Compressive oscillations in hot coronal loops: Are sloshing oscillations and standing slow waves independent?

TL;DR

This study provides the first observational evidence that sloshing oscillations and standing slow waves in hot coronal loops are interconnected phenomena, with transformations between them influenced by plasma cooling and thermal conduction effects.

Contribution

It introduces a new analytical model for fitting sloshing oscillations and demonstrates the transformation from sloshing to standing waves in different EUV channels, confirming recent numerical predictions.

Findings

Sloshing oscillations transform into standing waves in the 94 Å channel.

Oscillation periods increase during the standing phase.

Cooling plasma and thermal conduction explain the observed oscillation behavior.

Abstract

Employing high-resolution EUV imaging observations from SDO/AIA, we analyse a compressive plasma oscillation in a hot coronal loop triggered by a C-class flare near one of its foot points as first studied by Kumar et al. We investigate the oscillation properties in both the 131{\,}{\AA} and 94{\,}{\AA} channels and find that what appears as a pure sloshing oscillation in the 131{\,}{\AA} channel actually transforms into a standing wave in the 94{\,}{\AA} channel at a later time. This is the first clear evidence of such transformation confirming the results of a recent numerical study which suggests that these two oscillations are not independent phenomena. We introduce a new analytical expression to properly fit the sloshing phase of an oscillation and extract the oscillation properties. For the AIA 131{\,}{\AA} channel, the obtained oscillation period and damping time are 608$\pm$4{\,}s and 431$\pm$20{\,}s, respectively during the sloshing phase. The corresponding values for the AIA 94{\,}{\AA} channel are 617$\pm$3{\,}s and 828$\pm$50{\,}s. During the standing phase that is observed only in the AIA 94{\,}{\AA} channel, the oscillation period and damping time have increased to 791$\pm$5{\,}s and 1598$\pm$138{\,}s, respectively. The plasma temperature obtained from the DEM analysis indicates substantial cooling of the plasma during the oscillation. Considering this, we show that the observed oscillation properties and the associated changes are compatible with damping due to thermal conduction. We further demonstrate that the absence of a standing phase in the 131{\,}{\AA} channel is a consequence of cooling plasma besides the faster decay of oscillation in this channel.