bluuebunny/crossref_metadata_2025_split huggingface dataset:
Three-dimensional non-linear instability of spontaneous fast magnetic reconnection
Authors: Shimizu T. , Kondoh K. , Ugai M. 
Published: August 2014

Abstract
Three-dimensional instability of spontaneous fast magnetic reconnection is studied using MHD (magnetohydro- dynamic) simulation. Previous two-dimensional MHD studies have demonstrated that, if a current-driven anomalous resistivity is assumed, two-dimensional fast magnetic reconnection occurs and two-dimensional largescale magnetic loops, i.e., plasmoids, are ejected from the reconnection region. In most two-dimensional MHD studies, the structure of the current sheet is initially one-dimensinal. On the other hand, in recent space plasma observations, fully three-dimensional magnetic loops frequently appear even in the almost one-dimensional current sheet. This suggests that the classical two-dimensional fast magnetic reconnection may be unstable to any three-dimensional perturbation, resulting in three-dimensional fast magnetic reconnection. In this paper, we show that a three-dimensional resistive perturbation destabilizes two-dimensional fast magnetic reconnection and results in three-dimensional fast magnetic reconnection. The resulting three-dimensional fast reconnection repeatedly ejects three-dimensional magnetic loops downstream. The obtained numerical results are similar to the pulsating downflows observed in solar flares. According to the Fourier analysis of the ejected magnetic loops, the time evolution of this three-dimensional instability is fully non-linear.
Development of the global simulation model of the heliosphere
Authors: Kamei Satomi , Nakamizo Aoi , Tanaka Takashi , Obara Takahiro , Shimazu Hironori 
Published: August 2014

Abstract
The heliospheric structure ranging from the solar surface to the earth’s orbit is self-consistently reproduced from a time-stationary three-dimensional (3D) magnetohydrodynamic (MHD) simulation. The simulation model incorporates gravity, Coriolis, and centrifugal forces into the momentum equation, and coronal heating and field-aligned thermal conduction into the energy equation. The heating term in the present model has its peak at 2.8 solar radius (R
s) and exponentially falls to zero at greater distance from the solar surface. The absolute value of heating depends on the topology of the solar magnetic field so as to be in inverse proportion with the magnetic expansion factor. The results of the simulation simultaneously reproduce the plasma-exit structure on the solar surface, the high-temperature region in the corona, the open- and closed-magnetic-field structures in the corona, the fast and slow streams of the solar wind, and the sector structure in the heliosphere.
Proxy for the ionospheric peak plasma density reduced by the solar zenith angle
Authors: Gulyaeva T. L. 
Published: August 2014

Abstract
The F2 layer peak plasma density N
m
F
2 is reduced by the factor constructed from the relative changes in the Sun’s zenith angle ξ for a particular local time and the local noon value χ0. Proposed transformation yields a proxy for the peak plasma density which coincides with the source observation at noon but apart from the latter is gradually reduced towards the night. Hourly observations at 8 ground based ionosondes for the solar maximum (2000) and minimum (2006) are analyzed for inter-stations and inter-seasonal correlation of the peak plasma density and the proxy values. The proxy values show improved correlation between the data at different locations and improved inter-seasonal correlations for a particular location due to greater homogeneity of results throughout the year contributing to improved evaluation of the ionospheric weather indices.
A new approach to the hourly mean computation problem when dealing with missing data
Authors: Marsal S. , Curto J. J. 
Published: August 2014

