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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.

Publication date: 15 November 2019

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

Author(s): Vladimir V. Zuev, Ekaterina Savelieva

The Arctic stratospheric polar vortex usually forms in autumn, reaches its peak intensity in mid-winter and decays in spring. The polar vortex strength and persistence in the winter–spring period play an important role in stratospheric ozone depletion with the return of solar radiation in late winter. The polar vortex breakdown in most cases occurs under the influence of vertically propagating planetary Rossby waves. The increased activity of planetary waves was observed in 1984/1985, 1998/1999 and 2012/2013 and led to the polar vortex breakdown in mid-winter, after which it was not observed for more than a month. In this study, Arctic sea ice loss is considered as the most likely cause of the increased activity of planetary waves resulting in the unusual weakening of the Arctic polar vortex. Arctic sea ice extent was a record low in autumn 1984, 1998 and 2012 in the Beaufort Sea, the Canadian Arctic Archipelago and the Central Arctic.

Publication date: 15 November 2019

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

Author(s): Vishal Chandole, Geeta S. Joshi, Shilpesh C. Rana

The present research aims to assess the climate change in Lower Tapi River Basin (LTRB) of the state of Gujarat in India. The Mann-Kendall (MK) test with Sen's slope estimator and Sen's Innovative Trend Analysis (ITA) have been used for detecting the trend of the hydro-meteorological parameters-temperature and rainfall. The temperature trend during the period 1979–2014 has been analyzed using MK test and ITA method. The annual maximum, annual minimum and annual mean temperature analysis is carried out for 5 weather stations in the basin. Also, the trend has been analyzed using MK test and ITA method for annual rainfall for 9 raingauging stations for three time periods i.e. 1928–2013, 1955–2013, and 1970–2013. Most of the results obtained from these two methods for trend detection of these hydro-meteorological parameters are found to be matching. Further, the results of this study shown that the annual maximum, minimum and mean temperature increases at trend value of 0.009 °C/yr, 0.0244 °C/yr and 0.0188 °C/yr, respectively, over the LTRB during 1979–2014. The annual rainfall increases at the trend value of 2.185 mm/yr, 4.181 mm/yr and 5.495 mm/yr for the period of 1928–2013, 1955–2013 and 1970–2013 respectively. The magnitude of upward (positive) trend of annual rainfall is greater for the period of 1955–2013 and 1970–2013 in comparison to the period of 1928–2013. The spatial distribution of temporal trend of these hydro-meteorological parameters have been represented in Arc-GIS 10.3.

The results of Spatio-Temporal trend distribution over the study area shown that the annual mean and annual minimum temperature indicates the upward (positive) trend over whole LTRB, while annual maximum temperature indicates the upward (positive) trend over the basin except west coastal region of LTRB. The annual rainfall exhibits upward (positive) trend in all three time periods over entire LTRB.

The trend (positive/negative) detected for mean temperature and annual rainfall is found to be consistent with the trend noticed by Indian Meteorological Department for the state of Gujarat. The spatial distribution of trend further needs to be analyzed to correlate it with changing pattern of land use – land cover over a period of time. The present case study for the assessment of climate change would be useful to the water resources engineers for planning and management and policy decisions for climate change adaptation in the study area.

Publication date: 15 November 2019

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

Author(s): N.S. Arshad, M. Abdullah, S.A. Samad, N.M. Hatta

High intensity lightning strike can lead to Very Low Frequency (VLF) signal reception. The lack of VLF signal feature characterisation creates the need for commercial lightning dataset subscription, so as to determine the causative lightning strike. This paper presents a signal feature analysis of VLF signal for high intensity lightning severity classification purpose. As such, the analysis was comprised of dual stages, namely feature extraction and feature classification, for two-class recognition. In the feature extraction stage, Discrete Wavelet Transform was implemented to decompose the VLF signal into four frequency bands. In each frequency band, five-signal parameter analysis was executed to yield twenty signal features. Next, the classification stage was executed to determine the best signal feature combination and the most dominant signal feature that generated the highest classification accuracy within the five classifiers. The simulation revealed that the most optimum signal feature combination was achieved via Boosted Trees classifier with classification accuracies of 70% and 50% for training and test datasets, respectively. Class prediction in the skewed test dataset appeared consistent with the test data classification accuracy. The Zero Crossing Rate between 30 and 60 kHz emerged as the most dominant signal feature due to its impact on all classifiers' best signal feature arrangement. The proposed recognition model enables the detection of high intensity lightning classes through the use of VLF signal features.

