Jodie Barker Ream
GEM Student Tutorial
June 15, 2014
SUBSTORMS
The Dynamic Magnetosphere
 If the solar wind has a negative Bz component,
magnetic reconnection can occur on the dayside
magnetopause and must then be balanced by
additional reconnection on the nightside. This
sets up the ‘Dungey Cycle’ (J.W. Dungey, 1958)
WHAT IS A SUBSTORM?
This is a question that people have been
trying to answer for over 50 years.
 Wikipedia’s answer: A substorm (aka.
magnetospheric substorm or auroral substorm) is
a brief disturbance in the Earth's magnetosphere
that causes energy to be released from the "tail"
of the magnetosphere and injected into the high
latitude ionosphere.
But how is the energy released??
 The reason for the long standing debate about
substorms goes back to the limitations on observations.
 The question: Inside-out or Outside-in?
Substorms And Related Phenomena
 Focusing on the outside-in
paradigm
 Several phenomena are
observed related to substorms
Substorms And Related Phenomena
Dipolarization and fast flows
 Focusing on the outside-in
paradigm
 Several phenomena are
observed related to substorms
Substorms And Related Phenomena
Dipolarization and fast flows
 Focusing on the outside-in
paradigm
 Several phenomena are
observed related to substorms
Substorm Current Wedge
Substorms And Related Phenomena
Dipolarization and fast flows
 Focusing on the outside-in
Substorm Current Wedge
paradigm
 Several phenomena are
observed related to substorms
Shear and Compressional Waves
Phenomenoloy of a Substorm
The Near-Earth Neutral Line (outside-in) paradigm
 Reconnection begins near -15 to -20 RE
 Fast Flows travel earthward
 Lead to wave generation and injection events
 A decrease is observed in the AL index
 Onset of magnetic pulsations
 The aurora brightens and breaks up
‘concurrently’ with the onset of magnetic
pulsations
Foster et al. 1999
McPherron
1995
Phenomenoloy of a Substorm
The Near-Earth Neutral Line (outside-in) paradigm
 Reconnection begins near -15 to -20 RE
 Fast Flows travel earthward
 Lead to wave generation and injection events
 A decrease is observed in the AL index
 Onset of magnetic pulsations
 The aurora brightens and breaks up
‘concurrently’ with the onset of magnetic
pulsations
Foster et al. 1999
McPherron
1995
What Is Substorm Onset?
 Typical features used to identify onset are
 The first brightening in an equatorward auroral
arc (This is the original method)
 The beginning of a sharp drop in the AL index
 The beginning of Pi2 pulsations on the ground at
mid latitudes
 But…
 The timing for each of these signatures can be
offset by several minutes
 This means that ‘onset’ is not well defined (or at
least not agreed upon)
Alternate Paradigm
 Inside-out
 The disturbance begins in the inner magnetosphere
or in the ionosphere
 Auroral Break-up
 There is a ballooning instability in the tail which
initiates reconnection and earthward flows
 The question is: What happens first?
So what are Substorms?
 Brief disturbances in the Earth's magnetosphere
that cause energy to be released from the
magnetotail and injected into the high latitude
ionosphere.
 Observed visually from the ground as a sudden
brightening of the equatorward auroral arc
 Observed as a change in the magnetic field measured
from the ground accompanied by the onset of magnetic
pulsations
 Substorm onset can be determined using auroral
brightening, Pi2 onset, or the beginning of a sharp
decrease in the AL index, however, this can lead to
confusion due to the limitations of existing
instruments (spatial and temporal resolution).