Multi Satellite Observation of a Foreshock Bubble Causing an Extreme Magnetopause Expansion

The interaction of a solar wind discontinuity with the backstreaming particles of the Earth's ion foreshock can generate hot, tenuous plasma transients such as foreshock bubbles (FB) and hot flow anomalies (HFA). These transients are known to have strong effects on the magnetosphere, distorting the magnetopause (MP), either locally during HFAs or globally during FBs. However, previous studies on the global impact of FBs have not been able to determine whether the response stems directly from the transverse scale size of the phenomenon or its fast motion over the magnetosphere. Here we present the observation of an FB and its impact on the magnetosphere from different spacecraft scattered over the dayside magnetosphere. We are able to constrain the size of the transverse scale of an FB from direct observations to be about 10 RE. We go on to discuss how the magnetosphere responds to this transient, which seems to have a similar scale across the dayside. The solar wind is a fast plasma flow of charged particles originating from the Sun. Earth's magnetic field diverts this flow around the planet forming the magnetosphere. The bow shock forms upstream of Earth to decelerate the solar wind and deflect the flow around Earth's magnetic field. A fraction of the solar wind particles are reflected back into the solar wind stream, forming the foreshock. Discontinuities in the solar wind can concentrate these backstreaming particles forming a bubble of hot and tenuous plasma called a foreshock bubble. These transients structures move with the solar wind, arriving at and modifying the bow shock and leading to an expansion of the magnetosphere due to lower pressure within the transients' core. Such a response was reported before in different studies which conclude that foreshock bubbles have a global impact on the magnetosphere. In our study we report on another foreshock bubble observed by a multi-spacecraft constellation. The observations allowed us to constrain the size of the foreshock bubble. We observe that the large scale response of the magnetosphere seems to reflect with the size of the FB. Multi satellite observation of a large foreshock bubble (FB) hitting Earth's magnetosphere The transverse (y - z) scale size of the FB can be constrained from observations to be at least 8-10 RE fitting with simulations The magnetospheric response to the FB seems to be visible across almost 10 RE of the dayside, matching the FB size