17:15 〜 18:45
[PEM13-P18] Variation of molecular ions in the inner magnetosphere observed by the Arase satellite
キーワード：あらせ衛星、分子イオン、内部磁気圏
In the Earth’s magnetosphere, various ion species originate from both the solar wind and the
ionosphere. Molecular ions in the magnetosphere are originated in the Earth’s ionosphere.
The Arase satellite has observed various ion species since 2017 to the present, using two ion
analyzers, LEPi and MEPi, which together cover the energy range from 10 eV/q to 180 keV/q.
Using the data from the MEPi instrument, a previous study has investigated variations of
molecular ions in response to magnetic storms and solar wind conditions, and molecular ions
have been observed in the inner magnetosphere even during small magnetic disturbances
[Seki et al., 2019]. However, observations of molecular ions are still relatively limited
compared to other ion observations, and the mechanism of the outflow from the ionosphere
and the long-term variations are not well known. In this study, we analyzed the time-of-flight
(TOF) data from LEPi [Asamura et al., 2018] onboard Arase to investigate variations of
molecular ions in the inner magnetosphere. LEPi covers the energy range from 10 eV/q to 25
keV/q and obtains counts as a function of energy and TOF. The TOF measurements of LEPi
have been operated in the outbound passes every four revolutions around the Earth. We
derived counts of the molecular ions by subtracting the background contamination of oxygen
counts. We also evaluated time variations of the efficiency of LEPi in this analysis. We
investigated relationships between molecular ion counts and geomagnetic index as well as
solar wind parameters. The estimated molecular ion counts exhibited good correlation with
the solar wind dynamic pressure, the SYM-H index and the SML index. Long-term variations
of molecular ions were different from that of oxygen ions. Additionally, we propose a model
to discuss the importance of the solar wind dynamic pressure in causing the escape of
molecular ions into the magnetosphere through an increase in the convection electric field.
This mechanism results in distinct evolutions of oxygen ions and molecular ions.
ionosphere. Molecular ions in the magnetosphere are originated in the Earth’s ionosphere.
The Arase satellite has observed various ion species since 2017 to the present, using two ion
analyzers, LEPi and MEPi, which together cover the energy range from 10 eV/q to 180 keV/q.
Using the data from the MEPi instrument, a previous study has investigated variations of
molecular ions in response to magnetic storms and solar wind conditions, and molecular ions
have been observed in the inner magnetosphere even during small magnetic disturbances
[Seki et al., 2019]. However, observations of molecular ions are still relatively limited
compared to other ion observations, and the mechanism of the outflow from the ionosphere
and the long-term variations are not well known. In this study, we analyzed the time-of-flight
(TOF) data from LEPi [Asamura et al., 2018] onboard Arase to investigate variations of
molecular ions in the inner magnetosphere. LEPi covers the energy range from 10 eV/q to 25
keV/q and obtains counts as a function of energy and TOF. The TOF measurements of LEPi
have been operated in the outbound passes every four revolutions around the Earth. We
derived counts of the molecular ions by subtracting the background contamination of oxygen
counts. We also evaluated time variations of the efficiency of LEPi in this analysis. We
investigated relationships between molecular ion counts and geomagnetic index as well as
solar wind parameters. The estimated molecular ion counts exhibited good correlation with
the solar wind dynamic pressure, the SYM-H index and the SML index. Long-term variations
of molecular ions were different from that of oxygen ions. Additionally, we propose a model
to discuss the importance of the solar wind dynamic pressure in causing the escape of
molecular ions into the magnetosphere through an increase in the convection electric field.
This mechanism results in distinct evolutions of oxygen ions and molecular ions.