The SOHO mission, a joint programme between the European Space Agency (ESA) and NASA, studies the Sun’s interior, the Sun’s atmosphere and the solar wind.
SOHO moves in a halo orbit around the L1 Lagrange point, 1.5 million kilometres from the Earth. This location allows uninterrupted observation of the Sun, with all SOHO instruments observing continuously, 24-hours a day.
Over 3,700 papers using SOHO data have been published in refereed journals since the launch, representing the work of over 3,000 individual scientists. Virtually every living solar physicist has had access to SOHO data.
The UK has very strong involvement in SOHO, primarily through the Coronal Diagnostic Spectrometer (CDS) which was built at the Rutherford Appleton Laboratory, including contributions from the UCL Mullard Space Science Laboratory and several foreign partners.
The CDS instrument is managed from the operations centre at RAL Space. The CDS team, led by the Principal Investigator (PI), Dr Andrzej Fludra, receive and process observing requests from the world-wide research community, prepare daily science plans and send them to their instrument operators at NASA for uploading to the spacecraft. Over the 15 years, the RAL team have worked closely with UK research groups at 15 universities across the country, and also collaborated with 60 groups world-wide.
Dr Fludra, who has been involved in the SOHO CDS project for 16 years, including eight years at NASA Goddard Space Flight Center, said, “One of many things I enjoy in my PI role, is the opportunity to meet and interact with hundreds of users of the CDS instrument which include a significant fraction of the world-wide solar physics community. For many years, many of them were involved hands-on in preparing the observing plans for the instrument. A few years ago we streamlined the process and our project staff now do all the science planning.”
Recently, both NASA and ESA approved a further 2-year extension of SOHO operations. SOHO will continue to play a lead role in the early warning system for space weather, detecting mass ejections leaving the Sun’s corona and heading towards the Earth.
The UK instrument, CDS, remains as versatile as ever and continues observing all areas on the Sun, often focusing on regions of concentrated magnetic field emerging on the Sun’s surface, which contain hot plasma emitting strong EUV radiation, and are seats of violent flares. The numbers of these ‘active regions’ are finally picking up after a mysteriously prolonged solar minimum that lasted a year longer than expected. The almost unbroken 15-year record of CDS observations will help unravel this mystery if extended for a little longer and aid understanding of the solar activity cycle. Two or three more years of observations are needed to catch the moment when the Sun’s magnetic fields will reverse their polarity again (this happens every 11 years) and the magnetic field in the entire heliosphere will respond accordingly. This response is seen in such unusual ways as, for example, modulated numbers of galactic cosmic rays reaching the Earth.
What is CDS?
The Coronal Diagnostic Spectrometer records emission from the solar corona which is the hottest part of the solar atmosphere. CDS is sensitive to very short wavelengths of light, called extreme ultraviolet. A spectrometer separates individual wavelengths and measures intensities and profiles of spectral emission lines.
What is a spectrometer? The principle of observing the ultraviolet spectrum is similar to that when a prism separates white light into a rainbow of distinct colours. However, the extreme ultraviolet radiation is dispersed using reflective gratings instead of a prism. It is invisible to the human eye and can’t penetrate the Earth's atmosphere. It has to be observed from space. By analysing ultraviolet emission, recorded by the CDS spectrometer, we can learn a huge amount of detail about the Sun’s atmosphere and derive the temperature, density, chemical composition, and motion of plasma in the various atmospheric layers.
For more information please contact: RAL Space Enquiries