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Diamond and Related Materials     Hybrid Journal   (Followers: 12)
Differential Equations and Nonlinear Mechanics     Open Access   (Followers: 4)

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Journal Cover Annales Geophysicae (ANGEO)     [SJR: 1.151]   [H-I: 57]
   [6 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]
  • Observations and modeling of UHF-band scintillation occurrence probability
           over the low-latitude region of China during the maximum activity of solar
           cycle 24

    • Abstract: Observations and modeling of UHF-band scintillation occurrence probability over the low-latitude region of China during the maximum activity of solar cycle 24

      Annales Geophysicae, 33, 93-100, 2015

      Author(s): H. Zhang, Y. Liu, J. Wu, T. Xu, and D. Sheng

      The climatological characteristics of UHF-band scintillations over the low-latitude region of China were investigated by analyzing the observations recorded at three stations of our regional network of satellite-beacon-based scintillation monitoring in 2013. The three stations are Hainan (geographic 20.0° N, 110.3° E; geomagnetic 10.1° N, 177.4° W, dip 28.2°), Guangzhou (geographic 23.0° N, 113.0° E; geomagnetic 13.1° N, 174.8° W, dip 33.9°) and Kunming (geographic 25.6° N, 103.7° E; geomagnetic 15.7° N, 176.4° E, dip 39.0°), located at low latitudes of China. The variations of UHF-band scintillation occurrence with latitude, time and season are presented in detail to understand the morphology and climatology of ionospheric scintillations over the low-latitude region of China. An equinoctial asymmetry in the occurrences of scintillation and an obvious difference of the onset time of scintillations between Hainan and Kunming is noted in this data set. Subsequently, the ionosonde data are utilized to study the possible causes of the asymmetry between two equinoxes. The observations suggest that the mean critical frequency (foF2) at 20:00 LT (12:00 UT) in the autumnal equinoctial months (September and October) and the vernal equinoctial months (March and April) has a similar asymmetry. The ratio of the mean foF2 between two equinoxes is proportional to the ratio between the maximum scintillation occurrence in the autumnal equinox and in the vernal equinox. Therefore, this ratio can act as a proxy for the equinoctial asymmetry in the occurrences of scintillation over the low-latitude region of China, and can be used to model the equinoctial asymmetry in our empirical climatological model of scintillation occurrence probability (CMSOP). The CMSOP can provide the predictions of the occurrences of scintillation over the low-latitude region of China and was validated in this study.
      PubDate: 2015-01-16T00:00:00+01:00
  • Dipolarization fronts in the near-Earth space and substorm dynamics

    • Abstract: Dipolarization fronts in the near-Earth space and substorm dynamics

      Annales Geophysicae, 33, 63-74, 2015

      Author(s): I. I. Vogiatzis, A. Isavnin, Q.-G. Zong, E. T. Sarris, S. W. Lu, and A. M. Tian

      During magnetospheric substorms and plasma transport in the Earth's magnetotail various magnetic structures can be detected. Dipolarization fronts and flux ropes are the most prominent structures characteristic for substorm dynamics. However, they are treated as separate magnetotail features independent of each other. In this paper, we analyze a number of dipolarization fronts observed by the THEMIS (Time History of Events and Macroscale Interactions during Substorms) spacecraft at different geocentric distances by applying the magnetohydrostatic Grad–Shafranov (GS) reconstruction technique. Our analysis shows that there is a possibility of dipolarization fronts to originate from highly dissipated flux ropes which are in the late stage of their evolution, subjected to a continuous magnetic deterioration due to the reconnection process. These results may improve our understanding of magnetoplasma processes in Earth's magnetotail.
      PubDate: 2015-01-15T00:00:00+01:00
  • Radiation dose of aircrews during a solar proton event without
           ground-level enhancement

    • Abstract: Radiation dose of aircrews during a solar proton event without ground-level enhancement

