Notes on Medium Wave Propagation
By KN4LF

KN4LF daily solar info site I will try to keep things in simple to understand layman terms, as long complicated technical explanations can be boring and make one's eyes glaze over. Unfortunately though sometimes while trying to keep things simple certain definitions, meanings and technical aspects can get watered down or even lost, which tends to open me up criticism from well meaning individuals who just don't understand the KISS principle. Note: Though the propagation outlooks are primarily directed towards mediumwave frequencies, all information contained within still apply to shortwave frequencies. Though this is a generalization, basically the only appreciable difference being that shortwave frequencies are effected to a lesser extent then mediumwave frequencies in relation to solar flare and attendant activity and more effected by ionospheric storms.

Medium wave frequencies encompass 300 to 3000 kc. The best way to look at medium wave frequencies as far as propagation issues, is to accept the fact that the majority of the time propagation is poor, especially past approximately 1050 miles, with occasional short lived good periods as far as 3100 miles.
Why?
D layer absorption!
At daytime the D layer, which is at an approximate height of 30-60 miles in the mesosphere totally absorbs medium wave rf signals most of the time. I say most of the time because during the winter season and especially at the low part of a sunspot cycle, penetration of RF signals through the D layer and then refraction via the E layer does occur. However the fly in the ointment is the fact that the D layer does not totally disappear at night. Most books that deal with radio wave propagation erroneously state the D and E layers disappear after sunset, totally incorrect.
Recently I saw a post on the Topband Reflector e-list, lamenting the seemingly unexplainable differences in propagation on certain paths from night to night. An explanation? Yes, unfortunately small increases in the density of the night time D layer, caused by smaller solar flares and also the general variability of the solar background x-ray level, can have a profound negative impact on absorption of high and even mid latitude medium wave signal paths, both on the AM broadcast band and 160 meters. The lower half of the AM broadcast is always affected first followed by the upper half of the AM broadcast band, then 160 and 90 meters. After much personal observation and research, it appears that TA and TP propagation tends to open up when the solar background x-ray level falls to C3 or lower on 160 meters and the broadcast band when solar background level falls below C2.

Also meteorological effects such as troposphere originating internal gravity waves (IGW), stratospheric level Quasi Biennial Oscillations (QBO) and warming (STRATWARM) have a negative effect on the D layer in the form of small to medium increased absorption variations. The QBO is a wind shift in the equatorial stratosphere, an oscillation from easterly to westerly and back on the time scale of approximately two years (26 months) and is a source of internal gravity waves (IGW) which create absorptive perturbations in the D and E layers and even possibly the F 1/2 layer. Thunderstorms, lightning, tornadoes and hurricanes and even man made activities such as rocket launches including the space shuttle are all sources of IGW's. Many times I've heard ham's lament that propagation was going to go to crap due to another space shuttle launch, they were correct.

Aurora Oval Blockage, Absorption And Refraction-
The aurora ovals generally have a negative impact on mediumwave propagation. If the path over which you are communicating lies along or inside one of the auroral ovals, you will experience degraded propagation in one of several different forms: strong signal absorption, brief periods of strong signal enhancement, which is mainly caused by tilts in the ionosphere that allow signals to become focused at your location or very erratic signal behavior in the form of strong and rapid fading, etc., caused by a variety of effects such as multipathing, anomalous and rapid variations in absorption, non-great-circle propagation (skewing) and polarization changes.
When the auroral oval zones are contracted and latitudinally thin coinciding with low geomagnetic activity, it is possible for a mediumwave frequency transmitted signal to propagate through the auroral oval zone without being heavily absorbed by skirting underneath it.
During periods of very low geomagnetic activity, areas of the auroral oval zones may only have a latitudinal thickness of approximately 300 miles. But radio signals reflected from the E layer can travel over distances of as much as 300 to 1100 miles at heights below the ionosphere for low take-off angles of between 0 and 25 degrees. When the geometry is just right, the mediumwave frequency transmitted signal can literally propagate underneath and through the auroral oval zones into the polar ionosphere which is less disturbed and from the polar ionosphere back into the middle latitude ionosphere, without ever coming in contact with the highly absorptive auroral ionosphere. This type of propagation is not as rare as you might think, and it can provide unusually stable openings to (TA) Transatlantic and (TP) Transpacific regions. But because the auroral oval zone expands and contracts constantly such conditions often do not last very long. More to come.

