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Last closed drift shell (LCDS) has been identified as a crucial parameter for investigating the magnetopause shadowing loss of radiation belt electrons. However, drift orbit bifurcation (DOB) effects have not been physically incorporated into the LCDS calculation. Here we calculate event‐specific LCDS using different approaches to dealing with the DOB effects, that is, tracing field lines ignoring DOB, tracing test particles rejecting field lines with DOB, and tracing particles including field lines with DOB, and then incorporate them into a radial diffusion model to simulate the fast electron dropout observed by Van Allen Probes in May 2017. The model effectively captures the fast dropout at high L* and exhibits the best agreement with data when LCDS is calculated by tracing test particles with DOB more physically included. This study represents the first quantitative modeling of the DOB effects on radiation belt magnetopause shadowing loss via a more physical specification of LCDS. Plain Language Summary: Magnetopause shadowing has been recognized as a significant loss mechanism contributing to the rapid dropouts of radiation belt electrons. The last closed drift shell (LCDS), a critical input in modeling the magnetopause shadowing loss, is typically calculated by tracing magnetic field lines with constant second adiabatic invariant and magnetic field strength at particle's mirror point (Bm). However, the effects of drift orbit bifurcation (DOB) have not been realistically incorporated into the LCDS estimation for dropout modeling. DOB occurs when the local maximum of the magnetic field strength on the equator exceeds Bm, causing particles to be trapped in one hemisphere and leading to the violation of the second adiabatic invariant. To evaluate the effects of DOB on shadowing loss, we implemented three different LCDS calculations, that is, tracing field lines ignoring DOB, tracing test particles rejecting field lines with DOB, and tracing test particles including field lines with DOB, in a one‐dimensional radial diffusion model. By reproducing the electron dropout observed at high L* during the geomagnetic storm in May 2017, we discovered, for the first time, that physically incorporating DOB effects into the LCDS calculation via test particle tracing can best capture the observed electron loss at high L*. Key Points: A radial diffusion model with event‐specific last closed drift shell (LCDS) is used to simulate the electron magnetopause shadowing lossDifferent approaches to dealing with the drift orbit bifurcation (DOB) effects are implemented in the calculation of LCDSThe model best reproduces the observations when LCDS is calculated by tracing test particles and physically including DOB [ABSTRACT FROM AUTHOR]

Abstract Last closed drift shell (LCDS) has been identified as a crucial parameter for investigating the magnetopause shadowing loss of radiation belt electrons. However, drift orbit bifurcation (DOB) effects have not been physically incorporated into the LCDS calculation. Here we calculate event‐specific LCDS using different approaches to dealing with the DOB effects, that is, tracing field lines ignoring DOB, tracing test particles rejecting field lines with DOB, and tracing particles including field lines with DOB, and then incorporate them into a radial diffusion model to simulate the fast electron dropout observed by Van Allen Probes in May 2017. The model effectively captures the fast dropout at high L* and exhibits the best agreement with data when LCDS is calculated by tracing test particles with DOB more physically included. This study represents the first quantitative modeling of the DOB effects on radiation belt magnetopause shadowing loss via a more physical specification of LCDS.