We survey radiation belt enhancement events during the Van Allen Probes era to determine what mechanism is the dominant cause of enhancements and where it is most effective. Two primary mechanisms have been proposed: (1) betatron/Fermi acceleration due to the Earthward radial transport of electrons, which produces monotonic gradients in phase space density (PSD), and (2) “local acceleration” due to gyro/Landau resonant interaction with electromagnetic waves, which produces radially localized growing peaks in PSD. To differentiate between these processes, we examine radial profiles of PSD in adiabatic coordinates using data from the Van Allen Probes and Time History of Events and Macroscale Interactions during Substorms satellites for 80 outer belt enhancement events from October 2012 to April 2017 This study shows that local acceleration is the dominant acceleration mechanism for MeV electrons in the outer belt, with 87% of the enhancement events exhibiting growing peaks. The strong correlation of the location of these with geomagnetic activity further supports this conclusion.

Key Points

Local acceleration is the dominant acceleration mechanism for most MeV electron enhancements in the outer belt
Local acceleration is observed for both storm time and non–storm time events
The radial location of phase space density peaks are well correlated with geomagnetic activity and the plasmapause location

Plain Language Summary

The radiation belts are regions of space around the Earth that are known for their high-energy particle populations. One of the key open questions to understanding how the radiation belts evolve is as follows: How do these particles get accelerated to such high energies? To address this question, two types of acceleration processes have been proposed. The first process takes particles located far from the Earth and moves them inward, energizing the particles as they move into the stronger magnetic field close to the Earth. The second process takes particles located in the heart of the radiation belts and energizes via interactions with electromagnetic waves in a process known as local acceleration. Differentiating between these processes has been historically difficult, and only recently are quality observations available for a long-enough time period to understand when and where and how often each process is dominant. In this paper, we examine observations of the radiation belts from the last 4.5 years and show that local acceleration is the dominant acceleration process in the outer radiation belt.

1 Introduction

The extreme variability of energetic (MeV) electrons in the outer radiation belt is due to the interplay of acceleration and loss processes (Reeves et al., 2003). These acceleration processes fall into two categories broadly referred to as radial transport and local acceleration. Radial transport takes a source population at large radial distance and moves it inward, violating its third invariant. Energization occurs through betatron/Fermi acceleration as the electrons conserve their first two invariants while moving into regions of stronger geomagnetic fields. This can be a diffusive process driven by ultralow-frequency waves (e.g., Ukhorskiy et al., 2005) or a rapid nondiffusive process involving rapid cross-field transport (e.g., Ingraham et al., 2001). By contrast, local acceleration involves accelerating particles in situ (i.e., at the same radial location) by violating their first and second invariants via gyro/Landau-resonant wave-particle interactions. This type of acceleration is most commonly associated with very low frequency chorus waves, which have been shown to be capable of rapidly accelerating electrons up to energies greater than a few MeV (e.g., Horne & Thorne, 1998; Reeves et al., 2013; Summers et al., 1998; Thorne et al., 2013).

The key observation needed to distinguish between these two processes (which may also occur simultaneously) is multisatellite measurements of phase space density (PSD) in adiabatic coordinates (μ,K, and L^∗; Green & Kivelson, 2004). These measurements must also have sufficient temporal cadence to resolve spatial and temporal changes in the radial gradients. Inward radial transport requires a source of the highest PSD at higher radial distance and therefore can only produce monotonic positive gradients of PSD. In contrast, local acceleration energizes electrons in a limited range of radial distances where the wave-particle resonances are effective. This produces a localized peak in PSD that grows as the electrons are energized.

Due to the difficulty in properly converting differential number flux (as a function of energy, pitch angle, and spatial location) to PSD (as a function of magnetic invariants), previous studies that examined PSD to identify local acceleration have primarily focused on individual events. Investigations using data from NASA's Polar spacecraft (Green & Kivelson, 2004; Selesnick & Blake, 2000), CRRES (Iles et al., 2006), and Time History of Events and Macroscale Interactions during Substorms (THEMIS; Turner et al., 2013, 2014) all observed peaks in PSD that could not be explained by inward radial diffusion alone. During the Van Allen Probes era, growing PSD peaks due to local acceleration have been observed for both large storms at L^∗<4 (Baker et al., 2014; Foster et al., 2014; Reeves et al., 2013) and for non–storm time events at L^∗>5 (Schiller et al., 2014). In one of the few surveys of PSD profiles, Chen et al. (2007) examined PSD observations from Polar, LANL-GEO, and GPS for a 6-month period and found that peaks in PSD due to local acceleration were commonly observed for L^∗≈ 5–6. However, the spatial coverage from these satellites was limited to the three radial positions where the satellites took equatorial measurements, making it difficult to resolve PSD peaks for L^∗<5. Together, these results have shown that local acceleration can be an important mechanism for at least some events but could not resolve the controversial question whether local acceleration is common, ubiquitous, or only dominates in a select handful of events.

