Stanislaw Grzedzielski

The influence of the anomalous cosmic-ray component on the dynamics of the solar wind

It is well known that both the galactic and anomalous cosmic rays show positive intensity gradients in the outer heliosphere which are connected with corresponding pressure gradients. Due to an efficient dynamical coupling between the solar wind plasma and these highly energetic media by means of convected MHD turbulences, there exists a mutual interaction between these media. As one consequence of this scenario the enforced pressure gradients influence the distant solar wind expansion. Here we concentrate in our theoretical study on the interaction of the solar wind only with the anomalous cosmic-ray component. We use the standard two-fluid model in which the cosmic-ray fluid modifies the solar wind flow via the cosmic-ray pressure gradient. Then we derive numerical solutions in the following steps: first we calculate an aspherical pressure distribution for the anomalous cosmic rays, describing their diffusion in an unperturbed radial solar wind. Second, we then consider the perturbation of the solar wind flow due to these induced anomalous cosmic-ray pressure gradients. Within this context we especially take account of the action of a non-spherical geometry of the heliospheric shock which may lead to pronounced upwinddownwind asymmetries in the pressures and thereby in the resulting solar wind flows. As we can show in our model, which fits the available observational data, radial decelerations of the distant solar wind by between 5 to 11% are to be expected, however, the deviations of the bulk solar wind flow from the radialdirections are only slightly pronounced.

Why are the Magnetic Field Directions Measured by Voyager 1 on Both Sides of the Heliopause so Similar

The solar wind carves in the interstellar plasma a cavity bounded by a surface, called the heliopause (HP), that separates the plasma and magnetic field of solar origin from the interstellar ones. It is now generally accepted that in August 2012 Voyager 1 (V1) crossed that boundary. Unexpectedly, the magnetic fields on both its sides, although theoretically independent of each other, were found to be similar in direction. This delayed the identification of the boundary as the heliopause and led to many alternative explanations. Here we show that the Voyager 1 observations can be readily explained and, after the Interstellar Boundary Explorer (IBEX) discovery of the ribbon, could even have been predicted. Our explanation relies on the fact that the Voyager 1 and the undisturbed interstellar field directions (which we assume to be given by the IBEX ribbon center (RC)) share the same heliolatitude (~34.5 degrees) and are not far separated in longitude (difference ~27 degrees). Our result confirms that Voyager 1 has indeed crossed the heliopause and offers the first independent confirmation that the IBEX ribbon center is in fact the direction of the undisturbed interstellar magnetic field. For Voyager 2 we predict that the difference between the inner and the outer magnetic field directions at the heliopause will be significantly larger than the one observed by Voyager 1 (~30 degrees, instead of ~20 degrees), and that the outer field direction will be close to the RC.

Can a charge-exchange induced density rise at the heliopause explain the frequency drift of the 3 kHz Voyager signal?

A physical model for the recently advanced interpretation of the 3 kHz data [Gurnett, Kurth et al., 1993] is proposed. Basing on the results of the flow dynamical model of the heliosheath [Czechowski and Grzedzielski, 1993, 1994] we conclude that the post-shock solar wind (SW) plasma, cooled down by charge-exchange with neutral hydrogen of LISM origin, should form a high density layer on the inner side of the surface of the heliopause. The model allows us to estimate the plasma density profile and the size of the layer. These we use as input for MHD calculation of (1) a transient shock moving through this layer towards the heliopause and (2) of resulting 2ωp (frequency) drift of VLF emissions generated at the shock. For the heliopause distance L= 150-180 A.U. our model reproduces both the time duration of the emissions and their typical frequency drifts.

THREE-DIMENSIONAL STRUCTURING OF THE DISTANT SOLAR WIND BY ANOMALOUS COSMIC RAY PARTICLES

It is well known that both the galactic and the anomalous cosmic rays show positive intensity gradients in the outer heliosphere which are connected with corresponding antiradial pressure gradients. Due to an efficient dynamical coupling between the solar wind plasma and these highly energetic media, by means of a scattering at convected MHD wave turbulences, the latter diffuse through the low energetic solar wind plasma flow, and thus there exists a mutual dynamic interaction of these media. As a prime consequence of this scenario, the diffusion-induced pressure gradients in the cosmic ray distributions influence the distant solar wind expansion. In this paper we concentrate on the interaction of the solar wind with the anomalous cosmic ray component giving a consistent formulation of the system of mutually interacting media. Then we derive numerical solutions in the following steps: First we calculate an aspherical pressure distribution for the anomalous cosmic rays describing their diffusion in an unperturbed radial solar wind. Second we consider the perturbation of the solar wind flow due to these induced anomalous cosmic ray pressure gradients. Within this context we especially take account of the action of an aspherical geometry of the heliospheric shock which may lead to a pronounced upwind/downwind asymmetry in the pressure distribution, and thereby in the resulting solar wind flows. As we can show decelerations of the distant solar wind by between 5 to 11 percent are to be expected, however, deviations of the bulk solar wind flow from the radial directions are only weakly pronounced.