Abstract: The ionosphere is the primary source for heavy ions which are ubiquitous in the terrestrial magnetosphere. Low-altitude energization in the auroral ionosphere results in bulk heating and transverse acceleration of the ions, which begin to upwell and/or be accelerated upward by the mirror force, starting upflow and leading to the outflow process. The details of the processes which seed ion outflow at low altitudes are difficult to measure in situ and thus remain an open question.

We examine the observational parameter regime in which ion upflow/outflow initiates. Emphasis is placed on making kinetic measurements of the thermal ion population, allowing for the accounting of the many processes which affect in situ plasma measurements. The instrument best-suited for making these measurements is the electrostatic plasma analyzer (ESA). The HT, a thermal ion ESA, is capable of making the measurements necessary to quantify the roles of various heating mechanisms in initiating ion upflow in the low-altitude auroral ionosphere. We present the difficulties associated with making these measurements and identify instrument design choices which mitigate some of these measurement challenges.

Analysis of HT measurements of the thermal ion distribution function taken on the MICA auroral sounding rocket is presented. Using a Maxwellian model to replicate possible measured spectra, we calculate moments from the model and compare with equivalent moments calculated from the in situ data. Liouville's theorem and the thin-sheath approximation allow us to couple the measured and model moments through a forward-modeling technique such that measurements inside the sheath provide information about the state of the plasma outside the sheath. Early in the MICA flight, ion upflow, with heating, is observed and attributed to wave-particle interactions at altitudes lower than previously reported in the literature. Later in the flight the ion temperature is driven by frictional heating. Ion upflow, without associated ion heating, is observed in an auroral arc, and interpreted as the seeds of the upflow and heating process. This upflow is part of a region of low-altitude plasma circulation, with ion upflow occurring in an auroral arc and downflow in the downward current region poleward of the arc. The low-altitude observations of the MICA case study will serve to inform future ionospheric modeling and simulations, specifically, (a) the importance of heating by wave-particle interactions at altitudes lower than previously considered viable, and (b) the occurrence of upflow on a Type 2 field line before/below higher altitude heating processes are expected.