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3D modeling of shock-induced trapping of solar energetic particles in the Earth's magnetosphere
ARTICLE IN PRESS
Journal of Atmospheric and Solar-Terrestrial Physics 66 (2004) 1389–1397 www.elsevier.com/locate/jastp
3D modeling of shock-induced trapping of solar energetic particles in the Earth’s magnetosphere M.K. Hudsona,, B.T. Kressa, J.E. Mazurb, K.L. Perrya, P.L. Slocumb a Physics and Astronomy Department, Dartmouth College, Hanover, NH 03755, USA Space Sciences Applications Laboratory, The Aerospace Corporation, Los Angeles, CA, USA
b
Received 2 February 2004; received in revised form 23 March 2004; accepted 24 March 2004
Abstract The prompt trapping of solar energetic particles (SEPs) in the inner magnetosphere around L ¼ 2–2.5, including protons and heavier ions, has been observed at both the Cycle 22 and 23 solar maxima, in association with high-speed interplanetary shocks and storm sudden commencements (SSCs). Recent observations include the Bastille Day 2000 CME-driven storm as well as two in November 2001, which produced a long-lived new proton belt, as well as trapping of heavy ions up to Fe in all three cases. A survey of such events around the most recent solar maximum, including high altitude measurements from Polar and HEO satellites along with low altitude measurements from ‘the Solar Anomalous and Magnetospheric Particle Explorer (SAMPEX)’, indicates similarities to the well-studied March 24, 1991 SSC event. In this event, electrons and protons in drift resonance with a magnetosonic impulse were transported radially inward. A requirement for such shock-induced acceleration is a high-speed CME-shock at 1 AU, which launches a perturbation with comparable velocity inside the magnetosphere. Secondly, there must be a source population which is drift-resonant with the impulse. The CME-shock itself is a source of solar energetic particles, both protons and heavy ions, with higher fluxes and harder spectra associated with faster moving CMEs. Arrival of the interplanetary shock compresses and changes the magnetosphere topology, leading to a reduction of the geomagnetic cutoff, initially around L ¼ 4 for SEP protons. This effect is modeled using a 3D Lorentz integration of SEP trajectories in electric and magnetic fields taken from the Lyon–Fedder–Mobarry (LFM) global MHD code, with solar wind input parameters taken from spacecraft measurements upstream from the bow shock, carried out for the November 24, 2001 SEP event. The results indicate that an enhancement in solar wind dynamic pressure for this event plays a role in the observed injection of ions to low L-values, to form a new proton belt which has lasted for more than 2 years. r 2004 Published by Elsevier Ltd. Keywords: Solar wind/magnetosphere interactions; Magnetosphere; Radiation belts; Particle acceleration; Solar energetic particles; Magnetic storms
1. Introduction
Corresponding author. Tel.: +1-603-646-2976; fax: +1-
603-646-1446. E-mail address: [email protected] (M.K. Hudson). 1364-6826/$ - see front matter r 2004 Published by Elsevier Ltd. doi:10.1016/j.jastp.2004.03.024
Solar energetic proton (SEP) events have been associated with solar activity since prior to the space age, with access of the most energetic ions to ground level at high latitudes, and the geomagnetic cutoff latitude determined by particle rigidity, defined as
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momentum per unit charge. ‘The Combined Radiation Release and Effects Satellite (CRRES)’, launched in July 1990, and operational until October, 1991 in a nearequatorial geosynchronous transfer orbit, provided the first clear evidence for the formation of a long-lived proton belt from the radial transport and trapping of an SEP source population, as well as transport and energization of outer zone electrons to form a new electron belt in the otherwise sparse slot region, L ¼ 223 (Blake et al., 1992). The CRRES satellite was fortuitously located around L ¼ 2:5 on the night side at the time of the injection. It was apparent in the arrival time of both 420 MeV protons and 413 MeV electrons, and subsequent drift echoes, that radial transport of both species resulted in the formation of new radiation belts on a particle drift time scale of a few minutes. Conserving the first adiabatic invariant, energy increases as L3 for protons (L3=2 for relativistic electrons), while the key feature for SEPs, already energized to significant flux levels at 410 MeV by the interplanetary shock (Reames, 2001), is the increase of their perpendicular relative to parallel energy, which changes their pitch angle and results in trapping. Lorentzen et al. (2002) provided a summary in their Table 1 of variations in the inner zone trapping boundary, which can be affected by SEP injections, trapping and de- trapping, based on observations beginning in the early 1960s through CRRES observations at the Cycle 22 solar maximum. Four events were seen up to and including the Bastille Day 2000 storm in the rising phase of Cycle 23 solar activity, summarized in their Table 2. These latter 4 events include August 26–28, 1998 and September 23–26, 1998, which were Geospace Environment Modeling storm events and moderate SEP events (http://leadbelly.lanl. gov/GEM_Storms/GEMstorms.html). Our Table 1 summarizes 2 years of geoeffective SEP events from the Cycle 23 solar maximum, including the April and July 2000 events studied by Lorentzen et al. This table was produced as a subset of those compiled in the Coordinated Data Analysis Workshop study of major SEP events of Cycle 23, see Gopalswamy (2003), Table 1. Included in the CRRES era catalog of events studied by Lorentzen et al. are changes in the outer trapping boundary of protons, also evident in Fig. 1, which shows the flux of 415 MeV protons measured by the HEO 1997-068 spacecraft, in a 12-h period, geosynchronous transfer orbit with a 63 inclination (Blake et al., 1997). This 18-month summary of energetic proton data shows three features which are common around solar maximum: (1) SEP events, (2) changes in the outer boundary of inner zone protons and (3) long term trapping of protons associated with SEP injection events. Aside from gray stripes corresponding to periods in which data is not available, vertical flux stripes are apparent
corresponding to individual SEP events. Abrupt changes in the inner zone outer boundary are also seen in association with a corresponding SEP event. However, the scale shown here masks the fact that the change in the trapping boundary is associated with buildup of the storm time ring current, delayed by 1–3 days relative to SEP arrival at geosynchronous orbit for most events (see Table 1). A significant perturbation in the background magnetic field due to buildup of the ring current causes a decrease in the geomagnetic field radius of curvature, similar to stretching of the tail due to increased cross-tail current (Anderson et al., 1997). This ring current modification of the background magnetic field results in loss of first invariant conservation when the gyroradius becomes comparable to magnetic field gradient scale lengths. This limitation on trapping is the basic cause of the outer boundary of the inner zone, which moves to a lower L-value during storm periods for these 415 MeV protons (Hudson et al., 1997; Young et al., 2002). The third feature to note in Fig. 1 is the injection of SEP protons to form a newly trapped population around L ¼ 2:5 on November 6, and again on November 24. These two injections are shown in more detail in Fig. 2, for 425 MeV protons observed by the same HEO spacecraft.
