High Pass TEC values from volcano

[Bill Liles]

This is from Cathryn MItchell.

https://people.bath.ac.uk/eescnm/Graphics/Volcano/015TNfilledshortfilt_fast_2022_01_15_04-23_Volcano_Response.m4v

starts about 10.15 UTC on the west coast which is about right for timescales of TIDs

[Black Michael]

What is High Pass TEC?

Mike W9MDB

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[Phil Erickson]

Hi Mike,

It is total electron content = number of electrons in a vertical column (in this case) over a given latitude/longitude pixel.  The high pass part refers to further processing of the TEC to remove a background, usually with some kind of moving window median filter in time with size 1-3 hours.  Not sure what Cathryn used here as there are a lot of different approaches with different benefits and drawbacks, but it's probably in that range.

You'll notice the color scale is very small - 0.2 TEC max when the daytime normal un-filtered TEC would be in the 5-20 TEC or even higher values, depending on conditions.  The high pass filtering brings out all the traveling ionospheric disturbances (TIDs) of various wavelengths and speeds that fly around in the ionosphere, triggered by such events as geomagnetic storms, atmospheric gravity waves, or in this case lower atmospheric forcing from the volcano pressure wave.  Probably the latter excited extra atmospheric gravity waves in the mesosphere that propagate up, break (sometimes multiple times) in the 80-110 km range, and then excite ionospheric TIDs.

I want to re-emphasize here though the magnitude of these fluctuations compared to the background: we are not talking 100% variations here on these time scales.  Important to clarify.

73

Phil W1PJE

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[James Secan]

(I sent this out to the group yesterday, but it seems to have not made it.)

I’ve been downloading and processing GPS data from Auckland, NZ, for evaluating the new SWPC WAM-IPE model, and today’s discussion moved me to look for eruption signals in those data. The attached plots are relative TEC measurements from two GPS satellites, PRNs 10 and 23, which were passing close to overhead at Auckland around the time of the eruption. The top panel in each is the relative vertical Total Election Content (TEC) for a four hour period including the eruption, the middle panel is the output of a high-pass filter on the data in the top panel with a cutoff at 1/512 Hz (including structures in the upper panel with time spans of 512 seconds or shorter), and the bottom two panels show the elevation-azimuth from Auckland to the satellite (left) and the location of the 275-km penetration point of the raypath from Auckland to the satellite.

The heavy red line in the top panel is the time of the main eruption, and the green and black lines are the estimated arrival time of a signal from the eruption for propagation velocities of 600 m/s (green) and 300 m/s (black). These velocities are the approx. velocities in the ionosphere and at ground level, respectively.

I think there is a clear signal of the eruption in the data from both satellites. I haven’t decided what to think about the arrival of more small-scale structure after the ground-speed arrival time, but it’s interesting.

Jim
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