The A-index: A Daily Summary of Geomagnetic Activity for Aurora Planners

Aurora forecasting involves a handful of indices that measure geomagnetic activity at different time scales. The Kp index updates every three hours. The A-index takes a different approach — summarizing an entire 24-hour period into a single daily number. It's less useful for real-time decision-making than Kp, but it serves a distinct purpose in longer-range aurora planning and historical analysis.

What the A-index Is

The A-index is a daily geomagnetic activity index derived from the eight three-hour K-index readings that cover a full 24-hour period. Each K-index reading is converted to an equivalent linear value, and those eight values are averaged to produce the daily A-index. Because the K-index scale is logarithmic and the A-index conversion linearizes it, the A-index is more sensitive to high-activity periods within a day than a simple average of K values would be.

What helped me picture the relationship between the indices: if Kp is like checking the temperature every three hours, the A-index is the daily high — a single number that captures how active the day was overall, weighted toward its most energetic moments. It doesn't tell you when during the day the activity peaked, but it tells you whether the day as a whole was quiet, moderate, or disturbed.

When the A-index Is Useful for Aurora Travelers

The A-index is most useful in two contexts. The first is historical review — looking back at past activity levels to understand patterns, identify active periods, and assess how a given destination performs over time. Aurora photographers building a database of productive nights often track A-index values alongside their shooting logs to correlate activity levels with image results.

The second is longer-range planning. NOAA's 27-day geomagnetic outlook includes projected A-index values alongside Kp forecasts, giving travelers a rough sense of which days within a planning window are expected to be more or less active. This is particularly useful when combined with knowledge of recurring coronal hole passages, which tend to produce elevated A-index values on a roughly 27-day cycle. For more on how solar activity patterns affect planning, see our overview of solar cycles and the northern lights.

For real-time decision-making on any given night, the A-index is less relevant than Kp or live magnetometer data. By the time the daily A-index is calculated, the night is over.

What the A-index Means for Photographers

For aurora photographers, the A-index is most valuable as a retrospective tool and a planning reference. After a productive shooting session, checking the A-index for that day gives a standardized activity level to attach to the night — useful for comparing sessions across different trips and different solar conditions.

In planning mode, an A-index forecast of 20 or above for a given day suggests moderate to active conditions are expected. Values above 50 indicate significant storm activity is projected. These thresholds aren't guarantees, but they give a rough calibration for how active a forecasted period is expected to be relative to a quiet baseline.

The Ap index — a planetary version of the A-index derived from the same global magnetometer network as Kp — is the variant most commonly cited in space weather reports and aurora planning resources. The two are closely related; Ap is the global average, while the local A-index reflects conditions at a specific station. Both are available through NOAA and most aurora tracking platforms.

Return to the full Northern Lights Glossary to continue through the Geomagnetic Indices and Measurements section.