Publication date: 15 November 2019

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

Author(s): Qiong Wu, Hui Li, Chi Wang

Galactic cosmic rays (GCRs), modulated by the heliospheric magnetic field (HMF), are speculated to provide a possible link between solar activities and the earth's lightning variation. To test this hypothesis, we investigate the correlation between the sudden decrease of GCR in a few hours to one day, known as Forbush Decrease (FD), and the lightning incidence in the tropics and subtropics. During the operating period of the TRMM Satellite, 28 FD events are identified with their decrease amplitudes (DAs) greater than 4%. For a typical FD event occurred on January 10, 2002, the daily cosmic ray (CR) intensity presents an intense decline from 5830.33 counts/min to 5675.96 counts/min in one day. Correspondingly, the daily lightning count decreases right after the FD's onset without any obvious time delay, specifically from 3474 day−1 to 672 day−1 in one day, and reaches its minimum of 355 day−1 another day later. Based on the superposed epoch analysis (SEA), similar statistical correlation is further confirmed. On average, the adjusted daily lightning anomaly decreases from 0.33 to −0.31 in three days after the FD's onset. The result of the Monte Carlo test indicates that such positive relevance between the CR intensity and the lightning incidence during a FD event is statistically significant.

Publication date: 15 November 2019

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

Author(s): Francesco Esposito, Mariarosaria Calvello, Giulia Pavese

A high-temporal resolution model for the identification of columnar aerosol composition was applied to two sets of radiometric measurements obtained one in a semi-rural site of South Italy and the other in an urban/traffic site of the Po Valley, northern Italy. Inputs of the model are the spectral Aerosol Optical Depths (AODs) from direct solar radiation data to derive the contribution to the total extinction of nine aerosol species, i. e Water Soluble (WS), Black Carbon (BC), Sea Salt accumulation (SSacc), Sea Salt coarse (SScoa), Biogenic (Bio), Mineral Coarse (Mcoa), Mineral Accumulation (Macc), Small Organic Matter (SOM), and Large Organic Matter (LOM). In order to better parameterize the absorptive properties of dust and organic particles, a wavelength dependent imaginary part of the refractive index was used for dust and organics. Despite the differences between the two sites, a prevalence of fine aerosols (organics, WS and BC) was found in both cases suggesting the relevance of anthropogenic sources such as traffic. The retrieved AOD BC component was compared to Equivalent BC (EBC) concentrations measured in situ by an aethalometer at both sites. For mean daily values good correlations were found (R2 = 0.7), whereas high-temporal resolution (hourly basis) columnar composition was verified to be more reliable in cases of surface-column coupling. The possibility to apply this simple model to direct irradiance data from portable radiometers in areas where no routine measurements are available, and obtaining results with an increased temporal resolution, can help to gather information on aerosol columnar composition to be used in radiative, climate and transport models.

Publication date: 15 November 2019

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

Author(s): Jialin Lin, Taotao Qian

This study examines the impact of the intraseasonal Madden-Julian Oscillation (MJO) on global land surface diurnal temperature range (DTR) using two daily surface air temperature datasets. The results show that the MJO significantly affects DTR over all the continents. The two datasets are highly consistent with each other, suggesting that the results are robust. Significant correlations exist in all seasons over the tropical continents, including Africa, India, southeast Asia, Australia, central America and Amazon. In the extratropics, the correlations change with season. The highest correlations occur in winter for Europe, in winter and spring for China, in autumn for United States, and in summer and winter for Canada. The surface DTR anomalies coincide well with the outgoing longwave radiation (OLR) anomalies at the top of the atmosphere, suggesting that variations of DTR are associated with variations of clouds, which is consistent with the results of previous studies on DTR trend and variability at seasonal to multi-decadal time-scales. The DTR variations are also associated with changes of deep convection, soil moisture and snow cover.

