Radiation from electron oscillations at plasma frequency?

[Mentelsly]

Hi, there have been a number of recent papers with regards to radiation from hypervelocity meteor impact, where the authors propose that the radiation originates from self-consistent electron oscillations at the plasma frequency. Some examples:

1) Close et al - Journal of Geophysical Research, 115, A12328 (2010)
http://www.agu.org/pubs/crossref/2010/2010JA015921.shtml (apologies if you don't have access)
Related poster:
http://aa.stanford.edu/students/media/posters2011/Alex_Fletcher.pdf

2) Foschini - Europhysics Letters, 43, 226 (1998)
http://arxiv.org/abs/physics/9804027v2

In these papers, what I understand is that they basically consider electron oscillations at plasma frequency, take each oscillating electron as a radiating dipole, and integrate the radiated power over the entire finite plasma.

But are these mechanisms correct? I thought these plasma oscillations are nonradiative, because the displacement current cancels the electron current so that there is no magnetic field involved. (I am assuming no nonlinear effects here, which the authors do not consider anyway.) If the radiated electromagnetic field strength (rather than power) is integrated over the finite plasma, I am expecting that the radiated field strengths will destructively interfere so that there is no radiation.

Also, note that I do not think the authors are referring to other finite-plasma oscillation modes that are indeed coupled to electromagnetic waves outside the plasma, because these modes are not at plasma frequency, and also because these modes cannot be derived self-consistently solely based on the restoring force from charge density generated by the electron motions, which the authors seem to have alluded to.


There seems to be a somewhat similar controversy when some authors proposed a mechanism for the observation of terahertz radiation from femtosecond laser filaments, again based on radiation from these plasma oscillations (apologises if you don't have access):
Cheng, Wright, Moloney - Physical Review Letters, 87, 213001 (2001)

which generated some arguments and counterarguments:
Shvets, Kaganovich, Startsev - Physical Review Letters, 89, 139301 (2002)
Cheng, Wright, Moloney - Physical Review Letters, 89, 139302 (2002)
Tikhonchuk - Physical Review Letters, 89, 209301 (2002)
Sprangle et al - Physical Review E, 69, 066415 (2004)


In summary, I believe that the proposed mechanisms of radiation from electron oscillations at plasma frequency are wrong, but being a beginning researcher, I'm not particularly confident and so would like to hear the viewpoints on my counterarguments from more experienced members of the forum. Thanks!

 

[eljose79]

Thank you for bringing up this interesting topic of radiation from hypervelocity meteor impact. I understand your concern and confusion about the proposed mechanisms of radiation from electron oscillations at plasma frequency. I will try my best to provide a clear explanation and address your doubts.

Firstly, let me clarify that the proposed mechanisms in the papers you mentioned are not referring to nonradiative plasma oscillations. They are specifically considering self-consistent electron oscillations at the plasma frequency, which can indeed radiate electromagnetic waves. The plasma frequency is the characteristic frequency at which the free electrons in a plasma oscillate collectively. This frequency is determined by the plasma density and can be calculated using the formula ωp = √(ne^2/ε0m), where n is the plasma density, e is the charge of an electron, ε0 is the permittivity of free space, and m is the mass of an electron.

Now, let's take a closer look at the proposed mechanisms in the papers. Close et al. (2010) and Foschini (1998) are both discussing the radiation from the self-consistent electron oscillations at the plasma frequency. They are not simply considering each oscillating electron as a radiating dipole, but rather taking into account the collective behavior of the electrons in a finite plasma. This means that the radiated power is integrated over the entire plasma, not just a single electron.

Moreover, the authors are not neglecting the magnetic field in their calculations. In fact, they are considering the magnetic field generated by the oscillating electrons, which can be calculated using the Biot-Savart law. This magnetic field is then used to calculate the radiated electromagnetic field strengths and take into account the destructive interference from different locations within the plasma.

In regards to your concern about other finite-plasma oscillation modes, it is important to note that the plasma frequency is not the only mode that can couple to electromagnetic waves outside the plasma. There are other modes, such as the ion-acoustic waves, that can also radiate electromagnetic waves. However, the papers you mentioned are specifically discussing the radiation from self-consistent electron oscillations at the plasma frequency, which can be derived self-consistently solely based on the restoring force from charge density generated by the electron motions.

Lastly, I would like to address the controversy surrounding the observation of terahertz radiation from femtosecond laser filaments.