Journal of Atmospheric and Solar-Terrestrial Physics

Analysis of solar cycle-like signal in the North Atlantic Oscillation index

Fri, 03/22/2019 - 19:10

Publication date: Available online 19 March 2019

Source: Journal of Atmospheric and Solar-Terrestrial Physics

Author(s): Aleksandr N. Gruzdev, Viacheslav A. Bezverkhnii

Abstract

Relation of decadal variations of the North Atlantic Oscillation index (NAOI) to the 11-year solar cycle in the sunspot number is analyzed for almost 200-year observational period with the use of cross-spectral and cross-wavelet techniques. Based on wavelet transforms of a pair of time series, local cross characteristics such as the local coherency and local correlation are used to study time evolution of the solar–NAOI relationship. The analysis of the NAOI is supplemented and confirmed by similar analysis of more than 300-year long data of Central England temperature (CET). Both the solar cycle–NAOI and solar cycle–CET relationships exhibit a quasi-periodic oscillation with the mean period of about 50 years, which includes a subinterval with positive coherency and a subinterval with negative coherency between the solar cycle and the 11-year modes of the NAOI and CET. The same multidecadal modulation of the solar–NAOI relationship is manifested in the temporal structure of the local correlation between the 11-year modes of the NAOI and the sunspot number at specific time lags of the NAOI relative to the sunspot number. It is distinct in the local correlations for the extended winter period (November–March) NAOI lagging by 7 years after the sunspot number. At this lag, the NAOI anticorrelates in general with the sunspot number and the local anticorrelation reinforces periodically with the aforementioned ∼50 year period. At the 1-year lag, which differs from the 7-year lag by a half solar cycle, the 11-year mode of the winter NAOI correlates in general with the solar cycle but the correlation is mainly associated with the last 70-year time interval. The change in the sign of the solar–NAOI relationship is also the case for the extended summer periods of the year (May–September). For this period, the approximately mirror image of the local correlations is observed at the 7-year and 1-year lags. The multidecadal modulation is likely associated with periods of enhanced solar impact on the NAO.

Role of winter jet stream in the middle atmosphere energy balance

Fri, 03/22/2019 - 19:10

Publication date: Available online 18 March 2019

Source: Journal of Atmospheric and Solar-Terrestrial Physics

Author(s): B.G. Shpynev, D.S. Khabituev, M.A. Chernigovskaya, O.S. Zorkal'tseva

Abstract

We consider physical mechanisms responsible for forming plain-layered jet streams in the winter stratosphere. Unlike the conventional notion about the balance between the energy of the solar UV radiation energy absorbed by the stratospheric ozone within the Hartley band and the energy of loss due to infrared emission from СО2, О3, and Н2О molecules, such a balance is shown not to persist. It is shown that the bias of these energies observed in satellite experiments can be explained by dynamic mechanism increasing the air gravity potential in the tropical stratosphere and forming equator/winter pole baroclinic instability, which generates the jet stream. Jet streams transport energy and pulse from equatorial to polar region and facilitate the descending part of the Brewer-Dobson global circulation. Potential energy release, when the stratospheric jet stream lowers, is ∼1018 W/day; the air mass transported by the jet stream to the winter tropopause region is estimated as being ∼1014 kg/day. Based on the ECMWF ERA-Interim reanalysis data, we analyzed the temporal characteristics of the stratospheric air motion from the region of gravity potential abundance generation in the summer tropical stratosphere to the polar winter tropopause altitudes, where the stratospheric air ends its motion, thus participating in cyclogenesis. Duration of the descending part of the Brewer-Dobson circulation in the winter stratosphere/troposphere averages 50–70 days.

Long-term variation of dust episodes over the United Arab Emirates

Fri, 03/22/2019 - 19:10

Publication date: June 2019

Source: Journal of Atmospheric and Solar-Terrestrial Physics, Volume 187

Author(s): Ghouse Basha, M. Venkat Ratnam, K. Niranjan Kumar, T.B.M.J. Ouarda, P. Kishore, Isabella Velicogna

