# CMAG: A Mission to Study and Monitor the Inner Corona Magnetic Field
## 2 Mission Concept  
### Mission Profile  
The mission consists of two spacecraft, the CMAG and the occulter spacecraft, flying in formation at the L5 Lagrangian point for a duration of 7 years. The occulter spacecraft must carry the necessary equipment for enabling formation flying, as well as the occulter disk. The CMAG spacecraft will carry the main scientific payload instrument: a coronal magnetograph. During observations, CMAG will be positioned approximately 430 m behind the occulter, significantly reducing solar disk scattered light by at least one order of magnitude as compared to the Proba-3 mission. Figure 3 shows the relevance of a high intersatellite distance. Unity represents the disk intensity. The regular bright profile without any instrument is plotted in green. The use of an external occulter such as that in Proba-3 at about 140 m from the coronagraph reduces the levels significantly (orange). However, if the occulter is at about 400 m, as in the case of CMAG, the background of scattered light decreases by one more order of magnitude, hence making the observation of the solar corona easier in a unique way. Moreover, the diffraction effects near the limb are much better suppressed, which allows for closer observation to the solar limb, much closer to the transition region of the Sun. This reduction is crucial to extract the polarimetric observations needed to measure the coronal vector magnetic field. Indeed, critical heights of CME onset and prominence instabilities take place from 1.03 to 1.1 R\({}_{\odot}\)[45], so those heights close to the solar limb are critical to observe if one wants to follow these events at all their evolutionary stages. The mission's choice of the Lagrangian L5 point, which positions CMAG at an angle of 60 degrees away from the Sun-Earth line, holds a significant advantage for solar physics and space weather research altogether. This positioning allows us to effectively identify the formation of potentially eruptive structures within the critical region of the corona, often referred to as the \"danger\" zone for Earth. Moreover, from this unique vantage point, we gain the capability to observe these structures during their gradual ascent in what is known as the \"slow-rise phase\". This level of observation and insight into solar phenomena is unattainable from the Earth-based perspective and is crucial for enhancing our understanding of solar activities that can impact our space environment. In any case, it is worth noting that the observation of magnetic fields in the inner corona from space remains a significant challenge in solar physics. Therefore, the majority of our (stand-alone) solar objectives could still be achievable from, for example, the Lagrangian L1 point. Figure 3: Solar scattered light intensity as a function of the distance to the solar disk center. CMAG observations can be performed simultaneously and without interference to any other instrumentation aboard the occulter spacecraft. Both spacecraft can be launched together and then transferred to the Lagrangian point. The CMAG spacecraft will travel as a small piggyback of the main primary mission. Both will have the necessary metrology equipment to maintain the right formation flying configuration with respect to one another. Earth-Sun system Lagrangian points, being in solar orbit, guarantee minimum propellant consumption for maintaining the formation due to their relatively small gravitational perturbations compared to any Earth-bound orbit. Preliminary analyses show that with current technology, mission performance needs can be accomplished. However, in addition, several opportunities have been identified that could potentially improve this further upon design consolidation. A radio-frequency link will be implemented between both spacecrafts providing two key services: on one hand as a data link, on the other hand as a first metrology step, determining relative position thanks to differential RF ranging technology.