A Census of Magnetospheric Electrons From Several eV to 30 keV

A survey of electrons with energies between ∼1 eV and 30 keV was conducted using measurements from the THEMIS spacecraft over radial distances between 3 and 12 Earth radii. Two distinct populations are observed, one with a peak energy near 10 eV and one at approximately 1 keV. These populations are present 88% of the time in the magnetosphere. The warm population (∼10 eV) is generally more dense (∼1 cm−3) and extends across the dayside. These warm electron characteristics are similar to the ion warm plasma cloak. The hot distribution (∼1 keV) peaks in number density (∼0.2 cm−3) near midnight and into the morning sector. Since the populations are transported through different evolutionary paths, there are spatial regions within the magnetosphere where the density ratio of the populations (warm to hot, nw/nh) is much larger or smaller than unity. At L shells greater than ∼6, near midnight and into the dawn sector, the density of the hot population is close to a factor of 10 larger than the warm populations. In other regions, the warm electron population is typically slightly more dense (∼2×) than the hot population. Key Points Multiple distinct electron populations are present in the Earth's magnetosphere The ratio of the density of electron populations depends on geomagnetic activity On the nightside of the Earth, there are often regions where the density of hot electrons is larger than that of warm electrons