bluuebunny/crossref_metadata_2025_split huggingface dataset:
Asymmetry of magnetosheath flows and magnetopause shape during low Alfvén Mach number solar wind
Authors: Lavraud B. , Larroque E. , Budnik E. , Génot V. , Borovsky J. E. , Dunlop M. W. , Foullon C. , Hasegawa H. , Jacquey C. , Nykyri K. , Ruffenach A. , Taylor M. G. G. T. , Dandouras I. , Rème H. 
Published: April 2013

AbstractPrevious works have emphasized the significant influence of the solar wind Alfvén Mach number (MA) on magnetospheric dynamics. Here we report statistical, observational results that pertain to changes in the magnetosheath flow distribution and magnetopause shape as a function of solar wind MA and interplanetary magnetic field (IMF) clock angle orientation. We use all Cluster 1 data in the magnetosheath during the period 2001–2010, using an appropriate spatial superposition procedure, to produce magnetosheath flow distributions as a function of location in the magnetosheath relative to the IMF and other parameters. The results demonstrate that enhanced flows in the magnetosheath are expected at locations quasi‐perpendicular to the IMF direction in the plane perpendicular to the Sun‐Earth line; in other words, for the special case of a northward IMF, enhanced flows are observed on the dawn and dusk flanks of the magnetosphere, while much lower flows are observed above the poles. The largest flows are adjacent to the magnetopause. Using appropriate magnetopause crossing lists (for both high and low MA), we also investigate the changes in magnetopause shape as a function of solar wind MA and IMF orientation. Comparing observed magnetopause crossings with predicted positions from an axisymmetric semi‐empirical model, we statistically show that the magnetopause is generally circular during high MA, while is it elongated (albeit with moderate statistical significance) along the direction of the IMF during low MA. These findings are consistent with enhanced magnetic forces that prevail in the magnetosheath during low MA. The component of the magnetic forces parallel to the magnetopause produces the enhanced flows along and adjacent to the magnetopause, while the component normal to the magnetopause exerts an asymmetric pressure on the magnetopause that deforms it into an elongated shape.
Direct detection of wavelike spatial structure at the bottom of the F region and its role on the formation of equatorial plasma bubble
Authors: Patra A. K. , Taori A. , Chaitanya P. P. , Sripathi S. 
Published: April 2013

AbstractWe present hitherto unrevealed wavelike spatial structures, as seen in the 630 nm airglow intensity using a scanning photometer, prior to the occurrence of equatorial plasma bubble (EPB). The dominant wavelengths in the spatial variation of airglow intensity have excellent correspondence with the spatial separations of periodic plume structures in the radar observations, and the amplitudes of the wave perturbations (6–12%) are significantly more than the commonly quoted 5% seed perturbation used in simulation studies. In contrast, no such airglow structure was observed on EPB free evening. Concurrent ionosonde observations made from the magnetic equator, although displayed satellite trace on some occasions, did not show one‐to‐one correspondence between the occurrence of satellite trace and EPB, a finding that is inconsistent with a recent suggestion that satellite trace can be used as a precursor of EPB occurrence. The new findings are discussed in the light of current understanding of the day‐to‐day variability in EPB including its prediction.
Midlatitude propagation of VLF to MF waves through nighttime ionosphere above powerful VLF transmitters
Authors: Lefeuvre F. , Pinçon J. L. , Parrot M. 
Published: April 2013

AbstractMidlatitude nighttime observations made by the DEMETER satellite in the very low frequency (VLF) to medium frequency (MF) bands (3 kHz to 3 MHz) have demonstrated the propagation of radio waves from the bottom of ionosphere up to the satellite altitude (~700 km). Propagation characteristics derived from the magneto‐ionic theory [Budden, ] are used to explain the absence of wave observations between ~1 and 2 MHz. Under hypotheses made for the Appleton and Hartree (or Appleton and Lassen) formula, studies of the vertical variations of the real and imaginary parts of the refractive index are performed to point out modifications in the propagation characteristics of the waves: (i) at the crossing of the plasma cutoffs regions, (ii) at the crossing of the ordinary and extraordinary mode resonance regions, and (iii) in the region where the product of the collision frequency (ν) and the electronic density (Ne) is maximum. It is shown that enhancements in the collision frequencies, produced by powerful VLF transmitters in the region where the product of ν and Ne is maximum, open the half angle of the MF wave transmission cones and increase the power densities of those waves at the DEMETER altitude.
Low‐latitude Es capable of controlling the onset of equatorial spread F
Authors: Joshi L. M. , Patra A. K. , Rao S. V. B. 
Published: April 2013

