Primaray Goal
Understand the physical conditions in the solar wind acceleration region

Question 1. Are there signatures of hot plasma released into the solar wind from previously closed fields?
CODEX measurements address two long-standing, inter-related issues in solar wind physics: the sources and acceleration mechanisms of the solar wind. Source regions of the solar wind have been studied as early as the 1950s (Parker 1958; Poletto 2013). The scientific consensus is that the fast wind originates from coronal holes (McComas et al. 2000), but the source of the slow wind is still undetermined and may be in the open field regions adjacent to the coronal streamers (e.g. the expansion factor model Wang & Sheeley, 1990; Cranmer et al. 2007), or in the closed field regions (e.g. the interchange reconnection model Fisk 2003; or the Separatrix-web model Antiochos et al. 2011), or a combination of both (Wang & Sheeley 2003; Stakhiv et al. 2015) (see also reviews by Cranmer (2009) and Abbo et al. (2016)). Furthermore, magnetic reconnection may also be contributing a significant amount of closed-field plasma to the fast wind through coronal hole jets (Raouafi et al. 2016). So for both fast and slow wind, it is actively debated how much wind forms via magnetic reconnection releasing plasma initially confined to closed loops vs. a wind entirely accelerated along quiescent open flux tubes (Axford et al. 1999; Cranmer & van Ballegooijen 2010). In situ evidence suggests that reconnection releases plasma that is hotter than its surroundings, even outside of helmet streamer tips (e.g., Geiss et al. 1995; Stansby & Horbury 2018), but the measurements necessary to unambiguously identify the hot plasma immediately after formation don’t exist. CODEX enables this revolutionary measurement.

Question 2. What are the velocities and temperatures of the density structures that are observed so ubiquitously within streamers and coronal holes?
The second issue is to determine how the solar wind is accelerated to several hundred kilometers per second within 10 R_sun. Two general mechanisms have been proposed for heating and acceleration: wave/turbulence (Cranmer et al. 2007; Velli & Grappin 1993; Hollweg & Isenberg 2002; Ofman 2010) and interchange reconnection between open fields and small closed flux regions embedded within the hole (e.g., Parker 1991; Axford & McKenzie 1992). Even with the reconnection mechanism, waves play a central role, because most of the energy released is believed to propagate away from the reconnecting site in the form of Alfven waves (Fisk, 2003). It is widely accepted that the solar wind acceleration region is in the heliocentric distance range ~2-8 R_sun (e.g., Withbroe et al. 1982; Cranmer 2009; Antonucci et al. 2012). Current information in the region ~3-10 R_sun for testing solar wind source and acceleration theories is limited to density from coronagraph images. The corona is now known to be highly structured in space and time on all scales down to the resolution of STEREO/SECCHI COR2 (DeForest et al. 2018). CODEX measurements within and between this ubiquitous structure provide crucial tests of the source of the structures and its effect on the resulting solar wind.

Secondary Goal
Enable and validate the next generation of space weather science models

Objective 1. Provide first observational constraints to empirical solar wind models of electron heating and acceleration in the key region of 3 – 8 R_sun.
The vast majority of coronal and solar wind models use partial information about the state and/or evolution of the solar surface as a lower boundary condition. Typically, surface magnetograms are extrapolated into the corona. The models also rely on simplifying hypotheses for the top boundary conditions, as well as for the physical processes leading to the heating and acceleration of solar wind flows. CODEX’s high resolution, global measurements over entire solar rotations allows for testing the full 3D validity of the models. Furthermore, the present global corona/heliosphere models are strictly steady-state. They do not include temporal variations of the photospheric field and, therefore, cannot capture the dynamics critical for the slow wind. CODEX will measure these fluctuations in density, temperature, and velocity, enabling daily characterization of their properties and for all types of structures. CODEX’s revolutionary new measurements of coronal and heliospheric dynamics will elevate space weather modeling to a dramatically new level of accuracy and rigor.