We plan to measure space weather phenomena that affect how radio waves propagate through the ionosphere with an instrument suite with two payloads, an ionosonde receiver and a photometer. These measurements will have the potential to advance satcom, OTH radar, and geomagnetics research. Our methods and the background science behind them are summarized here, and detailed proposals will be linked here as they become available. For more information about the RF payload, contact instrument lead Ryan Tse, and for more information about the optical payload, contact instrument lead Laura Cui.
RF Payload (Ionosonde Receiver)
The ionosphere is a region of the atmosphere that contains charged plasma. The charged material in this region can influence wireless communications since charged plasma refracts radio waves. This has a signficant effect on long distance radio communications and HF radar systems. Mapping the charge density of the ionosphere has been a longstanding scientific objective, as a live map of ionospheric conditions would allow radar systems to more precisely correct for ionospheric effects and would allow communications systems to exploit ionospheric refraction to perform longer range communication with less power. blair3sat’s RF payload will measure ionospheric charge density by observing the refraction of test signals transmitted from ground-based radar sounders called ionosondes.
Scientists currently measure charge density in the ionosphere by transmitting HF radio waves of different frequencies into the atmosphere and receiving the reflected and refracted waves. Since the refraction is a function of the frequency of the signal and the charge density, one may estimate the charge density at different places by considering the time delay between when each frequency was transmitted and when each frequency was received. These instruments are called ionosondes, and graphs of transmitted frequency versus time delay are called ionograms.
Unfortunately, ionosondes’ output data do not provide enough information to completely reconstruct the entire charge density gradient. Ionosonde can measure signals’ time of flight, but they cannot discern what paths the signals take. Existing ionosounding methods estimate charge density as a function of altitude by making simplifying assumptions about the path taken by the signal.
blair3sat will reduce ambiguity and improve mapping precision by receiving ionosonde signals in space before they reflect and refract back down to the surface. Signals that are totally refracted above blair3sat will be received by the RF payload twice: first from the direct path between the instrument and the ionsonde, and second from signals refracted back down from above the instrument. The time delay between the two times that the signal is received will be processed to map of the ionosphere above the instrument. Signals that are only received once by blair3sat and signals that are recieved by blair3sat but not the ground ionosonde do not allow us to generate complete maps of the ionosphere along their paths, but they do provide rough information about the path taken by the signal. blair3sat’s data will be combined with data from cooperating ionosonde stations, processed, and published.
Excited atoms in the ionosphere emit diffuse electromagnetic radiation corresponding to a characteristic spectrum. These emissions are produced by a variety of photochemical processes, and are specific to the ion species and conditions of the ionosphere. Optical measurements can be used to determine the concentration of different species as well as the electron density in a given region. With this data, we can also reconstruct three-dimensional structures, improving predictive models of dynamic processes in the ionosphere. blair3sat will use a limb-viewing photometer to measure variations in airglow intensity over the course of its mission.
Incoming solar radiation interacts with oxygen and nitrogen atoms in the neutral thermosphere to produce free ions and electrons, which can then participate in a variety of light-producing processes. In particular, during recombination, metastable or long-lasting electron configurations produce diffuse photons with distinct wavelengths. The intensity of the light is proportional to both ion and electron density as well as the emission rate of the transition, which depends on conditions such as temperature and neutral species density. Typically, this is taken as a line integral of the emission rate multiplied by the square of the electron density along the line of sight, allowing us to extract vertical electron density profiles from the data.
The use of the extreme ultraviolet airglow spectrum to measure ionospheric activity is well-established. While these emissions can be observed minimal background pollution, they are typically an order of magnitude less sensitive than visible spectrum measurements. Recently, the OI 557.8 nm line has received attention as another candidate for studying ionospheric activity, with the advantages of increased sensitivity and density dependency, which allow additional altitude information to be extracted from the measurements.
blair3sat will use a limb-viewing photometer design combined with a set of passband filters and visible range CCD sensor optimized for observing the 557.8 nm emission. The instrument will be placed at the back of the satellite, and oriented to look slightly downwards towards the horizon. The inclination of the satellite orbit, combined with its altitude near the peak of the daytime F layer, offer the opportunity to observe daily variations over a wide geographical area. Furthermore, by combining and correlating data collected by the optical and RF payloads, we will be able to better characterize the measurements made by each, allowing us to cross-validate our methods and to better understand the behavior of the 557.8 nm emission.