Spectroscopy of small scale magnetic structures in the solar atmosphere [Elektronische Ressource] = Spektroskopie kleinräumiger Magnetfeldstrukturen in der Sonnenatmosphäre / vorgelegt von Kai Christian Gerd Langhans

Kai Langhans

Sujets

• Physik
• Astronomie

Informations

• Publié par: albert-ludwigs-universitat_freiburg
• Publié le: 01 janvier 2003
• Nombre de lectures: 18
• Langue: English
• Poids de l'ouvrage: 10 Mo

Extrait

Spectroscopy of
small scale magnetic structures
in the solar photosphere
Kai Langhans
Fakultät für Mathematik und Physik
der Albert−Ludwigs−Universität
Freiburg im BreisgauSpectroscopy of small scale magnetic
structures in the solar atmosphere
Spektroskopie kleinraumiger Magnetfeldstrukturen in der Sonnenatmosphare
Inaugural-Dissertation
zur
Erlangung des Doktorgrades
der
Fakultat fur Mathematik and Physik
der
Albert-Ludwigs-Universitat,
Freiburg im Breisgau
vorgelegt von
Kai Christian Gerd Langhans
aus
Frankfurt am Main
2 0 0 3Dekan: Prof. Dr. R. Schneider
Leiter der Arbeit: PD Dr. W. Schmidt
Referent: PD Dr. W.
Korreferent: Prof. Dr. T. Heinzel
Tag der Verk undigung
des Pr ufungsergebnisses: 13.02.2003
Cover image: The solar photosphere observed in the G-band at 430.5 nm. The image with
2a eld-of-view of 12,000 30,000 km show many bright features at small scales that are
embedded in the granulation pattern close to an active region.
2’Daumenkino’: The 67 images (3700 3700 km ) cover a time period of about 18 minutes
illustrating the fast horizontal movements of the features called G-band bright points.
ivIn the context of this thesis the following articles have been published:
Journal papers
Langhans, K., Schmidt, W., 2002,
Center-to-limb-variation of the G-band lines at 430.5 nm
Astronomy & Astrophysics, v. 382, p. 312
Tritschler, A., Schmidt , W., Langhans , K., Kentischer, T. J., 2002
High-resolution Solar Spectroscopy with TESOS
Solar Physics, v. 211, p. 17
Langhans, K., Schmidt, W., 2002,
2D-spectroscopic observations of G-band bright structures in the solar photosphere
Astronomy & Astrophysics, v. 394, p. 1069
Conference proceedings and other publications
Michael Sigwarth, Thomas Rimmele, Kit Richards, Richard Radick, Klaus Hartkorn,
Kai Langhans, and Wolfgang Schmidt, 2001,
NSO Adaptive Optics Operating Successfully at the Dunn Solar Telescope and the
German VTT
in: NOAO Newsletter, No. 63
Langhans, K., Schmidt, W., Rimmele, T., Sigwarth, M., 2001,
Spectroscopic observations of G-band Bright Points
in: M. Sigwarth (ed.): \ASP Conf. Ser.236: Advanced Solar Polarimetry { Theory,
Observation, and Instrumentation"
Langhans, K., Schmidt, W., 2002,
Spectroscopic observations of G-band Bright Points
in: Proc. 10th. European Solar Physics Meeting, \Solar Variability: From Core to
Outer Frontiers", ESA SP-506, 397
Langhans, K., Schmidt, W., Tritschler, A., 2002,
2D-spectroscopic observations of G-band bright structures
in: Proc. 10th. European Solar Physics Meeting, \Solar Variability: From Core to
Outer Frontiers", ESA SP-506, 455
Langhans, K., Schmidt, W., Tritschler, A., 2003,
Observations of G-band bright structures with TESOS
Astronomical News, in press
vviAbstract
If the sun is imaged in the G-band (a spectral range at 430 nm that is dominated by
electronic transitions of the CH-molecule) at a su ciently high resolution, the presence
of the so-called G-band bright points is revealed. They are interpreted as small-scale
magnetic eld concentrations that are embedded in the convective ow eld of the solar
photosphere. As manifestations of small scale magnetic elds, bright points become
important for the understanding of the coronal heating process and the variability of the
solar irradiance.
Hitherto all observations of this ne structure in the G-band have been obtained with
broadband interference lters that integrate the spectral intensity over a variety of ab-
sorption lines. In this thesis the physical structure of the G-band bright points and the
properties of the G-band itself are investigated. The required spectral data at high spa-
tial resolution is in part obtained with the help of a low order adaptive optic system. A
comparison with synthesized spectra of di erent model atmospheres is done as well.
To investigate the G-band itself a center-to-limb analysis of the spectral range between
430.40 and 430.61 nm was carried out. Both the absolute line core intensities and the
continuum intensity at 430.41 nm can be described by the known functions of Neckel.
Based on the assumption of local thermodynamic equilibrium the behavior of the CH-
abundance in the range of the standard optical depth that is accessible by center-to-limb
observations, 0:05 < < 0:4, is investigated. The CH abundance decreases with500nm
height in accordance with recent numerical models.
