5:15 PM - 7:15 PM
[PCG19-P10] Observation of spatio-temporal variations in Mercury's sodium exosphere using the Haleakala T60 telescope with the fiber-fed spectrograph
Keywords:Mercury, ground observation, daytime observation
Mercury has a very thin exospheric atmosphere containing alkali metals. The resonant scattered emission from neutral Na D-lines (589.0 nm and 589.6 nm) is particularly bright and can be observed using small to medium aperture ground-based telescopes. Mercury’s magnetosphere, smaller than Earth’s, fluctuates on the order of a few minutes due to solar wind interactions, and the neutral exosphere is expected to vary on a similar timescale. Previous ground-based observations of Mercury’s Na exosphere have shown double-peak (DP) emissions at both poles, with the north-south brightness ratio fluctuating over tens of minutes. This timescale consistent with ion sputtering from the magnetosphere as a possible Na production source.
Conventional ground-based observations capture spatial and temporal variations using slit spectroscopy. This method requires slit movement to obtain the two-dimensional spatial distribution, taking about one hour for a global spectroscopic observation of Mercury. Variability on the order of a few minutes has been observed using a fixed slit position, but a new method is needed to capture global-scale fluctuations.
We are developing a fiber surface spectrograph and an adaptive optics (AO) system for the Tohoku University 60 cm telescope (T60) to observe Mercury’s Na exosphere with a temporal resolution of a few minutes on a global scale. After the BepiColombo spacecraft enters Mercury’s orbit in late 2026, we aim to coordinate our observations with its remote sensing of the exosphere and in-situ measurements of the solar wind and magnetosphere.
T60 is located at Haleakalā, Maui, Hawaii (3,040 m above sea level). It is equipped with a high-dispersion visible spectrograph (wavelength resolution: 67,000) and a fiber-field integrator for continuous remote-controlled observations. The fiber field-of-view integrator provides a 1.5” × 1.5” field per fiber, with a 10 × 9 array of 90 fibers covering 15” × 13.5”. An AO system compensates for daytime seeing. The system remains under development, but by fall 2024, it is expected to achieve a spatial resolution of ~2” FWHM in daylight.
A test observation of Mercury’s Na exosphere was conducted on November 12, 2024, from 03:34 to 08:02 (HST). The solar separation angle was 22.2°, Mercury’s altitude ranged from 26 to 88 degrees (solar altitude: 47 to 70 degrees) during the observation. Each observation cycle included (1) four frames of Mercury at the center of the field, (2) four frames of background sky near Mercury, and (3) four frames of white light for calibration, in the order (3) → (1) → (2) → (1). The integration time per frame was 1 minute, with four frames acquired continuously for one data point. One cycle lasted ~30 minutes. Over 10 consecutive cycles, 17 Mercury datasets were obtained, excluding failed data.
Spectroscopic data were analyzed by (a) removing scattered light from Earth’s atmosphere while accounting for bright variations, (b) separating Na emissions (NaD2 and NaD1 lines) from Mercury’s surface reflection spectrum, and (c) deriving two-dimensional distributions of surface-reflected light and Na emissions. As of January 2025, we are constructing an analysis framework to calibrate absolute emission intensity and determine disk location by comparing Mercury’s luminosity distribution model (Hapke model) with surface emissions. This presentation reports on the global distribution of Na emissions, particularly the north-south ratio, and comparisons with previous studies.
This test observation captured Mercury’s Na exosphere with a minimum interval of 10 minutes. Future observations aim to improve the sequence to maintain intervals of under 10 minutes. We also plan to refine spatial resolution and field of view by adjusting fiber optics based on insights from this test observation.
Conventional ground-based observations capture spatial and temporal variations using slit spectroscopy. This method requires slit movement to obtain the two-dimensional spatial distribution, taking about one hour for a global spectroscopic observation of Mercury. Variability on the order of a few minutes has been observed using a fixed slit position, but a new method is needed to capture global-scale fluctuations.
We are developing a fiber surface spectrograph and an adaptive optics (AO) system for the Tohoku University 60 cm telescope (T60) to observe Mercury’s Na exosphere with a temporal resolution of a few minutes on a global scale. After the BepiColombo spacecraft enters Mercury’s orbit in late 2026, we aim to coordinate our observations with its remote sensing of the exosphere and in-situ measurements of the solar wind and magnetosphere.
T60 is located at Haleakalā, Maui, Hawaii (3,040 m above sea level). It is equipped with a high-dispersion visible spectrograph (wavelength resolution: 67,000) and a fiber-field integrator for continuous remote-controlled observations. The fiber field-of-view integrator provides a 1.5” × 1.5” field per fiber, with a 10 × 9 array of 90 fibers covering 15” × 13.5”. An AO system compensates for daytime seeing. The system remains under development, but by fall 2024, it is expected to achieve a spatial resolution of ~2” FWHM in daylight.
A test observation of Mercury’s Na exosphere was conducted on November 12, 2024, from 03:34 to 08:02 (HST). The solar separation angle was 22.2°, Mercury’s altitude ranged from 26 to 88 degrees (solar altitude: 47 to 70 degrees) during the observation. Each observation cycle included (1) four frames of Mercury at the center of the field, (2) four frames of background sky near Mercury, and (3) four frames of white light for calibration, in the order (3) → (1) → (2) → (1). The integration time per frame was 1 minute, with four frames acquired continuously for one data point. One cycle lasted ~30 minutes. Over 10 consecutive cycles, 17 Mercury datasets were obtained, excluding failed data.
Spectroscopic data were analyzed by (a) removing scattered light from Earth’s atmosphere while accounting for bright variations, (b) separating Na emissions (NaD2 and NaD1 lines) from Mercury’s surface reflection spectrum, and (c) deriving two-dimensional distributions of surface-reflected light and Na emissions. As of January 2025, we are constructing an analysis framework to calibrate absolute emission intensity and determine disk location by comparing Mercury’s luminosity distribution model (Hapke model) with surface emissions. This presentation reports on the global distribution of Na emissions, particularly the north-south ratio, and comparisons with previous studies.
This test observation captured Mercury’s Na exosphere with a minimum interval of 10 minutes. Future observations aim to improve the sequence to maintain intervals of under 10 minutes. We also plan to refine spatial resolution and field of view by adjusting fiber optics based on insights from this test observation.