Evidence of interplanetary magnetic reconnection

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 100, NO. A10, PAGES 19,903-19,910, OCTOBER
1995
Ulysses observation of a noncoronal mass ejection flux rope:
Evidence of interplanetary
magnetic reconnection
M. ,B:. ~o1dwin,1 J. L. Phillips, J. T. Gosling, E. E. Scime, D. J. McComas,
and ~. J. .l:Same
Spaceand Atmospheric ScienceGroup, Los Alamos National Laboratory, Los Alamos, New Mexico
A. Balogh
and R. J. Forsyth
Spaceand Atmospheric Physics,Imperial College, London, England
Abstract. A well-defined, small-scale{~O.O5AU) magnetic flux rope was observedby
Ulyssesat about 5 AU in closeproximity to a heat flux dropout {HFD) at the
heliosphericcurrent sheet {HCS). This magnetic flux rope is characterizedby a
rotation of the field in the plane approximately perpendicular to the ecliptic {and
containing the Sun and the spacecraft)and with a magnetic field maximum centered
near the inflection point of the bipolar signature. The edgesof the flux rope are well
definedby diamagnetic field minima. A bidirectional electron heat flux signature is
coincidentwith the magnetic flux rope structure. The event occurredduring a time of
slightly increasing solar wind speed,suggestingthat the field and plasma were
locally compressed.Unlike most coronal mass ejections/magneticclouds,this event is
characterizedby high proton temperatures and densities, high plasma beta, no
significant alpha particle abundanceincrease,and a small radial size. We interpret
these observationsin terms of multiple magnetic reconnectionof previously open
field lines in interplanetary spaceat the HCS. Such reconnectionproducesa Ushapedstructure entirely disconnectedfrom the Sun {which we associatewith the
HFD), a closedmagnetic flux rope {which we associatewith the counterstreaming
electron event), and a closedmagnetic loop or tongue connectedback to the Sun at
both ends.Theseobservationssuggestthat magnetic reconnection,and its changesto
magneticfield topology, can occurwell beyondthe solar coronain interplanetary
space.
Introduction
Magnetic reconnection is thought to play an integral
role in the energization of solar plasma and the
reconfiguration of solar magnetic fields [e.g., Priest,
1984]. Heat flux dropouts (HFDs) in solar wind
suprathermal
electron
distributions
have been
interpreted as evidence of magnetic reconnection in the
solar corona [e.g., McComas et at., 1989]. One possible
interpretation ofHFDs is that they are due to spacecraft
passage through magnetic flux tubes that have been
completely disconnected from the Sun [e.g., McComas et
at., 1991] and that are connected to the outer
heliosphere at both ends. However, most HFDs
identified in ISEE 3 data by McComas et at. [1989] were
found to have streaming populations of more energetic
electrons (>2 keY) [Lin and Kahter, 1992], indicating
that HFDs are not necessarily unambiguous signatures
of disconnection from the Sun.
INow at Department of Physics and Space Sciences, Florida Institute
of Technology, Melbourne.
Copyright 1995by the American GeophysicalUnion.
Paper number 95]AOl123.
0148-0227/95/95]A.Oll23$05.00
19,903
Magnetic reconnectionin the solar corona [e.g., Kopp
and Pneuman, 1976; Hiei et al., 1993] has also been
suggestedto be responsiblefor producing magnetic flux
ropes/magnetic clouds that are observed in
interplanetary space [Gosling, 1990, 1993] .In this
picture, reconnection occurs on the sunward side of
expanding coronal loops to form the flux rope structure
(Figure 1). These magnetic structures have been shown
to be a subset of the class of solar ejecta called coronal
mass ejections (CMEs) [e.g., Klein and Burlaga, 1982;
Gosling, 1990]. Magnetic clouds, as definedby Klein and
Burlaga [1982], are regions with a radial dimension of
>0.25AU (at 1 AU) in which the magnetic field strength
is high and the magnetic field direction changes
appreciablyby means of rotation of one componentof B
nearly parallel to a plane. Magnetic clouds are also
inferred to be expanding at 1 AU due to a higher
pressure observedinside the structure comparedto the
surrounding medium.
