Journal of Atmospheric and Solar-Terrestrial Physics

Spectral characteristic of geomagnetically induced current during geomagnetic storms by wavelet techniques

Sun, 04/21/2019 - 19:10

Publication date: Available online 2 February 2018

Source: Journal of Atmospheric and Solar-Terrestrial Physics

Author(s): Binod Adhikari, Nirakar Sapkota, Subodh Dahal, Binod Bhattarai, Krishna Khanal, Narayan P. Chapagain

Abstract

An EMF (electromagnetic field) is induced over an incremental area when magnetized plasma from sun interacts with the Earth's magnetic field. This phenomenon delivers a Geomagnetically Induced Current (GIC) or induces geo-electric field at the Earth's surface and in the ground. GIC and horizontal component of geomagnetic field have been studied with respect to various geomagnetic events. Particularly, we have studied four events. The first one is geomagnetically quiet period (5 October 2003), the second one is weak storm (21 October 2003), the third one is moderate storm (14 October 2003) and the last one is an intense storm (30 October 2003). By comparing the development of GIC during geomagnetic storms, we found that intense geomagnetic storms show higher development on GIC magnitude. The GIC during storm events is several times greater than that during the quiet day. AE index shows more activity in the event of 30th October than other events and GIC is also more in this event. This can be accounted to the greater geomagnetic disturbance in this case. The power ranges of higher intensity are seen at various time scales on different events. We have analysed GIC signal associated with four geomagnetic storms and found distinct periodicities at the time when H component highly perturbed. The characteristic of GIC signal demonstrates high variability with time without presence of continuous periodicities. Discrete wavelet transform (DWT) analysis reveals that whenever the geomagnetic field is perturbed, there will be high possibility of detecting GIC. The singularities present in GIC signal are due to the peak value of electrical currents system in the ionosphere and magnetosphere, and corresponding high fluctuations in H component. In this work, we explore the remarkable ability of wavelets to highlight the singularities associated with discontinuities present in the GIC.

Modeling the global ionospheric variations based on complex network

Sun, 04/21/2019 - 19:10

Publication date: Available online 1 February 2018

Source: Journal of Atmospheric and Solar-Terrestrial Physics

Author(s): Shikun Lu, Hao Zhang, Xihai Li, Yihong Li, Chao Niu, Xiaoyun Yang, Daizhi Liu

Abstract

The modern science of networks has brought significant advances to our understanding of complex systems. We employ the probabilistic graphical model to build complex networks to model the global ionospheric variations. The global ionospheric maps (GIMs) of vertical total electron content (VTEC) for the 12 months in 2012 have been selected analyze the ionospheric variations from the perspective of complex network. The information flow in the networks represents the causal interactions between the ionospheric variations at different locations. The distributions of the edges' geospatial distances in the ionospheric networks show that the information flow in the ionosphere is mainly transmitted locally, almost obeying the geospatial proximity principle. The asymmetric distribution of the edges' distances probably elucidates the more efficient transmission of ionospheric variations in the westward and southward directions. The community topologies within the ionospheric networks indicate the effect of the geomagnetic field and geographical distance on the information flow in the ionosphere. The geomagnetic field has shown an enhanced effect on the meridional interaction in the ionosphere, causing the vertical community topologies within the ionospheric networks at middle and low latitudes. For the ionospheric cells located at high latitudes in GIM, the geographical distances result in the horizontal community topologies. The fractal analysis reveals the existence of self-similar structure in the ionospheric networks on the global scale. The fractality in the ionospheric information flow may indicate the reasonability of the VTEC's prediction at a certain location by spatial prediction based on the data obtained in known regions.

