Kinematic properties of globally-propagating waves in the solar corona

Abstract

Globally-propagating bright fronts in the solar corona are a poorly understood phenomena despite almost 15 years of research. Here, the kinematics and morphology of these disturbances are studied using Extreme UltraViolet (EUV) observations from the Solar Terrestrial RElations Observatory (STEREO) and Solar Dynamic Observatory (SDO) spacecraft. The first observations of a coronal bright front (CBF) made using STEREO are presented, with the pulse observed in all four EUV passbands, namely 171, 195, 284, and for the first time, 304 A. The pulse exhibited similar kinematics in all passbands studied, with a velocity peak of 238+-20 km s-1 within ~28 minutes of launch and acceleration varying from 76 m s-2 to -102 m s-2 observed in the 304 A passband. Similar kinematics are found in the 195 A passband, while lower values were found in the lower cadence 284 A passband. The higher cadence 171 A passband shows much stronger variation, with the velocity peaking at 475+-47 km s-1 within ~20 minutes of launch, and the acceleration varying from 816 m s-2 to -413 m s-2. The derived pulse velocity and acceleration were found to be strongly influenced by the observing cadence, implying that previous kinematics may have been underestimated. Several different image processing techniques for identifying CBFs were tested to determine the optimal method for CBF pulse analysis. It was found that the techniques traditionally used to identify CBFs are fundamentally awed and prone to undefined user-dependent errors. The kinematics of a simulated data-set were derived using several different numerical differencing techniques. These techniques were found to be inadequate for dealing with the typical data-set sizes encountered when studying CBFs. A semi-automated identification and tracking algorithm is proposed to provide a statistically rigorous analysis of CBFs. This algorithm was applied to STEREO observations of four CBF events and used to determine their kinematics. Following launch at initial velocities of ~240-450 km s-1 each of the four events studied showed significant negative acceleration ranging from ~ -290 to -60 m s-2. The spatial and temporal widths of the CBF pulses were also found to increase from ~50 Mm to ~200 Mm and ~100 s to ~1500 s respectively, indicating that the pulses are dispersive in nature. The variation in position-angle averaged pulse-integrated intensity with propagation was examined to identify evidence of dissipation, but exhibited no clear trend across the four events studied. A CBF pulse was also observed using contemporaneous data from both STEREO and SDO. The CBF exhibited a lower initial velocity with weaker deceleration in STEREO observations (~340 km s-1 and -72 m s-2) compared to SDO (~410 km s-1 and -279 m s-2). The CBF kinematics from SDO were found to be highly passband dependent, with an initial velocity ranging from 379 +-12 km s-1 to 460 +-28 km s-1 and acceleration ranging from -128 +-28 m s-2 to -431 +-86 m s-2 in the 335 A and 304 A passbands respectively. These kinematics were used to estimate a quiet coronal magnetic field strength range of ~1-2 G. Significant pulse broadening was also observed, with expansion rates of ~130 km s-1 (STEREO) and ~200 km s-1 (SDO). By treating the CBF as a linear superposition of sinusoidal waves within a Gaussian envelope, the resulting dispersion rate of the pulse was found to be ~8-13 Mm2 s-1. All of these results indicate that coronal bright fronts are best interpreted as fast-mode magnetoacoustic waves propagating in an inhomogeneous medium.