Valorization of 70 years of solar observations from the Royal Observatory of Belgium


The Sunspot Number is the longest scientific experiment still ongoing and a crucial benchmark to study solar activity, space weather and climate change. The Royal Observatory of Belgium (ROB) plays a central role in the continuation of this experiment, as it hosts the Sunspot Index and Long-term Solar Observations World Data Center (SILSO-WDC) . This World Data Center aims at collecting solar data, as well as producing and distributing the International Sunspot Number, which is used in about two hundred scientific publications on an annual basis.

This project hinges on two sunspot collections kept at ROB. The first collection consists of a series of about 20.000 digitized sunspot drawings acquired at the Uccle Solar Equatorial Table station (USET, you can see examples of these drawings on this page) since 1940 and a corresponding database. The second collection, the SILSO database, contains the numbers of spots and groups of spots on the Sun observed by a worldwide network since 1981 (more than 530.000 measurements). It is used on a monthly basis to compute the International Sunspot Number. A complete and consistent supervision of this database is important and as SILSO is the World Data Center for the determination of this index, it is our responsibility to bring the extracted Sunspot Number into the 21st century by exploiting this existing database to its full potential.


The objective to achieve on our two collections is threefold: (1) Bring our databases to the modern era by adding essential metadata, whether it is additional parameters or techniques for quality assessment and quality control. (2) Use the value of solar parameters recorded in these databases to address today’s scientific questions and (3) disseminate the collections and added value.


For the exploitation of our first collection, the USET drawings, we will first extract additional parameters through a mix of advanced programming techniques and semi-automatic techniques. Extracting such parameters is paramount to the production of the real-time bulletins used in the alert system of the Regional Warning Center (RWC) of the SIDC. As a RWC, we need an alternative source of data to feed our alert system. Second, we will assess the quality level of the existing and future associated metadata by comparing the extracted solar parameters catalogue to other overlapping catalogues.

For the exploitation of our second collection, the SILSO database, we will start by adding error-bars and then perform an extensive check for the stability and quality of past and present observers of the network, with the aim to define a robust subset of the network’s observation stations to be used as a reliable reference multi-station pool that will replace the current single pilot station.

For this, modern methods for statistical quality control must be used and adapted. As this is a growing database that still receives data on a daily basis, the process of determining a multi-station reference needs to be done dynamically. The current processing, done every month and every 3 months, is a historical heritage based on a very slow availability of data that is no longer relevant. Hence, those novel statistical methods need to be rendered dynamic in order to provide a method that is computable in near real-time.

This project is at the crossroads between Solar Physics, scientific programming and mathematical statistics. That is why this projects hinges on the close collaboration between the UCL and its statistical institute, and the ROB. In addition to that, the output of VAL-U-SUN presents a huge interest for domains outside of Solar Physics or Statistics such as Space weather and Climate Science.

Because of the importance of the output of this project for our understanding of the historical and future evolution of the Climate the modern evaluation of the Sunspot Number will have a non-negligible impact on decision-making. In addition, the use of our local data for warning bulletins is of paramount importance to space weather as existing real-time stations are getting more and more scarce because of a persistent lack of funding.

Scientific articles describing the studies conducted in the context of this project and their results will be published in well-known journals. During the course of the project, scientific participants will attend international conferences where they can directly interact with the follow-up comity members and present their results to the international community. The drawings and their metadata will be made available to the general public and a last quality check will be performed on these data in the context of a “citizen science” project. The sunspot number database will also be made available: in particular, each observer of the SILSO network will have access to his or her real-time quality assessment delivering a much needed feedback to our faithful observers.

You can help us

If you are interested in this project, and are willing to participate, we need you ! For statistical purposes, we need everyone with different backgrounds, a different knowledge and experience. Basically, you don't need to be a rocket scientist to help us. We've set up a citizen science website where you'll be guided through a fairly simple tutorial. You'll learn about the way people who actually work as solar observers draw and count sunspot groups. And finally you will be asked to count them as any other solar observer.