Recent studies have shown that the Low Energy Neutral Atom (LENA) imager on the IMAGE spacecraft in the magnetosphere detects neutral atoms coming from the direction of the high-altitude cusp. The primary source of the neutral atoms has been reported to be the entry of the ions in the cusp, but the evidence for this remains limited. Therefore we herein report the correlative variations of the neutral atom signal in the direction of the high-altitude cusp and the fast anti-sunward convection in the ionospheric cusp from an event of simultaneous observations from IMAGE/LENA and SuperDARN radar. During a period of IMF B _Z of −40 to −10 nT on 11 April 2001, LENA on IMAGE at (X _GSM, Y _GSM, Z _GSM) ∼ (4 R _E, 0 R _E, 4 R _E) observed significant enhancements of neutral atom signals in the direction of the high-altitude cusp. In concurrence with these enhancements, SuperDARN radars observed an increase in anti-sunward flow accompanied by equatorward shift of the polar cap boundary. Detailed analyses of the SuperDARN velocity data reveal that the fast anti-sunward flow region widens and shrinks longitudinally in correlation with the increase and decrease of the LENA signal, respectively. This correlation shows that the neutral atom emission in the direction of the high-altitude cusp for southward IMF is due to the ion entry caused by enhanced reconnection on the dayside magnetopause. Hence the present study provides evidence of the cusp ion injection being the primary source of the LENA high-altitude cusp signal for the southward IMF.