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Publication date: Available online 9 November 2019

Source: Journal of Atmospheric and Solar-Terrestrial Physics

Author(s): S. RavindraBabu, S.T.Akhil Raj, GhouseBasha, M.Venkat Ratnam

The Upper Troposphere and Lower Stratosphere (UTLS) region plays an important role in the climate system. Quantifying the processes that control UTLS represents a crucial task. We assess UTLS trends and associated tropopause parameters (Cold Point Tropopause temperature/altitude (CPT/CPH), Lapse Rate Tropopause temperature/altitude (LRT/LRH), Convective outflow level temperature/altitude (COT/COH) and Tropical Tropopause Layer (TTL) thickness). This study is based on high-resolution daily radiosonde data from 2006 to 2018 over a tropical station Gadanki in south India supported by satellite measurements. The results show an increase of CPH (LRH) of ∼0.06 km (∼0.10 km), decrease of CPT (LRT) of ∼1.09 K (1.16 K), increase of COH of ∼0.29 km and decrease of TTL thickness of ∼0.23 km in the recent decade. The vertical temperature trends show a strong cooling trend at lower stratosphere (17-19 km) with a maximum cooling rate of 1.3±0.86 K per decade at 19.4 km altitude unlike reported recently using global radiosonde network. A warming trend is observed in the entire troposphere (0-15 km) with maximum warming rate of 0.44±0.55 K at 11.6 km during the last decade. Distinct variability in the temperature is noticed below and above the tropopause with the strong seasonal change above the tropical tropopause (18 and 19 km) compared to the below the tropopause (15-17 km). The observed trends are explained in relation to the ozone (O3) and water vapor (WV) trends over Gadanki. Instead of ozone changes, the WV increasing trend was found strongly influencing the LS cooling trend in the recent decade over Indian monsoon region.

Publication date: Available online 5 November 2019

Source: Journal of Atmospheric and Solar-Terrestrial Physics

Author(s): Sarita Sharma, Raj Pal Singh, Devbrat Pundhir, Birbal Singh

The experimental data obtained from three different techniques such as (i) subsurface VLF electric field measured with the help of a borehole antenna (ii) total electron content (TEC) of the ionosphere measured with a GPS receiver and (iii) VLF amplitude of subionospheric fixed frequency (f = 19.8 kHz) NWC (Naval Communication Station) transmitter signals monitored with softPAL receiver are analysed in relation to two major Nepal earthquakes occurred on 25 April (M = 7.8) and 12 May (M= 7.3), 2015. The VLF electric field data and fixed frequency VLF transmitter signal data are taken from Mathura and Agra stations in India whereas TEC data are taken from three IGS stations, one of which Lucknow (Lck3) is located in India and others Lhasa (Lhaz) and Wuhan (Wuhan) are located in China, all within 800 km from epicenters of two earthquakes except Wuhan which is located at about 2865 km. The results of the analysis indicate that while borehole and TEC data show anomalies in normal variations within 8-15 days prior to the earthquake of 25 April, the same occur within 5 - 7 days prior to the earthquake of 12 May. The subionospheric variations of VLF amplitude also show an anomaly 4 days before the later earthquake. Occurrence of these anomalies is examined critically in the light of local lightning and magnetic storms. However, these factors are not found to influence the data. A brief description of generation and propagation mechanisms of seismogenic emissions is also presented.