Abstract

This paper deals with the investigation of long-term variability of atmospheric dust over the United Arab Emirates (UAE). The climatology of dust episodes (dust events, dust storms, and severe dust storms) is compiled based on the hourly observations and synoptic codes recorded at four different stations over UAE between the years 1983–2014. The diurnal, temporal, monthly, and inter-annual variations of dust episodes and their relation with the mean wind speed, maximum wind speed, and temperature are discussed. Dust episodes show a clear diurnal variation in all the stations. The duration of dust storms is large compared to dust events. For instance, dust events over the UAE persist for 2–5 h while dust storms last for about 5–11 h. Dust storms also show clear seasonal variability with the maximum occurring during winter and the minimum during summer whereas most of the dust events occur during the months of March and April. The inter-annual variation of dust events shows a significant decrease while dust storms depict a moderate increase over the UAE. The synoptic scale climatology of all dust storms is also analyzed and shows changes in wind direction to the south-west prior to 2 days of the dust storm generation. The climatology of wind direction and wind speed during the dust episode indicates that 90% of dust episodes are coming from the southwest direction. These observed results are discussed in light of the current global warming scenarios with a special emphasis on the role of dust episodes on the regional enhancement of temperature.

The solar wind-magnetosphere coupling and daytime disturbance electric fields in equatorial ionosphere during consecutive recurrent geomagnetic storms

Fri, 03/22/2019 - 19:10

Publication date: June 2019

Source: Journal of Atmospheric and Solar-Terrestrial Physics, Volume 187

Author(s): Thana Yeeram

Abstract

This paper investigates evolutions of geomagnetic and equatorial electrodynamic responses during three consecutive 27-day recurrent geomagnetic storms (RGSs) in 2007 by using the solar wind plasma, geomagnetic indices, and magnetometer data. The RGSs are classified as corotating interaction regions (CIR)-induced storms and High Intensity Long Duration Continuous AE Activity (HILDCAA). The RGSs show quasi-stable and diverge variabilities in storm-times that critically control the long-lasting and complex electrodynamic responses in equatorial ionosphere, particularly in CIR-storms. The correlations of Auroral Electrojet (AE)/Polar Cap (PC) indices are moderate for CIRs and HILDCAAs during local summer, which suggest to less effective coupling and/or different sources of PC and AE. The correlations of reconnection electric field with AE and Fourier analysis of the north-south component of the interplanetary magnetic field with AE reveal that the solar wind-magnetosphere-ionosphere coupling is more effective in the HILDCAAs than in the CIRs. Results indicate that disturbance electric fields are closely related to storm time evolution of the RGSs and are seasonal dependent. Disturbance dynamo electric fields (DDEFs) are related to each group of HSSs during HILDCAA periods. Justification of storm-time effect or seasonal effects is done. The substantial DDEFs are caused by the storm time effects of enhanced thermospheric wind due to the summer-to-winter winds in the late night sector. DDEFs are effective particularly in the morning-to-prenoon time in the summer, while they are less effective in equinoctial months due to the symmetric meridional winds and in winter months due to restriction of the equatorward motion.

Evaluation of some rain attenuation prediction models for satellite communication at Ku and Ka bands

Fri, 03/22/2019 - 19:10

Publication date: Available online 14 March 2019

Source: Journal of Atmospheric and Solar-Terrestrial Physics

Author(s): K.C. Igwe, O.D. Oyedum, M.O. Ajewole, A.M. Aibinu

Abstract

Propagation of radio waves between terrestrial and earth-space links at frequencies above 10 GHz are adversely affected by weather, especially rain. Rain-induced attenuation is an important propagation effect that has to be considered in satellite communication system design. Prediction of rain attenuation for earth-space links in North Central Nigeria at Ku and Ka bands is investigated using five rain attenuation models: The ITU-R P.618 model, Bryant model, Garcia-Lopez model, Svjatogor model and Simple attenuation model (SAM). The main objective is to determine the optimal rain attenuation prediction models for satellite communication in this region. 33 years (1983–2015) daily rainfall data obtained from the Nigerian Meteorological Agency (NIMET) were used. Three elevation angles were considered: 55° 42.5° and 23°. The results obtained showed that the ITU-R P.618, Garcia-Lopez and Bryant models performed best in this region. Also, attenuation ranged from 14 dB to 16 dB at 55° elevation angle, 15 dB–16 dB at 42.5° elevation angle and 20 dB–22 dB at 23° elevation angle for exceedance time percentage of 0.01% at Ku-band in all the stations. For the Ka-band, attenuation varied between 33 dB and 37 dB at 55° elevation angle, 33 dB and 37 dB at 42.5° elevation angle and between 42 dB and 48 dB at 23° elevation angle for same 0.01% exceedance time percentage. From the values of rain attenuation predicted for 0.01% time exceedance, availability of signal is possible at 42.5° and 55° elevation angles but impossible at 23° elevation angle at Ku-band. At Ka-band, the predicted rain attenuation values for 0.01% time exceedance have shown that availability of signal is impossible at all three elevation angles, which implies total signal fade out during such rainfall events in the region.