2. SEP proton events in November 2001 and August 1998 On the expanded scale of Fig. 2, one can see the arrival of 425 MeV protons, with both SEP injection events populating the magnetosphere into L ¼ 4: This prompt arrival is also seen by the GOES satellites at geosynchronous orbit (http://www.sec.noaa.gov), shortly following CMEs seen by the SOHO spacecraft at L1: Arrival of the corresponding interplanetary shock produced by each CME takes one to two days in both cases. Noteworthy in Fig. 2 is the shift in radial cutoff to lower L values which coincides with the arrival of the interplanetary shock at earth, within the resolution of the 12-h HEO orbit, sampling the SEP cutoff twice per orbit at a 9-h interval centered on apogee. These measurements are limited in temporal resolution by a greater portion of each 12-h orbit spent near apogee at 7:2RE : The black arrow indicates the arrival time of the forward shock at 0558 UT on November 24, at the location of the WIND spacecraft, which was on the dayside at (17.7, 74:5; 28:4RE ) in GSE coordinates. Solar wind density, V x and Bz components of the velocity and magnetic field measured by WIND instruments are plotted in Fig. 3, along with hourly averaged Dst : The driving solar wind conditions will be discussed further in the modeling section. Fig. 4 shows another class of proton trapping events for August 26–27, 1998 (day 238–239), where SEPs were present into L 4; prior to arrival of the interplanetary
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Table 1 Eleven Solar Proton Events selected for geoeffectiveness by NASA LWS SPE Workshop, August 2002, from Cycle 23 solar maximum catalog. (http://cdaw.gsfc.nasa.gov/LWS/data/event_list.html and Gopalswamy, 2003) for period April 2000–May 2002, excluding April 21–23, 2002 event, for which data is available at http://storms.jhuapl.edu SEP date
Max sw speed (km/s)
IP shocks— D.Berdichevsky
Bz
Ave shock speed H.Cane (km/s)
Max GOES8 410 MeV p+
SSC
DST min
DST val (-nT)
New belts
1
2000/04/04
606
4/6 1627@Wind, 1605@ACE, no mag cloud
Bz o0 long time
870
5:5 101
4/6 1639
4/7 0030
321
p, a
2
2000/07/14
1005
7/15 1435@Wind 1416@ACE, yes mag cloud
Bz o0 short time, density enhancement
1600
2:4x104
7/15 1437
7/15 2130
300
p, a, heavy
3
2000/11/08
900
11/11 0413@Wind 0400@ACE no cloud
density enhancement
1100
1:5 104
11/10 0628
11/10 1300
96
nothing
4
2001/03/29
848
03/30 2330@Wind 2150@ACE 3/31 0111@Wind 0030@ACE 3/31 2200@Wind
Bz o0 long time, density enhancement
690
3 101
03/31 0052
3/31 0900
387
p, a
5
2001/04/10
756
4/10 1056@Wind 4/11 1425@Wind 4/12 1743@Wind
Bz o0 long time density time density enhancement
1220
4 102
04/11 1343 04/11 1519
4/12 0000
236
p, a, heavy
6
2001/04/15
500
4/15 0140@Wind 0120@ACE
Bz o0 short time
behind limb
1 103 5 102
04/18 0046
4/18 0700
114
nothing
7
2001/09/24
740
9/25 2018@Wind 2003@ACE
Bz highly variable, waves at all freq.
1220
1 104
9/25 2025
9/26 0200
102
heavy
8
2001/10/01
575
10/1 2230@Wind 2150@ACE 10/3 0805@ACE
Bz variable
10/3 1500
166
nothing
9
2001/11/04
750
11/6 0145@Wind 0124 @ACE 11/6 1938@Wind 1905@ACE
Bz 70nT F Bz 20nT R
1240
5 104
11/6 0152
11/6 0700
292
p, heavy
10
2001/11/22 2100UT
1003
11/24 0452@Wind 0558@Wind 0540@ACE 1400 @Wind
Bz o0 post shocks, 40 between, density enhancement
1300
2 104
11/24 0556
11/24 1700
221
p, a, heavy
11
2002/5/22
950
5/23 1040@Wind 1010@ACE 2130@ACE
Bz o0 short time
8 102
5/23 1100
5/23 1800
108
p, a, heavy
3 103
Several control SEP events are included with no trapping of SEP ions, as indicated in the last column; average shock speed indicated is not available for all events. Information on maximum solar wind speed, shock arrival times, and IMF Bz supplied by Berdichevsky, shock speeds at L1 by H. Cane, new belts and ion species, by J. Mazur and P. Slocum from SAMPEX LICA measurements, and other data from http://www.sec.noaa.gov and links, e.g. to Kyoto final (00–01) and provisional (02) Dst :
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Fig. 1. Proton fluxes with E415 MeV from the HEO 1997-068 spacecraft for the period indicated, January 1, 2001–June 10, 2002. An IGRF field model is used to map off-equatorial plane fluxes to the L-shells indicated. Vertical gray stripes indicate periods when no data is available.
Fig. 3. November 24, 2001 Wind satellite data given in SM coordinates used to drive the LFM magnetospheric model. Measured values for the other magnetic and velocity field components and plasma temperature are also used but not shown here. Also shown is the Dst index from the WDC-C2 KYOTO Dst index service. The four dashed vertical lines correspond to times for which LFM MHD fields were used for SEP test particle trajectory calculations in Fig. 6 ( Kress et al., 2004).
Fig. 2. Proton fluxes with E425 MeV from the HEO 1997-068 spacecraft for the month of November 2001. The time resolution of the data is approximately 4 bins per day. The bin indicated by the arrow corresponds to an inward bound portion of the HEO orbit occurring between 6:30 and 9:30 UT on November 24 (Slocum et al., 2002).
shock on August 26 (left panel). The transient SEP flux at L44 has dropped by August 27 (right panel), and the new belt emerges at L ¼ 3:5: The observations suggest a diffusive transport mechanism, for example, driftresonant radial transport, which has been suggested for MeV electrons (Elkington et al., 1999, 2002, 2003) whose drift frequency is comparable to that of enhanced Pc 5 ULF waves observed during geomagnetic storms. Protons with energies shown in Fig. 4 have drift frequencies comparable to the ULF waves observed during the August 1998 storm period by the GOES 8 magnetometer (Hudson et al., 2001), suggesting that the same mechanism may operate for protons on a slower
time scale than the shock-drift acceleration mechanism (Perry et al., 2000). However, 10 MeV protons at L ¼ 3:5 will easily be perturbed from trapped orbits by ring current perturbations (Hudson et al., 1997; Young et al., 2002), so it is not surprising that new belts which form around L ¼ 3:5 are short-lived, as noted in Table 1 of Lorentzen et al. (2002).
3. Heavy ions Heavy ions provide further evidence for rapid radial transport and trapping, with the example of Fe counts plotted in Fig. 5, from ‘the Solar Anomalous and Magnetospheric Particle Explorer (SAMPEX)’ (Mason et al., 1993) satellite measurements in a low-altitude polar orbit, along with that of 20–29 MeV protons. Each blue circle corresponds to a measurement of one Fe count by the LICA instrument on SAMPEX in the 0.5–3 MeV/nucleon range. Charge state comparable to the average solar wind ionization state (+11) can be
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Fig. 4. Proton fluxes with E49; 17 and 26 MeV from the HEO 1997-068 spacecraft for August 26–27, 1998 separated by 18-h. Multiple traces correspond to inbound and outbound passes relative to perigee in a static IGRF magnetic field model used to map fluxes to the equatorial plane, while non-stationarity of the geomagnetic field on a 6-h (half-orbit) time scale results in additional traces. Note the CRAND population at low L in both panels, and the high SEP fluxes seen in the left panel outside L ¼ 4; prior to the shock arrival (SSC at 0942 UT; perigee time indicated by decimal day). The right panel shows that SEP fluxes have dropped the following day, while a new proton belt at L ¼ 3:5 is evident.
2:8: After arrival of the 0558 UT shock discussed previously (Fig. 2), they are further transported into L ¼ 2:5; with passage of the reverse shock. This appears to be strong evidence for drift-time scale transport and trapping, with trapped fluxes low, but apparent on a longer time scale plot for this and similar events (see Lorentzen et al., 2002, Fig. 6), also for November 6, 2000.
