G-band Images Research Articles

Motions of IsolatedG‐Band Bright Points in the Solar Photosphere

Magnetic elements on the quiet Sun are buffeted by convective flows that cause lateral motions on timescales of minutes. The magnetic elements can be observed as bright points (BPs) in the G band at 4305 A ˚ .W e present observations of BPs based on a long sequence of G-band images recorded with the Dutch Open Telescope and postprocessed using speckle-masking techniques. From these images we measured the proper motions of isolated BPs and derived the autocorrelation function of their velocity relative to the solar granulation pattern. The accuracy of BP position measurements is estimated to be less than 23 km on the Sun. The rms velocity of BPs (corrected for measurement errors) is about 0.89 km s � 1 , and the correlation time of BP motions is about 60 s. This rms velocity is about 3 times the velocity measured using cork tracking, almost certainly due to the fact that isolated BPs move more rapidly than clusters of BPs. We also searched for evidence of vorticity in the motions of G-band BPs. Subject headings: Sun: atmospheric motions — Sun: faculae, plages — Sun: magnetic fields — techniques: high angular resolution

Penumbral finestructure: need for larger telescopes

We obtained at the same time G-band images at the Dutch Open Telescope (DOT) on La Palma and spectropolarimetric data in the near infrared at the German Vacuum Tower Telescope (VTT) on Tenerife. The spectropolarimetric data show interesting correlations. Bright filaments have a smaller magnetic field strength, and higher Evershed velocities occur in dark structures. This result is in agreement with some previous observations, but also in contradiction to others. However, we suffer from the fact that the resolution limit of the VTT at a wavelength of 1.565 μm corresponds to 400 km. Spatial power spectra derived from the DOT data indicate a typical width of 250 km for penumbral filaments. Obviously a solar telescope with an aperture of at least 1.5 m is needed to obtain sophisticated results for penumbral structures.

Penumbral finestructure: need for larger telescopes

We obtained at the same time G-band images at the Dutch Open Telescope (DOT) on La Palma and spectropolarimetric data in the near infrared at the German Vacuum Tower Telescope (VTT) on Tenerife. The spectropolarimetric data show interesting correlations. Bright filaments have a smaller magnetic field strength, and higher Evershed velocities occur in dark structures. This result is in agreement with some previous observations, but also in contradiction to others. However, we suffer from the fact that the resolution limit of the VTT at a wavelength of 1.565 μm corresponds to 400 km. Spatial power spectra derived from the DOT data indicate a typical width of 250 km for penumbral filaments. Obviously a solar telescope with an aperture of at least 1.5 m is needed to obtain sophisticated results for penumbral structures.

The size of penumbral fine structure

I present power spectra of penumbral and granular intensity variations from a speckle-restored G-band image sequence of sunspot NOAA 9407 taken on April 1, 2001 with the Dutch Open Telescope on La Palma. I compare spatial power spectra of the sunspot penumbra with spatial power spectra from granulation with and without filigree. Relative to the granular power distribution, the penumbral power spectrum is enhanced over a wide range in spatial frequency peaking at 035. For smaller scales, the penumbral power distribution closely resembles that of the granular intensity variations. In contrast, the power spectrum of granulation with filigree exhibits increased power down to the resolution limit of 022, indicating the presence of unresolved magnetic elements.

G‐Band Spectral Synthesis in Solar Magnetic Concentrations

Narrowband imaging in the G band is commonly used to trace the small magnetic field concentrations of the Sun, although the mechanism that makes them bright has remained unclear. We carry out LTE syntheses of the G band in an assorted set of semiempirical model magnetic concentrations. The syntheses include all CH lines as well as the main atomic lines within the bandpass. The model atmospheres produce bright G-band spectra having many properties in common with the observed G-band bright points. In particular, the contrast referring to the quiet Sun is about twice the contrast in continuum wavelengths. The agreement with observations does not depend on the specificities of the model atmosphere; rather, it holds from single flux tubes to microstructured magnetic atmospheres. However, the agreement requires that the real G-band bright points are not spatially resolved, even in the best observations. Since the predicted G-band intensities exceed by far the observed values, we foresee a notable increase of contrast of the G-band images upon improvement of the angular resolution. According to the LTE modeling, the G-band spectrum emerges from the deep photosphere that produces the continuum. Our syntheses also predict solar magnetic concentrations showing up in continuum images but not in the G band. Finally, we have examined the importance of the CH photodissociation in setting the amount of G-band absorption. It turns out to play a minor role.

On the Relation of G‐Band Bright Points to the Photospheric Magnetic Field

Cotemporal observations in the 4305 A G-band and Ca II λ3933 K-line, Fe I 6302 A magnetograms, and 6563 A Hα images are used to study the relation of G-band bright points (GBPs) to magnetic elements in the photosphere. Angular resolution of 02 and 03 is achieved in the best G-band and magnetogram images, respectively. Single magnetogram sensitivity of 120-150 gauss (Φmin ~ 1016 Mx) is achieved. Small-scale GBPs appear both in intergranular lanes and on the edges of certain bright, rapidly expanding granules. The latter class of GBPs are nonmagnetic (at the flux limit) and are a source of confusion in magnetic element studies. The large-scale pattern of GBPs and magnetic flux in plage are highly correlated; GBPs occur preferentially on the periphery of extended plage regions, which are primarily demarcated by dark or neutral-contrast regions (e.g., pores or unresolved structures). On subarcsecond scales, GBPs are cospatial (to within an average of 024) and comorphous with magnetic elements in intergranular lanes; larger flux concentrations appear more diffuse than the associated GBP groupings. The average peak flux density of GBPs in the data set is ~160 gauss. No significant trend in GBP flux density as a function of either contrast or size is found. Rings of magnetic elements on 5'' scales remain very stable for periods on the order of hours. GBPs trace the temporal evolution of magnetic elements closely: there is no indication of a lead or lag (to within the 30-90 s precision of our data) in the appearance of GBPs relative to the appearance of magnetic elements. Pore formation via the accumulation of magnetic elements at a flow-field sink is seen in the data set. Magnetic elements and granules are continually advected into pores by the photospheric flow field.

Dynamics of Magnetic Flux Elements in the Solar Photosphere

The interaction of magnetic -elds and convection is investigated in the context of the coronal heating problem. We study the motions of photospheric magnetic elements using a time series of high-resolution G-band and continuum -ltergrams obtained at the Swedish Vacuum Solar Telescope at La Palma. The G-band images show bright points arranged in linear structures ( -ligree II) located in the lanes between neighboring granule cells. We measure the motions of these bright points using an object tracking tech- nique, and we determine the autocorrelation function describing the temporal variation of the bright point velocity. The correlation time of the velocity is about 100 s. To understand the processes that determine the spatial distribution of the bright points, we perform simulations of horizontal motions of magnetic Nux elements in response to solar granulation Nows. Models of the granulation Now are derived from the observed granulation intensity images using a simple two-dimensional model that includes both inertia and horizontal temperature gradients; the magnetic Nux elements are assumed to be passively advected by this granulation Now. The results suggest that this passive advection model is in reasonable agreement with the observations, indicating that on a timescale of 1 hr the Nux tubes are not strongly a†ected by their anchoring at large depth. Finally, we use potential--eld modeling to extrapo- late the magnetic and velocity -elds to larger height. We -nd that the velocity in the chromosphere can be locally enhanced at the separatrix surfaces between neighboring Nux tubes. The predicted velocities are several km s~1, signi-cantly larger than those of the photospheric Nux tubes. The implications of these results for coronal heating are discussed. Subject headings: convection E Sun: corona E Sun: granulation E Sun: magnetic -elds