University of Maryland
Solar Wind Heavy Ion Systematics
We present comparative measurements of solar wind He+2, O+6-8 and Fe+7-16 from the ACE/SWICS instrument and H+ from the SOHO/PM sensor. While it has been well established that alpha particles generally have a higher flow speed than protons, much less is known about the relative flow speeds of the heavier ions. Some previous workers have found that several heavy ion species have a slower flow speed than does He. In contrast, our results from a 6-year survey indicate that the average speeds of O and Fe are about 5 km/s above the He speed at the highest speeds, decreasing to near zero in the high-density low-speed solar wind. As expected, we find that the minor ions tend towards equal thermal speeds in high-speed coronal hole flows, and equal temperatures in low-speed flows. Our statistical survey also reveals a systematic behavior of the abundance ratios He/H, He/O, and Fe/O as functions of the parameter O+7/O+6 (a proxy for the coronal temperature at the location where the solar wind ions freeze-in). We compare these abundance ratios with photospheric values, and also explore possible solar cycle variations. Finally, a number of interesting time intervals have been identified during which He and the heavier ions show dramatic density decreases while the proton density remains relatively steady.