bluuebunny/crossref_metadata_2025_split huggingface dataset:
Orographic Effect on Extreme Precipitation Statistics Peaks at Hourly Time Scales
Authors: Marra Francesco , Armon Moshe , Borga Marco , Morin Efrat 
Published: February 2021

AbstractOrographic impact on extreme subdaily precipitation is critical for risk management but remains insufficiently understood due to complicated atmosphere‐orography interactions and large uncertainties. We investigate the problem adopting a framework able to reduce uncertainties and isolate the systematic interaction of Mediterranean cyclones with a regular orographic barrier. The average decrease with elevation reported for hourly extremes is found enhanced at subhourly durations. Tail heaviness of 10‐min intensities is negligibly affected by orography, suggesting self‐similarity of the distributions at the convective scale. Orography decreases the tail heaviness at longer durations, with a maximum impact around hourly scales. These observations are explained by an orographically induced redistribution of precipitation toward stratiform‐like processes, and by the succession of convective cores in multihour extremes. Our results imply a breaking of scale‐invariance at subhourly durations, with important implications for natural hazards management in mountainous areas.
Widespread Tissintite in Strongly Shock‐Lithified Lunar Regolith Breccias
Authors: Zhang Ai‐Cheng , Jiang Qin‐Ting , Tomioka Naotaka , Guo Yan‐Jun , Chen Jia‐Ni , Li Yang , Sakamoto Naoya , Yurimoto Hisayoshi 
Published: February 2021

AbstractShock‐lithification is a fundamental process of making rocks from fine‐grained and porous regoliths on the surface of the Moon. Previous investigations have constrained potential shock pressures during shock‐lithification based on experimental and numerical simulations. However, pressure and temperature conditions during shock‐lithification have not been directly inferred from natural lunar breccias. Here, we report the discovery of widespread tissintite in strongly shock‐lithified lunar meteorites, accompanied occasionally by Si‐rich corundum and coesite. The coexistence of tissintite and coesite and the absence of stishovite in molten regions indicate a shock pressure of 4–8 GPa, which might be the pressure boundary between weak and strong shock‐lithification. Meanwhile, the presence of Si‐rich corundum imposes a temperature constraint of ∼2,300 K for the intergranular melts. This temperature constraint has profound significance for interpreting the behaviors of volatile and moderately volatile elements and remanent magnetization records during strongly shock‐lithification of lunar breccias.
Second‐Harmonic Generation of Electromagnetic Emissions in a Magnetized Plasma: Kinetic Theory Approach
Authors: Yu Xiongdong , Yuan Zhigang , Xue Zuxiang 
Published: February 2021

AbstractSecond harmonics have been observed in various magnetospheric waves. Although particle simulations are able to produce the similar features as reported in satellite observations, the exact physical mechanism for the generation of second harmonics remains uncovered yet. In this letter, we first give wave equations describing the fundamental and second harmonics via multiscale analysis. Then, we apply a kinetic theory approach to obtain the conductivity tensor, nonlinear current density, and the necessary conditions for the second harmonic generation of electromagnetic emissions in a magnetized plasma. To verify our theoretical results, we also perform hybrid simulations to study the second harmonic generation of electromagnetic ion cyclotron waves for three cases in which the second harmonic deviates from the inherent modes by different degrees. Our results suggest the important role of the linear dispersion relation in the second‐harmonic generation.
A Large‐Scale Analysis of Pockets of Open Cells and Their Radiative Impact
Authors: Watson‐Parris D. , Sutherland S. A. , Christensen M. W. , Eastman R. , Stier P. 
Published: February 2021

AbstractPockets of open cells sometimes form within closed‐cell stratocumulus cloud decks but little is known about their statistical properties or prevalence. A convolutional neural network was used to detect occurrences of pockets of open cells (POCs). Trained on a small hand‐logged data set and applied to 13 years of satellite imagery the neural network is able to classify 8,491 POCs. This extensive database allows the first robust analysis of the spatial and temporal prevalence of these phenomena, as well as a detailed analysis of their micro‐physical properties. We find a large (30%) increase in cloud effective radius inside POCs as compared to their surroundings and similarly large (20%) decrease in cloud fraction. This also allows their global radiative effect to be determined. Using simple radiative approximations we find that the instantaneous global annual mean top‐of‐atmosphere perturbation by all POCs is only 0.01 W/m2.
e‐POP Observations of Suprathermal Electron Bursts in the Ionospheric Alfvén Resonator
Authors: Wu J. , Knudsen D. J. , Shen Y. , Gillies D. M. 
Published: February 2021

