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Abstract

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Two Magnetic Cloud Examples
  5. 3. Discussion
  6. 4. Summary
  7. Acknowledgments
  8. References

[1] A survey of Ulysses magnetometer data has been carried out between Oct 1990 (launch) and Apr 2003 (just after the second northern polar pass) cataloguing magnetic cloud (MC) signatures, i.e., a smooth magnetic field rotation and enhanced magnetic field magnitude. After confirming that each of these events was a true MC by checking for a low proton temperature signature, it was possible to fit a constant α, force-free flux rope model to each event. This allows parameters describing the global magnetic field configuration of the event in the region of the intersection with the spacecraft's trajectory to be determined. The survey has shown that associated with many MC field rotations there is a second, less well-defined, rotation. Usually this rotation is so poorly defined that it is impossible to fit the flux rope model to it. However, in two specific cases, presented here, it has been possible to produce good model fits to both rotations. This gives insights into the chirality and relative orientations of the two flux ropes, assuming that is what they are. The results suggest that the two rotations are part of the same, deformed, rope through which the spacecraft has passed at two different points.

1. Introduction

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Two Magnetic Cloud Examples
  5. 3. Discussion
  6. 4. Summary
  7. Acknowledgments
  8. References

[2] Coronal mass ejections (CMEs) are one of the most dynamic and spectacular processes in the corona. Observed most clearly through space based coronagraphs, e.g., the LASCO instrument on board SOHO [Brueckner et al., 1995], they are regularly observed with their occurrence rate increasing from ∼0.2 per day at solar minimum to ∼3.5 per day at maximum [Webb and Howard, 1994]. Thought to be the primary method by which the Sun rids its self of helicity, the cause and internal structure of these events is of great interest.

[3] In this paper we focus upon in-situ observations of CMEs once they have left the Sun and propagated out into the heliosphere, where they now become known as interplanetary CMEs or ICMEs, embedded in the nominal solar wind. In particular, we focus upon observations from the Ulysses spacecraft. Launched in 1990, Ulysses has a polar orbit of the Sun, ∼80° to the ecliptic plane, allowing investigations into the 3D Heliosphere to be undertaken for the first time. However, here we are more interested in Ulysses' 13 years of continual observations of the solar wind, and more specifically of ICMEs.

[4] Burlaga et al. [1981] were the first to define a subset of ICMEs termed magnetic clouds (MCs). These are events that exhibit a smooth magnetic field rotation, an enhanced magnetic field magnitude and a low proton temperature. Observations show that approximately one third to a half of ICMEs have associated MCs [e.g., Gosling, 1990; Bothmer and Schwenn, 1998].

[5] The Ulysses magnetometer [Balogh et al., 1992] dataset has been surveyed for the field signatures of MCs from Oct 1990 (launch) to Apr 2003, then each event has been checked for a temperature depression signature. The results of this survey are being prepared for publication elsewhere. In this paper we focus upon an interesting observation noted during the taking of the survey. Along with the primary 180° rotation, that normally indicates the passage of a flux rope like structure over the observing spacecraft, there often appeared a secondary 180° rotation. Almost without exception this secondary rotation occurred after the first and it was generally not as well defined, i.e., there were many fluctuations in the magnetic field similar to that of the non-ICME solar wind. The exact percentage of events that display this secondary rotation is very hard to determine due to ambiguity in defining this secondary rotation, but the authors believe that this percentage is greater than 50%.

[6] The cause of these secondary rotations is uncertain. It is possible that these are not flux ropes at all but spheromaks (Ivanov et al. [1989]; Vandas et al. [1991]; and for a good review see Farrugia et al. [1995]), completely detached magnetic bubbles. The presence, or absence, of bi-directional suprathermal electrons (BDEs) during the events will allow the distinction between these two magnetic configurations as the presence of superthermal BDEs cannot be explained in the spheromak case. It is also possible that the double rotations could simply be two interacting MCs that did not originate at the same time from the low corona but one has caught up with another due to a speed difference. Whereas this undoubtedly does happen in the 3D heliosphere we do not believe that this can account for all the double events observed in this survey as the radial velocity profiles do not show a fast trailing event, also an interaction/sheath like region would be expected between the two events.

[7] Work by Osherovich et al. [1999] theoretically examined possible configurations of the simplest bounded MHD states. One solution was that of two interacting helices, or flux ropes. This interaction caused the enhancement of one helix and the suppression of the other. A spacecraft passing through such a structure would observe two rotations, with the leading rotation being better defined. This is in agreement with the Ulysses observations. However, it is impossible with single spacecraft observations to uniquely identify this interacting helix model in interplanetary space.

