The LASCO Imager on the SOHO Satellite provides images of the Sun's corona. The most prominent feature is usually coronal streamers radiating out from the Sun. Occasionally, a coronal mass ejection (CME) can be seen as it is expelled away from the Sun. Sometimes the CME appears as an expanding halo around the Sun. These halo CME, if earth-directed, can be particularly significant for space weather since they represent clouds of electrically-charged matter (plasma) on the way to the Earth. If the solar-wind conditions are right, these clouds of plasma may cause magnetic storms and other spaceweather phenomena in the near-Earth environment.
The LASCO Team at NASA provides coded and plain language reports, describing halo coronal mass ejections (CME) from the Sun. A translation of the latest coded report gives simplified details of the latest halo CME. More complete details can be found at the Halo CME Mail Archive.
Spacecraft located near the L1 libration point provide data on several characteristics of the solar wind. The ACE (Advanced Composition Explorer) satellite provides near-real-time data on the solar wind speed, density, temperature, and on the Interplanetary Magnetic Field (IMF). The direction and intensity of the IMF are particularly important in determining the extent of the interaction and energy transfer between the solar wind and the Earth's magnetic field. Specifically, if the direction of the Bz component (vertical in the GSM coordinate system) is southward (negative) the conditions for energy transfer are favourable. Several of the data-sets from ACE play major roles in short-term geomagnetic activity forecasts . Another satellite (SOHO) near the L1 point also monitors the solar wind speed and other parameters.
The Sun's corona is continually emitting X-rays, but the distribution over the Sun is not uniform. Coronal holes are large relatively dark regions in the Sun's corona, as viewed in X-ray wavelengths. Magnetic field lines from coronal holes extend far out into the solar system and allow continuous flows of high-speed solar wind from these regions. Coronal holes are often long-lived and tend be be more numerous in the years following solar maximum. If one of these high-speed solar wind streams impacts the Earth's magnetic field, a magnetic storm may result. Mapping the coronal holes as they move across the Sun's disc provides a basis for long-term geomagnetic field activity forecasts.