IMAGE (Imager for Magnetopause-to-Aurora Global Exploration, Explorer 78 or MIDEX-1) was a NASA Medium Explorer mission that studied the global response of the Earth's magnetosphere to changes in the solar wind. It was believed lost but as of August 2018 might be recoverable. It was launched 25 March 2000, at 20:34:43.929 UTC, by a Delta II launch vehicle from Vandenberg Air Force Base on a two-year mission. Almost six years later, it unexpectedly ceased operations in December 2005 during its extended mission and was declared lost. The spacecraft was part of NASA's Sun-Earth Connections Program, and its data has been used in over 400 research articles published in peer-reviewed journals. It had special cameras that provided various breakthroughs in understanding the dynamics of plasma around the Earth. The principal investigator was Jim Burch of the Southwest Research Institute.
In January 2018, an amateur satellite tracker found it to be transmitting some signals back to Earth. NASA made attempts to communicate with the spacecraft and determine its payload status, but has had to track down and adapt old hardware and software to the current systems. On 25 February 2018, contact with IMAGE was again lost only to be reestablished on 4 March 2018. The signal disappeared once again on 5 August 2018. If recovery efforts succeed, NASA may decide to fund a restarted mission.
Overview
IMAGE was the first spacecraft dedicated to imaging the Earth's magnetosphere. IMAGE was a spacecraft developed by the Medium-class Explorer (MIDEX) program, and it was the first spacecraft dedicated to observing the magnetosphere of the Earth, producing comprehensive global images of plasma in the inner magnetosphere. The IMAGE craft was placed in a × orbit around the Earth, with an inclination of 90.01° (passing over the poles) and a 14.2 hour period.
By acquiring images every 2 minutes in wavelengths invisible to the human eye, it allowed detailed study of the interaction of the solar wind with the magnetosphere and the magnetosphere's response during a magnetic storm. From its distant orbit, the spacecraft produced a wealth of images of the previously invisible region of space in the inner magnetosphere, exceeded all its scientific goals. A senior review in 2005, just previous to its loss, described the mission as "extremely productive", having confirmed several theoretical predictions (e.g. plasmasphere plumes, pre-midnight ring-current injection, and continuous antiparallel reconnection), discovered numerous new and unanticipated phenomena (e.g. plasmasphere shoulders, subauroral proton arcs, and a secondary interstellar neutral atom stream), and answered a set of outstanding questions regarding the source region of kilometric continuum radiation, the role of solar wind pressure pulses in ionospheric outflow, and the relationship between proton and electron auroras during substorms. When the spacecraft went silent in December 2005, it had already been approved for a mission extension until 2010.
Costs for IMAGE are estimated at US$132 million, including the spacecraft, instruments, launch vehicle, and ground operations.
Payload Its science payload consists of three suites of instruments: Energetic neutral atom imagers (LENA, MENA, HENA) — These instruments use hydrogen, helium, and oxygen atoms in the exosphere to form images and determine the properties of their low-, medium- and high-energy parent ions. HENA, the High Energy Neutral Atom imager is a modified version of the MIMI/INCA (Magnetospheric Imaging Instrument/Ion and Neutral Camera) sensor flown on the Cassini–Huygens mission. Ultraviolet imagers (FUV and EUV) — The extreme ultraviolet (EUV) imager detects light from helium atoms in Earth's plasmasphere. The Far ultraviolet (FUV) imager employs three detectors to image the auroras in different wavelengths and measure the distribution of different ions. Radio Plasma Imager (RPI) — This instrument uses pulses of radio waves to image the entire volume of the Earth's magnetic field. With its -long antenna, it sends out a burst of radio waves, which reflect off of clouds of charged particles between the plasmasphere's outer boundary all the way out to the boundary where the Earth's magnetic field is impacted by the solar wind. RPI also measures naturally occurring plasma waves at the spacecraft's location.
The Central Instrument Data Processor (CIDP) as well as the Command & Data Handling Subsystem (main on-board computer) were built around the mission-proven IBM RAD6000 avionics processors.
Experiments Extreme Ultraviolet Imager (EUV)
The Extreme Ultraviolet Imager (EUV) of the IMAGE mission observes the distribution of He+ in Earth's plasmasphere by detecting its resonantly-scattered emission at 30.4-nm. It records the structure and dynamics of the cold plasma in Earth's plasmasphere on a global scale. The 30.4-nm feature is relatively easy to measure because it is the brightest ion emission from the plasmasphere, it is spectrally isolated, and the background at that wavelength is negligible. Measurements are easy to interpret because the plasmaspheric He+ emission is optically thin, so its brightness is directly proportional to the He+ column abundance. Effective imaging of the plasmaspheric He+ requires global "snapshops" in which the high apogee and the wide field of view of EUV provide in a single exposure a map of the entire plasmasphere. EUV consists of three identical sensor heads, each having a field of view of 30° in diameter. These sensors are tilted relative to one another to cover a fan-shaped field of 84° by 30°, which is swept across the plasmasphere by the spin of the satellite. EUV's spatial resolution is 0.6° of 0.1 RE in the equatorial plane seen from apogee. The sensitivity is 1.9 count/second-rayleigh, sufficient to map the position of the plasmapause with a time resolution of 10 minutes.
