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  Subjects -> PHYSICS (Total: 736 journals)
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PHYSICS (623 journals)                  1 2 3 4 5 6 7 | Last

Acoustical Physics     Hybrid Journal   (8 followers)
Acoustics Today     Hybrid Journal   (2 followers)
Acta Acustica united with Acustica     Full-text available via subscription   (7 followers)
Acta Mechanica     Hybrid Journal   (15 followers)
Acta Mechanica Sinica     Hybrid Journal   (4 followers)
Acta Physica Slovaca     Open Access   (3 followers)
Advanced Composite Materials     Hybrid Journal   (10 followers)
Advanced Electromagnetics     Open Access   (9 followers)
Advanced Functional Materials     Hybrid Journal   (29 followers)
Advanced Materials     Hybrid Journal   (202 followers)
Advances in Acoustics and Vibration     Open Access   (16 followers)
Advances In Atomic, Molecular, and Optical Physics     Full-text available via subscription   (4 followers)
Advances in Condensed Matter Physics     Open Access   (6 followers)
Advances in Exploration Geophysics     Full-text available via subscription   (3 followers)
Advances in Geophysics     Full-text available via subscription   (4 followers)
Advances in High Energy Physics     Open Access   (12 followers)
Advances in Imaging and Electron Physics     Full-text available via subscription   (1 follower)
Advances in Materials Physics and Chemistry     Open Access   (7 followers)
Advances in Natural Sciences: Nanoscience and Nanotechnology     Open Access   (13 followers)
Advances in Nonlinear Optics     Open Access   (2 followers)
Advances in OptoElectronics     Open Access   (2 followers)
Advances In Physics     Hybrid Journal   (6 followers)
Advances in Physics Theories and Applications     Open Access   (4 followers)
Advances in Remote Sensing     Open Access   (6 followers)
Advances in Synchrotron Radiation     Hybrid Journal   (1 follower)
AIP Advances     Open Access   (4 followers)
American Journal of Applied Sciences     Open Access   (27 followers)
American Journal of Condensed Matter Physics     Open Access   (2 followers)
Analysis and Mathematical Physics     Hybrid Journal   (2 followers)
Annalen der Physik     Hybrid Journal   (2 followers)
Annales Geophysicae (ANGEO)     Open Access   (3 followers)
Annales Henri PoincarĂ©     Hybrid Journal   (2 followers)
Annales UMCS, Physica     Open Access  
Annals of Nuclear Medicine     Hybrid Journal   (2 followers)
Annals of Physics     Hybrid Journal   (2 followers)
Annual Reports on NMR Spectroscopy     Full-text available via subscription   (1 follower)
Annual Review of Analytical Chemistry     Full-text available via subscription   (8 followers)
Annual Review of Condensed Matter Physics     Full-text available via subscription   (1 follower)
Annual Review of Fluid Mechanics     Full-text available via subscription   (11 followers)
Annual Review of Materials Research     Full-text available via subscription   (4 followers)
Annual Review of Nuclear and Particle Science     Full-text available via subscription   (1 follower)
APL : Organic Electronics and Photonics     Hybrid Journal   (1 follower)
APL Materials     Open Access   (1 follower)
Applied Acoustics     Hybrid Journal   (3 followers)
Applied Composite Materials     Hybrid Journal   (8 followers)
Applied Mathematics and Mechanics     Hybrid Journal   (2 followers)
Applied Physics A     Hybrid Journal   (9 followers)
Applied Physics Frontier     Open Access  
Applied Physics Letters     Hybrid Journal   (22 followers)
Applied Physics Research     Open Access   (5 followers)
Applied Physics Reviews     Hybrid Journal   (7 followers)
Applied Radiation and Isotopes     Hybrid Journal   (5 followers)
Applied Remote Sensing Journal     Open Access   (5 followers)
Applied Spectroscopy     Full-text available via subscription   (11 followers)
Applied Spectroscopy Reviews     Hybrid Journal   (2 followers)
Applied Thermal Engineering     Hybrid Journal   (4 followers)
Archive for Rational Mechanics and Analysis     Hybrid Journal   (3 followers)
Astronomy & Geophysics     Hybrid Journal   (1 follower)
Astrophysical Journal Letters     Full-text available via subscription   (2 followers)
Astrophysical Journal Supplement Series     Full-text available via subscription   (2 followers)
Atmospheric and Oceanic Optics     Hybrid Journal  
Atomic Data and Nuclear Data Tables     Hybrid Journal  
Atoms     Open Access  
Attention, Perception & Psychophysics     Full-text available via subscription   (4 followers)
Autonomous Mental Development, IEEE Transactions on     Hybrid Journal   (5 followers)
Axioms     Open Access  
Bangladesh Journal of Medical Physics     Open Access  
Bauphysik     Hybrid Journal   (1 follower)
Biomaterials     Hybrid Journal   (22 followers)
Biomedical Engineering, IEEE Reviews in     Full-text available via subscription   (14 followers)
Biomedical Engineering, IEEE Transactions on     Hybrid Journal   (11 followers)
Biomedical Imaging and Intervention Journal     Open Access   (4 followers)
Biophysical Reviews     Hybrid Journal  
Biophysical Reviews and Letters     Hybrid Journal   (3 followers)
BMC Biophysics     Open Access   (7 followers)
BMC Nuclear Medicine     Open Access   (4 followers)
Brazilian Journal of Physics     Hybrid Journal  
Broadcasting, IEEE Transactions on     Hybrid Journal   (5 followers)
Building Acoustics     Full-text available via subscription   (1 follower)
Bulletin of Materials Science     Open Access   (34 followers)
Bulletin of the Atomic Scientists     Full-text available via subscription   (3 followers)
Bulletin of the Lebedev Physics Institute     Hybrid Journal   (2 followers)
Bulletin of the Russian Academy of Sciences: Physics     Hybrid Journal   (1 follower)
Caderno Brasileiro de Ensino de FĂ­sica     Open Access  
Canadian Journal of Physics     Full-text available via subscription   (2 followers)
Cells     Open Access  
Central European Journal of Physics     Hybrid Journal   (2 followers)
Chinese Journal of Astronomy and Astrophysics     Full-text available via subscription  
Chinese Journal of Chemical Physics     Hybrid Journal   (1 follower)
Chinese Physics B     Full-text available via subscription  
Chinese Physics C     Full-text available via subscription  
Chinese Physics Letters     Full-text available via subscription  
Cohesion and Structure     Full-text available via subscription   (1 follower)
Colloid Journal     Hybrid Journal   (1 follower)
Communications in Mathematical Physics     Hybrid Journal   (3 followers)
Communications in Numerical Methods in Engineering     Hybrid Journal   (4 followers)
Communications in Theoretical Physics     Full-text available via subscription   (2 followers)
Composites Part A : Applied Science and Manufacturing     Hybrid Journal   (23 followers)
Composites Part B : Engineering     Hybrid Journal   (21 followers)
Computational Materials Science     Hybrid Journal   (15 followers)

