Observations of Radiation Belt Losses Due to Cyclotron Wave-Particle Interactions

Electron loss to the atmosphere plays a critical role in driving dynamics of the Earths Van Allen radiation belts and slot region. This is a review of atmospheric loss of radiation belt electrons caused by plasma wave scattering via Doppler-shifted cyclotron resonance. In particular, the focus is on observational signatures of electron loss, which include direct measurements of precipitating electrons, measured properties of waves that drive precipitation, and variations in the trapped population resulting from loss. We discuss wave and precipitation measurements from recent missions, including simultaneous multi-payload observations, which have provided new insight into the dynamic nature of the radiation belts

Asymmetric Genome Organization in an RNA Virus Revealed via Graph-Theoretical Analysis of Tomographic Data

Cryo-electron microscopy permits 3-D structures of viral pathogens to be determined in remarkable detail. In particular, the protein containers encapsulating viral genomes have been determined to high resolution using symmetry averaging techniques that exploit the icosahedral architecture seen in many viruses. By contrast, structure determination of asymmetric components remains a challenge, and novel analysis methods are required to reveal such features and characterize their functional roles during infection. Motivated by the important, cooperative roles of viral genomes in the assembly of single-stranded RNA viruses, we have developed a new analysis method that reveals the asymmetric structural organization of viral genomes in proximity to the capsid in such viruses. The method uses geometric constraints on genome organization, formulated based on knowledge of icosahedrally-averaged reconstructions and the roles of the RNA-capsid protein contacts, to analyse cryo-electron tomographic data. We apply this method to the low-resolution tomographic data of a model virus and infer the unique asymmetric organization of its genome in contact with the protein shell of the capsid. This opens unprecedented opportunities to analyse viral genomes, revealing conserved structural features and mechanisms that can be targeted in antiviral drug desig

Stretching Actin Filaments within Cells Enhances their Affinity for the Myosin II Motor Domain

To test the hypothesis that the myosin II motor domain (S1) preferentially binds to specific subsets of actin filaments in vivo, we expressed GFP-fused S1 with mutations that enhanced its affinity for actin in Dictyostelium cells. Consistent with the hypothesis, the GFP-S1 mutants were localized along specific portions of the cell cortex. Comparison with rhodamine-phalloidin staining in fixed cells demonstrated that the GFP-S1 probes preferentially bound to actin filaments in the rear cortex and cleavage furrows, where actin filaments are stretched by interaction with endogenous myosin II filaments. The GFP-S1 probes were similarly enriched in the cortex stretched passively by traction forces in the absence of myosin II or by external forces using a microcapillary. The preferential binding of GFP-S1 mutants to stretched actin filaments did not depend on cortexillin I or PTEN, two proteins previously implicated in the recruitment of myosin II filaments to stretched cortex. These results suggested that it is the stretching of the actin filaments itself that increases their affinity for the myosin II motor domain. In contrast, the GFP-fused myosin I motor domain did not localize to stretched actin filaments, which suggests different preferences of the motor domains for different structures of actin filaments play a role in distinct intracellular localizations of myosin I and II. We propose a scheme in which the stretching of actin filaments, the preferential binding of myosin II filaments to stretched actin filaments, and myosin II-dependent contraction form a positive feedback loop that contributes to the stabilization of cell polarity and to the responsiveness of the cells to external mechanical stimuli

The evolution of the cytoskeleton

The cytoskeleton is a system of intracellular filaments crucial for cell shape, division, and function in all three domains of life. The simple cytoskeletons of prokaryotes show surprising plasticity in composition, with none of the core filament-forming proteins conserved in all lineages. In contrast, eukaryotic cytoskeletal function has been hugely elaborated by the addition of accessory proteins and extensive gene duplication and specialization. Much of this complexity evolved before the last common ancestor of eukaryotes. The distribution of cytoskeletal filaments puts constraints on the likely prokaryotic line that made this leap of eukaryogenesis

A distributed lag-autoregressive model of geostationary relativistic electron fluxes: comparing the influences of waves, seed and source electrons, and solar wind inputs

Relativistic electron flux at geosynchronous orbit depends on enhancement and loss processes driven by ULF Pc5, chorus, and EMIC waves, seed electron flux, magnetosphere compression, the "Dst effect", and substorms, while solar wind inputs such as velocity, number density, and IMF Bz drive these factors and thus correlate with flux. Distributed lag regression models show the time delay of highest influence of these factors on log10 high energy electron flux (0.7 – 7.8 MeV, LANL satellites). Multiple regression with an autoregressive term (flux persistence) allows direct comparison of the magnitude of each effect while controlling other correlated parameters. Flux enhancements due to ULF Pc5 and chorus waves are of equal importance. The direct effect of substorms on high energy electron flux is strong, possibly due to injection of high energy electrons by the substorms themselves. Loss due to EMIC waves is less influential. Southward Bz shows only moderate influence when correlated processes are accounted for. Adding covariate compression effects (pressure and IMF magnitude) allows wave‐driven enhancements to be more clearly seen. Seed electrons (270 keV) are most influential at lower relativistic energies, showing that such a population must be available for acceleration. However, they are not accelerated directly to the highest energies. Source electrons (31.7 keV) show no direct influence when other factors are controlled. Their action appears to be indirect via the chorus waves they generate. Determination of specific effects of each parameter when studied in combination will be more helpful in furthering modelling work than studying them individually

Chorus-driven resonant scattering of diffuse auroral electrons in nondipolar magnetic fields

We perform a comprehensive analysis of resonant scattering of diffuse auroral electrons by oblique nightside chorus emissions present along a field line with an equatorial crossing of 6 R(E) at 00: 00 MLT, using various nondipolar Tsyganenko magnetic field models. Bounce-averaged quasi-linear diffusion coefficients are evaluated for both moderately and actively disturbed geomagnetic conditions using the T89, T96, and T01s models. The results indicate that inclusion of nondipolar magnetic field leads to significant changes in bounce-averaged rates of both pitch angle and momentum diffusion for 200 eV to 10 keV plasma sheet electrons. Compared to the results using a dipole field, the rates of pitch angle diffusion obtained using the Tsyganenko models are enhanced at all resonant pitch angles for 200 eV electrons. In contrast, for 500 eV to 10 keV electrons the rates of pitch angle scattering are enhanced at intermediate and/or high pitch angles but tend to be considerably lower near the loss cone, thus reducing the precipitation loss compared to that in a dipole field. Upper band chorus acts as the dominant cause for scattering loss of 200 eV to 2 keV electrons, while lower band chorus scattering prevails for 5-10 keV electrons, consistent with the results using the dipole model. The first-order cyclotron resonance and the Landau resonance are mainly responsible for the net scattering rates of plasma sheet electrons by oblique chorus waves and also primarily account for the differences in bounce-averaged diffusion coefficients introduced by the use of Tsyganenko models. As the geomagnetic activity increases, the differences in scattering rates compared to the dipole results increase accordingly. Nonnegligible differences also occur particularly at high pitch angles for the diffusion rates between the Tsyganenko models, showing an increase with geomagnetic activity level and a dependence on the discrepancy between the Tsyganenko model fields. The strong dependence of bounce-averaged quasi-linear scattering rates on the adopted global magnetic field model and geomagnetic activity level demonstrates that realistic magnetic field models should be incorporated into future modeling efforts to accurately quantify the role of magnetospheric chorus in driving the diffuse auroral precipitation and the formation of electron pancake distributions