      Annales Geophysicae, 33, 75-78, 2015

      Author(s): R. Kataoka, Y. Nakagawa, and T. Sato

      A significant enhancement of radiation doses is expected for aircrews during ground-level enhancement (GLE) events, while the possible radiation hazard remains an open question during non-GLE solar energetic particle (SEP) events. Using a new air-shower simulation driven by the proton flux data obtained from GOES satellites, we show the possibility of significant enhancement of the effective dose rate of up to 4.5 μSv h−1 at a conventional flight altitude of 12 km during the largest SEP event that did not cause a GLE. As a result, a new GOES-driven model is proposed to give an estimate of the contribution from the isotropic component of the radiation dose in the stratosphere during non-GLE SEP events.
      PubDate: 2015-01-15T00:00:00+01:00
  • Experimental test of the ρ(1-α) evolution for rotational
           discontinuities: cluster magnetopause observations

    • Abstract: Experimental test of the ρ(1-α) evolution for rotational discontinuities: cluster magnetopause observations

      Annales Geophysicae, 33, 79-91, 2015

      Author(s): A. Blagau, G. Paschmann, B. Klecker, and O. Marghitu

      Rotational discontinuities (RDs) are governed by two relations: the Walén relation predicting that the plasma velocity observed in the deHoffmann–Teller frame equals the local Alfvén velocity and another relation that connects the variation in plasma mass density, ρ, to variations in the pressure anisotropy factor, α, defined as α: ≡(p∥ − p⊥) μ0/B2, so that ρ(1 − α) is constant. While the Walén relation has become a standard tool for classifying magnetopause crossings as RDs , the ρ(1 − α)= const. condition has never been directly verified at the same time, largely due to problems with determining ρ when no ion composition measurements were available. In fact, to overcome the lack of composition information, the validity of the relation has often been assumed and the Walén relation reformulated so that variations in ρ are replaced by variations in α. In this paper we exploit the availability of high-time-resolution composition measurements on the Cluster spacecraft to directly test the ρ (1− α)= const. condition for three magnetopause crossings, identified as RDs from the application of the Walén relation to measurements of plasma ions and magnetic field by the CIS (Cluster Ion Spectrometry) and FGM (flux-gate magnetometer) instruments, respectively. We find that the relation is not fulfilled in either case. In one event, with a fairly large content of oxygen ions, the Walén test improved when the contribution from these ions was taken into account. Through comparisons of the measured ion densities with simultaneously measured total electron densities by the Waves of HIgh frequency and Sounder for Probing of Electron density by Relaxation (WHISPER) instrument, we were able to exclude the possibility that ion populations hidden to the CIS instrument because of their very low energies could have changed ρ to match the ρ(1 − α)= const. condition. We also excluded the possibility that energetic ions above the CIS energy range could have sufficiently changed the true α. It thus appears that the ρ(1 − α)= const. condition, for reasons not presently understood, is not valid for the kind of RD-like structures we observe.
      PubDate: 2015-01-15T00:00:00+01:00
  • Validation of GPS atmospheric water vapor with WVR data in satellite
           tracking mode

    • Abstract: Validation of GPS atmospheric water vapor with WVR data in satellite tracking mode

      Annales Geophysicae, 33, 55-61, 2015

      Author(s): M. Shangguan, S. Heise, M. Bender, G. Dick, M. Ramatschi, and J. Wickert

      Slant-integrated water vapor (SIWV) data derived from GPS STDs (slant total delays), which provide the spatial information on tropospheric water vapor, have a high potential for assimilation to weather models or for nowcasting or reconstruction of the 3-D humidity field with tomographic techniques. Therefore, the accuracy of GPS STD is important, and independent observations are needed to estimate the quality of GPS STD. In 2012 the GFZ (German Research Centre for Geosciences) started to operate a microwave radiometer in the vicinity of the Potsdam GPS station. The water vapor content along the line of sight between a ground station and a GPS satellite can be derived from GPS data and directly measured by a water vapor radiometer (WVR) at the same time. In this study we present the validation results of SIWV observed by a ground-based GPS receiver and a WVR. The validation covers 184 days of data with dry and wet humidity conditions. SIWV data from GPS and WVR generally show good agreement with a mean bias of −0.4 kg m−2 and an rms (root mean square) of 3.15 kg m−2. The differences in SIWV show an elevation dependent on an rms of 7.13 kg m−2 below 15° but of 1.76 kg m−2 above 15°. Nevertheless, this elevation dependence is not observed regarding relative deviations. The relation between the differences and possible influencing factors (elevation angles, pressure, temperature and relative humidity) are analyzed in this study. Besides the elevation, dependencies between the atmospheric humidity conditions, temperature and the differences in SIWV are found.
      PubDate: 2015-01-13T00:00:00+01:00
  • Three-dimensional multi-fluid model of a coronal streamer belt with a
           tilted magnetic dipole

