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The charged particle lunar environment experiment (CPLEE), a part of the Apollo 14 lunar surface package, is an ion‐electron spectrometer capable of measuring ions and electrons with energies between 40 ev and 50 kev. The instrument, with apertures 26 cm above the surface, has detected a photoelectron gas layer above the sunlit lunar surface. No detectable flux above 200 ev has been observed. Experimental data for periods while the moon was in the earth's magnetotail for electrons with energies 40 ev ≤ E ≤ 200 ev follow a power‐law spectrum j(E) = j0(E/E>0)−μ with 3.5 ≤ μ ≤ 4. In the absence of photoelectrons with E > 200, we assume that the surface potential is at least 200 volts. The modulation of this potential in the presence of intense plasma‐sheet fluxes has been observed. Also, a detailed history of the February 10, 1971, total lunar eclipse, to determine the source distribution of high‐energy solar photons, is presented. A classical penumbral‐umbral behavior indicates that at the time of the eclipse the emission of higher‐energy photons was uniform over the solar disc. Numerical solutions for the variation of electron density and potential as functions of height above the lunar surface were obtained. The solar photon spectrum I(hν), obtained from various experimental sources, and the photoelectron yield function of the surface materials, Y(hν), are two parameters of the solution. Energy spectra at the height of the measurements for various values of Y(hν) were computed until a fit to experimental data was obtained. Using a functional form Y(hν) = [Y0(hν − W) / (W / 2)] for 6 ev ≲ hν ≲, 9 ev and Y(hν) = Y0 for hν >9 ev, where the lunar‐surface work function W was set at 6 ev, we calculated a value of Y0 = 0.1 electrons/photon. The solution also showed that the photoelectron density falls by 5 orders of magnitude within 10 meters of the surface, but the layer actually terminates several hundred meters above this height. A hydrostatic model of the photoelectron layer has also been developed. It is shown that the numerically calculated pressure, density, and potential can be approximated by solving the hydrostatic equations with an equation of state P/n1/2 = constant out to 200 cm from the surface. Beyond this height, the equation of state shifts toward the isothermal case, P/n = constant.

The University of California at San Diego Auroral Particles Experiment on the ATS 6 satellite in synchronous orbit has detected a low-energy plasma population which is separate and distinct from both the ring current and the plasma sheet populations. The density and temperature of this low-energy population are highly variable, with temperatures in the range kT = 1-30 eV and densities ranging from less than I cm -3 to more than 10 cm -3. The occurrence of a dense low-energy plasma is most likely in the afternoon and dusk local time sectors, whereas n > I cm -a is seen in the local night sector only during magnetically quiet periods. These observations suggest that this plasma is the outer zone of the plasmasphere. During magnetically active periods this low-energy plasma is often observed flowing sunward. In the dusk sector, strong sunward plasma flow is often observed for 1-2 hours prior to the onset of a substorm-associated particle injection.

The design considerations that are necessary to optimize the performance of a tilting-filter photometer are presented. Such a photometer is described, and some typical measurements of aurora and airglow illustrate the application of this technique to low light level spectrophotometry. The digital approach used with photomultipliers at these low light levels is also discussed. Comparison of the tilting-filter photometer with other spectral scanning instruments reveals a superiority for many applications. A final section discusses possible space applications.

Energy spectra and pitch angle distributions of auroral electrons have been measured in a premidnight multiple arc auroral display by a Javelin rocket containing two identical payloads that separated in flight. The rocket (code-named Twins 2) was launched from Fort Churchill, Canada, at 0459 UT on March 2, 1968, and covered an altitude range up to 800 km. The electron energy spectra between 40 ev and 20 key show a 'continuum' spectrum with a superimposed energetic peak. The center energy of the peak was observed to shift from 10-12 key over the arcs to 2-3 key between the arcs. This spectral structure is shown to be similar to the inverted 'V' stYucture reported by other investigators. The in flight separation of the two payloads allowed investigation of spatial versus temporal effects in the auroral precipitation. In one interval of the flight an arc was observed to be moving northward with a velocity of 0.6 km/sec. Calculated backscattered spectra are compared with those actually measured. Good agreement was observed for electron energies above about 1.5 key, but below this energy the backscatter ratio was observed to be ~100%. Several explanations for this unusually high ratio are considered, and a likely possibility is shown to be a parallel downward-directed electric field between •150 and 500 km. Measurementsof auroral particle fluxes, their spectra, and pitch angle distributions. have been the object of numerous satellite and rocket investigations. A recent review of pertinent measurements has been given by Paulikas [1971]. The general features of auroral particle precipitation are now well known, although the details of the origins of the particles and of their interaction with the atmosphere are far from understood. Several investigators have reported 'anomalous' results in measurements of auroral particles. For example, Mozer and Bruston [1966a, b] reported energetic protons traveling upward from mirror points well within the atmosphere and anticorrelations between electron and proton fluxes. A recent study by R•me and Bosqued [1971] reported observations of anticorrelated proton and electron fluxes and proton pitch angle distributions peaked along field lines. These observations were interpreted as indicating the presence of electric fields parallel to magnetic field lines in the upper ionosphere with magnitudes ranging up • Permanent address: to a few hundred millivolts/meter. Furthermore, Mozer and Bruston [1967] reported direct experimental evidence by means of rocket-borne electric field probes of such parallel electric fields in the upper ionos.phere. Choy and Arnoldy [1971] have observed an anomalously high reflection coefficient or backscatter ratio for auroral electrons with energies below i kev. These results are indeed surprising in view of the relatively low altitude of the measurements (( 150 km, R. L. Arnoldy, private communication, 1972). Chappell [1968] has compared calculated and measured auroral electron backscatter ratios using data from the Twins I twin payload sounding rocket [Weste...