First Published in: Journal of Geophysical Research, 104, 28217-28223, 1999
1Institute of Geophysics and Planetary Physics, and the Department of Earth and Space Sciences,
University of California, Los Angeles
2Applied Physics Laboratory, Johns Hopkins University, Laurel, Maryland
3NASA Goddard Space Flight Center, Greenbelt, Maryland
4Los Alamos National Laboratory, Los Alamos, New Mexico
Abstract. Nearly 4 months of continuous interplanetary magnetic field measurements made as NEAR approached the Earth from September 1997 through December 1997 have allowed us to compare four interplanetary coronal mass ejection (ICME) events seen by the NEAR and Wind spacecraft. When the spacecraft are in close proximity (separated by in azimuth relative to the sun) the ICMEs seen by Wind and NEAR have similar signatures as expected for structures with dimensions along the solar wind flow of 0.2 AU. When the NEAR spacecraft is separated by in azimuth from the Earth the vector signature of ICMEs seen at NEAR begins to differ from those seen at Wind even though the magnitude of the field in the events and the background solar wind show similarities at the two spacecraft. When the spacecraft are separated by the magnetic signatures are quite different and sometimes ICMEs are seen only at one of the two locations. Nevertheless, in all cases the magnetic helicity of the cloud structures seen at NEAR is the same as at Wind. The radial speeds of the shock and ICME leading edge as they cross Wind and the time delays of those events, for which we have some assurance that they also arrived at NEAR, indicate that the ICMEs decelerate measurably as they travel near 1 AU.
Coronal mass ejections (CMEs) are believed to be produced from closed magnetic regions not previously participating in the solar wind expansion. At 1 AU they generally have a number of distinct plasma and field signatures that distinguish them from the ambient solar wind. Despite extensive in situ studies of the interplanetary manifestations of coronal mass ejections (ICMEs), the connection between ICMEs and their solar source regions is still not completely understood. Measurements made by a single spacecraft provide some information about the local magnetic topology, plasma composition, and connectivity of an ICME to the Sun but only along a radial vector to the Sun as the ICME convects across the spacecraft. In the specific cases of magnetic clouds, a subset of ICMEs Gosling(1990) having magnetic flux rope topology, we make simplifying assumptions about ICME structure and begin to extend that knowledge into three-dimensional space. Even though the observed helical magnetic configuration for a magnetic cloud is reasonably consistent with the cylindrically symmetric, constant-, force-free solutions Burlaga(1988)Goldstein(1983) Lepping et al.(1990), the extrapolations from single point measurements may still be misleading for those cases in which the structures are not entirely force free. Ivanov et al.(1989)and Vandas et al.(1993) argue that other ICME structures with spheroidal and toroidal field configurations are possible, although Burlaga et al.(1990)argue these geometries are unlikely since magnetic cloud signatures are not observed in pairs. Ideally, we would like to study ICMEs with multiple spacecraft over a wide range of separation distances in order to reveal the scale size and evolution of features seen in ICMEs. Fortunately, the interplanetary trajectory of the Near Earth Asteroid Rendezvous (NEAR) spacecraft culminating in an Earth flyby in early 1998 provides a unique opportunity to undertake such a study.
The NEAR spacecraft was launched on February 17, 1996, for a January 1999 rendezvous with
asteroid 433 Eros. Cheng et al.(1997) and McFadden et al.(1996) have described the NEAR mission and
spacecraft. The magnetometer is a triaxial fluxgate instrumment mounted on the antenna
feed horn Lohr et al.(1997). In June 1997, NEAR executed a flyby of asteroid 253 Mathilde
Veverka et al.(1997). On August 13, 1997, when the spacecraft power budget permitted, the
magnetometer was turned on and operated continuously until February 3, 1998. To bend
NEAR's trajectory out of the ecliptic plane toward its rendezvous with 433 Eros, the
spacecraft performed an Earth flyby maneuver. This maneuver occurred on January 23, 1998,
and the magnetometer data taken during and in the days before and after the flyby provided
a calibration and test of spacecraft contamination field correction procedures
demonstrating that the NEAR magnetic field data are accurate to 1 to 2 nT
Anderson et al.(1998), Izenberg and Anderson(1998). This level of accuracy is adequate to characterize the basic magnetic
features of ICMEs. During the period of September 1997 through December 1997, NEAR
encountered eight ICMEs, four of which were structures that appeared to be magnetic clouds. We
have examined the Wind magnetic field and plasma data and have
found six magnetic cloud events observed at the Earth during this period. Timing studies have established that observations of CMEs on the Sun by coronographs correspond to the type of structures observed in situ by spacecraft
that we classify herein as ICMEs Lindsay et al.(1999). Using a similar timing comparison and searching for similar magnetic structures, we find that four of the magnetic clouds seen by Wind correspond to those observed by the NEAR spacecraft. Some properties of key solar wind features
associated with these four events are summarized in Table 1.
