Abstract
Ionospheric heavy ions in the distant tail of the Earth’s magnetosphere
at lunar distances are observed using the ARTEMIS mission. These heavy
ions are originally produced in the terrestrial ionosphere. Using the
ElectroStatic Analyzers (ESA) onboard the two probes orbiting the Moon,
these heavy ions are observed as cold populations with distinct energies
higher than the baseline energy of protons, with the energy-per-charge
values for the heavy populations highly correlated with the proton
energies. We conducted a full solar cycle survey of these heavy ion
observations, including the flux, location, and drift energy, as well as
the correlations with the solar wind and geomagnetic indices. The
likelihood of finding these heavy ions in the preferred regions of
observation called “loaded” quadrants of the terrestrial magnetotail
is ~90%, regardless of the z orientation of the IMF. We
characterize the ratio of the heavy ion energy to the proton energy, as
well as the velocity ratio of these two populations, for events from
2010 to mid-2023. This study shows that the “common velocity”
assumption for the proton and heavy ion particles, as suggested in
previous work through the velocity filter effect, is not necessarily
valid in this case. Challenges in the identification of the mass of the
heavy ions due to the ESA’s lack of ion composition discrimination are
addressed. It is proposed that at the lunar distances the heavy ion
population mainly consists of atomic oxygen ions (O+) with velocities
~25% more than the velocity of the co-located proton
population.