(<4): Van Allen Probes Era Versus Previous Two Solar Cycles

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Journal of Geophysical Research: Space Physics

RESEARCH ARTICLE Radiation belt electron dynamics at low L (<4): Van Allen 10.1002/2017JA023924 Probes era versus previous two solar cycles

Key Points: X. Li1 , D. N. Baker1 , H. Zhao1 , K. Zhang1, A. N. Jaynes1 , Q. Schiller2 , S. G. Kanekal2, • ﬁ Demonstrated for the rst 3 4 time the ratio of low Earth orbit J. B. Blake , and M. Temerin

measurements of ~2 MeV 1 2 electrons versus measurements Laboratory for Atmospheric and Space Physics, University of Colorado Boulder, Boulder, Colorado, USA, Heliophysics 3 in a geotransfer-like orbit Laboratory, NASA Goddard Space Flight Center, Greenbelt, Maryland, USA, Space Sciences Department, The • No ~2 MeV electrons inside of Aerospace Corporation, Los Angeles, California, USA, 4Space Sciences Laboratory, University of California, Berkeley, L ~ 2.6 since 2005, while numerous California, USA penetrations of ~2 MeV electrons inside L of 2.6 occurred before • Three solar wind criteria are > fl presented to determine whether Abstract Long-term ( 2 solar cycles) measurements reveal that MeV electron uxes, solar wind speed, ~2 MeV electron enhancements and geomagnetic activity have been extremely low during this current solar cycle, including years before will occur inside L of 2.6 and during the Van Allen Probes era. This study examines solar wind speed, the geomagnetic storm index (Dst), >2 MeV electrons at geostationary orbit, and ~2 MeV electrons across various L shells measured by Solar Anomalous Magnetospheric Particle Explorer in low Earth orbit (LEO) and by the Van Allen Correspondence to: Probes/Relativistic Electron and Proton Telescope (REPT) in a geotransfer-like orbit; the latter measurements X. Li, are normalized to LEO based on comparison with Colorado Student Space Weather Experiment/Relativistic [email protected] Electron and Proton Telescope integrated little experiment (REPTile) measurements in LEO. The average ratio of REPTile/REPT varies in a systematic manner with L, ~16% at L = 2.7, decreasing with L and reaching Citation: ~0.7% at L = 4.7, and increasing again with L though with greater uncertainty. We show that there have been Li, X., D. N. Baker, H. Zhao, K. Zhang, A. N. Jaynes, Q. Schiller, S. G. Kanekal, no ~2 MeV electron enhancements inside L ~ 2.6 since 2006, prior to which numerous penetrations of J. B. Blake, and M. Temerin (2017), ~2 MeV electrons into L < 2.5 were measured during periods of stronger solar wind conditions (in terms Radiation belt electron dynamics at of high-speed solar wind, magnitude of interplanetary magnetic field, B, and a sustained southward B ) low L (<4): Van Allen Probes era versus z previous two solar cycles, J. Geophys. and thus stronger geomagnetic activity. We conclude that results from the Van Allen Probes, which have Res. Space Physics, 122, doi:10.1002/ been providing the finest measurements but in operation during a quiet solar activity period, may not be 2017JA023924. representative of radiation belt dynamics, particularly for the inner edge of the outer belt, during other solar cycle phases. Received 21 JAN 2017 Accepted 24 APR 2017 Accepted article online 28 APR 2017 1. Introduction

Earth’s magnetosphere, like any other planetary magnetosphere, is capable of producing and trapping energetic electrons and ions. The relativistic electrons trapped in the Earth’s magnetosphere nominally dwell in

two torus regions: the inner belt, centered near 1.5 Earth radii (RE) from the center of the Earth when measured in the equatorial plane, and the outer belt, which is most intense around 4–5 RE. The “slot” region, where the electron ﬂux is the low during geomagnetic quiet time, separates the two radiation belts. This description may give an impression of a static picture. In fact, the electron belts are constantly waning and waxing, as well as merging with each other, and have large variations during geomagnetic active times, a. k.a. geomagnetic storms. The radial extent, center location, and intensity of each belt can be different during and after such a magnetic storm.

