How it works

HUXt

HUXt is the solar-wind model behind the daily movie and the Kp bars. Each longitude and each latitude is its own ray. The rays do not push on each other. The flow is the time-dependent Burgers model of Owens et al. (2020), with a small magnetic correction added on top.

It is a stand-in for a full magnetohydrodynamic heliosphere, not a replacement for NASA’s WSA-ENLIL. A cone that leaves the Sun in the detector is dropped into this wind at 30 solar radii, and the movie shows whether that faster patch reaches Earth.

What the daily run contains

1. 1
   A HelioMAS wind at 30 solar radii The inner boundary is a Predictive Science HelioMAS map for the current Carrington rotation: radial speed and radial magnetic field, from 30° south to 30° north in steps of 5°. Outside that band the model has nothing to say.
2. 2
   Independent rays out to 240 solar radii 128 longitudes, a radial step of 1.5 solar radii. Each ray is stepped with an upwind scheme. A residual acceleration (strength 0.15, scale 50 solar radii) speeds the ambient wind as it leaves the boundary. That term is not a drag on a CME.
3. 3
   A CME is a cone of faster wind The cone’s direction, width, and speed come from the detector. It is inserted as a speed pulse at 30 solar radii. A fast cone slows because it runs into slower wind ahead. There is no separate drag coefficient.
4. 4
   Earth is longitude zero at 1 AU Arrival is the moment the front on the Earth ray crosses one astronomical unit. The homepage movie is that solution in three views: the ecliptic, the Earth-facing side, and a cross section at Earth.

The magnetic correction

The Burgers update itself is unchanged. Two radial forces are added, and both are capped at 5% of v²/r so they cannot dominate the flow. One is the radial Lorentz force of the ambient Parker spiral. The other is the magnetic-pressure jump at the edge of a flux rope: the nose is sped up and the tail is slowed. A force-free rope has no body force in its interior. This is not a transverse j×B, and it is not an MHD model.

While a cone is over Earth, the field drawn in the Kp tooltip is a Lundquist flux rope with cycle-25 Hale handedness and tilt. The axial scale is 18 nT at 400 km/s, and it is assumed. The coronagraph does not measure the rope. The most southward Bz in a 3-hour slot is that sample. It is not what sets Kp.

How speed becomes Kp

Speed Kp

(V − 330) / 80, rounded and held between 0 and 9. V is the HUXt speed at Earth in that 3-hour slot, ambient wind plus any cone that is overhead. Hatched bars on the chart are this number.

Glancing

One Kp less when the cone’s longitude is more than 0.65 times its half-angle. The flank is over Earth; the nose is not.

NASA Kp90

Solid bars use the Kp90 from a matching WSA-ENLIL run when one arrives in that slot. Speed Kp stays on the chart as the hatched alternative.

Where the cone comes from

The detector measures a plane-of-sky speed between about 2 and 6 solar radii. For a halo that number is the launch speed: the tracked edge is a flank, already close to the radial speed. For a narrower arc the model divides by the sine of the angle off the Sun–Earth line, and will not divide by less than sin 15°. A narrow arc is also placed on the solar limb, at ±90° longitude, so it never crosses Earth. A real Earth-directed storm that the cameras only show as a thin flank is missed for that reason. An arc of about 90° or wider at L1 is aimed near the Sun–Earth line instead, offset by at most 25°.

STEREO-A, about 70° ahead, can confirm a three-dimensional direction. An Earth detection is still run when Ahead does not see it.

Papers

1. Owens et al. 2020

   HUXt: a one-dimensional, time-dependent Burgers model of the solar wind, used as a fast surrogate for three-dimensional MHD. The daily run follows this hydrodynamic core. Solar Phys. 295, 43

2. Barnard & Owens 2022

   The open Python HUXt package: time-dependent boundaries and the same reduced-physics flow. Our daily movie is our own solver of that model, with the magnetic cap described above. Front. Phys. 10, 1005621

3. Predictive Science — HelioMAS

   The Carrington-rotation maps of radial speed and radial field that set the inner boundary. predsci.com

Limits. This is not a forecast you should treat as ENLIL. The domain is only ±30° of latitude. A limb-mapped cone misses Earth even when the storm is coming here. A plane-of-sky speed that is too low makes the arrival late, because the cone is still slow when it is inserted at 30 solar radii. The flux rope’s handedness, tilt, and strength are assumed. Cross-check an arrival against the WSA-ENLIL panel on the CME overview.