Expanding on previous studies, this study is the first to quantify how often and where local acceleration is observed across the entire outer radiation belt. With more than 4 years of data from the Van Allen Probes and THEMIS available, we now have a data set of PSD with sufficient temporal and spatial coverage to look in detail at a large number of enhancement events. Employing multisatellite observations, we quantify radial PSD profiles over a wide range in L^∗ for more than 80 enhancement events in order to determine how often and where growing peaks (and therefore local acceleration) are observed. Finally, we quantify how these peak locations relate to Dst and plasmapause location in order to understand the relative role that local acceleration plays in shaping the outer radiation belt.

2 Data and Event Selection

The primary source of PSD measurements for this study is from NASA's Van Allen Probes mission. The twin Van Allen Probes spacecraft were launched in August 2012 into near GEO-transfer, low-inclination orbits. Electron measurements are taken from two instruments in the Energetic Particle Composition and Thermal Plasma Suite (ECT; Spence et al., 2013): the Magnetic Electron Ion Spectrometer (Blake et al., 2013) and the Relativistic Electron Proton Telescope (Baker et al., 2013). Together, these instruments provide pitch angle resolved fluxes for 30 keV to 20 MeV electrons. These measurements are then combined with magnetic field measurements from Electric and Magnetic Field Instrument Suite and Integrated Science (Kletzing et al., 2013) to calculate PSD using the method outlined in Boyd et al. (2014).

Additional PSD data come from the Solid State Telescope onboard the THEMIS spacecraft (Angelopoulos, 2008). Since 2010, there are three THEMIS spacecraft, each in highly elliptical near-equitorial orbits. As described in Turner, Angelopoulos, et al. (2012), electron measurements for pitch angles 90 ± 15° are used to calculate PSD corresponding to K ≤0.025R_EG^1/2. As the Solid State Telescope instrument was not designed to measure the radiation belts, it suffers from shield-penetrating background contamination at low L shells. Therefore, only data from outside L = 6 are used in this study.

For both THEMIS and the Van Allen Probes, the TS04D magnetic field model (Tsyganenko & Sitnov, 2005) is used to calculate invariants K and L^∗. THEMIS and the Van Allen Probes have not been fully intercalibrated, so in order to get the PSD measurements to match at the Van Allen Probes apogee and produce a smooth PSD profile, the THEMIS data are multiplied by a constant factor of 1/3. This factor is applied equally at all times and across all L^∗ values to maintain the shape and gradient of the observed PSD profile. The details of how this factor is determined are shown in the supporting information.

In this study, we focus on enhancement events, defined as in Boyd et al. (2016), where the PSD at L^∗ = 5 increased by at least a factor of 2 within a 48-hr period. For the period from October 2012 through April 2017, there were 80 such events. These events are selected regardless of geomagnetic activity and represent a roughly equal mix of 42 moderate/strong storm events and 38 small/non–storm time events (where minimum Dst = −50 nT marks the boundary between small/moderate storms; Gonzalez et al., 1994). In this study, we examine μ = 700 MeV/G for both the Van Allen Probes and THEMIS, which corresponds to ≈1 MeV at L^∗ = 5. For the Van Allen Probes PSD, we examine K = 0.08R_EG^1/2, which corresponds to the smallest K value that is consistently observed by the Van Allen Probes, in order to get the best agreement with the THEMIS data.

3 Event Classification

We begin with an examination of the events using only data from the Van Allen Probes. Looking at the radial profiles of these events, they fall into three broad categories: peaked, flat gradients, and positive gradients. The details of how these events are categorized are shown in the supporting information. The first category, peaked events, exhibit a clearly resolved growing peak inside of the Van Allen Probes apogee. Several of these events have previously been studied in detail (e.g., Li et al., 2014; Reeves et al., 2013), and an example of this type of event is shown in Figure 1a. The second category, flat gradient events, have flat or only slightly negative gradients at the Van Allen Probes apogee. These profiles are suggestive of a peak at or near apogee, but without observations at higher L^∗, it is impossible to make that determination. An example of this type of event is shown in Figure 1b. Finally, the third category, positive events, have positive monotonic gradients going out to apogee. An example of a positive event is shown in Figure 1c.