AbstractTo investigate their role on the occurrence of equatorial spread F (ESF), the low‐latitude E region in the evening hours has been characterized in terms of sporadic E (Es) activity based on ionosonde observations from Sriharikota, a low‐latitude station in India. While prior to the sunset, either Chapman type E layer, or Es, or a combination of the two was observed, after sunset, when observed, the E layer was exclusively Es. Es, however, could be either blanketing or nonblanketing type. Observations revealed that while the occurrence of ESF had a broad relationship with the type of Es, a detailed investigation suggested that (1) ESF occurred (did not occur) irrespective of Es type if the virtual height of the F layer base (h'F) was greater than ~360 km (less than ~290 km), and (2) ESF occurrence depended strongly on the type of Es if 290 km < h'F < 360 km. Model computation shows that low‐latitude Es can significantly influence the growth of the Rayleigh‐Taylor instability for the cases corresponding to category 2 by increasing the total field‐line integrated Pedersen conductivity, consistent with the observations. The results presented in this paper highlight under what circumstances the low‐latitude Es would play a critical role in the occurrence of ESF, which can be used to understand the day‐to‐day variability of ESF.
Mercury's magnetopause and bow shock from MESSENGER Magnetometer observations
Authors: Winslow Reka M. , Anderson Brian J. , Johnson Catherine L. , Slavin James A. , Korth Haje , Purucker Michael E. , Baker Daniel N. , Solomon Sean C. 
Published: April 2013

AbstractWe have established the average shape and location of Mercury's magnetopause and bow shock from orbital observations by the MESSENGER Magnetometer. We fit empirical models to midpoints of boundary crossings and probability density maps of the magnetopause and bow shock positions. The magnetopause was fit by a surface for which the position R from the planetary dipole varies as [1 + cos(θ)]−α, where θ is the angle between R and the dipole‐Sun line, the subsolar standoff distance Rss is 1.45 RM (where RM is Mercury's radius), and the flaring parameter α = 0.5. The average magnetopause shape and location were determined under a mean solar wind ram pressure PRam of 14.3 nPa. The best fit bow shock shape established under an average Alfvén Mach number (MA) of 6.6 is described by a hyperboloid having Rss = 1.96 RM and an eccentricity of 1.02. These boundaries move as PRam and MA vary, but their shapes remain unchanged. The magnetopause Rss varies from 1.55 to 1.35 RM for PRam in the range of 8.8–21.6 nPa. The bow shock Rss varies from 2.29 to 1.89 RM for MA in the range of 4.12–11.8. The boundaries are well approximated by figures of revolution. Additional quantifiable effects of the interplanetary magnetic field are masked by the large dynamic variability of these boundaries. The magnetotail surface is nearly cylindrical, with a radius of ~2.7 RM at a distance of 3 RM downstream of Mercury. By comparison, Earth's magnetotail flaring continues until a downstream distance of ~10 Rss.
Long‐lasting daytime equatorial plasma bubbles observed by the C/NOFS satellite
Authors: Huang Chao‐Song , de La Beaujardiere O. , Roddy P. A. , Hunton D. E. , Ballenthin J. O. , Hairston M. R. 
Published: April 2013

AbstractWe report the first observations of long‐lasting daytime equatorial plasma bubbles with the Communication/Navigation Outage Forecasting System (C/NOFS) satellite. The most unusual features of the plasma bubbles are the persistence from the post‐midnight sector through the afternoon sector and the extremely long lifetime of 12 h. In one case, the plasma bubbles were generated at 02:00–03:00 LT near the end of the main phase of a moderate magnetic storm and detected by C/NOFS over eight successive orbits, and the decrease of the ion density inside the bubbles was still as large as ~30% at 14:00–15:00 LT. In another case, one group of plasma bubbles was generated near the sunset terminator and existed over the entire nighttime until the post‐sunrise sector (06:00–08:00 LT), and another group of plasma bubbles was first detected at 04:00–06:00 LT and lasted until ~11:00 LT. The latter group of bubbles occurred following a sharp northward turning of the interplanetary magnetic field (IMF) near the end of the main phase of a weak magnetic storm, and the overshielding electric field caused by the IMF northward turning and the storm time disturbance dynamo might both have contributed to the generation of the bubbles. The plasma bubbles reached 800 km or higher in altitude during daytime. The high altitudes may be critical for the long lifetime of the bubbles: the photo‐ionization rate decreases rapidly with altitude. The photo‐ionization process may take a long time to produce enough new plasma particles to fill the daytime bubbles at high altitudes.
Physical mechanisms of the ionospheric storms at equatorial and higher latitudes during the recovery phase of geomagnetic storms
Authors: Balan N. , Otsuka Y. , Nishioka M. , Liu J. Y. , Bailey G. J. 
Published: April 2013

