Sawtoothsubstorm connections: A closer look - Wiley Online Library

JOURNAL OF GEOPHYSICAL RESEARCH: SPACE PHYSICS, VOL. 118, 5136–5148, doi:10.1002/jgra.50440, 2013

Sawtooth-substorm connections: A closer look M. A. Noah1,2 and W. J. Burke 2 Received 5 January 2013; revised 3 July 2013; accepted 5 July 2013; published 28 August 2013.

[1] This paper seeks to improve understanding of sawtooth events (STEs) that occur during

the main phases of geomagnetic storms. We consider the dynamics of diverse space environments encountered during a magnetic storm on 24 March 2002 when four STEs were identified in energetic particle fluxes measured by Synchronous Orbit Particle Analyzers on Los Alamos National Laboratory satellites. Over the reported interval, 28 substorms were identified in magnetometer traces obtained via the SuperMAG international collaboration. Magnetic inclination variations sampled by the Geostationary Operational Environment Satellites 8 and 10 on the nightside indicate that STEs coincided with substorms marked by strong and prolonged dipolarizations. Significantly, the four STEs were accompanied by partial recoveries of the SYM-H index. Two SuperMAG substorms were marked by weak dipolarizations and energetic particle flux increases. The remaining 22 substorms showed no such signatures near 6.6 RE. We regard the latter SuperMAG events as indicating intensifications of the DP 2 current system driven by episodically enhanced reconnection at the distant X line. Presented data are shown to be consistent with the conjecture of Huang et al. (2009) that STEs occur only after open flux in the magnetotail exceeds a critical level near 109 Wb. During STEs, significant quantities of open flux were removed from the magnetotail at near-Earth reconnection lines. It took ~3 h to eject plasmoids and replenish open flux in the magnetotail to a critical level via continued dayside merging, thereby driving quasi-cyclic STE occurrences. Citation: Noah, M. A., and W. J. Burke (2013), Sawtooth-substorm connections: A closer look, J. Geophys. Res. Space Physics, 118, 5136–5148, doi:10.1002/jgra.50440.

1.

Introduction

[2] Borovsky et al. [1993] first noted the appearance of repeated energetic particle structures at geosynchronous orbit that Belian et al. [1995] later called “sawtooth events” (STEs). Phenomenologically, the phrase describes patterns in traces of directional differential fluxes versus time for energetic (50–450 keV) protons. As illustrated in Cai and Clauer [2009, Figures 1 and 2], over a wide range of energies, fluxes suddenly increase then smoothly decay over a few hours before repeating similar patterns that make the structures resemble teeth on a saw blade. STEs appear almost simultaneously at the local time locations of all geosynchronous satellites. Borovsky et al. [1993] reported average time intervals between particle injections of ~2.75 h and argued that this is very close to the periodicity of cyclical substorms. Cai and Clauer [2009] found average “sawtooth periods” of 179.6 min but pointed out that their occurrence is “quasi”

1 Center for Atmospheric Research, University of Massachusetts Lowell, Lowell, Massachusetts, USA. 2 Institute for Scientific Research, Boston College, Chestnut Hill, Massachusetts, USA.

Corresponding author: M. Noah, Institute for Scientific Research, Boston College, Chestnut Hill, MA 02764, USA. ([email protected]) ©2013. American Geophysical Union. All Rights Reserved. 2169-9380/13/10.1002/jgra.50440

rather than strictly periodic. In a comprehensive study, Cai et al. [2011] determined that the vast majority of STEs occur during magnetic storms. They also pointed out that STEs were not observed during many magnetic storms. Ohtani et al. [2007] used Cluster measurements to show that dipolarizations and particle injections can penetrate well inside geosynchronous orbit to at least 4.6 RE (Earth radii). [3] To better understand their origins, STEs have drawn significant theoretical and observational attention. For example, Taktakishvili et al. [2007] showed that under steadily southward interplanetary magnetic field (IMF) conditions, magnetohydrodynamic simulations yield quasiperiodic loading-unloading in the magnetotail and cyclical ring current enhancements. While making no mention of STE activity, Farrugia et al. [1993] provided observational support for a persistently high rate of repeating substorms during the passage of a large magnetic cloud. Taktakishvili et al.’s simulated cycling proceeds at much faster rates than are found in observational STE studies; however, the overall results suggest that STEs reflect internal dynamics of the magnetosphere. Other simulations by Brambles et al. [2011] indicate that fluxes of energetic O+ ions from the auroral ionosphere may trigger STE oscillations. Several observational studies support the internal triggering perception by showing that the total magnetic flux in the magnetotail and polar cap increases prior to onset and rapidly decreases during periodic substorms and STEs [Huang et al., 2009, Huang and Cai, 2009; Huang, 2011]. Huang [2009] argued that at the times

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of STE onsets, the total magnetic energy density in the magnetotail grows to about 3 times its quiet time value and suggested that a critical triggering condition develops in the magnetotail that is responsible for generating cyclical substorms and attendant STEs. Others argued that STEs are externally driven, either by sudden increases in solar wind dynamic pressure [Lee et al., 2004, 2005] or southward “dippings” of the IMF [Lui et al., 2004]. However, at this juncture, evidence supporting an external driving hypothesis appears to be more circumstantial than direct. [4] Currently, substorms are widely regarded as manifestations of magnetic flux loading-unloading processes in the magnetotail [Russell and McPherron, 1973; Hones, 1984]. As such, they represent fundamental disturbance responses activated by solar wind/IMF driving that abet energy transfer within the magnetosphere-ionosphere system [Akasofu, 2004]. These transient events typically last 1–3 h, initiate on the Earth’s nightside, and transfer significant energy to both the auroral ionosphere and the inner magnetosphere [Rostoker et al., 1980]. During the course of substorms, the strength of auroral electrojets increases from and returns to their original strength. In spite of ongoing controversy regarding the detailed sequence of substorm processes [cf. Sergeev et al., 1996, and references therein], there is general agreement that, first, after southward turning of the IMF north-south component BZ, magnetic flux is transferred from the dayside to the nightside at a faster rate than reconnection can return it to the dayside. This imbalance allows the magnetotail’s open flux content to increase [Coroniti and Kennel, 1972]. Consequently, the area of the polar cap increases [Siscoe, 1982], and the magnetic field configuration at geosynchronous altitude changes from quasi-dipolar to taillike. Second, onsets of substorm expansion phases are marked by returns toward dipolar magnetic configurations at geosynchronous altitude, the formation of near-Earth reconnection (X) lines (NEXL), and the earthward injection of energetic plasma. Based on simultaneous ground and Geotail measurements, Maynard et al. [1997] distinguished between weak substorms, in which only closed flux pinches off, and strong substorms, in which open flux also becomes engaged at a NEXL. Only in the latter case do aurorae in the midnight sector expand into the presubstorm polar cap. [5] This report summarizes an investigation of the magnetic disturbance environments in which STEs develop. We concentrate on events observed at geostationary/geosynchronous altitudes during the magnetic storm of 23–25 March 2002 because of the availability of magnetic field and energetic particle flux measurements from two Geostationary Operational Environment Satellites (GOES) and six Los Alamos National Laboratory (LANL) satellites in geosynchronous orbits, respectively. The study by Cai et al. [2011] identified four STEs during this storm. Their criteria for designating events as STEs demanded repeated and nearly simultaneous observations of increased 50–450 keV proton fluxes by LANL satellites within ±3 h of local midnight and noon. [6] Magnetic perturbations observed at high latitudes reflect the presence of large-scale ionospheric current systems called disturbances polar of the first (DP 1) or second kind (DP 2). The equivalent current associated with DP 1 consists of a single cell that has a strong westward component in the midnight sector. It is generally associated with the substorm current wedge driven by field-aligned currents consequent

