Scientists solve a decades-old mystery in the Earth’s upper atmosphere

From the University of California – Los Angeles

The top panel shows electron fluxes before (left) and after (right) a geomagnetic storm. The injection of low-energy plasma sheet electrons into the inner magnetosphere (1) causes chorus wave excitation in the low-density region outside the cold plasmasphere (2). Local energy diffusion associated with wave scattering leads to the development of strongly enhanced phase space density just outside the plasmapause (3). Subsequently, radial diffusion can redistribute the accelerated electrons inwards or outwards from the developing peak (4). Credit: Jacob Bortnik/UCLA

New research published in the journal Nature resolves decades of scientific controversy over the origin of the extremely energetic particles known as ultra-relativistic electrons in the Earth’s near-space environment and is likely to influence our understanding of planetary magnetospheres throughout the universe.

Discovering the processes that control the formation and ultimate loss of these electrons in the Van Allen radiation belts — the rings of highly charged particles that encircle the Earth at a range of about 1,000 to 50,000 kilometers above the planet’s surface — is a primary science objective of the recently launched NASA Van Allen Probes mission. Understanding these mechanisms has important practical applications, because the enormous amounts of radiation trapped within the belts can pose a significant hazard to satellites and spacecraft, as well astronauts performing activities outside a craft.

Ultra-relativistic electrons in the Earth’s outer radiation belt can exhibit pronounced variability in response to activity on the sun and changes in the solar wind, but the dominant physical mechanism responsible for radiation-belt electron acceleration has remained unresolved for decades. Two primary candidates for this acceleration have been “inward radial diffusive transport” and “local stochastic acceleration” by very low-frequency plasma waves.

In research published Dec. 19 in Nature, lead author Richard Thorne, a distinguished professor of atmospheric and oceanic sciences in the UCLA College of Letters and Science, and his colleagues report on high-resolution satellite measurements of high-energy electrons during a geomagnetic storm on Oct. 9, 2012, which they have numerically modeled using a newly developed data-driven global wave model.

Their analysis reveals that scattering by intense, natural very low–frequency radio waves known as “chorus” in the Earth’s upper atmosphere is primarily responsible for the observed relativistic electron build-up.

The team’s detailed modeling, together with previous observations of peaks in electron phase space density reported earlier this year by Geoff Reeves and colleagues in the journal Science, demonstrates the remarkable efficiency of natural wave acceleration in the Earth’s near-space environment and shows that radial diffusion was not responsible for the observed acceleration during this storm, Thorne said.

Co-authors of the new research include Qianli Ma, a graduate student who works in Thorne’s lab; Wen Li, Binbin Ni and Jacob Bortnik, researchers in Thorne’s lab; and members of the science teams on the Van Allen Probes, including Harlan Spence of the University of New Hampshire (principal investigator for RBSP-ECT) and Craig Kletzing of the University of Iowa (principal investigator for EMFISIS).

The local wave-acceleration process is a “universal physical process” and should also be effective in the magnetospheres of Jupiter, Saturn and other magnetized plasma environments in the cosmos, Thorne said. He thinks the new results from the detailed analysis of Earth will influence future modeling of other planetary magnetospheres.

The Van Allen radiation belts were discovered in the Earth’s upper atmosphere in 1958 by a team led by space scientist James Van Allen.

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The new research was funded by the NASA, which launched the twin Van Allen probes in the summer of 2012.

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23 thoughts on “Scientists solve a decades-old mystery in the Earth’s upper atmosphere

  1. Is there another side (a source) to the electron loss equation? It would seem that the Earth would build up a steady positive charge otherwise.

  2. What? And here I thought this was all settled science!!!

    I guess there are still rewards for doing science inspired by curiosity.

