Geomagnetic and Ionospheric disturbances (III)
HF communications can be subject to tens of phenomena that can affect sky wave propagation. Some perturbations occurs in normal conditions, other during so-called disturbed conditions.
When we run a propagation prediction program and set the circuit reliability to 90%, we expect that during those 27 days per month or so propagation conditions will be excellent. If this is well the case is average - we work with probabilities - we also know that on-the-air conditions can change and worsen in a few tens of minutes, fluctuations so fast that no program or data sounding network could never handle.
Under normal conditions, sky waves are subject to the next problems :
- Sporadic E
- Critical frequency variation at low latitudes
- Temporal gradient change on the gray line
- Field Alignment Irregularities (FAI)
- Auroral absorption
- Sub-auroral trough
- Polarization mismatch
- Noise and interference.
We will see in a second each of these perturbations. Don't be afraid of this list because most of these effects are temporary and last, at worst, one or two hours with highs and lows. Of course, if some of them rise together (e.g. fading, temporal gradient change, auroral absorption, noise and interference, this is not uncommon at high latitudes) they can force you to QRT and to spend your time to make another activity for some hours.
But these events are minors and nothing more than a strong QSB or some noise on frequencies compared to the effects of solar and geomagnetic disturbances.
Under disturbed conditions to normal communications, we look at the remaining 10% reliability, during which propagation conditions are sometimes so degraded that the least perturbation produces a deep QSB that can exceed 30 dB or 3 S-units. In the worst case, this is the blackout with an unbelievable noise on all frequencies.
The origin of these disturbances are associated to huge amounts of electrons and high-energy protons released by the sun and striking the Earth's upper atmosphere. During these events its structure and chemical compositions are affected, inducing severe perturbations of sky wave propagation in the ionosphere. These disturbances can be classified in several categories according the source of radiation :
Effects of X-rays :
- Shortwave Fadeout (SWF)
- Sudden Ionospheric Disturbances (SID)
Effects of heavy protons :
- Polar Cap Absorption (PCA)
of plasma clouds :
- Ionospheric storm
These events are complete by effects specific to solar features :
- High speed solar wind stream (HSSWS)
- Sudden Disappearing Filaments (SDF)
- Forbush effect and cosmic storm
At last several of these events can group and create geomagnetic effects :
- Geomagnetic storm
- Auroral E
- South Atlantic Anomaly (SAA).
Let's see in detail each of these phenomena, how they occur and what are their main effects on HF propagation.
Perturbations under normal conditions
There is unfortunately not only one sort of fading but at least six types of fading : normal, multipath fading, polarization fading, skip fading, absorption fading and scintillation.
The most known, the normal fading, occurs when a signal propagates via different paths to the receiver. As a result of slight differences in path taken across the ionospheric layers, signal can be slightly delayed. Although this delay last no more than about 10 ms, waves fronts have enough time to mutually add while others cancel. If the delay to receive a same signal under different pathes exceeds 5 ms you will already heard a light QSB.
If your working frequency is close to the MUF, there will have always more than one signal path and several hops depending on the distance to travel. The result will be a deep oscillation of signal strength. In these conditions you can experiment a QSB between 10-15 dB, even more on low bands, up to close temporary (few tens of minutes to one hour) some short distance circuits (local QSOs with bordering countries or states.
The reception is the most affected in AM transmissions where we hear intense distortions, and FAX images transmitted by shortwaves display smears in such occasions.
One of the most visible effects of the solar activity is without any doubt aurora borealis and australis, northern and southern lights.
During periods of strong geomagnetric activity, the shock of high-energy particles released by the sun and mainly constituted of fast electrons excite neutral atoms of the upper atmosphere, near 100 km aloft, at the upper edge of the D-layer, that free their additive energy in the form of visible light : auroras appear. They represent the charged particles precipitation to the ground. If the subject interest you I suggest you read this French page (using the translate module at left if necessary) to understand how "work" this phenomenon and see some vivid color pictures of some recent auroras.
During QSOs, radio amateurs working near polar regions (Scandinavia, Canada, Alaska) experiment a decreasing of the signal-to-noise ratio and increasing noise. If the situation worsen, HF blackouts may occur up to mid-latitudes. Sometimes at high latitudes, small but exceptional openings appear for DXing. However propagation in MW and LW are totally interrupted via the D-layer. While most HF bands are closed, this event increases the electronic density of D-layer, enhancing VLF propagation (see SID and SWF below).
After a high solar activity, after the emission of an eruptive prominence or a chromospheric ejection, we observe a decrease of cosmic rays known as Forbush effect, after american physicist Scott E. Forbush who studied cosmic rays in the 1930s and 40s.
When cosmic rays hit Earth's upper atmosphere, they produce a shower of secondary particles that can reach the ground. By monitoring these showers Forbush noticed, contrary to intuition, that cosmic ray doses dropped when solar activity was high.
How to explain this phenomenon ? The reason is simple. When an active solar region explodes and hurls massive clouds of hot gas (CME) into space, these masses contain not only gas but they also imprison knots of magnetic fields. Magnetic fields deflect charged particles (they are use in bubble chambers to identify charged particules), so when a CME sweeps past Earth, it also sweeps away many of the electrically-charged cosmic rays that would otherwise strike our planet.
The Forbush decrease last up to 24 hours and the come back occurs in a few days. This effect is sometimes called "cosmic storm" and it is associated with geomagnetic storms that preceed auroras. Such an event was observed during ISS mission in September 2005 as we can see in the above graph. During that time, the ISS crew absorbed about 30% less cosmic rays than usually.
