Monitoring the high-altitude cusp with the Low Energy Neutral Atom imager: Simultaneous observations from IMAGE and Polar

Abstract

[1] The Low Energy Neutral Atom (LENA) imager on the IMAGE spacecraft in the dayside magnetosphere can detect neutral particles that are emitted in the magnetosheath flow. During a period of dynamic pressure of 4–6 nPa and interplanetary magnetic field (IMF) Bz of −5 to 3 nT on 12 April 2001, LENA on IMAGE at (XGSM, YGSM, ZGSM) ∼ (4 RE, 0 RE, 6 RE) observed significant emission in the direction of the high-latitude magnetosheath. Detailed analyses have revealed that the high-latitude sheath emission consists of two parts: the stable emission at the higher latitudes and the lower-latitude emission that occurs on and off. During the interval of this event, the Polar spacecraft was located at somewhat lower latitudes than IMAGE in similar noon meridian, and the plasma observations with the Thermal Ions Dynamic Experiment showed that the entry of the cusp ions happens in concurrence with the appearance of the lower-latitude LENA emission. This coincidence strongly suggests that the cusp ions flowing earthward charge exchange with the hydrogen exosphere. For the higher-latitude emission, its stability suggests that the source is associated with the structure persistently existing, which is consistent with the recent result showing that the sheath flow in the cusp indentation can create neutral atom emissions. Comparison of the LENA emission and ACE solar wind suggests that the lower-latitude LENA emission occurs during the southward tilting of dawnward IMF, indicating that this emission is associated with the earthward ion flux along the newly reconnected field lines. Hence this unique event for the simultaneous observations strongly suggests that LENA monitors the entry of the ions in the cusp, which is triggered by the southward tilting of the IMF, and that the significant flux of the cusp ion entry occurs equatorward of and separately from the cusp indentation.