Intense solar magnetic activity and resulting geomagnetic storms observed on Earth, known as space weather, are becoming increasingly important due to their negative effect on satellite-based communication and navigation systems.
One type of space weather phenomena is electron density fluctuations in the ionized regions of the Earth’s atmosphere, known as ionospheric plasma turbulence. The intense flow of ionospheric electrical currents energized by solar wind and magnetospheric electromagnetic forcing often causes plasma turbulence between the altitudes of 100-500 km, which degrades or disrupts trans-ionospheric signals, causing signal fading or phase distortions. For example, a type of distortion called scintillation, which is conceptually similar to the twinkling of the stars, can make GPS signals unusable.
SRI's compact Radio Aurora Explorer (RAX) is a satellite known as a CubeSat—its dimensions are only 10x10x30 cm. Designed to investigate causes of upper atmospheric/ionospheric plasma turbulence, RAX is the first project under the National Science Foundation (NSF) CubeSat-based Space Weather and Atmospheric Research Program. The first RAX satellite was launched via the Department of Defense Space Test Program aboard a Minotaur-4 vehicle from Kodiak, Alaska, on November 20, 2010. The second RAX was launched through the NASA CubeSat initiative from Vandenberg AFB to a polar elliptical orbit (400x800 km).
A radar experiment aboard RAX uses a dynamic ground-to-space bi-static (i.e., transmission and reception occur at separate locations) scattering geometry that changes according to the orbital motion of the spacecraft. This dynamic geometry enables measurement of meter-scale ionospheric plasma irregularities with high spatial and angular resolution (with respect to the geomagnetic field). The fine angular measurements will quantify for the first time the energy deposition by plasma waves. Further, the signals incoherently scattered back to the ground are used to estimate, using the incoherent scatter theory, the plasma parameters (electric field, electron density) under which the turbulence occurred, effectively establishing a controlled, wall-less plasma laboratory in space.
The reason for, and the novelty of, the ground-to-space bi-static scattering geometry is that it is impossible for a CubeSat alone to have sufficient transmission power or a large-enough signal collecting area to illuminate and sense the turbulent structures. For RAX, the plasma turbulence is illuminated by megawatt-class incoherent scatter radars (ISRs). The scattered radiation, also known as “radio aurora,” is measured by the RAX ultra-high-frequency (UHF) radar receiver payload. The UHF receiver operates with five ISRs on the ground:
- PFISR at the Poker Flat Research Range, Alaska
- Resolute ISR (RISR) in Resolute Bay, Canada
- ESR in Svalbard, Norway
- Millstone Hill, Massachusetts
- Arecibo, Puerto Rico
The RISR facility enables observations of ionospheric turbulence driven directly by the Earth’s solar wind dynamo, the source that circulates the entire polar cap ionosphere.
The RAX I mission was terminated following a power system failure two months after launch. Meanwhile, the RAX II satellite, launched in October 2011, is investigating natural and artificial (by high-power, high-frequency ionospheric heating) ionospheric irregularities.