SDO will measure the irradiance of the Sun that produces the ionosphere. It will also measure the sources of that radiation and how they evolve.

Our Sun, a Weather Maker

Everyone knows (or at least complains) about the weather at the surface of the Earth. Water vapor condenses into clouds, falls to the ground, and flows to the oceans; only to be evaporated back into the atmosphere by solar radiation to complete the cycle. Space weather refers to changes in the magnetosphere, ionosphere, and thermosphere that can affect our technological systems and can endanger human life or health. These changes start at the Sun and move to the Earth. Creating, moving, and destroying charged particles is the cycle of Space Weather.

Charged particles are created in our atmosphere by the intense X-rays produced by a solar flare. The solar wind, a continuous stream of plasma (charged particles), leaves the Sun and fills the solar system with charged particles and magnetic field. There are times when the Sun also releases billions of tons of plasma in what are called coronal mass ejections. When these enormous clouds of material or bright flashes of X-rays hit the Earth they change the upper atmosphere. It is changes like these that make space weather interesting.

WHAT IS SPACE WEATHER?

The Parker Spiral shows how the solar wind pulls the magnetic field into a spiral shape. The orbits of Earth and Mars are drawn to show how field from one part of the Sun can onteract with the field from another when the solar wind velocity of the two regions is different.

Our Sun is an extremely active star. Solar activity expels radiation and atomic particles from the Sun during solar flares and coronal mass ejections. Space Weather is how we refer to the variations in the local space environment driven by the expelled radiation and particles and how those variations impact the Earth and human society. Those impacts include: electronic failures in satellites; communication and navigation problems in airplanes; radiation hazards to astronauts; and loss of satellites to atmospheric drag. Electrical power to our homes and businesses can be interrupted by geomagnetic storms driven by blasts from the Sun. The importance of these variations will become apparent as we begin to understand how Space Weather works.

How can there be weather in a vacuum? Compared to Earth's atmosphere, space is a very good vacuum, better than most vacuums we can create in the laboratory. Surface pressure is about 1013 mbar while the pressure 500 km (300 miles) above the surface is 3 x 10-9 mbar. Another example is the density of particles at different places. There are about 2 x 1019 (a 2 followed by 19 zeros) molecules in each cubic cm of the Earth's atmosphere near sea level (a mass density of 1.2 kg per cubic meter). This should be compared to a density of 1 to 10 particles per cubic cm in the solar wind as it flows by the Earth.

Another important difference is that, unlike the atmosphere we live in, the tenuous gas in space is ionized. This means that some (or all) of the electrons have been stripped from the atoms, resulting in a gas of positively charged ions and negative electrons called a plasma. These charged particles are steered and accelerated by the magnetic fields that pervade the solar system. Most of the plasma comes from the solar wind that escapes the Sun's gravity and sweeps through the solar system at speeds from about 400 km/s (1.4 million km per hour) for a quiet Sun to 2000 km/s (7.2 million km per hour) during solar storms. At the same time the Sun's magnetic field is swept out through the entire solar system where it interacts with the magnetic fields of the Earth and other planets. Because the Sun rotates the Sun's magnetic field expands outwards in a spiral pattern, the Parker Spiral, with the charged particles of the solar wind spraying out into the solar system like a garden sprinkler.