Formation of a solar Hα filament from orphan penumbrae
1 Max Planck Institute for Solar System Research Justus-von-Liebig-Weg 3 37077 Göttingen Germany
2 School of Space Research, Kyung Hee University Yongin 446-701 Gyeonggi Korea
Received: 15 October 2015
Accepted: 16 March 2016
Aims. The formation and evolution of an Hα filament in active region (AR) 10953 is described.
Methods. Observations from the Solar Optical Telescope (SOT) aboard the Hinode satellite starting from UT 18:09 on 27th April 2007 until UT 06:08 on 1st May 2007 were analysed. 20 scans of the 6302 Å Fe I line pair recorded by SOT/SP were inverted using the spatially coupled version of the SPINOR code. The inversions were analysed together with co-spatial SOT/BFI G-band and Ca II H and SOT/NFI Hα observations.
Results. Following the disappearance of an initial Hα filament aligned along the polarity inversion line (PIL) of the AR, a new Hα filament formed in its place some 20 h later, which remained stable for, at least, another 1.5 days. The creation of the new Hα filament was driven by the ascent of horizontal magnetic fields from the photosphere into the chromosphere at three separate locations along the PIL. The magnetic fields at two of these locations were situated directly underneath the initial Hα filament and formed orphan penumbrae already aligned along the Hα filament channel. The 700 G orphan penumbrae were stable and trapped in the photosphere until the disappearance of the overlying initial Hα filament, after which they started to ascend into the chromosphere at 10 ± 5 m/s. Each ascent was associated with a simultaneous magnetic flux reduction of up to 50% in the photosphere. The ascended orphan penumbrae formed dark seed structures in Hα in parallel with the PIL, which elongated and merged to form an Hα filament. The filament channel featured horizontal magnetic fields of on average 260 G at log (τ) = −2 suspended above the nearly field-free lower photosphere. The fields took on an overall inverse configuration at log (τ) = −2 suggesting a flux rope topology for the new Hα filament. The destruction of the initial Hα filament was likely caused by the flux emergence at the third location along the PIL.
Conclusions. We present a new interpretation of the Hα filament formation in AR 10953 whereby the mainly horizontal fields of orphan penumbrae, aligned along the Hα filament channel, ascend into the chromosphere, forming seed fragments for a new, second Hα filament. The orphan penumbral fields ascend into the chromosphere ~9–24 h before the Hα filament is fully formed.
Key words: Sun: filaments, prominences / Sun: magnetic fields
© ESO, 2016