Slash Boxes

SoylentNews is people

posted by mrpg on Tuesday June 08 2021, @12:13PM   Printer-friendly
from the what-about-aurora-australis? dept.

Physicists determine how auroras are created:

[...] In a new study, a team of physicists led by University of Iowa reports definitive evidence that the most brilliant auroras are produced by powerful electromagnetic waves during geomagnetic storms. The phenomena, known as Alfven waves, accelerate electrons toward Earth, causing the particles to produce the familiar atmospheric light show.

The study, published online June 7 in the journal Nature Communications, concludes a decades-long quest to demonstrate experimentally the physical mechanisms for the acceleration of electrons by Alfven waves under conditions corresponding to Earth's auroral magnetosphere.

"Measurements revealed this small population of electrons undergoes 'resonant acceleration' by the Alfven wave's electric field, similar to a surfer catching a wave and being continually accelerated as the surfer moves along with the wave," says Greg Howes, associate professor in the Department of Physics and Astronomy at Iowa and study co-author.

Journal Reference:
J. W. R. Schroeder, G. G. Howes, C. A. Kletzing, et al. Laboratory measurements of the physics of auroral electron acceleration by Alfvén waves [open], Nature Communications (DOI: 10.1038/s41467-021-23377-5)

Also at Nature

Original Submission

This discussion has been archived. No new comments can be posted.
Display Options Threshold/Breakthrough Mark All as Read Mark All as Unread
The Fine Print: The following comments are owned by whoever posted them. We are not responsible for them in any way.
  • (Score: 3, Insightful) by nostyle on Tuesday June 08 2021, @04:46PM (5 children)

    by nostyle (11497) Subscriber Badge on Tuesday June 08 2021, @04:46PM (#1143194) Journal

    Suppose you have a plasma, and you need to add some heat to it - say to bring it up to temperatures suitable for a fusion reactor.

    One way you might try is to hit that plasma with some high-energy radio waves, adding to agitation of the charged particles. A necessary condition is that you want the charged particles to be in resonance with (or opaque to) the radio waves you transmit. One resonant frequency is determined by the Alfven wave resonance. Then if you can find that resonant frequency and tune your RF excitation to it, your plasma can be heated efficiently - much like a microwave heats up food by being resonant with water molecules.

    I was a minor contributor to some research (long ago) into this use of this heating strategy. At the end of the day, though, there seem to be better ways to heat your plasma.

    "(Let's dance) to the song they're playing on the radio" -David Bowie

    • (Score: 2) by maxwell demon on Tuesday June 08 2021, @05:57PM (4 children)

      by maxwell demon (1608) Subscriber Badge on Tuesday June 08 2021, @05:57PM (#1143224) Journal

      much like a microwave heats up food by being resonant with water molecules.

      That's not the case [] (emphasis from the quoted original):

      The microwaves in a microwave oven are not tuned to a resonant frequency of water. In fact, the microwaves generated inside a microwave oven are not really tuned to any particular resonant frequency since the waves are broadband. A broadband electromagnetic wave contains many frequencies. You need a monochromatic wave (a nearly single-frequency wave) in order to tune to a specific frequency. Laser beams are monochromatic. Radio waves from simple antennas are monochromatic. Microwaves in an microwave oven are not monochromatic.


      So how do microwaves in an oven heat food if they are not tuned to a specific resonant frequency of water? They heat the food through simple dielectric heating. In dielectric heating, the electric field in the electromagnetic wave exerts a force on the molecules in the food, causing them to rotate in order to align with the field. Because of this rotating motion, the molecules collide into each other and convert their somewhat ordered rotational motion into disordered motion, which we macroscopically call heat. Many types of molecules in the food absorb energy from the microwaves in this way, and not just water molecules.


      There is no special significance of 2.45 GHz, except that it is allocated by the FCC as being allowable for microwave oven usage.

