Abstract
A stationary two-dimensional isothermal flow parallel to the magnetic lines of force is studied in connection with the hydrodynamic support of a spicule. Observed large extension into the corona (∼ 6000 km) and high velocities (∼ 25 km s-1) can be explained consistently if the effective kinetic temperature within a spicule could be about 104 K in the chromospheric region (z < 2000 km) and increase to about 2.5 × 104 K or more in the coronal region (z > 2000 km). In a special simple case, an analytic solution of equations of motion is obtained and is used for explaining why the pressure in a spicule can be higher than the normal surrounding pressure in upper levels.
Comparison between the effective kinetic temperatures for the spicule support and the empirical electron temperatures shows that they are about the same in lower levels (z < 2000 km) but contributions to the effective kinetic temperature other than the electron temperatures are necessary in higher levels (z > 2000 km). Thus, we postulate the role of acoustic waves that are enhanced by the presence of the magnetic field and are practically undamped in the accelerated flow in a spicule. The coupling between the acoustic waves and the outward expanding motion initiated at the foot of a spicule by the magnetic buoyancy and the solar oscillation is thought to be similar to the mechanism of a geyser in which the bubble formation in an ascending flow is fundamental. The magnetic field strength adequate to provide an appropriate circumstance for the occurrence of a spicule is considered to be about 200 G at the base of the chromosphere. Observational implications are briefly discussed.
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Unno, W., Ribes, E. & Appenzeller, I. On the structure and the motion of a spicule. Sol Phys 35, 287–308 (1974). https://doi.org/10.1007/BF00151950
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DOI: https://doi.org/10.1007/BF00151950