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COSMIC RAYS IN THE ATMOSPHERE At the entrance to the stratosphere, about 70,000 feet above Earth's surface, there is a broad layer of ionizing radiation called the "Pfotzer Maximum." Many airplanes fly just below it; satellites orbit high above it. The best way to penetrate this layer, and to study it, is using high-altitude balloons. Since Oct. 2013, Spaceweather and the students of Earth to Sky Calculus have been flying balloons into the Pfotzer Maximum to monitor its response to solar storms. Here is an example of data taken just last week.  The blue curve traces the increase in radiation as the balloon ascends toward the stratosphere. At the Pfotzer maximum, radiation levels are more than 100x higher than on the ground below. Monitoring this peak is important for many reasons. For example, radiation from the Pfotzer Maximum can leak down to aviation altitudes, affecting air travelers. It can also alter the chemistry of the upper atmosphere. During two years of monitoring, the students have seen the Pfotzer Maximum surge and subside in response to several solar storms. All of the data were collected high above the Sierra Nevada mountains of central California. This raises a question: What happens to the Pfotzer Maximum in other places? To find out, on July 20th the team launched two balloons, simultaneously, 2947 miles apart. Identical radiation sensors were flown to the stratosphere above California (+37 N latitude) and New Hampshire (+43 N). Despite the relatively small difference in geographic latitude between the two launch sites, there was a significant difference in the intensity of the Pfotzer Maximum. This graphic overlays data from the cross-country flights.  The two balloons were in flight for different amounts of time, almost 3 hours over California vs. little more than 2 hours over New Hampshire. That's why the curves appear to be stretched differently in the horizontal direction. Ignore that. Instead, pay attention to the amplitude of the curves: The stratosphere over New Hampshire was 25% more radioactive than the stratosphere over California. This shows that balloon flights over a single location can be informative, but they do not tell the full story. To learn more, we are planning additional cross-country flights in the months ahead. Stay tuned! What is the Pfotzer Maximum? When cosmic rays crash into Earth's atmosphere, they produce a spray of secondary particles. With increasing depth in the atmosphere, the primary cosmic radiation component decreases, whereas the secondary radiation component increases. This complex situation results in a maximum of the dose rate at an altitude of ~20 km, the so-called "Pfotzer maximum," named after physicist Georg Pfotzer who co-discovered the peak using balloons and Geiger tubes in the 1930s. . |
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