      The Tao of math: The numbers you can count are not the real numbers.
      • (Score: 2) by nostyle on Tuesday June 08 2021, @06:55PM (3 children)

        by nostyle (11497) Subscriber Badge on Tuesday June 08 2021, @06:55PM (#1143240) Journal

        Good point. I was being somewhat sloppy in my explanation. Still the microwave oven works because of "resonance" between the dipole moments of molecules and the frequency of the incident microwaves (AKA dielectric heating []). And the linked reference points out:

        Microwave frequencies penetrate conductive materials, including semi-solid substances like meat and living tissue. The penetration essentially stops where all the penetrating microwave energy has been converted to heat in the tissue. Microwave ovens used to heat food are not set to the frequency for optimal absorption by water. If they were, then the piece of food or liquid in question would absorb all microwave radiation in its outer layer, leading to a cool, unheated centre and a superheated surface. Instead, the frequency selected allows energy to penetrate deeper into the heated food. The frequency of a household microwave oven is 2.45 GHz, while the frequency for optimal absorbency by water is around 10 GHz.

        So the microwave frequency is not optimized (tuned) to the water, yet it is still resonant enough that one can heat a glass of pure water in the microwave. OTOH shining a 1000 watt beam of visible light through a glass of water will not produce nearly as much heating of the water since the frequency of the light is not in resonance with the dipole moment of the water (i.e. the water is transparent to that EM frequency).

        This detail does not change the fact that Alfven heating may be likened to commonly used microwave heating.

        • (Score: 2) by maxwell demon on Wednesday June 09 2021, @05:51AM (2 children)

          by maxwell demon (1608) Subscriber Badge on Wednesday June 09 2021, @05:51AM (#1143448) Journal

          There is no such thing as “resonant enough”. Either you are in resonance or you aren't.

          Absorption != resonance. While at resonance absorption is normally at its highest, you can have absorption even far away from any resonance. And that is exactly what happens in dielectric heating.

          The Tao of math: The numbers you can count are not the real numbers.
          • (Score: 2) by nostyle on Wednesday June 09 2021, @03:09PM (1 child)

            by nostyle (11497) Subscriber Badge on Wednesday June 09 2021, @03:09PM (#1143538) Journal

            You are mistaken [], sir.

            Resonance describes the phenomenon of increased amplitude that occurs when the frequency of a periodically applied force (or a Fourier component of it) is equal or close to a natural frequency of the system on which it acts. When an oscillating force is applied at a resonant frequency of a dynamic system, the system will oscillate at a higher amplitude than when the same force is applied at other, non-resonant frequencies.

            "There was voodoo in the vibes" - Atlanta Rhythm Section, So Into You

            • (Score: 0) by Anonymous Coward on Friday June 11 2021, @02:03PM

              by Anonymous Coward on Friday June 11 2021, @02:03PM (#1144244)

              Hear Hear!

              Furthermore, absorbtion isn't necessarily all-or-nothing either.

              It definitely is sometimes - like for sufficiency isolated quantum systems.

              But in the general case: continuous systems tend to have continuous allowable energy bands, maybe with gaps. (see, quite literally: Band gap structure - which is how semiconductors work).

              Transparent materials have gaps. Through which certain EM frequency 'fits'. You buy special lenses for long wave IR, for instance, that is opaque to visible light, since you need transparency elsewhere.

              You don't really think discrete 'allowable' electron orbital transfer quantum rules apply to all things, let alone the liberated electrons in a plasma, do you?

              Heat basically represents local 'pocket change' which allows broader emission/absorbtion bands.

              This works in gases - although there it's just due to the fact the hotter the gas is, the wider the distribution of velocities will be, and so some part of the population gets either a blue or red-shifted relationship.

              But whatever state your system is, it's basically always true that you will want to stabilise temperature (probably by applying a lot of cooling, because less heat means 'more quantum') when you want to make your spectrum sharper.

