Indications for particle precipitation impact on the ion-neutral collision frequency analyzed with EISCAT measurements

Abstract

The ion-neutral collision frequency is a key parameter for the coupling of the neutral atmosphere and the ionosphere. Especially in the mesosphere lower-thermosphere (MLT), the collision frequency is crucial for multiple processes, e.g., Joule heating, neutral dynamo effects, and momentum transfer due to ion drag. Few approaches exist to directly infer ion-neutral collision frequency measurements in that altitude range. We apply the recently demonstrated difference spectrum fitting method to obtain the ion-neutral collision frequency from dual-frequency measurements with the EISCAT incoherent scatter radars in Tromsø. A 60 h long EISCAT campaign was conducted in December 2022. Strong variations of nighttime ionization rates were observed with electron densities at 95 km altitude varying from Ne,95 = 10^9 to 10^11 m-3, which indicates varying levels of particle precipitation. A second EISCAT campaign was conducted on 16 May 2024, capturing a solar energetic particle (SEP) event, exhibiting constantly increased ionization due to particle precipitation in the lower E region: Ne,95 > 5 \cdot 10^10 m-3. We demonstrate variations of the ion-neutral collision frequency profile that we interpret as neutral particle uplift due to particle precipitation heating. Assuming a rigid-sphere particle model, we derive neutral density profiles which indicate a significant variation of neutral gas density between about 90-110 km altitude that correlate with the estimated strength of particle precipitation. However, the change in ion-neutral collision frequencies cannot be conclusively linked to the particle precipitation impact, and alternative interpretations are discussed. We additionally test the sensitivity of the difference spectrum method to various a priori collision frequency profiles.