Radial Evolution of Major Solar Wind Structures

Lan K. JianThanks to: C.T. Russell, J.G. Luhmann, R.M. Skou

g

Dept. of Earth and Space SciencesInstitute of Geophysics and Planetary Physics

University of California, Los Angeles

Whistler, Canada July 29, 2007

SHINE Student Research Talk:

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

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

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)

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)

Structure & Evolution of Interplanetary CMEs

• CMEs with the typical 3-part structure– a leading outward moving bright f

ront– a dark cavity– a bright core of filament plasma at

the trailing edge

• Generally assume: bright front sheath of compress

ed 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)

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 Ulysses near aphelion & within 10o of the ecliptic

plane (partial 1992, 1997 ~ 1998, 2003 ~ 2005)

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) = B2/2o + ∑jnjkTperp,j, where j = H+, e-, He++. The interaction force (Pt) drives the evolution.

SIR Identification• Criteria (by eye)

1. Increase of Vp2. A pile-up of Pt with gradual decr

eases at two sides3. Increase and then decrease of Np4. Enhancement of Tp5. Flow deflection 6. Compression of B, usually assoc

iated with B shear7. 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

ICME Identification• “Something of art”• Criteria (by eye)

1. Pt enhancement2. a stronger than ambient B3. a relatively quiet B4. relatively smooth rotations in B5. bidirectional superthermal electron fluxes

(BDE) 6. a declining of Vp7. low Tp8. abundance increase9. low 10.relatively small Np

• Generally, at least 3 signatures • None of the above features is necessary wh

en any 3 features in the criteria list are prominent

• For ambiguous events, check SOHO LASCO CME catalog to assure identification

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 distanc

es to the central flux rope

Group 2

Group 3

(Riley and Odstrcil MHD simulation)

Group 1

Three Groups of ICMEs

Group 1 Group 2 Group 3

Containing well defined MC with central maximum in Pt (probably self-balancedforces due to field curvature)

Containing magnetic obstacle with central Pt “plateau”

Poorly-defined magnetic obstacle with monotonic declining of Pt post shock

Variation of SIR (CIR) properties with heliocentric distance

Variation of ICME properties with heliocentric distance

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.

Comparison of the properties of SIRs and ICMEs at 1 AU

SIR ICME

(Jian et al., 2006a, 2006b)

Solar cycle variation of the fractional occurrence rates of ICMEs in 3 groups

1 AU

The fraction of Group 1 ICMEs decreases with solar activity, vice versathe fraction of Group 3. It suggests the possibility of encountering the central flux rope decreases with solar activity.

(min) (max)

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 inv

olve the same physical mechanisms At 5.3 AU, SIRs and ICMEs have interacted and merged much, causing mo

re 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”