A. D. Aylward

An investigation into the influence of tidal forcing on F region equatorial vertical ion drift using a global ionosphere‐thermosphere model with coupled electrodynamics

A recent development of the coupled thermosphere-ionosphere-plasmasphere model (CTIP) has been the inclusion of the electrodynamic coupling between the equatorial ionosphere and thermosphere. The vertical ion drifts which result are shown to be largely in agreement with empirical data, on the basis of measurements made at the Jicamarca radar and other equatorial sites [Scherliess and Fejer, 1999]. Of particular importance, the CTIP model clearly reproduces the “prereversal enhancement” in vertical ion drift, a key feature of the observational data. Inacurracies in the modeled daytime upward ion motion have been investigated with regard to changing the magnitude and phase of components of the lower thermospheric tidal forcing. The results show that daytime vertical ion motion is highly dependent upon both the magnitude and phase of the semidiurnal tidal component. In addition, the CTIP model shows the prereversal enhancement to be unaffected by changes in tidal forcing, but only for conditions of high solar activity. During periods of low solar activity the form of the prereversal enhancement is clearly dependant upon the magnitude and phase of the semidiurnal tide.

Ionospheric F 2 layer seasonal and semiannual variations

An extensive series of computations, using the Coupled Thermosphere-Ionosphere-Plasmasphere model (CTIP), has been undertaken to investigate the semiannual variation in peak noontime electron density, a common feature of the Fa-layer, particularly at low latitudes and in the southern hemisphere at mid-latitudes. Results from the model reveal such a variation, most prominently, at mid-latitudes, in the South American sector. An analysis of this phenomenon shows that it is intimately related to the large offset of the geomagnetic axis from Earths spin axis in the southern hemisphere. Because of this offset, a given geographic latitude in the South American sector corresponds to a lower magnetic latitude than in other sectors and is thus farther from the energy inputs associated with the auroral regions. As a result, the composition changes are much smaller during the winter months than at other longitudes, the mean molecular mass being essentially constant for a 4-month period centered on the winter solstice. This result is understood in terms of the global thermospheric circulation. In the absence of any composition changes, noon ionospheric density is influenced primarily by the solar zenith angle. This angle reaches a maximum at the winter Solstice, leading to diminished ion production, a minimum in N(m)F2, and therefore a semiannual variation overall. On the basis of the model results, the semiannual variation is seen as a feature of the midlatitude ionosphere at geographic longitudes opposite to the location of the geomagnetic pole. This phenomenon is seen in both northern and southern hemispheres, though the effect is much larger in the southern hemisphere as a result of the greater magnetic offset.

METHANE IN THE ATMOSPHERE OF THE TRANSITING HOT NEPTUNE GJ436B

We present an analysis of seven primary transit observations of the hot Neptune GJ436b at 3.6, 4.5, and 8 μm obtained with the Infrared Array Camera on the Spitzer Space Telescope. After correcting for systematic effects, we fitted the light curves using the Markov Chain Monte Carlo technique. Combining these new data with the EPOXI, Hubble Space Telescope, and ground-based V, I, H, and Ks published observations, the range 0.5–10 μm can be covered. Due to the low level of activity of GJ436, the effect of starspots on the combination of transits at different epochs is negligible at the accuracy of the data set. Representative climate models were calculated by using a three-dimensional, pseudospectral general circulation model with idealized thermal forcing. Simulated transit spectra of GJ436b were generated using line-by-line radiative transfer models including the opacities of the molecular species expected to be present in such a planetary atmosphere. A new, ab-initio-calculated, line list for hot ammonia has been used for the first time. The photometric data observed at multiple wavelengths can be interpreted with methane being the dominant absorption after molecular hydrogen, possibly with minor contributions from ammonia, water, and other molecules. No clear evidence of carbon monoxide and carbon dioxide is found from transit photometry. We discuss this result in the light of a recent paper where photochemical disequilibrium is hypothesized to interpret secondary transit photometric data. We show that the emission photometric data are not incompatible with the presence of abundant methane, but further spectroscopic data are desirable to confirm this scenario.

Modelling composition changes in F-layer storms

A coupled thermosphere-ionosphere-plasmasphere model CTIP is used to simulate storm changes in the ionosphere. The simulations cover a period of 72 hours, starting with imposed high-latitude energy inputs (particle precipitation and electric fields) that represent a moderately severe geomagnetic storm (Kp 5) lasting for 12 h. Equinox and solstice conditions are studied. We give particular attention to comparing changes in peak electron density, NmF2, to those of the [ON2] concentration ratio of the neutral air. During the first few hours of the storm, large perturbations are produced by strong meridional winds. After that initial phase, we find that the changes of NmF2 and of [ON2] ratio correspond closely, the composition changes being produced by the thermospheric “storm circulation”, as in the “composition bulge” theory of Fuller-Rowell el al. (1994). The simulations reproduce the general form of the seasonal variations in the changes of NmF2 at mid-latitudes as derived from worldwide ionosonde data. Some storm effects at sub-auroral latitudes are caused by movement and infilling of the ionospheric trough. We conclude that the composition change theory accounts for the major features of F-layer storm behaviour at midlatitudes.

Understanding space weather to shield society : A global road map for 2015-2025 commissioned by COSPAR and ILWS

There is a growing appreciation that the environmental conditions that we call space weather impact the technological infrastructure that powers the coupled economies around the world. With that co ...

