Spaceweather Products
GFZ Potsdam

Intense currents flowing in the ionosphere in association with the auroral oval, i.e. the polar/auroral electrojets, are believed to be the principal cause of magnetic disturbances giving rise to problems in power systems as a result of geomagnetically induced currents (GIC’s) [1]. Monitoring their location and intensity level is therefore an important matter for space weather. Intense currents and electric fields in the auroral region are also associated with ionospheric disturbances causing problems for navigation and communication.

Currently the electrojets are monitored using ground-based geomagnetic observations and derived indices, most prominently AE, AL and AU. The available magnetic stations naturally limit the ground-based monitoring. Their monitoring would therefore be well complemented by the polar orbiting Swarm satellites scanning all latitudes in 2 orbit planes.

Recent studies have shown that when a polar orbiting satellite, in an altitude range similar to the Swarm satellites, passes the auroral oval the latitude of the maximum PEJ encountered during the pass as well as its intensity level can be estimated from observing the absolute magnetic field intensity profile [2]. The intensity of PEJ depends both 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 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 [3]. Only the observed absolute magnetic field strengths and the satellite position is 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 [4]. Three levels of PEJ intensity are provided (green, yellow, red). The attributed intensity level is determined from the accumulative occurrence distribution of the PEJ intensities derived from the Swarm data base. Green is assigned to PEJ intensity values that fits to the lowest 80% of the PEJ intensity values in the statistical data base, red is assigned to PEJ intensity values that fits to the highest 0.8% of the PEJ intensity values in the statistical data base, 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, 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.

References

[1] "The complex-image method for calculating the magnetic and electric fields produced at the surface of the earth by the auroral electrojet"
Boteler, D.H., and R.J. Pirjola (1998), Geophys. J. Int., 132, 31–40, doi:10.1046/j.1365-246x.1998.00388.x

[2] "Monitoring auroral electrojets with satellite data"
Vennerstrom, S. and Moretto, T. (2013), Space Weather, 11, 509–519, doi:10.1002/swe.20090

[3] "Swarm Corrected Mag-L Preliminary Data Release Notes"
SW-RN-DTU-GS-002, rev. 3, 19 Mar 2015

[4] "Data Quality for L1B and L2 Cat-2 products"
https://earth.esa.int/web/guest/swarm/data-access/quality-of-swarm-l1b-l2cat2-products