Study of Hα Jets on the Quiet Sun

Abstract

High-speed jets of solar quiet regions have been observed at Big Bear Solar Observatory in Hα−1.0 Ú, and compared with high-resolution magnetograms. Over the whole Sun, the birthrate of the Hα−1.0 Ú jets is about 19±3 events s⁻¹, which is much lower than the birthrate of spicules. The average lifetime of these jets is 2±1 min. Hα−1.0 Ú jets are very different from spicules, in the sense of birthrate, lifetime, and shape. Jets tend to recur in the same sites, always located in boundaries of supergranules. Under the best observing conditions, we found that 80% of the major jet sites are associated with converging magnetic dipoles – mainly the sites where intranetwork elements are canceling with opposite polarity network elements. In order to establish a possible relationship between the disk Hα jets and limb macrospicules, we have also obtained time sequences of Hα center-line images at the limb. These images are enhanced by median filtering so that jet structures over the limb are easily studied. We found that these limb Hα jets (above the spicule forest) repeatedly occur in the same sites, which is the property shared by the disk Hα−1.0 Ú jets. However, their mean lifetime is 10 min, substantially longer than that of disk jets. Comparison with simultaneous SOHO/EIT Heii 304 Ú images shows that every Heii 304 Ú jet over the limb coincides with an Hα jet, although Heii 304 Ú jets extend much farther out. Some Hα jets do not have associated He jets, probably due to the difference in image resolutions. Hα spectra of selected jets are analyzed, and we found that jets are not simply blue-shifted; instead, the line profiles are broadened with significantly larger broadening on the blue side. Two-component fitting finds that the velocity of the blue-shifted component (an optically-thin component) is around 20 to 40 km s⁻¹.