The main objective of this paper is to put recent Van Allen Probes [Mauk et al., 2012; Kessel et al., 2012] measurements of relativistic electrons into context by comparing them with long-term measurements from other missions during previous solar cycles, with an emphasis on the inner edge of the outer belt. It is fortunate that the Colorado Student Space Weather Experiment (CSSWE) [Li et al., 2012] was launched at almost the same ©2017. The Authors. time as the Van Allen Probes. This concurrence allows us to compare relativistic electron measurements This is an open access article under the made by the Relativistic Electron Proton Telescope (REPT) experiment [Baker et al., 2012a; Spence et al., terms of the Creative Commons Attribution-NonCommercial-NoDerivs 2013] on the Van Allen Probes (with an inclination of 10° and apogee of 5.8 RE) with relativistic electron mea- License, which permits use and distri- surements made by the Relativistic Electron Proton Telescope integrated little experiment (REPTile) on the bution in any medium, provided the CSSWE [Li et al., 2012] at low altitudes. This then allows us to compare REPT measurements with long-term original work is properly cited, the use is non-commercial and no modiﬁcations measurements made by Solar Anomalous Magnetospheric Particle Explorer (SAMPEX) [Cook et al., 1993] at or adaptations are made. low altitudes.

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Figure 1. (top and middle) Daily averaged electron ﬂuxes from REPTile (1.63–3.8 MeV, including only from the Southern Hemisphere between À60 and 50° longitude, mostly trapped population. White areas are due to data gaps) and REPT (2.1 MeV) are L sorted and color coded and plotted versus time, from 5 October 2012 to 20 February 2013 (with different color bars). (bottom) The daily average ﬂux ratio (asterisks) and vertical envelope obtained using the middle 80% of the data points. The top and bottom 10% of the data points are excluded to remove outliers. Data between 28 October 2012 and 8 December 2013 where REPTile measurements were affected by thermal noise are also excluded.

Some results from the Van Allen Probes mission that are interesting to compare with previous longer-term measurements are the lack of measurable >1 MeV electrons in the inner belt [Li et al., 2015; Fennell et al., 2015]. This result is also supported by measurements from CSSWE [Li et al., 2013, 2015]. Another result from the Van Allen Probes measurements is the existence of a seemingly impenetrable barrier to ultrarelativistic

electrons that prevents >5 MeV electrons from migrating below L = 2.8 RE [Baker et al., 2014a]. These results are very clear in the REPT data and are also supported by, e.g., Magnetic Electron Ion Spectrometer (MagEIS) measurements [Blake et al., 2013] and by REPTile measurements [Baker et al., 2014a; Li et al., 2015].

2. Normalizing REPT Measurements to Low Earth Orbit In order to compare REPT measurements with SAMPEX measurements, REPT measurements need to be normalized to reflect measurements taken at an orbit similar to SAMPEX’s (550 km × 650 km, 82° inclination). We did this using concurrent measurements between REPT and REPTile on CSSWE, which had a similar low Earth orbit (480 km × 780 km, 65° inclination) to SAMPEX’s. REPTile is a simplified and miniaturized version of REPT [Li et al., 2012]. Its channel 2 (1.63–3.8 MeV) electron data are similar to SAMPEX’s measurements as previously demonstrated in Figure 1 of Li et al. [2013]. REPTile’ s look direction slowly wobbles near 90° with respect to the local background magnetic field, thus measuring locally mirroring particles. However, as the field of view of the REPTile is large (52° full width), the measured particles are a combination of trapped and precipitating

