19,308 research outputs found
Observations of Sunspot Oscillations in G band and Ca II H line with Solar Optical Telescope on Hinode
Exploiting high-resolution observations made by the Solar Optical Telescope
onboard Hinode, we investigate the spatial distribution of power spectral
density of oscillatory signal in and around NOAA active region 10935. The
G-band data show that in the umbra the oscillatory power is suppressed in all
frequency ranges. On the other hand, in Ca II H intensity maps oscillations in
the umbra, so-called umbral flashes, are clearly seen with the power peaking
around 5.5 mHz. The Ca II H power distribution shows the enhanced elements with
the spatial scale of the umbral flashes over most of the umbra but there is a
region with suppressed power at the center of the umbra. The origin and
property of this node-like feature remain unexplained.Comment: 7 pages, 8 figures, accepted for publication in PASJ (Hinode Special
Issue
A distinct magnetic property of the inner penumbral boundary
A sunspot emanates from a growing pore or protospot. In order to trigger the
formation of a penumbra, large inclinations at the outskirts of the protospot
are necessary. The penumbra develops and establishes by colonising both umbral
areas and granulation. Evidence for a unique stable boundary value for the
vertical component of the magnetic field strength, ,
was found along the umbra-penumbra boundary of developed sunspots. We use
broadband G-band images and spectropolarimetric GFPI/VTT data to study the
evolution of and the vertical component of the magnetic field on a forming
umbra-penumbra boundary. For comparison with stable sunspots, we also analyse
the two maps observed by Hinode/SP on the same spot after the penumbra formed.
The vertical component of the magnetic field, , at the
umbra-penumbra boundary increases during penumbra formation owing to the
incursion of the penumbra into umbral areas. After 2.5 hours, the penumbra
reaches a stable state as shown by the GFPI data. At this stable stage, the
simultaneous Hinode/SP observations show a value comparable to
that of umbra-penumbra boundaries of fully fledged sunspots. We confirm that
the umbra-penumbra boundary, traditionally defined by an intensity threshold,
is also characterised by a distinct canonical magnetic property, namely by
. During the penumbra formation process, the inner
penumbra extends into regions where the umbra previously prevailed. Hence, in
areas where , the magneto-convection
mode operating in the umbra turns into a penumbral mode. Eventually, the inner
penumbra boundary settles at , which hints toward the
role of as inhibitor of the penumbral mode of
magneto-convection.Comment: Accepted as a Letter to A&A. Reproduced with permission from
Astronomy & Astrophysics, \copyright ES
- …