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A mission proposal led by the Solar Physics Branch of the Naval Research Laboratory (NRL) Space Science Division is one of six selected for further study as part of the NASA Small Explorer (SMEX) Program. The NRL-led team successfully proposed the innovative Coronal Physics Explorer (CPEX), a mission that will address three fundamental questions in the field of heliophysics:

• What are the physical processes responsible for heating and accelerating the primary (proton, electron, helium) and secondary (ion) plasma components of the fast and slow solar wind?
• How are Coronal Mass Ejections (CMEs) heated and accelerated?
• How are the geoeffective properties of CMEs (including Solar Energetic Particles, or SEPs) established in the extended corona?

The CPEX science payload builds on discoveries by the NRL Large Angle Spectrometric Coronagraph (LASCO) and the Smithsonian Astrophysical Observatory (SAO) Ultraviolet Coronagraph Spectrometer (UVCS), which are currently operational on the NASA/ESA Solar and Heliospheric Observatory (SOHO) mission. CPEX consists of a next-generation solar coronagraph telescope situated 11 meters behind an external occulter that blocks direct solar disk light while allowing the faint outer solar atmosphere, or corona, to be observed. The telescope feeds two innovative focal plane packages, a visible light coronagraph polarimeter (VLCP) and an extreme ultraviolet spectrograph (UVS). The VLCP design will allow density measurements of the faint solar corona plasma in unprecedented detail and return images of the inner corona of a quality previously possible only during brief, natural solar eclipses. The UVS will have significantly greater spectral sensitivity to the line emission arising from coronal plasma ions and much higher spatial resolution than previous similar instruments. The UVS will also provide a new capability to detect suprathermal ions in the corona. These suprathermal ions are believed to be the seed particles for high-energy SEPs, which are a significant radiation hazard for astronauts, satellites, and high-altitude aircraft. The combined white light (VLCP) and extreme ultraviolet (UVS) observations will enable a major breakthrough in our understanding of the coronal conditions that allow a CME to become an effective accelerator of SEPs.

The CPEX Principal Investigator, Dr. Dennis Socker, leads a Co-investigator Team that includes personnel from NRL and SAO. NRL will manage and develop the science payload, coronagraph telescope, VLCP, as well as the instrument structure and electronics. SAO will provide the UVS focal plane package. The payload operations control center will be located on-site at NRL during the mission.

The Co-investigator Team will be funded during the mission to analyze the new data set in pursuit of answers to the fundamental science questions addressed by CPEX. A spacecraft and associated services will be procured by NRL from an industrial partner.

The NASA Explorer Program Office, located at the NASA Goddard Space Flight Center in Greenbelt, MD, provides overall management of the mission. The Small Explorer missions provide focused scientific flight opportunities at relatively moderate cost compared to large observatories.

*Link for NASA SMEX award announcement: [http://www.nasa.gov/home/hqnews/2008/may/HQ_C08029_SMEX_Awards.html]

Figure 1: CPEX will provide unique data on the formation and acceleration of coronal mass ejections (CMEs), enormous eruptions of plasma from the sun that can interfere with satellites, communications and navigation at Earth. The image opposite shows a NRL LASCO CME image (red) and a computer model of the three-dimensional structure of the CME (Lin & Forbes(2000); blue). The solar corona - the Sun's hot outer atmosphere - is shown in the lower-left corner.

Figure 2: CPEX will determine how the coronal magnetic field drives the solar wind, a continuous and variable outflow of plasma from the Sun. The image opposite shows a solar eclipse image (with the Sun blocked by the solid black disk), allowing views of the fainter outer-atmosphere structures. CPEX will enable studies of the primary features affecting the solar wind (labeled a though d), including fluctuations in the solar plasma, coronal magnetic fields and fine structures such as jets.

Figure 3: A schematic view of solar energetic particle (SEP) production in the corona, looking down on the ecliptic plane. As a fast CME plows through the corona, it establishes as a shock wave. Wave-particle interactions at the shock front efficiently produce high-energy SEPs by acting on the population of coronal suprathermal ions. The characteristics of the shock and the suprathermal population, both of which will be measured by CPEX, are keys to understanding and modeling this process.

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