Surprisingly complex: The plasma bubble of the solar system is shaped differently than previously thought. According to this, the heliosphere is not comet-shaped, but rather round and in some aspects even resembles a curved croissant, as researchers report in the journal “Nature Astronomy”. This also explains the initially surprising measurement data from the Cassini and Voyager space probes.
The Heliosphere covers an area that extends from the sun about twice as far into space as the orbit of Pluto. This bubble, formed by the solar magnetic field and the solar wind, shields us from high-energy, interstellar particle flows. For a long time, astronomers assumed that this heliosphere extended into a long plasma tail – similar to the earth’s magnetic field on the side facing away from the sun or the tail of a comet.
Contradicting observations
However, data from NASA’s Cassini probe brought up this picture a few years ago to falter. They showed that fast, neutral atoms, which are thrown back from the outer border of the heliosphere, travel the same length from all directions. This speaks against a comet-shaped and for a rather round shape of the heliosphere. This is supported by data from the two Voyager probeswho the Border to interstellar space at almost the same distance from the sun – although their trajectories are almost 90 degrees apart.
The strange thing is that these observations do not fit newer astrophysical models. Because according to these, the heliosphere should neither be comet-shaped nor round. Instead, it would have to be shaped more like a sickle or croissant – with two plasma tails pulling backwards to the side.
Two types of fast ions
But how can this discrepancy be explained? Merav Opher from Boston University and her colleagues may have found an answer to this. Because, as you found out, the heliosphere is characterized by two different particle populations. The first group consists of charged particles that come directly from the solar wind.
The second group of particles in the heliosphere are so-called “pick-up” ions (PUI). As neutral atoms, they flow largely unhindered from the interstellar medium into the heliosphere. There, however, they are accelerated by the solar wind and heated up so much that they lose electrons and become ionized. Evidence of this has been provided by data from the NASA New Horizons spacecraft.
Roundishly compact and curved at the same time
Opher and her team have now developed a new three-dimensional model of the heliosphere that takes these two particle populations into account for the first time. “The presence of the pick-up ions as a separate fluid changes the energetics of the heliosphere shell and its structure in two ways,” the researchers report. On the one hand, they weaken the termination shock – the bow wave of the heliosphere – and thus allow greater compression of this border region. On the other hand, the secondary ions flow to the side faster along the magnetic field lines than the ions from the solar wind.
Taken together, the heliosphere assumes a compact, rather round shape. In certain energy ranges, however, there are curved particle flows that protrude to the rear. Overall, the heliosphere thus resembles a cross between a ball and a croissant, as Opher and her colleagues explain.
The existing data alone cannot tell whether your model is correct. “However, future measurements from afar and on site will be able to check whether the rounder heliosphere corresponds to reality,” said the researchers. Decades will pass before a space probe reaches this outer limit of the solar system. (Nature Astronomy, 2020; doi: 10.1038 / s41550-020-1036-0)
Source: Boston University
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