Low ionization state plasma in CMEs

TL;DR

This study investigates the small spatial extent of low ionization plasma in CMEs observed by UV spectroscopy and explains its rarity in in situ measurements, finding that low ionization regions are confined to less than 10% of the CME volume.

Contribution

It provides the first quantitative analysis of the spatial coverage of low ionization plasma in CMEs and compares UV observations with in situ data to explain their infrequent detection.

Findings

Low ionization plasma covers less than 10% of CME volume.

Faster CMEs have smaller low ionization covering factors.

The rarity of low ionization plasma in in situ data is due to small spatial coverage.

Abstract

The Ultraviolet Coronagraph Spectrometer on board the Solar and Heliospheric Observatory (SOHO) often observes low ionization state coronal mass ejection (CME) plasma at ultraviolet wavelengths. The CME plasmas are often detected in O VI (3x10^5K), C III (8x10^4K), LyA, and LyB, with the low ionization plasma confined to bright filaments or blobs that appear in small segments of the UVCS slit. On the other hand, in situ observations by the Solar Wind Ion Composition Spectrometer (SWICS) on board Advanced Composition Explorer (ACE) have shown mostly high ionization state plasmas in the magnetic clouds in interplanetary coronal mass ejections (ICME) events, while low ionization states are rarely seen. In this analysis, we investigate whether the low ionization state CME plasmas observed by UVCS occupy small enough fractions of the CME to be consistent with the small fraction of ACE ICMEs that show low ionization plasma, or whether the CME plasma must be further ionized after passing the UVCS slit. To do this, we determine the covering factors of low ionization state plasma for 10 CME events. We find that the low ionization state plasmas in CMEs observed by UVCS show average covering factors below 10%. This indicates that the lack of low ionization state ICME plasmas observed by the ACE results from a small probability that the spacecraft passes through a region of low ionization plasma. We also find that the low ionization state plasma covering factors in faster CMEs are smaller than in slower CMEs.