    • Abstract: Three-dimensional multi-fluid model of a coronal streamer belt with a tilted magnetic dipole

      Annales Geophysicae, 33, 47-53, 2015

      Author(s): L. Ofman, E. Provornikova, L. Abbo, and S. Giordano

      Observations of streamers in extreme ultraviolet (EUV) emission with SOHO/UVCS show dramatic differences in line profiles and latitudinal variations in heavy ion emission compared to hydrogen Ly-α emission. In order to use ion emission observations of streamers as the diagnostics of the slow solar wind properties, an adequate model of a streamer including heavy ions is required. We extended a previous 2.5-D multi-species magnetohydrodynamics (MHD) model of a coronal streamer to 3-D spherical geometry, and in the first approach we consider a tilted dipole configuration of the solar magnetic field. The aim of the present study is to test the 3-D results by comparing to previous 2.5-D model result for a 3-D case with moderate departure from azimuthal symmetry. The model includes O5+ ions with preferential empirical heating and allows for calculation of their density, velocity and temperature in coronal streamers. We present the first results of our 3-D multi-fluid model showing the parameters of protons, electrons and heavy ions (O5+) at the steady-state solar corona with a tilted steamer belt. We find that the 3-D results are in qualitative agreement with our previous 2.5-D model, and show longitudinal variation in the variables in accordance with the tilted streamer belt structure. Properties of heavy coronal ions obtained from the 3-D model together with EUV spectroscopic observations of streamers will help understanding the 3-D structures of streamers reducing line-of-sight integration ambiguities and identifying the sources of the slow solar wind in the lower corona. This leads to improved understanding of the physics of the slow solar wind.
      PubDate: 2015-01-12T00:00:00+01:00
  • The nightside magnetic field line open–closed boundary and polar
           rain electron energy-latitude dispersion

    • Abstract: The nightside magnetic field line open–closed boundary and polar rain electron energy-latitude dispersion

      Annales Geophysicae, 33, 39-46, 2015

      Author(s): S. Wing and Y. L. Zhang

      The polar rain electrons near the open–closed field line boundary on the nightside often exhibit energy-latitude dispersion, in which the energy decreases with decreasing latitude. The solar wind electrons from the last open-field line would E × B drift equatorward as they move toward the ionosphere, resulting in the observed dispersion. This process is modeled successfully by an open-field line particle precipitation model. The existing method for determining the magnetotail X line distance from the electron dispersion underestimates the electron path length from the X line to the ionosphere by at least 33%. The best estimate of the path length comes from using the two highest energy electrons in the dispersion region. The magnetic field line open–closed boundary is located poleward of the highest energy electrons in the dispersion region, which in turn is located poleward of Defense Meteorological Satellite Program (DMSP) b6, b5e, and b5i boundaries. In the four events examined, b6 is located at least 0.7–1.5° equatorward of the magnetic field line open–closed boundary. The energy-latitude dispersion seen in the electron overhang may result from the plasma sheet electron curvature and gradient drifts into the newly closed field line.
      PubDate: 2015-01-12T00:00:00+01:00
  • Spectrum analysis of short-period K index behaviour at
           high and mid-latitudes

    • Abstract: Spectrum analysis of short-period K index behaviour at high and mid-latitudes