[8] The two rotations could also be associated with two flux ropes originating from the same source region on the Sun one after the other, i.e., a nested flux rope eruption (Figure 1a) [Vandas et al., 1999]. Alternatively, the double rotation could be an artefact of the geometry of the flux rope. If the rope has been deformed and bent back onto itself in a Parker spiral like fashion (Figure 1b), it is possible that the spacecraft has passed through the same rope twice. This scenario has been suggested before, once by Crooker et al. [1998], to explain the structure of MCs at sector boundaries and once by Marubashi [1997], to explain a double rope like signature observed by IMP 8. Vandas et al. [2002] have recently performed 3D MHD simulations of a loop like MC in the solar wind and have shown that with favourable spacecraft trajectories it is indeed possible to pass through the same deformed flux rope twice.

image

Figure 1. Schematic diagram illustrating two possible causes of the double rotation signature. A) two nested flux ropes that erupted at almost the same time as part of the same event. B) a deformed flux rope that the spacecraft cuts through twice.

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[9] To determine parameters describing the large scale structure of an MC close to the interception with the observing spacecraft's trajectory, a constant α, force-free flux rope, similar to that used by Lepping et al. [1990], was applied to all MCs observed by Ulysses. Usually this flux rope model was only applied to the leading rotation as this was the only part of the event that was well enough defined to allow such modelling. The secondary rotations were generally too poorly defined to accept the fit of a second flux rope, in all but two cases throughout the entire dataset. It is these two events that are discussed below, although event B resembles the other double rotations more so than event A.

[10] Other authors have also commented upon double rotations. Osherovich and Burlaga [1997] noted a double rotation during a Ulysses observation of a MC (June 9–15 1993). However, the first rotation occurred in the sheath region of this MC. This is interesting but it should be noted that this is a separate phenomenon to the two observations that we present below, as neither of the rotations in the two examples below occur in the sheath region.

2. Two Magnetic Cloud Examples

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Two Magnetic Cloud Examples
  5. 3. Discussion
  6. 4. Summary
  7. Acknowledgments
  8. References

2.1. Example Event A – August 1997

[11] The first event, hereafter referred to as event A, (Figure 2), was observed in August 1997 (DOY 227–232), when Ulysses was at ∼6°N and at a radial distance of ∼5.6 AU. Figure 2 shows the data taken during this time, along with the flux rope fits for the leading and trailing rotations over-plotted. The panels from top to bottom represent the three components of the magnetic field in the RTN coordinate system (where R is defined as an axis pointing radially away from the Sun, T is the vector product of the solar rotation axis with R, and N is the vector product of R with), the two corresponding angles of the field, the field magnitude, the radial velocity profile, the pressure, the proton plasma beta, the proton temperature profile, the number density of protons and the ratio of the number density of alpha particles to that of protons. From this figure the signatures of a MC can be clearly seen across both rotations. Other signatures of ICMEs can also be noted, low proton plasma beta and enhanced alpha to proton number density ratio.

image

Figure 2. The first example of a double flux rope in the Ulysses dataset. Observed in August 1997 when Ulysses was ∼6°N. The panels from top to bottom represent the three field components (RTN coordinate system), field angles, field magnitude, radial velocity profile, pressure profile, plasma beta, Proton temperature (upper and lower bounds), number density of protons and the ratio of the number density of alphas to protons. The lighter smooth lines represent the two model flux ropes.

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[12] The two 180° rotations in this particular event are unusual as they appear as one complete 360° rotation (Figure 2, 5th panel from the top). No other event in the entire dataset displays this property. Also, as can be seen in Figure 2 there is a small shock like feature embedded in the event towards the end of DOY 228. The origin of this shock is unknown.

[13] The fitting of the two flux rope models show that the leading flux rope has a right handed chirality, a radius of ∼0.20AU and axis orientation of (−0.37, 0.74, −0.57) in the RTN coordinate system, as are all following axis orientations. The trailing flux rope also displays a right handed chirality, with a radius of ∼0.25AU and an axis orientation of (0.51, −0.59, 0.62). These results will be discussed in section 3 below.

