Despite the ubiquity and importance of cosmic magnetic fields, their measurements are very challenging. The contributions to this research topic include reviews and articles on recently developed novel techniques and improved traditional techniques of measuring magnetic fields in the interstellar medium, supernova remnants, molecular clouds, star-forming regions, and on the roles of (turbulent) magnetic fields in star and disk formation and cosmic ray propagation.The progress made in magnetohydrodynamic (MHD) turbulence theories enable the development of new techniques of measuring turbulent magnetic fields. Their application to the observables in diverse astrophysical environments further improves our understanding on turbulence and magnetic fields. Turbulence induces fluctuations in magnetic fields, synchrotron intensity, and polarized synchrotron intensity. Both Zhang & Wang (2022) and Shimoda (2022) discuss probing turbulent magnetic fields with synchrotron emission. Zhang & Wang (2022) review the recent development on the Synchrotron Fluctuation Technique. When polarization measurements are available, the application of the polarization spatial analysis (PSA) to diffuse synchrotron emission and the application of the polarization frequency analysis (PFA) to a point source observed at multiple frequencies can reveal the underlying properties of MHD turbulence in both diffuse astrophysical media and high-energy astrophysical systems. With only synchrotron intensity measurements available, the synchrotron gradient technique provides a promising way to map magnetic field orientations in faint radio sources detected by current and next generation radio telescopes (e.g., LOFAR, SKA).The turbulent magnetic fields in supernova remnants (SNRs) are important for understanding the confinement and acceleration of cosmic rays (CRs), and the synchrotron emissions from the accelerated electrons. Shimoda (2022) reviews the recent investigation on the magnetic field structure in SNRs obtained by the radio synchrotron polarimetry and the statistical properties of turbulent magnetic fields in SNRs diagnosed by a new method using the spatial two-point correlation analysis of synchrotron intensity. The latter, in combination with the future X-ray atomic line observations, can impose observational constraint on the CR injection efficiency at the shock.Turbulent magnetic fields play an important role not only for the shock acceleration of CRs, but also for their propagation in interstellar media (ISM). In the multiphase ISM, with turbulence-induced density and magnetic fluctuations, the Alfvén speed can have a significant spatial variation. By using a large ensemble of compressible MHD turbulence simulations, Beattie et al. (2022) studies the effect of the spatially varying Alfvén speed on the transport of streaming CRs, which can have their streaming speed close to the Alfvén speed. The turbulence-dependent propagation of streaming CRs has important implications on modeling the macroscopic diffusion coefficient of CRs in the complex ISM.Mapping the magnetic fields in molecular clouds and star-forming regions is essential for quantifying their roles in star formation. The principal way of tracing magnetic fields in such regions is based on the polarization of thermal emission and absorption by aligned dust grains. The alignment of dust grains is far from trivial, and its understanding for decades presented one of the astrophysical puzzles of the longest standing. The solution came as the RAdiative Torques (RATs), which were identified as the dominant alignment mechanism, and Analytical MOdel (AMO) was introduced to describe the process of grain alignment. It is important to note that RATs not only align grains but also can disrupt grains by centrifugal stress, which consitutes a newly discovered effect called RAdiative Torque Disruption (RAT-D). The latter effect occurs as aligned grains get into the critical rotational state with centrifugal stress exceeding the binding energy of grain constituents. The alignment and disruption of grains by RATs produce key signatures on dust polarization, which were observationally tested. Tram & Hoang (2022) review these important developments in both theory and observational studies, and discuss the broader implications of the RAT paradigm on dust evolution, time-domain astrophysics and astrochemistry.On the plane-of-sky component of magnetic field orientations in the ISM and molecular clouds, Liu, Zhang, & Qiu (2022) review the advances, limitations, and applications of the Davis-Chandrasekhar-Fermi (DCF) technique of obtaining magnetic field strength, as well as recently developed Histogram of Relative Orientations (HRO) and the polarization-intensity gradient (KTH) method. It is pointed out that the energy nonequipartition can be the biggest uncertainty in the DCF. The angular dispersion function (ADF) accounts for the beam-smoothing effect, interferometric filtering effect, and the ordered field structure. The differential measure approach (DMA) represents a recent important improvement of the DCF. Compared to the DCF, the KTH method has the advantage that it does not require the information on the velocity dispersion, but more tests are required for its uncertainty. Tahani (2022) reviews the progresses recently achieved in improving the observational techniques in measuring either the line-ofsight component or the plane-of-the-sky component of magnetic fields, and their integration to obtain 3D ordered magnetic fields. The forthcoming Zeeman measurements, Faraday rotation measures, starlight polarization observations, and their synergy hold a great promise in providing 3D magnetic fields of many molecular clouds. Down to smaller scales, Maury, Hennebelle, & Girart (2022) review the progresses made in the past decade on the properties and roles of magnetic fields in shaping young protostars, their envelopes, disks, and stellar systems. The recent progresses made in both observations and simulations enable detailed confrontation between observed protostellar properties and magnetized model predictions and improved understanding on the roles of magnetic fields in star and disk formation.This Research Topic provides the reader the recent progresses and new insights in both the methods of measuring multi-scale magnetic fields and their importance in studying diverse astrophysical problems.
