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Faraday tomography of radio polarimetric data below 200 MHz from LOFAR areproviding us with a new perspective on the diffuse and magnetized interstellarmedium (ISM). Of particular interest is the discovery of Faraday-rotatedsynchrotron polarization associated with neutral gas, as traced by atomichydrogen (HI) and dust. Here we present the first in-depth numerical study ofthese LOFAR results. We produce and analyze comprehensive syntheticobservations of low-frequency synchrotron polarization from MHD simulations ofcolliding super shells in the multiphase ISM. We define five distinct gasphases over more than four orders of magnitude in gas temperature and density,ranging from hot, and warm, fully ionized gas to cold neutral medium. We focuson the contribution of each gas phase to synthetic observations of bothrotation measure and synchrotron polarized intensity below 200 MHz. We alsoinvestigate the link between the latter and synthetic observations of opticallythin HI gas. We find that, not only the fully ionized gas but also the warmpartially ionized and neutral phases strongly contribute to the total rotationmeasure and polarized intensity. However, the contribution of each phase to theobservables strongly depends on the choice of integration axis and theorientation of the mean magnetic field with respect to the shell-collisionaxis. Strong correlation between HI synthetic data and synchrotron polarizedintensity, reminiscent of LOFAR results, is obtained with lines of sightperpendicular to the mean magnetic field direction. Our study suggests thatmultiphase modelling of MHD processes is needed in order to interpretobservations of the radio sky at low frequency. This work is a first steptoward understanding the complexity of low-frequency synchrotron emission thatwill be soon revolutionized by large-scale surveys with LOFAR and the SKA.

First look at the multiphase interstellar medium with synthetic observations of low-frequency Faraday tomography