We have measured the root-mean-square ( RMS ) amplitude of intensity fluctuations , \Delta I , in plume and interplume regions of a polar coronal hole .
These intensity fluctuations correspond to density fluctuations .
Using data from the ( SWAP ) on Project for Onboard Autonomy ( Proba2 ) , our results extend up to a height of about 1.35 R _ { \sun } .
One advantage of the RMS analysis is that it does not rely on a detailed evaluation of the power spectrum , which is limited by noise levels to low heights in the corona .
The RMS approach can be performed up to larger heights where the noise level is greater , provided that the noise itself can be quantified .
At low heights , both the absolute \Delta I , and the amplitude relative to the mean intensity , \Delta I / I , decrease with height .
However , starting at about 1.2 R _ { \sun } \Delta I / I increases , reaching 20–40 % by 1.35 R _ { \sun } .
This corresponds to density fluctuations of \Delta n _ { \mathrm { e } } / n _ { \mathrm { e } } \approx 10–20 % .
The increasing relative amplitude implies that the density fluctuations are generated in the corona itself .
One possibility is that the density fluctuations are generated by an instability of Alfvén waves .
This generation mechanism is consistent with some theoretical models and with observations of Alfvén wave amplitudes in coronal holes .
Although we find that the energy of the observed density fluctuations is small , these fluctuations are likely to play an important indirect role in coronal heating by promoting the reflection of Alfvén waves and driving turbulence .