Transcript BRIEFING TITLE - ALL CAPS 30 Jan 01

Space Weather: Magnetic Substorms
7 November 2011
William J. Burke
Air Force Research Laboratory/Space Vehicles Directorate
Boston College Institute for Scientific Research
CRESS
C/NOFS
DMSP
Space Weather
Course Overview
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Lecture 1:
Overview and Beginnings
Lecture 2:
The Aurorae
Lecture 3:
Basic Physics (painlessly administered)
Lecture 4:
The Main Players
Lecture 5:
Solar Wind Interactions with the Earth’s Magnetic Field
Lecture 6:
Magnetic Storms
Lecture 7:
Magnetic Substorms
Lecture 8:
Magnetosphere – Ionosphere Coupling
Lecture 9
The Satellite Drag Problem
Lecture 10:
Verbindung (to help make up for your rash decision not to take
Wollen Sie Deutch Sprechen?)
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Space Weather
Magnetic Substorms
Overview
The subject matter of Lectures 6 through 8 are closely linked
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• In the previous two lectures we introduced the concepts of a super sonic
• The consequent, distorted magnetic field of Earth
no longer looks like a dipole. Two clefts or cusps
develop on the dayside that separate magnetic flux
that close on day and night side. Solar wind plasma
has direct access to the ionosphere through the cusps.
Dayside cusps
solar wind carrying a weak interplanetary magnetic field (IMF) that
interact with the Earth’s magnetic field. The interaction produces a
magnetic cavity in the solar wind called the magnetosphere.
• In Dungey’s model the cusp separates closed and open flux
• Today we first consider ionospheric and magnetospheric convection
patterns during periods of southward IMF and how they are disrupted
during geomagnetic substorms.
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Space Weather
Magnetic Substorms
Dungey model of magnetic
merging-reconnection cycle
Ionospheric footprint of
convecting plasma & field
• Simplified representation of high-latitude
convection pattern.
• Ionospheric plasma drifts on equipotentials
away from the Sun across polar cap and
back toward the Sun through auroral oval.
• Associated two-cell current system: DP2.
• Stations rotating under DP2 current see
+/- BH before/after local midnight.
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Space Weather
Magnetic Substorms
• Kristian Birkeland was first to describe a pattern of nightside magnetic
disturbance that he called “polar elementary storms.”
- Signatures appear in high- but not low-latitude magnetometer
data (unlike storms).
- Note that all stations saw negative magnetic perturbations => DP1.
- Initial slope was gently downward: => “growth phase” (30 min)
- Traces steepened sharply:
=> “expansion phase”
- Minimum excursion in BH :
=> “maximum epoch”
- Positive slope:
=> “recovery phase”
- Typically all over in < 2 hours
• Sydney Chapman introduced the now standard term “substorm” (~1960).
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Space Weather
Magnetic Substorms
Auroral Substorm: In 1964 Syun-Ichi Akasofu
pieced together all-sky images taken during the
1958 IGY that showed the development of
aurorae during substorms.
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T < 0 min:
quiet-time, dim auroral arcs
T = 0-5 min: equatorward arc brightens
T = 5-10 min: poleward auroral expansion
T = 10-30 min: auroral breakup
T = 30-60 min: early recovery phase
T = 60-120 min: late recovery phase
• After more than 20 years of satellite imaging
of aurora, Akasofu’s summary still accepted.
• Figure from Akasofu Polar and Magnetospheric
Substorms, (D. Reidel, 1968), summarizing
IGY ground observations of discrete aurora.
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Space Weather
Magnetic Substorms
All the different sets of visible and
extreme ultraviolet images of the
high-latitude ionosphere acquired
during magnetic substorms confirm
the picture that Akasofu assembled
in the mid 1960s.
Integration times of 6 to 12 minutes
Do not allow us to see the dynamics
seen by the human eye on the ground.
- Point made earlier by Størmer.
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Space Weather
Magnetic Substorms
• The first indication that something very
different from predictions of the simple
Dungey model came from auroral/plasma
sheet particle measurements acquired by
VELA satellites at about 14 RE in the
magnetotail.
• Ed Hones of LANL noticed that if VELA
satellites were in the plasma sheet and
ground magnetometers indicated that a substorm growth phase
onset occurred, the satellites suddenly shifted to the open-field
line lobes of the magnetotail and stayed there until early recovery.
When VELA returned to the plasma sheet it was much hotter than
was measured before the substorm process began.
• He suggested an new model for loading the lobes with new flux
in the growth phase with a thinning of the plasma sheet and that
a new reconnection line forms close to Earth.
VELA
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Space Weather
Magnetic Substorms
About the same time Bob McPherron of
UCLA argued that the single polarity
of BH perturbations could be explained
if a substorm system of field-aligned currents
flow into the ionosphere on the morning side
of local midnight and out on the evening side.
- The development of McPherron’s postulated FACs have been verified
in both satellite and ground based measurements.
- Model correctly predicts that magnetic fields near local midnight
at geostationary altitude should change from stretched and tail-like
in the growth phase to nearly dipolar at the onset of the expansion
phase. This too has been verified in CRRES measurements.
- After 40 years, controversy still exists as to whether the near-Earth
reconnection line or the new current system forms first.
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Space Weather
Magnetic Substorms
This controversy is not within the observational community but among
theoretical modelers. Everyone agrees the picture of substorm morphology
in the magnetosphere is correct.
Still they squabble endlessly (and I suspect enjoying every minute of it)
concerning the nature of the underlying physics connected with triggering
magnetotail current diversion to the ionosphere and the formation of a nearEarth reconnection. No one wants to admit we just don’t know!
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Space Weather
Magnetic Substorms
• This spectrogram shows the history of energetic electron fluxes sampled
by the geosynchronous satellite ATS 6 on 29 July 1974.
• Jagged peaks mark the arrival of electrons in 3 substorms, and they
gradually drift away again.
• Lower energy (~ 1000 ev) electrons that persist belong to the plasma
sheet in which the satellite is immersed for about half of its orbit.
• The high-energy substorm injection population can lead to severe
charging of spacecraft, especially it is in eclipse at substorm onset
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Space Weather
Magnetic Substorms
Summary and Conclusions
• During the GROWTH PHASE of substorms, the magnetotail configuration
becomes stretched, and density of the cross-tail current becomes very large.
– Energy is stored in the tail lobes during the growth phase.
• Energy release begins suddenly at the onset of the EXPANSION PHASE.
- Field lines that were very stretched and tail-like collapse to dipolar shapes.
– Aurora suddenly brighten, and the ionospheric conduction currents
in the westward electrojet intensifies greatly near local midnight.
– As expansion phases proceed, regions of depolarization, bright aurorae
and intense electrojet currents expand to fill nightside auroral oval.
- High-energy electrons cause severe charging of geosynchronous satellites.
• Ionospheric currents/auroral activity gradually die in RECOVERY PHASE.
– Post-substorm plasma sheet is hotter than it was before.
- One or more substorms can occur in main phase of magnetic storms.
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