Several criteria have been used to identify CMEs in
the solar wind. Table 1 presents someof the signatures
that have been used to identify CMEs (adapted from
Gosling [1990]).Not all events identified as CMEs have
all of these signatures [e.g., Zwickl et al., 1983]. It is our
experiencethat the most robust signature of a CME at 1
AU is the presence of counterstreaming solar wind
MOLDWIN
19,904
Rising -
CME
Loops
ET AL.: EVIDENCE
OF INTERPLANETARY
I
\\
\\
fl
Figure 1. A schematic showing the formation of a
magnetic flux rope in interplanetary space by magnetic
reconnection of the closed loops embedded within a CME
[from Gosling, 1993].
suprathermal electrons [Gosling et al. ,1987]. However,
studies of fast CMEs that drive interplanetary shocks
have found that other signatures are just as good as, if
not better than, the presence of counterstreaming
electrons in identifying CMEs. Richardson and Cane
[1993] found that plasma proton temperature
depressionswere the best signature of fast CMEs, while
Zwickl et al. [1983] found that low-variance, enhanced
magnetic fields [e.g., Tranquille et al. , 1987] were
equally as goodas counterstreamingelectron signatures.
It should be noted that the Richardson and Cane and
Zwickl et al. studies were of fast CMEs that drove
shocks.Thesefast CMEs make up only about one-third
of all CMEs [Gosling et al., 1991]. Adding to the
difficulty of using counterstreaming electron signatures
to identify CMEs is that beyond2 AU and in the vicinity
of Jupiter there are several other sources of
counterstreaming electron fluxes, including magnetic
connectionto interplanetary shocks[Gosling et al., 1993]
and to the Jovian bow shock [Moldwin et al. , 1993].
Though the signatures used to identify CMEs are not
universal amongall CMEs, the physical placeof origin of
Table
I. Common Signatures
MAGNETIC
RECONNECTION
CMEs is well understood (i.e., the solar corona). CMEs
are ejections of coronal material containing closed
magnetic field lines not previously participating in the
solar wind expansion [e.g., Hundhausen, 1988; Gosling,
1993].
During the in-ecliptic phase of its mission, Ulysses
observed a sequence of plasma and magnetic field
signatures at about 5 AU that, in some ways, were
reminiscent of aCME/magnetic cloud [e.g., Klein and
Burlaga, 1982;Burlaga, 1991; Farrugia et al. , 1993].
Thesesignatures included high magnetic field strength,
a smooth rotation of the magnetic field, and the
coincident
observation
of counter streaming
suprathermal halo electrons.However, many features of
this event are not consistent with a coronal origin of the
structure. Here we proposethat this event resulted from
magnetic reconnection in interplanetary space of
previously open magnetic field lines. It has been
suggestedthat magnetic reconnectionsometimesoccurs
aheadof fast CMEs in interplanetary space[McComaset
al. , 1988]. If the CME is moving faster than the
surrounding solar wind, interplanetary magnetic field
(lMF) lines will drape about the CME. If within the
draping region there is a region in which the magnetic
fields have components that are antiparallel to each
other (such as at a swept up heliospheric current sheet),
magnetic reconnection could be initiated. A possible
example of such a reconnection scenario was presented
recently [McComas et al., 1994]. The present
observations suggest that magnetic reconnection can
occur in interplanetary space at a heliospheric current
sheet even without a CME driver. In addition, the flux
rope nature of the event argues for multiple magnetic
reconnection as opposed to a single reconnection site
that cannot produce a topologically distinct flux rope
from previously openmagnetic field lines.
Observations
This study uses ion and electron measurementsfrom
the in-ecliptic phaseof the Ulyssesmission. The Ulysses
Solar Wind Plasma Experiment's electron instrument is
a spherical-section electrostatic analyzer which uses
channel electron multipliers (CEMs) to count particles
discretely. The energy range of the instrument is 1.6 to
862 eV. Each electron spectrum takes 2 min to measure
and includes 20 logarithmically spacedenergy steps.The
ion instrument is also a spherical-section electrostatic
analyzer which returns three-dimensionalvelocity space
measurements of the solar wind proton and helium
of CMEs and Their References.
Signature
Counterstreaming suprathermal electrons
Counterstreaming energetic ions
Helium abundance enhancement
Ion and electron temperature depressions
Strong magnetic fields
Low magnetic field variance
Anomalous field rotations
Low plasma beta
Unusual ionization states
Reference
Montgomeryet al., [1974],Goslinget al., [1987]
Marsdenet al., [1987],Palmer et al., [1978]
Hirshberg et al., [1972],Borrini et al., [1982]
Goslinget al., [1973],Montgomeryet al., [1974]
Hirshberg and Colbum, [1969],Burlaga and King, [1979]
Pudovkin et al., [1979],Marsden et al., [1987]
Burlaga et al., [1981],Klein and Burlaga, [1982]
Goslinget al., [1987]
Bame et al., [1979], Fenimore, [1980]
MOLDWIN ET AL.: EVIDENCE OF INTERPLANETARY
distributions over an energy range of 0.248 to 12.2
keV/q. A full description of the plasma instruments is
given by Bame et al. [1992]. The magnetometer data
used in this study are from the Ulysses magnetic field
instrument [Balogh et al., 1992]. In this study we use
256-saveragedvector magnetic field observations.
