Satoshi Taguchi

Overwhelming O+ contribution to the plasma sheet energy density during the October 2003 superstorm: Geotail/EPIC and IMAGE/LENA observations

[1]xa0We studied dynamics of O+ ions during the superstorm that occurred on 29–31 October 2003, using energetic (9–210 keV/e) ion flux data obtained by the energetic particle and ion composition (EPIC) instrument on board the Geotail satellite and neutral atom data in the energy range of 10 eV to a few keV acquired by the low-energy neutral atom (LENA) imager on board the Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) satellite. Since the low-energy neutral atoms are created from the outflowing ionospheric ions by the charge exchange process, we could examine variations of ionospheric ion outflow with the IMAGE/LENA data. In the near-Earth plasma sheet of XGSM ∼ −6 RE to −8.5 RE, we found that the H+ energy density showed no distinctive differences between the superstorm and quiet intervals (1–10 keV cm−3), while the O+ energy density increased from 0.05–3 keV cm−3 during the quiet intervals to ∼100 keV cm−3 during the superstorm. The O+/H+ energy density ratio reached 10–20 near the storm maximum, which is the largest ratio in the near-Earth plasma sheet ever observed by Geotail, indicating more than 90% of O+ in the total energy density. We argued that such extreme increase of the O+/H+ energy density ratio during the October 2003 superstorm was due to mass-dependent acceleration of ions by storm-time substorms as well as an additional supply of O+ ions from the ionosphere to the plasma sheet. We compared the ion composition between the ring current and the near-Earth plasma sheet reported by previous studies and found that they are rather similar. On the basis of the similarity, we estimated that the ring current had the O+/H+ energy density ratio as large as 10–20 for the October 2003 superstorm.

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

[1]xa0The 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.

Neutral atom emission in the direction of the high‐latitude magnetopause for northward IMF: Simultaneous observations from IMAGE spacecraft and SuperDARN radar

[1]xa0During a northward interplanetary magnetic field on 27 March 2001, the Low Energy Neutral Atom (LENA) imager on the Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) spacecraft in the magnetosphere observed an enhanced emission in the direction of the very high-latitude magnetopause. Simultaneous observations from IMAGE/LENA and SuperDARN radar show that the LENA emission appears concurrently with the enhancement of the sunward flow of the reverse convection in the ionosphere. The field line mapping from the magnetosphere to the ionosphere suggests that the source ions for the LENA emission are in the sunward flow region. Although the direction of the emission is relatively stable, its direction changes slightly so that the emission may shift poleward or equatorward. From these observations, we suggest that LENA can monitor the ion entry caused by cusp reconnection and that the reconnection site moves on a timescale of several minutes.