M. Galand

Time variability and heterogeneity in the coma of 67P/Churyumov-Gerasimenko

Comets contain the best-preserved material from the beginning of our planetary system. Their nuclei and comae composition reveal clues about physical and chemical conditions during the early solar system when comets formed. ROSINA (Rosetta Orbiter Spectrometer for Ion and Neutral Analysis) onboard the Rosetta spacecraft has measured the coma composition of comet 67P/Churyumov-Gerasimenko with well-sampled time resolution per rotation. Measurements were made over many comet rotation periods and a wide range of latitudes. These measurements show large fluctuations in composition in a heterogeneous coma that has diurnal and possibly seasonal variations in the major outgassing species: water, carbon monoxide, and carbon dioxide. These results indicate a complex coma-nucleus relationship where seasonal variations may be driven by temperature differences just below the comet surface.

Ionospheric electrical conductances produced by auroral proton precipitation

Prom incoherent scatter radar observations and space-borne particle detector data, it appears that energetic proton precipitation can sometimes, for some locations, be a major source of ionization in the auroral ionosphere and contribute significantly to the electrical conductances. Here we propose a simple parameterization for the Pedersen and Hall conductances produced by proton precipitation. The derivation is based on a proton transport code for computing the electron production rate and on an effective recombination coefficient for deducing the electron density. The atmospheric neutral densities and temperatures and the geomagnetic-field strength are obtained from standard models. The incident protons are assumed to have a Maxwellian distribution in energy with a mean energy 〈E〉 in the 2–40 keV range and an energy flux Q0. The parameterized Pedersen and Hall conductances are functions of 〈E〉 and Q0, as well as of the geomagnetic-field strength. The dependence on these quantities is compared with those obtained for electron precipitation and for solar EUV radiation. To add the contribution of proton precipitation to the total conductances for electrodynamic studies in auroral regions, the conductances produced by electron and proton precipitations can be combined by applying a root-sum-square approximation.

Electron temperature of Titan's sunlit ionosphere

Titans upper atmosphere is ionized by solar radiation and particle bombardment from Saturns magnetosphere. The induced ionosphere plays a key role in the coupling of Titans atmosphere with the Kronian environment. It also provides unique signatures for identifying energy sources upon Titans upper atmosphere. Here we focus on observations from the first, close flyby by the Cassini spacecraft and assess the ionization and electron heating sources in Titans sunlit ionosphere. We compare CAPS electron spectra with spectra produced by an electron transport model based on the INMS neutral densities and a MHD interaction model. In addition, we compare RPWS electron temperature against the models. The important terms in the electron energy equation include loss through excitation of vibrational states of N-2 and CH4, Coulomb collisions with suprathermal electrons, and thermal conduction. Our analysis highlights the important role of the magnetic field line configuration for aeronomic studies at Titan.

Introduction to special section: Proton precipitation into the atmosphere

Energetic protons, precipitating into the atmosphere, interact with the ambient neutrals, leading to excitation, ionization, elastic scattering, and, for MeV protons, dissociation. In addition, a proton can capture an electron, producing an energetic H atom, which has enough energy to interact, in turn, with the ambient neutrals. This H atom can also get stripped of its electron, becoming a proton again. Because of these charge-changing reactions, the incident proton beam penetrating the atmosphere becomes a mixture of protons and H atoms [see, e.g., Rets, 1989].

Aerosol growth in Titan’s ionosphere

Photochemically produced aerosols are common among the atmospheres of our solar system and beyond. Observations and models have shown that photochemical aerosols have direct consequences on atmospheric properties as well as important astrobiological ramifications, but the mechanisms involved in their formation remain unclear. Here we show that the formation of aerosols in Titan’s upper atmosphere is directly related to ion processes, and we provide a complete interpretation of observed mass spectra by the Cassini instruments from small to large masses. Because all planetary atmospheres possess ionospheres, we anticipate that the mechanisms identified here will be efficient in other environments as well, modulated by the chemical complexity of each atmosphere.

Diurnal variations of Titan's ionosphere

of � 700 cm �3 below � 1300 km. Such a plateau is a combined result of significant depletion of light ions and modest depletion of heavy ones on Titan’s nightside. We propose that the distinctions between the diurnal variations of light and heavy ions are associated with their different chemical loss pathways, with the former primarily through ‘‘fast’’ ion-neutral chemistry and the latter through ‘‘slow’’ electron dissociative recombination. The strong correlation between the observed night-to-day ion density ratios and the associated ion lifetimes suggests a scenario in which the ions created on Titan’s dayside may survive well to the nightside. The observed asymmetry between the dawn and dusk ion density profiles also supports such an interpretation. We construct a time-dependent ion chemistry model to investigate the effect of ion survival associated with solid body rotation alone as well as superrotating horizontal winds. For long-lived ions, the predicted diurnal variations have similar general characteristics to those observed. However, for short-lived ions, the model densities on the nightside are significantly lower than the observed values. This implies that electron precipitation from Saturn’s magnetosphere may be an additional and important contributor to the densities of the short-lived ions observed on Titan’s nightside.