AbstractThe paper studies the physical mechanisms of the ionospheric storms at equatorial and higher latitudes, which are generally opposite both during the main phase (MP) and recovery phase (RP) of geomagnetic storms. The mechanisms are based on the natural tendency of physical systems to occupy minimum energy state which is most stable. The paper first illustrates the recent developments in the understanding of the mechanisms during daytime MPs when generally negative ionospheric storms (in Nmax and TEC) develop at equatorial latitudes and positive storms occur at higher latitudes, including why the storms are severe only in some cases. The paper then investigates the relative importance of the physical mechanisms of the positive ionospheric storms observed at equatorial latitudes (within ±15°) during daytime RPs when negative storms occur at higher latitudes using CHAMP Ne and GPS‐TEC data and Sheffield University Plasmasphere Ionosphere Model. The results indicate that the mechanical effect of the storm‐time equatorward neutral winds that causes plasma convergence at equatorial F region could be a major source for the positive storms, with the downwelling effect of the winds and zero or westward electric field, if present, acting as minor sources.
Low shear velocities in the sub‐lithospheric mantle beneath the Indian shield?
Authors: Kumar M. Ravi , Saikia Dipankar , Singh Arun , Srinagesh D. , Baidya P.R. , Dattatrayam R. S. 
Published: April 2013

AbstractEver since its breakup from the Gondwanaland ~140 Myr ago, the Indian plate was ravaged by four hot spots. Although the surface manifestations of such deep processes are evident in terms of large igneous provinces like the Deccan and the fast drift of the Indian plate, the modifications to the deep structure remain to be grasped. In this study, we investigate the mantle transition zone (TZ) structure beneath the Indian shield region using ~14,000 teleseismic receiver functions from 77 broadband stations sited on diverse geologic terrains. The arrival times of the P‐to‐s (Ps) conversions from the 410 km discontinuity at most cratonic stations appear to be delayed by ~2 s in comparison with the times observed for other Precambrian shield regions like Africa, Australia, and Canada. Such delays in the conversions from the 410 km discontinuity below the Indian shield suggest low shear wave speeds in the lithospheric and sub‐lithospheric mantles due to higher temperatures, together with a thinner high velocity lid that contrasts with a thicker one found beneath most Archean cratons. A thin transition zone beneath most of the cratonic stations lends support to the enhanced temperatures within the TZ itself. Also, a further delay of the TZ discontinuities is observed for stations on the southern granulite terrain, which was under the influence of the Marion plume that is responsible for the separation of Madagascar from India. Although the data do not conclusively show evidence for a 520 km discontinuity, an LVL atop the 410 cannot be ruled out beneath certain geological provinces of the Indian shield.
Real episodic growth of continental crust or artifact of preservation? A 3‐D geodynamic model
Authors: Walzer Uwe , Hendel Roland 
Published: April 2013