Coronal Mass Ejection (CME)
A coronal mass ejection is the name given to an ejection of a large amount of matter from the Sun's outer atmosphere or corona. These ejections typically comprise millions of tons of material in the form of charged particles, and can be seen because the material reflects sunlight. When one of these ejections is directed towards the Earth (or conversely, directly away from the Earth), it looks like a roughly circular "halo" surrounding the Sun. The "Halo CME's" then are those CME's which are more likely to impact the Earth than those which are shot out at right angles to the Earth-Sun line. Energetic protons emitted during CME's play a major role in D layer absorption of mediumwave frequencies. More to come.

Correlation Of Energetic Protons, Solar Flux and A & K Indices With MW Frequencies
I've been watching energetic proton levels, as well as the A & K indices for a long time and can see a direct correlation between high energetic proton levels and poor propagation on high latitude MW paths, where as A & K don't as readily correlate. For a further explanation see the last paragraph above under Aurora Oval Blockage, Absorption And Refraction.
High solar flux values are generally considered to be detrimental to mediumwave frequency signals both domestic and TA/TP, as more absorption can be present via as the transmitted signal makes two trips through the D layer, near sunrise and sunset. However most medium wave frequency RF signals in excess of 3100 miles are propagated via the E/F layer ducting and/or E valley propagation mechanism and a high solar flux value ensures a strong E and F layer duct mechanism. More to come.

E-F Layer Propagation Ducting Mechanism/Chordal Hop Propagation
Most mediumwave frequency RF signals in excess of 3100 miles is via the E/F layer ducting and/or E Valley propagation mechanism. High solar flux values can aid in long haul medium wave propagation in excess of 3100 miles, as a high solar flux value ensures a strong E and F layer duct mechanism. Typically a transmit antenna takeoff angle must be under 30 degrees to enter the E-F layer duct.
A note: high solar activity in the form of increase ionization created by ultraviolet and x-ray radiation can fill in the E Valley region with absorptive ionization and interfere with the E/F layer ducting mechanism. In a sense the E/F layer duct is shut down and the mediumwave frequency RF signal only propagates between the E layer and land/ocean surface, with a higher angle and more loss. More to come.

Electron Gyrofrequency Absorption
Unfortunately mediumwave frequencies fall within the electron gyrofrequency which is in the approximate range of 630 to 1630 kHz and of course the AM broadcast band and 160 meter band is very close to these electron gyrofrequencies.
Basically, the electron gyrofrequency is a measure of the interaction between an electron in the Earth's atmosphere and the Earth's magnetic field. The closer a transmitted a mediumwave carrier wave frequency is to the electron gyrofrequency, the more energy that is absorbed by the gyro electrons from that carrier wave frequency. This is especially true for mediumwave frequency radio signals traveling perpendicular to the Earth's magnetic field, meaning high latitude NW and NE propagation paths. More to come.

High Latitude Propagation Path Skewing
Basically the simplest way to look at mediumwave frequency signal skewing is that the transmitted RF signal will always seek the path of least absorption. A signal transmitted from directly Norway to New England, which is a polar great circle path, will be absorbed with the remaining mediumwave frequency signal skirting south and then west on a darkness path, arriving in New England from say the SE rather then the more normal NE path. More to come.

Ionospheric Or Geomagnetic Storm
A worldwide disturbance of the earth's magnetic field, distinct from regular diurnal variations.
Minor Geomagnetic Storm: A storm for which the Ap index was greater than 29 and less than 50, Kp 4-5.
Major Geomagnetic Storm: A storm for which the Ap index was greater than 49 and less than 100, Kp 6.
Severe Geomagnetic Storm: A storm for which the Ap index was 100 or more, Kp 7+.
Initial phase of a geomagnetic storm is that period when there may be an increase of the middle latitude horizontal intensity.
Main phase of a geomagnetic storm is that period when the horizontal magnetic field at middle latitudes is generally decreasing.
Recovery phase of a geomagnetic storm is that period when the depressed northward field component returns to normal levels.

