Click Here GEOPHYSICAL RESEARCH LETTERS, VOL. 33, L14107, doi:10.1029/2006GL026294, 2006 for Full Article Correlation between the inner edge of outer radiation belt electrons and the innermost plasmapause location Xinlin Li,1,2 D. N. Baker,1 T. P. O’Brien,3 L. Xie,4 and Q. G. Zong5 Received 13 March 2006; revised 26 May 2006; accepted 5 June 2006; published 27 July 2006. [1] A remarkable correlation between the inner edge of the outer radiation belt electrons and the innermost plasmapause location is demonstrated by using 12 years of measurements of MeV electrons from the Solar, Anomalous, and Magnetospheric Particle Explorer (SAMPEX) and the Combined Radiation and Release Experiment Satellite (CRRES) during its entire mission (July 1990 – October 1991). An empirical model of the plasmapause location, Lpp, by O’Brien and Moldwin (2003) as well as the measured Lpp during the CRRES mission is used for the comparison. This correlation suggests that the innermost Lpp is the innermost limit of the outer radiation belt penetration. We discuss the possible scenarios that might explain this correlation. Citation: Li, X., D. N. Baker, T. P. O’Brien, L. Xie, and Q. G. Zong (2006), Correlation between the inner edge of outer radiation belt electrons and the innermost plasmapause location, Geophys. Res. Lett., 33, L14107, doi:10.1029/2006GL026294. 1. Introduction [2] While the outer radiation belt is comprised of electrons with energies of 100s’ keV to MeV, the plasmasphere consists of cold (1 eV) and dense (103 cm 3) plasma. The inner edge of the outer radiation belt, Lrb, can be anywhere between L = 2 and L = 5 (see Figure 1), where L is the radial distance in RE at the equator if Earth’s magnetic field is approximated as a dipole. The outer boundary of the plasmasphere, usually referred to as the plasmapause, is where the plasma density has a steep gradient. This boundary can be at a wide range of L-values, as small as at L = 2 and as high as L = 7 and beyond, strongly dependent on the level of geomagnetic activity [Chappell, 1972]. Early in space era, it was reported that the position of the plasmapause was related to the region of enhanced trapped energetic electrons [e.g., Carpenter et al., 1971]. [3] There has been renewed interest in understanding the plasmapause and especially its relation to the outer radiation belt. Moldwin et al. [2002] found that only 16% of the 1 Laboratory for Atmospheric and Space Physics and Department of Aerospace Engineering Sciences, University of Colorado, Boulder, Colorado, USA. 2 Also affiliated with Laboratory for Space Weather, Chinese Academy of Sciences, Beijing, China. 3 Space Science Department-Chantilly, The Aerospace Corporation, Chantilly, Virginia, USA. 4 Department of Geophysics, Peking University, Beijing, China. 5 Center for Atmospheric Research, University of MassachusettsLowell, Lowell, Massachusetts, USA. Copyright 2006 by the American Geophysical Union. 0094-8276/06/2006GL026294$05.00 plasmapause crossings encountered by CRRES satellite (altitude: 350 km 5.2 RE, inclination: 18°) were clean and sharp ‘classic’ isolated steep density gradients, and that plasmapause observations with significant structure or density cavities are more common. Nonetheless, based on the CRRES data, O’Brien and Moldwin [2003] derived an empirical plasmapause model to describe the averaged plasmapause location, Lpp, as a function of magnetic indices. Carpenter and Lemaire [2004] suggested that the plasmapause region be called plasmasphere boundary layer, PBL, because the term PBL represents the outer boundary region of the plasmasphere more objectively and accurately. [4] It is understood that a broad-band whistler mode emission known as plasmaspheric hiss inside plasmasphere is the primary cause of the quiet-time loss of outer radiation belt electrons, precipitating them into atmosphere due to pitch angle scattering [Lyons et al., 1972]. During storm times, the pitch angle scattering of the radiation belt electrons is more prominent near and just outside the plasmapause where electromagnetic ion cyclotron, EMIC, waves are also contributing [Albert, 2003]. O’Brien et al. [2003] showed that the statistical relationship between the plasmapause and the Dst index resembles that of the location of the peak outer zone flux and the Dst index [Tverskaya et al., 