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doi: lO.l016/SO273-1177(03)00339-9

SOLAR MAXIMUM LOW-ENERGY PARTICLE OBSERVATIONS AT HELIOGRAPHIC LATITUDES. ABOVE 75 DEGREES

D. Lario’, E. C. Roelof’, R. B. Decker’, and D. B. Reisenfeld2

‘Applied Ph$Gcs Laboratory, Johns Hopkins University, 11100 Johns Hopkins Rd., Laurel, MD 20723, USA %.os Alamos National Laboratory, Los Alamos, NM 87545, USA

ABSTRACT

We present a preliminary analysis of low-energy particle observations by the Ulysses and ACB spacecraft during the time intervals that Ulysses spent at heliographic latitudes above 75 degrees during its solar maximum orbit. The most ‘southerly and northerly latitudes (80.2”) were reached by Ulysses in November 2000 and October 2001, respectively. These two periods were characterized by high levels of solar activity. During its passage through the highest southern latitudes, Ulysses encountered complex magnetic field and solar wind configurations, with numerous energetic particle events of either solar origin or associated with solar wind stream interaction regions. By contrast, during its high northern latitude passage, Ulysses remained immersed in the high-speed (-700 km s-l) solar wind coronal hole flow bearing magnetic field of negative polarity and detected three intense solar energetic particle (SEP) events. We correlate observations from Ulysses at high heliographic latitudes with 1 AU measurements in the ecliptic from the ACE spacecraft. Large SEP events were observed simultaneously by both spacecraft. Periods with comparable particle intensities at ACE and Ulysses occurred during the decay phase of large SEP events. We discuss the formation of these periods of nearly identical particle intensities (i.e., “reservoirs*‘) in terms of both fresh injections of solar energetic particles and the formation of compressed magnetic field regions in the heliosphere beyond the Earth orbit (-2-5 AU). We discuss the implications of these observations for models of solar energetic particle propagation to both high and low heliographic latitudes. 0 2003 COSPAR. Published by Elsevier Ltd. All rights reserved.

‘INTRODUCTION

The Ulysses spacecraft has provided the opportunity to analyze the heliosphere at high heliographic latitudes. In this paper we describe low-energy proton and electron observations made by the Heliosphere Instrument for Spectra Composition and Anisotropy at Low Energies (HI-SCALE) on board Ulysses (Lanzerotti et al., 1992). We also use Ulysses energetic particle observations from the Low-Energy Telescope (LET) of the Cosmic Ray and Solar Particle Investigation (COSPIN; Simpson et al., 1992) and solar wind measurements from the Ulysses solar wind experiment (SWOOPS; Bame et al., 1992). We compare and contrast low-energy particle observations made by the Ulysses spacecraft during the two high (>75”) latitude portions of its solar maximum orbit. hr this paper we refer to the two periods, >75”S and >75”N, as southern high-latitude passage and northern high-latitude passage, respectively. The southern high-latitude passage extended over days 280-3652000, and the northern high-latitude passage from days 254-319, 2001. The highest southern latitude (S80.2’) was reached by Ulysses on day 332 of 2000 and the highest northern latitude (N80.2’) on day 287 of 2001. We compare low-energy particle observations at Ulysses with in-ecliptic measurements from the Electron Proton and Alpha Monitor (BPAM) on board the ACE spacecraft (Gold et al., 1998) at the heliocentric distance of 1 AU. We correlate the observation of solar energetic particle (SEP) events ,at both spacecraft during these two time intervals. Analyses of Ulysses data over longer periods of time have been presented elsewhere (e.g., Lario et al., 2001; McKibben et al., 2OOla). By limiting the study to the highest latitudes, explored under different solar wind and magnetic field configurations, we establish the extreme cases that models of particle propagation in the three-dimensional heliosphere must deal with.

