ULYSSES-VOYAGER STUDY OF ENERGETIC PARTICLES ASSOCIATED WITH THE INTENSE SOLAR ACTIVITY OF MARCH/JUNE 1991

F. B. McDONALD 1, E. KEPPLER 2 and M. K. REUSS 2 1Institute for Physical Science and Technology, University of Maryland, College Park, MD

USA 2Max Planck Institute far Aeronomie, Katlenburg-Lindau, Germany

Abstract. The intense solar activity centered in March and June 1991 produced some of the largest interplanetary disturbances over the past several solar cycles. For these events the Ulysses EPAC energetic particle observations near 3 AU are compared with those of the Voyager 2 CRS experiment near 35 AU. At Voyager 2 there is a single long-lived event extending over a period of some 6 months while the Ulysses data shows the imprint of individual events as well as the formative stages of the longer lived structure. The average intensity gradient is - -17% AU between the 2 spacecraft. At both locations the energy spectra can be represented by an exponential in momentum. The characteristic momentum for protons, (Po)H is on the average 4-5 times larger at 35 AU than at 3 AU and there is a significant change in the (Po)He/(Po)/_i ratio. However the average H to He ratio is in the range 20-25 for both sets of measurements.

1. Introduction Solar/interplanetary energetic particle (SLIP) events evolve with increasing heliocen- tric distance; inside some 5 AU they retain the characteristics of individual solar particle events; (Hamilton, 1977; Zwickl and Webber, 1977), by 10 AU the particle enhancements may extend over a solar rotation and display more pronounced shock-associated features; (Drfge et al., 1992). Beyond some 25 AU the individual increases in particle intensity observed in the inner heliosphere have lost their identity and there emerges a new type of particle event that represents the evolution and superposition of multiple energetic panicle events from the inner heliosphere that have occurred over several solar rotations and the continuing acceleration of these ions by large scale interplanetary disturbances known as Global Merged Interaction Regions (GMIRs) (Van Allen and Mihalov, 1990; McDonald and Selesnick, 1991; Decker et at., 1991). The energetic particle events reflect the evolution and development of these large scale disturbances in the outer heliosphere.

Originally identified by Burlaga et at. (1984, 1985, 1993) GMIRs evolve with increasing heliocentric distance through the coalescence of interplanetary shocks produced by CMEs and high speed solar wind streams. These systems are a major element in producing the long term 11 year cosmic modulation (le Roux & Potgieter, 1993; McDonald et at., 1993; Burlaga et al. 1993) and they take on added importance with their role as the probable trigger for the strong outburst of low frequency ( - 2 - 3 K Hz) radio emission recently observed by the Voyager 1/2 Plasma Wave System (PWS) (Gurnett et al. 1993). A key question is whether the MeV ions and electrons observed in the outer heliosphere several months after an outburst of solar activity are the solar energetic particles accelerated in the inner heliosphere and partially re-energized by the shocks within the GMIR or are they more recently accelerated by these shocks from a seed population of superthermal

Space Science Reviews 72: 359-364. �9 1995 Kluwer Academic Publishers. Printed in the Netherlands.

360 F.B. McDONALD ET AL.

solar wind ions. In March 1991, some 20 months after the period of maximum solar activity there

was a new increase in activity with 35 major flares (x-ray classification M-5 or higher) during-the month. The great majority of these events occurred in the southern hemisphere of the Sun. This activity decreased over the next several months but in June the premier active region of cycle 22 appeared, producing in a single passage 6 X-10 (or greater) flares, all of them in the northern hemisphere. The relative temporal isolation of this intense outburst of solar activity and its marked north-south asymmetry combined with the data from Ulysses in the inner heliosphere and Voyager 2 in the outer heliosphere provide a unique opportunity to study the generation of a GMIR and the acceleration and transport of low- energy ions over a wide range of heliocentric distances.

In this paper the low energy H and He data from the Ulysses EPAC experiment near 3 AU and the Voyager 2 CRS experiment near 35 AU are used to study the evolution from many events in the inner solar system to a single event lasting almost 6 months in the outer heliosphere. Special emphasis is placed on both the spectral information and the H/He ratio measured at the 2 locations as a possible tracer of the origin of the ions observed near 3 AU.

