Soft X-ray emission in kink-unstable coronal loops

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Revision as of 16:30, 4 March 2015

Context

Solar flares liberate large amounts of energy in short periods of time and heat up the plasma in magnetic coronal loops. Theoretical explanations for the triggering and progression of flares usually invoke the presence of geometrical features such as strongly twisted magnetic field-lines, large-scale magnetic writhe or regions of strong magnetic shear and currents in the flaring regions. These elements can make the magnetized coronal plasma susceptible to the development of a certain number of magneto-hydrodynamical instabilities which can provide the means for the rapid energy liberation in flares. These hypothesis (and correspondent flare scenarii), although well laid out theoretically, are very hard/unlikely to be observed directly. Flare observations show that soft X-ray emission show a quick growth of volume-filling thermal emission in the flaring loop volume without being able to reveal the fine-structure of the underlying magnetic complex. Other techniques such as coronal magnetic field reconstruction/extrapolation can only give clues about the general large-scale geometry of the field, and cannot grasp the non-potential (and possibly non-force-free) nature of the intrinsically unstable fields. As such, it is necessary to test flare scenarii and models by simulating their temporal evolution and deducing observable signatures from them. Here, we ponder over the possibility of the kink-instability being at the origin of B and C class flares. This instability is prone to develop in strongly twisted magnetic flux-ropes in the corona, and several of its properties make it a good candidate for explaining solar flares. The major objection if faces lies in that flux-ropes with sufficiently high twist are rarely seen in the corona.

Magnetic twist: how much do we see and how much there is

Our simulations suggest that observations may underestimate the actual magnetic twist present in the flaring regions. This happens mainly for two reasons: 1) the magnetic threads which become “illuminated” at a given moment do not necessarily correspond to the most strongly twisted magnetic field-lines, 2) the erupting loops have already lost a large fraction of their initial twist at the point when thermal emission becomes substantial. Figure 1 shows a few snapshots of the X-ray emissivity at 5 keV superposed over a sample of magnetic field-lines. This is an important point: the lack of observational evidence of strongly twisted magnetic flux-ropes was seen as a major hurdle faced by flare scenarii relying on the kink instability.

Figure 1. Continuum emissivity at 5 keV and some magnetic field-lines from a kink-unstable twisted flux-rope.

Emission measures: how much hot plasma, and how dense

The amount of heated plasma which is susceptible of emitting significantly in the soft X-ray range can be represented by the emission measure of the plasma above a certain temperature (9 MK here). Figure 2a shows the temporal evolution of the emission measure of the “hot” and the “cold” plasma components. Figure 2b shows the distribution of the emission measure as a function of plasma temperature at about the flare peak.

Flux emitted as a function of time

Figure AA shows a series of light-curves corresponding to different photon energy bands. All curves are normalized to their maximum value (for easier representation). Thermal emission grows rapidly during the initial phases of the simulated flare (i. e., during the linear phase of the kink-instability). The initial growth is followed by a slower decay (i. e., during the relaxation phase of the instability). This asymmetry occurs because small sporadic heating events happen repeatedly after the main initial burst of ohmic heating. The asymmetry is more evident in the lower energy bands than on the higher energy bands, as these corresponds to the hotter bits of the plasma which are cooled more efficiently by thermal conduction.