to disruptions of the cross-tail current system and magnetic dipolarizations [Clauer and McPherron, 1974]. DP 2 consists of two cells that roughly correspond to convection patterns described by Heppner and Maynard [1987]. Clauer and Kamide [1985] showed that DP 2 is the dominant current system during the growth phase of storm. This gives way to DP 1 dominance at the onset of the substorm expansion phase. Late in expansion phases, the DP 2 currents again assume dominant roles [Cai et al., 2006a]. For later reference, we stress the critical connection between ionospheric DP 1 currents and inner magnetospheric magnetic dipolarizations. [7] The remainder of the paper has four sections and two appendices. The following section summarizes the satelliteand ground-based resources used to develop a comprehensive description of the geophysical environments encountered during the March 2002 storm. The observation section describes the STE-relevant events detected near the first libration (L1) point, at geosynchronous orbit and on the ground. The discussion section focuses on an interpretation of the reported measurements to improve understanding of the cyclic nature of STEs. We also suggest a simple method to estimate lower bounds on energetic particle penetration inside geosynchronous orbit after STE injections. The final section briefly summarizes our empirical results and the conclusions drawn from them. The appendices develop simple relationships used to estimate the quantity of open flux in the magnetotail and drift times of energetic particles.

2.

Data Resources

[8] Measurements of the interplanetary and geostationary/ geosynchronous environments used in this study were obtained from the Space Physics Data Facility (SPDF) at the Goddard Space Flight Center (http://cdaweb.gsfc.nasa.gov/istp_public). Solar wind and IMF data were measured by the Solar Wind Electron, Proton, and Alpha Monitor [McComas et al., 1998] and the Magnetic Field Experiment [Smith et al., 1998] aboard the Advanced Composition Explorer (ACE) satellite in a complex Lissajous orbit, around L1 [Stone et al., 1998]. ACE data are reported at a 1 min cadence in universal time after estimated propagation to the nose of the Earth’s bow shock [Weimer and King, 2008]. [9] Geostationary magnetic field environments were specified using data from three-axis fluxgate magnetometers located on 3 m booms extending from the GOES 8 and 10 satellites. Each fluxgate covers a ±1000 nT range. Measurements are analog-to-digital converted using 16 bit words to attain accuracy of ~1 nT [Singer et al., 1996]. Here data are reported at a 1 min cadence. Following Henderson [2004] and Rodriguez [2012], we use magnetic inclination angle (I) measured by GOES spacecraft to identify dipolarization signatures. This parameter is defined as I ¼ tan

1

BZ pffiffiffiffiffiffiffiffiffiffiffi 2 2 BX þBY

where the symbols BX, BY, and BZ represent measured components of the Earth’s magnetic field in geocentric solar magnetospheric (GSM) coordinates. I maintains relatively high values during quiet times. On the nightside, I decreases toward zero when the magnetic field stretches into a taillike configuration and then increases rapidly toward 90° as the

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360

Figure 1. Distribution in geographic longitude of the six LANL geosynchronous and two GOES geostationary satellites (filled circles) in the equatorial plane used in this study. The solid line represents the locus of the IGRF 2010 magnetic equator at the Earth’s surface.

30

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magnetic field relaxes during dipolarizations [McPherron, 1970; Coroniti and Kennel, 1972]. [10] Geosynchronous energetic particle environments were measured by Synchronous Orbit Particle Analyzers (SOPA) [Belian et al., 1995] on board six LANL satellites. The satellites have spin periods of 10 s with spin axes that point toward the Earth. SOPA consists of three solid-state telescopes pointed in three directions relative to the spin axis. Data for 23 March 2002 came from the Virtual Radiation Belt Observatory, a domain-specific virtual observatory in the NASA Heliophysics Data Environment program that provides access to data visualizations related to the Earth’s radiation belt. Spin-averaged fluxes are available in the NASA common data format for ftp download at ftp://virbo. org. Here we only use measurements from proton and electron detectors that cover energy ranges in the 50–450 and 50–500 keV ranges, respectively. They are presented as directional differential number fluxes averaged over 1 min intervals. [11] For reference below, Figure 1 shows the distribution in longitude in the geographic equatorial plane of the LANL and GOES satellites used in this study. Also marked in the figure is the locus of the International Geomagnetic Reference Field (IGRF) 2010 magnetic equator at the Earth’s surface. While the 1990-095 and 1991-080 LANL and GOES 10 satellites fly close to the magnetic equator, the other four LANL satellites are at southern magnetic latitudes. The GOES 8 satellite is positioned well north of the magnetic equatorial plane. [12] To help understand geophysical responses in the magnetotail to STE onsets, we have also consulted databases acquired by the magnetic field fluxgate instrument [Kokubun et al., 1994] and the low-energy particle (LEP) experiment [Mukai et al., 1994] on the Geotail satellite, as well as the far ultraviolet (FUV) camera [Mende et al., 2000a, 2000b] on board the Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) satellite. Geotail is a spin-stabilized satellite whose spin axis is maintained within 5° perpendicular to the ecliptic plane. During the March 2002 magnetic storm, the spacecraft was near GSM coordinates (28, 10, 1) RE.

