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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]

Drift orbit bifurcation (DOB) has been suggested to play a major role in the loss and transport of radiation belt electrons since it violates the second adiabatic invariants of particles and makes the third invariant undefined. Results from our guiding‐center test particle simulations using the Tsyganenko‐1989c magnetic field model show that the DOB could affect a broad region of the outer radiation belt, which can penetrate inside the geosynchronous orbit at Kp ≥ 3, and its effects are more significant further away from the Earth, at a higher Kp and for higher electron energies. Specifically, the short‐term simulation results after one electron drift show both traditional and nontraditional DOB transport of electrons, with the nontraditional DOB, caused by a third minimum of the magnetic field strength near the equator, reported here for the first time. Moreover, our results show large ballistic jumps in the second invariant and radial distance for electrons at high equatorial pitch angles after one drift. In addition, long‐term DOB transport coefficients of electrons over many drifts are calculated based on our simulation results. We find that the pitch angle and radial diffusion coefficients of electrons due to DOB could be comparable to or even larger than those caused by electron interactions with chorus and Ultra‐Low‐Frequency waves, respectively. In sum, our results demonstrate that DOB could cause effective loss and transport of radiation belt electrons even in the absence of waves. Key Points: Test particle simulations are performed to quantify the effects of drift orbit bifurcation (DOB) on radiation belt electronsShort‐term simulations after one drift show electron transport due to both traditional and nontraditional DOBLong‐term transport coefficients by DOB could be comparable to or larger than those by wave‐particle interactions [ABSTRACT FROM AUTHOR]

Dropout events are dramatic decreases in radiation belt electron populations that can occur in as little as 30 minutes. Loss to magnetopause due to a combination of magnetopause shadowing and outward radial transport plays a significant role in these events. We examine the dropout of relativistic electron populations during the October 2012 geomagnetic storm using simulated electron phase space density, evaluating the contribution of different processes to losses across the magnetopause. We compare loss contribution from outward transport calculated using a standard empirical radial diffusion model that assumes a dipolar geomagnetic field to an event-specific radial diffusion model evaluated with a non-dipolar geomagnetic field. We additionally evaluate the contribution of Shabansky type 1 particles, which bounce along magnetic field lines with local equatorial maxima, to the loss calculated during this event. We find that the empirical radial diffusion model with a dipolar background field underestimates the contribution of radial diffusion to this dropout event by up to 10% when compared to the event-specific, non-dipolar radial diffusion model. We additionally find that including Shabansky type 1 particles in the initial electron phase space density, that is, allowing some magnetic field lines distorted from the typical single-minima configuration in drift shell construction, increases the calculated loss by an average of 0.75%. This shows that the treatment of the geomagnetic field significantly impacts the calculation of electron losses to the magnetopause during dropout events, with the non-dipolar treatment of radial diffusion being essential to accurately quantify the loss of outer radiation belt populations., (©2022. The Authors.)