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populations, dominated by the trapped population but still only a fraction of the total trapped population [Li et al., 2012, 2013]. For example, the largest electron equatorial pitch angle that can be measured by REPTile is ~17° at L = 2.5 and ~5° at L = 5. However, REPTile data have gaps and the mission ended in early 2015. In order to normalize REPT data to REPTile and thus to SAMPEX data, we first created two data arrays containing daily average fluxes from REPT (2.1 MeV) and REPTile (1.63–3.8 MeV) from 5 October 2012 to 20 February 2013 (after which, REPTile had more frequent and larger data gaps), as shown in Figure 1. Where either array had data gaps, we removed these data from both arrays. Then we determined the average ratio of the concurrent measurements between REPTile and REPT for each L (L bin: 0.1, L = 1.1–6.6. The L for REPTile is determined from the IGRF model at LEO; the L for REPT is determined from Olson-Pfitzer 77 quiet model. The difference between these two L is less than 0.1 for daily averages in the region of L < 4, which is the focus of this paper.), using the middle 80% of the data points in order to avoid outliers. The vertical lines in Figure 1 (bottom) are the envelope of the 80% remaining data, and the asterisks represent the averaged ratio. We excluded data between 28 October and 8 December of 2012 when REPTile data were affected by thermal noise. Finally, we applied this average ratio (asterisks) to each L to normalize REPT fluxes from 5 September 2012 to 31 August 2016. It is interesting to note that the ratio has a systematic behavior with L, which is a significant result by itself despite the appearance of the large variations, which are partially due to the incomplete coverage of REPTile for some regions. The largest value of the average ratio, 16%, is near L = 2.7 (with an equatorial loss cone of ~14°), decreasing toward lower and higher L. The decreased ratio at lower L can be understood as caused by the increase in the background noise in REPT as it becomes fully exposed to inner belt protons (at and below L = 2.7 [see Li et al., 2015, Figure 7]), while REPTile, in a low-altitude, high-inclination orbit, is only exposed to inner belt protons in the South Atlantic Anomaly [Li et al., 2013, 2015]. The decreased ratio at higher L can be understood as due to the decrease in the equatorial loss cone with increasing L and so a smaller percentage of electrons can reach lower altitudes. The average ratio reaches ~0.7% around L = 4.8. However, this ratio increases with L after L = 5. There are several possible explanations for this increase: (1) REPT measurements at higher L are normally at higher magnetic latitudes (Van Allen Probes have a 10° inclination, thus can reach 21.5° in magnetic latitude), (2) the electron pitch angle distributions (PADs) may be more like butterfly shape distributions at higher L (fewer electrons mirroring around the equator), (3) the energy spectrum at higher L may be softer (fewer electrons at higher-energy at higher L), and (4) poor statistics at higher L because both the Van Allen Probes and CSSWE could reach higher L (>6) only for limited time, leading to bigger uncertainties for this normalization ratio at higher L. Some more detailed discussion of (2) and (3) listed above are noteworthy. At higher L shells, the PADs tend to assume a butterfly shape (i.e., minimum flux at 90°) as can be seen in recent studies using Van Allen Probes data [e.g., Artemyev et al., 2015; Baker et al., 2016] as well as earlier studies using CRRES data [e.g., Gannon et al., 2007] and using CRESS, Polar, and GEO spacecraft data [e.g., Chen et al., 2014]. Since one spacecraft is in LEO while the other is in a geotransfer-like orbit, CSSWE and the Van Allen Probes measure different portions of the PAD. REPTile observes more of the off-90° particles, and therefore, one would expect a higher REPTile/REPT ratio during times of butterfly PADs. Additionally, the butterfly PADs appear to be most preva- lent at higher L near the nightside [e.g., Gannon et al., 2007; Chen et al., 2014; R. Friedel, personal communica- tion, 2016]. Van Allen Probes apogee was near the nightside during the time period of Figure 1. Regarding explanation (3) listed above, going outward from the heart of the outer radiation belt, the energy spectrum tends to be increasingly softer with L shell (see long-term figures in Baker et al. [2014b, 2016]). Since the CSSWE energy channel is slightly lower than the REPT channel, with a steeply falling spectrum one would enhance this difference at higher L.

