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Cosmic rays trace Sun's journey through space
Scientists may have a new tool to help them work out what sort of environment the solar system's been experiencing during its journey through our galaxy, the Milky Way.

It takes the Sun and its family of planets about 220 million years to complete each orbit of the galaxy, but it can be quite difficult to detect the local interstellar medium which the solar system's travelling through, because it's so tenuous and emits very little light.

During this galactic journey the solar system passes through different types of interstellar medium with different compositions, densities and ionization levels such as galactic clouds of atomic hydrogen, with densities of just two atoms per ten square centimetres.

As tenuous as it sounds, this can still affect the size and shape of the heliosphere, the Sun's ionized area of influence dominated by the solar wind and magnetic field.
Tracing the journey in the rocks

Now Priscilla Frisch from the University of Chicago and Hans-Reinhard Mueller from Dartmouth College in Hanover, believe changes in the isotopes of some elements in Earth's geologic record could hold clues about this journey and consequently the local galactic environment.

Writing on the pre-press website arXiv.org, Frisch and Mueller say radioisotopes like carbon 14 and beryllium 10 are created when cosmic rays from outside our solar system slam into atomic nuclei in elements on Earth.

But factors such as the strength and shape of the Earth's magnetic field and the solar wind emanating from the Sun, affect the number of cosmic rays reaching Earth.
Galactic clouds

Frisch and Mueller hypothesise if solar winds can do this, other things like galactic clouds in the interstellar medium (which the solar system goes through) could also show up in the paleorecord.

They say measuring these isotopes may let scientists know where the solar system has been.

Frisch and Mueller admit their conclusions are speculative, but believe there's evidence in the past 2 to 3 thousand years of a change in the local flux which seems to correlate with when the solar system entered the local interstellar cloud-cluster, a grouping of tiny clouds of atomic hydrogen which we are still travelling through.

Frisch and Mueller also claim there's evidence of an earlier event about 18 thousand to 24 thousand years ago when the radio isotope levels also changed; and which seems to correspond with another cloud the solar system travelled through at the time.

Professor Naomi McClure-Griffiths from the University of Sydney and the CSIRO says it's a neat paper and an idea which demands follow up.

"The most interesting aspect of the work is how the shape of the heliosphere and heliopause relate to the interstellar environment around us.

She says travelling through the different galactic clouds changes the heliosphere.

"If the Sun travels through a very dense cloud of gas, the heliosphere would shrink down to something so small, the Earth's orbit could actually take us outside the heliopause, the boundary between the heliosphere and the interstellar medium", says McClure-Griffiths.

In that case, only the Earth's magnetic field would protect the planet from cosmic rays.

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