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A sort-of follow-up to Are the Voyager probes still sending us a signal ?

Still following on the news report referenced in the above question, this report indicates further magnetic surprises at, or near to, the interface of Sol's family with the rest of the universe. I recall reading there are such things as constructive/destructive interference which serve to amplify/attenuate an EM wave respectively. By this I assume there is a limit on the minimum amount of power required to penetrate a given magnetic field (I know it's possible to operate QRP 1W CW across antipodes!)

What is the minimum strength a signal must be at the edge of the Solar System to penetrate the magnetic field/s and be audible on Earth?



  1. The discovered magnetic region is of uniform strength, spherical in nature (centred around Sol), and
  2. The source magnetic field is in the planetary plane


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From where outside the solar system? In the plane with all the planets (and thus potentially also past magnetic active objects such as Jupiter?) –  Hennes Mar 3 '13 at 18:24
@Hennes: Didn't think of that but assuming it lies in the plane of the planets - say the source is one of the Voyagers; outside this field, and close to it! –  Everyone Mar 4 '13 at 2:35

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It's not so much about "penetrating magnetic fields" as it is about seeing the signal above background noise (assuming your transmission is above the background plasma frequency). The intensity of any given signal drops off as $\propto$ r$^{-2}$. This means that a signal sent from r = 1 will be 16 times as strong as a signal from r = 4. All receiving dish networks have a finite amount of background noise, which we often call a noise floor. This is below the lower bound of any signal that we could every hope to detect.

In the case of the Voyager probes, the limiting factor will most likely be the power of the antenna (assuming nothing critical fails). Eventually, the spacecraft will be too far away for the deep space network (DSN) to detect. All spacecraft communicate/transmit at frequencies well above the background plasma frequency, so their transmissions are effectively immune (thus, high frequency EM radiation is called free emission or free modes) from the changes in the background plasma and magnetic field.

I think their signal is strong enough that we can detect them for at least another decade, but my memory of specific numbers like this is not the most reliable. All NASA spacecraft and data products are subject to a senior review and the Freedom of Information Act, both of which are open to the public.

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Yes - it is a signal to noise question; and thus a question of bandwidth. If you modulate sufficiently slowly you can operate in an arbitrarily narrow bandwidth and thus get excellent nose rejection - but if "audible" means bandwidth of 1 kHz that immediately sets a limit. Mostly you need to be concerned about inverse square power drop off - every time you double the distance you quarter (approximately) the bandwidth/transmission speed. –  Floris Oct 5 '14 at 20:29

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