Geomagnetic field now

The state of Earth's magnetic field and magnetosphere at this hourly update (Sat Oct 31, 8:58 PM ET), from live NOAA and Kyoto data. Measured values and the model estimate are kept separate below. Not for operational use: for official alerts see NOAA SWPC.

What each number means

Measured

• Kp is a 0 to 9 index of global geomagnetic disturbance over each 3-hour interval, from ground magnetometers. NOAA's G-scale starts at Kp 5 (G1, minor) and runs to Kp 9 (G5, extreme). NOAA Space Weather Scales.
• Dst (nT) tracks the ring current, a belt of charged particles that weakens the field at the equator during storms. More negative means a stronger storm. Gonzalez et al. (1994) class −30 to −50 nT as weak, −50 to −100 nT as moderate and below −100 nT as intense. Gonzalez et al. (1994), J. Geophys. Res. 99(A4), 5771. Realtime Dst is a provisional quicklook value from the WDC for Geomagnetism, Kyoto.
• Solar wind and IMF Bz are measured upstream at the L1 point, about 1.5 million km sunward, so changes usually reach Earth 30 to 60 minutes later. A sustained southward (negative) Bz lets solar wind energy into the magnetosphere and drives storms and aurora.
• GOES Hp is the north-south field component at geostationary orbit (6.6 Earth radii). On the dayside it is normally well above zero. If it drops to near zero or below on the dayside, the magnetopause may have been pushed inside geostationary orbit, which happens mainly in very disturbed conditions (NOAA SWPC). This page flags any dayside minute with Hp under 10 nT in the last 6 hours.

Estimated (model)

The magnetopause standoff distance r₀, the distance from Earth's centre to the boundary on the Sun–Earth line, is computed from the solar wind dynamic pressure D_p and IMF B_z with the Shue et al. (1998) model:

r₀ = [10.22 + 1.29 tanh(0.184 (B_z + 8.14))] · D_p^−1/6.6   (Earth radii)
flaring α = (0.58 − 0.007 B_z) · [1 + 0.024 ln D_p],   r(θ) = r₀ (2 / (1 + cos θ))^α

D_p (nPa) ≈ 1.6726 × 10⁻⁶ · n · v², with proton density n in cm⁻³ and speed v in km/s. We use protons only and ignore helium, which typically adds a few percent to the pressure. Inputs are 10-minute means of the active L1 spacecraft data. Shue et al. (1998), J. Geophys. Res. 103(A8), 17691. Typical quiet values are around 10 to 11 Earth radii. The model is an empirical average and can be off by an Earth radius or more, especially in extreme conditions.

The picture

The schematic is redrawn at every update. The dayside magnetopause uses the computed r₀ and the model's shape, and the dotted blue ring is geostationary orbit, both at the same scale. The bow shock (drawn with its nose at about 1.3 r₀, a typical ratio), the field lines and the tail are illustrative only. Colours follow the activity level: green quiet, amber unsettled to active, orange G1 to G2, red G3 or higher. The arrow shows the current IMF Bz direction. Original drawing, released CC0.

Related: Aurora tonight · What the Kp index means for seeing the aurora · Geomagnetic Field & Pole Shift claims · Geomagnetic & Pole Shift Books (Both Sides)