Slow EIT waves as gravity modes

TL;DR

This paper proposes that slow EIT waves observed in the solar corona are better explained as gravity modes, specifically internal and surface gravity waves, rather than the traditionally assumed fast magneto-acoustic modes.

Contribution

It introduces a gravity wave model to explain slow EIT wave speeds, contrasting with the standard fast magneto-acoustic wave interpretation.

Findings

Gravity modes can account for low propagation speeds of EIT waves.

Internal gravity waves are transverse and dispersive.

Surface gravity waves also match observed slow wave velocities.

Abstract

The EIT waves [named after the Extreme-ultraviolet Imaging Telescope (EIT) onboard the Solar and Heliospheric Observatory (SOHO)] are in the literature usually described as fast magneto-acoustic (FMA) modes. However, observations show that a large percentage of these events propagate with very slow speeds that may be as low as 20 km/s. This is far below the FMA wave speed which cannot be below the sound speed, the latter being typically larger than $10^2$ km/s in the corona. In the present study it is shown that, to account for such low propagation speed, a different wave model should be used, based on the theory of gravity waves, both internal (IG) and surface (SG) ones. The gravity modes are physically completely different from the FMA mode, as they are essentially dispersive and in addition the IG wave is a transverse mode. Both the IG and the SG mode separately can provide proper propagation velocities in the whole low speed range.