PUBLICATIONS Journal of Geophysical Research: Space Physics RESEARCH ARTICLE 10.1002/2014JA020049 Key Points: • Energetic electron PSDs inside the radiation belt are much lower than outside • PSD changes are frequent and occur in a timescale of a few hours • Inward transport of PSD seen with gradient broadening with time
Correspondence to: A. T. Y. Lui,
[email protected]
Citation: Lui, A. T. Y., D. G. Mitchell, and L. J. Lanzerotti (2014), Comparison of energetic electron intensities outside and inside the radiation belts, J. Geophys. Res. Space Physics, 119, 6213–6230, doi:10.1002/2014JA020049. Received 7 APR 2014 Accepted 19 JUL 2014 Accepted article online 28 JUL 2014 Published online 7 AUG 2014
Comparison of energetic electron intensities outside and inside the radiation belts A. T. Y. Lui1, D. G. Mitchell1, and L. J. Lanzerotti2 1
JHU/APL, Laurel, Maryland, USA, 2Center for Solar Terrestrial Research, Department of Physics, New Jersey Institute of Technology, Newark, New Jersey, USA
Abstract
The intensities of energetic electrons (~25–800 keV) outside and inside Earth’s radiation belts are reported using measurements from Time History of Events and Macroscale Interactions during Substorms and Van Allen Probes during nongeomagnetic storm periods. Three intervals of current disruption/dipolarization events in August 2013 were selected for comparison. The following results are obtained. (1) Phase space densities (PSDs) for the equatorially mirroring electron population at three values of the first adiabatic invariant (20, 70, and 200 MeV/G) at the outer radiation belt boundary are found to be 1 to 3 orders of magnitude higher than values measured just inside the radiation belt. (2) There is indication that substorm activity leads to PSD increases inside L = 5.5 in less than 1 h. (3) Evidence for progressive inward transport of enhanced PSDs is found. (4) Reductions and enhancements in the PSDs over L shells from 3.5 to 6 are found to occur rapidly in ~2–3 h. These results suggest that (1) continual replenishments are required to maintain high levels of PSD for electrons at these energies and (2) inward radial transport of these electrons occurs in a fast timescale of a few hours.
1. Introduction The perpetual presence of Earth’s radiation belts emphasizes the importance of one or more sources for the particle population in the radiation belts that can replenish losses through various sinks (e.g., precipitation, scattering, and charge exchange). An early favorite source was inward radial diffusion from the magnetopause and/or magnetotail plasma sheet population [e.g., Fälthammar, 1965; Schulz and Lanzerotti, 1974]. For radial diffusion, the levels of electric and magnetic fluctuations in the magnetosphere determine to a large extent the timescale for particle transport to the outer radiation belt and associated acceleration. Recent research efforts have shown that a much faster timescale than diffusion can be achieved with direct penetration of energized particles from the plasma sheet into the outer radiation belt through injection pulses or wave-particle resonance [e.g., Ingraham et al., 2001; Li et al., 2003; Zaharia et al., 2004; Taylor et al., 2004; Mithaiwala and Horton, 2005; Ukhorskiy et al., 2009; Clilverd et al., 2012; Ganushkina et al., 2013, 2014]. These previous studies constitute the basic idea for an external source providing the maintenance of energetic particles in the outer radiation belt. These findings do not diminish the important fact that internal sources involving local wave-particle acceleration can play a major role in populating the outer radiation belt, especially during geomagnetic storm periods [Horne and Thorne, 1998; Summers et al., 1998; Liu et al., 1999; Albert, 2000; Brautigam and Albert, 2000; Horne et al., 2005; Hudson et al., 2008]. The successful launch of the Van Allen Probes mission and the ongoing multisatellite THEMIS (Time History of Events and Macroscale Interactions during Substorms) [Angelopoulos, 2008] mission offer a unique once in a lifetime opportunity to investigate the relationship between one external source of energetic particles at the inner edge of the magnetotail and their intensity within the radiation belts in more detail than before. In a recent article, Lui et al. [2012] used observations from THEMIS to show that the phase space density (PSD) of energetic electrons associated with current disruption/dipolarization (CDD) events during nonstorm periods outside the radiation belts is high enough to account for the population of relativistic electrons in the outer radiation belt if they could be transported without significant loss. This study was followed by another study [Lui, 2013] showing similar results for the 5 April 2010 magnetic storm previously examined for different aspects of the storm by others [e.g., Connors et al., 2011; McComas et al., 2012; Goldstein et al., 2012a, 2012b; Clilverd et al., 2012]. These two studies by Lui emphasized that the results do not dispute the importance of an internal source in the production of energetic electrons in the radiation belts during geomagnetic storms. Rather, the main result was that an external source should not be ignored, especially during nonstorm intervals.
