J-Filtering: A Novel Multipoint Technique for Current Distribution Analysis in Space Plasmas

Both the interplanetary medium and the near-Earth space are filled with plasmas and a key remaining question in space physics is the understanding of the processes governing the energization of both particles and waves in space plasmas. Measurements of the fields in space plasmas exhibit temporal and spatial variations across all observed scales. Single-satellite measurements provide only a partial picture because they cannot capture the details of these variations. Multipoint missions, particularly the four-satellite tetrahedron configurations of CLUSTER  and MMS , were launched to overcome this limitation. Specialized techniques for multipoint data analysis have been developed. Among them, the Curlometer exploits the magnetic field measurements of the individual spacecraft magnetometers and uses Maxwell-Ampere's law to estimate the current density (J) through the tetrahedron formed by the four-spacecraft constellation. However, it assumes a linear spatial variation of the magnetic field across the spacecrafts, which actually seriously limits its applicability in space plasmas. To overcome the limitations of the Curlometer, we are proposing a new technique called J-Filtering (where J represents current density) for measuring and visualizing local current distributions in space. The idea behind J-Filtering is to borrow the principle of optimal filter determination from the K-filtering method, which was developed for the CLUSTER mission. Here, the filters are defined to allow identification of the current structures that are responsible for the magnetic fields measured by the spacecrafts of the constellation . We will present the principles of J-Filtering and its first applications to spacecraft data from CLUSTER, showing in particular its validation by comparison with the Curlometer results when the linear spatial variation condition is assumed. We will also present results obtained by applying the techniques to MMS data specifically for thin current sheets at reconnection sites where the Curlometer method can be not valid.