Transcript Storms

ESS 200C
Lecture 18
• An isolated substorm is caused by a brief (30-60 min) pulse of
southward IMF.
• Magnetospheric storms are large, prolonged disturbances of the
magnetosphere caused by variations in the solar wind.
– Many storms follow coronal mass ejections.
– Storms also can be caused by high speed streams (interplanetary
shocks).
• The impulse from the interplanetary disturbance impulsively
compresses the magnetosphere.
– The sudden compression rapidly increases the magnetopause current
increasing the H- component of the magnetic field.
– The sudden commencement can be seen in midlatitude
magnetograms.
– The rise time is a few minutes and corresponds to the propagation
time of MHD waves from the magnetopause to the point of
observations.
– The compressive phase of the storm lasts 2 to 8 hours.
– When not followed by the other phases of the storm this part is called
a sudden impulse
• The ring current causes decreases in the horizontal component of
the magnetic field at the Earth’s surface.
• The disturbance storm time (Dst) index measures these
differences.
Sudden Commencement
Main Phase
Recovery Phase
• Extended periods (several
hours) of southward IMF lead to
the main phase of the magnetic
storm.
– Southward IMF leads to
magnetic reconnection.
– Northward IMF has only
minimal dayside reconnection.
• The increased dayside
reconnection increases the
penetration of the solar wind
into the magnetosphere.
• The enhanced duskward electric
field increases the number of
particles injected into the ring
current.
– Stronger electric fields lead to
earthward expansion of the ring
current region.
– Heavy ionospheric particles
also are added to the ring