4. Cutoff and radial transport modeling
Fig. 5. SAMPEX count rates for 20–29 MeV protons (labeled PLO) from the PET instrument, black line, while individual counts of 0.5–3 MeV/nucleon Fe from the LICA instrument on SAMPEX (Mason et al., 1996) appear as blue circles. Shock arrival times at WIND are indicated with red lines. Note that inward transport of Fe ions is well-correlated with shock arrival at 0558 UT, the clear jump in solar wind density n; V x and Bz in Fig. 3.
inferred from the geomagnetic cutoff (Mason et al., 1995) prior to trapping. Also plotted are two forward and one reverse shock crossings at the WIND spacecraft, as reported by Berdichevsky et al. (2001). One again sees, at the 96 min cadence of the SAMPEX orbit, radial transport of energetic Fe ions from their preshock arrival cutoff value, between L ¼ 3:5–4, to L ¼
We have modeled surfaces of constant cutoff rigidity in 3D for 425 MeV protons for the November 24, 2001 storm using measured solar wind parameters at WIND (Fig. 3) as input to the LFM MHD code. A relativistic Lorentz trajectory integrator has been developed for use with the 3D MHD fields, and proton access calculated using sequential snapshots of the MHD fields, since 425 MeV protons transit 1=4 of the test particle simulation domain in a time comparable to the MHD simulation time step of 0:25 s: In the 2D noon–midnight meridional plane shown in Fig. 6, 106 particles were launched isotropically from each of 18,000 points on a uniform grid over a 14RE 14RE earth-centered region in the noon–midnight meridian, i.e. with a resolution of 0:1RE : SEP access is determined by following time reversed particle trajectories. Thus, an escaped particle indicates SEP access to the point from which the particle is launched (see e.g., Kress et al., 2004; Smart et al., 2000; Smart and Shea, 2001). The results
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Fig. 6. Modeled 25 MeV proton cutoff in SM coordinates at four snapshots in time from the MHD simulation. The boundary of the black shaded region is the cutoff surface. The cutoff is determined by following reverse particle trajectories. An escaped particle is one which exits a sphere at r ¼ 15RE before its total trajectory path length exceeds 1000RE (Kress et al., 2004).
are shown in Fig. 6, with the color bar indicating the number of escaped particles out of 106 at each point, and black indicating zero escaped particles. The boundary of the black region is the cutoff surface. The test-particle simulation domain is a subset of the MHD domain extending to 30RE upstream and 300RE downstream from the earth (Fedder and Lyon, 1995). Previous cutoff calculations have been performed using static magnetic field models, for example, Tsyganenko, 1989 (Smart and Shea, 2001). Fig. 6 shows that prior to arrival of the interplanetary shock at 0558 UT, a magnetosphere slightly compressed from dipolar is seen in panel (a), as compared with the corresponding calculation in a dipole model (Kress et al., 2004, Fig. 2). The most extreme change in SEP access is apparent in panel (c), which corresponds to arrival of the high density impulse in Fig. 3, where the solar wind density jumps to 70 cm3 : This sudden change in the solar wind dynamic pressure appears to have a greater effect on the calculated SEP cutoff than the shock transition defined by a jump in V x ; Bz and density at 0558 UT in Fig. 3. Buildup of the ring current, as indicated by Dst in the bottom panel of Fig. 3, following the extended period of southward IMF Bz ; appears to have a smaller effect on suppressing the SEP
cutoff, as apparent in panel (d) of Fig. 6, corresponding to the last vertical dash in the WIND data. SEP access into L ¼ 2:5 is seen in the HEO spacecraft data plotted in Fig. 2. Fig. 2 uses an IGRF magnetic field model to transform off-equatorial plane particle flux data to the equator, and again to convert to a reference L-value not adjusted for disturbed geomagnetic conditions, for example, incorporated into T96 (Tsyganenko, 1996) and recent models by Tsyganenko et al. (2003). Fig. 6 shows proton access in geocentric radial distance calculated as the number of escaping protons, with reverse integration of trajectories in time for the 106 protons launched isotropically from each pixel. While a direct comparison of IGRF L-values in Fig. 2 and geocentric radial distance in Fig. 6 is not possible, it is clear that cutoffs undergo the most dramatic change when the solar wind density impulse arrives, vs. on the time scale of buildup of the ring current over hours, as indicated by Dst in the bottom panel of Fig. 3.
5. Discussion It is significant from a space weather viewpoint that the magnetosphere responds as rapidly as indicated in
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Fig. 6 from the MHD simulations, separated in time by only 1 h of solar wind input conditions between panels (b) and (c); or the 96-min resolution of SAMPEX data shown in Fig. 5, and the 6–12-h resolution HEO data plotted in Fig. 2. All indicate a response in solar energetic ion access inside undisturbed cutoffs which is much faster than diffusive time scales (Spjeldvik, 1988). Widely-used radiation belt models do not adequately specify this kind of event. The simulations indicate a faster response than the buildup time of Dst during geomagnetic storm main phase, which is typically several hours. The correlation in depression of cutoff latitude with Dst has been noted previously (Mason et al., 1995; Leske et al., 2001). However, an examination of the November 2001 events (R. Leske, private communication), along with the July 2000 (Bastille Day) and April 10–12, 2001 events, see Table 1 and http://storms.jhuapl.edu/posters/Labrador/SAMPEX_ Storms2_Cutoffs/slide08.html, shows that the measured cutoff invariant latitude drops with decreasing Dst ; but recovers much more quickly. Dst recovers on the time scale of collisonal loss of ring current ions, with an initially rapid recovery rate of hours on the Oþ –O charge exchange time scale, and a slower recovery over 1–2 days with the slower Hþ –H charge exchange rate determining that time scale (Kozyra, 1989). The initial drop in Dst is correlated with enhanced convection and opening up of the polar cap, which affects cutoff invariant latitude (Leske et al., 2001). Fig. 6 indicates that the magnetosphere may respond rapidly to increased solar wind dynamic pressure, with cutoff suppression to L ¼ 3:5 in the MHD fields driven by input solar wind parameters. However, trapping still requires a change in pitch angle, which can be achieved with perpendicular acceleration by the azimuthal electric field accompanying an enhancement in magnetospheric Bz on the dayside due to magnetopause compression (Li et al., 1993). For both electrons and ions, the compression of the dayside magnetopause launches a magetosonic impulse propagating around the flanks of the inner magnetosphere (Wilken et al., 1982), which contains an azimuthal electric field component due to the timevarying Bz of the magnetopause compression. Key to the March 24, 1991 compression, the largest reported by Kakioka ground magnetograms between 1957–1994 (Araki et al., 1997), was the magnitude of the dBz =dt impulse, exceeding 200 nT. This magnitude far exceeds typical Storm Sudden Commencement (SSC) values. Comparing multi-spacecraft magnetometer data, Araki et al. showed that the March 1991 impulse propagated at an average velocity of 800–900 km/s through the dayside magnetosphere, consistent with the average magnetosonic speed in this region. Li et al. (1993) developed a simple propagating compressional Bz and bipolar E j impulse that reproduced, via test particle simulations, the electron transport and acceleration
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evident in measurements at CRRES, using a larger constant pulse speed within the magnetosphere of 2000 km/s in their model. This model was less successful at reproducing SEP trapping (Hudson et al., 1997), and was replaced by MHD simulations of this event using the Lyon–Fedder–Mobarry code (Lyon et al., this issue) to advance guiding center proton trajectories in the equatorial plane (Hudson et al., 1997). Both approaches to modeling this event confirm the particle drift-time scale transport over more than 1RE radially, in minutes, in contrast to a process like radial diffusion which occurs over many hours to days. All necessary ingredients for radial transport and trapping of SEPs at L ¼ 2:5 were again present in the November 24, 2001 event studied: the solar energetic ions which preceed the arrival of the interplanetary shock, serving as the source population; their access to lower L-values with arrival of a high density solar wind impulse; and trapping at lower L-values due to nonadiabatic effects, i.e. violation of the third invariant with rapid radial transport on a drift time scale. The latter prevents the restoration of fluxes to higher L-value when the magnetosphere relaxes, as it clearly has by the fourth panel of Fig. 6. A caveat is required regarding use of L-value to organize particle data during highly disturbed geomagnetic conditions. What has been used in Figs. 1, 2, 4, and 5 is