Publication date: December 2019

Source: Journal of Atmospheric and Solar-Terrestrial Physics, Volume 196

Author(s): Aliihsan Sekertekin

Land Surface Temperature (LST) is a key criterion in the physics of the Earth surface that controls the interactions between the land and atmosphere. The objective of this study is to evaluate the performance of physics-based Radiative Transfer Equation (RTE) method on LST retrieval using Landsat 8 satellite imagery and simultaneous in-situ LST data. In order to validate the satellite-based LST, in-situ LST measurements were obtained from Surface Radiation Budget Network (SURFRAD) stations simultaneous with satellite data acquisitions. In the study, four SURFRAD stations (BND, FPK, TBL and GWN) and five images for each SURFRAD station, totally twenty cloud-free images, were used for RTE-based LST validation. RTE method uses the atmospheric parameters acquired from radiosounding data simultaneous with satellite pass; however, these parameters were retrieved from NASA's atmospheric correction parameter calculator since radiosounding data are not available every time. Thus, this situation is another contribution of this study. As a result of the validation process of all data, the statistical measures, namely, coefficient of determination (R2), Root Mean Square Error (RMSE), Mean Absolute Error (MAE) and RMSE-observations standard deviation ratio (RSR) were calculated as 0.96, 3.12 K, 2.30 K and 0.33, respectively. However, the accuracy of RTE method on LST retrieval increased (R2 = 0.97, RMSE = 2.17 K, MAE = 1.44 K and RSR = 0.25) after removing TBL station from the analysis, since LST differences in this station were high for all scenes. RSR (ranging from 0 to high positive vlues) is an important measure for model evaluation, and the lower RSR value means high performance of the model. The obtained results revealed that physics-based RTE method is an effective and practical way for LST retrieval from Landsat 8 data despite using interpolated atmospheric parameters instead of radiosounding data.

Publication date: 15 November 2019

Source: Journal of Atmospheric and Solar-Terrestrial Physics, Volume 195

Author(s):

Publication date: Available online 23 October 2019

Source: Journal of Atmospheric and Solar-Terrestrial Physics

Author(s): B.S. Murthy, R. Latha, Arpit Tiwari, Aditi Rathod, Siddhartha Singh, G. Beig

Air quality in Delhi is largely dependent on concentration of aerosol particulate matter with aerodynamic diameter less than 2.5 μm (PM2.5). Diurnal variation of PM2.5 is mainly determined by rates of emission, deposition, chemical reactions and turbulent mixing caused by vertical wind shear and buoyancy. Continuous observations of PM2.5, NOx and O3 along with mixing layer height (MLH) by ceilometer (Vaisala CL51) in Delhi (Lodi road) during December 2017, January and February 2018 are analyzed. Comparison of MLH in winter (December) obtained from ceilometer and radiosonde showed good agreement in 66% of the cases for stable conditions and 36% of the cases for convective conditions. Diurnal variation revealed relatively low PM2.5 and NOx during convective period coinciding with high MLH while O3 registered higher concentration proportional to solar radiation. PM2.5 mass density was found to be inversely correlated to MLH during convective period. The mass density decreased by 14, 13 and 7 μgm-3 in December, January and February respectively for every 100 m increase of MLH. Variation of PM2.5 normalized by MLH qualitatively illustrated the influence of MLH on air quality in terms of dilution and accumulation during convective and statically stable conditions respectively. NOx and O3 had better correlations with MLH as compared to that of PM2.5. Air quality index in Delhi during winter with very low wind speed is mainly determined by MLH except during morning transition from stable to unstable condition.

Publication date: December 2019

Source: Journal of Atmospheric and Solar-Terrestrial Physics, Volume 196

Author(s): X. Wang, X. Zhao, H. Cai, G. Liu, M. Liao, L. Qu

The propagation characteristics of one downward positive leader with mainly four forked branches in one natural positive lightning have been investigated based on high-speed video observations. One branch develops horizontally; one branch travels downward eventually reaching the ground, and two faint branches are reilluminated by recoil leader (RL) activity multiple times. Four branches share some interesting correlation during their development. At the early phase, the horizontal branch and descending branch are inactive or propagate slowly when RL activity is more active. After the appearance of RL activity each time, the horizontal branch propagates slightly faster. After that, the two luminous branches propagate more actively and faster, while RL process becomes diminished and suppressed. Both downward and horizontal branches speed up as the downward one is approaching the ground. The two-dimensional (2-D) propagation speed for the downward branch ranges from 4.25 × 104 m/s to 3.85 × 105 m/s, with an average value of 1.01 × 105 m/s, and it ranges from 2.69 × 104 m/s to 3.61 × 105 m/s with an average speed of about 1.22 × 105 m/s for the horizontal one.

