Statistical Analysis of Spicule Inclinations and chromosphere thickness variations

Document Type : Research Article


1 Ph.D. Student, Department of Physics, Payame Noor University (PNU), Tehran, Iran

2 Associate Professor, Department of Physics, Payame Noor University (PNU), Tehran, Iran


Spicules are intermittently rising above the surface of the Sun eruptions; EUV jets are now also reported immediately above surface layers. The orientation of spicules is a valuable parameter in the absence of direct magnetic field measurements with a sufficient spatial resolution in the chromospheric region because it is presumably determined by the confined flow of plasma, which should occur along the magnetic field lines, especially where the solar magnetic field pressure dominates the gas pressure. Of course, all these measurements suffer from the overlapping effect of spicules seen along each line of sight, the effect of which will be more critical when we look near the solar limb. In the case of macrospicules as well imaged by AIA of the SDO mission using the 304 filter recording the emissions of the HeII, resonance line, an additional effect arises due to the optical thickness of the line, especially on disk and also above the limb in the inner chromospheric shell.
The primary purpose of this paper is to determine automatically and objectively the apparent tilt angle of spicules, using the best available highly processed observations, from the Solar Optical Telescope (SOT) limb imaging experiment by using an H CaII line, onboard Hinode mission. Furthermore, the Hough transform is applied to the resulting images for making a statistical analysis of spicule orientations in different regions around the solar limb, from the pole to the equator. A technique for the automatic detection off-limb spicules was implemented, and statistical measurements were conducted to determine the tilt angle for spicules at different heliocentric angles.
We apply and develop a method with the following steps: (1) To increase the visibility of spicules, a radial logarithmic scale is applied; (2) To enhance linear features, while the Madmax operator is used. We investigated in more detail the apparent inclination of spicules and found the statistically average values for different locations around the solar limb for tilt angle. The results show a large difference of spicule apparent tilt angles in (i) the solar pole regions, (ii) the equatorial regions, (iii) the active regions, and (iv) the coronal hole regions. Analytically, during the minimum solar magnetically activity, from the equator to the poles, the inclination angles of the spicules are getting smaller and their lengths increase. As a result, the chromosphere thickness in this case is thicker than that of the solar maximum activities. When the spicules in the polar coronal holes are significantly inclined, the chromosphere and even transition regions thickness is thinner. Numerically, spicules are visible in a radial direction in the polar regions with a tilt angle <200. The tilt angle is even reduced to 10 degrees inside the coronal hole with open magnetic field lines and at the lower latitude, the tilt angle reaches values over 50 degrees. Usually, around an active region, they show a wide range of apparent angle variations from -60 to +60 degrees, which is in close resemblance to the rosettes that are made of dark mottles and fibrils in projection on the solar disk. However, large-scale activities with short life-time do not play a significant role in the thickness of the chromosphere, and they are removed for long term measurements by averaging. Therefore, this study considers the most statistical population of spicules in the minimum (and maximum) solar activity in the polar regions, and in lower latitude, to be considered as their inclination angles. While at the maximum laps of solar cycle, the opposite result will be expected, and fully confirmed, and give us a topological reason for the chromospheric prolateness at minimum activities.


Main Subjects

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