10 0 -10 -20 50 0 -50

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1994-084

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(nT) N

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0

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-20

LANL-097

-10

LANL-02A

0

Magnetic field vectors are presented in GSM coordinates at a 3 s cadence. Particle measurements are presented as moments of measured distribution functions at a 12 s cadence. The IMAGE satellite is in a high-inclination, eccentric orbit designed to hover over the northern high-latitude ionosphere. The FUV experiment has three components called the Wideband Imaging Camera (WIC) as well as two Spectral Imagers (SI-12 and SI-13). WIC is sensitive to radiation in the Lyman-Birge-Hopfield (140–160 nm) band; SI-12 and SI-13 detect emissions in the 119–124 and 133–138 nm bands, respectively. Methods used to extract locations of open-closed field boundaries (OCBs) are described by Boakes et al. [2008]. Polar cap areas estimated via IMAGE OCBs are made available by the Natural Environment Research Council British Antarctic Survey at http:// www.antarctica.ac.uk/bas_research/ our_research/ -az/ magnetic_reconnection/auroral_boundary_data.html. [13] Gjerloev [2009] described SuperMAG as a “virtual laboratory” through which data collected at more than 200 magnetic observatories around the world are made available for use by the scientific community. SuperMAG may be accessed through the http://supermag.jhapl.edu server that provides data at a 1 min cadence after rotations into local magnetic coordinate systems specified by time-dependent declination angles and automated baseline subtractions [Newell and Gjerloev, 2011a; Gjerloev, 2012]. Data from about 100 stations are used to construct the SuperMAG Lower (SML) and SuperMAG Upper (SMU) indices. They are analogous to the standard AL and AU indices obtained from the 12 auroral electrojet

Sym H (nT)

GOES 8

GOES 10

10

LANL-01A

Geographic Latitude

20

-100 -150 82:00:00

83:00:00

84:00:00

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Figure 2. (top) Solar wind parameters NSW (red), PSW (black), and VSW (blue). (middle) The GSM Y (blue) and Z (red) components of the IMF. (bottom) The SYM-H index. Vertical dashed lines are guide to the eye for comparing data acquired near the times of the SSC as well as the beginnings of the main and recovery phases. Four vertical red lines at 22:13 on 23 March (day 82) and at 01:59, 06:31, and 08:53 on 24 March (day 8) indicate approximate times of STE onsets [Cai et al., 2011].

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-150 82:00

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Figure 3. Comparison between the (top) SuperMAG indices with (bottom) SYM-H. The upper and lower traces represent the SMU and SML indices. Vertical dashed and red lines have the same significance as in Figure 2. Blue vertical lines mark times of substorm onsets indentified with an automated algorithm [Newell and Gjerloev, 2011a]. stations [Davis and Sugiura, 1966]. However, due to the higher density of observations, the SML index usually allows greater precision in identifying times of substorm onset than do AL traces. Newell and Gjerloev [2011a] established criteria for identifying magnetic disturbances at auroral latitudes as substorms. They require that the SML index undergoes a sharp decrease by at least 45 nT in 3 min and remains below 100 nT for the subsequent half hour. These criteria for substorm identifications are more relaxed than are used within the substorm research community [e.g., Rostoker et al., 1980]. Siscoe and Petschek [1997] pointed out that the word “substorm” carries two meanings: (1) the widely used loading-unloading event picture and (2) all the relatively small magnetic disturbances that constitute magnetic storms [Chapman, 1962]. Thus, as used by Newell and Gjerloev [2011a], the word substorm encompasses both meanings. [14] It should also be pointed out that after examining the statistics of time intervals Δt between substorm onsets, Newell and Gjerloev [2011b] questioned the concept of periodic substorms. They demonstrated power law spectra with a break in the spectral index at Δt ≈ 3 h. In the observation section below, we present traces of the SMU and SML indices marked with times of substorm onsets derived from SML traces and provided by the SuperMAG website.

3.

Observations

[15] Clarifying the geophysical environment in which the studied STEs occurred, Figure 2 provides the interplanetary drivers and the large-scale geomagnetic response observed during the magnetic storm of 23–25 March 2002 (days 82–84). Plots found in the top panel show the solar wind speed VSW (blue), the density NSW (red), and the dynamic pressure PSW (black). Traces in the middle panel give the Y (blue) and Z (red) components of the IMF in GSM coordinates [Russell, 1971]. One storm time response, in the form of the SYM-H index [Iyemori, 1990], is shown in the bottom panel. The four vertical red lines superposed on the SYM-H trace mark the times of the four identified STE onsets [Cai

et al., 2011]. An examination of these data identifies four empirical results. [16] 1. Three (four) sharp increases in VSW (NSW) were observed over the reported interval. Only the first increase occurred simultaneously in both VSW and NSW traces. This caused PSW to increase from ~1 to 5.6 nPa and led to the storm sudden commencement (SSC) seen on day 82. Lin et al. [2010] showed that the magnetopause subsolar distance can be fit to a function of IMF BZ, PSW, and solar wind magnetic pressure. Applied to OMNI inputs for this storm, this distance is never less than 7.27 RE. Dmitriev et al. [2011] modeled geosynchronous magnetopause crossings (GMCs) and geosynchronous magnetosheath intervals as functions of GSM magnetic local time (MLT), GSM magnetic latitude, Dst/SYM-H, IMF BZ GSM, and PSW. Applying this model to the GOES orbit and to the subsolar point using the OMNI solar wind data, there were no GMCs during this storm period. Available data from these satellites show no signs of magnetosheath encounters during this storm. [17] 2. In the first 3 h after the sudden increase in solar wind parameters, IMF BZ behaved erratically, rapidly alternating between northward and southward polarities. At ~15:00 UT on day 82, it turned more steadily southward but was still marked by northward turnings. After ~22:00 UT on day 83, it assumed a persistently northward polarity. [18] 3. Although the SSC occurred near 11:40 UT, the main phase, identified as a generally downward trend in SYM-H, did not begin until ~15:00 UT. The storm’s SYM-H minimum of 114 nT occurred at 08:40 UT on day 83 and was followed by a long period in which the trace shows a sequence of sporadic increases and decreases. Sustained recovery began at 22:09 UT on day 83. The vertical red lines in the lower panel in Figure 2 indicate that the four observed STEs occurred during the storm’s main phase. [19] 4. As previously noted by Henderson et al. [2006], each STE onset during the present storm was followed by a partial recovery in SYM-H. [20] For reference below, it is useful to note that, first, in descending order, the three most significant contributors to the main-phase SYM-H index are the cross-tail current, the asymmetric ring current, and the region 1 field-aligned currents [Tsyganenko and Sitnov, 2005]. Second, Iyemori and Rao [1996] reported weakening in the slopes of storm time Dst traces during the expansion phases of embedded substorms. Commenting on these counterintuitive outcomes, Siscoe and Petschek [1997] argued that during substorm unloading processes, more magnetic energy is lost to ionospheric dissipation and precipitation than is transferred to magnetospheric particles. We return to this interpretation in the discussion section. [21] Before examining LANL and GOES measurements, it is useful to first consider the SuperMAG SMU and SML indices acquired slightly before and during the March 2002 storm period. They are plotted in the top panel in Figure 3. Times of 28 substorm onsets, identified in SuperMAG data using an automated algorithm [Newell and Gjerloev, 2011a], are marked by vertical blue lines. To facilitate comparisons, the SYM-H index traces along with vertical dashed lines found in Figure 2 are repeated. The SMU/SML traces indicate that auroral activity began near the time of the SSC, intensified during the storm’s main phase, and relaxed as recovery began. Taking SuperMAG identifications at face

