David Rodgers

Problems with models of the radiation belts

The current standard models of the radiation-belt environment have many shortcomings, not the least of which is their extreme age. Most of the data used for them were acquired in the 1960s and early 1970s. Problems with the present models, and the ways in which data from more recent missions are being or can be used to create new models with improved functionality, are described. The phenomenology of the radiation belts, the effects on space systems, and geomagnetic coordinates and modeling are discussed. Errors found in present models, their functional limitations, and problems with their implementation and use are detailed. New modeling must address problems at low altitudes with the south Atlantic anomaly, east-west asymmetries and solar cycle variations, and at high altitudes with the highly dynamic electron environment. The important issues in space environment modeling from the point of view of usability and relationship with effects evaluation are presented. New sources of data are discussed. Future requirements in the data, models, and analysis tools areas are presented.

SPIS Open-Source Code: Methods, Capabilities, Achievements, and Prospects

In this paper, recent improvements in the modeling capabilities of the Spacecraft Plasma Interaction Software (SPIS) code are presented. New developments still in progress are also reported. They should in particular allow modeling of fast dynamical phenomena, including processes as challenging as the second part of ESDs, i.e., the vacuum arc and its related flashover plasma expansion. The first, electronic, part of ESDs is already modeled. The range of SPIS application domains and studies is reviewed. An interesting study case, the assessment of charging at multiple-scale levels, is presented here in more detail. Charging in geostationary-Earth-orbit conditions is simulated from the spacecraft scale down to a solar-cell-gap (hence, decameters to millimeter) scale. This self-consistent computation shows that macroscopic inverted-voltage-gradient (IVG) cases may differ at microscopic scales close to a solar-cell gap, due to the local blocking of secondary emission by the small-scale electric-field configuration. We consider this effect as the likely origin of the different ESD triggering thresholds, depending whether IVG is obtained by electrons or plasma.

ESA's tools for internal charging

Electrostatic discharges, caused by bulk charging of spacecraft insulating materials, are a major cause of satellite anomalies. A quantitative knowledge of the charge build-up is essential in order to eliminate these problems in the design stage. This is a presentation of ESAs tools to assess whether a given structure is liable to experience electrostatic discharges or not. A study has been made of the physical phenomenon, and an engineering specification has been created to be used to assess a structure for potential discharge problems. The specification has been implemented in new software, DICTAT. The implementation of tests in dedicated facilities is an important part of the specification, and tests have been performed to validate the new tool.

Observations of Internal Charging Currents in Medium Earth Orbit

The Galileo global navigation system will employ an array of satellites in medium Earth orbit (MEO). Internal charging is one of the primary hazards for any spacecraft in MEO, and accordingly, the Galileo test satellite, Giove-A, carries a detector, SURF, to undertake the measurement of internal charging currents at three different shielding depths. The currents are due to electrons stopped in three aluminum plates: the first plate is 0.5 mm thick and located under 0.5-mm Al-equivalent shielding, the second is 0.5 mm thick and located underneath the first plate, and the third is 1.0 mm thick and located underneath the second plate. Giove-A was successfully launched on December 28, 2005 into a 23 300-km circular 56deg inclination orbit. In this paper, we provide data on the internal charging currents observed in 2006, with particular emphasis on two large charging events, one in mid-April and one in mid-December. Comparisons are made to predictions using the DICTAT internal charging tool and the FLUMIC ldquoworst caserdquo trapped electron belt model. In general, the charging currents observed are safely within the standard DICTAT 3.5/FLUMIC 3.0 predictions but are exceeded in the most shielded plate on five calendar days during December 2006. The December event was unique in 2006 in that it was triggered by the arrival of a coronal mass ejection (CME) rather than being due to the effects of a fast solar wind stream flowing from a coronal hole. The currents measured in the December event were, however, an order of magnitude lower than those predicted using the ldquoanomalously large eventrdquo supplement to FLUMIC which models the most extreme electron enhancements associated with CMEs.

Modeling of Plasma Probe Interactions With a PIC Code Using an Unstructured Mesh

A new Spacecraft Plasma Interaction Software has been developed in the frame of the Spacecraft Plasma Interaction Network (SPINE). This software is designed to simulate the kinetic processes of ions and electrons, taking into account their space charge and their interaction with spacecraft surfaces. It is freely available worldwide in open source. While the development and the qualification of the software functionalities were under the responsibility of a consortium led by ONERA under contract with the European Space Agency, the test and validation of the applicability of the code for solving problems in plasma physics remained under SPINE responsibility. The validation program includes step-by-step applications of the software to sheath modeling in simple geometry, artificial plasma injection, and spacecraft charging. We report here on the progress along this program, including Langmuir probe tests with spherical and cylindrical geometry and comparison with other numerical methods.

