Earth and Planetary Physics最新文献

An important population of the dayside Martian ionosphere are photoelectrons that are produced by solar Extreme Ultraviolet and X-ray ionization of atmospheric neutrals. A typical photoelectron energy spectrum is characterized by a distinctive peak near 27 eV related to the strong solar HeII emission line at 30.4 nm, and an additional peak near 500 eV related to O Auger ionization. In this study, the extensive measurements made by the Solar Wind Electron Analyzer on board the recent Mars Atmosphere and Volatile Evolution spacecraft are analyzed and found to verify the scenario that Martian ionosphere photoelectrons are driven by solar radiation. We report that the photoelectron intensities at the centers of both peaks increase steadily with increasing solar ionizing flux below 90 nm and that the observed solar cycle variation is substantially more prominent near the O Auger peak than near the HeII peak. The latter observation is clearly driven by a larger variability in solar irradiance at shorter wavelengths. When the solar ionizing flux increases from 1 mW·m^-2 to 2.5 mW·m^-2, the photoelectron intensity increases by a factor of 3.2 at the HeII peak and by a much larger factor of 10.5 at the O Auger peak, both within the optically thin regions of the Martian atmosphere.

Electron pitch angle distributions similar to bidirectional electron conics (BECs) have been reported at Mars in previous studies based on analyses of Mars Global Surveyor measurements. BEC distribution, also termed “butterfly” distribution, presents a local minimum flux at 90° and a maximum flux before reaching the local loss cone. Previous studies have focused on 115 eV electrons that were produced mainly via solar wind electron impact ionization. Here using Solar Wind Electron Analyzer measurements made onboard the Mars Atmosphere and Volatile Evolution spacecraft, we identify 513 BEC events for 19–55 eV photoelectrons that were generated via photoionization only. Therefore, we are investigating electrons observed in regions well away from their source regions, to be distinguished from 115 eV electrons observed and produced in the same regions. We investigate the spatial distribution of the 19–55 eV BECs, revealing that they are more likely observed on the nightside as well as near strong crustal magnetic anomalies. We propose that the 19–55 eV photoelectron BECs are formed due to day-to-night transport and the magnetic mirror effect of photoelectrons moving along cross-terminator closed magnetic field lines.

Profiles of the Martian dayside ionosphere can be used to derive the neutral atmospheric densities at 130 km, which can also be obtained from the Mars Climate Database (MCD) and spacecraft aerobraking observations. In this research, we explain the method used to calculate neutral densities at 130 km via ionosphere observations and three long-period 130-km neutral density data sets at northern high latitudes (latitudes > 60°) acquired through ionospheric data measured by the Mars Global Surveyor (MGS) Radio Occultation Experiment. The calculated 130-km neutral density data, along with 130-km density data from the aerobraking observations of the MGS and Mars Odyssey (ODY) in the northern high latitudes, were compared with MCD outputs at the same latitude, longitude, altitude, solar latitude, and local time. The 130-km density data derived from both the ionospheric profiles and aerobraking observations were found to show seasonal variations similar to those in the MCD data. With a negative shift of about 2 × 10 ¹⁰ cm⁻³, the corrected 130-km neutral densities derived from MCD v4.3 were consistent with those obtained from the two different observations. This result means that (1) the method used to derive the 130-km neutral densities with ionospheric profiles was effective, (2) the MCD v4.3 data sets generally overestimated the 130-km neutral densities at high latitudes, and (3) the neutral density observations from the MGS Radio Science Experiment could be used to calibrate a new atmospheric model of Mars.

The first Mars exploration mission of China (Tianwen-1) is scheduled to be launched in 2020; a charged particle telescope, the Mars Energetic Particle Analyzer (MEPA), is carried as one of the payloads on the orbiter. The MEPA is designed to measure solar energetic particles (SEPs) and galactic cosmic rays (GCRs) in the near-Mars space and in the transfer orbit from Earth to Mars. Before the launch, the MEPA was calibrated in ground experiments with radioactive sources, electronic pulses, and accelerator beams. The calibration parameters, such as energy conversion constants, threshold values for the triggers, and particle identification criteria, were determined and have been stored for onboard use. The validity of the calibration parameters has been verified with radioactive sources and beams. The calibration results indicate that the MEPA can measure charged particles reliably, as designed, and that it can satisfy the requirements of the Tianwen-1 mission.

