bluuebunny/crossref_metadata_2025_split huggingface dataset:
Production of very high potential differences by intracloud lightning discharges in connection with terrestrial gamma ray flashes
Authors: Mallios Sotirios A. , Celestin Sebastien , Pasko Victor P. 
Published: April 2013

AbstractTerrestrial gamma‐ray flashes (TGFs) have been correlated with an early development stage of high altitude positive intracloud (+IC) flashes in which the negative leader propagates up toward the upper positive charge region, while the positive leader propagates down toward the lower negative charge region. The resultant bidirectional leaders develop electrical potential differences in the vicinity of their heads with respect to the ambient potential distribution created by the thundercloud charges. These potential differences are believed to be of essential importance for the generation of TGFs. Using electrostatic calculations and a three‐dimensional Cartesian fractal model, we quantify these potential differences produced in a developing +IC lightning discharge for given thunderstorm electric configurations. We present a case of a +IC lightning discharge in a realistic thunderstorm configuration that leads to a very high (∼300 MV) potential difference and show how a delay in the development of the negative leader with respect to the positive one in a bidirectional leader system can facilitate a high potential difference in the negative leader head region.
Direct observations of the role of convection electric field in the formation of a polar tongue of ionization from storm enhanced density
Authors: Thomas E. G. , Baker J. B. H. , Ruohoniemi J. M. , Clausen L. B. N. , Coster A. J. , Foster J. C. , Erickson P. J. 
Published: April 2013

AbstractWe examine the relationship of convection electric fields to the formation of a polar cap tongue of ionization (TOI) from midlatitude plumes of storm enhanced density (SED). Observations from the geomagnetic storm on 26–27 September 2011 are presented for two distinct SED events. During an hour‐long period of geomagnetic activity driven by a coronal mass ejection, a channel of high‐density F region plasma was transported from the dayside subauroral ionosphere and into the polar cap by enhanced convection electric fields extending to middle latitudes. This TOI feature was associated with enhanced HF backscatter, indicating that it was the seat of active formation of small‐scale irregularities. After the solar wind interplanetary magnetic field conditions quieted and the dayside convection electric fields retreated to higher latitudes, an SED plume was observed extending to, but not entering, the dayside cusp region. This prominent feature in the distribution of total electron content (TEC) persisted for several hours and elongated in magnetic local time with the rotation of the Earth. No ionospheric scatter from SuperDARN radars was observed within this SED region. The source mechanism (enhanced electric fields) previously drawing the plasma from midlatitudes and into the polar cap as a TOI was no longer active, resulting in a fossil feature. We thus demonstrate the controlling role exercised by the convection electric field in generating a TOI from midlatitude SED.
Two models of cross polar cap potential saturation compared: Siscoe‐Hill model versus Kivelson‐Ridley model
Authors: Gao Ye , Kivelson Margaret G. , Walker Raymond J. 
Published: April 2013

AbstractThe cross polar cap potential is considered an instantaneous monitor of the rate at which magnetic flux couples the solar wind to the Earth's magnetosphere‐ionosphere system. Studies have shown that the cross polar cap potential responds linearly to the solar wind electric field under nominal solar wind conditions but asymptotes to the order of 200 kV for large electric field. Saturation of the cross polar cap potential is also found to occur in MHD simulations. Several mechanisms have been proposed to explain this phenomenon. Two well‐developed models are those of Siscoe et al. (2002), herein referred to as the Siscoe‐Hill model, and of Kivelson and Ridley (2008), herein referred to as the Kivelson‐Ridley model. In this study, we compare the mathematical formulas as well as the predictions of the two models with data. We find that the two models predict similar saturation limits. Their difference can be expressed in terms of a factor, which is close to unity during a saturation interval. A survey of the differences in the model predictions show that, on average, the potential of the Kivelson‐Ridley model is smaller than that of the Siscoe‐Hill model by 10 kV. Measurements of AMIE, DMSP, PC index, and SuperDARN are used to differentiate between the two models. However, given the uncertainties of the measurements, it is impossible to conclude that one model does a better job than the other of predicting the observed cross polar cap potentials.
Characteristics of global plasmaspheric TEC in comparison with the ionosphere simultaneously observed by Jason‐1 satellite
Authors: Lee H.‐B. , Jee G. , Kim Y. H. , Shim J. S. 
Published: April 2013

