Numerical calculations of relativistic electron drift loss effect

Abstract

[1] It has been suggested that drift loss to the magnetopause can be one of the major loss mechanisms contributing to relativistic electron flux dropouts. In this study, we examine details of relativistic electrons' drift physics to determine the extent to which the drift loss through the magnetopause is important to the total loss of the outer radiation belt. We have numerically computed drift paths of relativistic electrons' guiding center for various pitch angles, various measurement positions, and different solar wind conditions using the Tsyganenko T02 model. We specifically demonstrate how the drift loss effect depends on these various parameters. Most importantly, we present various estimates of relative changes of the omnidirectional flux of 1 MeV electrons between two different solar wind conditions based on a simple form of the directional flux function. For a change of the dynamic pressure from 4 nPa to 10 nPa with a fixed IMF B Z = 0 nT, our estimate indicates that after this increase in pressure, the equatorial omnidirectional flux at midnight near geosynchronous altitude decreases by $56 to $97%, depending on the specific pitch angle dependence of the directional flux. The effect rapidly decreases at regions earthward of geosynchronous orbit and shows a general trend of decrease away from midnight. For a change of the IMF B Z from 0 nT to À15 nT with a fixed dynamic pressure of 4 nPa, the relative decrease of the omnidirectional flux at geosynchronous altitude on the nightside is much smaller than that for the pressure increase, but its effect becomes substantial only beyond geosynchronous orbit. Possibilities exist that our results may change to some extent for a different magnetospheric model than the one used here.

The Major Geoeffective Solar Eruptions of 2012 March 7: Comprehensive Sun-to-Earth Analysis

“…The significant compression of the magnetopause along with the extended intervals of southward B z suggest that the main cause of the dropout is magnetopause shadowing (Kim et al 2008;Kim & Lee 2014). On 2012 March 10 this population is enhanced above the pre-storm levels (yellow and brown curves) but the peak of the PSD distribution is now located at 6<L * <6.5 (instead of L * =5.3), indicating the existence of chorus activity which accelerates the seed population following the scenario of Horne et al (2005).…”

Quantifying the effect of magnetopause shadowing on electron radiation belt dropouts

“…On the other hand, radiation belt numerical modeling (e.g., Desorgher et al, 2000;Brautigam and Albert, 2000;Kim et al, 2008Kim et al, , 2010Kim et al, , 2012Su et al, 2011b;Subbotin et al, 2011) has the advantage for researchers that by manipulating the code and switching on/off the mechanisms of interest, their relative importance in the flux dropout can be revealed. For example, Kim et al (2008) numerically studied the extent of the drift loss/magnetopause shadowing under different solar wind pressure or IMF Bz conditions through drift path tracing, using the guiding-center method, and estimated the relative decrease of the MeV electron flux when pressure or IMF Bz is enhanced.…”

“…Therefore the following steps are carried out to obtain the outer boundary at a certain K parameter (K 0 ): (1) a bisection iterative method is applied in tracing each drift shells from the midnight meridian until the last closed drift shell L * max (α) is determined for a set of equatorial pitch angle α ranging from 20 to 90 • ; (2) the above procedure also provides the corresponding K parameter at the last closed drift shell for a specific equatorial pitch angle, i.e., K(α); (3) these L * max (K(α)) are then interpolated into L * max (K 0 ). (Shabansky, 1971;Öztürk and Wolf, 2007;Ukhorskiy et al, 2011) in which it does not come across the equator but bounces within one hemisphere, allowing a larger drift shell until it encounters the magnetopause boundary (see Kim et al, 2008, for an illustration). The quasi-periodic daily evolution of the last drift shell is caused by the warping of the tail current sheet across the magnetic equator in the T89 magnetic field model (Tsyganenko, 1989) that is used to account for the geodipole tilt angle.…”

“…For example, Kim et al (2008) numerically studied the extent of the drift loss/magnetopause shadowing under different solar wind pressure or IMF Bz conditions through drift path tracing, using the guiding-center method, and estimated the relative decrease of the MeV electron flux when pressure or IMF Bz is enhanced. Using a variable boundary condition from the satellite CRRES, Shprits et al (2006) simulated the main-phase depletion and concluded that the radial diffusion can effectively propagate the nonadiabatic boundary flux variations down to L * = 4 and that the possible magnetopause loss in the variable boundary conditions together with the radial transport can account for the mainphase flux dropout.…”

The role of drift orbit bifurcations in energization and loss of electrons in the outer radiation belt

“…Another effect which adds to the complexity of outer belt dynamics is drift orbit bifurcations, also referred to as Shabansky orbits after Shabansky [1971]. Known for a long time [Northrop and Teller, 1960;Northrop, 1963;Roederer, 1970;Antonova et al, 2003], it recently received renewed attention [Öztürk and Wolf, 2007;Kim et al, 2008;McCollough et al, 2010;Wan et al, 2010] due to development of new particle tracing techniques and improved geomagnetic field models. In this paper we investigate the implications of drift orbit bifurcations for acceleration, transport and loss of the outer belt electrons.…”