Earth-directed CME: what it really means for aurora
Calling a CME Earth-directed means estimating that at least part of its structure propagates toward the Sun–Earth line. It does not imply a central hit or reveal its internal Bz in advance.
What it really means
For aurora, what matters is the transfer of energy from the solar wind into the magnetosphere. No single number can summarise the whole process: some measurements describe what is arriving, while others measure Earth’s response already under way. The link with aurora is also not linear: rigid thresholds work poorly because the magnetosphere retains a memory of prior conditions and responds dynamically.
Why it matters
A responsible forecast also separates probability from observation. A model may indicate favourable conditions, but real-time measurements and local cloud cover remain decisive for someone actually standing outside. In practice it should be compared with the other available parameters and with how they evolve over time. One isolated value can be interesting; a coherent sequence of measurements is far more informative.
How to interpret it without oversimplifying
Space weather describes how solar activity propagates through space and interacts with Earth’s environment. Earth-directed CME is useful only in context: speed, magnetic field, density and geomagnetic response do not always evolve at the same time.