P.-L. Blelly

Modeling the OI 630.0 and 557.7 nm thermospheric dayglow during EISCAT‐WINDII coordinated measurements

The 630.0 and 557.7 nm thermospheric dayglow was modeled at high-latitude using an eight-moment fluid model, from measurements coordinated between the European Incoherent Scatter (EISCAT) radar and the Wind Imaging Interferometer (WINDII). The emission computation in particular included the electron (suprathermal and thermal) impact whose cross sections have been updated, the dissociative recombination of O2+ ion described for the first time by a theoretical rate coefficient, the photodissociation of molecular oxygen, and some relevant chemical reactions. The neutral atmosphere was adjusted by calibrating the ionospheric model outputs to EISCAT data. Slight adjustments were needed in order to reach a good agreement. The results were successfully compared to WINDII observations. Our present study shows that simultaneous EISCAT-WINDII measurements can be used to reduce uncertainties due to the neutral composition and that new observations of the EUV solar spectrum are still needed.

The TEC and F2 parameters as tracers of the ionosphere and thermosphere

Abstract The more common measurements of the ionosphere are the total electron content (TEC) (measured with the positioning systems) and the parameters of the F-region peak, namely its altitude (hmF2) and its density (NmF2). In this paper, we address the question of using these reduced parameters as tracers of the ionosphere and thermosphere. We make use of a 19 h daylight experiment by the EISCAT incoherent scatter radar based in Tromso, Norway. We reduce the full profile measurement of the electron density (between 80 and 425 km ) to the TEC, hmF2 and NmF2. Using a 1-D fluid/kinetic model, we reproduce these parameters. Then, by inversion, we retrieve the ionospheric profiles of the electron density, electron and ion temperature and, to a smaller extent, of the ion velocity. With this method, we show that it is in principle possible to evaluate a correction factor for the atomic oxygen. This factor evolves with time. We study the different uncertainties (heat flow, molecular densities, limitations of the model). Finally, we evaluate the contribution of the upper ionosphere (up to 3000 km) to the calculation of the TEC and show that 90% of the TEC is reached when the integration is made up to 1200 km .

Numerical modelling of intermittent ion outflow events above EISCAT

Abstract EISCAT observations with the UHF and VHF radars of the dynamics of the upper ionosphere have revealed the occurrence of intermittent ion outflows with velocities reaching several hundred m s −1 . It was previously shown that, during such events, the topside downward electron heat flux, inferred from the analysis of the vertical (field-aligned) structure of the electron temperature profiles, increases drastically up to values of about 10 μW m −2 . The numerical models described in a companion paper are used here to simulate the effects of energy inputs driven by the magnetosphere. Three main effects are simulated separately: effects of frictional heating related to E × B drifts, effects due to topside heat flux perturbations and effects of upward field-aligned currents of a few tens of μA m −2 . It is shown that field-aligned currents and topside heat flux perturbations produce very similar effects and that combining field-aligned currents with frictional heating allows us to model the overall characteristics of the perturbations of the electron density, of the electron and ion temperatures, and of the ion vertical velocity. A good agreement with observations is found.

Time-dependent models of the auroral ionosphere above EISCAT

Abstract Two numerical one-dimensional and time-dependent models of the topside auroral ionosphere in the altitude range 200–3000 km are presented: they are based on two different numerical schemes to solve the eight-moment approximation of Boltzmanns equation, namely the Flux Corrected Transport (FCT) and the Method of Lines (ML). The transport equations for densities, velocities, temperatures and heat fluxes are simultaneously solved along the magnetic field lines for each constituent of the ionospheric plasma assumed to be composed of electrons and of O + and H + ions. These models, using the MSIS-86 neutral atmosphere model, include solar EUV photoionization, chemical and collisional processes between the various charged and neutral species. Steady-state results for both near-summer and winter conditions as well as for diurnal evolution are presented and compared to experimental data from the European Incoherent SCATter (EISCAT) VHF radar. It is shown that independently of the numerical scheme, the ionospheric structure is very well reproduced, given realistic external sources (solar ionization and heating, magnetospheric energy input). On the basis of the comparisons, the eight-moment approximation is validated up to 3000 km altitude. Furthermore, simulation of a diurnal evolution shows terminator effects on the enhancement of the downward F2 electron heat flow.

Solar cycle variations in the ionosphere of Mars as seen by multiple Mars Express datasets

The response of the Martian ionosphere to solar activity is analyzed by taking into account variations in a range of parameters during four phases of the solar cycle throughout 2005–2012. Multiple Mars Express data sets have been used (such as Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) in Active Ionospheric Sounding, MARSIS subsurface, and MaRS Radio Science), which currently cover more than 10 years of solar activity. The topside of the main ionospheric layer behavior is empirically modeled through the neutral scale height parameter, which describes the density distribution in altitude, and can be used as a dynamic monitor of the solar wind-Martian plasma interaction, as well as of the mediums temperature. The main peak, the total electron content, and the relationship between the solar wind dynamic pressure and the maximum thermal pressure of the ionosphere with the solar cycle are assessed. We conclude that the neutral scale height was different in each phase of the solar cycle, having a large variation with solar zenith angle during the moderate-ascending and high phases, while there is almost no variation during the moderate-descending and low phases. Between end-2007 and end-2009, an almost permanent absence of secondary layer resulted because of the low level of solar X-rays. Also, the ionosphere was more likely to be found in a more continuously magnetized state. The induced magnetic field from the solar wind, even if weak, could be strong enough to penetrate more than at other solar cycle phases.

Evidence of scale height variations in the Martian ionosphere over the solar cycle

Solar cycle variations in solar radiation create density changes in any planetary ionosphere, which are well established in the Earths case. At Mars, however, the ionospheric response to such changes is not well understood. We show the solar cycle impact on the topside ionosphere of Mars, using data from the Mars Advance Radar for Subsurface and Ionospheric Sounding (MARSIS) on board Mars Express. Topside ionospheric variability during the solar cycle is analyzed through neutral scale height behavior. For moderate and high solar activity phases, the topside electron density profile is reproduced with an altitude-variable scale height. However, for the period of extremely low solar activity in 2008 and 2009, the topside was smaller in density than in the other phases of the solar cycle, and there is evidence that it could be reproduced with either a constant scale height or a height-variable scale height with lower electron density. Moreover, the ionosphere during this time did not show any apparent dependence on the EUV flux. This singular behavior during low solar activity may respond to the presence of an induced magnetic field which can penetrate to lower ionospheric altitudes than in other phases of the solar cycle due to the reduced thermal pressure. Numerical simulations of possible scenarios for two different solar cycle phases indicate that this hypothesis is consistent with the observations.

Storm effects on the ion composition

Abstract It is well known that the ion composition of the F1-region is strongly affected by electric fields, electron precipitations and change in the neutral atmosphere composition. The transition altitude between molecular and oxygen ions can therefore be raised or lowered depending on local conditions. Ion composition measurements by the EISCAT facility during a substorm are presented. Emphasis is put on results obtained during a period of high electric field with a new analysis program of incoherent scatter spectra that allows for non-Maxwellian ion velocity distribution functions. These results are discussed in the light of numerical simulation, using TRANSCAR 1D-ionospheric model. This model couples a kinetic description of precipitating electrons with a fluid description of the thermal plasma (6 ions and electrons) and is therefore well adapted to a quantitative description of storm effects on the transition altitude between molecular and oxygen ions.