188 research outputs found
What do iris observations of Mg II k tell us about the solar plage chromosphere?
We analyze observations from the Interface Region Imaging Spectrograph of the
Mg II k line, the Mg II UV subordinate lines, and the O I 135.6 nm line to
better understand the solar plage chromosphere. We also make comparisons with
observations from the Swedish 1 m Solar Telescope of the H{\alpha} line, the Ca
II 8542 line, and Solar Dynamics Observatory/Atmospheric Imaging Assembly
observations of the coronal 19.3 nm line. To understand the observed Mg II
profiles, we compare these observations to the results of numerical
experiments. The single-peaked or flat-topped Mg II k profiles found in plage
imply a transition region at a high column mass and a hot and dense
chromosphere of about 6500 K. This scenario is supported by the observed
large-scale correlation between moss brightness and filled-in profiles with
very little or absent self-reversal. The large wing width found in plage also
implies a hot and dense chromosphere with a steep chromospheric temperature
rise. The absence of emission in the Mg II subordinate lines constrain the
chromospheric temperature and the height of the temperature rise while the
width of the O I 135.6 nm line sets a limit to the non-thermal velocities to
around 7 km/s
What causes the high apparent speeds in chromospheric and transition region spicules on the Sun?
Spicules are the most ubuiquitous type of jets in the solar atmosphere. The
advent of high-resolution imaging and spectroscopy from the Interface Region
Imaging Spectrograph (IRIS) and ground-based observatories has revealed the
presence of very high apparent motions of order 100-300 km/s in spicules, as
measured in the plane of the sky. However, line-of-sight measurements of such
high speeds have been difficult to obtain, with values deduced from Doppler
shifts in spectral lines typically of order 30-70 km/s. In this work we resolve
this long-standing discrepancy using recent 2.5D radiative MHD simulations.
This simulation has revealed a novel driving mechanism for spicules in which
ambipolar diffusion resulting from ion-neutral interactions plays a key role.
In our simulation we often see that the upward propagation of magnetic waves
and electrical currents from the low chromosphere into already existing
spicules can lead to rapid heating when the currents are rapidly dissipated by
ambipolar diffusion. The combination of rapid heating and the propagation of
these currents at Alfv\'enic speeds in excess of 100 km/s leads to the very
rapid apparent motions, and often wholesale appearance, of spicules at
chromospheric and transition region temperatures. In our simulation, the
observed fast apparent motions in such jets are actually a signature of a
heating front, and much higher than the mass flows, which are of order 30-70
km/s. Our results can explain the behavior of transition region "network jets"
and the very high apparent speeds reported for some chromospheric spicules.Comment: 8 pages, 5 figures, accepted for publication in ApJ Letter
Chromospheric thermodynamic conditions from inversions of complex Mg II h & k profiles observed in flares
The flare activity of the Sun has been studied for decades, using both space- and ground-based telescopes. The former have mainly focused on the corona, while the latter have mostly been used to investigate the conditions in the chromosphere and photosphere. The Interface Region Imaging Spectrograph (IRIS) instrument has served as a gateway between these two cases, given its capability to observe quasi-simultaneously the corona, the transition region, and the chromosphere using different spectral lines in the near- and far-ultraviolet ranges. IRIS thus provides unique diagnostics to investigate the thermodynamics of flares in the solar atmosphere. In particular, the Mg II h&k and the Mg II UV triplet lines provide key information about the thermodynamics of low to upper chromosphere, while the C II 1334 & 1335 Å lines cover the upper-chromosphere and low transition region. The Mg II h&k and the Mg II UV triplet lines show a peculiar, pointy shape before and during the flare activity. The physical interpretation, i.e., the physical conditions in the chromosphere, that can explain these profiles has remained elusive. In this paper, we show the results of a non-LTE inversion of such peculiar profiles. To better constrain the atmospheric conditions, the Mg II h&k and the Mg II UV triplet lines are simultaneously inverted with the C II 1334 & 1335 Å lines. This combined inversion leads to more accurate derived thermodynamic parameters, especially the temperature and the turbulent motions (micro-turbulence velocity). We use an iterative process that looks for the best fit between the observed profile and a synthetic profile obtained by considering non-local thermodynamic equilibrium and partial frequency redistribution of the radiation due to scattered photons. This method is computationally rather expensive (≈6 CPU-hour/profile). Therefore, we use the k-means clustering technique to identify representative profiles and associated representative model atmospheres. By inverting the representative profiles with the most advanced inversion code (STiC), in addition to recover the main physical parameters, we are able to conclude that these unique, pointy profiles are associated with a large gradient in the line-of-sight velocity along the optical depth in the high chromosphere
Numerical Simulations of Coronal Heating through Footpoint Braiding
Advanced 3D radiative MHD simulations now reproduce many properties of the
outer solar atmosphere. When including a domain from the convection zone into
the corona, a hot chromosphere and corona are self-consistently maintained.