AbstractWe compared the global plasmaspheric total electron content (pTEC) with the ionospheric TEC (iTEC) simultaneously measured by Jason‐1 satellite during the declining phase of solar cycle 23 (2002–2009) to investigate the global morphology of the plasmaspheric density in relation to the ionosphere. Our study showed that the plasmaspheric density structures fundamentally follow the ionosphere, but there are also significant differences between them. Although the diurnal variations are very similar to each region, the plasmasphere shows much weaker variations, only approximately 1 TECU day‐night difference. By analyzing the day‐night differences in the plasmasphere, we found that the plasmaspheric contribution to the nighttime ionosphere does not increase with solar activity and the largest contribution occurs during June solstice. The plasmasphere shows similar seasonal variations to the ionosphere, except for the semiannual variation, which is essentially absent in the plasmasphere. There is also an important difference in the annual variation: although the annual variation in the ionosphere exists regardless of longitude, it occurs only at American sector in the plasmasphere. As solar activity increases to moderate level, the pTEC substantially enhances from approximately 2 to 4 TECU at the initial increase of solar activity below F10.7p = 100 and then quickly slows down while the iTEC almost linearly enhances. Although it is well known that magnetic storms are the major source of plasmaspheric density depletion, pTEC does not show this aspect of the plasmasphere probably due to the relatively small Kp values for high magnetic activity (Kp > 2.5) in the current study.
Dynamics of the foreshock compressional boundary and its connection to foreshock cavities
Authors: Omidi N. , Sibeck D. , Blanco‐Cano X. , Rojas‐Castillo D. , Turner D. , Zhang H. , Kajdič P. 
Published: April 2013

AbstractWe use several global hybrid (kinetic ions, fluid electrons) simulation runs for steady and time‐varying interplanetary magnetic field (IMF) conditions to examine the dynamics of the foreshock compressional boundary (FCB) and its connection to foreshock cavities. The results demonstrate that for steady IMF conditions, the FCB forms and evolves over a long period of time due to the dynamics of the bow shock and ion foreshock. Formation of the FCB is tied to the generation and nonlinear evolution of ULF waves associated with large‐amplitude fluctuations in magnetic field and density within the foreshock. As a result, even during steady IMF conditions, the transitions in the magnetic field strength and direction across an FCB evolve. Although the FCB itself is associated with increases in the magnetic field strength and density, these quantities are reduced on the turbulent side of the FCB as compared to the pristine solar wind. Hybrid simulations with time‐varying IMF have been performed to examine the relationship between the FCB and foreshock cavities generated under two possible scenarios. In the first scenario, a bundle of field lines connects to an otherwise quasi‐perpendicular bow shock and results in the formation of a finite‐sized foreshock region that travels with this bundle of field lines as it connects to different parts of the bow shock surface. Two FCBs bound the traveling foreshock region. In the second scenario, solar wind discontinuities cause the IMF cone angle (angle between the IMF and the solar wind flow direction) to vary and thereby modify the foreshock geometry and the position of the FCB. We demonstrate that structures similar to foreshock cavities bounded by FCBs form in both scenarios. We show that the two scenarios cannot be distinguished based on convecting or nonconvecting FCBs. We also demonstrate that depending on spacecraft location and the nature of the solar wind discontinuities, foreshock cavities may be bounded by an FCB on one side and a foreshock bubble on the other.
An improved dispersion relation for parallel propagating electromagnetic waves in warm plasmas: Application to electron scattering
Authors: Chen Lunjin , Thorne Richard M. , Shprits Yuri , Ni Binbin 
Published: April 2013

