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Ring Current Coupling: a Comparison of Isolated and Compound Substorms

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Ring Current Coupling: a Comparison of Isolated and Compound Substorms manuscript submitted to JGR: Space Physics 1 Substorm - Ring Current Coupling: A comparison of 2 isolated and compound substorms 1 1 2 3 1 3 J. K. Sandhu , I. J. Rae ,M.P.Freeman , M. Gkioulidou , C. Forsyth ,G. 4 5 6 4 D. Reeves ,K.R.Murphy , M.-T. Walach 1 5 Department of Space and Climate Physics, Mullard Space Science Laboratory, University College 6 London, Dorking, RH5 6NT, UK. 2 7 British Antarctic Survey, Cambridge, CB3 0ET, UK. 3 8 Applied Physics Laboratory, John Hopkins University, Maryland, USA. 4 9 Los Alamos National Laboratory, Los Alamos, USA. 5 10 University of Maryland, USA. 6 11 Lancaster University, LA1 4YW, UK. 12 Key Points: 13 • Quantitative estimates of ring current energy for compound and isolated substorms 14 are shown. 15 • The energy content and post-onset enhancement is larger for compound compared 16 to isolated substorms. 17 • Solar wind coupling is a key driver for di↵erences in the ring current between iso- 18 lated and compound substorms. Corresponding author: Jasmine K. Sandhu, [email protected] –1– manuscript submitted to JGR: Space Physics 19 Abstract 20 Substorms are a highly variable process, which can occur as an isolated event or as part 21 of a sequence of multiple substorms (compound substorms). In this study we identify 22 how the low energy population of the ring current and subsequent energization varies 23 for isolated substorms compared to the first substorm of a compound event. Using ob- + + 24 servations of H and O ions (1 eV to 50 keV) from the Helium Oxygen Proton Elec- 25 tron instrument onboard Van Allen Probe A, we determine the energy content of the ring 26 current in L-MLT space. We observe that the ring current energy content is significantly 27 enhanced during compound substorms as compared to isolated substorms by 20 ⇠ − 28 30%. Furthermore, we observe a significantly larger magnitude of energization (by 40 ⇠ − 29 50%) following the onset of compound substorms relative to isolated substorms. Anal- 30 ysis suggests that the di↵erences predominantly arise due to a sustained enhancement 31 in dayside driving associated with compound substorms compared to isolated substorms. 32 The strong solar wind driving prior to onset results in important di↵erences in the time 33 history of the magnetosphere, generating significantly di↵erent ring current conditions 34 and responses to substorms. The observations reveal information about the substorm 35 injected population and the transport of the plasma in the inner magnetosphere. 36 1 Introduction 37 Substorms are an impulsive phenomenon associated with the storage and release 38 of energy in the Earth’s magnetosphere. Based on auroral observations, it was proposed 39 that substorms can be described as the occurrence of three separate phases: the growth 40 phase, the expansion phase, and the recovery phase (Akasofu, 1968; R. L. McPherron, 41 1970). Overall, a substorm typically lasts 2-4 hours (Tanskanen, 2009). During the growth 42 phase, low latitude dayside reconnection with the IMF (Interplanetary Magnetic Field) 43 dominates over the nightside reconnection rate, resulting in an accumulation of open field 44 lines in a highly stretched magnetotail (Kokubun & McPherron, 1981; R. McPherron, 45 1972; R. L. McPherron, 1970; Milan, Provan, & Hubert, 2007). Substorm onset marks 46 the beginning of the substorm expansion phase, and during the onset process rapid bursts 47 of nightside reconnection close significant amounts of flux in the magnetotail (e.g., Hones Jr. 48 & Schindler, 1979; Hubert et al., 2006). The dipolarization of the magnetic field and the 49 destabilisation of the near-Earth tail act to energize particles and drive intense electric 50 currents (e.g., Forsyth et al., 2014; R. L. McPherron, Russell, & Aubry, 1973). The mag- 51 netosphere then enters the substorm recovery phase, where the nightside reconnection 52 rate gradually subsides and the system returns to its original state. The occurrence of 53 a substorm has wide ranging and substantial implications for the global magnetosphere 54 and ionosphere. In this study we focus on the impact of substorms on the inner mag- 55 netosphere, specifically on the ring current population. 