| Propagating quasi-periodic intensity disturbances (PDs) are the phenomena that the coronal emission shows quasi-periodic fluctuations. In the polar coronal holes, PDs normally have periodicities of 10 to 30 minutes. They may propagate along the magnetic field lines to at least two solar radii with a velocity of about 100 km s-1 that is equivalent to the local sound speed. Although PDs have been studied intensively for more than three decades, their mechanism and nature are still debated. Since PDs propagate at velocities equivalent to local sound speeds, many authors suggest they are slow magneto-acoustic waves in the coro-na. However, the source of the slow magneto-acoustic waves is unclear. If they are triggered in the photosphere, they cannot propagate to the corona because they will be reflected by the chromosphere (cut-off effect). This is why previous simulations of PDs normally assume their sources in the transition region or coro-na, which needs observational evidences though. Another scenario suggests that PDs are repeating plasma flows, but this requires further confirmations because plasma flows and waves are hard to be distinguished by observations.Nevertheless, it is believed that PDs are important for the coronal heating and/or mass supplement of the solar wind. If they are waves, their propagation and damping in the corona might contribute to heating the lower corona. If they are plasma flows, they might carry plasma into the solar wind. Therefore, it is crucial to carry out a deep-insight study on these phenomena. In this thesis, we analyze high-resolution multi-wavelength observations of polar coronal holes taken by the Solar Dynamics Observatory (SDO) to investigate the morphology, triggers and propagating properties of PDs. Our studies will put a step toward understanding the mechanism and the nature of these phenomena.Firstly, we analyze three datasets observed in the polar coronal holes by SDO/AIA 171A passband. By applying FFT to the time-series of each pixel in the field-of-view, we are able to create a 2D amplitude/power map at a certain frequency or frequencies. We found that PDs are much easier to be identified on a 2D amplitude/power map, which enables us to recognize 42 PDs. Because a particular PD that might last for a short time during the full observation sequence, we use wavelet analysis to obtain its period. Statistically, we found that the periods of the PDs are 20.8 ± 6.8 minutes. Most PDs show two different velocities at different heights, a smaller one at the bottom of the structure and a much larger one at the propagating-front part. With 42 events, the velocities at their bottom part are 42.6 ± 14.6 km s-1 and at their propagating-front part are 117.2 ± 25.3 km s-1. We suggest that the smaller velocities at the bottom of the PDs might represent the properties of their sources.In order to follow the evolution of the PDs seen in AIA 171A passband, we introduced an easy-to-used method based on simple smooth of the time-series. To investigate how the PDs are generated in the solar atmosphere, we compare the evolution of PDs seen in AIA 171A passband and the spicular activities viewed in the AIA 304 A passband. We found convincing evidence that spicular activities seen in the solar transition region are responsible for PDs in the corona. We conclude that spicules are an important source that triggers coronal PDs.We then study the propagation properties of the PDs in the polar coronal hole to provide more insights into their physical nature. The damping and power spectra of the PDs with frequencies from 0.07 mHz to 10.5 mHz are investigated. The damping of the PDs tends to be stronger at lower frequencies, and their damping behavior below 980" (for comparison, the limb is at 945") is different from what happens above. This indicates that the PDs may experience a two-step spatial damping rather than being damping by a single mechanism operating at all heights. We found that the power spectra of the PDs show power-laws with roughly the same index in different regions in a chosen AIA passband. An additional enhanced component is present in the power spectra in a period range of 8-40 minutes at lower heights, which suggests a possible contribution of upward propagating component from lower parts of the solar atmosphere. The observed damping and power spectra of the PDs are found to be at variance with the interpretation that coronal PDs are a continuous spectrum of slow waves excited by broadband drivers. While the power spectrum of spicule is highly correlated with its associated PD, it suggests that the power spectra of the PDs might be a mixture of spicules and wave activities. We suggest that each PD in the polar coronal hole is possibly a series of independent slow magneto-acoustic waves triggered by spicular activities. |
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