Measurements of the polarization and line shape of the Lyman &agr; hydrogen geocoronal emission obtained on board the satellite D2A Tournesol are compared with theoretical predictions using Kockarts-Nicolet (K-N) profiles. From these data the value of the hydrogen density at the exobase &rgr;500 can be inferred without a large error, whatever the model; the value of the exospheric temperature Tex, however, depends on the value of the gradient Δ&rgr;/Δz at the exobase level. Data obtained over one orbit (April 27, 1971) provide some values and features in agreement with other experimenter's measurements as follows: (1) &rgr;500 density at noon near the equator (11.5¿2.5)104 atoms cm-3, (2) &rgr;500 density variation consistent with a diurnal variation of 2 between 6 and 17 h LT, and (3) maximum exospheric temperature Tex late in the evening (1140¿¿100¿K). However, the present data show two new features: (1) a density increase of 2 between southern latitudes (22¿S-46¿S) and northern latitudes (46¿N to the equator) and (2) an altitude gradient of hydrogen density different from the gradient predicted by K-N theory in a region between 46¿N to the equator.