Microwave Emission from Twisted Magnetic Fields

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Revision as of 07:23, 12 November 2016

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 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].

Figure 1. Panel a: a magnetic loop with regular twist, green lines/arrows show magnetic field and its direction. Panel b: the scheme of circular polarisation of optically-thin GSMW emission produced by energetic electrons in a twisted loop observed from the top (upper sketch) and from the side (lower sketch).

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.

Figure 2. Microwave emission from a twisted loop with nearly contant cross-section at the beginning of the impulsive phase. The foot-point magnetic field is 320 G, loop plasma temperature about 18 MK, non-thermal electron density at the loop axis is about 4x10⁷cm^-3, and non-thermal electron spectrum a power law with index -1.4.