32178161
Exclusion of Tiny Interstellar Dust Grains From the Heliosphere
The distribution of interstellar dust grains (ISDG) observed in the Solar System depends on the nature of the interstellar medium‐solar wind interaction. The charge of the grains couples them to the interstellar magnetic field (ISMF) resulting in some fraction of grains being excluded from the heliosphere while grains on the larger end of the size distribution, with gyroradii comparable to the size of the heliosphere, penetrate the termination shock. This results in a skewing the size distribution detected in the Solar System. 
 
We present new calculations of grain trajectories and the resultant grain density distribution for small ISDGs propagating through the heliosphere. We make use of detailed heliosphere model results, using three‐dimensional (3‐D) magnetohydrodynamic/kinetic models designed to match data on the shape of the termination shock and the relative deflection of interstellar H° and He° flowing into the heliosphere. We find that the necessary inclination of the ISMF relative to the inflow direction results in an asymmetry in the distribution of the larger grains (0.1 μm) that penetrate the heliopause. Smaller grains (0.01 μm) are completely excluded from the Solar System at the heliopause.
18721076
A practical approach to coronal magnetic field extrapolation based on the principle of minimum dissipation rate
We present a newly developed approach to solar coronal magnetic field extrapolation from vector magnetograms, based on the principle of minimum dissipation rate (MDR). The MDR system was derived from a variational problem that is more suitable for an open and externally driven system, like the solar corona. The resulting magnetic field equation is more general than force-free. Its solution can be expressed as the superposition of two linear (constant-α) force-free fields (LFFFs) with distinct α parameters, and one potential field. Thus, the original extrapolation problem is decomposed into three LFFF extrapolations, utilizing boundary data. The full MDR-based approach requires two layers of vector magnetograph measurements on the solar surface, while a slightly modified practical approach only requires one. We test both approaches against three-dimensional MHD simulation data in a finite volume. Both yield quantitatively good results. The errors in the magnetic energy estimate are within a few percent. In particular, the main features of relatively strong perpendicular current density structures, representative of the non-force-freeness of the solution, are well recovered.
118571843
Effect of collective neutrino oscillations on the neutrino mechanism of core-collapse supernovae
In the seconds after collapse of a massive star, the newborn protoneutron star (PNS) radiates neutrinos of all flavours. The absorption of electron-type neutrinos below the radius of the stalled shockwave may drive explosions (the ‘neutrino mechanism’). Because the heating rate is proportional to the square of neutrino energy, flavour conversion of μ and τ neutrinos to electron-type neutrinos via collective neutrino oscillations (CνO) may in principle increase the heating rate and drive explosions. In order to assess the potential importance of CνO for the shock revival, we solve the steady-state boundary value problem of spherically symmetric accretion between the PNS surface (rν) and the shock (rS), including a scheme for flavour conversion via CνO. For a given rν, PNS mass (M), accretion rate () and assumed values of the neutrino energies from the PNS, we calculate the critical neutrino luminosity above which accretion is impossible and explosion results. We show that CνO can decrease the critical luminosity by a factor of at most ∼1.5, but only if the flavour conversion is fully completed inside rS and if there is no matter suppression. The magnitude of the effect depends on the model parameters (M, and rν) through the shock radius and the physical scale for flavour conversion. We quantify these dependencies and find that CνO could lower the critical luminosity only for small M and , and large rν. However, for these parameter values CνO are suppressed due to matter effects. By quantifying the importance of CνO and matter suppression at the critical neutrino luminosity for explosion, we show in agreement with previous studies that CνO are unlikely to affect the neutrino mechanism of core-collapse supernovae significantly.
12895171
Dark radiation alleviates problems with dark matter halos.
We show that a scalar and a fermion charged under a global U(1) symmetry can not only explain the existence and abundance of dark matter (DM) and dark radiation (DR), but can also imbue DM with improved scattering properties at galactic scales, while remaining consistent with all other observations. Delayed DM-DR kinetic decoupling eases the missing satellites problem, while scalar-mediated self-interactions of DM ease the cusp versus core and too big to fail problems. In this scenario, DM is expected to be pseudo-Dirac and have a mass 100 keV ≲ m(χ) ≲ 10 GeV. The predicted DR may be measurable using the primordial elemental abundances from big bang nucleosynthesis, and using the cosmic microwave background.