at immersion and at emersion, at latitudes ranging from 46• S to 20• N, and are thus global features of the stratosphere. The profiles of temperature gradients exhibit a clear cutoff at the adiabatic lapse rate, indicating that fluctuations lead to marginal convective instabilities. Although ray crossing can also cause an apparent cutoff of the temperature gradients, we estimate it probably does not play an important role in the observed cutoff, at least for the larger structures under study. The vertical power spectra of fluctuations show a general power law behavior, with an exponent close to −3, between vertical wavelengths of ∼5 and 50 km. The finite stellar diameter and ray crossings can distort the real spectra, and we can only conclude that the original power spectra have slopes between −2 and −3. The horizontal structure of the atmosphere exhibits typical aspect (horizontal-to-vertical) ratios of 15-45, with a tail in the distribution with values as high as 100-200 for some structures. Finally, the horizontal spectrum of fluctuations is a power law with an exponent close to −4 (between horizontal wavelengths of ∼25 and 250 km), if we assume it is separable from the vertical spectrum.
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