“The Sun from SOHO, and First Glimpses of Parker Solar Probe and Solar Orbiter“ with Daniel Müller (Solar Orbiter Project Scientist, ESA – ESTEC, The Netherlands)

SOHO, the Solar and Heliospheric Observatory, is a cooperative project of ESA and NASA to study the Sun, from its deep core to the outer corona, the solar wind, and energetic particles. Together with Cluster it forms the Solar-Terrestrial Science Programme (STSP), the first cornerstone of ESA’s long-term science program “Horizon 2000.” SOHO was launched on 2 December 1995 and inserted into a halo orbit around the L1 Lagrangian point in February 1996. The twelve instruments on SOHO have provided an unparalleled breadth and depth of information about the Sun, from its interior, through the hot and dynamic atmosphere, out to the solar wind and its interaction with the interstellar medium. SOHO’s findings have been documented in over 5900 scientific publications in the refereed literature, authored by more than 4000 scientists worldwide. SOHO provided the first images of structures and flows below the Sun’s surface and of activity on the far side of the Sun. It discovered sunquakes and has shed new light on a number of structural and dynamic phenomena in the solar interior, such as the absence of differential rotation in the radiative zone, subsurface zonal and meridional flows, sub-convection zone mixing, and very slow polar rotation. It provided evidence for upward transfer of magnetic energy from the surface to the corona through a “magnetic carpet” and revealed an extremely dynamic solar atmosphere where plasma flows play an important role. It discovered new dynamic phenomena such as coronal waves, measured the acceleration profiles of the slow and fast solar wind, and identified the source regions and acceleration mechanisms of the latter. It revolutionized our understanding of solar-terrestrial relations and dramatically boosted space weather forecasting capabilities by providing, in a near-continuous stream, a comprehensive suite of images covering the dynamic atmosphere and extended corona. SOHO observed and characterized over 40,000 coronal mass ejections and, as a byproduct, became the most prolific discoverer of comets in astronomical history, with over 4000 comets found in SOHO observations, most of them by citizen scientists accessing SOHO real-time data via the Internet.

Control of the spacecraft was lost in June 1998 due to an unfortunate series of events during a spacecraft maneuver, but restored three months later in a dramatic recovery operation. Miraculously, all twelve instruments were still usable, most with no ill effects, despite the enormous temperatures they were exposed to during the time contact with SOHO was lost. Despite subsequent failures of all three gyroscopes (the last in December 1998), new gyroless control software installed by February 1999 allowed the spacecraft to return to full scientific operations. This made SOHO the first three-axis stabilized spacecraft operated without gyroscopes.

While SOHO is still operating today with 7 of its 12 instruments at almost full capability, two new solar missions have joined it recently. In 2018 the Parker Solar Probe was launched by NASA and already has approached the Sun closer than any spacecraft before. In February 2020 ESA launched its Solar Orbiter that will approach the Sun to as close as 0.28 AU and will use a series of Venus fly-bys to work its way to higher and higher latitudes, thus obtaining an unprecedented view onto the solar polar regions. The future game changers are underway, but they stand on the shoulders of a giant – SOHO.

Dr. Daniel Müller holds a Ph.D. in Physics from the Albert-Ludwigs-Universität Freiburg, Germany. After a Marie Curie Fellowship at the University of Oslo, Norway, he joined ESA’s SOHO team at NASA’s Goddard Space Flight Center, where he became Deputy Project Scientist of SOHO. In 2010, he moved to ESTEC to start working on Solar Orbiter, and has been serving as the mission’s Project Scientist since 2012. In addition to his scientific and project management work, he has a strong interest in high-performance scientific data visualization. In particular, he is coordinating the development of the open-source Helioviewer software and is the ESA Lead of the ESA/NASA Helioviewer Project.

This seminar was recorded on October 1, 2020