Electron precipitation (also called energetic electron precipitation or EEP) is an atmospheric phenomenon that occurs when previously trapped
electron
The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family,
and are generally thought to be elementary particles because they have no kn ...
s enter the
Earth's atmosphere
The atmosphere of Earth is the layer of gases, known collectively as air, retained by Earth's gravity that surrounds the planet and forms its planetary atmosphere. The atmosphere of Earth protects life on Earth by creating pressure allowing for ...
, thus creating communications interferences and other disturbances.
Electrons trapped by Earth's magnetic field spiral around
field line
A field line is a graphical visual aid for visualizing vector fields. It consists of an imaginary directed line which is tangent to the field vector at each point along its length. A diagram showing a representative set of neighboring field l ...
s to form the
Van Allen radiation belt. The electrons are from the
solar wind
The solar wind is a stream of charged particles released from the upper atmosphere of the Sun, called the corona. This plasma mostly consists of electrons, protons and alpha particles with kinetic energy between . The composition of the sola ...
and may remain trapped above Earth for an indefinite period of time (in some cases years). When
broadband very low frequency (VLF) waves propagate the radiation belts, the electrons exit the radiation belt and "precipitate" (or travel) into the
ionosphere
The ionosphere () is the ionized part of the upper atmosphere of Earth, from about to above sea level, a region that includes the thermosphere and parts of the mesosphere and exosphere. The ionosphere is ionized by solar radiation. It plays an ...
(a region of Earth's atmosphere) where the electrons will collide with
ions
An ion () is an atom or molecule with a net electrical charge.
The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conven ...
.
Electron precipitation is regularly linked to
ozone depletion
Ozone depletion consists of two related events observed since the late 1970s: a steady lowering of about four percent in the total amount of ozone in Earth's atmosphere, and a much larger springtime decrease in stratospheric ozone (the ozone l ...
. It is often caused by
lightning strike
A lightning strike or lightning bolt is an electric discharge between the atmosphere and the ground. Most originate in a cumulonimbus cloud and terminate on the ground, called cloud-to-ground (CG) lightning. A less common type of strike, ground- ...
s.
Process
An electron's
gyrofrequency
Cyclotron resonance describes the interaction of external forces with charged particles experiencing a magnetic field, thus already moving on a circular path. It is named after the cyclotron, a cyclic particle accelerator that utilizes an oscillati ...
is the number of times it revolves around a field line.
VLF waves traveling through the
magnetosphere
In astronomy and planetary science, a magnetosphere is a region of space surrounding an astronomical object in which charged particles are affected by that object's magnetic field. It is created by a celestial body with an active interior dynam ...
, caused by lightning or powerful transmitters, propagate through the radiation belt. When those VLF waves hit the electrons with the same frequency as an electron's gyrofrequency, the electron exits the radiation belt and "precipitates" (because it will not be able to re-enter the radiation belt) throughout the Earth's atmosphere and ionosphere.
Often, as an electron precipitates, it is directed into the upper atmosphere where it may collide with neutral particles, thus depleting the electron's energy.
If an electron makes it through the upper atmosphere, it will continue into the ionosphere. Groups of precipitated electrons can change the shape and conductivity of the ionosphere by colliding with atoms or molecules (usually oxygen- or nitrogen-based particles
) in the region.
When colliding with an atom, the electron strips the atom of its other electrons creating an ion. Collisions with the air molecules also release
photons
A photon () is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are massless, so they alway ...
which provide a dim "
aurora
An aurora (plural: auroras or aurorae), also commonly known as the polar lights, is a natural light display in Earth's sky, predominantly seen in high-latitude regions (around the Arctic and Antarctic). Auroras display dynamic patterns of bri ...
" effect.
Because this occurs at such a high altitude, humans in aircraft are not affected by the radiation.
The ionization process, caused by electron precipitation in the ionosphere, increases its electrical conductivity which in turn brings the bottom of the ionosphere to a lower altitude.
When this happens,
ozone depletion
Ozone depletion consists of two related events observed since the late 1970s: a steady lowering of about four percent in the total amount of ozone in Earth's atmosphere, and a much larger springtime decrease in stratospheric ozone (the ozone l ...
occurs and certain communications may be disrupted.
The lowered altitude of the ionosphere is temporary (unless electron precipitation is steady) while the ions and electrons rapidly react to form neutral particles.
Ozone depletion
Electron precipitation can lead to a substantial, short-term loss of
ozone
Ozone (), or trioxygen, is an inorganic molecule with the chemical formula . It is a pale blue gas with a distinctively pungent smell. It is an allotrope of oxygen that is much less stable than the diatomic allotrope , breaking down in the lo ...
(capping out at around 90%). However, this phenomenon also correlates to some long-term ozone depletion as well.
Studies have revealed that 60 major electron precipitation events occurred from 2002 to 2012. Different measurement tools (see below) read different ozone depletion averages ranging from 5-90%. However, some of the tools (specifically the ones that reported lower averages) did not take accurate readings or missed a couple of years. Typically, ozone depletion resulting from electron precipitation is more common during the winter season. The largest EEP event from the studies during 2002 to 2012 was recorded in October 2003. This event caused an ozone depletion of up to 92%. It lasted for 15 days and the ozone layer was fully restored a couple of days afterwards. EEP ozone depletion studies are important for monitoring the safety of Earth's environment and variations in the
solar cycle
The solar cycle, also known as the solar magnetic activity cycle, sunspot cycle, or Schwabe cycle, is a nearly periodic 11-year change in the Sun's activity measured in terms of variations in the number of observed sunspots on the Sun's surfa ...
