Corona II
Presiding: R Moore, Marshall Space Flight Center; H Gilbert, High Altitude Observatory
SP14A-01 15:30h
HYDRAD: a Hydrodynamics Code for Time-Dependent Ionisation and Forward Modelling of EUV Emission from the Solar Atmosphere
Observations in the Extreme Ultra-Violet have shown the solar atmosphere to be highly dynamic and time-dependent. The high-spatial resolution that these observations achieve has revealed the fundamental component of the solar atmosphere to be a loop-like structure: a tube of magnetic flux confining plasma which emits at EUV and X-ray wavelengths. The high-temporal cadence of these observations reveal activity time-scales for solar loops on the order of seconds. The emission spectrum of the solar atmosphere provides all of our information regarding the key physics that takes place. The properties of the individual spectral lines allow the calculation of plasma temperatures, densities and velocities. The spectral lines are critically dependent upon the population of emitting ions and in traditional hydrodynamic models these have been assumed to remain in equilibrium throughout the evolution of the plasma. However, the time-scale for ionisation in the solar atmosphere can be on the order of minutes. Thus, given activity time-scales on the order of seconds in solar loops, it is easy to see that the assumption of equilibrium ion populations may not be valid. This has extremely important consequences for the correct interpretation of observations, the accurate treatment of optically-thin radiation in numerical models and forward modelling studies derived from model results. The HYD(rodynamic) and RAD(iation) code (HYDRAD) has been developed specifically to handle non-equilibrium ion populations in the solar atmosphere and the consequences for optically-thin radiation, in a self-consistent way. The equations of hydrodynamics and ionisation balance are coupled together at each time-step during the evolution of the plasma. The HYDRAD package also provides tools for forward modelling and 3D visualisation. The results of some investigations carried out using HYDRAD will be presented and the specific consequences of a non-equilibrium ionisation balance will be discussed in each case.
SP14A-02 15:45h
Predicting Coronal Emissions with Multiple Heating Rates
A variety of proposed coronal heating mechanisms remain prominent in the literature, with insufficient observational constraints to distinguish between them. In an attempt to add further constraints, we create predicted coronal emission maps of several active regions using simple parametrizations of differing theoretical heating mechanisms and compare the results to the observed coronal emissions. The results are interpolated to a 3-d grid, convolved with instrument response function, and integrated over line of sight to simulate satellite observation of the modeled loops. We also discuss those factors which dominate the differences in observed and predicted coronal emission.
SP14A-03 16:00h
Highly Efficient Modeling of Dynamic Coronal Loops
It now seems clear that many coronal loops, especially those observed by TRACE and EIT, are inherently dynamic and composed of large numbers of impulsively-heated strands. Modeling these loops in full detail is extremely challenging, and modeling entire active regions or the whole Sun is completely out of the question unless approximate techniques are used. We have developed a simplified set of equations that is remarkably accurate at describing the evolution of the thermodynamic variables (T, P, n, v) averaged along the magnetic field of an individual strand. The equations can be solved ten thousand times more quickly than the full 1D hydro equations. This "0D" model relaxes two key assumptions of Cargill's (1994) nanoflare model: (1) the heating can have any time-dependent profile and need not be instantaneous; and (2) thermal conduction cooling and radiation cooling occur together at all times, in varying proportions. We here describe the essential features of the model and show examples of how well it works.
SP14A-04 16:15h
Heating of Coronal Active Regions: Statistics of Dissipation Events from Reduced MHD Simulations
Within the reduced MHD approximation, we numerically simulate the dynamics of a coronal loop driven by a stationary velocity field at the photospheric boundaries. After several photospheric turnover times, a turbulent stationary regime is reached, characterized by a broadband power spectrum and heating rate levels compatible with the heating requirements of active region loops. The energy dissipation rate as a function of time displays a complex superposition of impulsive events, which we associate to the so-called nanoflares. A statistical analysis of these events yields a power law distribution as a function of their energies, which is consistent with those obtained for flare energy distributions reported from X-ray observations. We also study the distributions of peak dissipation rates, durations, and waiting times between events.
SP14A-05 16:30h
Shell-Model Simulations of MHD in a Solar Coronal Loop
Statistics may be necessary to keep a global view of the complexity of astrophysical turbulence, in particular the effects of non-linear interactions over a wide range of scales. However, from the numerical point of view, a statistical approach to turbulence has the contradictory needs for computing speed and for a good description of the solutions of the MHD equations. This problem can be addressed by simplified models like cellular automata or shell-models. In the shell-models, the low number of well-chosen modes allows to keep the most possible of the complex and non-linear physics of the MHD equations while running sufficiently fast to produce statistics of fields, of structures, and of "events". The model we present here is designed to represent a magnetic loop in the solar corona. It consists of a pile of shell-models, which allows to reach a wide range of wavenumbers in cross-sections of the loop and model the non-linear couplings between these modes. The shell-models are also coupled by Alfvén waves propagating along the loop. We study the statistical properties of intermittent energy dissipation and of the velocity and magnetic fields produced by this model. These statistical properties can be compared to statistics issued from observations, like structure functions or events distributions.
SP14A-06 16:45h
Are Coronal Loops Self-organized Critical Systems?
In Lopez-Fuentes et al. 2004 we studied a set of loops observed by GOES/SXI and found that loop evolution can be separated in three phases (rise, main and decay). We found that the time scales of all three phases are long compared to a cooling time. In this work we explore whether this evolution is consistent with the development of a self-organized critical (SOC) system. We compare the observed soft X-ray light curves with light curves predicted by a simple SOC model, and we examine how the rise and decay times are related to the properties of the system driver. The physical picture we have in mind is the shuffling of elemental flux tubes by photospheric motions, as first advocated by Parker. Work funded by NASA and ONR. Lopez Fuentes, M.C., Mandrini, C.H., & Klimchuk, J.A., 2004, American Astronomical Society Meeting Abstracts, 204, 5602L