Abstract
Abstract. Oxygen ion outflow associated with the cusp and cleft give rise to persistent oxygen ion beams which can be observed over the polar cap. For high altitude spacecraft such as Cluster these beams are often observed for several hours on each occasion. This allows for a study of typical temporal structures on the time scale of minutes. We have used 3 years of data from spring, January to May of years 2001 to 2003, for a study of the oxygen number flux variation in the polar cap ion outflow. The source of these oxygen ion beams is the cusp and cleft, and variations in ionospheric upflow on time scales of around 8 min have been reported from ground based studies using incoherent scatter radar. Such upflows typically do not reach escape velocity, and further energization above the ionosphere is required for outflow to occur. Our study shows that a typical time scale between sudden number flux enhancements observed by Cluster in a geocentric distance range of 5 RE to 12 RE is 5 to 10 min. A superposed epoch study does not reveal any significant convection velocity or temperature changes around the flux enhancement events. Sudden temperature enhancements occur with a typical time interval of about 4 min, A superposed epoch study does not reveal any number flux enhancements associated with the temperature enhancements. The clear modulation of the high altitude number flux in a manner which resembles the modulation of the ionospheric upflow indicates that this is the main limiting factor determining the total outflow. The process behind transient upflow events in the ionosphere is therefore important for the total ionospheric outflow. Subsequent heating above the ionosphere appears to be common enough in the cusp/cleft region that it does not significantly modulate the oxygen ion number flux.
Highlights
Heavy ion escape from the Earth’s ionosphere appears to take place in a two step process
This is very similar to what has been reported for cusp transient events and upflow associated with cusp transient events (Moen et al, 2004)
It is worth to point out that whereas many indications on how dynamic the cusp is have come from ground based observations (e.g. Lockwood et al, 1993; Moen et al, 2004), when we study a parameter which is strongly coupled to the ionosphere, the oxygen ion number flux, we do see signs of the same dynamic phenomena in the high altitude spacecraft data
Summary
Heavy ion escape from the Earth’s ionosphere appears to take place in a two step process. First an upflow from the ionosphere occurs. The flows are typically of the order of 100 m/s to 1000 m/s, and the ion velocity does not reach escape velocity. In order to escape the ions must experience further energization at higher altitude. It is not obvious if the ionospheric upflow or the subsequent energization is the most important process in determining the total number flux of outflowing heavy ions. The upflows are often associated with energy input into the ionosphere through enhanced magnetospheric electric fields and particle precipitation. Changing high altitude boundary conditions, i.e. the pressure profile of the plasma above the ionosphere, may play an important role. Changing high altitude boundary conditions, i.e. the pressure profile of the plasma above the ionosphere, may play an important role. Yau and Andre (1997) provides a recent review of the mechanisms involved in ion outflow
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