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 large, prolonged V x B z in solar wind. Many storms follow coronal mass ejections (CMEs; during solar max). Solar Wind has increased magnetic field (flux rope) and high speeds Bipolar signature in Bz depends on solar cycle (+/- versus -/+) Storms can be caused by high speed streams (during solar min). Recur with solar rotation period, 27 days, due to coronal hole recurrence CME or fast wind generate interplanetary shock as overtaking slow wind Impulse from the interplanetary disturbance impulsively compresses the magnetosphere. The sudden compression rapidly increases the magnetopause current and brings it closer to Earth increasing the H- component on the ground. 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 ring current.
RC The ring current will grow and D st will decrease ( = ) and eˆ z B0 3 Wmag approach a saturation level when particle sources and losses balance. Ion Losses: charge exchange, scattering into the loss cone and outward diffusion + magnetopause loss Electron Losses: pitch angle scattering into loss cone ΔB r 2 W 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(er) hydrogen atoms of the extended exosphere. 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 Ion + + Thermal Neutral Atom Energetic Neutral Atom (leaves the system)
Images in Extreme UltraViolet (EUV) (30.4nm line) of the He + resonant absorption and scattering of sunlight provides a global snapshot of the plasmaspheric He + density. This is produced by photoionization of exospheric He, which provides an image of the exospheric density. This image is taken from the EUV instrument on the IMAGE satellite (Sandel et al., 2000).
Charge exchange of energetic ions with exospheric neutrals produces ENA images which image the charged particle density of the ring current in the energy range of the neutrals. This image is taken from the High Energy Neutral Atom instrument on the IMAGE satellite, imaging the Oxygen between 50-200keV, the peak contributors to the energy density of the storm time ring current (Mitchell et al., 2000).
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 (D st <-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.
Examples of magnetic storms: On September 24, 1998 a strong interplanetary shock reached the Wind spacecraft 185R E 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)
THEMIS recently discovered that (+/-) North-then-South polarity of CMEs results in 20 times more solar wind plasma penetrating Earth's magnetosphere ahead of a storm This causes a cold-dense plasma sheet prior to the storm Results in intense storm time ring current compared to (-/+) CMEs Boundary Layer under Northward IMF Boundary Layer under Southward IMF
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. Bremsstrahlung can also ionize atoms or molecules. A rad (Radiation Absorbed Dose) is the amount of ionizing radiation corresponding to 0.01 Joule absorbed by one kilogram of material. The SI unit for radiation is 1gray (1Gy) which is equal to 1J/kg = 1m 2 /s 2 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 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 to produce the equivalent biological effectiveness for the radiation being considered. For 200keV X- or γ-rays RBE=1. The SI unit is Sievert (1SV) but the rem is used (Radiation Equivalent in Man). 1 rem = 0.01SV. 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 energy deposition in matter is quantified by the Linear Energy Transfer (LET). This energy deposition LET is equal to the radiation energy loss per unit distance, except when bremsstrahlung is emitted (travels far from particle track). For biological applications LET is measured in kev/μm For electronics applications LET is measured in MeV cm 2 /mg; i.e., energy per distance divided by mass density.
The average person in the US gets ~170mr per year from radioactive elements around us, cosmic rays, 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 receive several times the average annual dose in one short mission. At higher apogees astronauts can get hundreds of milli rads. The dose is lower at low latitudes than above 50 o. Doses are higher for extra-vehicular activities since space suits have little shielding. Protons >10 MeV penetrate typical suits. Spacecraft exteriors have several grams per cm 2 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 Break down thermal coatings (kapton, blanket layers, Dag). 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, and poor conductance paths result in kilovolt potentials. Electrons with energies of a few kev can penetrate the skin of the spacecraft and charge that portion of the spacecraft negatively. Differential charging relative to the rest of the spacecraft develops. In shadow, the entire spacecraft becomes negatively charged.
Surface Charging: SCATHA Satellite Observations. SCATHA (Reagan et al., 1981)
Electrons with energies between 2 and 10 MeV have enough energy to get deep into satellite surfaces and results in component malfunction 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 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
Single Event Effects (SEEs) Apply to both ground (cell phones, computers) and space Are the result of individual penetrating ion hits causing ionization that can temporarily or permanently damage a component Single Event Upsets or soft errors bit flips can result in I/O error Caused by:» Galactic cosmic rays» Solar Energetic Particles» Radiation Belts Result in:» Damage to stored data» Damage to software» Cause processor to write over critical data tables.» Create faulty commands. Single Event Latchups result in shortcircuit of power rails, and over-current, possibly permanently damaging component.
Background caused by Solar Energetic Particles
Other Hazards from Space Weather: 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