Small-scale loops heated to transition region temperatures and their chromospheric signatures in the simulated solar atmosphere

Context. Recent observations have revealed loop-like structures at very small scales visible in observables that sample the transition region (TR) and even coronal temperatures. These structures are referred to as either 'unresolved fine structures', 'dynamic cool loops', 'miniature hot loops' or 'campfires' depending on the observables in which they are detected. Their formation remains unclear. Aims. Realistic magnetohydrodynamic simulations and forward synthesis of spectral lines are used to investigate how these features occur. Methods. Computations were carried out using the MURaM code to generate model atmospheres. The synthetic H alpha and Si IV spectra are calculated at two angles (mu=1, mu=0.66) using the Multi3D code. We traced magnetic field lines in the model and examined the evolution of the underlying field topology. Results. The synthetic H alpha Dopplergrams reveal loops that evolve dramatically within a few minutes. The synthetic H alpha line profiles show observed asymmetries and Doppler shifts in the line core. However, they also show strong emission peaks in the line wings, even at the slanted view. The synthetic Si IV emission features partly coincide with structures visible in H alpha Dopplergrams and partly follow separate magnetic field threads. Some are even visible in the emission measure maps for the lg(T/K) = [5.8, 6.2] temperature interval. The emission areas trace out the magnetic field lines rooted in opposite polarities in a bipolar region. Conclusions. The model shows that a loop-like structure in a bipolar system with footpoints undergoing rapid movement and shuffling can produce many small-scale recurrent events heated to high temperatures. It demonstrates that heating to different temperatures occurs and can be confined to a small part of the loop, at the location where resistive and viscous heating increases. The model largely reproduces the observed features in terms of size, lifetime and morphology in chromospheric, TR and coronal observables. The morphology and evolution of the resulting observable features can vary depending on the viewing angle.