Publication date: 15 November 2019

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

Author(s): Samson Tilahun, Yekoye Asmare Tariku

This study verifies the ionospheric perturbation induced scintillation in L-band range of frequency using HF/VHF bands over the African equatorial and low latitude regions with a focus over Ethiopia. The study was conducted using the SCINDA, ionosonde and beacon receivers installed at different regions in Ethiopia. In the F-layer height (hmF2), a reverse decrease of foF2 in combination with significant background TEC reaching up to 10 TECU prior to the depletion was observed during the evening hours. This creates conducive environment for the generation of ionospheric irregularities that can be manifested by Rate of TEC Index (ROTI) and scintillation index (S4) enhancement. Vertical Perturbation Index (PI) was computed using vertically integrated ion density profiles to verify the irregularities from independent source and found that HF inferred PI can potentially identify the existence of ion density irregularity and further verify L-band frequency scintillation. On the other hand, simultaneous observation of S4 in VHF and UHF bands showed excellent agreement despite VHF scintillates more than UHF due the difference of transmitting frequency bands. Besides, Power spectrum density (PSD) was computed using the FFT algorithm for S4 data measured while receiving L-band and VHF signals for selected PRNs observed by the receivers. The calculated slope (m) and the strength of irregularity (beta) using the power spectral law has enabled us to produce a model which better fits the observed spectrum. It shows beta higher for VHF and lower for UHF; while, PSD modelling better matches for UHF than VHF spectral observation.

Publication date: December 2019

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

Author(s): Tong Xu, Michael Rietveld, Jian Wu, Guanglin Ma, Yanli Hu, Jun Wu, Qingliang Li

Modulated high frequency heating of the polar ionosphere in the presence of a polar electrojet current (PEJ) can generate extremely/very low frequency (ELF/VLF, 3 Hz–30 kHz). In order to overcome the dependence of the radiation intensity on the variable PEJ, researchers have focused on electrojet-independent approaches. One potential method is so-called “beat-wave” (BW) ELF/VLF generation invoking nonlinearity of the F-region ionospheric plasma, but this F-region source mechanism remains controversial. We perform polarization analysis of BW generated ELF/VLF waves using EISCAT heating facility near Tromsø. The results show that ELF/VLF waves can be decomposed into right- (R) and -left (L) handed circularly polarized waves with comparable strengths, and the polarization ellipse tends to be parallel to the well-known dominant background electric field, which is fully consistent with the recognized features of ELF/VLF waves generated by PEJ modulation. Hence, the results demonstrate that the BW generated current is located in the D/E region, and not the F region.

Publication date: 15 November 2019

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

Author(s): Daniel Atnafu Chekole, Nigussie Mezgebe Giday, Melessew Nigussie

This paper investigates the performance of NeQuick-2, the latest version of International Reference Ionosphere IRI extended to the Plasmasphere (IRI-Plas 2017), and Global Ionospheric Map (GIM) models over four stations deployed in Ethiopia during medium, high, and low solar activity periods. The monthly and seasonal mean diurnal variations of Vertical Total Electron Content (VTEC) derived from NeQuick-2, IRI-Plas 2017 and GIM models were compared with the GPS-VTEC values in terms of Root-Mean-Square Errors (RMSE) and correlation coefficients (r). It was found that the monthly and seasonal mean diurnal VTEC variations agree fairly well with the GPS-VTEC at all stations although with some offsets. For the entire years of the study, the highest monthly mean VTEC variation is observed during equinox months, whereas minimum in the solstice months. The NeQuick-2 model monthly and seasonal mean VTEC output were generally better than IRI-Plas 2017 and GIM models output in most of the stations during medium solar activity year of 2012. For all models, the significant discrepancy (i.e, overestimations or underestimations) is observed during daytimes and seasons of high solar activities. In some cases, the IRI-Plas 2017 model displayed poor performance during medium and low solar activity periods, while it showed a better agreement for some stations (e.g. asos, armi, and bdmt) during high solar activity phase (especially in March equinox and December solstice). The performance of GIM model was good for most of the stations during low-solar activity year of 2016 and during the study years for adis, while it has poor performance during the periods of high solar activity year of 2014. All models correlated well, although GIM model was highly correlated with the GPS-VTEC value (r ≥0.99). The IRI-Plas 2017 model correlated better with the GPS-VTEC than NeQuick-2 model during the study years. Moreover, the performance of IRI-Plas 2017 model was better during high than low and medium periods of solar activity years. The GIM model has better performance for most stations (e.g. adis, asos, and bdmt) over the study region during low-solar activity period.

