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Radio Aurora Explorer (RAX) mission is a CubeSat-based radar experiment designed to investigate the causes of upper atmospheric/ionospheric turbulence driven by solar wind and magnetospheric electromagnetic forcing. RAX is the first CubeSat being constructed under the NSF CubeSat-based Space Weather and Atmospheric Research Program. The project is being carried out jointly by SRI International and the University of Michigan, since September 2008. The satellite was launched via the Department of Defense Space Test Program aboard a Minotaur-4 vehicle from Kodiak, Alaska, on November 20, 2010 at 01:25 UTC.

The RAX radar experiment utilizes a dynamic ground-to-space bistatic scattering geometry that is changing by 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 plasma turbulence is illuminated by megawatt-class incoherent scatter radars, and the scattered radiation, also known as "radio aurora", is measured by RAX UHF radar receiver.

The primary scientific objective of the RAX 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. Unlike the FAI in the equatorial ionosphere, high horizontal and altitudinal resolution measurements of high latitude FAI have not been possible mainly because it is difficult to achieve a scattering geometry normal to the magnetic field lines, which are nearly vertical in the high latitudes. Coherent scatter radars can be pointed to very low elevations to scatter off normally, however, these radars cannot make height-resolved measurements because the beam width in the vertical direction is too wide or because refraction causes source location ambiguity. Moreover, ionospheric altitudes below 300 km are beyond the reach of orbiting spacecraft. Sounding rockets measured FAI with the highest altitude resolution, yet they are not sufficiently sensitive to sort out wave energy in magnetic aspect angle.

The RAX mission is specifically designed to remotely measure, with high angular resolution (0.5 degrees), the 3-D k-spectrum (spatial Fourier transform) of about 1 m scale FAI as a function of altitude, in particular measuring the magnetic field alignment of the irregularities. The spacecraft will measure radio aurora, the Bragg scattering from FAI that are illuminated with a narrow beam incoherent scatter radar (ISR) on the ground. The scattering locations are determined using a GPS-based synchronization between ISR transmissions and satellite receptions and the assumption that the scattering occurs only inside the narrow ISR beam.


Figure 1. Experiment description
The Figure above shows a drawing of the irregularities (red wiggles), the magnetic field lines, the radar beam, radio aurora (cones) and the satellite (cubes), and the satellite tracks. The irregularities inside the narrow radar beam and at a given altitude scatter the signals in a hollow cone shape. The thickness of the wall of each cone is a measure of magnetic aspect sensitivity, which is also a measure of plasma wave energy distribution in the parallel and perpendicular directions with respect to the geomagnetic field.

The main RAX science data product is I(E, Ne, Te, Ti, h, theta), that is, irregularity intensity (I) as a function of convection electric field (E), electron density (Ne), electron and ion temperatures (Te, Ti), altitude (h), and magnetic aspect angle (theta). RAX measures I,the ISR on the ground measures the plasma parameters, and (h, theta) are given by the experimental geometry.


Table 1. Plasma waves responsible for radar scattering.

Table 1 shows the list of potential plasma waves making up the meter-scale irregularities in the ionospheric E and F regions. Radar returns will be analyzed to identify the waves responsible for radar scatter and to determine the ionospheric conditions leading to the corresponding plasma instabilities.

SYSTEM CAPABILITIES

The payload radar receiver operates in a snapshot acquisition mode collecting raw samples at 1 MHz for 300 s over the experimental zone. Following each experiment, the raw data is post-processed for range-time-intensity and, in certain modes, Doppler spectrum. The snapshot raw data acquisition enables flexibility in forming different radar pulse shapes and patterns. In addition, PFISR electronic beam steering capability can be utilized for simultaneous multiple beam position experiments. The system angular and spatial resolutions depend on the scattering geometry which is a function of the satellite position. The radar resolutions at optimal satellite position are 3-5 km spatial and 0.5 degree angular. The image below shows altitude-aspect angle ambiguity functions assuming a 1 us pulse.
Spatial-angular ambiguity functions for different parts of a scattering zone.

In the scattering zone (magnetic aspect angles less than 3 degrees), radar Bragg wavelength also depends on the satellite position and ranges between 0.4-2.0 m, with a concentration near 0.5 m. Also, for a given altitude and magnetic aspect angle, the satellite will cross a cone at two points during a single experiment. The two crossings will provide measurements at two different radar Bragg wavelengths (and flow angles), corresponding to measurements at a pair of k-spectrum points.

A picture of the payload radar receiver is shown below. A detailed description can be found here: UHF radar receiver

RAX payload: UHF radar receiver


The payload radar receiver is designed to operate with the radars shown below.


Table 2. UHF Incoherent scatter radars


(PFISR, Poker Flat, Alaska (left) and RISR, Resolute, Canada (right)
ESR, Svalbard, Norway (left) and Millstone Hill Radar, Westford, MA (right)
Arecibo, Puerto Rico

SENSITIVITY TO RADAR SCATTERING


Figure 2. Parameters of the radar equation on the left and, on the right, the statistical distribution of RAX received power (scaled from HOMER radar)

The radar sensitivity is nearly (to a few dB) equivalent to that of the mono-static Homer UHF radar which operated from Homer, AK in 1970s. Figure 2 shows the two statistical distributions of Homer radar received power from auroral electrojet irregularities for two different observation periods. The bottom axis is rescaled for RAX.

NUMBER OF EXPERIMENTAL PASSES


Figure 3. The loci of perpendicularity (< 3 degrees) and 1 min satellite tracks over the experimental zone for 30 days.

The satellite can perform 2-3 conjunction experiments per day. Figure 3 shows the 1-min satellite tracks that pass through the scattering zone. There are ~1000 passes good for experiments in the 1- year mission lifetime. However, based on DE-2 electric field statistics, approximately 1 out of 5 experimental passes will have an electrojet speed above the two-stream instability threshold, resulting in electrojet irregularities.