By the way effects of the solar wind on the magnetosphere decreases as we approach the solstices. Why? basically it's the orientation of Earth's magnetic field with respect to the interplanetary magnetic field within the solar wind. When solar material and shock waves reach Earth their effects may be enhanced or dampened depending on the angle at which they arrive. More to come.

Meteorological Effects On The Ionosphere
Meteorological effects such as troposphere originating internal gravity waves (IGW), stratospheric level Quasi Biennial Oscillations (QBO) and warming (STRATWARM) have a negative effect on the D layer in the form of small to medium increased absorption variations. The QBO is a wind shift in the equatorial stratosphere, an oscillation from easterly to westerly and back on the time scale of approximately two years (26 months) and is a source of internal gravity waves (IGW) which create absorptive perturbations in the D and E layers and even possibly the F 1/2 layer. Thunderstorms, lightning, tornadoes and hurricanes and even man made activities such as rocket launches including the space shuttle are all sources of IGW's. Many times I've heard ham's lament that propagation was going to go to crap due to another space shuttle launch, they were correct. More to come.

Mid Winter Anomaly
As if we didn't have enough problems with absorption of mediumwave frequencies, the mid winter anomaly represents increased mediumwave frequency signal absorption at high and mid latitudes. More to come.

Polar Cap Absorption (PCA)
An anomalous condition of the polar ionosphere whereby MF (300-3000 kc) and HF (3000-30000 kc) radiowaves are absorbed, and LF and VLF (3-300 kHz) radiowaves are waveguided at lower altitudes than normal. In practice, the absorption is inferred from the proton flux at energies greater than 10 MeV, so that PCA's, Polar Radio Blackouts and Proton Events are interrelated and often simultaneous.
Note: high latitude radio propagation paths may still be disturbed for days, up to weeks, following the end of a proton event.

Solar Flares
A day side earthward bound M5 or higher class solar flare will move the proton flux above 10o (10 MeV) and initiate large scale high latitude propagation path absorption but increasingly I suspect that even smaller M4 class flares and weaker, including C7-4 flares are probably the culprit behind night to night variations in signal strength on the AM broadcast band and 160 meters, both stateside and DX. This transfer of increased density and RF signal absorption from the day side D layer to night side of the ionosphere occurs through high level neutral winds.
X-Ray Class Solar Flare. The Rank of a solar flare based on its x-ray energy output. Flares are classified by the order of magnitude of the peak burst intensity (I) measured at the earth in the 1 to 8 angstrom band as follows:
Class (in Watt/sq. Meter)
B- I less than (l.t.) 10.0E-06
C- 10.0E-06 l.e.= I l.t.= 10.0E-05
M- 10.0E-05 l.e.= I l.t.= 10.0E-04
X- I g.e.= 10.0E-04
Basically a C class solar flare possesses energy 1/10 the level of an M class solar flare and an M class solar flare possesses energy 1/10 the level on an X class solar flare.

Sporadic E (Es) Absorption, Blocking & Refraction
Yes, just as the E layer is the main refraction medium for medium wave signal propagation within approximately 3100 miles, so is Sporadic E (Es). Like stratospheric level warming and troposphere level temperature and moisture discontinuities, Sporadic E can depending on the circumstances absorb, block and refract medium wave RF signals in an unpredictable manner. More to come.

Stratospheric Warming (Stratwarm Alert)
A major temperature change of the winter time polar and middle atmosphere from the tropopause (where the troposphere transitions into the stratosphere) to the base (D layer)of the ionosphere, lasting for many days at a time and characterized by a warming of the stratospheric temperature by some tens of degrees.
As the stratosphere lies below the ionosphere, which is at mesosphere and thermosphere height, you would not expect to see stratospheric warming effect mediumwave frequency propagation in any way BUT mediumwave frequency signals do refract off of temperature inversions and moisture discontinuities and a temperature inversion is involved with stratospheric warming. So it's possible that a mediumwave frequency signal could do any number of things when refracting off of a temperature inversion, at any height. Also stratospheric warming (STRATWARM) has an effect on the D layer by increasing radio wave absorption.
Also I've observed many times that stratospheric warming can coincide with major jetstream circulation pattern changes and movement of Arctic air from Siberian Russia across the pole to Canada and the U.S. More to come.