40 MHz (8 m) TEP propagation from ZS6WAB – Part 2

 

40MHz (8m) TEP propagation from ZS6WAB – Part 2

Solar drivers, event behaviour and what the data can (and cannot) prove

Introduction

In Part 1, the ZS6WAB 40.675 MHz CW beacon dataset demonstrated that transequatorial propagation (TEP) into Europe is:

  • Repeatable
  • Strongly seasonal
  • More prominent during the stronger years of Solar Cycle 25

The next question is straightforward:

What governs individual TEP openings?

This article examines how the observed events relate to solar and geomagnetic conditions, while remaining grounded in what the dataset can realistically support.

A note on methodology

Before looking at the data, several limitations must be recognised:

  • CW spotting is not continuous
  • Event start/end times are approximate
  • Mode labels are operator assigned
  • No direct ionospheric measurements are included

This means that the dataset is well suited to identifying patterns and correlations, but not sufficient on its own to establish causation.

Solar Cycle 25 context

The strongest TEP activity in the dataset occurs during:

  • 2024
  • 2025
  • Early 2026

This coincides with the recognised peak phase of Solar Cycle 25.

From the dataset:

  • TEP spot counts increase significantly from 2022 → 2025
  • Average spots per active day also increases

Interpretation

This supports the view that:

  • Increased solar activity enhances the probability of 40MHz TEP

However:

  • Not all high solar activity days produce openings
  • Some openings occur under only moderate solar conditions

So, Solar activity appears to be an important contributing factor, but not the sole determinant of TEP events.

Event behaviour

1. Apparent sharp onset

TEP events often appear suddenly in the logs:

  • Few or no earlier reports
  • Rapid increase in spots within a short period

However this apparent sharp onset may partly reflect the intermittent nature of human spotting rather than purely propagation physics.

2. Moderate duration windows

From the dataset:

  • Most observed events last ~1–2 hours
  • Some extend to several hours

Interpretation

  • The supporting ionospheric structure is transient
  • Observed durations are likely underestimates

3. Daylight alignment

Most TEP events occur:

  • Late morning to early afternoon UTC

Interpretation

  • Consistent with a daytime ionisation-dependent mechanism
  • No evidence for dominant nocturnal behaviour in this dataset

Solar indices and event correlation

F10.7 solar flux

  • Proxy for solar EUV output and ionisation potential

Dataset interpretation:

  • Strong TEP seasons coincide with elevated F10.7
  • Individual events do not map cleanly to daily peaks. F10.7 appears to set the background ionospheric capability rather than control event timing.

Kp index (geomagnetic activity)

  • Indicator of geomagnetic disturbance

From the dataset:

  • No clear evidence that high Kp drives TEP
  • Many events are consistent with quiet or mildly disturbed conditions. The dataset is consistent with TEP favouring relatively stable geomagnetic conditions, but does not demonstrate a strict dependence.

Solar events (flares / CMEs)

  • Can affect ionisation and geomagnetic stability

Dataset limitation:

  • No time-aligned solar event data included therefore, no direct association between individual TEP events and solar flares can be established from this dataset.

A working interpretation model

A layered interpretation consistent with the data:

  1. Solar cycle phase
    Controls overall likelihood of 40MHz support
  2. Season (equinox periods)
    Strongly favours TEP
  3. Daily ionospheric state
    Determines whether an opening occurs
  4. Short-term dynamics
    Governs onset, duration and strength

Is Sporadic-E involved?

The dataset shows:

  • Very few ES-labelled spots
  • Some mixed TEP/F2 days but there is insufficient evidence in this dataset to confirm Sporadic-E involvement.

Representative event behaviour

Typical TEP day characteristics:

  • Sudden appearance late morning UTC
  • Rapid spread across multiple European regions
  • Peak within ~1–2 hours
  • Decline or abrupt disappearance

This suggests:

  • Once established, the path is geographically broad
  • But temporally unstable

What would improve understanding

1. WSPR beacon deployment

Would provide:

  • Continuous monitoring, 24/7/365
  • Accurate timing
  • Detection of weak propagation

2. Solar data overlays

Including:

  • F10.7
  • Kp
  • Solar wind / X-ray

Would allow:

  • Better correlation analysis
  • Identification of triggering conditions

3. Ionosonde data (foF2 / foEs)

Would provide:

  • Direct measurement of ionospheric state
  • Stronger physical interpretation

Final conclusions

From this dataset:

  • TEP at 40MHz is influenced by:
    • Solar cycle phase
    • Seasonal effects
  • Individual events:
    • Are short-lived
    • Occur in daylight
    • Appear rapidly
  • Solar activity:
    • Increases likelihood
    • Does not uniquely determine events
  • Mixed modes:
    • May occur
    • Cannot be resolved in detail

This article reflects a combination of traditional amateur radio experimentation and modern AI assisted analysis, where original data and direction are provided by the author and advanced tools enable deeper exploration and presentation of the results. 

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