AbstractUtilizing the suprathermal electron imager onboard the e‐POP satellite, we performed a statistical study of groups of repetitive suprathermal electron bursts (STEBs) with 52 events (each with ≥3 STEBs) during 21 e‐POP orbits. The STEBs are observed on both the day side and night side, and within a wide range of geomagnetic latitudes, from 64° to 78°. We interpret these data as temporally varying signatures; we suggest the repetition is due to the ionospheric Alfvén resonator (IAR), since the statistical time separation of adjacent STEBs in the satellite frame is 0.5–1.5 s, which is consistent with the characteristic period of the IAR. Furthermore, the STEBs are associated with low‐frequency magnetic field perturbations. The statistical occurrence of repetitive STEBs shows a preference toward postnoon and midnight sectors at high magnetic latitudes. We also present one example event of four successive STEBs associated with auroral rays in conjugate imagery. The repetition period of STEBs is similar to the oscillation period of magnetic fields observed by Enhanced Polar Outflow Probe (e‐POP). One of the STEBs in an example event exhibits “normal” dispersion (high energy particles with field‐aligned pitch angle arriving first) and the adjacent STEB exhibits “inverse” dispersion (low energy particles with broad pitch angles arriving first). With a test particle simulation, we propose one possible explanation of inverse dispersion in terms of a suprathermal electron population and a downward‐propagating parallel electric field.
Conjugate Properties of Magnetospheric Pc5 Waves: Antarctica‐Greenland Comparison
Authors: Pilipenko Vyacheslav A. , Martines‐Bedenko Valery A. , Coyle Shane , Fedorov Evgeny N. , Hartinger Michael D. , Engebretson Mark J. , Edwards Thom R. 
Published: February 2021

AbstractAn electric circuit analogy for magnetosphere‐ionosphere current systems has two extremes for their driver: a current generator and voltage generator. In the case of a magnetospheric current generator the ground magnetic effect must be nearly the same under low‐conductivity or high‐conductivity ionospheres, whereas in the case of a voltage generator the magnetic response must be larger under a high‐conductivity ionosphere. Theoretical consideration of the field‐aligned current generation by a magnetospheric driver within the framework of the magnetosphere “plasma box” model with asymmetric ionospheres showed that excitation of resonant Alfvenic oscillations should correspond rather to a current generator regime. The asymmetry of Pc5 waves (frequencies about several mHz) recorded at conjugate magnetometers in Greenland and Antarctica has been analyzed from the viewpoint of the voltage/current generator dichotomy. We have selected Pc5 events under contrasting ionospheric conditions, during summer and winter. For resonant frequencies, we have plotted the latitudinal profiles of spectral power and compared the amplitudes of spatial maxima in opposite hemispheres. Along the same profile, latitudinal dependences of the ionospheric height‐integrated conductivities have been reconstructed with the use of the OVATION‐prime (OP) and solar illumination models. During both winter and summer events, the amplitudes of the spectral power peak in Greenland and Antarctica have been found to be about the same, though the OP model predicts a contrast between ionospheric conductivities of 5–10 times. Thus, the interhemispheric properties of Pc5 waves correspond to the current generator regime, as predicted by the magnetospheric plasma box model.
The Influence of Magnetic Field Topology and Orientation on the Distribution of Thermal Electrons in the Martian Magnetotail
Authors: Nauth Murti , Fowler Christopher M. , Andersson Laila , DiBraccio Gina A. , Xu Shaosui , Weber Tristan , Mitchell David 
Published: February 2021

