GEM 2014 Summer Workshop
Student Tutorial
__ _ Sunday 15 June 2014
Geomagnetic Storms
David A. Mackler
PhD Candidate Student
The University of Texas at San Antonio
Southwest Research Institute
mackler.david@gmail.com
Overview
Student Tutorial
__ _ Sunday 15 June 2014
• What is a storm?
• What causes a storm?
• How can you tell the storm source?
• Why is it important to study storms?
What is a Storm ?
Student Tutorial
From Wikipedia:
__ _ Sunday 15 June 2013
“A geomagnetic storm is a temporary disturbance of the Earth's magnetosphere
caused by a solar wind shock wave and/or cloud of magnetic field which interacts
with the Earth's magnetic field.”
Gonzalez et al. [1994] ‘What is a Geomagnetic Storm?’:
• A geomagnetic storm is defined by changes in the Dst index
So what is the Dst index?
• The Dst (disturbance, storm time) is a measure of the change in the horizontal
component of the global surface equatorial magnetic field
• Computed from four ground magnetometer stations
• More importantly: The Dst is a measure of the RING CURRENT
What is the Ring Current?
http://wdc.kugi.kyoto-u.ac.jp/dstdir/
What is a Storm ?
Student Tutorial
__ _ Sunday 15 June 2013
Single particle motion in a non-uniform magnetic field:
1. Gyromotion about B Field
Lorentz Force
Gyrofrequency
𝐹 = 𝑞𝑣 × 𝐵
Ω𝑐 = 𝑞𝐵 𝑚
Gyroradius
𝑅𝑐 = 𝑚𝑣⊥ 𝑞𝐵
2. Bounce Motion – First Adiabatic Invariant Conservation
Magnetic Moment
1
2
𝑚𝑣
⊥
𝜇≡2
𝐵
What is a Storm ?
Student Tutorial
3. Bounce Motion – Gradient & Curvature
Magnetic Gradient (Grad B) Drift
Curvature Drift
__ _ Sunday 15 June 2013
What is a Storm ?
Student Tutorial
__ _ Sunday 15 June 2013
RING CURRENT:
• Roughly equatorial distribution of (mostly) protons and electrons circling
the Earth, creating a current
• Within L = 2 – 9; maps to 45° - 70.5° invariant latitude (most dense L ≈ 3 – 5)
• During storms J x B force moves ring current Earthward
• Storm time current density ~ 7 nA/m2 (average ~1-4 nA/m2 )
• Energies of ~10 – 100’s of keV
Taken from ‘Basic Space Plasma Physics’, Baumjohann & Treumann
Figure is for a 1 keV particle, computed using bounce
averaged grad curvature drift – proportional to E
10 keV particle encircles Earth in ~24 hours
What is a Storm ?
Student Tutorial
__ _ Sunday 15 June 2013
~500 km/s
Main Phase
Storm Sudden Commencement
Recovery Phase
http://wdc.kugi.kyoto-u.ac.jp/aeasy/index.html (SYM-H Index, 1 min time cadence)
What Causes a Storm ?
Student Tutorial
Briefly: Magnetic Reconnection
• Magnetic flux brings oppositely directed B
fields together: May have a current sheet
• Diffusion region becomes small compared to
particle gyroradii
• Non-Adiabaticity: Ideal MHD breaks down
• B-Field reconnects, magnetic topology is
carried out the orthogonal direction
__ _ Sunday 15 June 2013
What Causes a Storm ?
Student Tutorial
Briefly: The Dungey Cycle
• Reconnection drives plasma convection;
ExB drift [Dungey, 1961]
• Dayside reconnection leads to
antisunward convection (FAC)
• Magnetic flux builds up in the tail, X-line
forms
• Reconnection in the tail convects
plasma to the inner-magnetosphere
• Ring Current forms
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What Causes a Storm ?
Student Tutorial
Coronal Mass Ejection (CME):
• Massive burst of solar corona plasma
• Has an embedded magnetic field
(Large CMEs may have a ‘magnetic
cloud’)
• May have a shock, Storm Sudden
Commencement
• “Geoeffectiveness”: Storm strength
linked to IMF clock angle, Dst, and
solar wind pressure
__ _ Sunday 15 June 2013
What Causes a Storm ?
Student Tutorial
__ _ Sunday 15 June 2013
Corotating Interaction Region (CIR):
• During solar min, Fast (Slow) winds originate from High
(Low) latitudes
• Solar magnetic pole is not always near the spin pole;
Parker Spiral
• Fast solar wind (high speed streams from coronal holes)
catches up with Slow solar wind
• Combined can cause the IMF clock angle to turn south; Geoeffectiveness (w/Dst, V)
Fast wind deflected counter-rotation
Slow wind deflected along-rotation
How Can You Tell The Storm Source ?
Student Tutorial
__ _ Sunday 15 June 2013
CME:
• More CMEs occur during Solar Max (~3 CMEs/Day) vs Solar
Min (~1 CME/5 Days)
• Typically stronger storms (lower Dst) are
CME driven
• May occur at irregular intervals
• Main phase of the storm is steeper, shorter
CIR:
• No well defined Parker Spiral during Solar Max
• Can persist periodically over multiple solar
rotations (~27 days)
• Main phase of the storm is less steep,
longer
How Can You Tell The Storm Source ?
Student Tutorial
__ _ Sunday 15 June 2013
CME Storm
15 May 2005
|B|
Bz
V
np
T
Xmp
How Can You Tell The Storm Source ?
Student Tutorial
__ _ Sunday 15 June 2013
CIR Storm
10 March 1998
|B|
Bz
V
np
T
Xmp
Why are Storms Important?
Student Tutorial
__ _ Sunday 15 June 2013
• Most significant geomagnetic event in the Sun-Earth connection
• Includes many other sub topics:
o Magnetic Reconnection; Dayside and Night
o High Latitude Bz North Reconnection
o Ionospheric Convection, Conductivity, and TEC (GPS!)

o Thermospheric Energy Balance, Density, and S/C Drag
o Aurora; Discrete and Diffuse
o Dipolarization Fronts, Bursty Bulk Flows
o Energetic Neutral Atom Emissions
o Resources; Satellites, Power Grids, Pipelines
Why are Storms Important?
Student Tutorial
__ _ Sunday 15 June 2013

24 Oct 2011
Why are Storms Important?
Student Tutorial
__ _ Sunday 15 June 2013
Great White North

Perry Lake, KS
39.1676°N
Enjoy GEM and Portsmouth!