An Interstellar Compass?
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A magnetic field in
the interstellar medium could produce a distorted, non-axisymmetrical
shape for the heliosphere, because a magnetic field exerts a
non-isotropic (i.e. different in different directions) pressure. In
the picture, the interstellar flow ("IS FLOW") is being deflected on
the distorted "nose" of the heliosphere. This could well be the
explanation for a controversy over recent Voyager 1 data.
The Sun moves through the local interstellar medium, and is currently crossing the Local Galactic Cloud at a speed of about 25.5 km/s, or almost 100 000 km/h. Streaming out from the Sun, the solar wind creates a "bubble" around the solar system where the conditions of the interstellar medium are barely felt. The front of this bubble is defined by the "solar wind termination shock", where the undisturbed solar wind's outflow slams into a "heap" of solar wind material that have been slowed down to much lower speeds. Somewhere outside that region is the "heliopause", the interface between material originating from the Sun and material originating from the interstellar matter. Outside the heliopause again is the "bow shock", where interstellar matter collides with material scooped up by (and sliding around) the advancing heliosphere.
But neutral atoms from the interstellar cloud can pass unscathed through these regions, into the heliosphere. The SWAN instrument on SOHO observes the glow of interstellar hydrogen atoms as they absorb and reemit ultraviolet light from the Sun. But a fraction of the hydrogen atoms come in disguise: They started their interaction with the bow shock as ionized hydrogen. Through a process known as charge exchange, they then became neutral and passed into the heliosphere. But they retain the qualities they had at the moment they became neutral, such as the direction of travel, speed, and temperature.
In the absence of a magnetic field, the "nose" of the heliosphere is axially symmetric around the direction of motion of the solar system. And so is the stream of neutral hydrogen atoms that started out their interaction with the heliosphere as ions. In other words, the mixing in of these camouflaged atoms do not affect the direction of the incoming flow - just the speed and temperature.
But the SWAN instrument on SOHO has detected that the stream of neutral interstellar hydrogen appears to come from a slightly different direction than other neutral matter (such as helium, measured by other instruments not on board SOHO). This finding has been published as a report in the latest issue of Science magazine (4 March 2005).
The results indicate that the nose of the heliosphere is likely to be distorted due to the presence of a magnetic field in the "local interstellar cloud", not aligned with (nor perpendicular to) the direction of motion of the solar system through the cloud. Through modelling and further observations, this line of research may enable a determination of the orientation and strength of the magnetic field in the local interstellar medium - in other words, it's an "interstellar compass".
One reason for the interest in such a result is that the Voyager 1 spacecraft has sent back some surprising results: some instruments (measuring energetic particles) indicate that the spacecraft has crossed the solar wind termination shock, whereas the magnetometers report the opposite. In order to explain this controversy, models involving a distorted front-side of the heliosphere have been invoked, and the recent SWAN results seem to back this explanation.
- Science magazine (4 March 2005, Volume 307 No 5714)
- Rosine Lallement, Rosine.Lallement@aerov.jussieu.fr, Service d' Aeronomie du CNRS, +33 1 64 47 42 35
Picture credits: Illustration adapted from Ratckiewicz et al (2002) and Pogorelov and Zank (2004).
Instrument: SWAN (Solar Wind Anisotropies)