|ICME Associated Features||Event 1||Event 2||Event 3||Event 4|
|ICME feature start Wind||1830 Dec. 10||1500 Nov. 22||0500 Nov. 7||2200 Sept. 21|
|ICME feature start NEAR||1900 Dec. 11||2200 Nov. 23||0800 Nov. 8||1000 Sept. 24|
|Radial separation, AU||0.18||0.28||0.37||0.63|
|Azimuthal separation, deg||1.2||5.4||11.3||33.4|
|Wind plasma ICME speed, km/s||359||458||470||459|
|Apparent radial ICME speed, km/s||306||377||572||438|
|Postshock Wind plasma speed, km/s||365||447||835||NA|
|Postshock Wind plasma density, cm-3||29.1||33.3||78.5||NA|
|Shock Wind||0430 Dec. 10||0915 Nov. 22||2230 Nov. 6||NA|
|Shock NEAR||2330 Dec. 10||1145 Nov. 23||1930 Nov. 7||NA|
|Apparent radial shock speed, km/s||395||440||743||NA|
|Calculated radial shock speed, km/s||405||502||891||NA|
|Pre-shock Wind plasma speed, km/s||290||347||341||NA|
|Pre-shock Wind plasma density, cm-3||10.1||12.2||8.1||NA|
|Figure 1: Plot of NEAR spacecraft positions in the ecliptic plane during the four ICME events observed at NEAR and at Earth. Radial distances from the Sun are given in AU and azimuthal separation in degrees. In the plot the Earth's position is fixed and NEAR moves along an arc toward it.|
During the ICME observed on December 10, 1997, at the Wind spacecraft, NEAR is located at a
radial distance from
the Sun of 1.18 AU and to the east relative to the Earth's position
(see Figure 1). At Wind upstream of the shock the plasma is flowing at 290
km/s. A shock passes over Wind and the observed solar wind speed jumps to 365 km/s.
Assuming radial propagation and conservation of mass the speed of the shock is calculated
to be 405 km/s. We call this the calculated radial shock speed in
Table 1. By the time of the arrival of the leading edge of the ICME the
observed speed has fallen to 359 km/s. We call this the Wind plasma ICME speed. At
this speed the same ICME should be observed at NEAR approximately a day later.
Figure 2 shows the magnetograms for both spacecraft for this event.
|Figure 2: Three-day plots of magnetic field observations for the December 10-11 event. (a) Data for the ICME observed at Wind. (b) Data for the ICME observed at NEAR. In both Figures 2a and 2b, lines indicate the shock and beginning of the ICME, respectively.|
Figure 2a contains data from Wind, while Figure 2b contains data from NEAR. The first vertical line in the top panel marks the shock seen at Wind. A vertical line in the lower panel indicating the shock at NEAR is also shown. A second vertical line in Figure 2a shows the approximate start time of the ICME at Wind determined by the magnetic field jump and proton temperature depression. The start time for this ICME is also shown at NEAR. Comparing Figures 2a and 2b we see that the shock is further ahead of the ICME at NEAR than at Wind. This could occur if the Mach number of the shock at NEAR is less than that at Wind so that the plasma is not as compressed. That the strength of the jump in magnetic field is ~30% weaker at NEAR than at Wind and that the field orientation at both spacecraft upstream of the shock is similar suggests this is the case. It could also occur if the radius of curvature of the ICME increases as it propagates outward. This would cause the shock to move a little faster than the ICME but would not cause a weakening of the shock.
The start time of the ICME at Wind occurs 24.5 hours before that seen at NEAR. This delay time and the radial separation distance between observations of 0.18 AU gives an apparent radial speed of 306 km/s. We call this the apparent radial ICME speed in Table 1 because the true speed depends upon the geometry of the event. A similar calculation using the shock arrival at NEAR to determine its delay time gives a radial speed for the shock of 395 km/s. This is the apparent radial shock speed in Table 1. It is clear from the consistency of the event associated speeds there may be up to a 30% deceleration of the structure over 0.18 AU. Such a deceleration is consistent with the broader magnetosheath and weaker shock at NEAR noted above.
