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Geophysical Research Letters

D Schmid, R Nakamura, M Volwerk, F Plaschke, Y Narita, W Baumjohann, W Magnes, D Fischer, H U Eichelberger, R B Torbert, C T Russell, R J Strangeway, H K Leinweber, G Le, K R Bromund, B J Anderson, J A Slavin, E L Kepko
We present a statistical study of dipolarization fronts (DFs), using magnetic field data from MMS and Cluster, at radial distances below 12 R E and 20 R E , respectively. Assuming that the DFs have a semicircular cross section and are propelled by the magnetic tension force, we used multispacecraft observations to determine the DF velocities. About three quarters of the DFs propagate earthward and about one quarter tailward. Generally, MMS is in a more dipolar magnetic field region and observes larger-amplitude DFs than Cluster...
June 28, 2016: Geophysical Research Letters
J P Eastwood, T D Phan, P A Cassak, D J Gershman, C Haggerty, K Malakit, M A Shay, R Mistry, M Øieroset, C T Russell, J A Slavin, M R Argall, L A Avanov, J L Burch, L J Chen, J C Dorelli, R E Ergun, B L Giles, Y Khotyaintsev, B Lavraud, P A Lindqvist, T E Moore, R Nakamura, W Paterson, C Pollock, R J Strangeway, R B Torbert, S Wang
New Magnetospheric Multiscale (MMS) observations of small-scale (~7 ion inertial length radius) flux transfer events (FTEs) at the dayside magnetopause are reported. The 10 km MMS tetrahedron size enables their structure and properties to be calculated using a variety of multispacecraft techniques, allowing them to be identified as flux ropes, whose flux content is small (~22 kWb). The current density, calculated using plasma and magnetic field measurements independently, is found to be filamentary. Intercomparison of the plasma moments with electric and magnetic field measurements reveals structured non-frozen-in ion behavior...
May 28, 2016: Geophysical Research Letters
T D Pering, A J S McGonigle, M R James, G Tamburello, A Aiuppa, D Delle Donne, M Ripepe
Recent gas flux measurements have shown that Strombolian explosions are often followed by periods of elevated flux, or "gas codas," with durations of order a minute. Here we present UV camera data from 200 events recorded at Stromboli volcano to constrain the nature of these codas for the first time, providing estimates for combined explosion plus coda SO2 masses of ≈18-225 kg. Numerical simulations of gas slug ascent show that substantial proportions of the initial gas mass can be distributed into a train of "daughter bubbles" released from the base of the slug, which we suggest, generate the codas, on bursting at the surface...
May 28, 2016: Geophysical Research Letters
Nicholas J Byrne, Theodore G Shepherd, Tim Woollings, R Alan Plumb
Lagged correlation analysis is often used to infer intraseasonal dynamical effects but is known to be affected by nonstationarity. We highlight a pronounced quasi 2 year peak in the anomalous zonal wind and eddy momentum flux convergence power spectra in the Southern Hemisphere, which is prima facie evidence for nonstationarity. We then investigate the consequences of this nonstationarity for the Southern Annular Mode and for eddy momentum flux convergence. We argue that positive lagged correlations previously attributed to the existence of an eddy feedback are more plausibly attributed to nonstationary interannual variability external to any potential feedback process in the midlatitude troposphere...
April 28, 2016: Geophysical Research Letters
J Dong, X Xiao, G Zhang, M A Menarguez, C Y Choi, Y Qin, P Luo, Y Zhang, B Moore
Paddy rice in monsoon Asia plays an important role in global food security and climate change. Here we documented annual dynamics of paddy rice areas in the northern frontier of Asia, including Northeastern (NE) China, North Korea, South Korea, and Japan, from 2000-2014 through analysis of satellite images. The paddy rice area has increased by 120% (2.5 to 5.5 million ha) in NE China, in comparison to a decrease in South Korea and Japan, and the paddy rice centroid shifted northward from 41.16 °N to 43.70 °N (~310 km) in this period...
April 28, 2016: Geophysical Research Letters
L Shen, L J Mickley, E Gilleland
We develop a statistical model using extreme value theory to estimate the 2000-2050 changes in ozone episodes across the United States. We model the relationships between daily maximum temperature (Tmax) and maximum daily 8-hour average (MDA8) ozone in May-September over 2003-2012 using a Point Process (PP) model. At ~20% of the sites, a marked decrease in the ozone-temperature slope occurs at high temperatures, defined as ozone suppression. The PP model sometimes fails to capture ozone-Tmax relationships, and so we refit the ozone-Tmax slope using logistic regression and a Generalized Pareto Distribution model...