2003]. Recently, Baker et al. [2004] showed that the Lrb was closely related to the plasmapause location even during the extreme distortion of the outer radiation belt during the October – November 2003 storm. Later, Goldstein et al. [2005] investigated the spatial relationship between the outer radiation belt and the plasmapause during the two month period 30 March – 30 May 2001. Focusing on the loss mechanism of the MeV electrons, they found that the Lrb matched with a 3-day running average of the innermost plasmapause location, implying a 3-day loss time scale from pitch angle scattering by plasmaspheric hiss. [5] In this letter, we will demonstrate that the penetration of the outer radiation belt, which usually occurs during the recovery phase of a magnetic storm, is closely related to the minimum plasmapause location, which usually occurs during the main phase of a magnetic storm. We will then discuss the most promising explanations of this correlation regarding the acceleration and loss mechanisms of the outer radiation belt electrons. 2. Observations [6] The bottom plot of Figure 1 shows monthly windowaveraged fluxes of 2 – 6 MeV electrons (#/cm2-s-sr) from PET instrument on SAMPEX [Cook et al., 1993] since its launch (July 3, 1992) into a low-altitude and highly inclined orbit. The superimposed white curve represents L14107 1 of 4 L14107 LI ET AL.: RADIATION BELT AND PLASMAPAUSE LOCATION Figure 1. (top) Variations of yearly window-averaged sunspot numbers (black curve) and weekly windowaveraged solar wind speed (km/s, red curve). (bottom) Monthly window-averaged, color-coded in logarithm, and sorted in L (L bin: 0.1) electron fluxes of 2– 6 MeV (#/cm2s-sr) by SAMPEX since its launch (July 3, 1992) into a lowaltitude (550 600 km) and highly inclined (82°) orbit. The superimposed white curve represents every 10-day’s minimum Lpp based on an empirical model [O’Brien and Moldwin, 2003]. The yellow vertical bars on the horizontal axis are marks of equinoxes. every 10-day’s minimum Lpp, based on an empirical model using the Dst index as input [O’Brien and Moldwin, 2003]. The top plot of Figure 1 shows yearly window-averaged sunspot numbers and weekly window-averaged solar wind speed (km/s). [7] A remarkable correlation is evident in Figure 1 between the Lrb and the innermost position of the Lpp over a long term period, during which the outer radiation belt also exhibits a strong solar cycle and seasonal variation. The outer radiation belt was most intense, on average, during the descending phase of the sunspot cycle (1993 –1995 and 2003 – 2004) when recurrent high speed solar wind streams emanating from persistent trans-equatorial coronal holes were prominent and long-lasting, as shown in the top plot. The outer belt was weakest during sunspot minimum (1996 – 1997) when the solar wind speed was also low. The outer belt became more intense during the ascending phase of the solar cycle (1997 – 1999). Interestingly, the outer belt was not most intense approaching or at sunspot maximum. Seasonally, the outer belt is most intense and penetrates the deepest around the equinoxes [Baker et al., 1999; Li et al., 2001], as demonstrated by its correlation with the yellow bars (marks of equinoxes) on the horizontal axis. Despite the variations in the time scale and magnitude of the outer electron belt, the depth of its penetration into the inner magnetosphere remains closely tied to the innermost Lpp. [8] Figure 2 shows daily-averaged electron fluxes of 2 – 6 MeV (#/cm2-s-sr) by SAMPEX during 1998. The superimposed white curve represents the daily minimum Lpp. The superimposed black curve represents the Lrb, which is defined as the L-value where the electron intensity is 40% of its peak. It is evident that the initial penetration of outer radiation belt correlates well with the innermost location of the Lpp and stays inside the Lpp even when the Lpp recovers to large L. The deep penetration of the outer L14107 radiation belt usually corresponds to an overall enhancement of the electrons since the innermost location of Lpp and the value of the Lrb correlates well during enhancements of the outer radiation belt. If we define an enhancement event as a total electron flux (integrating over all L values) increase over 30% from the previous day’s value, then 92 days in 1998 satisfy the criteria. During