SOHO marks 30 years of solar observation
SOHO is a joint mission between the European Space Agency (ESA) and NASA. ESA provided the spacecraft and nine of its twelve instruments, while NASA contributed three instruments, the launcher and mission operations at the Goddard Space Flight Center. The precise 1995 launch required no planned course corrections, enabling operations far beyond the originally expected three years and making it possible for the remaining fuel to support operations for more than 70 years.
Finland has played a key role from the beginning. The University of Turku developed the Energetic and Relativistic Nuclei and Electron Experiment (ERNE). The Finnish Meteorological Institute (FMI), together with VTT and Finnish industry partners, built major parts of the Solar Wind Anisotropy (SWAN) instrument in cooperation with a French research institute, which leads the instrument and provides detector systems and data analysis. FMI developed the remaining hardware and control systems and has operated SWAN throughout the mission.
Transforming solar and space weather research
SOHO has profoundly advanced knowledge of the Sun’s interior, magnetic activity and their effects on the solar system. Its measurements have revealed temperature layers, subsurface flows and complex rotation patterns, and improved understanding of energy transfer processes that drive sunspots, magnetic storms, fast solar wind streams and coronal mass ejections. These discoveries have significantly enhanced space-weather forecasting, helping operators of power grids and satellites prepare for solar storms. Monitoring solar irradiance and variations in extreme ultraviolet radiation has also supported climate-related studies of solar variability.
Since late 1995, the SWAN instrument has produced three-dimensional maps of the solar wind by observing how the Sun’s ultraviolet radiation interacts with interstellar hydrogen. The long observational record, covering several solar cycles, shows hemispheric asymmetries, cycle-dependent variations and provides insights into the structure of the heliosphere. Early in the mission, SWAN also detected traces of water vapour escaping Earth as far as the Moon’s orbit.
Comet discoveries and new early-warning capability – and continuing
SOHO has discovered more than 5,000 previously unknown comets, most via the LASCO instrument, but over 3,700 SWAN images have also contained comet signatures. Several comets discovered from SWAN data carry the instrument’s name. These observations have enabled studies of cometary water production rates and offered detailed insights into the disintegration of comets such as PanSTARRS and LEONARD.
SWAN’s observations of ultraviolet outflows and high-energy particle ejections also allow the detection of strong solar events occurring on the far side of the Sun. Combined with knowledge of solar rotation, these measurements can provide several days of advance warning which is longer than the typical few hours offered by Earth-based observations.
With most instruments still operational and no full replacement currently in orbit, ESA and NASA have extended the SOHO mission at least into 2026.
Further information
Walter Schmidt, retired Research Manager, Finnish Meteorological Institute, tel. +358 50 324 3107. Email address is in the format forname.surname@saunalahti.fi.
Ari-Matti Harri, Head of Unit, Finnish Meteorological Institute, tel. +358 29 539 4632. Email address is in the format forename.surname@fmi.fi.
ESA: Solar and Heliospheric Observatory mission (SOHO)
Scientific review article in Nature Astronomy: SOHO’s 30-year legacy of observing the Sun
