PROPAGATION OF THE BASTILLE DAY 2000 CME SHOCK IN THEOUTER HELIOSPHERE

CHI WANG1,2, JOHN D. RICHARDSON1 and LEN BURLAGA3

1Center for Space Research, Massachusetts Institute of Technology, Cambridge, MA 02139, U.S.A.2Laboratory for Space Weather, Chinese Academy of Sciences, Beijing, China

3Goddard Space Flight Center, Greenbelt, MD 20771, U.S.A.

(Accepted 6 September 2001)

Abstract. The Bastille Day (14 July) 2000 CME is a fast, halo coronal mass ejection event headedearthward. The ejection reached Earth on 15 July 2000 and produced a very significant magneticstorm and widespread aurora. At 1 AU the Wind spacecraft recorded a strong forward shock with aspeed jump from ∼ 600 to over 1000 km s−1. About 6 months later, this CME-driven shock arrivedat Voyager 2 (∼ 63 AU) on 12 January 2001 with a speed jump of ∼ 60 km s−1. This provides a goodopportunity to study the shock propagation in the outer heliosphere. In this study, we employ a 2.5-DMHD numerical model, which takes the interaction of solar wind protons and interstellar neutrals intoaccount, to investigate the shock propagation in detail and compare the model predictions with theVoyager 2 observations. The Bastille Day CME shock undergoes a dramatic change in character fromthe inner to outer heliosphere. Its strength and propagation speed decay significantly with distance.The model results at the location of Voyager 2 are in good agreement with in-situ observations.

1. Introduction

One of the most powerful solar flares of the current solar cycle was recorded byNOAA satellites and Solar and Heliospheric Observatory (SOHO) on Bastille Day(14 July) 2000. Soon after the solar flare, images from the Large-Angle Spectro-metric Coronagraph instrument on SOHO showed that a large, fast-moving coronalmass ejection (CME) was headed earthward at ∼ 1300 to 1800 km s−1. The ejec-tion reached Earth on 15 July and was very geo-effective, producing a magneticstorm and widespread aurora. The solar wind variables, including the plasma andmagnetic field observed by the IMP 8 and Wind spacecraft at 1 astronomical unit(AU), show very dramatic changes within a few days. Figure 1 shows the hourlyaverage plasma and magnetic field measurements (speed, density, temperature, andmagnetic field magnitude) from 9 to 22 July 2000 (day of year (DOY) 191 to DOY204 of 2000) from Wind. On 14 July (DOY 196) the plasma parameters stay fairlyflat until about 15:30 UT when a forward shock with a speed increase to over700 km s−1 passes the spacecraft. On 15 July (DOY 197) the speed declines untila large forward shock arrives near 14:37 UT. The shock is clearly identified by theabrupt speed increase from ∼ 600 to over 1000 km s−1. This shock also exhibitsstrong density, temperature and magnetic field enhancements. After 16:00 UT, thespeed increases further while the density, temperature and magnetic field decline

Solar Physics 204: 411–421, 2001.© 2001 Kluwer Academic Publishers. Printed in the Netherlands.

CD

ROM

412 CHI WANG ET AL.

Figure 1. Solar wind parameters observed at 1 AU by the Wind spacecraft from 9 to 22 July 2000(DOY 191–204): (top) speed, (second) density, (third) temperature, and (bottom) magnetic fieldmagnitude. A strong forward shock was observed on DOY 197.

markedly. The plasma data from IMP 8 show a similar behavior, but have a slightlyhigher density.

While Wind and IMP 8 observe the solar wind at 1 AU, Voyagers 1 and 2 con-tinue to explore the outer heliosphere. At the time of this event, Earth and Voyager 2were at nearly identical heliolongitudes (∼ 287◦), but were separated by ∼ 61 AUin radial distance and ∼ 25◦ in heliolatitude. After about six months, this CME-driven shock was observed by Voyager 2 on 12 January 2001 with a speed jumpof ∼ 60 km s−1 (see the paper by Burlaga et al., 2001, for a detailed descriptionof this event). This provides a good opportunity to study shock propagation in theouter heliosphere.