April 28, 2016: Geophysical Research Letters
E Spagnuolo, S Nielsen, M Violay, G Di Toro
Empirically based rate-and-state friction laws (RSFLs) have been proposed to model the dependence of friction forces with slip and time. The relevance of the RSFL for earthquake mechanics is that few constitutive parameters define critical conditions for fault stability (i.e., critical stiffness and frictional fault behavior). However, the RSFLs were determined from experiments conducted at subseismic slip rates (V < 1 cm/s), and their extrapolation to earthquake deformation conditions (V > 0...
April 16, 2016: Geophysical Research Letters
Shin-Chan Han, Jeanne Sauber, Fred Pollitz
Large earthquakes often trigger viscoelastic adjustment for years to decades depending on the rheological properties and the nature and spatial extent of coseismic stress. The 2006 Mw8.3 thrust and 2007 Mw8.1 normal fault earthquakes of the central Kuril Islands resulted in significant postseismic gravity change in GRACE but without a discernible coseismic gravity change. The gravity increase of ~4 µGal, observed consistently from various GRACE solutions around the epicentral area during 2007-2015, is interpreted as resulting from gradual seafloor uplift by ~6 cm produced by postseismic relaxation...
April 16, 2016: Geophysical Research Letters
R Somavilla, C González-Pola, U Schauer, G Budéus
Prior to the 2000s, the North Atlantic was the basin showing the greatest warming. However, since the mid-2000s during the so-called global warming hiatus, large amounts of heat were transferred in this basin from upper to deeper levels while the dominance in terms of atmospheric heat capture moved into the Indo-Pacific. Here we show that a large transformation of modal waters in the eastern North Atlantic (ENA) played a crucial role in such contrasting behavior. First, strong winter mixing in 2005 transformed ENA modal waters into a much saltier, warmer, and denser variety, transferring upper ocean heat and salt gained slowly over time to deeper layers...
March 16, 2016: Geophysical Research Letters
Daniel R Parsons, Robert J Schindler, Julie A Hope, Jonathan Malarkey, Jaco H Baas, Jeffrey Peakall, Andrew J Manning, Leiping Ye, Steve Simmons, David M Paterson, Rebecca J Aspden, Sarah J Bass, Alan G Davies, Ian D Lichtman, Peter D Thorne
Biologically active, fine-grained sediment forms abundant sedimentary deposits on Earth's surface, and mixed mud-sand dominates many coasts, deltas, and estuaries. Our predictions of sediment transport and bed roughness in these environments presently rely on empirically based bed form predictors that are based exclusively on biologically inactive cohesionless silt, sand, and gravel. This approach underpins many paleoenvironmental reconstructions of sedimentary successions, which rely on analysis of cross-stratification and bounding surfaces produced by migrating bed forms...
February 28, 2016: Geophysical Research Letters
Pierre Gentine, Alix Garelli, Seung-Bu Park, Ji Nie, Giuseppe Torri, Zhiming Kuang
The role of surface heat fluxes underneath cold pools is investigated using cloud-resolving simulations with either interactive or horizontally homogenous surface heat fluxes over an ocean and a simplified land surface. Over the ocean, there are limited changes in the distribution of the cold pool temperature, humidity, and gust front velocity, yet interactive heat fluxes induce more cold pools, which are smaller, and convection is then less organized. Correspondingly, the updraft mass flux and lateral entrainment are modified...
January 28, 2016: Geophysical Research Letters
Chien Wang
The climate response of precipitation to the effects of anthropogenic aerosols is a critical while not yet fully understood aspect in climate science. Results of selected models that participated the Coupled Model Intercomparison Project Phase 5 and the data from the Twentieth Century Reanalysis Project suggest that, throughout the tropics and also in the extratropical Northern Hemisphere, aerosols have largely dominated the distribution of precipitation changes in reference to the preindustrial era in the second half of the last century...
December 28, 2015: Geophysical Research Letters
B R Anderson, R M Millan, G D Reeves, R H W Friedel
Past studies of radiation belt relativistic electrons have favored active storm time periods, while the effects of small geomagnetic storms (D s t > -50 nT) have not been statistically characterized. In this timely study, given the current weak solar cycle, we identify 342 small storms from 1989 through 2000 and quantify the corresponding change in relativistic electron flux at geosynchronous orbit. Surprisingly, small storms can be equally as effective as large storms at enhancing and depleting fluxes...
