Observations and modeling of the onset of fast reconnection in the solar transition region

Magnetic reconnection is a process in which the magnetic topology is changed and magnetic energy is released to heat plasma, drive Alfvénic flows, induce Alfvén as well as magneto-acoustic waves and energize particles. It is responsible for energetic phenomena such as geomagnetic storms and aurora (Lui, 1996; Sitnov et al., 2019), solar flares and coronal mass ejections (Tsuneta, 1996; Shibata & Magara, 2011; Kliem et al., 2000; Karlický & Bárta, 2007; Li et al., 2016; Lin & Forbes, 2000), X-ray flares in magnetars (Hurley et al., 2005), and magnetic interactions between neutron stars and their accretion disks (Kato et al., 2004). It is also crucially important in laboratory plasma physics because it triggers the sawtooth crashes that can potentially cause disruptive instability in magnetically confined fusion plasmas (Taylor, 1986; Bhattacharjee et al., 2001; Yamada et al., 2010). A better understanding of reconnection is thus a common challenge confronting laboratory, space, and astrophysical plasma physicists.

Recently, observational and experimental works have provided new insights into reconnection mechanisms. For example, NASA’s Magnetospheric Multiscale Mission (MMS) has found direct evidence for electron demagnetization and acceleration at reconnection sites in Earth’s magnetosphere (Burch et al., 2016), while laboratory experiments have revealed structures in the diffusion region that trigger fast reconnection (Ji et al., 2004; Katz et al., 2010; Dorfman et al., 2013). Observations from the Cluster spacecraft in the Earth’s magnetotail (Chen et al., 2008) and more recently the Parker Solar Probe have found magnetic islands in the inner solar wind (Howard et al., 2019). However, many questions remain about how the onset of fast reconnection occurs. For example, transition region explosive events, rapid brightenings thought to be driven by reconnection and associated with strong flows visible in spectral lines formed at temperatures of a few hundred thousand Kelvin (Brueckner, 1981; Dere et al., 1989), occur on timescales of minutes, much shorter than classic reconnection mechanisms predict. Recent progress in reconnection theory (Shibata & Tanuma, 2001; Loureiro et al., 2007; Bhattacharjee et al., 2009) suggests that under certain circumstances fast reconnection can be triggered by the so-called plasmoid instability, when a thin current sheet breaks up into secondary current sheets and plasmoids (or magnetic islands). Recent theoretical studies (e.g., Pucci & Velli, 2013; Comisso et al., 2016; Uzdensky & Loureiro, 2016; Huang et al., 2017) suggest that the thinning of the current sheet and growth of various tearing modes proceed simultaneously before fast reconnection is triggered. Nevertheless, these predictions for the onset of the fast reconnection, namely a transition from a slow quasi-continuous phase to fast, plasmoid-mediated, reconnection, have not yet found definitive support from observations, despite observations of the presence of plasmoid-like features (Lin et al., 2005; Singh et al., 2012; Takasao et al., 2012; Rouppe van der Voort et al., 2017). In addition, alternative theories have suggested that strong turbulent motions in solar microflares lead to the onset of fast reconnection (Chitta & Lazarian, 2020).

UV bursts, sudden and compact brightenings in UV light, provide an opportunity to study magnetic reconnection on the Sun (Young et al., 2018). A variety of phenomena such as transition region explosive events and IRIS bombs are considered to be UV bursts: we shall refer to all these events as UV bursts in this paper. These bursts are often characterized by broad spectral line profiles and the more violent ones show a strong enhancement of emission (Dere et al., 1991). They are thought to be driven by magnetic reconnection (Dere et al., 1991; Peter et al., 2014; Innes et al., 1997), because of their characteristic spectral profiles that indicate strong bidirectional flows, which typically occur where magnetic flux concentrations of opposite polarity meet (Hansteen et al., 2019). Other explanations for explosive events exist as well (Curdt & Tian, 2011; Curdt et al., 2012), but the majority of explosive events are still considered to be driven by reconnection. As mentioned, high-resolution observations of UV bursts have provided tantalizing indications for the presence of plasmoids (Rouppe van der Voort et al., 2017), but they have not shown the transition from a slow to fast phase. In this paper we present observational evidence for such a transition, and compare it with numerical simulations of magnetic reconnection, transitioning from a slow phase to a plasmoid-mediated phase in which reconnection occurs much faster.

In § 2 we describe the observations, while we provide details of the numerical simulations in § 3. Our results are described in § 4 and we finish with a discussion and conclusions in § 5.

