H. Fichtner

A self‐consistent determination of the heliospheric termination shock structure in the presence of pickup, anomalous, and galactic cosmic ray protons

A self-consistent time-dependent model is used to study the modification of the heliospheric termination shock in the upwind direction under the influence of three suprathermal particle populations, namely pickup, anomalous and galactic cosmic ray protons. By ensuring that the resulting modulated cosmic ray proton spectra are consistent with those observed by the Voyager 2 and Pioneer 10 spacecraft during the solar activity minimum in 1987, two alternative modifications of the shock can be identified. While the first is characterized by a low injection efficiency of pickup protons into the process of diffusive shock acceleration and mainly determined by those particles, the second, resulting from a high injection efficiency, is clearly dominated by anomalous cosmic rays.

The Influence of Pickup, Anomalous, and Galactic Cosmic-Ray Protons on the Structure of the Heliospheric Shock: A Self-Consistent Approach

A self-consistent time-dependent model is used to study the modification of the heliospheric shock in the upwind direction under the influence of three suprathermal particle populations, namely, pickup, anomalous, and Galactic cosmic-ray protons. By ensuring that the resulting modulated cosmic-ray proton spectra are consistent with those observed by the Voyager and Pioneer spacecraft during the solar activity minimum in 1987, two alternative modifications of the heliospheric shock can be identified. While the first is characterized by a low injection efficiency of pickup protons into the process of diffusive shock acceleration and is mainly determined by those particles, the second, resulting from a high injection efficiency, is clearly dominated by anomalous cosmic rays.

Self‐consistent acceleration of multiply reflected pickup ions at a quasi‐perpendicular solar wind termination shock: a fluid approach

The purpose of this paper is to report about a self-consistent study of acceleration of multiply reflected pickup protons at a quasi-perpendicular solar wind termination shock and the influence of these protons on the shocks structure. A time-dependent formulation is developed by treating the solar wind and low-energy pickup protons as one fluid under the influence of the accelerated pickup protons as a second fluid. The results suggest that the termination shock would experience significant mediation by such accelerated pickup protons if the width of the termination shocks (sub)ramp would be smaller than four times the thermal electron inertial length Le. There are examples in observations of the Earths bow shock with a width close to such values. For a shock width of 2Le and a multiply reflected ion (MRI) distribution function fMRI ∝ v−a with a = 4, the shock compression ratio is reduced to ∼2.4 compared with the initial value of ∼3.1 and the fraction of reflected pickup protons is ∼11%. For this width the accelerated pickup protons reach a maximum energy of up to ∼ 51 keV, if a e [3,4]. Such extended MRI spectra for pickup protons, if realistic, might enable a direct injection of these particles into the process of diffusive shock acceleration at the termination shock. Therefore the model also provides a theoretical basis for a study of the formation of anomalous cosmic rays.

Longitudinal gradients of the distribution of anomalous cosmic rays in the outer heliosphere

Recent results, both from observation and theory, suggest that the termination shock of the solar wind is not spherical. First, observations made with the Ulysses spacecraft have confirmed that the solar wind momentum flux increases with latitude, implying an elongation of the shock above the suns poles. Second, results of modeling the global structure of the heliosphere indicate that a so-called upwind-downwind asymmetry of the shock is probable. In view of these findings as well as the observed upwind-downwind asymmetry in pick-up ion fluxes, one might suspect the existence of longitudinal gradients in the distributions of anomalous cosmic rays, which are supposedly identical with the pick-up ions accelerated at the termination shock. We demonstrate that such longitudinal gradients should indeed be expected in the outer heliosphere and estimate their magnitude using a three-dimensional modulation model.

The acceleration time of anomalous cosmic rays: Observational constraints from Pioneer 10 data

Crosscorrelations of solar Lyman α and cosmic ray data from 1981 until 1990 recorded by instruments on Pioneer 10 during its interplanetary cruise are presented. Using the Lyman α radiation as a proxy for the long-term solar activity, a comparison of the correlation functions of the anomalous and galactic cosmic ray data reveals a significantly greater phase shift in the time variation of the former. This additional time lag is increasing with increasing particle energy and is attributed to the time required to energize pickup ions at the heliospheric termination shock and thus allows a determination of the characteristic acceleration time of anomalous cosmic rays. This, in turn, results in an estimate of the efficiency of spatial diffusion of energetic particles perpendicular to the magnetic field just upstream of the heliospheric termination shock at low heliographic latitudes.

THE INFLUENCE OF THE LOCAL INTERSTELLAR MEDIUM ON THE SOLAR WIND DYNAMICS IN THE INNER HELIOSPHERE

The consequences of the interaction between the solar wind and the local interstellar medium for the wind region enclosed by the heliospheric shock are reviewed. After identifying the principal mechanisms to influence the dynamics of the solar wind, an approach allowing the simultaneous incorporation of neutral atoms, pick-up ions, cosmic rays and energetic electrons into a multifluid model of the expanding wind plasma is outlined. The effects of these particle species are discussed in detail, with special emphasis on the electron component which behaves more like a quasi-static hot gas rather than an expanding fluid. This electron gas is effectively trapped within a three-dimensional trough of a circumsolar electric potential whose outer fringes are possibly determined by the density distribution of anomalous cosmic rays. The electrons are proven to be a globally structered component of great importance for the solar wind momentum flow contributing to a triggering of the solar wind dynamics by asymmetric interstellar boundary conditions. Finally, the consequences for the relative motion of the Sun and the local interstellar medium as well as for the solar system as a whole are described.