3. Long-Term Observations Figure 2 shows, from top to bottom, the 27 day (solar rotation period) window-averaged >2 MeV electron

ﬂux measured at geostationary orbit (r = 6.6 RE) by GOES spacecraft, and the monthly minimum Dst index (nT), the yearly window-averaged sunspot numbers (black curve) and the weekly window-averaged solar wind speed (km/s, red curve), and ~2 MeV electron ﬂux(#/cm2 s sr MeV) measured by SAMPEX since its launch on 3 July 1992 to 5 September 2012. The electron measurements are 27 day window-averaged, color coded in logarithm, and sorted in L (L bin: 0.1). After 5 September 2012 the normalized 2.1 MeV REPT electron

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Figure 2. (first and second rows) The 27 day window-averaged >2 MeV electron fluxes measured GOES at geostationary orbit (r = 6.6 RE) and the monthly minimum Dst index (nT). (third row) The yearly window-averaged sunspot numbers (black curve) and the weekly window-averaged solar wind speed (km/s, red curve). The light-color shaded area indicates the Van Allen era (5 September 2012 to 31 August 2016). (fourth row) The 27 day window-averaged, color coded in logarithm, and sorted in L (L bin: 0.1) ~2 MeV electron fluxes (#/cm2 s sr MeV) measured by SAMPEX since its launch on 3 July 1992 into a low-altitude (550 km × 700 km) highly inclined (82°) orbit (it reentered on 13 November 2012) and REPT measurements of ~2.1 MeV electron fluxes starting on 5 September 2012 but normalized based on CSSWE/REPTile measurements of 1.63–3.8 MeV electron fluxes (5 October 2012 to 20 February 2013) from a similar orbit to SAMPEX.

measurements are plotted until 31 August 2016 (light-color shaded area in Figure 2 (ﬁrst to third rows)). The SAMPEX mission was launched into a 550 km × 700 km altitude, 82° inclination orbit. The average altitude of SAMPEX decayed slowly for the ﬁrst 8 years (~580 by 2000) and faster afterward, reaching ~480 km by 2005, ~440 km by 2010, ~340 km by the launch of Van Allen Probes, and reentered on 13 November 2012 [Baker et al., 2012b]. Several previous efforts on long-term observations of the radiation belt electrons should be mentioned here before we discuss Figure 2 in detail. Rodger et al. [2016] used SAMPEX, GOES, and POES satellites but only showed 1998–2013. Reeves et al. [2011] showed data from 1989 to 2009, but only showed GEO data and did not cover the Van Allen Probes era. Morley et al. [2017] showed electron radiation belt dynamics from 2001 to 2016 using GPS, missing solar cycle 22 and the ascending phase of solar cycle 23, and only for L > 4. Therefore, Figure 2 presents the most comprehensive long-term observation of the radiation belt electrons to date. We note several outstanding features in Figure 2: 1. Geomagnetic activity as indicated by the Dst has been low during this current solar cycle, including years before and during Van Allen Probes era (see Riley and Love [2017] for the frequency and amplitude of geomagnetic storms over the last six decades). 2. The intensity of relativistic electrons measured by GOES in geostationary orbit and by SAMPEX in LEO and by REPT on the Van Allen Probes in a geotransfer-like orbit was very low during periods of low geomagnetic activity and low solar wind speed, notably in the middle of 1996, during 2009–2010, in early 2013, and in 2014. The low intensity of radiation belt electrons during these periods was also studied by other researchers [e.g., Kataoka and Miyoshi, 2010; Reeves et al., 2011; Rodger et al., 2016].

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3. The intensity of outer belt electrons is well correlated with solar wind speed, most notable enhancements occurred during the declining phases of the two previous solar cycles: 1994–1995 and 2003–2005. This correlation has been well recognized before [e.g., Paulikas and Blake, 1979]. 4. The inner edge (or barrier) of outer belt ~2 MeV electrons seems well correlated with Dst, as will be further discussed. During 2009–2010, the barrier moved up to L ~ 3.5. During this period, the total intensity of outer belt electrons was also very low as shown by both GOES and SAMPEX measurements. 5. Deep penetrations (enhancements at L < 2.6) of ~2 MeV electrons occurred rather frequently before 2006, when there were 17 geomagnetic storms with Dst below À200 nT. Only two such storms occurred after 2006, namely, on 17 March and 23 June of 2015, and deep penetrations of MeV electrons were visible for both. It is also evident that ~2 MeV electron enhancements even occurred near L = 1.5 on several occa- sions before 2006. This point will also be further discussed. 6. Since uncertainties associated with the normalization ratio (Figure 1) are bigger at higher L, one should be cautious in comparing the normalized REPT measurements at higher L with SAMPEX and GOES measurements.

