46 research outputs found
Forward modeling of emission in SDO/AIA passbands from dynamic 3D simulations
It is typically assumed that emission in the passbands of the Atmospheric
Imaging Assembly (AIA) onboard the Solar Dynamics Observatory (SDO) is
dominated by single or several strong lines from ions that under equilibrium
conditions are formed in a narrow range of temperatures. However, most SDO/AIA
channels also contain contributions from lines of ions that have formation
temperatures that are significantly different from the "dominant" ion(s). We
investigate the importance of these lines by forward modeling the emission in
the SDO/AIA channels with 3D radiative MHD simulations of a model that spans
the upper layer of the convection zone to the low corona. The model is highly
dynamic. In addition, we pump a steadily increasing magnetic flux into the
corona, in order to increase the coronal temperature through the dissipation of
magnetic stresses. As a consequence, the model covers different ranges of
coronal temperatures as time progresses. The model covers coronal temperatures
that are representative of plasma conditions in coronal holes and quiet sun.
The 131, 171, and 304 \AA{} AIA passbands are found to be least influenced by
the so-called "non-dominant" ions, and the emission observed in these channels
comes mostly from plasma at temperatures near the formation temperature of the
dominant ion(s). On the other hand, the other channels are strongly influenced
by the non-dominant ions, and therefore significant emission in these channels
comes from plasma at temperatures that are different from the "canonical"
values. We have also studied the influence of non-dominant ions on the AIA
passbands when different element abundances are assumed (photospheric and
coronal), and when the effects of the electron density on the contribution
function are taken into account.Comment: 48 pages, 14 figures, accepted to be publish in Ap
On the origin of the magnetic energy in the quiet solar chromosphere
The presence of magnetic field is crucial in the transport of energy through
the solar atmosphere. Recent ground-based and space-borne observations of the
quiet Sun have revealed that magnetic field accumulates at photospheric
heights, via a local dynamo or from small-scale flux emergence events. However,
most of this small-scale magnetic field may not expand into the chromosphere
due to the entropy drop with height at the photosphere. Here we present a study
that uses a high resolution 3D radiative MHD simulation of the solar atmosphere
with non-grey and non-LTE radiative transfer and thermal conduction along the
magnetic field to reveal that: 1) the net magnetic flux from the simulated
quiet photosphere is not sufficient to maintain a chromospheric magnetic field
(on average), 2) processes in the lower chromosphere, in the region dominated
by magneto-acoustic shocks, are able to convert kinetic energy into magnetic
energy, 3) the magnetic energy in the chromosphere increases linearly in time
until the r.m.s. of the magnetic field strength saturates at roughly 4 to 30 G
(horizontal average) due to conversion from kinetic energy, 4) and that the
magnetic features formed in the chromosphere are localized to this region.Comment: 12 pages, 14 figures, accepted to be published in Ap
Spicule-like structures observed in 3D realistic MHD simulations
We analyze features that resemble type i spicules in two different 3D
numerical simulations in which we include horizontal magnetic flux emergence in
a computational domain spanning the upper layers of the convection zone to the
lower corona. The two simulations differ mainly in the preexisting ambient
magnetic field strength and in the properties of the inserted flux tube. We use
the Oslo Staggered Code (OSC) to solve the full MHD equations with non-grey and
non-LTE radiative transfer and thermal conduction along the magnetic field
lines. We find a multitude of features that show a spatiotemporal evolution
that is similar to that observed in type i spicules, which are characterized by
parabolic height vs. time profiles, and are dominated by rapid upward motion at
speeds of 10-30 km/s, followed by downward motion at similar velocities. We
measured the parameters of the parabolic profile of the spicules and find
similar correlations between the parameters as those found in observations. The
values for height (or length) and duration of the spicules found in the
simulations are more limited in range than those in the observations. The
spicules found in the simulation with higher preexisting ambient field have
shorter length and smaller velocities. From the simulations, it appears that
these kinds of spicules can, in principle, be driven by a variety of mechanisms
that include p-modes, collapsing granules, magnetic energy release in the
photosphere and lower chromosphere and convective buffeting of flux
concentrations.Comment: 31 pages, 9 figures. accepted the 23 of June in Ap
Twisted flux tube emergence from the convection zone to the corona II: Later states
3D simulations of magnetic flux emergence are carried out in a computational
domain spanning the upper layers of the convection zone to the lower corona. We
use the Oslo Staggered Code to solve the full MHD equations with non-grey and
NLTE radiative transfer and thermal conduction along the magnetic field lines.
In this paper we concentrate on the later stages of the simulations and study
the evolution of the structure of the rising flux in the upper chromosphere and
corona, the interaction between the emerging flux and the weak coronal magnetic
field initially present, and the associated dynamics.
