ALBERT

Description: A multievent study was performed using conjugate measurements of the Time History of Events and Macroscale Interactions during Substorms (THEMIS) spacecraft and an all-sky imager during periods of intense lower-band chorus waves. The thirteen identified cases support our previous finding, based on two events, that the intensity modulation of lower-band chorus near the magnetic equator is highly correlated with quasiperiodic pulsating auroral emissions near the spacecraft's magnetic footprint, indicating that lower-band chorus is the driver of the pulsating aurora. Furthermore, we identified a fortuitous measurement made simultaneously by two THEMIS spacecraft with small spatial separation. The two spacecraft were found to be located in a single pulsating chorus patch and the spacecraft footprints were in the same pulsating auroral patch when intense chorus bursts were measured simultaneously, whereas only one of the spacecraft's footprints was in a patch when the other spacecraft did not detect intense chorus. On the basis of this event, we can estimate the pulsating chorus patch size by mapping the pulsating auroral patches from the ionosphere toward the magnetic equator, giving a roughly circular region of ∼5000 km diameter for corresponding azimuthally elongated patches with ∼100 km size in the ionosphere. Using a ray-tracing-based calculation of the divergence of chorus raypaths from a point source, together with the corresponding resonant energies, we found that the chorus patch size is most probably not a result of ray divergence but a property of the wave excitation region.

Description: Electromagnetic ion cyclotron (EMIC) waves have been proposed to cause efficient losses of highly relativistic (〉1 MeV) electrons via gyroresonant interactions. Simultaneous observations of EMIC waves and equatorial electron pitch angle distributions, which can be used to directly quantify the EMIC wave scattering effect, are still very limited, however. In the present study, we evaluate the effect of EMIC waves on pitch angle scattering of ultra-relativistic (〉1 MeV) electrons during the main phase of a geomagnetic storm, when intense EMIC wave activity was observed in situ (in the plasma plume region with high plasma density) on both Van Allen Probes. EMIC waves captured by THEMIS probes and on the ground across the Canadian Array for Real-time Investigations of Magnetic Activity (CARISMA) are also used to infer their MLT coverage. From the observed EMIC wave spectra and local plasma parameters, we compute wave diffusion rates and model the evolution of electron pitch angle distributions. By comparing model results with local observations of pitch angle distributions, we show direct, quantitative evidence of EMIC wave-driven relativistic electron losses in the Earth's outer radiation belt.

Description: We estimate gravity changes due to co-seismic horizontal deformation and inclined bathymetry (referred to as topographic effects in this study) associated with the 2011 Tohoku-Oki earthquake, using a layered half-space dislocation model. As an approximation, we represent inclined topography with ramp-body models in the near field of the earthquake, and calculate topographic effects from both an inclined seafloor and Moho. The Moho effect partly compensates that from the seafloor. An additional contribution comes from seawater column variations as the sloped seafloor moves trench-ward. Topographic effects are first-order contributions to co-seismic gravity changes for the 2011 Tohoku-Oki earthquake. After 300 km Gaussian smoothing, comparable to the spatial resolution of the Gravity Recovery and Climate Experiment (GRACE), topographic effects create a dominantly negative gravity pattern with amplitudes up to 1.1 μGal for oceanic areas surrounding the epicenter. There are discrepancies between model predictions (-10.4 to +6.9 μGal) and GRACE observations (-10.1 to +1.7 μGal). Errors in the adopted fault-slip model and difficulties in separating co- and post-seismic deformation may contribute to these, but topographic effects, currently neglected in dislocation models, are also responsible. Topographic effects ought to be included in co-seismic fault slip and related gravity modeling for the 2011 Tohoku-Oki earthquake and similar events in regions of steep topography.

