COronal Diagnostic EXperiment (CODEX)

Understanding solar wind sources and acceleration mechanisms is an overarching solar physics goal. Current models are highly under-constrained due to the limitations of the existing data, particularly in the ~3-10 Rsun range. COronal Diagnostic EXperiment (CODEX) is designed to deliver the first global, comprehensive data sets that will impose crucial constraints and answer targeted essential questions, including: Are there signatures of hot plasma released into the solar wind from previously closed fields? What are the velocities and temperatures of the density structures that are observed so ubiquitously within streamers and coronal holes?

To provide these crucial measurements, NASA’s Goddard Space Flight Center, in collaboration with the Korea Astronomy and Space Science Institute, and the Italian National Institute for Astrophysics have developed a next-generation coronagraph hosted on the International Space Station starting November 2024. This imaging solar coronagraph uses multiple narrow passband filters to obtain simultaneous measurements of electron density, temperature, and velocity of the nascent solar wind. CODEX provides comprehensive data sets that test theories of solar wind formation and provide crucial constraints on predictive solar wind models, taking multiple daily measurements in this critical solar wind formation region.

The CODEX coronagraph has the following unique technological features driven by the science objectives:

1. All externally occulted coronagraphs in the past, such as, SOHO/LASCO-C2, STEREO/SECCHI-COR2 etc. suppress the diffraction using 3 elements: External Occulter, Internal Occulter, and Lyot stop. To reach the signal-to-noise ratio required for coronal temperature and velocity measurements, an additional occulter at telescope focus, i.e., the focal mask, is added. The diffraction analysis, verifies that the added focal mask indeed further suppresses the diffraction efficiently, which decreases the noise without compromising the signal and the field of view (FOV). Additionally, the vignetting is also reduced, which implies the signal increase. Together, the S/N ratio has been significantly increased.
2. Current space- and ground-based white-light coronagraphs, including STEREO/COR1 and COR2, SOHO/LASCO, and MLSO/Mark IV, are capable of imaging the coronal environment in polarized brightness, which can easily be inverted to create synoptic maps of the coronal electron density. However, for speed measurements, existing coronagraphs can only observe moving structures and have little sensitivity for the background solar wind nor temperature information. The coronal electron temperature and velocity are measured using the filter-ratio technique. This new technology imaging coronagraph uses multiple narrow passband filters to isolate key areas of the spectrum and obtain simultaneous measurements of electron density, temperature, and velocity within a single instrument. This will be the first time all three have been measured simultaneously for this field of view.
3. A polarization camera, utilizing a Sony IMX253MZR CMOS detector, is used to replace the polarizer mechanism used in most coronagraphs. This greatly simplifies the design, and still provides the detailed polarization measurements that enable us to remove the light from the F-corona (dust in interplanetary space) and only see the desired K-corona (solar wind electrons).

CODEX launched on November 4, 2024 on board the SpaceX Falcon 9 Commercial Resupply mission CRS-31. CODEX was installed on the International Space Station Express Logistics Carrier 3 on November 11. CODEX completed commissioning and is currently in science operations. The Baseline Mission extends through 30-June-2025. All systems on CODEX are functioning nominally and returning ~500 images per day (~12 GB/day uncompressed). Science analysis is ongoing. It is expected the data (Stokes vectors I, Q, U, polarization brightness pB, calibration) will be publicly released in summer 2025.

CODEX supports NASA's strategic objectives to understand the Sun and its interactions with Earth and the solar system, including space weather. This will be achieved by developing/demonstrating instrumentation technology. Specifically, by applying these technologies, CODEX will use these measurements of density, temperature and radial speed to test fundamental theories of solar wind formation and provide crucial constraints on predictive models of the solar wind. Specifically, CODEX will determine

1. the low coronal source of different types of solar wind streams
2. how much solar wind plasma is released through magnetic reconnection of closed magnetic fields, and
3. the amount of energization and acceleration of the plasma as a function of height through the middle and upper corona.

However, the CODEX coronagraph has added features that can measure some very important properties of the solar wind, which can travel as fast as a million miles per hour as it flows off the Sun carrying charged particles or plasma and embedded magnetic fields outward across the solar system. Although scientists know that the solar wind originates in the corona, they don’t know precisely how it forms or accelerates.

This question is of particular importance to scientists. Understanding the source of the solar wind, which determines how space weather-causing coronal mass ejections, or CMEs, propagate between the Sun and Earth, can help improve space-weather forecasts, particularly in the near-Earth environment where changes can sometimes interfere with radio communications or GPS. During particularly strong geomagnetic storms, sparked by the release of tons of charged particles during a CME, particles that make up the solar wind can flow along magnetic fields through Earth’s protective magnetosphere onto the surface where they can disrupt power grids and electronics.

There have been two long-standing inter-related issues in solar wind physics. The first issue regarding the origin of solar wind on the Sun; in other words, regarding the source regions of the solar wind. 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 1970s (Fu et al. 2015). 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, 1996; Cranmer et al. 2007), or in the closed field regions (e.g. the interchange reconnection model Fisk et al. 1998; 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).

The second issue is to determine how the solar wind is accelerated to several hundred kilometers per second within 10 Rsun. 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 1992; Axford and McKenzie 1992). Velocity, temperature, and density measurements in this range are crucial to validate the current theories

All solar wind source and acceleration models are under-constrained: they only explain the large-scale, long-term structure in the solar wind that results in the average differences between fast and slow wind. Due to the scarcity of simultaneous measurements of velocity, temperature, and density, current data cannot distinguish between the models. CODEX’s regular, global measurements in the acceleration region between ~3-10 Rsun provide the crucial missing measurements to determine which solar wind acceleration and source theories are valid.

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