Abstract
Geomagnetic observatory records are unavoidably affected by primary data interruptions which, in turn, may have possible effects on the accuracy of the definitive data derived from them. One of the products most widely used by the scientific community is the mean hourly values, immediately obtained from the primary minute values of the geomagnetic field. Although some precepts have already been proposed and used, a definitive criterion regarding the procedure to follow when dealing with missing data has not yet been established. This could be seen in the last IAGA meetings and workshops, where several constructive opinions were put forward in this respect. The present discussion is devoted to analyzing the effects that different amounts of missing data have upon the accuracy of the means, a necessary step before establishing a definitive rule as to how to deal with these situations. In this statistical approach, we propose a new criterion based on the relative value of the root mean square error (between actual and computed means) with respect to the natural magnetic field variations of the original hourly interval.
Co-seismic EM signals in magnetotelluric measurement—a case study during Bhuj earthquake (26th January 2001), India
Authors: Azeez K. K. Abdul , Manoj C. , Veeraswamy K. , Harinarayana T. 
Published: August 2014

Abstract
Significant changes in amplitude and frequency characteristics were observed in the magnetotelluric (MT) time series recorded during Bhuj earthquake (∼7.6 M
w), at a site ∼350 km from the epicenter. The telluric and magnetic signals recorded in the frequency range (10−1-101 Hz) of MT spectrum show considerable variations in their spectral characteristics during the earthquake event compared to the data recorded before and after the earthquake. The spectral analysis brings out sharp changes in amplitude of low-frequency signals during the earthquake as compared to the typical flat spectrum observed before and after the earthquake. The wavelet analysis of the electric and magnetic field data reveals two different spectral regimes; (1) the flat spectrum related to the natural MT signals, and (2) localized, high amplitude signals (in time and frequency) related to the onset of main shock. Three more high amplitude events are noted in the wavelet spectrum, after the main shock event, and can be speculated to be associated with the after-shock events. The MT impedance estimates clearly show scattered apparent resistivity and phase values during the earthquake suggesting that the high amplitude electric and magnetic signals were not related by an MT transfer function. The MT impedance estimates made before and after the earthquake are strikingly normal and smooth. The Pearson’s correlation coefficients between the orthogonal electric and magnetic fields show a drastic drop for the data measured during earthquake, while the MT fields recorded before and after the earthquake are well correlated. The observed MT signals during the seismic activity do not show any external geomagnetic origin and may be attributed to co-seismic EM phenomena. The probable mechanisms responsible for the co-seismic EM phenomena could be electro-kinetic and seismic dynamo effects.
Characteristics of gravity waves observed with intensive radiosonde campaign during November–December 2005 over western Sumatera
Authors: Ratnam M. Venkat , Alexander Simon P. , Kozu T. , Tsuda T. 
Published: August 2014

Abstract
Characteristics of gravity waves are studied using radiosonde campaign conducted during November–December 2005 at Koto Tabang (KT, 0.2°S, 100.32°E). Intensive sounding with hourly launches was also conducted on seven days to study the characteristics of short period (2–6 hours) waves along with EAR data. Gravity waves with period (τ) of 3 hours and vertical wavelength (λ
z
) of 10 km seemed to be generated due to localized convection around KT, which is inferred from X-band Doppler radar. The energy of the gravity wave with period of 2–3 days and vertical wavelength of 3–5 km is largest between 15 and 20 km and 25 and 30 km. We also report the comparison of the wave activity and its interaction with background wind between the three campaigns (CPEA-I, CPEA-II and Nov. 2002). Most of the time waves are propagating towards east and the source of gravity waves is strongly related to the slowly eastward-advecting tropospheric convection, implying that the wave activity was generated at far distant sources located west of KT. A key finding of this study is neither short period nor long-period gravity waves are generated during stationary type of convection. The change in the propagation direction of the short period waves within the event is observed which is not expected.
Local differences in great magnetic storms observed at middle and low latitudes
Authors: Li Qi , Gao Yufen , Wang Jianjun , Han De-Sheng 
Published: August 2014

