ESS 200CESS 200CLecture 18Lecture 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 current population.

• 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.

zmag

RC eW

W

B

Bˆ

3

2

0

• 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 ring-current 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 green-line 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.

Dst

Bz GSM (nT)

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.– 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:– 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.

SC

AT

HA

(R

ea

ga

n e

t a

l., 1

98

1)

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.

3. Influx of electrons increases to levels higher than the leakage rate

2. Electrons slowly leak out of the insulator

1. Electrons bury themselves in the insulator 4. Electrons build up faster than they leak off

5. Discharge (electrical spark) that damages or destroys the material

High Energy Electrons: Deep-Dialectric Charging

• Additional hazards that affect spacecraft systems.– 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.

Transformer exit lead overheatingTransformer winding failure

Transformers destroyed by induced currents.

Area affected by blackout.