The shock propagation and interaction in the heliosphere has been studied in-tensively (see the work by, e.g., Hundhausen, 1985; Burlaga et al., 1986; Whang,1991, and references therein). However, these studies could not take advantage ofrecent spacecraft observations made out to 63 AU and did not include the effect

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of pickup ions, which play an important role in the distant heliosphere. It hasbeen suggested by Axford (1972), Holzer (1972), Vasyliunas and Siscoe (1976),and many others that interstellar neutrals should penetrate into the heliosphere,where they are ionized by photoionization, charge exchange, and electron impactionization and picked up by the interplanetary magnetic field (IMF), introducinghot pickup ions into the solar wind. Pickup ions have now been observed directly atheliocentric distances less than 5.6 AU (Möbius et al., 1985; Gloeckler et al., 1993,1994). Observational evidence of an increased pressure associated with pickupions in the outer heliosphere comes from studies of pressure-balanced structures(Burlaga et al., 1994). The slowdown of the solar wind due to pickup ions havebeen investigated by Richardson et al. (1995) and Wang, Richardson, and Gosling(2000a,b). Since the pickup ions dominate the internal energy of the solar windbeyond the ionization cavity (∼ 6–10 AU), the properties of shocks propagatingin the inner and outer heliosphere are very different. The most obvious differenceis the change in propagation speed due to the increase of the sound speed/fastmagneto-acoustic speed. The impact of pickup ions on shock propagation and solarwind structures from the inner to outer heliosphere has been investigated by variousauthors (Zank and Pauls, 1997; Rice and Zank, 1999; Whang, Lu, and Burlaga,1999; Wang, Richardson, and Gosling, 2000b).

We have already used a 1-D MHD model, which includes the effect of pickupions, to predict the Voyager 2 observations of the Bastille Day CME shock (Wang,Richardson, and Paularena, 2001). The prediction turned out to be in fair agree-ment with the actual measurements made by Voyager 2. However, the model isone dimension and the input data did not include the magnetic field, which wasunavailable at that time. In this study, we extend our 1-D model to 2.5-D (3-D with azimuthal symmetry) including the three components of the velocity andmagnetic field. Furthermore, we take advantage of the recently-recovered plasmaand magnetic field measurement of the Bastille Day 2000 CME shock from Wind(Figure 1). The model equations and methods of solution are presented in Section 2.The numerical results and their comparison with the Voyager 2 observations areshown in Section 3. Section 4 contains the summary and concluding remarks.

2. Numerical Model

In this study, a time-dependent, 2.5-D MHD model, which includes the interactionof the solar wind protons with the interstellar neutral hydrogen, is used. We assumethat all variables are independent of the azimuthal angle φ. In spherical coordinates(r, θ, φ) the equations of mass continuity, momentum, and energy conservation intwo spatial dimensions (r, θ) are

∂ρ

∂t+ 1

r2

∂

∂t(r2ρVr) + 1

r sin

∂

∂θ(sin θρVθ) = Qn , (1)

414 CHI WANG ET AL.

∂

∂t(ρVr) + 1

r2

∂

∂r

[r2(ρV 2

r + p + 1

8π(B2

θ + B2φ − B2

r )

]+

+ 1

r sin θ

∂

∂θ

[sin θ

(ρVrVθ − BθBr

4π

)]−

−ρ(V 2θ + V 2

φ )

r− 2p

r− B2

r

4πr+ ρGM�

r2= Qmr ,

(2a)

∂

∂t(ρVθ) + 1

r2

∂

∂r

[r2

(ρVrVθ − BrBθ

4π

)]+

+ 1

r sin θ

∂

∂θ

[sin θ

(ρV 2

θ + p + B2 − 2B2θ

8π

)]− ρV 2

φ

rcot θ+

+ρVθVr

r− BθBr

4πr−

(p + B2 − 2B2

θ

8π

)cot θ

r= Qmθ,

(2b)

∂E

∂t+ 1

r2

∂

∂r

[r2

((E + P ∗)Vr − Br

(B · V)4π

)]+

+ 1

r sin θ

∂

∂θ

[sin θ

((E + P ∗)Vθ − Bθ

(B · V)4π

)]+

+ρGM�Vr

r2= Qe,

(3)

where

P ∗ = p + B2

8π, E = 1

2ρV2 + p

γ − 1+ B2

8π. (4)

The variation of the magnetic field is governed by Faraday’s law of induction

∂B∂t

= × (V × B) (5)

and

· B = 0. (6)

All notation is conventional and is used without description.In our model, we follow the approach of Zank and Pauls (1997) in the treatment

of the interaction of the solar wind protons with the interstellar neutral hydro-gen. The terms Qn,Qm,Qe represent charge exchange source terms of number,momentum and energy, and were summarized in the work by Wang and Belcher(1999). The cold neutral density distribution nH(r) is taken as (Vasyliunas andSiscoe, 1976)

