Thermal Expansion Effects on F-region Height Changes During Geomagnetic Storms

The increased high-latitude energy input during geomagnetic storms, mainly resulting from Joule heating, causes the atmosphere to heat and expand at thermospheric heights. As a consequence, a global wind surge is generated and propagates from both polar regions to low latitudes and into the opposite hemisphere. Those winds are driven by the pressure inequalities due to temperature differences between high and equatorial regions. Divergence in horizontal winds drive vertical upward winds across pressure surfaces, the so-called "divergence velocity". Conversely, convergent horizontal winds are associated with a downward "divergence wind". The circulation is closed by a return flow in the lower thermosphere. At the same time, the expansion and contraction of a fixed pressure level atmospheric parcel cause vertical winds, the so-called "barometric velocity". Barometric winds are related to the thermal expansion of the atmosphere, while vertical divergence winds are associated to the conservation of mass relative to the levels of fixed pressure. In this study, the relative contribution of the horizontal thermospheric winds, the divergence winds and the barometric winds in the thermosphere-ionosphere response to geomagnetic storms is examined and analysed using the global, three- dimensional, time-dependent, non-linear coupled model of the thermosphere, ionosphere, plasmasphere, and electrodynamics (CTIPe). In order to simulate a thermospheric storm-time heating which does not create the global wind surge at high latitudes that propagates towards the equator, the neutral temperature in the thermosphere is uniformly enhanced. By doing this, it is expected no changes in the horizontal thermospheric winds, so the uplift of hmF2 by the horizontal wind mechanism is due to the vertical movement of the pressure level caused by the heating of the thermosphere. The neutral molecules on this pressure level move upwards, colliding with ions and electrons and driving them up along the geomagnetic field lines, as long as they are inclined. Ionosonde data from various mid-latitude stations are used to compare and support results provided by the physical model.