AbstractWe investigate whether the observed zircon age distribution of continental crust (CC) is produced by real crustal growth episodes or is only an artifact of preservation. In connection with the second alternative of this question, other authors proposed that there was little episodicity in the production of new CC and that modeling corroborates this opinion. We conclude that a combination of the two answers might be possible. In matters of modeling, however, we ascertain that adynamicmodeling of the convection‐differentiation system of the mantle reveals the high probability of magmatic episodes. We solve the full set of balance equations in a 3‐D spherical‐shell mantle. The heat‐producing elements are redistributed by chemical differentiation. A realistic solidus model of mantle peridotite is essential for an applicable model. The solidus depends not only on depth but also on the volatile concentration. Furthermore, we introduced realistic profiles of Grüneisen parameter, viscosity, adiabatic temperature, thermal expansivity, and specific heat. Our model automatically produces lithospheric plates and growing continents. Regarding number, size, form, distribution, and surface velocity of the continents, no constraints have been prescribed. Regions of the input parameter space (Ra,σy,k,f3) that are favorable with respect to geophysical quantities showsimultaneouslynot only episodicity of CC growth but also a reproduction of the observed zircon‐age maxima referring to the instants of time. We also obtain Archean events for ages greater than 3000 Ma that are not or scarcely visible in the observed zircon ages. Sinusoidal parts of the evolution curve ofqob,Ur, andEkinare superposed with a monotonous decrease. The volumetrically averaged mantle temperature,Tmean, however, decreases smoothly and slowly, nearly without pronounced variations. Therefore, we can dismiss catastrophic mechanisms thatsimultaneouslyincorporate thewholemantle.
Quasi‐periodic slow slip events in the afterslip area of the 1996 Hyuga‐nada earthquakes, Japan
Authors: Yarai Hiroshi , Ozawa Shinzaburo 
Published: April 2013

AbstractThe time evolution of afterslip on a plate boundary experiencing interplate earthquakes is expected to show logarithmic decay. The global positioning system network in Japan has been monitoring transient deformation since the occurrence of two large interplate earthquakes with moment magnitudes of 6.8 and 6.7 in the Hyuga‐nada area, southwest Japan, in 1996. The spatial and temporal evolution of aseismic interplate slip based on crustal deformation data indicates that afterslip followed the two earthquakes and gradually declined to background rates by around 2004 with total moment magnitude of 7.3. However, quasi‐periodic slow slip events suddenly began within the afterslip area in 2005 with approximately one year duration and two‐year recurrence interval. The moment magnitudes of the three slow slip events since January 2005 range from 6.7 to 6.8. This differs greatly from the expected behavior of logarithmic decay over time. Both velocity‐strengthening and velocity‐weakening rate‐and‐state modes have been implicated as the cause of afterslip, whose location is complementary to the main shock area of velocity‐weakening, while a slow slip event occurs in the velocity‐weakening area with different frictional properties from those of an afterslip area. In light of the seemingly different frictional properties, the coexistence of afterslip and slow slip events in the same area would provide additional information about precisely how the plate interface is behaving. The monitoring of these slow slip events should give the clues to understanding the coexistence of long‐term afterslip and slow slip events and the increasing risk of earthquakes in neighboring areas.
Anisotropic amplitude variation of the bottom‐simulating reflector beneath fracture‐filled gas hydrate deposit
Authors: Sriram G. , Dewangan P. , Ramprasad T. , Rama Rao P. 
Published: April 2013

AbstractFor the first time, we report the amplitude variation with angle (AVA) pattern of bottom‐simulating reflectors (BSRs) beneath fracture‐filled gas hydrate deposits when the effective medium is anisotropic. The common depth point (CDP) gathers of two mutually perpendicular multichannel seismic profiles, located in the vicinity of Site NGHP‐01‐10, are appropriately processed such that they are fit for AVA analysis. AVA analysis of the BSR shows normal‐incidence reflection coefficients of −0.04 to −0.11 with positive gradients of 0.04 to 0.31 indicating class IV pattern. The acoustic properties from isotropic rock physics model predict class III AVA pattern which cannot explain the observed class IV AVA pattern in Krishna‐Godavari basin due to the anisotropic nature of fracture‐filled gas hydrate deposits. We modeled the observed class IV AVA of the BSR by assuming that the gas hydrate bearing sediment can be represented by horizontally transversely isotropic (HTI) medium after accounting for anisotropic wave propagation effects on BSR amplitudes. The effective medium properties are estimated using Backus averaging technique, and the AVA pattern of BSRs is modeled using the properties of overlying HTI and underlying isotropy/HTI media with or without free gas. Anisotropic AVA analysis of the BSR from the inline seismic profile shows 5–30% gas hydrate concentration (equivalent to fracture density) and the azimuth of fracture system (fracture orientation) with respect to the seismic profile is close to 45°. Free gas below the base of gas hydrate stability zone is interpreted in the vicinity of fault system (F1).
Kinematics of the Pamir and Hindu Kush regions from GPS geodesy
Authors: Ischuk Anatoli , Bendick Rebecca , Rybin Anatoly , Molnar Peter , Khan Shah Faisal , Kuzikov Sergey , Mohadjer Solmaz , Saydullaev Umed , Ilyasova Zhyra , Schelochkov Gennady , Zubovich Alexander V. 
Published: April 2013

