Very active auroras, however, are sometimes visible even close to the equator. This un-predictable behavior has attracted the interest of some of the greatest scientists in the world. Early theories explained the auroras as an evaporation of air which then became incandescent at high altitude or, alternatively, as air clouds illuminated by sunlight. It was not until the eighteenth century that magnetic disturbances were found to be associated with auroras. This discovery marked an important step in the physics of auroras.
Magnetic disturbances remain one of the main tools to characterize auroral activity today.
The second major step forward in understanding auroras concerned the effect of solar activity. In the nineteenth century it was realized that there was a clear correlation between the number of sunspots and the intensity and number of geomagnetic disturbances. Then in the twentieth century with the International Geophysical Year, the first spacecraft, carrying scientific experiments to measure electric and magnetic fields, were launched into space. These in-situ measurements gave an unprecedentedly detailed view of the plasma surrounding the Earth, namely the magnetosphere. The direct influence of the solar wind was also observed, with multi-point measurements made simultaneously in the solar wind and in the Earth's ionosphere.