        1 2 3 4 5 6 7 | Last

Annales Geophysicae (ANGEO)    [5 followers]  Follow    
  This is an Open Access Journal Open Access journal
     ISSN (Print) 0992-7689 - ISSN (Online) 1432-0576
     Published by European Geosciences Union Homepage  [8 journals]   [SJR: 1.151]   [H-I: 57]
  • Observation of shocks associated with CMEs in 2007
    • Abstract: Observation of shocks associated with CMEs in 2007

      Annales Geophysicae, 32, 223-230, 2014

      Author(s): H. Aryan, M. A. Balikhin, A. Taktakishvili, and T. L. Zhang

      The interaction of CMEs with the solar wind can lead to the formation of interplanetary shocks. Ions accelerated at these shocks contribute to the solar energetic protons observed in the vicinity of the Earth. Recently a joint analysis of Venus Express (VEX) and STEREO data by Russell et al. (2009) have shown that the formation of strong shocks associated with Co-rotating Interaction Regions (CIRs) takes place between the orbits of Venus and the Earth as a result of coalescence of weaker shocks formed earlier. The present study uses VEX and Advanced Composition Explorer (ACE) data in order to analyse shocks associated with CMEs that erupted on 29 and 30 July 2007 during the solar wind conjunction period between Venus and the Earth. For these particular cases it is shown that the above scenario of shock formation proposed for CIRs also takes place for CMEs. Contradiction with shock formation resulting from MHD modelling is explained by inability of classical MHD to account for the role of wave dispersion in the formation of the shock.
      PubDate: 2014-03-12T00:00:00+01:00
       
  • Influence of water vapour on the height distribution of positive ions,
           effective recombination coefficient and ionisation balance in the quiet
           lower ionosphere
    • Abstract: Influence of water vapour on the height distribution of positive ions, effective recombination coefficient and ionisation balance in the quiet lower ionosphere