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The previous studies on external source strength for electrons did not have simultaneous comprehensive measurements of electron intensity inside the radiation belts for direct comparison. This study complements the previous studies by examining energetic electron intensity measurements both outside and inside the radiation belts. Measurements of energetic electrons and magnetic field from THEMIS for CDD events near the neutral sheet outside the radiation belts are used to compare with the daily measurements of energetic electron PSDs within the radiation belts.
2. Data Energetic electron data from the solid state telescope (SST) [Angelopoulos, 2008] and magnetic field data from the fluxgate magnetometer (FGM) [Auster et al., 2008] on THEMIS satellites are used to compute the electron PSD using the formula by Chen et al. [2006]. More precisely, the following formulas are used to calculate the PSD f(μ):
f ðμÞ ¼ 3:3249 10–8 j= p2 c2 ;
μ ¼ 104 p2 c2 = 2m0 c2 B ;
2 2 p c ¼ K l K l þ 2m0 c2 þ K u K u þ 2m0 c2 =2: Here f(μ) is in (c/MeV cm)3, B is the magnetic field strength in nanotesla, μ is in MeV/G, m0 is the electron rest mass, c is the speed of light in vacuum, m0c2 = 0.511 MeV is the electron rest mass, j is the differential intensity in (cm2 s sr keV)1, and Kl and Ku are the lower and upper limits of the energy channel in MeV, respectively. PSD inside the radiation belts are similarly calculated based on energetic electron data from the Radiation Belt Storm Probes Ion Composition Experiment (RBSPICE) [Mitchell et al., 2013] and magnetic field data from the Electric and Magnetic Field Instrument Suite and Integrated Science (EMFISIS) instrument [Kletzing et al., 2013] on Van Allen Probes (VAPs). Three CDD events near the neutral sheet observed in three consecutive days by THEMIS satellite P5 at downtail distances of ~6–9 RE in 2013 are presented in this work. CDD events have the characteristics of rapid variations in magnetic field components near the neutral sheet [Takahashi et al., 1987; Lui and Najmi, 1987]. These field variations are then followed by the Bz component changing from small values to sustained high values. This change indicates reconfiguration of the magnetic field from tail like to dipolar like due to a reduction of the cross-tail current [e.g., Lui et al., 1988]. In this study, electrons with pitch angles 90° ± 10° near the equator/neutral sheet are examined. This is because the first adiabatic invariant for equatorial mirroring electrons is considered to be conserved for an external source as the origin of energetic electrons in the outer radiation belt if no nonadiabatic process intervenes along their inward transport. Furthermore, it can be determined precisely without invoking a magnetic field model, unlike the other two adiabatic invariants. The radial profile of the phase space density is given in Roederer parameter L* defined by L* ¼ 2πM=ΦRE ; where M is the Earth’s magnetic moment and Φ is the magnetic flux through the electron’s drift orbit around the Earth based on OP-quiet magnetic field model [Olson and Pfitzer, 1977].
3. Observations 3.1. Overview of Observations for This Study Figure 1a shows the locations of THEMIS P5 projected on the equatorial plane during these events (different colors to distinguish these events) in relation to the orbits of Van Allen Probes (blue). P5 orbit segments are labeled with month and day for the three consecutive days from 14 to 16 August 2013. Van Allen Probes (VAP) had their apogee near 19 magnetic local time (MLT) while the THEMIS locations were in the premidnight sector with YGSM ~ 3 RE. August 2013 was the prime month for THEMIS tail pass in the premidnight sector, aligning favorably with the premidnight orbits of VAP. The chosen days are the only three consecutive days in August in which CDD events near the neutral sheet just outside the radiation belts (XGSM ≈ 6 to 9 RE) were observed by THEMIS on each day.
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Figure 1. (a) The equatorial projections of the daily trajectory locations of Van Allen Probes and the orbit segments of three THEMIS current disruption/dipolarization events; (b) SYM-H for the month of August 2013; (c) solar wind dynamic pressure for 14–16 August 2013; (d) components of the interplanetary magnetic field in different colors for 14–16 August 2013.
During these 3 days, no geomagnetic storm took place, as shown in Figure 1b. The minimum SYM-H during this period reached 53 nT briefly on 16 August, probably as a result of considerable substorm activity. The solar wind dynamic pressure was nominal with a brief high value of ~6 nPa on 15 August (Figure 1c). The interplanetary magnetic field did not show any unusual behavior either, with the Bz component changing sign frequently (Figure 1d). 3.2. CDD Event on 14 August 2013 Figure 2 shows the CDD event on this day when THEMIS satellite P5 was in the premidnight sector. The satellite was at (XGSM, YGSM) = (8.1, 3.1) RE at the start of the interval and at (9.1, 2.7) RE at the end. Figure 2a shows the magnetic field magnitude (BT ) in red and the Bz component in GSM coordinate in black. Figure 2b shows the spin-averaged fluxes of energetic electrons in 10 energy channels of the SST with different colors LUI ET AL.
