An empirical model of the occurrence rate of low latitude post-sunset
plasma irregularities derived from CHAMP and Swarm magnetic observations
Abstract
The prediction of post-sunset equatorial plasma depletions (EPDs), often
called ionospheric plasma bubbles, has remained a challenge for decades.
In this study, we introduce the Ionospheric Bubble Probability (IBP), an
empirical model predicting the occurrence probability of EPDs derived
from 9 years of CHAMP and 8.5 years of Swarm magnetic field
measurements. The model predicts the occurrence probability of EPDs for
a given longitude, day of year, local time and solar activity, for the
altitude range 350-500 km, and low geographic latitudes of ± 45◦. IBP
has been found to successfully reconstruct the distribution of EPDs as
reported in previous studies from independent data. IBP has been further
evaluated using one-year of partly untrained data of the Ionospheric
Bubble Index (IBI). IBI is a Level 2 product of the Swarm satellite
mission used for EPD identification. The relative operating
characteristics (ROC) curve shows positive excursion above the no-skill
line with Hanssen and Kuiper’s Discriminant (H&KSS) score of 0.66,
0.73, and 0.65 at threshold model outputs of 0.22, 0.18, and 0.18 for
Swarm A, B, and C satellites, respectively. Additionally, the
reliability plots show proximity to the diagonal line with a fairly
decent Brier Skill Score (BSS) of 0.317, 0.320, and 0.316 for Swarm A,
B, and C respectively. These tests indicate that the model performs
significantly better than a no-skill forecast. The IBP model offers a
compelling glimpse into the future of EPD forecasting, thus
demonstrating its potential to reliably predict EPD occurrences. The IBP
model is made publicly available.