55 research outputs found
Observations of the Kelvin-Helmholtz instability driven by dynamic motions in a solar prominence
Prominences are incredibly dynamic across the whole range of their observable
spatial scales, with observations revealing gravity-driven fluid instabilities,
waves, and turbulence. With all these complex motions, it would be expected
that instabilities driven by shear in the internal fluid motions would develop.
However, evidence of these have been lacking. Here we present the discovery in
a prominence, using observations from the Interface Region Imaging Spectrograph
(IRIS), of a shear flow instability, the Kelvin-Helmholtz sinusoidal-mode of a
fluid channel, driven by flows in the prominence body. This finding presents a
new mechanism through which we can create turbulent motions from the flows
observed in quiescent prominences. The observation of this instability in a
prominence highlights their great value as a laboratory for understanding the
complex interplay between magnetic fields and fluid flows that play a crucial
role in a vast range of astrophysical systems.Comment: 7 pages, 4 figures, accepted for publication in ApJ
Fan Loops Observed by IRIS, EIS and AIA
A comprehensive study of the physical parameters of active region fan loops
is presented using the observations recorded with the Interface Region Imaging
Spectrometer (IRIS), the EUV Imaging Spectrometer (EIS) on-board Hinode and the
Atmospheric Imaging Assembly (AIA) and the Helioseismic and Magnetic Imager
(HMI) on-board the Solar Dynamics Observatory (SDO). The fan loops emerging
from non-flaring AR~11899 (near the disk-center) on 19th November, 2013 are
clearly discernible in AIA 171~{\AA} images and those obtained in \ion{Fe}{8}
and \ion{Si}{7} images using EIS. Our measurements of electron densities reveal
that the footpoints of these loops are approximately at constant pressure with
electron densities of 10.1 cm at
(\ion{O}{4}), and 8.9 cm at
(\ion{Si}{10}). The electron temperature diagnosed across the fan loops by
means of EM-Loci suggest that at the footpoints, there are two temperature
components at and 5.95, which are picked-up by IRIS lines
and EIS lines respectively. At higher heights, the loops are nearly isothermal
at , that remained constant along the loop. The measurement
of Doppler shift using IRIS lines suggests that the plasma at the footpoints of
these loops is predominantly redshifted by 2-3~km~s in \ion{C}{2},
10-15~km~s in \ion{Si}{4} and 15{--}20~km~s in \ion{O}{4},
reflecting the increase in the speed of downflows with increasing temperature
from to 5.15. These observations can be explained by low
frequency nanoflares or impulsive heating, and provide further important
constraints on the modeling of the dynamics of fan loops.Comment: Accepted for publication in The Astrophysical Journal; 8 Figures, 11
page
A Statistical Study of IRIS Observational Signatures of Nanoflares and Non-thermal Particles
Nanoflares are regarded as one of the major mechanisms of magnetic energy
release and coronal heating in the solar outer atmosphere. We conduct a
statistical study on the response of the chromosphere and transition region to
nanoflares, as observed by the Interface Region Imaging Spectrograph (IRIS), by
using an algorithm for the automatic detection of these events. The initial
atmospheric response to these small heating events is observed, with IRIS, as
transient brightening at the footpoints of coronal loops heated to high
temperatures (>4 MK). For four active regions, observed over 143 hours, we
detected 1082 footpoint brightenings under the IRIS slit, and for those we
extracted physical parameters from the IRIS Mg II and Si IV spectra that are
formed in the chromosphere and transition region, respectively. We investigate
the distribution of the spectral parameters, and the relationship between the
parameters, also comparing them with predictions from RADYN numerical
simulations of nanoflare-heated loops. We find that these events, and the
presence of non-thermal particles, tend to be more frequent in flare productive
active regions, and where the hot Atmospheric Imaging Assembly 94 \AA\ emission
is higher. We find evidence for highly dynamic motions characterized by strong
Si IV non-thermal velocity (not dependent on the heliocentric x coordinate,
i.e., on the angle between the magnetic field and the line-of-sight) and
asymmetric Mg II spectra. These findings provide tight new constraints on the
properties of nanoflares, and non-thermal particles, in active regions, and
their effects on the lower atmosphere.Comment: 18 pages, 8 figures, Accepted to Ap
Multi-wavelength observations and modeling of a microflare: constraining non-thermal particle acceleration
In this work we analyze a small B-class flare that occurred on 29 April 2021 and was observed simultaneously by the Interface Region Imaging Spectrograph (IRIS) and the Nuclear Spectroscopic Telescope Array (NuSTAR) X-ray instrument. The IRIS observations of the ribbon of the flare show peculiar spectral characteristics that are typical signatures of energy deposition by non-thermal electrons in the lower atmosphere. The presence of the non-thermal particles is also confirmed directly by fitting the NuSTAR spectral observations. We show that, by combining IRIS and NuSTAR multi-wavelength observations from the corona to the lower atmosphere with hydrodynamic simulations using the RADYN code, we can provide strict constraints on electron-beam heated flare models. This work presents the first NuSTAR, IRIS and RADYN joint analysis of a non-thermal microflare, and presents a self-consistent picture of the flare-accelerated electrons in the corona and the chromospheric response to those electrons
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