Significant observational and theoretical evidence suggests that coronal heating operates at sub-arcsecond, currently unresolved, spatial scales and is impulsive in time. We demonstrate that the most sensitive diagnostic for this type of heating is provided by spectroscopic observations in the early phase of such events. We demonstrate that the spectra of hot lines (> 5 MK), observed at arcsecond resolution with the EIS spectrometer onboard the recently launched Hinode mission, hold the signature of the impulsive heating process via the development of asymmetric profiles. Solar Orbiter (SolO) will provide a unique opportunity to directly view the postulated sub-arcsecond impulsive energy releases. We demonstrate that the superior spatial resolution of the Orbiter EUV remote sensing instrumentation should be tuned to high temperature plasmas in order to resolve the individual strands.
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