Multi-stranded Loops Research Articles

Multi-Stranded Coronal Loops: Quantifying Strand Number and Heating Frequency from Simulated Solar Dynamics Observatory (SDO) Atmospheric Imaging Assembly (AIA) Observations

Coronal loops form the basic building blocks of the magnetically closed solar corona yet much is still to be determined concerning their possible fine-scale structuring and the rate of heat deposition within them. Using an improved multi-stranded loop model to better approximate the numerically challenging transition region, this article examines synthetic NASA Solar Dynamics Observatory’s (SDO) Atmospheric Imaging Assembly (AIA) emission simulated in response to a series of prescribed spatially and temporally random, impulsive and localised heating events across numerous sub-loop elements with a strong weighting towards the base of the structure: the nanoflare heating scenario. The total number of strands and nanoflare repetition times is varied systematically in such a way that the total energy content remains approximately constant across all the cases analysed. Repeated time-lag detection during an emission time series provides a good approximation for the nanoflare repetition time for low-frequency heating. Furthermore, using a combination of AIA 171/193 and 193/211 channel ratios in combination with spectroscopic determination of the standard deviation of the loop-apex temperature over several hours alongside simulations from the outlined multi-stranded loop model, it is demonstrated that both the imposed heating rate and number of strands can be realised.

Wave Heating in Simulated Multistranded Coronal Loops

Abstract It has been found that the Kelvin–Helmholtz instability (KHI) induced by both transverse and torsional oscillations in coronal loops can reinforce the effects of wave heating. In this study, we model a coronal loop as a system of individual strands, and we study wave heating effects by considering a combined transverse and torsional driver at the loop footpoint. We deposit the same energy into the multistranded loop and an equivalent monolithic loop, and then observe a faster increase in the internal energy and temperature in the multistranded model. Therefore, the multistranded model is more efficient in starting the heating process. Moreover, higher temperature is observed near the footpoint in the multistranded loop and near the apex in the monolithic loop. The apparent heating location in the multistranded loop agrees with the previous predictions and observations. Given the differences in the results from our multistranded loop and monolithic loop simulations, and given that coronal loops are suggested to be multistranded on both theoretical and observational grounds, our results suggest that the multistrandedness of coronal loops needs to be incorporated in future wave-based heating mechanisms.

Broadening of the differential emission measure by multi-shelled and turbulent loops

Context. Broad differential emission measure (DEM) distributions in the corona are a sign of multi-thermal plasma along the line-of-sight. Traditionally, this is interpreted as evidence of multi-stranded loops. Recently, however, it has been shown that multi-stranded loops are unlikely to exist in the solar corona, because of their instability to transverse perturbations. Aims. We aim to test if loop models subject to the transverse wave-induced Kelvin-Helmholtz (TWIKH) instability result in broad DEMs, potentially explaining the observations. Methods. We took simulation snapshots and compute the numerical DEM. Moreover, we performed forward-modelling in the relevant AIA channels before reconstructing the DEM. Results. We find that turbulent loop models broaden their initial DEM, because of the turbulent mixing. The width of the DEM is determined by the initial temperature contrast with the exterior. Conclusions. We conclude that impulsively excited loop models have a rather narrow DEM, but that continuously driven models result in broad DEMs that are comparable to the observations.

THE INSTABILITY AND NON-EXISTENCE OF MULTI-STRANDED LOOPS WHEN DRIVEN BY TRANSVERSE WAVES

ABSTRACT In recent years, omni-present transverse waves have been observed in all layers of the solar atmosphere. Coronal loops are often modeled as a collection of individual strands in order to explain their thermal behavior and appearance. We perform three-dimensional (3D) ideal magnetohydrodynamics simulations to study the effect of a continuous small amplitude transverse footpoint driving on the internal structure of a coronal loop composed of strands. The output is also converted into synthetic images, corresponding to the AIA 171 and 193 Å passbands, using FoMo. We show that the multi-stranded loop ceases to exist in the traditional sense of the word, because the plasma is efficiently mixed perpendicularly to the magnetic field, with the Kelvin–Helmholtz instability acting as the main mechanism. The final product of our simulation is a mixed loop with density structures on a large range of scales, resembling a power-law. Thus, multi-stranded loops are unstable to driving by transverse waves, and this raises strong doubts on the usability and applicability of coronal loop models consisting of independent strands.