AbstractAn improved dispersion relation, with thermal corrections retained, for parallel propagating electromagnetic waves in a warm plasma is developed for both left‒hand (L‒) and right‒hand (R‒) polarized modes. Compared with the cold plasma dispersion relation, the derived dispersion relation is in much better agreement with the full hot plasma dispersion relation (including the wave growth rate). The pitch angle scattering rates of energetic electrons are compared between using cold and full dispersion relation. Significant differences are found when evaluating pitch angle scattering rate of MeV electron caused by He+ band waves in multiple ion plasmas. Due to He+ ion cyclotron absorption, the He+band electromagnetic ion cyclotron waves, which are able to resonate with MeV electrons (at large wave number), tend to be strongly suppressed. Less significant differences in scattering rate of electrons between using cold and hot dispersion relation are found in the case of L‒mode waves in single H+ ion plasma and in the case of R‒mode waves.
Gravity wave coupling between the mesosphere and thermosphere over New Zealand
Authors: Smith S. M. , Vadas S. L. , Baggaley W. J. , Hernandez G. , Baumgardner J. 
Published: April 2013

AbstractAll‐sky images obtained with the Boston University all‐sky imaging system located at the Mount John University Observatory, New Zealand (43.98°S, 170.42°E) show clear evidence of dynamic coupling between the mesosphere and thermosphere. Gravity wave (GW) breaking events in the upper mesosphere at altitudes (z) of 80 to 100 km were observed in the 557.7 nm emission on the evening of 4 March 2009 from 08:40 to 13:50 UT. During this time, unusual oppositely propagating weak northwestward (NW‐ward) and strong southeastward (SE‐ward) GWs were observed in the all‐sky images of the thermospheric atomic oxygen O(1D) 630.0 nm emission at 250 km altitude. The waves appeared to originate from the same location over New Zealand, with phase fronts nearly parallel to the landmass axis of the South Island of New Zealand. Additionally, the southern portion of the wave train ends abruptly at the southern tip of New Zealand. The SE‐ward GWs were stronger and appeared for ~5 h, while the NW‐ward GWs were weaker and only appeared for ~1.75 h. We provide evidence that mountain waves were likely generated in the troposphere that evening. Momentum deposition from GW breaking excites secondary GWs. We model these secondary GWs and show that these GWs have a similar morphology and behavior as observed. Wind filtering in the thermosphere can account for the larger amplitudes and persistent appearance of the SE‐ward GWs and the smaller amplitudes and less persistent appearance of the NW‐ward GWs. Thus, the morphology and behavior of the 630.0 nm GWs suggest that they were secondary GWs generated from mountain wave breaking in the upper mesosphere. We also show that similar SE‐ward GWs have occurred in the 630.0 nm emission on other occasions.
Energetic electron (>10 keV) microburst precipitation, ~5–15 s X‐ray pulsations, chorus, and wave‐particle interactions: A review
Authors: Tsurutani Bruce T. , Lakhina Gurbax S. , Verkhoglyadova Olga P. 
Published: April 2013

AbstractThe fundamental features of ~0.1–0.2 s duration ~0.5 s spaced ionospheric electron precipitation “microbursts,” ~5 to 15 s microburst “trains,” and 5–15 s electron precipitation pulsations are reviewed in light of similar temporal structures of electromagnetic whistler mode “chorus” waves detected in the outer magnetosphere. Past observations of microbursts point to extremely rapid (ms timescale) wave‐particle interactions, probably between lower band chorus subelements (durations of ~10 to 100 ms) and energetic ~10 to 100 keV electrons. A recent theory explaining such rapid interaction rates observed in microbursts is briefly reviewed. Arguments are given why ~5–15 s X‐ray (and optical) pulsations are also associated with chorus scattering of energetic electrons. Comments about relativistic (E > 1 MeV) microbursts are also provided. There are, however, many other unsolved problems of outer zone energetic electron precipitation. The authors will attempt to indicate several of these for the interested reader. Finally, an appendix is provided for a brief review of two‐frequency chorus and some current problems with that topic.
Simultaneous conjugate observations of small‐scale structures in Saturn's dayside ultraviolet auroras: Implications for physical origins
Authors: Meredith C. J. , Cowley S. W. H. , Hansen K. C. , Nichols J. D. , Yeoman T. K. 
Published: April 2013

