Detection of a large-scale organized 2 kG order magnetic field in the Herbig Ae star HD179218
While about two dozen Herbig Ae/Be stars have been reported to be magnetic, only two of them, HD101412 and HD190073, have had their magnetic field geometries studied in the past. The knowledge of the magnetic field structure is important to understanding how magnetospheric accretion works in these stars. We aim to study in detail the spectral and magnetic variability of HD179218, which is necessary to put constraints on its magnetic field geometry. We measured the mean longitudinal magnetic field, $\langle B_{\rm z}\rangle$, from newly acquired and archival high-resolution spectropolarimetric observations of HD179218 using the least-squares deconvolution technique. Additionally, we studied the spectral variability of the hydrogen lines using dynamical spectra. Based on our analysis of the Stokes V spectra of HD179218, we report for the first time the definite detection of a magnetic field. Using a slightly refined rotation period of P$_{rot}$ = 1.34102 d, we constrained its geometry as follows: an estimated magnetic obliquity angle of $β$= 79.9 $\pm$ 0.7 deg and a dipole strength of B$_d$ = 2142 $\pm$ 52 G. The $\langle B_{\rm z}\rangle$ variation is best fitted by the superposition of a sine wave and of its first harmonic, but more spectropolarimetric observations are necessary to test the impact of the limited measurement precision and the uneven coverage of the rotation cycle. The strongest emission in the H$α$ and H$β$ line profiles in the medium-resolution spectra acquired in 2025 was detected close to the phases of the best visibility of the magnetic poles. HD179218 is the second Herbig Ae/Be star after HD190073 for which a first snapshot of a magnetosphere is presented.