Multivariate analysis of combined GPS/GLONASS point positioning performance in Brazilian regions under different ionospheric conditions

Fri, 03/22/2019 - 19:10

Publication date: June 2019

Source: Journal of Atmospheric and Solar-Terrestrial Physics, Volume 187

Author(s): Gabriel Oliveira Jerez, Daniele Barroca Marra Alves, Vilma Mayumi Tachibana

Abstract

The development of Global Navigation Satellite Systems (GNSS) was a revolution in activities related to positioning. Currently, GLONASS (Global'naya Navigatsionnaya Sputnikovaya Sistema) and GPS (Global Positioning System) are the main systems with full constellation. The use of GPS and GLONASS combined data gained renewed attention after the GLONASS restoration and modernization plan, which enabled the system to reach full constellation in 2011. In addition to the use of combined data, several other factors can influence positioning quality, such as the methods applied and errors that can affect the transmitted signals. Concerning errors, the ionosphere is an important source, particularly for users of single frequency receivers. This requires special attention, because, in addition to degradation of positioning accuracy, there is a great interdependency between signal loss and ionospheric irregularities, such as ionospheric scintillation. In this paper, multivariate analysis techniques were applied to investigate the influence of ionospheric activity, specifically ionospheric scintillation, in positioning error. An experiment was carried out applying the point positioning method considering stations located in different places during periods of high and low ionospheric activity. The results showed high similarity between GPS and GLONASS data and significant ionosphere influence in the positioning error. S4 indexes presented correlations higher than 0.75 when considering GPS and GLONASS data. Positioning error using GPS or GPS/GLONASS data presented correlations higher than 0.93 for all stations considered. Techniques such as clustering, correspondence analysis and factor analysis were also applied in this study.

Positive and negative feedbacks in the magnetosphere-ionosphere coupling

Fri, 03/22/2019 - 19:10

Publication date: June 2019

Source: Journal of Atmospheric and Solar-Terrestrial Physics, Volume 187

Author(s): V.M. Mishin, V.V. Mishin, M.A. Kurikalova, L.A. Sapronova, Yu. A. Karavaev

Abstract

We study dynamics of field-aligned currents (FACs) during the expansion phase (EP) of two selected summer and winter substorms, by applying the magnetogram inversion technique (MIT) to the data of the world network of ground-based stations. The empirical MIT data and the modified Iijima and Potemra (I-P) model are presented. It is supposed a sharp increase in the upward FAC in I-P Region 1 and associated upward inter-hemispheric current in the dusk sector of the summer hemisphere during EP – in correspondence with theoretical models. Simultaneously (relative to the substorm onset time), and with the same value range, the downward inter-hemispheric current should enhance in the conjugate sector of the winter hemisphere. This FACs restructuring scenario in two hemispheres may clarify some of the unresolved issues in substorm physics. E.g., we describe the strong seasonal variation in the magnetic and auroral activity that is practically absent or unknown in literature even for the Northern hemisphere. Trying to answer this and other questions debated in literature, we study and describe strong feedbacks between the intensity of night FACs flowing in/out of each hemisphere, on the one hand, and the ionosphere conductivity or electric field, on the other. These magnetosphere-ionosphere feedbacks cause, in particular, the above inter-hemispheric currents and consequences prior and during the EP of the addressed substorms.

Dynamical properties of acoustic-gravity waves in the atmosphere

Fri, 03/22/2019 - 19:10

Publication date: May 2019

Source: Journal of Atmospheric and Solar-Terrestrial Physics, Volume 186

Author(s): Animesh Roy, Subhrajit Roy, A.P. Misra

Abstract

We study the dynamical behaviors of a system of five coupled nonlinear equations that describes the dynamics of acoustic-gravity waves in the atmosphere. A linear stability analysis together with the analysis of Lyapunov exponents spectra are performed to show that the system can develop from ordered structures to chaotic states. Numerical simulation of the system of equations reveals that an interplay between the order and chaos indeed exists depending on whether the control parameter s1, associated with the density scale height of acoustic-gravity waves, is below or above its critical value.