these 92 days the correlation coefficient between the innermost model Lpp and the Lrb is 0.510. Including all days in 1998, the correlation coefficient is 0.354. [9] We must note that the empirical model of Lpp [O’Brien and Moldwin, 2003] does not reproduce the detailed structure of the plasmapause motion and there is considerable scatter between the model and the data from which the model was derived [see O’Brien and Moldwin, 2003, Figure 1]. Nonetheless, here we demonstrate that there exists a close relationship between the outer radiation belt and the modeled outer boundary of plasmasphere even though it may be only in an averaged sense. [10] Figure 3 shows orbitally-averaged electron fluxes from CRRES during its entire mission, July 1990– October 1991. The superimposed white curve represent every twoday’s minimum Lpp from the same plasmapause model and the superimposed black curve is a five day window-averaged daily minimum Lpp measured by CRRES [Moldwin et al., 2002]. The modeled Lpp and the measured Lpp do not always track each other well (as discussed by O’Brien and Moldwin [2003]) having a typical root-mean-square (RMS) error of about 1 L. Also there are significant data gaps for the measured Lpp [Moldwin et al., 2002]. Nonetheless, the correlation between the inner edge of the outer belt and the inner most Lpp, both measured and modeled, is evident. The new belt centered at L 2.5 starting on March 24, 1991 (DOY starting 1990: 448) was caused by a strong interplanetary shock impacting on the Earth [Blake et al., 1992; Li et al., 1993]. Lower energy electrons (not shown here) usually penetrate deeper than the electrons shown (1.13– 1.23 MeV). 3. Discussion [11] Several significant points from the above observations warrant more discussion. As noted above, the Lpp shown in Figures 1 – 3 and used in the correlation calcula- Figure 2. Daily electron fluxes of 2 – 6 MeV (#/cm2-s-sr) by SAMPEX, color-coded in logarithm, and sorted in L (L bin: 0.1) for 1998. The superimposed white curve represents daily minimum L of the empirical Lpp. The superimposed black curve represents the inner edge of the outer radiation belt (see text for details). 2 of 4 L14107 LI ET AL.: RADIATION BELT AND PLASMAPAUSE LOCATION Figure 3. Orbitally averaged, color-coded in logarithm, and sorted in L (L bin: 0.1) electron fluxes (#/cm2-s-sr) from CRRES for its entire mission, July 1990 – October 1991. The superimposed white curve represents every 2-day’s minimum Lpp based on an empirical model [O’Brien and Moldwin, 2003] and the black curve is the 5-day windowaveraged daily minimum Lpp measured from CRRES [Moldwin et al., 2002]. tion is the averaged L-value calculated from an empirical model and there is considerable scatter between the model and the data [O’Brien and Moldwin, 2003]. If the local time dependence is included, the minimum Lpp would be a little smaller. It is important to emphasize that the Lpp values shown in Figures 1 – 3 are heavily smoothed. The inner edge of the outer radiation belt does not follow the plasmapause all the time. Only the initial penetration of the outer belt follows the plasmapause erosion. After the plasmapause moves outward, the most inward electrons stay at small L shells and slowly decay inside the plasmasphere. This also means that the outer radiation belt co-exists with the plasmasphere after a deep penetration. It is most visible from Figures 2 and 3 that the electron flux inside plasmasphere continues to diffuse inward and slowly decay. [12] We consider three possible explanations for the observed correlation. First, as examined by Baker et al. [2004] and Goldstein et al. [2005], the plasmasphere may shape the outer zone by providing enhanced storm-time losses immediately interior to the plasmapause. The dominant loss process would be storm-enhanced EMIC and hiss waves, combining to deplete the MeV electrons [Albert, 2003]. Both EMIC and hiss are enhanced during the storm, but would not remain a strong loss process during and after the recovery phase. [13] The electrons in Figures 1– 3 are spin-averaged and cover all local pitch angles, both trapped or precipitating. Xie et al. [2006], separating measured electrons (30 keV to >300 keV) into trapped and precipitating during Sept. 10– Nov. 10 of 2003 from the NOAA-17 satellite (in a low altitude polar