Static Loop Models Research Articles

HOW GAS-DYNAMIC FLARE MODELS POWERED BY PETSCHEK RECONNECTION DIFFER FROM THOSE WITH AD HOC ENERGY SOURCES

Aspects of solar flare dynamics, such as chromospheric evaporation and flare light-curves, have long been studied using one-dimensional models of plasma dynamics inside a static flare loop, subjected to some energy input. While extremely successful at explaining the observed characteristics of flares, all such models so far have specified energy input ad hoc, rather than deriving it self-consistently. There is broad consensus that flares are powered by magnetic energy released through reconnection. Recent work has generalized Petschek's basic reconnection scenario, topological change followed by field line retraction and shock heating, to permit its inclusion into a one-dimensional flare loop model. Here we compare the gas dynamics driven by retraction and shocking to those from more conventional static loop models energized by ad hoc source terms. We find significant differences during the first minute, when retraction leads to larger kinetic energies and produces higher densities at the loop top, while ad hoc heating tends to rarify the loop top. The loop-top density concentration is related to the slow magnetosonic shock, characteristic of Petschek's model, but persists beyond the retraction phase occurring in the outflow jet. This offers an explanation for observed loop-top sources of X-ray and EUV emission, with advantages over that provided by ad hoc heating scenarios. The cooling phases of the two models are, however, notably similar to one another, suggesting observations at that stage will yield little information on the nature of energy input.

Diagnostics of Coronal Heating in Solar Active Regions

We study the relationship between EUV spectral line intensities and the photospheric magnetic field in solar active regions, using magnetograms from SOHO-MDI and EUV spectra of the Fe XVI 360.8 Â line (2 × 106 K) and the O V 629.7 A line (220,000 K) from the Coronal Diagnostic Spectrometer on SOHO, recorded for several active regions. We overlay and compare spatial patterns of the O V emission and the magnetic flux concentrations, with a 4″ x 4″ spatial resolution, and search for a relationship between the local O V line intensity and the photospheric magnetic flux density in each active region. While this dependence exhibits a certain amount of scatter, it can be represented by a power law fit. The average power index from all regions is 0.7 ± 0.2. Applying static loop models, we derive the dependence of the heating rate on the magnetic flux density, Eh ∝ B0.8, and compare it to the dependence predicted by the coronal heating models.

Transition Region and Coronal Explorerand Soft X‐Ray Telescope Active Region Loop Observations: Comparisons with Static Solutions of the Hydrodynamic Equations

Active region coronal loop observations with broadband X-ray instruments have often been found to be consistent with the predictions of static loop models. Recent observations in the EUV, however, have discovered a class of active region loops that are difficult to reconcile with static loop models. In this paper, we take a comprehensive look at how coronal loops compare with static models. We select 67 loops with a large range of apex temperatures and half-lengths observed with either the Transition Region and Coronal Explorer or the Soft X-Ray Telescope. We compare these observations to static loop models using both uniform and nonuniform heating. We find that only 2 of the 67 loops are fully consistent with static solutions with uniform heating and a filling factor of unity. We further find that long, cool (<3 MK) loops are as much as 2500 times "overdense," while short, hot (>3 MK) loops are as much as 63 times "underdense" when compared to the static solutions with uniform heating. We then consider the possibility that the disparity in the density could be due to steady, nonuniform heating along the loop and find that footpoint heating can increase densities only by a factor of 3 over density solutions with uniform heating while loop-top heating results in density solutions that are, at most, a factor of 2.5 smaller than the density solutions with uniform heating. Only 19 of the 67 loops in this data set could be fully consistent with hydrodynamic solutions with steady heating. Hence, we conclude that static loop models are poor representations of most active region loops.

Hydrodynamic Modeling of Active Region Loops

Recent observations with the Transition Region and Coronal Explorer (TRACE) have shown that many apparently cool (Te ~ 1-1.5 MK) active region loops are much brighter and have flatter temperature profiles than static loop models predict. Observations also indicate that these loops can persist much longer than a characteristic cooling time. Using time-dependent hydrodynamic simulations, we explore the possibility that these active region loops are actually a collection of small-scale filaments that have been impulsively heated and are cooling through the TRACE 171 A (Fe IX/X) and 195 A (Fe XII) bandpasses. We find that an ensemble of independently heated filaments can be significantly brighter than a static uniformly heated loop and would have a flat filter ratio temperature when observed with TRACE.

Coronal Loops in Binary Stars

Einstein and IUE observations on main sequence close binaries are fitted with approximate power-laws of type F ∝ Ωαfβ where Ω is the angular velocity (period-1) and f the convective efficiency (∼ Teff-7). The data suggest the reducing nature of binarity for the X- and UV-activity (tidal effects, mass and energy transfer in contact binaries). The whole surface of the most active stars can be visualized to be entirely filled with structures similar to those found in strong solar active regions. The results are roughly compatible with the constancy of the differential emission measure between 105 K and Tmax in static loop-models. The total radiative losses from the upper atmosphere may be simply proportional to the angular velocity, in spite of different Ω-dependences in separate wavelength-bands. The peculiar behaviour of physical-contact binaries of W UMa-type is remarked upon.

Coordinated Einstein and IUE observations of a disparitions brusques type flare event and quiescent emission from Proxima Centauri

view Abstract Citations (131) References (49) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Coordinated Einstein and IUE observations of a disparitions brusques type flare event and quiescent emission from Proxima Centauri. Haisch, B. M. ; Linsky, J. L. ; Bornmann, P. L. ; Stencel, R. E. ; Antiochos, S. K. ; Golub, L. ; Vaiana, G. S. Abstract The Einstein Imaging Particle Counter observed a major X-ray flare in its entirety during a 5-hr period of simultaneous observations, with the IUE, of the dM5e flare star Proxima Centauri in August, 1980. The detailed X-ray light curve, temperature determinations during various intervals, and UV line fluxes obtained before, during, and after the flare indirectly indicate a 'two-ribbon flare' prominence eruption. The calculated ratio of coronal to bolometric luminosity for the event is about 100 times the solar ratio. The Proxima Cen corona is analyzed in the context of static loop models, in light of which it is concluded that less than 6% of the stellar surface seems to be covered by X-ray emitting active regions. Publication: The Astrophysical Journal Pub Date: April 1983 DOI: 10.1086/160866 Bibcode: 1983ApJ...267..280H Keywords: Flare Stars; Heao 2; Iue; Stellar Coronas; Stellar Flares; X Ray Sources; Coronal Loops; Solar Corona; Stellar Luminosity; Stellar Temperature; Ultraviolet Spectra; Astrophysics full text sources ADS | data products SIMBAD (1) MAST (1) INES (1)