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 BZ in terms of the arrival direction and the IMF clock angle θCA. Results of statistical analyses show that the occurrence frequency is high on the postnoon side when θCA is between ∼20° and ∼50°. This is ascribed to ion injection caused by cusp reconnection typical of positive IMF BZ. Our results also show that there is another situation of high occurrence frequency, which can be identified with θCA of ∼30° to ∼80°. When θCA 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 θCA decreases. When θCA 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 BY > 0 and BZ > 0.