# CMAG: A Mission to Study and Monitor the Inner Corona Magnetic Field
## 1 Introduction
### Top Science Requirements  
To achieve its science goals, CMAG must collect as many photons as possible to make the required measurements of all four Stokes parameters of the selected spectral line(s) to achieve the required polarimetric performance. The necessary spatial, spectral, and temporal resolutions will be possible through full vector spectropolarimetric observations using a coronal magnetograph as the sole instrument on board a spacecraft, hereafter referred to as the CMAG spacecraft. This will be combined with an external occulter installed on another spacecraft, hereafter referred to as the occulter spacecraft. These two spacecraft will maintain a formation flight distance of 430 m for coronagraphic observations. At that distance, the radius of the occulter has to be around two meters. The observational periods will be of, at least, 2.5 h every twelve hours (this assumes a mission to L5 and a certain solid fuel capacity that may vary). The main scientific and design requirements are listed in Table 1. Since polarimetry demands precise pointing stability (Performance Drift Error, PDE, must be better than 0.125\({}^{\prime\prime}\) in one minute), both spacecraft will adhere to stringent requirements (see next Section).  
CMAG's measurements complement those of METIS aboard the Solar Orbiter mission by providing access to the coronal magnetic field. While our goals are similar to those of the (on-ground) large coronagraph of COSMO (the COronal Solar Magnetism Observatory project [41]), we use a much smaller aperture telescope (10 cm vs. 170 cm). Nonetheless, we expect to obtain sharper coronal structure maps due to the absence of atmospheric seeing and a larger throughput, resulting in high S/N observations in less time. Moreover, externally occulted coronagraphy ensures significantly lower scattered light levels (by three orders of magnitude [42]) when the occulter is positioned at 430 m, increasing our polarimetric sensitivity and allowing us to observe much closer to the solar limb (at 1.02 R\({}_{\odot}\)) where scattered light effects are more detrimental (see Figure 3).  
The approved NASA's PUNCH mission [43], expected to be launched in 2025, follows a loose formation flying concept and aims to explore the corona and solar wind by imaging light scattered from free electrons from 6 to 180 solar radii. Missions such as PUNCH will complement the science being carried out now by other missions such as Solar Orbiter, enabling space weather scientists to trace solar phenomena from their origin to their potential impacts on Earth. CMAG's expected benefits will be a significant leap in this endeavor, provided that it is operational in a reasonable time to overlap with the above missions. DKIST will also benefit from pointing its extremely high spatial resolution instrumentation to targets provided by CMAG. Additionally, CMAG is expected to be a full member of the battery of instruments enabling multimessenger solar astronomy, combining its measurements not only with remote-sensing coronagraphs but also with in situ instruments aboard missions such as Solar Orbiter and Parker Solar Probe, ushering in a new era of solar physics [44].