      Annales Geophysicae, 32, 207-222, 2014

      Author(s): V. Barabash, A. Osepian, and P. Dalin

      Mesospheric water vapour concentration effects on the ion composition and electron density in the lower ionosphere under quiet geophysical conditions were examined. Water vapour is an important compound in the mesosphere and the lower thermosphere that affects ion composition due to hydrogen radical production and consequently modifies the electron number density. Recent lower-ionosphere investigations have primarily concentrated on the geomagnetic disturbance periods. Meanwhile, studies on the electron density under quiet conditions are quite rare. The goal of this study is to contribute to a better understanding of the ionospheric parameter responses to water vapour variability in the quiet lower ionosphere. By applying a numerical D region ion chemistry model, we evaluated efficiencies for the channels forming hydrated cluster ions from the NO+ and O2+ primary ions (i.e. NO+.H2O and O2+.H2O, respectively), and the channel forming H+(H2O)n proton hydrates from water clusters at different altitudes using profiles with low and high water vapour concentrations. Profiles for positive ions, effective recombination coefficients and electrons were modelled for three particular cases using electron density measurements obtained during rocket campaigns. It was found that the water vapour concentration variations in the mesosphere affect the position of both the Cl2+ proton hydrate layer upper border, comprising the NO+(H2O)n and O2+(H2O)n hydrated cluster ions, and the Cl1+ hydrate cluster layer lower border, comprising the H+(H2O)n pure proton hydrates, as well as the numerical cluster densities. The water variations caused large changes in the effective recombination coefficient and electron density between altitudes of 75 and 87 km. However, the effective recombination coefficient, αeff, and electron number density did not respond even to large water vapour concentration variations occurring at other altitudes in the mesosphere. We determined the water vapour concentration upper limit at altitudes between 75 and 87 km, beyond which the water vapour concentration ceases to influence the numerical densities of Cl2+ and Cl1+, the effective recombination coefficient and the electron number density in the summer ionosphere. This water vapour concentration limit corresponds to values found in the H2O-1 profile that was observed in the summer mesosphere by the Upper Atmosphere Research Satellite (UARS). The electron density modelled using the H2O-1 profile agreed well with the electron density measured in the summer ionosphere when the measured profiles did not have sharp gradients. For sharp gradients in electron and positive ion number densities, a water profile that can reproduce the characteristic behaviour of the ionospheric parameters should have an inhomogeneous height distribution of water vapour.
      PubDate: 2014-03-11T00:00:00+01:00
       
  • Automated identification and tracking of polar-cap plasma patches at solar
           minimum
    • Abstract: Automated identification and tracking of polar-cap plasma patches at solar minimum

      Annales Geophysicae, 32, 197-206, 2014

      Author(s): R. Burston, K. Hodges, I. Astin, and P. T. Jayachandran

      A method of automatically identifying and tracking polar-cap plasma patches, utilising data inversion and feature-tracking methods, is presented. A well-established and widely used 4-D ionospheric imaging algorithm, the Multi-Instrument Data Assimilation System (MIDAS), inverts slant total electron content (TEC) data from ground-based Global Navigation Satellite System (GNSS) receivers to produce images of the free electron distribution in the polar-cap ionosphere. These are integrated to form vertical TEC maps. A flexible feature-tracking algorithm, TRACK, previously used extensively in meteorological storm-tracking studies is used to identify and track maxima in the resulting 2-D data fields. Various criteria are used to discriminate between genuine patches and "false-positive" maxima such as the continuously moving day-side maximum, which results from the Earth's rotation rather than plasma motion. Results for a 12-month period at solar minimum, when extensive validation data are available, are presented. The method identifies 71 separate structures consistent with patch motion during this time. The limitations of solar minimum and the consequent small number of patches make climatological inferences difficult, but the feasibility of the method for patches larger than approximately 500 km in scale is demonstrated and a larger study incorporating other parts of the solar cycle is warranted. Possible further optimisation of discrimination criteria, particularly regarding the definition of a patch in terms of its plasma concentration enhancement over the surrounding background, may improve results.
      PubDate: 2014-03-11T00:00:00+01:00
       
  • Is there a one-to-one correspondence between ionospheric anomalies and
           large earthquakes along Longmenshan faults?
    • Abstract: Is there a one-to-one correspondence between ionospheric anomalies and large earthquakes along Longmenshan faults?

      Annales Geophysicae, 32, 187-196, 2014

      Author(s): L. M. He, L. X. Wu, A. De Santis, S. J. Liu, and Y. Yang

      On 12 May 2008, a destructive M8.0 earthquake struck Wenchuan County (31.0° N, 103.4° E) in the Longmenshan fault zone of southwestern China. Five years later, on 20 April 2013, another terrible M7.0 earthquake struck Lushan County (30.3° N, 103.0° E) in the same fault area, only 87 km away from the epicenter of the Wenchuan earthquake. In this paper, an integrated wavelet analysis methodology is proposed to detect and diagnose ionospheric total electron content (TEC) anomalies related to seismic activities. Analytic wavelet transform is used to detect ionospheric perturbations, and then cross-wavelet analysis is used to diagnose ionospheric anomalies by gaining further insights into the dynamic relationship between the anomaly variability of ionospheric TEC and geomagnetic indices for the same set of observations. The results show that a significant ionospheric disturbance occurred on 9 May 2008 above the forthcoming epicenter, 3 days prior to the Wenchuan earthquake. However, we did not observe an ionospheric anomaly over the epicenter of the Ya'an earthquake during the 1 month period before the shock. Finally, we discuss the possible interpretations of the different seismo-ionospheric effects for the two similar earthquakes.
      PubDate: 2014-02-26T00:00:00+01:00
       
  • Rolls of the internal gravity waves in the Earth's atmosphere
    • Abstract: Rolls of the internal gravity waves in the Earth's atmosphere

      Annales Geophysicae, 32, 181-186, 2014

      Author(s): O. Onishchenko, O. Pokhotelov, W. Horton, A. Smolyakov, T. Kaladze, and V. Fedun

      The effect of the wind shear on the roll structures of nonlinear internal gravity waves (IGWs) in the Earth's atmosphere with the finite vertical temperature gradients is investigated. A closed system of equations is derived for the nonlinear dynamics of the IGWs in the presence of temperature gradients and sheared wind. The solution in the form of rolls has been obtained. The new condition for the existence of such structures was found by taking into account the roll spatial scale, the horizontal speed and wind shear parameters. We have shown that the roll structures can exist in a dynamically unstable atmosphere.
      PubDate: 2014-02-25T00:00:00+01:00
       
