Plasma diagnostics of an EIT wave observed by HINODE/EIS and SDO/AIA

Veronig, A.M.¹, Gömöry, P.^2,3, Kienreich, I.W.¹, Muhr, N.¹, Vrsnak, B.⁴, Temmer, M.¹, Warren, H.P.⁵

¹ Institute of Physics, University of Graz, Universitaetsplatz 5, A-8010 Graz, Austria
² Astronomical Institute, Slovak Academy of Sciences, SK-05960 Tatranská Lomnica, Slovakia
³ Kanzelhoehe Observatory/Institute of Physics, University of Graz, A-9521 Treffen, Austria
⁴ Hvar Observatory, Faculty of Geodesy, Kaciceva 26, 1000 Zagreb, Croatia
⁵ Space Science Division, Naval Research Laboratory, Washington, DC 20375, USA

Abstract:
We present plasma diagnostics of an Extreme-Ultraviolet Imaging Telescope (EIT) wave observed with high cadence in Hinode/Extreme-Ultraviolet Imaging Spectrometer (EIS) sit-and-stare spectroscopy and Solar Dynamics Observatory/Atmospheric Imaging Assembly imagery obtained during the HOP-180 observing campaign on 2011 February 16. At the propagating EIT wave front, we observe downward plasma flows in the EIS Fe XII, Fe XIII, and Fe XVI spectral lines (log T ~ 6.1-6.4) with line-of-sight (LOS) velocities up to 20 km/s. These redshifts are followed by blueshifts with upward velocities up to -5 km/s indicating relaxation of the plasma behind the wave front. During the wave evolution, the downward velocity pulse steepens from a few km/s up to 20 km/s and subsequently decays, correlated with the relative changes of the line intensities. The expected increase of the plasma densities at the EIT wave front estimated from the observed intensity increase lies within the noise level of our density diagnostics from EIS Fe XIII 202/203 A line ratios. No significant LOS plasma motions are observed in the He II line, suggesting that the wave pulse was not strong enough to perturb the underlying chromosphere. This is consistent with the finding that no H-alpha Moreton wave was associated with the event. The EIT wave propagating along the EIS slit reveals a strong deceleration of a ~ -540 m/s^2 and a start velocity of v0 ~ 590 km/s. These findings are consistent with the passage of a coronal fast-mode MHD wave, pushing the plasma downward and compressing it at the coronal base.