Exploring the Origin of Coronal Mass Ejection Plasma from In Situ Observations of Ionic Charge State Composition.

Abstract

Solar wind ionic composition measurements are powerful tools in discriminating between different sources of solar wind as well as identifying interplanetary coronal mass ejections (ICMEs). First, we present a new charge state evolution model which estimates the coronal electron environment from in situ ionic composition measurements. The coronal electron profile is not well measured, as direct observations are difficult to obtain due to the extreme heat and radiation near the sun. Using this model, we show that the unique bi-modal charge states, observed in the iron charge state distribution, may be a direct result of the heating and expansion characteristics of a coronal mass ejection (CME). We next turn our attention to very cool charge states which are sometimes observed concurrently with hot charge states during ICMEs. We show that these observations are a result of simultaneous observations of hot plasma and the remnants of an embedded prominence within the same ICME.

We then use the charge state distribution to explore the origin of suprathermal plasma observed during ICMEs. Suprathermal plasma is known to be an important seed population for solar energetic particles (SEPs) which are accelerated at the CME-driven shock, but the plasma which is being accelerated to the suprathermal energies is not well understood. Using in situ measurements from the Suprathermal Ion Composition Spectrometer (STICS) onboard the Wind spacecraft and the Solar Wind Ion Composition Spectrometer (SWICS) on the Advanced Composition Explorer (ACE), we compare the suprathermal ionic composition to the bulk solar wind plasma during ICMEs. We present a comparison of suprathermal iron and oxygen to the co-located bulk plasma distribution during ICMEs as well as the bulk plasma upstream of the CME-driven shock. This is one of the first studies to present the suprathermal composition of heavy ions observed in ICME plasma. We find that there is a strong correlation between the suprathermal plasma and the co-located bulk plasma and not with the upstream bulk plasma. This implies a local acceleration mechanism is energizing the local bulk plasma to suprathermal energies and not due to shock acceleration acting on the heliospheric plasma upstream of the ICME.