133 research outputs found
Simulating coronal condensation dynamics in 3D
We present numerical simulations in 3D settings where coronal rain phenomena
take place in a magnetic configuration of a quadrupolar arcade system. Our
simulation is a magnetohydrodynamic simulation including anisotropic thermal
conduction, optically thin radiative losses, and parametrised heating as main
thermodynamical features to construct a realistic arcade configuration from
chromospheric to coronal heights. The plasma evaporation from chromospheric and
transition region heights eventually causes localised runaway condensation
events and we witness the formation of plasma blobs due to thermal instability,
that evolve dynamically in the heated arcade part and move gradually downwards
due to interchange type dynamics. Unlike earlier 2.5D simulations, in this case
there is no large scale prominence formation observed, but a continuous coronal
rain develops which shows clear indications of Rayleigh-Taylor or interchange
instability, that causes the denser plasma located above the transition region
to fall down, as the system moves towards a more stable state. Linear stability
analysis is used in the non-linear regime for gaining insight and giving a
prediction of the system's evolution. After the plasma blobs descend through
interchange, they follow the magnetic field topology more closely in the lower
coronal regions, where they are guided by the magnetic dips.Comment: 47 pages, 59 figure
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Stellar energetic particles in the magnetically turbulent habitable zones of TRAPPIST-1-like planetary systems
Planets in close proximity to their parent star, such as those in the
habitable zones around M dwarfs, could be subject to particularly high doses of
particle radiation. We have carried out test-particle simulations of ~GeV
protons to investigate the propagation of energetic particles accelerated by
flares or travelling shock waves within the stellar wind and magnetic field of
a TRAPPIST-1-like system. Turbulence was simulated with small-scale
magnetostatic perturbations with an isotropic power spectrum. We find that only
a few percent of particles injected within half a stellar radius from the
stellar surface escape, and that the escaping fraction increases strongly with
increasing injection radius. Escaping particles are increasingly deflected and
focused by the ambient spiralling magnetic field as the superimposed turbulence
amplitude is increased. In our TRAPPIST-1-like simulations, regardless of the
angular region of injection, particles are strongly focused onto two caps
within the fast wind regions and centered on the equatorial planetary orbital
plane. Based on a scaling relation between far-UV emission and energetic
protons for solar flares applied to M dwarfs, the innermost putative habitable
planet, TRAPPIST-1e, is bombarded by a proton flux up to 6 orders of magnitude
larger than experienced by the present-day Earth. We note two mechanisms that
could strongly limit EP fluxes from active stars: EPs from flares are contained
by the stellar magnetic field; and potential CMEs that might generate EPs at
larger distances also fail to escape.Comment: 17 pages, 12 figures, ApJ in pres
An Earth-like stellar wind environment for Proxima Centauri c
A new planet has been recently discovered around Proxima Centauri. With an
orbital separation of au and a minimum mass of about
, Proxima c is a prime direct imaging target for atmospheric
characterization. The latter can only be performed with a good understanding of
the space environment of the planet, as multiple processes can have profound
effects on the atmospheric structure and evolution. Here, we take one step in
this direction by generating physically-realistic numerical simulations of
Proxima's stellar wind, coupled to a magnetosphere and ionosphere model around
Proxima c. We evaluate their expected variation due to the magnetic cycle of
the host star, as well as for plausible inclination angles for the exoplanet
orbit. Our results indicate stellar wind dynamic pressures comparable to
present-day Earth, with a slight increase (by a factor of 2) during high
activity periods of the star. A relatively weak interplanetary magnetic field
at the distance of Proxima c leads to negligible stellar wind Joule heating of
the upper atmosphere (about of the solar wind contribution on Earth) for
an Earth-like planetary magnetic field ( G). Finally, we provide an
assessment of the likely extreme conditions experienced by the exoplanet
candidate Proxima d, tentatively located at au with a minimum mass of
.Comment: 9 Pages, 4 Figures, 1 Table. Accepted for publication in The
Astrophysical Journal Letters (ApJL
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