EVIDENCE FOR ENHANCED THERMOSPHERIC HYDROGEN DENSITIES FROM THE UKRAINIAN INCOHERENT SCATTER RADAR DATA

Abstract:

This presentation investigates the cause of anomalously high night time topside H⁺ densities that were observed by the Kharkiv, Ukraine incoherent scatter radar (49.6° N, 36.3° E, L=2.1) during the last solar minimum. The FLIP physical model underestimates the H⁺ densities by a factor of 2-3 between March 2006 and December 2010 for all seasons and low to moderate solar activity. The calculations indicate that the higher measured H⁺ densities are most likely due to higher neutral hydrogen densities. This could be the result of weaker than usual magnetic activity, which would reduce the energy input to high latitudes. Prolonged low magnetic activity periods could cause a global redistribution of hydrogen and also allow more neutral hydrogen to settle down from the exosphere into the mid latitude thermosphere. The finding of the need for higher H densities agrees well with recent exosphere measurements. The higher H densities can help explain the pronounced nighttime increases in NmF2 that are seen in winter through increased ion flow from the plasmasphere. Accurate H densities are also important for accurate modelling of plasmasphere refilling rates. Our calculations with the FLIP model show that the refilling rate is increased by a factor of 2 when the NRLMSISE H density is increased by a factor of 3 and no other changes made to the model inputs.