  • Signatures of Rossby wave modulations in aerosol optical depth over the
           central Himalayan region
    • Abstract: Signatures of Rossby wave modulations in aerosol optical depth over the central Himalayan region

      Annales Geophysicae, 32, 175-180, 2014

      Author(s): D. V. Phanikumar, K. Niranjan Kumar, K. K. Shukla, H. Joshi, M. Venkat Ratnam, M. Naja, and K. Reddy

      Long-period modulations are shown in aerosol optical depth measured by the Microtops II Sun photometer over a high-altitude site the central Himalayan region (Nainital, 29.4° N, 79.5° E, 1958 m a.m.s.l.) for the first time. Fourier analysis of aerosol optical depth showed dominant 25–45 day oscillations observed in MODerate-resolution Imaging Spectro radiometer data. Further, a Hovmiller diagram showed westward (northward) propagation at a different longitude (latitude), confirming that the modulations are associated with Rossby waves. It is also shown that the Rossby wave amplitude causes an additional warming of 4.16 ± 0.98 W m−2 over the observational site. Hence, the present study illustrates the importance of wave-induced aerosol dynamics and the corresponding radiative effects.
      PubDate: 2014-02-21T00:00:00+01:00
       
  • A model of the magnetosheath magnetic field during magnetic clouds
    • Abstract: A model of the magnetosheath magnetic field during magnetic clouds

      Annales Geophysicae, 32, 157-173, 2014

      Author(s): L. Turc, D. Fontaine, P. Savoini, and E. K. J. Kilpua

      Magnetic clouds (MCs) are huge interplanetary structures which originate from the Sun and have a paramount importance in driving magnetospheric storms. Before reaching the magnetosphere, MCs interact with the Earth's bow shock. This may alter their structure and therefore modify their expected geoeffectivity. We develop a simple 3-D model of the magnetosheath adapted to MCs conditions. This model is the first to describe the interaction of MCs with the bow shock and their propagation inside the magnetosheath. We find that when the MC encounters the Earth centrally and with its axis perpendicular to the Sun–Earth line, the MC's magnetic structure remains mostly unchanged from the solar wind to the magnetosheath. In this case, the entire dayside magnetosheath is located downstream of a quasi-perpendicular bow shock. When the MC is encountered far from its centre, or when its axis has a large tilt towards the ecliptic plane, the MC's structure downstream of the bow shock differs significantly from that upstream. Moreover, the MC's structure also differs from one region of the magnetosheath to another and these differences vary with time and space as the MC passes by. In these cases, the bow shock configuration is mainly quasi-parallel. Strong magnetic field asymmetries arise in the magnetosheath; the sign of the magnetic field north–south component may change from the solar wind to some parts of the magnetosheath. We stress the importance of the Bx component. We estimate the regions where the magnetosheath and magnetospheric magnetic fields are anti-parallel at the magnetopause (i.e. favourable to reconnection). We find that the location of anti-parallel fields varies with time as the MCs move past Earth's environment, and that they may be situated near the subsolar region even for an initially northward magnetic field upstream of the bow shock. Our results point out the major role played by the bow shock configuration in modifying or keeping the structure of the MCs unchanged. Note that this model is not restricted to MCs, it can be used to describe the magnetosheath magnetic field under an arbitrary slowly varying interplanetary magnetic field.
      PubDate: 2014-02-21T00:00:00+01:00
       
  • Variations in the occurrence of SuperDARN F region echoes
    • Abstract: Variations in the occurrence of SuperDARN F region echoes

      Annales Geophysicae, 32, 147-156, 2014

      Author(s): M. Ghezelbash, R. A. D. Fiori, and A V. Koustov

      The occurrence of F region ionospheric echoes observed by a number of SuperDARN HF radars is analyzed statistically in order to infer solar cycle, seasonal, and diurnal trends. The major focus is on Saskatoon radar data for 1994–2012. The distribution of the echo occurrence rate is presented in terms of month of observation and magnetic local time. Clear repetitive patterns are identified during periods of solar maximum and solar minimum. For years near solar maximum, echoes are most frequent near midnight during winter. For years near solar minimum, echoes occur more frequently near noon during winter, near dusk and dawn during equinoxes and near midnight during summer. Similar features are identified for the Hankasalmi and Prince George radars in the northern hemisphere and the Bruny Island TIGER radar in the southern hemisphere. Echo occurrence for the entire SuperDARN network demonstrates patterns similar to patterns in the echo occurrence for the Saskatoon radar and for other radars considered individually. In terms of the solar cycle, the occurrence rate of nightside echoes is shown to increase by a factor of at least 3 toward solar maximum while occurrence of the near-noon echoes does not significantly change with the exception of a clear depression during the declining phase of the solar cycle.
      PubDate: 2014-02-20T00:00:00+01:00
       
