Ion density fluctuations and the ion velocity distribution function in the presence of the current‐driven electrostatic ion cyclotron instability are determined using resonance fluorescence of the ions. The optical line intensity modulation shows that the ion density modulation can be as large as 90%. From the Doppler broadening of the lines it is found that the distribution function of ions heated by the unstable ion cyclotron waves is Maxwellian with an uncertainty of 5%.