Observations of Solar Spicules at Millimeter and Ultraviolet Wavelengths

Abstract:

Solar spicules are a ubiquitous chromospheric phenomenon in which multitudes of dynamic jets with temperatures of order 10⁴ K extend thousands of kilometers into the solar atmosphere. Recent progress has been made refining the observational characteristics of spicules using the Hinode Solar Optical Telescope (SOT) and the Interface Region Imaging Spectrograph (IRIS) observations at optical and ultraviolet wavelengths, respectively. Two types of spicule have been identified. Type I spicules, prevalent in solar active regions, have upward speeds of order 25 km/s and lifetimes of 3-7 min. They may be the limb counterpart to shock-wave-driven fibrils commonly seen against the solar disk in active regions. In contrast, type II spicules, more common in quiet regions and coronal holes, display upward speeds of 50-150 km/s, lifetimes of 30-110 s, and appear to be partially heated to temperatures of 10⁵ K and higher. These observations have provoked intense interest in spicules and have led to proposals that type II spicules play a central role as a source of hot plasma in the corona. Nevertheless, their role in mass and energy transport between the lower and upper layers of the solar atmosphere remains an outstanding problem.

Here, we report imaging observations of solar spicules at millimeter wavelengths using the Atacama Large Millimeter-submillimeter Array (ALMA) with arcsecond angular resolution. Continuum millimeter wavelength radiation forms under conditions of local thermodynamic equilibrium, thereby providing a complementary tool to UV lines, which form under non-LTE conditions. The observations were made on 2018 December 24-25 at λ=1.25 mm and λ=3 mm. The ALMA observations pose special challenges, particularly at 1.25 mm, where the limited field of view of the instrument motivated us to use a novel mosaic imaging technique: multiple pointings were assembled to form a single map with an angular resolution of 1” x 0.7” on a cadence of roughly 2 min. In contrast, we were able to image at 3 mm continuously, with a map cadence of 2 s and an angular resolution of 2.3” x 1.3”.

We compare and contrast the morphology and dynamics of mm-λ observations of spicules with those obtained by IRIS at UV wavelengths and place constraints on spicule temperatures and masses using the joint millimeter-wavelength observations.