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Figure 2. (a) Magnetic field measured by FGM on THEMIS P5 for the CDD event on 14 August 2013. The Bz component and BT are shown by the black and red traces, respectively; (b) the electron energy fluxes measured by 10 channels (shown in different colors) of SST on P5; (c) the electron PSDs for three values of the first adiabatic invariant (20, 70, and 200 MeV/G) in different colors.
denoting different channels. Figure 2c shows the electron PSD for 90° ± 10° pitch angle electrons. Three values of the first adiabatic invariant μ are shown, namely, 20, 70, and 200 MeV/G. These invariant values are chosen to ensure adequate comparison with those measured by RBSPICE within the L* range of 3.5 to 6 (see below). During some short intervals, the energy for 20 MeV/G falls below SST energy range and data from the Electrostatic Analyzer [McFadden et al., 2008] are used. The small differences between Bz and BT values in Figure 2a indicate that P5 was indeed within the neutral sheet region with Bz being the dominant component. At ~0432 UT, large magnetic field fluctuations occurred, indicating the start of a local CDD event. Eventually, these large fluctuations subsided at ~0458 UT, leading to sustained dipolarization with relatively stable values of Bz. Prior to the CDD onset, the energetic electron fluxes as seen in Figure 2b had slight reductions for ~5 min. At the CDD onset, the magnetic field fluctuations were enhanced significantly and abruptly, followed by episodic increases thereafter. The largest electron fluxes in these energy channels were reached at ~0452 UT, around the time of the largest magnitude of Bz and BT. The three PSDs at 90° pitch angle all showed small increases prior to CDD onset. At CDD onset, there were significant reductions of all PSDs. This is due to the large increases in BT overcompensating for the increases LUI ET AL.
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Figure 3. (a) Variation of electron intensities at six representative energy channels of RBSPICE on 14 August 2013; (b) magnetic field strength measured by EMFISIS; (c) the Roederer’s L* values for the trajectory of VAP A (black) and the provisional AE index (red); (d) the variation of the PSD for 20 MeV/G electrons along the orbits, different colors are used to distinguish different orbit segments for comparison with the L* profile; (e) the L* profile of the PSD for 20 MeV/G electrons at different orbit segments with colors matching those given in Figure 3b. The time of THEMIS injection is marked by the vertical dashed line in panels Figures 3a–3d.
in the energy fluxes, leading to PSD for the same μ being determined by electron fluxes at higher energies. The PSDs also showed significant variations in later intervals, especially for μ = 200 MeV/G in association with large fluctuations in BT. After ~0457 UT, PSDs for the two lower μ values gradually attained higher values, possibly as a sign of recovery to quiet condition. The highest values of PSDs for these electrons occurred at ~0450 UT, with values of ~437, 158, and 21 (c/MeV cm)3 for 20, 70, and 200 MeV/G, respectively. At the end of the interval, the corresponding PSDs were ~45, 7, and 0.02 (c/MeV cm)3. At the start of the interval, the corresponding PSDs were ~523, 156, and 14 (c/MeV cm)3. The values at the end of the event represent reductions by about 1 to 3 orders of magnitude, with the highest μ having the largest reduction. Electron data from RBSPICE and magnetic field strength from EMFISIS during 14 August 2013 are shown in Figure 3. The daily variations of electron intensity from six representative energy channels of RBSPICE and magnetic field strength are shown in Figures 3a and 3b, respectively. Electron data at L* < 3.4 are not shown LUI ET AL.
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due to contamination from higher-energy electrons. The L* shell traversed by VAP A (black) and the provisional AE index (red) on this day are shown in Figure 3c. The 20 MeV/G PSD as a function of time and L* are shown in Figures 3d and 3e, respectively. Different colors are used to distinguish PSDs in different segments of the orbits. There was considerable AE activity on this day. The largest AE index was 608 nT during the first 6 h and was 661 nT during the last 5 h of the day, indicating moderate size substorms during these intervals. Noticeable enhancements of electron intensity were seen during these two intervals whereas the magnetic field strength did not show substantial variations associated with the electron intensity enhancements. This indicates that the increases in PSD were mainly due to intensity increases and not decrease in magnetic field strength. At the first inbound pass (black), the PSD was ~8 × 103 (c/MeV cm)3 for L* from ~4 to ~3.5. At the subsequent outbound pass (blue), the PSD decreased significantly to ~3 × 103 (c/MeV cm)3 at these L shells, a drop of about a factor of 3 in ~3 h. The PSD started to increase gradually from L* ~ 4.2. When VAP A reached L* = 5.2 at ~0451 UT, the PSD had a value of ~8 × 102 (c/MeV cm)3 (Figure 3e). However, the PSD was more than 2 orders of magnitude lower than that seen in the outer boundary at THEMIS P5 location (Figure 2c). At the subsequent inbound pass (cyan), the PSD had higher values than the previous outbound pass (blue). The high values were maintained until L* ~ 4.5 at which it started to drop, producing a significant gradient in the L shell profile. At