Publication date: 15 November 2019

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

Author(s): N. Jeni Victor, Devendraa Siingh, R.P. Singh, Rejesh Singh, A.K. Kamra

Continuous measurements of the concentration of Radon (222Rn) and its progenies were made with a Radon detector, RTM 2200, at a tropical site, Pune (18° 31′ N, 73° 55′ E, 560 m), India from January to December 2012. Simultaneous measurements of small ions of <1.6 nm diameter were made with a Neutral Air Ion Spectrometer (NAIS). Annually - averaged diurnal variation curves of 222Rn concentration showed a peak (~9.9 Bq m−3) at 0800 Local Time (LT) and a minimum (~5.4 Bq m−3) at 1800 LT. Diurnal variations of 222Rn concentration during winter, post-monsoon, and pre-monsoon seasons were almost similar in shape but with decreasing amplitudes in these seasons. However, during monsoon season, 222Rn concentration was the lowest and remained almost constant throughout the day. Monthly mean value of 222Rn concentration was the highest in December and the lowest in June. Polonium (218Po) concentration did not show any systematic diurnal variation. Results are explained in terms of surface meteorology, soil moisture, soil temperature, vertical wind velocity, local topography and development of atmospheric boundary layer. 222Rn concentration increased with soil moisture content of <16%, rapidly dropped down to dry soil values and then remained almost constant with increasing soil moisture. On the other hand, 222Rn concentration decreased with the increase of upward vertical wind velocity in the atmospheric boundary layer. The diurnal variations of ionization rate, directly estimated from 222Rn measurements, and calculated from ion concentration measurements showed some similarities but differed in their amplitudes. Annual effective dose of 222Rn (~0.075 mSv) as estimated from our measurements at Pune was found below the value prescribed by World Health Organization.

Publication date: 15 November 2019

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

Author(s): R. Morrow

It is shown how ball and bead lightning can originate from a narrow lightning channel which has been expanded by an ionizing wave. The resulting plasma structure development is followed for 10 s, and it is shown that the electric field developed can act as an electrostatic trap for negative burning particles. Previously it has been shown that a secondary lightning stroke current travelling down the expanded channel is necessary to explain the strength of the channel luminosity. This same secondary current can constrict the channel irregularly, via the magnetic pinch effect, to produce elongated segments of the channel; these become ball and bead lightning. All the plasma balls/beads will be “phantom plasma” structures unless some negatively-charged burning particles (fuel such as soil, copper wire or smoke) are trapped in the electric field distribution providing illumination.

Publication date: Available online 23 July 2019

Source: Journal of Atmospheric and Solar-Terrestrial Physics

Author(s): Jiangnan Li, Chenghui Ding, Fangzhou Li, Wenshi Lin, Yerong Feng

The WRF model was adopted to perform simulations of Typhoon (TC) Sarika (2016). Two local (QNSE and MYJ) and three nonlocal (YSU, ACM2, and GFS) closure Planetary boundary layer (PBL) schemes were selected to perform five groups of simulation tests. (1) PBL schemes have a minor influence on TC track but exert a major influence on both the intensity and structure of TC. TC track, intensity, and structure simulated according to the nonlocal PBL schemes are close to the observations, while the TC intensities simulated by the local PBL schemes are slightly higher than the actual observed value. The nonlocal GFS scheme generates the best simulation result. (2) The TC intensity is closely related to variations in the TC structure. Of these schemes, the TC simulated by the local QNSE scheme has the largest intensity, the smallest eye zone, a complete eyewall, the strongest convection, the largest reflectivity, and the strongest tangential wind. (3) Differences in the TC intensities occur under the joint action of the thermal and dynamic factors. Regarding the local closure schemes, the simulated values of the surface enthalpy flux, water vapor flux, and momentum flux are slightly higher and the TC can be stronger accompanied by a higher PBL, stronger entrainment at the top of the PBL, and more intense vertical mixing. Moreover, more intense vertical mixing can bring surface energy into the upper air, resulting in the development of deep convection in a stronger TC, an intensified warm core structure, and a higher warm center.