      Annales Geophysicae, 33, 31-37, 2015

      Author(s): P. B. Kotzé

      Geomagnetic activity levels during the declining phase and solar minimum period of the solar cycle are considerably different from those during the solar maximum phase. Previous studies revealed variations in the pattern of recurrent activity from cycle to cycle as well as variations in the average geomagnetic activity levels during a solar cycle. During the declining phase of a solar cycle (and solar minimum), the solar and interplanetary causes of geomagnetic activity are substantially different from those during the solar maximum phase. Co-rotating fast solar wind streams originating from large polar coronal holes, extending towards the Sun's equator, interact with the Earth's magnetosphere, resulting in recurrent geomagnetic activity particularly during solar cycle minimum periods. This is a well-known phenomenon with respect to 27.0- and 13.5-day recurrence geomagnetic activity, and it is well-known to be related to sectorial (non-axial) poloidal magnetic field structure in the Sun. Published results of the recent solar-cycle-23 minimum showed that the presence of 9.0- and 6.7-day recurrent geomagnetic activities can be attributed to the sectorial spherical harmonic structure present in the solar magnetic field. In this study we performed a wavelet and Lomb–Scargle analysis of the geomagnetic activity K index at Lerwick (LER), Hermanus (HER) and Canberra (CNB) for the period between 1960 and 2010, overlapping with solar cycles 20 to 23. Daily mean K indices are used to identify how several harmonics of the 27.0-day recurrent period change during each solar cycle when comparing high and mid-latitude geomagnetic activity, applying a 95% confidence level. In particular the behaviour of the second (13.5-day), third (9.0-day) and fourth (6.7-day) harmonics are investigated by doing a wavelet analysis of each individual year's K indices at each location. Results obtained show that particularly during solar minima the 27.0-day period is no longer detectable above the 95% confidence level, and that geomagnetic activity is in fact dominated by higher harmonics like 13.5-, 9.0- and 6.7-day periods. These findings in fact are in line with previous investigations and confirm the results obtained by researchers using other geomagnetic activity indices like \textit{aa} and C9. The wavelet-spectrum analysis also reveals that during the downward phase of cycle 23 and the very long minimum of 23–24 between 2002 and 2008, the 27.0-day activity period drops below the 95% confidence level. This is confirmed by Lomb–Scargle analyses of every year's K index activity. Results obtained in this study support evidence by other investigations that this can be attributed to the lack of coronal-mass ejection (CME)-dominated solar activity during solar minima, periods characterized by strong solar dipolar magnetic fields, less sunspot numbers than at solar maxima, and multiple prominent co-rotating solar wind streams present. This analysis further confirms previous studies by other authors that the pattern of recurrent activity is dictated by the configuration of coronal holes which give rise to related high-speed streams during a solar cycle by analysing K indices at both high- and mid-latitude magnetic observatories.
      PubDate: 2015-01-09T00:00:00+01:00
  • Identification of slow magnetosonic wave trains and their evolution in 3-D
           compressible turbulence simulation

    • Abstract: Identification of slow magnetosonic wave trains and their evolution in 3-D compressible turbulence simulation

      Annales Geophysicae, 33, 13-23, 2015

      Author(s): L. Zhang, L.-P. Yang, J.-S. He, C.-Y. Tu, L.-H. Wang, E. Marsch, and X.-S. Feng

      In solar wind, dissipation of slow-mode magnetosonic waves may play a significant role in heating the solar wind, and these modes contribute essentially to the solar wind compressible turbulence. Most previous identifications of slow waves utilized the characteristic negative correlation between δ B and δρ. However, that criterion does not well identify quasi-parallel slow waves, for which δ B is negligible compared to δρ. Here we present a new method of identification, which will be used in 3-D compressible simulation. It is based on two criteria: (1) that VpB0 (phase speed projected along B0) is around ± cs, and that (2) there exists a clear correlation of δv and δρ. Our research demonstrates that if vA > cs, slow waves possess correlation between δv and δρ, with δρ / δv ≈ ± ρ0 / cs. This method helps us to distinguish slow-mode waves from fast and Alfvén waves, both of which do not have this polarity relation. The criteria are insensitive to the propagation angle θk B, defined as the angle between wave vector k and B0; they can be applied with a wide range of β if only vA > cs. In our numerical simulation, we have identified four cases of slow wave trains with this method. The slow wave trains seem to deform, probably caused by interaction with other waves; as a result, fast or Alfvén waves may be produced during the interaction and seem to propagate bidirectionally away. Our identification and analysis of the wave trains provide useful methods for investigations of compressible turbulence in the solar wind or in similar environments, and will thus deepen understandings of slow waves in the turbulence.
      PubDate: 2015-01-07T00:00:00+01:00
  • Physics of outflows near solar active regions