TIME-RESOLVED EMISSION FROM BRIGHT HOT PIXELS OF AN ACTIVE REGION OBSERVED IN THE EUV BAND WITH SDO/AIA AND MULTI-STRANDED LOOP MODELING

ABSTRACT Evidence of small amounts of very hot plasma has been found in active regions and might be an indication of impulsive heating released at spatial scales smaller than the cross-section of a single loop. We investigate the heating and substructure of coronal loops in the core of one such active region by analyzing the light curves in the smallest resolution elements of solar observations in two EUV channels (94 and 335 Å) from the Atmospheric Imaging Assembly on board the Solar Dynamics Observatory. We model the evolution of a bundle of strands heated by a storm of nanoflares by means of a hydrodynamic 0D loop model (EBTEL). The light curves obtained from a random combination of those of single strands are compared to the observed light curves either in a single pixel or in a row of pixels, simultaneously in the two channels, and using two independent methods: an artificial intelligent system (Probabilistic Neural Network) and a simple cross-correlation technique. We explore the space of the parameters to constrain the distribution of the heat pulses, their duration, their spatial size, and, as a feedback on the data, their signatures on the light curves. From both methods the best agreement is obtained for a relatively large population of events (1000) with a short duration (less than 1 minute) and a relatively shallow distribution (power law with index 1.5) in a limited energy range (1.5 decades). The feedback on the data indicates that bumps in the light curves, especially in the 94 Å channel, are signatures of a heating excess that occurred a few minutes before.

Properties of multistranded, impulsively heated hydrodynamic loop models

Aims. We investigate the capability of multistranded loop models subject to nanoflare heating to reproduce the properties recently observed in coronal loops at extreme ultraviolet (EUV) wavelengths. Methods. One-dimensional hydrodynamic simulations of magnetic loop strands were performed with an impulsive, footpointlocalised heating, with a moderate asymmetry between the two loop halves that was produced either by a sequence of identical nanoflares with a given cadence time tC or by a single energy pulse. The temporal evolution of the emission of a multistranded loop was modelled by simply combining the results of independent single-strand simulations, neglecting any spatial interaction among the strands, and was compared with TRACE and SDO/AIA light curves. The density excess with respect to hydrostatic equilibrium (the ψ factor) was evaluated with the filter-ratio technique. Results. Both loop models exhibit a density excess compared with hydrostatic equilibrium models, which agrees well with the observed values (1 ψ 12). However, in the single-pulse model the light curve and density excess maxima do not match. On the other hand, the models with a sequence of nanoflares predict strong emission at lower temperatures that cannot be reconciled with the available observations.

GROWING TRANSVERSE OSCILLATIONS OF A MULTISTRANDED LOOP OBSERVED BY SDO /AIA

The first evidence of transverse oscillations of a multistranded loop with growing amplitudes and internal coupling observed by the Atomspheric Imaging Assembly (AIA) onboard the Solar Dynamics Observatory (SDO) is presented. The loop oscillation event occurred on 2011 March 8, triggered by a CME. The multiwavelength analysis reveals the presence of multithermal strands in the oscillating loop, whose dynamic behaviors are temperature-dependent, showing differences in their oscillation amplitudes, phases and emission evolution. The physical parameters of growing oscillations of two strands in 171 A are measured and the 3-D loop geometry is determined using STEREO-A/EUVI data. These strands have very similar frequencies, and between two 193 A strands a quarter-period phase delay sets up. These features suggest the coupling between kink oscillations of neighboring strands and the interpretation by the collective kink mode as predicted by some models. However, the temperature dependence of the multistarnded loop oscillations was not studied previously and needs further investigation. The transverse loop oscillations are associated with intensity and loop width variations. We suggest that the amplitude-growing kink oscillations may be a result of continuous non-periodic driving by magnetic deformation of the CME, which deposits energy into the loop system at a rate faster than its loss.

