► Japanese

► DARTS of the Month

Figure 1. A sunspot observed with Hinode Solar Optical Telescope.
A bright filamentary structure at the center of the square is "light bridge."(JAXA/NAOJ)

Sunspots - New chromospheric activities revealed in the light bridges -

keywords: hinode, solar

Sunspots (Figure 1) are the cross section of magnetic flux bundles at the solar surface, originally emerged from the solar interior. The strong magnetic flux prevents the heat from transporting convectively from the interior to the surface, resulting in lower temperature and visually dark.
The solar observing satellite "Hinode" has been investigating sunspots in details with high spatial resolution never achieved so far. The sunspot movies sometimes show the formation of light bridges in the sunspot umbrae. The light bridges are one of the fundamental magnetic structures in sunspots, separating two umbral regions with filamentary structure or photospheric-like morphological features. Hinode's Ca II H movie observations have revealed that a sunspot light bridge produces chromospheric plasma ejections intermittently and recurrently for more than 1 day (Figure 2).

With detailed analysis of high-resolution magnetic field measurements, we found very strong vertical electric current density along the light bridge. The observations suggest that current-carrying highly twisted magnetic flux tubes are trapped below a cusp-shaped magnetic structure along the light bridge. The presence of trapped current-carrying flux tubes is essential for causing long-lasting chromospheric plasma ejections at the interface with pre-existing vertically oriented umbral fields. Quantitative understanding of sunspot magnetic field behaviors may give some hits for understanding various explosive events observed in the universe and space plasma
For details, refer to Shimizu et al.
"Hinode observation ofthe magnetic fields in a sunspot light bridge accompanied by long-lasting chromospheric plasma ejections," The Astrophysical Journal, Vol. 696, L66-L69, 2009

Figure 2. Some Ca II H snapshots of chromospheric jets, which were observed intermittently and recurrently at the sunspot light bridge. The field of view is same as the square in Figure 1.(JAXA/NAOJ)

Toshifumi Shimizu (ISAS/JAXA)

August,2009