Editorial09 January 2023
Editorial: Multi-scale magnetic field measurements in the multi-phase interstellar medium
In this review paper, we aim at providing a global outlook on the progresses made in the recent years to characterize the role of magnetic fields during the embedded phases of the star formation process. Thanks to the development of observational capabilities and the parallel progress in numerical models capturing most of the important physics at work during star formation, it has recently become possible to confront detailed predictions of magnetized models to observational properties of the youngest protostars. We provide an overview of the most important consequences when adding magnetic fields to state-of-the-art models of protostellar formation, emphasizing their role to shape the resulting star(s) and their disk(s). We discuss the importance of magnetic field coupling to set the efficiency of magnetic processes, and provide a review of observational works putting constraints on the two main agents responsible for the coupling in star-forming cores: dust grains and ionized gas. We recall the physical processes and observational methods allowing to trace the magnetic field topology and its intensity in embedded protostars, and review the main steps, success and limitations in comparing real observations to synthetic observations from the non-ideal MHD models. Finally, we discuss the main threads of observational evidence that suggest a key role of magnetic fields for star and disk formation, as well as the remaining tensions that exist between models and observations.
Modern understanding of dust astrophysics reveals that RAdiative Torques (RATs) arising from the radiation-dust interaction can induce two fundamental effects, including grain alignment and rotational disruption. Here we review the recent progress in the theoretical development and observational testing of these effects using dust polarization observed toward star-forming regions (SFRs). We first review the basic theory of the RAT alignment and RAT disruption, which are referred to as RAT-A and RAT-D effects, respectively. We then briefly describe the numerical method we use to model polarized thermal dust emission by accounting for both RAT-A and RAT-D and theoretical predictions of dust polarization for observations. Next, we review our observational efforts to search for observational evidence of the RAT-A and RAT-D effects using thermal dust polarization toward SFRs. Finally, we discuss magnetic fields inferred from dust polarization observed toward these SFRs and implications of the RAT paradigm for different astrophysical conditions, including protostellar environments, dust evolution, and time-domain astrophysics.
Linearly polarized emission from dust grains and molecular spectroscopy is an effective probe of the magnetic field topology in the interstellar medium and molecular clouds.
The longstanding Davis-Chandrasekhar-Fermi (DCF) method and the recently developed Histogram of Relative Orientations (HRO) analysis and the polarization-intensity gradient (KTH) method are widely used to assess the dynamic role of magnetic fields in star formation based on the plane-of-sky component of field orientations inferred from the observations. We review the advances and limitations of these methods and summarize their applications to observations. Numerical tests of the DCF method, including its various variants, indicate that its largest uncertainty may come from the assumption of energy equipartition, which should be further calibrated with simulations and observations. We suggest that the ordered and turbulent magnetic fields of particular observations are local properties of the considered region. An analysis of the polarization observations using DCF estimations suggests that magnetically trans-to-super-critical and averagely trans-to-super-Alfv\'{e}nic clumps/cores form in sub-critical clouds. High-mass star-forming regions may be more gravity-dominant than their low-mass counterparts due to higher column density. The observational HRO studies clearly reveal that the preferential relative orientation between the magnetic field and density structures changes from parallel to perpendicular with increasing column densities, which, in conjunction with simulations, suggests that star formation is ongoing in trans-to-sub-Alfv\'{e}nic clouds. There is a possible transition back from perpendicular to random alignment at higher column densities. Results from observational studies using the KTH method broadly agree with those of the HRO and DCF studies.