Spectral properties of mesoscale fluctuations in interplanetary coronal mass ejections

Magnetic fields in interplanetary coronal mass ejections (ICMEs) often display a flux rope structure at large scales and a turbulent cascade of fluctuations at smaller scales. However, the nature of fluctuations at transition scales between the flux rope and turbulence - i.e., at mesoscales - has not been previously examined in detail. Mesoscale fluctuations of the magnetic field in 142 ICME intervals with clear flux rope signatures at 1 au have been examined using wavelet power spectra, with "mesoscale" defined here as spacecraft-frame frequencies $10^{-4}\ \text{Hz}<f_\mathrm{sc}< 10^{-3}\ \text{Hz}$, equivalent to timescales $2.8\ \text{hr}\gtrsim \tau \gtrsim 17\ \text{min}$. The spectra were found to be significantly less steep at mesoscales than at larger and smaller scales, with an average spectral slope close to $-1$ at these intermediate scales. The mesoscale fluctuations had relatively high cross helicity magnitudes and low compressibility, suggesting a significant degree of Alfv\'enicity, while also having more negative residual energy than the turbulent fluctuations found at smaller scales. Uncertainties in the spectral slope values at mesoscales were used to distinguish between spectra showing a relatively smooth power-law behavior or (less commonly) a more bumpy trend; the smoother power-law intervals had a balanced cross helicity distribution while the more bumpy spectra had a predominantly antisunward cross helicity. Slow wind intervals were similarly analyzed, and showed similar properties to the ICMEs. These results indicate that a spectrally narrow $1/f$ range is typically present in ICMEs at mesoscales.