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NOAH AND BURKE: SAWTOOTH-SUBSTORM CONNECTIONS CASE STUDY Table 1. Local Times of the STEs at the LANL Satellites STE 1 2 3 4

UT

1990-095

1991-080

1994-084

LANL-97A

LANL-01A

LANL-02A

82:22:13:57 83:01:59:07 83:06:31:25 83:09:53:01

19:40:28 23:25:38 03:57:56 07:19:32

11:11:55 14:57:05 19:29:23 22:50:59

07:52:55 11:38:05 16:10:23 19:31:59

05:06:45 08:51:55 13:24:13 16:45:49

22:47:00 02:32:10 07:04:28 10:26:04

02:53:02 06:38:12 11:10:30 14:32:06

value, we see that only a small fraction of the main-phase substorms occurred at the same times as the four STEs. We defer comment on this discrepancy to the discussion section below. [22] Table 1 summarizes the distributions in local time of the six LANL satellites at the universal times of the four STE detections. The first and second columns list the STE by number and the universal time of its onset as identified by Cai et al. [2011]. The top row of the following six columns lists official designation of the LANL satellites operating in March 2002. Below each satellite is listed its geographic local time at the universal time of given STEs. Local time entries for the satellites positioned closest to noon and midnight are highlighted with the colors red and blue,

respectively. Note that while waiting times between STEs appear to be periodic on the compressed scales in Figures 2 and 3, entries in Table 1 indicate Δt1, 2 ≈ 3.58 h, Δt2, 3 ≈ 4.53 h, and Δt3, 4 ≈ 3.53 h. These temporal separations are all higher than the average values cited by Cai and Clauer [2009]. [23] Actual local time distributions of geostationary and geosynchronous spacecraft at the universal times of the four STEs are shown in Figure 4. At the time of STE 3, satellite 1991-080 was outside the normal ±3 h range around local midnight normally used by Cai et al. [2011] to identify STEs. However, the longitudinal distribution of these satellites shown in Figure 1 indicates that GOES 8 and 10 were within the specified local time band during STEs 3 and 4. In all cases, local noon and midnight are indicated at the

(1)

(2)

(3)

(4)

Figure 4. Approximate locations in local time of the six geosynchronous LANL and two geostationary GOES satellites near the universal times of the four STEs identified by Cai et al. [2011]. 5140

LANL-01A Flux

NOAH AND BURKE: SAWTOOTH-SUBSTORM CONNECTIONS CASE STUDY 108

1

107 106 105 104 1000 82:12

82:15

82:18

82:21

83:00

Day: UT 2002

Figure 5. Fluxes of energetic protons measured by the LANL-01A satellite near local midnight at the time of STE 1. top and bottom of the diagrams, respectively. As indicated in Figure 4 and Table 1, LANL spacecraft were near local noon and midnight during STEs 1 and 2. The locations of GOES satellites during events 3 and 4 near local midnight were serendipitous in that they allow magnetic field observations during substorms that both did and did not result in STEs. [24] Figure 5 shows traces of proton directional differential fluxes versus universal time as measured in four energy channels centered near 90, 130, 190, and 270 keV during the last half of day 82 (23 March). We note that four vertical blue lines indicating substorm activations identified in SuperMAG data streams occurred between 12:00 and 15:30 UT but produced little to no perturbations in the observed particle fluxes. Also, LANL-01A detected several short-lived flux enhancements between 17:00 and 20:00 UT when no substorm activity was identified. The vertical red line at ~22:14 UT marks the initiation of the first STE. Shortly thereafter, a substorm onset was identified in SuperMAG measurements. The distribution of satellites shown in the upper left panel of Figure 4 indicates that both GOES satellites were in the afternoon local time sector. There they had no access to information about whether the nightside magnetic field went through a stretching-dipolarization cycle at this time. [25] Figures 6a–6e show directional differential fluxes measured by multiple LANL satellites during the first half of 24 March 2002 (day 83) for comparisons with magnetic inclination traces from GOES 8 (green) and GOES 10 (black) found in Figure 6f. Data plotted in Figure 6a came from the the electron detector on 1990-095. Fluxes plotted in Figures 6b–6e were measured by proton detectors on 1994-084, 1991-080, LANL-02A, and LANL-01A, respectively. No proton fluxes are available from 1990-095 on 24 March 2002. [26] The upper right panel of Figure 4 indicates that the 1990-095 and 1994-084 satellites were near local midnight and noon, respectively, at the time of STE 2. Note that the red line marking STE 2 covers a blue line marking a simultaneous substorm onset identified in the SuperMAG database. Plots shown in Figures 6a, 6b, and 6f indicate that at the time of STE 2, the following are observed. [27] 1. Electron fluxes at the location of 1990-095 rose sharply and maintained high values until ~04:00 UT when they decreased precipitously. [28] 2. Proton fluxes observed near noon show a sharp but brief increase at the time of STE 2. Ion fluxes rose again prior to the substorm blue line at 02:17 UT. Rise times for fluxes measured in the different channels indicate dispersions, with the highest energy protons being first to reach 1994-084. [29] 3. The GOES 8 and 10 traces show inclination decreases before and a sustained recovery lasting ~1.5 h after the time of STE 2. Prior to recovery, minimum inclinations

were about 10° lower than those reported by Cai et al. [2006b] preceding similar STE onsets. Unfortunately, our database is too small to determine whether this discrepancy is physically significant or simply an artifact of differences between the GSM coordinate system used here or the vertical, declination, and horizontal (VDH) system employed by Cai et al. [2006b]. [30] The lower panels of Figure 4 (and Table 1) show that 1991-080 was the LANL satellite closest to midnight at the times of STEs 3 and 4. LANL-02A and LANL-01A were closest to noon at the times of STEs 3 and 4, respectively. Figures 6c–6e provide traces of proton fluxes measured by these satellites. Time differences between STE onsets on the nightside and dayside were typically about 10 min. Looking first at GOES data in Figure 6f, we see that STEs 107 106 105 104 1000 100 10 105 104 1000 100 10 1 105 104 1000 100 10 1 105 104 1000 100 10 1 105 104 1000 100 10 1 80

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Figure 6. Fluxes of energetic particles measured during the first half of 24 March 2002 by LANL satellites: (a) 1990-095 (electrons) near local midnight and (b) 1994-084 near local noon (protons) at the time of STE 2. (c–e) Proton fluxes measured by 1991-80, LANL-02A, and LANL-01A, respectively. During STEs 3 and 4, 1991-80 was in the evening-midnight local time sectors, while LANL-02A and LANL-01A were near local noon. (f) Magnetic inclinations sampled by GOES 8 (green) and GOES 10 (black). Green triangles mark two minor dipolarizations.