Plate 1 shows a color-coded spectrogram of the
log(counts)of suprathermal halo electrons(78-175eV) as
a function of time and look direction for December 22,
1991.At this time, Ulysseswas approachingJupiter and
was at a heliocentric distance of 5.01 AU. Two regions of
interest to this study are marked. The first region,
beginning at about 0430 UT, is an HFD. Note the
absenceof a suprathermal electron flux. At 1600 UT a
bidirectional heat flux event begins, as evidencedby the
appearanceof flux at two azimuths separated by 180°.
The normal solar wind unidirectional heat flux is
evident betweenthe two events and prior to the HFD.
Figure 2 showsmagnetic field data (Be,B., B total) for
these events in RTN spherical coordinates(where R is
parallel to the Sun-spacecraftvector, T is parallel to .0.x
R, where .0.is parallel to the solar rotation axis, and N
completesthe right-handed set; Be is the polar angle and
is definedto be 90° along Nand 0° in the R-T plane, and
MAGNETIC RECONNECTION
19,905
B. is the azimuthal angle and is defined to be 00 along
R). The two regions identified with the electron
spectrogramalso have signatures in the magnetic field,
namely, the HFD is coincident with a heliospheric
current sheet crossing, as was also found with most of
the HFDs identified by McComaset al. [1989], and the
region of bidirectional electron heat flux is coincident
with a rotation of the magnetic field.
Figure 3 shows selectedfield and plasma parameters
for the secondhalf of December 22, 1991. The panels
display the values of the total magnetic field, proton
density, proton temperature, bulk speed,alpha particle
abundance (Nr/N p), total plasma pressure, and total
plasma beta. The boundaries of the counterstreaming
suprathermal electron event are defmedby the magnetic
field minimum. A magnetic field strength maximum is
evident at the center of the event. This maximum is
larger than the surrounding interplanetary magnetic
field (IMF) value. The value of the proton density triples
within the event, and the proton temperature has an
intermediate value from the solar wind before and after
the event. The event is superimposed upon a slowly
increasing solar wind velocity profile. There is no alpha
abundanceenhancementduring this event, as shown in
E
00
02
04
06
HFD
08
10
12
14
16
18
20
22
24
Bidirectional
Electron
Event
Plate I. A color plot showing the number of electron counts (color intensity) in the suprathermal
(77 eV to 124 eV) energy band for December22, 1991, as a function of time and spacecraft look
direction (in the R-T plane). The HFD and flux rope intervals are marked.
MOLDWIN
19,906
ET AL.: EVIDENCE
OF INTERPLANETARY
MAGNETIC
in this study. The similarities are striking, including the
well-defined diamagnetic field minima at the
boundaries, the magnetic field maximum, and the
bipolar signature. The magnetic field minima at the
boundaries of geotail flux rope plasmoids have been
interpreted to be due to plasma sheet-like plasma inside
the flux rope plasmoid [e.g., Moldwin and Hughes,
1991].The diamagnetic field minima observed on the
boundaries of the Ulysses flux rope therefore could
analogously be due to the entry of HCS plasma sheetlike plasma.
~
m
~
CD
Discussion
if
Non-CME-like
3
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4:00
8:00
12:00
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16:00
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0.5
0
0:00
20:00
Qualities
The 4-hour-long magnetic signature (Figure 3b) is
consistent with a magnetic flux rope. Since magnetic
clouds can be modeled as magnetic flux ropes [e.g.,
Burlaga, 1988] and since magnetic clouds are a subset of
CMEs [e.g., Gosling, 1990], it is reasonable to assume
that this event is a CME, especially since the structure
is also marked by a bidirectional electron heat flux
2.5
ID
RECONNECTION
0:00
01
"'
E
ID
Figure 2. The magnetic field in RTN spherical
coordinates for December 22, 1991. The boundaries of
the HFD and bidirectional heat flux events are marked.
0.
z
the fifth panel. The event is marked by high plasma
beta, particularly during the diamagnetic minimums at
the boundary.
Figure 4a showsthe magnetic field data for 1600-2000
UT in principal axis analysis coordinates (PAA).
