438 research outputs found
Dynamic fibrils in H-alpha and C IV
Aim: To study the interaction of the solar chromosphere with the transition
region, in particular active-region jets in the transition region and their
relation to chromospheric fibrils. Methods: We carefully align image sequences
taken simultaneously in C IV with the Transition Region and Coronal Explorer
and in H-alpha with the Swedish 1-m Solar Telescope. We examine the temporal
evolution of "dynamic fibrils", i.e., individual short-lived active-region
chromospheric jet-like features in H-alpha. Results: All dynamic fibrils appear
as absorption features in H-alpha that progress from the blue to the red wing
through the line, and often show recurrent behavior. Some of them, but not all,
appear also as bright features in C IV which develop at or just beyond the apex
of the H-alpha darkening. They tend to best resemble the H-alpha fibril at +700
mA half a minute earlier. Conclusions: Dynamic chromospheric fibrils observed
in H-alpha regularly correspond to transition-region jets observed in the
ultraviolet. This correspondence suggests that some plasma associated with
dynamic fibrils is heated to transition-region temperatures.Comment: 8 pages, 8 figure
Heating of ions by low-frequency Alfv\'{e}n waves in partially ionized plasmas
In the solar atmosphere, the chromospheric and coronal plasmas are much
hotter than the visible photosphere. The heating of the solar atmosphere,
including the partially ionized chromosphere and corona, remains largely
unknown. In this paper we demonstrate that the ions can be substantially heated
by Alfv\'{e}n waves with very low frequencies in partially ionized low beta
plasmas. This differs from other Alfv\'{e}n wave related heating mechanisms
such as ion-neutral collisional damping of Alfv\'{e}n waves and heating
described by previous work on resonant Alfv\'{e}n wave heating. In this paper,
we find that the non-resonant Alfv\'{e}n wave heating is less efficient in
partially ionized plasmas than when there are no ion-neutral collisions, and
the heating efficiency depends on the ratio of the ion-neutral collision
frequency to the ion gyrofrequency.Comment: Published as Letter
On the multi-threaded nature of solar spicules
A dominant constituent in the dynamic chromosphere are spicules. Spicules at
the limb appear as relatively small and dynamic jets that are observed to
everywhere stick out. Many papers emphasize the important role spicules might
play in the energy and mass balance of the chromosphere and corona. However,
many aspects of spicules remain a mystery. In this Letter we shed more light on
the multi-threaded nature of spicules and their torsional component. We use
high spatial, spectral and temporal resolution observations from the Swedish
1-m Solar Telescope in the H{\alpha} spectral line. The data targets the limb
and we extract spectra from spicules far out from the limb to reduce the
line-of-sight superposition effect. We discover that many spicules display very
asymmetric spectra with some even showing multiple peaks. To quantify this
asymmetry we use a double Gaussian fitting procedure and find an average
velocity difference between the single Gaussian components to be between 20-30
km s for a sample of 57 spicules. We observe that spicules show
significant sub-structure where one spicule consists of many 'threads'. We
interpret the asymmetric spectra as line-of-sight superposition of threads in
one spicule and therefore have a measure for a perpendicular flow inside
spicules which will be important for future numerical model to reproduce. In
addition we show examples of {\lambda}-x-slices perpendicular across spicules
and find spectral tilts in individual threads providing further evidence for
the complex dynamical nature of spicules.Comment: Accepted by APJ Letter
Observational Signatures of Simulated Reconnection Events in the Solar Chromosphere and Transition Region
We present the results of numerical simulations of wave-induced magnetic
reconnection in a model of the solar atmosphere. In the magnetic field geometry
we study in this article, the waves, driven by a monochromatic piston and a
driver taken from Hinode observations, induce periodic reconnection of the
magnetic field, and this reconnection appears to help drive long-period
chromospheric jets. By synthesizing observations for a variety of wavelengths
that are sensitive to a wide range of temperatures, we shed light on the often
confusing relationship between the plethora of jet-like phenomena in the solar
atmosphere, e.g., explosive events, spicules, blinkers, and other phenomena
thought to be caused by reconnection.Comment: 13 pages, 22 figures. Submitted to The Astrophysical Journa
Wave Propagation and Jet Formation in the Chromosphere
We present the results of numerical simulations of wave propagation and jet
formation in solar atmosphere models with different magnetic field
configurations. The presence in the chromosphere of waves with periods longer
than the acoustic cutoff period has been ascribed to either strong inclined
magnetic fields, or changes in the radiative relaxation time. Our simulations
include a sophisticated treatment of radiative losses, as well as fields with
different strengths and inclinations. Using Fourier and wavelet analysis
techniques, we investigate the periodicity of the waves that travel through the
chromosphere. We find that the velocity signal is dominated by waves with
periods around 5 minutes in regions of strong, inclined field, including at the
edges of strong flux tubes where the field expands, whereas 3-minute waves
dominate in regions of weak or vertically oriented fields. Our results show
that the field inclination is very important for long-period wave propagation,
whereas variations in the radiative relaxation time have little effect.
