FM DX with Aurora

http://radiacja.blogspot.com/2015/12/aurora.html
Northern lights as visible from Poland on 15-12-20
(by Konrad)
On Sunday, Dec 20th for the first time I observed auroral propagation on FM radio.
Although i've been familiar with its theory for years now, and - thanks to other Polish FM-DXers' achievements, I learned how it can look and sound like, in my over 15 years of DX history it was the first time to observe Au personally. 

In this post i'm sharing my impressions about this type of reception and while describing it, try to reflect best the properties of this quite exotic mode of propagation.
It is also available in Polish.

For the record: The phenomenon of December, 20th 2015 was an off-season event. Typically, Au occurence is expected during weeks preceeding and following the equinoxes (twice a year: around March and September). But this time... December? Off-season exceptions occur mostly after solar storms releasing huge amounts of solar wind (causing geomagnetic storms). Which was of course the case just before Dec, 20th.

According to most descriptions of Aurora-DX that I've read, direct visibility of aurora borealis, aka northern lights, is not necessary in place of observation to make Au DX possible there. (Potentially aurora can be radio-observed at latitudes even southern of 50 deg N). But, as the matter of fact, in at least 2 out of 5 cases of auroral propagation (on FM) in Poland, northern lights were actually either visible here (this itself is a rare phenomenon) or detectable on photographs taken with long exposures.
The illustration to this article is a beautiful pic taken on Dec, 20th by Konrad, an Au-FM-DX-veteran, who encouraged me to attempt catching aurora on radio that day. His own log of the evening is available in English on FMDX.pl)

Aurora (Au) has a number of features and characteristics, which taken together distinguish it from other, more common propagation modes. But it is easy to confuse Aurora with tropo or Es, especially if we are not aware of Au's existence.

What at first glance can make Au signals be taken as tropospheric ducting is their stability - they don't fade in & out quickly like typical Sporadic E signals. Instead, they appear faint, grow slowly but steadily, reach maximum and, slowly again, vanish. Just like during a developing tropo. "Maximum" does not mean perfect quality. Even at peak, auroral signals are typically noisy and distorted (an external, directional antenna is required for most Au FM DX observations). Direct distance to a transmitter received may resemble those of tropo ducting or of a typical Sporadic E. The span is quite broad though - varying from as low as 200 kms to 2000 kms, with median around 1000 to 1500 kms - a ionospheric standard. (But the real distance, with Au reflection taken into account is always greater. More on that later.)

Now let's take a closer look at some features that are exclusive to Aurora. 
Firstly, reception direction and area. In majority of cases: northern. Today for example, with my antenna pointing precisely north, I picked couple frequencies with Norwegian NRK P1 and P2 - although direct line to their transmitters lays to northwest. But this is how reflection of Aurora works like: it is always a backscatter, never a direct link. Whether a transmitter is situated westerly or easterly of us, we don't receive it directly, but a fraction of its signal beamed north, coming onto northern light curtain and reflected back at us. As a result, the reception direction is either north or up to 45 degrees east and west of it.
Even reception of transmitters situated slightly southerly of us is sometimes possible (in Europe no southern than 45deg.), but again... from northern direction!
What is also significant, only high-powered (tens to hundreds of kilowatts) transmitters take this journey. And it is the best to check for Au DX on frequencies that are not occupied with tropospheric signals. The strength of Au DX can be insufficient to override them.

Moving on, we encounter THE most peculiar feature of auroral FM DX signals: blurred and smeary modulation. If you are familiar with multipath tropospheric reception, this may sound like one, but on a greater scale, heavier. And multipath is not the cause of such sound. Due to constant, chaotic movement of particles charged by solar wind in high atmosphere (aurora often occurs as a result of solar storm), VHF radio frequencies are constantly shifted and changed up to hundreds Hertz from original one - so called Doppler Shift. Because it is happening extremely rapid, human ear can't hear the shift, but the result of it: a distorted audio of a distant station.