The flux tube injected at the bottom boundary rises to the photosphere where
it largely remains. However, some parts of the flux tube become unstable and
expand in patches into the upper chromosphere. The flux rapidly expands towards
the corona, pushing the coronal and transition region material aside, lifting
and maintaining the transition region at heights greater than 5 Mm above the
photosphere for extensive periods of time. The pre-existing magnetic field in
the corona and transition region is perturbed by the incoming flux and
reoriented by a series of high Joule heating events. Low density structures
form in the corona while at later times a high density filamentary structure
appears in the lower part of the expanding flux. The dynamics of these and
other structures is discussed. While Joule heating due to the expanding flux is
episodic, it increases in relative strength as fresh magnetic field rises and
becomes energetically important in the upper chromosphere and corona at later
times. Chromospheric, transition region and coronal lines are computed and
their response to the perturbation caused by the expanding emerging flux is
discussed.Comment: 31 pages, 12 figures, accepted in Ap
Two-dimensional Radiative Magnetohydrodynamic Simulations of Partial Ionization in the Chromosphere. II. Dynamics and Energetics of the Low Solar Atmosphere
We investigate the effects of interactions between ions and neutrals on the
chromosphere and overlying corona using 2.5D radiative MHD simulations with the
Bifrost code. We have extended the code capabilities implementing ion-neutral
interaction effects using the Generalized Ohm's Law, i.e., we include the Hall
term and the ambipolar diffusion (Pedersen dissipation) in the induction
equation. Our models span from the upper convection zone to the corona, with
the photosphere, chromosphere and transition region partially ionized. Our
simulations reveal that the interactions between ionized particles and neutral
particles have important consequences for the magneto-thermodynamics of these
modeled layers: 1) ambipolar diffusion increases the temperature in the
chromosphere; 2) sporadically the horizontal magnetic field in the photosphere
is diffused into the chromosphere due to the large ambipolar diffusion; 3)
ambipolar diffusion concentrates electrical currents leading to more violent
jets and reconnection processes, resulting in 3a) the formation of longer and
faster spicules, 3b) heating of plasma during the spicule evolution, and 3c)
decoupling of the plasma and magnetic field in spicules. Our results indicate
that ambipolar diffusion is a critical ingredient for understanding the
magneto-thermo-dynamic properties in the chromosphere and transition region.
The numerical simulations have been made publicly available, similar to
previous Bifrost simulations. This will allow the community to study realistic
numerical simulations with a wider range of magnetic field configurations and
physics modules than previously possible.Comment: 13 figures. Accepted to be published in Ap
Thermal Instability–Induced Fundamental Magnetic Field Strands in the Solar Corona
Thermal instability is a fundamental process of astrophysical plasmas. It is expected to occur whenever the cooling is dominated by radiation and cannot be compensated for by heating. In this work, we conduct 2.5D radiation MHD simulations with the Bifrost code of an enhanced activity network in the solar atmosphere. Coronal loops are produced self-consistently, mainly through Joule heating, which is sufficiently stratified and symmetric to produce thermal nonequilibrium. During the cooling and driven by thermal instability, coronal rain is produced along the loops. Due to flux freezing, the catastrophic cooling process leading to a rain clump produces a local enhancement of the magnetic field, thereby generating a distinct magnetic strand within the loop up to a few Gauss stronger than the surrounding coronal field. These strands, which can be considered fundamental, are a few hundred kilometers in width, span most of the loop leg, and emit strongly in the UV and extreme UV (EUV), thereby establishing a link between the commonly seen rain strands in the visible spectrum with the observed EUV coronal strands at high resolution. The compression downstream leads to an increase in temperature that generates a plume-like structure, a strongly emitting spicule-like feature, and short-lived brightening in the UV during the rain impact, providing an explanation for similar phenomena seen with IRIS. Thermal instability and nonequilibrium can therefore be associated with localized and intermittent UV brightening in the transition region and chromosphere, as well as contribute to the characteristic filamentary morphology of the solar corona in the EUV
Twisted flux tube emergence from the convection zone to the corona
3D numerical simulations of a horizontal magnetic flux tube emergence with
different twist are carried out in a computational domain spanning the upper
layers of the convection zone to the lower corona. We use the Oslo Staggered
Code to solve the full MHD equations with non-grey and non-LTE radiative
transfer and thermal conduction along the magnetic field lines. The emergence
of the magnetic flux tube input at the bottom boundary into a weakly magnetized
atmosphere is presented. The photospheric and chromospheric response is
described with magnetograms, synthetic images and velocity field distributions.
The emergence of a magnetic flux tube into such an atmosphere results in varied
atmospheric responses. In the photosphere the granular size increases when the
flux tube approaches from below. In the convective overshoot region some 200km
above the photosphere adiabatic expansion produces cooling, darker regions with
the structure of granulation cells. We also find collapsed granulation in the
boundaries of the rising flux tube. Once the flux tube has crossed the
photosphere, bright points related with concentrated magnetic field, vorticity,
high vertical velocities and heating by compressed material are found at
heights up to 500km above the photosphere. At greater heights in the magnetized
chromosphere, the rising flux tube produces a cool, magnetized bubble that
tends to expel the usual chromospheric oscillations. In addition the rising
flux tube dramatically increases the chromospheric scale height, pushing the
transition region and corona aside such that the chromosphere extends up to 6Mm
above the photosphere. The emergence of magnetic flux tubes through the
photosphere to the lower corona is a relatively slow process, taking of order 1
hour.Comment: 53 pages,79 figures, Submitted to Ap