Background:

1. Scale Size of Flux Ropes in the Solar Wind Cartwright, ML & Moldwin, MB IGPP/UCLA, Los Angeles, CA (mcartwri@ucla.edu) Background: The solar wind has long been probed for magnetic structures. These efforts have resulted in the discovery of large-scale (~day) magnetic clouds or magnetic flux ropes (Klein and Burlaga, 1982). These flux ropes have been associated with magnetic reconnection occurring in the solar corona with the macroscopic structure being the interplanetary coronal mass ejection (e.g., Hundhausen, 1988). Recent work has discovered a new class of flux ropes with much smaller time (~hr) and length scales (Moldwin et al., 1995; 2000). It was suggested that small-scale flux ropes provide evidence of reconnection in the solar wind across the heliospheric current sheet. An automated survey of interplanetary flux rope structures in the solar wind has been carried out by Shimazu and Marubashi (2000), where they covered length scales down to two hours. They found the majority of the flux ropes where small-scale (2-10 hours) rather than large-scale (1-2 day). This implies that there is at minimum a bi-modal scale size distribution of flux ropes in the solar wind. How these scales are connected and what it means to the Earth’s geomagnetic activity is unknown at present. Figure 4: Time scale-size of flux ropes observed in solar wind from automated routine for the years of 1995-2001 from the spacecraft Wind (blue). The red line indicates flux ropes found by R.P. Lepping (http://lepmfi.gsfc.nasa.gov/mfi/mag_cloud_pub1.html). Introduction: The difficulty with flux rope identification in the past is that it is usually through visual identification which tends to vary from author to author. To get around this problem and to be able to process large amounts of data, an automated technique is preferred. Shimazu and Marubashi (2000) wrote an algorithm to do this using the feature that the flux rope’s magnetic field components move in a slow and smooth rotation, so the third derivative of any field component with respect to time will be smaller than a certain value. The shortcoming of this approach is the arbitrary definition of ‘smooth rotation’. We have developed an automated technique that identifies small-scale flux ropes through recognizing a peak in total field coincident with a bipolar signature in the y or z component of the GSE magnetic field. The physical nature of this condition is based on the ‘core’ field of the flux rope. This method differs from Shimazu and Marubashi (2000) in that it has no arbitrary definition of ‘smooth rotation’ but classifies flux ropes according to strength of the core field and the bipolar amplitude. Figure 1: Small-scale flux rope on May 14, 1996 ( ~ 1.4 hr). Results & Conclusions: • This method can automatically identify flux ropes of small-scale sizes, see Figures 1-2. Currently, this method has a hard time finding large-scale flux ropes because the peak in the core field doesn’t always coincide with the inflection point in By and/or Bz. Figure 3 is an example of a large-scale flux rope found by R.P. Lepping. R.P. Lepping’s list of visual inspected/modeled flux ropes is (http://lepmfi.gsfc.nasa.gov/mfi/mag_cloud_pub1.html). • For the years of 1995-2001 our algorithm finds that the majority of flux ropes are small-scale (~ 1-10 hrs) verses large-scale (~ days) structures, see Fig. 4. In the figure, I included the flux ropes from R.P. Lepping’s list (in red) to get the full scale-size of the solar wind. This is consistent with Shimazu and Marubashi (2000) result. • We validated this method with a visual survey and by doing a least-squares fit to the flux ropes using a force-free cylindrically symmetric flux rope model. Algorithm/Methodology: We used key parameter data in GSE coordinates obtained by the WIND spacecraft over the years 1995 to 2001 excluding data when the spacecraft was in the magnetosphere or in the magnetosheath. We then averaged the data into 10 and 60min resolution data sets. Our algorithm is as follows: • It finds every peak in the total magnetic field, defined as a point which is greater than the surrounding four points. It then finds the minima associated with the peak, averages the two minima and finds the change in total field associated with the peak (B). • In this study we are finding strong core fields so we require B to be greater than the standard deviation of the 24 surrounding points of the peak. We used the criteria below for “strong core field” because it was the most accurate in identifying flux ropes from our visual survey of the data, as the data resolution became lower the noise level decreased accordingly. • B > 3* St Dev for 10 min resolution data • B > 2* St Dev for 60 min resolution data • The algorithm finds bipolar signatures associated with the peaks by examining the peak point and the two surrounding points. If the points are in linear order and go through zero, the algorithm identifies it as a bipolar signature. It then finds the amplitude of the bipolar signature. If this bipolar amplitude is larger than the standard deviation of the 24 surrounding points of the bipolar signature then it is saved as a candidate flux rope and computes the timescale of the structure. • The algorithm then excludes all candidate flux ropes which show bipolar signatures in the x component of magnetic field, as to exclude current sheet crossings. We validate our method by comparing the results of the automated routine to a visual survey of the data, to published events in Moldwin et al. (2000) and by modeling the flux ropes using a force-free cylindrically symmetric flux rope model. Figure 2: Medium-scale flux rope on May 21, 1996 ( ~ 4 hr). Future Work: • Improve the modeling of the flux ropes to better understand the flux rope boundaries. • Include the velocity and plasma properties . • Expand data set to include IMP-8, and ACE spacecraft data. • Compare the small-scale flux ropes occurrence rates to geomagnetic activity indexes. Figure 3: Large-scale flux rope on July1-2, 1996 (~26 hr). This flux rope was found and modeled by R.P. Lepping (see text). Special thanks to J. Weygand for use of the solar wind dataset. • References: • Hundhausen, A.J., The origin and propagation of coronal mass ejections in Proceedings of the Sixth International Solar Wind Conference, edited by V. Pizzo, T.E. Holzer, and D.G. Sime, NCAR/TN-306+Proc, Boulder, Colo., pp.181-214, 1988. • Klein, L. W. and Burlaga, L. F., Interplanetary Magnetic Clouds at 1 AU, J. Geophys. Res., 87, A2, pp. 613-624, 1982. • Lepping, R.P. et al., Magnetic Field Structure of Interplanetary Magnetic Clouds at 1 AU. J. Geophys. Res., 95, A8, pp. 11957-11965, 1990. • Moldwin, M.B. et al., Small-scale magnetic flux ropes in the solar wind, Geophys. Res. Lett., 27, 1, pp. 57-60, 2000. • Moldwin, M.B., et al., Ulysses observations of a noncoronal mass ejection flux rope: Evidence of interplanetary magnetic reconnection, J. Geophys. Res., 100, A10, pp. 19,903-19,910, 1995. • Shimazu, H. and Marubashi, K., New Method for detecting interplanetary flux ropes, J. Geophys. Res., 105, A2, pp. 2365-2373, 2000.