R. Muller

Structure of the solar granulation

The structure of the solar granulation has been analysed using computer-processed images of two very high resolution (0″.25) white-light pictures obtained at the Pic-du-Midi Observatory.The narrow dispersion in the distribution of granule sizes is not confirmed. On the contrary, it is found that the number of granules increases continuously toward smaller scales; this means that the solar granulation has no characteristic or mean scale. Nevertheless, the granules appear to have a critical scale of 1″.37, at which drastic changes in the properties of granules occur; in particular the fractal dimension changes at the critical scale. The granules smaller than this scale could be of turbulent origin.

Variability of the quiet photospheric network

High-resolution photographs of the photospheric network taken in the Caii K 3933 Å line and at λ4308 Å are analysed in order to study the variation, in latitude and over the sunspot cycle, of its density (the density is defined as the number of network elements - also called facular points - per surface unity). It appears that the density of the photospheric network is not distributed uniformly at the surface of the Sun: on September 1983, during the declining phase of the current activity cycle, it was weakened at both the low (equatorial) and high (polar) active latitudes, while it was tremendously enhanced toward the pole. The density at the equator is varying in antiphase to the sunspot number: it increases by a factor 3 or more from maximum to minimum of activity. As a quantum of magnetic flux is associated to each network element, density variations of the photospheric network express in fact variations of the quiet Sun magnetic flux. It thus results that the quiet Sun magnetic flux is not uniformly distributed in latitude and not constant over the solar cycle: it probably varies in antiphase to the flux in active regions.The variation over the solar cycle and the latitude distribution of photospheric network density are compared to those of X-ray bright points and ephemeral active regions: there are no clear correlations between these three kinds of magnetic features.

Formation of Network Bright Points by granule compression

Network Bright Points (NBPs) are tiny, subarcsecond, bright features, visible in high-resolution filtergrams taken in white light as well as in photospheric and chromospheric absorption lines. They form the photospheric network and are associated with kilogauss, concentrated magnetic fields. Their behaviour is studied in a 3-hour, high-resolution granulation movie recorded at the Pic-du-Midi Observatory and processed at Lockheed Palo Alto Research Laboratory. The movie shows the important role played by granules. It appears that NBPs are formed in dark spaces when surrounding granules converge to fill this space. The formation is a fast phenomenon which lasts only 4 min. The lifetime of NBPs is 18 min on the average. About 15% of them split when they are squeezed between two expanding granules. Some consequences concerning the strength of the magnetic field during the formation of NBPs are discussed.

Relation between families of granules, mesogranules and photospheric network

The analysis of a solar granulation 3-h time sequence obtained at the Pic du Midi Observatory confirms the existence of Trees of Fragmenting Granules (TFG). The TFG lifetime histogram is fitted by a power law with an exponent equal to −1.68. The positive divergences (i.e. mesogranules) come from several families that are visible at different stages of the mesogranule evolution. We observe a good coincidence between the cork (passive scalars) distribution and the TFG boundaries. Thus, TFGs seem to play a role in the diffusion of the magnetic elements on the Sun surface.

Perturbation of the granular pattern by the presence of magnetic flux tubes

The presence of flux tubes, visible as Network Bright Points (NBP) at the surface of the Sun outside active regions, disturbs the granular pattern in a similar way as it can be observed in laboratory convection experiments. Around an NBP, granules are smaller, more numerous than around a normal intergranular space without NBP; they are elongated and pointing towards it. Such a perturbation is visible several minutes before the appearance of the NBP, indicating that the magnetic flux, although not yet visible as a bright point, is already interacting with the convection pattern. The perturbation reaches its maximum within one minute after the NBP appearance; and the granular pattern returns to normal just after the NBP disappearance.

High resolution solar magnetometry with the spectrograph of the Pic du Midi Turret Dome

We present arguments which show that in all likelihood mesogranulation is not a true scale of solar convection but the combination of the effects of both highly energetic granules, which give birth to strong positive divergences (SPDs) among which we find exploders, and averaging effects of data processing. The important role played by SPDs in horizontal velocity fields appears in the spectra of these fields where the scale

Phase diversity restoration of sunspot images I. Relations between penumbral and photospheric features

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THE LARGE-SCALE PATTERN FORMED BY THE SPATIAL DISTRIBUTION OF GRANULES

4 Mm is most energetic; we illustrate the effect of averaging with a one-dimensional toy model which shows how two independent non-moving (but evolving) structures can be transformed into a single moving structure when time and space resolution are degraded. The role of SPDs in the formation of the photospheric network is shown by computing the advection of floating corks by the granular flow. The coincidence of the network bright points distribution and that of the corks is remarkable. We conclude with the possibility that supergranulation is not a proper scale of convection but the result of a large-scale instability of the granular flow, which manifests itself through a correlation of the flows generated by SPDs.We present the flrst results obtained with a new Ferroelectric Liquid Crystal (FLC) polarimeter operating with the spectrograph of the Pic du Midi Turret Dome, since September 2003. We observed the solar granulation around active region NOAA 0459 with the Multichannel Subtractive Double Pass (MSDP) operating mode and analysed the circular polarization of the Na D1 589.6 nm spectral line in terms of longitudinal magnetic flelds in a 2D fleld of view (16 x 142 arc sec). Image quality was fairly good and limited to 0.4 arc sec due to the pixel sampling. This observation reveals the presence of magnetic concentrations of several hundred Gauss which are mainly located in the intergranular lanes. Data analysis performed at two difierent optical depths (line core and line wings) also suggests that magnetic lines are curved and diverge with increasing altitude.

High spatial resolution capabilities of Doppler measurements with the Pic du Midi MSDP spectrograph

We investigate the dynamics of and the relations between small-scale penumbral and photospheric features near the outer penumbral boundary: penumbral grains (PGs), dark penumbral fibrils, granules, and photospheric G-band bright points. The analysis is based on a 2 h time sequence of a sunspot close to disc center, taken simultaneously in the G-band and in the blue continuum at 450.7 nm. Observations were performed at the Swedish Vacuum Solar Telescope (La Palma) in July 1999. A total of 2564 images (46 �� ×75 �� ) were corrected for telescope aberrations and turbulence perturbations by applying the inversion method of phase diversity. Our findings can by summarized as follows: (a) one third of the outward-moving PGs pass through the outer penumbral boundary and then either continue moving as small bright features or expand and develop into granules. (b) Former PGs and G-band bright points next to the spot reveal a different nature. The latter have not been identified as a continuation of PGs escaping from the penumbra. The G-band bright points are mostly born close to dark penumbral fibrils where the magnetic field is strong, whereas PGs stem from the less-magnetized penumbral component and evolve presumably to non-magnetic granules or small bright features.

Results from high resolution solar images and spectra obtained at the Pic du Midi Observatory (1986–1990)

The spatial distribution of granule sizes at the surface of the sun is investigated. Granules have been separated into two classes: those larger than 1″.37 and those smaller, where 1″.37 is the critical scale defined by Roudier and Muller (1986). It is found that granules are not distributed at random: large granules appear to be clustered, forming a cellular pattern with a characteristic scale of 7″; small granules form a similar and complementary pattern. These patterns are probably related to the mesogranulation.