OBSERVING THE FINE STRUCTURE OF LOOPS THROUGH HIGH-RESOLUTION SPECTROSCOPIC OBSERVATIONS OF CORONAL RAIN WITH THE CRISP INSTRUMENT AT THE SWEDISH SOLAR TELESCOPE

We present here one of the first high resolution spectroscopic observations of coronal rain, performed with the CRISP instrument at the Swedish Solar Telescope. This work constitutes the first attempt to assess the importance of coronal rain in the understanding of the coronal magnetic field in active regions. A large statistical set is obtained in which dynamics (total velocities and accelerations), shapes (lengths and widths), trajectories (angles of fall) and thermodynamic properties (temperatures) of the condensations are derived. Specifically, we find that coronal rain is composed of small and dense chromospheric cores with average widths and lengths of 310 km and 710 km respectively, average temperatures below 7000 K, displaying a broad distribution of falling speeds with an average of 70 km/s and accelerations largely below the effective gravity along loops. Through estimates of the ion-neutral coupling in the blobs we show that coronal rain acts as a tracer of the coronal magnetic field, thus supporting the multi-strand loop scenario, and acts as a probe of the local thermodynamic conditions in loops. We further find that the cooling in neighboring strands occurs simultaneously in general suggesting a similar thermodynamic evolution among strands, which can be explained by a common footpoint heating process. Constraints for coronal heating models of loops are thus provided. Estimates of the fraction of coronal volume with coronal rain give values between 7% and 30%. Estimates of the occurrence time of the phenomenon in loops set times between 5 and 20 hours, implying that coronal rain may be a common phenomenon, in agreement with the frequent observations of cool downflows in EUV lines. The coronal mass drain rate in the form of coronal rain is estimated to be on the order of 5x10^9 g/s, a significant quantity compared to the estimate of mass flux into the corona from spicules.

CAN THERMAL NONEQUILIBRIUM EXPLAIN CORONAL LOOPS?

Any successful model of coronal loops must explain a number of observed properties. For warm (~ 1 MK) loops, these include: 1. excess density, 2. flat temperature profile, 3. super-hydrostatic scale height, 4. unstructured intensity profile, and 5. 1000--5000 s lifetime. We examine whether thermal nonequilibrium can reproduce the observations by performing hydrodynamic simulations based on steady coronal heating that decreases exponentially with height. We consider both monolithic and multi-stranded loops. The simulations successfully reproduce certain aspects of the observations, including the excess density, but each of them fails in at least one critical way. Monolithic models have far too much intensity structure, while multi-strand models are either too structured or too long-lived. Our results appear to rule out the widespread existence of heating that is both highly concentrated low in the corona and steady or quasi-steady (slowly varying or impulsive with a rapid cadence). Active regions would have a very different appearance if the dominant heating mechanism had these properties. Thermal nonequilibrium may nonetheless play an important role in prominences and catastrophic cooling events (e.g., coronal rain) that occupy a small fraction of the coronal volume. However, apparent inconsistencies between the models and observations of cooling events have yet to be understood.

The Temporal Evolution of Coronal Loops Observed byGOESSXI

We study the temporal evolution of coronal loops using data from the Solar X-Ray Imager (SXI) on board the Geosynchronous Operational Environmental Satellite 12 (GOES-12). This instrument provides continuous soft X-ray observations of the solar corona at a high temporal cadence permitting detailed study of the full lifetime of each of several coronal loops. The observed light curves suggest three evolutionary phases: rise, main, and decay. The durations and characteristic timescales of these phases [I/(dI/dt), where I is the loop intensity] 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 (uniformly filled) 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 relates to 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, with a constant of proportionality of approximately 1.5 for monolithic loops and 1.0 for multistranded loops. 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. This provides further support for a trend suggesting that all loops are heated in a similar way.

Modeling the Radiative Signatures of Turbulent Heating in Coronal Loops

The statistical properties of the radiative signature of a coronal loop subject to turbulent heating obtained from a three-dimensional (3D) magnetohydrodynamics (MHD) model are studied. The heating and cooling of a multistrand loop is modeled and synthetic spectra for Fe XII 195.12, Fe XV 284.163, and Fe XIX 1118.06 ? are calculated, covering a wide temperature range. The results show that the statistical properties of the thermal and radiative energies partially reflect those of the heating function in that power-law distributions are transmitted, but with very significant changes in the power-law indices. There is a strong dependence on the subloop geometry. Only high-temperature radiation (?107 K) preserves reasonably precise information on the heating function.