AbstractSmall‐scale features in Saturn's dayside UV auroras are examined using images obtained on 32 Hubble Space Telescope visits close to Saturn equinox when both northern and southern emissions were simultaneously observed, allowing their interhemispheric conjugacy to be investigated. Eastward‐propagating patches in the dawn‐to‐noon sector were observed on ~70% of visits, which when present were nearly always observed both north and south. The patches were generally not closely conjugate, however, but typically displaced in local time by ~0.5–1 h, with maxima in one hemisphere falling near minima in the other. Averaged angular velocities were ~80% of rigid corotation, larger than plasma angular velocities reported in the outer magnetosphere to which the emissions are likely conjugate. We suggest the patches are associated with field‐aligned currents of eastward‐propagating ULF waves, specifically second harmonic Alfvén resonances with typical azimuthal wave numbers m ≈ 20 and plasma rest frame periods ~80 min, plausibly driven by drift‐bounce resonance with hot magnetospheric water ions. Transient dusk sector emissions of ~10–30 min duration were also observed on ~40% of visits, and found to be strictly nonconjugate, with enhancements in one hemisphere, north or south, being unaccompanied by enhancements in the other. We suggest an association with open flux tubes, and discuss one scenario where hemispheric symmetry is broken on newly opened flux tubes via the interplanetary magnetic field Y component, plausibly consistent with nonconjugate events north and south, preferential postnoon occurrence, and time scales of a few tens of minutes, though the expected relationship with the Y component remains to be established.
Artificial plasma cusp generated by upper hybrid instabilities in HF heating experiments at HAARP
Authors: Kuo Spencer , Snyder Arnold 
Published: April 2013

AbstractThe High Frequency Active Auroral Research Program digisonde was operated in a fast mode to record ionospheric modifications by the HF heating wave. With the O mode heater of 3.2 MHz turned on for 2 min, significant virtual height spread was observed in the heater off ionograms, acquired beginning the moment the heater turned off. Moreover, there is a noticeable bump in the virtual height spread of the ionogram trace that appears next to the plasma frequency (~ 2.88 MHz) of the upper hybrid resonance layer of the HF heating wave. The enhanced spread and the bump disappear in the subsequent heater off ionograms recorded 1 min later. The height distribution of the ionosphere in the spread situation indicates that both electron density and temperature increases exceed 10% over a large altitude region (> 30 km) from below to above the upper hybrid resonance layer. This “mini cusp” (bump) is similar to the cusp occurring in daytime ionograms at the F1–F2 layer transition, indicating that there is a small ledge in the density profile reminiscent of F1–F2 layer transitions. Two parametric processes exciting upper hybrid waves as the sidebands by the HF heating waves are studied. Field‐aligned purely growing mode and lower hybrid wave are the respective decay modes. The excited upper hybrid and lower hybrid waves introduce the anomalous electron heating which results in the ionization enhancement and localized density ledge. The large‐scale density irregularities formed in the heat flow, together with the density irregularities formed through the parametric instability, give rise to the enhanced virtual height spread. The results of upper hybrid instability analysis are also applied to explain the descending feature in the development of the artificial ionization layers observed in electron cyclotron harmonic resonance heating experiments.
Large‐scale structures in the equatorial ionosphere and their connection to the generalized Rayleigh‐Taylor instability
Authors: Dao E. , Kelley M. C. , Pfaff R. F. , Roddy P. A. 
Published: April 2013

AbstractFor the first time, electric field and density fluctuations have been simultaneously measured on the Communications/Navigation Outage Forecasting System satellite at large scales (>300 km). The ratio of their spectral power rises as k2, unlike scales from 20 m to 300 km where the ratio is independent of k. We suggest that gravity waves dominate these large scales and mix with density gradients via self‐generated electric fields to form δn/n0.
Elastic laboratory measurements and modeling of saturated basalts
Authors: Adam Ludmila , Otheim Thomas 
Published: April 2013

