Automatic Detection And Statistical Analysis Of Solar Filaments

Solar filaments, called prominences when they appear above the solar limb, are important magnetized structures containing cool and dense plasma embedded the hot solar corona. Typically, a filament is 100 times cooler and denser than its surrounding corona. They are particularly visible in Ha observations, where they often appear as elongated dark features with several barbs. Filaments are always aligned with photo-spheric magnetic-polarity inversion lines and are located at a wide range of heliocentric latitudes. This characteristic makes filaments suitable for tracing and analyzing the so-lar magnetic fields. Moreover, filaments sometimes undergo large-scale instabilities, which break their equilibria and lead to eruptions, so they are often associated with flares and coronal mass ejections (CMEs). Therefore, both case study and statistical analysis of filaments are important and significant. With the rapid development of the telescopes, both time cadences and spatial resolutions of the observations are becoming higher and higher. As a consequence, we have to deal with a vast amount of data, and automated detection is an efficient way to derive the features of interest in the observa-tions. We developed a method to automatically detect and trace solar filaments in Ha full-disk images. The program is able not only to recognize filaments and determine their properties, such as the position, the area, the spine, the tilt angle, the barb and other relevant parameters, but also to trace the daily evolution of the filaments. The pro-gram consists of three steps:First, preprocessing is applied to correct the original im-ages; Second, the Canny edge-detection method combined with connected component labeling algorithm is used to detect filaments; Third, filament properties are recognized through the morphological operators. To test the algorithm, we applied it to the obser-vations from various observatories and telescopes, and the program is demonstrated to be robust and efficient.Our filament automated detection method was then applied to process the full disk Ha data mainly obtained by Big Bear Solar Observatory (BBSO) from 1988 to 2013, spanning nearly 3 solar cycles. We attempt to derive the statistical properties of the solar filament features. The butterfly diagrams of the filaments, showing the information of the distributions of the filament number, filament area, and the spine length, are obtained. The variations of these features, the filament tilt angle and the barb number with calendar years and latitude bands are analyzed. The drift velocities of the filaments in different latitude bands and its variations with the filament area and spine length are calculated and studied. We also investigate the north-south (N-S) asymmetries of the filament numbers, the filament numbers in each subclass classified according to the filament area, spine length, tilt angle, and the cumulative areas and spine lengths. The latitudinal distribution of the filament number is found to be bimodal. Over 80% of all the detected filaments in the three solar cycles have areas less than 1.0 x 109 km2. The filament spine lengths are mainly less than 1.0 x 105 km for 90% of all the detected filaments in the solar cycle 22 and 80% of all the detected filaments in solar cycles 23 and 24. About 85% filaments have less than 5 identifiable barbs. About 80% of all the filaments have tilt angles within [0°.60°]in both solar hemispheres. For the filaments within latitudes lower than 50° in both hemispheres the northeast direction is dominant in the northern hemisphere and the southeast direction is dominant in the southern hemisphere. For the filaments within latitudes higher than 50° in both hemispheres the northwest direction is dominant in the northern hemisphere and the southwest direction is dominant in the southern hemisphere. The latitudinal migrations of the filaments have three trends in solar cycles 22 and 23:from the beginning of the solar cycle to the solar maximum the drift velocity is high. From the solar maximum to a few years before the solar minimum the drift velocity becomes lower. After that the migration becomes divergent and the appearance of the filaments in high latitudes represents the start of a new cycle. Most filaments in latitudes lower than 50°migrate towards the equator and most filaments in latitudes higher than 50°migrate towards the polar region. Filaments in the medium latitudes around 50°can migrate towards both directions and some filaments in high latitudes around 75°can also migrate towards the equator. The N-S asymmetry indices of the total filament numbers, the numbers of filaments with various areas, spine lengths, the cumulative areas and spine lengths indicate that the southern hemisphere is the dominant hemisphere in solar cycle 22 and the northern hemisphere is the dominant one in solar cycle 23, though the asymmetry is weak. The variation of the N-S asymmetry indices of the filaments with different tilt angles in the latitude band [0°,50°] are similar to the sine functions and these results do not depend on solar cycle.We attempt to adopt our automated program to detect the solar filament chirality and barb bearing. We test the automatic detection method with Ha filtergrams from the BBSO Ha archive and magnetograms observed with the Helioseismic and Magnetic Imager (HMI) on board the Solar Dynamics Observatory (SDO). Four filaments are automatically detected and illustrated to show the results. The barbs in different parts of a filament may have opposite bearings. The filaments in the southern hemisphere (northern hemisphere) mainly have left-bearing (right-bearing) barbs and positive (neg-ative) magnetic helicity, respectively. With the help of the vector magnetograms, the helicity of a filament can be determined. However, if we assume the empirical wire model for the filament, the contradicting sign of helicity might be obtained. The tested results demonstrate that our method is efficient and effective in detecting the bearing of filament barbs. A filament with a unique axis chirality may have different barb bear-ings in its different parts. It infers that some of the quiescent filaments have a complex magnetic structure that formed partly by a flux rope and partly by a dipped arcade. One could not determine the filament axis chirality and magnetic helicity only by the barb bearing. The correct detection of the filament axis chirality should be done by com-bining both imaging morphology and magnetic field observations. In the condition of lacking vector magnetic filed data, the method of determining the filament axis chirality based on the magnetic field signs of the two footpoints is not valid, which could result in errors.