[1] We have developed a numerical model that solves the time-dependent, onedimensional, coupled continuity and momentum equations for an arbitrary number of charged and neutral particle species. The model includes production and loss of particles due to ionization, recombination, and attachment of ions and electrons by heavy aerosol particles, and transport due to gravity and multipolar diffusion. The model is used to study the response of the mesopause plasma to small-scale, aerosol particle density perturbations. We find that for aerosol structures on the order of a few meters, electron attachment and ambipolar diffusion are the dominant processes, leading to small-scale electron perturbations that can cause polar mesosphere summer echoes (PMSEs). Moreover, for small aerosol particles, with radii on the order of 10 nm or less, ambipolar diffusion leads to an anticorrelation between electron and ion densities, which is in agreement with most rocket observations. These small-scale structures persist as long as the aerosol layer persists, which will be limited by aerosol particle diffusion. For 10-nm particles, this diffusive lifetime will be on the order of hours. The few instances where rocket observations find instead a correlation between electron and ion densities can be explained either by the aerosol particles becoming large, on the order of 50 nm or more, in which case ion attachment becomes important, or by rapid evaporation of aerosol particles. In the latter case, evaporation must be sufficiently fast to overcome ambipolar diffusion. Citation: Lie-Svendsen, Ø., T. A. Blix, U.-P. Hoppe, and E. V. Thrane, Modeling the plasma response to small-scale aerosol particle perturbations in the mesopause region,

Abstract. We study the evolution of the electron velocity distribution function in high-speed solar wind streams from the collision-dominated corona and into the collisionless interplanetary space. The model we employ solves the kinetic transport equation with the Fokker-Planck collision operator to describe Coulomb collisions between electrons. We use a test particle approach, where test electrons are injected into a prescribed solar wind background. The density, temperature, and electric field associated with the background are computed from fluid models. The test electrons are in thermal equilibrium with the background at the base of the corona, and we study the evolution of the velocity distribution of the test electrons as a function of altitude. We find that velocity filtration, due to the energy dependence of the Coulomb cross section, is a small effect and is not capable of producing significant beams in the distribution or a temperature moment that increases with altitude. The distribution function is mainly determined by the electric field and the expanding geometry and consists of a population with an almost isotropic core which is bound in the electrostatic potential and a beam-like high-energy tail which escapes. The trapped electrons contribute significantly to the even moments of the distribution function but almost nothing to the odd moments; the drift speed and energy flux moments are carried solely by the tail. In order to describe the high-speed solar wind observed near 0.3 AU by the Helios spacecraft, we use a multifluid model where ions are heated preferentially. The resulting test electron distribution at 0.3 AU, in this background, is in very good agreement with the velocity distributions observed by the Helios spacecraft.

The paper reviews the main advantages and limitations of the kinetic exospheric and fluid models of the solar wind (SW). The general theoretical background is outlined: the Boltzmann and Fokker-Planck equations, the Liouville and Vlasov equations, the plasma transport equations derived from an ''equation of change''. The paper provides a brief history of the solar wind modeling. It discusses the hydrostatic model imagined by Chapman, the first supersonic hydrodynamic models published by Parker and the first generation subsonic kinetic model proposed by Chamberlain. It is shown that a correct estimation of the electric field, as in the second generation kinetic exospheric models developed by Lemaire and Scherer, provides a supersonic expansion of the corona, reconciling the hydrodynamic and the kinetic approach. The modern developments are also reviewed emphasizing the characteristics of several generations of kinetic exospheric and multi-fluid models. The third generation kinetic exospheric models consider kappa velocity distribution function (VDF) instead of a Maxwellian at the exobase and in addition they treat a non-monotonic variation of the electric potential with the radial distance; the fourth generation exospheric models include Coulomb collisions based on the Fokker-Planck collision term. Multi-fluid models of the solar wind provide a coarse grained description of the system and reproduce with success the spatio-temporal variation of SW macroscopic properties (density, bulk velocity). The main categories of multi-fluid SW models are reviewed: the 5-moment, or Euler, models, originally proposed by Parker to describe the supersonic SW expansion; the 8-moment and 16-moment fluid models, the gyrotropic 123Surv ) 32:1-70 DOI 10.1007 approach with improved collision terms as well as the gyrotropic models based on observed VDFs. The outstanding problem of collisions, including the long range Coulomb encounters, is also discussed, both in the kinetic and multi-fluid context. Although for decades the two approaches have been seen as opposed, in this paper we emphasize their complementarity. The review of the kinetic and fluid models of the solar wind contributes also to a better evaluation of the open questions still existent in SW modeling and suggests possible future developments.

We present a solar wind fluid model extending from the chromosphere to Earth. The model is based on the gyrotropic approximation to the 16‐moment set of transport equations, in which we solve for the density, drift speed, temperature parallel and perpendicular to the magnetic field, and transport of parallel and perpendicular thermal energy along the magnetic field (heat flux). The solar wind plasma is created dynamically through (photo) ionization in the chromosphere, and the plasma density in the transition region and corona is computed dynamically, dependent on the type of coronal heating applied, rather than being set arbitrarily. The model improves the description of proton energy transport in the transition region, where classical heat conduction is only retrieved in the collision‐dominated limit. This model can serve as a “test bed” for any coronal heating mechanism. We consider heating of protons by a turbulent cascade of Alfvén waves in rapidly expanding coronal holes. The resulting high coronal proton temperatures lead to a downward proton energy flux from the corona which is much smaller than what classical transport theory predicts, causing a very low coronal density and an extremely fast solar wind with a small mass flux. Only when some of the wave energy is forcibly deposited in the lower transition region can a realistic solar wind be obtained. Because of the poor proton heat transport, in order to produce a realistic solar wind any viable heating mechanism must deposit some energy in the transition region, either directly or via explicit heating of coronal electrons.