  • The structure of strongly tilted current sheets in the Earth magnetotail
    • Abstract: The structure of strongly tilted current sheets in the Earth magnetotail

      Annales Geophysicae, 32, 133-146, 2014

      Author(s): I. Y. Vasko, A. V. Artemyev, A. A. Petrukovich, R. Nakamura, and L. M. Zelenyi

      We investigate strongly tilted (in the y–z GSM plane) current sheets (CSs) in the Earth magnetotail using data from the Cluster mission. We analyze 29 CS crossings observed in 2001–2004. The characteristic current density, magnetic field at the CS boundary and the CS thickness of strongly tilted CSs are similar to those reported previously for horizontal (not tilted) CSs. We confirm that strongly tilted CSs are generally characterized by a rather large northward component of the magnetic field. The field-aligned current in strongly tilted CSs is on average two times larger than the transverse current. The proton adiabaticity parameter, κp, is larger than 0.5 in 85% of strongly tilted CSs due to the large northward magnetic field. Thus, the proton dynamics is stochastic for 18 current sheets with 0.5 < κp < 3 and protons are magnetized for 6 sheets with κp > 3, whereas electrons are magnetized for all observed current sheets. Strongly tilted CSs provide a unique opportunity to measure the electric field component perpendicular to the CS plane. We find that most of the electric field perpendicular to the CS plane is due to the decoupling of electron and ion motions (plasma polarization). For 27 CSs we determine profiles of the electrostatic potential, which is due to the plasma polarization. Drops in the potential between the neutral plane and the CS boundary are within the range of 200 V to 12 kV, while maximal values of the electric field are within the range of 0.2 mV m−1 to 8 mV m−1. For 16 CSs the observed potentials are in accordance with Ohm's law, if the electron current density is assumed to be comparable to the total current density. In 15 of these CSs the profile of the polarization potential is approximately symmetric with respect to the neutral plane and has minimum therein.
      PubDate: 2014-02-20T00:00:00+01:00
       
  • Quasi-16-day periodic meridional movement of the equatorial ionization
           anomaly
    • Abstract: Quasi-16-day periodic meridional movement of the equatorial ionization anomaly

      Annales Geophysicae, 32, 121-131, 2014

      Author(s): X. H. Mo, D. H. Zhang, L. P. Goncharenko, Y. Q. Hao, and Z. Xiao

      Based on the daytime location of the equatorial ionization anomaly (EIA) crest derived from GPS observations at low latitude over China during the 2005–2006 stratospheric sudden warming (SSW), a quasi-16-day periodic meridional movement of EIA crest with the maximum amplitude of about 2 degrees relative to the average location of EIA crest has been revealed. In addition, periodic variations that are in phase with the meridional EIA movement are also revealed in the equatorial electrojet (EEJ) and F2 layer peak height (hmF2) over Chinese ionosonde stations Haikou and Chongqing. The quasi-16-day periodic component in Dst index is weak, and the 16-day periodic component does not exist in F10.7 index. Such large-scale periodic meridional movement of EIA crest is likely related to the globally enhanced stratospheric planetary waves coupled with anomalous stratospheric zonal wind connected with SSW. In addition, such large-scale periodic movement of EIA should be global, and can affect the ionospheric morphology around the low-latitude belt near the EIA region. Further case analysis, simulation and theoretical studies must proceed in order to understand the periodic movements of EIA connected with the different periodic atmospheric variations.
      PubDate: 2014-02-18T00:00:00+01:00
       
  • Long-term changes in thermospheric composition inferred from a spectral
           analysis of ionospheric F-region data
    • Abstract: Long-term changes in thermospheric composition inferred from a spectral analysis of ionospheric F-region data

      Annales Geophysicae, 32, 113-119, 2014

      Author(s): C. J. Scott, R. Stamper, and H. Rishbeth

      A study of ionospheric data recorded at Slough/Chilton, UK, from 1935 to 2012, has revealed long-term changes in the relative strength of the annual and semi-annual variability in the ionospheric F2 layer critical frequencies. Comparing these results with data from the southern hemisphere station at Stanley in the Falkland Islands between 1945 and 2012 reveals a trend that appears to be anti-correlated with that at Chilton. The behaviour of foF2 is a function of thermospheric composition and so we argue that the observed long-term changes are driven by composition change. The ionospheric trends share some of their larger features with the trend in the variability of the aa geomagnetic index. Changes to the semi-annual/annual ratio in the Slough/Chilton and Stanley data may therefore be attributable to the variability in geomagnetic activity which controls the average latitudinal extent of the auroral ovals and subsequent thermospheric circulation patterns. Changes in ionospheric composition or thermospheric wind patterns are known to influence the height of the F2 layer at a given location. Long-term changes to the height of the F2 layer have been used to infer an ionospheric response to greenhouse warming. We suggest that our observations may influence such measurements and since the results appear to be dependent on geomagnetic longitude, this could explain why the long-term drifts observed in F2 layer height differ between locations.
      PubDate: 2014-02-17T00:00:00+01:00
       