Examining the internal magnetic structure of the ICME in Figure 2 reveals similar rotations in all three components of the field as well as similar local magnetic features at both spacecraft. Similar magnetosheath structure ahead of the ICME is also apparent, particularly in the By component and in the scalar field, even though the magnitude is lower at NEAR. Interpreted in terms of the flux rope model of ICMEs the field rotations indicate left-handed helicity at both the Wind and NEAR spacecraft.
November 22-23 Event
The next shortest baseline over which an ICME is seen by both spacecraft occurs at Wind on November 22, 1997. During this event, NEAR was located at a radial distance of 1.28 AU from the Sun and to the east relative to the Earth's position (again see Figure 1). Plasma data from Wind indicates the observed speed of the leading edge of the ICME to be 458 km/s. For this speed the ICME should arrive at NEAR more than a day later.
|Figure 3: Two-day magnetic field observations at both (a) Wind and (b) NEAR for the November 22-23 event. The first line in each panel indicates the shock. A second line indicates a distinguishing feature seen in the By component at both spacecraft.|
Figure 3 shows the magnetic traces for both spacecraft for this event. Again, Figure 3a contains data from Wind and Figure 3b contains data from NEAR. The first vertical line in Figure 3a indicates the shock seen at Wind. Again assuming radial propagation and conservation of mass the shock speed at Wind is calculated to be 502 km/s. The shock is also indicated in the NEAR data. The second vertical line in Figure 3a marks a distinguishing feature in the By component in the sheath ahead of the ICME in the Wind data. The observed speed for this particular feature is 465 km/s. This same feature is readily identified in the NEAR data and is also indicated by a vertical line. A third vertical line in Figure 3a indicates the leading edge of the ICME at Wind. A similar line also approximates the leading edge of the ICME at NEAR.Comparing the two panels we see that the shock at NEAR occurs 27 hours after that at Wind and the sheath feature occurs nearly 31 hours later at NEAR. Using these delay times of the shock and the feature in By arriving at NEAR give an apparent radial speed of 440 km/s for the shock and 377 km/s for the ICME sheath feature. The shorter duration of the magnetosheath region at Wind could be due to Wind sampling the ICME closer to the ``nose'' of the ICME while NEAR (5.4 to the east) could go through the sheath away from the nose where it is thicker. However, deceleration and expansion of the ICME could also contribute to the apparent greater thickness of the sheath seen at NEAR. Again the speeds at NEAR further from the Sun are lower than at Wind. Despite the larger separation, distinct structural similarities still exist within the sheath at the two spacecraft. However, examination of the internal magnetic structure of the ICME reveals very dissimilar signatures in the three components of the field. Starting at approximately 2100 UT on November 22 at Wind and at 1000 UT on November 24 at NEAR both the Bx and Bz components in the Wind data have the opposite polarity when compared to the Bx and Bz components in the NEAR data. These intervals are indicated by arrows in the figure. Also, at these times the By component is different, rotating from positive to negative in the Wind data, but negative to positive in the NEAR data. However, interpreted in terms of a magnetic rope the field within the ICME indicates right-handed helicity at the Wind spacecraft. The NEAR data appear to be less rope-like, and hence it is difficult to assign a NEAR helicity for this event.
November 7-8 Event
Moving to even longer baselines, we find that both spacecraft detect an ICME that occurs at Wind on November 7, 1997. During this event, NEAR was located at a radial distance of 1.37 AU from the Sun and to the east relative to the Earth's position (see Figure 1). Plasma data from Wind indicate the observed ICME speed to be approximately 470 km/s. For this speed the expected arrival time at NEAR would be 32 hours later. Magnetograms for Wind and NEAR for this event are shown in Figure 4.