Here we study two realistic models, with different simulated area, magnetic
field strength and topology, and numerical resolution. These are compared in
order to characterize the heating in the 3D-MHD simulations which
self-consistently maintains the structure of the atmosphere. We analyze the
heating at both large and small scales and find that heating is episodic and
highly structured in space, but occurs along loop shaped structures, and moves
along with the magnetic field. On large scales we find that the heating per
particle is maximal near the transition region and that widely distributed
opposite-polarity field in the photosphere leads to a greater heating scale
height in the corona. On smaller scales, heating is concentrated in current
sheets, the thicknesses of which are set by the numerical resolution. Some
current sheets fragment in time, this process occurring more readily in the
higher-resolution model leading to spatially highly intermittent heating. The
large scale heating structures are found to fade in less than about five
minutes, while the smaller, local, heating shows time scales of the order of 2
minutes in one model and 1 minutes in the other, higher-resolution, model.Comment: 20 pages, accepted by Ap
The Effects of Spatio-temporal Resolution on Deduced Spicule Properties
Spicules have been observed on the sun for more than a century, typically in
chromospheric lines such as H-alpha and Ca II H. Recent work has shown that
so-called 'type II' spicules may have a role in providing mass to the corona
and the solar wind. In chromospheric filtergrams these spicules are not seen to
fall back down, and they are shorter-lived and more dynamic than the spicules
that have been classically reported in ground-based observations. Observations
of type II spicules with Hinode show fundamentally different properties from
what was previously measured. In earlier work we showed that these dynamic type
II spicules are the most common type, a view that was not properly identified
by early observations.The aim of this work is to investigate the effects of
spatio-temporal resolution in the classical spicule measurements. Making use of
Hinode data degraded to match the observing conditions of older ground-based
studies, we measure the properties of spicules with a semi-automated algorithm.
These results are then compared to measurements using the original Hinode data.
We find that degrading the data has a significant effect on the measured
properties of spicules. Most importantly, the results from the degraded data
agree well with older studies (e.g. mean spicule duration more than 5 minutes,
and upward apparent velocities of about 25 km/s). These results illustrate how
the combination of spicule superposition, low spatial resolution and cadence
affect the measured properties of spicules, and that previous measurements can
be misleading.Comment: Accepted for publication in ApJ. 5 pages, 3 figures. Movies of
figures 1 and 3 available via Data Conservanc
The Role of Partial Ionization Effects in the Chromosphere
The energy for the coronal heating must be provided from the convection zone.
The amount and the method by which this energy is transferred into the corona
depends on the properties of the lower atmosphere and the corona itself. We
review: 1) how the energy could be built in the lower solar atmosphere; 2) how
this energy is transferred through the solar atmosphere; and 3) how the energy
is finally dissipated in the chromosphere and/or corona. Any mechanism of
energy transport has to deal with the various physical processes in the lower
atmosphere. We will focus on a physical process that seems to be highly
important in the chromosphere and not deeply studied until recently: the
ion-neutral interaction effects (INIE) in the chromosphere. We review the
relevance and the role of the partial ionization in the chromosphere and show
that this process actually impacts considerably the outer solar atmosphere. We
include analysis of our 2.5D radiative MHD simulations with the Bifrost code
(Gudiksen et al. 2011) including the partial ionization effects on the
chromosphere and corona and thermal conduction along magnetic field lines. The
photosphere, chromosphere and transition region are partially ionized and the
interaction between ionized particles and neutral particles has important
consequences on the magneto-thermodynamics of these layers. The INIE are
treated using generalized Ohm's law, i.e., we consider the Hall term and the
ambipolar diffusion in the induction equation. The interaction between the
different species affects the modeled atmosphere as follows: 1) the ambipolar
diffusion dissipates magnetic energy and increases the minimum temperature in
the chromosphere; 2) the upper chromosphere may get heated and expanded over a
greater range of heights. These processes reveal appreciable differences
between the modeled atmospheres of simulations with and without INIE.Comment: 25 pages, 3 figures, accepted to be published in Royal Societ
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