    • Abstract: Physics of outflows near solar active regions

      Annales Geophysicae, 33, 25-29, 2015

      Author(s): D. J. Price and Y. Taroyan

      Hinode/EIS observations have revealed outflows near active regions which remain unexplained. An outflow region observed by the EUV Imaging Spectrometer (EIS) that appears slightly redshifted at low temperatures and blueshifted at higher temperatures is presented. We conduct simulations and use those to create synthetic line profiles in order to replicate the observed line profiles of an apparent open structure. The results of the forward modelling support a scenario whereby long loops consisting of multiple strands undergo a cyclical process of heating and cooling on timescales of approximately 80 min.
      PubDate: 2015-01-07T00:00:00+01:00
  • Internally and externally induced deformations of the magnetospheric
           equatorial current as inferred from spacecraft data

    • Abstract: Internally and externally induced deformations of the magnetospheric equatorial current as inferred from spacecraft data

      Annales Geophysicae, 33, 1-11, 2015

      Author(s): N. A. Tsyganenko, V. A. Andreeva, and E. I. Gordeev

      Based on a data pool of 79 yearly files of space magnetometer data by Polar, Cluster, Geotail, and THEMIS satellites between 1995 and 2013, we developed a new quantitative model of the global shape of the magnetospheric equatorial current sheet as a function of the Earth's dipole tilt angle, solar wind ram pressure, and interplanetary magnetic field (IMF). This work upgrades and generalizes an earlier model of Tsyganenko and Fairfield (2004) by extending the modeling region to all local times, including the dayside sector. In particular, an essential feature of the new model is the bowl-shaped tilt-related deformation of the equatorial surface of minimum magnetic field, similar to that observed at Saturn, whose existence in the Earth's magnetosphere has been demonstrated in our recent work (Tsyganenko and Andreeva, 2014).
      PubDate: 2015-01-06T00:00:00+01:00
  • TID characterised using joint effort of incoherent scatter radar and GPS

    • Abstract: TID characterised using joint effort of incoherent scatter radar and GPS

      Annales Geophysicae, 32, 1511-1532, 2014

      Author(s): M. van de Kamp, D. Pokhotelov, and K. Kauristie

      Travelling Ionospheric Disturbances (TIDs), which are caused by Atmospheric Gravity Waves (AGWs), are detected and characterised by a joint analysis of the results of two measurement techniques: incoherent scatter radar and multiple-receiver GPS measurements. Both techniques to measure TIDs are already well known, but are developed further in this study, and the strengths of the two are combined, in order to obtain semi-automatic tools for objective TID detection. The incoherent scatter radar provides a good vertical range and resolution and the GPS measurements provide a good horizontal range and resolution, while both have a good temporal resolution. Using the combination of the methods, the following parameters of the TID can be determined: the time of day when the TID occurs at one location, the period length (or frequency), the vertical phase velocity, the amplitude spectral density, the vertical wavelength, the azimuth angle of horizontal orientation, the horizontal wavelength, and the horizontal phase velocity. This technique will allow a systematic characterisation of AGW-TIDs, which can be useful, among other things, for statistical analyses.

      The presented technique is demonstrated on data of 20 January 2010 using data from the EISCAT incoherent scatter radar in Tromsø and from the SWEPOS GPS network in Sweden. On this day around 07:00–12:00 UT, a medium-scale TID was observed from both data sets simultaneously. The TID had a period length of around 2 h, and its wave propagated southeastward with a horizontal phase velocity of about 67 m s−1 and a wavelength of about 500 km. The TID had its maximum amplitude in Tromsø at 10:00 UT. The period length detected from the GPS results was twice the main period length detected from the radar, indicating a different harmonic of the same wave. The horizontal wavelength and phase velocity are also estimated from the radar results using Hines' theory, using the WKB approximation to account for inhomogeneity of the atmosphere. The results of this estimate are higher than those detected from the GPS data. The most likely explanation for this is that Hines' theory overestimated the values, because the atmosphere was too inhomogeneous even for the WKB approximation to be valid.
      PubDate: 2014-12-17T00:00:00+01:00
  • Correlation studies for B-spline modeled F2 Chapman parameters obtained
           from FORMOSAT-3/COSMIC data

    • Abstract: Correlation studies for B-spline modeled F2 Chapman parameters obtained from FORMOSAT-3/COSMIC data

      Annales Geophysicae, 32, 1533-1545, 2014

      Author(s): M. Limberger, W. Liang, M. Schmidt, D. Dettmering, M. Hernández-Pajares, and U. Hugentobler