SHOCK PROPAGATION 415

nH(r) = nH∞e−λ�/r sin�, uH = uH∞, (7)

where λ = 4 AU and uH∞ = 20 km s−1. The angle � is the angle relative to thedirection of the incoming interstellar neutral hydrogen flow, which has an eclipticlatitude β of 7◦ and longitude λ of 252◦ (Lallement et al., 1990). The subscriptinfinity refers to the boundary values (the boundary being the termination shock).Throughout the calculation, the electron temperature is assumed to be equal to thetemperature of the protons, and the ratio of specific heat index γ is taken to be 5

3 .The MHD equations (1)–(6) are solved using the piecewise parabolic method

(PPM) algorithm (Collella and Woodward, 1984; Dai and Woodward, 1995). ThePPM algorithm is a high-order extension of the Godunov scheme (third-order ac-curate in smooth regions of flow) which can capture shocks and discontinuitieswithin 1–2 grid points. The simulation box extends from 0◦ to 180◦ in θ-direction,and from 1 AU to 70 AU in r-direction. The angular grid spacing is constant at 2◦,and the radial grid spacing increases as a numerical series �ri = (1 + α)�ri−1

with �r1 = 0.05 AU and α = 0.05. All quantities can be specified independentlyat the inner boundary (r = 1 AU), since it belongs to an inflow boundary with theradial velocity larger than the fast magneto-acoustic speed. At the top (r = 70 AU)the radial magnetic flux (r2Br ), the proton flux (r2nVr), velocity Vr, Vθ , and thetemperature T are evaluated by linear extrapolation.

3. Numerical Results and Comparison with Voyager Observations

The simulation procedure consists of two steps: First the values of solar wind para-meters at the inner boundary are fixed and we run the simulation until a steady stateis reached. This solution thus represents a pickup ion mediated heliosphere. Then,we introduce the Bastille Day CME disturbances observed by Wind (Figure 1) asinput into the steady state heliosphere and follow the subsequent evolution of thisCME shock through the outer heliosphere until it passes the location of Voyager 2.

The physical quantities in the solar wind at 1 AU used for calculating the steadystate are the velocity Vr = 400 km s−1, Vθ = 0, the number density ne = 7 cm−3,the temperature T = 1.5 × 105 K, and the magnetic field magnitude B = 6 nT.These values are applied across the entire inner boundary; no dependence on thelatitude has been considered. This approximation is reasonable near solar maxi-mum. A comparison of solar wind near Earth and at Ulysses at high latitudes showthat the latitudinal variation is small (McComas, 2000). Figure 2 shows the radialvelocity Vr , temperature T, number density n, and the azimuthal component ofmagnetic field Bφ, profiles after the solar wind solution has relaxed to a dynamicequilibrium. The solid lines are results along the equatorial direction (θ = 90◦),and the dotted lines are along the polar direction (θ = 0◦). As expected, the pickupions slow down and heat the solar wind in the outer heliosphere. The slowdownof the solar wind in the equator is slightly larger and the temperature is slightly

416 CHI WANG ET AL.

Figure 2. Steady state heliosphere. From top to bottom: shown are the radial velocity, temperature,number density, and azimuthal component of the magnetic field profiles (solid lines: θ = 90◦; dottedlines: θ = 0◦).

higher than in the polar direction, since the equator is closer to the direction ofthe incoming interstellar neutrals. However, the difference is very small. FromEquation (7), the interstellar neutral hydrogen distribution is not sensitive to theangle � in the upwind direction (−90◦ < � < 90◦), so the profiles along differentdirections in the upwind hemisphere are actually very similar. Thus, we expect theshock will propagate more or less symmetrically in the upwind hemisphere in theouter heliosphere.

After the steady state heliosphere has been reached, we are in a position tostudy the propagation of the Bastille Day CME shock. The Wind data are thenintroduced at the inner boundary at 1 AU for all directions. Figure 3 shows thepropagation and evolution of the Bastille Day CME shock in the outer heliosphere.Shown are solar wind speed contours with contour values equally spaced from420 to 500 km s−1 in 20 km s−1 increments at four different times (10, 50, 100,and 150 days after the CME passed Earth) for a portion of the simulation time

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Figure 3. Propagation of the Bastille Day CME shock in the outer heliosphere. Shown are speedcontours with contour values equally spaced from 420 to 500 km s−1 in 20 km s−1 increments. Thelocation of Voyager 2 is also labeled (�).

