# CMAG: A Mission to Study and Monitor the Inner Corona Magnetic Field
## 3 A Mission Opportunity  
The CMAG mission was proposed to ESA in February 2022 in response to ESA's call for a \"Fast\" mission opportunity. In that proposal, the main platform was assumed to be the current ESA's Space Safety Vigil mission, which, with a few configuration modifications and additional equipment, can act as an external occulter and enable formation flying. The combination with the Vigil mission at the L5 Lagrangian vantage point ensures a unique, low-risk, cost-effective, and interdisciplinary scientific return considerably improving current coronal models and space weather predictions with a clear benefit for both the Science and the Space Safety programs of ESA. The use of Vigil as an external occulter presents an exceptional opportunity for the solar physics and space weather communities, as it allows establishing a connection between photospheric magnetic activity (observed with the Photospheric Magnetic Field Imager instrument, PMI, aboard Vigil [50]) and the corona through CMAG's continuous monitoring of the magnetic field and plasma velocities in the inner corona. Combining CMAG with Vigil's solar instrumentation ensures optimal scientific return and increased space weather forecasting capabilities, which are not expected from any other space agency in the near future, resulting in a significant qualitative leap. However, CMAG can be adapted to any other space mission, provided it fulfills the science requirements. Modifications to the main mission will be necessary. The main spacecraft needs to carry an occulter disk, which can come from redesigned solar panels or a retractable structure, ensuring the mission's remote sensing payload remains unperturbed. The intersatellite distance, estimated to be 430 m, requires a 2.5 m disk radius. The distance between the two spacecraft, as well as the final external occulter's disk radius, has been defined based on scientific requirements. Additionally, careful consideration has been given to ensure that the mission accommodates the dimensions of the Ariane 6 fairing. The chosen distance is also sufficiently large to prevent any protrusions from potential additional instrumentation on board the occulter spacecraft. This meticulous planning aims to optimize the scientific objectives while ensuring compatibility with the spacecraft's launch configuration. The metrology suite of instruments for formation flight does not necessarily interfere with the main AOCS system design, resulting in an estimated total mass increment of 24 kg, including the occulter disk. CMAG can be used as a science data relay, transmitting data through an intersatellite link to the main mission, which downloads them to the Ground Station. Operationally, CMAG observations will be performed autonomously, providing the necessary data products. It can also be used on demand if the scientific team needs particular observations. Conceptualization, D.O.S., J.C.d.T.I, F.J.B.M., M.B.J., D.A.G., D.S., L.F.P., L.R.B.R., J.A., J.T.B., R.E., J.A.D., L.M.G., S.A.M., I.P.G., I.T.G., E.S.K., J.L.G.B., D.M.L., M.M., C.K., J.L., S.F. and E.S. ; methodology, D.O.S., J.C.d.T.I, F.J.B.M., M.B.J., D.A.G., D.S., L.F.P., L.R.B.R. and J.A. ; software, A.V.-R., D.H.E., F.J.B.M. and B.R.C. ; investigation, D.O.S., J.C.d.T.I, F.J.B.M., D.A.G., D.S., L.F.P. and L.R.R.R. ; writing--original draft preparation, D.O.S. and J.C.d.T.I; writing--review and editing, F.J.B.M., M.B.J., D.A.G., D.S., L.F.P., L.R.B.R., J.A.V.-R., I.P.G., I.T.G., E.S.K., J.L.G.B., D.H.E., B.R.C., J.T.B., R.E., J.A.D., L.M.G., S.A.M., D.M., M.C.K., J.L., S.F. and E.S. ; visualization, D.O.S., F.J.B.M., J.A., A.V.-R. and D.S. ; supervision, D.O.S., J.C.d.T.I and M.B.J. ; project administration,J.C.d.T.I. and M.B.J. ; funding acquisition, D.O.S. and J.C.d.T.I. All authors have read and agreed to the published version of the manuscript.