Neutrons Detected on Commercial Airplane Flights

March 13, 2019: Long lines. Narrow seats. Baggage fees. You recognize this list. It’s the downside of flying on modern commercial airlines. And now we have a new item to add: cosmic ray neutrons. and Earth to Sky Calculus have just completed a 5-continent survey of neutron radiation at aviation altitudes. From December 2018 through February 2019, Hervey Allen of the University of Oregon’s Network Startup Resource Center carried  Earth to Sky radiation sensorsincluding neutron bubble chambers–onboard commercial flights from North America to Europe, Africa, South America and Asia.


Hervey logged 83 hours in the air as he traveled 41,500 miles above 30,000 feet. For reference, that’s almost twice the circumference of the Earth. The entire time, he gathered data on X-rays, gamma-rays and neutrons in an energy range (10 keV to 20 MeV) similar to that of medical radiology devices and “killer electrons” from the Van Allen Radiation Belts.

The results were eye-opening. During the trip, Hervey recorded 230 uGy (microGrays) of cosmic radiation. That’s about the same as 23 panoramic dental x-rays or two and a half chest X-rays. Moreover, 41% of the dose came in the form of neutrons. This confirms that cosmic-ray neutrons are abundant at aviation altitudes and must be considered in any discussion of “Rads on a Plane.”


Each bubble represents a cosmic ray neutron penetrating the chamber. Left: Neutrons on board a flight from San Francisco to South Korea. Right: Neutrons detected while crossing the Atlantic Ocean from New Jersey to Brussels.

Researchers have long known that cosmic rays penetrate airplanes. Our own 3-year survey of global radiation shows that X-rays and gamma-rays at aviation altitudes are typically 50 times stronger than sea level. This new survey focuses on neutrons, a more potent type of radiation from deep space. Studies show that neutrons can be ten times more effective at causing biological damage compared to X-rays and gamma-rays in the same energy range. Neutrons are so effective, they are used for cancer therapy, killing tumors better than other forms of radiation.

Should we be worried about Hervey? Although he absorbed a lot of radiation during the survey, he did so slowly. Hervey’s whole body dose was spread out over 14 flights and 3 months–unlike, say, a dental X-ray which is localized to the jaw and delivered in a split-second. Slow delivery gives the body time to respond, repair damage, and move on without obvious health effects. On the other hand, at least one study shows that low-dose radiation received over a long period of time may slightly increase the risk of leukaemia, while flight attendants have been found to have a higher risk of cancer than the general population.


Left: Neutrons detected while flying from Europe to Accra, Ghana. Right: Ground-level measurements in Ghana with giant ant mounds in the background.

Our survey also revealed some geographical variations. Generally speaking, neutron radiation was stronger near the Arctic Circle and weaker near the equator. It was weakest of all, however, in flights over Chile as the aircraft skirted the South Atlantic Anomaly. We will be investigating these variations with additional flights in the near future.

Stay tuned!

The CMEs Are Coming. (Maybe)

March 9, 2019: Earth is about to be sideswiped by a pair of coronal mass ejections (CMEs). Maybe. The two solar storm clouds left the sun on March 8th when sunspot AR2734 erupted, producing a C1-class solar flare. NASA’s Solar Dynamics Observatory recorded the blast:

Above: This movie comes from SDO’s extreme ultraviolet telescope–hence the strangely beautiful colors.

The explosion and its ultraviolet afterglow lasted for more than an hour. Such long-duration flares are notorious for producing “solar tsunamis” and CMEs. Indeed, in the global movie, below, a shadowy shockwave may be seen billowing away from the blast site like a ripple in a giant pond. That wave hurled two faint coronal mass ejections (CMEs) into space.

NOAA analysts have modeled the eruption and reached the following conclusions: On March 11th, one CME will pass just behind Earth while the other passes just in front. Both could deliver glancing blows to our planet’s magnetic field. Minor G1-class geomagnetic storms are possible when Earth splits the gap between these two solar storm clouds.

In recent months, geomagnetic storms have been caused mainly by streams of solar wind flowing from holes in the sun’s atmosphere. CMEs tend to be more effective instigators of geomagnetic storms and auroras. This is because of intense shocks and strong magnetic fields CMEs often contain.

There’s no guarantee these CMEs will hit Earth. Just in case, Arctic sky watchers should be alert for bright lights on Monday night. Subscribers to our Space Weather Alert service will receive an instant text message when the CMEs arrive. Aurora Alerts: SMS text, email.

