Recruitment and Integration of Observers in the SILSO Network

1. Overview of the SILSO observing network

The Sunspot Index and Long-term Solar Observations (SILSO) service operates a global network of observing stations contributing sunspot observations used to compute the International Sunspot Number. The network consists of both professional observatories and experienced amateur astronomers who regularly observe the solar disk and report sunspot counts.

Each participating observer or observing site is referred to as a station. A station typically corresponds to a specific observing setup, which may include a particular telescope, observing method, and observer (or group of observers). The station identifier allows SILSO to track observations, assess observer consistency, and compute individual scaling factors used in the construction of the international sunspot number.

The SILSO network currently includes more than 300 observing stations worldwide, among which 80 active stations which can be seen as red dots in the figure below.

2. Network governance

The SILSO observing network is coordinated by the network manager (O. Lemaître) and the Director of SILSO (L. Lefèvre), and is hosted at the Royal Observatory of Belgium.

The network manager is responsible for:

The director of SILSO is responsible for:

The SILSO Advisory Panel is responsible for:

This governance structure ensures that the observing network is actively maintained and that the composition of the network remains scientifically appropriate.

In addition, SILSO runs its own observing station from the network, the USET (Uccle Solar Equatorial Table: https://www.sidc.be/uset/). It is a professional observing station run in shifts of one week. The SILSO director and network manager are regular observers of USET as well as the head of USET (Dr. S. Bechet). The local station enables SILSO to understand all the challenges faced by the observers and answer all technical questions from a user perspective.

3. Initiation of participation

New observers generally contact SILSO on their own initiative, expressing interest in contributing observations to the network. Upon receiving such a request, the SILSO network manager responds with information about the requirements and expectations for participation. Observers are informed that regular observations are required and that participation is ideally long-term.

In particular, SILSO requests approximately 8–10 observations per month on a sustained basis to ensure meaningful statistical comparison between observers. Observers are also encouraged to commit to observations over several years, since long-term consistency is important for the calibration of the Sunspot Number.

4. Information requested from prospective observers

Before admission to the network, prospective observers are asked to provide information about their observing setup and practices.

Typical information requested includes:

This information allows SILSO to evaluate whether the observing setup is suitable for contributing useful data to the international dataset.

5. Evaluation and acceptance

The SILSO network manager reviews the information provided by the candidate observer.

The following aspects are considered:

The network manager then discusses the application with the SILSO director. If the observer is deemed suitable, the observer is admitted into the network.

6. Creation of observing station credentials

Once accepted, the observer is assigned:

Observers submit their observations through the SILSO web interface: http://sidc.be/WOLF

Each station enters its daily observations through this interface, which includes validation checks to detect common data-entry errors and automatically integrates the observations into the SILSO processing system.

7. Data submission and operational rules

Observers are asked to submit observations as soon as possible after the observation, since rapid reporting contributes to the daily estimated Sunspot Number.

The submission system performs automatic checks, such as verifying that the Wolf number is consistent with the reported sunspot counts.

Observers can correct their entries for up to three months after submission.

Detailed observing definitions and rules are publicly available at: https://sidc.be/WOLF/definitions.php

8. Training and learning resources

To support new observers and ensure consistency of observations, SILSO provides training and reference material. These include:

These tools allow observers to learn how to identify sunspot groups and count sunspots according to the SILSO methodology.

9. Quality control of observations

Observations submitted by stations are monitored statistically as part of the computation of the International Sunspot Number. Each observing station is associated with an individual scaling factor (k-factor), which is determined from long-term comparisons with reference observations. This factor accounts for differences between observers due to instrumentation, observing techniques, and local conditions.

Stations with irregular observation cadence or inconsistent observations can be flagged and their contributions evaluated. When necessary, observations from a station may be temporarily excluded from the computation until the issue is clarified.

This statistical monitoring ensures the scientific consistency and long-term stability of the sunspot number time series.

10. Role of the observing network in the SILSO data production chain

The observing network is the primary data source for the International Sunspot Number.

Individual station observations are combined using statistical methods to compute the daily, monthly, and yearly sunspot numbers. The long-term continuity and diversity of the network are essential to ensure robustness and stability of the time series.

The network therefore plays a central role in the operation of SILSO as a World Data Center for the Sunspot Number.

11. Network robustness and continuity

The SILSO observing network has operated for many decades and has historically relied on a combination of professional observatories and experienced amateur astronomers. The distributed nature of the network provides significant resilience: the International Sunspot Number is derived from the combination of observations from many stations rather than relying on a single source.

At any given time, multiple observing stations contribute daily observations, which ensures continuity even if individual stations temporarily stop observing due to weather, technical issues, or personal circumstances.

The SILSO team continuously maintains and renews the network by recruiting new observers when appropriate and by supporting existing observers through documentation, training tools, and direct communication. This ongoing management ensures the long-term sustainability of the observing network and the continuity of the sunspot number time series.

Definition of a station in the SILSO network

In the SILSO observing network, the term “station” refers to a specific observing setup used to report sunspot observations.

A station is typically defined by:

Each station is assigned a unique station identifier used in the SILSO database. This identifier allows the SILSO team to track observations from that particular setup and to compute the individual scaling factor used in the calculation of the International Sunspot Number.

If an observer changes instrument or observing location significantly, a new station identifier may be created, since the observational characteristics may differ.

This station-based approach allows SILSO to monitor the statistical behaviour of each observing configuration and maintain the long-term consistency of the sunspot number series.

SILSO - 2026