Cassini Reveals We Live In A Smaller Bubble Than We Thought

New evidence suggests that the trailing end of our solar system may not be stretched out in a long tail. Instead the heliosphere is, in fact, a sphere. From top left counterclockwise: LLOrionis, BZ Cam, and Mira. NASA/HST/R.Casalegno/GALEX

If you feel like you live in a bubble, cut off from the wider universe, you're right. The bubble is also smaller and more rounded than many suspected. The heliosphere, caused by the Sun's solar wind, really is almost spherical, rather than having a long trailing tail.

The Sun spits out high-speed charged particles, whose motion drags out a component of the Sun's magnetic field. The pressure from this wind produces a bubble known as the heliosphere, holding out the interstellar medium. Eventually, however, the answering pressure from the gas between the stars brings the wind's motion to a halt. In 2012 the Voyager 1 spacecraft passed beyond the heliosphere to enter interstellar space, followed by Voyager 2.

Although this gave us a precise measurement of the location of the heliosphere's boundary, it was only in two directions. The Sun orbits the galactic center, and a popular model had the heliosphere squashed in the direction of travel, while trailing out in a long tail behind the Sun.

However, a paper in Nature Astronomy contradicts this. "Instead of a prolonged, comet-like tail, this rough bubble-shape of the heliosphere is due to the strong interstellar magnetic field – much stronger than what was anticipated in the past – combined with the fact that the ratio between particle pressure and magnetic pressure inside the heliosheath is high," said lead author Dr Kostas Dialynas of the Academy of Athens in a statement.

The conclusion is based on evidence from the Cassini spacecraft. Cassini's travels around Saturn take it nowhere near the edges of the heliosphere. Nevertheless, it is far enough from the distractions of the inner Solar System to study the fast-moving neutral atoms produced when charged particles bounce off the heliosphere's boundaries. The capacity to do so is an unexpected application of the Ion and Neutral Camera, intended to enable Cassini to study particles trapped by Saturn's magnetosphere.

The strength of the solar wind varies with the Sun's 11-year cycle, which is reflected, after a 2-3 year delay for traveling time, in the number of neutral atoms bouncing back. Astronomers expected the response to this cycle would be delayed in atoms coming from the trailing edge of the heliosphere compared to the lead.

This view was challenged by a 2009 study reporting that responses appeared almost simultaneous from all directions, suggesting a shape not far off from a sphere. Nevertheless, astronomers regarded that conclusion as preliminary, based as it was on Cassini data from just a snapshot in time.

Now, however, as Cassini prepares to end its mission, the craft has been orbiting Saturn for an entire solar cycle and Dialynas concludes the results confirm the near-circular shape, particularly when combined with observations from other spacecraft.

The bubble created by the solar wind is almost circular (left), rather than having a long tail behind the Sun as previously thought (right). Dialynas, et al. (left); NASA (right)

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