Observation of a non-radial penumbra in a flux emerging region under chromospheric canopy fields

Eun Kyung Lim, Vasyl Yurchyshyn, Philip Goode, Kyung Suk Cho

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26 Scopus citations


The presence of a penumbra is one of the main properties of a mature sunspot, but its formation mechanism has been elusive due to a lack of observations that fully cover the formation process. Utilizing the New Solar Telescope at the Big Bear Solar Observatory, we observed the formation of a partial penumbra for about 7 hr simultaneously at the photospheric (TiO; 7057 Å) and the chromospheric (Hα-1 Å) spectral lines with high spatial and temporal resolution. From this uninterrupted, long observing sequence, we found that the formation of the observed penumbra was closely associated with flux emergence under the pre-existing chromospheric canopy fields. Based on this finding, we suggest a possible scenario for penumbra formation in which a penumbra forms when the emerging flux is constrained from continuing to emerge, but rather is trapped at the photospheric level by the overlying chromospheric canopy fields.

Original languageEnglish (US)
Article numberL18
JournalAstrophysical Journal Letters
Issue number1
StatePublished - May 20 2013

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science


  • Sun: magnetic topology
  • Sun: photosphere
  • Sun: surface magnetism
  • sunspots


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