Until recently, auroral propagation was better known in HAM radio, where narrow modulations are in use - CW (tones, like: morse code) and SSB (for transmitting voice). They suffer the most distortion, making sometimes the audio unintelligible, or - at least ...creepy. Take a look at this QSO with human voice in SSB distorted by aurora:https://www.youtube.com/watch?v=6Ba-pxs_ZQI

Wideband WFM modulation used for VHF broadcasting band II. (88-108 MHz) is heavily distorted by Aurora, but because of wider frequency span used for a single channel, it is not as severe as SSB in HAMradio. Distortion applies to all reflected signals - no matter what strength.

In WFM, the higher acoustic frequency, the more it becomes affected. Pop music stations with entire spectrum of sound are the best too see the effect: bass remanis merely untouched, 10-15kHz sounds (treble, sopranos) sounds like a noise shifting in rhythm of an initial sound. Stereo reception (19kHz pilot tone) is impossible, not to mention RDS (57kHz subcarrier).
Konrad's recording of Au reception of Norwegian pop station NRK P3 is a great example of what i'm trying to explain here. It sounds as if bass was boosted. But mind you - the whole band was recorded flat!.

Today, Dec 20th, a bit later when the reception area moved from Scandinavia to British Isles, out of pop music stations I received BBC Radio 1. It sounded much like Konrad's NRK P3. It is no wonder that the best quality is provided by talk stations - 93,7 BBC Radio 4 from Holme Moss performed the best today.


Although still affected by distortion, the overall strength was quite high. What unfortunately cannot be said about the reception of irish RTE1 (89,6 Mount Leinster tx), with roughly 1900 kms direct distance my furthest DX today. To make sure, I compared the sound to RTE1 web stream, and it matched.
And it also seems that aurora does not save original polarisation of the signal. With comparable levels I received Norway (H-pol.) and the UK (V-pol or mixed H+V), when my antenna was installed for Vertical.

Below is my log of all received stations. I already received most all of them in the past - whenther by Es (all) or even one via tropo (Pontop Pike, 92,9).

LOG Au-FM-DX  of 20-12-2015 (17:20-18:20 and 19:45-19:50 CET):
88.8 NO NRK P1 Greipstad/Eidsåvegen
89.3 UK BBC Radio 2 Holme Moss
89.6 IR RTÉ Radio 1 Mount Leinster (1894km direct)
91.3 NO NRK P1 Kongsberg
92.1 UK BBC Radio 3 Black Hill
92.5 UK BBC Radio 4 FM Sandale
92.9 UK BBC Radio 4 FM Pontop Pike
93.1 UK BBC Radio Scotland Meldrum
93.5 NO NRK P1 Bokn/Grønnestadvegen
93.5 UK BBC Radio Scotland Ashkirk
93.7 UK BBC Radio 4 FM Holme Moss
93.8 unid (SWE? SR P1 Skelleftea or NRK P1 Kautokeino)
94.2 NO NRK P2 Bjerkreim
94.3 UK BBC Radio Scotland Black Hill
96.1 unid Dido - White Flag
96.8 UK BBC Radio Cymru Wenvoe
97.3 NO NRK P2 Bokn/Grønnestadvegen
99.3 UK BBC Radio 1 Tacolneston
99.5 UK BBC Radio 1 Black Hill
99.7 UK BBC Radio 1 Divis
99.9 UK Classic FM Sandale

Identification of stations reflected by Au is relatively simple. Seek for matches among high-power transmitters on a given frequency. They propagate better with Au.

- I have previously received all above stations multiple times with Sporadic E, so what's the point of Au DX if they all can be received more simple, with much better quality by more common propagation mode?
Aurora is quite rare phenomenon mid-latitudes (52 deg N for Poland) - either on the sky or on the radio, it is our duty as DXers to investigate and try to understand better this heavenly phonomenon. Note which signal was of most eastern located transmitter, most western, most northern and even southern. Which appeared as first, which of them disappeared as last one? Which carried the clearest signal. Maybe you received a transmitter of less than 50kW power?
With these factors we can better estimate aurora's properties, structure and... possible "records" to beat next time we see Au.

73!

PS. Here is some footage of previous, regular Au season observations in Poland:
http://fmdx.pl/2015/03/aurora-fm-dx-17-03-2015/
and http://fmdx.pl/2013/03/aurora-fm-dx-17-03-2013/

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