Science Seminars

3/02/2017 – Bill Lotko; What causes high-latitude thermospheric density anomalies?

Speaker: Bill Lotko (Dartmouth College and NCAR/HAO)
Date: Tuesday, Mar 02, 2021
Time: 4:00 PM
Location: SPCS W120

Seminar Abstract:

Air density anomalies were recorded by the CHAMP satellite (nominal orbit at 400 km altitude) on 40% of its traversals through the dayside magnetic cusp. The anomalies are substantial density enhancements (>20%) over predictions of the MSIS empirical atmosphere model. They are typically accompanied by intense, small-scale magnetic fluctuations, which are commonly assumed to be induced by quasi-static electrical currents that enhance thermospheric Joule heating and upwelling. Earlier DE-2 satellite measurements at altitudes sampled by CHAMP suggest that the magnetic fluctuations are actually signatures of small-scale Alfvén waves – magnetically guided, ultralow frequency electromagnetic waves. I will review observed properties and models for anomaly formation and present new model results showing that the altitude profile of Alfvén wave energy deposition in the ionosphere and thermosphere is very different from that of quasi-static electromagnetic power flows. Inductive (Alfvén wave) fields with transverse scales of order 1 km or greater deposit Joule heat near the F-region peak in plasma density, where it is needed to produce the observed anomalies. Fluctuations at scales < 1 km deposit more distributed and diffuse Ohmic heat throughout the F region. These effects are due to Alfvén wave trapping in the so-called ionospheric Alfvén resonator.