Bibcode
Bethge, C.; Beck, C.; Peter, H.; Lagg, A.
Referencia bibliográfica
Astronomy and Astrophysics, Volume 537, id.A130
Advertised on:
1
2012
Revista
Número de citas
22
Número de citas referidas
20
Descripción
Context. Siphon flows that are driven by a gas pressure difference
between two photospheric footpoints of different magnetic field strength
connected by magnetic field lines are a well-studied phenomenon in
theory, but observational evidence is scarce. Aims. We investigate the
properties of a structure in the solar chromosphere in an active region
to find out whether the feature is consistent with a siphon flow in a
magnetic loop filled with chromospheric material.
Methods. We derived the line-of-sight (LOS) velocity of several
photospheric spectral lines and two chromospheric spectral lines, Ca II
H 3968.5 *Aring; and He I 10830 Å, in spectropolarimetric
observations of NOAA 10978 done with the Tenerife Infrared Polarimeter
(TIP-II) and the POlarimetric LIttrow Spectrograph (POLIS). The
structure can be clearly traced in the LOS velocity maps and the
absorption depth of He I. The magnetic field configuration in the
photosphere is inferred directly from the observed Stokes parameters and
from inversions with the HELIX+ code. Data from the full-disk
Chromospheric Telescope (ChroTel) in He I in intensity and LOS velocity
are used for tracking the temporal evolution of the flow, along with
TRACE Fe IX/X 171 Å data for additional information about coronal
regions related to the structure under investigation.
Results. The inner end of the structure is located in the penumbra of a
sunspot. It shows downflows whose strength decreases with decreasing
height in the atmosphere. The flow velocity in He I falls abruptly from
above 40 km s-1 to about zero further into the penumbra. A
slight increase of emission is seen in the Ca II H spectra at the
endpoint. At the outer end of the structure, the photospheric lines that
form higher up in the atmosphere show upflows that accelerate with
height. The polarization signal near the outer end shows a polarity
opposite to that of the sunspot, the magnetic field strength of 580 G is
roughly half as large as at the inner end. The structure exists for
about 90 min. Its appearance is preceeded by a brightening in its middle
in the coronal TRACE data.
Conclusions. The observed flows match theoretical predictions of
chromospheric and coronal siphon flows, with accelerating upflowing
plasma at one footpoint with low field strength and decelerating
downflowing plasma at the other end. A tube shock at the inner end is
probable, but the evidence is not conclusive. The TRACE data suggest
that the structure forms because of a reorganization of field lines
after a reconnection event.
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