Abstract
The dependence of the storm-time amplitude on longitude and latitude was analyzed by statistically investigating great magnetic storms observed at different observatories. First, we compared the storm-time ranges observed at Beijing Observatory (BJI) and San Juan Observatory (SJG) to reveal their longitudinal dependence. It was found that the difference between BJI and SJG could be fitted by the 4-order Fourier series approximation, and the storm-time H ranges at dawn were less than those at dusk. Second, we carried out a case study of two typical storms, and analyzed the focused local time when peak storm-time H ranges occurred. The results confirmed the above conclusion. Third, a statistical study was conducted for all magnetic storms observed at the 120°E magnetic chain in eastern China for 1995–2004 to reveal the latitudinal dependence of storm-time ranges. It was found the relationship depended on the activity of analyzed storms and became complicated for giant storms with Dst ≤ −300 nT.
Monitoring the global-scale winter anomaly of total electron contents using GPS data
Authors: Huo X. L. , Yuan Y. B. , Ou J. K. , Zhang K. F. , Bailey G. J. 
Published: August 2014

Abstract
The winter anomaly phenomenon of Total Electron Contents (TEC) at latitudes 15°–60°N and 15°S–60°S is presented using GPS carrier-phase data obtained from GPS stations during 2002. The correlation between the [O/N2] ratio estimated using the NRLMSISE-00 atmospheric model and the TEC winter anomaly is also investigated. The numerical results show that the TEC winter anomaly in different regions of the world tends to be dominated by different factors. In North America, the TEC winter anomaly is strongly affected by the magnetospheric processes in high latitudes and the [O/N2] ratio. In the Euro-Africa and Russia-Asia regions, the TEC winter anomaly depends mainly on the [O/N2] ratio at the latitude band of 30°–60°N, and the extent of the TEC winter anomaly gradually decreases from 60°N to 30°N. The extent of the TEC winter anomaly increases at the latitude band of 15°–30°N due to the influence of the meridional neutral wind and the seasonal changes of the subsolar point. However, the TEC winter anomaly was not observed in southern hemisphere in 2002. The TEC equinoctial asymmetries in the northern and southern hemisphere are also presented using GPS TEC values collected in March and September 2002.
Long-term prediction of solar and geomagnetic activity daily time series using singular spectrum analysis and fuzzy descriptor models
Authors: Mirmomeni Masoud , Kamaliha E. , Shafiee Masoud , Lucas Caro 
Published: August 2014

AbstractOf the various conditions that affect space weather, Sun-driven phenomena are the most dominant. Cyclic solar activity has a significant effect on the Earth, its climate, satellites, and space missions. In recent years, space weather hazards have become a major area of investigation, especially due to the advent of satellite technology. As such, the design of reliable alerting and warning systems is of utmost importance, and international collaboration is needed to develop accurate short-term and long-term prediction methodologies. Several methods have been proposed and implemented for the prediction of solar and geomagnetic activity indices, but problems in predicting the exact time and magnitude of such catastrophic events still remain. There are, however, descriptor systems that describe a wider class of systems, including physical models and non-dynamic constraints. It is well known that the descriptor system is much tighter than the state-space expression for representing real independent parametric perturbations. In addition, the fuzzy descriptor models as a generalization of the locally linear neurofuzzy models are general forms that can be trained by constructive intuitive learning algorithms. Here, we propose a combined model based on fuzzy descriptor models and singular spectrum analysis (SSA) (FD/SSA) to forecast a number of geomagnetic activity indices in a manner that optimizes a fuzzy descriptor model for each of the principal components obtained from singular spectrum analysis and recombines the predicted values so as to transform the geomagnetic activity time series into natural chaotic phenomena. The method has been applied to predict two solar and geomagnetic activity indices: geomagneticaaand solar wind speed (SWS) of the solar wind index. The results demonstrate the higher power of the proposed method—compared to other methods—for predicting solar activity.
Influences of Venus’ topography on fully developed superrotation and near-surface flow
Authors: Yamamoto Masaru , Takahashi Masaaki 
Published: August 2014

