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Erofeev, D. V.

doi:10.1023/a:1012741904366

Cited by 7 publications

(5 citation statements)

References 9 publications

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“…The absence of sunspots during this long period clearly indicates the necessity of alternative indexes to characterize minimum activity times. Among the solar features observed in white light, polar faculae may be a good choice to describe the minimum, since they have a variation anti-correlated with the sunspot cycle (Sheeley 1964;Makarov & Makarova 1996) and their number reflects the mean magnetic flux in the polar region (Sheeley 1991;Erofeev 2001). Recently, the position of polar faculae was correlated to the intense magnetic field patches that can reach kG intensities (Okunev & Kneer 2004;Tsuneta et al 2008).…”

Section: Introductionmentioning

confidence: 99%

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

Selhorst¹,

Castro²,

Válio³

et al. 2011

ApJ

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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.

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Section: Introductionmentioning

confidence: 99%

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

Selhorst¹,

Castro²,

Válio³

et al. 2011

ApJ

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“…Faculae are mainly observed within active regions and near the poles. Several authors have reported a significant increase in the number of faculae toward the poles (Ortiz et al 2002;Erofeev 2001), and the intensity contrast of faculae has been shown to increase near the poles (Wang & Zirin 1987;Lawrence 1988;Unruh et al 1999 and references therein). Several authors have reported the existence of regions of enhanced temperatures at radio wavelengths in the polar zones of the Sun (Kundu & McCullogh 1972;Efanov et al 1980;Riehokainen et al 1998Riehokainen et al , 2001Shibasaki 1998;Nindos et al 1999;Gelfreikh et al 2002).…”

Section: Introductionmentioning

confidence: 99%

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

Selhorst

Silva

Costa

et al. 2003

A&A

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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.

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“…Gaizauskas (1984) observed that there are no spicules over faculae as well as over plages (Zirin 1988). Moreover, faculae are known to be more abundant near the poles (Erofeev 2001;Unruh et al 1999;Ortiz et al 2002) and the radio polar bright regions are close to the polar faculae regions (Riehokainen et al 1998;Gelfreikh et al 2002).…”

Section: Model With Spicule-less Regionsmentioning

confidence: 97%

What determines the radio polar brightening?

Selhorst

Silva

Costa

2005

A&A

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Abstract. In order to explain the bright patches of emission near the poles of 17 GHz solar maps, we have applied a previously developed atmospheric model based on radio observations. This 2-D model, which includes spicules, yields results in good agreement with brightness temperature values at the disk center, radius, and limb brightening 17 GHz observations at equatorial and intermediate regions. Nevertheless, the intensity of discrete bright patches observed near the poles in 17 GHz maps (as bright as 40% over the quiet Sun), can only be explained by holes in the spicule forest. These regions without spicules probably reflect the presence of polar faculae, which inhibit the appearance of spicules. Moreover, the absence of spicules over faculae explains the anti-correlation between the mean polar limb brightening and the solar cycle, because the polar faculae are known be anti-correlated with the solar cycle. Results from the model with spicule-less regions showed that: 1) for bare regions of the same size (5 • ), the limb brightening increases with the pole proximity; 2) larger regions yield more intense limb brightening; 3) the average height of the spicules greatly influences the solar radius; 4) the maximum excess brightness temperature of 40% is in very good agreement with the polar limb observations above bright patches.

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Untitled

Cited by 7 publications

References 9 publications

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

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

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

What determines the radio polar brightening?

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