Blog/Vlog by Anna Herbst
Hello cosmic ray enthusiasts! I’m a founding member of the student research group Earth to Sky Calculus. My job is to reduce the data from all of our balloon launches and airplane flights. We’re really excited about our research and we want to share it with everyone! My goal for these blogs is to make that information accessible to anyone interested.
Watch the story here or read the blog below.
Between December 6, 2018 and January 14, 2019, Rads on a Plane programmer and globetrotter extraordinaire Hervey Allen embarked on an epic adventure that spanned 30,210 miles, 4 continents, and clocked in at upwards of 60 airborne hours.
Not only was Hervey’s journey epically long (more than 5,000 miles longer than the Earth’s entire circumference!) it was likewise epically rad! During those sixty plus hours he absorbed a total radiation dose of 153.3 uGy (micrograys).
Of course, if you don’t spend your free time immersed in the minutiae of cosmic ray terminology, those units probably don’t mean much to you. So, in order for that dose to impart the desired effect, let’s back up. (Note: if in fact you are a cosmic ray whiz feel free to skim the next section while you’re recovering from your shock.)
Let’s define our terms:
(1) rad (n): (in our context) an abbreviation for ‘radiation’; specifically cosmic radiation
(2) rad (adj): describes a most excellent adventure
cosmic ray (n): Cosmic rays are made up of high-energy particles, most originating beyond the Solar System in distant supernovae and a smaller number from solar activity. These high-energy particles collide with Earth’s outer atmosphere and create a shower of secondary radiation.
The term cosmic radiation actually refers to a cocktail of many different types of radiation including X-radiation, gamma radiation, neutron radiation and ultraviolet (UV) radiation, to name a few.
On his Big Rad Adventure, Hervey measured three types: X-rays, gamma rays and neutrons. We describe these in units called micrograys (uGy).
What happens when we fly
The Earth’s atmosphere acts like a shield against most types of cosmic radiation, so at ground level cosmic rays pose only a minor risk to your health (just remember to wear sunscreen!). However, as your altitude increases and the atmosphere thins, you are exposed to higher levels of cosmic radiation.
Commercial flights generally reach a cruising altitude between 25,000 and 45,000 ft where levels of cosmic radiation range from between 10 to 50 times stronger than at sea level, as seen in the figure below. Of course, altitude isn’t the only factor that affects radiation levels (more on that in a minute).
What happened when Hervey flew
Now, back to the numbers. Hervey absorbed a total cosmic radiation dose of 153.3 uGy. To put that in perspective, you get a dose of approximately 10 uGy when you visit the dentist for a panoramic X-ray. A trip to the doctor’s office for a chest scan is around 100 uGy.
That means Hervey’s trip was the equivalent of 15 dental X-rays or 1.5 chest scans!
Or, if we want to go cosmic: in September 2017, a solar storm hitting Mars resulted in surface radiation averaging around 400 uGy per day. In just 60 hours, Hervey’s radiation dose was approaching 40% of a daily surface dose on Mars during a solar storm.Keep in mind that Mars is a planet without a protective magnetosphere and is subject to much higher levels of cosmic radiation than Earth’s surface. By this point, you have enough context to understand that 153.3 uGy is a relatively high dose of cosmic radiation. (I’ll give you a moment to collect your jaw from the floor.)
Now that you’re suitably dazzled – and possibly weighing the pros and cons of a life relegated to international sea-travel – allow me to ease your concerns. A dose of 153 uGy is hardly lethal, especially over an extended period of time. Remember that Hervey’s 60 hours were spread out over 5 weeks. (All that airplane food was probably of more immediate concern to his health.)
That’s not to say that prolonged, repeated exposure to cosmic radiation is risk free. Pilots and flight attendants are at greater risk of developing radiation-related health issues. Luckily there are some ways to mitigate your radiation exposure, like which route you choose to fly – but I digress. Let’s get back to Hervey’s adventure.