an IGRF field model for mapping fluxes, while Fig. 6 simply plots proton access as a function of radial distance in the electric and magnetic fields calculated by the 3D LFM-MHD code. It is expected that L-shells become more highly distorted during extreme solar wind driving conditions than is captured by IGRF, or the T96 model, which contains a Dst -dependent term to model ring-current evolution. The Tsyganenko model has recently been extended to model extreme storm conditions, Dst o 100 (Tsyganenko et al., 2003). However, neither particle flux measurements nor static field cutoff calculations have yet incorporated this more perturbed parameterization of the geomagnetic magnetic field. Finally, an outstanding research question to investigate is: what distinguishes the March 24, 1991 and November 6 and 24, 2001 class of events, which result in long-lived trapping of SEPs at low L-value ðL 2:5Þ; from those such as August 26–28, 1998, and the Bastille Day 2000 storms? Phenomenologically, they are distinguished by depth of penetration of the SEPs upon arrival of the CME-shock at the magnetosphere, since prior to shock arrival, both classes of events are characterized by cutoffs around L ¼ 4 for SEPs of energy 10 MeV; as measured for example by CRRES (Hudson et al., 1997) and the HEO spacecraft, Fig. 1. No solar wind data was available for the March 24, 1991 event, but comparison of ground magnetometer data for all three deep-penetration events using the 210 Magnetometer Chain indicate an unusually strong dBz =dt
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compression of the dayside magnetopause as inferred from the measured H-component (Blake et al., 2004). The driving feature for the strongest perturbation of the magnetosphere in the 24 November 2001 event, as determined from the MHD simulations, was the high solar wind density impulse seen in Fig. 3. The November 6, 2001 event (minimum Dst ¼ 292 nT) shows a lower trapping boundary in L-value in Fig. 1 than the November 24, 2001 event (minimum Dst ¼ 221 nT), which is consistent with the effect of ring current perturbation on first invariant conservation (Young et al., 2002). However, injection and trapping of Fe ions at lower L-value in the earlier event (Slocum et al., 2002) suggests that rigidity cutoff or radial transport or both were deeper in the inner magnetosphere for the earlier November 2001 event. Table 1 summarizes parameters for these and a total of eleven geoeffective SEP events between April 2000 and May 2002. Simulations of the November 6, 2001 and Bastille Day 2000 SEP events are planned for comparison with the Novemeber 24, 2001 event examined here. Improved temporal resolution of solar wind data is needed in future upstream spacecraft missions, or improved dynamic solar wind models, in order to better resolve dB=dt input to the magnetosphere. It has been noted, for example, that ULF oscillations in the 5–10min period range in the solar wind are well-correlated with those seen at geosynchronous (Kepko et al., 2001), and likely play a role in radial transport on diffusive time scales. However, the magnitude of dB=dt; which is a key driver for shock-drift acceleration on time scales faster than radial diffusion, is not well resolved by current solar wind measurements, when compared to the rapid rise time seen by ground magnetograms for the March 24, 1991 and November 2001 events (Blake et al., 2004). In addition, improved coverage of L ¼ 2–4 will be necessary to capture other shock- injection events with the temporal resolution that the CRRES satellite provided for the March 24, 1991 event.
Acknowledgements We thank NSSDC for making WIND data available on OMNIWeb and NASA SPE Workshop organizers, Nat Gopalswami, Barbara Thompson and Barbara Giles who co-organized along with MKH the Magnetospheric Effects working group which produced Table 1; D. Berdichevsky and H. Cane provided data in Table 1, as did coauthors JEM and PLS. Special thanks to J.B. Blake for providing HEO data and K. Shiokawa for access to 210 Magnetometer Chain data for the March 91, August 98 and November 2001 events. This material is based upon work supported in part by CISM, which is funded by the STC Program of the National Science Foundation under Agreement Number ATM-0120950,
NSF GEM grant ATM-0201624 and NASA grants NAG5-12202 and NAG5-11735. References Anderson, B.J., Decker, R.B., Paschalidis, N.P., 1997. Onset of nonadiabatic particle motion in the near-Earth magnetotail. Journal of Geophysical Research 102, 17553. Araki, T., Fujitani, S., Emoto, M., Yumoto, K., Shiokawa, K., Ichinose, T., Luehr, H., Orr, D., Milling, D.K., Singer, H., Rostoker, G., Tsunomura, S., Yamada, Y., Liu, C.F., 1997. Anomalous sudden commencement on March 24, 1991. Journal of Geophysical Research 102, 14075. Berdichevsky, D.B., Szabo, A., Lepping, R.P., Lazarus, A.J., 2001. A preliminary list of shocks seen by WIND from launch till August 2001. Eos Trans. of the American Geophysical Union 81, 48. Blake, J.B., Kolasinski, W.A., Fillius, R.W., Mullen, E.G., 1992. Injection of electrons and protons with energies of tens of MeV into L44 on 24 March 1991. Geophysical Research Letters 19, 821. Blake, J.B., Baker, D.N., Turner, N., Oglivie, K.W., Lepping, R.P., 1997. Correlation of changes in the outer-zone relativistic-electron population with upstream solar wind and magnetic field measurements. Geophysical Research Letters 19, 927. Blake, J.B., Slocum, P.L., Mazur, J.E., Looper, M.D., Selesnick, R.S., Shiokawa, K., 2004. Geoeffectiveness of Shocks in Populating the Radiation Belts In: Lui, A.T.Y., Kamide, Y., Consolini, G., (Eds.), Multiscale Coupling of Sun-Earth Processes. Elkington, S.R., Hudson, M.K., Chan, A.A., 1999. Acceleration of relativistic electrons via drift-resonant interaction with toroidal mode Pc-5 ULF oscillations. Geophysical Research Letters 26, 3273. Elkington, S.R., Hudson, M.K., Wiltberger, M.J., Lyon, J.G., 2002. MHD/Particle simulations of radiation belt dynamics. Journal of Atmospheric and Solar Terrestial Physics 64, 607. Elkington, S.R., Hudson, M.K., Chan, A.A., 2003. Resonant acceleration and diffusion of outer zone electrons in an asymmetric geomagnetic field. Journal of Geophysical Research 108, art. no. 1029. Fedder, J.A., Lyon, J.G., 1995. The Earth’s magnetosphere is 165RE long: self-consistent current convection magnetospheric structure processes for northward interplanetary magnetic field. Journal of Geophysical Research 100, 3623. Gopalswamy, N., 2003. Solar and geospace connections of energetic particle events. Geophysical Research Letters 30, 8013. Hudson, M.K., Kotelnikov, A.D., Li, X., Roth, I., Temerin, M., Wygant, J., Blake, J.B., Gussenhoven, M.S., 1997. Simulations of proton radiation belt formation during the March 24, 1991 SSC. Geophysical Research Letters 22, 291. Hudson, M.K., Elkington, S.R., Lyon, J.G., Marchenko, V.A., Roth, I., Temerin, M., Blake, J.B., Gusenhoven, M.S., Wygant, J.R., 1997. Simulation of radiation belt formation during storm sudden commencements. Journal of Geophysical Research 102, 14087. Hudson, M.K., Elkington, S.R., Lyon, J.G., Wiltberger, M., Lessard, M., 2001. Radiation belt electron acceleration by
ARTICLE IN PRESS M.K. Hudson et al. / Journal of Atmospheric and Solar-Terrestrial Physics 66 (2004) 1389–1397 ULF wave drift resonance: simulation of and storms. In: Song, P.S., Singer, H.J., Siscoe, G.L. (Eds.), Space Weather, Geophysical Monograph 125. AGU, Washington, DC, pp. 289. Kepko, L., Spence, H.E., Singer, H.J., 2001. ULF waves in the solar wind as direct drivers of magnetospheric pulsations. Geophysical Research Letters 29, 1197. Kozyra, J.U., 1989. Sources and losses of ring current ions: an update. Advances in Space Research 9 (12), 171. Kress, B.T., Hudson, M.K., Perry, K.L., Slocum, P.L., 2004. Dynamic modelling of geomagnetic cutoff for the November 23–24, 2001 solar energetic particle event. Geophysical Research Letters 31, L04808. Leske, R.A., Mewaldt, R.A., Stone, E.C., 2001. Observations of geomagnetic cutoff variations during solar energetic particle events and implications for the radiation environment at the Space Station. Journal of Geophysical Research 106, 30011. Li, X., Roth, I., Temerin, M., Wygant, J.R., Hudson, M.K., Blake, J.B., 1993. Simulations of the prompt energization and transport of radiation belt particles during the March 24, 1991 SSC. Geophysical Research Letters 20, 2423. Lorentzen, K.R., Mazur, J.E., Loper, M.E., Fennell, J.F., Blake, J.B., 2002. Multisatellite observations of MeV ion injections during storms. Journal of Geophysical Research 107, 1231. Mason, G.M., Hamilton, D.E., Walpole, P.H., Heureman, K.F., James, T.L., Lennard, M.H., Mazur, J.E., 1993. A low energy ion composition analyzer for the study of solar and magnetospheric heavy ions. IEEE Transaction on Geoscience and Remote Sensing 31, 549. Mason, G.M., Mazur, J.E., Looper, M.D., Mewalt, R.A., 1995. Charge state measurements of solar energetic particles observed with SAMPEX. Astrophysics Journal 452, 901. Perry, K.L., Hudson, M.K., Elkington, S.R., 2000. Driftresonant acceleration of protons via global ULF waves. Eos Transaction of the American Geophysical Union 81, 48.