Internal Charging Measurements in Medium Earth Orbit Using the SURF Sensor: 2005–2014

The SURF internal charging monitor that measures deposited currents in three shielded aluminum plates is one of the sensors within the Merlin radiation monitoring instrument launched on Giove-A in December 2005 into a 23300-km circular 56° inclination medium earth orbit. The instrument has now completed 8.5 years in orbit despite being originally intended for a two-year operational life. Here, we report on the instruments health, provide further recent data, and review the overall results in terms of their engineering significance. It is concluded that the instrument remains in good health, with no significant deterioration, and a near continuous data set from December 2005 to April 2014 is now available. The maximum plate currents to date were recorded over the period April 6-8, 2010, during a significant outer electron belt enhancement (also observed by Geostationary Operational Environmental Satellite (GOES)-12 in geostationary orbit), which persisted for several days. From April 7 to April 9, 2010, the current in the most shielded plate (1.0-mm thickness with 1.5-mm shielding) exceeded the widely used 0.1-pA cm-2 safety threshold for internal charging (the only days in the mission where this occurred), and on these days, it also exceeded the reasonable worst case current predications given by the DICTAT internal charging tool (the other plate currents remained within DICTAT predictions). The 0.02-pA cm-2 safety threshold used in European ECSS charging standards for dielectrics below 25 °C has been exceeded on a far larger number of days; for example, in the most shielded plate, it has been exceeded on 55 days (8% of the total).

Results From the Galileo Giove—A Radiation Monitors and Comparison With Existing Radiation Belt Models

The radiation monitors on board the Galileo Giove-A satellite, CEDEX and Merlin, and their data are presented. The instruments include energetic proton and ion detectors, an internal charging monitor, RADFETs and experimental dose-rate photodiodes. A comparison of the data with existing monitors and models is presented.

New SPIS Capabilities to Simulate Dust Electrostatic Charging, Transport, and Contamination of Lunar Probes

The spacecraft-plasma interaction simulator has been improved to allow for the simulation of lunar and asteroid dust emission, transport, deposition, and interaction with a spacecraft on or close to the lunar surface. The physics of dust charging and of the forces that they are subject to has been carefully implemented in the code. It is both a tool to address the risks faced by lunar probes on the surface and a tool to study the dust transport physics. We hereby present the details of the physics that has been implemented in the code as well as the interface improvements that allow for a user-friendly insertion of the lunar topology and of the lander in the simulation domain. A realistic case is presented that highlights the capabilities of the code as well as some general results about the interaction between a probe and a dusty environment.

A New Model of Outer Belt Electrons for Dielectric Internal Charging (MOBE-DIC)

The outer electron belt poses a significant radiation hazard to spacecraft due to internal charging and dose effects. We present a new user-friendly model to characterise the worst-case environment built on data from the Giove-A spacecraft. We use instrument data based on a novel technique of direct charging current measurements, uncontaminated by protons and unaffected by dead-time, to create the model. The model provides integral or differential electron flux spectra with the following input parameters: percentile 90%, 99% or 100%; L-shell range 3-8; Magnetic Latitude ( B/B0) range 1-100; Energy range 0.5-3 MeV. Comparisons with other models and independent data sets show that our “Model of Outer Belt Electrons for Dielectric Internal Charging” (MOBE-DIC) provides a sound worst-case specification for mission planners and design engineers. We find that, in medium Earth orbit particularly, MOBE-DIC indicates a harder electron spectrum than either AE9 or FLUMIC.

Simulation and Analysis of Spacecraft Charging Using SPIS and NASCAP/GEO

Spacecraft charging in GEO particularly concerns dielectric surfaces that may charge to significant voltages relative to spacecraft ground because of the space environment. Testing materials helps to define the level of risk and to maintain confidence in a spacecrafts immunity to damaging effects. Another factor defining the risk involves numerical simulation of spacecraft charging. Several tools aim to calculate surface charging, which is particularly hazardous in harsh environments produced by geomagnetic sub storms, where particles in the energy range of a few to hundreds of kiloelectronvolts are present. The main codes include Nascap-2k, Spacecraft plasma Interaction Software (SPIS), MUSCAT, and Coulomb-2. They use different numerical and sometimes physical models and cross checking their results is a necessary process to achieve better confidence in simulations performed by spacecraft prime manufacturers. The objective of this paper is to simulate different GEO spacecraft configurations with NASA Charging Analyzer Program at geosynchronous orbits (a 1980s to 1990s predecessor to Nascap-2k) and SPIS and to compare the results, both in terms of absolute and differential potentials. The first section concerns the SCATHA spacecraft. The second part of this paper compares efforts to model a modern telecom spacecraft. Finally, we conclude on the reliability of the simulations performed and possible areas for modeling improvement.