AbstractThermal (<1 eV) electron density measurements, derived from the Mars Atmosphere and Volatile Evolution's (MAVEN) Langmuir Probe and Waves (LPW) instrument, are analyzed to produce the first statistical study of the thermal electron population in the Martian magnetotail. Coincident measurements of the local magnetic field are used to demonstrate that close to Mars, the thermal electron population is most likely to be observed at a cylindrical distance of ∼1.1 Mars radii (RM) from the central tail region during times when the magnetic field flares inward toward the central tail, compared to ∼1.3 RM during times when the magnetic field flares outward away from the central tail. Similar patterns are observed further down the magnetotail with greater variability. Thermal electron densities are highly variable throughout the magnetotail; average densities are typically ∼20–50 cm−3 within the optical shadow of Mars and can peak at ∼100 cm−3 just outside of the optical shadow. Standard deviations of 100% are observed for average densities measured throughout the tail. Analysis of the local magnetic field topology suggests that thermal electrons observed within the optical shadow of Mars are likely sourced from the nightside ionosphere, whereas electrons observed just outside of the optical shadow are likely sourced from the dayside ionosphere. Finally, thermal electrons within the optical shadow of Mars are up to 20% more likely to be observed when the strongest crustal magnetic fields point sunward than when they point tailward.
Radar Observations of Flows Leading to Substorm Onset Over Alaska
Authors: Lyons Larry R. , Liu Jiang , Nishimura Yukitoshi , Reimer Ashton S. , Bristow William A. , Hampton Don L. , Shi Xueling , Varney Roger H. , Donovan Eric F. 
Published: February 2021

AbstractAbout 40 years after the discovery of the substorm, auroral observations were used to propose that substorm onset was triggered in the inner plasmas sheet (equatorward portion of the auroral oval) by an intrusion of the low entropy plasma by plasma sheet flow channels. Longitudinal localization makes such flow channels difficult to observe with sparse spacecraft, but they can be seen in the ionosphere via the broader, two‐dimensional coverage by ground‐based radars. We have analyzed all eight substorm auroral onset events with appropriate auroral and Poker Flat radar viewing during a radar campaign. These, together with five previously analyzed events provide evidence that intruding flow channels play an important role in substorm onsets. That the flux tube integrated entropy of this new plasma must be lower than that of the surrounding plasma implies that the change in the entropy distribution within the plasma sheet gives rises to the substorm onset instability that is seen via the growing auroral and electromagnetic waves seen with substorm auroral onset. Our analysis also adds to previous evidence that the flow channels leading to onset move into the plasma sheet/auroral oval from the open polar cap region of the magnetotail lobes/polar caps, that these flow channels move across the polar cap following their origin near the dayside boundary of the polar cap, and that substantial ground magnetic depressions associated with substorm do not occur with substorm onset but instead occur a few to several minutes later in association with expansion phase streamers.
Radar Observations of Flows Leading to Longitudinal Expansion of Substorm Onset Over Alaska
Authors: Lyons Larry R. , Liu Jiang , Nishimura Yukitoshi , Wang Chih‐Ping , Reimer Ashton S. , Bristow William A. , Hampton Don L. , Shi Xueling , Varney Roger H. , Donovan Eric F. 
Published: February 2021

AbstractLongitudinal expansion of brightening of the substorm onset arc likely represents longitudinal expansion of the substorm onset instability. We used Poker Flat radar campaign auroral observations to identify seven substorm onsets seen east or west of the radar that expanded into the radar field‐of‐view. The radar observations were used to show flows relative to the expanding onset aurora. Flow enhancements were found equatorward (poleward) of the westward (eastward) longitudinal expansion of the onset arc brightening that agree with plasma sheet bubble modeling. This lends significant additional supporting evidence that an incoming reduced entropy flow channel leads to onset. As a flow channel enters the auroral oval (plasma sheet) from the polar cap (magnetotail lobes) and starts to move equatorward (earthward in the plasma sheet) as a bubble, it simultaneously expands azimuthally due to the combination of electric drift along dawnward tilted equipotentials and westward magnetic drift of more energetic ions. As the azimuthally expanding bubble reaches near the equatorward boundary of the electron auroral oval, it leads to the onset instability indicated by the auroral beading and associated flows if conditions are appropriate. The instability grows in the initial onset region, as it simultaneously extends longitudinally with the continued azimuthal expansion of the bubble within the plasma sheet. Additionally, after onset, the auroral bulge expands poleward and can lead to a westward traveling surge. We observe strong flows heading toward the front of the surge that appear to bring low entropy plasma, perhaps from the polar cap, into the surge.
Thermospheric Impact on the Magnetosphere Through Ionospheric Outflow
Authors: Pham K. H. , Lotko W. , Varney R. H. , Zhang B. , Liu J. 
Published: February 2021

