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 Metsahovi 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.

中文翻译：

---

在太阳活动最小值期间识别 8 毫米射电亮度

过去对强烈的太阳射电增亮进行了广泛的研究，并且它们与太阳黑子和活动区域的相关性已经众所周知。但即使太阳表面上是安静的，实际上总有一些活动可以在无线电领域被检测到。在本文中，我们使用阿尔托大学梅萨霍维射电天文台的射电望远镜在 8 毫米处调查这些半主动特征。观测是在 2019 年 5 月至 2019 年 9 月之间进行的，当时太阳活动非常低，为了我们的详细研究，我们选择了国家海洋和大气管理局未在太阳表面确定活动区域的日期。在太阳活动的这个静止期，这些射电区的亮温最高比太阳静止水平高出大约 250 K。我们将毫米波数据与极紫外数据进行比较，发现这些微弱的射电增亮主要与日冕孔特征和磁亮点有关。我们还发现有两类不同的亮点：有和没有磁通管结构的亮点。弱射电增亮的形成与强射电增亮相当：来自弱光球特征的上升通量可以被检测为射电源。