L* = 3.5, its value decreased to ~9 × 103 (c/MeV cm)3. The gap in PSD between the blue and cyan traces in Figure 3d is present because 20 MeV/G electrons correspond to electrons with energies below the energy threshold (~25 keV) of RBSPICE in that portion of the orbit. The correspondence between energy range and first adiabatic invariant range will be shown later in Figure 6 for three specific L shells. At the fourth orbit segment (green), the PSD maintained the previous values down to L* = 3.5. The radial gradient around L* = 4.4 seen in the previous inbound pass became smoother by then. The PSD at the subsequent inbound pass (brown) showed no significant decrease from the previous outbound pass (green) for the range of L shells shown. For the last outbound pass of the day (red), the PSD values were lower by nearly 1 order of magnitude than the previous inbound pass (brown) in all these L shells. However, as shown later in Figure 5, this last orbit segment occurred at higher magnetic latitudes than the other orbit segments. This satellite trajectory may have contributed to the overall reduction in the measured PSD. The L shell profiles during these passes show an interesting development. The largest PSD in the first outbound pass (blue) at ~0450 UT occurred soon after the CDD was detected at ~0432 UT outside the radiation belts by P5 (Figure 2a). Furthermore, there was an inward progression of enhanced PSD for the subsequent orbit segment (from blue to cyan). This was followed later by a reduced radial gradient in PSD around L* = 4.4 in the subsequent orbit segment (green). The data show that the PSD can change in a timescale of a few hours. For example, comparing the PSD at L* = 3.6 during the first pass (black) with that at the second pass (blue) shows that the drop in PSD occurred in ~3 h. Comparing the PSD at L* = 4.7 during the third pass (cyan) with that at the second pass (blue) shows a large PSD increase by nearly an order of magnitude occurring in ~4 h. In addition, the change in the radial gradient around L* = 4.4 took place within ~4 h. This suggests that radial diffusion in smoothing a radial gradient occurs also in a timescale of a few hours. Figure 4 shows that there are similar trends in the time profile and L profile developments of the PSD for 70 and 200 MeV/G electrons. The 70 MeV/G PSD as a function of time and L shell are shown in Figures 4a and 4b, respectively. The L shell profile for 200 MeV/G is shown in Figure 4c. These L shell profiles resemble those for 20 MeV/G well, with the exception of the values being lower, as would be expected due to the higher μ values. Small disjoints seen at the apogees in the transition from outbound and inbound passes are due to the larger time intervals residing at the outer L shells (70 MeV/G electrons have energies within the energy range of RBSPICE even at apogees). The similarity between the different PSD electrons over this range of μ suggests that the entire electron population was transported inward nearly as a whole over the range of μ measured. Comparison of these PSDs at higher μ with those seen at the outer boundary by P5 indicates that all PSD are lower by about 2 and 1 orders of magnitude, respectively, than those measured at P5 for 70 and 200 MeV/G. Figure 5 shows the measured PSD versus MLT on the left and versus magnetic latitude (MLAT) on the right. As before, different colors are used to distinguish different orbit segments. Large PSD changes by about 1 order of magnitude can be seen for both inbound and outbound portions of the orbits at different MLT. With the
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Figure 4. (a) The variation of PSD for 70 MeV/G electrons along the orbits on 14 August 2013 in the same format as Figure 3b; (b) the L* profile of the PSD for 70 MeV/G electrons at different orbit segments in the same format as Figure 3c; (c) the L* profile of the PSD for 200 MeV/G electrons at different orbit segments in the same format as Figure 3c.
exception of the last orbit segment (red) occurring at higher MLAT with noticeable PSD reduction throughout, the MLAT graph indicates that the PSD changes can be severe for both high and low MLAT. Therefore, PSD enhancements and reductions do not appear to be dependent on MLT alone nor on MLAT alone. Figure 6 presents a comparison between the energy spectra and the corresponding PSD spectra of electrons at three different L shells in order to provide an insight on how the energy changes at different L shells for a fixed μ value. These spectra are from the third orbit segment (cyan). The PSD spectra have steeper slopes since the range of PSD is typically two decades more than the range of electron intensity. Also, the local peaks in the energy spectra appear smoother in the PSD spectra. At L* = 5, 20 MeV/G corresponds to the low end of the energy range sampled by RBSPICE. At L* = 4, 200 MeV/G corresponds to the high end of the RBSPICE energy range. At L* = 4.5, the three values of μ lie well within the energy range of RBSPICE. These plots provide the underlying reason for the choice of μ values used for this study. Unfortunately, there were no RBSPICE data from VAP B on this day, and so a comparison between the two VAPs cannot be made.
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Figure 5. (a) The variation of PSD for 20 MeV/G electrons versus magnetic local time during different orbit segments and (b) the PSD variation for 20 MeV/G electrons versus magnetic latitude.