              What this is about, is that plasmas of temperature enough for fusion, are actually amazingly broadband light sources.
              Synchrotron light - which is described as being 'ultra-broadband' and 'has photon energies from 0 to 100 eV' doesn't compare well to a plasma hot enough for a practical nano-miniature sun.
              We're talking a few more orders of magnitude there -- perhaps (in the case of p-B11 fusion) up to around four more.

              The problem is that plasma that hot (and dense enough to do the high Fusion power intensity demanded) makes 'Bremsstrahlung' radiation.

              It glows with X-ray-white heat.

              X-ray optics are interesting: You can basically only make the equivalent of a 'grating' by using the nuclei of solid (and perfect) crystals, and you can just 'deflect some' not really make true mirrors or lenses at all.

              This means you can't actually make a 'cavity' for X-rays. They just won't stay inside your box - they'll leave.

              Probably to slightly warm the metal your box is made of. Volumetricaly, like a microwave oven, not able to be coralled/focused to a place you can conveniently put a heat-exchanger to drive your steam engine.

              nano-scale fusion (which is what the goal is - fusion on scales of metres, not the ~0.7 Giga metre radius the sun has) is *always* fighting this to keep the plasma hot.

              And BTW: Power for radiating heat from a hot thing (T) to a 'cold' thing (Tc) goes like this: P[Watts] = emissivity[some 0-1 thing] x 5.6703e-8 x Surface Area [m^2] x (TxTxTxT - TcxTcxTcxTc)[both in K]

              And gosh, darn it - the Tc for this penetrative, unreflectable X-ray heat is basically always 'how hot is your building?'. At best. Likely more 'what's the average temperature of the universe?'. So basically temperture in Kelvins to the fourth power. That constant at 10^-8 goes away quickly.

              There's even a chance that you just can't build a fusion chain-reaction energy system this way - it's always going to make far more of this nearly-useless radiation than it is fusion power. But hey, if you include the full (unusable) power of all the Xrays, you might be able to say 'more power came out than we put in' like JET did a while ago.

              Doing fusion by brute force compressed heat - and expecting it to behave like fire - with a chain reaction, on a conveniently nano-ized scale?
              Probably wishful thinking.

              Stars don't work that way - they use distance -- and plenty of it.

              But the average fusion power of the core of our star is only on the order of maybe 1 W per cubic metre? Chemistry in your body is higher than that - closer to 60 W /m^3 at rest. Combustion chamber of Diesel engines are up in the MW/m^3, also just chemicals. Guns/bombs and generally explody things far, far higher.

              They can 'close the loop' to make it a chain-reaction, basically because the surface-area to volume ratio is low enough. There needs to be enough distance of hot plasma to give the Xrays (and fusion waste particles) enough chance to finally hit something else, so as to pass the heat on to the next generation fusion reaction in the chain.

              This also means that stars are pretty special, as far as matter goes. Most of the matter in the universe is probably in hydrogen interstellar cold gas giants. You usually only get a star forming when gravity happens to collect enough of those to finally kick off fusion in the core - and the first thing that happens is that most of the mass blows away.

              What about H-bombs, I hear you ask? Don't they, at least, do a fusion chain reaction?

              Well, Sure, they do fusion: Pumped by having all the light of a detonating A-bomb focused on D-T fuel.

              But each individual fusion reaction happens basically separately from any of the others, and most of the energy goes into a very hot neutron - which then leaves the fuel, as fast neutrons like that basically are penetrating radiation also. 80% of the 'yield' of a H-bomb comes from the fission made possible because those neutrons are just so damn high energy, able to split even depleted uranium, which, BTW, isn't able to sustain a chain reaction itself *either*. But those neutrons themselves do not contribute much back to the fusion reaction.

              So, we probably can't build a fusion machine as a chain reactor. Does this mean fusion is hopeless?

              No - just use a particle accelerator, and design it to be efficient at not losing expensive accelerated particles that haven't fused yet.

  • (Score: 2) by MIRV888 on Wednesday June 09 2021, @04:32AM

    by MIRV888 (11376) on Wednesday June 09 2021, @04:32AM (#1143425)

    I want to see auroras proper.