We report on a cosmic ray energy spectrum measurement by the Telescope Array Low-Energy extension (TALE) air fluorescence detector {FD). This spectrum exhibits two features, the second knee in the lower part of the 10^17 eV decade, and a hardening in the lower part of the 10^16 eV decade. The energies of these features, for the first time, are tied to the energy scale of fluorescence detectors of ultra-high energy cosmic rays.
The TALE FD is sensitive to both fluorescence light and to the Cherenkov light produced by shower particles. Since Cherenkov light is very bright, we measure the energy spectrum from a low energy of ~ 4 PeV to an energy greater than 100 PeV in Cherenkov light. Above 100 PeV the detection of cosmic rays by their air fluorescence emission becomes more efficient (larger air volume) and it allows for the extension of the measurement to energies greater than a few EeV.