On the explosive nature of auroral substorms and solar flares: The electric current approach

Fri, 03/22/2019 - 19:10

Publication date: May 2019

Source: Journal of Atmospheric and Solar-Terrestrial Physics, Volume 186

Author(s): Syun-Ichi Akasofu, Lou-Chuang Lee

Abstract

Both auroral substorms and solar flares have explosive nature. Since both are mostly various manifestations of electromagnetic energy dissipation processes, it is basic to consider both phenomena as a chain of electromagnetic processes, which consists of a dynamo process as the power supply, power transmission and manifestations of dissipative processes (auroral substorms and solar flares)--the electric current approach. Based on this view, since both phenomena are powered by a dynamo and have an explosive component, they are likely to have the component directly driven (DD) by the dynamo and the component which results from the unloading (UL) process of accumulated energy (generated also by the dynamo). Thus, it is crucial to identify and distinguish the dynamo-produced power/energy/dissipation into two components in both morphology and theory. In this paper, since we have taken this approach for auroral substorms, we attempt to apply it for solar flares by adopting a photospheric dynamo theory by Lee et al. (1995) and Choe and Lee (1996a, 1996b); their dynamo process takes place on the photosphere under a magnetic arcade. It is shown that their dynamo can supply both the power and energy needed for flares, providing the arcade field-aligned currents for the two-ribbon Hα emission (DD) and the current loop along the dark filament above the arcade for the explosive process (UL).

The performance of IRI-2016 in the African sector of equatorial ionosphere for different geomagnetic conditions and time scales

Fri, 03/22/2019 - 19:10

Publication date: May 2019

Source: Journal of Atmospheric and Solar-Terrestrial Physics, Volume 186

Author(s): Endalkachew Mengistu, Mark B. Moldwin, Baylie Damtie, Melessew Nigussie

Abstract

This paper presents the performance of IRI-2016 model in describing the East African sector of equatorial ionosphere during different geomagnetic conditions and time scales. The analysis is carried out by taking six years (2008 and 2011–2015) of the vertical total electron content (vTEC) data from GPS receiver located at Arba Minch (Geographic: 6.06°N and 37.56°E; Geomagnetic: −3.03°N and 109.29°E) in Ethiopia. The IRI-2016 model either underestimates or overestimates observations depending on time of day, months, season, solar and geomagnetic conditions. An underestimation becomes more pronounced from the first to third quartiles, while an overestimation is nearly constant. The diurnal variation pattern of measured vTEC (vTECo) shows a single peak mostly occurring between 1000 and 1400 UT (1230 and 1630 LT), while the modeled vTEC (vTECm) is characterized by two remarkable peak values with the first peak occurring between 0500 and 0700 UT (0730 and 0930 LT), and the second peak occurring between 1300 and 1400 UT (1530 and 16300 LT). The nighttime monthly peak vTECm is lower compared to observations during 2008, but it is reversed during 2011–2015. Seasonally, the vTECo variations show a semiannual variation pattern with maximum values during March equinox and December solstice months and minimum during June solstice and September equinox months. This trend equally explains the semiannual variability noted on the seasonal percentage deviation in vTECm variability. The mean and median annual observations show better agreement with vTECm during daytime, but opposite at nighttime hours (except in 2008). The response of the equatorial ionosphere for storm, which was observed clearly in the observations, did not show in the model results. The vTECo shows a significant relation with both merging and interplanetary electric fields during storm. This suggests that a linkage of those parameters with storm-time vTEC needs to be established for a variety of geophysical conditions to improve the forecasting capability of the model. Generally, the IRI-2016 model shows better agreement with observations during solar minimum compared to other solar activity phases. We suggest that the results from this study would complement in the model improvement towards near real time predictions of different ionospheric parameters over the equatorial and low latitude regions like East Africa.

Evening corotating patches (ECP) observed by DMSP/SSUSI

Fri, 03/22/2019 - 19:10

Publication date: May 2019

Source: Journal of Atmospheric and Solar-Terrestrial Physics, Volume 186

Author(s): Jiansheng Yao, Yongzhi Cai, Yuzhang Ma, Su Zhou

Abstract

Evening corotating patches (ECP) are a special type of aurora, which occur at subauroral latitude. In the present work we have used the images observed by DMSP/SSUSI instruments to investigate the ECP in a global perspective. This study indicates that the ECP occur in the recovery phase of magnetic storms. The ECP were observed between 58° and 68° magnetic latitude (MLAT) in the evening sector between 16:00 and 20:00 magnetic local time (MLT). The ECP cororate with the Earth but they occur at same MLAT, this suggest that the source region related to ECP is inside the plasmasphere. Particle observations indicate that ECP are produced by energetic ions with energies above ∼20 keV, without much electron precipitations. We argue that the energetic ions producing ECP originates from the ring current, which is assumed to be filled with hot ions during the recovery phase of magnetic storms.

