Abstract
Context. Jets often have a helical structure containing ejected plasma that is both hot and also cooler and denser than the corona. Various mechanisms have been proposed to explain how jets are triggered, primarily attributed to a magnetic reconnection between the emergence of magnetic flux and environment or that of twisted photospheric motions that bring the system into a state of instability. Aims. Multi-wavelength observations of a twisted jet observed with the Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics Observatory and the Interface Region Imaging Spectrograph (IRIS) were used to understand how the twist was injected into the jet, thanks to the IRIS spectrographic slit fortuitously crossing the reconnection site at that time. Methods. We followed the magnetic history of the active region based on the analysis of the Helioseismic and Magnetic Imager vector magnetic field computed with the UNNOFIT code. The nature and dynamics of the jet reconnection site are characterised by the IRIS spectra. Results. This region is the result of the collapse of two emerging magnetic fluxes (EMFs) overlaid by arch filament systems that have been well-observed with AIA, IRIS, and the New Vacuum Solar Telescope in Hα. In the magnetic field maps, we found evidence of the pattern of a long sigmoidal flux rope (FR) along the polarity inversion line between the two EMFs, which is the site of the reconnection. Before the jet, an extension of the FR was present and a part of it was detached and formed a small bipole with a bald patch (BP) region, which dynamically became an X-current sheet over the dome of one EMF where the reconnection took place. At the time of the reconnection, the Mg II spectra exhibited a strong extension of the blue wing that is decreasing over a distance of 10 Mm (from −300 km s−1 to a few km s−1). This is the signature of the transfer of the twist to the jet. Conclusions. A comparison with numerical magnetohydrodynamics simulations confirms the existence of the long FR. We conjecture that there is a transfer of twist to the jet during the extension of the FR to the reconnection site without FR eruption. The reconnection would start in the low atmosphere in the BP reconnection region and extend at an X-point along the current sheet formed above.
Highlights
In the 1990s, jets had already been observed across all temperature ranges (104 K–107 K) in multi-wavelength observations from Hα with ground-based instruments (Gu et al 1994; Schmieder et al 1994, 1995; Canfield et al 1996) to X-rays with Yohkoh (Shibata et al 1992)
Looking at the direction of the magnetic field vectors between JN2 and JP1, we find that they are oriented from the negative polarity to the positive polarity, which is evidence that it is a bald patch (BP) region, more generally, that it is a region with magnetic field lines that exhibit a dip grazing the surface at the PIL (Fig. 11e)
With the same twist in both polarities, the weaker fields result in weaker current densities in the positive polarity. Another difference is that in the MHD simulation, some strong quasi-separatrix layers (QSLs)-related current sheets surround the flux rope (FR) footpoint related extended patches (see Janvier et al (2013), Aulanier & Dudík (2019). These are not visible with HMI and we argue that this is due to the limitations of the HMI data, from which current sheets can only be extracted during flares and with some processing of the data, as in Janvier et al (2014), Barczynski et al (2020)
Summary
In the 1990s, jets had already been observed across all temperature ranges (104 K–107 K) in multi-wavelength observations from Hα with ground-based instruments (Gu et al 1994; Schmieder et al 1994, 1995; Canfield et al 1996) to X-rays with Yohkoh (Shibata et al 1992). The cool plasma is advected over the emergence domain without passing near the reconnection site and flows along the reconnected magnetic field lines These models fit with the observations of X-ray jets observed with Hinode and with Hα jets from the Swedish 1-m Solar Telescope (SST) (Nóbrega-Siverio et al 2017). The system becomes unstable with a disruption of the field lines via reconnection occurs and material expelled along the open magnetic field lines This mechanism is based on a twisted flux rope (FR) formation during the shear in the jet region (Yeates & Hornig 2011; Pariat et al 2015, 2016; Raouafi et al 2016).
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