3.3. CDD Event on 15 August 2013 Figure 7 shows another CDD event when THEMIS satellite P5 was also in the premidnight sector, moving from (XGSM, YGSM) = (6.3, 3.5) RE at the start to (7.1, 3.4) RE at the end of the interval. Again, the small differences between Bz and BT values in Figure 7a indicate that P5 was indeed very close to the neutral sheet except for a short interval from ~0150 to 0157 UT during which the differences were quite noticeable. The energetic electron fluxes in Figure 7b were reduced during the same short period when noticeable differences between Bz and BT existed, suggesting that the fluxes were lower with increasing distances from the neutral sheet, which is an expected trend. The local CDD onset was at ~0204 UT, and magnetic field fluctuations lasted for only ~1 min, followed by a longer duration of sustained dipolarization. The transient and sharp jump in both Bz and BT are probably the features of the front of sustained dipolarization. The energetic electron fluxes were noticeably enhanced prior to the CDD onset and by about an order of magnitude at CDD onset. On the other hand, PSD reductions occurred with CDD onset, as shown in Figure 7c. For the 20 and 70 MeV/G electrons, the PSD values for the two lower μ gradually recovered after CDD onset. At the end of the interval, their PSD values were ~23, 4, and 4 × 103 (c/MeV cm)3 for 20, 70, and 200 MeV/G, respectively. At the start of the interval, the corresponding PSDs were ~162, 2, and 4 × 103 (c/MeV cm)3. These values indicate a reduction by about 1 order of magnitude only for the lowest μ, with no notificable reduction for the two higher μ. The L* traversal by VAPs A and B, the provisional AE index, the measured electron intensity and PSD at 20 MeV/G, and the magnetic field strength at both VAPs on this day are given in Figure 8. The basic display format of Figure 8 is the same as that of Figure 3 except for the additional column of VAP B data. With the availability of RBSPICE data from VAP B, comparison between PSD values from both satellites can be made. The AE index shows four episodes of major substorm activity, with the peak value at 931, 927, 785, and 671nT, respectively. The substorm intensity appeared to be decreasing with time. As in the previous day, the variations of magnetic field strength were dominated by the orbit location of VAP and not by substorm activity, indicating that increases in PSD were dominated by increases in intensity. For the first inbound pass (black) of VAP A (Figure 8d), the PSD at apogee was ~2 × 102 (c/MeV cm)3, lower by 3 orders of magnitude than the PSD of 20 MeV/G at the outer boundary location of P5. Comparing the L profile of PSD for this pass with that for the last orbit segment of the previous day (red in Figure 3e) indicates an enhancement by a factor of ~4 in the inner L shells (L* < 4.0). At the first outbound pass (blue), there were substantial enhancements (about an order of magnitude) of PSD in the inner L shells. Again, this difference is not due to sampling at a different MLT sector since similar increases were seen in the subsequent inbound pass (cyan) at a different MLT sector. As before, the PSDs as a function of MLT and MLAT were examined, and the result reinforces the independence of PSD changes on these parameters alone. As noted in the event
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Figure 6. (a) The electron energy spectrum at L* = 5 during the second inbound pass on 14 August 2013; (b) the electron PSD spectrum at L* = 5 corresponding to the interval in Figure 6a; (c) the electron energy spectrum at L* = 4.5 during the second inbound pass; (d) the electron PSD spectrum at L* = 4.5 corresponding to the interval in Figure 6c; (e) the electron energy spectrum at L* = 4 during the second inbound pass; (f) the electron PSD spectrum at L* = 4 corresponding to the interval in Figure 6e.
of 14 August, the PSD gap near the apogee in Figure 8d is due to 20 MeV/G corresponding to energies lower than the energy range of RBSPICE. The PSD increase during the first outbound pass (blue) from L* = 4.2 to L* = 4.7 produced a noticeable sharp radial gradient (Figure 8e). On the subsequent pass (cyan), this radial gradient appeared to move further inward to the region from L* = 3.9 to L* = 3.4. At the next outbound pass (green), there were a local peak at L* = 4.8 and a noticeable radial gradient from L* = 4.5 to L* = 4.0. At the subsequent inbound pass (brown), two local peaks appeared, one at L* = 5.0 and the other at L* = 4.0. At the subsequent outbound pass (red), the measured PSD had considerable lower values.
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Figure 7. (a) Magnetic field measured by FGM on THEMIS P5 for the CDD event on 15 August 2013; (b) the electron energy fluxes measured by SST on P5; (c) the electron PSDs for three values of the first adiabatic invariant.