56 The terrestrial ring current is generated predominantly by ions with energies rang- 57 ing from tens to hundreds of keV and resides mainly between 4 to 7 Earth Radii (RE) 58 (Daglis, Thorne, Baumjohann, & Orsini, 1999; Le, Russell, & Takahashi, 2004; Sandhu 59 et al., 2018). Substorm dipolarization following substorm onset is associated with the 60 injection of plasma to the inner magnetosphere, typically a↵ecting ring current ions with 61 100s eV to 10s keV energies (Yue et al., 2018). However, previous work has shown that 62 the injection of plasma into the inner magnetosphere is highly variable. It has been iden- 63 tified that only approximately 30% of substorms are associated with an observed clas- 64 sical injection signature in the inner magnetosphere (Boakes et al., 2011; Takada et al., 65 2006). Despite the variability of the injections, a study conducted by Sandhu et al. (2018) 66 demonstrated that, on average, the ring current experiences statistically significant en- 67 hancements following substorm onset. It was establised that the global energy content, 68 estimated from an energy range up to 100s keV covering the bulk population, increased 69 by 12% relative to the pre-onset value, with the enhancement predominantly occurring 70 within the substorm expansion phase. Sandhu et al. (2018) showed that the low energy –2– manuscript submitted to JGR: Space Physics + + 71 population of H and O ions with energies ranging up to 50 keV exhibited significant 72 enhancements following substorm onset, with the energy content of these ions increas- 73 ing by more than 50%. The energy range is consistent with the expected energy range 74 of substorm-associated plasma injections in the inner magnetosphere (Yue et al., 2018), 75 as well as the ion plasma sheet population convected earthwards. 76 In this paper, we extend the analysis of Sandhu et al. (2018) to explore whether 77 some substorm characteristics are more favourable to ring current energisation than oth- 78 ers. Specifically, we categorise substorms according to the level of additional substorm 79 activity prior to and following an event. We define isolated substorms as those where there 80 is no substorm activity prior to the event and after the event. Compound substorms are 81 defined as occurring as part of a sequence of substorms where the recovery phase leads 82 directly to the expansion phase of a succeeding substorm. Previous work has demonstrated 83 di↵erences in the solar wind driving and auroral evolution during compound substorms 84 in comparison to isolated substorms (e.g., Kim, Lee, & Lyons, 2008; Liou, Newell, Zhang, 85 & Paxton, 2013; Newell & Gjerloev, 2011), such that compound substorms are typically 86 associated with periods of high solar wind - magnetosphere coupling. However, the quan- 87 titative aspect of how a sequence of compound substorms as opposed to an isolated sub- 88 storm can a↵ect the inner magnetosphere, specifically the ring current population, re- 89 mains poorly understood. In this study, we examine whether both types of substorms 90 enhance the ring current population, focusing on the evolution and morphology of the 91 low energy ring current ion population. 92 2 Data and Method 93 The Van Allen Probes mission consists of two identically instrumented spacecraft: 94 probe A and probe B (Mauk et al., 2013). The elliptical orbits have an inclination of 10◦, 95 a perigee of 600 km altitude, and an apogee of 6 R geocentric radial distance. The ⇠ E 96 orbital period is 9 hours and the precession of the orbital apogee allows sampling of all 97 local times in less than 2 years. The coverage and low inclination of the Van Allen Probes 98 orbit are highly suited to studying the ring current region. The probes are equipped with 99 the Helium Oxygen Proton Electron (HOPE) mass spectrometers (Funsten et al., 2013; 100 Spence et al., 2013). In this study we use the Level 3 HOPE observations of omnidirec- + + 101 tional energy fluxes for H and O ions, with an energy range from 1 eV to 50 keV. For 102 this statistical study we take all observations obtained during 2012 to 2018. We note that 103 the energy range was selected to focus on the injected and convected ion population that 104 is particularly sensitive to substorm onset (Sandhu et al., 2018; Yue et al., 2018), and 105 the energy range is not representative of the full ring current population. It is known that 106 higher energies also exhibit energisation, as injections are observed to range up to sev- 107 eral hundred keV (e.g., Sandhu et al., 2018; D. L. Turner et al., 2017). 108 The omnidirectional ion fluxes obtained from HOPE are used to estimate the en- + + 109 ergy content for both the H and O data sets. The same method as detailed by Sandhu 110 et al. (2018) (adapted from Gkioulidou, Ukhorskiy, Mitchell, and Lanzerotti (2016)) is 111 applied to the data and will now be briefly summarised. The omnidirectional ion energy 112 flux, j(Ech), at the instrument energy channels, Ech, is taken for a given data set. The 113 partial energy density, ", is then calculated with a temporal resolution of 5 minutes us- 114 ing the following equation: " = 2⇡ 2Echmj(Ech)∆Ech (1) E Xch p 115 where ∆Ech is the energy channel bin width, and m is the ion mass. For a given 116 5 minute time bin, we then consider the volume, δV (L), which is the volume of the dipole 117 magnetic field intersecting the area defined by the range of L shells traversed in the time –3– manuscript submitted to JGR: Space Physics 118 interval and 6 hours of magnetic local time (MLT).
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