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

Abstract

A spectacular manifestation of solar activity is the appearance of transient brightenings in the far wings of the Hα line, known as Ellerman bombs (EBs). Recent observations obtained by the Interface Region Imaging Spectrograph have revealed another type of plasma "bombs" (UV bursts) with high temperatures of perhaps up to 8×10 4 K within the cooler lower solar atmosphere. Realistic numerical modeling showing such events is needed to explain their nature. Here, we report on 3D radiative magnetohydrodynamic simulations of magnetic flux emergence in the solar atmosphere. We find that ubiquitous reconnection between emerging bipolar magnetic fields can trigger EBs in the photosphere, UV bursts in the mid/low chromosphere and small (nano-/micro-) flares (10 6 K) in the upper chromosphere. These results provide new insights intothe emergence and build up of the coronal magnetic field and the dynamics and heating of the solar surface and lower atmosphere.

RADYN Simulations of Non-thermal and Thermal Models of Ellerman Bombs

“…However, using a semi-empirical model, Fang et al (2017) argued that EB temperatures cannot be higher than 10,000 K in order to reproduce the observed line profiles and continuum. On the other hand, recent 3D simulations suggest that most UV bursts may originate from low/mid-chromospheric plasma (Hansteen et al 2017).…”

“…In observations, EBs are always associated with other activities including moat flows (Watanabe et al 2011;Vissers et al 2013), newly emerged magnetic flux (Pariat et al 2007;Watanabe et al 2008;Yang et al 2013Yang et al , 2016Reid et al 2016;Danilovic et al 2017), jets (Watanabe et al 2011;Reid et al 2015), or surges (Matsumoto et al 2008a;Yang et al 2013;Pasechnik 2016). Numerical simulations show that magnetic reconnection of a U-shaped or Ω-shaped loop in the lower atmosphere can be the cause of an EB (Isobe et al 2007;Archontis & Hood 2009;Hansteen et al 2017). Different methods to invert the spectral lines including Hα, Ca II 8542 Å, and Fe I lines have been used to obtain the local temperature enhancement in the lower atmosphere.…”

“…The line wings of Hα are enhanced, while the line center remains undisturbed. Observations have shown that EBs are also visible in other spectral lines and continua, including the Ca II 8542 Å line (Yang et al 2013;Kim et al 2015;Rezaei & Beck 2015), the Ca II H and K lines (Matsumoto et al 2008a;Rezaei & Beck 2015), the G band (Herlender & Berlicki 2011;Nelson et al 2013), the ultraviolet continuum at 1600 and 1700 Å Rezaei & Beck 2015;Tian et al 2016), Mg II triplet lines Hansteen et al 2017;Hong et al 2017), and the He I D 3 and 10830 Å lines (Libbrecht et al 2017). The typical lifetime of EBs is a few minutes and the typical size is about 1″-2″ in diameter (Georgoulis et al 2002;Rutten et al 2013).…”

A Cancellation Nanoflare Model for Solar Chromospheric and Coronal Heating

“…Third, the driving of magnetic reconnection by flux emergence or cancellation has different observational consequences depending on the location in height of the reconnection, which in turn depends on the magnitudes of the flux source and the overlying field strength (see Section 2). Thus, energy release can produce: an Ellerman bomb in the wings of Hα low down in the atmosphere around sunspots or in the Quiet Sun (Rouppe van der Voort et al 2016;Hansteen et al 2017); ultraviolet (UV) bursts in the chromosphere of an active region (Peter et al 2014); explosive events in the transition region (Brueckner & Bartoe 1983;Innes et al 2011), blinkers (Harrison 1997); and X-ray bright points and X-ray jets in the corona (Shibata et al 1992;Shimojo et al 2007). Recent studies further emphasize the possible, widespread role of reconnection during flux cancellation as the source of coronal loop brightenings.…”

Chromospheric Inversions of a Micro-flaring Region

“…A recent study by Hansteen et al (2017) simulates reconnection events in the solar atmosphere and shows the difference in atmospheric conditions for EBs, micro-flares, and UV bursts. They found EBs at localized reconnection sites in the photosphere, with temperatures below 10,000K and bidirectional flows of up to ∼20 km s −1…”