Furthermore, we find that atmospheric conditions can vary significantly on
timescales of a few minutes, meaning that a Fourier analysis of wave
propagation can be misleading. Wavelet techniques take variations with time
into account and are more suitable analysis tools. Finally, we investigate the
properties of jets formed by the propagating waves once they reach the
transition region, and find systematic differences between the jets in inclined
field regions and those in vertical field regions, in agreement with
observations of dynamic fibrils.Comment: 27 pages, 29 figures; accepted for publication in Astrophysical
Journa
The formation of IRIS diagnostics II. The formation of the Mg II h&k lines in the solar atmosphere
NASA's Interface Region Imaging Spectrograph (IRIS) small explorer mission
will study how the solar atmosphere is energized. IRIS contains an imaging
spectrograph that covers the Mg II h&k lines as well as a slit-jaw imager
centered at Mg II k. Understanding the observations requires forward modeling
of Mg II h&k line formation from 3D radiation-MHD models.
We compute the vertically emergent h&k intensity from a snapshot of a dynamic
3D radiation-MHD model of the solar atmosphere, and investigate which
diagnostic information about the atmosphere is contained in the synthetic line
profiles. We find that the Doppler shift of the central line depression
correlates strongly with the vertical velocity at optical depth unity, which is
typically located less than 200 km below the transition region (TR). By
combining the Doppler shifts of the h and the k line we can retrieve the sign
of the velocity gradient just below the TR. The intensity in the central line
depression is anticorrelated with the formation height, especially in subfields
of a few square Mm. This intensity could thus be used to measure the spatial
variation of the height of the transition region. The intensity in the
line-core emission peaks correlates with the temperature at its formation
height, especially for strong emission peaks. The peaks can thus be exploited
as a temperature diagnostic. The wavelength difference between the blue and red
peaks provides a diagnostic of the velocity gradients in the upper
chromosphere. The intensity ratio of the blue and red peaks correlates strongly
with the average velocity in the upper chromosphere. We conclude that the Mg II
h&k lines are excellent probes of the very upper chromosphere just below the
transition region, a height regime that is impossible to probe with other
spectral lines.Comment: 15 pages, 12 figures, accepted for ApJ, astro-ph abstract shortened
to confirm to submission requirement
The formation of IRIS diagnostics I. A quintessential model atom of Mg II and general formation properties of the Mg II h&k lines
NASA's Interface Region Imaging Spectrograph (IRIS) space mission will study
how the solar atmosphere is energized. IRIS contains an imaging spectrograph
that covers the Mg II h&k lines as well as a slit-jaw imager centered at Mg II
k. Understanding the observations will require forward modeling of Mg II h&k
line formation from 3D radiation-MHD models. This paper is the first in a
series where we undertake this forward modeling. We discuss the atomic physics
pertinent to h&k line formation, present a quintessential model atom that can
be used in radiative transfer computations and discuss the effect of partial
redistribution (PRD) and 3D radiative transfer on the emergent line profiles.
We conclude that Mg II h&k can be modeled accurately with a 4-level plus
continuum Mg II model atom. Ideally radiative transfer computations should be
done in 3D including PRD effects. In practice this is currently not possible. A
reasonable compromise is to use 1D PRD computations to model the line profile
up to and including the central emission peaks, and use 3D transfer assuming
complete redistribution to model the central depression.Comment: 13 pages, 13 figures, accepted for Ap
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