Adv. Space Res. Vol. 32, No. 4, pp. 579-584,2003 Q 2003 COSPAR. Published by Elsevier Ltd. All rights reserved Printed in GreatBritain 0273-1177/$30.00 + 0.00

580 D. Lario et al

SOUTHERN VS. NORTHERN HIGH-LATITUDE PASSAGES

Figure 1 shows the energetic ion intensities and solar wind speed for the southern (Figure la) and northern (Figure lb) high-latitude passages. During the high-latitude southern passage, Ulysses observed low-speed solar wind (-400 km s-l, even below 300 km se1 at the highest latitudes on days 326-330) with occasional streams of faster (-500 km s”) wind (McComas et al., 2002a). The iriteractions between both types of solar wind have been labeled stream interaction regions (SIRS) in Figure la. The last two SIRS in Figure la were bounded by forward and reverse shocks, whereas the first SIR was preceded by a forward shock o&y. These SIRS were accompanied by low-energy ion intensity increases that peaked at the shock passages. We distinguish in Figure la four different energetic particle events (lS, 2S, 3s and 4s). High-energy ion intensities (McKibben et al., 2OOlb), near- relativistic electron intensity increases (see below) and 1 AU observations allow us to classify these events as of solar origin. The onsets of the events 1s and 3s were affected by the presence of SIRS. The event 4s developed over a high pre-existing background level. By contrast, the onset of the event 25 occurred in a period nearly devoid of solar wind structures and with relatively low pre-event intensities. Its risk-time, however, was slow compared to other SEP events observed at 1 AU or to the two intense SEP events 1N and 3N described below.

R [AU]= 2.50 ,) ( 2.40 2.30 2. 0

1874-4752 keV

Y 3N

c WU

E 600 t 400 A. >

HLoNG%60* 320 340 HLONG%i2* 2i$I2319 280 290 300 310 HLONG=WBP 320 HLAT=-76.5' 2000 HLAT=-74.6' HLAT=t76.5O 2001 HLAT=+73.7'

Fig. 1. Hourly averages of ion intensities measured by, top four traces, HI-SCALE/LEMS120 (Lanzerotti et al., 1992) and, two lower traces, COSPIN/LET (Simpson et al., 1992) and of the so&r wind’speed measured by SWOOPS (bottom panels; Bame et al., 1992) for the time intervals 290-365 of 2000 (a) and 260-324 of 2001 (b). Solid vertical lines mark the arrival of interplanetary shocks (J. T. Gosling and R. J. Forsyth, private communication) and black rectangles in the bottom panel the passage of CMEs as identified’by Reisenfeld et al. (2003).

During the northern high-latitude passage (Figure lb), Ulysses was immersed in the high-speed (i700 km s‘l) polar solar wind stream in which only an inward magnetic field polarity was observed (McComas et al., 2002b). Four coronal mass ejections (CMEs) were clearly observed in the solar wind plasma even at these high latitudes and under these high-speed solar wind conditions (Reisenfeld et al., 2003). The passage of these CMEs was characterized by enhanced low-energy ion intensities, whose characteristics wilI be studied in a future work. We distinguish in Figure lb three different SEP events (designated lN, 2N and 3N). Increases in the intensities of high-energy ions and near-relativistic electron (see below) allow us to classify these events as of solar origin. The high electron intensities during the rising phases of the events 1N and 3N contaminated the ion channels of the HI- SCALFYLEMS120 detector. We indicate those periods by dotted traces in Figure lb. Note that throughout the southern high-latitude passage low-energy (cl MeV) proton intensities were high (either due to SIRS, SEP events and/or local acceleration at the interplanetary shocks), whereas during the northern passage, energetic particle intensities dropped to instrumental background levels between the SEP events.

Low-Energy Particles at High Heliolatitudes 581

IN-ECLIPTIC VS. HIGH-I-IELIOLATITUDE OBSERVATIONS

The EPAM instrument on board ACE allows us to compare particle intensities at high latitudes with those in the ecliptic. Figure 2 compares hourly averages of the 175-315 keV electron intensities at Ulysses and ACE for the southern (a) and northern (b) high-latitude passages. Compared to electrons, low-energy ion intensities at ACE (not shown here) are heavily modified by local interplanetary structures, and their comparison will not be addressed here (for a comparison of high-energy ion intensities during the southern high-latitude passage see McKibben et al., 2OOlb). Figure 2 shows that during the two high-latitude passages, there were more electron increases at ACE than at Ulysses. For events observed by both spacecraft, the peak intensity at ACE is always higher than at Ulysses. At Ulysses the typical duration of the events is longer than at ACE. Table 1 gives details of the solar origin’of the SEP events observed at ACE, including the time of observation of CMEs, their propagation direction (taken from the LASCO web site), and the occurrence of the temporally associated X-ray flares (indicated by vertical arrows in Figure 2). Note that the SEP events observed at both spacecraft were always preceded by the occurrence of a halo CME at the Sun. Other striking features of the intensity profiles in Figure.2 are (1) the period from day 294 to 299 in 2000 (event 1s) when Ulysses observed higher intensities than ACE, (2) the periods during the decaying phase of the large events (e.g., events 3S, 4s and 3N) when intensities at both spacecraft were comparable and evolved similarly, and (3) the different intensity level during the decay phase of the event 1N.

