920 research outputs found
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.Comment: 34 pages, 8 figures, accepted by Astrophysical Journal (minor
revisions of original submitted version
Coronal heating in multiple magnetic threads
Context. Heating the solar corona to several million degrees requires the
conversion of magnetic energy into thermal energy. In this paper, we
investigate whether an unstable magnetic thread within a coronal loop can
destabilise a neighbouring magnetic thread. Aims. By running a series of
simulations, we aim to understand under what conditions the destabilisation of
a single magnetic thread can also trigger a release of energy in a nearby
thread. Methods. The 3D magnetohydrodynamics code, Lare3d, is used to simulate
the temporal evolution of coronal magnetic fields during a kink instability and
the subsequent relaxation process. We assume that a coronal magnetic loop
consists of non-potential magnetic threads that are initially in an equilibrium
state. Results. The non-linear kink instability in one magnetic thread forms a
helical current sheet and initiates magnetic reconnection. The current sheet
fragments, and magnetic energy is released throughout that thread. We find
that, under certain conditions, this event can destabilise a nearby thread,
which is a necessary requirement for starting an avalanche of energy release in
magnetic threads. Conclusions. It is possible to initiate an energy release in
a nearby, non-potential magnetic thread, because the energy released from one
unstable magnetic thread can trigger energy release in nearby threads, provided
that the nearby structures are close to marginal stability
Modelling interplanetary CMEs using magnetohydrodynamic simulations
International audienceThe dynamics of Interplanetary Coronal Mass Ejections (ICMEs) are discussed from the viewpoint of numerical modelling. Hydrodynamic models are shown to give a good zero-order picture of the plasma properties of ICMEs, but they cannot model the important magnetic field effects. Results from MHD simulations are shown for a number of cases of interest. It is demonstrated that the strong interaction of the ICME with the solar wind leads to the ICME and solar wind velocities being close to each other at 1 AU, despite their having very different speeds near the Sun. It is also pointed out that this interaction leads to a distortion of the ICME geometry, making cylindrical symmetry a dubious assumption for the CME field at 1 AU. In the presence of a significant solar wind magnetic field, the magnetic fields of the ICME and solar wind can reconnect with each other, leading to an ICME that has solar wind-like field lines. This effect is especially important when an ICME with the right sense of rotation propagates down the heliospheric current sheet. It is also noted that a lack of knowledge of the coronal magnetic field makes such simulations of little use in space weather forecasts that require knowledge of the ICME magnetic field strength
Inference of heating properties from "hot" non-flaring plasmas in active region cores. I. Single nanoflares
The properties that are expected of âhotâ non-flaring plasmas due to nanoflare heating in active regions are investigated using hydrodynamic modeling tools, including a two-fluid development of the Enthalpy Based Thermal Evolution of Loops code. Here we study a single nanoflare and show that while simple models predict an emission measure distribution extending well above 10 MK, which is consistent with cooling by thermal conduction, many other effects are likely to limit the existence and detectability of such plasmas. These include: differential heating between electrons and ions, ionization non-equilibrium, and for short nanoflares, the time taken for the coronal density to increase. The most useful temperature range to look for this plasma, often called the âsmoking gunâ of nanoflare heating, lies between 10 6.6 and 10 7 K. Signatures of the actual heating may be detectable in some instances.Publisher PDFPeer reviewe
First Cluster results of the magnetic field structure of the mid- and high-altitude cusps
International audienceMagnetic field measurements from the four Cluster spacecraft from the mid- and high-altitude cusp are presented. Cluster underwent two encounters with the mid-altitude cusp during its commissioning phase (24 August 2000). Evidence for field-aligned currents (FACs) was seen in the data from all three operating spacecraft from northern and southern cusps. The extent of the FACs was of the order of 1 RE in the X-direction, and at least 300 km in the Y-direction. However, fine-scale field structures with scales of the order of the spacecraft separation (300 km) were observed within the FACs. In the northern crossing, two of the spacecraft appeared to lie along the same magnetic field line, and observed very well matched signals. However, the third spacecraft showed evidence for structuring transverse to the field on scales of a few hundred km. A crossing of the high-altitude cusp from 13 February 2001 is presented. It is revealed to be a highly dynamic structure with the boundaries moving with velocities ranging from a few km/s to tens of km/s, and having structure on timescales ranging from less than one minute up to several minutes. The cusp proper is associated with the presence of a very disordered magnetic field, which is entirely different from the magnetosheath turbulence