  • Statistical visualization of the Earth's magnetotail and the implied
           mechanism of substorm triggering based on superposed-epoch analysis of
           THEMIS data
    • Abstract: Statistical visualization of the Earth's magnetotail and the implied mechanism of substorm triggering based on superposed-epoch analysis of THEMIS data

      Annales Geophysicae, 32, 99-111, 2014

      Author(s): S. Machida, Y. Miyashita, A. Ieda, M. Nosé, V. Angelopoulos, and J. P. McFadden

      To investigate the physical mechanism responsible for substorm triggering, we performed a superposed-epoch analysis using plasma and magnetic-field data from THEMIS probes. Substorm onset timing was determined based on auroral breakups detected by all-sky imagers at the THEMIS ground-based observatories. We found earthward flows associated with north–south auroral streamers during the substorm growth phase. At around X = −12 Earth radii (RE), the northward magnetic field and its elevation angle decreased markedly approximately 4 min before substorm onset. Moreover, a northward magnetic-field increase associated with pre-onset earthward flows was found at around X = −17 RE. This variation indicates that local dipolarization occurs. Interestingly, in the region earthwards of X = −18 RE, earthward flows in the central plasma sheet (CPS) reduced significantly approximately 3 min before substorm onset, which was followed by a weakening of dawn-/duskward plasma-sheet boundary-layer flows (subject to a 1 min time lag). Subsequently, approximately 1 min before substorm onset, earthward flows in the CPS were enhanced again and at the onset, tailward flows started at around X = −20 RE. Following substorm onset, an increase in the northward magnetic field caused by dipolarization was found in the near-Earth region. Synthesizing these results, we confirm our previous results based on GEOTAIL data, which implied that significant variations start earlier than both current disruption and magnetic reconnection, at approximately 4 min before substorm onset roughly halfway between the two regions of interest; i.e. in the catapult current sheet.
      PubDate: 2014-02-17T00:00:00+01:00
       
  • Characteristics of equatorial ionization anomaly (EIA) in relation to
           transionospheric satellite links around the northern crest in the Indian
           longitude sector
    • Abstract: Characteristics of equatorial ionization anomaly (EIA) in relation to transionospheric satellite links around the northern crest in the Indian longitude sector

      Annales Geophysicae, 32, 91-97, 2014

      Author(s): A. Das, K. S. Paul, S. Halder, K. Basu, and A. Paul

      The poleward gradient of the equatorial ionization anomaly (EIA) introduces more intense propagation effects on transionospheric satellite links in comparison to the equatorward gradient. Characterization of the poleward gradient was performed during March–April, August–October 2011 and March–April 2012 using GPS total electron content (TEC) recorded from a chain of stations located more or less along the same meridian (88.5° E) at Calcutta, Baharampore, Farakka and Siliguri. The poleward gradients calculated on magnetically quiet days at elevation in excess of 50° at 14:00, 15:00 and 16:00 LT were found to have a strong correlation with GPS S4 observed from Calcutta during post-sunset-to-midnight hours. A threshold value of poleward TEC gradient is calculated above which there is a probability of scintillation at Calcutta with S4 ≥ 0.4.
      PubDate: 2014-02-12T00:00:00+01:00
       
  • Ion drift simulation of sudden appearance of sub-keV structured ions in
           the inner magnetosphere
    • Abstract: Ion drift simulation of sudden appearance of sub-keV structured ions in the inner magnetosphere

      Annales Geophysicae, 32, 83-90, 2014

      Author(s): M. Yamauchi, Y. Ebihara, H. Nilsson, and I. Dandouras

      Energy–latitude dispersed structured sub-keV ions in the inner magnetosphere often show significant development or intensification (by more than factor of 3) within 1–2 h near noon or afternoon where the azimuthal ion drift velocity at the sub-keV range is expected to be near zero. To examine whether such sudden appearances in the dayside can be explained by the drift motion of ions that are formed during substorm-related injections, we numerically simulated two such examples, one at noon and the other in the afternoon, based on the ion drift model. For both cases, the ion drift model with finite duration of proton source in the nightside can explain the observed large inbound–outbound differences in the sub-keV proton population without any new sources. Ion drift motion can thus cause rapid changes of complicated ion populations, at remote places from the source a long time after the substorm activities.
      PubDate: 2014-02-12T00:00:00+01:00
       
  • Terrestrial aurora: astrophysical laboratory for anomalous abundances in
           stellar systems
    • Abstract: Terrestrial aurora: astrophysical laboratory for anomalous abundances in stellar systems

      Annales Geophysicae, 32, 77-82, 2014

      Author(s): I. Roth

      The unique magnetic structure of the terrestrial aurora as a conduit of information between the ionosphere and magnetosphere can be utilized as a laboratory for physical processes at similar magnetic configurations and applied to various evolutionary phases of the solar (stellar) system. The most spectacular heliospheric abundance enhancement involves the 3He isotope and selective heavy elements in impulsive solar flares. In situ observations of electromagnetic waves on active aurora are extrapolated to flaring corona in an analysis of solar acceleration processes of 3He, the only element that may resonate strongly with the waves, as well as heavy ions with specific charge-to-mass ratios, which may resonate weaker via their higher gyroharmonics. These results are applied to two observed anomalous astrophysical abundances: (1) enhanced abundance of 3He and possibly 13C in the late stellar evolutionary stages of planetary nebulae; and (2) enhanced abundance of the observed fossil element 26Mg in meteorites as a decay product of radioactive 26Al isotope due to interaction with the flare-energized 3He in the early solar system.
      PubDate: 2014-02-11T00:00:00+01:00
       
