Applying 2D motion magnification to EUV observations of the low-amplitude kink oscillations
This page is online version of my poster on RADIOSUN-5 workshop and Summer school.
We introduce a new approach for the investigation of the low-amplitude transverse oscillations of coronal loops recently discovered in SDO/AIA data. Typical amplitude of these oscillation is found to be about 0.2 Mm, that is less than SDO/AIA pixel size. Measuring parameters of such a small motion is a challenging task. The technique called motion magnification can help here and simplify the analysis. Motion magnification acts like a microscope for low amplitude motions in image sequences (i.e. data cubes). It artificially amplifies small displacements making them much better visible in time-distance plots and animations. This approach is found to give good results for harmonic oscillations, including the modulated signals, such as exponentially decaying, multi-modal and frequency modulated signals. The method was tested on an artificial data set, and using the EUV observations of a non-flaring active region, clearly demonstrating the presence of low-amplitude decay-less oscillations in the majority of coronal loops associated with the active region.
- Input: a sequence of 2D images (data cube)
- Compute spatial 2D dual tree complex wavelet transform (DTℂWT) of each image
- Calculate slow trend of DTℂWT components phases by smoothing them along the time dimension
- Magnify phase variation of DTℂWT components
- Reconstruct output images using inverse DTℂWT
- Output: a data cube with magnified transverse motions
Testing on an artificial model
The method has been tested on a sequence of artificial images, imitating typical kinds of kink oscillations in a system of four loops.
Loop 1: harmonic oscillation with constant period and amplitude.
Loop 2: decay-less oscillation in an expanding loop. Period is changing in time.
Loop 3: exponentially decaying standing oscillation
Loop 4: coexistence of two harmonics, global mode and third harmonic.
The built-in oscillations are clearly visible in the processed data for all four kinds of the kink oscillatory patterns that are difficult to see in the original data.
Testing on SDO/AIA data
The developed method was tested on the SDO/AIA 171 Å observations of a coronal loop bundle previously analysed by Nistico, Anfinogentov, and Nakariakov (2014). This set of coronal loops was observed by SDO/AIA on the south-western limb of the Sun on 21 January 2013 and was not associated with any NOAA active region. In the original time-distance plot, the low amplitude oscillations could only be noticed by an experienced eye knowing where and when to look at. But, even a small motion magnification by a factor of 3 makes the oscillatory patterns visible to everyone without difficulty. Larger magnification (×9) allows one to investigate oscillatory profiles in detail, revealing possible amplitude evolution and coexistence of multiple harmonics.