Publication date: Available online 15 October 2019

Source: Journal of Atmospheric and Solar-Terrestrial Physics

Author(s): Sara Karami, Nasim Hossein Hamzeh, Khan Alam, Abbas Ranjbar

Dust plays an important role in the modification of microphysical and optical properties of clouds. The presence of dust at an elevated level significantly increases snow mass and rain concentration. A rare phenomenon of rain and snowfall with dust was occurred simultaneously in the eastern Iraq, Syria, and west and south west of Iran on 19 and 20 January 2018. In the meantime, while the dust impact was dominating over a large area within Syria, Iraq, Saudi-Arabia, Kuwait and Iran, it consequently caused extreme temperature drop, rain and snowfall problems for the residents living in the Middle East. Likewise, a visibility drop together with rainfall was also observed in the same region. The visibility was reduced to less than 500 m over most of the region. Further, an image showing dust mass taken from the satellite with higher amount of AOD (≥1) indicated the severe dust activity. It is worth to mention that a dynamic low pressure system covering the surface, and also the cold and warm fronts and the relevant occluded front caused such a phenomenal problem. The dust storm on 19th January was mainly prefrontal typed and transported from Horolazim Lagoon, Ad Dahna and An Nafud deserts, under the impact of the south easterly winds. The cold front was displaced to the eastward with a consequent dust storm on 20th January, originated from the vaster parts in Iraqian and Syrian deserts and Ad Dahna and An Nafud deserts. The back trajectory analysis of HYSPLIT MODEL shows the different sources of dust in the study region during 19-20th January. On the whole, it can be concluded that the dust concentration and atmospheric quantities, including the wind, the precipitation, and the cold and warm fronts having a permissible effect on simulation processes, are well simulated by the WRF Chem model. The comparison of WRF-Chem and NAAPS outputs revealed that the dust pattern and the procedure of vertical transfer of particles of each model are very identical, but the amounts of dust in the Model NAAPS are underestimated.

Publication date: December 2019

Source: Journal of Atmospheric and Solar-Terrestrial Physics, Volume 196

Author(s): Hosik Kam, Yong Ha Kim, Nicholas J Mitchell, Jeong-Han Kim, Changsup Lee

We have evaluated the reliability of two methods for estimating mesospheric temperatures from all-sky VHF meteor radar data. The first method utilizes the decay time of meteor trails, and the other method takes advantage of the linear relation between temperatures and the full width at half maximum (FWHM) of the observed meteor echoes height distribution. We estimated the temperatures from two meteor radar datasets of King Sejong Station (62.22°S, 58.78°W), Antarctic and Esrange, Sweden (67.90°N, 21.10°E) during a period of 2007–2017 and 2003 to 2013, respectively. We devised an improved decay time method of temperature estimation that utilizes careful selection of detected echoes by reflecting seasonal change in height range where ambipolar diffusion is dominant in meteor decay. Applying the improved method, we achieved temperature estimation on average within 6.2 and 5.4% from Aura/MLS temperatures around 90 km at Esrange and KSS, respectively. In comparison, temperatures estimated by the FWHM method have mean differences of 5.1 and 3.6% from the MLS temperatures at Esrange and KSS, respectively. The FWHM temperatures show significantly less discrepancy from MLS temperatures and temporal fluctuations than the temperatures estimated by the decay time for both sites. This may indicate that the FWHM method is more robust to estimate mesospheric temperatures from meteor radar data.

Publication date: December 2019

Source: Journal of Atmospheric and Solar-Terrestrial Physics, Volume 196

Author(s): António A.S. Magalhães, Gabriel Guerche, Jean-Pierre Raulin

In this paper we analyse the effects of 25 solar flare events over long VLF propagation paths, one of them with the very remarkable property of crossing nearly the centre with the lowest magnetic field intensity in the South Atlantic Magnetic Anomaly (SAMA). The phase of the VLF transmitter signal (NPM: 21.4 kHz) on Lualualei, Hawaii, was recorded at the stations Punta Lobos (PLO, Peru) and Atibaia (ATI, Brazil) between 2007 March and 2011 September. Both paths NPM-PLO and NPM-ATI are collinear, and the comparison of the recorded phases suggests a descent of the lower ionosphere quiescent reflection height, possibly associated with the weakening of the Earth magnetic field in the SAMA region.