orbit), found that the inner edge of the precipitating electrons traces the empirical Lpp well and stays outside of the Lpp. However, the inner edge of the trapped electrons often stays inside the empirical Lpp. This suggests that pitch angle scattering of electrons by VLF waves responsible for precipitation is more significant just outside the Lpp. This is also consistent with the theoretical analysis that the combined EMIC waves, often occurring outside plasmapause, and plasma hiss lead to enhanced radiation belt electron precipitation [Albert, 2003]. [14] The second explanation for the plasmapause role in shaping the outer zone is a possible role of the plasma- L14107 spheric mass density in modifying the ULF waves that cause radial transport and energization. What role the PBL plays in terms of the distribution of global ULF wave activity is still an unresolved question [Carpenter and Lemaire, 2004]. However, the plasmapause certainly plays some role in modifying ULF waves [Takahashi and Anderson, 1992; Lee, 1996], although not necessarily those in the Pc5 band. The latter are most often associated with acceleration of MeV electrons [e.g., Rostoker et al., 1998]. There is certainly plenty of empirical evidence for the necessary outward radial gradient in the pre-storm radiation belt phase-space density [e.g., Hilmer et al., 2000]. Also, even though ULF power decreases with L, storm-time ULF power is considerably enhanced over a wide range of L values [Mathie and Mann, 2001]. A positive radial gradient of the phase-space density and enhanced ULF power would lead to enhanced inward radial diffusion. [15] Finally, the VLF acceleration mechanism may move inward as the plasmapause is eroded, because of the coincidence of a local minimum in the ratio of the electron plasma to cyclotron frequencies with strong lower-band VLF chorus [e.g., O’Brien et al., 2003]. This mechanism would lead to a phase-space density peak outside the plasmapause. Selesnick and Blake [2000] and Green and Kivelson [2004] have shown evidence for storm-time phasespace density peaks growing out of the more usual outward gradient. In this scenario, VLF acceleration would be effective and occur just beyond the plasmapause, as was argued by Horne et al. [2005] and Shprits et al. [2006] for the October –November 2003 storm case. The phase-space density peak would be smoothed out over a range of nearby L values by radial diffusion. Some of the particles would be carried to lower L, into the plasmasphere, gaining a small amount of extra energy. 4. Summary [16] A remarkable correlation between the inner edge of the outer radiation belt electrons and the innermost plasmapause location is demonstrated using 12 years of measurements of MeV electrons from SAMPEX as well as from CRRES for its entire mission (July 1990 – October 1991), and an empirical model of the Lpp [O’Brien and Moldwin, 2003]. This correlation suggests that the depth of penetration of the outer radiation belt electrons into the inner magnetosphere is associated with the innermost Lpp and the inner part of the outer radiation belt co-exists with the plasmasphere after the recovery of the plasmapause to a larger L. The electrons inside the plasmasphere may continue to diffuse inward and slowly decay. Lower energy electrons penetrate deeper than higher energy electrons. Pitch-angle scattering of electrons by VLF and EMIC waves is probably responsible for the precipitation occurs mostly near the Lpp. Recent observational evidence is consistent with this explanation [Xie et al., 2006]. [17] However, we have also outlined two mechanisms by which the plasmapause may play a role in modifying the acceleration processes as well: the plasmasphere may modify the characteristics of the ULF waves that diffuse particles radially inward and to higher energies, and acceleration by VLF chorus may produce a flux peak just outside the plasmasphere because chorus is strongest just beyond the plasmapause. We must note that these mechanisms are 3 of 4 L14107 LI ET AL.: RADIATION BELT AND PLASMAPAUSE LOCATION not mutually exclusive. More likely they act in concert to produce the above correlation. [18] Determining the relative contribution of the mechanisms discussed above to the observed correlation will require further study. 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