Two azimuthally separated regions of cusp ion injection observed via energetic neutral atoms

Abstract

[1] The low-energy neutral atom (LENA) imager on the IMAGE spacecraft can detect energetic neutral atoms produced by ion injection into the cusp through a charge exchange with the Earth's hydrogen exosphere. We examined the occurrence of the LENA cusp signal during positive IMF B Z in terms of the arrival direction and the IMF clock angle CA . Results of statistical analyses show that the occurrence frequency is high on the postnoon side when CA is between ∼20°and ∼50°. This is ascribed to ion injection caused by cusp reconnection typical of positive IMF B Z . Our results also show that there is another situation of high occurrence frequency, which can be identified with CA of ∼30°t o ∼80°. When CA is relatively large (60°-80°), occurrence frequencies are high at relatively low latitudes over a wide extent spanning both prenoon and postnoon sectors. This feature suggests that the ion injection is caused by reconnection at the dayside magnetopause. Its postnoon side boundary shifts toward the prenoon as CA decreases. When CA is less than ∼50°, the high occurrence frequency exists well inside the prenoon sector, which is azimuthally separated from the postnoon region ascribed to cusp reconnection. The prenoon region, which is thought due to ion injection caused by dayside reconnection, may explain the recent report that proton aurora brightening occurs in the unanticipated prenoon sector of the northern high-latitude ionosphere for IMF B Y > 0 and B Z > 0.Citation: Abe, M., S. Taguchi, M. R. Collier, and T. E. Moore (2011), Two azimuthally separated regions of cusp ion injection observed via energetic neutral atoms,

Cassini observations of Saturn's southern polar cusp

“…From Figure it can be seen that the cusp and boundary layer are encountered quasiperiodically. Such a morphology is also seen in the terrestrial cusp and has been interpreted as a spatial structure and as the consequence of a strong B y component of the IMF [ Wing et al ., ; Pitout et al ., ; Abe et al ., ] or as a temporal effect when the magnetosphere oscillates causing the location of the cusp to also oscillate [ Zong et al ., ]. In support of the former interpretation for these data at Saturn, the nominal spiral angle of the IMF at Saturn's location is 87° [ Jackman and Arridge , ] producing a nominally large B y component.…”

“…The effects of magnetopause reconnection and its time variability can be identified via latitude‐energy and pitch angle‐energy dispersions [e.g., Reiff et al ., ; Burch et al ., ; Lockwood and Smith , ]. The orientation of the interplanetary magnetic field (IMF) plays a significant role in determining the motion of newly opened flux tubes at the dayside magnetopause [e.g., Cowley and Owen , ; Cooling et al ., ] which contributes to the complex spatial morphology of the cusp, which can be separated into multiple entry regions [e.g., Wing et al ., ; Zong et al ., ; Pitout et al ., ; Abe et al ., ].…”