Low-energy electron intensities at large distances over the earth's polar cap

The eccentric-orbiting satellite Imp 5 penetrated the distant polar magnetosphere at positions corresponding to those for magnetic field lines which intersect the earth's northern polar cap. Measurements of electron intensities with E> or =250 eV in these regions of extremely low plasma densities were gained with an electrostatic analyzer, a Lepedea. The observational period utilized here was January--October 1970. Electron intensities within the energy range 250 eV< or =E< or =50 keV were less by orders of magnitude than those typically encountered within the plasma sheet and over the auroral oval. However, dramatic temporal variations of average electron intensities in the polar cap region were found for orbit-to-orbit comparisons. Differential intensities at 400 eV varied from 1.5 x 102 to 7.5 x 103 el(cm2s sr eV)-1, and electron densities within the above energy range fluctuated from 1.5 x 10-3 to 4.5 x 10-2 el(cm)-3. These intensity variations showed a remarkable correlation with the polarity of the magnetic sector structure in inerplanetary medium: high intensities for 'away from the sun' sectors and low intensites of 'toward' sectors