The impact of particle precipitation 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 transport due to ion drag. Very 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-hour-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 N_e,95 ∼ 10⁹ − 10¹¹ 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 N_e,95 ≳ 5 · 10¹⁰ m⁻³. We demonstrate that the particle precipitation significantly impacts the ion-neutral collision frequency profile. Assuming a rigid-sphere particle model, we derive neutral density profiles and show that the particle precipitation heating causes a significant uplift of neutral gas between about 90–110 km altitude. We additionally test the sensitivity of the difference spectrum method to different a priori collision frequency profiles.