The Case for Ultra-High Spatial Resolution (~0.2'' or better) EUV Solar Spectroscopy: Spatial Scales in the Transition Region and Corona Derived from SOHO/SUMER and Hinode/EIS Spectra

EUV spectrometers on SOHO (SUMER) and Hinode (EIS) have 1” pixels which translate to a spatial resolution at the Sun of about 1400 km. We discuss spectroscopic results from SUMER demonstrating that current orbital solar instrumentation greatly under-resolves the transition region at temperatures ranging from about 20,000 K to 1 million degrees K. From EIS spectra, we show with specific examples using active region and coronal hole bright point data that coronal structures may be close to being resolved at 0.2-0.3” and that heating to active region temperatures can occur over regions with spatial scales well-below 1-2”. These results are obtained from electron density measurements that allow filling factors and path lengths to be determined, from spatial images constructed from the spectra, and also from line widths in some transition region cases that allow path lengths to be estimated from opacity. The EIS results could be considerably improved by new and detailed calculations of the atomic structure and electron impact excitation data for EUV emission lines of Fe XII and Fe XIII. Putting together the SUMER and EIS data implies that in order to trace the flow of energy into the corona from lower temperature regions significantly higher spatial resolution instrumentation is required. It is difficult to imagine how problems such as the coronal heating problem can be solved without such instrumentation.