|Figure 4: Magnetograms at both Wind and NEAR spacecraft for the November 7-8 event. (a) Data for the ICME observed at Wind. (b) Data for the ICME observed at NEAR. As in the previous figures, the first line in each panel indicates the shock. The second line indicates where the By component reverses sign.|
The first vertical line in Figure 4a marks the shock seen by Wind. The speed calculated for the shock assuming radial propagation and using mass conservation is 891 km/s. This shock, evidenced by a much weaker magnetic disturbance, is probably also detected at NEAR and is indicated by the first vertical line in Figure 4b. This disturbance arrives at NEAR over 19 hours later. Again, assuming radial propagation and using this time delay, we get an apparent shock speed of 743 km/s. The second vertical line in Figure 4a marks the point at which the By component reverses sign, which also approximately marks the beginning of the helical rotation of the field within the ICME. The point at which the By component reverses sign in the NEAR data is also indicated. This reversal reaches NEAR 27 hours later. Assuming that the ICME is moving radially outward a similar calculation using this delay time gives an apparent radial speed for the ICME of 572 km/s. Again, comparing the Wind and NEAR data shows that as in the previously discussed cases the shock at NEAR is further away from the ICME, as would occur if NEAR were further away from the nose of the ICME than is Wind. The broader sheath region observed at NEAR is also consistent with the shock being weaker at this spacecraft. On the other hand, the greater ICME speed deduced from the NEAR-Wind timing (the apparent speed) relative to the observed speed from the Wind plasma data suggests that NEAR was closer to the ICME nose. Since the ICME seems to be moving significantly slower than the shock in this event it appears that the ICME is decelerating significantly with radial distance as well as having different sheath thicknesses at the two locations 11.3 apart.
Although the background fields prior to the shock in each component are similar at the two spacecraft, the magnetic disturbances that follow are quite dissimilar. In the Wind data the By component within the ICME has a bipolar rotation (from a negative to a positive polarity), which begins on November 7 at 1600 UT and ends on November 8 around 1400 UT. The Bz component also undergoes a rotation beginning at 1600 UT on November 7, during which the field rotates from a zero Bz component, through a maximum positive Bz, and then ends back at zero on November 8 (a unipolar rotation) at approximately the same time as the By component rotation ends (1400 UT). Arrows in the figure indicate rotations in the field components. The short discontinuity seen in both field components labeled the ``spike feature'' approximately at 0400 UT on November 8 at Wind at first appears to be a local disturbance having too small a scale size to be part of the overall structure of the ICME. These ICMEs are indicated in Figure 4. However, the fact that it is also seen in the NEAR data at 2200 UT on the same day indicates that it is a large-scale feature of the ICME. The By and Bz components in the NEAR data also show coherent rotations, but the signatures of these two components seem almost reversed when compared to the Wind data. The By component at NEAR has the more unipolar signature in which the field rotates from zero to a negative maximum and back to zero beginning 0745 UT on November 8, whereas the Bz component has a bipolar signature beginning at about the same time that rotates from negative to positive. The intervals of rotation are again labeled by arrows. Again, interpretation of the ICME magnetic field at each spacecraft in terms of a flux rope model indicate the helical signature within the ICME is right-handed at both locations.
September 21-24 Event
|Figure 5: Ten-day plots of magnetic field observations for the September 21-24 event as seen by (a) Wind and (b) NEAR. Lines in both panels indicate the leading edge of the ICME.|
The final observation in the series involves two ICMEs. For these events NEAR was located at a radial distance of 1.63 AU from the Sun and almost to the east relative to the Earth's position. Figure 5 shows 10-day plots of the magnetic field data from both Wind and NEAR during mid-September. It is immediately apparent from the intervals of enhanced field magnitude and coherent field rotations that Wind sees two ICMEs during this interval, whereas only one is observed at NEAR. These ICMEs are labeled in the figure. However, the first ICME observed by Wind occurs over six days prior to that seen in the NEAR data, therefore we can say the ICME observed at NEAR does not correspond to the one Wind observes on September 18 at 0000 UT. Considering that the ICME beginning on September 21, 2200 UT at Wind has an observed speed of 459 km/s, the same ICME should be observed at NEAR more than 57 hours later. The leading edge of this ICME seen at both Wind and NEAR is indicated in Figure 5. The ICME at NEAR occurs 59 hours after the September 21 ICME at Wind. Again, assuming radial propagation and using this time delay results in an apparent ICME speed of 438 km/s. Unfortunately, there does not appear to be any shock at either Wind or NEAR to help determine which spacecraft passed closer to the ICME nose. However, the field magnitude enhancement at NEAR is small relative to that observed by Wind, suggesting that Wind observed the much larger field compression associated with the nose of the ICME.