Publication date: 15 November 2019

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

Author(s): Yekoye Asmare Tariku

This paper investigates the validation of the performance of the latest versions of the International Reference Ionosphere (IRI 2016), the IRI extended to the plasmasphere (IRI-Plas 2017) and NeQuick 2 models in the estimation of the variation of the Total Electron Content (TEC) over the West Pacific regions during the 2015–2017 years. This has been performed employing the GPS-derived TEC data obtained from stations located at Observation Rock, OBSR (geog 46.90°N, 238.18°W, Geom. 52.46°N), Husband, HUSB (44.12°N, 238.15°W, Geom. 49.73°N). It has been revealed that both the GPS-derived VTEC and modelled (IRI 2016, IRI-Plas 2017 and NeQuick 2 VTEC) variations attain their minima at about 13:00 UT (05:00 LT) and maxima at about 20:00 UT (12:00 LT). Moreover, because of the enhancement of photo-ionization process in the region as a result of exposure of the ionosphere for direct radiation in the summer months, large measured VTEC values are seen in the June solstice months during 2015–2017. It has also been shown that, because of the variation of the Sunspot number (SSN) and solar radio flux 10.7 cm (F10.7), the VTEC variations for both the GPS-derived and models show decrease at transition from 2015 to 2017, with some exceptions observed in the June solstice months. In addition, the root-mean-square deviations (RMSD) between the GPS-derived and modelled diurnal VTEC variations are generally less than 0.5TECU in using both the SSN and F10.7 indices. This shows that all the three models are good in TEC estimation with the IRI-Plas 2017 and NeQuick 2 perform the best on most of the hours, in comparison with the IRI 2016 model with IRI2001 option, especially in using the F10.7 index when the solar activity increases. However, when the solar activity decreases, utilizing the SSN for all the three models (especially for the IRI-Plas 2017 and NeQuick 2) shows the better performance. Moreover, during the geomagnetic storm condition, both the IRI 2016 and IRI-Plas 2017 models with the storm option “on” do not adequately reflect to the sharp increase or decrease of the GPS-derived VTEC values.

Publication date: Available online 15 May 2019

Source: Journal of Atmospheric and Solar-Terrestrial Physics

Author(s): Oindrila Nath, S. Sridharan

The volume mixing ratios (VMRs) of Nitrous Oxide (N2O) and NOx [Nitric Oxide (NO) + Nitrogen-di-Oxide (NO2)] derived from the radiance measured by the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) Fourier infrared spectrometer, for the period 2007–2011 have been used to study their seasonal, interannual as well as Sudden Stratospheric Warming (SSW) related variations in the equatorial (5°N-5°S) stratosphere. Both N2O and NOx VMR show a clear seasonal variation around 45 km. The N2O VMR exhibits minimum (around 200 ppbv) during July–August and remains at 250–300 ppbv during the rest of the months. The NOx VMR is found to be minimum (around 10 ppbv) during May–July, whereas it shows higher values (∼15–17 ppbv) in the other months. Higher values are found in N2O and NOx VMRs in the years 2008 and 2010 in the northern tropics (10–30°N) when the QBO is in westward phase favouring high residual meridional circulation. During the SSW of 2009, NOx VMR is found to increase by 3 ppbv. Using a primitive chemistry-transport model, the increase in NOx during the SSW is demonstrated to be due to the enhanced vertical upwelling as well as meridional circulation prior to the onset of the SSW in response to the larger planetary wave activity.

Publication date: Available online 26 April 2019

Source: Journal of Atmospheric and Solar-Terrestrial Physics

Author(s): Yu Zhao, Qi Feng, Aigang Lu

A systematic investigation and greater understanding of the precipitation chemistry with respect to the altitude gradient in mountainous areas is necessary to promulgate the detrimental consequences of pollution on various ecosystems. Studies on the chemical compositions of wet precipitation were performed on the northern slopes of Mt. Taibai, Qinling Mountains, one of the highest mountains in East China, from different lateral monitoring stations (i.e., with elevations between 610 and 3511 m) over the period from 2011 to 2014. All samples were analyzed for major ions (SO42−, NO3−, Cl−, F−, NH4+, Ca2+, Mg2+, Na+, and K+). The results revealed that the total ion concentration and average measured ions concentrations in precipitation decreased with the increase of altitude in the Mt. Taibai. The concentrations of precipitation ions are always higher during non–monsoon period compared to the monsoon period, except for the altitude above 3000 m a.s.l. A negative gradient of approximately −6.58 and −34.04 μeq·L−1/100 m of the total ion concentrations was obtained during the monsoon and the non–monsoon period, respectively. NH4+ was the most promising species for completely neutralize the acidity at the altitude of 500–1000 and 3000–3500 m a.s.l., while Ca2+ was the potential species for completely neutralize the acidity at the altitude of 1000–3000 m a.s.l. The crustal–derived species (Ca2+, Mg2+, Na+, K+ and Cl−) exhibit statistically significant correlations with each other at the altitude from 500 to 2500 m a.s.l. (r = 0.31–0.91). The contributions of sea salt source and terrestrial source to SO42− and NO3− are small, which is mainly contributed by human activities. For Na+, Mg2+, and K+, there was both the contribution of the terrestrial and human activities. The inorganic pollutants (major ions such as the SO42−, NO3−, Cl−, F−, NH4+, Ca2+, Mg2+, Na+, and K+) in the Mt. Taibai appear to be derived from the regional crustal dusts, anthropogenic emissions, and the long–range transported from the sea.