AbstractWe have taken a key step in evaluating the importance of ionospheric outflows relative to electrodynamic coupling in the thermosphere’s impact on geospace dynamics. We isolated the thermosphere’s material influence and suppressed electrodynamic feedback in whole geospace simulations by imposing a time‐constant ionospheric conductance in the ionospheric Ohm’s law in a coupled model that combines the multifluid Lyon‐Fedder‐Mobarry magnetosphere model with the Thermosphere Ionosphere Electrodynamic General Circulation Model and the Ionosphere Polar Wind Model that includes both polar wind and transversely accelerated ion species. Numerical experiments were conducted for different thermospheric states parameterized by F10.7 for interplanetary driving representative of the stream interaction region that swept past Earth on March 27, 2003. We demonstrate that thermosphere through its regulation of ionospheric outflows influences magnetosphere‐ionosphere (MI) convection and the ion composition, symmetries, x‐line perimeter and magnetic merging of the magnetosphere. Feedback to the ionosphere‐thermosphere from evolving MI convection, and Alfvénic Poynting fluxes and soft (∼few 100 eV) electron precipitation originating in the magnetosphere, in turn, modify the evolving O+ outflow properties. The simulation results identify a variety of observed magnetospheric features that are attributable directly to the thermosphere’s material influence: Asymmetries in O+ outflow fluxes and velocities in the pre/postnoon low‐altitude magnetosphere, dawn/duskside lobes and pre/postmidnight plasmasheet; O+ distribution of the plasmasheet; magnetic x‐line location and reconnection rate along it. O+ outflows during solar maximum conditions (high F10.7) tend to counteract the plasmasheet’s pre/postmidnight asymmetries caused by the night‐to‐day gradient in ionospheric Hall conductance.
Spatiotemporal Variations of Quiet Time Equatorial Ionosphere Longitudinal Structure Under Low Solar Activity
Authors: Hou Weijun , Xu Buyun , Wang Hongxia , Li Yihong , Li Xihai , Liu Daizhi 
Published: February 2021

AbstractThe low‐latitude longitudinally periodic density structure with three or four peaks at a fixed local time is well known. Both harmonic analysis of observations and numerical simulations have been extensively performed to ascribe the periodic structures to different atmospheric tides. But to date, no research has been done to explain how the two structures transform between seasons. Besides, other different wave structures have also been reported. To clarify these uncertainties, the COSMIC electron density data during 2006–2019 are used to further examine the spatiotemporal variations of the longitudinal structure. The results show that a wavenumber‐4 structure is clearly distinguishable during equinoxes and the four peaks are basically the same size, but in two solstices, a wavenumber‐3 structure is finally observed and the three peaks vary greatly in size. The wavenumber‐4 is well established before noon while the stable wavenumber‐3 form cannot be reached until after noon. Due to the blurred boundary caused by plasma transportation, the density distribution can be characterized by other wave structures before reaching its stable form during solstices. Subdividing the data, we find that from vernal (autumn) equinox to June (December) solstice, the peak in American sector gradually merges with its west (east) side peak into a super‐large one, its east (west) side peak weakens into a small one, and the peak in Southeast Asian sector gradually shrinks (expands). We speculate that the transition from wavenumber‐4 to wavenumber‐3 is due to the modulation of transequatorial winds and magnetic declination.
Evolution of Microstructural Properties in Sheared Iron‐Rich Olivine
Authors: Qi Chao , Zhao Yong‐Hong , Zimmerman Mark E. , Kim Daeyeong , Kohlstedt David L. 
Published: February 2021

