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Coronal loop widths and pressure scale heights  

Gordon Petrie   Submitted: 2006-07-03 12:19

The scale heights of stratification and the widths of steady solar coronal loops exhibit properties unexplained by standard theory: observed scale heights are often much greater than static theory predicts, while the nearly-constant widths of loop emission signatures defy theoretical expectations for large flux tubes in stratified media. In this work we relate the cross-sectional profile of a coronal flux tube to its density scale height in steady-state plasma flow regimes. Steady flows may shorten or lengthen the scale height according to how the tube cross-sectional area varies with arclength. In a near-potential corona the flux tubes are expected to be sufficiently expansive in many active regions for scale heights to be increased by steady flows. On the other hand, cases where scale lengths are actually increased to observed sizes form a small part of the solution space, close to regimes where density profiles reverse. Therefore, although steady flows are the only steady process known to be capable of extending scale heights significantly, they are not expected to be not responsible for the majority of extended active region scale heights.

Authors: G.J.D. Petrie
Projects: None

Publication Status: ApJ, to appear October 2006
Last Modified: 2006-07-03 14:49
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Filamentary Loop Systems and Spontaneous Current Sheets in the Solar Corona  

Gordon Petrie   Submitted: 2006-07-03 12:15

The solar coronal plasma is too hot and too highly conducting thermally to be static in the absence of powerful inward forces. Sufficiently strong and closed magnetic field configurations do, however, contain the plasma in the low corona in spite of its high temperature. Very many of these closed loop structures, with greatly varying physical characteristics, coexist in quasi-static mutual equilibrium in the corona. We model 3D loop systems by horizontally stacking many diverse loops into a 3D equilibrium solution, where individual loops are separated from each other by tangential discontinuities. Simulated narrow-band filter images of this solution reproduce the filamentary appearance of loop systems familiar from observations. A simple 3D time-dependent model of horizontal shearing motions is also presented. We then investigate the dissipation and field reconnection across these tangential discontinuities that must form spontaneously in coronal magnetic fields. The post-diffusion dynamics are dictated by the outward cross-field gas pressure gradient forces which are increasing functions of pressure and temperature. Thus in isothermal systems a net plasma outflow results from each field diffusion while in non-isothermal systems relatively hot loops tend to expand as a result of field diffusion processes which may allow hot plasma ultimately to escape into the solar wind. Accompanying cool downflows predicted by the calculations may be identified with observed persistent mass downflows in active region loops in cool coronal lines. In addition, current sheet dissipation in a sheared loop system may create field-aligned counter-flow patterns in neighboring loops, a phenomenon that has also been observed.

Authors: G.J.D. PETRIE
Projects: None

Publication Status: ApJS, to appear September 2006
Last Modified: 2006-07-03 14:49
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2D MHD modelling of compressible and heated coronal loops obtained via nonlinear separation of variables and compared to TRACE and SoHO observations  

Gordon Petrie   Submitted: 2003-07-04 05:42

An analytical MHD model of coronal loops with compressible flows and including heating is compared to observational data. The model is constructed via a systematic nonlinear separation of the variables method used to calculate several classes of exact MHD equilibria in Cartesian geometry and uniform gravity. By choosing a particularly versatile solution class with a large parameter space we are able to calculate models whose loop length, shape, plasma density, temperature and velocity profiles are fitted to loops observed with TRACE, SoHO/CDS and SoHO/SUMER. Synthetic emission profiles are also calculated and fitted to the observed emission patterns. An analytical discussion is given of the two-dimenional balance of the Lorentz force and the gas pressure gradient, gravity and inertial forces acting along and across the loop. These models are the first to include a fully consistent description of the magnetic field, 2D geometry, plasma density and temperature, flow velocity and thermodynamics of loops. The consistently calculated heating profiles which are largely dominated by radiative losses and concentrated at the footpoints are influenced by the flow and are asymmetric, being biased towards the upflow footpoint.

Authors: G.J.D. Petrie, C. Gontikakis, H. Dara, K. Tsinganos, M.J. Aschwanden
Projects: None

Publication Status: A&A (in press)
Last Modified: 2003-07-04 05:42
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2D MHD modelling of compressible and heated coronal loops obtained via nonlinear separation of variables and compared to TRACE and SoHO observations  

Gordon Petrie   Submitted: 2003-03-18 05:39

An analytical MHD model of coronal loops with compressible flows and including heating is compared to observational data. The model is constructed via a systematic nonlinear separation of the variables method used to calculate several classes of exact MHD equilibria in Cartesian geometry and uniform gravity. By choosing a particularly versatile solution class with a large parameter space we are able to calculate models whose loop length, shape, plasma density, temperature and velocity profiles are fitted to loops observed with TRACE, SoHO/CDS and SoHO/SUMER. Synthetic emission profiles are also calculated and fitted to the observed emission patterns. An analytical discussion is given of the two-dimenional balance of the Lorentz force and the gas pressure gradient, gravity and inertial forces acting along and across the loop. These models are the first to include a fully consistent description of the magnetic field, 2D geometry, plasma density and temperature, flow velocity and thermodynamics of loops. The consistently calculated heating profiles which are largely dominated by radiative losses are influenced by the flow and are asymmetrical being concentrated at the upflow footpoint.

Authors: G.J.D. Petrie, C. Gontikakis, H. Dara, K. Tsinganos & M.J. Aschwanden
Projects:

Publication Status: submitted to A&A
Last Modified: 2003-03-18 05:46
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Abstracts by Author
Coronal loop widths and pressure scale heights
Filamentary Loop Systems and Spontaneous Current Sheets in the Solar Corona
2D MHD modelling of compressible and heated coronal loops obtained via nonlinear separation of variables and compared to TRACE and SoHO observations
2D MHD modelling of compressible and heated coronal loops obtained via nonlinear separation of variables and compared to TRACE and SoHO observations

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