A comparison between resonant and nonresonant heating at EISCAT

Abstract

We compare the height‒integrated electron heating rates from O‒ and X‒mode HF pump waves to extract the components due to resonant and nonresonant heating mechanisms in the ionosphere. We present results from a November 2011 campaign in Norway, using the European Incoherent Scatter (EISCAT) heater facility and UHF radar. We show that the theoretical nonresonant, ohmic heating due to the electromagnetic pump wave electric field agrees with observations for X‒mode pumping. For O‒mode pumping, the observed height‒integrated heating rate exceeds the theoretical ohmic electron heating rate by a factor of 2–5, the excess being attributed to resonant heating mechanisms. In addition, a persistent UHF ion‒line enhancement is observed for O‒mode and, more unusually, X‒mode pumping. We attribute the latter to O‒mode leakage in the X‒mode pulse. For O‒mode, we see a descent in altitude of the ion‒line enhancement and show that this is most likely due to ionization from pump‒induced fluxes of suprathermal electrons. We estimate the ionization rate and determine an enhanced electron flux showing that approximately 10–20% of the pump power is transferred to high energy suprathermal electrons.