Microwave Emission from Twisted Magnetic Fields
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Number: | 286 |
1st Author: | Mykola Gordovskyy |
2nd Author: | and Phillippa Browning |
Published: | 13 November 2016 |
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Introduction
In the accepted picture of the magnetic activity of the solar corona, free energy gradually accumulates in the coronal volume via body currents, expressed as geometrical twist within the magnetic field. Magnetic twist in the solar corona can thus be a key factor in many phenomena, from flares and eruptions to loop oscillations and coronal wave propagation. The instability of a twisted coronal loop has been invoked by many as a viable mechanism for fast energy release and solar flares (e.g., Ref. [1]). Such an instability would be particularly attractive in explaining "failed" eruptions and flares in small isolated loops. Furthermore, with the reconnection and energy release locations evenly distributed within the loop volume, they are also attractive as particle accelerators [2].
In order to investigate the role of magnetic twist in coronal events we need to be able to detect it. The EUV and soft X-ray emission from thermalized plasmas captured in coronal magnetic flux tubes ought to reveal some visible twist. Flares create hot plasma, and its emission can thus be used for twist detection; however, this hot plasma appears only towards the end of the impulsive phase, when the twist can be substantially reduced [9]. The hard X-ray emission produced by non-thermal electrons in a flaring twisted loop could reveal its presence by structurally changing the X-ray sources. This generally is an ambiguous signature, though, and in any individual flare this feature alone cannot be used for twist detection. Hence the question of twist detection, particularly during the early stages of a flare, remains open.
Gyrosynchrotron emission produced by energetic electrons is sensitive to the magnetic field direction: it is left- or right-polarised depending on the sign of the line-of-sight magnetic field. This effect is often observed as opposite polarisations of microwave emission coming from opposite footpoints of coronal loops. Therefore, gyrosynchrotron circular polarisation might be used to determine the sign of the line-of-sight magnetic field component and, hence, to detect the twisted magnetic fluxtubes in the corona.
The sketch in Figure 1 provides a graphic description of this method. A magnetic flux-tube that is regularly twisted (panel a), when viewed from its side, will show oppositely-polarised upper and lower parts (panel b), a pattern called cross-loop polarisation gradient (CLPG). A twisted flux-tube viewed from the top also demonstrates the CLPG pattern. Ideally, the line separating two polarisations will run as a diagonal, crossing loop's mid-plane as shown in Figure 1b (upper part). However, in reality, the visibility of the CLPG pattern can be affected by many factors, from loop temporal evolution to the optical thickness of the generated microwave emission. In order to evaluate viability of GSMW polarisation for magnetic twist detection in the corona, we calculate synthetic maps of polarized microwave emission (Stokes I and V) produced by hot plasma and energetic particles in an evolving twisted magnetic loop following a kink instability followed via MHD/test-particle simulations [2].
Synthetic microwave maps
The GX Simulator allows us to synthesize the model loop's microwave image properties, as shown in the animation of Figure 2.
Figures 3 and 4 then show snapshots of models with different magnetic field configurations. We find the emission to be optically thin at frequencies above the spectral peak frequency, which varies between 2-20 GHz (depending on the beam spectral index). At the optically-thin frequencies, CLPG patterns can be seen near loop-tops when observed from the top, and along a whole loop when observed from the side. However, in the strongly converging loop (Figure 4), the foot-points are much brighter than the loop-top, which may reduce the CLPG pattern visibility. At the optically-thick frequencies the gradient of Stokes V across the loop can still be observed; however, it has a more complicated structure which would be difficult to interpret.
RHESSI Nugget Date | 13 November 2016 + |
RHESSI Nugget First Author | Mykola Gordovskyy + |
RHESSI Nugget Index | 286 + |
RHESSI Nugget Second Author | and Phillippa Browning + |