  3. Solar activity ‘products’ mainly coronal mass ejections (CMEs) exert pressure on the Earth’s magnetosphere, which in turn is maintained by the Earth’s magnetic field.
    It is well known that the solar activity is not constant but oscillates with relatively regular periodicity known as the ‘sunspot cycle’. It is less known that the Earth’s field is not constant either, it shows similar decadal variability, as shown in the data from number of distinguished scientists and researchers (Jault Gire, LeMouel, J. Bloxham, D. Gubbins, A.Jackson, R. Hide, D. Boggs, J. Dickey etc,)
    Since changes in either of two fields affect strength of the magnetosphere, it would be expected that the ‘magnetospheric variability’ time function could be produced by combining two sets of available data.
    That is exactly what I did some months ago introducing terms ‘Geo-Solar Oscillation’ and ‘Geo-Solar Cycle’.
    Comparing the GSC to two well known climatic sets of data opens a way into an unexpected and fascinating direction for climatologists’ research

    http://www.vukcevic.talktalk.net/GSC1.htm

  4. The diagram is confusing.

    I wonder why no mention that the Earth’s magnetic field is in fact a quadrupole, with 0.5% of the total field emanating from around the equator? It is bound to have an effect on the Van Allen belts, without having to resort to invoking relativistic electrons.

  5. vukcevic,

    I think you may have just sent me an early Christmas present. Going to your link provided earlier, I discovered what appears to be hundreds of graphs of all sorts of good things.

    I was recently trying to find a comprehensive source of magnetic field graphs/data(for earth) plotted with other measures and this looks like the jackpot………..with much more than that.
    Thanks

    http://www.vukcevic.talktalk.net/Contents1.htm

  6. “Their analysis reveals that scattering by intense, natural very low–frequency radio waves known as “chorus” in the Earth’s upper atmosphere is primarily responsible for the observed relativistic electron build-up.”

    Which is it?
    Scattering or build up?

  7. “Ultra-relativistic electrons….”???

    Are these electrons accelerated to some significant percentage (say 80%?) of the speed of light? I did a quick search but could not find a definition of ‘ultra- relativistic’.

  8. Until they realise that the universe is fundamentally electrical in nature, it’s all just mumbo-jumbo. The press release even says, “The team’s detailed modeling”. I don’t trust modelling.

  9. vukcevic says:
    December 20, 2013 at 3:35 pm

    Vukcevic,
    Do you prefer ‘Vuk’ or ‘Vukcevic’?

    Thanks for the link! That stretches the calculus, differential equations, and ‘modern physics’ I had 30 years ago! The Appendix: Hamiltonian Representation provided the insights to help me draw up some rusty memories. Also, a trip to http://en.wikipedia.org/wiki/Hamiltonian_%28quantum_mechanics%29. THAT reminded me of why I pursued engineering degrees instead of quantum physics degrees!

    After a bit of ‘sussing out the intent of the equations’ I arrived at this conclusion: The electrons are accelerated to speeds approaching ‘C’, the speed of photonic (massless) electromagnetic radiation in a vacuum. That is amazing…. and adds an even higher degree of potential harm to humans, space craft, and satellites passing through or working with the spatial regions of the Van Allen belts, than was previously known.

  10. Tenuc says December 20, 2013 at 6:13 am
    The diagram is confusing.

    I wonder why no mention that the Earth’s magnetic field is in fact a quadrupole, with 0.5% of the total field emanating from around the equator? …

    Are you familiar with the term RSS (root sum of squares)? It means the the bigger term dominates when two different quantities are summed …

  11. It’s nice to know this has been solved. Forget hypotheses and theories. This article makes clear this is a fact. I bow down to your factualness.

  12. “Ultra-relativistic electrons in the Earth’s outer radiation belt can exhibit pronounced variability in response to activity on the sun and changes in the solar wind’

    Oh and now the sun is now actually being noticed? (other than scorching the lily white skins of the UK’s meteorologists? sorry to a few UK met guys)
    “Ultra-relativistic electrons in the Earth’s outer radiation belt can exhibit pronounced variability in response to activity on the sun and changes in the solar wind”
    Is there a possibility that some one can explain a few words in this sentence? And beforehand thanks you, I am confused.
    What is an “Ultra-relativistic” electron? Lets say compared to a “Regular Relativistic electron” ? ( I can just hear Einstein’s brain sloshing in the jar).
    “can exhibit pronounced variability”, So if it so “pronounced” why is there a “can”, “variability”, and changes in the now all of a sudden previously unremarkable phenomenon called the SUN?,
    Merry Christmas and a Great (the as normal weather wise) New Year to all of you on this site Thanks AW. Love the place and all of you on it..

