Цікаві дослідi Genosse нашего Herr-a Markus-a DF6NM та колег його

[rw3adb]

Недосуг видать было ему самому сюда писать, ну, да ладно.
Мы занесём.

Весьма интересный эксперимент провел Маркус и коллеги за 411 дней!

Hi Joe, all,
 
now we have more than a year of (more or less) continuous data. Inspired by DF3LP's beautiful annual plots of HBG and HGA22, I tried to generate a similar image (albeit of course with much lower SNR and time resolution). Individual days are plotted as vertical columns (0 to 24 UT bottom to top), and the horizontal axis goes from Jan 14, 2021 to Feb 28, 2022 (left to right). Carrier amplitude is shown as brightness and phase as colour, advancing red-green-blue-red.
 
Each day, we see repeatable amplitude and phase changes at sunrise and sunset. During the summer season, sunrise happens earlier and days are longer, and afternoons are often contaminated by high QRN levels.
 
I also attempted to plot a high-resolution spectrum of the whole period, with resolution 1 / 411 days = 28.9 nHz. To compensate for recording gaps, data were adjusted by their timestamps before going into the Fourier transform. At this resolution, we find that the diurnal peaks at +-11.57 uHz are no longer perfectly sharp, mostly due to the seasonal changes. I also looked for tidal periods at +-0.784 uHz (2 / 29.53 d), but lunar effects are obviously much too weak to show up in this kind of measurement.
 
The collated raw data is here: https://vlf.u01.de/files/vo1na_dl0ao_411days.zip
 
PS. regarding phase "glitches": I still think that phase ambiguity should only be "corrected" in multiples of whole cycles, ie. 2pi and not pi steps. This should affect only the visual appearance of phase-plot rollovers, but have no effect on the complex data values or calculated spectra.
 
73, Markus

[rw3adb]

Hello Joe,

(Sorry, forgot to press Send last week)

Thanks for the valuable report, and for maintaining an exceptional reference
signal.

Your data seems to support this 2017 viewpoint from two Space Science
Institute members:
"Many studies choose Dst as a proxy for storms simply because of its
continuous scale, its high cadence, and its simple interpretation; however,
as argued in this commentary such use of this index may often be
inappropriate or not optimal for identification of geomagnetic storms and
representation of storms and their effects."
From:
"Is the Dst Index Sufficient to Define All Geospace Storms?"
https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/2017JA024679
(authors encourage development of better indices)

Thanks to you and Markus for valuable contribution to the community.

73,
Jim AA5BW

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Hi Markus, Jim and Group,

That is a pretty image and informative as well. Thank-you for
your efforts and the data. Perhaps one day we will find lunar
periodicities -- if the ERP can be improved!

I could not find any diurnal harmonics in the collated raw data but will
be looking for other features such as seasonal effects.

Thanks for the pi vs 2pi 'correction' advice. It is remarkable that
we can even get phase measurements with such a weak signal.

There is freezing drizzle at the moment and phase lagging by 60
degrees because of a layer of ice on the aerial -- let's hope it stays up!

73
Joe VO1NA

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When preparing the coloured annual plot, I had tried to correct for Joe's 180° phase step on Mar 2 last year. But I had applied the step at the wrong date, which let the nighttime signal appear green instead of purple for a couple of weeks. In the corrected image (attached), nights (ie. bottom and top sections) are now always more-or-less red. But we still see a gradual phase change from purple-red towards orange-red, which is likely caused by subtle slow changes in the hardware setup or antenna environment at the receive or transmit site. This phase retardation is also obvious when comparing the orientation of the" analemma" loops in previous posts (vertical in March 2021, versus more horizontal in Dec 2021 and Jan 2022).
 
I also attempted to improve the spectrum by excluding high-noise periods (rms noise > 50e-9), filling in averaged data from the same hour of previous and following days. This has brought down the baseline significantly, such that the weaker diurnal harmonics are now clearly visible. I also shortened the analyzed period to 365 days (Mar 1, 2021 to Feb 28, 2022), avoiding a seasonal-cycle discontinuity at the end of the dataset.
 
Regarding tidal effects, I am not sure whether we should expect an effect twice per synodic month (2/29.53 days = 0.784 mHz), or rather sidereal month (2/27.32 days = 0.847 mHz). When thinking about this, it seems that I can find arguments for either. Can we let the experiment decide?
 
Interestingly, the spectrum indeed shows a pair of small peaks approximately at the synodic tidal period (zoomed section at the right). The SNR is probably not good enough to definitely claim that we have discovered a lunar effect on propagation, but it might well be an indication.
 
Best 73,
Markus

[rw3adb]

Hi Jean-Marie,

thanks for the reply,and good to hear that RTL is loud and clear again. Looking on the 24-h-spectrogram from Enschede, we find that there was only a temporary outage from 23 to 4 UTC.

> expected to close end of year
Oh, not nice. It's a shame to see all those great LW / MW stations being taken down.

73, Markus

[rw3adb]

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Thank you Jean-Marie for your very interesting comment and animation!
 
However I have to say that I am doubtful whether the pair of lines around Joe's carrier is really related to lunar tides. The SNR is low, and the peaks are near but not exactly on the expected frequencies.
 
To analyze the modulation, I generated a filtered spectrum (red) and transformed it back to time domain. Time scale is now shown in synodic months since full moon on Feb 27, 8:19 UT, so the green grid lines correspond to full and new moons. We can see deep amplitude and phase modulation, but the cycles do not stay perfectly aligned over the year. Also it seems hard to believe that subtle tidal changes could lead to such a strong modulation. I would not exclude that similar cycles could be caused by other factors, e.g. related to space weather, or local weather affecting our antenna environment.
 
It may be easier and more reliable to try to extract tidal cycles from strong VLF signals with good SNR, like time signals or military MSK stations. The DL0AO grabber usually shows amplitude and phase plots of Alpha (when present), NWC and JJY, but unfortunately we did not save all our data. Possibly there are SID monitoring stations who have been keeping continuouus logs?
 
73, Markus


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Markus, I do agree with your explanation because detection of Na+ influence is done a different way.
busy at hospital for exams, I will come back on that

73 JM