North Pole

0’)

175-315 keV electrons

300 R-2.56 AU HLONG-WI 60’ HLAT--76.5’

320 340 360 270 280 290 300 310 320 2000 R-2.03 Au R- 1.56 AU 2001 R-2.31 AU

HLONG-E, 62. HLONG-W31’ HLONG-W62’ HLAT--74.6. MAT-+76.5’ HLAT-+73.7’

Fig. 2. Hourly averages of the 175-315 keV electron intensities as measured by Ulysses/HI-SCALE (black traces) and ACE/EPAM (gray traces) for the southern high-latitude passage (a) and the northern high-latitude passage (b). Arrows indicate the occurrence of the X-ray flares and CMEs in Table 1.

At the analyzed energies, the event 1s at Ulysses did not start until day 294, just after the passage of a CME and a SIR (Figure. 1). At higher proton energies (30-70 MeV) this event started on day 290 and therefore has been related by McKibben et al. (2OOlb) to the event observed at ACE on day 290. The arrival of low-energy protons at Ulysses, as well as the arrival of near-relativistic electrons, were probably delayed by the presence of the CMF and SIR. Intensities rose at Ulysses when the intensities at ACE had already decayed to instnmrental background level. This is an example of how spatial structures may affect the free streaming of energetic particles through the heliosphere and hence modulate the appearance of the SEP events. In the same way, the onset of the event on day 303 in 2000 at ACE was delayed with respect to the onset of the event 2s at Ulysses probably because of the passage of a CME by ACE.

Periods ‘of equal intensities at both spacecraft, i.e., periods with small longitudinal, latitudinal and radial gradients of particle intensities were termed “reservoirs” by Roelof et al. (1992). The interpretation for the formation of these reservoirs given by these authors was that the outer boundaries of the reservoirs were formed by the merging of several plasma disturbances (CMEs) launched during periods of intense solar activity. The magnetic field magnitude increases formed at these boundaries may affect the%ee propagation of energetic particles, resulting”in flux tubes globally filled inside -2.5 AU (Roelof et al., 1992). Note that in order to explain these periods of nearly equal intensities at different heliospheric locations, very efficient and uniform re- distribution of particles must occur (see discussion below). The zero-gradient reservoirs were observed during the decaying phases of the SEP events 3S, 4s and 3N. For the event 3N, a CME driving a strong interplanetary shock was observed at both Ulysses on days 312-314 and at ACE on days 310-311 .of 2001, presumably from the same solar origin (Reisenfeld et al., 2003). Therefore, the formation of the reservoir for this specific event may well

582 D. Lario et al.

occur behind the CME front that generated the SEP event. For events 3s and 4s no local plasma signatures of CMEs were observed at Ulysses; however, the onset of both events occurred during periods of intense solar activity (Nitta and Hudson, 2001; and Table 1). In addition, the presence of SIRS observed by both Ulysses and ACE may also produce the interplanetary disturbances necessary to generate these reservoirs over large stretches of the heliosphere. By contrast, during the decay phase of the large SEP event 1N particle intensities at ACE and Ulysses were quite different. Several new solar injections were observed at ACE but were not discernable above the high intensities during the decay phase of event 1N at Ulysses. The fluence of these additional events at ACE was too small to add significantly to the electrons injected into the reservoir by the main event. Note that, although Figure 2 shows a remarkable similarity of the ACE and Ulysses electron intensities during event decay phases, the ,fluences at Ulysses during the periods in panels (a) and (b) were only -2-48 of those measured at ACE.

