An article published in the journal Nature Astronomy describes a research on the shape of the heliosphere, the “bubble” in which the solar wind density is greater than that of interstellar matter. A team led by Kostas Dialynas of the Academy of Athens used data collected by four space probes – Cassini, the two Voyagers and IBEX (Interstellar Boundary Explorer) – to prove that the heliosphere has an approximately spherical shape and not extended with a tail as seemed much more likely.
The problem of the shape of the heliosphere created by the solar wind that fills the solar system has been debated for decades. The hypothesis of the extended shape was due to the fact that the solar system moves in interstellar space along with the heliosphere, which could therefore have a shape that remembers that of a comet with a rounded head and a tail. The observations of other stars showed such structures, supporting that theory.
In 2013, NASA confirmed that its Voyager 1 space probe entered interstellar space in 2012, so its detection of the heliosphere’s boundaries are crucial and Voyager 2 is following it. The IBEX space probe specifically aims to map the heliosphere’s boundaries from the Earth’s orbit. Using the Cassini space probe’s data considering that it just started the last phase of its mission in the Saturn system, can be a surprise but in addition to the data gathered on the planet, its rings and its moons, it also studied the behavior of solar wind with its INCA (Ion and Neutral Camera) instrument.
The heliosphere’s border area is called heliopause and the charged particles coming from the Sun can exchange charges with neutral atoms encountered in the interstellar medium. In these cases, some atoms can be pushed back into the interior of the solar system in the form of high speed neutral atoms. The INCA instrument was designed to study ions in the planet Saturn’s magnetosphere but it also proved useful in measuring neutral atoms back from the heliopause.
These atoms take years to travel from the Sun to the heliopause and more years to come back before they’re intercepted by the various space probes’ instruments. Their amount is influenced by the 11-year solar activity cycle so the measurement of neutral atoms can provide an idea of the distance they traveled. Cassini’s measurements indicate that the particles coming from the heliopause’s “head” reflect the changes in the solar cycle almost exactly as those from its “tail”.
If the heliosphere was extended with a tail similar to that of a comet the effects would be measurable with different times in reflecting the changes in the solar cycle. Instead, the measurements indicate that the heliopause has a shape much more like a sphere. This is a surprising result.
One possible explanation is that the interstellar magnetic field interacts with the solar wind in such a way as to push the heliopause to the Sun symmetrically. This magnetic field proved to be more intense than expected given the measurements carried out in recent years by the Voyager space probes and that could make the difference in the heliosphere’s shape.
The data collection from the edges of the heliosphere continues to verify its shape and build new models that match the measurements carried out. This type of research helps to understand how this “shield” works, a protection also for the Earth from part of the cosmic rays and other potentially harmful particles coming from interstellar space.