Prediction of the effects of coronal mass ejections ( CMEs ) on Earth strongly depends on knowledge of the interplanetary magnetic field southward component , B _ { z } .
Predicting the strength and duration of B _ { z } inside a CME with sufficient accuracy is currently impossible , which forms the so-called B _ { z } problem .
Here , we provide a proof-of-concept of a new method for predicting the CME arrival time , speed , B _ { z } and the resulting Dst index at Earth based only on magnetic field data , measured in situ in the inner heliosphere ( < 1 AU ) .
On 2012 June 12–16 , three approximately Earthward-directed and interacting CMEs were observed the by the STEREO imagers , and by Venus Express ( VEX ) in situ at 0.72 AU , 6 degree away from the Sun Earth line .
The CME kinematics are calculated using the drag–based and WSA–Enlil models , constrained by the arrival time at VEX , resulting in the CME arrival time and speed at Earth .
The CME magnetic field strength is scaled with a power law from VEX to Wind .
Our investigation shows promising results for the Dst forecast ( predicted : -96 and -114 nT ( from 2 Dst models ) , observed : - 71 nT ) , for the arrival speed ( predicted : 531 \pm 23 km s ^ { -1 } , observed : 488 \pm 30 km s ^ { -1 } ) and timing ( 6 \pm 1 hours after actual arrival time ) .
The prediction lead time is 21 hours .
The method may be applied to vector magnetic field data from a spacecraft at an artificial Lagrange point between the Sun and Earth , or to data taken by any spacecraft temporarily crossing the Sun–Earth line .