VAP B traveled slightly behind VAP A along the same orbit. This time delay allows a temporal assessment of PSD when compared with VAPA data. Overall, the VAP B (Figures 8f–8j) data reveal mostly similar PSD developments as VAP A but also some slight differences. The first inbound pass (black) had similar PSD values as the first inbound pass of VAP A except near the apogee where the PSD values were about a factor of 3 below that at VAP A. For the next pass (blue), the PSD values were quite similar to that at VAPA (Figure 8i). The subsequent inbound pass (cyan) shows a similar drop in PSD from L* = 4.3 at ~6 × 102 to ~9 × 103 (c/MeV cm)3 at L* = 3.8 (Figures 8i–8j). For the subsequent outbound pass (green), the local peak seen by VAP A at L* = 4.8 appeared to move to L* = 4.4. For the last inbound pass (brown), there was only one broad local peak around L* = 4.5 (seen at ~2100 UT). It is plausible that the electron population at this local peak may be the result of inward transport of the population at the local peak around L* = 5.0 seen by VAP A (at ~1948 UT) during its last inbound pass. The developments in time and L profile for 70 and 200 MeV/G electrons are shown in Figure 9 in the same format as Figure 4. The highest PSD values for 70 and 200 MeV/G during this day were, respectively, ~2 × 102 and ~6 × 104 (c/MeV cm)3, which are lower by 2 and 1 orders of magnitude than those values measured at P5. The PSDs near the apogees of both VAP A and B for the 70 MeV/G electrons show no gap as for the prior event. These plots show great similarity between VAP A and B as well as between 70 and 200 MeV/G electrons
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Figure 8. (a) Electron intensity measured at six representative energy channels of RBSPICE on VAP A on 15 August 2013; (b) magnetic field strength measured by EMFISIS on VAP A; (c) the Roederer’s L* values for the trajectory of VAP A (black) and the provisional AE index (red); (d) the variation of the PSD for 20 MeV/G electrons along the orbits of VAP A; (e) the L* profile of the PSD for 20 MeV/G electrons at different orbit segments of VAP A; (f) electron intensity measured by VAP B; (g) magnetic field strength measured by VAP B; (h) the Roederer’s L* values for the trajectory of VAP B; (i) the variation of the PSD for 20 MeV/G electrons along the orbits of VAP B; (j) the L* profile of the PSD for 20 MeV/G electrons at different orbit segments of VAP B. The time of THEMIS injection is marked by the vertical dashed line in Figures 8a–8d and 8f–8i.
although the PSD of 200 MeV/G is typically 1 order of magnitude lower than that of 70 MeV/G. These L profiles show less structure than that for 20 MeV/G. 3.4. CDD Event on 16 August 2013 THEMIS data from P5 satellite for this CDD event are shown in Figure 10. P5 was in the premidnight sector at (XGSM, YGSM) = (8.6, 3.1) RE at the start to (9.5, 2.7) RE at the end of the interval. Figure 10a shows that the local CDD onset was at ~0300 UT with ~7 min of large magnetic field fluctuations preceding the eventual LUI ET AL.
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Figure 9. (a) The variation of PSD for 70 MeV/G electrons along the orbits of VAP A on 15 August 2013; (b) the L* profile of the PSD for 70 MeV/G electrons at different orbit segments of VAP A; (c) the L* profile of the PSD for 200 MeV/G electrons at different orbit segments of VAP A; (d) the variation of PSD for 70 MeV/G electrons along the orbits of VAP B; (e) the L* profile of the PSD for 70 MeV/G electrons at different orbit segments of VAP B; (f) the L* profile of the PSD for 200 MeV/G electrons at different orbit segments of VAP B.
sustained dipolarization. Prior to this onset, the Bz and BT traces had similar values for ~15 min, indicating the close proximity of P5 to the neutral sheet. The energetic electron fluxes in Figure 10b showed a small decrease for ~8 min before CDD onset and were enhanced by more than 1 order of magnitude at onset. As before, there were considerable reductions of the PSDs after CDD onset, as shown in Figure 10c. At the end of the interval, the PSDs attained values of ~9, 2, and 0.08 (c/MeV cm)3 for 20, 70, and 200 MeV/G, respectively. At the start of the interval, the corresponding PSDs values were 173, 49, 5 (c/MeV cm)3, representing reductions of 1 to 2 orders of magnitude. The L* traversal by VAPs A and B, the provisional AE index, the measured electron intensity and PSD at 20 MeV/G, and the magnetic field strength at both VAPs on this day are given in Figure 11. There was intense substorm activity during the first part of the day with the peak AE index of over 1200 nT at ~0320 UT and LUI ET AL.
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Figure 10. (a) Magnetic field measured by FGM on THEMIS P5 for the CDD event on 16 August 2013; (b) the electron energy fluxes measured by SST on P5; (c) the electron PSDs for three values of the first adiabatic invariant.
followed by intermittent high activity throughout the rest of the day. The magnetic field variations during these substorm intervals were rather minimal, consistent with previous indication on the dominance of electron intensity in PSD variations. For the first outbound pass of VAP A (black), the highest PSD was ~0.06 (c/MeV cm)3 at the apogee. On its inbound (blue), its value near the apogee jumped by more than an order of magnitude to ~1.6 (c/MeV cm)3, which is still a factor of ~6 lower than that seen at P5. This abrupt increase occurred at ~0348 UT, shortly after the CDD onset at ~0300 UT as seen by P5 and the highest AE index for the day at ~0341 UT. Later in that pass, there were significant enhancements also in the inner L shells. The following outbound pass (cyan) showed significant PSD increases reaching to the outer L shells as well. However, these values were not maintained in the subsequent passes (green, brown, and red). In addition, local PSD peaks can be seen for the last two passes (brown and red). The PSD profiles seen at VAP B provide the temporal variations following the measurements by VAPA. For instance, for the first outbound pass of VAP B (black), the PSD at apogee was higher at ~0.4 (c/MeV cm)3 than that seen earlier by VAPA (black), indicating that the PSD was increasing in time. Further PSD increases were seen at the start of the first inbound pass (blue). The L profiles resemble closely to that of corresponding VAP A orbit segments. For instance, there was a local peak for the last outbound pass (brown) similar to the last outbound pass of VAP A.
