5,825 research outputs found
Coronal rain in magnetic bipolar weak fields
We intend to investigate the underlying physics for the coronal rain
phenomenon in a representative bipolar magnetic field, including the formation
and the dynamics of coronal rain blobs. With the MPI-AMRVAC code, we performed
three dimensional radiative magnetohydrodynamic (MHD) simulation with strong
heating localized on footpoints of magnetic loops after a relaxation to quiet
solar atmosphere. Progressive cooling and in-situ condensation starts at the
loop top due to radiative thermal instability. The first large-scale
condensation on the loop top suffers Rayleigh-Taylor instability and becomes
fragmented into smaller blobs. The blobs fall vertically dragging magnetic
loops until they reach low beta regions and start to fall along the loops from
loop top to loop footpoints. A statistic study of the coronal rain blobs finds
that small blobs with masses of less than 10^10 g dominate the population. When
blobs fall to lower regions along the magnetic loops, they are stretched and
develop a non-uniform velocity pattern with an anti-parallel shearing pattern
seen to develop along the central axis of the blobs. Synthetic images of
simulated coronal rain with Solar Dynamics Observatory Atmospheric Imaging
Assembly well resemble real observations presenting dark falling clumps in hot
channels and bright rain blobs in a cool channel. We also find density
inhomogeneities during a coronal rain "shower", which reflects the observed
multi-stranded nature of coronal rain.Comment: 8 figure
Solar flares and Kelvin-Helmholtz instabilities: A parameter survey
Hard X-ray (HXR) sources are frequently observed near the top of solar flare
loops, and the emission is widely ascribed to bremsstrahlung. We here revisit
an alternative scenario which stresses the importance of inverse Compton
processes and the Kelvin- Helmholtz instability (KHI) proposed by Fang et al.
(2016). This scenario adds a novel ingredient to the standard flare model,
where evaporation flows from flare-impacted chromospheric foot-points interact
with each other near the loop top and produce turbulence via KHI. The
turbulence can act as a trapping region and as an efficient accelerator to
provide energetic electrons, which scatter soft X-ray (SXR) photons to HXR
photons via the inverse Compton mechanism. This paper focuses on the trigger of
the KHI and the resulting turbulence in this new scenario. We perform a
parameter survey to investigate the necessary ingredients to obtain KHI through
interaction of chromospheric evaporation flows. When turbulence is produced in
the loop apex, an index of -5/3 can be found in the spectra of velocity and
magnetic field fluctuations. The KHI development and the generation of
turbulence are controlled by the amount of energy deposited in the
chromospheric foot-points and the time scale of its energy deposition, but
typical values for M class flares show the KHI development routinely. Asymmetry
of energy deposition determines the location where the turbulence is produced,
and the synthesized SXR light curve shows a clear periodic signal related to
the sloshing motion of the vortex pattern created by the KHI.Comment: 12 pages, 14 figure
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