Hinode EUV Imaging Spectrometer Observations of Active Region Loop Morphology: Implications for Static Heating Models of Coronal Emission
Theoretically, magnetic fields are expected to expand as they rise above the photosphere and into the corona, so the apparent uniform cross-sections of active region loops are difficult to understand. There has been some debate as to whether coronal loops really have constant cross-sections, or are actually unresolved and composed of expanding threads within the constant cross-section envelopes. Furthermore, loop expansion is critical to the success or failure of hydrostatic models in reproducing the intensities and morphology of observed emission. We analyze Hinode EIS (EUV Imaging Spectrometer) observations of loops in active region 10953 and detect only moderate apex width expansion over a broad range of temperatures from log Te / K = 5.6 to 6.25. The expansion is less than required by steady-state heating models of coronal emission suggesting that such models will have difficulty reproducing both low and high temperature loop emission simultaneously. At higher temperatures (> log Te / K = 6.3) the apex widths increase substantially, but the emission at these temperatures likely comes from a combination of multiple loops. These observations demonstrate the advantage of EIS over previous instruments. For the first time, active region loops can be examined over a broad temperature range with high temperature fidelity and the same spatial resolution. The results therefore provide further clues to the coronal heating timescale and thus have implications for the direction of future modeling efforts.