TGFs - "Storm Activity" relationship

In the first two years of ASIM operations from June 2018 till the end of 2019 486 TGFs have been observed, with a TGF rate of 0.84 per day. Their geographical distribution is consistent with the three main lightning chimneys Central America, Central Africa, and South East of Asia. Figure 1 displays the ISS footprint positions when the TGFs were detected.

Figure 1: ISS position for the 2018-2019 ASIM TGFs. Red circles marked those within 4 minutes of the previous TGF detected.

If the TGF occurrence follows a stochastic process (each TGF is not related to the next one), the time-difference distribution between a TGF detection and the next one should fit an exponential distribution. For a Δt < 4 minutes the number of TGFs following the exponential distribution is 16. Opposite we got 85 in groups of 2-3 TGFs displayed in Figure 1 in red circles. Analyzing the apparent strong discrepancy in the number of detection in less than 4 minutes (Figure 2) and the number derived from the exponential distribution is one of the motivations of this study.

We build a grid of variable dimension cell size to keep the same ISS observing time for each cell in a Monte Carlo code to simulate the TGF generation that has into account the frequency and the anisotropy distribution of the TGFs over the earth.

To preserve the total number of TGF observed in Δt < 4 minutes we need to add a parameter related to the “Storm Activity” defined as the time in a cell available to generate a TGF. The model fits observations when this parameter is 7%±1%. The good correlation between model/observation is displayed in Figure 2.

Figure 2: The predicted distribution of the TGF pairs (Orange) in 15s bins fits the observations (Blue).

The scope of this work is to check the adopted “Storm Activity” value using WWLLN sferics database as a good indicator of storm activity.