A stability limit for the atmospheres of giant extrasolar planets

Recent observations of the planet HD209458b indicate that it is surrounded by an expanded atmosphere of atomic hydrogen that is escaping hydrodynamically. Theoretically, it has been shown that such escape is possible at least inside an orbit of 0.1 au (refs 4 and 5), and also that H3+ ions play a crucial role in cooling the upper atmosphere. Jupiter’s atmosphere is stable, so somewhere between 5 and 0.1 au there must be a crossover between stability and instability. Here we show that there is a sharp breakdown in atmospheric stability between 0.14 and 0.16 au for a Jupiter-like planet orbiting a solar-type star. These results are in contrast to earlier modelling that implied much higher thermospheric temperatures and more significant evaporation farther from the star. (We use a three-dimensional, time-dependent coupled thermosphere–ionosphere model and properly include cooling by H3+ ions, allowing us to model globally the redistribution of heat and changes in molecular composition.) Between 0.2 and 0.16 au cooling by H3+ ions balances heating by the star, but inside 0.16 au molecular hydrogen dissociates thermally, suppressing the formation of H3+ and effectively shutting down that mode of cooling.

The thermosphere of Titan simulated by a global three-dimensional time-dependent model

We present three-dimensional numerical simulations for dynamics and energetics of Titans thermosphere. In so doing, we distinguish between the dynamics driven by solar insolation and those driven by vertical coupling to winds in Titans middle atmosphere. Our calculations reveal that the solar-driven thermospheric dynamics are characterized by the balance between pressure gradients and viscosity, while the super-rotating zonal winds detected in Titans stratosphere set up a balance between the pressure gradients, curvature and Coriolis forces. The day to night temperature gradients in the upper thermosphere (around 1300 km) typically lie around 20 (10) K for solar maximum (minimum), with peak solar-driven winds of around 60 (30) m/s. This difference decreases with height and virtually disappears below 1000 km as a result of dayside adiabatic cooling and nightside adiabatic heating. The model highlights unique features about the thermosphere on Titan, such as the important nighttime heating from mid-latitudes to high-latitudes caused by the relatively small size of the planets shadow, leading to features in the wind profiles which are not found on Earth. Although the lack of measurement constraints prevents us from making predictions of actual wind profiles on Titan, the model does illustrate the physical processes driving the dynamics and suggests that anticipated thermospheric measurements from the Cassini spacecraft may provide constraints also for the dynamics at lower altitudes.

An unexpected cooling effect in Saturn's upper atmosphere.

The upper atmospheres of the four Solar System giant planets exhibit high temperatures that cannot be explained by the absorption of sunlight. In the case of Saturn the temperatures predicted by models of solar heating are ∼200 K, compared to temperatures of ∼400 K observed independently in the polar regions and at 30° latitude. This unexplained ‘energy crisis’ represents a major gap in our understanding of these planets’ atmospheres. An important candidate for the source of the missing energy is the magnetosphere, which injects energy mostly in the polar regions of the planet. This polar energy input is believed to be sufficient to explain the observed temperatures, provided that it is efficiently redistributed globally by winds, a process that is not well understood. Here we show, using a numerical model, that the net effect of the winds driven by the polar energy inputs is not to heat but to cool the low-latitude thermosphere. This surprising result allows us to rule out known polar energy inputs as the solution to the energy crisis at Saturn. There is either an unknown—and large—source of polar energy, or, more probably, some other process heats low latitudes directly.

the driver of giant planet atmospheres

We present a review of recent developments in the use of molecular ion as a probe of physics and chemistry of the upper atmospheres of giant planets. This ion is shown to be a good tracer of energy inputs into Jupiter (J), Saturn (S) and Uranus (U). It also acts as a ‘thermostat’, offsetting increases in the energy inputs owing to particle precipitation via cooling to space (J and U). Computer models have established that is also the main contributor to ionospheric conductivity. The coupling of electric and magnetic fields in the auroral polar regions leads to ion winds, which, in turn, drive neutral circulation systems (J and S). These latter two effects, dependent on , also result in very large heating terms, approximately 5×1012 W for Saturn and greater than 1014 W for Jupiter, planet-wide; these terms compare with approximately 2.5×1011 W of solar extreme UV absorbed at Saturn and 1012 W at Jupiter. Thus, is shown to play a major role in explaining why the temperatures of the giant planets are much greater (by hundreds of kelvin) at the top of the atmosphere than solar inputs alone can account for.

Tidal oscillations in the thermosphere: a theoretical investigation of their sources

Abstract Measurements of tidal oscillations in the mid to high latitude thermosphere reveal a dependency of diurnal and semidiurnal wind amplitudes on geomagnetic activity. This cannot be explained on the basis of the classical assumption that tides in the lower thermosphere originate primarily from below the mesopause. We use an updated version of the Coupled Thermosphere–Ionosphere–Plasmasphere model (CTIP) to numerically simulate the thermospheric wind oscillations, distinguishing between those propagating upwards through the mesopause and those generated in situ. These simulations suggest that in situ diurnal and semidiurnal oscillations generated at mid to high latitudes by ion-neutral interactions such as ion drag and Joule heating are comparable in magnitude and, towards higher latitudes, stronger than the upwards propagating tides with which they interact through both destructive and constructive interference. Due to their geomagnetic origin, the in-situ oscillations strongly depend on K p and thus cause an overall K p dependency in the observed diurnal and semidiurnal winds. We predict the occurrence of measurable in-situ tides also for mid-latitude sites with higher geomagnetic latitude, such as Millstone Hill, during geomagnetically disturbed conditions.