Cosmic Chorus: Magnetic mid-tail whistler surprises "beyond the traditional expectation"

Tiassa

Tiassa

Let us not launch the boat ...
Valued Senior Member
Perturbation Chorus

liuetal-2025-nature-fieldparticlechorusemissionsspace-fig1schematicwhistlermode.png

The news summary from Associated Press↱

Scientists have detected cosmic waves that sound like birds chirping in an unexpected place.

These bursts of plasma, called chorus waves, ripple at the same frequency as human hearing. When converted to audio signals, their sharp notes mimic high-pitched bird calls.

Researchers have captured such sounds in space before, but now they have sensed the chirping waves from much farther away: over 62,000 miles (100,000 kilometers) from Earth, where they've never been measured before ....

.... Scientists still aren't sure how the perturbations happen, but they think Earth's magnetic field may have something to do with it ....

.... The newfound chorus waves were detected in a region where Earth's magnetic field is stretched out, which scientists didn't expect. That raises fresh questions about how these chirping waves form.​

And the more formal version, per Liu, et al.↱:

Chorus waves have hitherto been studied mainly in the near-planet region (L-shell less than or equal to 9 in geospace), and it was believed that they are governed by the magnetic dipolar field. The presence of repetitive, rising-tone chorus waves in the mid-tail (L ≈ 25), where there is no effect due to the magnetic dipolar field, is beyond the traditional expectation and provides important, new insights into understanding their generation and propagation in space. Interestingly, the observed rising-tone chorus waves have durations close to 0.1 s and chirping rates close to 100 Hz s⁻¹, close to those observed in the near-planet region. Considering that the magnetic field and plasma populations in the radiation belts and the mid-tail neutral sheet are very different, such similarities indicate that their generation is not uniquely determined by the local environment and that they can develop anywhere in space ....

One point under intense debate for chorus waves is the role of the magnetic field gradient in their chirping formation. Using MMS four-point measurements, we directly calculated the inhomogeneity factor S in the second-order resonance equation ('The inhomogeneity factor' in Methods). For the chorus element studied above, we found S≈-0.51. The obtained S is not only within the [−1, 0] range for the second-order resonance condition to be satisfied but close to the value of −0.41 that corresponds to the maximum energy transfer condition. We found that the main contributions to S are from the nonlinear term in equation (6), R₁≈ 0.46, which indicates that the magnetic field gradient is not important ....

.... In summary, this study presents the observation of repetitive, rising-tone chorus emissions in the terrestrial neutral sheet, suggesting that chorus waves may be ubiquitous in planetary magnetospheres, and it provides direct measurements of nonlinear energy transfer inside the waves, showing evidence of strong wave–particle interactions and the development of electron holes. The results not only establish pathways for studying chorus waves using state-of-the-art measurements provided by MMS to diagnose electron dynamics inside chorus waves but also provide important insights for understanding nonlinear wave–particle interactions in space and astrophysical plasmas.​

Also, just because: The AP article does go on to make clear it's not just Earth's magnetic field; the phenomenon has been observed at Mars, Jupiter, and Saturn.
____________________

Notes:

Liu, C. M., et al. "Field–particle energy transfer during chorus emissions in space". Nature. 22 January 2025. Nature.com. 24 January 2025. https://www.nature.com/articles/s41586-024-08402-z

Ramakrishnan, Adithi. "Scientists detect chirping cosmic waves in an unexpected part of space". Associated Press. 22 January 2025. APNews.com. 24 January 2025. https://apnews.com/article/chirping...gnetic-field-8f4846687863cdc9d88f3d7f2d6744e3
 
Tiassa said:
Perturbation Chorus

liuetal-2025-nature-fieldparticlechorusemissionsspace-fig1schematicwhistlermode.png

The news summary from Associated Press↱

Scientists have detected cosmic waves that sound like birds chirping in an unexpected place.​
​
These bursts of plasma, called chorus waves, ripple at the same frequency as human hearing. When converted to audio signals, their sharp notes mimic high-pitched bird calls.​
​
Researchers have captured such sounds in space before, but now they have sensed the chirping waves from much farther away: over 62,000 miles (100,000 kilometers) from Earth, where they've never been measured before ....​
​
.... Scientists still aren't sure how the perturbations happen, but they think Earth's magnetic field may have something to do with it ....​
​
.... The newfound chorus waves were detected in a region where Earth's magnetic field is stretched out, which scientists didn't expect. That raises fresh questions about how these chirping waves form.​