3.1. High-Speed Solar Streams and Outer Belt Electron Intensity It has been known for decades that the intensity of the outer belt is positively correlated with solar wind speed [e.g., Paulikas and Blake, 1979; Baker et al., 1979; Li et al., 2005; Lyatsky and Khazanov, 2008; Reeves et al., 2011; Kellerman et al., 2013; Osthus et al., 2014]. Also required for the enhancement of outer belt electrons is geomagnetic activity, which requires a southward component of the interplanetary magnetic ﬁeld (IMF) [e.g., Miyoshi and Kataoka, 2008; McPherron et al., 2009; Li et al., 2011; Miyoshi et al., 2013; Jaynes et al., 2015]. However, a southward component of the IMF almost always occurs during high-speed solar wind streams since the streams are long lasting (days), while the IMF sector polarity after the stream interface shifts often between northward and southward; the latter case is associated with enhanced geomagnetic activity and dynamic variations of the outer belt electron intensity [e.g., Miyoshi and Kataoka, 2008]. Thus, it is not surprising that the outer belt intensity has been relatively weak during the Van Allen Probes era since solar wind speed has been relatively low, although not as low as during 2009–2010 when the solar wind speed and electron ﬂuxes were even lower. Nonetheless, Figure 2 is noteworthy for showing this correlation with long-term GOES, SAMPEX, and REPT measurements.

4. Deep Penetration of MeV Electrons and the Dst Index How deeply the outer belt penetrates is correlated with the magnitude of geomagnetic storms (Dst) [e.g., Tverskaya, 1986; Tverskaya et al., 2003; Zhao and Li, 2013a], which can be expressed as a function of the solar wind with strongest weights on the southward IMF and the solar wind speed [Temerin and Li, 2002, 2006, 2015]. Figure 3 illustrates the relationship between minimum Dst and the lowest L in which the MeV electron flux changed by at least one order of magnitude during the storm, from 1 day before to 3 days after the time of minimum Dst, following an isolated geomagnetic storm, defined as no other storm 7 days before or 4 days after the time of minimum Dst [Zhao and Li, 2013a]. Figure 3a uses daily-averaged SAMPEX PET/ELO (~2 MeV) electron fluxes from 1995 to 2004 (adapted from Zhao and Li [2013a]), and Van Allen Probes electron data from October 2012 to August 2016 for three different energies are in Figures 3b–3d. Only isolated storms with

Dstmin < À50 nT are used. There were 117 storms between 1995 and 2004 and 48 storms between October 2012 and August 2016 that met these criteria. Several outstanding features of Figure 3 are noted: 1. The greater the storm, the deeper the electron penetration. This overall trend is clear for all energies and years. 2. Lower energy electrons penetrate deeper for given storms, as illustrated in Figures 3b–3d. 3. There were only two storms with Dst < À200 nT during the Van Allen Probes era. However, there were many more in the period covered by SAMPEX measurements, as shown in Figure 3a. Despite the scatter and the limited statistics (particularly for the Van Allen Probes measurements), Figure 3 shows that deep penetration of approximately MeV electrons happens only during intense magnetic storms and that lower energy electrons always penetrate deeper, which has been discussed earlier [Zhao and Li, 2013b, Zhao et al., 2016; Reeves et al., 2016]. Thus, it is not surprising that MeV electrons

