[time-nuts] BOAT ionospheric effects

[Phil Erickson]

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

  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