Spectroscopic Detection of Turbulence in Post-CME Current Sheets

• Published in: The Astrophysical journal Vol. 689; no. 1; pp. 572 - 584
• Main Author: Bemporad, A
• Format: Journal Article
• Language: English
• Published: Bristol IOP Publishing 10.12.2008; IOP
• Subjects: Astronomy; Earth, ocean, space; Exact sciences and technology; Ultraviolet radiation; Turbulence; Coronal mass ejection; Current layers; Sun: corona; Sun: coronal mass ejections (CMEs); Sun: UV radiation; Sun
• ISSN: 0004-637X; 1538-4357
• DOI: 10.1086/592377

Plasma in post-CME current sheets (CSs) is expected to be highly turbulent because of the tearing and coalescence instability and/or local microscopic instabilities. For this reason, in the last decade the inconsistency between the observed ([image] km) and the expected ([image]1-10 m) CS thickness has been tentatively explained in many MHD models as a consequence of plasma turbulence that should be able to significantly broaden the CS. However, from the observational point of view, little is known about this subject. A few post-CME CSs have been observed in UVCS spectra as a strong emission in the high-temperature [Fe xviii] line, usually unobservable in the solar corona. In this work, published data on post-CME CSs observed by UVCS are reanalyzed, concentrating for the first time on the evolution of turbulence derived from the nonthermal broadening of the [Fe xviii] line profiles. Derived turbulent speeds are on the order of [image]60 km s super(-1) a few hours after the CME and slowly decay down to [image]30 km s super(-1) in the following 2 days. From this evolution the anomalous diffusivity due to microinstabilities has been estimated, and the scenario of multiple small- scale reconnections is tested. Results show that the existence of many ([image]10 super(-11) to 10 super(-17) [mu]CS m super(-3)) microscopic CSs ([mu]CSs) of small sizes ([image] m) could explain not only the high CS temperatures but also the much larger observed thickness of macroscopic CSs, thanks to turbulent broadening.