AbstractIron‐rich olivine is mechanically weaker than olivine of mantle composition, ca. Fo90, and thus is more amenable to study under a wide range of laboratory conditions. To investigate the effects of iron content on deformation‐produced crystallographic preferred orientation (CPO) and grain size, we analyzed the microstructures of olivine samples with compositions of Fo70, Fo50, and Fo0 that were deformed in torsion under either anhydrous or hydrous conditions at 300 MPa. Electron backscatter diffraction (EBSD) observations reveal a transition in CPO from D‐type fabric, induced by dislocation glide on both the (010)[100] and the (001)[100] slip systems, at low strains, to A‐type fabric, caused by dislocation glide on the (010)[100] slip system, at high strains for all of our samples, independent of iron content and hydrous/anhydrous conditions. A similar evolution of fabric with increasing strain is also reported to occur for Fo90. Radial seismic anisotropy increases with increasing strain, reaching a maximum value of ∼1.15 at a shear strain of ∼3.5 for each sample, demonstrating that the seismic anisotropy of naturally deformed olivine‐rich rocks can be well approximated by that of iron‐rich olivine. Based on EBSD observations, we derived a piezometer for which recrystallized grain size decreases inversely with stress to the ∼1.2 power. Also, recrystallized grain size increases with increasing iron content. Our experimental results contribute to understanding the microstructural evolution in the mantle of not only Earth but also Mars, where the iron content in olivine is higher.
Ground Deformation After a Caldera Collapse: Contributions of Magma Inflow and Viscoelastic Response to the 2015–2018 Deformation Field Around Bárðarbunga, Iceland
Authors: Li Siqi , Sigmundsson Freysteinn , Drouin Vincent , Parks Michelle M. , Ófeigsson Benedikt G. , Jónsdóttir Kristín , Grapenthin Ronni , Geirsson Halldór , Hooper Andrew , Hreinsdóttir Sigrún 
Published: February 2021

AbstractImprovement of our understanding of the role of ground deformation due to viscoelastic relaxation following eruptions is important, as the generated signals can resemble renewed magma inflow. We study post‐eruptive unrest at the subglacial Bárðarbunga volcano, Iceland, after a caldera collapse and major magma drainage in 2014–2015. Elevated seismicity began about 6 months after the eruption ended, including nine Mlw > 4.5 earthquakes. Global Navigation Satellite System and Sentinel‐1 Interferometric Synthetic Aperture Radar geodesy are applied to evaluate post‐eruptive ground deformation from 2015 to 2018. Horizontal velocities locally exceed 10 cm/year and rapidly decay with distance away from the caldera. We explore two end‐member models and their combination to explain the post‐eruptive deformation field: 1) viscoelastic relaxation caused by the co‐eruptive caldera collapse and magma withdrawal, and 2) renewed magma inflow. We find parameter combinations for each model that explain the observed ground deformation. The purely viscoelastic relaxation model, consisting of a half‐space composed of a 7‐km thick elastic layer on top of a viscoelastic layer with a viscosity of 3.0 × 1018 Pa s reproduces broadly the observations. A simple magma inflow model consisting of a single point source with an inflow rate of 1 × 107 m3/year at 0.7 km depth broadly fits the observations, but may be unrealistic. A more elaborate model of magma inflow into a 10‐km deep sill combined with slip on the caldera ring fault explains the observations well. Our results suggest that the co‐eruptive deformation field is likely influenced by viscoelastic relaxation, renewed magma inflow, or a combination of both processes.
Overlapped Postseismic Deformation Caused by Afterslip and Viscoelastic Relaxation Following the 2015 Mw 7.8 Gorkha (Nepal) Earthquake
Authors: Diao Faqi , Wang Rongjiang , Xiong Xiong , Liu Chengli 
Published: February 2021

AbstractStudying transient postseismic deformation is one of the key approaches to probe fault kinematics and rheological behavior of rocks surrounding tectonic faults. Previous studies have shown that transient postseismic deformation is dominantly induced by afterslip and postseismic viscoelastic relaxation. However, effects of these two mechanisms are generally overlapped in space and time, making the analysis of the observations challenging. Here, we investigate the overlapping effects of afterslip and viscoelastic relaxation following the 2015 Gorkha earthquake based on the first 4‐years’ Global Positioning System observations and finite element models. Separated mechanisms either fail to explain the observations or lead to unreasonable model parameters. A combined model that incorporates stress‐driven afterslip and viscoelastic relaxation can overcome the limitations of separated mechanisms. Our results indicate that only short‐term (<1 year) postseismic deformation in near‐to middle‐fields is caused mainly by afterslip, whereas that in far‐field or long‐term are dominated by the viscoelastic relaxation effect. The stress‐driven afterslip is mainly confined to a narrow zone adjacent to the coseismic rupture asperities, in contrast to the widespread deep afterslip as suggested by previous studies. The total seismic moment released by the afterslip is ∼5% of that released by the Mw 7.8 main shock. The optimal viscosity of the lower crust beneath the south Tibet is estimated to be 2.5 × 1018 Pa∙s, which is at least one order lower than that beneath Indian plate, suggesting a significant lateral variation across the Himalayan fault. Validations by using independent InSAR observations further confirm the reliability of the combined model.