Publication date: Available online 29 March 2019

Source: Journal of Atmospheric and Solar-Terrestrial Physics

Author(s): Daniele Barroca Marra Alves, Eniuce Menezes de Souza, Tayná Aparecida Ferreira Gouveia

GNSS (Global Navigation Satellite Systems) can provide high accuracy positioning at low cost. But, depending on the sources of error, e.g. the atmospheric effects, this accuracy can be degraded. The ionosphere is one of the most important error sources in GNSS positioning. Among several effects caused by the ionosphere, irregularities such as ionospheric scintillations are very relevant. They can cause cycle slips, degrade the positioning accuracy and, when severe enough, can even lead to a complete loss of signal lock. Brazil, in particular, is located in one of the regions most affected by ionospheric scintillations and these effects were intensified during the last solar maximum. The main goal of this paper is to evaluate the impact of scintillation effects on the degradation of positioning during the last solar maximum. Data from 2012 to 2014 of three reference stations located in different regions of Brazil was used. Statistically significant correlations were identified from Spearman's correlation coefficient. Using Odds Ratio, an effect-size statistic, it was possible to see that the chance of large discrepancies in 3D positioning coordinates could be three times greater under strong scintillation effects (S4 ≥ 1) than under moderate ones (0.5<S4<1).

Publication date: Available online 18 October 2019

Source: Advances in Space Research

Author(s): Kewei Xia, Hungsun Son

The guaranteed performance control problem of spacecraft attitude tracking with control constraint, disturbance and time-varying inertia parameters is investigated. A new saturation function is designed to satisfy different magnitude constraints by introducing a piecewise smooth asymmetric Gauss error function. Based on the mean-value theorem, the constrained problem is transformed into an unconstrained control design subject to an unknown bounded coefficient matrix. To satisfy the constraints by performance functions, a tracking error constrained control is developed based on a hyperbolic arc-tangent asymmetric barrier Lyapunov function (BLF). In the backstepping framework, an adaptive robust control law is proposed by employing a smooth robust term simultaneously counteracting the parametric and non-parametric uncertainties, where the unknown coefficient matrix resulting from the control constraint is compensated by a Nussbaum function matrix. Rigorous stability analysis indicates that the proposed control law realizes the asymptotically tracking of spacecraft attitude and that the tracking error remains in a prescribed set which implies the achievement of the guaranteed transient performance. Numerical simulations validate the proposed theoretical results.

Publication date: Available online 16 October 2019

Source: Advances in Space Research

Author(s): Jia Huang, James D. Biggs, Naigang Cui

Solar sail halo orbits designed in the Sun-Earth circular restricted three-body problem (CR3BP) provide inefficient reference orbits for station-keeping since the disturbance due to the eccentricity of the Earth’s orbit has to be compensated for. This paper presents a strategy to compute families of halo orbits around the collinear artificial equilibrium points in the Sun-Earth elliptic restricted three-body problem (ER3BP) for a solar sail with reflectivity control devices (RCDs). In this non-autonomous model, periodic halo orbits only exist when their periods are equal to integer multiples of one year. Here multi-revolution halo orbits with periods equal to integer multiples of one year are constructed in the CR3BP and then used as seeds to numerically continue the halo orbits in the ER3BP. The linear stability of the orbits is analyzed which shows that the in-plane motion is unstable while the out-of-plane motion is neutrally stable and a bifurcation is identified. Finally, station-keeping is performed which shows that a reference orbit designed in the ER3BP is significantly more efficient than that designed in the CR3BP, while the addition of RCDs improve station-keeping performance and robustness to uncertainty in the sail lightness number.