Numerical calculations of relativistic electron drift loss effect

Numerical calculations of relativistic electron drift loss effect Kyung Chan Kim, 1 D.-Y. Lee, 1 H.-J. Kim, 2 L. R. Lyons, 3 E. S. Lee, 4 M. K. O ¨ zt u ¨ rk, 5 and C. R. Choi 1 Received 29 December 2007; revised 13 May 2008; accepted 16 June 2008; published 9 September 2008. [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 chan ges of the omnidir ectio nal flux of 1 MeV elec trons betwee n 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. Citation: Kim, K. C., D.-Y. Lee, H.-J. Kim, L. R. Lyons, E. S. Lee, M. K. O ¨ ztu ¨ rk, and C. R. Choi (2008), Numerical calculations of relativistic electron drift loss effect, J. Geophys. Res., 1 13, A09212, doi:10.1029/2007JA013011. 1. Introduc tion [2] It has been sugge sted that ther e are several pro cesses which can contribute to relativistic electron flux dropouts. They can be divided into two classes depending on whether the process of flux dropouts is adiabatic or non-adiabatic. The most well-known adiabatic process is a fully-adiabatic flux change (or the Dst effec t) that leads to a global dropout during the storm main phase [e.g., Kim and Chan, 1997; Li et al ., 1997]. During storm-time s, relat ivisti c electrons respond to the changing magnetospheric field because of the enhanced ring current by conserving all three adiabatic invariants. This reduces relativistic electron fluxes as mea- sured at consta nt energy and locati on, but they are expect ed to return to pre -storm levels when the storm recovers . However, there have been observations during storms where the reduced fluxes of relativistic electrons during the storm main phase never recovered to their pre-storm levels at the end of a storm [e.g., O’Brien et al., 2001; Onsager et al., 2002; Reeves et al. , 2003]. These suggest that other loss pr ocesses occurr ed dur ing the storm- time leading to a permanent loss of relativistic electrons within the radiation belt. [3] In fact, the re are severa l other , either adiabatic or no n- adiabatic, mechanisms proposed to explain flux dropouts. They include the substorm growth phase-like flux dropout [e.g., Ree ves et al. , 1998], atmo spheri c pre cipitat ion by wave-particle interaction or by pitch angle scattering under severe ly curved magneti c field [e.g., Horn e and Thorne, 2003; Green et al., 2004; Lee et al. , 2006; Bortnik et al., 2006; Mil lan et al. , 2007], and dr ift loss through the magnetopause [e.g., Li et al., 1997; Desorgher et al., 2000]. [4] This paper deals wi th the dr if t loss ef fect wher e electr ons are lost perman ently through the magnet opause when their drift shell is opened to the magnetopause [e.g., Li et al., 1997; Desorgher et al. , 2000] . Recentl y , this process has been reported as an important cause of flux dropouts. Ukhorskiy et al. [2006] suggested, on the basis on the test parti cle simula tion, that contin uous intensifica tion of the ring current during storm-times leads to a drift loss to the JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 113, A09212, doi:10.1029/2007JA013011, 2008 Click Here for Full Article 1 Depart ment of Astrono my and Space Scien ce, Chungbuk National University, Chungbuk, South Korea. 2 Department of Astronomy and Space Science, Kyunghee University, Yongin, Gyeonggi, South Korea. 3 Depart ment of Atmosph eric Sciences, Univer sity of Calif ornia Los Angeles, Los Angeles, California, USA. 4 Space Sciences Labor atory , Univer sity of Calif ornia Berkel ey, Berkeley, California, USA. 5 Department of Information Technologies, Is ¸ık University, I : stanbul, Turkey. Copyri ght 2008 by the American Geophysic al Union. 0148-0227/08/2007JA013011$09.00 A09212 1 of 14

Numerical calculations of relativistic electron drift loss effect

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