Solar disturbances affect the Earth not only from changes in intensity of radiation, but also through the solar wind - a gas of ionized, charged particles streaming away from the sun at an average speed of 400 kilometers per second. The solar wind is tenuous, containing only a few particles in a cubic centimeter, mainly protons and electrons. This gas of charged particles - called a plasma - can support electric currents. As a result, the Sun's magnetic field is essentially "frozen in" to the moving plasma and is carried out to the planets. The solar wind plasma flow impacts the Earth, where our own geomagnetic field acts as a shield - forming the magnetopause and magnetosphere, where most solar wind particles are unable to penetrate.

Despite this shielding effect, the Earth-magnetosphere system does react to suddentstrong changes in the solar wind plasma and magnetic field. Sudden changes in solar wind density may cause the magnetosphere to "balloon" out or be compressed inwards, while magnetic fields in the solar wind and "reconnect" with magnetic fields generated by the Earth and lead to an exchange of particles and energy. Sudden changes in density or velocity may indicate we are in a magnetic storm. If these increases are within days of a Halo CME, it is likely that was the cause of the solar wind changes.

The ACE spacecraft monitors the solar wind conditions "upstream" of the Earth, before the flow arrives. A sudden change in the sign of Bz - the z-component of the solar wind measured at ACE, essentially the north-south direction of the magnetic field - can signal the beginning of a geomagnetic substorm, in which energy begins to be transferred from the solar wind into the Earth's near-space environment. Within hours this energy is released through Aurora, when ionospheric currents become intensified and the Earth's magnetic field rapidly reconfigures itself into an equilibrium.

CURRENT CONDITIONS
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