Dynamics of photosphere in presence of magnetic field

Abstract

In this paper we report the results of the preliminary analysis of observations carried out with the panoramic monochromator IBIS, installed at the Dunn Solar Telescope, Sacramento Peak (NM), in the spectral lines Ca II 854.2 nm, Fe I 709.0 nm and Fe II 722.4 nm. We analyzed the dynamical properties of a quiet region centered on a large scale (˜ 30 Mm) structure of magnetic network.

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Mem. S.A.It. Suppl. Vol. 9, 100
c
 SAIt 2006
Memorie della
Supplementi
Dynamics of photosphere in presence of
magnetic field
S. Giordano, F. Berrilli, D. Del Moro
Department of Physics, University of Rome Tor Vergata
Received September 15, 1996; accepted March 16, 1997
Abstract.
In this paper we report the results of the preliminary analysis of observations carried
out with the panoramic monochromator IBIS, installed at the Dunn Solar Telescope,
Sacramento Peak (NM), in the spectral lines Ca II 854.2 nm, Fe I 709.0 nm and Fe II
722.4 nm. We analyzed the dynamical properties of a quiet region centered on a large scale
(∼ 30 Mm) structure of magnetic network.
1. Introduction
Solar surface shows a wide variety of magnetic
structures, ranging from the largest sunspots
(tens of Mm across) down to the 100 km
scale magnetic elements, the smallest observ-
able forms of magnetic flux in the photosphere.
The distribution of magnetic structures on the
solar surface and their dynamical evolution
are influenced by the solar plasma which, un-
der the control of convective motions, concen-
trates or diffuses the magnetic field emerging
on the solar surface. In quiet areas magnetic
field concentrations appear as single bright
points, whose dynamics is determined by gran-
ular flows. These bright points are passively
advected towards the borders of supergranu-
lar cells, where they gather and produce the
magnetic network, a useful proxy for super-
granules. In this paper we present preliminary
results about the interaction between photo-
spheric velocity fields and magnetic elements
obtained from high spatial, temporal and spec-
tral resolution observations, which allowed a
3-D reconstruction of the photospheric veloc-
ity field.
2. Observations and data analysis
The data set contains observations of a
roundish network cell collected by the IBIS
2-D spectrometer on October, 16 2003 (from
14:24 UT to 17:32 UT) in the spectral lines
Ca II 854.2 nm, Fe I 709.0 nm and Fe II
722.4 nm. The temporal interval between suc-
cessive images was ∼300 ms. Each monochro-
matic image was acquired with 25 ms exposure
time by a CCD detector, whose pixel scale was
0.17”pixel
−1
. Each image was corrected for
CCD non linearity effects, dark current, gain
table and blue shift (Reardon et al. 2003). The
vertical velocity fields were computed for the
Fe I and Fe II lines by means of a Doppler shift
evaluated, pixel by pixel, using a Gaussian fit
of the line profile. The line core and width
fields were obtained by the amplitude and
width, respectively, of the same Gaussian func-
tion. In order to identify the small magnetic
fields we use the strong correlation between

Giordano et al.: Dynamics of photosphere in presence of magnetic field 101
Fig.1. Upper left panel: continuum image; upper central panel: Fe I 709.0 nm Doppler velocity
field; upper right panel: Fe II 722.4 nm Doppler velocity field; lower left panel: Ca II wing
intensity image with superimposed the tracked granule displacements relative to < h >= 0 km;
lower central panel: Fe I 709.0 nm line core intensity; lower right panel: Fe II 722.4 nm line core
intensity.
magnetic field signal and brightness in the Ca
II 854.2 nm line wing. These ”chromospheric
magnetograms” are used to derive a mask to
apply on Fe I 709.0 nm velocity (Doppler) and
line core intensity fields.
An analysis of the histograms of Fe I 709.0
plasma line-of-sight velocity (Fig. 2) and line
core intensity (Fig. 3) associated with magnetic
features outlines that magnetic regions corre-
spond to downflows and to region of reduced
contrast in the line core. We apply the TST
procedure (Del Moro 2004) on the continuum
image series and on both the Fe II 722.4 nm
and Fe I 709.0 nm velocity field series in order
to retrieve the horizontal velocity field at dif-
ferent depths of the solar atmosphere. In fact,

102 Giordano et al.: Dynamics of photosphere in presence of magnetic field
Fig.2. Histogram of the 709.0 nm Doppler ve-
locity values associated to magnetic regions.
the major contribution to these Doppler fields
comes from the two layers at about 140 km
and 70 km above the photospheric surface, for
the 722.4 nm and 709.0 nm lines respectively.
For each time series the TST produced two
horizontal velocity fields, associated with the
first and second 30 minutes of the series (see
Fig. 1). The tracked granules seem to gather on
the borders of the supergranule, with the ex-
ception of an extended cluster of bright struc-
tures.
This behaviour is also confirmed by the
mean divergence image, extracted from the
horizontal velocity field, which shows that the
bright magnetic area is not a region of conver-
gence. This seems to suggest that granules on
the edge of the supergranule are in some way
different from the granules in the centre.
Fig.3. Histogram of the contrast value for the
quiet sun (black continuous) and the magnetic
region (grey continuous). The crosses repre-
sent the Gaussian fit to the distributions
References
Berrilli, F., Del Moro, D., Giordano, S.,
Pietropaolo, E. 2005, Magnetic Network
Dynamics in photosphere, in Advances in
S pace Research : COS PAR04, submitted
Del Moro, D. 2004 A&A, 428, 107
Reardon, K. and Cavallini, F. 2003,
Characterization of the IBIS Trasmission
Profile, Memorie della Societ
´
a Astronomica
Italiana, 74-815-818.