* Astronomy

Title: Why are the magnetic field directions measured by Voyager 1 on both sides of the heliopause so similar?
Author: Jolanta Grygorczuk, Andrzej Czechowski, Stanislaw Grzedzielski

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.

Read more (151kb, PDF)

Title: Propagation into the heliosheath of a large-scale solar wind disturbance bounded by a pair of shocks
Authors: E. Provornikova, M. Opher, V. Izmodenov, G. Toth

After the termination shock (TS) crossing, the Voyager 2 spacecraft has been observing strong variations of the magnetic field and solar wind parameters in the heliosheath. Anomalous cosmic rays, electrons, and galactic cosmic rays present strong intensity fluctuations. Several works suggested that the fluctuations might be attributed to spatial variations within the heliosheath. Additionally, the variability of the solar wind in this region is caused by different temporal events that occur near the Sun and propagate to the outer heliosphere. To understand the spatial and temporal effects in the heliosheath, it is important to study these effects separately. In this work we explore the role of shocks as one type of temporal effects in the dynamics of the heliosheath. Although currently plasma in the heliosheath is dominated by solar minima conditions, with increasing solar cycle shocks associated with transients will play an important role. We used a 3D MHD multi-fluid model of the interaction between the solar wind and the local interstellar medium to study the propagation of a pair of forward-reverse shocks in the supersonic solar wind, interaction with the TS, and propagation to the heliosheath. We found that in the supersonic solar wind the interaction region between the shocks expands, the shocks weaken and decelerate. The fluctuation amplitudes of the plasma parameters vary with heliocentric distance. The interaction of the pair of shocks with the TS creates a variety of new waves and discontinuities in the heliosheath, which produce a highly variable solar wind flow. The collision of the forward shock with the heliopause causes a reflection of fast magnetosonic waves inside the heliosheath.

Read more (1652kb, PDF)

 IBEX Reveals a Missing Boundary At the Edge Of the Solar System

For the last few decades, space scientists have generally accepted that the bubble of gas and magnetic fields generated by the sun - known as the heliosphere - moves through space, creating three distinct boundary layers that culminate in an outermost bow shock. This shock is similar to the sonic boom created ahead of a supersonic jet. Earth itself certainly has one of these bow shocks on the sunward side of its magnetic environment, as do most other planets and many stars. A collection of new data from NASA's Interstellar Boundary Explorer (IBEX), however, now indicate that the sun does not have a bow shock.
In a paper appearing online in Science Express on May 10, 2012, scientists compile data from IBEX, NASA's twin Voyager spacecraft, and computer models to show that the heliosphere just isn't moving fast enough to create a bow shock in the tenuous and highly magnetised region in our local part of the galaxy.
Read more

The edge of the solar system is filled with a turbulent sea of magnetic bubbles, astronomers said Thursday.
The finding changes ideas about the distant region and how the rest of the galaxy interacts with the solar system.
The two Voyager spacecraft, which have spent more than three decades travelling toward the outer boundaries of our solar system, found unexpected changes in the magnetic field that extends outward from the Sun. This discovery was made once they reached the heliosheath, as the outer part of the solar system is called.
Read more