Table 1. Solar origin of the major SEP events at ACE

Flare Location X-ray Onset CMEnASCO (direction)

M2l c4/ M4/2B C2l

M7/3F X2/3B M8/2N

Southern High-Latitude Passage 2000 Beyond W90 290/0640 Halo-290/0727 (W) Beyond W90 29910845 Halo-299/0826 (W) S25E35 303/0130 No data s17E39 30611551 Halo-306/1626 (SE)

307/1626 (W) NlOW77 313/2300 Halo-3 i3/2306 (NW) N20W05 32910457 Halo-329/0530 (NW) NO7E50 330/0100 Halo-330/013 1 (E)

X2/2B M9/

M1/2F X1/2B X1/2B’

M6/2N X1/2B X1/3B M2/1N

Northern High-Latitude Passage 2001 S16E23 26710932 Halo-267/1031 (SE) S22W9 1 274/04 11 Halo-274/0530 (SW)

Halo-278/1030 (SW) S28EO8 282/1059 Halo-282/l 130 (SW) N16W18 292/0049 292/0127 (Earth) N15W29 292./1614 Halo-29U1650 (W)

Halo-293/0226 (Earth) S21E18 29511425 Halo-2951 1506 (SE) S16W21 29811442 298/1526 (SW) NO6W18 308/1603 Halo-308/1635 (W) ) S13E42 321/0448 Halo-321/0530 (E)

During solar maximtmr the situation is more complicated since

DISCUSSION

Since the discovery of the recurrent energetic particle intensity enhancements observed by Ulysses during the solar minimum orbit at latitudes well above the region where the ClRs form (~30’ around the solar equator), two models were proposed to explain the access of energetic particles to high latitudes. Simnett and Roelof (1995) argued for a direct magnetic connection between Ulysses and distant (>lO AU) CIRs that led Fisk (1996) ’ to develop a model of heliospheric magnetic field that allows magnetic connection between Ulysses at high latitudes and CLRs at lower latitudes and large distances (>lO AU). On the other hand, K&a and Jokipii (1995) argued for the latitudinal propagation of energetic particles in a diffusive transport across the average magnetic field due to a random walk or braiding of field lines in the latitudinal direction. the relativeiy simple structure of the

heliosphere is replaced by a much more complex magnetic field configuration (Smith et al., 2001; Sanderson et al., 2001). The good correlation between huge SEP events at ACE and Ulysses suggests an easy access of SEPs at both low and high latitudes. Smith et al. (2001) suggested that SEPs may access high latitudes via the complex magnetic field topology existing during solar maximum where field lines originating at low latitudes may reach the polar regions. Sanderson et al. (2001) suggested that energetic particles may access high latitudes due to the highly tilted current sheet during solar maximum. Whereas these situations may apply to the southern high-latitude passage, the simultaneous observation of SEP events when Ulysses was immersed in the northern high-speed coronal hole solar wind flow with uniform magnetic field (where only one magnetic sector was observed) seems to exclude the possibility that magnetic field lines originating at low latitudes reached Ulysses.

In order to study the access of energetic particles to high latitudes during the northern high-latitude passage, it is essential to analyze events with clean onsets. Figure 3 shows 178-290 keV electron intensities during the onsets of the events 1N (Figure 3a) and 3N (Figure 3b). Pre-event background intensities have been subtracted to better visualize the onset of the events. For the event lN, Ulysses was at the heliocentric distance of 1.90 AU and heliographic latitude of N78”; while for the event 3N Ulysses was at 2.19 AU and N77O. The time delay between the arrival of the 178-290 keV electrons at ACE and the onset time of the soft X-ray flare emission (vertical arrows

Low-Energy Particles at High Heliolatitudes 583

in Figure 3) was 1.4 hours and 0.6 hours for the events 1N and 3N, respectively; while for Ulysses the delays were 6.3 hours for the event 1N and only 3.0 hours for the event 3N. Those time delays are longer than those expected for energetic electrons (-250 keV) traveling along a nominal Parker’spiral connection. Pitch-angle distributions at the onset of these events (insets in Figure 3) show at ACE a very strong antisunward anisotropic flow, while at Ulysses the onsets were just weakly antisunward anisotropic and evolving rapidly (in a few hours) to isotropic. The rise-times of the events at ACE were faster than at Ulysses. Thus, the onsets of these SEP events at Ulysses suggest a more diffusive transport to high latitudes,than to ACE. Frop these observations, however, we cannot distinguish if the delayed onsets at Ulysses were due to either (1) a late injection of partiCles into thobe field lines connecting to Ulysses, (2) different transport conditions at high and low latitudes, (3) the possibility of particle diffusion ‘perpendicular to the mean magnetic field able to spread the particles in the latitudinal direction, or (4) the arrival of energetic particles at Ulysses resulting from filling the reservoir of the inner heliosphere (Roelof et al., 1992).

