Response of neutral atom emissions in the low‐latitude and high‐latitude magnetosheath direction to the magnetopause motion under extreme solar wind conditions

Abstract

On 11 April 2001 the high velocity and density of the solar wind and the strong southward interplanetary magnetic field moved the dayside magnetopause inside of geosynchronous orbit. The Low Energy Neutral Atom (LENA) imager on the Imager for Magnetopause‐to‐Aurora Global Exploration (IMAGE) spacecraft in the magnetosphere observed significant emission in the magnetosheath direction. The total neutral atom flux from the dayside region, ignoring the neutral solar wind flux directly from the Sun, shows a threefold enhancement, and each of the three increases is coincident with the occurrence of the magnetopause inside 6.6 RE. Observations by LENA also show that emission in the direction of the low‐latitude and high‐latitude magnetosheath is modulated in such a manner that the sources shift earthward/sunward and equatorward/poleward in the low‐latitude and high‐latitude sheath, respectively. A model based on the distributions of the sheath flux and of the number density of the hydrogen exosphere explains these characteristics as a result of the motion of the magnetopause having an indentation at the cusp, suggesting a means for monitoring the cusp motion using IMAGE/LENA.

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

“…When the spacecraft is situated as is shown in Figure 2, the spacecraft would observe clearly neutral hydrogen emitted in the direction of the arrow shown as Emission A. A simple model by Taguchi et al [2004, Figure 5] shows that this kind of emission can be observed as a distinct peak in the distribution of the hydrogen count.…”

“…Emission B can be parallel to the Sun‐Earth line, and would overlap the Sun signal when the spacecraft is located downstream of Emission B. Emission C can also overlap stronger emission when the spacecraft is located downstream of Emission C. The stronger emission may come from the magnetosheath near the equatorial plane [ Collier et al , 2005; Taguchi et al , 2004]. Hence a spacecraft residing at a somewhat smaller Z than the possible location of the indentation would be suitable for monitoring the cusp.…”

“…In Figure 3a, two arrows from the starting location on the IMAGE orbit indicate the approximate projection of a 120° field of view of LENA to the Y GSM = 0 meridian. The upward arrow indicates the upper boundary of the 120°, which was decided as an 8° bin whose line of sight (LOS) makes the minimum angle from the z axis [ Taguchi et al , 2004]. The other arrow represents the lower boundary, which corresponds to the 8° bin that covers from 97° to 105°.…”

Imaging Plasma Density Structures in the Soft X-Rays Generated by Solar Wind Charge Exchange with Neutrals

“…Furthermore, the energies, composition, flux, and direction of the ENAs arriving at the observing location provide important information concerning the processes occurring at remote magnetospheric locations (Taguchi et al 2004;Collier et al 2005a;Hosokawa et al 2008).…”

Low‐energy neutral atom signatures of magnetopause motion in response to southward Bz

“… Taguchi et al [2004a] have shown for an event on 11 April 2001 that an increase in solar wind ram pressure and a strong southward IMF moved the magnetopause inward, resulting in enhanced charge exchange with the Earth's hydrogen exosphere and increasing the flux of the postshocked neutral solar wind by a factor of three. They suggest that low‐energy neutral atoms may be a means of monitoring cusp motion.…”