Abstract
Plasma inpost-CMEcurrentsheets(CSs)isexpectedtobehighly turbulent because of the tearing andcoalescence instability and/or local microscopic instabilities. For this reason, in the last decade the inconsistency between the observed (� 10 4 Y10 5 km) and the expected (� 1Y10 m) CS thickness has been tentatively explained in many MHD modelsasaconsequenceof plasmaturbulencethatshouldbeabletosignificantlybroadentheCS.However,fromthe 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 evolutionof turbulencederivedfromthenonthermalbroadeningof the[Fe xviii]lineprofiles.Derivedturbulentspeeds are on the order of � 60 km s � 1 a few hours after the CME and slowly decay down to � 30 km s � 1 in the following 2days.Fromthisevolutiontheanomalousdiffusivityduetomicroinstabilitieshasbeenestimated,andthescenarioof multiple small-scale reconnections is tested. Results show that the existence of many (� 10 � 11 to 10 � 17 � CS m � 3 ) microscopic CSs (� CSs) of small sizes (� 10Y10 4 m) could explain not only the high CS temperatures but also the much larger observed thickness of macroscopic CSs, thanks to turbulent broadening. Subject headingg Sun: corona — Sun: coronal mass ejections (CMEs) — Sun: UV radiation — turbulence
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