11.2 The Ionosphere, Solar Radiation, and Solar Cycles

11.2 The Ionosphere, Solar Radiation, and Solar Cycles

Long-distance, intercontinental High Frequency (HF) communication without artificial satellites or undersea cables is made possible by the ionosphere—a dynamic shell of partially ionised plasma enveloping the Earth between altitudes of approximately $60\text{ km}$ and $500\text{ km}$. Operating as a vast, solar-powered radio reflector, the ionosphere refracts HF electromagnetic waves back to the Earth's surface. Understanding the photoionisation mechanisms that create this plasma, the unique diurnal behavior of its discrete layers, and the periodic solar cycles that govern its density is fundamental to mastering HF radio propagation.

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Formation of the Ionosphere: Photoionisation Mechanics

The upper atmosphere consists of neutral gas molecules and atoms, primarily molecular nitrogen ($N_2$), molecular oxygen ($O_2$), atomic oxygen ($O$), and nitric oxide ($NO$). The ionosphere is created through photoionisation, driven by solar electromagnetic radiation in the Extreme Ultraviolet (EUV) spectrum (wavelengths $\lambda \approx 10\text{ to }121\text{ nm}$) and soft/hard X-ray spectrum ($\lambda < 10\text{ nm}$).

When a high-energy solar photon strikes a neutral gas atom or molecule, it transfers its quantum energy ($E = h\nu$). If this photon energy exceeds the first ionisation threshold of the atom (typically $12\text{ to }15\text{ eV}$), it ejects a bound orbital valence electron, creating a free electron and a positively charged ion:

$$O + h\nu \to O^+ + e^-$$ $$N_2 + h\nu \to N_2^+ + e^-$$ $$O_2 + h\nu \to O_2^+ + e^-$$ $$NO + h\nu \to NO^+ + e^-$$

                     Photoionisation production profile

High altitude:          Strong sunlight, few neutral particles
                        -> relatively few ion pairs produced
Intermediate altitude:  Enough photons and neutral particles
                        -> a maximum in production rate q(z)
Lower altitude:         Much ionising radiation absorbed above
                        -> reduced photoionisation production

q(z) is ion pairs produced per cubic metre per second.
Electron density Ne is a concentration, not a production rate.
Its profile also depends on electron loss and plasma transport.

The Chapman Ionisation Profile

Chapman's model explains how competition between decreasing neutral density and absorption of incoming solar radiation produces an altitude of maximum photoionisation rate. Electron density is not the same quantity: it depends on production, chemical loss and plasma transport.

1. High Altitudes: Ionising radiation is plentiful, but few neutral particles are available to absorb it, limiting production per unit volume.
2. Lower Altitudes: The dense overlying atmosphere absorbs much of the ionising radiation, reducing photoionisation. This is a gradual, wavelength-dependent effect, not a universal zero-ionisation boundary at $60\text{ km}$.
3. Intermediate Altitudes: Sufficient photons and neutral particles coexist to produce peaks in ionisation. Different absorption and loss processes contribute to the D, E and F regions.

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The Stratified Ionospheric Layers: D, E, F1, and F2

Atmospheric composition changes with altitude, different gases absorb different solar wavelengths, and the rates of electron loss also change. Together with plasma transport, these processes produce the D, E, F1 and F2 structure. The regions are not formed by gravitational separation alone, and their heights and densities vary with time and location.

           Diurnal Ionospheric Structure and Electron Density Profile

   Altitude (km)
        ^
    500 |             DAYTIME                             NIGHTTIME
        |
    400 |            /-------\
        |           /   F2    \  (210-450 km)           /-------\
    300 |          |           |                       /    F    \  (250-350 km)
        |          |    F1     | (140-210 km)         |   Merged  |
    200 |           \---------/                        \---------/
        |               |
    100 |           /-------\ E (90-140 km)                 : (Residual E)
        |          |=========| D (60-90 km) (Absorber)      : (D vanishes)
      0 +----------+---------+------------------------------+--------->
               10^8       10^11      10^12              10^8       10^11
                          Electron Density N_e (electrons / m^3)

1. The D-Layer ($60\text{ to }90\text{ km}$)

• Ionisation Sources: Hard X-rays ($\lambda < 1\text{ nm}$) during solar flares and solar Lyman-alpha radiation ($\lambda = 121.6\text{ nm}$) ionising trace amounts of nitric oxide ($NO$), along with high-energy galactic cosmic rays.
• Electron Density ($N_e$): Modest daytime density, typically $10^8\text{ to }10^9\text{ electrons/m}^3$ at solar noon.
• Atmospheric Environment: The D-region lies relatively low in the atmosphere, where neutral particles are abundant. Electrons therefore undergo frequent collisions with neutral molecules, much more readily than in the higher F-region.
• RF Attenuation Mechanism (The Absorption Blanket): When an electromagnetic wave passes through the D-layer, its alternating electric field imparts kinetic energy to the free electrons, causing them to oscillate. Before these vibrating electrons can reradiate their energy, they collide with neutral gas molecules. The electron's kinetic energy is converted into thermal heat in the neutral gas. This process is known as non-deviative absorption.

