Sergey Borovikov

Three-Dimensional Features of the Outer Heliosphere Due to Coupling Between the Interstellar and Interplanetary Magnetic Fields. III. The Effects of Solar Rotation and Activity Cycle

We investigate the effects of the 11 year solar cycle and 25 day rotation period of the Sun on the interaction of the solar wind (SW) with the local interstellar medium (LISM). Our models take into account the partially ionized character of the LISM and include momentum and energy transfer between the ionized and neutral components. We assume that the interstellar magnetic field vector belongs to the hydrogen deflection plane as discovered in the SOHO SWAN experiment. This plane is inclined at an angle of about 60° toward the ecliptic plane of the Sun, as suggested in recent publications relating the local interstellar cloud properties to the radio emission observed by Voyager 1. It is assumed that the latitudinal extent of the boundary between the slow and fast SW regions, as well as the angle between the Suns rotation and magnetic-dipole axes, are periodic functions of time, while the polarity of the interstellar magnetic field changes sign every 11 years at the solar maximum. The global variation of the SW-LISM interaction pattern, the excursions of the termination shock and the heliopause, and parameter distributions in certain directions are investigated. The analysis of the behavior of the wavy heliospheric current sheet in the supersonic SW region shows the importance of neutral atoms on its dynamics.

PLASMA NEAR THE HELIOSHEATH: OBSERVATIONS AND MODELING

Sound numerical modeling is capable of providing important predictive information about the solar wind interaction with the local interstellar medium. The results of our three-dimensional simulation show a good agreement with Voyager observations from 2007 to 2010. We analyze the termination shock properties at the Voyager crossing points and juxtapose them with the observed data. The heliospheric current sheet structure in the inner heliosheath is examined.

VOYAGER 1 NEAR THE HELIOPAUSE

Recent observations from the Voyager 1 spacecraft show that it is sampling the local interstellar medium (LISM). This is quite surprising because no realistic, steady-state model of the solar wind (SW) interaction with the LISM gives an inner heliosheath width as narrow as ~30 AU. This includes models that assume a strong redistribution of the ion energy to the tails in the pickup ion distribution function. We show that the heliopause (HP), which separates the SW from the LISM, is not a smooth tangential discontinuity, but rather a surface subject to Rayleigh-Taylor-type instabilities which can result in LISM material penetration deep inside the SW. We also show that the HP flanks are always subject to a Kelvin-Helmholtz instability. The instabilities are considerably suppressed near the HP nose by the heliospheric magnetic field in steady-state models, but reveal themselves in the presence of solar cycle effects. We argue that Voyager 1 may be in one such instability region and is therefore observing plasma densities much higher than those in the pristine SW. These results may explain the early penetration of Voyager 1 into the LISM. They also show that there is a possibility that the spacecraft may start sampling the SW again before it finally leaves the heliosphere.

Radial Velocity along the Voyager 1 Trajectory: The Effect of Solar Cycle

As Voyager 1 and Voyager 2 are approaching the heliopause (HP)—the boundary between the solar wind (SW) and the local interstellar medium (LISM)—we expect new, unknown features of the heliospheric interface to be revealed. A seeming puzzle reported recently by Krimigis et al. concerns the unusually low, even negative, radial velocity components derived from the energetic ion distribution. Steady-state plasma models of the inner heliosheath (IHS) show that the radial velocity should not be equal to zero even at the surface of the HP. Here we demonstrate that the velocity distributions observed by Voyager 1 are consistent with time-dependent simulations of the SW-LISM interaction. In this Letter, we analyze the results from a numerical model of the large-scale heliosphere that includes solar cycle effects. Our simulations show that prolonged periods of low to negative radial velocity can exist in the IHS at substantial distances from the HP. It is also shown that Voyager 1 was more likely to observe such regions than Voyager 2.

THE SOLAR WIND AS A POSSIBLE SOURCE OF FAST TEMPORAL VARIATIONS OF THE HELIOSPHERIC RIBBON

We present a possible source of pickup ions (PUIs) the ribbon observed by the Interstellar Boundary EXplorer (IBEX). We suggest that a gyrating solar wind and PUIs in the ramp and in the near downstream region of the termination shock (TS) could provide a significant source of energetic neutral atoms (ENAs) in the ribbon. A fraction of the solar wind and PUIs are reflected and energized during the first contact with the TS. Some of the solar wind may be reflected propagating toward the Sun but most of the solar wind ions form a gyrating beam-like distribution that persists until it is fully thermalized further downstream. Depending on the strength of the shock, these gyrating distributions can exist for many gyration periods until they are scattered/thermalized due to wave-particle interactions at the TS and downstream in the heliosheath. During this time, ENAs can be produced by charge exchange of interstellar neutral atoms with the gyrating ions. In order to determine the flux of energetic ions, we estimate the solar wind flux at the TS using pressure estimates inferred from in situ measurements. Assuming an average path length in the radial direction of the order of a few AU before the distribution of gyrating ions is thermalized, one can explain a significant fraction of the intensity of ENAs in the ribbon observed by IBEX. With a localized source and such a short integration path, this model would also allow fast time variations of the ENA flux.