The statistical investigation of two-dimensional spectra of two nearby lines of the G-
band, namely the Fe II-line at 430.32 nm and the CH-line at 430.34 nm, suggests the
existence of two classes of G-band bright features which di er in their spectroscopic
signature: (1) Bright structures caused by signi cant weakening of the absorption lines
of the CH molecule and (2) bright structures only caused by an enhanced continuum
intensity. In order to distinguish between those two classes the Bright Point Index
(BPI) is introduced, de ned by the ratio of the relative line depressions of the low-
excitation line from the abundant single-ionized iron and the CH-line minus one. The
bright structures caused by weakening of the CH-lines have high BPI values and are
embedded in the down ow regions of the intergranular lanes. They do not show a
signi cant enhancement in the continuum intensity in comparison with the ‘undisturbed’
sun reference. The remaining G-band bright structures have low BPI and are related to
granules.
The detailed analysis of one-dimensional spectra obtained in three di erent wavelength
regions (at 430 nm, 526 nm and 569 nm) of spatially resolved G-band bright points con-
rm the results of the two-dimensional data. The averaged extent of the bright points
with enhanced BPI is 0.38 arcsec. They show a average G-band contrast of 11 % with re-
spect to the ‘undisturbed’ sun reference, with values varying between -2.3 % and 28.3 %.
The CH-lines are weakened in the bright point interior which nds expression in decreased
values for the line depth and the equivalent width with respect to the bright points’ im-
mediate surroundings and the undisturbed sun as well. Also the atomic lines, such as
the Fe I-line at 569.15 nm, weaken in the bright point interior. In contrast the absorption
line of the single ionized iron at 526.48 nm remains almost constant between bright point
viiinterior and the immediate surroundings. The measurement of the line-of-sight velocities
show that the down ow within the bright points is stronger than in the close environ-
1ment. A net down ow of around 80 ms is measured. Some bright points in the sample
show a clear velocity enhancement with respect to their immediate surroundings. In a
direct comparison with synthesized spectra for di erent uxtube models lling factors are
calculated, based on the measured and the computed values for the BPI and the G-band
contrast. The resulting lling factors vary between 0.08 and 0.55. These lling factors
that are applied to estimated the ‘true’ velocity in bright points with respect to their
1immediate surroundings. The estimated velocities mount up to the order of kms in
both up- and down ows. This is in agreement with the magneto-convective picture of
the formation and dispersal of magnetic uxtubes and con rms former observations of
zero-crossing wavelengths of StokesV pro les in network and plage regions.
viiiZusammenfassung
Beobachtet man die Photosphare der Sonne bei ausreichender raumlicher Au osung im
G-Band, einem Spektralbereich bei 430 nm, der von Absorptionslinien des CH-Molek uls
gepragt ist, zeigt sich eine Vielzahl heller Punkte bei au ergew ohnlich hohem Kontrast.
Diese hellen Punkte im G-Band werden als Manifestation kleinraumiger Konzentrationen
magnetischen Feldes angesehen, die in das Str omungsfeld der Konvektionszone der Sonne
eingebettet sind. Es wird angenommen, dass sie einen wichtigen Beitrag zum Verstandnis
der Heizung der solaren Korona sowie der Variation der Sonnenleuchtkraft mit dem Ak-
tivitatszyklus liefern onnen.k
Alle bisherigen Beobachtungen dieser Feinstruktur im G-Band wurden mit breitbandigen
Interferenz ltern gemacht, welche die Intensitat ub er viele Absorptionslinien des CH-
Molek uls integrieren. In der vorliegenden Arbeit werden anhand hochaufgel oster Spek-
tren, die zum Teil mit Hilfe adaptiver Optik gewonnen wurden, die physikalische Struk-
tur der hellen Punkte im G-Band und die Eigenschaften des G-Bandes selbst untersucht
und mit Daten, die aus synthetisierten Spektren unterschiedlicher Modellatmosph aren
gewonnen wurden, verglichen.
Zur Untersuchung des G-Bandes wurde die Mitte-Rand-Variation des Spektralbereichs
zwischen 430.40 und 430.61 nm vermessen. Die Messung zeigt, da sich sowohl die
absoluten Linienkernintensit aten als auch die Kontinuumsintensit at durch die bekann-
ten Funktionen von Neckel darstellen lassen. Die Berechnung der CH-H au gkeit im
Bereich optischer Tiefe, der durch die Mitte-Rand-Variationsmessung zuganglich ist,
0:05 < < 0:4, zeigt einen Abfall der Hau gkeit mit zunehmender H ohe in500nm
Ubereinstimmung mit theoretischen Berechnungen.
In einer statistischen Analyse zweidimensionaler Spektren eines Linienpaares im G-
Band, bestehend aus der Fe II-Linie bei 430.32 nm und der CH-Linie bei 430.34 nm,
wurde die spektroskopische Signatur de