Credit: Image courtesy of M. Van Zeeland
Spectrograms showing the change in wave activity with and without microwave heating. With no heating (top), two types of Alfvén waves are observed. With heating near the wave location (bottom), no frequency sweeping waves are expected or observed. The black curves indicate that the electron heating leads to closure of a theoretically predicted window (gray shading) in which these waves exist. The plots show the frequency of the waves in kilohertz and the time in milliseconds.