Like a frontal trough creates poor weather conditions, a sub-auroral trough do the same but from an electromagnetical point of view. Its presence decreases the MUF and affects the great-circle propagation (short or long path). During the strongest events, close to the auroral oval area, the great circle can be shifted of a spectacular angle ranging from 50 to 100°.
All these auroral-related events are strongly depending on the conductivity and electron density, which models can forecast such disturbances.
These are rapid fluctuations of the geomagnetic field which period is between a fraction of a second and tens of minutes. They last from some minutes to several hours. There are two main patterns : a continuous, almost sinusoidal pattern called "Pc", and an irregular pattern called "Pi". Pulsations occur during magnetically quiet as well as disturbed days.
According to their physical and morphological properties, Pc's are grouped into five categories :
- Pc1 : periods 0.2-5 sec. They may occur in bursts called "pearls" or in consecutive groups with sharply decreasing frequency
- Pc2 : periods 5-10 sec. They do not seem to be physically related to Pc1 or Pc3
- Pc3 : periods 10-45 sec. They are observed over a wide range of latitudes
- Pc4 : periods 45-150 sec. They are also known as Pc II or Pc
- Pc5 : periods 150-600 sec. They are sometimes called "giant micropulsations".
Listen to the sounds of space as you never heard them:
We have explained in other chapters dealing with antenna properties that the polarization plan of incident radio waves had to match the one of the receive antenna. If the wave incident to the receive antenna is normal (at right angle) to the polarization plane of the transmit antenna, the voltage induced in the receive antenna will be zero, even if a small fraction of RF actually reach the antenna. Result is that no signal will be picked up by your receiver.
In some circumstances, depending on the working frequency and whether the radio wave couples or not with the geomagnetic field, only one of the characteristic wave (ordinary of extraordinary) will travel through the ionosphere to the receiver. If this happens, it is more than probable that a wave polarized horizontally will be completely converted into an ordinary wave and that no energy will go into the extraordinary wave. The resulting wave will show a circular polarization and the antenna polarized horizontally will only pick up half the incident energy.
Worse, if all the energy is converted into the extraordinary wave you have chance that the signal is lost at receive. On low frequencies (0.3-3 MHz) this extraordinary wave can even be totally absorbed before arriving at the receiver.
Taking into account the orientation of the geomagentic field, if you experiment or expect a polarization mismatch, the next rule is generally applied successfully :
- At low latitudes (below 30°) and in E-W propagation, use horizontal polarized antennas
- At mid latitudes (30-60°) is case of geomagnetic disturbances or polarization mismatch, prefer the vertical polarized antenna
- At high latitudes (over 60°), use vertical polarized antennas.
This is to prevent such problems that some amateurs working at high latitudes use quad antennas to get directivity and a high gain but they feed their antenna such a way to get a vertical polarization.
Polarization effects become less important as the frequency increases because the absorption of the extraordinary component of the wave also decreases.
Noise and interference
Radio communications need to be clear to be readable, and loud enough to not request to repeat the same sentence ten times, the famous "again ?...", Hi ! We must made the distinguo between the natural noise, QRN, and the man-made noise, QRM or RFI.
Radio noise arises from four main sources :
- Atmospheric : or sferics, this is the noise generated by lightnings striking within a thousand kilometers or so of the receiver. As they travel like ordinary radio waves they cannot be eliminated before reaching the receiver. Hopefully, at daytime low frequencies sferics are absorbed by the D-layer that helps eliminating most of them but it is of a short respite as they will come back at night if there is an unstable frontal system over your region (up to about 500 km around your receiver).
- Galactic : Karl Jansky and Grote Reber discovered in the 30's the radio emission of the galaxy and some other radiosources using relatively modest techniques. This noise is of low amplitude and much weaker than spherics. The ionosphere being impenetrable to low frequencies, only noise at frequencies over foF2 can affect HF signals. However, in very remote locations, at distance of 100 km or more from any human infrastructure (in deserts of Chile, Sahara, Australia, in ranges or at very high latitudes), the galactic noise can be a source of perturbations at daytime at frequencies above about 10 MHz.
- Local : this is the noise generated by local sferics that mainly affects MF broadcastings from 10 MHz and below.
- Interference : this is the too famous man-made noise, QRM. If we exclude the QRM Pro and other exotic sources, Hi!, it concerns any electrical equipment emitting RF and unprotected like the motor ignition system, remote controller, neon lamp, dimmer, welding machine, fridge, transformer, PC screen, high power line and any leaking device like an old TV or a VCR. Even some high-end transceivers have their image frequency in amateur bands...
Interference or RFI is a special kind of QRM that concerns a device properly designed but that transmits noise on frequencies allocated to other services, like a remote controller or a microwave oven that radiates in HF or UHF bands. In some circumstances, the manufacturer is not to blame because there is lack in the regulatory body that has forget to protect some frequencies at primary or secundary use for specific activities. Only a national or an international recommendation or a law can solve the problem. In the meantime there is no other solution to eliminate such RFI than to decrease the signal strength of the RFI source or to stop its operations. However most of the time it is impossible or to expensive and users victims of QRM have no other choice than moving the receiver away from the interference (e.g. with BPL) or to add in front of their receiver some special pass-band filters or to reduce the receiver bandwidth. In worst cases the only solution is to move away from the "civilization", to the country, a few tens of kilometres from cities where the noise level is lower and often set by atmospheric noise.
At last, a large part of the broadband QRM can be eliminated using a horizontally polarized antenna, including a beam, as most RFI come from the industry and buildings that all are erected in height.