When the numbers don’t behave as expected
Hervey began his journey at Portland International Airport in Oregon on December 6, 2018. Between Portland and Newark, New Jersey Hervey recorded a neutron dose of 12.9 uGy. This value represented the only instance during Hervey’s adventure where the neutron dose was greater than the X-ray/gamma ray dose (9.2 uGy).
From Newark, Hervey spent the next 6.4 hours crossing the Atlantic to Brussels, Belgium. This flight followed and unusually low latitude route (averaging 45.5°) perhaps to avoid strong winds. The next day Hervey boarded a flight to Accra, Ghana. This is when things started to get weird.
When altitude increases, it’s a pretty safe bet that radiation rates increase, as well. This trend is reflected in the figure below:
On Hervey’s flight from Brussels to Accra a very strange thing happened. As the plane’s elevation went up, radiation went down!
As counterintuitive as this event may seem, there’s an easy explanation. Remember I mentioned that other factors besides altitude influence radiation levels? At the same time Hervey’s plane was ascending, it was also approaching the Equator. Even at high altitudes, as you approach the equator, cosmic rays decrease. This is an effect of the dipole shape of Earth’s magnetic field.
The mystery explained: A magnetic actor
The magnetic field is positioned vertically with relation to a person standing on Earth’s poles. Charged particles pointing in the direction of the poles move parallel to the magnetic field and stream down with relative ease towards Earth’s surface. (This is the reason we see auroras near the poles.)
The reverse is true at the Equator where Earth’s magnetic field is horizontal to a person standing on the ground. Cosmic rays aiming at the Equator move perpendicular to the magnetic field and often get flung away from the Earth. The important takeaway is this: where the magnetic field has a strong horizontal component (like at the Equator) cosmic radiation is less.
When Hervey flew back from Accra to Brussels 10 days later, he witnessed the inverse phenomenon – as altitude decreased, cosmic rays increased! (We should thank the pilots for their cooperation in confirming this trend!)
Between Brussels and Washington D.C., Hervey absorbed 18.1 uGy, the strongest X-ray/gamma ray dose of the trip. This flight resumed the usual high latitude North Atlantic route, which accounts for such a high dose. From D.C., our intrepid traveller journeyed south to Houston, Texas where he promptly hopped on yet another international flight to Santiago, Chile.
Although this was the longest continuous flight of Hervey’s adventure, his total combined dose of X-ray/gamma ray and neutrons (17.5 uGy) was still less than just the X-ray/gamma ray measurement for Hervey’s North Atlantic journey. Hervey crossed over the Equator on his way from North America to South America. Like the Equator, Chile is another place where Earth’s magnetic field has a strong horizontal component. This explains the relatively lower cosmic ray dose on this flight.
While in Chile, Hervey ventured a bit further south to Temuco for a few days. On the quick return to Santiago, Hervey measured the trip’s peak elevation as the pilot cruised at around 40,000 ft! Even at this relatively high altitude, the uGy was comparatively low; its largest single value measured only 1.8 uG per hour.
Changes in the radiation cocktail
At this point in Hervey’s Big Rad Adventure, we can identify a clear trend: as you travel from high latitudes to low latitudes the cocktail of cosmic radiation changes. At higher latitudes, the ratio of X-rays and gamma rays to neutrons is comparable, while at lower latitudes neutrons are scarcer.
This would seem to indicate that horizontal magnetic fields repel neutrons much better than the particles that give rise to X-rays and gamma rays. In other words, in our cosmic ray cocktail, high latitudes taste like an even blend of X-rays/gamma rays and neutrons, and low latitudes taste strongly of X-rays/gamma rays.
Hervey departed from Santiago on January 13, 2019, beginning the final leg home of his epic adventure. He flew from Santiago to Miami, Florida, Miami to Los Angeles, California, and Los Angeles to Portland, Oregon, racking up another of 29.8 uGy!
Hervey finally touched down in Portland the following day, marking the end of the biggest, raddest (dare I say most excellent) adventure since two high school seniors got their hands on a time machine.