      The determination of ionospheric key quantities such as the maximum electron density of the F2 layer NmF2, the corresponding F2 peak height hmF2 and the F2 scale height HF2 are of high relevance in 4-D ionosphere modeling to provide information on the vertical structure of the electron density (Ne). The Ne distribution with respect to height can, for instance, be modeled by the commonly accepted F2 Chapman layer. An adequate and observation driven description of the vertical Ne variation can be obtained from electron density profiles (EDPs) derived by ionospheric radio occultation measurements between GPS and low Earth orbiter (LEO) satellites. For these purposes, the six FORMOSAT-3/COSMIC (F3/C) satellites provide an excellent opportunity to collect EDPs that cover most of the ionospheric region, in particular the F2 layer. For the contents of this paper, F3/C EDPs have been exploited to determine NmF2, hmF2 and HF2 within a regional modeling approach. As mathematical base functions, endpoint-interpolating polynomial B-splines are considered to model the key parameters with respect to longitude, latitude and time. The description of deterministic processes and the verification of this modeling approach have been published previously in Limberger et al. (2013), whereas this paper should be considered as an extension dealing with related correlation studies, a topic to which less attention has been paid in the literature. Relations between the B-spline series coefficients regarding specific key parameters as well as dependencies between the three F2 Chapman key parameters are in the main focus. Dependencies are interpreted from the post-derived correlation matrices as a result of (1) a simulated scenario without data gaps by taking dense, homogenously distributed profiles into account and (2) two real data scenarios on 1 July 2008 and 1 July 2012 including sparsely, inhomogeneously distributed F3/C EDPs. Moderate correlations between hmF2 and HF2 as well as inverse correlations between NmF2 and HF2 are reflected from the simulation. By means of the real data studies, it becomes obvious that the sparse measurement distribution leads to an increased weighting of the prior information and suppresses the parameter correlations which play an important role regarding the parameter estimability. The currently implemented stochastic model is in need of improvement and does not consider stochastic correlations which consequently cannot occur.
      PubDate: 2014-12-17T00:00:00+01:00
  • The numerical simulation on ionospheric perturbations in electric field
           before large earthquakes

    • Abstract: The numerical simulation on ionospheric perturbations in electric field before large earthquakes

      Annales Geophysicae, 32, 1487-1493, 2014

      Author(s): S. F. Zhao, X. M. Zhang, Z. Y. Zhao, and X. H. Shen

      Many observational results have shown electromagnetic abnormality in the ionosphere before large earthquakes. The theoretical simulation can help us to understand the internal mechanism of these anomalous electromagnetic signals resulted from seismic regions. In this paper, the horizontal and vertical components of electric and magnetic field at the topside ionosphere are simulated by using the full wave method that is based on an improved transfer matrix method in the lossy anisotropic horizontally stratified ionosphere. Taken account into two earthquakes with electric field perturbations recorded by the DEMETER satellite, the numerical results reveal that the propagation and penetration of ULF (ultra-low-frequency) electromagnetic waves into the ionosphere is related to the spatial distribution of electron and ion densities at different time and locations, in which the ion density has less effect than electron density on the field intensity. Compared with different frequency signals, the minimum values of electric and magnetic field excited by earthquakes can be detected by satellite in current detection capability have also been calculated, and the lower frequency wave can be detected easier.
      PubDate: 2014-12-09T00:00:00+01:00
  • Energy exchange and wave action conservation for magnetohydrodynamic (MHD)
           waves in a general, slowly varying medium

    • Abstract: Energy exchange and wave action conservation for magnetohydrodynamic (MHD) waves in a general, slowly varying medium

