Science and Technology Demonstration

The Small Satellite Program is currently developing a scientific mission of recognized value along with significant nano-satellite technology demonstrations. The primary scientific mission is to understand the complex behavior of the low latitude ionosphere. This goal is similar to that of the Air Force Communication/Navigation Outage Forecasting Satellite (C/NOFS) which is being deployed to locate and forecast scintillations in the low latitude ionosphere. Scintillations are caused by naturally occurring irregularities and lead to fluctuations in communication signals. Scintillations are generally responsible for decreased performance in UHF and L-band satellite-to-ground communications systems.

The Current Science Mission

Ionospheric densities are highly variable and a complex function of a variety of driving mechanisms [1]. Global characterization of the ionosphere remains a significant challenge. The low latitude ionosphere has been a recent focal point of ionospheric study. A noted feature of this region is the equatorial anomaly (EA) [2,3,4] where the globally highest plasma densities can occur. Large scale plasma density depletions that are often referred as equatorial plasma bubbles (EPB) [5,6,7,8,9] are also typical of this region. Understanding the processes that form the EA and control plasma disturbances is a significant component of the larger study of the Sun-Earth system.

TIMED/GUVI 1356Å night side airglow measurements

The equatorial zonal electric field is the driver of the plasma transport process that creates the EA. Extensive studies of equatorial vertical plasma drifts [10,11,12] have lead to recent strides in understanding this process on a global scale. EA morphology and the distribution of plasma as a function of the zonal electric field is not as well understood. This is due in large part to the difficulty of measuring plasma density across the latitudinal extent of the EA at a given zonal sector. Similarly, plasma disturbances such as spread F and EPBs have been studied extensively [5,6,7,8,9,13]. Here again, due to the difficulty in measuring plasma density over large spatial regions the relationship between plasma distribution in the EA and it's affect on EPB growth conditions is lacking.

There are several important studies that have been able to analyze these relationships with high space-time resolution in the Western-American sector (75o W) [18,19,20,21]. The limited data used in these studies was collected from ground-based instruments and represents a coarse longitudinal sampling. One on going study uses FUV limb scans from GUVI on NASA’s TIMED spacecraft to estimate vertical electron density profiles parallel to the satellite trajectory. This technique however assumes plasma density is horizontally constant over a 1000 km range. Further, any occurrence of a plasma irregularity, such as an EPB, causes significant estimation error. Thus, the technique fails to provide quality measurements during critical events.

The purpose of this proposed mission is to put an instrument into orbit that can measure vertical electron density profiles with 20 km resolution in the nightside ionosphere at all longitudes and local times. The technique will provide quality measurements during plasma disturbance events. In doing so, the mission builds directly on the previous works of Scherliess and Fejer [16] and Whalen [18] to understand EA and EPB morphology on a global scale.

The proposed mission and instrumentation are very similar to that originally attempted as part of the NASA Student Explorer Demonstration Initiative by Boston University. The TERRIERS satellite was to produce unprecedented three-dimensional tomographic maps of the ionosphere but unfortunately the spacecraft’s control system failed shortly after launch in 1999 and no data was returned. We intend to follow their analytical methods but with a simpler instrument and mission.

The TOROID Science Instrument

The measurement methodology is a technique similar to TERRIERS for obtaining altitude profiles of the low-latitude nighttime ionosphere and accompanying large scale irregularities. The primary science instrument, TOROID - TOmographic Remote Observer of Ionospheric Disturbances, is a photometer to measure the 135.6 nm airglow of electron and oxygen ion recombination. The data will be gathered in such a way that tomographic techniques can be used to construct the altitude profiles with a 20 km resolution of ionospheric density along the spacecraft trajectory.

The TOROID instrument will sample FUV emission from the nightside ionosphere. It is based on the design of the well proven normal incidence single element Rowland spectrograph [24,25]. The optical layout is shown in Figure 1. The light first passes through a collimator to eliminate off-axis radiation through the slit. It is then reflected off of a diffractive grating with curvature specified by the Rowland circle. A photomultiplier tube based photon counting detector is then placed behind a slit on the Rowland circle opposite the grating. The location is at the point of the desired wavelength. The present design calls for a sampled non-simultaneous 360° field of view which will be accomplished by means of a mirror mounted to the space craft body. The mirror will rotate continuously in 360° and direct light into the spectrograph. Other designs have made use of a rotating mirror for imaging purposes .

Figure 1: Optical layout of the TOROID Photometer

We propose to use ionospheric tomography [27] to estimate volume emission levels in the intersection of the ionosphere and the satellites orbital plane. To do this, the mirror rotation axis will be parallel to the angular momentum of the satellites orbit. A diagram of the sampling geometry is given in Figure 2. Intensity is sampled through several consecutive revolutions of the mirror. The set of observations effectively breaks the volume under study into a segmented patch work of smaller volumes. If emissions within each segment are assumed uniform then they can be thought of as point sources whose intensity is a function of the known volume size and distance to the satellite. Observations can then be modeled as linear combinations of grouped point sources and least squares techniques can be used to estimate pixel intensities.

Figure 2: Natural Pixel Ionospheric Tomography

The close affiliation of USU with SDL provides a wealth of resources for design and construction of the TORIOD Photometer. One recent series of instruments that has great relevance to the presently proposed spectrograph is ATOX. Dr. Charles Swenson, the PI, in association with Blake Crowther and Pat Patterson of SDL supported the development and flight of the Atomic Oxygen (ATOX) sensor on multiple sounding rockets. ATOX uses a vacuum ultraviolet (VUV) atomic oxygen lamp to measure the density of atomic oxygen along the sounding rocket trajectory. The atomic oxygen in the space environment resonantly scatters the energy from the lamp which is observed by photomultiplier tube based detectors. An increase in oxygen density is correlated with an increase in the detector photon count. This system operates at the same 135.6 nm FUV wavelength as the proposed TORIOD Photometer. Thus, we propose to use the ATOX photometer design as a basis for TORIOD.

Dr. Swenson also has experience with remote sensing of oxygen airglow from radiative recombination. He is currently involved in analysis of data collected by the Global Ultraviolet Imager (GUVI) on NASA's TIMED spacecraft. This work involves characterization of the EA, detection of EPBs, and global morphology studies.

The Small Satellite Program is actively soliciting small, low-power scientific payloads to contribute to this mission. Sufficient mass, volume, power and telemetry margins exist for a limited secondary science payload that is consistent with the primary science mission.


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