EGU General Assembly 2011 - PowerPoint Presentation

1. EGU General Assembly 2011Occurrence FrequencyofInterplanetary Magnetic Flux RopesK. Marubashi, Y.-H. Kim, K.-S. Cho, Y.-D. Park, K.-C. Choi,S. Choi, and J.-H. Baek(KASI: Korea Astronomy and Space Science Institute)

2. OUTLINEWe searched for magnetic field structures which can be well fitted to the force-free flux rope models, in the solar wind data from WIND/ACE (1995 – 2009).The most important finding is: “The number of such structures is far, far, far more than those implied by previous surveys.”Why it happened? How it is possible?  Identification of large impact parameter events  Events that can be explained only by torus model

3. Lepping et al. (2006) Occurrence frequency: very low MCs and MC-like structures: follow the change of SSN.Previous MC SurveysKlein & Burlaga (1982)Zhang & Burlaga (1988)Lepping et al. (1990)Bothmer & Schwenn (1998)Marubashi (2000)Lynch et al. (2005)Huttunen et al. (2005)Lepping et al. (2006) ------- -------

4. Present Result: Year-to-year variation of occurrence number (Conditions: Duration >= 7 hours, Erms < 0.35)toruscylinder(some part: torus) Occurrence : (1) much higher than hitherto believed (2) yearly change in parallel to solar activityCYLINDER, CYLINDER & TORUS: 440

5. Two mathematical models (cylinder & torus)S/C passing near the apex:Local Structure can be approximated by a cylinder shown by dashed line. S/C passing near the flank:Curvature effects should be taken intoaccount, and the simplest approximationIs given by a torus geometry.Torus model: to describe local geometry, not indicating the wholestructure were torusCylinder modelTorus model

6. An example: Only “torus model” can reproduce the observations (Duration = 43 hours, Rotation of the filed = 330 deg)

7. Statistical Distribution of “cylinder parameters”(Conditions: Duration >= 7, Erms < 0.35, Cone angle < 10) Finding: Occurrence increases with impact parameter. Note: Circled portions need further consideration.

8. Further Selection (Exclude extreme cases)1. Impact parameter (p): |p| < 0.98 Geometries may not be reliable. This condition excludes many events of R > 0.2 AU.2. Duration (Td): Td < 30 hours Many of long-duration events are better fitted to torus model.3. Cone Angle (Ac): Ac > 10 degrees (already adopted) Small Ac events need torus-fitting. This condition excludes many events of small R.(Note: Criteria for p, Td, Ac need further consideration.)

9. Statistical Distribution of Cylinder Parameters(Modified: |p| < 0.98, Td < 30 hrs, Ac > 10 deg) excludedmostly excludedmostly excludedIncrease with |p|: main reason for the large occurrence

10. Lepping and Wu (2010): occurrence vs. Impact ParameterWe need to admit that only small I.P. cases were studied so far.(They are easy to identify due to large angle rotation of B vector)

11. Possible|p|dependence of detected event number p (dark blue) < p (light blue) Cylinder axes correspond to tangent lines to 2 circles.Consider in 2-D (YZ-plane): If angular distributin is uniform, number is propotional to r (p).

12. Distribution of the cylinder axis direction (Lat. & Long.)N = 325(to Sun)

13. Distribution of Cone Angles 7 hrs =< Td < 30 hrs |p| < 0.98 Cone angle > 10 degIf the axis orientation is uniformly distributed, theevent numbers should be constant in this diagram.At cone angles 20~75, it looks to be satisfied.

14. Most probable distribution of MC cylinder radii (241 events: 20 deg =< Cone angle < 75 deg)We expect a similar radius distribution for torus events.

15. Concluding Remarks1. We could identify ~ 500 flux rope structures in the solar wind data of 1995 -2009, their occurrence frequency changes with sunspot activity. (much, much larger number)2. The flux rope detection rate increases with the impact parameter, in agreement with simple geometrical consideration.We are preparing a website to provide all the fitting results.

16. Thank youforyour attention!