Determination of the orientation of the local interstellar magnetic field at the outer edge of the solar system.

A new study by a European team, led by a scientist from IAP (CNRS, University Paris-6 Pierre & Marie Curie), combines three-dimensional numerical simulations with Voyager spacecrafts observations to clarify plasma and magnetic field conditions at our solar system’s outermost frontiers.

The local interstellar cloud
Our Solar System entered an interstellar cloud 10,000 years ago. Today it is speeding through this nebulosity at Mach 2 behind a supersonic shock wave in much the same way that a Concorde crosses the Atlantic at supersonic speed. Since its formation 4.6 billion years ago, our Solar System has encountered numerous interstellar clouds, knots, filaments, shells, and bubbles of different sizes and contents on its path through the Milky Way. For more than 80 years astronomers have been attracted by these past and future encounters and have tried to understand the physics behind them in order to decipher the dynamic interplay between the interstellar material and the solar system, a process in which the interstellar magnetic field plays a major role.

The heliosphere (fig. 1)
Charged particles from the Sun spiral outward into space and form the solar wind. The solar wind particles follow the lines of the solar magnetic field and fill a region of space called the heliosphere that encloses the solar system. The solar particles at the edge of the heliosphere form a barrier, the heliopause, to deflect other incoming charged particles and magnetic fields, thereby partially protecting the inner solar system from the surrounding interstellar medium. The motion of the solar system through the dust, gas, and nebulosity that make up the interstellar medium gives the heliosphere a comet-like shape with a head and a tail. At the leading edge of the heliosphere, ions from the interstellar medium slow as they approach the head and form a shock wave known as the interstellar bow shock. Consequently, incoming neutral particles pile up, through a cascade of charge exchanges with decelerated ions, forming a hydrogen wall upstream of the heliopause. A decade ago, owing to sophisticated three-dimensional magneto-hydrodynamic simulations, the European team predicted that the region upstream of the heliosphere may suffer strong distortions, the strength of which depends on the orientation of the local interstellar magnetic field that wraps the heliosphere (fig. 2). The unknown interstellar magnetic field will thus be uncovered by the detection of these distortions.

The observations
Seven years ago, by combining measurements from the Hubble Space Telescope with Voyager 2 measurements, the team not only located the interstellar bow shock but also discovered that the nose of the heliosphere points 12^o away from the direction from which the local cloud is approaching (1, 2). The resulting deviation of the interstellar gas was recently confirmed by independent observations of the SOHO/SWAN experiment (Lallement et al., 2005) and by Voyager measurement of the magnetic field near the termination shock (Opher et al., 2007). All three observations point to the existence of an interstellar magnetic field that causes all detected asymmetries. Unfortunately, past theoretical models were not yet fully consistent and observations did not yet cover the whole medium, and thus they were not able to efficiently constrain the direction of the interstellar magnetic field.

Since then, new computer resources acquired by IAP and SRC have powered the simulation capabilities, allowing accurate mapping of particles and magnetic fields [interplanetary (known) and interstellar (unknown)] near the heliopause (3). In addition, continuing deep observations of the sky background at Lyman-a (UV resonance line of atomic hydrogen) by Voyagers 1 & 2 allow an extensive coverage of space far from the Sun’s influence. By combining refined numerical simulations with these recent data sets, the team concluded that the asymmetry observed in the Voyager 1 & 2 data reflects the asymmetry predicted by the model for the particle distribution near the heliopause (fig. 2). The direction of the local interstellar magnetic field was thus derived to be (latitude = -41^o to -58^o, longitude = 203^o -231^o) in galactic coordinates, confirming initial estimates made by the team in 2000 (Ben-Jaffel et al., 2000) (1, 2), and consistent with constraints obtained in the past by other independent techniques. These new results were presented at the June 2007 Astronum meeting (Ratkiewicz, Ben Jaffel & Grygorczuk, 2007) (3).