2.2. Example Event B – December 2000

[14] Ulysses observed the second event, hereafter referred to as event B, (Figure 3), when it was almost at it's highest southerly latitude (∼80°S) in December 2000 (DOY 341–346) and at a radial distance of ∼2.2AU. Figure 3 shows this event in detail in a format similar to that of Figure 2 with the two flux rope fits over-plotted on the data. The signatures of an MC are again present across both rotations. However in this case, unlike event A, there is a small gap between the two rotations. The two flux rope fits show that the leading rotation displays a right handed chirality, has a radius of ∼0.16AU and an axis orientation of (−0.63, 0.71, −0.30). The trailing event also displays a right handed chirality, has a radius of ∼0.14AU and an axis orientation of (0.19, −0.27, 0.95).

image

Figure 3. The second example of a double flux rope event. Observed by Ulysses in December 2000 whilst it was at its highest southerly latitudes, ∼80°S. The panel format is the same as that of Figure 2.

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3. Discussion

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Two Magnetic Cloud Examples
  5. 3. Discussion
  6. 4. Summary
  7. Acknowledgments
  8. References

[15] Before discussing the chirality and orientation of the four fitted flux ropes we should first briefly address the spheromak and colliding scenarios described in the introduction. The presence of BDEs during both events [Forsyth et al., 2003; J. Gosling, personal communication, 2002] indicates that the field lines threading the events are still connected at both ends to the low corona, discounting the spheromak geometry [Vandas et al., 1999]. It also appears that there is no interaction/sheath region between the two flux ropes suggesting that these events are not simply colliding MCs. The fairly flat velocity profile of event A also suggests a non-colliding scenario. The velocity profile of event B shows faster plasma ahead than behind the rotations. This is not what would be expected when a faster event catches a slower one. Thus, in these two cases we believe we are not simply dealing with two colliding MCs.

[16] Examining the chirality and axis orientations may allow us to distinguish between the two remaining scenarios. It is clear that in the deformed flux rope scenario both observed flux ropes should have the same chirality. However, in the case of the nested flux ropes this is not necessarily true. Thus, if we observe two different chiralities we can discount the deformed flux rope scenario. The observations show that the in both events both flux ropes show the same chirality (right handed). This then does not help distinguish between the two scenarios.

[17] Next consider the orientations of the various flux rope axes. Figure 4 shows, for the two events, the projection of the two axes onto the R-T plane (bottom two plots of Figure 4) and the angle of axes out of the R-T plane (top two plots of Figure 4). It can be seen that the axis orientations of the flux ropes in event A are almost completely anti-parallel, ∼168° between the two axes. This suggests that the deformed flux rope scenario is applicable in this case, as it is hard to imagine two nested flux ropes being produced one immediately after the other having almost completely anti-parallel axes.

image

Figure 4. The angle of the axes of the two flux ropes of event A out of the R-T plane (a) and the axes projected onto the R-T plane (c). Similarly, the angle of the axes of the two flux ropes of event B out of the R-T plane (b) and the axes projected onto the R-T plane (d). Solid lines represent the axes of the leading rotations and the dotted lines that of the trailing rotations.

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[18] The axes of the two flux ropes in event B are not anti-parallel, ∼127° between the two axes (Figure 4). However, their projections onto the R-T plane do appear to be anti-parallel, ∼174° between the two projections. Suggesting that this flux rope has only been deformed in this particular plane, in a similar effect to that of the Parker spiral. If the travel time from the Sun to Ulysses is not insignificant, relative to the rotational motion of the Sun, then there would indeed be a deformation. The evidence for this is admittedly very circumstantial. It can only be said that both of these examples are relatively slow events, with event A moving radially at ∼365 km/s and event B at ∼400 km/s. Only multi-spacecraft observations can establish the true magnetic configuration of these events.

4. Summary

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Two Magnetic Cloud Examples
  5. 3. Discussion
  6. 4. Summary
  7. Acknowledgments
  8. References

[19] The two examples of double flux ropes presented in this paper give clues to the large 3D structure of MCs in the Heliosphere. The cause of these events have been discussed. The spheromak geometry has been discounted due to the presence of bi-directional electrons. It is unlikely that these events are interacting MCs due to the lack of a sheath like region between the rotations. A force free flux rope model has been fitted to both rotations in both events in order to determine chirality and axis orientation of each. If the chirality of the two rotations in a single event were different then we could conclusively discount the deformed flux rope scenario. However, in both events, both chiralities were the same.

[20] Examining the axes of the two rotations in both events has shown that for event A the axes are approximately anti-parallel. In event B only the projection of the axes onto the R-T plane are anti-parallel. These results suggest that the flux ropes have been deformed in the R-T plane in a Parker spiral like fashion.

[21] These results are by no means conclusive and further study of other double flux rope events, particularly multi-spacecraft observations are required in order to completely distinguish between the two scenarios described above.