2. Observations The time history of 0.5-1.0 MeV protons (24 hr avg)

R S /C-Sun 1.20 2.49 3 .69 4 .74 I I I 0 4 , , f , ~ , , , ,

H (0,52-0.99 Me.V) ULYSSES _

o~ '" ..... i,,"

n ' ' ; He (0.40-3.30 MeV/N) ,f

I I 1991.8

136.8 AU t - >

= 1 ~ ................................. r ............. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

10 -2

~o~ -- " �9 w i ~ ~ .~-J~

1991.2 1991.7 1992.2

Fig. 1: (a) Intensity of 0.52-0.99 MeV H and 0.4-3.30 MeV/n He measured by the Ulysses EPAC experiment between 1.2 and 4.74 AU. (24 hr Averages); (b) Intensity of 1.8-2.6 and 4.245.2 MeV H measured by the Voyager 2 CRS Experiment between 34.1 and 36.8 AU

103

102

101

lO o

10-=

10-2i 10 -3

-4: I I I I I I I O D o Y 1991.0 1991.4

I0~ 34.~ Au ] i

from the Ulysses EPAC experiment for the period 1990.9 to 1991.9 are shown in Figure 1 along with the Voyager 2 data near 35 AU for 1.8-2.6 and 4.2-6.2 protons for a time period that has been displaced by 0.3 years (this would correspond to a plasma transit velocity of -550 km/s b e t w e e n t h e 2 spacecraft). At V-2 the abrupt increase in both energy intervals on 1991.4 is associated with a strong shock as revealed in the solar wind [Belcher et al., 1993] and interplanetary magnetic field data. These MeV ions are clearly under the control

U L Y S S E S - V O Y A G E R S T U D Y O F E N E R G E T I C P A R T I C L E S 361

of the large-scale disturbances in the interplanetary medium. The 3 small increases in 1.8-2.6 MeV H at 1991.35, 1992.05 and 1991.12 appear to be produced by co- rotating interaction regions. In the inner heliosphere the Ulysses 0.5-1.0 MeV proton data show both the individual solar events and the nascent structure of the long-lived event observed in the outer heliosphere. The two periods in the EPAC data that extend from (a) 1991.15-1991.29 and (b) 1991.35-1991.47 reflect the intense activity centered around March and June of that year. Previously Dr6ge et al. (1992) had observed the intermediate state of these events at heliocentric distances out to 30 AU. 10 ~

The main diagnostics available 1 0 4 from these joint data sets for

understanding the interplanetary z lo-2 t r anspo r t and accelera t ion ~. processes are the H and He ~ 1~ energy spectra and the H/He ratio ~ 10" and their changes with time and

50.0 wi th he l iocent r i c distance. A t V-2, "~ the H and He spectra were best fit 40.0

by either an exponential in v momentum/nucleon (i.e., velocity ~ 30.0 at these low energies) or in kinetic energy (McDonald et al., 1994). 20.0 The characteristic values for (Po)rI

10.0 and (Po)He (generally averaged 48.0 over a 10 day period) are plotted in Figure 2 along with the H/He ~ 38.0 ratio for 1.8-2.6 MeV/n ions. The representation as a function of Y:

28.0 momentum used in this plot has the interesting feature that (Po)n ~" (Po)He immediately at or just 18.0 after the passage of 2 of the 3 largest interplanetary disturbances in this GMIR (Fig. 2). Over the rest of the event, the ratio of (Po)He/(Po)i{ = 0.67 which is very close to the average value of 0.75 reported by Mewaldt (1979) for a number of CIR events at 1 AU. [Mewaldt et al. expressed their ratios in terms of magnetic rigidity, R with (Po)ite/(Ro)H 1.5 which is equivalent to

, ' i T , !

v.2! . , .e 188-2.3 MeV,0!c I I ...................

I i I [

.................. + .......................... % i ................. § ................... i ....................

1991.3 1991.6 1991,9 Year

Fig. 2.: The characteristic values (Po)rI and (Po)He obtained by fitting the V-2 H and He data as an exponential in momentum/nucleon (i.e., proportional to particle velocity at these energies), The averages are generally taken over a 10 day interval. Also shown for reference is the time history of 1.88-2.38 MeV/n H and He and of 4.2- 6.2 MeV H and the H/He ratio for 1.9-2.4 MeV/n ions. The vertical - - - lines mark the passage of the 3 largest interplanetary shocks in the GMIR.

362 F.B. McDONALD ETAL.

(Po)He/(Po)H = 0.67 since (Ro)He = 2(Po)He ]. The H/He ratio in Figure 2 is generally in the range 19-28 (except at the time of the intensity minimum in 1991.5), with an average value of 25.3.

At Ulysses the characteristic spectral values and the H/He ratios are obtained from 24 hr averages. (Figure 3). This value of (Po)H for period a is 4.6 MV and

25 2 0 - 1 5 - I0-- 5 -

2~ 2t) -- I5-- I0--

ULYSSES l l ) r l ) r l ) , l , , f , , l , , l , , l , , l , , [ , , l , , l , ) ) ) , [ , , l , ) l , , l , ) l , , ) , , l * , l , , I , , I [

l~,(cxponemlal lit 1o momentum spectnam) IMe V/el - vrotons EPAC

. . . . .

H/He ratio (0.7 Me V/'N) - -

1 0 2 8 " ~ ---------"

/ ~o~_ ~ ( . .OM m)C':) . . . . . ' . p( - ",9 ................