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Figure 11. (a) Electron intensity measured at six representative energy channels of RBSPICE on VAP A on 16 August 2013; (b) magnetic field strength measured by EMFISIS on VAP A; (c) the Roederer’s L* values for the trajectory of VAP A (black) and the provisional AE index (red); (d) the variation of the PSD for 20 MeV/G electrons along the orbits of VAP A; (e) the L* profile of the PSD for 20 MeV/G electrons at different orbit segments of VAP A; (f) Electron intensity measured by VAP B; (g) magnetic field strength measured by VAP B; (h) the Roederer’s L* values for the trajectory of VAP B; (i) the variation of the PSD for 20 MeV/G electrons along the orbits of VAP B; (j) the L* profile of the PSD for 20 MeV/G electrons at different orbit segments of VAP B. The time of THEMIS injection is marked by the vertical dashed line in Figures 11a–11d and 11f–11i.
The developments of PSDs for 70 and 200 MeV/G are given in Figure 12. As noted for the other two events, the PSD gaps near apogee evident for 20 MeV/G electrons were absent. The highest PSD values reached by 70 and 200 MeV/G for the day are ~2 × 102 and ~2 × 103 (c/MeV cm)3, respectively. They amount to about 2 and 1 orders of magnitudes lower than that seen at P5. One major difference in comparing these L profiles with those for 20 MeV/G is the absence of large increases seen at apogees for both VAP A and B. This feature suggests that the substorm activity in this case did not lead to significant energization for electrons at these higher μ electrons. LUI ET AL.
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Figure 12. (a) The variation of PSD for 70 MeV/G electrons along the orbits of VAP A on 16 August 2013; (b) the L* profile of the PSD for 70 MeV/G electrons at different orbit segments of VAP A; (c) the L* profile of the PSD for 200 MeV/G electrons at different orbit segments of VAP A; (d) the variation of PSD for 70 MeV/G electrons along the orbits of VAP B; (e) the L* profile of the PSD for 70 MeV/G electrons at different orbit segments of VAP B; (f) the L* profile of the PSD for 200 MeV/G electrons at different orbit segments of VAP B.
4. Summary and Discussion Comparisons have been made of the electron phase space densities inside the radiation belts measured by RBSPICE on Van Allen Probes with those outside the radiation belts during three CDD events near the neutral sheet as measured by THEMIS P5. These CDD events occurred at downtail distances of ~6 to 9 RE, just outside the radiation belts. Equatorially mirroring electrons with 90° ± 10° pitch angle are studied. Three values of the first adiabatic invariant, 20, 70, and 200 MeV/G, are selected for detailed comparison. The following results are obtained with PSD below referring to these three selected values although the features are quite common over a broader range of μ for ~25–800 keV electrons. In terms of orders of magnitude value, the PSDs measured by THEMIS after a CDD event are representative of any injection from an external source just outside the radiation belts. This is supported by the general agreement in orders of magnitude of PSDs
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among all three CDD events. There is no intention of matching a THEMIS injection with a particular PSD enhancement except for the rather obvious case on 16 August 2013 (see Figure 11). Therefore, it does not matter whether or not there were many other injections occurring on the same day as the one detected by THEMIS. 1. After dipolarization intervals, the electron PSDs may decrease by 1 to 3 orders of magnitude from the quiet time values at the outer boundary of the radiation belts, with the largest difference typically occurring for 200 MeV/G electrons. The PSD reduction arises from the increase in magnetic field magnitude overcompensating the increase in energy fluxes at a fixed μ. 2. The PSDs inside the radiation belts are generally 1 to 3 orders of magnitude lower than those observed outside after the CDD event, with the largest difference for 20 MeV/G electrons typically. 3. There is indication that substorm activity leads to PSD enhancements inside L = 5.5 in less than 1 h. 4. Sequential L profiles of PSD for these energetic electrons show evidence for radial inward transport of PSD in a timescale of a few hours. 5. PSD reductions and enhancements by more than 1 order of magnitude appear frequently. These PSD changes occur on a fast timescale of ~2–3 h. This short timescale for PSD reduction suggests that continual replenishment is required to maintain high PSD values in the L shells from 3.5 to 6. The previous study of Lui et al. [2012] showed that the electron PSD outside the radiation belt after CDD is typically 1 to 3 orders of magnitude higher than those inside. However, the comparison was not made at the same time. This study complements the previous study by arriving at the same conclusion with comparison made at the same time. In the past, this issue was mainly addressed by examining the PSD radial gradient. For example, Chen et al. [2005] examined the gradient for the μ range of 167–2083 MeV/G over a limited L* range (6.0–7.0). They found positive gradients for μ = 167–1051 MeV/G but not for μ = 2083 MeV/G. Turner et al. [2012] showed from THEMIS data that the gradients for electrons to be μ dependent also, positive