  • Topside equatorial zonal ion velocities measured by C/NOFS during rising
           solar activity
    • Abstract: Topside equatorial zonal ion velocities measured by C/NOFS during rising solar activity

      Annales Geophysicae, 32, 69-75, 2014

      Author(s): W. R. Coley, R. A. Stoneback, R. A. Heelis, and M. R. Hairston

      The Ion Velocity Meter (IVM), a part of the Coupled Ion Neutral Dynamic Investigation (CINDI) instrument package on the Communication/Navigation Outage Forecast System (C/NOFS) spacecraft, has made over 5 yr of in situ measurements of plasma temperatures, composition, densities, and velocities in the 400–850 km altitude range of the equatorial ionosphere. These measured ion velocities are then transformed into a coordinate system with components parallel and perpendicular to the geomagnetic field allowing us to examine the zonal (horizontal and perpendicular to the geomagnetic field) component of plasma motion over the 2009–2012 interval. The general pattern of local time variation of the equatorial zonal ion velocity is well established as westward during the day and eastward during the night, with the larger nighttime velocities leading to a net ionospheric superrotation. Since the C/NOFS launch in April 2008, F10.7 cm radio fluxes have gradually increased from around 70 sfu to levels in the 130–150 sfu range. The comprehensive coverage of C/NOFS over the low-latitude ionosphere allows us to examine variations of the topside zonal ion velocity over a wide level of solar activity as well as the dependence of the zonal velocity on apex altitude (magnetic latitude), longitude, and solar local time. It was found that the zonal ion drifts show longitude dependence with the largest net eastward values in the American sector. The pre-midnight zonal drifts show definite solar activity (F10.7) dependence. The daytime drifts have a lower dependence on F10.7. The apex altitude (magnetic latitude) variations indicate a more westerly flow at higher altitudes. There is often a net topside subrotation at low F10.7 levels, perhaps indicative of a suppressed F region dynamo due to low field line-integrated conductivity and a low F region altitude at solar minimum.
      PubDate: 2014-02-04T00:00:00+01:00
       
  • Investigation of convectively generated gravity wave characteristics and
           generation mechanisms during the passage of thunderstorm and squall line
           over Gadanki (13.5° N, 79.2° E)
    • Abstract: Investigation of convectively generated gravity wave characteristics and generation mechanisms during the passage of thunderstorm and squall line over Gadanki (13.5° N, 79.2° E)

      Annales Geophysicae, 32, 57-68, 2014

      Author(s): M. Arunachalam Srinivasan, S. V. B. Rao, and R. Suresh

      This study illustrates the convectively generated gravity wave generation mechanisms during the passage of thunderstorms and squall line using Indian MST radar. For the first time, it has been shown that all three generation mechanisms have been involved in the generation of gravity waves during the passage of squall line event. It is observed that the periodicities in the range of 8–80 min in the tropospheric and 8–32 min in the lower stratospheric regions and vertical wavelengths in the range of 3.2–4.8 km in the tropospheric and 1.2–1.92 km in the lower stratospheric regions are found to be dominant in the present study and are distinctly different during initial, mature and dissipative phases of convection. Amplitude of vertical wind has been weakened (from ~ 4–6 m s−1 to ~ 1 m s−1) considerably after 10–30 min of a convection event. It appears that the wind shear associated with the convective clouds acted like an obstacle to the mean background flow during the squall line passage generated gravity waves. The phase profiles corresponding to the dominant period show both downward and upward propagation of waves. The vertical extent of heating is found to be deeper during squall line event compared with thunderstorm event. From the phase profiles, during 27 September 2004, two peaks of constant phase region are observed. One is due to convective elements and the other is due to strong background wind shear; however, only one peak is observed on 29 September 2004, which is only due to convective processes.
      PubDate: 2014-01-30T00:00:00+01:00
       
  • China's dimming and brightening: evidence, causes and hydrological
           implications
    • Abstract: China's dimming and brightening: evidence, causes and hydrological implications

      Annales Geophysicae, 32, 41-55, 2014

      Author(s): Y. W. Wang and Y. H. Yang

      There is growing evidence that, corresponding to global dimming and brightening, surface solar radiation and sunshine hours over China have undergone decadal fluctuations during the 1960s–2000s. The results of a number of these analyses are, however, very different. In this study, we synthesize reliable results and conclusively address recent advances and insufficiencies in studies on dimming and brightening in China. A temporally and spatially prevalent dimming trend is noted in surface solar radiation, direct solar radiation and sunshine hours since the 1960s. Meanwhile, the changing trend in diffuse solar radiation is less pronounced. Increasing anthropogenic aerosol loading is regarded as the most plausible explanation for China's dimming. The brightening trend since 1990, which mainly occurs in southeastern China and in the spring season, is weak and insignificant. The reverse in the solar radiation trend is associated with climate change by cloud suppression and slowdown in anthropogenic emissions. The future solar radiation trend in China could largely depend on the development of air quality control. Other potential driving factors such as wind speed, water vapor and surface albedo are also non-negligible in specific regions of China. Hydrological implications of dimming and brightening in China lack systematic investigation. However, the fact that solar radiation and pan evaporation trends in China track a similar curve in 1990 further suggests that the pan evaporation paradox could be partly resolved by changes in solar radiation.
      PubDate: 2014-01-28T00:00:00+01:00
       
