Resolving uncertainty in the solar wind driving of the magnetosphere

Magnetospheric dynamics at Earth are primarily driven by coupling between the magnetised solar wind and the magnetospheric magnetic field and plasma. This coupling is highly variable and strongly controlled by the relative orientation of the interplanetary and magnetospheric magnetic fields. Our understanding of this cause-and-effect relationship has advanced significantly since the dawn of the space age due to the availability of in situ measurements of the interplanetary environment.

Earth is the only planet in the solar system at which measurements of upstream solar wind/IMF conditions are routinely available. These are enabled by spacecraft orbiting the L1 Lagrange point, the gravitational equilibrium position located on the Sun-Earth line, approximately 1.5 million kilometres upstream of the Earth. The spacecraft measure the magnetic field and plasma within the solar wind as it streams towards the Earth.

Based on the solar flow speed, it is possible to estimate the conditions arriving at the magnetosphere and the time at which they will arrive there (usually about 1 hour after observation at L1). By comparing these in situ measurements of the solar wind and indices of magnetospheric activity derived from measurements at Earth, the relationship between the drivers and responses of the coupled solar wind/magnetosphere system can be explored.

Many studies have revealed that the magnetosphere has a non-linear response to solar wind drivers. During periods of enhanced solar wind driving, magnetospheric activity ceases to increase in line with increasing driving conditions, an effect known as "saturation". Several physical mechanisms for this effect have been proposed, but no consensus has been reached over the cause.

Recently, it has also been suggested that the saturation effect is not real, but an artefact of uncertainties in the propagation of upstream measurements from the L1 position to the Earth. It is also noted that the magnetospheric response to solar wind drivers observed at L1 can be highly variable. Sometimes a given set of driving conditions results in significantly higher levels of magnetospheric activity than at other times.

The reasons for this are not wholly understood, but some of the variability is likely to be caused by uncertainties in the propagation of upstream measurements from L1 to the Earth leading the wrong solar wind driving conditions to be associated with the observed magnetospheric response.

In this project, we shall circumvent these uncertainties by exploiting measurements nearer to the magnetosphere, made by the ESA Cluster spacecraft, to confirm or discount the non-linear response of the system to interplanetary drivers. The 20+ year Cluster dataset is the product of decades of UKRI/UKSA investment and represents an invaluable scientific resource.

Cluster measurements in the solar wind and magnetosheath (within 20 R_E of the Earth) will enable us to examine driver/response relationships in more detail and with less uncertainly than has previously been possible. The available distribution of magnetosheath and solar wind measurements from Cluster will also allow us to segregate our analyses to assess the significance of the modification of upstream drivers as they transit the bowshock that stands upstream of the magnetosphere on the correlation with the magnetospheric response.

We will also resample (average) the spin-averaged Cluster data over timescales (e.g. 1 min to 1 hour) to study the impact of the temporal variability of the driver on the apparent variability of the response. Different correlations are likely at different averaging timescales because some magnetospheric activity indices capture responses which include dynamics in the magnetotail.

Correlations with such indices, which respond several tens of minutes after the solar wind input changes, could be especially sensitive to the assignment of an incorrect driver due to uncertainties in the upstream conditions.