Features of the inter-hemispheric field-aligned current system over Malaysia ionosphere

Sun, 04/21/2019 - 19:10

Publication date: Available online 31 January 2018

Source: Journal of Atmospheric and Solar-Terrestrial Physics

Author(s): Zamri Zainal Abidin, M.H. Jusoh, M. Abbas, O.S. Bolaji, A. Yoshikawa

Abstract

Magnetic records of the declination (D) component for the solar quiet year 2011–2013 obtained from Magnetic Data Acquisition System (MAGDAS) at Langkawi (Geog. Lon. 99.68∘ E, Geog. Lat. 6.30∘N), Malaysia were utilized in this study. The minutes averages were used to delineate the diurnal (Sq(D)) variation. The monthly mean (MSq(D)) and their seasonal variabilities (SVq(D)) were also analysed. The Sq(D) and their MSq(D) exhibit smooth regular occurring pattern in the month of April–September and became highly perturbed in October–March across the years. The highest positive (∼3.5 arc-min) and the negative (∼−3.0 arc-min) values were observed in August 2011 during the dawn and noon sectors. These maxima shifted to July and September in 2012 with peaks ∼3.2 and −3.0 arc-min. In 2013, the positive maximum (∼3.0 arc-min) and its negative (∼−2.5 arc-min) were again seen in August. This implies that the dawn and noon sectors of August 2011 and 2013 are strongly influenced by IHFACs and this effect shifted to July and September in 2012. IHFACs through the years flow from the winter to summer hemisphere during the noon and dusk sectors and flow in opposite direction during the dawn sector. The day-to-day magnitudes of Sq(D) and MSq(D) seems to suggest the inter-hemispheric imbalance of the ionospheric Sq current earlier established by Van Sabben as the cause of IHFACs is not strongly affected by the changes in annual solar variation. Dusk-side IHFACs were observed to be northbound in all the seasons with the exception of June solstice. The direction of IHFACs does not change except in April and November. The current intensity is not large in solstices except in August 2011 and 2013 but it shifted to July in 2012. The result further showed that the magnitude of the duskside IHFACS is determined to some extent by the strength of the noontime IHFACs. IHFACs were generally observed to be greater during the daytime than night-time hours.

Application of solar quiet (Sq) current in determining mantle conductivity-depth structure in Malaysia

Sun, 04/21/2019 - 19:10

Publication date: Available online 31 January 2018

Source: Journal of Atmospheric and Solar-Terrestrial Physics

Author(s): Zamri Zainal Abidin, M.H. Jusoh, M. Abbas, A. Yoshikawa

Abstract

The mantle electrical conductivity-depth structure of Malaysia was determined for the first time using solar quiet day ionospheric current (Sq) variations. Spherical harmonic analysis (SHA) was employed to separate the external and internal field contribution to the Sq variations. A transfer function was applied in estimating the conductivity-depth profile for the paired of external and internal coefficients of the SHA. We observed a downward increase in electrical conductivity with initial magnitude of 0.0065 S/m at a depth of ∼ 56 km which gradually rose to 0.0106 and 0.0140 S/m at 118 and 180 km. Subsequently, the conductivity profile rose to about 0.0228 S/m at 380 km (near the base of the upper mantle) and reached 0.0260 S/m at 435 km, after which a sharp steep increase was observed at 450 km with conductivity profile of 0.0278 S/m. Consequently, the conductivity profile increases significantly to about 0.1367 S/m at a depth of 973 km and reached its peak value 0.1975 S/m at the depth of 1097 km in the lower mantle with no indication of leveling off. An evidence of discontinuity was observed near 390–460 km and 675–746 km. A slight increase in conductivity values at depth between 150 and 300 km corresponds to the region of unusual global low velocity zone with high electrical conductivity. The conductivity profile showed a less steep increase above 450 km below which a steep increase was observed. The present profile showed the deepest penetration depth which may be attributed to the influence of equatorial electrojet current (EEJ) that is actively supported by the conductive properties of the Earth's interior within the study region.