  • Correction of artificial jumps in the historical geomagnetic measurements
           of Coimbra Observatory, Portugal
    • Abstract: Correction of artificial jumps in the historical geomagnetic measurements of Coimbra Observatory, Portugal

      Annales Geophysicae, 32, 19-40, 2014

      Author(s): A. L. Morozova, P. Ribeiro, and M. A. Pais

      The Coimbra Magnetic Observatory (International Association of Geomagnetism and Aeronomy code COI) in Portugal has a long history of observation of the geomagnetic field, spanning almost 150 yr since the first geomagnetic measurements in 1866. These long instrumental geomagnetic records provide very important information about variability of geomagnetic elements and indices, their trends and cycles, and can be used to improve our knowledge on the sources that drive variations of the geomagnetic field: liquid core dynamics (internal) and solar forcing (external).

      However, during the long life of the Coimbra Observatory, some inevitable changes in station location, instrument's park and electromagnetic environment have taken place. These changes affected the quality of the data collected at COI causing breaks and jumps in the series of geomagnetic field components and local K index. Clearly, these inhomogeneities, typically shift-like (step-like) or trend-like, have to be corrected or, at least, minimized in order for the data to be used in scientific studies or to be submitted to international databases.

      In this study, the series of local K index and declination of the geomagnetic field are analysed: the former because it allows direct application of standard homogenization methods and the latter because it is the longest continuous series produced at COI. For the homogenization, visual and statistical tests (e.g. standard normal homogeneity test) have been applied directly to the local geomagnetic K index series (from 1951 to 2012). The homogenization of the monthly averages of declination (from 1867 to 2012) has been done using visual analysis and statistical tests applied to the time series of the first differences of declination values, as an approximation to the first time derivative. This allowed not only estimating the level of inhomogeneity of the studied series but also detecting the highly probable homogeneity break points. These points have been cross-checked with the metadata, and the COI series have been compared with reference series from the nearest geomagnetic stations and, in the case of declination series, from the recent geomagnetic field model COV-OBS to set up the required correction factors. As a result, the homogenized series measured in COI are considered to be essentially free of artificial shifts starting from the second half of the 20th century, and ready to be used by the scientific community.
      PubDate: 2014-01-27T00:00:00+01:00
       
  • Low-latitude scintillation occurrences around the equatorial anomaly crest
           over Indonesia
    • Abstract: Low-latitude scintillation occurrences around the equatorial anomaly crest over Indonesia

      Annales Geophysicae, 32, 7-17, 2014

      Author(s): P. Abadi, S. Saito, and W. Srigutomo

      We investigated low-latitude ionospheric scintillation in Indonesia using two GPS receivers installed at Bandung (107.6° E, 6.9° S; magnetic latitude 17.5° S) and Pontianak (109.3° E, 0.02° S; magnetic latitude 8.9° S). This study aimed to characterise climatological and directional ionospheric scintillation occurrences, which are useful not only for the physics of ionospheric irregularities but also for practical use in GNSS (global navigation satellite system)-based navigation. We used the deployed instrument's amplitude scintillation (S4 index) data from 2009, 2010, and 2011; the yearly SSN (sunspot-smoothed numbers) were 3.1, 16.5, and 55.9, respectively. In summary, (1) scintillation occurrences in the post-sunset period (18:00–01:00 LT) during equinox months (plasma bubble season) at the two sites can be ascribed to the plasma bubble; (2) using directional analyses of the two sites, we found that the distribution of scintillation occurrences is generally concentrated between the two sites, indicating the average location of the EIA (equatorial ionisation anomaly) crest; (3) scintillation occurrence enhancements for the two sites in field-aligned directions are herein reported for the first time by ground-based observation in a low-latitude region; (4) distribution of scintillation occurrences at Pontianak are concentrated in the southern sky, especially in the southwest direction, which is very likely associated with the plasma bubble tilted westward with increasing latitude; and (5) scintillation occurrence in the post-midnight period in the non-plasma-bubble season is the most intriguing variable occurring between the two sites (i.e. post-midnight scintillations are observed more at Bandung than Pontianak). Most of the post-midnight scintillations observed at Bandung are concentrated in the northern sky, with low elevation angles. This might be due to the amplitude of irregularities in certain directions, which may be effectively enhanced by background density enhancement by the EIA and because satellite–receiver paths are longer in the EIA crest region and in a field-aligned direction.
      PubDate: 2014-01-16T00:00:00+01:00
       
 
 
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