A simplified collisional model gives the absorption dependence

$$\alpha_D\propto\frac{N_e\nu}{\nu^2+\omega^2}.$$

When $\omega=2\pi f\gg\nu$, this approaches $\alpha_D\propto N_e\nu/f^2$. Here $\nu$ is the electron-neutral collision rate. This explains the usual trend toward greater D-region absorption at lower HF frequencies; it is not a fixed loss value for each band.

• After Sunset: Electron loss rapidly weakens the normal D-region absorption, allowing lower frequencies to reach the higher refracting regions more readily. This improves many nighttime MF/HF paths, but does not by itself guarantee a received signal or a fixed recovery time.

2. The E-Layer ($90\text{ to }140\text{ km}$)

• Ionisation Sources: Soft X-rays ($1\text{ to }10\text{ nm}$) and solar EUV radiation ($80\text{ to }103\text{ nm}$) ionising molecular oxygen ($O_2$).
• Electron Density ($N_e$): Moderate density, peaking around $10^{11}\text{ electrons/m}^3$ at solar noon.
• Operational Role: Atmospheric density at $110\text{ km}$ is sufficiently low that collision frequency drops significantly. The E-layer can refract signals in the $2\text{ to }7\text{ MHz}$ range back to Earth over single-hop distances of $1000\text{ to }2000\text{ km}$.
• Diurnal Behavior: Recombination proceeds rapidly after dusk, though a very weak residual E-layer persists through the night ($N_e \approx 10^9\text{ m}^{-3}$).
• Sporadic-E ($E_s$): During summer months, intense, localized clouds of metallic ions (meteoric ablation deposits of $\text{Fe}^+, \text{Mg}^+, \text{Si}^+$) are compressed into ultra-dense, paper-thin sheets ($1\text{ to }2\text{ km}$ thick) by upper-atmospheric wind shears. These Sporadic-E clouds can achieve plasma frequencies exceeding $15\text{ to }30\text{ MHz}$, permitting spectacular multi-hop skip openings on the $28\text{ MHz}$, $50\text{ MHz}$, and occasionally $144\text{ MHz}$ amateur bands.

3. The F-Layer ($140\text{ to }500\text{ km}$)

• Ionisation Sources: Solar EUV photons ionise the upper atmosphere, including atomic oxygen. Production depends on the solar spectrum, the relevant absorption/ionisation cross-sections and the local neutral composition; it is not specified by one solar emission line alone.
• Atmospheric Environment: At altitudes above $200\text{ km}$, atmospheric pressure is negligible. Free electron collision frequency is exceedingly low ($\nu \ll 10^3\text{ s}^{-1}$), rendering the F-layer a virtually lossless refractive medium for RF waves.
• Daytime Splitting (F1 and F2): Under the influence of intense solar overhead illumination, the F-region divides into two distinct zones:
  • F1-Layer ($140\text{ to }210\text{ km}$): Daytime peak electron density reaches $2\text{ to }5 \times 10^{11}\text{ m}^{-3}$. It acts as an intermediate refracting layer for lower HF signals, causing distinct cusps on vertical ionograms. The F1-layer recombines rapidly after sunset and merges into the composite night F-layer.
  • F2-Layer ($210\text{ to }450\text{ km}$): The highest, thickest, and most heavily ionised layer of the Earth's atmosphere. Peak electron density reaches $10^{12}\text{ to }3 \times 10^{12}\text{ electrons/m}^3$ during solar maximum. The F2-layer is the primary reflector for all intercontinental amateur DX communications on HF.
• Nighttime Electron Loss: Direct radiative recombination of $O^+$ is slow. Important loss paths first convert atomic ions to molecular ions through reactions with $N_2$ or $O_2$, followed by faster dissociative recombination with electrons. One example is $O^++N_2\to NO^++N$, followed by $NO^++e^-\to N+O$. Because molecular neutral densities are low in the F2-region, overall electron loss is relatively slow. Plasma transport also helps determine the nighttime density.

Comprehensive Ionospheric Layer Matrix

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Solar Activity, Sunspots, and the 11-Year Schwabe Cycle

The electron density of the ionosphere is not static; it is directly coupled to the magnetic dynamism of the Sun. Solar activity undergoes a well-documented cyclic variation discovered in 1843 by Heinrich Schwabe: the 11-year solar cycle.

