Abstract
The MAVEN SupraThermal And Thermal Ion Compostion (STATIC) instrument is designed to measure the ion composition and distribution function of the cold Martian ionosphere, the heated suprathermal tail of this plasma in the upper ionosphere, and the pickup ions accelerated by solar wind electric fields. STATIC operates over an energy range of 0.1 eV up to 30 keV, with a base time resolution of 4 seconds. The instrument consists of a toroidal “top hat” electrostatic analyzer with a $360^{\circ} \times 90^{\circ}$ field-of-view, combined with a time-of-flight (TOF) velocity analyzer with $22.5^{\circ}$ resolution in the detection plane. The TOF combines a $-15~\mbox{kV}$ acceleration voltage with ultra-thin carbon foils to resolve $\mathrm{H}^{+}$ , $\mathrm{He}^{++}$ , $\mathrm{He}^{+}$ , $\mathrm{O}^{+}$ , $\mathrm{O}_{2}^{+}$ , and $\mathrm{CO}_{2}^{+}$ ions. Secondary electrons from carbon foils are detected by microchannel plate detectors and binned into a variety of data products with varying energy, mass, angle, and time resolution. To prevent detector saturation when measuring cold ram ions at periapsis ( $\sim10^{1 1}~\mbox{eV/cm}^{2}\,\mbox{s}\,\mbox{sr}\,\mbox{eV}$ ), while maintaining adequate sensitivity to resolve tenuous pickup ions at apoapsis ( $\sim10^{3}~\mbox{eV/cm}^{2}\,\mbox{s}\,\mbox{sr}\,\mbox{eV}$ ), the sensor includes both mechanical and electrostatic attenuators that increase the dynamic range by a factor of $10^{3}$ . This paper describes the instrument hardware, including several innovative improvements over previous TOF sensors, the ground calibrations of the sensor, the data products generated by the experiment, and some early measurements during cruise phase to Mars.
Highlights
IntroductionThe MAVEN (Mars Atmosphere and Volatile EvolutioN) mission was designed to measure the loss of volatiles from the Mars atmosphere to space, and to combine these observations
The MAVEN (Mars Atmosphere and Volatile EvolutioN) mission was designed to measure the loss of volatiles from the Mars atmosphere to space, and to combine these observations with modeling to understand the histories of Mars’ atmosphere and climate, liquid water, and planetary habitability
Ion scattering in the carbon foil is large enough to spread out the pencil beam and eliminate any microchannel plate (MCP) droop in Stop events. (Note: inflight measurements of efficiency will not experience saturation problems until rates exceed ∼ 200 kHz because naturally occurring beams are much broader in angle.) Second, typical background event rates due to cosmic rays and radioactive decay in the MCP were observed at ∼ 100–200 Hz
Summary
The MAVEN (Mars Atmosphere and Volatile EvolutioN) mission was designed to measure the loss of volatiles from the Mars atmosphere to space, and to combine these observations. In order to effectively model the loss of Martian ions, MAVEN needs to measure the source density of ions in the collisional lower ionosphere, the creation of a suprathermal tail of these ions as the spacecraft transitions above the collisional regime, and the eventual acceleration of these ions to escape velocity by solar wind electric fields and other processes. These measurements will be provided by the SupraThermal And Thermal Ion Compostion (STATIC) instrument described in this paper. STATIC’s inflight calibration data analysis will utilize observations from the other MAVEN sensors including the Magnetometer (Connerney et al 2015) for pickup ion analysis, and LPW for analyzing ion heating and spacecraft potential
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