M. N. Vlasov

Modeling of the electron density depletion in the storm-time trough on April 20, 1985

Abstract Multi-instrument experimental data are analyzed to determine the main processes forming a deep trough in the electron density at F-peak altitudes during a strong magnetic storm ( K p ⩾5). Previous attempts to explain the observations were not successful. The model we use to interpret the data includes production of vibrationally excited N 2 in the region poleward of the trough and its transport into the trough region by a southward wind. The main source of the vibrationally excited N 2 is secondary electrons created by precipitating electrons. Joule heating and dissipation of precipitating electron energy create a pressure gradient and induce the southward wind. According to the model calculations, such a system of processes can cause the very strong electron density depletion observed by the Millstone Hill incoherent scatter radar on April 20, 1985. An important additional condition for such a deep trough is a decrease in the [O]/[N 2 ] ratio. The total energy flux and average energy of precipitating electrons just poleward of the trough is also a factor.

Impact of vibrational excitation on ionospheric parameters and artificial airglow during HF heating in the F region

[1] Vibrational excitation by the impact of electrons on molecular nitrogen in the energy range of 1.8-3.2 eV is well known. Electrons heated by high-power HF radio waves can effectively lose energy due to this vibrational excitation, which, in turn, creates a sharp energy barrier. Electrons must overcome this barrier in order to reach higher energies. A model for this vibrational barrier has been developed, and deviations of the electron energy distribution from a Maxwellian distribution have been estimated under different conditions. The depletion of electrons with energy higher than 2 eV decreases the excitation rate of the optical emissions (for example, 630.0 nm airglow) observed during HF heating. We show that the vibrational barrier can explain why the 630 nm airglow reported by Gustavsson et al. [2001] was only slightly enhanced during HF heating, in spite of the measured electron temperature being higher than 3000 K. In some cases, vibrationally excited molecules can accumulate in the ionosphere due to slow deactivation. These species increase the recombination coefficient, which, in turn, may result in the decrease of electron density. This effect can also contribute to the saturation of the optical emission strength as a function of heater power, as reported by Pedersen et al. [2003].

Crucial discrepancy in the balance between extreme ultraviolet solar radiation and ion densities given by the international reference ionosphere model

[1] Molecular ions play an important role in the balance of charged particles below the peak density in the F2 region. Balance is determined by ionization of the main neutral constituents by extreme UV solar radiation (EUVSR) and by dissociative recombination of the molecular ions. Using the empirical EUVSR model and the neutral densities given by the MSISE-90 (Mass Spectrometer-Incoherent Scatter) model, ionization rates can be calculated using well-developed, textbook-level analysis and then compared with the recombination rates corresponding to the molecular ion and electron densities given by the IRI-2001 and IRI-2007 empirical models. Our results show that the recombination rates calculated using the IRI model are larger by a factor of 5 to 30 than the ionization rates corresponding to the EUVSR empirical model at altitudes above 190 km at middle and low latitudes and low solar activity. Previously, and for similar reasons, we showed that the IRI predicted 630 nm and 557.7 nm emissions, which were found to be much higher than the observations (Vlasov et al., 2005; Nicolls et a]., 2006). The discrepancy greatly increases from noon to dusk and decreases with increasing solar activity. Our results thus show a contradiction between the EUVSR model and the IRI model. Since empirical data on molecular ions are scarce, we argue that straightforward ion chemistry calculations should be added to the IRI to correct the discrepancy.

On the basic processes forming the long-term variations of the exospheric temperature

The analytical expression for the long-term variations of the exospheric temperature, T∞, has been derived by solving the simplified equation of the thermal balance and the hydrostatics equation. This expression describes the T∞ variations with the solar activity and season in a very good agreement with empirical and physical models. Excellent agreement has also been found between the exospheric temperatures calculated by the analytical expression and the physical model for the long-term cooling of the thermosphere due to the CO2 infrared radiation increase. Further analysis indicates that the T∞ long-term changes depend on the input energy, molecular thermal conductivity, and the temperature at the lower boundary of the thermosphere. The analytical expression makes it possible to compare the experimental data on the long-term cooling of the lower thermosphere and the exosphere. A strong discrepancy between these data has been revealed.