Acknowledgments

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Two Magnetic Cloud Examples
  5. 3. Discussion
  6. 4. Summary
  7. Acknowledgments
  8. References

[22] Ulysses support at Imperial College London is provided by the U.K. Particle Physics and Astronomy Research Council. We thank the Ulysses SWOOPS team (PI, D. J. McComas) for providing the plasma data shown in Figures 2 and 3.

References

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Two Magnetic Cloud Examples
  5. 3. Discussion
  6. 4. Summary
  7. Acknowledgments
  8. References
  • Balogh, A., T. J. Beek, R. J. Forsyth, P. C. Hedgecock, R. J. Marquedant, E. J. Smith, D. J. Southwood, and B. T. Tsurutani (1992), The magnetic field investigation on the Ulysses mission: Instrumention and preliminary scientific results, Astron. Astrophys. Suppl. Ser., 92, 221236.
  • Bothmer, V., and R. Schwenn (1998), The structure and origin of magnetic clouds in the solar wind, Ann. Geophys., 16, 194.
  • Brueckner, G. E., et al. (1995), The large angle spectroscopic coronagraph (LASCO), Sol. Phys., 162, 357.
  • Burlaga, L. F., E. Sittler, F. Mariani, and R. Schwenn (1981), Magnetic loop behind an interplanetary shock: Voyager, Helios, and IMP 8 observations, J. Geophys. Res., 86, 66736684.
  • Crooker, N. U., J. T. Gosling, and S. W. Kahler (1998), Magnetic clouds at sector boundaries, J. Geophys. Res., 103, 301306.
  • Farrugia, C. J., V. A. Osherovich, and L. F. Burlaga (1995), Magnetic flux rope versus the spheromak as models for interplanetary magnetic clouds, J. Geophys. Res., 100, 12,29312,306.
  • Forsyth, R. J., A. Rees, D. B. Reisenfeld, S. T. Lepri, and T. H. Zurbuchen (2003), ICME observations during the Ulysses fast latitude scan, in Proceedings of Solar Wind 10, edited by M. Velli, R. Bruno, and F. Malara, p. 715720, AIP, New York.
  • Gosling, J. T. (1990), Coronal mass ejections and magnetic flux ropes in interplanetary space, in Physics of Magnetic Flux Ropes, Geophys. Monogr. Ser., vol. 58, edited by C. T. Russell et al., pp. 343364, AGU, Washington, D. C.
  • Ivanov, K. G., A. F. Harshiladze, and E. G. Eroshenk (1989), Configuration, structure, and dynamics of magnetic clouds from solar-flares in light of measurements on board Vega-1 and Vega-2 in January–February 1986, Sol. Phys., 120, 407419.
  • Lepping, R. P., J. A. Jones, and L. F. Burlaga (1990), Magnetic field structure of interplanetary magnetic clouds at 1 AU, J. Geophys. Res., 95, 19571965.
  • Marubashi, K. (1997), Interplanetary magnetic flux ropes and solar filaments, in Coronal Mass Ejections, Geophys. Monogr. Ser., vol. 99, edited by N. Crooker, J. A. Joselyn, and J. Feynman, pp. 147156, AGU, Washington, D. C.
  • Osherovich, V. A., and L. F. Burlaga (1997), Magnetic cloud, in Coronal Mass Ejections, Geophys. Monogr. Ser., vol. 99, edited by N. Crooker, J. A. Joselyn, and J. Feynman, pp. 157168, AGU, Washington, D. C.
  • Osherovich, V. A., J. Fainberg, and R. G. Stone (1999), Multi-tube model for interplanetary magnetic clouds, Geophys. Res. Lett., 26(3), 401404.
  • Vandas, M., S. Fischer, and A. Geranios (1991), Spherical and cylindrical models of magnetized plasma clouds and their comparison with spacecraft data, Planet. Space Sci., 39(8), 11471154.
  • Vandas, M., S. Fischer, and A. Geranios (1999), Double flux rope structure of magnetic clouds, in Proceedings of Solar Wind Nine, edited by S. R. Habbal, R. Esser, J. V. Hollweg, and P. A. Isenberg, 127130, AIP, New York.
  • Vandas, M., D. Odstrčil, and S. Watari (2002), Three-dimensional MHD simulation of a loop-like magnetic cloud in the solar wind, J. Geophys. Res., 107(A9), 1236, doi:10.1029/2001JA005068.
  • Webb, D. F., and R. A. Howard (1994), The solar cycle variation of coronal mass ejections and the solar wind mass flux, J. Geophys. Res., 99, 42014220.