Simultaneous observations of meteoric Ca and Ca+ by employing the Ti:sapphire-laser-based resonance-scattering Ca/Ca+ lidar

The upper atmosphere located at an altitude of 80 - 120 km above the ground is a crucial region for comprehensively understanding the behavior of Earth's entire atmosphere, because it is the region where the atmosphere transitions from neutral to ionospheric. In this transitional region, meteoroids are continually supplying metallic atoms and ions. The resonant-scattering lidar, which emits laser beams from the ground and then detect on the ground again how much atoms and ions cause resonant scattering of the laser radiations, is one of the significant measurement methods of observing such transitional region. While Fe and Na are selected as the major targets, we have focused on Ca and have developed a specific lidar system to detect it. This is because Ca has uniquely preferable resonance transitions for neutral atoms and ions (Ca: 422.7918 nm and Ca+: 393.4770 nm) for performing lidar measurements from the ground. The core of the developed resonant-scattering Ca/Ca+ lidar system is the injection-locked Ti:sapphire solid-state laser, which has the remarkable ability to simultaneously emit the two laser beams from a single resonator at a variety of combinations of two wavelengths, including the above resonant transitions of neutral Ca and Ca+.
    Here, we report on the first results of the long-term observations, where the developed resonant-scattering Ca/Ca+ lidar system was operated for an entire night. The averaged laser power, time resolution, and altitude resolution of the Ca/Ca+ lidar system are set to 0.2 W, 30 s, and 15 m, respectively, for Ca, and 0.4 W, 30 s, and 30 m , respectively, for Ca+ in this operation. Both neutral Ca and Ca ions distributed in the identical spatio-temporal regions could be measured in detail over an entire night. It was clearly observed that the neutral Ca and Ca ions had almost the same spatio-temporal structures with complex time and space dependences in the main layer at an altitude of 80 - 100 km, and Ca ions also had an additional high-density thin layer with a few kilometers deep at the highest altitude in the main layer. This high-density layer of Ca ions, which was not seen with the neutral Ca, suggests that it is to be related to the sporadic E layer. In our presentation, we will also report on the progress of this ongoing project.