This HOP aims to understand the statistical properties of propagating waves along spicules. We obtained observations of off-limb spicules at the polar coronal hole boundary (at the footpoints of open field lines) with very high cadence (1.6 sec) in January 2011, and found that there are numerous high-frequency propagating waves along spicules (Okamoto and De Pontieu 2011). For the analysis, we developed a technique for automatic detection of spicules and high-frequency waves along them. We detected numerous wave packets on 89 different spicules. Statistically, the observed waves have several features: (1) The occurrence rates of upward propagation, downward propagation, and standing waves are 59%, 21%, and 20%, respectively. (2) The apparent phase velocity increases as a function of height. (3) The wave packets and their properties are highly time dependent. Statistically speaking, higher phase velocities (a signature of standing waves) are dominant in the early and late phases of the spicules, while lower phase velocities (a signature of upward propagating waves) dominate the middle phase of the spicules. (4) The median amplitude, period, and velocity amplitude are 55 km, 45 sec, and 7.4 km/s, respectively. (5) The estimated Poynting flux is 2.5x10^5 erg/cm^2/s.
We would like to investigate whether the properties mentioned above are universal or peculiar in the polar coronal hole. For the purpose, we propose to observe off-limb regions of a coronal hole, the quiet Sun, and an active region. We note that the detection method used in Okamoto and De Pontieu (2011) can be easily applied to these observations. The ratios of upward, downward, and standing waves and height-dependence of phase velocity will provide constraints on numerical models of spicule formation, and will provide insight into the complex and dynamic atmospheric structuring at the interface between the chromosphere and corona. By obtaining large statistical samples for these different magnetic regions, we will address energy transfer into the corona by high-frequency waves. We will also study whether seismology of spicules (e.g., Verth et al., 2011) is feasible or too problematic because of the prevalence of reflection and (counter)propagating waves in spicules. |
|