Details are in the caption following the image — **Figure 1**
Open in figure viewer PowerPoint

Examples of three types of events observed by the Van Allen Probes. Panel (a) shows an event 21 December 2015 with a growing peak at L^∗ = 3.8, Panel (b) shows an event from 01 December 2015 with flat gradients near apogee, and Panel (c) shows an event from 08 December 2013 that has positive gradients. All panels are for μ = 700 MeV/G and K = 0.08R_EG^1/2. Inbound passes are marked with downward triangle, outbound passes with upward triangle. RBSP = Radiation Belt Storm Probes.

Of the 80 enhancement events, 24 have growing peaks, 33 have flat gradients, and 23 have positive gradients. Therefore, using only observations from the Van Allen Probes, 30% of enhancement events have evidence of local acceleration in the form of growing peaks in PSD. However, from the flat and positive gradient events, it is clear that observations of PSD beyond the Van Allen Probes apogee are needed in order to unambiguously classify these events.

In particular, incorporating data at higher L^∗ from THEMIS can completely change the interpretation of an event. The top panel of Figure 2 shows an event from 13 Jan 2013. The PSD profiles from the Van Allen Probes from 22:50 on 13 January onward show a clear increase at apogee and consistent positive gradients. Therefore, based on Van Allen Probe observations alone, the observed enhancement might be interpreted as due to radial transport. However, during this same time period, all the THEMIS spacecraft observed negative gradients beyond L^∗ = 5.2. Together, these observations show that there is a growing peak at L^∗ = 5.2. Therefore, as was also shown in Schiller et al. (2014), the observed enhancement is due to local acceleration that is occurring just beyond the Van Allen Probes apogee but the identification could not have been confirmed by the Van Allen Probes alone.

Additionally, observations at high L^∗ can also give context to the source of observed peaks at low L^∗ as for the case for an event on 7 December 2014, which is shown in the bottom panel of Figure 2 . From 14:30 UT to 23:35 UT on 7 December 2014, the Van Allen Probes observed a growing peak around L^∗ = 5. However, during this time, the THEMIS spacecraft observed a mixture of flat or positive gradients. Therefore, while a growing peak is observed at low L^∗, the THEMIS observations indicate that the peak may not be due to local acceleration, as there seems to be a source at high L^∗.

Examining the data from the Van Allen Probes and THEMIS together gives the statistics shown in Table 1. 70 of the 80 events had growing peaks and consistent negative PSD gradients at high L^∗. In addition, these features are observed across all levels of geomagnetic activity, including 32 small events (min Dst > −50 nT). As noted earlier, growing peaks are the signature of local acceleration. However, there are a few other processes that can also produce peaked distributions. The first is a loss at high L^∗, for example, due to magnetopause shadowing (e.g., Turner, Shprits, et al., 2012). This would produce a peak in PSD but alone cannot produce a growing peak, as is observed during the 70 identified events. The second process is where the population at high L^∗ first increases, then decreases. This “on-off” process can produce a growing peak at low L^∗. However, such a process would produce fluctuations in PSD near the source at high L^∗. These are observed for the 10 “other” type events where the gradients measured by THEMIS are periodically flat or positive. However, for the 70 identified events, THEMIS observes consistently negative gradients throughout the acceleration processes. This makes such an on-off process very unlikely and local acceleration remains the best explanation for the observed growing peaks.

Table 1. Statistics for the Studied Enhancement Events, Where “Local Acceleration Dominant” Events Have Clear Observations of Growing PSD Peaks Due to Local Acceleration and for the “Other” Events, the Dominant Acceleration Mechanism for the Observed Enhancements Is Unclear

Event type	Van Allen Probes only	THEMIS & Van Allen Probes
Local acceleration dominant	24 (22)	70 (38)
Other	56 (20)	10 (4)
Total	80 (42)	80 (42)

Note. The columns show the statistics for (1) using only observations from the Van Allen Probes and (2) using observations from both Van Allen Probes and THEMIS. In each column, the numbers in parenthesis show the number of moderate/strong storm time events (minimum Dst < −50 nT). THEMIS = Time History of Events and Macroscale Interactions during Substorms.