And the more formal version, per Liu, et al.↱:

Chorus waves have hitherto been studied mainly in the near-planet region (L-shell less than or equal to 9 in geospace), and it was believed that they are governed by the magnetic dipolar field. The presence of repetitive, rising-tone chorus waves in the mid-tail (L ≈ 25), where there is no effect due to the magnetic dipolar field, is beyond the traditional expectation and provides important, new insights into understanding their generation and propagation in space. Interestingly, the observed rising-tone chorus waves have durations close to 0.1 s and chirping rates close to 100 Hz s⁻¹, close to those observed in the near-planet region. Considering that the magnetic field and plasma populations in the radiation belts and the mid-tail neutral sheet are very different, such similarities indicate that their generation is not uniquely determined by the local environment and that they can develop anywhere in space ....​
​
One point under intense debate for chorus waves is the role of the magnetic field gradient in their chirping formation. Using MMS four-point measurements, we directly calculated the inhomogeneity factor S in the second-order resonance equation ('The inhomogeneity factor' in Methods). For the chorus element studied above, we found S≈-0.51. The obtained S is not only within the [−1, 0] range for the second-order resonance condition to be satisfied but close to the value of −0.41 that corresponds to the maximum energy transfer condition. We found that the main contributions to S are from the nonlinear term in equation (6), R₁≈ 0.46, which indicates that the magnetic field gradient is not important ....​
​
.... In summary, this study presents the observation of repetitive, rising-tone chorus emissions in the terrestrial neutral sheet, suggesting that chorus waves may be ubiquitous in planetary magnetospheres, and it provides direct measurements of nonlinear energy transfer inside the waves, showing evidence of strong wave–particle interactions and the development of electron holes. The results not only establish pathways for studying chorus waves using state-of-the-art measurements provided by MMS to diagnose electron dynamics inside chorus waves but also provide important insights for understanding nonlinear wave–particle interactions in space and astrophysical plasmas.​

Also, just because: The AP article does go on to make clear it's not just Earth's magnetic field; the phenomenon has been observed at Mars, Jupiter, and Saturn.
____________________

Notes:

Liu, C. M., et al. "Field–particle energy transfer during chorus emissions in space". Nature. 22 January 2025. Nature.com. 24 January 2025. https://www.nature.com/articles/s41586-024-08402-z

Ramakrishnan, Adithi. "Scientists detect chirping cosmic waves in an unexpected part of space". Associated Press. 22 January 2025. APNews.com. 24 January 2025. https://apnews.com/article/chirping...gnetic-field-8f4846687863cdc9d88f3d7f2d6744e3
Click to expand...
OK so waves set up in the stream of plasma from the sun, in the wake of the Earth, i.e. downstream from the planet. I'm not what the significance of these waves is, apart from them just being an interesting phenomenon.

Thing that caught my eye, though, was the reference at the end to "electron holes". To me, the concept of holes is from solid state physics, representing gaps in the valence band of a semiconductor where an electron has been removed, which can be treated as quasi-particles of +ve charge. I do not immediately see how it is possible to apply this concept to a plasma, given that plasmas consist of streams of ionised particles. The Nature article is behind a paywall, unfortunately.
 
exchemist said:
Thing that caught my eye, though, was the reference at the end to "electron holes". To me, the concept of holes is from solid state physics, representing gaps in the valence band of a semiconductor where an electron has been removed, which can be treated as quasi-particles of +ve charge. I do not immediately see how it is possible to apply this concept to a plasma, given that plasmas consist of streams of ionised particles. The Nature article is behind a paywall, unfortunately.
Click to expand...

Well, I'm hardly the expert, but ... oh.