Characteristics of high-frequency gravity waves generated during pre-monsoon season over a tropical location: A case study

Fri, 03/22/2019 - 19:10

Publication date: May 2019

Source: Journal of Atmospheric and Solar-Terrestrial Physics, Volume 186

Author(s): Priyanka Ghosh, Thokuluwa Krishnamurthy Ramkumar, Viswanadhapalli Yesubabu, Som Sharma

Abstract

Mesoscale convection weather events, associated with deep cumulonimbus clouds transport moisture, energy and momentum to upper troposphere and lower stratosphere (UTLS), affect the energetics and general circulation of the middle atmosphere. Therefore, it is necessary to understand the atmospheric dynamics during pre–monsoon periods (when strong mesoscale convective events occur) for improving the parameterization of model physics. The present study attempts to investigate the characteristics of high-frequency gravity waves (GWs) generated over Gadanki (13.5° N, 79.2° E), India during one such severe convection event in the pre–monsoon period. For this purpose, the Mesosphere Stratosphere Troposphere (MST) radar at Gadanki was operated continuously for the period of ∼10 h during 27–28 May 2015. A wide spectrum of high frequency GWs with different generation mechanisms are found during this convective event whose vertical propagation characteristics fulfill the non-hydrostatic GWs dispersion relation. The temperature and wind data of ERA (ECMWF Re-Analysis)-interim reanalysis and GPS radiosonde, launched at Gadanki, are also analyzed for ten days around the event day to determine the background atmospheric thermodynamical conditions. Strong up- and downdrafts are observed in the altitude range of ∼3.6–20 km (radar limited top height); presence of which even up to ∼20 km height is quite rare. The WRF (Weather Research and Forecasting) model simulated relative humidity on 27–28 May 2015 clearly depicts the mid-tropospheric moisture intrusion. Presence of moisture is also observed at the higher heights ∼17–18 km which could be due to strong low level convergence leading to high divergence value of deep cumulonimbus clouds in UTLS region. Furthermore, this is supported by the Doppler Weather Radar (DWR) and simulated reflectivity of WRF model reaching the tropopause height of ∼18 km. The altitude profiles of phase of some oscillations during convection period depict that the sources of these waves are near ∼20 km which is first of its kind outcome and it is corroborated with high resolution WRF model simulations.

Influence of sudden stratospheric warming on the mesosphere/lower thermosphere from the hydroxyl emission observations and numerical simulations

Fri, 03/22/2019 - 19:10

Publication date: Available online 23 February 2019

Source: Journal of Atmospheric and Solar-Terrestrial Physics

Author(s): I.V. Medvedeva, A.I. Semenov, A.I. Pogoreltsev, A.V. Tatarnikov

Abstract

We present the results of studying the behavior of temperature and of the atomic oxygen concentration in the mesopause region during the 2013 January major Sudden Stratospheric Warming (SSW). The data on the hydroxyl molecule OH(6-2), 834.0 nm emission intensity and rotational temperature were analyzed. These data were obtained through spectrometric measurements at the Geophysical Observatory of the Institute of Solar-Terrestrial Physics of the Siberian Branch of the Russian Academy of Sciences (51.8°N, 103.1°E, Tory), and at the Zvenigorod Station at the Obukhov Institute of Atmospheric Physics of the Russian Academy of Sciences (55.7°N, 36.8 E°). We calculated the concentration of atomic oxygen and its variations by using the data of the OH emission measurements. We revealed, that the response of the mesopause characteristics for two longitudinally spaced mid-latitude regions essentially differs. Thus, from the Tory Station data, the maximal increase in the OH emission intensity (by a factor of ∼2) and in the concentration [O] (by a factor of ∼3) occurred during the sudden stratospheric warming (SSW) evolution, whereas, from the Zvenigorod Station data, the OH emission intensity increase (by a factor of ∼3) and its concentration [O] increase (by a factor of ∼3.5) was observed at the SSW recovery phase. As a result of numerical modeling using the Middle and Upper Atmosphere Model (MUAM), it was shown, that the cause for the revealed effect may probably be longitudinal differences in the diurnal variation in the vertical wind at the mesopause heights over the indicated stations during the SSW. One may elucidate these differences through the generation of non-migrating tides due to a non-linear interaction between the intensified stationary planetary wave 1 (SPW1) and migrating tides and forcing a set of high-frequency PWs at the stratospheric heights. All these waves are capably of propagating into the MLT region and produce observed changes in behavior of the OH emission intensity, temperature and atomic oxygen concentration over Tory and Zvenigorod.

