4.5 Solar Activity, Sunspot Cycles, and VHF Anomalous Propagation

4.5 Solar Activity, Sunspot Cycles, and VHF Anomalous Propagation

The state of the ionosphere and lower atmosphere is dynamically tied to solar activity and meteorological events. Understanding space weather indicators and non-standard propagation modes allows operators to take advantage of rare long-distance openings on both HF and VHF bands.

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Solar Activity and the 11-Year Sunspot Cycle

Solar activity follows a cyclic variation averaging 11 years in duration (known as the Schwabe Cycle). Sunspots are cool, intense magnetic regions on the solar surface. The number of sunspots present correlates directly with the emission of Extreme Ultraviolet (EUV) and X-ray radiation.

                 11-Year Solar Sunspot Cycle
  Sunspot Number / Solar Flux
     ^
 High|       /\                 /\
  SFI|      /  \               /  \           Solar Max: High MUF (10m/6m open)
  150|     /    \             /    \          ---------------------------------
     |    /      \           /      \         
 Low |   /        \         /        \        Solar Min: Low MUF (Upper HF dead)
   70+--/----------\-------/----------\----->
     0     5.5     11     16.5        22  Years

• Solar Maximum: High sunspot counts and elevated EUV radiation create dense F2-layer ionisation. The MUF rises dramatically, opening the 28 MHz (10 m) band and occasionally the 50 MHz (6 m) VHF band for worldwide skywave propagation.
• Solar Minimum: Low sunspot activity reduces F2-layer ionisation. The MUF drops, limiting HF skywave communication to lower bands (below 14 MHz or 20 m).

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Key Solar and Geomagnetic Indices

Radio amateurs monitor three standard space weather indices to evaluate propagation conditions:

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Solar Disturbances and Radio Blackouts

1. Sudden Ionospheric Disturbances (SID) / Solar Flares

A solar flare is an intense burst of electromagnetic radiation on the sun. Hard X-rays travel to Earth at light speed (arriving in 8 minutes), penetrating deep into the atmosphere to heavily ionise the D layer.

• Effect: Total daytime HF radio blackout. Signals across the entire HF spectrum are absorbed by the dense D layer. SIDs last from 15 minutes to several hours.

2. Coronal Mass Ejections (CME) and Geomagnetic Storms

A Coronal Mass Ejection (CME) is a massive cloud of solar plasma and magnetic fields ejected from the sun, traveling slower than light (15 to 36 hours to reach Earth).

• Effect: When a CME strikes the Earth's magnetosphere, it triggers a geomagnetic storm ($K \ge 5$). This disrupts the F2 layer, drastically lowering HF MUFs and causing heavy polar cap absorption, while initiating auroral displays.

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VHF Anomalous Propagation Modes

While VHF signals normally travel via line-of-sight direct waves, specialised atmospheric and ionospheric phenomena produce remarkable long-distance propagation.

                    VHF Propagation Modes Comparison
  Mode                 Mechanism                        Typical Range    Bands Affected
  --------------------------------------------------------------------------------------
  Sporadic E (Es)      Dense ionised E-layer clouds     1000 - 2200 km   10m, 6m, (2m)
  Tropo Ducting        Tropospheric temp inversion      300 - 2500 km    2m, 70cm, 23cm
  Aurora Scatter       Polar auroral curtain reflection 500 - 1500 km    6m, 2m
  Transequatorial (TEP) Equatorial F2 ionisation crests  3000 - 5000 km   6m, 2m

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1. Sporadic E ($E_s$)

Sporadic E consists of small, highly dense clouds of ionisation that form unexpectedly in the E layer (altitude 90 km to 120 km).

• Seasonality: Prevalent during late spring and summer months in both hemispheres.
• Mechanism: Metallic ions (from microscopic meteor debris) are compressed into thin, intense ionised sheets by upper atmospheric wind shears and magnetic forces.
• Characteristics: Refracts VHF signals on 28 MHz (10 m), 50 MHz (6 m), and occasionally 144 MHz (2 m), achieving single-hop distances of 1000 km to 2200 km with extremely strong signal levels.

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2. Tropospheric Ducting (Tropo)

Tropospheric Ducting occurs in the lowest layer of the atmosphere (the troposphere) due to weather patterns.

                        Tropospheric Ducting
  ============================================================== Cool Dry Air
  ~~~~~~~~~~~~~~ Warm Dry Air Layer (Temperature Inversion) ~~~~~~~~~~~~~~~~~
  -> -> -> Radio Wave Trapped in Duct -> -> -> -> -> -> -> -> -> -> -> -> ->
  ~~~~~~~~~~~~~~ Cool Moist Surface Layer (Marine Air) ~~~~~~~~~~~~~~~~~~~~~~
  ============================================================== Earth / Sea

• Mechanism: Under normal conditions, air temperature decreases with altitude. During a temperature inversion (such as a warm dry air mass sliding over a cool moist marine layer or along weather fronts), the refractive index gradient changes sharply. Radio waves entering the boundary at a shallow angle are trapped inside an atmospheric "duct" between air layers or between an upper layer and the sea.
• Bands Affected: VHF, UHF, and microwave bands (144 MHz, 432 MHz, 1.2 GHz).
• Geographic Significance in Australia: Excellent coastal tropospheric ducts form frequently along the southern coast of Australia across the Great Australian Bight (connecting VK5/VK6 to VK3/VK7) and along the eastern seaboard (VK2 to VK4), enabling VHF/UHF contacts over 1000 to 2500 km.

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3. Aurora Propagation

High-energy solar particles during geomagnetic storms ionise vertical curtains of gas in the polar regions, creating auroral ovals.

• Mechanism: VHF radio signals directed toward the poles reflect off these ionised auroral curtains.
• Signal Distortion: Continuous movement and turbulence in the auroral display cause severe multipath dispersion and rapid phase shifts. Voice signals sound extremely raspy and distorted. Successful communication typically requires Continuous Wave (CW / Morse code).

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4. Transequatorial Propagation (TEP)

Transequatorial Propagation (TEP) occurs across the geomagnetic equator.

• Mechanism: Symmetrical ionised crests form in the F2 layer approximately $15^\circ$ to $20^\circ$ north and south of the magnetic equator during late afternoon and early evening (especially around the equinoxes).
• Application: Enables 50 MHz (6 m) and 144 MHz (2 m) contacts across the equator (e.g., Northern Australia VK4/VK8 to Japan and East Asia) over distances of 3000 km to 5000 km without intermediate ground reflections.