Earth’s Shields are Up

High above Earth's atmosphere, the Earth is surrounded by two belts of high energy particles mainly electrons moving a close to the speed of light and protons that are held in place by the magnetic fields. These belts were discovered in 1958, by James Van Allen. The Van Allen belts can wax and wane in response to incoming energy from the sun in the form of solar flares and coronal plasma ejections. A slot of fairly empty space typically separates the belts, but its width changes. Under extreme conditions, when a strong solar wind or a giant solar eruption such as a coronal mass ejection sends clouds of material into near-Earth space the electrons from the outer belt can be pushed into the usually-empty slot region between the belts.

In August 2012 the twin Van Allen Probes were launched into space to study the Van Allen Belts. The probes were built and are operated by the Johns Hopkins Applied Physics Laboratory in Laurel, Maryland for NASA’s Science Mission Directorate. While you and I go about our daily lives scientists at the University of Colorado, Massachusetts Institute of Technology (MIT), Johns Hopkins, University of California at Los Angeles and elsewhere are still learning about our planet using probes and satellites.

The electrons in the outer band of Van Allen’s belts are ultra-relativistic traveling near the speed of light and can circle the planet in just five minutes, bombarding anything in their path. Exposure to such high-energy radiation can disrupt on satellite electronics. These probes were designed to withstand constant radiation bombardment so it can measure the behavior of these ultra-relativistic electrons.

During the past decade scientists at MIT have studied plasma plume phenomena using radio signals transmitted from GPS satellites to more than 1,000 receivers on the ground. Large space-weather events can alter the incoming radio waves and allow scientists to see the concentration of plasma particles in the upper atmosphere during space weather events. Combining data from the ground based observations and the new space data from the Van Allen probes has given scientists a highly detailed picture of a natural defensive mechanism for Earth.

Now researchers at the University of Colorado, MIT, Johns Hopkins, University of California and elsewhere have found there is an absolute limit to how close ultra-relativistic electrons can get to the Earth. The team found that no matter where these electrons are circling around the planet's equator, they can get no further than about 6,800 miles from the Earth's surface. No matter how intense the energy of the particle there is a barrier that prevents it from penetrating our atmosphere. Earth has a shield.

This shield is created neither by the Earth's magnetic field nor long-range radio waves, but rather by extremely low frequency electromagnetic waves in the upper atmosphere. The Van Allen radiation belts are not the only particle structures surrounding Earth. A giant cloud of relatively cool, charged particles called the plasmasphere fills the outermost region of Earth's atmosphere, beginning at about 600 miles up and extending partially into the outer Van Allen belt. The particles at the outer boundary of the plasmasphere cause particles in the outer radiation belt to scatter, draining them and their energy from the belt. The scientists call this phenomenon "plasmaspheric hiss,” because when the low-frequency electromagnetic waves are played through a speaker sound like a static or a hiss.

This scattering effect is fairly weak and might not be enough to keep the electrons at the boundary of the outer Van Allen belt in place, except that the belt electrons move incredibly quickly, but not toward Earth. Instead all their speed is in loops around Earth. The Van Allen Probes data show that in the direction toward Earth, the ultra-relativistic electrons have only a slow drift towards earth that can be measured in months. This is a movement so slow and weak that it can be countered d by the scattering caused by the plasmasphere.

Based on their analysis published on Thanksgiving in the journal Nature believe that plasmaspheric hiss essentially deflects incoming electrons, causing them to collide with neutral gas atoms in the Earth's upper atmosphere, and ultimately disappear. This natural, impenetrable barrier appears to be extremely rigid, keeping high-energy electrons from coming no closer than about 6,800 from the Earth's surface, but a massive inflow of matter from the sun can erode the outer plasmasphere, pushing its boundaries inward and allowing electrons from the radiation belts to move further inward, too. So the shield is not rigid.

D.N. Baker et al, An impenetrable barrier toultrarelativistic electrons in the Van Allen radiation belts, Nature Volume:515, Pages: 531–534 Date published: (27 November 2014) doi:10.1038/nature13956.