EVIDENCE OF THE ZEEMAN SPLITTING IN THE 2(1)-]0(1) ROTATIONAL TRANSITION OF THE ATMOSPHERIC (OO)-O-16-O-18 MOLECULE FROM GROUND-BASED MEASUREMENTS

We present in this paper a set of spectra of the atmospheric (OO)-O-16 -O-18 molecule taken at 233.94611 GHz with the 2.5 m-radiotelescope PO M-2 located at Plateau de Bure (Lat. 44:38:02.00 N, Long. 5:54:28.50 E , Alt. 2550 m) in the French Alps. These spectra clearly show a Zeeman substructure over a +/-2.5 MHz range from the line centre. The centra l frequencies for each peak would indicate local mean values of the ge omagnetic field over Plateau de Bure for the altitude range 2.5-130 km of around 40 mu T that is a slight, but measurable, difference with r espect to a standard geomagnetic model based on measurements carried o ut in 1980, that gives values around 44 mu T. Depending on the directi on and polarization we see 2, 4, or 6 peaks that are identified as pi (Delta M = 0) or sigma +/- (Delta M = +/- 1) components. This effect c annot be seen from ground-based measurements in the millimetric lines of the main isotope O-16(2) for which very broad emission and high opa city of low layers eliminate any structure, but it must be taken into account for atmospheric temperature soundings from satellite at freque ncies close to these O-16(2) resonances.