              Approximate cycle timing (schematic, not measured data)

Phase             Minimum   Maximum   Minimum   Maximum   Minimum
Elapsed years        0        5-6        11       16-17      22
Relative activity   Low       High       Low      High      Low

A cycle runs from one minimum to the next, averaging about 11 years.
Peak strength and exact timing vary. SSN and SFI are different
indices and do not share a single numerical scale.

The Solar Dynamo and Sunspot Formation

The Sun does not rotate as a rigid body. Because it is a ball of incandescent gas and plasma, its equator rotates once every $25\text{ days}$, while its polar regions rotate once every $35\text{ days}$. This differential rotation twists and stretches the Sun's internal poloidal magnetic field lines into concentrated east-west toroidal magnetic ropes wrapped tightly beneath the solar surface.

When magnetic tension becomes extreme, buoyant magnetic flux tubes ($0.1\text{ to }0.4\text{ Tesla}$) erupt through the visible solar surface (the photosphere). The intense magnetic fields choke off the upward convective transport of superheated plasma from the solar interior. Consequently, the surface within the magnetic footpoints cools from its normal temperature of $\approx 5800\text{ K}$ down to $\approx 3500 - 4000\text{ K}$. Because the cooler area emits significantly less optical light than the surrounding photosphere, it appears visually dark by contrast—forming a sunspot.

Crucially for radio propagation, sunspots themselves are not the source of ionizing radiation. Rather, the intensely active magnetic areas immediately surrounding sunspots—termed plages and faculae—emit colossal torrents of ionizing extreme ultraviolet (EUV) and X-ray radiation.

Sunspot Quantification: The International Sunspot Number ($R_i$)

To quantify solar activity over historical epochs, Swiss astronomer Rudolf Wolf formulated the standard Sunspot Number ($R_i$, or Wolf Number):

$$R_i = k(10g + s)$$

where:

• $g$ is the number of identified sunspot groups.
• $s$ is the total count of individual visible spots across all groups.
• $k$ is an observatory scaling factor that standardizes observations across varying telescope apertures and atmospheric viewing conditions.

Daily sunspot numbers vary with solar rotation and the evolution of active regions. The standard smoothed monthly sunspot number uses a centred $13$-month window: the two end months receive half weight, the other eleven receive full weight, and the sum is divided by $12$. This suppresses short-term fluctuations and shows the underlying cycle trend.

The 22-Year Hale Magnetic Cycle

Every 11 years, at the peak of each solar cycle, the Sun's global magnetic dipole completely reverses polarity: magnetic north becomes magnetic south. It requires two full 11-year Schwabe cycles—a 22-year Hale Cycle—for the Sun's global magnetic field to return to its original orientation.

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The Solar Flux Index (SFI) and HF Band Openings

Visual sunspot counts provide a long historical record. The Solar Flux Index supplies an independent, instrumentally measured indicator of solar activity that can be used as a proxy for ionising solar output; it does not directly measure EUV irradiance.

What Does the Solar Flux Index Measure?

The Solar Flux Index is the Sun’s spectral flux density at $10.7\text{ cm}$, approximately $2800\text{ MHz}$. The Canadian monitoring programme measures it at the Dominion Radio Astrophysical Observatory near Penticton, British Columbia. Precise daily determinations include the local-noon measurement.

SFI is expressed in Solar Flux Units (SFU):

$$1\text{ SFU} = 10^{-22}\text{ W}\cdot\text{m}^{-2}\cdot\text{Hz}^{-1}$$

Why SFI is the Primary Propagation Metric

1. Ground Measurability: Solar extreme ultraviolet (EUV) radiation—the direct driver of F2-layer photoionisation—is entirely absorbed by the upper atmosphere. Consequently, direct EUV monitoring requires orbital satellites. In contrast, $10.7\text{ cm}$ ($2800\text{ MHz}$) microwave emissions penetrate cleanly through Earth's atmosphere, enabling continuous, highly reliable ground-based monitoring regardless of terrestrial weather.
2. Activity Proxy: The $2800\text{ MHz}$ emission originates in the solar atmosphere and correlates with active-region magnetic activity and EUV output. This makes SFI useful in ionospheric modelling. It is a proxy rather than an exact measurement of EUV, and local HF conditions also depend on season, time, path and geomagnetic activity.

Solar Cycle Metrics and Amateur HF Band Conditions

These are trends, not a schedule or fixed SFI threshold. Check the intended path and time: no solar-cycle category guarantees $14\text{ MHz}$ around the clock or zero absorption on $28\text{ MHz}$. Suitable high-HF openings can nevertheless give strong intercontinental signals with ordinary amateur power and antennas.