Author name code: heggland ADS astronomy entries on 2022-09-14 author:"Heggland, Lars" ------------------------------------------------------------------------ Title: Wave Propagation and Jet Formation in the Chromosphere Authors: Heggland, L.; Hansteen, V. H.; De Pontieu, B.; Carlsson, M. Bibcode: 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. Title: Wave propagation and jet formation in the chromosphere and transition region Authors: Heggland, Lars Bibcode: 2011PhDT.......322H Altcode: No abstract at ADS Title: Observational Signatures of Simulated Reconnection Events in the Solar Chromosphere and Transition Region Authors: Heggland, L.; De Pontieu, B.; Hansteen, V. H. Bibcode: 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. Title: Numerical Simulations of Shock Wave-driven Chromospheric Jets Authors: Heggland, L.; De Pontieu, B.; Hansteen, V. H. Bibcode: 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. Title: Mode Conversion in Magneto-Atmospheres Authors: Bogdan, T. J.; Carlsson, M.; Hansteen, V.; Heggland, L.; Leer, E.; McMurry, A. D.; Stein, R. F. Bibcode: 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.