A&A 422, L63-L66 (2004)
DOI: 10.1051/0004-6361:200400019
E. Wiehr1 - B. Bovelet 1 - J. Hirzberger 2
1 - Universitäts-Sternwarte,
Geismarlandstraße 11, 37083 Göttingen, Germany
2 -
Institut für Geophysik, Astrophysik und Meteorologie,
Universitätsplatz 5, 8010 Graz, Austria
Received 13 April 2004 / Accepted 22 June 2004
Abstract
The new 1 m Swedish Solar Telescope SST on La Palma allows to
observe inter-granular G-band bright points (igBP) in solar active regions
at an unprecedented spatial resolution. The igBP are reasonably assumed to
be small-scale magnetic flux-concentrations. A sample of more than 1500 igBP
shows tight relations of diameter and brightness in the G-band and in the
continuum; it covers a diameter range of 100 km to 300 km, with a most
frequent value near 160 km. Features larger than 300 km formerly reported,
evidently result from insufficient spatial resolution; that upper diameter
limit is close to the typical width of inter-granular lanes, and suggests a
"gap'' to small pores. The lack of igBP with sizes below 130 km is discussed
not to arise from the finite spatial resolution of the 1 m telescope.
Key words: Sun: photosphere - magnetic fields
The solar surface is widely covered by small magnetic flux concentrations, even outside active regions and in the minimum of the solar cycle. The true size and the magnetic flux of these features are still subject of scientific debate (Keller 1992; Sánchez Almeida 2000); another interesting question concerns the physical difference of bright and dark magnetic regions. A precise investigation of magnetic regions requires high spatial resolution Zeeman polarimetry (e.g., Domínguez Cerdeña et al. 2003) which, however, implies additional optical surfaces in the instrumental setup and diminishes the instrument's efficiency and the spatial resolution. Small-scale magnetic flux-concentrations can also be identified using their brightness excess in the 430 nm absorption lines of the CH molecule (e.g., Muller & Roudier 1984; Title & Berger 1996) which allows the use of filters instead of a spectrograph, yielding short exposure times and thus high spatial resolution (cf. Fig. 1).
![]() |
Figure 1:
Speckle reconstructed G-band image of an active region
at ![]() ![]() ![]() |
Open with DEXTER |
The G-band bright points (BP) mark either magnetic or non-magnetic structures (Berger & Title 2001); Langhans et al. (2002) identified the former as inter-granular downdraft motions, and the latter as granular edges with upward motions. For an automatic separation of both, Bovelet & Wiehr (2003) applied a "local contrast criterion'' making use of the larger contrast of magnetic structures to their inter-granular surroundings compared to non-magnetic ones. The thus identified inter-granular G-band bright points (hereafter referred to as "igBP'') were used as tracers for small-scale magnetic flux-concentrations. Here, we present a study of small-scale inter-granular G-band bright points at unprecedented spatial resolution.
We used the new 1 m Swedish Solar Telescope on La Palma (SST; Scharmer et al. 2003) on July 14 and 15, 2003, to observe active regions near the solar
disk center. Images in the "G-band'' at nm were simultaneously
taken with continuum images at
nm. The tip-tilt mirror of
the SST assured an effective correction for influences of image motion.
Bursts of some dozens of images each integrated over 4 ms in the G-band
and, respectively, 7 ms in the continuum were processed using the
speckle-masking method (Pelemann & von der Lühe 1989; de Boer 1996).
For a comparison of the reconstructed images in G-band and 587 nm continuum,
the latter were destreched using a "local correlation tracking algorithm''
(November & Simon 1988). Finally, the pattern recognition algorithm of
Bovelet & Wiehr (2001) was applied to the reconstructed G-band image to
automatically detect features brighter than the mean ambient photosphere (BP).
The 1528 inter-granular ones among these G-band bright points were
finally selected (cf. Fig. 2) using the above mentioned local contrast
criterion (Bovelet & Wiehr 2003). We tuned our algorithm in such a manner
that bright granular edges were certainly excluded, even at the price of
loosing some igBP.
In order to compare the G-band with the corresponding continuum image, we apply our pattern recognition to the continum image completely independent from the G-band analysis - this time without any selection criterion, since the small-scale inter-granular features do not show a pronounced continuum intensity excess. The thus identified continuum structures include numerous tiny features such as granular fragments, mini-granules, granular sub-structures, but also the (magnetic) "solar filigree'' (Dunn & Zirker 1973). Among these, we selected the inter-granular structures exclusively by spatial correspondence with the G-band-selected igBP sample, thus obtaining diameter and mean brightness independently from the G-bandanalysis.
Our algorithm assigns to each feature a specific size, given by the area of
the covered pixels, avoiding any explicit assumption for fitting an intrinsic
intensity profile. We set the lower detection limit to 9 pixels, e.g.
pixels (far from the Nyquist limit) corresponding to
(
km in July). We then approximate
each igBP feature by a corresponding ellipse of equal area, and establish that
the majority of igBP is largely circular (cf. Berger et al. 1995; Bovelet &
Wiehr 2003). We assign to each structure the diameter D of a circle covering
the same area.
Our simultaneous imaging of inter-granular small-scale features allows to compare their brightness in the G-band and continuum. Brightest igBP exceed the mean photospheric intensity up to a factor of 1.8 in G-band and 1.3 in the continuum; this agrees with findings by (Sánchez Almeida et al. 2001). We find a remarkably tight relation between both intensities with a gradient 2.4 (Fig. 3), in agreement with empirical models (Sánchez Almeida et al. 2001).
