Strong nonpermanent emissions are observed from two H Ly &agr; experiments place on board the satellite D2A. They are superimposed to the permanent emission due to resonant scattering of the Ly &agr; solar line by telluric atomic hydrogen. These emissions are detected both above and under the altitude of the satellite (~500 km), in a latitude range ¿40¿. The correlation observed between their occurrence frequency and the Kp index strongly suggests that the source of excitation is precipitation of particles. The intensities of these emissions observed under the satellite's altitude are of the order of a few hundred rayleighs. These intensities are in a ratio of about 10 relative to the strong Balmer &agr; emissions previously detected on board the same satellite, and they could be attributed to precipitation of protons in the range 10--300 keV. The intensities of the order of 1 kR measured above the satellite's altitude would require particle fluxes of lower energies too large to be provided by the ring current from double charge exchange. There remains then the possibility of an interference between the experiment and precipitating particles. Even in this eventuality our measurements show evidence of large fluxes of soft particles at low and middle latitudes, even for moderate magnetic activity.