.
Types
Electron precipitation can be caused by VLF waves from powerful transmitter based communications and lightning storms.
Lightning-induced Electron Precipitation (LEP)
Lightning-induced electron precipitation (also referred to as LEP) occurs when lightning strikes the Earth. When a bolt of lightning strikes the ground, an
electromagnetic pulse (EMP) is released which can hit the trapped electrons in the radiation belt. The electrons are then dislodged and "precipitate" into the Earth's atmosphere.
Because the EMP caused by lightning strikes is so powerful and occurs over a large range of spectrums, it is known to cause more electron precipitation than transmitter induced precipitation.
Transmitter-induced Precipitation of Electron Radiation (TIPER)
In order to cause electron precipitation, transmitters must produce very powerful waves with wavelengths from 10 to 100 km.
Naval communication arrays often cause transmitter-induced precipitation of electron radiation (TIPER) because powerful waves are needed to communicate through water. These powerful transmitters are operating at almost all times of the day. Occasionally, these waves will have the exact heading and frequency needed to cause an electron to precipitate from the radiation belt.
Measurement Methods
Electron precipitation can be studied by using various tools and methods to calculate its effects on the atmosphere. Scientists use
superposed epoch analysis Superposed epoch analysis (SPE or SEA), also called Chree analysis after a paper by Charles Chree
that employed the technique, is a statistical tool used in data analysis either to detect periodicities within a time sequence or to reveal a correl ...
to take into account the strengths and weaknesses of a large set of different measurement methods. They then use that collected data to calculate when an EEP event is taking place and its effects on the atmosphere.
Satellite measurements
In most cases, satellite measurements of electron precipitation are actually measurements of ozone depletion that is then linked to EEP events.
The different instruments use a wide variety of methods to calculate ozone levels. While some of the methods may provide significantly inaccurate data, the average of all of the data combined is widely accepted as accurate.
GOMOS
The
Global Ozone Monitoring by Occultation of Stars (GOMOS) is a measurement instrument aboard the European satellite
Envisat
Envisat ("Environmental Satellite") is a large inactive Earth-observing satellite which is still in orbit and now considered space debris. Operated by the European Space Agency (ESA), it was the world's largest civilian Earth observation satell ...
. It measures ozone amounts by using the emitted electromagnetic spectrum from surrounding stars combined with trigonometric calculations in a process called stellar
occultation
An occultation is an event that occurs when one object is hidden from the observer by another object that passes between them. The term is often used in astronomy, but can also refer to any situation in which an object in the foreground blocks ...
.
SABER
The Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) is a measurement instrument aboard NASA's Thermal Ionosphere Mesosphere Energetics Dynamics (TIMED) satellite.
The instrument measures ozone (and other atmospheric conditions) through an
infrared radiometer (with a spectral range from 1.27 μm to 17 μm).
MLS
The
Microwave Limb Sounder (MLS), an instrument aboard the
Aura satellite, measures microwave emission from the Earth's upper atmosphere. This data can help researchers find the levels of ozone depletion to an accuracy of 35%.
MEPED
The Medium Energy Proton Electron Detector (MEPED) measures electrons in the Earth's radiation belt and can estimate the amount of precipitating electrons in the ionosphere.
Sub-ionospheric Detection
With Sub-ionospheric Detection, a signal is sent from a VLF transmitter through the radiation belt to a VLF receiver on the other end.
The VLF signal will cause some electrons to precipitate, thus disturbing the VLF signal before it reaches the VLF receiver on the other end. The VLF receiver measures these disturbances and uses the data to estimate amount of precipitated electrons.
PIPER
PIPER is a Stanford-made photometer specifically designed for capturing the photons emitted when ionization occurs in the ionosphere.
Researchers can use this data to detect EEP events and measure the amount of precipitated electrons.
X-rays
X-ray
An X-ray, or, much less commonly, X-radiation, is a penetrating form of high-energy electromagnetic radiation. Most X-rays have a wavelength ranging from 10 picometers to 10 nanometers, corresponding to frequencies in the range 30&nb ...
equipment can be used in conjunction with other equipment to measure electron precipitation.
Because x-rays are emitted during electron collisions, x-rays found in the ionosphere can be correlated to EEP events.
VLF Remote Sensing
VLF Remote Sensing is a technique of monitoring electron precipitation by monitoring VLF transmissions from the U.S. Navy for "Trumi Events" (large changes of phase and amplitude of the waves).
Although this method can monitor electron precipitation, it cannot monitor the ionization of said electrons.
History
James Van Allen
James Alfred Van Allen (September 7, 1914August 9, 2006) was an American space scientist at the University of Iowa. He was instrumental in establishing the field of magnetospheric research in space.
The Van Allen radiation belts were named afte ...
from the State University of Iowa with his group, were the first to use vehicles with sensors to study electron fluxes precipitating in the atmosphere with
rockoon
A rockoon (from ''rocket'' and ''balloon'') is a solid fuel sounding rocket that, rather than being immediately lit while on the ground, is first carried into the upper atmosphere by a gas-filled balloon, then separated from the balloon and ign ...
rockets. The rockets would reach a maximum height of 50 km. The
soft radiation detected was later named after Van Allen in 1957.
The next advancement of research of electron precipitation was performed by Winckler with his group from the university of Minnesota. They used balloons that carried detectors into the atmosphere.
References
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Electron