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As discussions and answers to How large does refraction become in radioastronomy? point out, it is difficult to do radio astronomy much below 30 MHz (or 10 MHz depending on how aggressive you are in correcting for ionospheric effects like wavefront distortion and scintillation) from Earth's surface due to our ionosphere, which has made AM and short-wave radio listeners and ham radio operators happy for almost a century by being so reflective.

But that doesn't necessarily mean that there's no interesting radio astronomy way below 30 MHz.

Question(s):

  • What can be learned from low frequency radio astronomy available outside of Earth's ionosphere?
  • Are there currently any proposed or planned space-based low-frequency radio-telescope efforts?
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    $\begingroup$ You might want to look into the research done by Grote Reber, an American radio astronomer who moved to Tasmania in the 1954 to study radio signals in the 0.5 to 3 MHz range. $\endgroup$
    – Fred
    Dec 20 '19 at 10:58
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    $\begingroup$ [History of Low Frequency Radio Astronomy in Australia 1](www.narit.or.th/en/files/2015JAHHvol18/2015JAHH...18..312G.pdf), [History of Low Frequency Radio Astronomy in Australia 2](www.narit.or.th/en/files/2015JAHHvol18/2015JAHH...18...14G.pdf) & History of Low Frequency Radio Astronomy in Australia 3. Podcast $\endgroup$
    – Fred
    Dec 20 '19 at 11:19
  • $\begingroup$ @Fred I have quite an interest in early radio astronomy but don't know much about it, thanks! $\endgroup$
    – uhoh
    Dec 20 '19 at 11:25
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    $\begingroup$ How low is "low"? The plasma frequency in the interplanetary medium near the Earth is about 100 kHz. physics.stackexchange.com/a/519164/43351 $\endgroup$
    – ProfRob
    Dec 20 '19 at 19:53
  • $\begingroup$ @RobJeffries that's good to know, thanks! In this case "low" was defined as below what can be observed from inside the ionosphere. $\endgroup$
    – uhoh
    Dec 21 '19 at 3:14
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The Wind, STEREO, Parker Solar Probe, and Solar Orbiter spacecraft all carry radio instruments that observe radio frequency emissions from a few kHz up to ~10-20 MHz.

There's an entire section in Wilson et al. [2021] devoted to discussing the novel discoveries that resulted from the launch of Wind in radio astronomy, specifically solar radio observations. One of the primary things we examine in the kHz to few 10s of MHz range are called solar radio bursts. Most of these are generated indirectly by energized electrons. That is, electrons are energized to high enough energies to be unstable to something like a bump-on-tail instability which radiates Langmuir waves. These Langmuir waves then undergo some type of nonlinear wave-wave interaction (we still haven't figured out exactly which one) resulting in the emission of an ion acoustic wave and a so called free mode, which is usually an ordinary mode -- sometimes called an O-mode, these are left-hand polarized (with respect to the quasi-static magnetic field) electromagnetic waves with intrinsic frequencies above the local upper hybrid frequency (in the solar wind, this is basically the same as the plasma frequency).

In any case, the radio emission that we observe remotely with these spacecraft serves several purposes. The first being that it is a tracer of the local plasma density from which the emission arose (i.e., because the emitted radio wave is just slightly above the local plasma frequency). This can be used to determine the radial distance from the Sun where the emission originated or one can infer a total electron density model from these emissions.

The second purpose is that one of the solar radio burst types, Type II, has an interesting coexisting phenomena. That is, every time there are large solar energetic particle (SEP) events, there is an associated Type II radio burst. A Type II is useful because it results from electrons accelerated locally by a propagating interplanetary shock wave. Thus, we can trace the speed and distance to the incident shock and provide a (very small) warning lead time for potential SEP events and/or geomagnetic storms.

Type II radio bursts drift slowly in frequency versus time compared to, say, Type III radio bursts. The latter are thought to result from electrons accelerated in solar active regions and so the frequency drift rate can inform the user of the electron beam speed if one knows the radial electron density profile. With our recent in situ measurements from Parker Solar Probe, we have better constraints on the radial profile of the total electron density and the magnetic field strength.

Finally, if we are careful and properly calibrate things and make a meticulous effort to properly calibrate the noise levels of each frequency, we can make detailed and long-duration measurements of the galactic background, which is useful for radio astronomy.

Okay, one last point: We can also observe radio emissions arising from electrons in the Jovian magnetosphere.

Update
Note that the 25th to 75th percentile for the total electron density near Earth is ~5.7-13.0 cm-3 with a median of ~8.6 cm-3. This results in a plasma frequency of ~17.2-42.5 kHz with a median of ~26.3 kHz [e.g., see Table 6 in Wilson et al., 2021]. The total electron density in the solar corona is larger by ~5-8 orders of magnitude, depending on radial distance.

Also note that the electron cyclotron frequency near Earth is ~80-410 Hz with a median of ~162 Hz, i.e., much much smaller than the plasma frequency. Thus, the upper hybrid frequency in most of the solar wind is basically the same as the plasma frequency.

References

  • Bale, S.D., et al., "The FIELDS Instrument Suite for Solar Probe Plus," Space Sci. Rev. 204(1-4), pp. 49-82, doi:10.1007/s11214-016-0244-5, 2016.
  • Bougeret, J.-L., et al., "Waves: The Radio and Plasma Wave Investigation on the Wind Spacecraft," Space Sci. Rev. 71(1-4), pp. 231-263, doi:10.1007/BF00751331, 1994.
  • Bougeret, J.-L., et al., "S/WAVES: The Radio and Plasma Wave Investigation on the STEREO Mission," Space Sci. Rev. 136(1-4), pp. 487-528, doi:10.1007/s11214-007-9298-8, 2008.
  • Maksimovic, M., et al., "The Solar Orbiter Radio and Plasma Waves (RPW) instrument," Astron. & Astrophys. 642(A12), pp. 23, doi:10.1051/0004-6361/201936214, 2020.
  • Wilson, L.B., et al., "A Quarter Century of Wind Spacecraft Discoveries," Reviews of Geophysics 59(2), pp. e2020RG000714, doi:10.1029/2020RG000714, 2021.
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