R S/C-Sun 1.20 1.83 2.49 3.11 3.69 4.23 4.74

Fig. 3: The characteristic values (Po)H and (Po)He obtained by fitting the Ulysses EPAC data as an exponential in momentum/nut (24 hr averages) also plotted is the H/He ratio at 0.7 MeV/n and the time history of 0.52-0.99 MeV H and 0.4-3.30 MeV/n He.

5.6 MV for period b which is a factor of 4-5 smaller than that measured at V-2. The average value of the ratio of (Po)He/(Po)a = 2.22 and 1.45 for the 2 periods is very different from the ratio of 0.67 observed at 35 AU. However, at Ulysses the H/He ratio over the energy range 0.5-1 MeV/n is 21.4 and 25.4 for the 2 periods which is in good agreement with the Voyager rate of 25.3.

To obtain an average intensity gradient, Gr, between the 2 spaceacraft, it is necessary to extrapolate the 1.8-2.4 MeV intensity at V-2 t_o 0.5-1.0 MeV assuming an exponential in momentum for the spectral form with Po = 24 MV. Averaging the V-2 data over the 2 peak regions and the Ulysses data over the a and b periods gives a value of Gr between 3 and 35 of -17%/AU. This intensity gradient is comparable to the average of -45% AU between 1 and 10 AU and -15% AU from 10-30 AU for 11-21 MeV ions measured by Dr6ge et al. (1992) for a series of earlier S/IP events.

ULYSSES-VOYAGER STUDY OF ENERGETIC PARTICLES 363

3. Discussion At 35 AU, the interplanetary disturbances associated with the March/June 1991 solar activity have formed a Global Merged Interaction Region whose properties are reflected in the long-lived energetic particle event as observed for 2.5 and 5 MeV protons (Fig. 1). In the inner heliosphere the Ulysses EPAC data show both the individual solar energetic particle events as well as the formative stages of the long-lived event. There are four principle features that stand out in comparing the properties of the energetic particles at the 2 heliocentric distances:

(i) There is a strong negative gradient of - 17% AU for 0.5 - 1.5 MeV H (ii) The momentum spectra for both H and He become increasingly harder at

the larger heliocentric distances with (Po)H increasing by a factor of 4-5 between 3 and 35 AU.

(iii) Except in the spatial region near shocks (Po)He ------ 0"67(Po)H at 35 AU. However at Ulysses (Po)He ranges from 1.45 to 2.2 (Po)H"

(iv) Over most of this period the H/He ratio ~-. 25 at both spacecraft. Understanding the first 2 factors will require detailed modeling of the accelera-

tion and transport processes. The increase in (Po)H is very striking and provides evidence that these ions are continuing to be accelerated.

Of particular importance is the observation that (Po)H ~ (Po)He at V-2 near the time of the passage of 2 of the 3 strongest shocks in the GMIR. Such a velocity dependence is consistent with the predictions of the model of Fisk and Lee (1980) who considered shock acceleration in the forward and reverse shocks of co-rotating interaction regions. The principal loss process in this model are adiabatic energy losses in the interplanetary medium and the radial diffusion coefficient, Kr, was assumed to be of the form Kovr where K o is a constant, v is the particle velocity and r is the heliocentric distance. This leads to spectra of the form,

exp 6K~ f(~) (where f(/3) is a function of the shock parameters), which is in Vsw

agreement with the observed velocity dependence near the shock. Previously, particle acceleration by the major interplanetary shocks passing

Ulysses in the last half of March 1991 had been extensively studied using the EPAC data (Krupp et al. 1992; Naidu et al. 1992). In a detailed analysis of the March 23rd event Naidu et al. obtained a value of (Po)H = 3.8 MV and a diffusion mean free path of 0.29 AU at 700 kev.

There is a growing consensus that the solar energetic particles produced in large solar events are accelerated by shock waves generated by coronal mass ejections with the seed population being the ambient coronal plasma above the active region (Reames et al. 1991; Reames 1992). The Ulysses EPAC observations in March clearly establish that the CME driven shocks are still accelerating particles and at 35 AU this process continues.

It is interesting to note that the ratio of (Po)He/(Po)H and of ~ H e closely resemble that of CIR events (Mewaldt et al., 1979; McGuire et al., 1978). Previous measurements of the H/He ratio for solar energetic particles at higher energies had shown a large variation from event to event. However recent studies in the 1-4

364 F.B. McDONALD ET AL.

MeV/n range (Mazur 1991; Reames 1992) have found a much smaller variation and Reames finds a weighted mean average of 21.1 + 1.6.

The most plausible explanation for the Ulysses/Voyager observation is that the 0.5-1 MeV and even lower energy ions observed near 3 AU are the seed populat ion for the MeV ions at Voyager and this seed population is continually being replenished at least in the inner heliosphere. The observed (Po)rle/(Po)ri ratio at 35 AU would appear to require that additional loss processes be included in the Fisk-Lee (1980) model and these loss processes are similar for CIRs in the inner heliosphere and GMIRs in the outer heliosphere.

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