SOLAR ROTATION EFFECTS ON THE HELIOSHEATH FLOW NEAR SOLAR MINIMA

The interaction between fast and slow solar wind (SW) due to the Suns rotation creates corotating interaction regions (CIRs), which further interact with each other creating complex plasma structures at large heliospheric distances. We investigate the global influence of CIRs on the SW flow in the inner heliosheath between the heliospheric termination shock (TS) and the heliopause. The stream interaction model takes into account the major global effects due to slow-fast stream interaction near solar minima. The fast and slow wind parameters are derived from the Ulysses observations. We investigate the penetration of corotating structures through the TS and their further propagation through the heliosheath. It is shown that the heliosheath flow structure may experience substantial modifications, including local decreases in the radial velocity component observed by Voyager 1.

MHD heliosphere with boundary conditions from a tomographic reconstruction using interplanetary scintillation data

Observations of interplanetary scintillation (IPS) provide a set of data that is used in estimating the solar wind parameters with reasonably good accuracy. Various tomography techniques have been developed to deconvolve the line-of-sight integration effects ingrained in observations of IPS to improve the accuracy of solar wind reconstructions. Among those, the time-dependent tomography developed at the University of California, San Diego (UCSD) is well known for its remarkable accuracy in reproducing the solar wind speed and density at Earth by iteratively fitting a kinematic solar wind model to observations of IPS and near-Earth spacecraft measurements. However, the kinematic model gradually breaks down as the distance from the Sun increases beyond the orbit of Earth. Therefore, it would be appropriate to use a more sophisticated model, such as a magnetohydrodynamics (MHD) model, to extend the kinematic solar wind reconstruction beyond the Earths orbit and to the outer heliosphere. To test the suitability of this approach, we use boundary conditions provided by the UCSD time-dependent tomography to propagate the solar wind outward in a MHD model and compare the simulation results with in situ measurements and also with the corresponding kinematic solution. Interestingly, we find notable differences in proton radial velocity and number density at Earth and various locations in the inner heliosphere between the MHD results and both the in situ data and the kinematic solution. For example, at 1 AU, the MHD velocities are generally larger than the spacecraft data by up to 150 km s−1, and the amplitude of density fluctuations is also markedly larger in the MHD solution. We show that the MHD model can deliver more reasonable results at Earth with an ad hoc adjustment of the inner boundary values. However, we conclude that the MHD model using the inner boundary conditions derived from kinematic simulations has little chance to match IPS and in situ data as well as the kinematic model does unless it too is iteratively fit to the observational data and measurements.

Transient Phenomena in the Distant Solar Wind and in the Heliosheath

The difference in the heliocentric distances at which the Voyager 1 and Voyager 2 spacecraft crossed the termination shock (TS) is likely due to a combination of the heliospheric asymmetry caused by the interstellar magnetic field (ISMF) and time‐dependent boundary conditions at the Sun. We analyze both effects numerically. It is shown that an increase in the ISMF makes the termination shock more asymmetric, but results in an increasingly large discrepancy between the calculated and observed distributions of the solar wind (SW) velocity in the inner heliosheath. We analyze possible effects of the solar cycle on the SW properties in the inner heliosheath. The possibility is explored of the SW velocity to have a rather small gradient as V2 penetrates deeper into the heliosheath. To analyze the effect of the solar cycle we also perform the calculations of the SW—interstellar medium interaction with the inner boundary conditions extracted from the Ulysses data obtained during its first and third pole‐to‐pole ...

Relating IBEX and Voyager Data through Global Modeling of the Heliospheric Interface

The combination of the Interstellar Boundary Explorer (IBEX) all‐sky maps of the energetic neutral atom fluxes with the Voyager in situ measurements gives us a unique opportunity to learn more about the physics governing the solar wind (SW) interaction with the local interstellar medium (LISM). Moreover, since the position of the ribbon of an enhanced ENA flux in the sky strongly depends on the LISM properties, we are able to constrain those by comparing numerical simulations with the IBEX observations. In this paper, we discuss the current status of the Huntsville model of the SW‐LISM interaction, compare numerical results with the IBEX and Voyager observations, and discuss the importance of taking into account time‐dependent phenomena, particularly the solar cycle effects.

An Approach for Delaunay Tetrahedralization of Bodies with Curved Boundaries

Problem of tetrahedral meshing of three-dimensional domains whose boundaries are curved surfaces is wide open. Traditional approach consists in an approximation of curved boundaries by piecewise linear boundaries before mesh generation. As the result mesh quality may deteriorate. This paper presents a technique for Delaunay-based tetrahedralization in which a set of constrained facets is formed dynamically during face recovery and mechanisms for mutual retriangulation of the curved faces and the tetrahedralization are suggested. The proposed algorithm is constructed in such a way that a facet that was once added in the set of constrained facets is never split into small triangles. It allows retaining the high quality of surface mesh in the tetrahedralization, because during boundary recovery the surface mesh on the curved faces and the tetrahedralization are refined conjointly.