Tracking thermospheric wind

Earth's atmosphere at altitudes between 80 km and 110 km is a no man’s land, accessible to neither the highest research balloons nor the lowest orbiting satellites. But it is substantial enough to vaporize billions of meteors—most smaller than a grain of sand—that intersect Earth’s orbit every day. Tons of metal atoms ablated from those meteors circulate in pervasive winds that can reach hurricane speeds up to 150 m/s and create enormous sheers. Measurements of those speeds have been made over the past half century using rockets to disperse luminescent tracers that can be tracked as they’re swept up in the winds. Meers Oppenheim and colleagues from Boston University, Los Alamos National Laboratory, and Jicamarca Radio Observatory (JRO) east of Lima, Peru, now offer an alternative approach that avoids expensive rocket launches and can be performed nearly continuously: They use radar to track the meteors’ plasma trails that are also swept up in the winds. Four years ago and again in 2007, the team gained access to JRO's phased array of nearly 19 000 dipole antennas (shown here). Located at the geomagnetic equator, the radar facility provides sufficient power to capture nonspecular reflections from ionization trails as they evolve over many seconds. And interferometry allowed the team to build a vector profile of wind speeds at different altitudes using data from many meteors. The spatial resolution of the profile is a few hundred meters, comparable to that from the tracer experiments. (M. M. Oppenheim et al., Geophys. Res. Lett., in press.) — R. Mark Wilson