Inside the ICME strong coherent rotations in the By and Bz components of the magnetic field are observed by both spacecraft. Again, upon close inspection it is apparent that rotations in the field observed by Wind are different from those seen at NEAR. Similar to the November 22 event, the Bz component in the Wind data for this ICME has the opposite polarity when compared to the Bz component in the NEAR data having positive polarity at Wind and negative polarity at NEAR. The By component for this case is also similar to the November 22 ICME, rotating from positive to negative beginning at 0130 UT on September 22 in the Wind data, but rotating from negative to positive starting at 1830 UT on September 24 in the NEAR data as shown by the arrows in Figure 5. The field rotations observed at Wind and NEAR are consistent with flux ropes with left-handed helicity. Discussion and Conclusions
In this study we have presented observations of ICMEs at 1 AU by Wind for which similar, nearly contemporaneous ICMEs were also observed by NEAR over a range of azimuthal and radial separations. By comparing Wind and NEAR observations it is clear that when the radial and azimuthal separation distances between the spacecraft are small (i.e., 0.18 AU and respectively), the magnetic field data show strong similarities both in the sheath region and within the ICME itself even though the spacecraft are sampling the same magnetic structure at different locations relative to its center. However, as separation distances increase, dissimilar magnetic field signatures inside the ICME are evident, even when the background and sheath regions are similar (as is the case in the November 22 event). As the separation distances between the spacecraft increase further, we begin to see differences in the sheath magnetic structure and thickness as well as different magnetic signatures within the ICME. This is the case for the November 7 and September events when the spacecraft radial and azimuthal separation distances exceeded 0.37 AU and respectively.
The differences between observations made at Wind and NEAR stress the importance of multi-point measurements. Lindsay et al.(1999) has established the one to one relationship between CMEs and ICMEs using these types of measurements for a set of events in which the causative solar disturbance was unambiguous. It is argued here that the structures observed at Wind and NEAR are also ICME events originating in CMEs on the sun. When the two spacecraft are close to being radially aligned such as in the the December and November 22 events, the spacecraft appear to be passing through the same ICME but perhaps encountering different structures within the ICME. At greater separations, such as in the November 7 and September events, we cannot be as confident that both ICME observations represent the same structure seen at different locations. The observations are consistent with large scale magnetosheath signatures which retain characteristic features even if measurements are made as far as 0.30 AU apart. At first it may seem that these features broaden and expand as they propagate outward in the solar wind, but comparisons of calculated and apparent speeds of the shocks as well as comparisons of observed and apparent speeds of the ICMEs at the two spacecraft indicate these structures are decelerating as they propagate radially past the spacecraft. These observations are consistent with the deceleration noted for interplanetary shocks seen both at Venus and Earth Gosling et al.(1968),Mihalov et al.(1987), Vlasov(1988). Most shocks seen by Pioneer Venus were driven by ICMEs Lindsay et al.(1994).
A comparison of the vector field rotations inside these ICMEs also reveals differences between the observations at the two spacecraft. However, the reason for these differing rotations is more difficult to interpret. It may be that the fields within these ICMEs are not sufficiently close to being the force-free structures that categorize magnetic clouds. However, if we do assume a flux rope topology for the observations as described in Mulligan et al.(1998), Bothmer and Schwenn(1998), and Bothmer and Rust(1997), we deduce that the flux ropes at NEAR have the same helicity as those seen at Wind, but with axes that can be at large angles relative to one another when the spacecraft are separated in azimuth. Interpreting these observations in terms of flux ropes with rotated axial directions is suggestive of a bent flux rope configuration or perhaps of an overall toroidal configuration in which the direction of the axial field is dependent upon the location at which it is sampled, and in which the helicity remains the same. Ivanov et al.(1989) and Vandas et al.(1993) argue that such toroidal topologies for ICMEs as considered here are possible, although Farrugia et al.(1995) and Burlaga et al.(1990) argue that structures like these are highly unlikely to be found in the interplanetary medium. An alternate interpretation is that the scale of the CME is much greater than that of the magnetic flux ropes observed in these ICMEs and that a single CME may release several similar, nearly simultaneous flux ropes at different longitudes.
We are grateful to the many members of the NEAR team who successfully built and launched this first Discovery mission that has enabled these long baseline studies of ICMEs. This work was supported in part by the National Aeronautics and Space Administration through a grant from the Johns Hopkins University 730607 and supported at LANL by internal funding.
Hiroshi Matsumoto thanks the referees for their assistance in evaluating this paper.
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