AbstractUnderstanding the elastic behavior of basalt is important to seismically monitor volcanoes, subsea basalts, and carbon sequestration in basalt. We estimate the elastic properties of basalt samples from the Snake River Plain, Idaho, at ultrasonic (0.8 MHz) and seismic (2–300 Hz) frequencies. To test the sensitivity of seismic waves to the fluid content in the pore structure, measurements are performed at three saturation conditions: saturated with liquid CO2, water, and dry. When CO2 replaces water, the P‐wave velocity drops, on average, by 10%. Vesicles and cracks, observed in the rock microstructure, control the relaxation of pore‐fluid pressures in the rock as a wave propagates. The bulk and shear moduli of basalts saturated with liquid CO2 are not frequency dependent, suggesting that fluid pore pressures are in equilibrium between 2 Hz and 0.8 MHz. However, when samples are water saturated, the bulk modulus of the rock is frequency dependent. Modeling with Gassmann's equations predicts the measured saturated rock bulk modulus for all fluids for frequencies below 20 Hz but underpredicts the water‐saturated basalt bulk modulus for frequencies greater than 20 Hz. The most likely reason is that the pore‐fluid pressures are unrelaxed. Instead, the ultrasonic frequency rock moduli are modeled with high‐frequency elastic theories of squirt flow and Kuster–Toksöz (KT). Although KT's model is based on idealized pore shapes, a combination of spheres (vesicles) and penny‐shaped cracks (fractures) interpreted and quantified from petrographical data predicts the ultrasonic dry and saturated rock moduli for the measured basalts.
The instability of continental passive margins and its effect on continental topography and heat flow
Authors: Armitage J. J. , Jaupart C. , Fourel L. , Allen P. A. 
Published: April 2013

AbstractThe long geological history of passive margin evolution is complex yet typified by an initial ramp‐like tilting of the subaerial surface toward the continent‐ocean boundary, followed by episodic uplift and subsidence at a smaller wavelength. We argue that this behavior is due to changes in margin structure brought about by buoyancy‐driven lithospheric flow. Continental lithosphere is melt‐depleted, buoyant, and thick. It will resist convective breakdown into the asthenosphere below, but will be prone to lateral flow due to horizontal density contrasts. Changes in lithosphere thickness at the transition between continent and ocean will nucleate convection cells. Using a numerical model of viscous upper mantle flow, we show that stability or instability of the continental lithosphere at a passive margin is a function of the lithospheric rheology and composition. Increased compositional buoyancy leads to oceanward lateral flow of the continental lithosphere whereas decreased buoyancy has the opposite effect, causing landward lateral flow of the continental lithosphere. In model simulations, a continental lithosphere thought typical of Phanerozoic continental platforms experiences first a margin‐wide ramp‐like tilting, followed by topographic fluctuations due to an evolving array of convection cells in the mantle. The timing and magnitude of predicted changes in topography are similar to those observed at the eastern North American margin, suggesting that the tilting and episodic uplift and subsidence at continental passive margins are a natural consequence of the evolution of continental lithosphere after breakup and during mature seafloor spreading.
The use of GPS horizontals for loading studies, with applications to northern California and southeast Greenland
Authors: Wahr John , Khan Shfaqat A. , van Dam Tonie , Liu Lin , van Angelen Jan H. , van den Broeke Michiel R. , Meertens Charles M. 
Published: April 2013

AbstractWe describe how GPS measurements of horizontal crustal motion can be used to augment vertical crustal motion measurements, to improve and extend GPS studies of surface loading. We show that the ratio of the vertical displacement to the horizontal displacement, combined with the direction of the horizontal motion, can help determine whether nearby loading is concentrated in a small region (for example, in a single lake or glacier), and where that region is. We illustrate this method by applying it to two specific cases: an analysis of GPS data from northern California to monitor the level of Lake Shasta, and the analysis of data from a single GPS site in southeast Greenland to determine mass variability of two large, nearby outlet glaciers: Helheim Glacier and Midgaard Glacier. The California example serves largely as a proof‐of‐concept, where the results can be assessed by comparing with independent observations (Lake Shasta tide gauge data, in this case). Our Greenland results show that both Helheim and Midgaard have experienced notable interannual variations in mass loss rate over the last decade. Helheim's mass loss accelerated rapidly in mid‐2003, decelerated in late 2005, and increased again in 2008–2009 before returning to about its pre‐2003 rate in late 2010. Midgaard's mass loss accelerated in mid‐2004, and remained more‐or‐less constant before returning to its pre‐2003 rate in late 2008.