Sources of F-Region Height Changes During Geomagnetic Storms at Mid Latitudes

Abstract

The increased high-latitude energy input in the thermosphere during geomagnetic storms, mainly resulting from Joule heating, causes the atmosphere to heat and expand. The heating at high latitudes drives a global wind surge that propagates from both polar regions to low latitudes and into the opposite hemisphere. Those winds are driven by pressure inequalities due to temperature differences between high latitudes and equatorial regions. To balance the or convergence produced by large-scale horizontal wind systems, vertical motions of air must occur in the thermosphere. The vertical motion of the thermosphere due to the vertical wind velocity can be represented as the sum of the divergence and the components. The velocity (W D ) component of the vertical wind, so called because it arises from the of the horizontal winds, represents the flow across pressure surfaces. Conversely, the convergent horizontal wind is associated with a downward wind. The circulation is closed by a return flow in the lower thermosphere. The expansion of a fixed pressure level atmospheric parcel by the heating drives the second component of the vertical wind, the so-called barometric velocity ( W B ). The barometric component represents the rise and fall of constant pressure levels due to thermal expansion or contraction. Barometric winds are therefore related to the thermal expansion of the atmosphere, whereas vertical winds are associated with the conservation of mass relative to fixed pressure levels. The F 2 layer height can change during geomagnetic storms both from the change in horizontal winds pushing plasma parallel to the inclined magnetic field, and due to vertical winds from the thermal expansion of the neutral atmosphere. In this paper, numerical experiments are conducted using a global, three-dimensional, time-dependent, nonlinear coupled model of the thermosphere, ionosphere, plasmasphere, and electrodynamics to quantify the impact of the horizontal thermospheric wind and the thermal expansion on changes in the F 2 layer peak height (h m F 2 ). The results demonstrate that height changes in the neutral atmosphere from thermal expansion are clearly reflected in the changes of h m F 2. Comparisons between model results and mid-latitude ionosonde observations are carried out for the magnetic storm events on 31 March 2001 and 17 April 2002. The analysis of the horizontal thermospheric wind and thermal expansion's relative role during the 31 March 2001 event reveals that both processes contribute significantly to the F-region height changes. The relative importance of those physical mechanisms depends on the local time at the storm commencement, the spatial distribution of the energy input over the poles, and the storm development and recovery duration.