GUEST SPEAKER:  Bill Lotko, Ballard Professor of Engineering at Dartmouth College

TITLE:  What causes air density anomalies in the high-latitude thermosphere?

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.