Temporal characteristics of aerosol optical properties over the glacier region of northern Pakistan

Fri, 03/22/2019 - 19:10

Publication date: May 2019

Source: Journal of Atmospheric and Solar-Terrestrial Physics, Volume 186

Author(s): Bahadar Zeb, Khan Alam, Armin Sorooshian, Farrukh Chishtie, Ifthikhar Ahmad, Humera Bibi

Abstract

Glacier melting due to light-absorbing aerosol has become a growing issue in recent decades. The emphasis of this study is to examine aerosol loadings over the high mountain glacier region of northern Pakistan between 2004 and 2016, with sources including local emissions and long-range transported pollution. Optical properties of aerosols were seasonally analyzed over the glacier region (35–36.5°N; 74.5–77.5°E) along with three selected sites (Gilgit, Skardu, and Diamar) based on the Ozone Monitoring Instrument (OMI). The aerosol sub-type profile was analyzed with Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO). Hybrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT) model was used to understand the origin of air masses arriving in the study region. The highest values of aerosol optical depth (AOD) and single scattering albedo (SSA) occurred during spring, whereas aerosol index (AI) and absorption AOD (AAOD) exhibited maximum values in winter and summer, respectively. The minimum values of AOD, AI, AAOD, and SSA occurred in winter, autumn, winter, and autumn, respectively. The results revealed that in spring and summer the prominent aerosols were dust, whereas, in autumn and winter, anthropogenic aerosols were prominent. Trend analysis showed that AI, AOD, and AAOD increased at the rate of 0.005, 0.006, and 0.0001 yr−1, respectively, while SSA decreased at the rate of 0.0002 yr−1. This is suggestive of the enhancement in aerosol types over the region with time that accelerates melting of ice. CALIPSO data indicate that the regional aerosol was mostly comprised of sub-types categorized as dust, polluted dust, smoke, and clean continental. The types of aerosols defined by OMI were in good agreement with CALIPSO retrievals. Analysis of the National Oceanic and Atmospheric Administration Hybrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT) model revealed that air parcels arriving at the glacier region stemmed from different source sites.

Graphical abstract

Long term air ion monitoring in search of pre-earthquake signals

Fri, 03/22/2019 - 19:10

Publication date: May 2019

Source: Journal of Atmospheric and Solar-Terrestrial Physics, Volume 186

Author(s): Sheldon Warden, Tom Bleier, Karl Kappler

Abstract

Air ion concentrations at ground level have been measured both indoors and outdoors to address a wide variety of questions, such as estimating air quality or assessing the impact of ions on human health. While most studies rely on discrete measurements, other applications require continuous monitoring over extended periods of time. In this paper, we discuss the challenges posed by long term air ion monitoring within the frame of earthquake forecasting. We first provide a brief state of the art, outlining the main hypotheses advanced to explain anomalies in air ion concentrations that were reported prior to earthquakes, namely the p-hole theory and the radon exhalation theory. We then describe the array of Gerdien capacitors deployed by QuakeFinder in the US and abroad with the goal to identify pre-earthquake variations in air ion data: we discuss the characteristics of these instruments and derive the criticial mobility associated with their parallel-plate geometry. We then present the results of a Parallel Sensor Test (PST) during which positive and negative air ion counters were deployed side by side at increasing separation distances. This test provides insight about the sampling rate at which air ion concentrations should be measured with the QuakeFinder air ion counters.

The main processing steps applied to raw air ion data are then described. Particular emphasis is set on the unipolarity coefficient and how it may be used to identify faulty sensors. One of the key issues encountered while monitoring air ion concentrations for extended periods of time is the condensation forming on the electrodes due to increased relative humidity levels. It appears that the relative humidity working range provided by instruments manufacturers is not accurate for long term outdoors measurement and that sensors saturate at relative humidity values lower than the threshold advertised in the instrument specifications. We detail some of the strategies implemented to automatically reject data acquired during episodes of high relative humidity. Finally, we discuss the instrumental improvements that can be made to prevent moisture from forming on the plates of the Gerdien capacitors and that QuakeFinder plans to implement in the near future.

