Magnetic fields at the solar wind termination shock.

Abstract

A transition between the supersonic solar wind and the subsonic heliosheath was observed by Voyager 1, but the expected termination shock was not seen owing to a gap in the telemetry. Here we report observations of the magnetic field structure and dynamics of the termination shock, made by Voyager 2 on 31 August-1 September 2007 at a distance of 83.7 au from the Sun (1 au is the Earth-Sun distance). A single crossing of the shock was expected, with a boundary that was stable on a timescale of several days. But the data reveal a complex, rippled, quasi-perpendicular supercritical magnetohydrodynamic shock of moderate strength undergoing reformation on a scale of a few hours. The observed structure suggests the importance of ionized interstellar atoms ('pickup protons') at the shock.

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LETTERS
Magnetic fields at the solar wind termination shock
L. F. Burlaga1, N. F. Ness2, M. H. Acun ˜a1, R. P. Lepping1, J. E. P. Connerney1& J. D. Richardson3
A transition between the supersonic solar wind and the subsonic
heliosheath was observed by Voyager 1, but the expected termina-
tionshockwasnotseenowingtoagapinthetelemetry1–4.Herewe
report observations of the magnetic field structure and dynamics
of the termination shock, made by Voyager 2 on 31 August–1
September 2007 at a distance of 83.7 AU from the Sun (1 AU is the
Earth–Sun distance). A single crossing of the shock was expected,
with a boundary that was stable on a timescale of several days. But
the data reveal a complex, rippled, quasi-perpendicular supercrit-
ical magnetohydrodynamic shock of moderate strength under-
going reformation on a scale of a few hours. The observed
structure suggests the importance of ionized interstellar atoms
(‘pickup protons’) at the shock.
This Letter discusses three termination shock crossings (TS-2, TS-
3andTS-4)thatwereobserveddirectlybyVoyager2(Fig.1).Atleast
two additional termination shock crossings (TS-1 and TS-5)
occurred when there were gaps in the telemetry. The multiple cross-
ings imply motions of the termination shock, possibly caused by its
large-scale motion or ripples propagating along it5–7. The times
between crossings are of the order of magnitude expected for ripples
propagating on the shock.
Observations by the magnetic field experiment on Voyager 2 (ref.
8)show nosignificant change inthedirection ofthe magnetic field B
across any of the three termination shock crossings in Fig. 1, indi-
cating that the shock was quasi-perpendicular at each encounter.
This geometry is necessary to provide the return of reflected ions
from the shock9,10, which ultimately produce some of the heating
behind a high Mach number (supercritical) shock.
The structure of TS-3 is shown in Fig. 2. The foot, ramp and
overshoot of TS-3 (ref. 9) moved past Voyager 2 in ,23min,
,1.5min and ,17min, respectively. There is an increase in the
magnetic field strength B in the foot, where B is compressed as the
solar wind slows down11. The foot contains gyrating protons that are
reflectedfromtheshock9,12,13.Afurtherabruptdecreaseinspeedtoits
lowestvalueoccursattheramp,owingtotheelectricpotentialassoc-
iated with cross-field currents and gradients in B in the ramp14,15.
PlasmawaveswereobservedattherampofTS-3(ref.16).Theratioof
the maximum B in the overshoot to B downstream of the shock is
,2.5, indicating that the ratio of the magnetosonic Mach number
Mmsto the critical fast Mach number (Mc<2) is ,5 (ref. 17), sug-
gesting that Mms<10for TS-3,consistent withasupercritical shock.
From the passage time of the ramp (1.5min) and the estimated
shock speed (68617kms21; ref. 11), the thickness of the ramp is
,6,000km. The ramp size is usually expressed in terms of the ion
inertial length c/vpi,u, where c is the speed of light and vpi,uis the
plasma frequency of an ion upstream. For TS-3, c/vpi,u<6,000km
and the thickness of the ramp is ,1 c/vpi,u.
The strength of TS-3, measured by the ratio of the B in the solar
wind upstream of the shock (B1) to that behind the shock (B2) is
B2/B151.760.1. Thecorresponding
N151.460.2<B2/B1, consistent with a shock strength of 1.660.2.
densityratio isN2/
The moderate strength of TS-3 might have been produced by the
pickup protons and other suprathermal ions that were present18, as
considered in models9,19–21.
The magnetic field strength oscillated quasi-periodically within
the ramp (Fig. 3). The ramp moved past Voyager 2 in ,90s, and
each oscillation passed the spacecraft in ,12.3s on average. The
shock speed gives a characteristic length of the fluctuations in the
ramp, l<1,000km50.15 c/vpi,u. An oscillating ramp containing
substructures withascale of0.2c/vpi,uwas observed at1 AU(ref.22),
where shocks are not influenced by pickup protons.
AfewhoursaftercrossingTS-3,Voyager2crossedthetermination
shock (TS-4) back into the solar wind. The re-entry into the solar
wind might have been caused by a ripple propagating on the shock’s
1NASA/Goddard Space Flight Center, Greenbelt, Maryland 20771, USA.2The Catholic University of America, Washington DC 20064, USA.3Massachusetts Institute of Technology,
