[time-nuts] Re: BOAT ionospheric effects

[Bob kb8tq]

Hi

Single band or multi band, your GNSS device will struggle to tell you 
anything much below 10 ps and will likely not even display information
below 1 ps. That certainly true of the Trimble multiband gear as well as
the uBlox F9x and Septentrio Mosiac-T parts. 

So, it sounds like there likely is no cute GNSS based chart to put up along
with the VLF information related to the event ….

Bob

> On Oct 22, 2022, at 10:19 PM, Phil Erickson <phil.erickson at gmail.com> wrote:
> 
> Hi Bob and the list,
> 
>   [First of all, apologies to anyone who tried to follow the link to the Nature paper and hit a paywall.  There appears to not be an open access version out there.  Not sure how to solve that.]
> 
>   Now I understand the question.  Sure, single frequency devices far outnumber any dual frequency systems, although those new uBlox dual frequency chips might make a dent in that some day.  So seeing as this is a time-nuts mailing list, I did some calculations as follows:
> 
>   We need to start with an estimate of the electron density profile between 20 and 80 km in both normal conditions and under gamma ray burst impact.  The same Umran Inan at Stanford wrote a later paper - open access this time! - showing modeled profiles during a massive daytime GRB - Figure 4 of this paper gives profiles before ("ambient"), at flare time, 0.1 sec and 5 sec postflare.  (FYI, by 1000 seconds postflare, we're back to ambient.)
> 
> Inan, U. S., Lehtinen, N. G., Moore, R. C., Hurley, K., Boggs, S., Smith, D. M., and Fishman, G. J. (2007), Massive disturbance of the daytime lower ionosphere by the giant γ-ray flare from magnetar SGR 1806–20, Geophys. Res. Lett., 34, L08103, doi:10.1029/2006GL029145.
> https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/2006GL029145 <https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/2006GL029145>
> 
>   I used those profiles and integrated between 20 and 80 km altitude to calculate a total electron content value, expressed in m^-2 (e.g. number of electrons in a column with 1 m^2 cross sectional area).  You can convert that to an induced ionospheric group propagation delay in seconds using the formula at the bottom of the TEC page here:
> 
> https://en.wikipedia.org/wiki/Total_electron_content <https://en.wikipedia.org/wiki/Total_electron_content>
> 
>   Using a frequency of 1575.42 MHz (GNSS L1), the numbers work out as follows:
> 
> Over 20-80 km altitude:
> Ambient delay = 0.38 ps
> 0.1 s post-GRB delay = 0.71 ps (89% increase over ambient)
> 5 s post-GRB delay = 0.46 ps (23% increase over ambient)
> 
>   Looks like a large effect.  But: compare this with a typical mid-latitude total group delay from the whole ionosphere out to 20,000 km, using a nominal 20 TEC unit value typical of continental US values in the mid-afternoon this month (e.g.):
> 
> Total ionospheric delay = 10.8 ns
> 
>   So the GRB-impacted region generates delays that are negligible compared with the overall ionospheric delay which dominates.  Whether this GRB-induced increase of a fraction of picosecond makes any difference to a single frequency GNSS PLL or other timing loop is something I'm not qualified to answer, but perhaps others can respond.
> 
> Cheers
> Phil
> 
>   
> 
> 
> 
> 
> On Sat, Oct 22, 2022 at 8:01 AM Bob kb8tq <kb8tq at n1k.org <mailto:kb8tq at n1k.org>> wrote:
> Hi
> 
> Thanks !!!!!
> 
> There are a lot of devices that do “single band” GNSS for timing. 
> The reason is pretty simple: cost. On those devices, you don’t get
> the internal correction for atmosphere that you do on a multi band
> device. 
> 
> So, the rephrased / corrected / enhanced question becomes: Are
> single band issues likely in this case / did anybody see them?
> 
> Thanks again,
> 
> Bob
> 
> > On Oct 22, 2022, at 6:58 AM, Phil Erickson via time-nuts <time-nuts at lists.febo.com <mailto:time-nuts at lists.febo.com>> wrote:
> > 
> > Hi all,
> > 
> >  John Ackermann mentioned this thread.  I'm an ionospheric observational
> > physics person.
> > 
> >  The disturbance from a gamma ray flare is primarily on VLF propagation
> > (10s of kHz) because it penetrates so low in the atmosphere and enhances
> > the "sub-D region" between 40 and 80 km or so.  D region is highly
> > absorptive due to strong ion-neutral collisions in events where it gets
> > enhanced.
> > 
> >  VLF ionospheric propagation effects from gamma ray bursts weren't really
> > confirmed until this 1988 Nature paper by Fishman and Inan on a strong 1983
> > burst:
> > 
> > Fishman, G. J., and U. S. Inan. "Observation of an ionospheric disturbance
> > caused by a gamma-ray burst." Nature 331, no. 6155 (1988): 418-420.
> > https://www.nature.com/articles/331418a0.pdf?origin=ppub <https://www.nature.com/articles/331418a0.pdf?origin=ppub>
> > 
> >  There are a number of publications since that one.  Because VLF paths are
> > inherently transcontinental and interhemispheric in the earth-Ionosphere
> > waveguide (ground to bottom of the D region), this was seen only on a very
> > long path reception.  Check out Figure 2 - I think that's one of the
> > reasons why it took so long to conclusively identify it - as they say:
> > 
> > "Figure 2 shows a portion of the record from the three stations between
> > 21:40 UT and 23:00 UT on 1 August 1983. A clear indication of a disturbance
> > beginning at 22:14:10±10UT is seen in the radio station GBR signal. Weaker,
> > barely detectable decreases in amplitude are seen simultaneously in the two
> > other signals. Without the GBR signal, these other two signals alone would
> > have been considered uneventful as similar weak fluctu- ations are seen in
> > their records near the time of the burst. The disturbance in the GBR signal
> > differs in its rise-and-fall time from any other disturbances seen within
> > 60 h of the burst."
> > 
> >  So identifying the spike using multiple simultaneous receptions was
> > needed to disambiguate it from something like whistlers (lightning), flares
> > (SIDs), etc.  The SpaceWeather article that Bob KB8TQ mentioned shows a
> > similar type of detection of the recent super-GRB.
> > 
> >  Back to the topic though: the ionization deposit would be very wide
> > spread (not localized) and would however I think contribute not very much
> > to the total electron content (TEC), which is of course the critical thing
> > for dual frequency GNSS measurements at L band.  The way I could see
> > something occurring is if irregularities were created in the region of
> > enhanced ionization, but they wouldn't last too long.
> > 
> >  Consider also that the ionosphere's natural electron density variability
> > is 1 to a few % on any day of the year, and you can see this clearly in
> > differential TEC from things like traveling ionospheric disturbance (TID)
> > waves and the like (many many studies).  Those don't significantly impact
> > timing solutions due to the dual frequency nature of the GNSS system which
> > subtracts out ionospheric delay, so I can't imagine this event would change
> > those either.
> > 
> >  Of course, I could be wrong - please correct!
> > 
> > 73
> > Phil W1PJE
> > MIT Haystack Observatory
> > Westford, MA
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> 
> 
> -- 
> ----
> Phil Erickson
> phil.erickson at gmail.com <mailto:phil.erickson at gmail.com>