Temporal and angular variation of the solar limb brightening at 17 GHz

Abstract

Abstract. In order to better understand the atmosphere structure of the Sun, we have analyzed over 3000 daily maps of the Sun taken at 17 GHz from the Nobeyama Radioheliograph (NoRH) from 1992 through 2001, focusing on the excess brightness temperature observed near the limb. The purpose of this work is to characterize the limb brightness in two ways: (i) study the temporal variation of the intensity and radial width of polar brightening; and (ii) measure the brightness distribution along the limb as a function of position angle and compare it with data at other wavelengths throughout the solar cycle. The mean intensity of the polar regions were found to be approximately 13% and 14% above quiet Sun levels at the North and South poles, respectively. Moreover, the polar brightenings are strongly anti-correlated with solar activity (as measured by sunspot number). The radial width of the excess brightness is slightly over 1 arcmin for both polar regions. Only a small variation with the solar cycle was observed during the decline of last maximum, that is, the Southern polar brightening was found to be both wider and brighter than the Northern one for the 23rd cycle. As for the angular variation of the limb brightening, for a month during a period of minimum activity, it reaches 25% above quiet Sun levels at the poles, ∼15% near the equator, and 10% at intermediate regions. Hα images also show brightening enhancements at the polar regions for the same period. We also found a strong anti-correlation between the radio polar brightenings and the coronal holes seen in soft X-ray images from 1992 to 2001. There seems to be a strong association of the radio limb brightening at 17 GHz with faculae. The implications of these correlations are discussed.

Solar Cycle Variation of Microwave Polar Brightening and Euv Coronal Hole Observed by Nobeyama Radioheliograph and Sdo/Aia

“…Whereas, during the minimum, the frequency of flares and CMEs decreases and coronal hole tend to form at near the poles. There are several reports on various phenomena depending on the phase of the solar cycle, such as polar magnetic fields, polar crown filaments, surges near the poles, faculae, and so on (Selhorst et al 2003;Svalgaard et al 2005;Gopalswamy et al 2012;Shimojo 2013;Svalgaard & Kamide 2013;Altrock 2014;Silva et al 2016). For instance, the polar field reversal and flux migration to the poles are considered an indicator of the following solar maximum (Svalgaard & Kamide 2013) which is consistent with the dynamo model (Babcock 1961).…”

Radius variations over a solar cycle

“…If the atmosphere had a smooth profile, the brightest point at the solar limb would be close to the point where the optical depth is equal to unity at a certain frequency (τ ν = 1). Selhorst et al (2003), however, showed that the limb brightening intensity at 17 GHz is not uniformly distributed around the Sun, being larger near the polar region, indicating the influence of features such as spicules in the maximum brightening. In most cases, the Sun radius is defined as the point where the brightness temperature is equal to half of the quiet Sun brightness temperature or as the inflection point.…”

“…In a previous study of polar limb brightening seen in 17 GHz solar maps, Selhorst et al (2003) concluded that the • and 270…”

“…5b). Even though the solar poles are less affected by the solar maximum activity, they are the location of significant limb brightening at 17 GHz, that increases during solar minimum (Selhorst et al 2003). This indicates that the polar radius measurements of the Sun reflect the polar limb brightening at this frequency.…”

“…The point at half the mean intensity is indeed the radius of the Sun only for the case where the Sun may be accurately described by a flat disc of constant intensity. Observation at radio frequencies greater than 10 GHz (Selhorst et al 2003, and references therein) have shown the existence of solar limb brightening. Thus, the calculation of the solar limb at halfintensity value is affected by the convolution with the telescope beam.…”