Comparison of solar radio and extreme ultraviolet synoptic limb charts during the present solar maximum

Abstract

Aims. The present solar cycle is particular in many aspects: it had a delayed rising phase, it is the weakest of the last 100 yrs, and it presents two peaks separated by more than one year. To understand the impact of these characteristics on the solar chromosphere and coronal dynamics, images from a wide wavelength range are needed. In this work we use the 17 GHz radio continuum, which is formed in the upper chromosphere and the extreme ultraviolet (EUV) lines 304 and 171 Å, that come from the transition region (He ii, T ∼ 6−8 × 10 4 K) and the corona (Fe IX, X, T ∼ 10 6 K), respectively. We extend upon a previous similar analysis, and compare the mean equatorial and polar brightening behavior at radio and EUV wavelengths during the maximum of the present solar cycle, covering the period between 2010 and 2015. Methods. We analyze daily images at 304 and 171 Å obtained by the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory (SDO). The 17 GHz maps were obtained by the Nobeyama Radioheliograph (NoRH). To construct synoptic limb charts, we calculated the mean emission of delimited limb areas with 100 wide and angular separation of 5 • . Results. At the equatorial region, the results show a hemispheric asymmetry of the solar activity. The northern hemisphere dominance is coincident with the first sunspot number peak, whereas the second peak occurs concurrently with the increase in the activity at the south. The polar emission reflects the presence of coronal holes at both EUV wavelengths, moreover, the 17 GHz polar brightenings can be associated with the coronal holes. Until 2013, both EUV coronal holes and radio polar brightenings were more predominant at the south pole. Since then they have not been apparent in the north, but thus appear in the beginning of 2015 in the south as observed in the synoptic charts. Conclusions. This work strengthens the association between coronal holes and the 17 GHz polar brightenings as it is evident in the synoptic limb charts in agreement with previous case study papers. The enhancement of the radio brightness in coronal holes is explained by the presence of bright patches closely associated with the presence of intense unipolar magnetic fields. However, observations with better spatial resolution and also at different radio wavelengths will be necessary to fully understand the physical mechanisms that link these features.

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).…”

Solar Polar Brightening and Radius at 100 and 230 GHz Observed by ALMA

“…Nevertheless, they are frequently associated with the location of increased unipolar magnetic regions underlying the coronal holes (Gopalswamy et al 1999;Brajša et al 2007;Selhorst et al 2010). This association was reinforced by Oliveira e Silva et al (2016), who analyzed the polar behavior between 2010 and 2015 and found a good correspondence between the presence of coronal holes and polar brightening at 17 GHz.…”

Association of Radio Polar Cap Brightening with Bright Patches and Coronal Holes

“…The purpose of this work is to model the emission of the 17 GHz polar bright patches, which are frequently observed in the NoRH maps in association with coronal holes (Gopalswamy et al 1999;Selhorst et al 2003;Oliveira e Silva et al 2016). The simulations were based on the temperature and density distributions proposed in the SSC atmospheric model (Selhorst et al 2005a), with modifications to include a coronal hole atmospheric model and magnetic loops as the sources of the radio bright patches.…”

“…• of latitude are not able to reproduce the intense bright patches observed in the NoRH maps (Selhorst et al 2005b) and the presence of coronal holes reduce the expected limb brightening at 17 GHz (Figure 4), other solar features should be acting inside the coronal holes to increase their brightness temperature at 17 GHz (Gopalswamy et al 1999;Oliveira e Silva et al 2016). To simulate the observed 17 GHz bright patches, we introduced small magnetic loops inside coronal hole regions (Figure 2).…”

“…Through synoptic limb charts, Oliveira e Silva et al (2016) showed a good association between the presence of coronal holes and the 17 GHz polar brightening in the period of 2010-2015. Moreover, the authors attributed the enhancement of radio brightness in coronal holes to the presence of bright patches closely associated with the presence of intense unipolar magnetic fields.…”