Shock-induced magnetic reconnection driving Ellerman bomb emission and a spicule

TL;DR

This study addresses whether Ellerman bombs can causally trace magnetic reconnection responsible for type II spicules. Using a self-consistent 2.5D rMHD simulation with Bifrost, augmented by RH1.5D-based synthetic H-alpha spectra, the authors track shocks and current sheets to connect EB formation to reconnection events and spicule launching. They demonstrate a shock-induced current-sheet reconnection scenario that triggers an EB at a spicule footpoint and launches a type II spicule via reconnection outflows, establishing a causal EB–spicule link without requiring flux emergence. The findings offer a physical mechanism for observed EB–spicule associations and suggest EBs can serve as proxies for lower-atmosphere reconnection in solar dynamics, while highlighting limitations of 2D radiative transfer and the need for 3D validation and broader observational comparisons.

Abstract

The mechanism that forms dynamic type II spicules has remained elusive for many years. Their dynamical behaviour has long been linked to magnetic reconnection, yet no conclusive evidence has been provided. However, one recent observational study found signs of reconnection, as traced by Ellerman bombs (EBs), at the footpoints of many spicules. The triggering of EBs is generally linked to reconnection due to flux emergence and convective motions in the photosphere. We aim to explore whether we can connect EBs to type II spicules, and to what extent we can use EBs as an observational proxy to probe reconnection in this dynamic. We also aim to provide further insight into the mechanisms that trigger EBs. We used a simulation run with the radiative magnetohydrodynamics code Bifrost to track spicules and study the physical processes behind their formation. To detect EBs and classify the spicules, we synthesised the H-alpha line using the multilevel radiative transfer code RH1.5D. We also traced shocks and current sheets to decipher the origin of EBs and spicules. We selected one type II spicule with a strong EB near its footpoint and studied their formation in detail. A magnetoacoustic shock advects the magnetic field lines towards an oppositely directed ambient field, creating a current sheet. The current sheet accelerates dense plasma via a whiplash effect generated by reconnection into the inclined ambient field, launching the spicule. Several EB profiles trace shock- and magnetic-reconnection-induced dynamics during this process at the spicule footpoint. We present a new EB triggering mechanism in which a shock-induced current sheet reconnects, triggering an EB in the lower solar atmosphere. The shock-induced current sheet generates the launch of a type II spicule via reconnection outflows. These results provide a physical origin for the observed connection between EBs and spicules.

Figures (8)

• Figure 1: Spicule at the beginning of its ascending phase, indicated by the blue arrows. The left frame shows the temperature, saturated at 20 kK. The right frame shows the logarithm of the neutral hydrogen number density. The dashed lines represent the columns A and B used for calculating the $\textnormal{H}\alpha$$\lambda t$ diagrams, presented in Fig. \ref{['fig: lambda_t']}. An animation of this figure is available at http://tsih3.uio.no/lapalma/subl/recon_eb_spic/the_spicule_temp_dens.mp4.
• Figure 2: Temporal evolution of the $\textnormal{H}\alpha$ synthetic spectral profiles in $\lambda t$ diagrams for the EB (left panel) and the connected spicule (right panel). The EB and spicule spectra are calculated along the columns A and B marked by the dashed lines in Fig. \ref{['fig: spicule']}. The blue arrow indicates the spectral profile corresponding to the timeframe presented in Fig. \ref{['fig: spicule']}. The EB's beginning is marked by time $t = 0$, corresponding to 290 s in the simulation of 2020ApJ...889...95M.
• Figure 3: Formation of the $\textnormal{H}\alpha$ intensity profile for the EB at $t=0$ from column A marked in Fig. \ref{['fig: spicule']}. The contribution function to emergent intensity $C_I$ in the lower right panel can be calculated as the product of the three factors shown in the other panels. The height where $\tau_{\lambda} = 1$ (solid red) and the velocity profile in the $z$-direction (solid white) are plotted in all panels (negative velocities correspond to blueshift, i.e. upflows, while positive values are downflows). The top left panel shows the factor $\chi_{\lambda} / \tau_{\lambda}$, i.e. the ratio between the opacity and optical depth; the dotted cyan line shows the ionised to neutral hydrogen ratio as a function of height. The top right panel shows the monochromatic source function $S_\lambda$; the dotted green line shows the atmosphere's temperature profile. The lower left panel shows the factor $\tau_{\lambda} / e^{-\tau_{\lambda}}$, i.e. a function that peaks in the region where $\tau_{\lambda} \approx 1$. The lower right panel shows the contribution to emergent intensity $C_I$. The solid cyan line shows the $\textnormal{H}\alpha$ spectral profile of the EB, while the dotted white line shows an $\textnormal{H}\alpha$ background profile, i.e. the average profile over many columns and scans. The spectral profile is normalised to $I_{\text{wing}}$, which is the average intensity of the outermost wavelength points in the blue and red wings of the background profile.
• Figure 4: Time series illustrating the four main physical processes that trigger the EB and form the spicule. From left to right: propagation of a magnetoacoustic shock front; the generation of a current sheet by the shock; formation of a reconnecting current sheet; plasma ejection via reconnection outflows, ultimately leading to the formation of the rising spicule. Top: Normalised density fluctuation maps, $\delta\rho/\langle\rho\rangle_{x}$, with detected shocks and current sheets overlaid in magenta and green, respectively. Thick purple arrows indicate the direction of the shock's motion. The solid lines with arrows indicate the direction of the magnetic field. The vertical, dotted line marks the column that is used for integration of the synthetic spectrum; the horizontal, dotted line marks the formation height of the EB; the intersection between the dotted lines marks the location of the EB. Bottom: EB spectra (strong colour) corresponding to the column marked by the vertical dotted line, along with an $\textnormal{H}\alpha$ background profile (weak colour), i.e. the average profile over many columns and snapshots. An animation of the upper panels is available at http://tsih3.uio.no/lapalma/subl/recon_eb_spic/l2d90x40r_it_eb5_ShCsDyn2D5_320_355_1_5fps.mp4.
• Figure 5: Close-up of the mechanisms that trigger the EB and form the spicule. Thick green arrows indicate key flow directions for the plasma related to the current sheet (more details on the plasma flow directions are presented in Fig. \ref{['fig: Quiver']}). Everything else follows the convention of Fig. 4.