# A 2D Lithospheric Magnetic Anomaly Field over Egypt Using Gradient Data of Swarm Mission
## Data Set and Methodology  
### Data Set and Selection Criterion  
The Swarm mission consists of three satellites (Alpha, Bravo, and Charlie) used to study the Earth's geomagnetic field [28; 29]. The mission was launched on 25 November 2013 to study the ionospheric current systems and model the Earth's magnetic field [30]. Swarm A and C are flying side-by-side, separated by 1.4\({}^{\circ}\) (-150 km) at an altitude of 400 km, while Swarm B is flying at an altitude of 520 km with a higher inclination angle than Swarm A and C [6]. Our study depends on the Swarm A and C satellites. We have used MAG\({}_{\times}\)LR Level 1b Swarm data, which is available on the European Space Agency (ESA) website ([[https://swarm-diss.eo.esa.int/](https://swarm-diss.eo.esa.int/)]([https://swarm-diss.eo.esa.int/](https://swarm-diss.eo.esa.int/)) (accessed on 31 December 2021)). We have used the quiet-time magnetic data recorded by the two lower-altitude Swarm satellites, A and C, from January 2014 until August 2021. These data have been used to model the lithospheric magnetic anomaly field over Egypt and its surroundings. The quiet-time Swarm magnetic data has been selected according to the following criteria:  
1. Local nighttime data (\(23:00<Local\ Time\ (LT)<06:00\)).
2. \(\mathrm{Kp}\leq 1\).
3. The interplanetary magnetic field (IMF) within \(-3\leq\mathbf{B}_{z}\leq 4\).
4. The disturbance storm time index within \(-20<\mathrm{Dst}<20\) daily values.
5. The regional data observed within [0\({}^{\circ}\)-40\({}^{\circ}\) N] latitude and [0\({}^{\circ}\)-40\({}^{\circ}\) E] longitudes with observational error \(<0.3\) nT.  
The geomagnetic indices and solar wind parameters were obtained from the Omni website ([[https://omniweb.gsfc.nasa.gov/form/dx4.html](https://omniweb.gsfc.nasa.gov/form/dx4.html)]([https://omniweb.gsfc.nasa.gov/form/dx4.html](https://omniweb.gsfc.nasa.gov/form/dx4.html)) (accessed on 31 December 2021)). Figure 1 illustrates the relationship between the altitude and the number of data points within each 2 km. It shows that the maximum number of data points are collected at an altitude of about 440 km. Therefore, we have selected observations where the satellite is located within 440 \(\pm\) 2.5 km. These altitudinal limits have also been chosen to overcome the altitudinal variation of the lithospheric magnetic anomaly field.  
The gradient data from these satellites (Swarm A and C) have been calculated in the north-south (N-S) and the east-west (E-W) directions. It is worth noting that, we have first subsampled the 1 s into a 10 s time resolution. We have avoided subsampling it to 30 s in order to overcome gaps because the resolution of our plotted maps is 1\({}^{\circ}\)\(\times\) 1\({}^{\circ}\) (latxlon). Subsequently, we have followed the same criteria set as [17] in calculating the gradient data by confirming that the time lag between the two satellites (Swarm A and C) does not exceed the 50 s. Therefore, any path where Swarm A and Swarm C lagged by more than 50 s was excluded from the analysis. The N-S gradient data were calculated by subtracting two points separated by 15 s as observed by the same satellite [17]. Therefore, the amount of N-S gradient data is twice as large as the amount of E-W gradient data.