Stopping Jetpack Flames with ParticleSystem

I am trying to stop the jetpack flames (particlesystem) when character is on the ground. I continue to get the error "Consider storing value in temporary variable" as well as "Issue with converting float and MinMaxCurve." Help would be appreciated. Here is the code I have so far (I have attempted several adjustments, but none seem to work):

void adjustJetpack (bool jetpackActive) {

     /*var temp = jetpack.emission;
     temp.enabled = !grounded;
     jetpack.emission = temp;*/

     jetpack.emission .enabled  = !grounded;
     jetpack.emission .rate  = jetpackActive ? 300.0f : 75.0f;