AbstractGPS velocities measured in the Pamir and surrounding regions show a total of ~30 mm/yr of northward relative motion between stable Pakistan and Eurasia. The convergence budget is partitioned into 10–15 mm/yr of localized shortening across the Trans‐Alai Thrust, which bounds the Pamir on the north, consistent with southward subduction of intact lithosphere. Another 10–15 mm/yr of shortening is distributed across the Chitral Himalaya and Hindu Kush, suggesting that Hindu Kush seismicity might be related to northward subduction of Indian lithosphere. Modest shortening at <5 mm/yr occurs north of the Trans‐Alai Thrust, across the South Tien Shan and between the Ferghana Valley and Eurasia. Negligible north‐south shortening occurs within the high Pamir, but as much as 5 mm/yr, and perhaps 10 mm/yr, of east‐west extension occurs within this region. This extension is matched by a comparable amount of east‐west shortening in the Tajik Depression. The localization of shortening to the margins of the Pamir combined with observations of distributed internal extension implies that the east‐west vertically averaged, horizontal compressive normal stress is smaller than the north‐south compressive stress.
On the response of Southern Hemisphere subpolar gyres to climate change in coupled climate models
Authors: Wang Zhaomin 
Published: April 2013

We investigate the responses of the Southern Hemisphere subpolar gyres to projected climate changes over the 21st century by Coupled Model Intercomparison Project Phase 3 and 5 models. Under increased greenhouse gas forcing, the Southern Hemisphere westerly winds consistently become intensified, resulting in increased cyclonic wind forcing in the subpolar region in these models. Under such wind forcing changes, it is a robust feature that there are consistent increases in the westward flow close to the coast of Antarctica, with strong implications to the mass balance of the Antarctic ice shelves and ice sheets. However, there are large discrepancies in the responses of the gyre axes and overall gyre strengths. Some models show equatorward expansions of the southern gyre limbs, resulting in consistent and large gyre strength increases, while some other models show poleward contractions of the gyres and generally small and less consistent gyre strength changes. These uncertainties are primarily a result of the uncertain simulations of eddy‐driven circulations in the Antarctic Circumpolar Current. The associated buoyancy forcing changes play a secondary role in driving these oceanic responses.This study reveals that there are large uncertainties in the projections of the subpolar circulation in the current generation of coupled climate models, although CMIP5 models have considerably smaller inter‐model spreads in the present‐day and projected gyre strengths. To predict the subpolar circulation changes, future improved modelling studies need to particularly reduce the uncertainties in the projections of the westerly jet and to reduce the uncertainties in the eddy‐driven circulation responses to wind forcing changes.
Observations of mixed layer deepening during an Antarctic gale
Authors: Lane‐Serff G. F. , Stansfield K. L. 
Published: April 2013

AbstractObservations of mixed layer deepening made during a gale in February 2005 near an ice shelf, Fimbulisen, Antarctica, are reported. The observations were made from the RRS James Clark Ross in the lee of the ice shelf, using repeated downcasts (“yo‐yo”) of a conductivity‐temperature‐depth package, together with shipboard meteorological and other measurements. The mixed layer deepened from less than 40 m to over 120 m over the course of 27 h, with a very rapid deepening from 80 m to 120 m over a period of under 11 h. The mixed layer became both colder and fresher, with the change in salinity and heat content likely to be caused by melting ice. Oxygen isotope results suggest the source of the fresh water was melting sea ice rather than precipitation or ice shelf melt. The input of melt water at the surface stabilizes the mixed layer, so extra energy is required to deepen the mixed layer. The observations suggest that approximately 1.8% of the available “wind‐work” energy was used to mix the upper water column, while the stabilizing surface buoyancy flux inhibits the turbulence in the mixed layer, limiting the mixing length to 1.6 m. The eventual depth of the mixed layer is in line with estimates based on the planetary length scale u*/f. The rate of mixed layer deepening is given by Ue/u* = 0.035. The apparent peak ice melting rate was approximately 60 mm hr−1, although this is likely to be exaggerated by convergence and downwelling.