The Mid-Latitude Trough and Its HF Propagation Effects
The Mid-Latitude Trough and its HF Propagation Effects
By Frank Donovan, W3LPL (reprinted with his permission from the PVRC Newsletter)
We often experience brief European openings on 40 meters at night as the solar flux drops as we approach solar cycle minimum. This same phenomenon also causes the sudden loss of propagation on the higher bands as night-time arrives over the north Atlantic and north Pacific. It also causes the disappearance of our sunrise opening to Japan on 20 meters. The loss of propagation that often (but not always) impacts north Atlantic and north Pacific night-time propagation is caused by an electron depletion region in the F layer ionosphere known as the mid-latitude trough. The mid-latitude trough is primarily a night-time phenomenon that occurs almost exclusively during the winter and the equinoxes. Summer occurrences are very rare. The pole-ward wall of the mid-latitude trough is bounded by the low-latitude boundary of the auroral region. The width of the mid-latitude trough is typically five degrees or more, extending towards the equator from its pole-ward boundary with the aurora. During the night, the equator-ward edge of the mid-latitude trough steadily drifts to lower latitudes. The mid-latitude trough disappears by re-ionization when the sun rises over that part of the ionosphere.
During the onset of increased geo-magnetic activity, the low-latitude edge of the auroral region rapidly shifts towards the equator. Consequently, the entire mid-latitude trough also shifts toward lower latitudes and propagation to Europe and Japan is likely to fail completely.
What does this mean for upcoming ARRL DX contests? 40 meter propagation to Europe on 40 meters will be best during the hours near our sunset. Propagation to northern Europe will fade away within an hour or two after sunset, and only the southern tier of Europe may be workable by as early as 0100Z. Europe will often fade away by 0200Z, except for skew paths toward the south-east and south. On many (but not all) nights, propagation will return briefly during the hours near European sunrise, as the mid-latitude trough is re-ionized by the sun. Propagation to Europe will last longer into the night in North Carolina and Virginia than it does in New Hampshire and Massachusetts. Similarly, 15 meters will lose propagation to Europe shortly after European sunset, and 20 meters will lose propagation to Europe soon thereafter. On paths to Japan, direct path 40 meter propagation towards the northwest is unlikely except for a very brief period at 0800Z (Japanese sunset) and during the hour so around 1130-1230Z (our sunrise). During the remaining hours, propagation to Japan will be over the familiar (to 40 meter DXers) southwest skew path.
On 15 and 20 meters, evening openings to Japan will be brief, beginning shortly after 2130Z (sunrise in Japan) , and failing shortly on 15 meters after our sunset and soon afterwards on 20 meters. The morning 20 meter direct path openings to Japan we've become used to on 20 meters during high sunspot years will be only a distant memory as the mid-latitude trough kills that path during the Japanese night-time hours.
The mid-latitude trough can be very beneficial on long haul 160 meter paths at sunrise on the eastern end of the path. Ionospheric tilts occur in the vicinity of the trough, occasionally producing tremendously enhanced 160 meter propagation at our sunrise on our path to Japan. I've experienced several JA sunrise openings when the JAs are an honest "20 meter S9". It's amazing to listen to their S3 signals build up to S9 over about a 5 minute period, stay at that strength for 10-15 minutes, then steadily fade away as absorption takes over with the rising sun. On those occasions I've had runs of 20 or more JAs on 160, just as on 10 meters when propagation is good. Fig 1.6 at http://www.radtelnetwork.com.au/propagation/hfprop.htm shows an illustration of the MUF at the mid-latitude trough. Although the article says the mid-latitude trough occurs at 60 degrees north and latitude, recall that it's occurs during winter and in the vicinity of the equinoxes, so you would rarely experience a mid-latitude trough simultaneously at both north and south latitudes.
As we approach solar minimum, we should be prepared for shorter openings over the north Atlantic and north Pacific paths. Knowledge of propagation phenomenon will be more of a strategic advantage than during the high sunspot cycle, as openings become more fleeting. Operators who are aware of these patterns will be less likely to be disappointed by missed opportunities.