Centrifugal acceleration in the magnetotail lobes

H. Nilsson,P. A. Puhl-Quinn,E. Engwall,A. Eriksson,S. Arvelius

doi:10.5194/angeo-28-569-2010

Abstract

Abstract. Combined Cluster EFW and EDI measurements have shown that cold ion outflow in the magnetospheric lobes dominates the hydrogen ion outflow from the Earth's atmosphere. The ions have too low kinetic energy to be measurable with particle instruments, at least for the typical spacecraft potential of a sunlit spacecraft in the tenuous lobe plasmas outside a few RE. The measurement technique yields both density and bulk velocity, which can be combined with magnetic field measurements to estimate the centrifugal acceleration experienced by these particles. We present a quantitative estimate of the centrifugal acceleration, and the velocity change with distance which we would expect due to centrifugal acceleration. It is found that the centrifugal acceleration is on average outward with an average value of about of 5 m s−2. This is small, but acting during long transport times and over long distances the cumulative effect is significant, while still consistent with the relatively low velocities estimated using the combination of EFW and EDI data. The centrifugal acceleration should accelerate any oxygen ions in the lobes to energies observable by particle spectrometers. The data set also put constraints on the effectiveness of any other acceleration mechanisms acting in the lobes, where the total velocity increase between 5 and 19 RE geocentric distance is less than 5 km s−1.

Highlights

Solar wind interaction with a planetary atmosphere involves exchange of both mass and energy
We present a quantitative estimate of the centrifugal acceleration, and the velocity change with distance which we would expect due to centrifugal acceleration
In the ionospheric projection of the cusp the solar wind origin precipitation leads to ionospheric upflow which can be directly observed by incoherent scatter radar (Nilsson et al, 1996; Ogawa et al, 2003)

Summary

Introduction

Solar wind interaction with a planetary atmosphere involves exchange of both mass and energy. Whereas the magnetic field of Earth can be said to protect the low latitude atmosphere from the solar wind, the interaction is rather amplified in the cusp and polar cap region This is most clearly evident in the ionospheric projection of the magnetospheric cusp, the region of most direct entry of solar wind mass and energy into the magnetosphere. In the ionospheric projection of the cusp the solar wind origin precipitation leads to ionospheric upflow which can be directly observed by incoherent scatter radar (Nilsson et al, 1996; Ogawa et al, 2003) This upflow is still gravitationally bound and further heating is necessary in order to overcome gravity. An outflow of cold light ions (i.e. mainly protons) is expected on the open field lines of the polar cap.

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