Polar cap patches observed by the EISCAT Svalbard Radar: A statistical study of its dependence on the solar wind and IMF conditions

Sun, 04/21/2019 - 19:10

Publication date: Available online 16 January 2018

Source: Journal of Atmospheric and Solar-Terrestrial Physics

Author(s): Yuyan Jin, Zanyang Xing, Qinghe Zhang, Yong Wang, Yuzhang Ma

Abstract

Polar cap patches are common irregularities in the polar ionosphere, where their formation and evolution can directly affect satellite navigations and communications as well as over-the-horizon radar observations, etc. However, affected by the various dynamic processes during the solar wind-magnetosphere-ionosphere coupling, there is no fully accepted formation mechanism of polar cap patches. In this paper, a statistical analysis of 345 patches at the dayside sectors during 09:00–15:00 magnetic local time (MLT), observed by EISCAT Svalbard Radar (ESR) 42 m antenna from 2010 to 2013, has been performed. The dependence of their occurrence on solar wind and interplanetary magnetic field (IMF) conditions as well as their MLT distribution has been statistically investigated. The results show that the polar cap patches are preferentially formed during southward IMF conditions. In particular, the MLT dependence of the patches presents a clear IMF By-related prenoon-postnoon asymmetry, suggesting the patch formation is clearly modulated by the IMF By polarity. Moreover, our statistical results indicate that the patches should not be caused by the variations of the solar wind dynamic pressure or the solar wind velocity. All the results indicate that the pulsed dayside magnetic reconnection is possibly a significant formation mechanism of polar cap patches.

Understanding the global dynamics of the equatorial ionosphere in Africa for space weather capabilities: A science case for AfrequaMARN

Sun, 04/21/2019 - 19:10

Publication date: Available online 6 January 2018

Source: Journal of Atmospheric and Solar-Terrestrial Physics

Author(s): Hammed A. Lawal, Mark Lester, Stanley W.H. Cowley, S.E. Milan, T.K. Yeoman, Gabby Provan, Suzie Imber, A.Babatunde Rabiu

Abstract

The equatorial region of the Earth's ionosphere is one of the most complex ionospheric regions due to its interactions, instabilities, and several unresolved questions regarding its dynamics, electrodynamics, and physical processes. The equatorial ionosphere overall spans three continents with the longest region being that over the African continent. Satellite observations have demonstrated that very large differences exist in the formation of ionospheric irregularities over the African sector compared with other longitudinal sectors. This may be a consequence of the symmetric shape of the magnetic equator over the continent and the lack of variability in latitude. In this paper, we propose a science campaign to equip the African sector of the magnetic equator with ground-based instruments, specifically magnetometers and radars. The network of radars proposed is similar in style and technique to the high-latitude SuperDARN radar network, while the magnetometers will form an array along the equatorial belt. These two proposed space physics instruments will be used to study this region of the equatorial ionosphere over a long interval of time, at least one solar cycle. The deployment of an array of magnetometers (AfrequaMA) and a radar network (AfrequaRN) in the African sector of the magnetic equator is jointly called the Africa Equatorial Magnetometer Array and Radar Network (AfrequaMARN), which will provide simultaneous observations of both electric and magnetic variations over the African sector. We also examine the possible science questions such a magnetometer array and radar network would be able to address, both individually and in conjunction with other space-based and ground-based instrumentation. The proposed projects will clearly improve our understanding of the dynamics of the equatorial ionosphere and our understanding of its role in balancing the large-scale ionospheric current system, and will contribute to our ability to adequately model ionospheric and plasmaspheric densities. It will also enhance our understanding of global ionospheric processes, which will improve the space weather capabilities of the African and international space science communities.

Editorial Board

Sat, 04/13/2019 - 19:10

Publication date: June 2019

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

Author(s):

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

Sat, 04/13/2019 - 19:10

Publication date: June 2019

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

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.

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

Sat, 04/13/2019 - 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

Sat, 04/13/2019 - 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.

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

Sat, 04/13/2019 - 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

Sat, 04/13/2019 - 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.

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

Sat, 04/13/2019 - 19:10

Publication date: June 2019

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

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 the 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.

Editorial Board

Tue, 04/02/2019 - 19:10

Publication date: May 2019

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

Author(s):

Dynamical properties of acoustic-gravity waves in the atmosphere

Tue, 04/02/2019 - 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

Tue, 04/02/2019 - 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

Tue, 04/02/2019 - 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

Tue, 04/02/2019 - 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

Tue, 04/02/2019 - 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.

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

Tue, 04/02/2019 - 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

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