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Figure 3. Lowest L on which the electron ﬂuxes changed by more than 1 order of magnitude as a function of minimum Dst, using data of (a) SAMPEX PET/ELO (adapted from Zhao and Li [2013a]), ~2 MeV electrons; (b) MagEIS ~590 keV electrons; (c) MagEIS ~890 keV electrons; and (d) REPT 2.1 MeV electrons, during 117 isolated storms from 1995 to 2004 for Figure 3a and 48 isolated storms from October 2012 to August 2016 for Figures 3b–3d. The correlation coefﬁcient is also shown on each panel. The four horizontal dotted lines are a guide to the eye: L = 2, 2.5, 3, and 4.

did not penetrate inside L ~ 2.6 during the Van Allen Probes. No storms with Dst < À230 nT have occurred in the past 10 years, while 11 such storms occurred between October 1999 and May 2005, among them 4 storms with minimum Dst < À300 nT.

5. Deep Penetration of MeV Electrons and the Required Solar Wind Conditions Although Figure 3 demonstrates the correlation between the deep penetration of MeV electrons with the magnitude of the Dst index, the depth of penetration is not directly correlated with the Dst index in an obvious way. For example, Figure 3a shows that sometimes ~2 MeV electrons penetrated to ~L = 2.2 when the Dst index was barely below À200 nT, while some other times the ~2 MeV electrons penetrated only to ~L = 3 when the Dst index was already close to À300 nT. It is also known that the Dst index can be accurately predicted based on solar wind parameters [Temerin and Li, 2002, 2006, 2015]. Thus, we would also like to discuss the required solar wind conditions that would ensure the occurrence of injection of ~2 MeV to L = 2.5. Based on solar wind and SAMPEX data from January 1995 to June 2004, Zhao and Li [2013a] identiﬁed sufﬁ-

cient conditions for the injection of ~2 MeV electrons at L = 2.5 as: Bmax > 24 nT, Eymax > 9 mV/m, and Êy > 7 mV d/m. Here Ey is the solar wind electric field, VxBz; Êy is Ey integrated with time and calculated within 1.5 day before and after Eymax and calculated only when Ey > 0.5 mV/m, which means it only includes the positive part of Ey. For the period 1995–2004, there are a total of 10 intervals during which solar wind conditions met these sufficient conditions, and an injection of ~2 MeV at L = 2.5 occurred for each of these intervals. Here we plotted the solar wind conditions for all the intervals associated with storm events with the Dst index below À50 nT during the Van Allen Probe era (from 5 October 2012 to 31 August 2016) in Figure 4a. In order to meet the sufficient conditions, the dots (one dot for each interval) have to be in the upper right

hashed area and simultaneously satisfy Êy ≥ 7 mV d/m (at colored red). Obviously, no solar wind conditions during the Van Allen Probe era meet these three criteria, although several intervals met two of the three cri-

teria (several blue dots are located in the hashed area, but the magnitude of Êy is still lower than 7 mV d/m. Only two red dots in Figure 4a are located outside the hashed area). In Figure 4b, we plotted solar conditions for all the intervals of storm events with the Dst index below À100 nT from January 1995 (when continuous solar wind measurements became available) to September 2012. The contrast versus Figure 4a is evident.

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Figure 4. Solar wind conditions: Eymax, Bmax, and Êy, (a) during the intervals of total 48 storms with the Dst below À50 nT from October 2012 to August 2016; (b) during the intervals of total 92 storms with the Dst index below À100 nT from January 1995 to September 2012. The hashed area on the upper right corner represents that two of the three sufﬁcient solar wind conditions are met; see text for detailed description. The color of the dots represents that the third sufﬁcient solar wind condition is met only if the value is greater than 1 (red).

There are 12 red dots in the upper right hashed area in Figure 4b, leading to 12 injections of ~2 MeV electron at L = 2.5, which are discernable in Figure 2. There are two more deep injections between July 1992 and January 1995 (discernable in Figure 2), but the detailed and continuous solar wind measurements were not available prior to January 1995.

