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Author name code: heggland
ADS astronomy entries on 2022-09-14
author:"Heggland, Lars"
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Title: Wave Propagation and Jet Formation in the Chromosphere
Authors: Heggland, L.; Hansteen, V. H.; De Pontieu, B.; Carlsson, M.
2011ApJ...743..142H Altcode: 2011arXiv1112.0037H
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.
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Title: Wave propagation and jet formation in the chromosphere and
transition region
Authors: Heggland, Lars
2011PhDT.......322H Altcode:
No abstract at ADS
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Title: Observational Signatures of Simulated Reconnection Events in
the Solar Chromosphere and Transition Region
Authors: Heggland, L.; De Pontieu, B.; Hansteen, V. H.
2009ApJ...702....1H Altcode: 2009arXiv0902.0977H
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 paper, 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 spectra
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, and other phenomena thought to be caused by
reconnection. Our simulations produce spicule-like jets with lengths
and lifetimes that match observations, and the spectral signatures of
several reconnection events are similar to observations of explosive
events. We also find that in some cases, absorption from overlying
neutral hydrogen can hide emission from matter at coronal temperatures.
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Title: Numerical Simulations of Shock Wave-driven Chromospheric Jets
Authors: Heggland, L.; De Pontieu, B.; Hansteen, V. H.
2007ApJ...666.1277H Altcode: 2007astro.ph..3498H
We present the results of numerical simulations of shock wave-driven
jets in the solar atmosphere. The dependence of observable quantities
such as maximum velocity and deceleration on parameters such as the
period and amplitude of initial disturbances and the inclination of
the magnetic field is investigated. Our simulations show excellent
agreement with observations, and shed new light on the correlation
between velocity and deceleration and on the regional differences
found in observations.
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Title: Mode Conversion in Magneto-Atmospheres
Authors: Bogdan, T. J.; Carlsson, M.; Hansteen, V.; Heggland, L.;
Leer, E.; McMurry, A. D.; Stein, R. F.
2004AGUFMSH13A1162B Altcode:
Numerical simulations of wave propagation in a simple magneto-atmosphere
are employed to illustrate the complex nature of wave transformation
and conversion taking place in solar and stellar atmospheres. An
isothermal atmosphere threaded by a potential poloidal magnetic
field, and a superposed uniform toroidal field, is treated in a local
cartesian approximation. Spatial variations are restricted to the
two poloidal dimensions, but the toroidal field ensures that all
three MHD waves are present in the simulation. As in our previous
purely two-dimensional simulations (Bogdan et al. ApJ 599, 626-60,
2003), mode mixing and transformation take place at surfaces where
the magnetic and thermal pressures are equal. In the present case,
the upward propagating acoustic-gravity (MAG) wave is converted into
roughly equal parts transmitted fast, intermediate (Alfven), and
slow magneto-acoustic-gravity waves in passing through this mixing
layer. Unlike the fast and slow waves, the Alfven wave is weakly
damped, and is able to deposit its energy and momentum in the upper
chromosphere and corona. The fast and slow MAG waves are decoupled
on either side of mixing layer owing to their disparate propagation
speeds. Under certain fortuitous circumstances, the Alfven wave also
decouples from the fast and slow MAG waves.