Publication date: December 2019

Source: Journal of Atmospheric and Solar-Terrestrial Physics, Volume 196

Author(s): Tamás Bozóki, Ernő Prácser, Gabriella Sátori, Gergely Dálya, Kornél Kapás, János Takátsy

Schupy is an open-source python package aimed at modeling and analyzing Schumann resonances (SRs), the global electromagnetic resonances of the Earth-ionosphere cavity resonator in the lowest part of the extremely low frequency band (<100 Hz). Its very-first function forward_tdte applies the solution of the 2-D telegraph equation introduced recently by Prácser et al. (2019) for a uniform cavity and is able to determine theoretical SR spectra for arbitrary source-observer configurations. It can be applied for both modeling extraordinarily large SR-transients or “background” SRs excited by incoherently superimposed lightning strokes within an extended source region. Three short studies are presented which might be important for SR related research. With the forward_tdte function our aim is to provide a medium complexity numerical background for the interpretation of SR observations. We would like to encourage the community to join our project in developing open-source analyzing capacities for SR research as part of the schupy package.

Publication date: December 2019

Source: Journal of Atmospheric and Solar-Terrestrial Physics, Volume 196

Author(s): G.F. Remenets, V.A. Shishaev

In this paper we present a catalog of the new geophysical phenomena, Ultra-energetic Relativistic Electron Precipitations (UREP), and relying on our previous physics analysis of it, describe its specific quantitative characteristics. This full dataset was created based on the continuous very low frequency measurements of a polar radio path with three working frequencies during the years 1988–1992. As discussed in our previous works, the main feature of this phenomenon is that ultra-energetic relativistic electrons precipitate into the middle polar atmosphere and cause additional ionization at the altitudes 10–45 km by the bremsstrahlung X-rays and gamma rays which are generated by the ~100 MeV penetrating electrons. This new geophysical phenomenon was detected by the ground-based measurements of the Very Low Frequency (VLF) signal variations due to the signals' passing along the terrestrial waveguide with a sporadic layer of ionization created by the X-ray radiation in the middle atmosphere. Therefore, in all these abnormal cases the middle atmosphere and the processes of VLF wave propagation are the detector of this new effect.

In the dataset presented, each event is identified by the abnormal values of VLF signals variations which are not typical for the measurements in the distance zone of measurements. In a case of this moderate distance from a VLF ground based source (~900 km), which is investigated here, the abnormality manifests in the qualitatively similar negative type of variations in both phase and amplitude of the detected signals for all 3 used radio wave frequencies. Such similarity appears due to the signal of strait seeing turns into the diffraction wave, the 1-st hop (ray) becomes ~ 2 times less. The indicated abnormality in the experimental variations proves that the second sky wave does not reach a receiver and there is an anti-phase interference effect between two field terms at the middle distance zone, previously unknown. Over 1988–1992 years, 135 VLF disturbances have been observed (with 17 events during the night) with mentioned abnormal behavior: 7 powerful disturbances, 72 strong disturbances and 56 moderate disturbances. It is known that the corresponding minimal values of effective height (altitude) of these VLF disturbances are: 25–35 km for powerful, 35–45 km for strong, and 45–55 km for moderate disturbances.

Publication date: December 2019

Source: Journal of Atmospheric and Solar-Terrestrial Physics, Volume 196

Author(s): O.A. Troshichev, D.A. Sormakov

The polar cap magnetic activity (PC) index is regarded as a proxy for energy that input into the magnetosphere during the solar wind-magnetosphere coupling (Resolutions of XXII IAGA Assembly, 2013). The PC index variations usually follow the changes of the interplanetary electric field EKL, which is estimated by the solar wind parameters fixed in the Lagrange point and published in the OMNI database. However, the correspondence between EKL and PC is often violated under disturbed conditions. To reveal the regularity of violations, the correlation between the PC index and “estimated” EKL field was examined in course of magnetic disturbances, the magnetic substorms occurrence being regarded as independent evidence of the geoeffective solar wind affecting the magnetosphere. Analysis, carried out separately for PCN and PCS indices, showed that magnetic activity in the winter polar cap (PCwinter) ensures statistically more correct results than that in the summer polar cap (PCsummer). Correlation between the PCwinter index and “estimated” EKL field proved to be good (R > 0.5) in about 80% of the examined substorm events. In other 20% of events the correlation was poor or negative, even though the magnetic substorms occurred in evident association with the preceding PC growth. It implies that “estimated” EKL field did not impact on the Earth's magnetosphere in these cases. Thus, the PC index makes it possible to verify the actual EKL field affecting the magnetosphere, and to check, in such manner, whether or not the solar wind fixed in the Lagrange point (presented at the OMNI database) encountered the magnetosphere in actuality.