6. Discussion The Van Allen Probes have made many discoveries about radiation belt electron dynamics, thanks to their excellent instruments. Since the Van Allen Probes have been operating for more than 4 years, it seemed time to put some of these new results into the context of longer-term observations, such as SAMPEX and GOES and to caution against attempts to generalize some conclusions, in particular the lack of MeV electrons in the slot region and inner zone, which could be just a recent phenomenon due to the lack of strong storms during the Van Allen Probes era. One distinct discovery from the Van Allen Probes mission is the lack of measurable >1 MeV electrons in the inner belt [Li et al., 2015; Fennell et al., 2015]. However, there have been plenty of measurements of electrons with energies <0.8 MeV. A striking characteristic of such electrons, unveiled by Van Allen Probes measurements from the MagEIS instrument [Blake et al., 2013], is that they often have a 90° minimum in their pitch angle distribution in the slot region and inner belt [Zhao et al., 2014a, 2014b]. This result has inspired a great deal of theoretical interest in an attempt to explain such a peculiar pitch angle distribution by various wave-particle interactions [e.g., Albert et al., 2016; Li et al., 2016]. Such 90° minimum pitch angle distributions are also commonly observed in higher-energy electrons in the outer belt [Ni et al., 2016], which can be due to drift shell splitting [Selesnick and Blake, 2002] or nonadiabatic scattering [Artemyev et al., 2015]. The point here is to emphasize not only that plenty of sub-MeV electrons in the inner belt and slot region have been measured but also that detailed characteristics have also been revealed by Van Allen Probes/MagEIS measurements. Another striking result from Van Allen Probes mission is the existence of a seemingly impenetrable barrier to

ultrarelativistic electrons that prevents >5 MeV electrons from migrating below L = 2.8 RE [Baker et al., 2014a]. Baker et al. [2014a] have stated that earlier observations made by the Combined Release and Radiations Effects Satellite (CRRES) [Blake et al., 1992] and by SAMPEX [Baker et al., 2004, 2007; Zhao and Li, 2013a] sug- gest that the slot region and inner zone can be ﬁlled with electrons of many MeV in energy only following the most extreme solar wind conditions. Thus, without a strong perturbation from the solar wind, ultrarelativistic

(> 5 MeV) electrons cannot migrate past a barrier at ~2.8 RE [Baker et al., 2014a]. After the publication of Baker et al. [2014a], two larger magnetic storms occurred, with Dst values of À220 nT on 17 March 2015 and

À204 nT on 23 June 2015. During these two storms, >5 MeV electrons still did not penetrate below 2.8 RE, though lower energy electrons (~2 MeV) were seen down to ~2.6 RE [Baker et al., 2016]. The solar wind

