The discovery of nanojets by Antolin et al. (2021) provides strong evidence of reconnection-driven nanoflare heating in the solar corona. Nanojets are small-scale (< 1500 km in length, < 500 km in width), short-lived (< 20 s), and fast (~150 km/s) unidirectional bursts oriented perpendicular to the guide field, thought to be the dynamic counterparts of nanoflares as a result of small-angle (component) magnetic reconnection. Nanojets can therefore also be considered as direct evidence for the existence of braiding in coronal loops: The observations of nanojets to date reveal the prior existence of braiding in the structure, followed by an apparent reduction in the amount of braiding after nanojet storms. To investigate the nanojet’s underlying mechanism, we have simulated two straight and adjacent coronal flux tubes rooted in the chromosphere that are driven to form a small misalignment with an X-point configuration and introduce MHD waves into the tubes, following Antolin et al. (2021). This setup naturally creates an environment for reconnection events that would generate nanojets due to the local increase in the braiding angle produced either directly by the driver or by generated waves. We have conducted a parameter investigation of the effects of footpoint driving on the reconnection by varying the driving amplitudes between 2 km/s and 30km/s.
Results from our simulations show how reconnection occurring in misaligned flux tubes driven at their footpoints can produce structures with dynamic features similar to the nanojets. The left plot of Figure 1 shows an example of the field line configuration of the two flux tubes (in green and magenta) after the driving phase has finished, with velocity vector arrows showing the direction of the flows between the flux tubes, for the case with a footpoint driving amplitude of 20 km/s. We find that at the apex (as shown in the two xy cross-section plots at z=0 Mm), there are nanojet-like structures in the transverse velocity component (vy) with fast velocities above 100km/s. The reconnection is identified by the local increase in current density (|j|) between the two jets that occurs as they form. The jets from our simulations have lengths and widths ranging from 1400-3600 km and 180-550 km, respectively. The obtained dynamics and morphologies are very similar to those of the nanojets in the observations.
Our results show that driving the footpoints with amplitudes of 10km/s and larger produces a singular nanojet-like formation event characterised by the fast bi-directional flows, with an energy release of 1024 erg similar to Antolin et al (2021). In all such cases, the reconnection trigger appears to be MHD waves generated by the driver. Smaller driving amplitudes produce smaller scale and bursty reconnection events, or continuous energy release on the order of 1023 erg, without clear nanojet-like features. The simulations suggest that magnetic reconnection can be triggered by propagating MHD waves in a braided field which locally increase the current. Given that MHD waves are commonly found in the solar corona, if braiding is also common, the presence of waves must set a limit to the amount of current that can accumulate prior to reconnection. The project ID of this work is dp211 titled ‘Modelling of Coronal Nanojets’, and was run in DiAL3 at DiRAC@Leicester.
PIs: Ramada Sukarmadji, Patrick Antolin, Paolo Pagano, James McLaughlin