Medium Frequency

Some treatments group Medium Frequency (MF) and High Frequency (HF) together since both may utilize the skywave mode of propagation, and, with the possible exception of the lower MF band, operational utility is usually achieved through exploitation of this mode. Additionally, both MF and HF exhibit useful groundwave properties. For example, large-scale D-region absorption events, such as PCA or SWF, will reduce the competing noise at the groundwave receiver terminal arriving by skywave. Hence, a groundwave system will encounter reduced atmospheric noise, and this should increase the SNR and the system performance. This is even the case for absorption due to high altitude nuclear explosions (i.e., HANE events).

Figure 4-3: Phase and Amplitude Recordings during a PC A Event. From Galejs [1972], after Westerlund et al.. [1969].

Table 4-3: Ionospheric Effects at VLF

Event

Ionospheric Change

System Result

Solar Flare

Sudden Ionospheric Disturbances (SIDs): Solar x-rays cause excess ionization below the normal D-layer, on the sunlit side of the earth, changing the effective height of reflection, (wide-area and instantaneous event)

■ Sudden Phase Anomaly (SPA, or increase in wave phase)

■ Increase in signal strength above

16 kHz over sunlit paths

Solar Proton Event

Polar cap disturbance due to energetic particle event. This causes excess ionization in the D-layer within the polar cap. (wide-area and instantaneous event)

Phase lag and amplitude decreases:

■ several hours at midlatitudes

■ 10-20 days hi-lat/transpolar paths

Magnetic Storm and Auroral Phenomena

Precipitation in the auroral zone causes excess ionization in the D-region, but these events are more localized in space and time.

Irregular variations in phase and amplitude with periodicities of ~10s of minutes, especially at night and over long paths [ITU-R, 1998],

The variabilities in skywave field strength at MF have been outlined by Knight [1982]. While there are sound arguments for consideration of the MF and HF bands as a unified pair of bands, the ITU-R has chosen to group LF and MF together. Indeed, there are two recognized methods for predicting skywave field strength in the LF/MF bands, specifically within the range 150 kHz and 1700 kHz (i.e., [Wang, 1985, 1993] and ITU-R Rec.P.1147 [1997]). There are distinct magnetic and solar activity influences on LF and MF circuits. Figure 4-4 compares long-term MF field strength variations with SSN and Ap.

1800 1600 1400 1200 1000 800 600 400 200 0

Twelve Month Running Mean Smoothed Zurich Sunspot Number Ap Magnetic Index

Field Strength Monthly Medians (|iv/m)

Twelve Month Running Mean Smoothed Zurich Sunspot Number Ap Magnetic Index

Field Strength Monthly Medians (|iv/m)

40 80 120160200

Ap 10

1943 44 45 46 47 48 49 50 51 52 53 54 55 1956

Figure 4-4: Monthly median field strength for a Medium Frequency (MF path) between Cincinnati and Atlanta (600 km) compared with the Ap Index and the 12-month running mean sunspot number. (From Davies [1990]).

4.3.5 High Frequency (Shortwave)

The HF band is probably the most difficult regime to characterize, given the fact that it is the most precarious of the bands, vis-à-vis ionospheric propagation. This is because representative electron densities associated with various layers in the ionosphere correspond to critical frequencies (and plasma frequencies) that fall within the HF band. The layer critical frequency is the frequency above which radio waves will penetrate the ionosphere at vertical incidence, and this is the maximum plasma frequency of the layer. For example, a typical midlatitude daytime value of the F2 layer critical frequency is 10 MHz, but it varies considerably depending upon a number of geographical and solar epochal considerations (see Chapter 3). Space weather effects on HF are significant, and the effects include: solar flare-induced absorption, polar cap absorption, angle-of-arrival fluctuations, auroral scatter, multipath distortion, HF radar ranging errors, broadcast coverage variations, storm-driven MUF variations, and many more. One may refer to Goodman [1991] for additional information. Table 4-4 provides a synopsis of ionospheric disturbances on HF systems.

Table 4-4: Ionospheric disturbances that influence HF radio systems. The magnitude of the HF effects will depend critically upon system parameters.

Ionospheric Disturbances

Approximate Occurrence Frequency

Disturbance

Propagation Effects

Time and Duration

Solar Max

Solar Min

Probable Cause

a) Sudden Ionospheric Disturbance (SID)

In sunlit hemisphere, strong D layer absorption (shortwave fades), anomalous VLF-reflection, F-region effects.

All effects start approximately simultaneously. Duration ~ 1/2 hour.

2/Week

2/Year

Enhanced solar X-ray and EUV flux from solar flare.

b) Polar Cap Absorption (PCA)

Intense radiowave absorption in magnetic polar regions. Anomalous VLF-reflection.

Starts a few hours after flare. Duration one to several days.

1/Month

0

Solar protrons 1-100 MeV.

c) Magnetic Storm

F-region effects; increase of foF2 during first day, then depressed foF2, with corresponding changes in MUF.

May last for days with strong daily variations.