      Annales Geophysicae, 32, 1495-1510, 2014

      Author(s): A. D. M. Walker

      Magnetohydrodynamic (MHD) waves in the solar wind and magnetosphere are propagated in a medium whose velocity is comparable to or greater than the wave velocity and which varies in both space and time. In the approximation where the scales of the time and space variation are long compared with the period and wavelength, the ray-tracing equations can be generalized and then include an additional first-order differential equation that determines the variation of frequency. In such circumstances the wave can exchange energy with the background: wave energy is not conserved. In such processes the wave action theorem shows that the wave action, defined as the ratio of the wave energy to the frequency in the local rest frame, is conserved. In this paper we discuss ray-tracing techniques and the energy exchange relation for MHD waves. We then provide a unified account of how to deal with energy transport by MHD waves in non-uniform media. The wave action theorem is derived directly from the basic MHD equations for sound waves, transverse Alfvén waves, and the fast and slow magnetosonic waves. The techniques described are applied to a number of illustrative cases. These include a sound wave in a medium undergoing a uniform compression, an isotropic Alfvén wave in a steady-state shear layer, and a transverse Alfvén wave in a simple model of the magnetotail undergoing compression. In each case the nature and magnitude of the energy exchange between wave and background is found.
      PubDate: 2014-12-09T00:00:00+01:00
  • On the origin of falling-tone chorus elements in Earth's inner

    • Abstract: On the origin of falling-tone chorus elements in Earth's inner magnetosphere

      Annales Geophysicae, 32, 1477-1485, 2014

      Author(s): H. Breuillard, O. Agapitov, A. Artemyev, V. Krasnoselskikh, O. Le Contel, C. M. Cully, V. Angelopoulos, Y. Zaliznyak, and G. Rolland

      Generation of extremely/very low frequency (ELF/VLF) chorus waves in Earth's inner magnetosphere has received increased attention recently because of their significance for radiation belt dynamics. Though past theoretical and numerical models have demonstrated how rising-tone chorus elements are produced, falling-tone chorus element generation has yet to be explained. Our new model proposes that weak-amplitude falling-tone chorus elements can be generated by magnetospheric reflection of rising-tone elements. Using ray tracing in a realistic plasma model of the inner magnetosphere, we demonstrate that rising-tone elements originating at the magnetic equator propagate to higher latitudes. Upon reflection there, they propagate to lower L-shells and turn into oblique falling tones of reduced power, frequency, and bandwidth relative to their progenitor rising tones. Our results are in good agreement with comprehensive statistical studies of such waves, notably using magnetic field measurements from THEMIS (Time History of Events and Macroscale Interactions during Substorms) spacecraft. Thus, we conclude that the proposed mechanism can be responsible for the generation of weak-amplitude falling-tone chorus emissions.
      PubDate: 2014-12-08T00:00:00+01:00
  • Correlation of very low and low frequency signal variations at
           mid-latitudes with magnetic activity and outer-zone particles

    • Abstract: Correlation of very low and low frequency signal variations at mid-latitudes with magnetic activity and outer-zone particles

      Annales Geophysicae, 32, 1455-1462, 2014

      Author(s): A. Rozhnoi, M. Solovieva, V. Fedun, M. Hayakawa, K. Schwingenschuh, and B. Levin

      The disturbances of very low and low frequency signals in the lower mid-latitude ionosphere caused by magnetic storms, proton bursts and relativistic electron fluxes are investigated on the basis of VLF–LF measurements obtained in the Far East and European networks. We have found that magnetic storm (−150 < Dst < −100 nT) influence is not strong on variations of VLF–LF signals. The anomalies with negative amplitude were registered during the main and recovery phases for several magnetic storms (mainly for three northernmost paths). The correlation between VLF–LF signals and geomagnetic activity is rather weak even for these paths (≈ 12–18%). Also, the correlation between magnetic activity and VLF signal variations recorded onboard the DEMETER satellite is not found. The significant influence of outer-zone particles (energetic particle sensor on board/Geostationary Operational Environmental Satellite (GOES) measurements) on the VLF–LF signal variations is found for almost half of the sub-ionospheric paths.
      PubDate: 2014-12-04T00:00:00+01:00
  • A possible influence of the Great White Spot on Saturn kilometric
           radiation periodicity

    • Abstract: A possible influence of the Great White Spot on Saturn kilometric radiation periodicity

      Annales Geophysicae, 32, 1463-1476, 2014

      Author(s): G. Fischer, S.-Y. Ye, J. B. Groene, A. P. Ingersoll, K. M. Sayanagi, J. D. Menietti, W. S. Kurth, and D. A. Gurnett

