DOI: 10.5880/gfz.2.3.2024.003
title: SUA PEJ: Swarm Polar Electrojet

description: Recent studies have shown that when a polar-orbiting satellite, in an altitude range similar to the Swarm satellites (ESA's Earth Observation constellation mission), passes the auroral oval, the latitude of the maximum polar electrojet (PEJ) encountered during the pass, as well as its intensity level, can be estimated by observing the absolute magnetic field intensity profile. The intensity of PEJ depends on the coupling of the solar wind to the magnetosphere, the state of the magnetosphere and the ionospheric electrical conductance, which in turn depends on the season, local time and the intensity of auroral particle precipitation. Each orbit of one of the Swarm satellites provides data from four passages of the auroral oval - two in each hemisphere. 
 
The location and intensity level of the polar electrojets can be derived from the Swarm measurements of the magnetic field strength at 1 Hz sampling, currently provided as the Swarm L1b product MAGx_LR_1B. Only the observed absolute magnetic field strengths and the satellite position are required, which means that the algorithm is independent of both attitude observations and L1b processing of the vector data. The derivation also needs input from a model allowing for along-orbit estimation of the magnetic field created in the ionosphere. Currently, magnetic field contributions for the core, lithosphere and large-scale magnetosphere are predicted by respective magnetic field models provided as products from the Swarm mission. Three levels of PEJ intensity are provided (green, yellow, and red) with corresponding peak current values in kA. The attributed intensity level is determined from the accumulative occurrence distribution of the PEJ intensities derived from the Swarm database. Green is assigned to PEJ intensity values that fit the lowest 80% of the PEJ intensity values in the statistical database, red is assigned to PEJ intensity values that fit the highest 0.8% of the PEJ intensity values in the statistical database, and yellow to values between these two groups. The rationale for this choice of PEJ intensity levels was that the occurrence distribution of the Kp value in the intervals Kp = 0-2, 3-4, and 5-9 during this period is similar. However, it should be underlined that individual PEJ intensity levels cannot be directly translated into the associated Kp level for a given local pass. Although the two are related, it is far from a one-to-one correspondence between the PEJ intensity and the Kp value. 
 
The Swarm Utilisation Analysis (SUA) PEJ dataset is structured in ASCII format with daily files. The data files follow the naming convention "SW_OPER_PNMXTMSSWE_YYYY-MM-DD," where SW stands for Swarm, OPER for operational, PNM for a product name (e.g., PEJ), X for Swarm satellite name (A, B, or C), TMS for time series, SWE for Space Weather, and YYYY, MM, DD for year, month, and day, respectively. The dataset can be accessed at ftp://isdcftp.gfz-potsdam.de/swarm/SUA/PEJ_TMS for all three Swarm satellites. 
 
Each SUA PEJ data file contains nine variables (from column 1 to column 9) and starts with a header line: mjd2000; date [YYYY-MM-DDThh:mm:ss]; orb num; lat [deg]; lon [deg]; mlat [deg]; mlt; color code; Peak current intensity [kA]. The variable definitions are as follows: mjd2000 (modified Julian day that starts at midnight of 01/01/2000); date (YYYY-MM-DDThh:mm:ss, where hh:mm:ss represents hour, minute, and seconds expressed in UTC); orb num (Swarm Orbit Counter); lat (geocentric latitude in degrees); lon (geocentric longitude in degrees); mlat (magnetic latitude, modified apex, in degrees); mlt (magnetic local time in decimal hours); color code (string values of green [g], yellow [y], and red [r]); Peak current intensity (in kiloamperes).