Fred Herrero, Space Systems Research Corporation; Andrew Nicholas, U.S. Naval Research Laboratory; Daniel Perez, DP Consultants; Phillip Vu, OrboticSystems; Erik Long, OrboticSystems; Jane Ielmini, OrboticSystems
Keywords: WIND, Neutral Density, Wind Ion Neutral Density, Space Weather, Solar Flares, Conjunction Events, Space Weather Forecasting, Space Weather Modeling
Abstract:
Earth’s thermosphere and ionosphere are in thermal equilibrium. Therefore, satellites in this region are impacted by fluxes of neutrals and ions, derivable from their Maxwellian velocity distributions, that depend on the bulk velocities (neutral wind W and ion-drift VI), temperatures and densities. Thus, these fluxes have energy and angle distributions that when measured with a suitable spectrometer on the satellite can be analyzed to obtain the neutral wind W, ion-drift VI with neutral and ion temperatures and densities. These quantities and their gradients are the agents of neutral and ion transport that lead to density enhancements at a point or small region in the thermosphere. Their measurement requires precision on particle energies and angles, this presents several challenges in the performance of electron impact ion sources and ion energy analyzers as detailed below.
These challenges are met with the WIND instrument suite, a next-generation spectrometer designed to measure both neutral and ion energy-angle distributions with the same unprecedented precision demonstrated previously for ions with the WINCS instrument suite. WIND builds upon the WINCS heritage for its energy analyzer and goes further to solve the problem with the WINCS ion source that ionized the neutrals and completely distorted their energy and angular distributions to render them undetectable. Therefore, the WIND suite has a new ion source (AIS) that preserves the energy and angle distributions that the neutrals had prior to ionization and leverages on the WINCS energy analyzer.
The Wind-Ion-Neutral-Composition-Suite (WINCS) was developed explicitly to obtain neutral wind W and ion-drift VI vectors, temperatures and densities plus neutral/ion composition. A joint effort between the NASA Goddard Space Flight Center (GSFC) and the U.S. Naval Research Laboratory (NRL), WINCS has flown on DoD’s STP-Sat3, STP-H4, and NASA’s GPIM.
WIND inherits from WINCS its energy analyzer, the Small-Deflection-Energy-Analyzer (SDEA) that scans the energy of transmitted ions to measure their energy distributions simultaneously at multiple angles to generate the desired energy and angle distributions. WINCS used two SDEA chambers: one devoted to incident neutrals (which are, of course, ionized) and one to incident ions. One chamber is oriented in the horizontal plane, while a second chamber is oriented in the vertical plane, thus allowing WINCS to acquire complete energy and angular distributions using four SDEAs. As a result, the ionized neutrals and incident ions are analyzed with identical ion spectrometers.
WINCS successfully reproduced the predictable ion fluxes with point-by-point fidelity, providing error estimates for ion-drift, ion temperature, and densities using non-linear least squares fitting to match the predicted Maxwellian fluxes. This precision resulted from the clean ion-optics design of an ion spectrometer, which relies on just two limiting apertures: the collimator’s first aperture and the SDEA entrance aperture doubling as the second collimator aperture. This design eliminated electrostatic ion lenses, avoiding vignetting and distortion in the energy distribution. The sharp focus of the SDEA made possible a trapezoidal energy bandpass rather than the typical triangular bandpass to ensure stable transmission to the SDEA detector. While the ion spectrometer performed successfully, the neutral side of WINCS failed.
That failure is rooted in the electron impact ionization process. Since atoms in the neutral flux are most conveniently analyzed when ionized, WINCS used a conventional electron impact ion source for energy-angle measurements of the neutrals – the ion source being the source of failure of the neutral side of the spectrometer.
An analysis of this failure revealed that the likely cause was the effect of space-charge in the ionizing electron beam; similar to electron-electron repulsions that limit electron emission in vacuum tubes. In laboratory ion sources, this issue is often ignored since the neutral gas’s velocity distribution prior to ionization is not of interest. However, space-charge distortions significantly impacted WINCS and the solution is to restrict ionizing electron current and electron beam size. A quantitative analysis demonstrated that a 40 µA electron beam current, when confined to a 0.013 cm shallow dimension along the neutrals’ incidence line, would be sufficient to yield an ion signal of about 5×10⁵ ions/sec at 400 km altitude, where atomic oxygen density is ~10⁸ cm⁻³. This signal strength allows for precise energy distributions with space-charge distortions below 0.013 eV, leading to bulk velocity uncertainties of approximately ±10 m/s.
WIND has two primary instruments: (1) a neutral and ion spectrometer that uses the SDEA analyzer and the new ion source AIS for neutral wind W and only the SDEA for ion-drift VI vectors, obtaining neutral and ion temperatures and densities, and (2) the GEMS mass spectrometer, for composition of neutrals and ions with an expected mass resolution of 100.
The WIND instrument suite is designed to provide bulk velocity vectors (neutral wind W and ion-drift VI), temperatures, densities via its neutral and ion spectrometer, and composition data via its mass spectrometer, fulfilling the mission goals of its predecessor, WINCS.
Thus, a network of WIND instruments in orbit, will provide NOAA’s Space Weather Prediction Center (SWPC) with real-time data on neutral wind W and ion-drift VI vectors, corresponding temperatures and densities plus neutral/ion composition. This comprehensive dataset, fed into SWPC’s computer models, enables rapid predictions of density enhancements and satellite drag effects; ultimately leading to proactive orbit adjustments, thereby reducing the risk of satellite collisions in an increasingly congested space environment. Further, this data can also serve to benefit other computational models, both government and commercial.
Date of Conference: September 16-19, 2025
Track: Atmospherics/Space Weather