Evidence of m = 1 density mode (plasma cam) in Saturn's rotating magnetosphere

Cassini field and plasma data measured in the rotating Saturn Longitude System 3 (SLS3) coordinate system show positive evidence of structure whose dominant azimuthal wave number is m = 1: a long‐lived, nonaxisymmetric, cam‐shaped, global plasma distribution in Saturn's magnetosphere. Previous studies have identified evidence of this plasma cam in wave‐derived electron density data and in Cassini Plasma Spectrometer (CAPS) W + ion counts data. In this paper we report the first comprehensive analysis of CAPS ion moments data to identify the m = 1 density cam. We employ a multiyear, multispecies database of 685,678 CAPS density values, binned into a 1 R S by 4.8° discretized grid, spanning 4–19  R S . Fourier (harmonic) analysis shows that at most radial distances the dominant azimuthal mode is m = 1, for both W + and H + ion distributions. The majority (63%) of m = 1 ion peaks are clustered in an SLS3 quadrant centered at 330°. The plasma cam's existence has important implications for the global interchange‐driven convection cycle and is a clue to solving the mystery of the rotational periodicities in Saturn's magnetosphere.