Oblique absorption effects of the <em>D</em> region during HF waves heating

Fri, 03/22/2019 - 19:10

Publication date: May 2019

Source: Journal of Atmospheric and Solar-Terrestrial Physics, Volume 186

Author(s): Qi Cheng, Li-xin Guo, Hui-min Li, Jiang-ting Li, Dan Zhang

Abstract

With the development of heating research, the oblique heating effects of the D region are becoming increasingly important. For the vertical heating, the larger the beam width of heating wave is, the more evident the oblique effects are; For the oblique heating, the D region which also exhibits evident heating effects has received less attention than the F region in previous studies; Furthermore, the oblique heating effects of the D region should be considered for the studies on multi-beam modulation heating that has arisen recently. Therefore, the numerical model for oblique heating needs to be established to develop the heating study in the D region. In this paper, the classical model for vertical heating is improved by further considering the lossy characteristic of the D region. An added factor, the real part of the refractive index, is introduced to estimate the power density of heating wave. Based on this improved model, the oblique heating model is presented by applying the oblique extension of Wentzel-Kramers-Brillouin (WKB) method and “real pseudo ray” approximation. Considered that the D region presents horizontally discrete strata, oblique absorption effects are quantitatively studied by powerful high-frequency wave heating. Notably, the electron energy absorption occurs only along the vertical direction at the different incident angles due to the special spatial characteristic of the D region. Results show that with the increase of incident angle, the absorption index increases whereas the effective power density of oblique heating wave has a considerable reduction. Therefore, the larger the incident angle is, the less the electron temperature increases. Moreover, the electron temperature in the upper region is more sensitive to variations in incident angles than that in the lower region. In the upper D region, two additional factors severely restrict the enhancement of electron temperature. The self-absorption of the heating wave, well-known as one factor, decreases with the increase of incident angle. Another factor, the real part of refractive index which is neglected in past studies also restricts the increase of electron temperature in the upper D region.

Dynamical processes in the ionosphere following the moderate earthquake in Japan on 7 July 2018

Fri, 03/22/2019 - 19:10

Publication date: May 2019

Source: Journal of Atmospheric and Solar-Terrestrial Physics, Volume 186

Author(s): Qiang Guo, L.F. Chernogor, K.P. Garmash, V.T. Rozumenko, Yu Zheng

Abstract

The response of the ionosphere to the earthquake of a moderate magnitude (M ≈ 5.9) that occurred in Japan at 11:23:50 UT on 7 July 2018 has been studied using a newly developed coherent multi-frequency radio diagnostic system for remotely probing the ionosphere at oblique incidence. The seismic activity in Japan on 7 July 2018 was accompanied by aperiodic processes in the ionosphere at distances of no less than (1–2) × 103 km from the epicentre, with an enhancement in multiple-mode propagation, and a significant Doppler spectrum broadening. Three ways of transporting disturbances from the earthquake to the changes in the character of Doppler spectra variations have been identified by examination. First, the disturbances are generated by a surface Raleigh wave launched at the earthquake epicentre. They have been ascertained in the infrasonic range (a 3- to 4-min period) of oscillations. The relative amplitude of these quasi-periodic oscillations in the electron density is equal to 1.7–9%. The duration of the oscillation trains is found to be in the range of 24–55 min. The wave disturbance speed of propagation is approximately 3 km/s. Second, wave disturbances have also been ascertained in a 15- to 30-min period range. They could be generated in the vicinity of the epicentre and then propagated as atmospheric gravity waves modulating the electron density in the ionosphere. The relative amplitude of the quasi-periodic disturbances in the electron density is equal to 14 – 34%. The wave train attains a temporal duration of about 100 min and a speed of approximately 0.3 km/s. Third, the broadening of the Doppler spectra toward negative Doppler shifts with the time delay estimated to be 49–124 min, depending on the orientation of the propagation path, is the most pronounced Doppler signature of the disturbances caused by the earthquake. This time delay corresponds to a speed of about 0.3 km/s, and consequently, it suggests that this effect most likely is caused by the atmospheric gravity waves launched at the earthquake epicentre. Apparently, the rearrangement of the ionosphere acts to reverse the sign of the Doppler spectrum shift when the atmospheric gravity waves arrive at the reflection level.

The semidiurnal tide for individual nights derived consistently from O<sub>2</sub> and OH intensities and temperatures

Fri, 03/22/2019 - 19:10

Publication date: May 2019

Source: Journal of Atmospheric and Solar-Terrestrial Physics, Volume 186

Author(s): Esteban R. Reisin, Jürgen Scheer

Abstract

The semidiurnal tide is studied with 159 special cases from the large airglow database acquired at El Leoncito (31.8ºS, 69.3ºW). These cases correspond to nights which exhibit similar periods in the temperature and intensity variations of the OH(6-2) and O2b(0–1) emissions. We find that all the periods (except one) are between 9 and 15.5 h. The phase distributions of these cases are narrow enough to ensure the identification as the semidiurnal tide, and their progression with altitude is consistent with downward phase propagation. The mean temperature amplitudes are large for both emissions. We obtain new values for Krassovsky's ratio including its phase. The vertical wavelength is determined independently for each emission using the relation suggested by the Hines and Tarasick theory. Mean vertical wavelengths derived for O2 are longer than those for OH. The longest monthly mean wavelengths are observed from May to August. The mean ratio of temperature amplitudes between the two emissions corresponds to moderate wave attenuation during the upward propagation of the tide.