Abstract
We investigate the influence of topography on Venus’ atmospheric general circulation. Based on comparative simulations with and without the Venusian topography, we elucidate the role of the topography in the fully developed superrotation. Orographically forced stationary waves are predominant over Mt. Maxwell: slightly weakening the superrotation near the cloud top. Differently from previous GCM results, the orographically forced waves do not produce significant asymmetry between the northern and southern hemispheric superrotations in the present model. Weak surface flows from mountains to lowlands are caused by the pressure dependence of the Newtonian cooling. The pattern and magnitude of the near-surface flow are largely different from those simulated in the Herrnstein and Dowling (2007) model. This implies that the parameterizations of physical processes (such as Newtonian cooling, turbulence, diffusion, and surface drag) and the model resolution could significantly influence the pattern and magnitude of the near-surface flow and the orographical forcing of planetary-scale stationary waves.
Fuzzy mathematical model for the analysis of geomagnetic field data
Authors: Sridharan M. 
Published: August 2014

Abstract
The Indian network of magnetometers provides an opportunity to examine the pattern of geomagnetic field variations during magnetic storms. In this study, fuzzy transitive closure analysis, which is a powerful technique for pattern recognition, has been employed. The pattern of variation differs at the nonequatorial, equatorial and the observatory situated nearer to the geomagnetic S
q focus. The results of the analysis are compared with those of classical cluster analysis. The comparison confirms the validity of applying this model for the analysis of geomagnetic storms. The superiority of fuzzy concepts over the conventional method and the analytical techniques are presented here.
Deterministic and probabilistic seismic hazard analyses in Thailand and adjacent areas using active fault data
Authors: Pailoplee Santi , Sugiyama Yuichi , Charusiri Punya 
Published: August 2014

AbstractSeismic hazards in Thailand and adjacent areas were analyzed mainly on the basis of geological fault data. We identified 55 active fault zones using remote-sensing data on earthquake source parameters derived from both active fault data and earthquake catalogues. We selected strong ground-motion attenuation models by comparing the application of several candidate models with strong ground-motion data recorded in Thailand. Both deterministic (DSHA) and probabilistic (PSHA) approaches were used—DSHA for the design of critical construction and PSHA for the design of non-critical construction. We also applied two frequency-magnitude models in the PSHA approach: the exponential magnitude distribution model and the characteristic earthquake model. The seismic hazard results obtained using the deterministic and probabilistic approaches are not equivalent. The resulting DSHA map reveals extremely high seismic hazard levels in some areas of Thailand and in surrounding countries, while the PSHA map reveals a seismic hazard distribution similar to that of the DSHA but with lower seismic hazard levels. The areas of high seismic hazard include countries neighboring Thailand, such as Myanmar, Laos, Vietnam, and Indonesia (Sumatra Island), and areas within Thailand itself, primarily those areas in northern, western, and southern Thailand that are dominated by active fault zones.
Envelope syntheses of cylindrical vector-waves in 2-D random elastic media based on the Markov approximation
Authors: Sato Haruo , Korn Michael 
Published: August 2014

Abstract
Seismograms of microearthquakes are complex; however, their envelopes broaden as the travel distance increases. P-waves are recorded in transverse components, S-waves are recorded in the longitudinal component, and waves are observed at sites even in the nodal direction of the source radiation. These phenomena, which are typically found in short-period seismograms, can be interpreted to be the result of scattering due to lithospheric inhomogeneity. We report here our study of a simple statistical model in which the propagation of waves radiated from a point source in two-dimensional (2-D) random elastic media is characterized by a Gaussian autocorrelation function. For the case that the wavelength is shorter than the correlation distance, two methods based on the Markov approximation are introduced for the direct synthesis of vector wave envelopes. One is to analytically solve the stochastic equation for the two-frequency mutual coherence function; the validity of the solution is confirmed by using finite difference simulations. The second is to numerically solve the stochastic equation for the mutual coherence function. The two methods are equivalent, but the latter is applicable to nonisotropic source radiation. For the case of a point shear dislocation source, a peak delay from the onset and a smoothly decaying tail are found to be common to synthesized envelopes in all azimuths. Scattered waves are excited even at receivers in the nodal direction, and amplitudes become independent of the radiation pattern as lapse time increases.
High-resolution bathymetry using Alvin scanning sonar at the Southern East Pacific Rise and its implication to the formation of collapsed lava lakes
Authors: Tanaka Akiko , Rosat Severine , Kisimoto Kiyoyuki , Urabe Tetsuro 
Published: August 2014

