Solar Wind Transients

and SEPs

CSI 662 / ASTR 769 Lect. 06, March 20 Spring 2007

References: •Lecture•Gombosi: Chap. 12.5 – 12.7, P248 – P252 (supplement)•Tascione: Chap. 3, P31-P40 (supplement)•Prolss: 6.2, P300-P314 (supplement)

Solar Wind: BimodalFast wind originates from coronal hole,

Slow wind originates from regions close to streamer belts or heliospheric current sheet

SW heliographic latitudinalDistribution (Ulysses observation)

Slow Solar Wind:Speeds between 250-400km/sAverage density is ~ 8 ions/cm3 (1AU)Solar Minimum -slow wind originates from regions close to the heliospheric current sheetSolar Maxima - slow wind originates above the active regions in the streamer belt

Fast Solar Wind: originates in coronal holesHas flow speeds between 400-800km/s; average density is low ~ 3 ions/cm3 (1AU)The proton temperature is about 2x105 KThe electron temperature is about 1x105K

Solar Wind: Bimodalslow wind is denser and coolerfast wind is thinner and hotter

Heliospheric Current SheetIn a global sense, there is a huge current system flowing in a circumsolar disk, separating the two magnetic polarities

The current sheet is inclined with respect to the ecliptic plan•Solar rotation axis is 7° tilted•Solar magnetic dipole axis is tilted from the rotation axis

Magnetic Sector•The earth at one time above the current sheet, but at other times below the current sheet•During solar minima, current sheet is rather simple, resulting two magnetic sectors as seen from the Earth•During solar maximum, current sheet is complicated and highly distorted (warped), resulting in multiple magnetic sectors

Solar Wind Transients• The normal or background solar wind generally follows the

Archimedean spiral, characterized by the large scale sector magnetic structures and heliospheric current sheet

• They are usually steady and thus “quiet”; do not cause space weather disturbances

• Space weather is caused by solar wind transients, or highly disturbed solar wind.

• Solar wind transients are in two forms• Interplanetary CME (ICMEs)• Corotating interaction region (CIR)

• Solar wind transients are responsible for geomagnetic storms1. Increased IMF strength2. Increased solar wind speed3. Most importantly, the presence of southern IMF

Corotating Interaction Region (CIR)•When a low latitude coronal hole appears (across the heliographic equator), fast wind exists in the ecliptic plane.

Corotating Interaction Region (CIR)•The jetline of fast wind is less curved than that of slow wind•Fast streams “catching up” with slow streams, compressing the preceding stream and produce a high pressure region.•The interaction region is at the leading edge of the fast stream•Since low-latitude coronal holes can live over several solar rotations, this structure can recur several times•This is commonly called “corotating interaction region” or CIR•A pair of forward and reverse shocks forms

Interplanetary CME (ICME)•CME propagates into the interplanetary space, plowing through the ambient solar wind•The magnetic structure of ICME at 1 AU is similar to that in its solar origin, which is highly helical (called flux rope)•At 1 AU, it is called magnetic cloud

•highly organized magnetic field is observed, e.g., smooth rotation•Large scale, crossing the Earth for ~ 24 hours

Magnetic Cloud

Interplanetary CME (ICME)

•A Fast ICME pushes the interplanetary plasma, and produces a shock wave ahead of it.•A CME driven shock is efficient in accelerating energy particles•In addition to geomagnetic storms, CMEs are also responsible for energetic particle storms.

ICME driven shock

SW Observations•Direct solar wind observations are routine now•ACE (Advanced Composition Explorer) (1997-present) spacecraft at Lagrangian point 1•WIND (1994-present) spacecraft (complicated orbit, sampling different parts of space)

•Measuring•Magnetic field, 3-D•Plasma velocity, density, temperature•Particle energy, abundance, charge state, composition

ExampleDst

B/Bz

Vel

Np

TpTexp

β

Sun

#752004/07/27 storm (-182 nT)

Shock Front: discontinuity

ICME (ejecta): • B enhance• Bz rotation• Low Plasma β• Low Tp• High QFe

• -----

SH (Shock Sheath)

Solar Sources

Example

A Shock is a discontinuity separating two different regimes in otherwise continuous medium.

• It is associated with a disturbance moving faster than the signal speed in the medium (in a gas the signal speed is the speed of sounds; in space plasma: alfven speed)

• At the shock front the properties of the medium change abruptly. In a hydrodynamic shock, temperature and density increase- in a magnetohydrodynamic shock, magnetic field strength also increase.

Shock

Example of IP Shock

ShockSignal speed in the medium (Prolss Chap 6.3, P317-323)

•Sound speed or acoustic wave speed, caused by thermal pressure

P

Vs

•Alfven speed in magnetized plasma, caused by magnetic pressure

4

2 2BPV B

A

γ=5/3 for ideal gas

The Rankine-Hugoniot relations (Gombosi Chap 6.1, P103-106):

T2

T1

2 ( 1)M1

2 2M12 ( 1)

(1)2 M12

n2

n1

u1

u2

(1)M1

2

2 ( 1)M12

p2

p1

2M12 ( 1)

(1)

Shock

1: upper stream; 2: downsstream

M: Mach number (flow speed/sound speed)

Time-variation of SEP fluxes

Solar Energetic Particles (SEPs)

•SEPs with energies ranging from a few Kev to several Gev•Because traveling close to speed of light, they reach the Earth in tens of minutes of the eruption•Small SEPs are caused by flare related acceleration, lasting short (minutes)•Large SEPs from CMEs

Solar Energetic Particles (SEPs)•Large SEPs are accelerated by CME-related IP shocks.•They can last for several days because of the continuing driving of the shock•Particle energy is gained from the kinetic energy of the shock front.•Microscopic processes are complicated:

The End