Description: Abstract Hydrochlorofluorocarbons (HCFCs), the main substitutes of chlorofluorocarbons, are regulated by the Montreal Protocol. Chinese HCFC emissions increased fast from the beginning of this century. However, limit reports based on atmospheric measurement are available for years after 2011, an important period when significant changes are expected. Combining atmospheric observations at seven sites across China with a FLEXible PARTicle dispersion model‐based Bayesian inversion technique, we estimate emission magnitudes and changes of four major HCFCs in China during 2011–2017. The emissions of all four HCFCs reached peaks before 2015. Our results agreed well with the reported bottom‐up inventories. The Chinese ozone depletion potential (ODP)‐weighted emission of the three most abundant HCFCs accounted for 37% of global totals from 2011 to 2016. The total emission of HCFC‐22 from China, the European Union, and the United States accounted approximately a half of the global totals, suggesting large HCFC emission emitted from the rest of the world.

Description: Vegetation cover in dry regions is a key variable determining desertification. Soils exposed to rainfall by desertification can form physical crusts that reduce infiltration, exacerbating water stress on the remaining vegetation. Paradoxically, field studies show that crust removal is associated with plant mortality in desert systems, while artificial biological crusts can improve plant regeneration. Here, it is shown how physical crusts can act as either drivers of, or buffers against desertification depending on their environmental context. The behavior of crusts is first explored using a simplified theory for water movement on a uniform, partly vegetated slope subject to stationary hydrologic conditions. Numerical model runs supplemented with field data from a semiarid Long-Term Ecological Research (LTER) site are then applied to represent more realistic environmental conditions. When vegetation cover is significant, crusts can drive desertification, but this process is potentially self-limiting. For low vegetation cover, crusts mitigate against desertification by providing water subsidy to plant communities through a runoff-runon mechanism.

Description: Scale dependence of Hortonian rainfall-runoff processes has received much attention in the literature but has not been fully resolved. To further explore this issue, a recently developed model was applied to simulate rainfall-infiltration-runoff processes at multiple spatial scales. The model consists of the coupling between a two-dimensional runoff routing module and a two-layer infiltration module, thus accounting for spatial variability in soil properties, soil surface sealing, topography and partial vegetation cover. A 76 m 2 semiarid experimental plot with sparse cover of vegetation patches and a sealed soil surface in inter-patch bare areas was used as a representative elementary area (REA). A series of four larger artificial plots of different areas was created based on this REA to examine the scale dependence of rainfall-runoff relationships in the case of stationary heterogeneity. Results show that runoff depth (or runoff coefficient) decreases with increasing scale. This trend is more prominent at scales less than 10 times the REA length. Power-law relationships can quantitatively describe the scaling law. The major mechanism of the scale effect is run-on infiltration. However, rainfall intensity and soil properties can both affect the scaling trend through their interaction with run-on. Higher intensity and less temporal variability of rainfall can both reduce the scale effect. Temporally intermittent rainfall may produce spatially oscillating infiltration rates at large scales. Vegetation patterns are another factor that may affect the scaling. Random vegetation patterns, compared with regular patterns with similar statistical properties, change the spatial distributions, but do not significantly change either the total amount and statistical properties of infiltration and runoff or the scale dependence of the rainfall-runoff process. This article is protected by copyright. All rights reserved.

Description: Modulation of whistler mode chorus waves, which plays an important role in driving the pulsating aurora and other processes related to energetic electron dynamics, is an interesting but a long-standing unresolved problem. Here we utilize in situ observations from the THEMIS spacecraft to investigate the role of density variations in the modulation of the chorus wave amplitude, which forms a complementary study to the modulation of chorus by compressional Pc4–5 pulsations presented in a companion paper. We show that these density variations are correlated remarkably well with modulated chorus intensity and typically occur on a timescale of a few seconds to tens of seconds. Both density depletions (DD) and density enhancements (DE) are frequently correlated with increases in chorus wave amplitudes. Furthermore, density enhancements cause a lowering of the central frequencies of the generated chorus waves and vice versa. DD events are more likely to be related to quasi-periodic chorus emissions and thus may be related to the generation of the pulsating aurora. A systematic survey of both DD and DE events shows that DD events preferentially occur between premidnight and dawn, whereas DE events dominantly occur from dawn to noon. We also evaluate the growth rates of chorus waves using linear theory for both DD and DE events and show that both density depletions and enhancements can lead to an intensification of chorus wave growth. However, other potential mechanisms for chorus intensification caused by density variations such as wave trapping by density crests and troughs cannot be excluded.