4 Radial Locations of Growing Peaks

For each of the events, the location of the peak appears to be very well correlated with geomagnetic activity. As shown in Figure 3, the biggest events (in terms of minimum Dst) tend to produce the enhancements at smaller L^∗. The results have a very good agreement with the relationship between Dst and the L shell of the peak flux derived in Tverskaya et al. (2003), which is also shown in Figure 3. These results also confirm the findings of Moya et al. (2017), which found that Van Allen Probes flux observations agreed with the Tverskaya et al. (2003) relationship for enhancement events. This relationship is likely due to changes in the plasmapause location, which is also well correlated with geomagnetic activity (O'Brien et al., 2003). The relationship between the L^∗ of the peak and the average L value of the plasmapause from O'Brien and Moldwin (2003) is shown in Figure 3. The location of the peak for the local acceleration events is well correlated with plasmapause location with a correlation coefficient r = 0.8, which agrees with previous results (O'Brien et al., 2003; Shprits et al., 2012). Since we expect local acceleration to be most effective just outside of the plasmapause (Horne et al., 2005), these observations support the interpretation that these observed peaks are due to local acceleration.

5 Conclusions

We examined radial profiles of PSD for equatorially mirroring electrons with μ = 700 MeV/G in the outer belt from more than 4 years of observations from the Van Allen Probes and THEMIS, looking for when, where, and how often growing peaks were observed. We have three main conclusions:

Local acceleration is the dominant acceleration mechanism for MeV electrons in the outer radiation belt. Growing PSD peaks are observed for 87% of enhancement events during the Van Allen Probes era, and the combined observations from Van Allen Probes and THEMIS show that these peaks are due to local acceleration.
Local acceleration is observed across all different activity levels including small storms and non–storm time (Dst ≥ −50 nT).
The location of the peak is very well correlated with geomagnetic activity and the plasmapause location, with the strongest storms producing peaks at smaller L^∗ values.

The results presented here show that local acceleration is the dominant acceleration mechanism for the majority of the observed enhancement events. However, this does not suggest that radial transport does not play an important role in creating the dynamics observed in the outer belt. Of the 80 events presented here, 10 events had features consistent with inward radial transport playing an important if not dominant role in creating the observed enhancement. The observations from these events are more suggestive of an “on-off” source at the outer boundary driving the enhancement. In addition, for the 70 local acceleration dominated events, radial diffusion also plays an important role of redistributing the peak after the local enhancement is finished.

In this study, we have focused on μ = 700 MeV/G and small K values, corresponding to near equatorial, 1.5 MeV electrons. The results presented here are qualitatively similar for similar mu values, as shown in the supporting information. In a future study, we will explore in detail how these results depend on first invariant μ and second invariant K in order to understand how the different electron populations interact to produce the complex dynamics that we observe in the radiation belts.

Acknowledgments

Processing and analysis of the ECT data were supported by Energetic Particle, Composition, and Thermal Plasma (RBSP-ECT) investigation funded under NASA's Prime contract no. NAS5-01072. The authors acknowledge NASA contract NAS5-02099 and V. Angelopoulos for use of data from the THEMIS mission. All RBSP-ECT data are publicly available at http://www.RBSP-ect.lanl.gov/; THEMIS data are available at http://themis.ssl.berkeley.edu/index.shtml. The authors would like to thank the development teams of Spacepy (https://pythonhosted.org/SpacePy/) and the IRBEM-LIB toolset (http://craterre.onecert.fr/prbem/irbem/description.html).

Supporting Information

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Citing Literature

Volume45, Issue11

16 June 2018

Pages 5253-5259

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grl57363-sup-0001-supplementary.pdfPDF document, 647.9 KB	Supporting Information S1
grl57363-sup-0002-supplementary.xlsxapplication/unknown, 10.7 KB	Table S1

What Causes Radiation Belt Enhancements: A Survey of the Van Allen Probes Era

Abstract

Key Points

Plain Language Summary

1 Introduction

2 Data and Event Selection

3 Event Classification

4 Radial Locations of Growing Peaks

5 Conclusions

Acknowledgments

Supporting Information

References

Citing Literature

Figures

References

Information

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What Causes Radiation Belt Enhancements: A Survey of the Van Allen Probes Era

Abstract

Key Points

Plain Language Summary

1 Introduction

2 Data and Event Selection

3 Event Classification

4 Radial Locations of Growing Peaks

5 Conclusions

Acknowledgments

Supporting Information

References

Citing Literature

Figures

References

Related

Information

RESOURCES

PUBLICATION INFO