Spherical Harmonic Analysis of Jupiter's Cloud Features and Kinetic Energy from Cassini

Astronomy and Astrophysics – Astronomy

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Scientific paper

Recent power spectrum analysis of Jupiter's cloud features and atmospheric kinetic energy from a common Cassini imaging data set revealed suggestive evidence for an inverse energy cascade. We now report preliminary results from a re-analysis of our data utilizing spherical harmonics. Similar to our previous analysis, the typical power spectrum of Jupiter's cloud patterns and the kinetic energy of the atmosphere at the altitude of the ammonia cloud deck are broadly similar but contain some important differences. Power spectra of Jupiter's cloud patterns imaged with certain imaging filters resemble theoretical spectra of two-dimensional turbulence, with power-law slopes near -5/3 and -3 at low and high wavenumbers, respectively. The slopes of the kinetic energy power spectrum are also near -5/3 at low wavenumbers. At high wavenumbers, the spectral slopes appear to be relatively flatter than the theoretical prediction of -3, but the inferred slopes are sensitive to data analysis methods and may not represent the true spectral slope. In addition, the image mosaic and kinetic energy power spectra differ with respect to the location of the transition in slopes. The transition in slope is near planetary wavenumber 70 for the kinetic energy spectra, but is typically above 200 for the image mosaic spectra. We interpret the results overall as evidence suggesting the presence of an inverse energy cascade within Jupiter's atmosphere. Our results also show the importance of calculating spectral slopes from full 2D velocity maps rather than 1D zonal mean velocity profiles, as the spectral slopes obtained from 1D data sets differ greatly from those obtained from full 2D data sets. Furthermore, the difference between the image and kinetic energy spectra suggests caution in the interpretation of power spectrum results solely from image mosaics and its significance for the underlying dynamics.

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