Solar X-Band Imaging with the Arecibo 12-m Telescope: The Brightness Temperature and Magnetic Field of Active Regions

Solar radio observations provide a powerful diagnostic of the physical conditions of the solar atmosphere over a wide range of heights. In this paper, we report regular solar mapping made at the X-band (8.1 – 9.2 GHz) with the Arecibo 12-m radio telescope, covering a period between 13 December 2021 and 9 April 2023. This has demonstrated its potential for identifying active regions and tracking their brightness-temperature changes. The X-band results are discussed along with the near-simultaneous datasets available from space- and ground-based observations. A comparison of magnetic properties of active regions with their emission characteristics indicates that the X-band brightness temperature provides better information of the magnetic-field strength associated with the emission and a brightness temperature in excess of 13 000 K allows us to infer the possibility of intense flares (i.e., ≳ M1 class) and coronal mass ejections. The ‘latitude–time’ distribution of the brightness temperature reveals the three-dimensional evolution of quiet regions on the Sun, coronal holes, and eruptive sites, over many solar rotations in the ascending phase of the current Solar Cycle 25.