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Reames, D.V., 2001. SEPs: space weather hazard in interplanetary space. In: Song, P.S., Singer, H.J., Siscoe, G.L. (Eds.), Space Weather, Geophysical Monograph 125. AGU, Washington DC, pp. 101. Slocum, P.L., Lorentzen, K.R., Blake, J.B., Fennell, J.F., Hudson, M.K., Looper, M.D., Masson, G.M., Mazur, J.E., 2002. Observations of ion injections during large solar particle events. EOS Transaction AGU 83, SH61A-O501. Smart, D.F., Shea, M.A., 2001. A comparison of the Tsyganenko model predicted and measured geomagnetic cutoff latitudes. Advances in Space Research 28, 1733. Smart, D.F., Shea, M.A., Fluckiger, E.O., 2000. Magnetos pheric models and trajectory computations. Space Science Reviews 93, 305. Spjeldvik, W. N., 1988. Consequences of the duration of solar energetic particle- associated magnetic storms on the intensity of geomagnetically trapped protons. In: Moore, T.E., Waite, J.H. (Eds.), Modeling Magnetospheric Plasma, Geophysical Monograph 44. AGU, Washington DC, pp. 311. Tsyganenko, N.A., 1989. Determination of magnetospheric current system parameters and development of experimental geomagnetic field models based on data from IMP and HEOS satellites. Planetary and Space Science 37, 5. Tsyganenko, N.A., 1996. Effects of the solar wind conditions on the global magnetospheric configuration as deduced from data-based field models, European Space Agency Special Publication ESA SP-389, pp. 181. Tsyganenko, N.A., Singer, H., Kasper, J.K., 2003. Storm-time distortion of the inner magnetosphere: how severe can it get. Journal of Geophysical Research 108, art. no. 1209. Wilken, B., Goertz, C.K., Baker, D.N., Higbie, P.R., Fritz, T.A., 1982. The SSC on July 29, 1997 and its propagation within the magnetosphere. Journal of Geophysical Research 87, 5901. Young, S.L., Denton, R.E., Hudson, M.K., Anderson, B.J., 2002. A new empirical m- scattering model. Journal of Geophysical Research 107, art. no. 1069.
Journal of Atmospheric and Solar-Terrestrial Physics 66 (2004) 1389–1397 www.elsevier.com/locate/jastp
3D modeling of shock-induced trapping of solar energetic particles in the Earth’s magnetosphere M.K. Hudsona,, B.T. Kressa, J.E. Mazurb, K.L. Perrya, P.L. Slocumb a Physics and Astronomy Department, Dartmouth College, Hanover, NH 03755, USA Space Sciences Applications Laboratory, The Aerospace Corporation, Los Angeles, CA, USA
b
Received 2 February 2004; received in revised form 23 March 2004; accepted 24 March 2004
Abstract The prompt trapping of solar energetic particles (SEPs) in the inner magnetosphere around L ¼ 2–2.5, including protons and heavier ions, has been observed at both the Cycle 22 and 23 solar maxima, in association with high-speed interplanetary shocks and storm sudden commencements (SSCs). Recent observations include the Bastille Day 2000 CME-driven storm as well as two in November 2001, which produced a long-lived new proton belt, as well as trapping of heavy ions up to Fe in all three cases. A survey of such events around the most recent solar maximum, including high altitude measurements from Polar and HEO satellites along with low altitude measurements from ‘the Solar Anomalous and Magnetospheric Particle Explorer (SAMPEX)’, indicates similarities to the well-studied March 24, 1991 SSC event. In this event, electrons and protons in drift resonance with a magnetosonic impulse were transported radially inward. A requirement for such shock-induced acceleration is a high-speed CME-shock at 1 AU, which launches a perturbation with comparable velocity inside the magnetosphere. Secondly, there must be a source population which is drift-resonant with the impulse. The CME-shock itself is a source of solar energetic particles, both protons and heavy ions, with higher fluxes and harder spectra associated with faster moving CMEs. Arrival of the interplanetary shock compresses and changes the magnetosphere topology, leading to a reduction of the geomagnetic cutoff, initially around L ¼ 4 for SEP protons. This effect is modeled using a 3D Lorentz integration of SEP trajectories in electric and magnetic fields taken from the Lyon–Fedder–Mobarry (LFM) global MHD code, with solar wind input parameters taken from spacecraft measurements upstream from the bow shock, carried out for the November 24, 2001 SEP event. The results indicate that an enhancement in solar wind dynamic pressure for this event plays a role in the observed injection of ions to low L-values, to form a new proton belt which has lasted for more than 2 years. r 2004 Published by Elsevier Ltd. Keywords: Solar wind/magnetosphere interactions; Magnetosphere; Radiation belts; Particle acceleration; Solar energetic particles; Magnetic storms
1. Introduction
Corresponding author. Tel.: +1-603-646-2976; fax: +1-
603-646-1446. E-mail address: [email protected] (M.K. Hudson). 1364-6826/$ - see front matter r 2004 Published by Elsevier Ltd. doi:10.1016/j.jastp.2004.03.024
Solar energetic proton (SEP) events have been associated with solar activity since prior to the space age, with access of the most energetic ions to ground level at high latitudes, and the geomagnetic cutoff latitude determined by particle rigidity, defined as
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momentum per unit charge. ‘The Combined Radiation Release and Effects Satellite (CRRES)’, launched in July 1990, and operational until October, 1991 in a nearequatorial geosynchronous transfer orbit, provided the first clear evidence for the formation of a long-lived proton belt from the radial transport and trapping of an SEP source population, as well as transport and energization of outer zone electrons to form a new electron belt in the otherwise sparse slot region, L ¼ 223 (Blake et al., 1992). The CRRES satellite was fortuitously located around L ¼ 2:5 on the night side at the time of the injection. It was apparent in the arrival time of both 420 MeV protons and 413 MeV electrons, and subsequent drift echoes, that radial transport of both species resulted in the formation of new radiation belts on a particle drift time scale of a few minutes. Conserving the first adiabatic invariant, energy increases as L3 for protons (L3=2 for relativistic electrons), while the key feature for SEPs, already energized to significant flux levels at 410 MeV by the interplanetary shock (Reames, 2001), is the increase of their perpendicular relative to parallel energy, which changes their pitch angle and results in trapping. Lorentzen et al. (2002) provided a summary in their Table 1 of variations in the inner zone trapping boundary, which can be affected by SEP injections, trapping and de- trapping, based on observations beginning in the early 1960s through CRRES observations at the Cycle 22 solar maximum. Four events were seen up to and including the Bastille Day 2000 storm in the rising