Okay, the web article is Open Access in the U.S., and the pdf is similarly labeled, so here's the pdf:

• Liu, C.M., et al. "Field–particle energy transfer during chorus emissions in space". Nature 637. 22 January 2025. [link↱]​

Meanwhile, yeah, I'm hardly the expert on this, so, briefly:

• "Using high-cadence data from NASA’s MMS mission, we present ultrafast measurements of the wave fields and three-dimensional electron distributions within the waves, which provides evidence for chorus–electron interactions and the development of electron holes in the wave phase space." (Abstract, 813/1)

• In particular, the fast plasma investigation instrument onboard each MMS satellite can measure the three-dimensional electron distribution orders of magnitude faster than previous instruments and with a temporal resolution of 30 ms (ref. 36). The MMS mission was designed to study electron-scale processes, which are relevant for electron-scale chorus waves. Using MMS high-cadence data, we fully resolved electron distributions and electric currents within the waves. We present direct measurements of the nonlinear field–particle energy exchange inside the waves, showing that the waves were locally growing and associated with electron holes in the wave phase space. Our observations may open pathways for investigating chorus waves, and they set observational constraints for theoretical studies of nonlinear wave–particle interactions in collisionless plasmas." (814/2)

• "We found clear non-gyrotropy in the cyclotron plane, especially local depletion (for example, Fig. 4g(i),(iii)). These local depletions are also visible in the wave phase space , where ζ is the relative phase angle between the wave magnetic field and the electron perpendicular velocity with respect to the ambient magnetic field, as shown in Fig. 4h(i)–(iv). Near the resonance velocity, we observed localized depletions of the electron PSD. These depletions could be electron holes predicted by the nonlinear theory, whose existence within chorus waves has never been verified by observations so far (note that the local depletion of electron PSDs has been observed for lion roars in the magnetosheath). The identified electron holes have two main features that are consistent with the nonlinear theoretical expectations .... Note that the electron holes were both weak and local. Hence, they could easily fade into the thermal background when their centre speed approaches the electron thermal speed, as seen in Fig. 4h(4)." (817/5)

• "The measurement of three-dimensional electron distributions inside the waves led to some inspiring results, particularly the electron holes. These electron holes associated with chorus waves have been well studied by simulations and have now been observed in situ." (Discussion and summary, 819/7)​
 
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exchemist said:
OK thanks I've got it now, not sure why I couldn't before.
This about electron holes seems to be some kind of local deficit relative to expected electron denisty of the plasme. But this is all plasma physics that is beyond me.
Click to expand...
I don't really understand it. Sorry.
 
exchemist said:
I do not immediately see how it is possible to apply this concept to a plasma, given that plasmas consist of streams of ionised particles.
Click to expand...

But that almost sounds like the point: Some math tells physicists this must be going on; modeling validates the process, but it hadn't been observed, and now it has, i.e., "Using MMS high-cadence data, we fully resolved electron distributions and electric currents within the waves. We present direct measurements of the nonlinear field–particle energy exchange inside the waves, showing that the waves were locally growing and associated with electron holes in the wave phase space."

There's probably something I'm missing, but if I start with a Wikipedia↱ phrase, "A hole is the absence of a negative-mass electron", and consider the basic sketch—

Since in a normal atom or crystal lattice the negative charge of the electrons is balanced by the positive charge of the atomic nuclei, the absence of an electron leaves a net positive charge at the hole's location.

Holes in a metal or semiconductor crystal lattice can move through the lattice as electrons can, and act similarly to positively-charged particles.​

—it seems to me Liu et al. are saying they've observed a similar process occurring in streams of ionized particles where the lattice or matrix is the stream of plasma itself. That's the thing that stands out, "within the waves", "inside the waves".

But I could easily be wrong. For instance, I went to find out what a "collisionless plasma" is, and found myself at PNAS for Meyrand, Kanekar, Dorland, and Schekochichin (2019), "Fluidization of collisionless plasma turbulence"↱, and, right, I'll get back to you on that when, er ... uh ... right. I couldn't tell you if that one helps, or not.
 
exchemist said:
It’s pretty arcane. Seems to be just a curiosity of the magnetosphere.
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Dirac was obsessed with a hole thing, for his positron perhaps?
I saw the article on birds on phys.org but not read it yet. Another Hubble tension paper come out which took my interest.
I read both again and post later.
 
Pinball1970 said:
Dirac was obsessed with a hole thing, for his positron perhaps?
I saw the article on birds on phys.org but not read it yet. Another Hubble tension paper come out which took my interest.
I read both again and post later.
Click to expand...
Yes but not in a plasma. Holes are generally a solid state thing, to do with valence and conduction bands etc.
 
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