Observation of Kink Instability During Small B5.0 Solar Flare on 2007 June 4

Using multi-wavelength observations of SoHO/MDI, SOT-Hinode/blue-continuum (4504 ), G-band (4305 ), Ca II H (3968 ) and TRACE 171 , we present the observational signature of highly twisted magnetic loop in AR 10960 during the period 04:43 UT-04:52 UT at 4 June, 2007. SOT-Hinode/blue-continuum (4504 ) observations show that penumbral filaments of positive polarity sunspot have counter-clock wise twist, which may be caused by the clock-wise rotation of the spot umbrae. The coronal loop, whose one footpoint is anchored in this sunspot, shows strong right-handed twist in chromospheric SOT-Hinode/Ca II H (3968 ) and coronal TRACE 171 \, images. The length and the radius of the loop are L~80 Mm and a~4.0 Mm respectively. The distance between neighboring turns of magnetic field lines (i.e. pitch) is estimated as ≈ 10 Mm. The total twist angle, Φ~12π (estimated for the homogeneous distribution of the twist along the loop), is much larger than the Kruskal -Shafranov instability criterion. We detected clear double structure of the loop top during 04:47-04:51 UT on TRACE 171 \ images, which is consistent with simulated kink instability in curved coronal loops (T{\"o}r{\"o}k et al. 2004). We suggest, that the kink instability of this twisted magnetic loop triggered B5.0 class solar flare, which occurred between 04:40 UT and 04:51 UT in this active region.