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The Kp index — when a solar storm reaches your compass

A number between 0 and 9, updated every three hours, describing how hard the Earth's magnetic field is being shaken. Most days it is irrelevant. On the days it is not, it is the reason your aircraft cannot agree with itself about which way it is facing.

#Kp index #space weather #compass

What Kp actually measures

The sun continuously throws charged particles at us. When a burst of them hits the Earth's magnetic field, the field wobbles. Ground magnetometers around the world measure that wobble, and the results are condensed into a single planetary figure: the Kp index, from 0 (still) to 9 (severe), published every three hours.

Two things about that definition matter to a pilot. It is planetary, so it is an average and your location may be quieter or noisier than the number suggests. And it is magnetic, not atmospheric — it says nothing about wind, cloud or rain, which is exactly why it gets ignored.

Why it reaches your drone at all

Your aircraft carries a magnetometer — a compass — and it uses it to know which way it is pointing. That reference is the link between what the aircraft measures and the world you see. Disturb the field and you disturb the reference.

HeadingDegrades first. The compass is reading a field that is genuinely moving, so the aircraft's idea of north drifts.
PositionDegrades later and less. Satellite signals pass through a disturbed ionosphere, which adds error and, in strong events, reduces the number of usable satellites.
Compass calibrationBecomes unreliable. Calibrating during a disturbance teaches the aircraft a reference that will not be true an hour later.
Symptoms you seeYaw drift, circling ("toilet bowl") behaviour, compass warnings on the ground, a home point that will not settle.
This is the ordering that catches people out: heading goes before position. A pilot watching only the satellite count can be well into a geomagnetic problem before the count moves at all.

The thresholds that matter in practice

Kp 0–3Quiet to unsettled. No practical effect. Fly normally.
Kp 4Active. Usually still fine. Worth noticing if you are flying far, over water, or somewhere a heading error would be expensive.
Kp 5Minor storm. This is where pilots start reporting compass warnings and yaw oddities. Keep flights closer and treat calibration with suspicion.
Kp 6–7Moderate to strong. Expect real degradation, especially at high latitudes. Long-range and beyond-visual work should wait.
Kp 8–9Severe. Rare. Position and heading can both be unreliable. This is a day to charge batteries and do something else.

Latitude changes everything about that table. The same Kp value is far more disruptive close to the poles than near the equator, because the disturbance concentrates at high magnetic latitudes. A Kp 5 that is a non-event in southern Turkey can be a genuine problem in northern Norway.

What to change on an active day

  1. Do not calibrate the compassThis is the counter-intuitive one. A calibration performed during a disturbance stores a bad reference. If the aircraft is not demanding calibration, leave it alone.
  2. Take off away from metalOn any day this matters; on an active day the aircraft has less margin for a second source of error.
  3. Wait longer on the groundGive the fix and the heading time to settle, and watch whether they stay settled rather than just arriving.
  4. Keep it close and in sightHeading errors are recoverable when you can see the aircraft and fly it visually. They are not when it is a dot.
  5. Distrust automated returns more than usualA return depends on heading and position, which are exactly the two things being degraded.
  6. Postpone the ambitious flightLong-range, over-water and mapping missions are the ones that need a trustworthy heading for their whole duration.

Recognising it in the air

Slow circlingThe classic sign. The aircraft flies a wide arc instead of a straight line because its heading reference and its motion disagree.
Yaw that drifts on its ownYou are not asking for rotation and it rotates anyway, slowly and consistently.
Direction commands feel rotatedForward is not quite forward. The offset is often constant, which makes it flyable once you notice it.
Repeated compass warningsOne warning is a local metal object. Warnings that follow you across the field are something larger.
A home point that will not settleIf the recorded position keeps moving before take-off, the aircraft is not confident and neither should you be.

The recovery is the same as for any heading problem: stop travelling away, keep the aircraft in sight, fly it visually toward yourself, and land. Do not spend the flight trying to re-calibrate your way out of a planetary event.

What Kp does not tell you

Treat Kp the way you treat any other single number in aviation: as one input among several, useful for deciding whether today is the day for the ambitious flight, and not useful as a go/no-go switch on its own.

Where Fly4cast fits

Kp sits on the same screen as wind at altitude, airspace and interference reports, because on the ground they are one decision, not four. You do not need to remember to check a space-weather site — you need the number to be in front of you at the moment you are deciding whether to drive out.

See today's Kp and conditions

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Guidance only. You are the pilot in command — always confirm conditions and local rules before flying.