Abstract
A high-resolution bathymetric map of a collapsed lava lake on the ridge crest of the superfast-spreading Southern East Pacific Rise was constructed. The data were acquired during the MOAI’98 Cruise using a pencil-beam scanning sonar that was installed on the submersible Alvin. The map covers an area of 200 × 350 m at the Oasis hydrothermal site (17°25.4′S, 113°12.3′W) where low-temperature fluids are venting from a collapsed lava lake. The collapsed lava lake is 250 m long, with a mean width of 10 m in parallel to the spreading axis on the western flank of the ridge crest. The estimated volume of lava that has drained out is about 7000 m3 erupted from a single event. This small amount of melt and frequent contacts between “young” and “younger” flows indicate that volcanism at the superfast-spreading ridge system is characterized by frequent eruptions of very small volumes of magma.
Determining the parameters of ionospheric perturbation caused by earthquakes using the quasi-optimum algorithm of spatiotemporal processing of TEC measurements
Authors: Kiryushkin Vladislav V. , Afraimovich Edward L. 
Published: August 2014

Abstract
We present the quasi-optimum algorithm of localization of a source of coseismic ionospheric perturbation based on GPS receivers’ network data processing. The initial data of the algorithm are the series of variations of ionospheric total electron content (TEC), reconstructed using the measurements of the phase delays of GPS signals. In order to select the TEC increments caused by ionospheric perturbation due to the earthquake, the TEC series are filtered using a special procedure. The algorithm realizes coherent summation of all TEC series of GPS array reasoning from the maximization of the energy of the total signal of the ionospheric response to earthquake. The quasi-optimum algorithm allows determination of the perturbation propagation velocity as well as of the coordinates, height and “switch-on” time of a source of coseismic ionospheric disturbance without prior information about the perturbation form and the site and time of the main shock of earthquake. We used the algorithm for measuring the parameters of ionospheric perturbations which accompanied the earthquake in the vicinity of Hokkaido Island on September 25, 2003 and the earthquake near the south coast of Honshu Island on September 5, 2004. The results of these experiments show the high accuracy of the perturbation source coordinates estimation (33 km and 27 km respectively with reference of the earthquakes epicenters). The estimations of perturbations propagation velocity (820 ± 60 m/s and 460 ± 40 m/s), heights (340 ± 80 km and 370 ± 130 km) and “switch-on” delay (346 s and 507 s) of a source of the perturbation obtained in both experiments are in agreement with a theory according to which coseismic atmospheric disturbance propagates within a narrow cone of zenith angles up to ionospheric heights and then diverges in the form of a spherical wave with the radial velocity close to the speed of sound at these heights. It is also in agreement with the results of earlier researches.
Evolution of source characteristics of AE events during frictional sliding
Authors: Yabe Yasuo 
Published: August 2014

AbstractThe evolution of source characteristics of acoustic emission (AE) events and the surface topography of the fault surface during frictional sliding were investigated with the aim of improving current understanding of the microscopic process of friction. Both the AE source dimensions and the grain-scale topographies were well conserved against abrasion during the sliding, but the magnitude of stress drop was significantly reduced. The fractal-domain topographies, the wavelength of which is shorter than the fractal limit wavelength (λc), were significantly worn during the sliding. These results suggest that the grain-scale topography determined the AE source dimension, while the fractal-domain asperities controlled the magnitude of the stress drop. Since elastic wave radiation is one of the major energy consumption processes, the grain-scale and the fractal-domain topographies may play different roles, not only in the AE source process but also in the friction of rocks.