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Doppler Effect in Auroral Backscatter

Discussion of results

Volker Grassmann, DF5AI

April 23, 2003

Introduction

This webpage discusses the user feedback on the paper Doppler effect in Auroral backscatter, [1]. I am very much delighted that this paper encourages other radio amateurs to conduct Doppler measurements in Auroral backscatter. The paper does not consider the geophysical details of Auroral backscatter, i.e. all results and findings are actually based on geometrical considerations, more or less. In fact, discussing zero and maximum Doppler shifts, introducing a model calculation and examine the practice of Aurora dx communication only represents special applications of eqn. (5), i.e. the formula describing the Doppler effect in bistatic scattering. It is therefore important to compare the theoretical findings to experimental results and practical measurements. Please do not hesitate to contribute your findings, thoughts and ideas and please allow me to place your input on this webpage too.

Email from Graham, G3TCT, April 12, 2003

Hello Volker

I was most interested to find your paper on "Doppler effect in auroral backscatter" on your web site, as I have just been making some measurements of doppler backscatter in the recent openings 20-31 March. To do this I have used the vision carrier of the Greipstad TV transmitter in JO38 on 48.2524MHz. The interesting feature is that I can hear the carrier as well as the doppler shifted auroral signals!

In trying to explain my measurements I have read and re-read your paper but I still have some questions I cannot answer - perhaps you can help me!

I follow your reasoning and maths relating to zero doppler shift up to equation 9. What you are saying here, I think, is that for a specular reflection from the aurora, there is no change of path length and hence zero doppler. This is ok, but does it not ignore the situation of numerous random scatterers?

And it also does not consider the situation of a reflection from a "curtain" where a total internal reflection may take place?

Moving on to section 4.1 and the velocity vector field, I found it strange to suggest that the scatterers move from east to west. Do the scatterers not in fact move along the lines of the geomagnetic field "B"? And hence the theory of equation 9 would still hold, resulting in zero doppler? Or is Fig 4 depicting perhaps the wind speed at the height of the auroral scatterers?

Your model results are very intriguing and may well explain the measurements I have made. An example is shown in capt06.jpg. In this spectrogram, the transmitted carrier (not visible here) is at about 810Hz, with auroral returns centred at about 900Hz and 720Hz. Your Fig 9 suggests that yes, both positive and negative dopplers would be obtained from such a geometry (shifted to allow for receiver in IO91).

...

Best wishes

Graham Kimbell G3TCT (IO91TG)

G3TCT

 

Volker's (DF5AI) reply, April 19, 2003

Thanks to Graham's email we may address some aspects of Auroral backscatter and its Doppler shift in more detail. Please allow me to pick Graham's comments as follows:

AScissors"Zero Doppler shift in 'specular reflection' from the Aurora"

Zero Doppler shift is available only in special situations where the velocity vector is perpendicular to the difference of the transmitter and receiver wavevector. What does this mean? Imagine we would place a scissors at the scatter volume as shown in the picture on the right. The upper blade represents station A's wavevector and the lower blade denotes station B's wavevector. In this picture we may interpret the difference wavevector the vector which connects the tips of the two blades (see the red vector).

The Doppler shift is zero if the scatterer's velocity vector is orthogonal to red vector which means the velocity vector is directed along the scissors' line of symmetry, see the gray broken line (taking into consideration that this scenario is actually a three-dimensional problem, we may argue more precisely: the velocity vector lies in the plane perpendicular to the plane of this document where the above broken line represents the intersection of this two planes).

The scatterer's velocity vector will generally have a velocity component parallel to the difference vector and a component perpendicular to it. In this case, only the parallel component results in a Doppler shift in Auroral backscatter, i.e. the orthogonal component does not contribute any Doppler shift at all. Hence, zero Doppler shift is the exception and non-zero Doppler shift represents the general case.

