Estimation of magnetopause motion from low-energy neutral atom emission

A new method for deriving the position of the dayside equatorial magnetopause directly from the measured intensity of energetic neutral atom (ENA) emissions is presented. This approach makes it possible to track the position of the magnetopause using data observed by the low-energy neutral atom (LENA) imager on board the Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) spacecraft. The model is applied to data recorded during a period of high solar wind dynamic pressure on 13 April 2001. In this interval, significant ENA flux was observed originating from the dayside low-latitude magnetosheath. This ENA flux is primarily the result of enhanced charge exchange between the increased solar wind plasma and exospheric hydrogen neutrals. The temporal variation in the estimated magnetopause position is compared with in situ measurements of magnetopause crossings by the LANL-01A spacecraft in geosynchronous orbit and the results of a recent empirical magnetopause model. It is demonstrated that the subsolar distance of the magnetopause was successfully tracked for a period of more than 1 h. In this particular case example, the dayside magnetopause is closer to the Earth and fluctuates on a shorter timescale than predicted by the previous empirical model based on in situ data. It is also revealed that the subsolar magnetopause can move with speeds of 100-200 km s(-1) in response to marked dynamic pressure changes, and during periods of stable dynamic pressure can fluctuates with speeds of up to 50 km s(-1).