High-resolution Observational And Modeling Researches On Coronal Bright Points And Prominence Longitudinal Oscillations

Coronal bright points (CBPs) are long-lived small-scale brightenings in the lower solar corona. They are generally explained in terms of magnetic reconnection. However, the corresponding magnetic configuration and how the reconnection proceeds are not well understood. We carry out a detailed high-resolution multi-wavelength analysis of two neighboring CBPs on2007March16, one is bigger than the other, as revealed by the X-ray Telescope (XRT) aboard the Hinode satellite and the Extreme Ultraviolet Imager (EUVI) aboard the Solar TErrestrial RElations Observatory. It is seen that the soft X-ray (SXR) light curves present quasi-periodic flashes with an interval of～1hr superposed over the long-lived mild brightenings. We propose that the SXR brightenings of this type of CBPs might consist of two components:one is the gentle brightenings, and the other is the CBP flashes. It is found that the strong flashes of the bigger CBP are always accompanied by SXR jets. The potential field extrapolation based on photospheric magnetograms indicates that both CBPs are covered by a dome-like separatrix surface, with a magnetic null point above. We propose that the repetitive CBP flashes, as well as the recurrent SXR jets, result from the impulsive null-point reconnection, while the long-lived brightenings are due to the interchange reconnection along the separatrix surface. Although the EUV images at high-temperature lines (e.g.,284A) resemble the SXR appearance, the171A and195A channels reveal that the blurry CBP in SXR consists of a cusp-shaped loop and several separate bright patches, which are explained to be due to the null-point reconnection and the separatrix reconnection, respectively. Filament longitudinal oscillations have been observed on the solar disk in Ha. We carry out a multiwavelength data analysis of the active region prominence oscil-lations above the western limb on2007February8. The high-resolution observations by Hinode/SOT indicate that the prominence, seen as a concave-inward shape in lower-resolution Extreme Ultraviolet (EUV) images, actually consists of many concave-outward threads, which is indicative of the existence of magnetic dips. After being injected into the dip region, a bulk of prominence material started to oscillate for more than3.5hours, with the period being52min. The oscillation decayed with time, with the decay timescale being133min. The possible relation between the longitudinal oscillations and the later eruption of a prominence thread, as well as a coronal mass ejection (CME), is also discussed.To investigate the underlying physics for the coherent longitudinal oscillations of the entire filament body, such as the triggering and damping mechanisms, and the dominant restoring force, we carry out numerical simulations of prominence longitu-dinal oscillations using the MPI-AMRVAC code. First, a prominence is formed due to chromospheric evaporation and then coronal condensation due to thermal instability. We relax these finite-sized prominences to a thermal and force-balanced equilibrium state by stopping the chromospheric heating. The quiescent prominence starts to os-cillate subjecting to perturbations. Using the observed geometry of the prominence thread as the initial condition, our simulation can well reproduce the observed oscil-lation period, indicating that the gravity of the prominence is the restoring force for the oscillation. However, the damping timescale in the simulation is1.5times as long as the observations. We found that the radiative cooling is the dominant factor lead-ing to the damping, compared to heat conduction, but still insufficient to explain the rapid decay of the oscillations. Other mechanisms, such as wave leakage, have to be considered. Afterwards, we study the resulting oscillations along the dip with a survey of the parameters characterizing the magnetic field geometry. It is found that both the impulsive heating at one leg of the loop and a suddenly inherited velocity perturbation can propel the prominence to oscillate along the magnetic dip. The amplitude of oscil-lation increases with the strength of perturbations, but decreases with the depth of the magnetic dip. An extensive parameter survey shows that the period (P) of the oscil-lation, which is independent of the strength of the perturbations, scales with the depth (D) and the length (2w) of the magnetic dip as P～2ω/(?), a result consistent with the linear theory of a pendulum. We found that gravity acting along the geometry is the main restoring force for the oscillations. The prominence oscillations attenuate in the presence of non-adiabatic effects. The damping time and the ratio of damping to period decreases with period. Mass drainage also plays an important role in the attenuation of oscillations when the perturbation is so strong.