  Star formation is essential in galaxy evolution and the cycling of matter.
The support of interstellar clouds against gravitational collapse by magnetic
(B-) fields has been proposed to explain the low observed star formation
efficiency in galaxies and the Milky Way. Despite the Planck satellite
providing a 5-15' all-sky map of the B-field geometry in the diffuse
interstellar medium, higher spatial resolution observations are required to
understand the transition from diffuse gas to gravitationally unstable
filaments. NGC 2024, the Flame Nebula, in the nearby Orion B molecular cloud,
contains a young, expanding HII region and a dense filament that harbors
embedded protostellar objects. Therefore, NGC 2024 is an excellent opportunity
to study the role of B-fields in the formation, evolution, and collapse of
filaments, as well as the dynamics and effects of young HII regions on the
surrounding molecular gas. We combine new 154 and 216 micron dust polarization
measurements carried out using the HAWC+ instrument aboard SOFIA with molecular
line observations of 12CN(1-0) and HCO+(1-0) from the IRAM 30-meter telescope
to determine the B-field geometry and to estimate the plane of the sky magnetic
field strength across the NGC 2024. The HAWC+ observations show an ordered
B-field geometry in NGC 2024 that follows the morphology of the expanding HII
region and the direction of the main filament. The derived plane of the sky
B-field strength is moderate, ranging from 30 to 80 micro G. The strongest
B-field is found at the northern-west edge of the HII region, characterized by
the highest gas densities and molecular line widths. In contrast, the weakest
field is found toward the filament in NGC 2024. The B-field has a
non-negligible influence on the gas stability at the edges of the expanding HII
shell (gas impacted by the stellar feedback) and the filament (site of the
current star formation).