December 16, 2015: Geophysical Research Letters
Yu L Sasunov, M L Khodachenko, I I Alexeev, E S Belenkaya, E I Gordeev, I V Kubyshkin
The influence of average plasma energy E~ on the half thickness ℓ of a thin current sheet (TCS) is investigated for three cases of TCSs crossings. The value of ℓ was estimated from the magnetic field data by means of Cluster observations. The obtained scaling values for TCSs, Z~=ℓ/ρT, where ρ T is the thermal Larmor radius, were compared with the scaling Zμ=22E~/T, where E~ and T are the average plasma energy and the temperature of plasma, which assumes a specific dynamics (conservation of magnetic flux through the trajectory segment) of the current carriers...
November 28, 2015: Geophysical Research Letters
P R Mahaffy, M Benna, M Elrod, R V Yelle, S W Bougher, S W Stone, B M Jakosky
The Mars Atmosphere and Volatile EvolutioN (MAVEN) Neutral Gas and Ion Mass Spectrometer (NGIMS) provides sensitive detections of neutral gas and ambient ion composition. NGIMS measurements of nine atomic and molecular neutral species, and their variation with altitude, latitude, and solar zenith angle are reported over several months of operation of the MAVEN mission. Sampling NGIMS signals from multiple neutral species every several seconds reveals persistent and unexpectedly large amplitude density structures...
November 16, 2015: Geophysical Research Letters
Sophie M Cowie, John H Marsham, Peter Knippertz
Dust uplift is a nonlinear thresholded function of wind speed and therefore particularly sensitive to the long tails of observed wind speed probability density functions. This suggests that a few rare high-wind events can contribute substantially to annual dust emission. Here we quantify the relative roles of different wind speeds to dust-generating winds using surface synoptic observations of dust emission and wind from northern Africa. The results show that winds between 2 and 5 m s(-1) above the threshold cause the most emission...
October 16, 2015: Geophysical Research Letters
C A Cattell, A W Breneman, S A Thaller, J R Wygant, C A Kletzing, W S Kurth
We show the first evidence for locally excited chorus at frequencies below 0.1 f ce (electron cyclotron frequency) in the outer radiation belt. A statistical study of chorus during geomagnetic storms observed by the Van Allen Probes found that frequencies are often dramatically lower than expected. The frequency at peak power suddenly stops tracking the equatorial 0.5 f ce and f/f ce decreases rapidly, often to frequencies well below 0.1 f ce (in situ and mapped to equator). These very low frequency waves are observed both when the satellites are close to the equatorial plane and at higher magnetic latitudes...
September 28, 2015: Geophysical Research Letters
H Hietala, J F Drake, T D Phan, J P Eastwood, J P McFadden
A significant fraction of the energy released by magnetotail reconnection appears to go into ion heating, but this heating is generally anisotropic. We examine ARTEMIS dual-spacecraft observations of a long-duration magnetotail exhaust generated by antiparallel reconnection in conjunction with particle-in-cell simulations, showing spatial variations in the anisotropy across the outflow far (>100d i ) downstream of the X line. A consistent pattern is found in both the spacecraft data and the simulations: While the total temperature across the exhaust is rather constant, near the boundaries T i,|| dominates...
September 28, 2015: Geophysical Research Letters
Frans-Jan W Parmentier, Wenxin Zhang, Yanjiao Mi, Xudong Zhu, Jacobus van Huissteden, Daniel J Hayes, Qianlai Zhuang, Torben R Christensen, A David McGuire
The Arctic is rapidly transitioning toward a seasonal sea ice-free state, perhaps one of the most apparent examples of climate change in the world. This dramatic change has numerous consequences, including a large increase in air temperatures, which in turn may affect terrestrial methane emissions. Nonetheless, terrestrial and marine environments are seldom jointly analyzed. By comparing satellite observations of Arctic sea ice concentrations to methane emissions simulated by three process-based biogeochemical models, this study shows that rising wetland methane emissions are associated with sea ice retreat...
September 16, 2015: Geophysical Research Letters
N M Pilkington, N Achilleos, C S Arridge, P Guio, A Masters, L C Ray, N Sergis, M F Thomsen, A J Coates, M K Dougherty
For over 10 years, the Cassini spacecraft has patrolled Saturn's magnetosphere and observed its magnetopause boundary over a wide range of prevailing solar wind and interior plasma conditions. We now have data that enable us to resolve a significant dawn-dusk asymmetry and find that the magnetosphere extends farther from the planet on the dawnside of the planet by 7 ± 1%. In addition, an opposing dawn-dusk asymmetry in the suprathermal plasma pressure adjacent to the magnetopause has been observed. This probably acts to reduce the size asymmetry and may explain the discrepancy between the degree of asymmetry found here and a similar asymmetry found by Kivelson and Jia (2014) using MHD simulations...
September 16, 2015: Geophysical Research Letters
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