26/Year

22/Year

Interaction of solar low energy plasma (solar wind) with earth's magnetic field, causing energetic electron precipitation, auroral effects, heating, and TID generation.

d) Auroral Absorption

(AA)

Enhanced absorption along auroral oval in areas hundred to thousand kilometers in extent. Sporatic K may give enhanced MUF.

Complicated phenomena lasting from hours to days.

Essentially omnipresent

Precipitation of electrons with energies a few tens of keV within an oval ' region equatorward of the polar cap.

e) Travelling

Disturbances (TID)

Changes of foF2 with corresponding changes of MUF sometimes periodic.

Typically the periods are from tens of mnutes to hours.

Essentially omnipresent with larger scales enhanced during magnetic storms

Atmospheric waves.

HF signals penetrate the overhead ionosphere if the frequency is greater than the so-called critical frequency (see Equation 3-10). For practical systems, it is necessary to exploit the so-called skywave mode of propagation. Unless we simply want an "umbrella" type of coverage that is limited to 500 km or so, we actually want the transmission frequency to be higher than foF2.

This means we lose signals to outer space through an overhead "iris", but we gain the opportunity for greater coverage by oblique propagation. Long-haul skywave propagation is achieved if the transmission frequency is greater than foF2 but less than the Maximum Observable frequency (i.e., MOF) for the path length required. The MOF can be shown to be proportional to the quantity (k foF2 sec 0) where 0 is the ray zenith angle, foF2 is the ordinary ray critical frequency of the F2-layer, and A: is a number between 1 and 2 whose value is dependent upon the nature of the process involved. For classical HF skywave propagation, k = 1, but it approaches 2 for around-the-world propagation involving super-modes, chordal modes and ducted modes. For scatter modes, k > 1. Referring to the classical case, we find that as the elevation angle is reduced, a level is eventually reached at which reflection will occur, and it becomes possible to generate skywave modes. This effect is depicted in Figure 4-5.

Figure 4-5: Illustration of rays launched into the ionosphere. The numbers are sequenced from the lowest elevation angle labeled "1" at 0 degrees to "19" at 90 degrees. Notice that rays 1-9 participate in skywave propagation, and rays 10-19 escape through the ionospheric iris. A "skip zone" is also introduced. In practice this skip distance is weakly illuminated as the result of non-classical scatter modes. Groundwave and line-of-sight propagation will also provide "local" coverage.

Figure 4-5: Illustration of rays launched into the ionosphere. The numbers are sequenced from the lowest elevation angle labeled "1" at 0 degrees to "19" at 90 degrees. Notice that rays 1-9 participate in skywave propagation, and rays 10-19 escape through the ionospheric iris. A "skip zone" is also introduced. In practice this skip distance is weakly illuminated as the result of non-classical scatter modes. Groundwave and line-of-sight propagation will also provide "local" coverage.

The HF band is most sensitive to ionospheric effects. In fact, HF radiowaves experience some form of virtually every propagation mode or mechanism, as indicated in Table 4-5 and illustrated in Figure 4-6. It has been said that if one can understand radio propagation at HF, a comprehension of all other bands follows naturally.

Table 4-5: List of HF Propagation Mechanisms

Mode or Mechanism

Description

Groundwave

Propagates along surface of the earth or ocean (i.e., surface wave)

Spacewave

Superposition of LOS, earth-reflected signals, and secondary ionospheric modes

Terrestrial Line-of-Sight

Subionospheric rectilinear propagation

Earth-Space Line-of-Sight

Transionospheric quasi-rectilinear propagation

Reflected One-Hop

Earth-to-Earth propagation with intermediate reflection from an ionospheric layer.

Reflected Multi-Hop

Earth-to-Earth propagation with multiple hops using a single ionospheric layer or different layers

Ducted

Wave is trapped between two ionospheric layers (i.e., the E-F "valley" of ionization)

Chordal

Wave, launched from an earth terminal, skips along the base of an ionospheric layer without an intermediate ground reflection (i.e., transequatorial "supermodes")

Scatter

Wave is scattered from ionospheric mhomogeneities or features (i.e., auroral and sporadic-E sidescatter, spread-F scatter)

The major ionospheric layers possess characteristic plasma frequencies that lie within the HF band, and as a result, ionospheric interaction is pronounced. Accordingly, many of the traditional ionospheric diagnostic systems exploit this feature and use HF waveforms to probe the ionosphere and determine its structure. Figure 4-7 depicts the various possibilities. See Hunsucker [1990] for more details.

Propagation factors at HF have been widely explored, and numerous books are available on the subject with most being cited in Section 4.1. Naturally, the author's preference is the HF book by Goodman [1991], having an emphasis on applications, but the book by Davies [1990] is a well-rounded classic.

One of the major problem areas that arise in connection with HF system performance is the variability in coverage and reliability for a fixed transmitter site and a specified frequency. This variability mimics the ionospheric variability itself, and recently schemes have been developed to monitor the ionosphere and adjust certain system parameters in near real time to compensate for the system effects. The value of these Real-Time-Channel Evaluation (RTCE) schemes that may be central in specified adaptive-HF methodologies will be addressed later on in the chapter.

