1. Overview

Lagrange points in the Sun–Earth system (not to scale). By Xander89 - File:Lagrange_points2.svg, CC BY 3.0, https://commons.wikimedia.org/w/index.php?curid=36697081

File:Solar Blast.ogv To ensure an effective capability to monitor potentially dangerous solar events, ESA initiated a study of two potential future space weather satellites called Lagrange.^[1] The Lagrange mission concept is overseen by the Space Situational Awareness Programme at ESA. On 2 February 2018, ESA signed technological contracts (Phase A) to be led by Airbus UK and OHB SE of Germany to design the spacecraft specifications and the instruments' integration process.^[2] UK's Rutherford Appleton Laboratory and Mullard Space Science Laboratory will assess the requirements of the science payload.

This mission concept proposes positioning two spacecraft in orbit at the L1 and L5 Lagrangian points, respectively, where gravitational forces interact to create a stable location to save propellant and from which to make observations. L1 is in the solar wind 'upstream' from Earth, so measurements at L1 provide information about the space weather coming toward Earth. In contrast, the L5 point provides a way to monitor coronal mass ejections (CMEs) from the 'side' in order to estimate their speed and direction.^[3]

2. Objectives

The preliminary mission objectives are:^[3]

• the spacecraft at L1 is to provide observations of the solar wind speed, density, temperature and dynamic pressure, charged particle environment and the direction and strength of the interplanetary magnetic field.
• The spacecraft at L1 would also monitor the solar disk and solar corona and measure solar energetic particles that may be associated with solar flares and the onset of coronal mass ejections.
• The spacecraft at L5 would complement measurements made from L1 by providing a side-view of the propagation of plasma clouds emitted by the Sun toward Earth.
• The spacecraft at L5 would monitor of the solar disk and corona and carry out measurements of the interplanetary medium.

3. Payload

To achieve these objectives, the satellites at the L1 and L5 positions have to carry different types of remote-sensing and in-situ instruments. The suggested optical instruments take heritage from ESA and NASA science missions like SOHO, STEREO and Solar Orbiter, but the instruments would be optimized for operational space weather monitoring.^[4] The notional science payload may require:^[3][4]

Optical instruments

• Coronagraph – for onset and characterisation of coronal mass ejections (CMEs).
• Heliospheric imager (HI) – A wide-angle visible-light imaging system for the detection of coronal mass ejection events directed toward Earth.
• Magnetograph - would scan a selected solar spectrum to generate 3D maps of the magnetic field.
• EUV imager – Imaging of the complex solar corona (the Sun's atmosphere) will support monitoring of the magnetic complexity and activity in the corona and location of the flaring active regions.
• X-ray flux monitor – Detection of solar flares and quantification of the flare energy.

In situ instruments

• Magnetometer – Measurement of the interplanetary magnetic field.
• Plasma analyser – To measure angular velocity components of the solar wind approaching the Earth and estimates of geomagnetic storm strength.
• Radiation monitor – Monitoring radiation storms (Solar particle event) is crucial, as these can disrupt and damage spacecraft, aircraft and ground systems.
• Medium energy particle spectrometer – This can monitor clouds of medium-to-low energy ions approaching the Earth.