Credit: Katsuda et al., JGR Space Physics, 2023, Volume 128, Issue 2
Our Shrinking Atmosphere
Increasing levels of greenhouse gases (like carbon dioxide, methane and others) in the lowest layer of the earth's atmosphere (the troposphere), have caused a dramatic warming of the earth's surface. This surface warming causes the shrinking of the polar ice caps, the subsequent rise in sea level, desertification, more powerful and frequent hurricanes, and increasing migration of human and other populations seeking to escape these effects. The effect of greenhouse gas pollution on the earth's upper atmosphere (the mesosphere and thermosphere) at altitudes of 80-100 km above the earth's surface is less well understood. Now, using archival X-ray measurments of the Crab Nebula stretching back decades, scientists have attempted to measure the recent history of the upper atmosphere. The Crab Nebula is the remnant of a supernova which was observed to have exploded in 1054 CE and is a fairly constant source of X-rays. Thus it's an object that most X-ray satellite observatories use to calibrate their detectors. When the X-ray satellite is viewing the Crab Nebula through the earth's atmosphere, atoms in the earth's atmosphere (mostly nitrogen and oxygen) will absorb away some of the low-energy X-rays from the Crab, producing a tell-tail decrease in brightness at low energies. From the HEASARC, scientists obtained more than 28 years of X-ray archival observations of the Crab nebula from a variety of X-ray satellite observatories. Through careful analysis they were able to determine the changes in the amount of atmospheric absorption seen in the X-ray spectra, and thus determine the variation in the density of nitrogen and oxygen atoms in the earth's upper atmosphere over that timespan at a selected set of elevations. The image above left shows a typical spectrum of the Crab nebula from the ASCA X-ray observatory at three different elevations, showing that, at low elevations, where the atmosphere is thickest, the low energy emission from the Crab is much less than it is at higher elevations, where the atmosphere is much thinner. The center and right plots show the declining trend in atmospheric density over the last few decades at relatively low (center) and much higher (right) elevations. The shrinking of the atmosphere derived from the archival X-ray data analysis can be used to test models of climate change. This study highlights how the HEASARC's long-term archive of cosmic X-ray observations and X-ray satellite engineering data can provide totally new and unanticipated insights right here at home.
Published: December 18, 2023
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Page Author: Dr. Michael F. Corcoran
Last modified Tuesday, 27-Feb-2024 10:08:16 EST