"Situation of numerous random scatterers" and "reflection from a curtain"

In a real Aurora opening, backscatter generally originates from many scatterers at the same time. Thus, we need to apply our findings to a large number of scatterers resulting in a mixture of various Doppler components. Graham's spectrogram shows a nice example of various Doppler components, see the persistent and broad Doppler component centered at 900 Hz and the time-varying components at 680 and 750 Hz.

"Do the scatterers not in fact move along the lines of the geomagnetic field B?"

The concept of moving Auroral scatterers is actually problematic because it makes us believe rigid objects would travel through the ionosphere reflecting the radio waves in Auroral backscatter. Backscatter originates from inhomogenities in the ionosphere, i.e density fluctuations which may be described by a wave mixture comprising a large spectrum of wavelengths (similiar to the waves in the sea which comprises components of many kilometers wavelength as well as ripples of very short wavelength). Probing the ionosphere by using radio waves, we can only detect irregularities corresponding to wavelengths of half the radio wavelength. Thus, in 144 MHz Aurora dx communication (2 meter wavelength) we filter density fluctuations of 1 meter wavelength and in 432 MHz Auroral backscatter (70 cm wavelength), we can only detect 35cm-irregularities in the E-region of the ionosphere. Density fluctuations of other wavelengths do not contribute to Auroral radio echoes because of destructive interference. Auroral backscatter, by the way, corresponds to 'coherent backscatter' in contrast to the so-called 'incoherent scattering' (see, for example, [2] for more details).

Irregularities1In Auroral backscatter, the generation of irregularities ('plasma instabilities') is associated with electron precipitation into the ionosphere, i.e. high-energy electrons penetrating the ionosphere along the local Earth-magnetic field lines. However, the backscatter does not result from the presence of this electrons, i.e. the term 'scatterer' must not be considered identical to the penetrating electrons. Hence, the term 'scatterer' is misleading because there is no 'device' reflecting the radiowaves but a volume within the ionosphere which shows scattering properties. The picture on the right shows a schematic view of this scenario: the electron precipitation generates irregularities in the E-region of the ionosphere which may backscatter the incident radiowaves. Those irregularities are represented by a mixture of plasma waves where the red component corresponds to half the radio wavelength. Only this component contributes to Auroral backscatter on the actual radio frequency. Changing the transmitter frequency, we would pick another component in the spectrum of irregularities. Note that the excitation and dissipation of plasma waves varies with wavelength, i.e. we can indeed experience strong Auroral backscatter on 144 MHz but no backscatter at all in the 432 MHz band.

MovingIrr1aHowever, if the term 'scatterer' is synonymous for a volume capable to scatter radiowaves, what is the meaning of a 'moving scatterer'? Assuming an irregularity would exist somewhere within in the E-region of the ionosphere, see 'irregularity 1' at time t1 in the picture on the right. Assuming further, that this irregularity decays at the same time when another irregularity builds up, say, some tens of kilometers away from irregularity 1 (perhaps because the beam of high-energy electrons now selects another magnetic fieldline to penetrate the ionosphere). At the time t3, irregularity 1 has vanished but irregularity 2 has fully developed generating strong Auroral backscatter. The radio observer cannot distinguish between irregularity 1 and 2 in practice. However, he will notice a center of backscatter which has changed its position in the ionosphere, i.e. he may indeed interpret the scenario in terms of a moving scatterer.

The above example demonstrates that a moving scatterer is not necessarily identical to, say, a vehicle which travels from position A to B. In position B, the car is identical to the car we met in position A, of course. However, is there any material object in the above example travelling from one position to another? Let us accept the concept of moving scatterers a model which is useful in many applications. However, we have to be careful taking it literally.

Thanks for your email, Graham.

References

[1]

Doppler Effect in Auroral Backscatter

Grassmann, V., DF5AI, http://www.df5ai.net/ArticlesDL/ADoppler.pdf

[2]

Incoherent Scatter: Principles and Applications

Grassmann, V., DF5AI, http://www.df5ai.net/ArticlesDL/IScatter(E).pdf

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