Figure 4-6: Depiction of various propagation mechanisms (i.e., modes) at HF. The most prominent group is the set of skywave modes. These fall into three categories: (a) those associated with regular refraction from the ionospheric layers E, Ft, and F2; (b) those associated with scatter from sporadic-E, auroral forms, polar patches, blobs, etc.; and (c) those associated with ducted or chordal modes of propagation. There is also scatter from ionospheric inhomogeneities associated with above-the-MUF connectivity. Other non-skywave possibilities include groundwave, spacewave and line-of-sight. (From Goodman [1991]).

Figure 4-6: Depiction of various propagation mechanisms (i.e., modes) at HF. The most prominent group is the set of skywave modes. These fall into three categories: (a) those associated with regular refraction from the ionospheric layers E, Ft, and F2; (b) those associated with scatter from sporadic-E, auroral forms, polar patches, blobs, etc.; and (c) those associated with ducted or chordal modes of propagation. There is also scatter from ionospheric inhomogeneities associated with above-the-MUF connectivity. Other non-skywave possibilities include groundwave, spacewave and line-of-sight. (From Goodman [1991]).

Figure 4-7: Sounding and diagnostic techniques that use the HF band. From Goodman and Aarons, [1990].

Included among the quasi-global disturbance phenomena that may impact long-haul HF systems are: Sudden Frequency Deviations (SFD) and Short Wave Fades (SWF), both of which occur within seconds of the measurement (at earth orbit) of an x-ray flare on the solar surface. These events are important only on the sunlit portion of the ionosphere and the effects are diminished as the ionospheric distance from the subsolar point increases. Less immediate but near-term phenomena associated with energetic solar protons are also encountered. Polar Cap Absorption events (or PCAs) are perhaps the most catastrophic events in connection with HF radio propagation in the high latitude zone, with attenuation over skywave circuits in excess of 100 dB sometimes encountered. These absorption conditions may last from hours to days. Fortunately, they are rare events, which are not typically encountered at solar minimum, and are observed approximately once a month at solar maximum.

Probably the most interesting space weather phenomenon to be encountered at HF is the ionospheric storm, which gives rise to a hierarchy of effects at midlatitudes. Although the time history of ionospheric storms is also of importance in earth-space satellite applications, it may be devastating in the HF band. This is because it may limit ionospheric support in the frequency range < the undisturbed MUF, causing a non-absorptive "blackout" of HF trunks in the affected area. An ionospheric storm is an ionospheric manifestation of the geomagnetic storm whose basic phenomenology has been fully described by Akasofu [1977], and has been updated in Geophysical Monograph 98 [Tsurutani et al., 1997], and in a document edited by Daglis [2001], At HF, we are principally interested in the diminutions in the F2 region electron density, a phenomenon that is highly correlated with the temporal structure of the main phase of the geomagnetic storm. In concert with foF2 variations, Maximum Observable Frequency (MOF) reductions of up to 50% may be observed for the day of the disturbance with full recovery occurring over several days. (We also note that MOF enhancements are sometimes observed, and these events are designated positive storms.) It was once thought that ionospheric storms would be predominant during solar maximum conditions, but significant disturbances may be observed at any time, especially during the declining phase of the sunspot cycle.

Again, while there are many phenomena of interest, the most important concerns for HF systems vis-à-vis space weather relate to the delayed phenomena associated with particle eruptions or solar wind plasma. These events take a predictable amount of time to impact the earth, and as a consequence allow time to invoke countermeasures. The delayed events include energetic particles (viz., solar protons) that cause long-term fading in the polar region (i.e., PCA). Equatorward of the polar cap, the most important solar phenomena are those associated with expansion of the solar corona (i.e..

coronal holes) and coronal mass ejections (CMEs). As already described, these events give rise to magnetic storms and, more importantly ionospheric storms. Ionospheric storms are not only defined by large excursions in the electron density of the F layer, but can also introduce large-scale TIDs, enhanced spread-F, and other deleterious effects. Immediate events like solar flares have the potential to cause communication blackout (i.e., SWF) over a large area of the sunlit earth for up to an hour, and they can serve as markers for particle expulsion or solar wind enhancement. But, given this, the x-ray flare is still not as important an event for HF systems as the ionospheric storm.

Most HF systems are only partially adaptive, with the capability to compensate for relatively minor channel variations through equalization and diversity schemes. While HF systems are now designed to take care of traditional HF difficulties, they are ill-equipped to address massive changes in the propagation characteristics, such as total absorption of entire frequency families due to PCA and enhanced auroral absorption, loss of important circuits as the result of exaggerated F-layer depletion, etc. This puts a premium on the forecasting and early detection of magnetic storms. Such information allows the system manager, or possibly a computer controller, to activate new routing strategies and frequency plans, or possibly resort to the use of alternate media.

0 0

Post a comment