Far Ultraviolet Imager (FUV) The IMAGE satellite instrument complement includes three Far Ultraviolet (FUV) instruments. In the wavelength region 120-190 nm, a downward-viewing auroral imager is only minimally contaminated by sunlight, scattered from clouds and ground, and radiance of the aurora observed in a nadir viewing geometry can be observed in the presence of the high-latitude dayglow. The Wideband Imaging Camera (WIC) provides broadband ultraviolet images of the aurora for maximum spatial and temporal resolution by imaging the LBH N2 bands of the aurora. The Spectrographic Imager (SI), a monochromatic imager, images different types of aurora, filtered by wavelength. By measuring the Doppler-shifted Lyman-a, the proton-induced component of the aurora can be imaged separately. Finally, the GEO instrument observes the distribution of the geocoronal emission, which is a measure of the neutral background density source for charge exchange in the magnetosphere. The FUV instrument complement looks radially outward from the rotating IMAGE satellite and, therefore, it spends only a short time observing the aurora and the Earth during each spin (120-seconds period). Detailed descriptions of the WIC, SI, GEO, and their individual performance validations can be found in the January 2000 issue of the Space Science Reviews. One primary requirement of the FUV instrument is to maximize photon collection efficiency and use efficiently the short time available for exposures. The FUV auroral imagers WIC and SI both have wide fields of view and take data continuously as the auroral region proceeds through the field of view. To minimize data volume, multiple images are taken and electronically co-added by suitably shifting each image to compensate for the spacecraft rotation. In order to minimize resolution loss, the images have to be distortion -corrected in real time for both WIC and SI prior to co-adding. The distortion correction is using high speed look up tables that are pre-generated by least square fitting to polynomial functions by the on-orbit processor. The instruments were calibrated individually while on stationery platforms, mostly in vacuum chambers as described in the companion papers. Extensive ground-based testing was performed with visible and near UV simulators mounted on a rotating platform to estimate their on-orbit performance.
High-Energy Neutral Atom Imager (HENA) The High-Energy Neutral Atom (HENA) imager on IMAGE is one of three instruments designed to make observations of the Earth's magnetospheric environment using neutral atom imaging. The HENA instrument determines the velocity, trajectory, energy, and mass of ENAs in the 10-500 keV energy range and from these data generates images of ENA source regions in the inner magnetosphere. The two main HENA components are the sensor and the main electronics unit (MEU). The HENA sensor consists of alternately charged deflection plates mounted in a fan configuration in front of the entrance slit, three Microchannel plate detectors (MCP), a solid-state detector (SSD), two carbon-silicon-polyimide foils (one at the entrance slit, the other placed just in front of the back MCP), and a series of wires and electrodes to steer secondary electrons ejected from the foils (or the SSD) to the MCPs. Power for the MCPs and deflection plates and for secondary electron steering is provided by high-voltage power supplies that reside with the sensor. The MEU contains HENA's data processing unit (DPU), the analog electronics (which amplifies and processes signals from the sensor and performs housekeeping monitoring), analog-to-digital converters, and a low-voltage power supply.
HENA determines the velocity of the ENAs that it detects by measuring their time of flight (ToF) and trajectory through the sensor (from the entrance slit either to the back foil and two-dimensional imaging MCP detector or to the SSD. When an incoming ENA passes through the entrance foil, it produces secondary electrons, which are accelerated and steered to the front imaging MCP. This MCP, the "start" MCP, provides a start signal for the TOF analysis and registers the position at which the ENA penetrated the entrance slit. The ENA then continues through the sensor to the backplane and strikes either the foil in front of the 2-D imaging MCP or the SSD. In the first case, secondary electrons ejected from the back foil trigger a stop pulse in the 2-D imaging MCP, which also registers the position of the incident ENA. If the ENA strikes the SSD instead, the secondary electrons ejected by the impact are steered to the "coincidence" MCP, which provides the TOF stop signal; the position of impact is registered by the SSD. The start and stop signals are processed by the analog ToF electronics in the MEU and digitized for input into the DPU. The start and stop pulses give the ENA's time of flight, while the position measurements reveal its trajectory and thus its path length within the sensor. With these two pieces of information, time of flight and path length, HENA can calculate the ENA's velocity. The energy of the incident ENAs is measured with the SSD. When an ENA strikes the SSD, it generates a current pulse. The amplitude of this pulse (the pulse height) is directly proportional to the amount of energy that the ENA deposits in the SSD crystal. Thus, by analyzing the pulse height, HENA can determine the energy of an ENA incident on the SSD. And as mass is equal to the twice the energy divided by the velocity squared, once the energy and velocity of the ENA have been determined, its mass can be calculated. Calculating mass from the velocity and the SSD energy measurement is the primary technique used by HENA to determine composition of the ENAs. A second technique uses the pulse height of the MCP signal to distinguish between oxygen and hydrogen, the two most common neutral atoms expected in the magnetosphere.
Low-Energy Neutral Atom Imager (LENA)