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

TL;DR

This study uses high-cadence spectroscopy and imaging to analyze plasma motions during an EIT wave, revealing downward plasma flows and wave deceleration consistent with a fast-mode MHD wave in the solar corona.

Contribution

First detailed plasma diagnostics of an EIT wave combining Hinode/EIS spectroscopy and SDO/AIA imaging, revealing plasma flow dynamics and wave kinematics.

Findings

Downward plasma flows up to 20 km/s at the wave front.

Wave deceleration of approximately -540 m/s² with an initial speed of 590 km/s.

No significant chromospheric perturbation or Moreton wave observed.

Abstract

We present plasma diagnostics of an EIT wave observed with high cadence in Hinode/EIS sit-and-stare spectroscopy and SDO/AIA 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 red-shifts are followed by blue-shifts 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 XIII 202/203 AA 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 Halpha Moreton wave was associated with the event. The EIT wave propagating along the EIS slit reveals a strong deceleration of a ~ -540 m/s2 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.