phase of Cycle 23 solar activity, summarized in their Table 2. These latter 4 events include August 26–28, 1998 and September 23–26, 1998, which were Geospace Environment Modeling storm events and moderate SEP events (http://leadbelly.lanl. gov/GEM_Storms/GEMstorms.html). Our Table 1 summarizes 2 years of geoeffective SEP events from the Cycle 23 solar maximum, including the April and July 2000 events studied by Lorentzen et al. This table was produced as a subset of those compiled in the Coordinated Data Analysis Workshop study of major SEP events of Cycle 23, see Gopalswamy (2003), Table 1. Included in the CRRES era catalog of events studied by Lorentzen et al. are changes in the outer trapping boundary of protons, also evident in Fig. 1, which shows the flux of 415 MeV protons measured by the HEO 1997-068 spacecraft, in a 12-h period, geosynchronous transfer orbit with a 63 inclination (Blake et al., 1997). This 18-month summary of energetic proton data shows three features which are common around solar maximum: (1) SEP events, (2) changes in the outer boundary of inner zone protons and (3) long term trapping of protons associated with SEP injection events. Aside from gray stripes corresponding to periods in which data is not available, vertical flux stripes are apparent
corresponding to individual SEP events. Abrupt changes in the inner zone outer boundary are also seen in association with a corresponding SEP event. However, the scale shown here masks the fact that the change in the trapping boundary is associated with buildup of the storm time ring current, delayed by 1–3 days relative to SEP arrival at geosynchronous orbit for most events (see Table 1). A significant perturbation in the background magnetic field due to buildup of the ring current causes a decrease in the geomagnetic field radius of curvature, similar to stretching of the tail due to increased cross-tail current (Anderson et al., 1997). This ring current modification of the background magnetic field results in loss of first invariant conservation when the gyroradius becomes comparable to magnetic field gradient scale lengths. This limitation on trapping is the basic cause of the outer boundary of the inner zone, which moves to a lower L-value during storm periods for these 415 MeV protons (Hudson et al., 1997; Young et al., 2002). The third feature to note in Fig. 1 is the injection of SEP protons to form a newly trapped population around L ¼ 2:5 on November 6, and again on November 24. These two injections are shown in more detail in Fig. 2, for 425 MeV protons observed by the same HEO spacecraft.
2. SEP proton events in November 2001 and August 1998 On the expanded scale of Fig. 2, one can see the arrival of 425 MeV protons, with both SEP injection events populating the magnetosphere into L ¼ 4: This prompt arrival is also seen by the GOES satellites at geosynchronous orbit (http://www.sec.noaa.gov), shortly following CMEs seen by the SOHO spacecraft at L1: Arrival of the corresponding interplanetary shock produced by each CME takes one to two days in both cases. Noteworthy in Fig. 2 is the shift in radial cutoff to lower L values which coincides with the arrival of the interplanetary shock at earth, within the resolution of the 12-h HEO orbit, sampling the SEP cutoff twice per orbit at a 9-h interval centered on apogee. These measurements are limited in temporal resolution by a greater portion of each 12-h orbit spent near apogee at 7:2RE : The black arrow indicates the arrival time of the forward shock at 0558 UT on November 24, at the location of the WIND spacecraft, which was on the dayside at (17.7, 74:5; 28:4RE ) in GSE coordinates. Solar wind density, V x and Bz components of the velocity and magnetic field measured by WIND instruments are plotted in Fig. 3, along with hourly averaged Dst : The driving solar wind conditions will be discussed further in the modeling section. Fig. 4 shows another class of proton trapping events for August 26–27, 1998 (day 238–239), where SEPs were present into L 4; prior to arrival of the interplanetary
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Table 1 Eleven Solar Proton Events selected for geoeffectiveness by NASA LWS SPE Workshop, August 2002, from Cycle 23 solar maximum catalog. (http://cdaw.gsfc.nasa.gov/LWS/data/event_list.html and Gopalswamy, 2003) for period April 2000–May 2002, excluding April 21–23, 2002 event, for which data is available at http://storms.jhuapl.edu SEP date
Max sw speed (km/s)
IP shocks— D.Berdichevsky
Bz
Ave shock speed H.Cane (km/s)
Max GOES8 410 MeV p+
SSC
DST min
DST val (-nT)
New belts
1
2000/04/04
606
4/6 1627@Wind, 1605@ACE, no mag cloud
Bz o0 long time
870
5:5 101
4/6 1639
4/7 0030
321
p, a
2
2000/07/14
1005
7/15 1435@Wind 1416@ACE, yes mag cloud
Bz o0 short time, density enhancement
1600
2:4x104
7/15 1437
7/15 2130
300
p, a, heavy
3
2000/11/08
900
11/11 0413@Wind 0400@ACE no cloud
density enhancement
1100
1:5 104
11/10 0628
11/10 1300
96
nothing
4
2001/03/29
848
03/30 2330@Wind 2150@ACE 3/31 0111@Wind 0030@ACE 3/31 2200@Wind
Bz o0 long time, density enhancement
690
3 101
03/31 0052
3/31 0900
387
p, a
5
2001/04/10
756
4/10 1056@Wind 4/11 1425@Wind 4/12 1743@Wind
Bz o0 long time density time density enhancement
1220
4 102
04/11 1343 04/11 1519
4/12 0000
236
p, a, heavy
6
2001/04/15
500
4/15 0140@Wind 0120@ACE
Bz o0 short time
behind limb
1 103 5 102
04/18 0046
4/18 0700
114
nothing
7
2001/09/24
740
9/25 2018@Wind 2003@ACE
Bz highly variable, waves at all freq.
1220
1 104
9/25 2025
9/26 0200
102
heavy
8
2001/10/01
575
10/1 2230@Wind 2150@ACE 10/3 0805@ACE
Bz variable
10/3 1500
166
nothing
9
2001/11/04
750
11/6 0145@Wind 0124 @ACE 11/6 1938@Wind 1905@ACE
Bz 70nT F Bz 20nT R
1240
5 104
11/6 0152
11/6 0700
292
p, heavy
10
2001/11/22 2100UT
1003
11/24 0452@Wind 0558@Wind 0540@ACE 1400 @Wind
Bz o0 post shocks, 40 between, density enhancement
1300
2 104
11/24 0556
11/24 1700
221
p, a, heavy
11
2002/5/22
950
5/23 1040@Wind 1010@ACE 2130@ACE
Bz o0 short time
8 102
5/23 1100
5/23 1800
108
p, a, heavy
3 103
Several control SEP events are included with no trapping of SEP ions, as indicated in the last column; average shock speed indicated is not available for all events. Information on maximum solar wind speed, shock arrival times, and IMF Bz supplied by Berdichevsky, shock speeds at L1 by H. Cane, new belts and ion species, by J. Mazur and P. Slocum from SAMPEX LICA measurements, and other data from http://www.sec.noaa.gov and links, e.g. to Kyoto final (00–01) and provisional (02) Dst :
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Fig. 1. Proton fluxes with E415 MeV from the HEO 1997-068 spacecraft for the period indicated, January 1, 2001–June 10, 2002. An IGRF field model is used to map off-equatorial plane fluxes to the L-shells indicated. Vertical gray stripes indicate periods when no data is available.
