Heavy Ion Temperatures As Observed By ACE/Swics

Heavy ions observed near 1 AU, especially in fast solar wind, tend to have thermal speeds that are approximately equal, indicative of a mass proportional temperature. Additionally, observations near 1 AU have shown a streaming of heavy ions (Z>4) along the magnetic field direction at speeds faster than protons. The differential velocities observed are of the same order but typically less than the Alfven speed. Previous analysis of the behavior of ion thermal velocities with Ulysses-SWICS, focusing on daily average properties of 35 ion species at 5 AU, found only a small systematic trend with respect to q²/m.

Utilizing improved data processing techniques, results from the Solar Wind Ion Composition Spectrometer (SWICS) onboard the Advanced Composition Explorer (ACE) shed new light on the thermal properties of the heavy ion population at 1 AU. A clear dependence of heavy ion thermal behavior on q²/m has now been found in the recent ACE-SWICS two hour cadence data set at 1 AU. Examining the thermal velocities of about 70 heavy ion species relative to alpha particles (He²⁺) shows a distinct trend from equal thermal speed toward equal temperature with increasing q²/m. When examined for solar winds of different collisional ages, the observations indicate the extent of thermal relaxation present in different solar wind types. We explore this collisional dependence with a model for the collisional thermal relaxation of the heavy ions as the solar wind propagates out to 1 AU. This model is used to subtract out the collisional effects seen in the ACE-SWICS data, providing an estimate for the temperature distribution among heavy ions at the corona to be compared to remote sensing observations that have shown that heavy ions are preferentially heated at the corona. We will discuss how this new analysis elucidates the thermal behavior and evolution of heavy ions in the solar wind, along with implications for the upcoming Solar Probe Plus and Solar Orbiter missions.