Dynamical characteristics of small-scale Hα upflow events on the quiet Sun

Studies of small-scale dynamical features on the quiet Sun are crucial for understanding the energetics and mass supply in the chromosphere and corona. Recent works by Wang and coworkers revealed the existence of numerous short-lived upflow events in Hα on the quiet Sun. In this paper we examine in detail the dynamical characteristics of these Hα upflow events based on Hα spectrograph observations and compare them with those of elongated dark mottles, which are often considered to be the disk counterpart of limb spicules. Our observations were performed at the Big Bear Solar Observatory on 1997 August 26 using the Littrow Spectrograph on the 65 cm reflector. We repeated drift scans of a quiet Sun region, which produced a four-dimensional (x, y, t, λ) data array. We examine the spectral, temporal, and morphological characteristics of upflow events and determine their physical parameters like size, lifetime, birthrate, line-of-sight velocity, Doppler width, and optical thickness. Our results show that upflow events appear different from dark mottles in that (1) most of the line profiles of upflow events show absorption in the blue wing only, while those of dark mottles show absorptions in both wings, (2) the typical optical thickness of upflow events is smaller than that of dark mottles, and (3) unlike dark mottles, upflow events do not have returning flows at the same site. We discuss possible physical relationships among upflow events, elongated dark mottles, and spicules as observed at the limb.