Publication date: Available online 7 October 2019

Source: Journal of Atmospheric and Solar-Terrestrial Physics

Author(s): M.F. Camisay, J.A. Rivera, M.L. Mateo, P.V. Morichetti, M.V. Mackern

This study assesses the possibility of using Global Navigation Satellite System (GNSS) observations in combination with measurements of surface pressure and temperature to derive Integrated Water Vapor (IWVGNSS) estimates in Central-Western Argentina (CWA), a semi-arid region with complex topography. A significant agreement (coefficient of determination > 0.9) is observed between IWVGNSS and IWV estimates from radiosonde measurements, highlighting the capability of the GNSS stations to provide IWVGNSS estimates for a denser network. The variability of the IWV estimates, the atmospheric pressure and precipitation totals for a case study are compared. The results show that the occurrence of the increase in IWV values (positive IWV anomalies) precedes abundant precipitation over the CWA, in conjunction with the presence of mid-troposphere low-pressure anomalies acting as synoptic forcing. This kind of information provides a more comprehensive picture about the atmospheric processes involved in the development of deep convection, and it can be used for the development of contingency plans in the region. Heavy precipitation events and the difference in timing between positive IWV anomalies, both on-site and for their surroundings, should be considered.

Publication date: December 2019

Source: Journal of Atmospheric and Solar-Terrestrial Physics, Volume 196

Author(s): Muhammad Usman Alvi, Magdalena Kistler, Tariq Mahmud, Imran Shahid, Khan Alam, Farrukh Chishtie, Riaz Hussain, Anne Kasper-Giebl

Analysis of water soluble ions in PM10 in Pakistan is limited and therefore requires in-depth investigation. In this study composition and sources of ionic profile in PM10 were determined from an urban site of an Asian megacity (Faisalabad) in 2015–2016. The PM10 size fraction sampled on quartz filters was analyzed by ion chromatography for selected inorganic and organic ions. The daily mean PM10 mass concentration was found to be 744 ± 392 μgm−3, exceeding the limits proposed by Pak-EPA (150 μgm−3), US-EPA (150 μgm−3) and WHO (50 μgm−3). The ambient PM10 concentration was found to be highest in winter 2015-16 and autumn 2016, while the lowest in the monsoon 2016. The average total ion concentration was found to be 120 ± 51 μgm−3, which made about 16% of the total PM10 mass. CO32− was the most dominant specie followed by NH4+, Ca2+, K+, NO3−, SO42−, C2O42−, CH3COO−, Na+, Cl− and Mg2+. The ratio analysis of selected ions indicated dominant contribution of biomass burning during autumn and winter and higher impact of fossil fuel burning during spring and summer seasons. Positive Matrix Factorization identified traffic induced soil erosion, biomass burning, fugitive dust from construction activities, secondary aerosol formation processes and fossil fuel emissions from traffic and industry as major sources of particulates.

Publication date: December 2019

Source: Journal of Atmospheric and Solar-Terrestrial Physics, Volume 196

Author(s): Sridevi Jade, T.S. Shrungeshwara, Boddapati Anil

Precipitable Water Vapor (PWV) is estimated using MODIS (Moderate Resolution Imaging Spectro-radio-meter) Terra level 3 data with daily resolution i. e MOD08_D3 at 64 cGPS (continuous Global Positioning System) stations spatially spread over Indian subcontinent between geodetic latitude 5° to 35° N and geodetic longitude of 70° to 96° E. MODIS-PWV is compared with GPS-PWV estimated at these cGPS stations to check the validity of water vapor retrieved from MODIS data in Indian subcontinent. Correlation coefficient (R2) between daily values of MODIS and GPS water vapor is above 0.9 with RMSE (root mean square error) of 2–5 mm for 22 cGPS in peninsular India, above 0.9 with RMSE of 3–6 mm for 5 cGPS in northeast India and above 0.8 with RMSE of 1–9 mm for 26 cGPS in Himalayas. PWV time series at all the cGPS stations indicated distinct seasonal cycle for both MODIS and GPS PWV with high RMSE (~6 mm) in wet months and low RMSE (~3 mm) during dry months. Taking advantage of broad spatial spread of stations and long span of data, model for spatial variability of GPS-PWV for Indian subcontinent is proposed. Inter-annual and seasonal variability of GPS-PWV is discussed in detail for peninsular India, northeast India and Himalayas.

