The mechanism behind the modulation of polar mesospheric winter echoes (PMWE) observed in active HF heating experiments is considered. We propose that negative ion chemistry plays a role in the heater-induced modulation of the PMWEs. In the mesosphere, an increased electron temperature leads to a higher rate of electron to neutral attachment, and therefore to a decreased electron density. To a first-order approximation, scattering from Bragg-scale gradients caused by turbulence can be considered to be proportional to the electron density squared. Under this assumption, the electron density variation calculated by the detailed Sodankylä Ion Chemistry model (SIC) predicts correctly both the magnitude and the timescale of the modulation observed with the EISCAT VHF radar.