Fig. 3. November 24, 2001 Wind satellite data given in SM coordinates used to drive the LFM magnetospheric model. Measured values for the other magnetic and velocity field components and plasma temperature are also used but not shown here. Also shown is the Dst index from the WDC-C2 KYOTO Dst index service. The four dashed vertical lines correspond to times for which LFM MHD fields were used for SEP test particle trajectory calculations in Fig. 6 ( Kress et al., 2004).
Fig. 2. Proton fluxes with E425 MeV from the HEO 1997-068 spacecraft for the month of November 2001. The time resolution of the data is approximately 4 bins per day. The bin indicated by the arrow corresponds to an inward bound portion of the HEO orbit occurring between 6:30 and 9:30 UT on November 24 (Slocum et al., 2002).
shock on August 26 (left panel). The transient SEP flux at L44 has dropped by August 27 (right panel), and the new belt emerges at L ¼ 3:5: The observations suggest a diffusive transport mechanism, for example, driftresonant radial transport, which has been suggested for MeV electrons (Elkington et al., 1999, 2002, 2003) whose drift frequency is comparable to that of enhanced Pc 5 ULF waves observed during geomagnetic storms. Protons with energies shown in Fig. 4 have drift frequencies comparable to the ULF waves observed during the August 1998 storm period by the GOES 8 magnetometer (Hudson et al., 2001), suggesting that the same mechanism may operate for protons on a slower
time scale than the shock-drift acceleration mechanism (Perry et al., 2000). However, 10 MeV protons at L ¼ 3:5 will easily be perturbed from trapped orbits by ring current perturbations (Hudson et al., 1997; Young et al., 2002), so it is not surprising that new belts which form around L ¼ 3:5 are short-lived, as noted in Table 1 of Lorentzen et al. (2002).
3. Heavy ions Heavy ions provide further evidence for rapid radial transport and trapping, with the example of Fe counts plotted in Fig. 5, from ‘the Solar Anomalous and Magnetospheric Particle Explorer (SAMPEX)’ (Mason et al., 1993) satellite measurements in a low-altitude polar orbit, along with that of 20–29 MeV protons. Each blue circle corresponds to a measurement of one Fe count by the LICA instrument on SAMPEX in the 0.5–3 MeV/nucleon range. Charge state comparable to the average solar wind ionization state (+11) can be
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Fig. 4. Proton fluxes with E49; 17 and 26 MeV from the HEO 1997-068 spacecraft for August 26–27, 1998 separated by 18-h. Multiple traces correspond to inbound and outbound passes relative to perigee in a static IGRF magnetic field model used to map fluxes to the equatorial plane, while non-stationarity of the geomagnetic field on a 6-h (half-orbit) time scale results in additional traces. Note the CRAND population at low L in both panels, and the high SEP fluxes seen in the left panel outside L ¼ 4; prior to the shock arrival (SSC at 0942 UT; perigee time indicated by decimal day). The right panel shows that SEP fluxes have dropped the following day, while a new proton belt at L ¼ 3:5 is evident.
2:8: After arrival of the 0558 UT shock discussed previously (Fig. 2), they are further transported into L ¼ 2:5; with passage of the reverse shock. This appears to be strong evidence for drift-time scale transport and trapping, with trapped fluxes low, but apparent on a longer time scale plot for this and similar events (see Lorentzen et al., 2002, Fig. 6), also for November 6, 2000.
4. Cutoff and radial transport modeling
Fig. 5. SAMPEX count rates for 20–29 MeV protons (labeled PLO) from the PET instrument, black line, while individual counts of 0.5–3 MeV/nucleon Fe from the LICA instrument on SAMPEX (Mason et al., 1996) appear as blue circles. Shock arrival times at WIND are indicated with red lines. Note that inward transport of Fe ions is well-correlated with shock arrival at 0558 UT, the clear jump in solar wind density n; V x and Bz in Fig. 3.
inferred from the geomagnetic cutoff (Mason et al., 1995) prior to trapping. Also plotted are two forward and one reverse shock crossings at the WIND spacecraft, as reported by Berdichevsky et al. (2001). One again sees, at the 96 min cadence of the SAMPEX orbit, radial transport of energetic Fe ions from their preshock arrival cutoff value, between L ¼ 3:5–4, to L ¼
We have modeled surfaces of constant cutoff rigidity in 3D for 425 MeV protons for the November 24, 2001 storm using measured solar wind parameters at WIND (Fig. 3) as input to the LFM MHD code. A relativistic Lorentz trajectory integrator has been developed for use with the 3D MHD fields, and proton access calculated using sequential snapshots of the MHD fields, since 425 MeV protons transit 1=4 of the test particle simulation domain in a time comparable to the MHD simulation time step of 0:25 s: In the 2D noon–midnight meridional plane shown in Fig. 6, 106 particles were launched isotropically from each of 18,000 points on a uniform grid over a 14RE 14RE earth-centered region in the noon–midnight meridian, i.e. with a resolution of 0:1RE : SEP access is determined by following time reversed particle trajectories. Thus, an escaped particle indicates SEP access to the point from which the particle is launched (see e.g., Kress et al., 2004; Smart et al., 2000; Smart and Shea, 2001). The results
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Fig. 6. Modeled 25 MeV proton cutoff in SM coordinates at four snapshots in time from the MHD simulation. The boundary of the black shaded region is the cutoff surface. The cutoff is determined by following reverse particle trajectories. An escaped particle is one which exits a sphere at r ¼ 15RE before its total trajectory path length exceeds 1000RE (Kress et al., 2004).
are shown in Fig. 6, with the color bar indicating the number of escaped particles out of 106 at each point, and black indicating zero escaped particles. The boundary of the black region is the cutoff surface. The test-particle simulation domain is a subset of the MHD domain extending to 30RE upstream and 300RE downstream from the earth (Fedder and Lyon, 1995). Previous cutoff calculations have been performed using static magnetic field models, for example, Tsyganenko, 1989 (Smart and Shea, 2001). Fig. 6 shows that prior to arrival of the interplanetary shock at 0558 UT, a magnetosphere slightly compressed from dipolar is seen in panel (a), as compared with the corresponding calculation in a dipole model (Kress et al., 2004, Fig. 2). The most extreme change in SEP access is apparent in panel (c), which corresponds to arrival of the high density impulse in Fig. 3, where the solar wind density jumps to 70 cm3 : This sudden change in the solar wind dynamic pressure appears to have a greater effect on the calculated SEP cutoff than the shock transition defined by a jump in V x ; Bz and density at 0558 UT in Fig. 3. Buildup of the ring current, as indicated by Dst in the bottom panel of Fig. 3, following the extended period of southward IMF Bz ; appears to have a smaller effect on suppressing the SEP
cutoff, as apparent in panel (d) of Fig. 6, corresponding to the last vertical dash in the WIND data. SEP access into L ¼ 2:5 is seen in the HEO spacecraft data plotted in Fig. 2. Fig. 2 uses an IGRF magnetic field model to transform off-equatorial plane particle flux data to the equator, and again to convert to a reference L-value not adjusted for disturbed geomagnetic conditions, for example, incorporated into T96 (Tsyganenko, 1996) and recent models by Tsyganenko et al. (2003). Fig. 6 shows proton access in geocentric radial distance calculated as the number of escaping protons, with reverse integration of trajectories in time for the 106 protons launched isotropically from each pixel. While a direct comparison of IGRF L-values in Fig. 2 and geocentric radial distance in Fig. 6 is not possible, it is clear that cutoffs undergo the most dramatic change when the solar wind density impulse arrives, vs. on the time scale of buildup of the ring current over hours, as indicated by Dst in the bottom panel of Fig. 3.