Doubly charged positive ions (dications) are an important component of planetary ionospheres because of the large energy required for their formation. Observations of these ions are exceptionally difficult due to their low abundances; until now, only atomic dications have been detected. The Neutral Gas and Ion Mass Spectrometer (NGIMS) measurements made on board the recent Mars Atmosphere and Volatile Evolution mission provide the first opportunity for decisive detection of molecular dications, CO₂ ⁺⁺ in this case, in a planetary upper atmosphere. The NGIMS data reveal a dayside averaged CO₂ ⁺⁺ distribution declining steadily from 5.6 cm⁻³ at 160 km to below 1 cm⁻³ above 200 km. The dominant CO₂ ⁺⁺ production mechanisms are double photoionization of CO₂ below 190 km and single photoionization of CO₂ ⁺ at higher altitudes; CO₂ ⁺⁺ destruction is dominated by natural dissociation, but reactions with atmospheric CO₂ and O become important below 160 km. Simplified photochemical model calculations are carried out and reasonably reproduce the data at low altitudes within a factor of 2 but underestimate the data at high altitudes by a factor of 4. Finally, we report a much stronger solar control of the CO₂ ⁺⁺ density than of the CO₂ ⁺ density .

The main objective of the Mars Ion and Neutral Particle Analyzer (MINPA) aboard the Chinese Mars Exploration Mission (Tianwen-1) is to study the solar wind–Mars interaction by measuring the ions and energetic neutral atoms (ENAs) near Mars. The MINPA integrates ion and ENA measurements into one sensor head, sharing the same electronics box. The MINPA utilizes a standard toroidal top-hat electrostatic analyzer (ESA) followed by a time of flight (TOF) unit to provide measurement of ions with energies from 2.8 eV to 25.9 keV and ENAs from 50 eV to 3 keV with a base time resolution of 4 seconds. Highly polished silicon single crystal substrates with an Al₂O₃ film coating are used to ionize the ENAs into positive ions. These ions can then be analyzed by the ESA and TOF, to determine the energy and masses of the ENAs. The MINPA provides a 360°×90° field of view (FOV) with 22.5°×5.4° angular resolution for ion measurement, and a 360°×9.7° FOV with 22.5°×9.7° angular resolution for ENA measurement. The TOF unit combines a –15 kV acceleration high voltage with ultra-thin carbon foils to resolve H⁺, He²⁺, He⁺, O⁺, O₂ ⁺ and CO₂ ⁺ for ion measurement and to resolve H and O (≥ 16 amu group) for ENA measurement. Here we present the design principle and describe our ground calibration of the MINPA.

The background and scientific objectives of the Mars Climate Station (MCS) for Tianwen-1 are introduced, accompanied by a comparative review of the status of related meteorological observation missions and of advanced sensing technologies. As one of the China Tianwen-1 Mission’s principal scientific payloads, the MCS contains four measurement sensors and one electronic processing unit that are specially designed to measure local temperature, pressure, wind, and sound on the Martian surface. The MCS’s measurement principles, technical schemes, ground calibration techniques, and adaptability evaluation to the Mars surface environment of MCS are introduced in details. The conclusion presents measurement performance specifications of the MCS, based on ground test results, that will provide guidance to future research based on data from the Tianwen-1 and later Mars missions.

China's Mars probe, named Tianwen-1, including an orbiter and a landing rover, will be launched during the July-August 2020 Mars launch windows. Selected to be among the rover payloads is a Subsurface Penetrating Radar module (RoSPR). The main scientific objective of the RoSPR is to characterize the thickness and sub-layer distribution of the Martian soil. The RoSPR consists of two channels. The low frequency channel of the RoSPR will penetrate the Martian soil to depths of 10 to 100 m with a resolution of a few meters. The higher frequency channel will penetrate to a depth of 3 to 10 m with a resolution of a few centimeters. This paper describes the design of the instrument and some results of field experiments.