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Sym H (nT)

Inclination GOES 10

SML SMU (nT)

BZ (nT)

PSW (nPa)


Day UT 2002

Figure 7. Expanded views of (top) PSW, (second) IMF BZ, (third) the SuperMAG indices, and (bottom) GOES 10 inclination angles and SYM-H index measured during the first half of 24 March 2002 (day 83). The dashed line in the bottom plot represents the quiet time inclination angle as measured by GOES 10. Vertical red and blue lines have the same significance as in Figure 3. 3 and 4 were marked by sharp increases in inclination angles. Other sharp inclination increases, marked by green triangles, are apparent near 05:30 and 06:15 UT. The main differences seem to be that after STEs 2, 3, and 4, the inclination angles remained relatively high for about an hour before slowly retreating to low values indicative of field line stretching. STEs 3 and 4 were promptly followed by SuperMAG substorm signatures. We also note that two blue lines at 07:23 and 07:56 UT indicate substorm onsets derived from SuperMAG traces in which relatively brief increases in the particle fluxes and magnetic inclination angles occurred.

4.

Discussion

[31] The previous section summarized geophysical measurements acquired by sensors on the ground and on geostationary/geosynchronous satellites during the magnetic storm of 23–24 March 2002. Attention focused on magnetic signatures observed by GOES and SuperMAG near the times of four STE onsets identified in LANL fluxes by Cai et al. [2011]. Since the mid-1990s, studies of STE phenomenology have stressed their relationship with quasiperiodic substorms. Our present concern centers on their temporal relationship to independently identified substorm onsets. Three empirical results can be seen in the presented data. [32] 1. STEs 2, 3, and 4 were characterized by simultaneous dipolarizations of the nightside magnetic field at geostationary altitude. During STE 1, the GOES satellites were on the dayside, where magnetotail stretching and dipolarization cannot be observed. [33] 2. SuperMAG analyses identified four substorm expansions that began at about the same universal times as the four STE onsets. However, the SuperMAG algorithm

also identified 24 other magnetic disturbances as substorms. During only two of these (discussed below), briefly discernible effects appeared in traces of energetic particle fluxes and nightside magnetic inclinations. [34] 3. After dipolarizations, the nightside magnetic field remained relaxed. and SYM-H partially recovered for more than an hour before another cycle of magnetic stretching and diminishing particle fluxes initiated. The SYM-H trace in Figure 2 shows that a similar partial recovery followed the onset of STE 1. [35] The remainder of this section discusses implications of these observations for understanding substorm activity within magnetic storms and external versus internal STE triggering. [36] Cai et al. [2011] demonstrated that the vast majority (> 94%) of STEs occur during the magnetic storms. While they can occur in any phase of a storm, the majority were main-phase phenomena. During the March 2002 storm, STE detections were limited to the main phase when IMF BZ was persistently southward and magnetospheric dynamics were strongly driven by interplanetary forces (Figure 2). However, understanding of relationships between magnetic storms and substorms remains a much contended issue [cf. Sergeev et al., 1994; Siscoe and Petschek, 1997; Sharma et al., 2003, and references therein]. Data presented from the March 2002 storm in Figure 6 illustrate one related ambiguity. The identified STEs appear to be associated with dipolarizations consistent with the formation of a substorm current wedge [McPherron et al., 1973], also called a DP 1 current system. Most of the SuperMAG substorms are not. This leads us to suggest that such events reflect brief intensifications of the normal two-cell auroral current system (DP 2) indicative of episodic intensifications of reconnection rates at the distant X line [Sergeev et al., 1996]. [37] To help clarify the relationship between identified STEs and substorms with respect to proposed internal and external drivers, Figure 7 provides expanded plots of the solar wind’s dynamic pressure (top), IMF BZ (second), and the SMU and SML traces (third). Traces found in the bottom panel show magnetic inclinations measured by GOES 10 and the SYM-H index (orange). The dashed black trace in the bottom panel in Figure 7 represents quiet time averages of inclinations sampled by GOES 10. [38] As indicated above, previous investigators have offered empirical evidence that ties STE onsets to sources in the interplanetary medium such as sudden increases in the dynamic pressure of the solar wind [Lee et al., 2004, 2005] and southward “dippings” of IMF BZ [Lui et al., 2004]. Expanded plots of PSW and IMF BZ, respectively, in the top two panels in Figure 7 allow us to test these hypotheses. The plotted data show that none of the STEs coincides with a significant PSW increase; only one SuperMAG substorm (out of 13 during this period) reported at 83:03:07 UT coincided with a PSW increase. Only STE 3 appears to coincide with a southward dip in IMF BZ. STE 4 occurred after a brief (~3 min) northward swing in the IMF. However, uncertainties in calculating IMF propagation times from ACE at L1 to the nose of the bow shock [Weimer and King, 2008] render it difficult to regard these as unambiguous responses to southward dips of IMF BZ. We conclude that data acquired during the March 2002 storm do not appear to support either of the two proposed external triggering hypotheses.