Cambridge, Massachusetts 02139, USA.
a
b
0.08
0.06
0.04
0.02
0.00
90
270
180
360
0.0
0.1
0.2
0.3
0
–90
300
V (km s–1)
SD4min
d (deg)
l (deg)
B (nT)
200
100
18:00
Solar windHeliosheath
48 s
TS-2TS-3TS-4
48 s
192 s
4 min
HeliosheathSW
20:0022:00
Time (h:min)
00:00
Day 243 of 2007
Day 244 of 2007
02:00
c
e
d
Figure 1 | Three crossings of the termination shock, illustrating
reformation and the variability of its structure. The figure shows 48-s
averages of magnetic field strength B (a) and the standard deviation of B for
4-minute intervals (b), azimuthal angle l (c) and elevation angle d (d), and
the192-saverageofsolarwindspeedV(e)asafunctionoftimemeasuredin
days from the beginning of 2007. (Here the directions l and d of B are in
heliographic coordinates.) The time between the first and second of the
observed termination shock crossings (TS-2 and TS-3, respectively) is
,3.7h,andthatbetweenthesecondandthirdoftheobservedcrossings(TS-
3 and TS-4, respectively) is ,2.8h. The Larmor frequency of a proton with
the solar wind speed 300kms21gyrating in the magnetic field upstream of
theterminationshockisVL,u21<17min.ThetwoenhancementsinBatTS-
2 were separated in time by ,VL,u21. The uncertainty in B is 60.03nT.
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surface5,23. The structure of TS-4 (Fig. 1) is different from that of TS-
3. In the foot region, B(t) (where t is time) appears as a narrow peak,
and the overshoot is smaller than that of TS-3. Such a peak evolves
from the foot as a result of bunching of reflected solar wind ions
where they are turned back towards the shock6,12. The amplitude of
the overshoot decreases while the step-like foot evolves to a peak.
These changes were observed from TS-3 to TS-4 (Fig. 1). The struc-
ture of the termination shock evolved significantly within 2.7h. The
small peak in B at the front of TS-4 is expected to evolve to a new
ramp on this timescale, as part of the shock reformation pro-
cess10,12,24,25.
Reformation of the local structure of a supercritical quasi-perpen-
dicular shock was predicted by both hybrid and full particle simula-
tions10,13,25for a shock with large Mmsand/or a low b, where b is the
ratio of thermal pressure to magnetic pressure. Recall that we esti-
mated that Mms<10 for TS-3. Neglecting pickup protons, b50.04
in the solar wind upstream of TS-3, so that b might be small even if
the pickup protons contribute significantly to it. Reformation is a
patchy cyclic shock reformation process with a characteristic time of
the order of the downstream gyroperiod25.
Further evidence for local reformation of the termination shock is
provided by the qualitative transformation of the shock structure
during the ,3.9-h interval between TS-2 and TS-3 (Fig. 1). At the
frontofTS-2,thebulkspeedVincreasedcontinuously,ratherthanin
a step-like form. Instead of a simple ramp-overshoot structure in B,
there were two narrow enhancements in B resembling solitons, in
eachofwhichtherewasachangeinBcomparabletothatintheramp
of TS-3. A shock structure at 1 AU resembling that of TS-2 was
reported in fig. 5 of ref. 26.
Received 19 February; accepted 15 April 2008.
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9.
a
b
c
d
23:31
Time (h:min)
Day 244 of 2007
Day 243 of 2007
00:0000:28 00:57
192 s
48 s
Solar
wind
Foot
0.30
0.25
0.20
0.15
0.10
0.05
0.00
270
180
45
0
–45
–90
300
200
100
Overshoot
Undershoot
Heliosheath
Ramp
TS-3
01:26
V (km s–1)
d (deg)
l (deg)
B (nT)
Figure 2 | TS-3 is a supercritical quasi-perpendicular shock. The 48-s
averages of the magnetic field strength B (a) and its directions l (b) and d
(c) are shown here together with the 192-s average of the solar wind speed V
(d) across TS-3. The magnetic field strength profile shows the classical
features of a supercritical quasi-perpendicular shock: a ‘foot’, ‘ramp’,
‘overshoot’, ‘undershoot’ and smaller oscillations, in that order.
a
0.3
0.2
0.1
0.0
90
45
–90
00:09
–45
0
0.48 s
360
270
180
Foot Ramp
TS-3
Overshoot
b
c
00:10 00:11
Time (h:min, day 244 of 2007)
00:1200:13
d (deg)
l (deg)
B (nT)
Figure 3 | TheinternalstructureoftherampofTS-3. Thestructureisbased
on observations of the magnetic field strength B (a) and its directions l
(b) and d (c) at 0.48-s intervals. The magnetometer on Voyager 2 sampled
the vector magnetic fields in the termination shock at a rate of
2.08sampless21, and the spacecraft was able to transmit all of this
information,makingitpossibletodeterminethecomplexinternalstructure
of the ramp shown here.
LETTERS
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Acknowledgements We thank T. McClanahan and S. Kramer for support in the
processing of the data. We also thank D. Berdischevsky for computing the
instrument zero level corrections for the data in this paper, and for helping to solve
theproblemscreatedbytheerroneousdecodingofaspacecraftsystemscommand
senttoVoyager2in2006.N.F.N.waspartiallysupportedbyaNASAgranttoCUA.
Author Information Reprints and permissions information is available at
www.nature.com/reprints. Correspondence and requests for materials should be
addressed to L.F.B (Leonard.F.Burlaga@nasa.gov).
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