Principal axis coordinatesare defined by the direction of
minimum variance (see Sonnerup and Cahill [1967]for
details) and have been extensively used to order
magneticfield data of flux rope structures [e.g.,Elphic et
al. , 1980; Klein and Burlaga, 1982; Moldwin and
Hughes,1991].Hodogramsof the magnetic field data are
shown in Figure 4b. Table 2 gives the PAA coordinates
for this interval. The rotation is nearly in the R-N plane
with the minimum variance direction along the Taxis.
Thesehodogramsare very similar to onesfrom magnetic
clouds[Klein and Burlaga, 1982] and magnetic flux rope
plasmoidsobservedin the Earth's magnetotail [Moldwin
and Hughes, 1991] and suggestthat this structure is a
magneticflux rope.
It is instructive to comparethis flux rope observation
with flux rope plasmoid events observedin the distant
magnetotail. Suchevents have beeninterpreted as being
due to multiple magnetic reconnectionacrossthe Earth's
plasma sheet [e.g., Hones et al., 1979]. The magnetic
field behavior of the Ulysses event is very reminiscent of
many flux rope plasmoid observations in the deep tail
[e.g.,Moldwin and Hughes, 1992].Figure 5 compares
the magneticfield profile of a geomagnetictail plasmoid
observation made by ISEE 3 (at GSM x = -90 RE m
December28, 1982)with the flux rope event presented
-0:.
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,
6
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,...r
~-~
TiniUT
.
Figure 3. Selected field and plasma parameters for the
12 hours centered on the magnetic flux rope event. The
magnetic field boundaries of the event are coincident
with the suprathermal bidirectional electron heat flux.
MOLDWIN ET AL.: EVIDENCE OF INTERPLANETARY
MAGNETIC RECONNECTION
19,907
PAA Coordinates of Magnetic Flux Rope
~
,2.
~...
':
~
0-=
m
.~
N
m
:
.-
-2i
N
m
...,
a'""
,
-2
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,
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O
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2
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-3"1 ' , '.., ' , , , l' , , , , ' , , , " , .., ' , , , I
-3
-2
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2
3
81
Figure 4. (a) Time series of the magnetic field in PM coordinates for the flux rope event. (b)
Hodograms of the PM coordinates. The PM field behavior is consistent with a magnetic flux rope
structure and is similar to ones presented for magnetic clouds and flux rope plasmoids in the
Earth's magnetotail.
event. However, several features of this magnetic flux
rope are not consistent with a CME/magnetic cloud
interpretation. The first is the relatively small size. The
radial extent of this flux rope is estimated to be -0.05
AU. Sincewe assumeit is an off-axis pass, this value is
a minimum size; however, since there is a substantial
rotation of the field, it is not just a skimming pass. The
smallest magnetic cloud previously identified had a size
of 0.06 AU and was observed by Helios at 0.6 AU
[Bothmer, 1993].However, most CMEs/magneticclouds
observedbeyond1 AU have scalesizesof >0.2 AU [Klein
and Burlaga, 1982; Marsden et al., 1987; Bothmer,
1993].The averagesize of CMEs observedat 1 AU was
found to be 0.2 AU [Gosling et al., 1987]. The second
non-CME-like feature is the high proton temperature
observed inside the flux rope. Most CMEs/magnetic
clouds have plasma temperature depressions [e.g.,
Gosling et al. 1973; Montgomery et al., 1974; Zwickl et
al., 1983;Richardson and Cane, 1993],though there are
events identified as CMEs that do not [e.g., Gosling,
Table
2. The PAA Coordinates
1990]. The third non-CME-like feature of this event is
the relatively high plasma beta. Most magnetic clouds
and CMEs are low-beta objects [Burlaga, 1991; Gosling
et al., 1987;Farrugia et al. , 1993].The fourth non-CMElike feature is the lack of a substantial alpha abundance
increase. (Though alpha particle increases have often
beenused to identify CMEs, there are many examplesof
CMEs that do not have appreciable alpha abundance
increases [Gosling et al., 1987]).Another possiblepiece
of evidencethat argues against a coronal origin of this
structure is the total pressure profile (Figure 3). The
event is characterized by an overpressure, which,
coupledwith the relatively small size, suggeststhat the
flux rope could not have propagated out to 5 AU and
remained so small, unless the magnetic "twist" or
tension is strong enough to confine the overpressure.
The observedazimuthal field, however, is insufficient to
confine the overpressure. Most CMEs/magnetic clouds
are observedto be expanding and grow to considerable
size by 1 AU. For these five reasonswe argue against E'
for the Flux Rope
B,
B2
B3
*Var is the variance of the field along the PAA direction.