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conditions during these two storm time periods correspond to the two red dots in Figure 4a, and only met one of the three criteria given in section 5. A recent theory to explain the existence of this barrier is that VLF transmitter-induced waves scatter electrons into the atmospheric loss cone before they can be transported farther inward [Foster et al., 2016]. VLF transmitter-induced waves may indeed expedite the loss of MeV electrons; however, we have numerous observations of deep penetrations of MeV electrons into L < 2.5 before the Van Allen Probes era. This is evidence that extreme solar wind conditions can cause such fast transport to stronger magnetic field regions that even the transmitter-induced waves cannot result in loss quickly enough. Other evidence of deep penetration of relativistic electrons should be noted. A new electron belt with energy >13 MeV and a proton belt with energy >20 MeV were formed at L ~ 2.5 immediately following the impact of an extremely strong interplanetary shock on 24 March 1991 [Blake et al., 1992]. This event was well measured by instruments on CRRES, which had an orbit similar to the Van Allen Probes. The formations of the new belts were investigated and well reproduced with test particle simulations [Li et al., 1993; Hudson et al., 1995]. The new electron belt lasted for years and slowly moved into L ~ 1.5 before decaying away and was well moni- tored by SAMPEX measurements [Looper et al., 2005]. This well-documented example shows that given appropriate external driver, in this case an extremely strong interplanetary shock, multiple MeV electrons will penetrate deep into the inner magnetosphere. Another deep penetration of multiple MeV electrons into the slot region and inner belt occurred following the 2003 Halloween storm, which was well measured by SAMPEX [Baker et al., 2004]. Independently, rapid enhancements of the 4.5 MeV electron flux at L = 2.5 were observed by the Polar spacecraft following the 2003 Halloween storm and successfully modeled by the fast inward radial transport mechanism [Li et al., 2009]. The 2003 Halloween storm was one of the largest storms in the previous 26 years in terms of the Dst index, reaching À353 nT, caused by strong coronal mass ejections (CMEs). This is another well-documented example showing that given strong enough solar wind conditions as defined in the previous section, ~2 MeV electron enhancements will occur inside L < 2.5. Outer belt electrons have their largest variations during geomagnetic storms. During the main phase of storms, outer belt electron fluxes decrease due to a combination of the “Dst effect” (the outward adiabatic motion of electrons due to the magnetic field configuration changes), magnetopause shadowing [e.g., Li et al., 1997], and enhanced precipitation loss [e.g., Tu et al., 2010]. The subsequent increase of the outer belt electrons depends on multiple factors: (1) continued plasma sheet convection and substorm activity, leading to enhanced chorus waves and increased tens to hundreds of keV electrons; (2) the shrinking of the plasmasphere, enabling chorus waves to energize relativistic electrons further inside the inner magnetosphere; and (3) enhanced ultralow-frequency waves, transporting relativistic electrons farther inward and thus energizing them. In general, the larger the storm, the smaller the plasmasphere, and the deeper relativistic electrons can penetrate [e.g., Baker et al., 2004; Li et al., 2006]. Additionally, studies have shown that the evolution of outer belt electrons differs depending on whether the storm is driven by a CME or by a high-speed solar wind stream [e.g., Miyoshi and Kataoka, 2005; Kanekal et al., 2015]. All storms with Dst < À200 nT during solar cycle 23 (1996–2005) were driven by CMEs [Zhang et al., 2007]. The two storms with Dst < À200 nT after 2006 (17 March and 23 June of 2015) were also driven by CMEs.

7. Summary Simultaneous measurements of relativistic electrons in the radiation belts at low altitudes by CSSWE and near the equator by the Van Allen Probes are used to ﬁnd the average ratio between low-altitude and equatorial radiation belt electron ﬂuxes. The ratio varies in a systematic manner with L, peaking at L = 2.7 with a value of ~16%, decreasing with L with a value of ~0.7% at L = 4.7, and then increasing with L again reaching a value of ~1.2% at L = 6 (the uncertainty of this ratio also increases with increasing L). Using this ratio, previous low-altitude measurements by SAMPEX are compared with Van Allen Probes measurements in order to put current measurements of relativistic electrons into a longer-term perspective, with an emphasis on the inner part of the outer radiation belt. Based on long-term measurements, we have shown that sunspot

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number, the solar wind speed, and geomagnetic activity have been extremely low during the current solar cycle, including years before and during Van Allen Probes era, which have confirmed and extended previous works [e.g., Rodger et al., 2016; Morley et al., 2017; Riley and Love, 2017]. Consequently, radiation belt electron fluxes, as measured by GOES, SAMPEX, and the Van Allen Probes, have been low and there have been no ~2 MeV electron enhancements inside L ~ 2.6 since 2006. This has led to the appearance of a barrier around L = 2.6 during the Van Allen Probes era preventing ~2 MeV electrons from moving farther inward [Baker et al., 2014a, 2016]. However, deep penetrations of ~2 MeV electrons into the slot region and inner belt occurred previously during extreme solar wind conditions [e.g., Blake et al., 1992; Baker et al., 2004; Zhao and Li, 2013a]. No solar wind activity during the Van Allen Probe era have met these conditions. Thus, we should be careful about interpreting Van Allen Probes data, showing the lack of MeV electrons in the slot region and inner zone [Li et al., 2015; Fennell et al., 2015], as applicable to other time periods since Van Allen Probes have been operating during a quiet solar activity period. During more active times this may change significantly, particularly at the inner edge of the outer belt. It would be most interesting to observe extreme radiation belt enhancements with the exquisite tools afforded by the Van Allen Probes mission.

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