Use of L5 Data in CME Propagation Models

Introduction

ESA/A. Baker, CC BY-SA 3.0 IGO

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 predictions 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 is being made of data from the NASA STEREO-B spacecraft during the period 2009 to 2011 and STEREO-A during 2020 to 2021 when their orbits placed them 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.

Tasks

The activity is split into four sequential tasks each consisting of a set of work packages many of which run concurrently.

Task-1 Requirements Analysis and Elaboration of Use Cases

The requirements analysis task includes work packages to review the current state of the art, establish whether further elaboration of the SWE Network baseline requirements is needed to support traceability of multi-viewpoint techniques and assessment of the specific Vigil measurement requirements and anticipate product developments based on the baseline payload. A preliminary elaboration of the Use Cases that will be used to quantify and validate the benefits of multi-viewpoint measurements and that will provide the basis for the bulk of the work in the remaining tasks is also developed at this point.

Task-2 Analysis Environment Design and Preparation

This is the preparation task for the Use Case experiments that are scheduled within Task-3. The primary aim of this task is to establish the consistent methodology that will be used for the execution and assessment of the Use Case experiments to ensure that quantitative comparisons can be made. A key element of this task in the finalisation of the event list that will be used to drive the Use Case experiments taking account of data availability and event characteristics to ensure that they are suitable for supporting the analysis and that considerations such as selection bias are adequately understood. This task also finalises the elaboration of the Use Case experiments which will assess the impact of the inclusion of multi-viewpoint measurements in the analysis pipeline and how measurement properties such as measurement resolution and cadence can affect the quality of the forecast.

Task-3 Multi-viewpoint Assessment

This involves the systematic implementation and application of the specified Use Cases to the agreed set of events and analysis intervals. The list of the Use Cases follows below.

There are necessarily some interactions and inter-dependencies between the Use Cases as shown below. These have been assessed as part of the planning activities and are taken into account to ensure that there is efficient scheduling and exchange of the necessary inputs and outputs.

The study will assess the benefits that can be realised from multi-point observations for operational forecasting when applied to different models and techniques. For example two techniques, Cone model and the more advanced Graduated Cylindrical Shell (GCS) model, will be used for the initial near-Sun characterisation of CMEs which is a vital input to the subsequent propagation models. The figure below shows a snapshot of the multiviewpoint GCS analysis for one of the selected events.

The study will also make use of different sensitivity experiments to investigate how properties of the data such as spatial resolution and cadence affect the ability and quality of the operational forecasts. For example for figure below shows two views of the Heliospheric time-elongation maps used for CME tracking. In this case the same input data is shown for two sensitivity experiments, the original STEREO-A observations on the right (best case) and the equivalent reduced cadence and resolution on the left (worst case).

Task-4 Roadmap for Vigil Service Products and Tools

The final task of the activity will draw together the key findings of the Use Case analysis in order to develop a roadmap for the future utilisation of the Vigil mission data within the context of the SWE services and the exploitation of multi-viewpoint techniques to which Vigil can contribute.

The roadmap will provide recommendations and development steps that will need to be taken in order to build on the potential improvements on CME propagation provided by the L5 data from the Vigil mission. It will also identify capabilities that are not currently mature enough to have been included in the Task-3 assessment activity but have the potential to become operational on the timescale of the Vigil mission.

Study Team

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