IRIS is a NASA small explorer that contains a high-resolution spectrograph and slit-jaw imager, both sensitive to near- and far-UV light. The instrument has spectral passbands that cover emission lines and continua that originate at temperatures covering the solar atmosphere from the photosphere to the corona (De Pontieu et al., 2014). In this paper, we concentrate on the Si IV 1403Å line originating in the transition region between the chromosphere and corona at about 80,000K. The spatial resolution of IRIS is 0.33″, while the spectral sampling corresponds to a velocity sampling of around 2.7 km/s. We also use the slit-jaw images of the 1400Å passband from the IRIS slit-jaw imager which are dominated by the Si IV lines at 1394Å and 1403Å  as well as the far ultraviolet continuum (De Pontieu et al., 2014). We analyzed four IRIS data sets (so-called sit-and-stare rasters) to search for UV bursts sharing similar spectral properties: 13-Aug-2013 at 13:35 UTC, 4-Feb-2014 at 00:29 UTC, 15-Apr-2014 09:59 UTC, and 4-May-2014 12:09 UTC. These datasets have cadences short enough (3s for all, except for the latter, which has 5s cadence) to observe fast-evolving UV bursts. All were targeted on active regions.

We also use a magnetogram obtained by the Helioseismic and Magnetic Imager (Scherrer et al., 2012, HMI,) onboard the Solar Dynamics Observatory (SDO, Pesnell et al., 2012) along with an IRIS slit-jaw image to show an active region with several UV bursts in Figure 1, with sample spectra shown in Fig. 2. Co-alignment of the HMI and IRIS data was done by cross-correlation between the 1600Å continuum obtained by the Atmospheric Imaging Assembly (Lemen et al., 2012, AIA) onboard SDO and IRIS 2796Å slit-jaw images that show the chromosphere in the Mg II k line. We first perform co-alignment using the coordinates provided in the headers of the data sets and then use a cross correlation method to determine the pointing offset between the IRIS telescope and the AIA telescope. The HMI and AIA data are both full-disk and easily co-aligned.

The simulations in this letter solve single-fluid, resistive MHD equations with anisotropic heat conduction and radiative cooling, using a state-of-the-art simulation code (Huang et al., 2017). Similar to Innes et al. (2015), we use a Harris sheet equilibrium as the initial configuration, which is given by $B_{x}(y)=B_{0}\tanh(y/\delta)$ (Harris, 1962) where $\delta=10km$ is the half width of the current sheet; we adopt a Lundquist number of $10^{5}$, which is defined by $S=Lv_{A}/\eta$ with L the spatial scale, $v_{A}$ the Alfvén speed and $\eta$ the resistivity. We adopt the radiative loss estimated in Colgan et al. (2008). Assuming the simulation domain is in thermal equilibrium initially, background heating is added to balance the radiative loss at t=0 s. The resistive MHD equations are solved in a two-dimensional domain 2 Mm $\times$ 2 Mm on a grid of 2064 $\times$ 2048 grid points. In Figure 3 we show a subdomain enclosing the current sheet ($<1$ Mm) instead of the whole simulation domain. The $x$ direction is along the long axis of the current sheet and the $y$ direction perpendicular to the current sheet. Viscosity is not included in this qualitative study. Initially, at the asymptotic region, the Alfvén speed is 200 km/s, the number density $10^{10}$ cm^-3, the temperature $3\,10^{4}$ K and the plasma beta = 0.1. We did not include gravity and a stratified atmosphere in the current study. The fact that the spatial extent of a UV burst, and thus our computational domain, is smaller than the pressure scale height in the transition region at around 100,000 K justifies this assumption. Gravity and stratification would cause a density gradient along the outflow direction and possibly decrease the speed of reconnection outflows toward the Sun and slightly alter the red wings of line profiles. Nevertheless, we do not expect that including these two terms will qualitatively change our results.

Note that the simulation is conducted over a relatively narrow range of plasma parameters, which are chosen specifically to focus on the observed events in the temperature range of Si IV formation. The initial temperature is chosen to be 30,000 K so that the current sheet is at the lower range of the Si IV contribution function (weaker emission) in the slow phase and at the peak of the Si IV contribution function (stronger emission) in the fast phase. If we use lower initial temperatures (e.g. 10,000 K), the slow phase is less visible in Si IV but the emission in the fast phase will still be strong. It certainly is possible to imagine other scenarios in which the slow phase is too cool to be visible in the Si IV passband, the setup of our simulation in this paper is focused on UV bursts observed in Si IV with both phases visible.

In this paper, we present rare spectroscopic observations of the transition of a reconnecting current sheet from a slow quasi-continuous phase to a fast and impulsive phase. We find that the observations of events showing bidirectional flows followed quickly by triangular shaped-profiles are compatible with the growth of the plasmoid instability, which is an important prediction of theory but had not yet been validated by experimental or observational evidence until now. While theoretical work on this mechanism has been extensive, observational studies are typically more limited because of practical considerations (e.g., limitations of remote sensing). Thanks to the high temporal resolution and high sensitivity of the IRIS instrument, we can now, for the first time, shed light on the events preceding the impulsive phase of transition region UV bursts or explosive events. The new observations presented here have the special feature of capturing both the slow and the impulsive phases of the events, and the close correspondence with the simulation results make a compelling case for the interpretation based on the nonlinear plasma instability. These observations, which go further than previous reports (e.g., Innes et al., 1997, 2015; Hara et al., 2011) on plasmoids on the Sun, demonstrate that IRIS spectroscopic data can be used for diagnosing rapid reconnection dynamics on the Sun, thereby opening up opportunities for future studies that constrain theoretical models of magnetic reconnection.