Radial Evolution of Major Solar Wind Structures

1. SHINE Student Research Talk: Radial Evolution of Major Solar Wind Structures Lan K. Jian Thanks to: C.T. Russell, J.G. Luhmann, R.M. Skoug Dept. of Earth and Space Sciences Institute of Geophysics and Planetary Physics University of California, Los Angeles Whistler, Canada July 29, 2007

2. Outline • Motivation • Introduction • Stream interaction region (SIR) • Interplanetary coronal mass ejection (ICME) • Approach • Data set • Parameters under use • SIR identification • ICME identification (magnetic cloud, 3 Groups) • Results • Variation of SIR and ICME properties with heliocentric distance • Comparison of SIR and ICME properties at 1 AU • Solar cycle variation of fractional occurrence rate of ICMEs in 3 groups • Summary

3. Motivation • Two large-scale solar wind structures • Stream interaction region (SIR) • Interplanetary CME (ICME) Both can cause shocks, generate or accelerate energetic particles, and affect magnetic activity of the Earth and maybe also of other planets • A key element of successful space weather forecast: being able to predict how these structures evolve radially • Our focus: 0.7 ~ 5.3 AU around the ecliptic plane • This study will quantify our empirical understanding of the evolution of solar wind structures, and also provide constraints for existing heliospheric models

4. Formation and Evolution of Stream Interaction Region (SIR) • Magnetic structure of the corona controls solar wind velocity • Solar magnetic field is roughly a dipole tilted with respect to the solar rotation axis • The tilt angle between solar rotation axis and magnetic axis varies during one solar cycle, so the magnetic structure is not symmetric around the rotational equator • Fast and slow streams originating from different sources can collide and interact with each other as the Sun rotates • SIR: a compression in the rising-speed portion of the slow stream, and a rarefaction in the trailing part of the fast stream, with a pressure ridge at the stream interface (SI)

5. If the flow pattern emanating from the Sun is roughly time-stationary  these compression regions form spirals in the solar equatorial plane that corotate with the Sun  corotating interaction region (CIR) • SIR = CIR + transient and localized stream interactions • The pressure waves associated with the collision steepen with radial distance, eventually forming shocks, sometimes a pair of forward-reverse shocks • Compression and shocks can heat the plasma within the SIR (Sanderson,SHINE 2005)

6. Structure & Evolution of Interplanetary CMEs • CMEs with the typical 3-part structure • a leading outward moving bright front • a dark cavity • a bright core of filament plasma at the trailing edge • Generally assume: • bright front  sheath of compressed solar wind • dark cavity  flux rope, low  • cool and dense filament  ? • Some signatures of CMEs may have been washed out as they evolve from the Sun (Hudson et al., 2006)

7. Data Set • 1 AU – baseline • Wind [93-s time resolution] (1995 ~ 2004) • Advanced Composition Explorer (ACE) [Level 2 data, 64-s] (1998 ~ 2004) • 0.72 AU • Pioneer Venus Orbiter (PVO) during Jan. 1979 ~ Aug. 1988 • 5.3 AU • Ulyssesnear aphelion & within 10o of the ecliptic plane (partial 1992, 1997 ~ 1998, 2003 ~ 2005)

8. Parameters under Use • Vp: proton bulk velocity • Np: proton number density • Tp: proton temperature • Te: electron temperature • N/ Np: density ratio of  particle to proton • superthermal electron velocity distribution • B and B: magnetic field vector and magnitude • entropy: defined as ln (Tp3/2 / Np), hint of the sources of different plasmas • : plasma thermal pressure / magnetic pressure • Total perpendicular pressure (Pt) = B2/2o + ∑jnjkTperp,j, where j = H+, e-, He++. The interaction force (Pt) drives the evolution.

9. SIR Identification • Criteria (by eye) • Increase of Vp • A pile-up of Pt with gradual decreases at two sides • Increase and then decrease of Np • Enhancement of Tp • Flow deflection • Compression of B, usually associated with B shear • Change of entropy • Stream interface (SI) at the peak of Pt, where usually Vp and Tp increase and Np begins to drop after a plasma compression region

10. ICME Identification • “Something of art” • Criteria (by eye) • Pt enhancement • a stronger than ambient B • a relatively quiet B • relatively smooth rotations in B • bidirectional superthermal electron fluxes (BDE) • a declining of Vp • low Tp •  abundance increase • low  • relatively small Np • Generally, at least 3 signatures • None of the above features is necessary when any 3 features in the criteria list are prominent • For ambiguous events, check SOHO LASCO CME catalog to assure identification

11. (Riley andOdstrcil MHD simulation) Group 3 Group 2 Magnetic Clouds & ICME Grouping • Magnetic clouds (MCs), a specific subset of ICMEs, characterized by • enhanced magnetic field strength • smooth magnetic field rotations through a relatively large scale • Low  • Our hypothesis: each ICME has a central flux rope • Sort ICMEs into 3 groups depending on Pt temporal profiles • 3 groups of Pt profiles are probably due to different approach distances to the central flux rope Group 1

12. Three Groups of ICMEs Group 1 Group 2 Group 3 Containing well defined MC with central maximum in Pt (probably self-balanced forces due to field curvature) Containing magnetic obstacle with central Pt “plateau” Poorly-defined magnetic obstacle with monotonic declining of Pt post shock

13. Variation of SIR (CIR) properties with heliocentric distance

14. Variation of ICME properties with heliocentric distance

15. Radial extent (W) of SIRs & ICMEs • W of SIRs increases linearly with heliocentric distance R, but the angular width decreases continuously. • W of ICMEs increases as ~ 0.37×R0.85 within 1 AU, and then the expansion slows greatly farther out.

16. Comparison of the properties of SIRs and ICMEs at 1 AU SIR ICME (Jian et al., 2006a, 2006b)

17. Solar cycle variation of the fractional occurrence rates of ICMEs in 3 groups 1 AU (min) (max) The fraction of Group 1 ICMEs decreases with solar activity, vice versa the fraction of Group 3. It suggests the possibility of encountering the central flux rope decreases with solar activity.

18. Summary • CIR fraction decreases with heliocentric distance • From Venus to Jupiter orbit, the radial extent of SIRs spreads by a factor of 3 (0.34 → 1.16), but the angular width actually decreases continuously. • Most shocks associated with SIRs start forming at 1 AU • Properties of CIRs mimic those of SIRs, suggesting that SIRs and CIRs involve the same physical mechanisms • At 5.3 AU, SIRs and ICMEs have interacted and merged much, causing more hybrid events than 1 AU • From 0.72 to 5.3 AU, the ICME expansion weakens, as the expansion speed decreases by a half, and the radial extent is much smaller than the expectation of the power law fit of radial extents within 1 AU • Declining rate of Pmax and Bmax: ICME > general ambient solar wind > SIR • For more detail, please see Jian et al.’s posters • “Radial evolution of stream interactions from 0.72 to 5.3 AU” • “Radial evolution of interplanetary coronal mass ejections from 0.72 to 5.3 AU”