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[39] Nearly simultaneous observations of dipolarizations at the times of STEs 2, 3, and 4 may be viewed as manifestations of repeated loading-unloading cycles in the storm time magnetotail. Huang et al. [2009] argued that in the buildup (loading) to STE detections, the magnetic flux in the polar cap (and magnetotail) increases to a critical value near 1 GWb and then decreases rapidly after onset. As long as flux loading remains below the critical level, even large changes in the solar wind do not induce STE onsets [Huang, 2011]. Once criticality has been reached, relatively small perturbations may serve to induce onsets. In agreement with these conclusions, we note that inclination angles presented in Figure 7 indicate that the field at geosynchronous altitude remained stretched, indicating continuous loading for more than an hour prior to red lines marking the onsets of STEs 2, 3, and 4. Huang and Cai [2009] showed that during STEs, the amount of open flux threading the magnetotail and polar cap diminishes significantly. For this to happen, the auroral oval must expand into what had been the polar cap prior to STE onsets. [40] This conclusion is consistent with an interpretation of simultaneous Geotail/ground observations reported by Maynard et al. [1997], who distinguished between two substorm scenarios in the magnetotail. In both scenarios, substorm onsets are associated with the pinching off of previously closed magnetic flux at a near-Earth X line (NEXL). In the first instance, reconnection ceases before open flux becomes engaged at a newly formed NEXL. During this interval, both the near and distant X lines may operate simultaneously. In the second case, previously open flux becomes engaged so that the distant X line disconnects from Earth and a plasmoid is ejected down the distant magnetotail [Slavin et al., 1985]. In the case studied by Maynard et al. [1997], Geotail data indicated that after the plasmoid passage, the NEXL expanded down the magnetotail to become a new distant X line. Optical emissions from the aurorae were observed to expand poleward, indicating that a region of previously open flux changed topology to become closed. [41] The two minor increases in magnetic inclination angles that occurred near the times of SuperMAG substorm onsets at 05:34 and 06:16 UT on 24 March appear to correspond to the first scenario suggested by Maynard et al. [1997], in which NEXLs form but turn off before engaging open flux in the magnetotail. These shifts toward quiet time inclinations coincided with brief particle flux enhancements but were not sustained long enough to qualify as full-blown STEs. [42] The four STEs occurred during the main phase while magnetospheric electrodynamics were driven by a dawn-todusk interplanetary electric field and consequent magnetic merging along the dayside magnetopause. For more than an hour after each STE onset, the magnetic inclination at geosynchronous altitude remained high, suggesting that reconnection must have been proceeding at a faster rate than dayside merging. [43] At first glance, simultaneous partial recoveries in SYM-H seem paradoxical [Siscoe and Petschek, 1997]. Energetic charged particles injected toward Earth should contribute to an intensification of the ring current and thereby drive SYM-H toward more negative values. The fact that the opposite SYM-H behavior occurred repeatedly [Henderson et al., 2006] provides an important clue about the quasi-

cyclic nature of STEs. In reflecting upon these unexpected responses [Iyemori and Rao, 1996], Siscoe and Petschek [1997] considered their implications in the light of the generalized Dessler-Parker-Sckopke (DPS) relationship [Dessler and Parker, 1959; Sckopke, 1966]. They argue that during substorm unloading events, in which Dst/SYM-H relaxes, DPS requires that more than half of the magnetic energy converted during dipolarization dissipates in the ionosphere as Joule heat and particle precipitation rather than being spent energizing charged particles in the magnetosphere. In support of their conjecture, Siscoe and Petschek [1997] pointed to calculations by Harel et al. [1981] as indicating that ionospheric Joule heating exceeds the kinetic energy gained by magnetospheric particles. [44] Independently, Burke et al. [2009] analyzed orbitaveraged accelerometer measurements from the polar-orbiting Gravity Recovery and Climate Experiment (GRACE) satellite [Tapley et al., 2004] to show that, indeed, the storm time thermosphere receives about 4 times the energy allotted to magnetospheric particles as derived from the DPS relationship. Since the ~100 min orbit of GRACE greatly exceeds the lifetimes of typical substorm expansion phases, Burke et al. [2009] regarded the observed thermospheric heating as a general storm time effect. We agree with Siscoe and Petschek [1997] that the storm time thermosphere-ionosphere dissipates more energy than magnetospheric particles gain. However, we do not regard this as necessarily the full story. First, the partition between interplanetary and magnetospheric energy sources for the storm time thermosphere-ionosphere heating currently remains undetermined. Second, in their consideration of DPS, Siscoe and Petschek [1997] made no mention of the large quantities of particle kinetic and magnetic field energies that are lost to the magnetosphere in the form of plasmoids during unloading event substorms. As illustrated in Hones [1984, Figure 4], during the period of plasmoid formation and ejection, the cross-tail current must weaken dramatically. Since the cross-tail current contributes more to Dst during a storm’s main phase than does the ring current [Tsyganenko and Sitnov, 2005], a fuller analysis of DPS during SYM-H relaxation events ought to include the contributions of distant magnetotail dynamics. Such an analysis, however, lies beyond the scope of this mainly observations report. [45] In the Hones [1984] scenario, as the NEXL expands downtail and dayside merging continues at a high rate, the cross-tail current grows, inclination angles at geosynchronous orbit decrease, and SYM-H intensifies. Once a distant X line is reestablished, the main-phase merging rates on the dayside magnetopause again exceed the reconnection rates, and magnetotail lobe flux can grow toward a critical level. This cycle of open-flux loading and unloading should continue generating STEs for as long as the magnetosphere is externally driven. Unlike the ~1 h periods that emerge from simulations by Taktakishvili et al. [2007] and substorm repletion rates reported by Farrugia et al. [1993], STE cycles observed during the March 2002 storm had periods longer than 3 h. [46] Two pieces of evidence from widely spaced sources support these conjectures. Appendix A contains a simple method to estimate the area of the polar cap APC and the amount of open flux Φ. Within this approximation, Φ ≈ 0.0586APC, wherein Φ is given in gigawebers, and APC is in millions of square kilometers. Thus, 1 GWb

