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We study horizontal supergranule-scale motions revealed by TRACE observationof the chromospheric emission, and investigate the coupling between thechromosphere and the underlying photosphere. A highly efficientfeature-tracking technique called balltracking has been applied for the firsttime to the image sequences obtained by TRACE (Transition Region and CoronalExplorer) in the passband of white light and the three ultraviolet passbandscentered at 1700 {\AA}, 1600 {\AA}, and 1550 {\AA}. The resulting velocityfields have been spatially smoothed and temporally averaged in order to revealhorizontal supergranule-scale motions that may exist at the emission heights ofthese passbands. We find indeed a high correlation between the horizontalvelocities derived in the white-light and ultraviolet passbands. The horizontalvelocities derived from the chromospheric and photospheric emission arecomparable in magnitude. The horizontal motions derived in the UV passbandsmight indicate the existence of a supergranule-scale magnetoconvection in thechromosphere, which may shed new light on the study of mass and energy supplyto the corona and solar wind at the height of the chromosphere. However, it isalso possible that the apparent motions reflect the chromospheric brightnessevolution as produced by acoustic shocks which might be modulated by thephotospheric granular motions in their excitation process, or advected partlyby the supergranule-scale flow towards the network while propagating upwardfrom the photosphere. To reach a firm conclusion, it is necessary toinvestigate the role of granular motions in the excitation of shocks throughnumerical modeling, and future high-cadence chromospheric magnetograms must bescrutinized.

Horizontal supergranule-scale motions inferred from TRACE ultraviolet observations of the chromosphere