Reconstructing space climate

By Matt Owens

The approximately 11-year cycle in the number of sunspots visible on the Sun was first identified more than 150 years ago, by Samuel Schwabe, and has been well-observed ever since. In fact, with the power of hindsight, the sunspot cycle can be traced back through historical records, all the way to Galileo’s first telescopic experiments in 1610. This is by far the longest record of direct solar observation, making it invaluable to understanding solar variability.

2015 01 29 Matt Owens Fig 1

Figure 1. Space climate reconstructed from a range of proxies. The bottom panel shows the heliospheric magnetic field (HMF) reconstructed from sunspot observations (black), geomagnetic observations (blue) and spacecraft data (red).

Thanks to spectroscopic observing techniques, sunspots are known to result from strong concentrations of solar magnetic field. The solar surface is a teeming mass of convection cells driven by heating from the solar interior, but the magnetic fields in sunspots inhibit this convection, allowing the plasma to cool and making them appear “dark,” at least relative to the rest of the solar surface. Thus the sunspot record potentially presents information about long-term changes in solar magnetism.

The solar wind is a constant outflow of solar plasma from the hot solar atmosphere, which drags the solar magnetic field out into space. Spacecraft launched beyond the Earth’s own magnetosphere can make direct measurements of the heliospheric magnetic field (HMF), shown in red in the bottom panel of Figure 1. Since the start of observations in the mid-1960s, the HMF has varied in phase with sunspot number and exhibited similar cycle-to-cycle trends, though the magnitude of HMF variations are considerably smaller than those of sunspot number.

2015 01 29 Matt Owens Fig 2

Figure 2. A coronal mass ejection erupting off the north pole of the Sun.

Evolution of the HMF is not smooth and continuous, but proceeds in episodic bursts. New HMF is added by large solar eruptions, called Coronal Mass Ejections (CMEs), while old HMF is removed by magnetic restructuring close to the Sun. Observed CME rates can be used to model the HMF, providing a strong match with the spacecraft observations. Furthermore, as CME rates vary with sunspot number, such a model of the HMF can be extended back to 1610 (though there are no Renaissance spacecraft observations with which to perform validation!).

Proxies for the HMF, however, do allow the pre-space age HMF to be inferred. One such proxy is the intensity of galactic cosmic rays (GCRs) reaching Earth. GCRs are near relativistic charged particles which originate far outside the solar system. As charged particles are deflected by magnetic fields, the HMF partially shields Earth from GCRs. When GCRs do enter the Earth’s atmosphere, they collide with air molecules and create a shower of exotic fundamental particles (like CERN, but cheaper to run). Since the 1950s, GCR intensity has been measured using ground-based neutron monitors, which hit record high values in 2009. But GCRs also produce isotopes which do not naturally occur, such as Carbon-14 and Beryllium-10. These “cosmogenic” isotopes are removed from the atmosphere and deposited in biomass and ice sheets, respectively, providing natural records of GCR intensity, and hence HMF strength, over the last 9,400 years or so. This proxy for the HMF shows excellent agreement with the sunspot-driven model back to 1610.

This entry was posted in Environmental physics, Space and tagged , . Bookmark the permalink.