Observations of Ellerman bomb emission features in He i D₃and He i 10 830 Å

Abstract

Context. Ellerman bombs (EBs) are short-lived emission features, characterized by extended wing emission in hydrogen Balmer lines. Until now, no distinct signature of EBs has been found in the He i 10830 Å line, and conclusive observations of EBs in He i D 3 have never been reported. Aims. We aim to study the signature of EBs in neutral helium triplet lines. Methods. The observations consist of 10 consecutive SST/TRIPPEL raster scans close to the limb, featuring the Hβ, He i D 3 and He i 10830 Å spectral regions. We also obtained raster scans with IRIS and make use of the SDO/AIA 1700 Å channel. We use Hazel to invert the neutral helium triplet lines. Results. Three EBs in our data show distinct emission signatures in neutral helium triplet lines, most prominently visible in the He i D 3 line. The helium lines have two components: a broad and blue-shifted emission component associated with the EB, and a narrower absorption component formed in the overlying chromosphere. One of the EBs in our data shows evidence of strong velocity gradients in its emission component. The emission component of the other two EBs could be fitted using a constant slab. Our analysis hints towards thermal Doppler motions having a large contribution to the broadening for helium and IRIS lines. We conclude that the EBs must have high temperatures to exhibit emission signals in neutral helium triplet lines. An order of magnitude estimate places our observed EBs in the range of T ∼ 2 · 10 4 − 10 5 K.

Frequently Occurring Reconnection Jets from Sunspot Light Bridges

“…It turns out that for most jets this height is in the range of 250-750 km, indicating that the reconnection takes place in the upper photosphere or lower chromosphere. Considering the height of occurrence, these reconnection events are similar to Ellerman bomb (Ellerman 1917), which are believed to be formed by reconnection taking place around the TMR or in the upper photosphere (e.g., Ding et al 1998;Watanabe et al 2011;Yang et al 2013;Vissers et al 2013;Nelson et al 2015;Rezaei & Beck 2015;Rouppe van der Voort et al 2016;Rutten 2016;Nelson et al 2017;Danilovic et al 2017;Libbrecht et al 2017). It is worth noting that Ellerman bombs are usually observed outside sunspots in the areas of emerging flux and that they are normally believed to be generated by magnetic reconnection at the upper part of U-shaped magnetic structures (e.g., Isobe et al 2007;Archontis & Hood 2009;Georgoulis et al 2002;Pariat et al 2009;Cheung et al 2010;Schmieder et al 2014).…”

Magnetic Reconnection at the Earliest Stage of Solar Flux Emergence

“…Thus, the association of some UV bursts with EBs suggests that reconnection around the TMR may heat the cool materials to a temperature of ∼80,000 K, the formation temperature of the Si iv lines under ionization equilibrium. A recent investigation of EB emission signals in the He i D 3 and 10830Å lines also indicates a similar temperature enhancement (Libbrecht et al 2017). Such a scenario appears to challenge existing models of EBs, in which the TMR could not be heated to a temperature higher than 10,000 K (e.g., Fang et al 2006Fang et al , 2017Bello González et al 2013;Berlicki & Heinzel 2014;Hong et al 2014Hong et al , 2017aLi et al 2015;Reid et al 2017).…”

Bombs and Flares at the Surface and Lower Atmosphere of the Sun

“…Reconnection at the current layers leads to plasma heating via Joule dissipation (panels c, f, i, and the online animated version, which shows the time evolution of these quantities) and to the emission of bi-directional flows (b, e, h) with velocities comparable to the local Alfvén speed. Note that the velocities are asymmetric, with the upward propagating portion having substantially higher velocity (see,e.g., Libbrecht et al 2016). The aforementioned events have lifetimes of at least 120-180s.…”