B
2 WRC

eˆz
B0
3 Wmag
• The ring current will grow and Dstwill decrease (
)
and approach a saturation level when particle sources and
losses balance.
• The period during which the ring current increases is the main
phase.
• As the southward component of
the IMF weakens or disappears,
the ring current starts to decay.
This is the recovery phase of
the storm.
• The recovery phase has several
steps.
– The reduction of the southward
IMF causes the reconnection
rate to decrease.
– The reduction of the southward
IMF results in a decreasing
electric field which leads to a
reduction in the injection of new
particles into the ring current.
– The convection boundary moves
outward.
– The ionosphere fills the depleted flux tubes within this expanded
boundary with cold ionospheric particles.
– The interaction between the two plasma populations (hot ring
current and cold ionospheric) causes plasma waves which scatter
the ring current particles into the loss cone. This causes a loss of
ion ring current particles.
– Another loss mechanism for ring current particles is charge
exchange.
 Charge exchange occurs between energetic ring-current ions and
cold hydrogen atoms.
 The result is energetic neutral atoms and cold ions.
 Detectors which can detect the energetic neutral atoms are have been
developed. They enable us to image the ring current in three
dimensions.
–
The result of the last two processes is a gradual decrease of the
ring current over several days.
Energetic Ring Current Ion
Thermal Neutral Atom
+
+
Thermal Ion
Energetic Neutral Atom
(leaves the system)
• During quiet times the solar wind
provides ~65% of the ring current
energy density and the ionosphere
only ~35%. (H+ dominant).
• During small and moderate storms
the ionospheric contribution
becomes ~50% (H+ dominant).
• During intense storms (Dst<-150
nT) the ionospheric contribution
increases to ~70%. (O+ dominant).
• The O+ dominance during intense
storms is greater during solar
maximum.
– Increased solar EUV irradiation
causes increases ionospheric and
atmospheric scale heights which
favors the escape of O+.
– Increased heating of neutral
atmosphere and increased
ionization rates.
• Ring current injection can be
explained primarily in terms
of inward transport of plasma
sheet and pre-existing ringcurrent particles.
• None of the models currently
includes the ionosphere.
• Diffusion has been used
successfully to study the
injection of radiation belt
particles during a storm (see
figure at the right). However,
the diffusion calculations
don’t seem to work for the
lower energies of the ring
current.
New Radiation Belt formed by October
2003 Magnetic Storm
• During magnetic storms
precipitation of auroral
particles expands
toward lower latitudes.
• Intense red and greenline auroral emissions
are found at the
equatorward most part
of the expanded auroral
oval.
• Magnetic storms can be caused by high speed solar wind.
• On September 24, 1998 a strong interplanetary shock reached
the Wind spacecraft 185RE upstream of the Earth.
– When this hit the Earth the pressure at the nose of the
magnetosphere went from 2nPa to 15nPa.
– The x-component of velocity was -900 km/s
– The IMF initially was horizontal but after 2 hours it turned southward
and a strong storm began.
Bz GSM (nT)
Dst
Vx (km/s)
• Solar variability effects human activities in three ways.
– Space travelers can be exposed to potentially lethal radiation
especially when carrying out activities outside of the spacecraft.
– Technology both in space and on the ground can be damaged
especially during some magnetic storms.
• Satellites are damaged by energetic ions and electrons.
• High frequency communications used by airplanes (30-300MHz) can
be disrupted.
• The Wide Area Augmentation System that uses Global Positioning
Satellites to aid aircraft navigation can be disrupted by events that
effect the GPS satellites and make precise approaches impossible.
• The power grid can be disrupted by induced currents during storms.
– There may be a relationship between terrestrial climate and solar
activity.
• When high energy particles encounter atoms or molecules
within the human body, ionization may occur.
– Ionization can occur when the particle is stopped by an atom or
molecule. The resulting radiation can ionize nearby atoms or
molecules.
– Bremstrahlung (radiation released by a “near” miss) can also ionize
atoms or molecules.
• A rad is the amount of ionizing radiation corresponding to 0.01
Joule absorbed by one kilogram of material.
– The rad unit is independent of the type of radiation.
– ~100 rads will cause radiation sickness (1Gy = 100 rads).
– 1 Gy has a high probability of killing a cell by producing a lesion in
its DNA.
– 1 rad received from x-rays is less harmful than 1 rad from high
energy protons.
• The relative biological effectiveness (RBE) of radiation is
normalized to 200 keV x-rays.
– The biological damage is measured in rem (rem=dose(rad)X RBE).
– Electrons, protons, neutrons and alpha particles are the most
damaging because they penetrate deeply into human tissue.
• The average person in the US gets ~170mr per year from
radioactive elements around us, cosmic rays and our food and
water.
• Astronauts must worry about a number of sources.
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–
–
–
Solar energetic particle events (SEPs)
Relativistic electron events (REE)
Passages through the south Atlantic anomaly
Radiation belts.
• Astronauts can received several times the average annual dose
in one short mission.
– At higher apogees astronauts can get hundreds of mr.
– The dose is lower at low latitudes than above 50o.
– Doses are higher for extra-vehicular activities since space suits
don’t have much shielding. Protons with greater than 10 MeV can
penetrate the suits.
– Spacecraft exteriors have several grams per cm2 of aluminum
shielding and can stop higher energy particles.
• Spacecraft charging is a variation of the electrostatic potential of
the spacecraft surface with respect to the surrounding plasma.
The resulting discharges can:
–
–
–
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Cause spurious electronic switching
Breakdown vehicle thermal coatings.
Degrade amplifiers and solar cells
Degrade optical sensors.
• Photoionization frees electrons from the spacecraft and it
develops a positive charge.
– Electrons may form a negative cloud near the spacecraft.
– If the entire surface was a homogeneous conductor this would not be
a problem but this isn’t the case.
– Differential charging of the sunlit surface with respect to the dark
surface.
• Electrons with energies of a few keV can penetrate the skin of the
spacecraft and charge it negatively.
SCATHA (Reagan et al., 1981)
Surface Charging: SCATHA Satellite
Observations.
• Electrons with energies between 2 and 10 MeV have enough
energy to get deep into satellite surfaces.
• The excess charge spreads out evenly on conducting surfaces
but the charge accumulates on dielectric surfaces resulting in
potential differences between different parts of the satellite.
• Eventually static discharges will occur. This can happen on
electron circuitry.
• Plot shows count rate of 3 MeV electrons versus time. Arrows
show times when the spacecraft star tracker had anomalies.
High Energy Electrons: Deep-Dialectric Charging
1. Electrons bury themselves in the insulator
4. Electrons build up faster than they leak
off
2. Electrons slowly leak out of the insulator
5. Discharge (electrical spark) that damages
or destroys the material
3. Influx of electrons increases to levels higher than the leakage rate
• Additional hazards that affect spacecraft systems.
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Variable atmospheric drag
Enhanced ionospheric ionization
Solar x-ray (SX) and Energetic Particle Events (SEPs).
Relativistic electron events (REE)
Magnetospheric particles and fields.
• Single Event Upsets
– Single event upsets are bit flips in digital micro-electronic circuits.
 Damage to stored data.
 Damage to software.
 Stop central processing unit (CPU).
 Cause CPU to write over critical data tables.
 Create faulty commands.
– Caused by high energy ions ionizing silicon electronics.
 Galactic cosmic rays.
 SEPs
 Radiation belts.
Background caused by Solar Energetic Particles
• Spacecraft operating below a few thousand kilometers encounter
a significant number of atmospheric particles during each orbit.
• Any mechanism that heats the atmosphere will produce density
increases above the level heated.
– Geomagnetic storms
– Changes in solar extreme ultraviolet (EUV) radiation.
• Heating during magnetic storms
– Strong field-aligned currents and enhanced electrojets contribute to
atmospheric heating.
– Most of the heating is in the auroral zone so polar orbiting satellites
experience the greatest effects.
• Enhanced drag can cause satellites to reenter the atmosphere.
– Enhanced drag at perigee will cause the orbit to become more
circular and increase the interval with drag.
– Even a single density increase will alter all future orbits.
• At auroral latitudes currents induced by magnetospheric activity
can interfere with the transmission of electrical power. One
major blackout was caused by this in Quebec.
• Numerous studies have suggested that solar events can effect
terrestrial atmospheric weather.
– The Maunder minimum was a little ice age.
– There is some evidence that penetrating particles can influence
cloud formation.
– These ideas are highly controversial.
• Precipitating high energy electrons may contribute to ozone
depletion.
Area affected by blackout.
Transformers destroyed by induced
currents.
Transformer
winding failure
Transformer exit lead overheating