New AO Lidar Observations of Ca+ in the Mesosphere and Thermosphere

Forty-eight tons (44,000 kilograms) of material from space fall onto the Earth each year. These meteoroids experience ablation and sputtering in the atmosphere, leaving behind metal and ion deposits that form thin layers in the ionosphere.

Calcium ions (Ca+) are the only metal ions that can be observed from the ground, so detecting these metals helps scientists understand how much meteoritic material is deposited in the atmosphere and how that material is then redistributed in the skies.

“For the first time, these calcium ions have been studied in the atmospheric layers between 90 - 111 miles (140 - 180 km) altitude at mid-latitudes,” stated AO scientist Dr. Shikha Raizada, who led the study titled New Lidar Observations of Ca+ in the Mesosphere and Lower Thermosphere Over Arecibo, published in the American Geophysical Union’s Geophysical Research Letters.

Dr. Raizada explained, “Only the Arecibo Observatory can perform direct ground-based metal ion measurements simultaneously with Incoherent Scatter Radar experiments, allowing a unique opportunity to study electron-ion interactions.”

“Only the Arecibo Observatory can perform direct ground-based metal ion measurements simultaneously with Incoherent Scatter Radar experiments, allowing a unique opportunity to study electron-ion interactions.” - Dr. Shikha Raizada, Senior Scientist at Arecibo Observatory

The intriguing observations of Ca+ at altitudes exceeding 75 miles (120 km) during dawn time opens up new opportunities to explore this undersampled region often referred to as “F-region” valley. This led the researchers to determine that the column concentration of these ions is half of those occurring in the lower altitudes (within the meteor zone between 50 - 75 miles, or 80 - 120 km). Additionally, the absence of meteoric metal during the pre-midnight hours is also surprising.

“Another key finding of our study was the detection of thin (<500 ft, or 150 m) calcium ion structures that are part of Sporadic-E (“Es”), which are thin layers of ionization in the E-region of the ionosphere,” Dr. Raizada related. This detection was enabled by the instrumentation upgrade that improved the range and height resolution of the Ca+ lidar. The simultaneous incoherent scatter radar observations revealed broader Es layers.

Dr. Raizada emphasized that “the studies reported in the paper represent a few cases, but more observations are required to get seasonal variability.” She encourages others to read the publication for more detail.

“Comparisons of lidar and Incoherent Scatter Radar data, which can only be done simultaneously at Arecibo, provide new insights about the ionospheric processes that are discussed in the paper. This further highlights the importance of the highly unique instrument cluster at AO,” Dr. Raizada concluded.

This work was led by Dr. Shikha Raizada and a team of researchers that included several other AO scientists: Dr. Jens Lautnebach; Dr. Nestor Aponte; Mr. Phil Perrilat; and Dr. Mike Sulzer; as well as J. Smith from NASA Langley Research Center and J. D. Mathews from the Pennsylvania State University.