Reconnection Microjets in Solar Coronal Loops

Coordinated observations with SDO/AIA, IRIS and Hinode/SOT were combined with state-of-the-art numerical simulations to reveal the direct observational signature of a nanoflare produced by magnetic reconnection in the solar corona. A swarm of these elemental heating events is observed with unprecedented detail, leading to the coronal heating of a loop structure. The loop is initially subject to thermal instability leading to the appearance of coronal rain that allows the dynamic and minute tracing of the coronal magnetic field during the reconnection process and heating to multi-million degree temperatures. Single and clustered reconnection events can be distinguished and are characterised by a strong transverse displacement of an internal loop strand, a localised intensity burst in the strand and, in particular, the presence of a microjet – a fast (~200 km/s), bursty, very short lived (<10 s) and small-scale (500 km width, 1500 km in length) jet-like structure stemming from the rain strand and perpendicular to it, accelerated by magnetic tension (slingshot effect) during the magnetic reconnection process at small angles. The strongest events are accompanied by the ejection of small-scale plasmoids along the jet axis, perpendicular to the loop. The observed microjets have an estimated energy of the order of 10^25 erg. The spatial and temporal evolution of the reconnection events show characteristics of a magnetohydrodynamic (MHD) avalanche: the events are first clustered and highly localised near the loop apex. They expand within minutes across and along the loop. Bi-directional flows along the loop, internal rotational motions, a decrease in the amount of braiding of the loop and an increase in the overall loop intensity accompany the entire process. Supported by our numerical simulations, these localised, telltale observational signatures of magnetic reconnection that we attribute to the elemental nanoflares allow to clearly differentiate reconnection-driven heating mechanisms with the coronal heating candidates for which reconnection does not play a central role.