Filling the Gap: Viewing the Corona from the Disk to the Source Surface
Abstract:Only recently has it become clear that the solar corona viewed in the EUV is detected out to far greater distances above the limb than had been expected (Seaton etal 2013). The reasons for the visibility of streamers out to >2 R_sol are unclear (Goaryaev etal 2014), but the fact that such structures can be seen is indisputable (Masson etal 2014). The EUV imaging instruments that have been used for these observations, such as TRACE, SWAP, STEREO and AIA, were not specifically designed to detect the extended corona, and such detection typically requires multiple exposures and mosaics. An instrument designed to obtain views of the extended EUV corona out to the nominal source surface with high efficiency is straightforward to build, and would provide 100X to 1000X higher sensitivity than current instruments. The main advantages of such EUV observations are: 1. the coronal dynamics and eruptions (such as CMEs) and the locations and effects of nearby streamers are visible from the start of the event on-disk; 2. the initial conditions of an event, including initial acceleration, changes of path due to interaction with nearby coronal structures, and propagation of coronal waves from the eruption site, are all directly seen; 3. the evolution of the eruption from the solar surface out to the inner edge of a white-light coronagraph can be seen, thus filling the gap in such WL images (Byrne etal 2014).
We will present a class of simple EUV wide-field imager designs for implementation on a low-cost platform. Operation in the EUV implies that Sun-centered pointing tolerances can be relaxed, dust and scattering are negligible issues, and simple optics will fulfill all requirements. Implementation on a Sun-pointed spinning spacecraft is possible without great difficulty.