THE APPLICATION OF 3-D MHD MODELS OF THE HELIOSPHERE TO UNDERSTANDING OF CENTURY-SCALE VARIATIONS IN GALACTIC COSMIC RAYS

Galactic Cosmic Rays are highly energetic particles which move at very close to the speed of light. They are generated at the shock fronts produced when a star explodes - a supernova. These particles constantly bombard the Earth and are important for a number of reasons. 1. They cause malfunctions in the electronics on board aircraft and spacecraft and they are also a health hazard for astronauts and even for passengers in aircraft. . We call the study of these effects 'space weather'. 2. Cosmic rays have an influence on climate which we do not yet understand. They produce ionisation in the atmosphere which allows currents to flow and a global circuit of driven by thunderstorms. There have also been suggestions that cosmic rays assist in the formation of clouds. This is still highly controversial, but we do know that variations in cosmic rays since the last ice age are connected with detected changes in climate. This may well be because the brightness of the Sun is linked to how many cosmic rays reach Earth, but this connection is far from being understood. 3. When cosmic rays smash into atmospheric particles, they generate characteristic products that we call cosmogenic isotopes. These are washed out of the atmosphere in snow and rain and are stored in ice sheets, tree trunks and ocean sediments. Taking cores into these reservoirs allows us to study how much of an isotope was deposited at a given time which can be dated by counting the tree rings, or the layers of clean and dirty snow in the ice sheet (corresponding to winter and summer), or from the small fossils and volcanic dust found in an given layer of ocean sediment. These give us a unique record that extends back over several millennia. Because they are produced by cosmic rays which are shielded from away from Earth's atmosphere by the magnetic field of the Sun and of the Earth, these cosmogenic isotopes can tell us much about how these fields have changed in recent millennia. This proposal is aimed at letting us use these isotopes to study how the Sun has changed in the past few centuries. On these timescales, the variation of Earth's magnetic field is slow and the changes in cosmic rays and cosmogenic isotopes are caused by a magnetic field that is produced in the Sun and the pulled out of it by a continuous outflow of charged particles called the solar wind. We are particularly interested in recent centuries for two reasons: 1. Understanding any influences of the Sun on climate over these timescales is important for the correct prediction of how much our climate will be changed in the future by greenhouse gasses. 2. Around 1700, the Sun behaved in a very unusual way. We call this period the Maunder minimum after the scientist who first noted the almost complete lack of sunspots at this time. There was much debate as to whether or not this was a real effect or if wars and famine had just caused the early astronomers not to abandon their studies. Cosmogenic isotopes tell us that the Sun was indeed different in the Maunder minimum compared to today and so give us a key insight into how and why the Sun has changed. If we are to interpret cosmogenic isotopes in terms of what the Sun was doing in the past, we must understand how the solar magnetic field modulates the fluxes of cosmic rays arriving at Earth. The area around our Sun dominated by this magnetic field is called the heliosphere and in recent years complex models of how the heliosphere behaves and varies have been developed. Our previous work allows us to see that these models can be used to understand most of the shielding of cosmic rays from the Sun and this proposal is aimed at letting us be the first to exploit this. Our results also show that the widely-accepted theory of how this happens is inadequate in some important respects. The results of our work will have great implications for our understanding of how the Sun has varied and what effects cosmic rays have on us.