The discovery of the direction of the interstellar magnetic field from the tilt observed in the heliosphere is highly significant. Indeed, for many years it was thought that the charged particles from the interstellar medium were hitting the heliosphere head-on. Now we see that these ions are deflected by the interstellar magnetic field. Owing to sophisticated numerical simulations, now we are better able to understand the processes in play in the outer edges of the solar system, thereby improving, in the near future, the evaluation of the impact of the interstellar medium on our planet and on other planets and exoplanets. This will be one of the key goals of project ESINPLE (Environnements Stellaire et INterstellaire des PLanètes et Exoplanètes) under development at IAP within the “Simulations” project and the “Exoplanetes” team.

The next step - an interstellar probe and extensive simulations
Despite their limited technology, continuous reports from Voyagers 1 & 2 from deep space observations confirm the inestimable role of a space probe observing in situ both the heliosphere and the local interstellar medium. This dream may well come true. Scientists are currently investigating the different particles of interstellar origin that have reached the inner heliosphere using the ESA/NASA solar explorers Ulysses and SOHO. In the long-term, NASA is working on plans to send a probe that will fly into the region of the bow shock closest to Earth and investigate the boundary between the Solar System and the interstellar medium. The results obtained by the IAP team will help define this forthcoming mission.

Meanwhile, another approach would be to use three-dimensional simulations to explore, understand, and predict the distribution of the different components of the medium (neutrals, ions, pick-up ions, energetic neutrals, cosmic rays of heliospheric origin, etc.) under the governing effects of the ambient magnetic fields and physical processes in play. This very important conceptual effort requires heavy computer resources at the level of a “meso-machine.”

Figure 1

This artist’s impression shows the nearest surroundings of the solar system as it would look after a new discovery by European scientists using observations from the Hubble Space Telescope and Voyager.

The yellow Sun with the planets is in the center. The heliosphere is a bubble blown in space by charged particles from the Sun, and it encloses the solar system. The motion of the Sun relative to the surrounding medium – the so-called interstellar medium – and the interaction between particles from the Sun and particles from the interstellar medium makes the heliosphere slightly elongated (similar to the head of a comet). A shock wave, known as the interstellar bow shock, is also created in front of the solar system. The heliopause is our solar system’s outermost frontier in the interstellar medium. The interstellar magnetic field, indicated at the top left of the image by an arrow pointing 30° to 50° from the interstellar wind direction, strangely distorts the heliopause upward and the interstellar shock (bow shock) in the opposite direction.

These are the spatial distortions that numerical simulations predicted a decade ago and which the European team, led by the French astrophysicist Lotfi Ben Jaffel, successfully used to uncover the direction of the interstellar magnetic field from sky background observations made by Voyager spacecrafts far in the outer regions of the heliosphere.

Artist’s impression: ESA & Lotfi Ben Jaffel (Institut d'astrophysique de
Paris, CNRS-UPMC), Martin Kornmesser et Lars Lindberg Christensen (Space
Telescope-European Coordination Facility).
 

Figure 2

The figure shows isocontours of magnetic pressure in the ecliptic plane for different angles of the interstellar magnetic field with the interstellar wind direction (horizontal). The results of this simulation have been confirmed by three independent observations.

© Ben Jaffel et al.


References:
(1) http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=12651
(2) Ben-Jaffel, Puyoo, Ratkiewicz, Astrophys. J., v. 533, 924, 2000. (http://www.iap.fr/users/lotfi/bjl_apj2000.pdf)
(3) Ratkiewicz, Ben-Jaffel, et Grygorczuk, ASP conference series, 2008, in press. (http://www.iap.fr/users/lotfi/astronum2007.pdf)

Acknowledgements:
The Centre National de la Recherche Scientifique (CNRS) and the Academy of Sciences of Poland (PAN) recently celebrated the 50th anniversary of collaboration between the two institutions. The European team acknowledges continuous support in this frame, from the France-Pologne jumelage, the LEA Astro-PF, and directly from CNRS-PAN grants.