A Really Weird Solar Eclipse

March 7, 2019: Earlier today, NASA’s Solar Dynamics Observatory (SDO) observed an eclipse of the sun–a strange kind of eclipse that you can only see while orbiting Earth. The black disk of the New Moon passed in front of the sun, reversed course, and did it again:

During the eclipse, which lasted just over 4 hours, as much as 82% of the sun was covered. Technically, that makes it an annular solar eclipse, not total. At maximum, an annulus or “ring of fire” completely surrounded the Moon.

The strange “double-dip” motion of the Moon across the sun is a result of orbital mechanics. Both SDO and the Moon are orbiting Earth, but at different speeds. SDO’s velocity of ~3 km/s is faster than the Moon’s velocity of 1 km/s. SDO thus overtakes the Moon first in one direction, then the other, during the long eclipse.

High-resolution images of the eclipse reveal that the Moon is not perfectly smooth. The little bumps and irregularities you see are lunar mountains backlit by solar plasma:

Images like these have practical value to the SDO science team. The sharp edge of the lunar disk helps researchers measure the in-orbit characteristics of the telescope–e.g., how light diffracts around the telescope’s optics and filter support grids. Once these are calibrated, it is possible to correct SDO data for instrumental effects and sharpen images of the sun even more than before.

Realtime Space Weather Photo Gallery

A Month Without Sunspots

March 1, 2019: There are 28 days in February. This year, all 28 of them were spotless. The sun had no sunspots for the entire month of Feb. 2019. This is how the solar disk looked every day:

The last time a full calendar month passed without a sunspot was August 2008. At the time, the sun was in the deepest Solar Minimum of the Space Age. Now a new Solar Minimum is in progress and it is shaping up to be similarly deep. So far this year, the sun has been blank 73% of the time–the same as 2008.

Solar Minimum is a normal part of the solar cycle. Every ~11 years, sunspot counts drop toward zero. Dark cores that produce solar flares and CMEs vanish from the solar disk, leaving the sun blank for long stretches of time. These minima have been coming and going with regularity since the sunspot cycle was discovered in 1859.

However, not all Solar Minima are alike. The last one in 2008-2009 surprised observers with its depth and side-effects. Sunspot counts dropped to a 100-year low; the sun dimmed by 0.1%; Earth’s upper atmosphere collapsed, allowing space junk to accumulate; the pressure of the solar wind flagged while cosmic rays (normally repelled by solar wind) surged to Space Age highs. All these things are happening again.

How does this affect us on Earth? The biggest change may be cosmic rays. High energy particles from deep space penetrate the inner solar system with greater ease during periods of low solar activity. Indeed, NASA spacecraft and space weather balloons are detecting just such an increase in radiation. Cosmic rays can alter the flow of electricity through Earth’s atmosphere, trigger lightning, potentially alter cloud cover, and dose commercial air travelers with extra “rads on a plane.”

As February ended, March is beginning … with no sunspots. Welcome to Solar Minimum!

What Kind of Sunspot is That?

March 5, 2019: Today, a tiny sunspot is struggling to form in the sun’s northern hemisphere. It is so small, it has not yet been numbered, and it may fade away before the day is done, leaving the sunspot number technically zero. Even if it vanishes, though, this funny little sunspot is worth mentioning because of its tilted magnetic field:

This is a magnetogram (magnetic map) of the sun obtained on March 5th by NASA’s Solar Dynamics Observatory. The sunspot is inset. Note how its magnetic field is almost orthogonal to other patches of magnetism elsewhere on the solar disk.

Sunspots are islands of magnetism floating on the surface of the sun. Like all magnets, they have two poles, plus (+) and minus (-). Usually these poles are aligned almost parallel to the sun’s equator. Today’s sunspot is almost perpendicular.

Could this be a sunspot from the next solar cycle? Right now, Solar Cycle 24 is decaying into a deep Solar Minimum. Solar Cycle 25 is still in the offing. According to Hale’s Law, sunspot magnetic fields reverse polarity between solar cycles. If this sunspot continues to grow–and if its magnetic axis tilts a bit to the right–Hale’s Law would tag it as a member of Solar Cycle 25.

Postscript: The sunspot did continue to grow, and its magnetic field remained ambiguous. Based on its relatively low latitude, we believe it is probably a member of old Solar Cycle 24.