PPARCセミナー (2025/12/5)
PPARCセミナー (2025/12/5)
(1)
[Name]
Haruto Yamanaka
[Title]
Long-term Statistical Analysis of Auroral Kilometric Radiation Observed by Geotail: Applied Automated Detection Technique
[Abstract]
Auroral Kilometric Radiation (AKR) is the most intense radio emission from the Earth. It is generated by precipitating energetic electrons along auroral field lines by electron cyclotron resonance. Its intensity correlates with the field-aligned current density, while its frequency is related to the source altitude through the local electron cyclotron frequency. Consequently, AKR provides a valuable diagnostic of both the auroral electron precipitations and their acceleration altitude. The objective of this study is to investigate these statistical characteristics, including their dependence on solar activity.
We performed a long-term statistical analysis of AKR, covering nearly three decades (September 1992–June 2022) using the Plasma Wave Instrument (PWI) onboard the Geotail satellite. This dataset enables us to investigate AKR occurrence and characteristics over 3 solar cycles.
Since Solar type-III bursts also overlap the AKR frequency range (from several 10s kHz up to 500–600 kHz), their automatic removal is essential for robust statistics. In previous studies by the Wind spacecraft, a method using 3-second standard deviations of the spin-axis antenna (Waters et al., 2021) and its extension with the criteria combining frequency and temporal continuity (Fogg et al., 2022).
We modified those methods and applied the long-term dataset of high-quality measurements from the Geotail Plasma Wave Instrument (PWI) Sweep Frequency Analyzer (SFA), covering nearly 30 years from September 1992 to June 2022. Compared with Wind, Geotail needs two major challenges. (1) It lacks a spin-axis antenna, so the angle between the antenna and the radio wave direction varies with the spin (period: ~3 second), producing artificial spin modulations which Wind spin-antenna data does not have. (2) The SFA Band-5 (100–800 kHz), the most suitable for AKR, requires 8 seconds for one frequency sweep, so it limits the ability to get short-term fluctuations used in Wind. In order to overcome these issues, we identified AKR with following three criteria: (A) Larger time variability: To mitigate spin-induced fluctuations, we deleted the spin modulation by averaging a longer averaging window (168 s: three 8-s cycles and 8 spin periods), and picked up AKR with a threshold based on the standard deviation of time variations (e.g., 4.65×10⁻³ W/m²/Hz). (B) Larger frequency variability: Since AKR shows stronger frequency fluctuations than solar type-III bursts, we set an additional threshold on the standard deviation across frequency (e.g., 2.65×10⁻³ W/m²/Hz, 32 channels, 175 kHz). (C) Larger temporal continuity: To exclude short-lived type-III bursts, we require events to satisfy a wide frequency wave (e.g., 30 channels, 160 kHz) with the continuity of 528 seconds (allowing short gaps of up to 168 seconds). By combining these criteria, contamination from solar type-III bursts was substantially reduced.
Next, we applied a distance correction (normalized to 30 Re, assuming an r² decay) and start the investigation how the factors controlling AKR appearance and strength are affected with solar activity. The analysis covers the full 30-year Geotail mission period (1992–2022). We started the solar maximum (2000 and 2014) and the solar minimum (1996 and 2009), to examine correlations with the solar cycle. Our preliminary analysis yields the following results. (1) AKR occurrence frequency (time fraction of AKRs above a threshold intensity) tends to increase during periods of low solar activity. (2) AKR intensity (frequency-integrated power) does not show a simple or strong correlation with solar activity.
Solar activity can affect AKR through ionospheric conductivity and magnetospheric activity. For examples, it is known that the appearance, strength, and frequency range of AKR show diurnal and seasonal variations associated with the angle of the magnetic axis relative to the Sun (Morioka et al., 2013). These variations are thought to arise from changes in ionospheric conductivity and the configuration of the magnetosphere, and clarifying their relationship with solar activity may provide further insights into these processes. Moreover, frequency variations in AKR could shed light on the dependence of auroral electron acceleration altitudes—linked to magnetosphere–ionosphere coupling—on.
Our analysis will address the dependence of AKR intensity and occurrence frequency in (i) the entire frequency range (78.125–625 kHz), (ii) the high-altitude region (low-frequency range, ≤200 kHz), and (iii) the low-altitude region (high-frequency band around 500 kHz), with the following three factors:(A) Local time dependence: Since AKR is more observed on the nightside, the local time effect will be distinguished.(B) Magnetic latitude dependence: AKR is emitted above both polar region and is more readily observed at high latitudes (|MLAT| ≥ 20°). Near the magnetic equator, propagation shadow can also occur near the Earth. (C) Dipole tilt dependence. The inclination of the magnetic dipole axis serves as an indicator for verifying seasonal dependence. Using this tilt angle, we confirm differences in AKR behavior between seasons.
(2)
[Name]
Masato Kagitani
[Title]
Daytime observation of Mercury with a visible AO System installed on Tohoku 60cm telescope at Haleakala observatory
[Abstract]
We report on observation of Mercury’s potassium exosphere using the Tohoku 60-cm telescope (T60) at the Haleakala Observatory in
Hawaii, equipped with a visible adaptive optics (AO) system and a high-dispersion imaging spectrograph. Our current goal is
to provide ground-based support observations for the ESA-JAXA joint Mercury mission, BepiColombo, which will be in orbit around
Mercury in 2026.
Mercury is known to have an exosphere consisting of alkali metals. The resonance scattering emissions of neutral sodium (Na D2
589.0 nm, D1 589.6 nm) and potassium (K D2 766.5 nm, D1 769.9 nm) are bright enough to be observable from the ground with high-dispersion
spectroscopy, enabling studies of their spatial distribution and velocity fields. Sodium and potassium, both alkali elements, are
expected to share common release and transport processes from Mercury’s surface. However, previous studies have shown that the
Na/K abundance ratio exhibits very large spatio-temporal variability, ranging from 30 to 400, and their emission distributions differ as
well. Particularly, Na often shows enhancements at cusp regions, whereas K has been reported to increase at low latitudes and small
solar zenith angles (Lierle et al., 2022), drawing attention to the differences in their source and transport processes. Nevertheless,
because the potassium exosphere is about two orders of magnitude fainter than that of sodium, observations are still very limited,
and its variabilities remain unclear. Our study aims to monitor the potassium exosphere continuously during daytime ground-based
observations with AO, achieving unprecedented spatial resolution.
The observations employed the T60 telescope at Haleakala, combining with the visible AO system and a high-dispersion spectrograph
(R = 60,000) equipped with a fiber integral-field unit (IFU, fov=15×18”). These instruments are operated remotely and in automated
mode from Japan, contributing to long-term monitoring of solar system bodies.
From the 11–12 Nov 2025 observations, Na column density shows a typical pattern with maxima in the northern and southern cusp regions,
whereas the K column density peaks at southern high latitudes. The Na/K ratios was 40-90 on 11–12 Nov 2025.
Those emission intensities and the Na/K ratio fall within the variability ranges reported in previous studies.
In this presentation, we will discuss the spatial distributions of both potassium and sodium emissions in detail.
