arXiv subjects

J. A. Klimchuk

Publications and source records attributed to J. A. Klimchuk.

24 records · Page 2Linked to original sources

arXiv2008

Highly Efficient Modeling of Dynamic Coronal Loops

Observational and theoretical evidence suggests that coronal heating is impulsive and occurs on very small cross-field spatial scales. A single coronal loop could contain a hundred or more individual strands that are heated quasi-independently by nanoflares. It is therefore an enormous undertaking to model an entire active region or the global corona. Three-dimensional MHD codes have inadequate spatial resolution, and 1D hydro codes are too slow to simulate the many thousands of elemental strands that must be treated in a reasonable representation. Fortunately, thermal conduction and flows tend to smooth out plasma gradients along the magnetic field, so "0D models" are an acceptable alternative. We have developed a highly efficient model called Enthalpy-Based Thermal Evolution of Loops (EBTEL) that accurately describes the evolution of the average temperature, pressure, and density along a coronal strand. It improves significantly upon earlier models of this type--in accuracy, flexibility, and capability. It treats both slowly varying and highly impulsive coronal heating; it provides the differential emission measure distribution, DEM(T), at the transition region footpoints; and there are options for heat flux saturation and nonthermal electron beam heating. EBTEL gives excellent agreement with far more sophisticated 1D hydro simulations despite using four orders of magnitude less computing time. It promises to be a powerful new tool for solar and stellar studies.

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arXiv2007

Are constant loop widths an artifact of the background and the spatial resolution?

We study the effect of the coronal background in the determination of the diameter of EUV loops, and we analyze the suitability of the procedure followed in a previous paper (López Fuentes, Klimchuk & Démoulin 2006) for characterizing their expansion properties. For the analysis we create different synthetic loops and we place them on real backgrounds from data obtained with the Transition Region and Coronal Explorer (\textit{TRACE}). We apply to these loops the same procedure followed in our previous works, and we compare the results with real loop observations. We demonstrate that the procedure allows us to distinguish constant width loops from loops that expand appreciably with height, as predicted by simple force-free field models. This holds even for loops near the resolution limit. The procedure can easily determine when loops are below resolution limit and therefore not reliably measured. We find that small-scale variations in the measured loop width are likely due to imperfections in the background subtraction. The greatest errors occur in especially narrow loops and in places where the background is especially bright relative to the loop. We stress, however, that these effects do not impact the ability to measure large-scale variations. The result that observed loops do not expand systematically with height is robust.

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arXiv2006

The temporal evolution of coronal loops observed by GOES-SXI

We study the temporal evolution of coronal loops using data from the Solar X-ray Imager (SXI) on board of GOES-12. This instrument allows us to follow in detail the full lifetime of coronal loops. The observed light curves suggest three somewhat distinct evolutionary phases: rise, main, and decay. The durations and characteristic timescales of these phases are much longer than a cooling time and indicate that the loop-averaged heating rate increases slowly, reaches a maintenance level, and then decreases slowly. This suggests that a single heating mechanism operates for the entire lifetime of the loop. For monolithic loops, the loop-averaged heating rate is the intrinsic energy release rate of the heating mechanism. For loops that are bundles of impulsively heated strands, it is an indication of the frequency of occurrence of individual heating events, or nanoflares. We show that the timescale of the loop-averaged heating rate is proportional to the timescale of the observed intensity variation. The ratios of the radiative to conductive cooling times in the loops are somewhat less than 1, putting them intermediate between the values measured previously for hotter and cooler loops. Our results provide further support for the existence of a trend suggesting that all loops are heated by the same mechanism, or that different mechanisms have fundamental similarities (e.g., are all impulsive or are all steady with similar rates of heating).

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arXiv2005

The Magnetic Structure of Coronal Loops Observed by TRACE

Previous studies have found that coronal loops have a nearly uniform thickness, which seems to disagree with the characteristic expansion of active region magnetic fields. This is one of the most intriguing enigmas in solar physics. We here report on the first comprehensive one-to-one comparison of observed loops with corresponding magnetic flux tubes obtained from cotemporal magnetic field extrapolation models. We use EUV images from TRACE, magnetograms from the MDI instrument on SOHO, and linear force-free field extrapolations. For each loop, we find the particular value of the force-free parameter (alpha) that best matches the observed loop axis and then construct flux tubes using different assumed cross sections at one footpoint (circle and ellipses with different orientations). We find that the flux tubes expand with height by typically twice as much as the corresponding loops. We also find that many flux tubes are much wider at one footpoint than the other, whereas the corresponding loops are far more symmetric. It is clear that the actual coronal magnetic field is more complex than the models we have considered. We suggest that the observed symmetry of loops is related to the tangling of elemental magnetic flux strands produced by photospheric convection.

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arXiv2004

Coronal Seismology and the Propagation of Acoustic Waves Along Coronal Loops

We use a combination of analytical theory, numerical simulation, and data analysis to study the propagation of acoustic waves along coronal loops. We show that the intensity perturbation of a wave depends on a number of factors, including dissipation of the wave energy, pressure and temperature gradients in the loop atmosphere, work action between the wave and a flow, and the sensitivity properties of the observing instrument. In particular, the scale length of the intensity perturbation varies directly with the dissipation scale length (i.e., damping length) and the scale lengths of pressure, temperature, and velocity. We simulate wave propagation in three different equilibrium loop models and find that dissipation and pressure and temperature stratification are the most important effects in the low corona where the waves are most easily detected. Velocity effects are small, and cross-sectional area variations play no direct role for lines-of-sight that are normal to the loop axis. The intensity perturbation scale lengths in our simulations agree very well with the scale lengths we measure in a sample of loops observed by TRACE. The median observed value is 4.35x10^9 cm. In some cases the intensity perturbation increases with height, which is likely an indication of a temperature inversion in the loop (i.e., temperature that decreases with height). Our most important conclusion is that thermal conduction, the primary damping mechanism, is accurately described by classical transport theory. There is no need to invoke anomalous processes to explain the observations.

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arXiv1998

A Model for Solar Coronal Mass Ejections

We propose a new model for the initiation of a solar coronal mass ejection (CME). The model agrees with two properties of CMEs and eruptive flares that have proved to be very difficult to explain with previous models. a) Very low-lying magnetic field lines, down to the photospheric neutral line, can open toward infinity during an eruption. b) The eruption is driven solely by magnetic free energy stored in a closed, sheared arcade; consequently, the magnetic energy of the closed state is well above that of the post-eruption open state. The key new feature of our model is that CMEs occur in multi-polar topologies, in which reconnection between a sheared arcade and neighboring flux systems triggers the eruption. In this ``magnetic breakout'' model, reconnection removes the unsheared field above the low-lying, sheared core flux near the neutral line, thereby allowing this core flux to burst open. We present numerical simulations which demonstrate that our model can account for the energy requirements for CMEs. We discuss the implication of the model for CME/flare prediction.

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