Near-Ultraviolet and Visible Spectroscopy of HAYABUSA Spacecraft Re-entry

HAYABUSA is the first spacecraft ever to land on and lift off from any celestial body other than the moon. The mission, which returned asteroid samples to the Earth while overcoming various technical hurdles, ended on June 13, 2010, with the planned atmospheric re-entry. In order to safely deliver the sample return capsule, the HAYABUSA spacecraft ended its 7-year journey in a brilliant "artificial fireball" over the Australian desert. Spectroscopic observation was carried out in the near-ultraviolet and visible wavelengths between 3000 and 7500 \AA at 3 - 20 \AA resolution. Approximately 100 atomic lines such as Fe I, Mg I, Na I, Al I, Cr I, Mn I, Ni I, Ti I, Li I, Zn I, O I, and N I were identified from the spacecraft. Exotic atoms such as Cu I, Mo I, Xe I and Hg I were also detected. A strong Li I line (6708 \AA) at a height of ~55 km originated from the onboard Li-Ion batteries. The FeO molecule bands at a height of ~63 km were probably formed in the wake of the spacecraft. The effective excitation temperature as determined from the atomic lines varied from 4500 K to 6000 K. The observed number density of Fe I was about 10 times more abundant than Mg I after the spacecraft explosion. N2+(1-) bands from a shock layer and CN violet bands from the sample return capsule's ablating heat shield were dominant molecular bands in the near-ultraviolet region of 3000 - 4000 \AA. OH(A-X) band was likely to exist around 3092 \AA. A strong shock layer from the HAYABUSA spacecraft was rapidly formed at heights between 93 km and 83 km, which was confirmed by detection of N2+(1-) bands with a vibration temperature of ~13000 K. Gray-body temperature of the capsule at a height of ~42 km was estimated to be ~2437 K which is matched to a theoretical prediction. The final message of the HAYABUSA spacecraft and its sample return capsule are discussed through our spectroscopy.Comment: Accepted for publication in PASJ, 22 pages, 7 figures, 6 table

Correlation between electron density and temperature in the topside ionosphere

The correlation between the electron temperature (Te) and electron density (Ne) at 600 km height at magnetic dip latitudes (MLat) less than about ± 40° measured by the Hinotori satellite from February 1981 to June 1982 is presented. The results show the well-known negative correlation between daytime Ne and Te when Ne is low. However, when the daytime Ne is significantly high (>10^[6] cm^[-3]), the correlation turns positive irrespective of latitude, longitude, season, solar flux levels, and magnetic activity levels. The positive correlation is most clear during 1100-1500 local time and around the magnetic dip equator (|MLat| < 10°) where high values of Ne with high Te are most abundant. Te also increases with increasing MLat in the same Ne range. Since the cooling through Coulomb collisions increases with the increase of Ne, the results suggest that an additional heat source(s) is involved for the positive correlation between Ne and Te. The additional heat source seems to be related to the integrated Ne along the magnetic field lines from the ground to 600 km altitude in one hemisphere, which is found to decrease with increasing MLat. Although the mechanism for the positive correlation is not well understood, the results imply that the Te in the topside ionosphere is controlled more by the integrated Ne than by in situ Ne or F2 peak Ne

Observation and simulation of atmospheric gravity waves exciting subsequent tsunami along the coastline of Japan after Tonga explosion event

Abstract Tsunamis are commonly generated by earthquakes beneath the ocean floor, volcanic eruptions, and landslides. The tsunami following the Tonga eruption of 2022 is believed to have been excited by atmospheric pressure fluctuations generated by the explosion of the volcano. The first, fast-traveling tsunami was excited by Lamb waves; however, it has not been clarified observationally or theoretically which type of atmospheric fluctuations excited more prominent tsunami which followd. In this study, we investigate atmospheric gravity waves that possibly excited the aforementioned subsequent tsunami based on observations and atmosphere-ocean coupling simulations. The atmospheric fluctuations are classified as Lamb waves, acoustic waves, or gravity waves. The arrival time of the gravity wave and the simulation shows that the gravity wave propagated at a phase speed of 215 m/s, coinciding with the tsunami velocity in the Pacific Ocean, and suggesting that the gravity wave resonantly excited the tsunami (Proudman resonance). These observations and theoretical calculations provide an essential basis for investigations of volcano-induced meteotsunamis, including the Tonga event

Validation of electron density and temperature observed by DEMETER

Measuring electron density (Ne) and temperature (Te) using a DC Langmuir probe in the ionosphere is very often degraded by the electrode contamination. In order to examine the accuracy of DEMETER observations, we compared DEMETER Ne and Te with several other satellites observations and IRI2012 as reference data. DEMETER Ne and Te show well-known dependencies on the solar irradiance except for the range of F10.7 > 100. However, DEMETER Ne are about 70% lower than those of IRI in day time data and its solar irradiance dependency is consistent with the reference data in night time data. It was confirmed that the negative slope appears in deep solar minimum solar cycle 23/24. DEMETER Te are higher than IRI data by 500-1500 K in day time and by 800 K in night time. The relation between Ne and Te is well defined by a negative slope both in DEMETER and IRI during day time, while such a similarity is not recognized in night time data. DEMETER Te is 700 K higher than IRI Te for the same value of Ne. When Ne is less than 10(4) cm(-3) in night time, significant reductions in DEMETER Te are observed, which is close to expected values. Such discrepancies from the reference data and some peculiar behaviors of DEMETER Te and Ne data necessitate a careful attention in using them in consideration of their data alterations. However, their relative variations and averaged behavior in time contain useful information for scientific studies such as dependencies on solar irradiance and wave-4 longitudinal structure under certain conditions (Ne > 10(4) cm(-3) and F10.7 < 100). (C) 2013 COSPAR. Published by Elsevier Ltd. All rights reserved