Enthalpy-Based Thermal Evolution of Loops: III. Comparison of Zero-Dimensional Models
Zero dimensional (0D) hydrodynamic models, provide a simple and quick way to study the thermal evolution of coronal loops subjected to time-dependent heating. This paper presents a comparison of a number of 0D models that have been published in the past and is intended to provide a guide for those interested in either using the old models or developing new ones. The principal difference between the models is the way the exchange of mass and energy between corona, transition region and chromosphere is treated, as plasma cycles into and out of a loop during a heating-cooling cycle. It is shown that models based on the principles of mass and energy conservation can give satisfactory results at some, or, in the case of the Enthalpy Based Thermal Evolution of Loops (EBTEL) model, all stages of the loop evolution. Empirical models can lead to low coronal densities, spurious delays between the peak density and temperature, and, for short heating pulses, overly short loop lifetimes
The energetics of the gradual phase
Reseachers compare results with those in the chapter by Moore et al. (1980), who reached five main conclusions about the gradual phase: (1) the typical density of the soft X-ray emitting plasma is between 10 to the 11th power and 10 to the 12th power cm-3 for compact flares and between 10 to the 10th power and 10 to the 11th power cm-3 for a large-area flare; (2) cooling is by conduction and radiation in roughly equal proportions; (3) continual heating is needed in the decay phase of two-ribbon flares; (4) continual heating is probably not needed in compact events; (5) most of the soft-X-ray-emitting plasma results from chromospheric evaporation. The goal was to reexamine these problems with the data from the Solar Maximum Mission (SMM) and other supporting instruments as well as to take advantage of recent theoretical advances. SMM is capable of measuring coronal temperatures more accurately and with a better cadence than has been possible before. The SMM data set is also unique in that the complete transit of an active region was observed, with soft X-ray and UV images being taken every few minutes. Researcher's were therefore able to establish the pre-flare conditions of the region and see whether anything has changed as a result of the flare. The assumptions made in attempting to determine the required plasma parameters are described. The derived parameters for the five prime flares are presented, and the role of numerical simulations is discussed
Coronal loop widths and pressure scale heights
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
A new approach for modelling chromospheric evaporation in response to enhanced coronal heating : II. Non-uniform heating
This project has received funding from the Science and Technology Facilities Council (UK) through the consolidated grant ST/N000609/1 and the European Research Council (ERC) under the European Unionâs Horizon 2020 research and innovation program (grant agreement No 647214).We proposed that the use of an approximate âjump conditionâ at the solar transition region permits fast and accurate numerical solutions of the one dimensional hydrodynamic equations when the corona undergoes impulsive heating. In particular, it eliminates the need for the very short timesteps imposed by a highly resolved numerical grid. This paper presents further examples of the applicability of the method for cases of non-uniform heating, in particular, nanoflare trains (uniform in space but non-uniform in time) and spatially localised impulsive heating, including at the loop apex and base of the transition region. In all cases the overall behaviour of the coronal density and temperature shows good agreement with a fully resolved one dimensional model and is significantly better than the equivalent results from a 1D code run without using the jump condition but with the same coarse grid. A detailed assessment of the errors introduced by the jump condition is presented showing that the causes of discrepancy with the fully resolved code are (i) the neglect of the terms corresponding to the rate of change of total energy in the unresolved atmosphere; (ii) mass motions at the base of the transition region and (iii) for some cases with footpoint heating, an over-estimation of the radiative losses in the transition region.PostprintPeer reviewe
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