# Perpendicular Electrical Conductivity in the Topside Ionosphere Derived from Swarm Measurements
## 2 Data and Methods  
### Swarm Observations and Preliminary Analysis  
We used eight years of Level 1b data acquired at 1 Hz sampling by both the Langmuir probes (LPs) of the Electric Field Instrument (EFI, [63]) and the Absolute Scalar Magnetometer (ASM) instruments on board the European Space Agency's (ESA's) Swarm A satellite [54]. All data span the time interval from 1 April 2014 to 31 March 2022, just before the occurrence of the manoeuvres in April 2022 that substantially changed the satellite's altitude. During the selected period, the satellite flew in a low and nearly circular orbit and a polar orbit with an inclination of \(\sim\)87.4\({}^{\circ}\) and an average altitude of \(\sim\)450 km. Due to its orbit, in the selected period, the Swarm A satellite took about 133 days to cover all local times. Thus, the selected eight-year time window ensures adequate coverage of the globe with a robust number of observations at each local time within a 24 h day, even when parcelling out the data by making selections, for instance, on season or solar activity. Level 1b data were downloaded from the LATEST_BASELINES folder of the ESA dissemination server. Specifically, 1 Hz Langmuir probe data are those identified by the label \"EFlxLPI\" and are obtained from data measured at 2 Hz in harmonic mode [64], (which are identified by the label \"EFlx_LP\") by interpolation at full UTC seconds. LP data have been the subject of several calibration and validation studies aimed at understanding the effectiveness of comparing and inter-calibrating data from different satellites [65; 66; 67; 68; 69; 70; 71; 72; 73]. These studies used for the comparison of incoherent scatter radars, GPS radio occultation, and ionosonde data and pointed out, for example, that LP data generally underestimate values of electron density, while they tend to overestimate observations during low solar activity, particularly at nighttime. Differently, LP electron temperature values show an overall overestimation, particularly at low latitudes. However, we decided to use the original Level 1b data to be consistent with previous work by the same authors aimed at studying electrical conductivity in the topside ionosphere using Swarm's LP data. Level 1b provides UTC time, the position of the satellite in geographical latitude and longitude, and in situ electron density and temperature observations. The latter were filtered out based on the quality flags provided by the mission team. Swarm LP observations recorded in High-Gain mode are those considered as 'nominal' and are recommended for scientific purposes. According to the Swarm's product definition document (Swarm L1b Product Definition, 2018; available online: [[https://earth.esa.int/eogateway/documents/20142/37627/swarm-level-1b-product-definition-specification.pdf](https://earth.esa.int/eogateway/documents/20142/37627/swarm-level-1b-product-definition-specification.pdf)]([https://earth.esa.int/eogateway/documents/20142/37627/swarm-level-1b-product-definition-specification.pdf](https://earth.esa.int/eogateway/documents/20142/37627/swarm-level-1b-product-definition-specification.pdf)), accessed on 23 August 2024), we selected nominal LP observations with the following choice of the flags--Flag_LP=1 and Flags_Te or Flags_Ne parameters-- equal to 10, 19, or 20. The remaining part was discarded as 'not nominal' according to the Swarm's product definition document. Data missed or discarded after the quality flags check were replaced by not-a-number (NaN) values to guarantee the continuity of the time series. This procedure allowed us to consider only high-gain data from the Langmuir probes. Successively, magnetic data were prepared. We used those identified by the label 'MAGx_LR' available in the LATEST_BASELINES folder as conducted for LP data. Then, for UTC time, geographical latitude and longitude of the selected LP data, we picked the corresponding value of the total intensity of the geomagnetic field, namely \(B\), as provided by Swarm A ASM.