or flat for μ < 200 MeV/G and flat or negative for μ > 200 MeV/G, in general. Results from these two studies are generally consistent with the findings here. However, the result of Turner et al. has a slight difference from our result in that PSD for μ = 200 MeV/G outside the radiation belt was at least 1 order of magnitude higher than inside in this work. One plausible cause is that this difference arises from the choice of CDD events near the neutral sheet here to determine the external PSD, rather than not seeking the occurrence of CDD near the neutral sheet to determine the PSD gradient as was done in Turner et al. Another plausible cause is that the transition of positive to negative/flat radial gradient is at a higher μ than indicated by Turner et al. Further study will be needed to verify the cause of this slight difference. Since VAP data in this study are all taken in the premidnight sector and energetic electrons drift toward the dawn sector, one question that can be raised is whether the electron intensity is generally lower in the premidnight sector than in the dawn sector. To examine this possibility, we have examined VAP data for 14–30 November 2012 when VAP orbits were in the dawn sector. The highest PSD in all these dawn orbits for the μ range examined here were comparable to the values found on the three premidnight days. Therefore, the result obtained is not altered if simultaneous comparison could be made with VAP data in the dawn sector. This also implies that the PSD for low-μ electrons is rather independent of MLT for nonstorm periods. The 16 August 2013 event is a clear example of dipolarization at ~9 RE in the outer magnetosphere leading to significant increases in electron PSD in the inner magnetosphere at L* = 5.6. There is no doubt that substorm injections can be a significant source for low-energy radiation belt electrons. This constitutes clear evidence of inward transport of substorm-associated electron population to the inner magnetosphere. The rapid (timescale of ~2–3 h) reductions and enhancements in radiation belt electron PSD suggest continual supply of these electrons from the outer magnetosphere/plasma sheet. However, this result cannot be used to imply the external source to be dominant in the generation of low-energy electrons in the outer radiation belt because the loss processes during inward transport are not examined in this study. The result only emphasizes that the external source cannot be ignored during nonstorm periods. The PSD outside the radiation belts found in this study can serve as an appropriate outer boundary condition for simulations of radiation belt electron intensity enhancements due to substorm injections. For instance, Ganushkina et al. [2013] used the Inner Magnetosphere Particle Transport and Acceleration model to compare the modeled electron fluxes with the observed fluxes in energy range of 50–225 keV measured by Synchronous Orbit Particle Analyzer onboard the Los Alamos National Laboratory spacecraft. They found that
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the modeled fluxes are 1 to 2 orders of magnitude lower than the observed values even without the inclusion of loss processes. They attributed this difference to the inaccuracy of electron outer boundary condition. Adopting the electron outer boundary condition reported here may help to resolve this disagreement.
5. Conclusion To the best of our knowledge, this is the first comparison of electron phase space densities between THEMIS and RBSPICE on Van Allen Probes. From the comparison of the phase space densities for equatorially mirroring electrons inside and outside the radiation belts for three consecutive nonstorm days, it is found that their phase space densities outside are generally 1 to 3 orders of magnitude higher than inside for electrons with low values of first adiabatic invariant (from ~20 to ~200 MeV/G). There is compelling evidence of inward transport of enhanced phase space densities from substorm injections. Rapid changes in phase space densities for these electrons inside the outer radiation belt imply the necessity of continual replenishment in order to maintain high-density levels during nonstorm periods.
Acknowledgments This work was supported by the NASA contract to New Jersey Institute of Technology with subcontracts to the Johns Hopkins University Applied Physics Laboratory by NSF grant AGS-1250634 and NASA grant NNX12AP62G directly to the Johns Hopkins University Applied Physics Laboratory and by NAS5-02099 for THEMIS mission support. We acknowledge NASA contract NAS5-02099 and V. Angelopoulos for use of data from the THEMIS mission through AIDA at Institute of Space Science, National Central University in Taiwan; C.W. Carlson and J.P. McFadden for use of ESA data; D. Larson and R.P. Lin for use of SST data; and K-H. Glassmeier, U. Auster, and W. Baumjohann for use of FGM data provided under the lead of the Technical University of Braunschweig and with financial support through the German Ministry for Economy and Technology and the German Center for Aviation and Space (DLR) under contract 50 OC 0302, and NASA CDAWeb for EMFISIS and OMNI data, and the provisional AE index from the WDC for Geomagnetism, Kyoto. M. Balikhin thanks the reviewers for their assistance in evaluating this paper.
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