What is the Regener-Pfotzer Maximum?

June 7, 2018: About once a week, Spaceweather.com and the students of Earth to Sky Calculus launch a helium balloon with radiation  sensors to the stratosphere over California. This is a unique monitoring program aimed at tracking the cosmic ray situation in Earth’s atmosphere. During each flight, our balloon passes through something called the Regener-Pfotzer Maximum, a layer of peak radiation about 20 km above Earth’s surface. This plot of radiation vs. time taken during a July 2015 balloon flight illustrates the peak:

Image source: Phillips, T., et al. (2016), Space Weather Ballooning, Space Weather, 14, 697–703, doi: 10.1002/2016SW001410.

What is this peak? To understand it, let us begin in deep space. Cosmic rays are, essentially, the subatomic debris of dying stars, accelerated to nearly light speed by supernova explosions. They travel across space and approach Earth from all directions, peppering our planet 24/7. When cosmic rays crash into Earth’s atmosphere, they produce a spray of secondary particles and photons that is most intense at the entrance to the stratosphere. Physicists Eric Regener and Georg Pfotzer discovered the maximum using balloons in the 1930s and it is what we are measuring today.

In some ways, secondary cosmic rays are like froth on the ocean. By watching the froth, you can learn a lot about the underlying water. Likewise, by watching secondary cosmic rays, we learn a lot about primary cosmic rays hitting the top of the atmosphere. Indeed, our balloon measurements have recently confirmed what NASA spacecraft are finding: The cosmic ray situation is worsening.

For many years, the Regener-Pfotzer Maximum was called, simply, the “Pfotzer Maximum.” Regener’s name is less recognized by present-day physicists largely because in 1937 he was forced to take early retirement by the National Socialists as his wife had Jewish ancestors. This interesting story weaving science, politics, and human nature has recently been told by historians of science P. Carlson and A. A. Watson. Ref: Hist. Geo Space. Sci., 5, 175-182, 2014.

Technical note: The radiation sensors onboard our helium balloons detect X-rays and gamma-rays in the energy range 10 keV to 20 MeV. These energies span the range of medical X-ray machines and airport security scanners.