  13. Not having link to the full article, I read the above twice to try to understand their conclusion, there is much uncertainty and even worse the researchers could be wrong.
    Here is my take: Van Allen radiation belts are well within magnetosphere (see link)

    so how the ultra-relativistic electrons in the Earth’s outer radiation belt can be accelerated to such velocities ?
    In the labs that may be achieved under control conditions where powerful sources of energy (MeV & GeV) may be available. Having in mind that 1 MeV is equivalent (I think) somewhere around 10 G Kelvin; are these energies readily available in the magnetosphere ?
    Magnetic reconnection is a frequent energetic event in the magnetosphere, when enormous amount of magnetic energy is released, and I would suggest it may be the prime candidate for acceleration via ‘instantaneous’ magnetic shock-wave (see Nasa video link)

    http://www.nasa.gov/mpg/262351main_reconnect.mpg

    The above (nonsense?) may tempt Dr. S to come out and weigh in with some more reasoned science.

    Mac the Knife says:
    Do you prefer ‘Vuk’ or ‘Vukcevic’?
    either is OK, but ‘Vukcevic’ may land you in the mod’s bin.

    happy Xmas to all

  14. “”Two primary candidates for this acceleration have been “inward radial diffusive transport” and “local stochastic acceleration” by very low-frequency plasma waves.””

    I think they have been looking at the “wave family” for a while now. As in, EMIC, Electromagnetic Ion Cyclotron Wave, ULF and VLF Ultra Low + Very Low Frequency Waves, as the acceleration mechanism.

    Well now that we have those pesky high energy, ultra-relativistic, near light speed electrons moving, where might they be moving to?
    Let’s see um..
    First article has them down to between 44 km and 54 km altitude at 65±5°N geomagnetic latitude.”

    Mesospheric NO2 production due to relativistic electron precipitation from 2007 till 2011

    Friederich, Felix; Sinnhuber, Miriam; Funke, Bernd; von Clarmann, Thomas; Stiller, Gabriele; Orphal, Johannes

    http://adsabs.harvard.edu/abs/2013EGUGA..15.4879F

    EGU General Assembly 2013, held 7-12 April, 2013 in Vienna, Austria, id. EGU2013-4879

    Electrons of the radiation belts and the aurora can precipitate into the Earth’s atmosphere. Depending on their energy they intrude into different altitudes and can excitate, ionize, and dissociate molecular nitrogen. Subsequent (ion-)chemical reactions result in an effective NOx-production (NOx=NO+NO2). NOx is produced mostly by auroral electrons in the thermosphere at ca. 110 km altitude. But relativistic electrons from the radiation belts can also reach the stratosphere..

    Next one to the equator, anomaly, note “mirror point.”

    Evidence of coherent drift-resonant acceleration of radiation belt particles by ULF waves

    Sauvaud, Jean-Andre.; Walt, Martin; Delcourt, Dominique; Benoist, Clément; Penou, Emmanuel; Chen, Yue; Russell, Christopher T.

    http://adsabs.harvard.edu/abs/2013EGUGA..15.1830S

    EGU General Assembly 2013, held 7-12 April, 2013 in Vienna, Austria, id. EGU2013-1830

    Evidence is presented for frequent, coherent and powerful accelerations of radiation belt electrons and protons during magnetic storms. Geomagnetic storms are indeed frequently associated with the formation of well developed, multiple bands of energetic electrons inside the inner radiation belt at L=1.1-1.9 and with prominent similar energy structures of protons inside the slot region at L=2.2-3.5. These structures typically from 100 keV up to the MeV range result from coherent interactions of energetic particles with quasi- monochromatic Ultra Low Frequency waves (ULF). These waves are induced by magnetospheric changes due to the arrival of dense solar material and related nightside injections of particles from the outer magnetosphere that destabilize field lines in the inner magnetosphere down to L=1.1. We show that, at the low altitudes of the Demeter spacecraft, these structures are best seen near the South Atlantic Anomaly because of lowering of the belt particle mirror point. As evidenced from ground measurements,…

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