Interplanetary Transients

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Introduction to Space Weather
The Heliosphere:
Interplanetary Transients
March 08, 2012
Jie Zhang
Copyright ©
CSI 662 / PHYS 660
Spring, 2012
Roadmap
•Part 1: Sun
•Part 2: Heliosphere
•Part 3: Magnetosphere
•Part 4: Ionosphere
•Part 5: Space Weather
Effects
CH5: The Solar Wind
CH6: Interplanetary Transients
CSI 662 / PHYS 660
Mar. 08, 2012
CH6: Interplanetary Transients
6.1. Corotating Interaction Regions
6.2. Interplanetary Coronal Mass Ejections
6.3. Shock Waves
6.4. Solar Energetic Particles
Plasma-10: Shock Wave Models
CH6: Interplanetary Transients
References and Reading Assignment:
•KAL CH 6.3.4
(on CIRs)
•KAL CH 6.7.7
(on ICMEs)
•KAL CH 6.7.8
(on Interplanetary Shocks)
•KAL CH 6.8
(on Shock Theory)
Introduction
• The magnetic field of 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
• Corotating interaction region (CIR)
• Interplanetary CME (ICMEs)
• Solar wind transients are responsible for geomagnetic storms
1. Increased IMF strength
2. Increased solar wind speed
3. Most importantly, the presence of southward IMF (Bz)
CH 6.1. CIR
•When a low latitude coronal hole appears
(across the heliographic equator), fast
wind exists in the ecliptic plane. The
magnetic field of the fast wind is less
curved than that of the slow wind
•The fast stream “catches up” with the
slow stream, compressing the preceding
stream and producing a high pressure
region.
•This interaction region is at the leading
edge of the fast stream, commonly called
“corotating interaction region” or CIR
•Since low-latitude coronal holes can live
over several solar rotations, this structure
can recur several times.
CH 6.1. CIR
CH 6.2. Interplanetary CME (ICME)
•A CME propagates into the interplanetary space, plowing
through the ambient solar wind
•The magnetic structure of an ICME at 1 AU is similar to that
in its solar origin, which is a highly helical flux rope.
•At 1 AU, it is also called magnetic cloud
•highly organized magnetic field is observed, e.g.,
smooth rotation
•Large scale, crossing the Earth for ~ 24 hours
Magnetic Cloud
CH 6.2. ICME
•A Fast ICME pushes through 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 (3-D), density, temperature
•Particle energy, abundance, charge state, composition
Example
In Situ Observations of the 2008 Feb. 4 CME
Credit: Brian Woods
12
CH 6.3. Shock Waves
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 increases.
Example
of IP Shock
Shock Wave Theory
Kallenrode: Chap 6.8
•Rankine-Hugoniot Equations:
•Relate parameters between upstream and downstream
•M: Mach number (shock speed/signal speed)

CH 6.4. Solar Energetic
Particles (SEP)
•SEP with energies ranging
from a few Kev to several
Gev
•Because traveling close to
the speed of light, they reach
the Earth in tens of minutes
of the eruption
•Small SEPs are caused by
flare related acceleration
•Large SEPs from CMEdriven shocks
Time-variation of SEP fluxes
CH 6.4. SEP
•Large SEPs are accelerated by CME-driven 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.
•Refer to Kallenrode Chap 7 for details, if you are interested.
The End
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