H. Schmidt

Cometary gas and plasma flow with detailed chemistry

Abstract This paper describes recent hydrodynamic and magnetohydrodynamic (MHD) simulations for the gas and plasma flow around a comet with detailed photo and chemical reaction network of 59 neutral and 76 ionized chemical species. The method allows for a separate energy balance of the electrons, separate flow of neutral gas, fast neutral atomic and molecular hydrogen, and plasma with momentum exchange by elastic collisions, mass-loading of the plasma flow by ion pick-up, and Lorentz-forces of the advected magnetic field. The contact surface or magnetopause is resolved. The simulation is applied to comet Halley at the time of the Giotto encounter. The results can be compared with the data from the Giotto mission as they become available. The chemical composition of the nucleus can be inferred from these data by iteration.

Effect of biogenic volatile organic compound emissions on tropospheric chemistry during the Atmospheric Pollution Over the Paris Area (ESQUIF) campaign in the Ile-de-France region

[1]xa0This paper investigates the impact of biogenic isoprene and terpene emissions on photochemical species levels in the French Ile-de-France region during several photooxidant pollution episodes in summer 1998 and 1999 during the Atmospheric Pollution Over the Paris Area (ESQUIF) project. The effect of biogenic emissions on both ozone produced on a continental scale and advected in Ile-de-France and on ozone locally formed are assessed. For this purpose, simulations with and without biogenic emissions are performed with a nested version of the CHIMERE model. This chemistry transport model includes both a continental (western European) domain with 0.5° horizontal resolution and a regional domain (Ile-de-France) of 150 × 150 km extension with a horizontal resolution of 6 km. An emissions database for biogenic isoprene and terpene emissions from forests has been set up. These emissions are estimated using emission factors for different tree species recently revised by Simpson et al. [1999] and for different land use data sets, including highly resolved (1 km) satellite measurements for Ile-de-France. Good agreement has been found between modeled and measured (by aircraft) isoprene levels (overall bias <10%), which lends confidence to the use of the emissions database for subsequent simulations. The comparison between runs with and without biogenic volatile organic compound (VOC) emissions indicates a significant difference in ozone in Ile-de-France, up to 40 ppb for one extreme day. Biogenic VOC emissions from Ile-de-France along with those from outside the Ile-de-France region have an approximately equal responsibility for the additional ozone buildup. The main reason for this increased ozone formation is the enhancement of radicals due to larger concentrations of carbonyl species and ozone and their subsequent photolysis. Biogenic emissions lead to a shift in the sensitivity to emissions (toward more “NOx-limited”). However, generally, either with or without biogenic emissions, a “VOC-limited” regime is simulated over the Ile-de-France region.

A Dynamical Model for the Penumbral Fine Structure and the Evershed Effect in Sunspots

Relying on the assumption that the interchange convection of magnetic flux tubes is the physical cause for the existence of sunspot penumbrae, we propose a model in which the dynamical evolution of a thin magnetic flux tube reproduces the Evershed effect and the penumbral fine structure such as bright and dark filaments and penumbral grains. According to our model, penumbral grains are the manifestation of the footpoints of magnetic flux tubes, along which hot subphotospheric plasma flows upward with a few km s-1. Above the photosphere the hot plasma inside the tube is cooled by radiative losses as it flows horizontally outward. As long as the flowing plasma is hotter than the surroundings, it constitutes a bright radial filament. The flow confined to a thin elevated channel reaches the temperature equilibrium with the surrounding atmosphere and becomes optically thin near the outer edge of the penumbra. Here the tube has a height of approximately 100 km above the continuum, and the flow velocity reaches up to 14 km s-1. Such a flow channel can reproduce the observed signatures of the Evershed effect.

Cometary MHD and chemistry

Our magneto-hydrodynamical (MHD) and chemical comet-coma model has been applied to describe and analyze the plasma flow, the magnetic field and the ion abundances in comet P/Halley in a consistent manner. We assume the volatile composition to consist of 80% water and 20% carbon-, nitrogen-, oxygen-, and sulfur-compounds. The radius of the nucleus is 3.36 km. With hemispherical illumination, this is equivalent to the active area on the nucleus during the Giotto encounter. The physics and chemistry of the coma are modeled in great detail, including photoprocesses, gas-phase chemical kinetics, energy balance with a separate electron temperature, multifluid hydrodynamics with a transition to free molecular flow, fast-streaming atomic and molecular hydrogen, counter and cross-streaming of the ionized species relative to the neutral species in the coma-solar wind interaction region with momentum exchange by elastic collisions, mass-loading through ion pick-up, and Lorentz-forces of the advected magnetic field. A comparison of the results is made with the data from the Giotto mission, especially from the HIS ion mass spectrometer. We resolve the contact surface (magnetopause); its position is in agreement with observations. We also find the enhancement of the ion density just outside of the contact surface and agreement for the three groups in the ion mass spectra, particularly for the first group up to 21 amu.

Solar cycle signal in a general circulation and chemistry model with internally generated quasi‐biennial oscillation

[1]xa0Simulations with the HAMMONIA general circulation and chemistry model are analyzed to improve the understanding of the atmospheric response to solar cycle variations and the role of the quasi-biennial oscillation of equatorial winds (QBO) for this response. The focus is on the Northern Hemisphere winter stratosphere. Owing to the internally produced QBO, albeit with a too short period of 24 months, the model is particularly suited for such an exercise. The simulation setup with only solar and QBO forcing allows an unambiguous attribution of the simulated signals. Two separate simulations have been performed for perpetual solar maximum and minimum conditions. The simulations confirm the plausibility of dynamical mechanisms, suggested earlier, that propagate the solar signal from the stratopause region downward to the troposphere. One feature involved in this propagation is a response maximum of temperature and ozone in the lower equatorial stratosphere. In our model, this maximum appears as a pure solar signal independent of the QBO and of other forcings. As observed, the simulated response of the stratospheric polar vortex to solar cycle forcing depends on the QBO phase. However, in the model this is statistically significant only in late winter. The simulation for early and mid winter suffers probably from a too strong internal variability of the polar vortex in early winter.