418 CHI WANG ET AL.

Figure 4. (a) The shock strength (density compression ratio) and (b) the speed decay with distanceof the leading forward shock.

domain. The whole history of the evolution as a movie is available at our Webpage (http://space.mit.edu/∼cw/bastille.html) and at the accompanying CD-ROM.After 10 days, the speed structure is relatively simple and confined to a limitedregion (∼ 1.5 AU ) between a forward and reverse shock pair (Figure 3(a)). Af-ter they encounter the ionization cavity, new shocks are introduced in a manneranalogous to the withdrawal of a piston in a tube (Zank and Pauls, 1997), sincethe plasma temperature starts increasing and so does the fast magneto-acousticspeed. The evolution and interaction of these disturbances (shocks and discon-tinuities) result in a complicated solar wind speed pattern as illustrated in Fig-ure 3(b), and the disturbance region expands dramatically. As these solar windstructures continue to propagate into the distant heliosphere, they dampen sig-nificantly or disappear, resulting in relatively simple speed structure again in thedistant heliosphere (Figures 3(c) and 3(d)). The leading forward shock decayssignificantly while propagating in the outer heliosphere. Figure 4 shows that theshock strength indicated by the density compression (a) and propagation speed (b)decrease with distance in the direction of Voyager 2 (∼ 25◦ south). A very strongleading forward shock with a compression ratio of close to 4 forms in the innerheliosphere. The compression ratio decreases steadily to ∼ 1.8 at the location ofVoyager 2 (∼ 63 AU). The propagation speed decreases from above 700 km s−1

to ∼ 520 km s−1. Except for the leading forward shock, the shocks/discontinuitieshave virtually disappeared at the location of Voyager 2 about 6 months later. On 12January 2001, Voyager 2 observed the Bastille Day CME shock. The speed jumpedfrom ∼ 380 km s−1 to ∼ 440 km s−1. The magnetic field magnitude also jumpedfrom ∼ 0.06 to ∼ 0.13 nT. Figure 5 shows the comparison of the model results(dotted lines) with the Voyager 2 observations. From the top to bottom panels, weshow the speed, number density and the magnetic field magnitude, respectively.Overall, the model results match the observations very well. The timing, the speedand magnetic field profiles are in good agreement with the observations. However,there is a discrepancy in the density profile. The density depletion region right afterthe shock front is not expected from the model. One possible explanation is that the

SHOCK PROPAGATION 419

Figure 5. Comparison of model prediction at the location of Voyager 2 with the observations. Fromtop to bottom panels: shown are the speed, number density and the magnetic field magnitude,respectively.

ambient solar wind density decreased at nearly the same time as the shock arrived,so that the actual density change was small. Another possibility is that non-MHDeffects are important; density variations are observed behind other shocks seenby Voyager 2, although the density structure at the shock in this case is unique.Examination of other strong shocks observed by Voyager 2 in the outer heliospherehas not revealed a single case with the same characteristics. It is unclear whatprecise mechanism is responsible, and this point needs future investigation.

4. Summary

The Bastille Day (14 July) 2000 solar flare is the most powerful solar event of thecurrent solar cycle observed to date. It was followed by a large, fast-moving haloCME heading for Earth at a speed of ∼ 1300 to 1800 km s−1. At 1 AU the Windand IMP 8 spacecraft recorded a large forward shock with a strong speed increasefrom about 600 to over 1000 km s−1. This CME shock is associated with a strongdensity, temperature and magnetic field enhancement. About 6 months later, this

420 CHI WANG ET AL.

CME-driven shock arrived at Voyager 2 (∼ 63 AU) on 12 January 2001 with aspeed jump of ∼ 60 km s−1. The magnetic field jumped from ∼ 0.06 to ∼ 0.13 nT.

In this paper, we use a 2.5-D MHD numerical model to study the propagation ofthe strong CME-driven shock in the outer heliosphere in the r−θ plane. This modeltakes into account the interaction of the solar wind protons with the interstellarneutrals. The effect of pickup ions is important in the outer heliosphere. They notonly slow down and heat the solar wind flow, but play an important role in shockpropagation in the outer heliosphere. In the upwind hemisphere facing the incom-ing interstellar neutrals, the characteristics of shock propagation are not sensitiveto the angle relative to the direction of the interstellar wind. The Bastille Day CMEshock undergoes a dramatic change in character from the inner to outer heliosphere.The leading forward shock continues to decay dramatically with distance as itpropagates outward through the heliosphere. All other shocks/discontinuities re-sulting from shock evolution and interaction dissipated by the location of Voyager 2(∼ 63 AU). The model results are in good agreement with the in-situ observations.

Acknowledgements

This work was supported under NASA contract 959203 from the Jet PropulsionLaboratory to the Massachusetts Institute of Technology. C. Wang is grateful tothe one-hundred talent program of the Chinese Academy of Sciences. The authorsthank A. J. Lazarus and A. Szabo for providing the Wind plasma data and magneticfield data, respectively.

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