Description: Due to having strong anisotropy in their polarization state and spatial structure, it is believed that kinetic Alfvén waves (KAWs) can play an important role in various energization phenomena of plasma particles and the fine-structure formation in magneto-plasma environments. The filamentous fine-structures are a kind of the common density inhomogeneity phenomena in magneto-plasmas, and hence, the density gradient is one of the sources of free energy that can lead to KAWs instabilities. In this paper, based on the two-fluid model in which ions and electrons are treated as separate fluids, we investigate the effect of density gradient inhomogeneous on the dispersion and instability of KAWs in a magneto-plasma. The results show that KAW instability can be excited effectively by the density gradient. Especially, both the real frequency ω R and the growth rate ω I of KAWs are dramatically dependent on the spatial position x in the presence of an inhomogeneous density gradient. The results also show that the real frequency increases with the characteristic spatial scale of inhomogeneity , while the growth rate of KAWs has maximum in the growing ranges of . On the other hand, the excited KAWs are weakly dispersive with λ e k x  〈 1. The results have potential importance for better understanding the microphysics of the filamentous fine-structure formation since the phenomena of density gradient inhomogeneous are ubiquitous in various magneto-plasmas, such as in the laboratory plasma as well as in both the solar coronal and terrestrial auroral plasmas.

Description: [1]  The characteristics of the Poynting flux and wave normal vectors of whistler-mode waves outside the plasmapause are investigated for the lower (0.1-0.5 f ce ) and upper bands (0.5-0.8 f ce ), where f ce is the equatorial electron cyclotron frequency. To analyze the wave properties, we utilized high-resolution waveform data from multiple THEMIS spacecraft in the near-equatorial magnetosphere from June 2008 to November 2012. Full measurements of the wave electric and magnetic fields are used to calculate the Poynting fluxes and construct the wave normal vectors, which are then used to calculate the polar and azimuthal angles with respect to the background magnetic field. Statistical results show that the majority of whistler-mode waves propagate away from the magnetic equator, suggesting that the major source region is very close to the equator. The lower-band wave normal angle distribution shows a major peak close to the field line direction and a secondary peak near the resonance cone. In contrast, the wave normal distribution of upper-band waves exhibits a broad distribution between 0° and 60° with the largest probability at ~0°. The azimuthal component of the wave normal vector predominantly points radially outward for both lower and upper band waves, but a tendency for azimuthal propagation is observed for lower band waves in the day and dusk sectors probably due to pronounced azimuthal density gradients in the afternoon sector. Our statistical results provide crucial information on the Poynting fluxes and wave normal vectors of whistler-mode waves, which play a significant role in radiation belt electron dynamics.

Description: Chorus-like whistler-mode waves that are known to play a fundamental role in driving radiation-belt dynamics are excited on the Large Plasma Device by the injection of a helical electron beam into a cold plasma. The mode structure of the excited whistler wave is identified using a phase-correlation technique showing that the waves are excited through a combination of Landau resonance, cyclotron resonance and anomalous cyclotron resonance. The dominant wave mode excited through cyclotron resonance is quasi-parallel propagating, whereas wave modes excited through Landau resonance and anomalous cyclotron resonance propagate at oblique angles that are close to the resonance cone. An analysis of the linear wave growth rates captures the major observations in the experiment. The results have important implications for the generation process of whistler waves in the Earth's inner magnetosphere.