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83:12:00

Figure 8. Area of the northern polar cap in square kilometers estimated using images obtained from three EUV sensors on the IMAGE satellite during the last half of 23 March 2002 and the first half of 24 March 2002. For reference, the dashed black line marks APC = 1.7 × 107 km2. of open magnetic flux requires a polar cap area of ~17 Mkm2 = 1.7 × 107 km2. Figure 8 shows APC in square kilometers plotted as a function of time, estimated using IMAGE FUV measurements of the OCB from WIC, SI-12, and SI-13 and published by the British Antarctic Survey. Boundary locations are provided for 1 h magnetic local time (MLT) segments, ascending order from 00:30 to 23:30 MLT. Missing points were filled by interpolations, and polar cap areas were estimated by integrating 2πR2E ð1 cosΘÞ= 24:0 for all 24 MLT segments. Since IMAGE is in a highly elliptical orbit, there are ~3.5 h gaps during which the northern polar cap lies below the sensor’s horizon. As it enters or leaves the sensor’s field of view, the images have partial or distorted perspectives that degrade our ability to estimate the APC. Except near the time of STE 2, when imagery data were unavailable, onsets were marked by significant decreases. We also see that the critical open flux 1 GWb estimated by Huang [2011] appears to be qualitatively consistent with the maximum preonset values of APC shown in Figure 8. [47] Plasmoid ejections are consequences of magnetic reconnection that engages previously open flux at a NEXL. To demonstrate that this indeed was the case, we have plotted a subset of Geotail plasma and field measurements acquired near the times of STEs 2, 3, and 4 in Figure 9. During the period of interest, Geotail was near XGSM = 28 RE and within 2 RE of the GSM neutral sheet. Attention is first directed to the plasma density plots (Figure 9, second panel). Before events 2 and 4, Ni was relatively high, indicating that Geotail was in the plasma sheet. Shortly after STE 2 and 4 onsets, the local magnetic field developed a southward component (BZ < 0) of ~10 nT. Consistent with a plasmoid hypothesis, the plasma jetted in the antisunward (VX < 0) direction with speeds of ~1500 and ~500 km/s after STEs 2 and 4, respectively. Prior to STE 3, Geotail entered the northern lobe of the magnetotail where the plasma density fell to very low values and BX reached 37 nT. After STE onsets, Ni increased to ~0.4 cm3, BX decreased to 18 nT, and BZ turned weakly negative. However, no sign of antisunward plasma motion was detected. Within a short time, Geotail returned to a pure, northern lobe environment. The entry of Geotail into the lobe suggests that the magnetotail was experiencing a windsock effect [Russell and Brody, 1967] that rendered the detection of a plasmoid, if one was present, impossible. During the two instances when Geotail was in the plasma sheet prior to STE onsets, plasmoid signatures were clearly evident.

2000 1000

2

3

4

0

X

83:06:00

V (km/s)

83:00:00

-1000

Geotail

24 March 2002

-2000 1 -3

0 82:18:00

0.5

i

1 107

N (cm )

2 107

[48] Empirical results presented here and by Huang et al. [2009] and Huang [2011] tell us little about possible triggering mechanism once criticality has been reached. For example, Brambles et al. [2011] suggested that storm time–generated O+ ions upwelling from the auroral ionosphere may act to destabilize the magnetotail current sheet. While this mechanism may be plausible, like externally applied perturbations, it too appears ineffective until a critical level of magnetotail loading has been achieved. [49] Ohtani et al. [2007] demonstrated that dipolarizations and particle injections can reach to 4.6 RE. While this information is important for modeling the dynamics of the inner magnetosphere during magnetic storms, observing satellites like Cluster are seldom in positions to provide it. However, LANL satellites with sensor payloads capable of monitoring STE onsets may provide a reliable basis for estimating lower bounds on the depth of energetic particle penetration inside geosynchronous orbit. During the March 2002 storm, the typical time delay between STE onsets near midnight and noon is about 10 min. The equatorial distance travelled along nearly circular gradient-curvature drift paths between spacecraft was ~20 RE. Using ACE measurements and the method

0 40 20

X

4

B (nT)

3

0 -20 -40 20 10 0

Z

2

B (nT)

1

Inclination

3 107

-10 GOES 8 GOES 10

Polar Cap Area (km2)


-20 80 60 40 20 0 83:00

83:03

83:06

83:09

83:12

Day: UT 2002

Figure 9. Geotail measurements acquired during the first 12 h of 24 March 2002. The top two panels show the local plasma’s sunward drift component of plasma VX and the ion density Ni, respectively. Traces in the third and fourth panels give the GSM magnetic field components BX and BZ, respectively. Magnetic inclinations from GOES 8 and 10 as well as times of STE 2, 3, and 4 onsets are repeated for reference.

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Figure A1. (a) Schematic used to estimate area APC in millions of square kilometers and open flux Φ in gigawebers of the polar cap. (b) Area (blue) and open magnetic flux (red) plotted as functions of the magnetic colatitudinal width of polar cap Θ.

described by Burke [2007], we estimate that the main-phase electric field at the times of the three STEs shown in Figure 7 is ~0.5 mV/m. With a magnetic field of ~100 nT, this translates into an E × B drift of ~5 km/s or about 0.5 RE in 10 min. However, calculations provided in Appendix B show that gradient drifts of energetic particles are consistent with observed time delays between onsets near local midnight and noon. For simplicity, we limited consideration to particles that mirror near the magnetic equatorial plane → → where the gradient drift V G ¼ qBμ 2 B ∇B dominates. The symbol μ ¼ 12 mv2⊥ =B represents a particle’s magnetic moment. In Appendix B, we also assumed a cylindrically symmetric dipolar magnetic field. [50] More information can be gained if this symmetry is relaxed to reflect the fact that equatorial magnetic fields at 6.6 RE are stronger on the dayside. The guiding centers of gradient drifting energetic particles are confined to move along surfaces of constant magnetic field B. For a particle to reach a dayside satellite at 6.6 RE, it must begin its postinjection journey at a nightside location that is earthward of location. To estimate the injection location of energetic particles reaching a given LANL satellite, we used the T05 model [Tsyganenko and Sitnov, 2005] to project the magnetic field at the spacecraft location to the magnetic equatorial plane and determine the value of Beq. For STE 4, the particle flux enhancement was measured on the nightside in LANL 1991-080 about 11 min before it was detected in LANL-01A on the dayside. At the time of the measurement, the value at LANL-01A projected into the magnetic equator is about 110 nT. Likewise, we go to the location of the nightside LANL satellite that first detected the STE onset, project its location to the equatorial plane, and calculate Beq at that location. If Beq on the nightside is smaller than its dayside analog, calculate Beq along a radially inward straight line until it matches the dayside magnetic field value. For STE 4, the location of 110 nT is around 6.1 RE in the equatorial plane

for the satellite local time of 1991-080. The injection had to penetrate to at least that distance from Earth.

5.

