TGFs - "Storm Activity" relationship

<p lang="en-US" align="justify">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.</p> <p lang="en-US" align="justify"><img src="" alt="" /></p> <p><em>Figure 1: ISS position for the 2018-2019 ASIM TGFs. Red circles marked those within 4 minutes of the previous TGF detected.</em></p> <p lang="en-US" align="justify">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 &#916;t < 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.</p> <p lang="en-US" align="justify">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.</p> <p lang="en-US" align="justify">To preserve the total number of TGF observed in &#916;t < 4 minutes we need to add a parameter related to the &#8220;Storm Activity&#8221; defined as the time in a cell available to generate a TGF. The model fits observations when this parameter is 7%&#177;1%. The good correlation between model/observation is displayed in Figure 2.</p> <p lang="en-US" align="justify"><img src="" alt="" /></p> <p><em> </em></p> <p><em>Figure 2: The predicted distribution of the TGF pairs (Orange) in 15s bins fits the observations (Blue). </em></p> <p lang="en-US" align="justify"><span id="Marco1" dir="ltr"></span></p> <p lang="en-US" align="justify">The scope of this work is to check the adopted &#8220;Storm Activity&#8221; value using WWLLN sferics database as a good indicator of storm activity.</p> <p align="justify">&#160;</p> <p align="justify">&#160;</p> <p align="justify">&#160;</p> <p align="justify">&#160;</p> <p align="justify">&#160;</p> <p align="justify">&#160;</p> <p>&#160;</p>