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Saturn's largest icy moon, Rhea, hosts a tenuous surface-sputtered exosphere composed primarily of molecular oxygen and carbon dioxide. In this Letter, we examine Cassini Plasma Spectrometer velocity space distributions near Rhea and confirm that Cassini detected nongyrotropic fluxes of outflowing CO$_2^+$ during both the R1 and R1.5 encounters. Accounting for this nongyrotropy, we show that these possess comparable alongtrack densities of $\sim$2$\times$10$^{-3}$ cm$^{-3}$. Negatively charged pickup ions, also detected during R1, are surprisingly shown as consistent with mass 26$\pm$3 u which we suggest are carbon-based compounds, such as CN$^-$, C$_2$H$^-$, C$_2^-$, or HCO$^-$, sputtered from carbonaceous material on the moons surface. These negative ions are calculated to possess alongtrack densities of $\sim$5$\times$10$^{-4}$ cm$^{-3}$ and are suggested to derive from exogenic compounds, a finding consistent with the existence of Rhea's dynamic CO$_2$ exosphere and surprisingly low O$_2$ sputtering yields. These pickup ions provide important context for understanding the exospheric and surface-ice composition of Rhea and of other icy moons which exhibit similar characteristics., Comment: 6 pages, 4 figures, submitted to Geophysical Research Letters on 29th November 2017 and accepted on 24th January 2018

We calculate the spin transport of hydrogenated graphene using the Landauer-B\"uttiker formalism with a spin-dependent tight-binding Hamiltonian. The advantages of using this method is that it simultaneously gives information on sheet resistance and localization length as well as spin relaxation length. Furthermore, the Landauer-B\"uttiker formula can be computed very efficiently using the recursive Green's function technique. Previous theoretical results on spin relaxation time in hydrogenated graphene have not been in agreement with experiments. Here, we study magnetic defects in graphene with randomly aligned magnetic moments, where interference between spin-channels is explicitly included. We show that the spin relaxation length and sheet resistance scale nearly linearly with the impurity concentration. Moreover, the spin relaxation mechanism in hydrogenated graphene is Markovian only near the charge neutrality point or in the highly dilute impurity limit.

In order to use graphene for semiconductor applications, such as transistors with high on/off ratios, a band gap must be introduced into this otherwise semimetallic material. A promising method of achieving a band gap is by introducing nanoscale perforations (antidots) in a periodic pattern, known as a graphene antidot lattice (GAL). A graphene antidot barrier (GAB) can be made by introducing a 1D GAL strip in an otherwise pristine sheet of graphene. In this paper, we will use the Dirac equation (DE) with a spatially varying mass term to calculate the electronic transport through such structures. Our approach is much more general than previous attempts to use the Dirac equation to calculate scattering of Dirac electrons on antidots. The advantage of using the DE is that the computational time is scale invariant and our method may therefore be used to calculate properties of arbitrarily large structures. We show that the results of our Dirac model are in quantitative agreement with tight-binding for hexagonal antidots with armchair edges. Furthermore, for a wide range of structures, we verify that a relatively narrow GAB, with only a few antidots in the unit cell, is sufficient to give rise to a transport gap.

The electronic properties of graphene may be changed from semimetallic to semiconducting by introducing perforations (antidots) in a periodic pattern. The properties of such graphene antidot lattices (GALs) have previously been studied using atomistic models, which are very time consuming for large structures. We present a continuum model that uses the Dirac equation (DE) to describe the electronic and optical properties of GALs. The advantages of the Dirac model are that the calculation time does not depend on the size of the structures and that the results are scalable. In addition, an approximation of the band gap using the DE is presented. The Dirac model is compared with nearest-neighbour tight-binding (TB) in order to assess its accuracy. Extended zigzag regions give rise to localized edge states, whereas armchair edges do not. We find that the Dirac model is in quantitative agreement with TB for GALs without edge states, but deviates for antidots with large zigzag regions., Comment: 15 pages, 7 figures. Accepted by Journal of Physics: Condensed matter

Electrons can be accelerated to ultrarelativistic energies at strong (high-Mach number) collisionless shock waves that form when stellar debris rapidly expands after a supernova. Collisionless shock waves also form in the flow of particles from the Sun (the solar wind), and extensive spacecraft observations have established that electron acceleration at these shocks is effectively absent whenever the upstream magnetic field is roughly parallel to the shock surface normal (quasi-parallel conditions). However, it is unclear whether this magnetic dependence of electron acceleration also applies to the far stronger shocks around young supernova remnants, where local magnetic conditions are poorly understood. Here we present Cassini spacecraft observations of an unusually strong solar system shock wave (Saturn's bow shock) where significant local electron acceleration has been confirmed under quasi-parallel magnetic conditions for the first time, contradicting the established magnetic dependence of electron acceleration at solar system shocks. Furthermore, the acceleration led to electrons at relativistic energies (~MeV), comparable to the highest energies ever attributed to shock-acceleration in the solar wind. These observations demonstrate that at high-Mach numbers, like those of young supernova remnant shocks, quasi-parallel shocks become considerably more effective electron accelerators., Comment: 16 pages, 4 figures, accepted for publication in Nature Physics. Following journal guidelines, this is the submitted version (the accepted version will be uploaded 6 months after publication)

The experimental data on GOES magnetic measurements and plasma measurements on LANL geosynchronous satellites is used for selection of 169 case events containing 638 geosynchronous magnetopause crossings (GMCs) in 1995 to 2001. We study the necessary conditions for the geosynchronous magnetopause crossings using scatter plot of the GMCs in the coordinate space of Psw versus Bz. In such representation the upstream solar wind conditions demonstrate sharp envelope boundary under which no GMCs are occurred. The boundary has two strait horizontal branches where Bz does not influence on the magnetopause location. The first branch is located in the range of Psw=21 nPa for large positive Bz and is associated with an asymptotic regime of the pressure balance. The second branch asymptotically approaches to the range of Psw=4.8 nPa under very strong negative Bz and it is associated with a regime of the Bz influence saturation. We suggest that the saturation is caused by relatively high contribution of the magnetosphere thermal pressure into the pressure balance on the magnetopause. The intermediate region of the boundary for the moderate negative and small positive IMF Bz can be well approximated by a hyperbolic tangent function. We interpret the envelope boundary as a range of necessary upstream solar wind conditions required for GMC in the point on the magnetopause located mostly close to the Earth ("perigee" point). We obtain that the dipole tilt angle and dawn-dusk asymmetry influence on the "perigee" point location. We find that the aGSM latitude of this point depends linearly on the dipole tilt angle with the slope about -0.5. The aGSM longitude of the "perigee" point decreases with IMF Bz with a rate of about 2 angular minutes per 1 nT. An empirical model predicting the magnetopause crossing of the geosynchronous orbit in the "perigee" point is proposed., Comment: Review of the previous results, 68 pages, 16 figures

Using a model of force balance in Saturn's disk-like magnetosphere, we show that variations in hot plasma pressure can change the magnetic field configuration. This effect changes (i) the location of the magnetopause, even at fixed solar wind dynamic pressure, and (ii) the magnetic mapping between ionosphere and disk. The model uses equatorial observations as a boundary condition-we test its predictions over a wide latitude range by comparison with a Cassini high-inclination orbit of magnetic field and hot plasma pressure data. We find reasonable agreement over time scales larger than the period of Saturn kilometric radiation (also known as the camshaft period)., Comment: 15 pages, 3 figures, accepted for publication in GRL