Small scale energy release can play an important role in many phenomena: solar flares, coronal heating, fast solar wind etc. However, microwave observations of small scale features, in particularly, coronal (UV, SXR) bright points remains episodic. Coronal bright points (CBPs): angular size is 10-40” (5-10” bright cores), T~10^6 K, n = 10^9-10^10 cm^-3 ). Lifetime – hovers-days, but sometimes brightness can be significantly increased during few minutes These features are uniformly distributed over the solar disk and their number does not depend on the solar cycle. CBPs correspond to bipolar features on the photospheric magnetograms Model : CBPs are coronal loops with jets Microwave Observations of Compact Radio Sources During Solar Eclipses and Ballooning Instability of Coronal Loops Yu.T. Tsap*1,2, L.I. Tsvetkov *2, S.A. Samisko*2 Pulkovo Observatory, Saint-Petersburg, Russia CrimeanAstrophysical Observatory, Crimea, Ukraine

Image of the Sun obtained with the Hinode/XRT on 01 March 2007.

Movie

Microwave observations of small scale radio sources Kundu et al. (1988), VLA (6 cm), AR = 4’’: AS = 5-15’’,Tb = 2x10^4 K. However, observations are not reliable problems with calibration and side lobes. Kundu et al. (1994), Nobeyama Radioheliograph (1.76 cm), AR = 15’’: Tb = 10^4 K, correlations between CBPs (Yohkoh) and MWBPs Nindos et al. (1999): AS=15-50’’, Tb =10^4-10^5 K, bed correlation CBPs(EIT/SOHO) and MWBPs VLA NR

Maksimov et al. (2001) : SSRT (5.2 cm), AR = 21’’: AS=60’’, Tb=4x10^4K SSRT+Yohkoh – good correlation, SSRT+Nobeyama – bed correlation Main result: thermal mechanism is the main mechanism of MW emission of BP beause of the low brightness temperature

Radio observations of solar eclipses is the powerful tool for investigations of the solar fine structure Aug. 1, 2008 EIT/SOHO =195 Ǻ, 10:24 UT Total solar eclipse Aug. 1, 2008, near Jiugan, China Photo K. Shibata

RT-22 Crimean Astrophysical Observatory Waves: 2.0, 2.3, 2.8, 3.5 cm with the angular resolutions: 3’.6, 4’.1, 5’.0, 6’.0 However the angular resolution during solar eclipse can achieve 2”.0 - 4”.0

Example of images of the solar region with a coronal hole, obtained on 01 August 2008 with EIT/SOHO (195 A) near the North pole just before the solar eclipse. Numbers denote coronal BP. The cross corresponds to the maximum of the antenna directivity diagram. Denoted CBPs are located inside the antenna directivity diagram. Three solar eclipses on 03 October 2005, 29 Murch 2006, and 01 August 2008

Example of derivatives of eclipsing curves at 2.0 cm during covering and opening of the solar disk by the moon on 01 August 2008 and the antenna directivity diagram. Numbers denote radio sources corresponding to UV brightenings.

Example of eclipsing curves obtained at 2.8 cm for radio sources N 1 (a) and N 2 (b) during the solar disk opening.

λParameters/ Sources№ 1№ 2№ 3№ сm Flux drop, dF, s.u.f. Angle size, θ ’’ Brightness temperature Тb х 10^6, K сm Flux drop, dF, s.u.f. Angle size, θ ’’ Brightness temperature Тb х 10^6, K сm Flux drop, dF, s.u.f. Angle size, θ ’’ Brightness temperature Тb х 10^6, K Average sizes, θ ’’

Average spectrum of the compact radio sources

Interpretation Thermal bremsstrahlung mechanism. At L = 10 9 сm, n e = cm -3 and λ =3.5 cm Thermal magnetobremsstahlund mehanism. At f = 15.4 GHz (2.0 cm), l=3 Estimates suggest that radio emission is detemined by non-thermal gyrosynchrotron mechanism! Small scale loops are filled by non-thermal electrons!

Standard solar model ? Filament is needed for compact flares

Loop-loop interaction (Hanaoka 1999) But brightenings of XBPs occur without moving or change of magnetic fragments (Kotoku, 2007).

Ballooning instability as a trigger of jets and the energy release. Cusp-shaped coronal loops

Models of energy release current sheet

Nanoflares and microflares Parker's Model (1972) Photosphere

Conclusions 1. Microwave radiation of CBPs can be determined by the gyrosynchrotron radiation of non-thermal electrons at least for a time. 2. Ballooning instability can result in CBPs and microjets. 3. The Sun is always active, the solar activity is determined by the scales of the energy release.

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