Markus,

From your message:

“Is this not just an electron density gradient, vertically measured?”

Yes, as you concluded in a message that you sent later.

“I am also wondering how to make quantitative use of the long / short path fringe patterns, which are so easily observed in spectrograms of NWC…”

Likewise. The valuable data being generated seems to be ahead of tools. A simple modeler that could make qualitative (save quantitative for later) sense out of conspicuous fringe patterns, diurnals, and spatial sensitivities would be helpful. If a tool of that sort hinted at a path sensitivity that could be mitigated by (for example) a frequency change, or operation at an atypical time of day, it could save days of effort in some cases. I’ve started assembling a ray-based tool that is simple enough to accept different types of ionospheric and earth-surface parameters, and provide visibility into their effects. This could take a while, but the front end looks like it’s working.

“BTW Jim are you planning to start transmitting on VLF yourself, either 29.5 or below 9 kHz ?”

I’m thinking of switching efforts to < 9 kHz. Work, travel and HW/SW issues have slowed my progress on 29.5 kHz (lots of almost-working 29.5 kHz equipment). On the other hand, my < 9 kHz TX and Rx have been operational for years, and I just need to install the recently-built pulse stretcher in line with the GPSDO, to operate at DX bandwidths (I’d been operating at > 1 mHz bandwidth without a GPSDO).      

I’ve been enjoying propagation analysis in the meantime, but running < 9kHz and 29.5 kHz concurrently would provide interesting propagation data and put the TX equipment to good use.

73,  Jim AA5BW

Markus,

Thanks for the DHO diurnal plot.

It’s a memorable image, with interesting symmetries.

I remain hopeful of finding or making a model that qualitatively represents VLF amplitude and phase diurnals for a substantial percentage of cases. This would be very helpful in preparing for DX, QRP and other activities.

Thomson has done some great work in this area (VLF diurnals). I was surprised to find that he reverted (prior to 2011) to Wait’s 2-parameter ionospheric reflection model (using h and beta*, and adjusting them empirically), instead of using LWPC or ModeFinder (which he mentions require more knowledge of ionospheric parameters than is currently available:  http://www.physics.otago.ac.nz/space/Thomson_LongPath_paper_JGR_2011.pdf ; I thought Paul might also find this point interesting)

To make matters more interesting, Han (http://people.ee.duke.edu/~cummer/reprints/132_Han11_JGR_DaytimeDRegionSharpness.pdf  ) recently showed considerable and substantial disagreement (qualitative and quantitative) between beta* parameters values derived (empirically) by a number of expert sources including Thomson. Han’s findings are well-summarized on the last page of his paper. The kind of discrepancies shown by Han seem (to me) suggest a lot of work remaining on the way to good diurnal amplitude and phase models. Among the four expert sources on D-region beta cited by Han, there were three different findings on the polarity of the change in beta with zenith angle:  decreasing beta with increasing zenith angle (1 of 4), increasing beta with increasing zenith angle (2 of 4), relatively constant beta with increasing zenith angle (1 of 4); not to mention considerable disagreement between the four sources in magnitude of sensitivity of beta to zenith angle. All of this is clearly summarized on the last page of the paper.

I thought you might find the paragraph above interesting because it shows a significant gap in the understanding of some basics that affect (at some distances and on some days) whether VLF daytime signal strength increases or decreases. In his conclusion, Han describes the magnitude of  disagreement in derived values for beta as surprising. He suggests that the two-parameter model may not be sufficient (a circumstance that Thomson hoped to avoid, see above)

On a related topic, Han’s method of deriving beta (using broadband signals) is interesting and appears to have substantial merit, but I’m guessing that the available broadband sources used by Han (sferics) come with their own issues in this type of measurement.

I think I can end on a light note: if you have a chance to look at one figure and one paragraph in this paper by Volland: http://nvlpubs.nist.gov/nistpubs/jres/68D/jresv68Dn2p225_A1b.pdf , Adobe Reader page 4 (document page 228), including Figure 3 and the paragraph including “A remarkable exception from this rule has been observed in Lindau (Germany), Figure 3 shows two successive daily phase variations of GBR… Such phase inversions are rare…”, I think you may find something interesting, odd or even humorous.

  *  (D-region electron density sharpness)

Regards,

Jim