On the Origin of Solar Photospheric Turbulence: Granular Velocity Shear

We trace the Doppler velocity and turbulent velocity elds at diierent heights in the solar overshoot layers using absorption lines of diierent strengths in spec-trograms taken at the center of the solar disk. We show the results for one regular and one exploding granule as two representative examples. The convec-tive velocity patterns do not change with the height up to 200km above 5000 = 1, neither for the regular nor for the exploding granule. The turbulent velocity eld, however, shows a change with the height in the photosphere. At the border of both granules shear ows are strongly associated with the enhanced turbulence. Global oscillations and p-modes excita-tion depended sensitively on the dynamical state of the granulation, according to some recent investigations. Acoustic events in the photosphere are found preferentially in the intergranular space, the place of enhanced turbulence. The origin of acoustic oscillations is also localized at places of strong downwards velocities. We hope that this investigation provides some insight into the physics of the origin of the turbulence in the deep photosphere, and its interaction with helioseismology. 1. INTRODUCTION First observational indications of strong velocity gradients at the granular borders came from spectro-grams of high spatial and spectral resolution taken at the center of the solar disk with the German Vacuum Tower Telescope (VTT) in Iza~ na (Tenerife, Spain; cf. Nesis et al. 1992). The same spectrograms provided also observational evidence of unresolved turbulent velocity elds in the photospheric layers: Nesis et al. (1993) found enhanced turbulence in the inter-granular space located preferentially at the granu-lar borders, where the large gradients of the con-vective downnow occur. New observations taken in 1994 provided further evidence of shear ows and turbulent spots at the granular borders (Nesis et al. 1996), which manifest the association of the turbulence in the intergranular space with the granular shear ow. Large shear ows are commonly connected with turbulent velocity and pressure variations (Tritton 1988). Christensen-Dalsgaard (1996)noted the innuence of the dynamics of the granulation layers on the global oscillations. The direct eeect of the granular shear ow and the associated turbulence on solar oscillations was demonstrated recently by means of numerical models (Rosenthal 1996, Brun et al. 1996).