This project is a collaboration between a space science Principal Investigator (PI) at SRI International and an engineering PI at University of Michigan. The objective of this three-year cross-disciplinary team effort is to build and operate a tiny, so-called CubeSat, spacecraft carrying a UHF radar receiver payload. Launch of the satellite will be as a secondary on a Department of Defense launch scheduled for December 2009. The satellite will be operated in coordination with the AMISR incoherent scatter radar from the ground to investigate the radio aurora from field-aligned irregularities in the high-latitude ionosphere. The primary scientific objective of the Radio Aurora Explorer (RAE) mission is to understand the microphysics of plasma instabilities that lead to field-aligned irregularities (FAI) of electron density in the polar lower (80-300 km) ionosphere. The RAE mission is specifically designed to remotely measure, with extremely high angular resolution (~0.5 degree), the wave spectrum of ~1 m scale FAI as a function of altitude, in particular measuring the magnetic field alignment of the irregularities. Due to the magnetic field geometry at high latitudes this bi-static (ground radar to satellite) configuration, in which a narrow radar beam is scattered off the FAI and then observed by the CubeSat receiver, is the only way to perform these measurements. Better understanding of ionospheric irregularities and their role in ionospheric dynamics is an important space weather research objective because plasma structures in the ionosphere can have an adverse effect on communications via satellite, HF and VHF radio and as well as an adverse effect on navigation, tracking, and positioning. The project has a very strong educational component; it relies on extensive undergraduate and graduate student involvement through all aspects of the mission. The new, largely unproven technology involved in cubesat missions, inherently makes the project associated with significant risks. On the other hand, however, the project has tremendous potential to be transformational not only within its own research area but also for the larger field of space science and atmospheric research as well as within aerospace engineering and education.
The NSF funded the Radio Aurora Explorer (RAX) space weather mission to determine the causes of auroral turbulence in the Earth’s upper atmosphere. Auroral turbulence is a space weather phenomenon, comprised of geomagnetic field-aligned electron density irregularities (FAI), which affects trans-ionospheric communication and navigation signals like GPS. Large-amplitude FAI make the trans-ionospheric path less transparent to these signals, reducing their quality or disrupting them completely. Furthermore, these FAI are capable of scattering radio signals, which has a deleterious effect on space-based remote sensing. Understanding the processes that lead to the generation and distribution of auroral turbulence will enable short-term forecasting of when disruptions will occur. RAX is unique in its ability to provide measurements of FAI with unprecedented resolution, and it is the only way to measure meter-scale FAI at high latitudes. Traditionally, FAI have been studied with ground-based radars, but the geomagnetic geometry at high latitudes causes the radar signals to bounce off the nearly vertical geomagnetic lines and escape to space, making it difficult to probe the Earth’s upper-atmospheric turbulence. RAX is designed to catch these bounced-off signals. Each RAX experiment is conducted with a ground-based radar, most often the Poker Flat Incoherent Scatter Radar (PFISR), located in the University of Alaska’s Poker Flat Research Range. This megawatt-power NSF research radar transmitter, operated by SRI International (SRI), is used to illuminate the turbulent structures. The radar sends electromagnetic pulses in an extremely narrow beam (about 1 degree), and the FAI acts like a mirror, reflecting some of the energy of the beam into space. RAX experimental passes are timed to coincide with this radiation and measure its intensity along the spacecraft’s path. The FAI location is determined using the directly arriving signals to time the echoes. Some of the energy sent by the ground radar is scattered back to itself as incoherent scatter. These signals are used to measure the background states of the ionosphere, such as its electron density and electric fields. The RAX mission is a joint effort between SRI’s Center for Geospace Studies and the Michigan Exploration Laboratory (MXL) at the University of Michigan. Instruments were developed at SRI and MXL designed, built, and integrated the spacecraft at MXL. Work on RAX began in late 2008 and two satellites are currently in orbit: RAX-1, launched in November 2010, and RAX-2, launched in October 2011. Science operations are managed at SRI and satellite operations take place at MXL. Ground stations around the world have partnered to downlink science and telemetry data from the RAX satellites. RAX-1 demonstrated on-orbit operation of the radar and satellite systems; however, its mission ended prematurely due to shadow-induced solar panel failure several months after launch. RAX-2 is operational and is performing routine science measurements; it will be operated until its end of life, which is expected to be in three to four years. The RAX team has been running experiments on a weekly basis since the launch of RAX-2 in October 2011. However, extremely quiet geomagnetic conditions precluded the detection of echoes—some of the experimental data were completely discarded without even processing. Finally, for the first time on the 18th experiment, the RAX satellite detected PFISR’s radio waves bounced off FAI located in the Earth’s upper atmosphere around the altitude of 100 km. This happened during the solar storm that struck Earth on 7 March 2012. The echoes occurred exactly where we expected them, at about 300 microseconds after the arrival of the direct pulse. Meanwhile, the electric fields were measured at about 70 mV/m—way exceeding the threshold for the Farley-Buneman instability (an instability of ionized gas operating around the altitude of 100 km) that is responsible for the turbulence. During this event, the Poker Flat magnetometer recorded a 550 nT magnitude for the horizontal component, which is a large magnetic deflection indicative of strong ionospheric currents. Therefore, the conditions were optimal for echo generation. The RAX data from this 18th experiment provides information on the distribution of FAI at altitude and on their alignment with the geomagnetic field, directly addressing the original scientific objectives of the project. Currently, these measurements are under study. Overall, RAX, the first NSF space weather satellite project, has demonstrated a unique capability of CubeSats: that they can be launched rapidly to complete a space mission in only a few years and at low cost. RAX has shown that a science-quality mission implemented on a CubeSat is a very effective tool for scientific research. The RAX CubeSat has already contributed immensely to the science and engineering education of more than 50 undergraduate and graduate students, primarily at the University of Michigan. The data from the first-time detection of auroral turbulence from a CubeSat-based radar receiver proves that this space mission is not just educational, but that it is also a unique method of performing space research.
