Measurements of the hydrogen auroral emissions have been made by rocket‐borne photometers simultaneously with observations of the energy spectra and pitch angle distributions of the precipitating energetic particles. The rocket was flown through a postbreakup auroral glow known to be associated with proton precipitation on the basis of optical measurements from the ground. The observed Hβ profiles are compared with the profiles derived from the measured primary proton beam and obtained by using Eather's (1967) values for the pertinent excitation cross sections. Good agreement between measured and computed profiles is obtained. The measured height profiles had the same shape during the ascent and descent parts of the flight, although the intensity of the Hβ emission varied by a factor of 2. The maximum volume emission rate was found to be at 112±1 km in both cases, whereas the half value emission rates were at 107 and 119 km. The atmospheric density above 120‐km altitude had to be reduced by a factor of 2 in comparison with the Cira (1965) model atmosphere in order to account for the observed energy degradation of the incoming proton beam.

The pitch-angle distribution of 100-to 300-key protons in the early momingflate afternoon local magnetic-time sector has been studied by using data from the early lifetime of the Esro IB (Bo.reas) satellite. The pitch-angle distroebution is described as a function of invariant latitude for different geomagnetic activity levels. On the nightside, going poleward, the particle fluxes were found to peak at 90 ø, followed by a latitude range with isotropy. On the dayside, two precipitation zones were found during slight disturbances, the poleward zone showing gn isotropic pitch-angle distribution and the equatorward zone showing an anisotropic distribution peaked at 90 ø . During disturbed conditions, the two zones merge and the region of isotropic poetch-angle distribution expands. The particle loss cones were determined and compared with theoretical loss cones in a realistic field model. The phenomenon of energetic protons impinging on the atmosphere may be described by giving, for instance, the flux values and energy and pitch-angle distributions, for different geomagnetic activity levels, as a function of invariant latitude and magnetic local time.In this paper we concentrate on the pitchangle distribution of protons with energy above 100 key. The knowledge of the pitch-angle distribution is necessary to get a quantitative understanding of the processes going on when the energetic particles penetrate the upper atmosphere. To estimate properly the interaction between the incoming particles and the atmosphere, a detailed knowledge of the pitch-angle distribution of the particles is of prime importance. Knowing the pitch-angle distribution, one can also extract information on acceleration processes for the energetic particles and get evidence concerning electromagnetic fields in the precipitation region.Only a few direct measurements of pitchangle distribution of energetic protons in the ionosphere at higher latitudes have been reported. Information of proton pitch-angle distribution obtained by sounding rockets has been reported by, for instance, S•raas and Trumpy [1966] and more recently by Whalen and McDiarmid [1970] and Whalen et al. [1971]. The evidence is generally indicative of a flux maximum perpendicular to the magnetic-field lines or near isotropy over the upper hemisphere. By satellite-borne experiments, Sharp et al. [1967] found isotropy within a factor of 2 for protons with energy above 20 key. Recently Hultqvist et al. [1971] reported on both 'depleted loss-cone' type of anisotropy, to the equatorward edge of the particle zone, and field-aligned anisotropy, to the poleward edge, for ions of below 10 kev of energy. By studying the red shift of auroral hydrogen line profiles, one has found indication of anisotropy in the incident proton flux with a peak 90 ø to the magnetic-field lines. See review by Eather [1967]. Precipitation of protons above 100 key of energy has been studied by using data from the S71C experiment on board the satellite Esro IA (Aurorae) [Aarsnes et al., 1970a]..A slight modification ...

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