Spectral and magnetic properties of the jet base in NGC 315

The dynamics of relativistic jets in inner parsec regions are deeply affected by the nature of magnetic fields. The level of magnetization of the plasma as well as the geometry of these fields on compact scales have not yet been fully constrained. In this paper, we employ multi-frequency and multi-epoch very long baseline interferometry observations of the nearby radio galaxy NGC 315. We aim to derive insights into the magnetic field properties on sub-parsec and parsec scales by examining observational signatures such as the spectral index, synchrotron turnover frequency, and brightness temperature profiles. This analysis was performed by considering the properties of the jet acceleration and collimation zone, which can be probed thanks to the source vicinity as well as the inner part of the jet conical region. We observed remarkably steep values for the spectral index on sub-parsec scales ($ -2$, $S_ which flatten around $ -0.8$ on parsec scales. We suggest that the observed steep values may result from particles being accelerated via diffusive shock acceleration mechanisms in magnetized plasma and subsequently experiencing cooling through synchrotron losses. The brightness temperature of the 43\,GHz cores indicates a dominance of the magnetic energy at the jet base, while the cores at progressively lower frequencies reveal a gradual transition toward equipartition. Based on the spectral index and brightness temperature along the incoming jet and by employing theoretical models, we derived that the magnetic field strength has a close-to-linear dependence with distance going from parsec scales up to the jet apex. Overall, our findings are consistent with a toroidal-dominated magnetic field on all the analyzed scales.