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Use of L5 Data in CME Propagation Models


Figure 1, Illustration of ESA's Lagrange L5 mission.

Current operational space weather forecasting is largely constrained by availability of assets located at the Lagrange 1 (L1) vantage point, in geocentric orbit or on the ground. These provide effectively a single viewpoint and limit the forecaster's ability to view solar disk features, such as active regions that, as the Sun rotates, will influence space weather along the Sun-Earth Line (SEL).

When an earthward directed Coronal Mass Ejection (CME) is launched it is observed by near-Earth coronagraphs as a halo signature, which helps to resolve the propagation direction but leaves a degree of ambiguity between the CME width and its radial propagation speed. As a result of these factors, the accuracy of CME forecasting from a single viewpoint is associated with an average uncertainty of typically +/- 12 hours but this belies large spreads such that an individual event may have a forecast with an error more than twice this average figure.

Locating additional instrumentation away from the SEL provides alternative viewpoints that can be combined with SEL measurements to remove or reduce the ambiguities of single viewpoint observations. The Lagrange 5 (L5) vantage point (trailing the Earth by 60 degrees) is ideal from this perspective as it will provide both an early view of solar disc features before they appear on the visible disk as seen from Earth and also a side-on view of Earthward directed CMEs as they propagate out from the Sun.

Study Objectives

The "Use of L5 Data in CME Propagation Models" study is taking a systematic and quantitative approach to assessing the anticipated benefits that the combination of L5 with L1 and near-Earth data will have on operational space weather capabilities. The main focus of the activity is on CME arrival time predications at Earth as socio-economic studies have demonstrated that these have the largest net impact as a result of their potential to disrupt terrestrial technological infrastructure. However, other space weather factors are also being considered, such as the prediction of high-speed solar wind streams and the profile of Solar Energetic Particle (SEP) events. The main assessment activity will take a step-wise approach, looking at the potential gains from combined L5 and L1 observations in, for example, CME characterisation and background solar wind estimation before looking at the overall improvements in end-user products such as forecasts of CME arrival time. Extensive use will be made of data from the NASA STEREO-B spacecraft during the phase of the mission where its orbit placed it close to the L5 location. The work shall take account of the fact that its purpose is for the assessment of improvements in an operational forecasting capability and will therefore take into account the cadence and timeliness of the input data that is used to drive the models.

Study Team

The study is being undertaken by an experienced team of space weather model developers, researchers, service providers and forecasters.


STFC, RAL Space (RAL Space)

Science and Technology Facilities Council (STFC)

United Kingdom


UK Met Office (UKMO)

United Kingdom

University of Graz, Institute of Physics (UNIGRAZ)

University of Graz (UNIGRAZ)


Institute for Astrophysics (UGOE)

University of Göttingen (GAU)


University of Central Lancashire (UCLan)

United Kingdom

University of Helsinki


University of Reading

United Kingdom

Austrian Academy of Sciences


Mullard Space Science Laboratory

University College London

United Kingdom

University of St Andrews

United Kingdom

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