News and Views on Fang et al. 2017, RAA 2017, Vol 17, No. 4, p31

Hui Tian¹ and Lei Ni²

1. School of Earth and Space Sciences, Peking University, China

2. Yunnan Astronomical Observatory, Chinese Academy of Sciences, China

Solar Ellerman bombs (EBs) were discovered by Ellerman (1917) and they are usually identified as intense short-lived brightening of the extended wings of the H\(\alpha\) line at 6563 Å. They are often observed in emerging active regions, marking local magnetic reconnection in the photosphere (e.g., Watanabe et al. 2011). Non-local thermodynamic equilibrium (non-LTE) modeling of EBs generally suggests a temperature enhancement by a few hundred to \(\sim\)3000 K around the temperature minimum region (TMR, e.g., Fang et al. 2006, Isobe et al. 2007, Hong et al. 2014, Li et al. 2015).

Recent observations by the Interface Region Imaging Spectrograph (IRIS; De Pontieu et al. 2014) have revealed numerous compact brightenings at typical transition region temperatures. These so-called IRIS bombs (IBs) are characterized by chromospheric absorption lines superimposed on the greatly broadened transition region line profiles. Such spectral features indicate the possible presence of hot materials in the lower chromosphere or even the photosphere (Peter et al. 2014). The hot materials likely result from local heating caused by reconnection during magnetic flux emergence (Toriumi et al. 2017, Zhao et al. 2017). Joint observations between IRIS and ground-based telescopes, e.g., the Chinese New Vacuum Solar Telescope (Liu et al. 2014), have clearly shown that some IBs and EBs are co-spatial and co-temporal (Vissers et al. 2015, Kim et al. 2015, Tian et al. 2016). Based on this connection, these authors concluded that some EBs could be heated to \(1-8\times10^4\) K, much higher than previously thought. To understand this new observational result, Ni et al. (2016) performed 2.5-D MHD simulations and found that reconnection around the TMR can indeed heat some of the materials to such high temperatures if the plasma beta is low.

Fang et al. (2017) also attempted to understand how much EBs can be heated, but adopting a different approach. They performed detailed non-LTE calculations of the H\(\alpha\) and Ca II 8542 Å line profiles, as well as the continuum emissions, for three EB models with different temperatures around the TMR. They found that the calculated line profiles and continuum intensity are much stronger than what is observed if the EB temperature is higher than 10000 K. Thus, their results imply that EBs are unlikely to be heated to a temperature higher than 10000 K around the TMR, apparently not in line with the proposal of hot EBs mentioned above. They also noticed that a hot EB should have a very short lifetime due to the high radiative losses. However in observations, the IBs could last for a few to ~20 minutes, much longer than the expected lifetime of a hot EB.

To explain the observational fact that some IBs are closely connected to EBs, Fang et al. (2017) proposed two alternative scenarios: (1) IBs and EBs may reflect different temperature components produced by a common magnetic reconnection process; (2) EB-associated jets may heat the upper atmosphere via waves or shocks, leading to the formation of IBs in higher layers. These two new proposals appear promising. Recent progress in reconnection theory (e.g., Bhattacharjee et al. 2009, Daughton et al. 2011) suggests that a magnetic reconnection process is usually very turbulent and many small-scale structures appear inside the reconnection region. Small-scale shocks and other physical mechanisms may play important roles in plasma heating. The numerical simulations by Ni et al. (2016) show that there are multiple components of plasma with different temperatures even in one small magnetic island during a low beta reconnection process around the TMR, which may support the first proposal and be consistent with the observations that some IBs are indeed connected to EBs (Tian et al. 2016). The second proposal implies different formation heights of the two types of bombs. In this scenario, one may see a systematic spatial offset between the IBs and EBs in high-resolution observations close to the limb, due to the projection effect. Future coordinated observations between IRIS and a large-aperture ground-based telescope should be designed to test this idea.

The heating of EBs is essential for the understanding of magnetic reconnection in the partially ionized lower solar atmosphere. Fang et al. (2017) has made a first attempt to compare calculated spectra and observed ones to investigate the heating of EBs after the discovery of IBs. The obtained results have shed new light on the problem of EB heating, and will definitely lead to intensive discussion in the community. In the future, one may consider continuous energy release through recurrent reconnection and investigate whether this process can result in a relatively long lifetime of IBs. The non-equilibrium ionization effect and more realistic radiative cooling should be included in this type of investigation. Three-dimensional non-LTE modeling of EBs may also need to be performed as the solar atmosphere is highly inhomogeneous and dynamic. EB heating should also be an important scientific objective for future large-aperture telescopes such as DKIST, the Chinese 2.5-m Imaging and Spectroscopic Multi-Application Telescope and the 8-m Chinese Giant Solar Telescope.

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