Publication date: 15 November 2019

Source: Journal of Atmospheric and Solar-Terrestrial Physics, Volume 195

Author(s): Hasssen Babaousmail, Rongtao Hou, Brian Ayugi, Gnim Tchalim Gnitou

This study concentrated on the assessment of two operational high-resolution satellite-based rainfall products, Climate Hazards Group InfraRed Precipitation with Stations (CHIRPS) and NOAA CPC Morphing Technique (CMORPH) over Algeria. The assessment was carried out for a duration of 19-years ranging between 1998 and 2016 using 20 rain gauge datasets. The study area can be divided into five regions (zones) according to Köppen climate classification (CSA, BSK, BWK, BSH, and BWH). Both satellite derived rainfall estimates (SRE); (CHIRPS and CMORPH) were evaluated over these regions. Continuous statistics was employed to measure their performance in evaluating and reproducing rainfall while categorical statistics employed on daily time scale to assess the ability of SRE in detecting rain/no rain events. Correlation Coefficient (R), Bias and Root Mean Square Error (RMSE) are employed on daily, monthly as well as on annual timescales. At daily scale, most SRE showed unsatisfactorily performance over all the five regions with exception for CMORPH products exhibiting better performance. However, as the time step augmented, the performance of SRE enhanced. At monthly time scale a higher agreement was observed. The CHIRPS performed the best (R = 0.9), whereas CMORPH-CRTv1.0 showed relatively weaker but good correlation (R = 0.83). At annually time scale CHIRPS has a better performance in CSA, BSK regions, on the other hand CMORPH demonstrate a better results BSH, BWK, BWH regions. The study demonstrate that reliable evaluation of rainfall in Algeria with different climate zones employing satellite-based precipitation estimation products remains a challenge.

Publication date: 15 November 2019

Source: Journal of Atmospheric and Solar-Terrestrial Physics, Volume 195

Author(s): Xing Li, Weixing Wan, Jinbin Cao, Xianghui Xue, Zhipeng Ren

Based on the daily DE3 temperature tidal component from Li et al. (2015), which is extracted from the TIMED/SABER observations in the mesosphere/lower thermosphere region (70–108 km altitudes) and at the low-latitude and mid-latitude (45°S–45°N) from 2002 to 2012, we decompose the 1-day resolution tidal component into some orthogonal Hough modes and the climatological (annual and inter-annual variations) and meteorological (day-to-day variability) characteristics of the first two ranks of the Hough modes are presented. The results show that the first four rank of the Hough modes can well reconstruct the latitudinal and altitudinal variation of the daily DE3 tidal component. Wherein, the first symmetric and antisymmetric Hough modes ((3, 3) ＆ (3, 4)) play an important role in the higher altitudes, meanwhile, the second symmetric and anti-symmetric Hough modes ((3, 5) ＆ (3, 6)) mainly contribute in the all altitude range, especially in the lower altitudes. The properties of the large time scale variation of the Hough coefficients are similar to those of previous work, which use the data with 60-day resolution. These properties are that the first symmetric Hough mode (3, 3) presents an obvious annual variation that they are stronger during boreal summer; the relative contribution of the first anti-symmetric Hough mode (3, 4) is much larger during boreal winter. Practically, the higher resolution data were used to reveal the day-to-day variabilities of the DE3 tidal Hough modes. It is found that the first two ranks Hough coefficients all dominate in high altitudes from 100 to 108 km and display an obvious semi-annual variation that they are larger in solstices than that in equinoxes. The day-to-day variability of each rank Hough coefficient may be explained by the variance of the absolute amplitudes and the contribution of the phases. It is found that the contribution from the absolute amplitudes plays an important role in the day-to-day variability of the first rank Hough coefficient; the contributions from the absolute amplitudes and phases are both important to the day-to-day variability of the second rank Hough coefficient.

