The Solar Cycle

Solar Cycle 25 started in December 2019 and is projected to reach its peak in 2025, according to predictions from an international panel of experts co-sponsored by NOAA and NASA. December 2019 marked the “Solar Minimum,” the period of lowest solar activity within the Sun’s approximately 11-year cycle.

A- and K-Index:

The A and K indices measure geomagnetic activity, with higher values indicating more turbulent conditions in Earth’s magnetic field. When these indices are high (K > 5 or A > 20), geomagnetic storms occur, leading to unstable radio propagation and potential signal blackouts. This effect is most pronounced at high latitudes and in polar regions, where propagation may completely vanish.

Severe geomagnetic disturbances can also trigger aurora-related propagation, though this typically results in degraded long-distance communications. Conversely, when these indices are low, radio wave propagation tends to be more stable and favorable, particularly for transpolar communication paths.

The K-index ranges from 0 to 9, with classifications as follows:

• K0 – Inactive
• K1 – Very quiet
• K2 – Quiet
• K3 – Unsettled
• K4 – Active
• K5 – Minor storm
• K6 – Major storm
• K7 – Severe storm
• K8 – Very severe storm
• K9 – Extremely severe storm

Similarly, the A-index categorizes geomagnetic activity, with values ranging from 0 to over 400:

• A0 – A7 – Quiet
• A8 – A15 – Unsettled
• A16 – A29 – Active
• A30 – A49 – Minor storm
• A50 – A99 – Major storm
• A100 – A400 – Severe storm

Background X-ray Levels:

X-ray radiation levels from the Sun are classified as B (very low), C (low to moderate), M (moderate to high), and X (high to extremely high). The number following the letter indicates intensity—for example, an X0.1 flare is more intense than an M9.9. Increased X-ray activity can cause strong ionization in the D-layer of the atmosphere, leading to significant absorption of HF signals.

Solar Flux:

Solar flux measures the intensity of solar radiation at a wavelength of 10.7 cm (2800 MHz) and correlates with ultraviolet radiation, which helps generate the ionosphere. The lowest recorded value for solar flux is 63.75. At levels around 70, single-hop propagation can occur in lower latitude regions. Global long-distance (DX) propagation can begin at around 120, while an average solar flux of 170 is considered optimal for 10m-20m bands, allowing for worldwide communication using low-power (QRP) setups, even with a simple dipole antenna running as low as 5 watts.

Other Solar and Geomagnetic Indicators:

• 304A – Measures the Sun’s radiation at a wavelength of 304 Angstroms (30.4 nm). This loosely correlates with the Solar Flux Index.
• Proton Flux (PTN Flux) – Measures proton activity in the solar wind, primarily affecting the E-layer.
• Electron Flux (ELC Flux) – Measures electron activity in the solar wind, which also influences the E-layer.
• Aurora Activity (N Number) – Indicates the probability of auroral activity, rated on a scale from 0 to 5, with 0 indicating low probability and 5 indicating high probability.