5:15 PM - 6:30 PM
[PPS05-P05] Whistler-mode waves with ‘chirping’ observed by ARTEMIS around the Moon
Keywords:Moon, whistler-mode waves, ARTEMIS
Whistler-mode chorus emissions are a type of narrow band electromagnetic waves with a typical frequency range of 0.2–0.8 fce, where fce is the electron cyclotron frequency. When converted to audio, they sound like chirping of birds due to their frequent repetition of rising or falling tones, hence the name. Chorus emissions are mainly observed in the terrestrial inner magnetosphere, and also found in the magnetospheres of Jupiter, Saturn and Mars. However, they have not been reported around airless bodies without magnetospheres like the Moon.
Although the Moon does not have a dense atmosphere nor a global intrinsic magnetic field, it is known that the plasma of the solar wind and the Earth’s magnetotail interacts with the lunar surface and the lunar crustal magnetic field, resulting in various plasma phenomena around the Moon. Whistler-mode waves can be excited by cyclotron resonance of waves traveling toward the Moon and upcoming electrons magnetically reflected from the lunar surface. In the process, a free energy source is provided by effective temperature anisotropy in electron velocity distribution function caused by absorption of parallel electrons at the lunar surface and magnetic reflection of perpendicular electrons. As these lunar whistler-mode waves can have as large amplitudes as those occur in the terrestrial inner magnetosphere, they might possibly grow like chorus emissions.
We report two events of whistler-mode waves with discrete rising tone elements observed by ARTEMIS along with results of two ways of analysis: two-point observations and data-theory comparison. Simultaneous two-point data of electrons and the magnetic field reveal that these events are related to the Moon. Furthermore, predicted sweeps of rising tone elements based on nonlinear growth theory of chorus emissions by Omura et al. (2008) show good agreement with the observations. These results imply that whistler-mode waves even in the airless environment without an intrinsic magnetic field around the Moon can grow nonlinearly in a similar way with those in the planetary magnetospheres. We will also discuss the relationship between the magnetic irregularities on the lunar surface and the occurrence of the events.
Although the Moon does not have a dense atmosphere nor a global intrinsic magnetic field, it is known that the plasma of the solar wind and the Earth’s magnetotail interacts with the lunar surface and the lunar crustal magnetic field, resulting in various plasma phenomena around the Moon. Whistler-mode waves can be excited by cyclotron resonance of waves traveling toward the Moon and upcoming electrons magnetically reflected from the lunar surface. In the process, a free energy source is provided by effective temperature anisotropy in electron velocity distribution function caused by absorption of parallel electrons at the lunar surface and magnetic reflection of perpendicular electrons. As these lunar whistler-mode waves can have as large amplitudes as those occur in the terrestrial inner magnetosphere, they might possibly grow like chorus emissions.
We report two events of whistler-mode waves with discrete rising tone elements observed by ARTEMIS along with results of two ways of analysis: two-point observations and data-theory comparison. Simultaneous two-point data of electrons and the magnetic field reveal that these events are related to the Moon. Furthermore, predicted sweeps of rising tone elements based on nonlinear growth theory of chorus emissions by Omura et al. (2008) show good agreement with the observations. These results imply that whistler-mode waves even in the airless environment without an intrinsic magnetic field around the Moon can grow nonlinearly in a similar way with those in the planetary magnetospheres. We will also discuss the relationship between the magnetic irregularities on the lunar surface and the occurrence of the events.