Variation in MERRA-2 aerosol optical depth and absorption aerosol optical depth over China from 1980 to 2017

Fri, 03/22/2019 - 19:10

Publication date: May 2019

Source: Journal of Atmospheric and Solar-Terrestrial Physics, Volume 186

Author(s): Enwei Sun, Xiaofeng Xu, Huizheng Che, Zhiwei Tang, Ke Gui, Linchang An, Chunsong Lu, Guangyu Shi

Abstract

4258 instantaneous 550 nm Aerosol Optical Depth (AOD) values were compared between the second Modern-Era Retrospective analysis for Research and Applications (MERRA-2) and the Aerosol Robotic Network (AERONET) in four seasons at 12 AERONET sites across China. The correlation coefficients (R) in spring, summer, autumn and winter were 0.88, 0.92, 0.91 and 0.87, respectively. The MERRA-2 AOD was compared with the Moderate resolution Imaging Spectroradiometer (MODIS/Aqua) AOD over China from 2003 to 2017, and good agreement was obtained. 4501 daily AOD values were compared between MERRA-2 and MODIS at 16 sites over China. Spatial distribution and temporal variation of MERRA-2 AOD over China were analyzed from 1980 to 2017. Mean values of MERRA-2 AOD indicated that high AOD mainly appeared in the eastern, southeastern and central China, while low AOD mostly occurred in the western and northeastern China. Mean AOD values over China during this study period in each month was also discussed, and similar spatial distribution (high AOD in developed areas, low AOD in rural and less developed areas) was found in each month. Slight AOD increase could be observed in the 1980s and 1990s, and a rapid increase happened from 2001 to 2010, followed by an AOD decrease between 2011 and 2017. Annual variation of mean AOD of the whole China (the Yangtze River Delta) showed a slight increase of 0.0010 (0.0045) per year from 1980 to 1999, a rapid increase of 0.0096 (0.0271) per year between 2000 and 2009, and a decrease of −0.0089 (−0.0206) per year from 2010 to 2017. Annual AOD Variation in Jing-jin-ji and Pearl River Delta is similar to that in the Yangtze River Delta. In four seasons, AOD change was similar to the annual AOD variation except winter with a slight decrease of −0.0012 per year between 1980 and 1999. AOD variation over China before and after the Pinatubo volcanic eruption in the Philippines in June 1991 was analyzed to get a better understanding of the transport and the impact on the environment from volcanic pollutants. AOD variation trend over China was studied in two periods. The increasing trend appeared in most part of China from 1980 to 2009, while a decreasing trend could be found in almost the entire China between 2010 and 2017. Spatial and temporal variation of MERRA-2 Absorption Aerosol Optical Depth (AAOD), Black Carbon Absorption Aerosol Optical Depth (BCAAOD) and Dust Absorption Aerosol Optical Depth (DUAAOD) were analyzed during the period between 1980 and 2017. BCAAOD (66.30%) and DUAAOD (30.56%) offered the major contribution to total AAOD in China. 38 years of variation of AAOD over China is mainly due to the variation of BCAAOD. BCAAOD experienced an increase (1980–2007) and a decrease (2008–2017) during the 38 years in China.

Seasonal variations of sea breeze and its effect on the spectral behaviour of surface layer winds in the coastal zone near Visakhapatnam, India

Fri, 03/22/2019 - 19:10

Publication date: May 2019

Source: Journal of Atmospheric and Solar-Terrestrial Physics, Volume 186

Author(s): N.V.P. KiranKumar, K. Jagadeesh, K. Niranjan, K. Rajeev

Abstract

Seasonal variation of sea breeze (SB) characteristics and its effect on turbulence spectra at Visakhapatnam (17.7°N, 83.3°E) located at east coast of Peninsular India are investigated by considering 244 sea breeze events during December 2012 to March 2014. The delayed onset of backdoor SB during winter occurs due to the southward component of shore-parallel background winds while the prevailing northward component of background wind enable the early onset of corkscrew SB during pre-monsoon and summer monsoon. The turbulence spectral peak of horizontal winds shifts to higher frequency side after SB onset.

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