820, 0.408), RH (r = 0.487, 0.799, 0.318), and precipitation (r = 0.388, 0.486, 0.784). There were strong correlations between RH and temperature (r = 0.741) and RH and wind (r = 0.977). This study suggested that individual meteorological factors had effects on PM10 concentration.

Role of anthropogenic emissions and meteorology on ultrafine particle bursts over a high altitude site in Western Ghats during pre-monsoon

Fri, 11/09/2018 - 19:10

Publication date: November 2018

Source: Journal of Atmospheric and Solar-Terrestrial Physics, Volume 179

Author(s): Sobhan Kumar Kompalli, S. Suresh Babu, C. Udayasoorian, R.M. Jayabalakrishnan

Abstract

The ultrafine particle number concentration and size distribution during pre-monsoon (spring) over a high altitude location, Ooty (11.3 °N, 74.4 °E, 2240 m amsl) in Western Ghats, the highest peak in South India, are examined using campaign based ground observations. The total number concentrations are in the range ∼1000–3000 cm−3 with significant increase (2–4 folds) during the periods of ultrafine particle (UFP) (diameter < 100 nm) bursts. The UFP burst happens mostly during afternoon/evening with significant enhancement in number concentrations of nucleation (Nnuc) and Aitken (NAitk) mode particles. Examination of the association of these events with prevailing meteorology and trace gas concentrations revealed weaker dependence in general. However, the association between NOX and the UFP concentration indicates the possibility of common source for both. During the high concentrations of UFP, local winds originated predominantly from south/south west directions of the study location where the valley region with significant anthropogenic activities is located. Time of occurrence of the UFP bursts, trace gas concentrations and direction of the winds points towards the role of valley winds in transporting plume of pollutants to the mountain top observatory during the daytime.

Extending a model for water vapor sounding by ground-based GNSS in the vertical direction

Fri, 11/09/2018 - 19:10

Publication date: November 2018

Source: Journal of Atmospheric and Solar-Terrestrial Physics, Volume 179

Author(s): Yibin Yao, Zhangyu Sun, Chaoqian Xu, Xingyu Xu, Jian Kong

Abstract

Weighted mean temperature (Tm) is a crucial parameter for retrieving precipitable water vapor (PWV) in Global Navigation Satellite System (GNSS) meteorology. The difference between the height of the GNSS receiver and the height of the derived Tm may cause large uncertainty in the value of the retrieved PWV. In this study, we analyze the vertical variation of Tm on a global scale and build a new vertical correction model using the operational reanalysis of the European Centre for Medium-Range Weather Forecasts (ECMWF). Another dataset from ECMWF and the radiosonde data derived from 678 globally distributed stations are used to independently validate the performance of the newly built model and demonstrate its superiority compared with the existing vertical correction method. The results show that the lapse rate of Tm has geographic and seasonal variations, and the Tm has large nonlinear variation along the vertical direction in the high-latitude regions, especially in the polar regions. A performance improvement of 15%–35% compared with the existing vertical correction method is achievable for the newly built model when tested with the ECMWF data, and the improvement is 10%–25% when tested with the radiosonde data.

The study on current of upward aborted leader initiated from Zi-Feng tower

Fri, 11/09/2018 - 19:10

Publication date: November 2018

Source: Journal of Atmospheric and Solar-Terrestrial Physics, Volume 179

Author(s): Xiaojun Luo, Qi Zhang, Yapeng Fu, Bo Yang, Fei Guo, Yantao Duan

Abstract

In this study, the characteristics analysis on current of three sets of upward aborted leaders initiated from Zi-Feng tower in response to nearby lightning is performed. All of them had only an initial stage, not followed by any return-stroke sequences. With duration time of 83.3μs–590.6 μs, the measured current train is composed of a series of pulses. The pulses are close to a double exponential wave with the pulse width of about 7.7 μs ∼18.7 μs, and the rising time is on the order of sub-microseconds. The time interval between the neighboring pulses is about 18.6 μs The peak of pulse current ranges from 1.6 A to 226 A. With respect to the transferred charge of pulse, it ranges from 19 μC to 195.3 μC. Moreover, the possible reason for how the upward aborted leader starts is discussed. The presented statistical results may be of assistance in understanding initial continuous current and the initiation of upward leader at tall grounded objects, as well as in building more appropriate protective measures against lightning hazards.

Atmospheric gravity wave ray tracing: Ordinary and extraordinary waves

Fri, 11/09/2018 - 19:10

Publication date: November 2018

Source: Journal of Atmospheric and Solar-Terrestrial Physics, Volume 179

Author(s): R. Michael Jones, Alfred J. Bedard

Abstract

Calculation of internal gravity-wave ray paths in the atmosphere using a general three-dimensional ray tracing computer program is discussed. To initialize a ray-path calculation, it is necessary to specify the initial values for the components of the wave vector at the source so that the dispersion relation is satisfied. For acoustic waves, or for gravity waves in the absence of wind, there is no ambiguity in determining the magnitude of the wave vector once the frequency and direction of propagation (wave-normal direction) have been specified. For gravity waves with wind, however, it is necessary to solve a quartic equation to specify the magnitude of the wavenumber. Two of the roots of the quartic reduce to the usual solutions in the absence of wind, and we designate these roots as ‘ordinary waves.’ The two new roots (whose values approach infinity as the wind speed approaches zero), we designate as ‘extraordinary waves.’ A section contrasts the properties of the different gravity wave types.

Comparison with some previously published examples of ray-path calculations of gravity waves shows that those previous examples were actually extraordinary waves, but their significance was not recognized at that time.

In the absence of wind, gravity waves are restricted to a fan of propagation directions centered about horizontal propagation, but asymptotic gravity waves (in the Bousinesq approximation) have a fixed propagation direction for a given frequency. In the presence of a horizontal wind, however, the wave-normal direction is restricted to a fan of directions in the upwind direction, but not in the downwind direction. The ray direction for upwind propagation has no restrictions. A short review of gravity wave theory and the gravity wave dispersion relation is included.

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