0.758
0.652
-0.003
-0.652
0.758
0.003
MOLDWIN
Flux
-~1U1
ET AL.: EVIDENCE
OF INTERPLANETARY
~\f\r-
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23_1;,_12:00:00
1__1__16:00:00Time
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RECONNECTION
Interplanetary Origin of the Flux Rope
Rope Plasmoid Observed
I§EE 13 in the ~ar1h's Ta~
15
5
MAGNETIC
1
--,
24:00:00
Figure
5. A comparison of a magnetic flux rope
plasmoid
observation
observed in the Earth's
magnetotail with ISEE 3 and the magnetic flux rope
structure observed in the solar wind with mysses.
coronal origin for this event. We propose that this
structure was generated by multiple magnetic
reconnectionof previously open magnetic fields at the
heliosphericcurrent sheetin interplanetary space.
An HFD was observed prior to the flux rope. HFDs
have been interpreted as evidence of flux tubes
completely disconnectedfrom the Sun [McComaset al.,
1989, 1994]. The flux rope is observed8 hours after the
HFD. Figure 6 shows an idealized schematic in the
plane perpendicular to the ecliptic of a suggested
scenario that we believe can best explain these
observations.As the solar wind propagatespast Ulysses,
the spacecraftfirst crossesthrough the HFD and across
the HCS. Owing to the IMF draping between the HFD
and the flux rope, Ulysses observes"normal" solar wind
prior to the entry into the flux rope. Note in Figure 2
that the magnetic field (e.g., the theta component)
betweenthe flux rope and the HFD rotates in away that
is consistent with the field draping between the HFD
and the flux rope as shown in Figure 6. This figure also
demonstrates a possible explanation for why this is the
only example of this type of sequenceof events in the
Ulyssesdata set identified so far, namely, the small size
of the flux rope (0.05 AU) and the alignment needed
betweenthe satellite trajectory and the different regions
make it a fortunate occurrenceto observethese events.
Recently, McComas et al. [1994] proposed a similar
origin for a sequence of plasma and field signatures
observed by Ulysses just beyond 1 AU, except it was
suggestedthat the reconnectionin that event was driven
by a fast CME overtaking slower ambient plasmas and
fields. The scenario developed in this study differs in
that no CME was observed following this sequenceof
plasma and field signatures and we observe a distinct
flux rope structure after the HFD. An interesting
implication of our interpretation of the magnetic event
as a magnetic flux rope is that multiple magnetic
reconnectionis required. The topological consequenceof
reconnecting open IMF lines at a single location is to
form a U-shaped HFD and a closed magnetic loop or
tonguethat is connectedback to the Sun at both ends.In
order to form a topologically distinct flux rope, multiple
magnetic reconnection is needed (or reconnection of
Figure 6. A schematic showing the possible magnetic field geometry surrounding this event.
MOLDWIN
ET AL.: EVIDENCE
OF INTERPLANETARY
closedmagnetic field lines, as illustrated in Figure 1).
Multiple magnetic reconnection forms three distinct
plasma structures: (1) a U-shaped HFD, (2) a flux rope
or magnetic island structure with the bidirectional
electron heat flux, and (3) a closed magnetic loop or
tongue. The third structure is not observedduring this
interval; however,if our interpretation of the formation
of a flux rope by multiple magnetic reconnection is
correct, it must have been created. Figure 6 shows a
possible explanation for why this third structure is not
observed, namely, the orientation of the HCS with
respectto the satellite trajectory was not favorable.
Summary
and Conclusions
The Ulyssesspacecraftobserveda sequenceof plasma
and magnetic field signatures that we interpret as
evidence for multiple magnetic reconnection of
previously open field lines in interplanetary space.The
signatures include an HFD followed closely by a nonCME magnetic flux rope. This study implies that
magneticislands or flux ropescan be producedalong the
HCS in interplanetary spacesomewhatanalogousto flux
rope plasmoids formed in the Earth's geomagnetictail.
In addition, if this interpretation is correct, a CME
driver is not required to initiate this magnetic
reconnection.These observations demonstrate that the
magnetic topologyof interplanetary magnetic field lines
canbe changedfar from the Sun.
Acknowledgments. One of the authors (M.M.) thanks Max
Hammond for helpful discussions and data analysis support.
The work at Los Alamos National Laboratory was performed
under the auspicesof the United States Department of Energy
with financial support ftom the National Aeronauticsand Space
Administration.
The Editor thanks two referees for their assistance in
evaluatingthis paper.
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M.B. Moldwin, Department of Physics and Space Sciences,
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(ReceivedAugust 26, 1994;revisedFebruary 24, 1995;
acceptedMarch 30, 1995.)