Highly ionized atomic physics systems are used to probe processes taking place in the upper solar atmosphere, a region that, when flares are included, encompasses some three orders of magnitude in the temperature (3 × 10⁴−3 × 10⁷K). A thorough review of spectral line intensities implies that plasmas in the different parts of the upper solar atmosphere are transiently ionizing and are consistently associated with increased electron density. A possible mechanism consistent with the observations is a current driven plasma compression.