01 March, 2010

Magnetospherically ducted echo

Magnetic field lines in the
magnetosphere (Ill. NASA)
Signals in the 1.8 - 4 MHz range may pass the ionosphere and be ducted in the magnetosphere out to a distance of several earth radii over to the opposite hemisphere where they will be reflected on top of the ionosphere.

The round-trip time varies with the latitude of the transmitter, or to be more accurate with the position relative to the magnetic Equator. Typical delay times are 140 - 300 ms. At my location near Oslo, Norway, the expected delay is about 308 ms, but unfortunately I have yet to hear such an echo.

210-220 ms delay time

The first echo is from a signal which has followed the path to the right out to a distance of about 1.5 times the radius of the earth over to the Southern hemisphere where it has been reflected back. The recording was done by G3PLX on 26. November 2006, at about 21 UTC (also local time), on 1998 kHz.

It has a delay of 210-220 ms, consistent with his location in Northern England. The transmitted signal was a chirp using a 100 Watt SSB transmitter with a 50 m longwire antenna. Note the stereo effect as the transmitter and receiver are in the right and left channels respectively. The chirps are exactly 5 sec apart, timed against GPS, so this will enable one to calculate the exact samplerate, which is about 8100Hz. Listen to the signal here (with kind permission from P. Martinez, G3PLX).

 Reference:
  • P. Martinez (G3PLX), Long Delayed Echoes, A Study of Magnetospheric Duct Echoes 1997-2007, Radcom, Oct 2007, pp. 60-63. 

165-168 ms delay time 

The next echo is from 17. February 2006, at 0345 UTC (2245 local time) when K4MOG heard his own 3.5 MHz signal coming back to him after 165-168 ms. He explained this as the travel time around the earth (138 ms) plus delay in the transmitter/receiver switching. 

I would like to offer an alternative explanation as it is very seldom that signals at this low frequency travel around the world. At his location in Georgia, US, the magnetospherically ducted delay was 143 ms at the time, only 5 ms more than the round-the-world travel time, so his observed delay can just as well have been a magnetospherically ducted echo. See the picture to the right which shows the path it could have taken, which in this case only goes out to about 1 earth radius. Listen to the signal here (with kind permission from G. Greneker, K4MOG).

References:
  • G. Greneker (K4MOG), “The Ultimate DX: An Around the Earth Path,” QST, June 2007.
  • S. Holm (LA3ZA), “Magnetospheric ducting as an explanation for delayed 3.5 MHz signals,” QST, March 2009.

214-219 ms delay time 

W2PA in NY heard similar echoes on February 15, 2008 on 3.530 MHz. He has documented this well, with audio examples, on his own web page. I ran my model for predicting the delay time based on his position and the position of the geomagnetic North at the time. It is shown on his page as well as here and fits quite nicely with the measured time.


Conclusion

The magnetospheric duct echo is the best understood mechanism for long delayed echoes (LDE) and corresponds to explanation no 1 in my article "Five Most Likely Explanations for Long Delayed Echoes". The expected delay can be predicted to what seems like about 5 ms accuracy using a simple model for the magnetic field lines.