The space above the Earth’s atmosphere, out to the Sun, and beyond, is populated by a tenuous gas made up of charged atomic particles, mostly, electrons, protons, and other ions. They are usually called “plasma”. The same space is also populated by magnetic and electric fields as well as waves of some form. A variety of disturbances occur in space around the Earth, which is mostly electromagnetic in nature. In such a case, several major physical variables including electric and magnetic fields, particle density, gas pressure, and gas flow velocity exhibit abrupt changes. This is what is called “space weather”.
  The space weather begins from the solar corona, the outer region of the Sun’s atmosphere. The corona can be seen during an eclipse of the Sun as bright streamers extending outward several times the radius of the Sun. The corona has a temperature of several million degrees. The corona emits outward ionized gas called the solar wind. This flows continuously at a supersonic speed away from the Sun to the orbit of the Earth and beyond – all the way to the outer reaches of the solar system.
  In the vicinity of the Earth, the solar wind encounters the Earth’s giant magnetic field. The solar wind is effectively blocked from the penetrating the magnetic field because the magnetic force turns aside the solar wind particles. This forms the magnetosphere of the Earth, a giant bullet-shaped region surrounding the Earth. However, the Earth’s magnetosphere is not a perfect barrier at all and some of the solar wind particles, their energy and momentum can penetrate into the magnetosphere. The penetration depends on the solar wind conditions, and when the condition is favorable for the penetration at a high rate, a particularly large electro-magnetic disturbance occurs inside the magnetosphere. This is called magnetic storm in space, the most representative element of space weather changes.
  The space weather can influence the performance and reliability of space-borne and ground-based technological systems and can endanger human life or health. For example, hundreds of satellites are currently in orbit, and they can be all exposed to severe space weather. On January 20, 1994, two Canadian geostationary communication satellites, Anik E1 and Anik E2, were damaged by a huge magnetic storm, and their recovery not only cost a huge amount of money but also required a long time delay, several months, back to full service.
  We cannot control space weather, but by trying to understand and eventually forecast storms in space, we can hope to minimize their impact. Nowadays this is possible to some extent, though still premature in many aspects, and serious activity is under way in many countries including Korea. Recent years have seen that many high tech facilities, particularly those based on advanced IT technologies – one of the main engines of the Korean economy – have become more vulnerable to the adverse effects of space weather. The need for space weather information is thus critical for the continued prosperity of the Korean economy.