Publication date: December 2019

Source: Journal of Atmospheric and Solar-Terrestrial Physics, Volume 196

Author(s): Mariano Fagre, Bruno S. Zossi, Jaroslav Chum, Erdal Yiğit, Ana G. Elias

High frequency (HF) electromagnetic wave propagation is commonly used in long-distance communication and detection. The ionosphere is a highly variable medium affecting this propagation. However, mean climatological conditions are useful in order to determine a “base level” for the design and operation of systems using HF waves that propagate in the ionosphere. Important variables that determine these conditions are the ionosphere peak height, hmF2, and F2 critical frequency, foF2. In the present work the effect of different hmF2 and foF2 model options in IRI-2016 and its spatial variability are analyzed through the analysis of the ground range and reflection height of HF ray paths using a numerical ray tracing method. The model options are M(3000)F2, AMTB and SDMF2 for hmF2, and URSI and CCIR for foF2. We perform this study for a quiet day, April 26 at 12 LT, and solar activity maximum conditions. Ground range and reflection height variation between values obtained with the different model options are on average not greater than ~40%, but can be higher for a Pedersen ray case. These variations are in general much stronger than those obtained when Earth's magnetic field is neglected in ray path assessments. However, while the magnetic field effect is of “physical” origin that has always the same sign, the effect of changing a model used for certain parameter's estimation depends on the model performance which may vary with location and time.

Publication date: 15 November 2019

Source: Journal of Atmospheric and Solar-Terrestrial Physics, Volume 195

Author(s): Mikhail L. Shmatov, Karl D. Stephan

Ball lightning is a rarely observed phenomenon whose existence is attested to by thousands of eyewitness reports, but which has so far evaded a widely accepted scientific explanation. This review paper summarizes theoretical, observational, and experimental work in the field since approximately 2000. In particular, several situations when mobile phone cameras as well as scientific instruments have been used to capture numerous events that are candidates for ball lightning sightings are considered. We evaluate recent experimental attempts to produce laboratory ball lightning, review what is known about possible ball lightning hazards, and conclude with recommendations for future research in this area.

Publication date: 15 November 2019

Source: Journal of Atmospheric and Solar-Terrestrial Physics, Volume 195

Author(s): A. Adero Ochieng, Geeta Vichare, Paul Baki, Pierre Cilliers, Pieter Kotze, Chao Xiong, Ashwini Kumar Sinha

Present paper studies Field aligned currents (FACs) estimated by employing Ampere's law to the magnetic field recorded by CHAMP satellite during 24 geomagnetic storms. Low-pass filtered FACs with a cutoff period of 20 s (scale size~150 km) are used to determine FAC range, which is defined as a peak-to-peak amplitude of FAC density. Thus we are considering only the strongest positive and negative FACs emerging either from Region 1, Region 2, Region 0, or substorm current wedge systems. It is known that the FACs significantly depend on the highly variable solar wind (SW) and interplanetary magnetic field (IMF) conditions and also on the processes internal to magnetospheric-ionospheric system such as substorm. The correlation analysis carried out here shows that sometimes the FAC range, correlates well with SymH, AsyH, AsyD, AL, am and Kp indices (>95% significance), but not always. The variation of the FAC range with magnetic local times shows distinctly different patterns during southward and northward IMF conditions, with peaks near dawn-dusk during southward IMF and near local noon-midnight during northward IMF. These results are in agreement with the earlier reports. However, the seasonal dependence reveals that the noon time peak is essentially associated with the summer season. We have determined a new parameter called ‘occurrence rate of FAC range ≥1 μA/m2’ and examined it under various solar wind and IMF conditions. It is found that the probability of FAC range ≥1 μA/m2 have a clear dependence on the clock angle, suggesting more frequent intensifications during southward IMF. Clear linear dependence on the cone angle demonstrates higher occurrence probability of FAC range ≥ 1 μA/m2 when the IMF is perpendicular to the Sun-Earth line (cone angle nearing 90°). All these results based on the newly defined parameters such as FAC range and probability of FAC range ≥1 μA/m2, for the storm time mesoscale FAC are consistent with the previous studies. The FAC ranges are found to have a linear dependence on the values of IMF BY, BYZ, BT and BZ, though saturation is apparent at higher values of the IMF parameters. FAC range shows distinctly different dependence for slow and fast solar wind, suggesting the importance of the composition and properties of SW in controlling the FAC strengths.