Summary and Conclusions

[51] This study used measurements from six LANL and two GOES satellites acquired during the magnetic storm of March 2002 to better understand physical relationships between STEs and substorms that occur during the main phase. Over the course of this storm, four STEs and 28 substorms were identified in high-energy charged particle flux [Cai et al., 2011] and in the SuperMAG [Newell and Gjerloev, 2011a] data streams, respectively. Both the STEs and substorms occurred during the storm’s main phase. All STE onsets were accompanied by identified substorms and dipolarizations of the nightside magnetic field. The magnetic field maintained quasi-dipolar configurations for about an hour before slowly returning to taillike geometries. In all four instances, the SYM-H index underwent partial recoveries that mimicked magnetic inclination traces from GOES 10 [Huang et al., 2004]. Finally, two SuperMAG substorms that did not have associated STEs were identified at the same times as partial dipolarization and increased particle fluxes. The particle flux enhancements were not of sufficient magnitude or global extent to be classified as STEs. Nonetheless, these two events and the SYM-H behavior provided critical clues for understanding the quasi-cyclic nature of STEs. [52] The two non-STE substorms are reminiscent of magnetotail behavior inferred from ground and Geotail observations [Maynard et al., 1997] in which a NEXL forms, pinching off previously closed flux but never engaging open flux. Prior to STE onsets, open flux in the tail and polar cap builds to about 3 times its quiet time value before a NEXL forms to remove much of the accumulation [Huang et al., 2009]. The partial recovery of SYM-H during ring current injections indicates the cessation of a large fraction of the predisturbance cross-tail current. This is accompanied by

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the rapid reconnection of open flux, ejection of plasmoids, and NEXL retreats downtail to reestablish distant X line configurations. Continuous merging along the dayside magnetopause transfers open flux to the magnetotail at faster rates than it is removed via reconnection at the new X line. Thereby, open flux accumulates in the tail and leads to magnetic field stretching on the nightside until a critical level is regained. Thus, the STE relaxation cycle continues.

Appendix A: Estimation of Open Magnetic Flux [53] Siscoe [1982] approximated the shape of the storm time polar cap as a circle whose radius grows while dayside merging rates exceed reconnection at the distant X line. As indicated in Figure A1a, we adopt this approximation by treating the Earth’s magnetic field as a centered dipole with the high-latitude boundary of the auroral oval represented by a circle of constant colatitude Θ. This allows a simple estimate of the polar cap area and the open magnetic flux → threading it. In this approximation, the magnetic field B is h i → BðRE ; θÞ ¼ -B0 2Cosθ ^r þ Sinθ θ^

(A1)

where B0 ≈ 3 × 104 nT represents the strength of the field strength at the Earth’s surface on the magnetic equator; θ is the magnetic colatitude; ^r and θ^ are unit vectors in the radial outward and increasing colatitude directions, respectively. The open magnetic flux Φ(Θ) and the area of the polar cap APC (Θ) may be approximated as Θ Θ → → ΦðΘÞ≈ ∫ B d A≈ 4πR2E B0 ∫ Cosθ Sinθdθ ≈ 2πR2E B0 Sin2 Θ (A2) 0

0

and Θ

APC ðΘÞ ≈2πR2E ∫ Sinθ dθ ≈ 2πR2E ð1 CosΘÞ

(A3)

0

[54] Plots in Figure A1b show solutions of equations (A2) and (A3) for Φ(Θ) and APC(Θ) in gigawebers and millions of square kilometers, respectively, plotted as functions of the polar cap’s colatitude radius. These plots indicate that to attain an open flux of 1 GWb, Θ must expand to ~20.6° in colatitude. At this radius, APC ≈ 1.7 × 107 km2. To an excellent approximation, this analysis showed that Φ(Θ) in gigawebers = 0.586 • APC(Θ) in millions of square kilometers.

Appendix B: Gradient Drift Approximation [55] Directional differential fluxes measured by LANL satellites in geosynchronous orbit near times of STE onsets show signatures of dispersionless injections near local midnight. Near local noon, onsets appear ~10 min later but are characterized by energy dispersions, with the highest energy particles arriving first. To help understand these observations, we approximate the Earth’s magnetic field as a centered dipole and only consider energetic particles with equatorial pitch angles of 90°, whose guiding center motions is dominated by

→ → the gradient drift V G ¼ qBμ 2 B ∇B. The symbol μ ¼ 12 mv2⊥ = B represents a particle’s magnetic moment, a constant of the motion. The equatorial magnetic field and its gradient → are B ¼ B0 =L3 θ^ and ∇B ¼ 3B0 =RE L4 ^r , where ^r and θ^ represent unit vectors in the directions of increasing radius and colatitude, respectively. B0 is the equatorial field strength at the Earth’s surface (~3 × 105 T ). Thus, → ^ ^ V G ¼ jq∓3μ jRE L ϕ , where ϕ is a unit vector in the direction ∓3E ðJ Þ of increasing azimuth. The term j∓3μ qjRE ¼ jqðC ÞjRE ðmÞB0 ðT Þ=L3 ≈ → ^ ∓15:6E ðkeVÞL3 , and V G ðm=sÞ ¼ ∓15:6E ðkeVÞL2 ϕ: [56] At L = 6.6, a 350 keV proton travels to the west at a speed of ~238 km/s or ~2.23 RE/min. Moving at this speed along a 6.6π RE semicircular path length requires ~9.3 min to travel from local midnight to noon. Lower/higher energy particles starting at the same initial location take proportionately longer/shorter times to reach local noon. Given the uncertainties about the exact locations and times of injections, we regard this simple estimate as being consistent with reported LANL observations.

[57] Acknowledgments. This work was supported by the National Science Foundation award 1003652 to Boston College Institute for Scientific Research. The authors thank Chaosong Huang of the Air Force Research Laboratory for providing conveniently formatted SOPA data acquired on 24 March 2002. For the ground magnetometer data, we gratefully acknowledge Intermagnet; USGS, Jeffrey J. Love; Danish Meteorological Institute; CARISMA, PI Ian Mann; CANMOS; the S-RAMP database, PI K. Yumoto and K. Shiokawa; the SPIDR database; AARI, PI Oleg Troshichev; the MACCS program, PI M. Engebretson, Geomagnetism Unit of the Geological Survey of Canada; GIMA; MEASURE, UCLA IGPP and Florida Institute of Technology; SAMBA, PI Eftyhia Zesta; 210 Chain, PI K. Yumoto; SAMNET, PI Farideh Honary; the institutes who maintain the IMAGE magnetometer array, PI Eija Tanskanen; PENGUIN; AUTUMN, PI Martin Conners; Greenland magnetometers operated by DTU Space; South Pole and McMurdo Magnetometer, PI Louis J. Lanzarotti and PI Alan T. Weatherwax; ICESTAR; RAPIDMAG; PENGUIn; British Antarctic Survey; McMac, PI Peter Chi; BGS, PI Susan Macmillan; Pushkov Institute of Terrestrial Magnetism, Ionosphere and Radio Wave Propagation (IZMIRAN); and SuperMAG, PI Jesper W. Gjerloev. Auroral boundary data were derived and provided by the British Antarctic Survey based on IMAGE satellite data (http:// www. antarctica.ac.uk/ bas_research/ our_research/az/magnetic_reconnection/ auroral_boundary_data.html). [58] Masaki Fujimoto thanks Xia Cai and another reviewer for their assistance in evaluating this paper.

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