Response of the upper atmosphere to auroral protons

A three-dimensional, time-dependent, coupled model of the thermosphere and ionosphere has been used to assess the influence of proton auroral precipitation on Earths upper atmosphere. Statistical patterns of auroral electron and proton precipitation, derived from DMSP satellite observations, have been used to drive the model. Overall, electrons are the dominant particle energy source, with protons contributing ∼ 15% of the total energy. However, owing to the offset of the proton auroral oval toward dusk, in certain spatial regions protons can carry most of the energy. This is the case particularly at the equatorward edge of the dusk sector and at the poleward edge of the dawn sector of the auroral oval. The increase in Pedersen conductivity raises the average Joule heating by ∼ 10%, so raising the E and F region temperature by as much as 7%. The enhanced E region ionization also drives stronger neutral winds in the lower thermosphere through ion drag, which alters the temperature structure through transport, adiabatic heating, and adiabatic cooling. The neutral wind velocity modifications in the E region can reach 40% in some sectors. In addition, the upwelling of neutral gas raises the N 2 /O ratio, depleting the F region and so reducing the ion-drag driven winds in this region. This study illustrates the modest yet significant impact of auroral proton precipitation on the upper atmosphere.

Latitudinal variations in Saturn's ionosphere: Cassini measurements and model comparisons

[1] We present a study of latitudinal variations in Saturn’s ionosphere using Cassini Radio Science Subsystem (RSS) measurements and Saturn‐Thermosphere‐Ionosphere‐Model (STIM) simulations. On the basis of Cassini RSS observations, the peak electron density (NMAX) and the total electron content (TEC) both exhibit a clear increase with latitude, with a minimum at Saturn’s equator. When compared with these RSS trends, current model simulations overestimate NMAX and TEC at low latitudes and underestimate those parameters at middle and high latitudes. STIM is able to reproduce the RSS values for NMAX and TEC at low latitude when an additional low‐latitude loss process, such as a water influx, is introduced near Saturn’s equator. The lack of auroral precipitation processes in the model likely explains some model/data discrepancies at high latitude; however, most of the high‐latitude RSS data are from latitudes outside of Saturn’s typical main auroral oval. Using Cassini RSS electron density altitude profiles combined with ion density fractions and neutral background parameters calculated in STIM, we also present estimates of the latitudinal variations of Saturn’s Pedersen conductance, SP. We find SP to be driven by ion densities in Saturn’s lower ionosphere and to exhibit a latitudinal trend with a peak at mid‐latitude. Model calculations are able to reproduce low‐latitude conductances when an additional loss process is introduced, as before, but consistently underestimate most of the mid‐ and high‐latitude conductances derived from Cassini observations, perhaps indicating a missing ionization source within the model.

Energy Deposition in Planetary Atmospheres by Charged Particles and Solar Photons

We discuss here the energy deposition of solar FUV, EUV and X-ray photons, energetic auroral particles, and pickup ions. Photons and the photoelectrons that they produce may interact with thermospheric neutral species producing dissociation, ionization, excitation, and heating. The interaction of X-rays or keV electrons with atmospheric neutrals may produce core-ionized species, which may decay by the production of characteristic X-rays or Auger electrons. Energetic particles may precipitate into the atmosphere, and their collisions with atmospheric particles also produce ionization, excitation, and heating, and auroral emissions. Auroral energetic particles, like photoelectrons, interact with the atmospheric species through discrete collisions that produce ionization, excitation, and heating of the ambient electron population. Auroral particles are, however, not restricted to the sunlit regions. They originate outside the atmosphere and are more energetic than photoelectrons, especially at magnetized planets. The spectroscopic analysis of auroral emissions is discussed here, along with its relevance to precipitating particle diagnostics. Atmospheres can also be modified by the energy deposited by the incident pickup ions with energies of eV’s to MeV’s; these particles may be of solar wind origin, or from a magnetospheric plasma. When the modeling of the energy deposition of the plasma is calculated, the subsequent modeling of the atmospheric processes, such as chemistry, emission, and the fate of hot recoil particles produced is roughly independent of the exciting radiation. However, calculating the spatial distribution of the energy deposition versus depth into the atmosphere produced by an incident plasma is much more complex than is the calculation of the solar excitation profile. Here, the nature of the energy deposition processes by the incident plasma are described as is the fate of the hot recoil particles produced by exothermic chemistry and by knock-on collisions by the incident ions.

Spectral morphology of the X-ray emission from Jupiter's aurorae

Simultaneous Chandra X-ray and Hubble Space Telescope FUV observations of Jupiters aurorae carried out in February 2003 have been re-examined to investigate the spatial morphology of the X-ray events in different energy bands. The data clearly show that in the Northern auroral region (in the main auroral oval and the polar cap) events with energy > 2 keV are located at the periphery of those with energy 2 keV events (similar to 45 MW emitted power) with the electron bremsstrahlung component recently revealed by XMM-Newton in the spectra of Jupiters aurorae, and the 2 keV X-ray and FUV (340 GW) powers measured during the observations shows that they are broadly consistent with the predicted emissions from a population of energetic electrons precipitating in the planets atmosphere, thus supporting our interpretation.