Identifying 8 mm Radio Brightenings During the Solar Activity Minimum

Abstract

Strong solar radio brightenings have been extensively studied in the past, and their correlation to the sunspots and active regions are already well known. But even when the Sun is ostensibly quiet, there is practically always some activity that can be detected in the radio domain. In this article we investigate these semi-active features at 8 mm using the radio telescope at Aalto University Metsähovi Radio Observatory. The observations were made between May and September 2019 when the solar activity was very low, and for our detailed study we chose dates when no active regions were identified on the solar surface by the National Oceanic and Atmospheric Administration. The brightness temperature of these radio regions during this quiescent period of solar activity is at maximum approximately 250 K above the quiet-Sun level. We compared our millimeter data with data taken in extreme ultraviolet, and we found that these weak radio brightenings are mostly related to coronal hole features and magnetic bright points. We also found that there are two different categories of bright points: those with and without flux tube structure. The formation of the weak radio brightenings is comparable to the stronger radio brightenings: the rising fluxes from the weak photospheric features can be detected as a radio source. Keywords Chromosphere, quiet • Solar cycle, observations 1. Introduction Even when the Sun is ostensibly quiet and there are no visible, classified active regions (ARs) on the solar disk, there is often some activity, such as coronal holes and magnetic bright points. Also solar radio observations practically always detect some activity. During B J. Kallunki

Solar Observations with Single-Dish INAF Radio Telescopes: Continuum Imaging in the 18 – 26 GHz Range

“…Comparing these AR features with those reported at other frequencies, 6 we note that AR details and substructures -like the complex AR morphology (McIntosh, 1990) and magnetic structures (Jaeggli and Norton, 2016) -are typically not resolved in our data (see Figure 1), while the bulk emission of many ARs is typically slightly larger than our spatial resolution (see radio beamwidth in Tables 1 and 2). On the other hand, we typically detect rich network structures on larger scales, compatible with the "semi ARs" described in a recent work by Kallunki, Tornikoski, and Björklund (2020), based on single-dish radio observations at 37 GHz; they claimed that these "semi ARs" -detected even during the minimum phase of the solar cycle -can be indicative of persistent solar activity in the radio domain, suggesting that these weak radio brightenings are mostly related to coronal hole features and magnetic bright points. The brightness excess temperatures with respect to the QS level (T ex ) are below ∼ 1000 K in most cases (see Figure 6); this suggests that in our data, the chromospheric network shows typical temperature fluctuations of <10%, except for sporadic ARs with total emission nearly doubling the QS brightness level.…”

“…The additional complex network of diffuse and weaker emission structures typically present in our images ("semi ARs" as defined by Kallunki, Tornikoski, and Björklund, 2020), as well as emission regions at lower temperatures than the QS level, possibly related to coronal holes, will require dedicated investigations. The cross-comparison of our simultaneous 7-feed images 8 provided by SRT observations clearly demonstrate the realm of such weak structures not related to instrumental gain fluctuations.…”

“…Features on the solar disk can evolve on very short timescales; thus, a daily or even more frequent monitoring is the optimal and typical choice for dedicated radio solar facilities, for example, NoRH (Shibasaki, 2013) and MRO (Kallunki, Tornikoski, and Björklund, 2020). SRT and Medicina perform an almost-weekly monitoring campaign.…”

Relationship Between Solar Millimeter and Soft X-Ray Emissions

“…The logarithmic scale data are used only for strong solar radio brightenings. The time between two consecutive solar radio maps is around 140 seconds at fastest [10]- [12].…”