Person:

Loeb, Abraham

Cold interstellar medium (ISM) clouds, 4-5 order of magnitude denser than the diffuse ISM, exist in the galaxy and our Solar System has most likely encountered at least one of them during its lifetime. However, evidence for such an encounter [[and its effects]] have not been studied in detail yet. Here we derive the velocity field of the Local Ribbon of Cold Clouds (LRCC) by modelling the 21-cm data from the HI4PI survey and find that the Solar System may have passed through the LRCC in the constellation Lynx between 2-3 million years ago. Using a state-of-the-art simulation of the heliosphere, we show that during the passage the heliosphere had shrunk to a scale of 0.22AU, smaller than the Earth’s orbit around the Sun. This would have put the Earth in direct contact with the dense ISM for a period of time and exposed it to a neutral hydrogen density above 3000cm-3. Such a scenario agrees with geological evidence from 60Fe and 244Pu isotopes. The encounter and related increased radiation from galactic cosmic rays flux might have had substantial impact on the Earth system and climate.

As the Sun moves through the surrounding partially-ionized medium, neutral hydrogen atoms penetrate the heliosphere, and through charge-exchange with the supersonic solar wind, create a population of hot pick-up ions (PUIs). Until recently, the consensus was that the shape of the heliosphere, is comet-like. The Termination Shock crossing by Voyager 2 (V2) demonstrated that the heliosheath (HS) (the region of shocked solar wind) pressure is dominated by PUIs; however, their impact on the global structure of the heliosphere has not been explored. Here we use a novel magnetohydrodynamic model that treats the PUIs as a separate fluid from the thermal component of the solar wind. The depletion of PUIs, due to charge exchange with the neutral H atoms of the ISM in the HS, cools the heliosphere, “deflating” it and leading to a narrower HS and a smaller and rounder shape, confirming the shape suggested by Cassini observations. The new model reproduces both the properties of the PUIs, based on the New Horizon, and solar wind ions, based on the V2 spacecraft observations, as well as the solar like magnetic field data outside the heliosphere at Voyager 1(V1) and V2.