# Polar Cap Patches Scaling Properties: Insights from Swarm Data
## 2 Data and Methods
### Swarm Data
#### 2.1.2 The PCP Flag  
Locating polar cap patches is made possible through the use of the PCP flag, which is contained in the Swarm IPDxIRR Level 2 product [31; 32; 33]. PCP flag, which is provided at 1 Hz rate, is calculated using 2 Hz electron density observations applying an algorithm developed by [PERSON] et al. [17] that represents a slightly modified version of that first introduced by [PERSON] and [PERSON] [34]. Besides that developed by [PERSON] and [PERSON] [34], other methods have been proposed to identify PCPs, for instance that proposed by [PERSON] et al. [15] or by [PERSON] et al. [35] who actually proposed a synthesis of previous methods with the purpose to overcome the limits inherent in both [PERSON] et al. [15] and [PERSON] and [PERSON] [34] algorithms. We are aware of the fact that all of them produce quite different results, especially concerning the spatial and temporal distributions of polar cap patches [35]. However, it is unclear whether discrepancies in results are due to the use of either different types of measurements or detection techniques. In fact, the correct way to identify, or better yet, define this type of irregularity is still debated. All of this, however, is beyond the purpose of this paper, so we decided to rely on the only PCP identification available along the track of Swarm A satellite, which is also distributed as an official ESA Swarm product. In practice, an electron density measurement is flagged to be taken inside a PCP when \(N_{\rm e}\) is at least twice the background electron density. One of the main criticisms to this procedure is that even small fluctuations can exceed the doubled value of the background when this is very low. To limit the drawbacks of the algorithm proposed by [PERSON] and [PERSON] [34], due to the times when low values of the background density are observed, we adopted the strategy proposed by [PERSON] et al. [36], i.e., to take only PCPs corresponding to electron density measurements higher than \(5\times 10^{4}\) cm\({}^{-3}\). Similarly, an electron density measurement is considered at the border of a PCP when \(N_{\rm e}\) measurements adjoining to a PCP are at least 30% the average background density within the PCP [17]. PCP flag is expected to have five values: 0 if the measurement is taken outside of a PCP, 1 if it is taken at the border of a PCP, either 2 or 3 if the border can be identified to be either a leading or trailing edge, respectively, and 4 if it is taken inside a PCP. However, at the present moment, PCP flag only takes three values (0, 1, 4), implying that the nature of the edges of the patches is unknown; so, we only consider \(N_{\rm e}\) measurements corresponding to a PCP flag equal to 1 and 4 in order to study processes occurring inside and at the borders of a PCP. So, in the following, PCP borders will be considered as part of the patch. We also want to draw the reader's attention to the fact that in the distributions that follow, PCPs are counted considering the single \(N_{\text{e}}\) measurements falling either inside or at the border of PCPs observed in the selected period and not considering as a whole the set of measurements belonging to a PCP. Since values of the PCP flag are available from 16 July 2014 we use this as the start time of our dataset. The temporal and spatial distribution (for both hemispheres) of the PCP dataset considered in this analysis perfectly matches that found by [PERSON] et al. [17], even though the time interval considered here is approximately doubled. Indeed, as shown in Figure 1 displaying the time distribution of all PCPs observed in the selected period, the maximum occurrence is observed in the local winter in both hemispheres, with an overall higher number observed in the Northern hemisphere than in the Southern one. As documented in the literature [36; 37], the occurrence of PCPs is deeply influenced by the difference in distance between the geographic and magnetic poles in the two hemispheres.