5. Discussion It is significant from a space weather viewpoint that the magnetosphere responds as rapidly as indicated in
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Fig. 6 from the MHD simulations, separated in time by only 1 h of solar wind input conditions between panels (b) and (c); or the 96-min resolution of SAMPEX data shown in Fig. 5, and the 6–12-h resolution HEO data plotted in Fig. 2. All indicate a response in solar energetic ion access inside undisturbed cutoffs which is much faster than diffusive time scales (Spjeldvik, 1988). Widely-used radiation belt models do not adequately specify this kind of event. The simulations indicate a faster response than the buildup time of Dst during geomagnetic storm main phase, which is typically several hours. The correlation in depression of cutoff latitude with Dst has been noted previously (Mason et al., 1995; Leske et al., 2001). However, an examination of the November 2001 events (R. Leske, private communication), along with the July 2000 (Bastille Day) and April 10–12, 2001 events, see Table 1 and http://storms.jhuapl.edu/posters/Labrador/SAMPEX_ Storms2_Cutoffs/slide08.html, shows that the measured cutoff invariant latitude drops with decreasing Dst ; but recovers much more quickly. Dst recovers on the time scale of collisonal loss of ring current ions, with an initially rapid recovery rate of hours on the Oþ –O charge exchange time scale, and a slower recovery over 1–2 days with the slower Hþ –H charge exchange rate determining that time scale (Kozyra, 1989). The initial drop in Dst is correlated with enhanced convection and opening up of the polar cap, which affects cutoff invariant latitude (Leske et al., 2001). Fig. 6 indicates that the magnetosphere may respond rapidly to increased solar wind dynamic pressure, with cutoff suppression to L ¼ 3:5 in the MHD fields driven by input solar wind parameters. However, trapping still requires a change in pitch angle, which can be achieved with perpendicular acceleration by the azimuthal electric field accompanying an enhancement in magnetospheric Bz on the dayside due to magnetopause compression (Li et al., 1993). For both electrons and ions, the compression of the dayside magnetopause launches a magetosonic impulse propagating around the flanks of the inner magnetosphere (Wilken et al., 1982), which contains an azimuthal electric field component due to the timevarying Bz of the magnetopause compression. Key to the March 24, 1991 compression, the largest reported by Kakioka ground magnetograms between 1957–1994 (Araki et al., 1997), was the magnitude of the dBz =dt impulse, exceeding 200 nT. This magnitude far exceeds typical Storm Sudden Commencement (SSC) values. Comparing multi-spacecraft magnetometer data, Araki et al. showed that the March 1991 impulse propagated at an average velocity of 800–900 km/s through the dayside magnetosphere, consistent with the average magnetosonic speed in this region. Li et al. (1993) developed a simple propagating compressional Bz and bipolar E j impulse that reproduced, via test particle simulations, the electron transport and acceleration
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evident in measurements at CRRES, using a larger constant pulse speed within the magnetosphere of 2000 km/s in their model. This model was less successful at reproducing SEP trapping (Hudson et al., 1997), and was replaced by MHD simulations of this event using the Lyon–Fedder–Mobarry code (Lyon et al., this issue) to advance guiding center proton trajectories in the equatorial plane (Hudson et al., 1997). Both approaches to modeling this event confirm the particle drift-time scale transport over more than 1RE radially, in minutes, in contrast to a process like radial diffusion which occurs over many hours to days. All necessary ingredients for radial transport and trapping of SEPs at L ¼ 2:5 were again present in the November 24, 2001 event studied: the solar energetic ions which preceed the arrival of the interplanetary shock, serving as the source population; their access to lower L-values with arrival of a high density solar wind impulse; and trapping at lower L-values due to nonadiabatic effects, i.e. violation of the third invariant with rapid radial transport on a drift time scale. The latter prevents the restoration of fluxes to higher L-value when the magnetosphere relaxes, as it clearly has by the fourth panel of Fig. 6. A caveat is required regarding use of L-value to organize particle data during highly disturbed geomagnetic conditions. What has been used in Figs. 1, 2, 4, and 5 is an IGRF field model for mapping fluxes, while Fig. 6 simply plots proton access as a function of radial distance in the electric and magnetic fields calculated by the 3D LFM-MHD code. It is expected that L-shells become more highly distorted during extreme solar wind driving conditions than is captured by IGRF, or the T96 model, which contains a Dst -dependent term to model ring-current evolution. The Tsyganenko model has recently been extended to model extreme storm conditions, Dst o 100 (Tsyganenko et al., 2003). However, neither particle flux measurements nor static field cutoff calculations have yet incorporated this more perturbed parameterization of the geomagnetic magnetic field. Finally, an outstanding research question to investigate is: what distinguishes the March 24, 1991 and November 6 and 24, 2001 class of events, which result in long-lived trapping of SEPs at low L-value ðL 2:5Þ; from those such as August 26–28, 1998, and the Bastille Day 2000 storms? Phenomenologically, they are distinguished by depth of penetration of the SEPs upon arrival of the CME-shock at the magnetosphere, since prior to shock arrival, both classes of events are characterized by cutoffs around L ¼ 4 for SEPs of energy 10 MeV; as measured for example by CRRES (Hudson et al., 1997) and the HEO spacecraft, Fig. 1. No solar wind data was available for the March 24, 1991 event, but comparison of ground magnetometer data for all three deep-penetration events using the 210 Magnetometer Chain indicate an unusually strong dBz =dt
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compression of the dayside magnetopause as inferred from the measured H-component (Blake et al., 2004). The driving feature for the strongest perturbation of the magnetosphere in the 24 November 2001 event, as determined from the MHD simulations, was the high solar wind density impulse seen in Fig. 3. The November 6, 2001 event (minimum Dst ¼ 292 nT) shows a lower trapping boundary in L-value in Fig. 1 than the November 24, 2001 event (minimum Dst ¼ 221 nT), which is consistent with the effect of ring current perturbation on first invariant conservation (Young et al., 2002). However, injection and trapping of Fe ions at lower L-value in the earlier event (Slocum et al., 2002) suggests that rigidity cutoff or radial transport or both were deeper in the inner magnetosphere for the earlier November 2001 event. Table 1 summarizes parameters for these and a total of eleven geoeffective SEP events between April 2000 and May 2002. Simulations of the November 6, 2001 and Bastille Day 2000 SEP events are planned for comparison with the Novemeber 24, 2001 event examined here. Improved temporal resolution of solar wind data is needed in future upstream spacecraft missions, or improved dynamic solar wind models, in order to better resolve dB=dt input to the magnetosphere. It has been noted, for example, that ULF oscillations in the 5–10min period range in the solar wind are well-correlated with those seen at geosynchronous (Kepko et al., 2001), and likely play a role in radial transport on diffusive time scales. However, the magnitude of dB=dt; which is a key driver for shock-drift acceleration on time scales faster than radial diffusion, is not well resolved by current solar wind measurements, when compared to the rapid rise time seen by ground magnetograms for the March 24, 1991 and November 2001 events (Blake et al., 2004). In addition, improved coverage of L ¼ 2–4 will be necessary to capture other shock- injection events with the temporal resolution that the CRRES satellite provided for the March 24, 1991 event.
Acknowledgements We thank NSSDC for making WIND data available on OMNIWeb and NASA SPE Workshop organizers, Nat Gopalswami, Barbara Thompson and Barbara Giles who co-organized along with MKH the Magnetospheric Effects working group which produced Table 1; D. Berdichevsky and H. Cane provided data in Table 1, as did coauthors JEM and PLS. Special thanks to J.B. Blake for providing HEO data and K. Shiokawa for access to 210 Magnetometer Chain data for the March 91, August 98 and November 2001 events. This material is based upon work supported in part by CISM, which is funded by the STC Program of the National Science Foundation under Agreement Number ATM-0120950,
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