![]() |
Figure 2:
Speckle reconstructed G-band ( upper) and simultaneous 587 nm
continuum ( lower) images of a 9.6 arcsec ![]() ![]() |
Open with DEXTER |
When plottting the intensity of each igBP versus its size (Fig. 4), we
find a largely flat relation showing that the igBP exhibit a mean
brightness excess, almost independent from their size. The slight intensity
decrease below 140 km stays within the width of the igBP brightness range and agrees with previous findings (Berger et al. 1995;
Bovelet & Wiehr 2003).
Our simultaneous imaging in G-band and continuum allows to compare the
diameters of inter-granular small-scale features in both formation heights.
We find (Fig. 5) that the sizes of the igBP are largely equal; deviations
from the line may readily be caused by slightly different intensity
"trends'' over the two images. These let the pattern recognition
algorithm segmentate slightly different contours (a few pixels) on its
given intensity thresholds. The poorer spatial resolution of in
the 587 nm continuum as compared to the 430 nm G-band can be seen as a
somewhat higher data point above the
line in Fig. 5. Our finding
of equal diameters in the G-band and the continuum indicates a rather
small difference in formation height and supports that the recognized
features are real.
![]() |
Figure 3:
Two dimensional histogram of the mean intensity of 1528 inter-granular
bright points recognized in the G-band (abscissa) and in the continuum
image (ordinate); the gray-scales give the number density of each
data point; circles give the weighted means, vertical bars the ![]() ![]() |
Open with DEXTER |
![]() |
Figure 4: Intensity-diameter relation of inter-granular features recognized in the G-band (upper) and in the continuum (lower part); x-bars mark samples of equal statistical weight; y-bars give standard deviations; light grey indicates the resolution range of an ideal 45 cm telescope; dark grey visualizes sizes below the SST's resolution; dotted lines give the total means. |
Open with DEXTER |
![]() |
Figure 5:
Two dimensional diameter histogram of 1528 inter-granular bright
points recognized in the G-band (abscissa) and in the continuum
image (ordinate); the gray-scales give the lower boundary of the
number density intervals; the total numbers of igBP for each sizebin
is indicated; circles give the weighted means, vertical bars the ![]() ![]() |
Open with DEXTER |
In Fig. 6, we present the size histogram for igBP segmentated in G-band.
It shows a narrow range between 100 km and 300 km with a most frequent
diameter of km, in good agreement with the 150 km fluxtubes
obtained from indirect line-ratio measurements (Stenflo 1973; Wiehr 1978).
![]() |
Figure 6: Diameter histogram for 1528 inter-granular G-band bright points fitted by a polynomial; light grey indicates the resolution range of an ideal 45 cm telescope; dark grey visualizes sizes beyond the resolution limit achieved. |
Open with DEXTER |
We find no structures larger than 300 km: the high spatial resolution of the 1 m SST allows a separation of structures which appear elongated at less spatial resolution (forming "filigree crincles''; cf. Dunn & Zirker 1973). Indeed, G-band images with less resolution yield igBP with larger diameters having the same mean brightness as in Fig. 4 - as expected for clusters of unresolved smaller igBP.
Our upper size limit near 300 km is close to the typical width of mean inter-granular lanes. This suggests that the corresponding flux-concentrations are "squeezed'' by neighboring granules (Muller & Roudier 1992), thus being elongated and fitting the inter-granular spacings. Such features ("filigree crincles'') might finally split due to inter-change instabilities (e.g. Schüssler 1984; Bünte 1993). Indeed, careful inspection of time series (Bovelet & Wiehr 2003) shows that chains of igBP mostly reveal splitting processes.
A maximum diameter of
km corresponds to a magnetic flux
of about 1018 Mx for a field of 1500 G; this value might be lower
if the filling of magnetic field within an igBP is smaller than unity. At
higher magnetic flux, solar magnetic regions appear dark. Calculations for
the stability of sunspots (Meyer et al. 1977) indicate a lower
limit of the sunspot magnetic flux near 1019 Mx, being ten times above
the upper flux limit for igBP. Assuming 2000 G magnetic field strength for
small dark features 1019 Mx would correspond to diameters of 800 km,
which seems a reasonable value for small pores (e.g., Hirzberger 2003).
The decrease of the number of igBP smaller than 130 km diameter is not an instrumental artifact; there is a true deficit of small igBP as inferred from Fig. 6. Our claim is based on the following arguments:
We find that igBP appear with diameters covering a limited range between 100 km and 300 km. The decrease near 130 km (Fig. 6) is close to the photon mean free path in the photosphere, so that smaller features would be laterally optical thin and therefore difficult to detect. The upper diameter limit indicates a gap between 1018 Mx and 1019 Mx for bright and dark flux-concentrations; indeed, the intensity-diameter relation (Fig. 4) gives no indication for a "smooth transition'' to dark magnetic structures (Spruit & Zwaan 1985; Knölker & Schüssler 1988; Soltau 1997). The bright magnetic flux-concentrations may then be intrinsically different from dark solar magnetic structures, all the more igBP show strong lateral motions (e.g., Bovelet & Wiehr 2003) in contrast to dark pores which may be deeper rooted.
Acknowledgements
We thank Drs. G. Scharmer and D. Kiselman for their kind support with the SST, Dr. J. Sánchez Almeida for valuable suggestions, and Drs. O. von der Lühe, M. Schüssler, F. Hessman for helpful discussions. The SST is operated by the Swedish Academy of Sciences at the Spanish Observatorio del Roque de los Muchachos (IAC). The data processing was done at the Institut für Geophysik, Astrophysik und Meteorologie Graz (IGAM) and at the Gesellschaft für wissenschaftliche Datenverarbeitung Göttingen (GWdG).