10’

% I 310 5

f L-2

E 3 910 i!

s P p) 10'

09 IO 11 12 13 14 15 16 17 18 19 20 21 15 16 17 18 19 20 21 22 23 24 01 02 03 267/2001 308/2001

Fig. 3: l-minute averages of 178-290 keV electron intensities as measured by HI-SCALEREFSGO (black traces) and EPAM/LEFSGO (gray traces) at the onset of events 1 N (a) and 3N (b). Vertical arrows indicate the onset of the associated soft X-ray flare emission. Insets show the pitch-angle .distributions for the 8 sectors of the detector LEFSGO in both instruments. Time units are in UT hours of the indicated day.

The study of the SEP events during the northern high-latitude passage has important implications for the, as yet, qualitative models proposed to explain the access of SEPs to high latitudes. [l] McKibben et al. (2OOlb) argued that the propagation of energetic particles across the mean magnetic field in the latitudinal direction was responsible for both the global filling of the heliosphere and the close correspondence between SEP events at high and low latitudes. Such an interpretation should consider both the transition between the non-uniform solar wind and complex magnetic field at low latitudes and the uniform solar wind and simpler magnetic field observed during the northern high-latitude passage, as well as realistic transport parameters to describe the transport of low-rigidity particles across the different magnetic field regimes (from low to high latitudes). [2] The interaction of multiple CMEs generated during the history of each of, the several active regions on the Sun during solar maximum may produce local increases in the magnitude &f the magnetic field and braiding of magnetic flux tubes beyond 1 AU, particularly where the field begins to fall off as l/r (where r is the heliocentric radial distance). Such structures may both delay the escape of energetic particles and redistribute them in both latitude and longitude as they re- enter the inner heliosphere, thus producing a leaky “reservoir” as suggested by Roelof et al. (1992). If this re- distribution of particles is responsible for the nearly simultaneous observation of SEP events at both spacecraft, it must act very efficiently in the latitudinal direction and very uniformly in order to produce the equal intensities observed in the decay phases of large SEP events. In addition, two observational facts that this model must explain are the weak antisunward anisotropies observed at the onset of the SEP events as well as the existence and extension of these reflecting boundaries before the onset of the SEP events. [3] The access of SEPs to high latitudes would be facilitated if particles were injected by broad solar sources of particles, probably associated with the halo CMEs observed at the origin of each one of these events. These sources may be able to inject energetic fiarticles to different latitudes, producing a global filling of the inner heliosphere, and consequently the observation of SEP events at widely separated locations. The different characteristics of the onsets of the events at ACE and Ulysses (including anisotropy, rise-times, and particle intensities) suggest different transport conditions at high and low latitudes. The lack of intensity gradients in the decay phase of the events, however, cannot be explained if only injection of particles from a single and exceptionally broad CMEdriven shock is assumed: (1) the shock is not

584 D.Lario etal.

always observed by both spacecraft, and (2) the particle transport in the downstream region of the shocks (between the converging magnetic field near the Sun and the moving shock) is most likely latitudinally dependent due to both the connection of the flux tubes to different points of the shock and the latitudinal variation of the particle source strength at the shock. Even when multiple CMEs may form a global moving disturbance; the particle population in the downstream region of these structures may depend on latitude, otherwise these turbulent regions must redistribute the pa&les uniformly and the conditions for particle transport must be the same at all latitudes. In summary, these observations clearly present a challenge to the models of particle propagation in the heliosphere.

ACKNOWLEDGMENTS

We acknowledge the use of the Ulysses Data System (UDS) and the COSPJN team for providing the data used in this paper. The work at JHU/APL was supported by ACE/EPAM under NASA contract NAS5-97271 and Ulysses/HI-SCALE under NASA research grant NAG561 13.

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E-mail address of D. Lario david.lario@jhuapl.edu Manuscript received 19 October 2002; revised 19 December 2002; accepted 19 December 2002