Notes on Medium
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.
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
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
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
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)
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
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
Main phase of a geomagnetic storm is that period
when the horizontal magnetic field at middle latitudes is
Recovery phase of a geomagnetic storm is that period
when the depressed northward field component returns to
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.
high latitude radio propagation paths may still be disturbed
for days, up to weeks, following the end of a proton event.
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
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.