1. Solar Wind Transients and SEPs
CSI 662 / ASTR Lect. 06, March Spring 2007
Solar Wind Transients and SEPs
References:
Lecture
Gombosi: Chap – 12.7, P248 – P252 (supplement)
Tascione: Chap. 3, P31-P40 (supplement)
Prolss: 6.2, P300-P314 (supplement)

2. Solar Wind: Bimodal Fast and Slow Wind
Fast wind originates from coronal hole,
Slow wind originates from regions close to streamer belts or heliospheric current sheet
SW heliographic latitudinal
Distribution (Ulysses observation)
Fast and Slow Wind

3. Solar Wind: Bimodal Fast and Slow Wind slow wind is denser and cooler
fast wind is thinner and hotter
Fast Solar Wind: originates in coronal holes
Has flow speeds between km/s;
average density is low ~ 3 ions/cm3 (1AU)
The proton temperature is about 2x105 K
The electron temperature is about 1x105K
Slow Solar Wind:
Speeds between km/s
Average density is ~ 8 ions/cm3 (1AU)
Solar Minimum -slow wind originates from regions close to the heliospheric current sheet
Solar Maxima - slow wind originates above the active regions in the streamer belt
Fast and Slow Wind

4. Heliospheric Current Sheet
In 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

5. 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

6. 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 storms
Increased IMF strength
Increased solar wind speed
Most importantly, the presence of southern IMF

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

8. 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

9. 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

10. 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

11. 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

12. Example Dst B/Bz Vel Np Tp Texp β Sun #75 2004/07/27 storm (-182 nT)
Shock Front: discontinuity
ICME (ejecta):
B enhance
Bz rotation
Low Plasma β
Low Tp
High QFe
-----
SH (Shock Sheath)
Solar Sources

13. Example

14. Shock
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.

15. Example of IP Shock

16. Shock Signal speed in the medium (Prolss Chap 6.3, P317-323)
Sound speed or acoustic wave speed, caused by thermal pressure
γ=5/3 for ideal gas
Alfven speed in magnetized plasma, caused by magnetic pressure

17. Shock The Rankine-Hugoniot relations (Gombosi Chap 6.1, P103-106):
1: upper stream; 2: downsstream
M: Mach number (flow speed/sound speed)

18. 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
Time-variation of SEP fluxes

19. 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:

20. The End