Smithsonian Astrophysical Observatory

About: Smithsonian Astrophysical Observatory is a facility organization based out in Cambridge, Massachusetts, United States. It is known for research contribution in the topics: Galaxy & Stars. The organization has 1665 authors who have published 3622 publications receiving 132183 citations. The organization is also known as: SAO.

Heating of coronal plasma by anomalous current dissipation

Abstract: We show that the observed high temperature and inhomogeneous structure of the solar corona, as well as the long-term spatial and temporal evolution of coronal features, is economically explained by in situ heating of the coronal plasma via anomalous current dissipation. The basic geometrical structure is a loop configuration heated by nearly field-aligned currents occupying a small fraction of the total loop volume. Energy is transferred from the turbulent convective zone and photosphere, where ..beta..> or approx. =1, into the low-..beta.. corona via the magnetic fields which link the two regimes. The coronal currents are generated initially by relaxation of emerging magnetic flux to the nearly force-free configuration, and subsequent quasi-steady deposition of energy is achieved via induction processes arising from the continual transfer of azimuthal flux to the corona and from direct generation of electric fields along the flux tube by subphotospheric changes in flux linkage. Laboratory experiments show that the current filamentation necessary for this model can occur if the effective resistivity and radiative losses are strongly temperature dependent, as is the case in the solar corona. As a result, local temperature increases lead, via a regenerative process, to further temperature enhancement; the relative ineffectiveness of cross-field thermalmore » trnasport leads to well-localized channels of current flow.« less

The three-dimensional structures of X-ray bright points

Abstract: Recently, the Converging Flux Model has been proposed for X-ray bright points and cancelling magnetic features. The aim of this peice of work is to try and model theoretically specific X-ray bright points in the framework of the Converging Flux Model. The observational data used includes a magnetogram showing the normal component of the magnetic field at the photosphere and a high-resolution soft X-ray image from NIXT showing the brightenings in the lower solar corona. By approximating the flux concentrations in the magnetograms with poles of the appropriate sign and sense, the overlying three-dimensional potential field structure is calculated. Deduction of plausible motions of the flux sources are made which produce brightenings of the observed shape due to reconnection between neighbouring flux regions. Also the three-dimensional separarix and separator structure and the way the magnetic field lines reconnect in three dimensions is deduced.

Stringent x-ray constraints on mass loss from proxima centauri

Abstract: We have analyzed data from two Chandra imaging observations of Proxima Centauri, searching for an X-ray halo arising from charge exchange between highly charged ions in its stellar wind and neutral gas in the surrounding interstellar medium. Based upon our model of Proxima Cen's charge exchange emission, the absence of any detectable charge exchange signal places a statistical 3 σ upper limit of ~3 × 10-13 M☉ yr-1 (14☉) on the mass-loss rate (9☉ for the 2 σ limit and 4☉ for 1 σ), with a model uncertainty of roughly a factor of 3. This is orders of magnitude smaller than the upper limits that have been placed on late-type dwarf stars using radio observations, and it supports a recent mass-loss result for Proxima Cen based on Lyα absorption profiles. We have also studied the coronal spectrum, both in quiescence and during a prominent flare. Results are consistent with those obtained in previous X-ray observations, but a firm determination of coronal metal abundances remains elusive.