A New Setup for Ground‐based Measurements of Solar Activity at 10 μm

Abstract

ABSTRACTSolar activity measurements in the far‐ to mid‐IR range are receiving renewed interest as part of an effort to complement recent results obtained at submillimeter wavelengths. A new setup has been developed to measure solar activity in the infrared spectral region centered at 10 μm (30 THz) by means of a camera with focal plane array of uncooled microbolometers. An optical arrangement of concave‐convex‐concave mirrors magnifies and focuses the full‐disk solar image to fit into the field of view of the camera. Techniques were developed to characterize the camera and calibrate the measurements in brightness temperature and flux units. Test observations were made at the Bernard Lyot Solar Observatory, Campinas, Brazil, and were continued at El Leoncito Astronomical Complex, San Juan, Argentina. The sky was found to be almost transparent at 10 μm at both sites. The first measurements of the solar disk have confirmed the presence of quiescent bright ring, or “plagelike,” regions around sunspots, and “disappearances” of sunspots coincident with small soft X‐ray bursts. Small mid‐IR flares were found, consisting of multiple rapid brightenings (tens of seconds to several minutes) at different locations in the solar active regions, corresponding to soft X‐ray bursts reported by GOES satellites. At a wavelength of 10 μm, the sizes of some flare sources were found to be smaller than the diffraction‐limited photometric beam of 25″ set by the 10.5 cm diameter objective lens. The intensities of such small mid‐IR flares were high, of the order of (8–14) × 10−19 W m−2 Hz−1 (or [8–14] × 104 solar flux units).