Spectroscopic evidence for helicity in explosive events

Abstract

Aims. We report spectroscopic observations in support of a novel view of transition region explosive events, observations that lend empirical evidence that at least in some cases explosive events may be nothing else but spinning narrow spicule-like structures. Methods. Our spectra of textbook explosive events with simultaneous Doppler flow of a red and a blue component are extreme cases of high spectroscopic velocities that lack apparent motion, to be expected if interpreted as a pair of collimated, linearly moving jets. The awareness of this conflict led us to the alternative interpretation of redshift and blueshift as a spinning motion of a small plasma volume. In contrast to the bidirectional jet scenario, a small volume of spinning plasma would be fully compatible with the observation of flows without detectable apparent motion. We suspect that these small volumes could be spicule-like structures and try to find evidence for this. We show observations of helical motion in macrospicules and argue that these features -if scaled down to a radius comparable to the slit size of a spectrometer -should have a spectroscopic signature similar to that observed in explosive events, which is admittedly not easily detectable by imagers. Despite of this difficulty, evidence of helicity in spicules has been reported in the literature. This led us to the new insight that the same narrow spinning structures may be the drivers in both cases, structures that imagers observe as spicules and that in spectrometers cross the slit and are seen as explosive events. Results. We arrive at a concept that supports the idea that explosive events and spicules are different manifestations of the same helicity-driven scenario. In contrast to the conventional view of explosive events as linear bidirectional jets that are triggered by a reconnection event in the transition region, this new interpretation is compatible with the observational results. Consequently, in this case a photospheric or subphotospheric trigger has to be assumed. Conclusions. We suggest that explosive events/spicules are to be compared to the unwinding of a loaded torsional spring.

Two Solar Tornadoes Observed with the Interface Region Imaging Spectrograph

“…Edison Pettit first noticed this type of prominences and categorized them as tornadoes (Pettit 1943). More recently, the term tornado has also been used to describe some other rotating solar phenomena including the macrospicules with rotating motions 1 School of Earth and Space Sciences, Peking University, 100871 Beijing, China; huitian@pku.edu.cn 2 Max-Planck Institute for Solar System Research, Justus-von-Liebig-Weg 3, D-37077 Göttingen, Germany 3 Key Laboratory of Dark Matter and Space Astronomy, Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing, Jiangsu, China (Pike & Mason 1998;Curdt & Tian 2011), chromospheric swirls which may channel energy to the corona (Wedemeyer-Böhm & Rouppe van der Voort 2009;Wedemeyer-Böhm et al 2012;Yang et al 2015) and fast-evolving tornado-like erupting structures (Chen et al 2017). Other rotating structures such as coronal cyclones rooted in the rotating network magnetic field (Zhang & Liu 2011) and rotating jet-like structures (e.g., Shen et al 2011Shen et al , 2012 have also received attention in recent years.…”

Energy and helicity budgets of solar quiet regions

“…There are no reports in the literature concerning the accumulation and expulsion of relative magnetic helicity in the magnetic network and quiet-Sun regions in general and this mechanism's role in quiet-Sun dynamics; only Zhao et al (2009) have investigated current helicity budgets in A&A 564, A86 (2014) network bright points. However, there exist reports (Jess et al 2009;Curdt & Tian 2011;De Pontieu et al 2012) that argue for torsional oscillations in fine structures, such as explosive events and spicules, thus suggesting the existence of twisting motions that could lead to expulsion or transfer of helicity to larger scales in the solar atmosphere.…”

Energy and helicity injection in solar quiet regions

“…With generation of both helicity and energy mostly concentrated along the network, our analysis suggests that there is sufficient free energy in the network to power fine-scale structures residing at the network via dissipative processes, such as magnetic reconnection. Although there is currently substantial observational evidence (Jess et al 2009;Curdt & Tian 2011;De Pontieu et al 2012;Tsiropoula et al 2012;De Pontieu et al 2014) for the presence of helical behavior and twist in small structures, such as mottles and spicules, complemented by some simulations on larger polar-jet structures (Pariat et al 2009) Tziotziou et al (2014a). Verification of these values is still pending and as the three-dimensional magnetic field configuration cannot be inferred from observations, it relies on a combination of high-resolution magneto-hydrodynamic helicity simulations in fine-scale structures with high-resolution chromospheric observations.…”

“…There are no quantitative estimations of the helicity stored and/or carried by fine-scale structures, such as mottles and spicules, although there exist reports in literature (Jess et al 2009;Curdt & Tian 2011;De Pontieu et al 2012;Tsiropoula et al 2012) that such structures often show a helical behavior. Recent observations with NASA's Interface Region Imaging Spectrograph (IRIS) have shown that even the quiet-Sun chromosphere and transition region, in sub-arcsec scales, are filled with twist or torsional motions (De Pontieu et al 2014).…”

“…Zhao et al (2009) have investigated current helicity budgets in network bright points, while studies by Welsch & Longcope (2003), Georgoulis et al (2009), andTziotziou et al (2014a) refer to total helicity budget derivations in the quiet Sun. However, observations suggest the presence of twist and torsional motions in fine structures, Article published by EDP Sciences A61, page 1 of 8 such as explosive events and spicules (Jess et al 2009;Curdt & Tian 2011;De Pontieu et al 2012) and the whole quiet Sun (De Pontieu et al 2014), which could infer the expulsion or transfer of helicity from small-scale structures to larger scales in the quiet solar atmosphere. The derivation of the free magnetic energy and relative magnetic helicity using classical formulas (Finn & Antonsen 1985;Berger 1999) requires an extrapolated, three-dimensional magnetic field, the corresponding current-free field, and their generating vector potentials.…”