Constraints to the drag-based reverse modeling

Jaša Čalogović

doi:10.1051/0004-6361/202346874

Constraints to the drag-based reverse modeling

2025-01-27

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DOI：10.1051/0004-6361/202346874

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Context. One of the most widely used space weather forecast models to simulate the propagation of coronal mass ejections (CMEs) is the analytical drag-based model (DBM). It predicts the CME arrival time and speed at Earth or at a specific target (planets, spacecraft) in the Solar System. The corresponding drag-based ensemble model (DBEM) additionally takes into account the uncertainty of the input parameters by making n ensembles and provides the most probable arrival time and speed as well as their uncertainty intervals. An important input parameter for DBM and DBEM is the drag parameter gamma, which depends on the CME cross-section and mass, as well as the solar-wind density. Aims. The reverse-modeling technique applied to the DBM allows us to derive gamma values that minimize transit time (TT) and arrival-speed (v(tar)) errors. The present study highlights the limitations and constraints of such a procedure. Methods. We searched for optimal gamma values that would yield the perfect TT within one hour of the actual observed CME transit time as well as perfect v(tar) within +/- 75 km s(-1). This optimal window for v(tar) was found by increasing v(tar) from +/- 10 to +/- 100 km s(-1), where the +/- 75 km s(-1) window gave the perfect TT and v(tar) in the case of 87% of CMEs compared to the +/- 10 km s(-1) window, which was used in some previous reverse-modeling studies and gave optimal results for only 45% of the events from our CME list. For our analysis, a 31 CME-ICME pair sample is used from the period spanning 1997-2018. The reverse-modeling method is applied using the DBEMv3 tool for different gamma ranges from 0.01 to 10 x 10(-7) km(-1). We tested whether and how the obtained optimal gamma depends on the chosen gamma range. Results. By increasing the gamma range, we find that the optimal gamma converges to a certain value for two thirds of the analyzed events. The highly constrained gamma ranges resulted in shifted and skewed gamma distributions. By using the largest gamma range (0.01-10 x 10(-7) km(-1)), the medians of the optimal gamma distributions are obtained for two thirds of the events in the common operational DBEMv3 range of 0.01-0.5 x 10(-7) km(-1). We also found that the important quantity in determining the range of gamma distribution and ability to find an optimal gamma is the difference between the CME launch speed and the solar-wind speed (v(0) - w), which together with gamma define the drag acceleration in the DBM. For small v(0) - w differences (e.g., < 200 km s(-1)), the reverse modeling may not be the appropriate method to find the optimal gamma due to large divergence of gamma values found, which may additionally be caused by larger input uncertainties and physical model limitations in turn leading to inappropriate gamma values.

Keyword:

Sun: corona

Sun: coronal mass ejections (CMEs)

Sun: heliosphere

solar wind

Constraints to the drag-based reverse modeling

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