MHD-calculations for cometary plasmas

Abstract We present in this paper a relatively simple method for the numerical solution of the MHD-equations on a curvilinear grid. We consider the full 3D equations as well as a modified axisymmetric version. The difference scheme is based on the principle of forming backward differences along characteristics in the spatial variables. This method is used to calculate the interaction of the interplanetary magnetic field with the plasma flow around a comet. The MHD-equations are modified by source terms, which describe the transfer of mass, momentum and energy from a given background (the comet) to the plasma.

Wintertime water vapor in the polar upper mesosphere and lower thermosphere: First satellite observations by Odin submillimeter radiometer

In this paper we present Odin submillimeter radiometer (Odin/SMR) water vapor measurements in the upper mesosphere and lower thermosphere with focus on the polar latitudes in winter. Measurements since 2003 have been compiled to provide a first overview of the water vapor distribution in this altitude range. Our observations show a distinct seasonal increase of the water vapor concentration during winter at a given altitude above 90 km. Above 95 km the observations exhibit the annual water vapor maximum during wintertime. Model simulations from the Hamburg Model of the Neutral and Ionized Atmosphere (HAMMONIA) and the Whole Atmosphere Community Climate Model version 3 (WACCM3) show results that are very similar to the observations. We suggest that the observed increase in water vapor during winter is mainly caused by a combination of upwelling of moister air from lower altitudes and diffusion processes. Distinct interhemispheric differences in the winter water vapor distribution in the upper mesosphere and lower thermosphere can be observed, both in the observations and the model results. The seasonal water vapor increase in the polar regions is much more pronounced in the Southern Hemisphere winter where higher concentrations can be observed. This observation is most likely due to interhemispheric differences in the underlying dynamics and diffusion processes.

Results from a gyrotropic two‐ion fluid model for the comet interaction with the solar wind

A two-and-a-half-dimensional two-ion fluid model (with gyrotropic pressure tensor) for the interaction of cometary pickup plasma with the solar wind has been developed. Whereas previous models have, in general, assumed a one-fluid plasma, this generalized model follows the evolution of two fluids: solar wind protons and cometary ions. The two fluids are coupled by the magnetic field and by coupling terms which are the velocity moments of the time relaxation model of Bhatnagar et al. [1954]. Furthermore, a gyrotropic pressure tensor replaces the isotropic pressure assumed in other models. Isotropization terms couple the pressures in the directions perpendicular and parallel to the magnetic field. Among the results received, the standoff distance of the shock increased significantly when the isotropization rate was reduced. The large cometary plasma pressure forces at the shock accelerated the cometary plasma along the shock. As a result, the cometary plasma drifted ahead of the solar wind along the shock flanks parallel to the magnetic field. Close to the nucleus of the comet in the far tail lobes and in the far inner tail the cometary plasma lagged behind the solar wind.

A two-ion fluid model with gyrotropic pressures for the comet interaction with the solar wind

Abstract A two-ion fluid model (with gyrotropic pressure tensor) for the interaction of cometary pickup plasma with the solar wind has been developed. Whereas previous models have, in general, assumed a single fluid plasma, this generalized model follows the evolution of two fluids — solar wind protons and cometary ions. The two fluids are coupled by the magnetic field and by coupling terms which are the velocity moments of the time relaxation model of Bhatnagar-Gross-Krook. Furthermore, a gyrotropic pressure tensor replaces the isotropic pressure assumed in other models. Isotropization terms couple the pressures in the directions perpendicular and parallel to the magnetic field. A Godunov type upstream method which uses an approximate Riemann solver adapted to the model equations is presented.

Nighttime Mesospheric/Lower Thermospheric Tropical Ozone Response to the 27‐Day Solar Rotational Cycle: ENVISAT‐GOMOS Satellite Observations Versus HAMMONIA Idealized Chemistry‐Climate Model Simulations

Global Ozone Monitoring by Occultation of Stars (GOMOS) satellite data are analyzed to estimate the first observation‐based night‐time (22:00 median local time) ozone response to the 27‐day solar rotational cycle in the tropical mesosphere/lower thermosphere (50‐110 km altitude). The ozone response to solar rotational variability is derived from linear correlation and regressions using Lyman‐⍺ line (121.6 nm) as solar index which varies by about 10‐15% over solar rotational cycles. In the lower mesosphere (50‐70 km), the GOMOS ozone is found to be correlated with the solar fluctuations and exhibits a sensitivity of ~0.1 (expressed in % change of ozone for 1% change in Lyman‐⍺). In the upper mesosphere/lower thermosphere (above 80 km), ozone variations become anti‐correlated with solar rotational variations. In this region, the vertical profile of ozone sensitivity to the 27‐day solar cycle exhibits a maximum of 1.8 at 81 km, a minimum of 0.3 at 100 km and a sharp increase above. Such high ozone sensitivities are observed for the first time. The observed ozone response is compared with chemistry‐climate simulations from the Hamburg model of the neutral and ionized atmosphere (HAMMONIA) which is forced with an idealized 27‐day solar spectral irradiance time series. Although observational and model results share some common features, substantial discrepancies are found. Namely, the altitude of transition from positive to negative solar‐ozone correlation signal in the model simulation is found about 10 km below the altitude of the observations and the amplitude of the ozone sensitivity is generally vastly underestimated by the model.