      The periodicity of Saturn kilometric radiation (SKR) varies with time, and its two periods during the first 5 years of the Cassini mission have been attributed to SKR from the northern and southern hemisphere. After Saturn equinox in August 2009, there were long intervals of time (March 2010 to February 2011 and September 2011 to June 2012) with similar northern and southern SKR periods and locked SKR phases. However, from March to August 2011 the SKR periods were split up again, and the phases were unlocked. In this time interval, the southern SKR period slowed down by ~ 0.5% on average, and there was a large jump back to a faster period in August 2011. The northern SKR period speeded up and coalesced again with the southern period in September 2011. We argue that this unusual behavior could be related to the so-called Great White Spot (GWS), a giant thunderstorm that raged in Saturn's atmosphere around that time. For several months in 2011, the visible head of the GWS had the same period of ~ 10.69 h as the main southern SKR modulation signal. The GWS was most likely a source of intense gravity waves that may have caused a global change in Saturn's thermospheric winds via energy and momentum deposition. This would support the theory that Saturn's magnetospheric periodicities are driven by the upper atmosphere. Since the GWS with simultaneous SKR periodicity measurements have only been made once, it is difficult to prove a physical connection between these two phenomena, but we provide plausible mechanisms by which the GWS might modify the SKR periods.
      PubDate: 2014-12-04T00:00:00+01:00
  • A comparison between VEGA 1, 2 and Giotto flybys of comet 1P/Halley:
           implications for Rosetta

    • Abstract: A comparison between VEGA 1, 2 and Giotto flybys of comet 1P/Halley: implications for Rosetta

      Annales Geophysicae, 32, 1441-1453, 2014

      Author(s): M. Volwerk, K.-H. Glassmeier, M. Delva, D. Schmid, C. Koenders, I. Richter, and K. Szegö

      Three flybys of comet 1P/Halley, by VEGA 1, 2 and Giotto, are investigated with respect to the occurrence of mirror mode waves in the cometosheath and field line draping in the magnetic pile-up region around the nucleus. The time interval covered by these flybys is approximately 8 days, which is also the approximate length of an orbit or flyby of Rosetta around comet 67P/Churyumov–Gerasimenko. Thus any significant changes observed around Halley are changes that might occur for Rosetta during one pass of 67P/CG. It is found that the occurrence of mirror mode waves in the cometosheath is strongly influenced by the dynamical pressure of the solar wind and the outgassing rate of the comet. Field line draping happens in the magnetic pile-up region. Changes in nested draping regions (i.e. regions with different Bx directions) can occur within a few days, possibly influenced by changes in the outgassing rate of the comet and thereby the conductivity of the cometary ionosphere.
      PubDate: 2014-11-28T00:00:00+01:00
  • Sporadic E layer at mid-latitudes: average properties and influence of
           atmospheric tides

    • Abstract: Sporadic E layer at mid-latitudes: average properties and influence of atmospheric tides

      Annales Geophysicae, 32, 1427-1440, 2014

      Author(s): A. Pignalberi, M. Pezzopane, and E. Zuccheretti

      This paper describes a study of the daily variability shown by the main characteristics of the sporadic E (Es) layer, that is the top frequency (ftEs) and the lowest virtual height (h'Es). The study is based on ionograms recorded by the Advanced Ionospheric Sounder by the Istituto Nazionale di Geofisica e Vulcanologia (AIS-INGV) ionosondes installed in the ionospheric stations at Rome (41.8° N, 12.5° E) and Gibilmanna (37.9° N, 14.0° E), Italy, during the summer (June, July, August and September) of 2013, a year falling in the ascending phase of solar cycle 24. The ftEs presents a diurnal variation characterized by two maxima, the first around noon is very well defined and the second in the evening/night is much less defined; the amplitude of both maxima decreases from June to September accompanied by a general decrease of the ftEs values which is more pronounced in the daytime than in the nighttime. h'Es also presents a diurnal variation characterized by two maxima but, unlike ftEs, these present the same amplitude which is independent from the considered month. Assuming that both ftEs and h'Es trends are influenced by the atmospheric tides, the height–time–intensity (HTI) technique was applied to deeply investigate how these waves control the Es dynamics. The HTI study, along with a fast Fourier transform analysis, show that a well-defined semidiurnal periodicity characterizes the Es layer dynamics most accurately in June and July, while in August and September the daytime semidiurnal periodicity becomes weaker and the role of the diurnal periodicity is consequently highlighted.
      PubDate: 2014-11-21T00:00:00+01:00
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