ON-ORBIT PERFORMANCE OF THE MICROWAVE ELECTROJET MAGNETOGRAM (MEM) RADIOMETER ON THE ELECTROJET ZEEMAN IMAGING EXPLORER (EZIE) MISSION

S. Misra¹, S. Padmanabhan¹, P. Kangaslahti¹, F. Werner¹, M. Schwartz¹, F. Ayoub¹, S. Lee¹, S. Yee², S. Brown¹

¹Jet Propulsion Laboratory, California Institute of Technology, ²Applied Physics Lab, Johns Hokins University

The Electrojet Zeeman Imaging Explorer (EZIE) mission was launched in March 2025 and is responsible for measuring the Earth’s ionospheric electrojet currents. EZIE exited its commissioning stage in May 2025 and will release calibrated brightness temperature data in November 2025. EZIE measures oxygen emissions from the Earth’s ionosphere at around an altitude of 80km. EZIE measures both Auroral and Equatorial Electrojets Currents (AEJ/EEJ respectively). The AEJ is derived from the solar and global current system that connects the solar winds, Earth’s magnetosphere, and Earth’s ionosphere. The EZIE mission is slated to explain the structure of the AEJ and help resolve several competing theories of the temporal and spatial nature of AEJ.

The EZIE mission consists of three CubeSats within the same polar orbit with an approximately 2-10 minute orbit lag to allow sufficient temporal sampling of the AEJ phenomenon. Spatial sampling is achieved via four microwave radiometers on each CubeSat known as the Microwave Electrojet Magnetogram (MEM) that are pointed at four separate incidence angles across track, allowing a 200-1000 km crossbeam swath coverage.

Each MEM radiometer itself measures V and H-pol emissions through an antenna feed with low cross-polarization coupling, that gets split via an ortho-mode transducer and sent to front-end InP LNA units, at 118.75GHz. The RF is mixed down via a common LO (amongst 4 receivers per MEM) to IF. One of the sidebands for each polarization and each receiver is then fed into a backend FPGA, that does digital polarimetry and spectrometry. The spectrometer measures the spectral split in the Oxygen line, which is a function of the magnetic field strength in the observed ionosphere, which in itself is a function of the electrojet currents.

We have presented MEMs unique architecture and EZIEs unique mission design before. Here we will present post-launch on-orbit performance of all 12 MEM radiometers on three spacecraft. We will present the unique calibration technique that MEM uses and its behavior. MEM radiometer does not have any internal calibration sources and uses a novel calibration technique. MEM measures cold-sky before and after every science orbit which is approximately 10 minutes over the magnetic pole. The cold sky has helped tune calibration coefficients post-launch. MEM uses a single point calibration technique, where the receiver temperature is known and derived from physical temperature of the radiometer. The other point of calibration is obtained from a radiative transfer forward model predicting temperatures 5-10MHz away from the Oxygen line center. The ratio of the spectra under calibration and the off-line calibration point allows any common mode variations (and 1/f noise) of the radiometer to be eliminated and the calibration to be transferred to the full spectra.

We will present challenges to this technique and necessary post-launch calibration tuning that occurred to obtain reliable measurements. The measured polarimetric brightness temperature spectra has several known physical behaviors when measuring a Zeeman phenomenon. We take advantage of these expected physical behaviors to calibrate the full spectra. We used an iterative and minimization technique to calibrate out inconsistencies or anomalies with respect to the known behavior to tune MEM calibration coefficients. Some of the expected brightness temperature behaviors that helped with calibration were – (1) Flat cold-sky brightness temperature spectra, (2) Unpolarized (zero) cold sky third and fourth Stokes, (3) Unpolarized (zero) off oxygen line atmospheric brightness temperature, (4) Symmetry of V, H, and 3rd Stokes brightness temperature spectra about the oxygen center line, (5) Asymmetry of the 4th Stokes brightness temperature spectra about the oxygen center line, (6) Predictable Doppler shift in line center due to space-craft movement, Earth’s rotation, Neutral winds, LO shifts, and MEM pointing. We took all of these into account in order to calibrate gain, receiver temperature, relative phase delay between V and H, antenna pattern matrix etc.

We will also present validation of the MEM radiometer calibration, which was done by three independent techniques. In the first technique, we compared the full radiance polarimetric spectra to that generated by the ARTS model (Atmospheric Radiative Transfer Simulator). The spectra shows good match across the board with ARTS generated model radiances, with minor differences in line widths etc. We also compared it with another independent radiative transfer model generated by the Microwave Limb Sounder (MLS) mission that shows even better match. The second technique is to look at the derived temperature profile product from the oxygen sounding brightness temperature. We compared EZIE T profiles to that measured by MLS or SABER around the same time and location to see good match and validate calibration consistency. Finally, we also do a comparison of calibration robustness by comparing derived magnetic field values with ground validation sites – that show good one to one comparison.

We will present new results from the EZIE mission as well as present an assessment of the unique calibration from the first year of mission data for all 12 radiometers.

This research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with NASA.