The A Index

A-Index Ranges: Update: 05:00
● Quiet (0-15) | ● Active (16-29) | ● Storm (30+) | ● Extreme (100+)
For HF operators: A-Index measures 24-hour geomagnetic activity. Lower values (0-15) indicate excellent HF propagation conditions with minimal ionospheric disturbance. Higher values suggest increased absorption and signal degradation, especially on higher frequency bands.

A-Index Reference Guide

Geomagnetic Activity Impact on HF Propagation (0–30 MHz)
A-Index Range Interpretation Impact on HF Bands
0–5 Very Quiet Excellent for all bands, including 160m and 80m. Optimal DX conditions with minimal absorption. Perfect for weak signal work and EME.
6–15 Unsettled to Active Slight degradation on higher bands. Mid/low bands (40m, 80m, 160m) still excellent. Good conditions for most HF activities.
16–30 Minor Storm Noticeable degradation on high bands (10m, 15m, 20m). Increased absorption and signal fading. Focus on 40m and below.
31–50 Moderate Storm Poor conditions overall. HF blackouts possible on higher bands. Enhanced auroral activity may benefit VHF/UHF operators.
50–99 Severe Storm Most bands unstable with severe signal degradation. Both NVIS and DX propagation significantly impaired. Consider QRT.
100+ Extreme Storm Ionospheric collapse likely. Expect major HF communication outages. Emergency frequencies may be affected. Rare but devastating events.
Excellent Conditions
Good Conditions
Moderate Disruption
Poor Conditions
Severe Disruption
Extreme Events

The A index

Exact Science? No, not exactly.

The A-index is a daily, averaged measure of geomagnetic activity derived from the more dynamic K-index values. Specifically, it’s a linearized version of 8 three-hour K-index readings taken over a 24-hour period, converted into a scale from 0 to 400—though values above 100 are rare and usually associated with strong geomagnetic storms. It’s derived from the K-index values recorded at a network of magnetic observatories worldwide.

Each 3-hour K-index (which ranges from 0 to 9) is first converted to a quasi-linear number using a fixed table. These 8 values are then averaged to form the A-index. There are actually different “A” values:

  • A-index (daily average, what you usually see reported)

  • Ap-index (planetary, weighted version used globally)

  • Ak-index (local A-index, tied to specific magnetometer stations)

For Ham Radio Operators: The A-index provides a broader view of geomagnetic conditions compared to the 3-hour K-index. Lower A-index values (0-15) generally indicate stable ionospheric conditions favorable for HF propagation, while higher values (30+) suggest increased absorption and signal degradation, particularly affecting long-distance communication paths through the polar regions.

Operational Notes: During moderate to severe geomagnetic storms (A ≥ 30), consider operating on lower frequency bands (40m, 80m, 160m) and using NVIS techniques for regional communication. Aurora may enhance VHF/UHF propagation during active periods.

The A index: The science of Avg.

Let’s unpack the A-index, the smoothed, linearized representation of global geomagnetic activity derived from the 3-hourly K-index values.

Where K is quasi-logarithmic and localized, A is linearized and averaged, making it ideal for assessing sustained geomagnetic conditions over a full 24-hour UTC period.

How It’s Computed

Each K-index value (ranging from 0–9) recorded at a geomagnetic observatory is mapped to a corresponding linear amplitude equivalent via a conversion table. These 8 values—one per 3-hour interval—are then arithmetically averaged to produce:

  • A (local): From a single observatory
  • Ap (planetary): Global average from selected mid-latitude stations
  • Ap real-time: Interpolated from preliminary Kp values

Typical range: 0–100+, though values >50 generally coincide with significant space weather events.

Why It’s Important to Hams

The A-index provides a quantitative retrospective of ionospheric stress over a given day. High A values suggest prolonged periods where the Earth’s magnetosphere has been distorted by enhanced solar wind pressure, southward Bz IMF orientation, or CME-driven shocks.

For HF propagation, especially below 30 MHz, this geomagnetic stress has several notable consequences:

Physical Effects on the Ionosphere

  1. Increased D-layer absorption:
    Sustained energy injection leads to increased ionization in the D-region (60–90 km), especially at mid-latitudes, resulting in higher absorption of lower HF frequencies (e.g., 160m, 80m, 40m). Even weak M-class solar flares layered on top of a high A-index day can black out lower bands completely.
  2. F-layer depletion and instability:
    A disturbed geomagnetic field disrupts thermospheric winds and reduces recombination rates in the F2 layer (~250–400 km), causing a drop in critical frequency (foF2) and thus MUF. Sporadic-E formation may be enhanced at mid-latitudes, but long-haul F-layer paths degrade.
  3. Auroral absorption and polar cap absorption (PCA):
    A prolonged A-index > 30 often correlates with elevated auroral zone absorption due to increased particle precipitation. Polar paths become highly attenuated or unusable, even at 20 MHz and higher.
  4. Ionospheric tilts and multi-path fading:
    High A conditions induce large-scale gradients and tilts in the ionosphere, increasing the likelihood of path distortion, Doppler shifts, and multi-hop interference. You’ll observe increased QSB (fading), phase jitter, and loss of frequency coherence.

 

 

Operational Summary for Hams

  • A-index is your climatological stability indicator. It’s not real-time, but it tells you whether the ionospheric medium was degraded leading into your operating window.
  • Combine it with Kp (for real-time disruption), Bz (for cause), and solar wind speed/density (for intensity) for complete situational awareness.
  • It’s especially critical for predicting recovery times after a geomagnetic storm. A slow return to A < 15 implies persistent ionospheric instability.

For advanced modeling, feed A and K values into tools like VOACAP, PHaRLAP, or PROPLAB to simulate usable frequency ranges.

 

In a Nutshell for the Radio Ham:

Kp tells you what’s happening right now.

A-index tells you how wrecked the ionosphere has been lately.

If you’re planning an evening net or weekend DXpedition, the A-index is your “is it even worth trying?” sanity check.

Quick Ham Guide to A-index Levels:

  • 0–5: Ionospheric Zen Mode. Great day for DX. Everything’s crisp.

  • 6–15: A little turbulence. You’ll feel it on 40m and up.

  • 16–30: Not good. Expect degraded conditions. F-layer gets flaky.

  • 31–50: Storm Level. High absorption, fading, weird skip zones.

  • 50-99: Time to clean the shack or grab a beer. The ionosphere’s closed for business.