THE BEHAVIOR OF THE 17 GHz SOLAR RADIUS AND LIMB BRIGHTENING IN THE SPOTLESS MINIMUM XXIII/XXIV

Abstract

The current solar minimum has surprised the entire solar community because the spotless period is presently almost 2-3 years longer than the usual minima. To better understand this, we studied the variation of the solar radius and the polar limb brightening at 17 GHz, comparing the results from the minimum at the end of cycle XXIII with those of the previous one. Daily maps obtained by the Nobeyama Radioheliograph (NoRH) from 1992 through 2010 were analyzed. Whereas the variation of the solar radius at radio frequencies indicates the heating of the solar atmosphere due to solar activity, the limb brightening intensity depends on the organization of the polar magnetic field of the Sun, including the global dipole and the features formed around it. These features are more prominent during minima periods. As a common result, researchers have observed a decrease in both radius and limb brightness intensity at 17 GHz during the present minimum when compared with the previous one. The mean solar radius is 0. 9 ± 0. 6 smaller and the limb brightening reduced its intensity by around 20%. Both decrements are interpreted in terms of the weaker solar chromospheric activity of the present cycle. Measurement of the radius and limb brightening at 17 GHz can be used as an alternative solar activity index and should be included in the set of parameters used to predict future cycles.

Center-to-limb observations of the Sun with ALMA

“…The inflection point of the CLV curve is at 976.7 ± 2.5 and 979.8 ± 4.5 for 239 and 100 GHz respectively, which is 1000 and 3300 km above the limb; reduced to 1 AU, the solar radio radius is 961.1 and 964.1 at 239 and 100 GHz respectively. As expected, these values are smaller than the one reported by Selhorst et al (2011) at 17 GHz for the same phase of the solar cycle, which was ∼ 975 .…”

Short-term variability of the Sun-Earth system: an overview of progress made during the CAWSES-II period

“…The rise phase of cycle 24 was already interesting because the Sun emerged from a deep solar minimum that gained particular interest among solar-terrestrial scientists (Selhorst et al 2011;Tsurutani et al 2011a;Dasso et al 2012;Gopalswamy et al 2012a;Solomon et al 2013;Lean et al 2014;Potgieter et al 2014). Solar activity is typically represented by the international sunspot number (SSN), but there are many other measures, which are needed for a complete understanding of the solar variability.…”

“…The polar field strength during the prolonged cycle 23/24 minimum was considerably smaller than that during the cycle 22/23 minimum. During solar minima, the polar field strength (B) reaches its peak values and vanishes at maxima, changing sign at the end of maxima (Selhorst et al 2011;Tsurutani et al 2011a;Gopalswamy et al 2012a;Shimojo 2013;Nitta et al 2014;Mordvinov and Yazev 2014). The maximum phase is indicated by the vanishing polar field strength.…”

Exoplanet transits with next-generation radio telescopes

“…Likewise, the baseline transit shape will differ significantly from the optical because where in the optical regime the Sun and sun-like stars are limb-darkened, they are actually limb-brightened at radio and millimetre wavelengths (Kundu et al 1979;Horne et al 1981;De la Luz 2016), so that instead of having a "u" shape as in the optical, the transit will actually have a "w" shape in the radio. Exoplanetary transits are the only way of detecting this effect in single stars more distant than the Sun without resolving them on extremely long baselines, a key observation for models of stellar chromospheres (Selhorst et al 2011). In this regime transit depths for a given star-planet pair are expected to be slightly shallower than in the optical case, owing to the increased stellar diameter.…”