PS2 - 2 Flashcards
Describe the 3 components of charged particle motion in the magnetosphere
Charged particles feel the Lorentz force which makes them obit around magnetic field lines, executing cyclotronic motion –typically at MHz frequencies. They move along the field lines until the convergence of the field causes the particle to be deflected back along the field line to the opposite hemisphere, where exactly the same thing occurs, thus causing the particles to “bounce” from “mirror-point” to “mirror–point” in each hemisphere –typically once per second. The third component of motion is a drift around the Earth due to the cyclotronic orbit of the particle causing it to experience a stronger field when it is closer to the Earth, and a weaker field when it is further away. Electrons drift eastwards, protons drift westwards –typically drifting around the Earth in a few hours
Considering the magnetic equator, what is the radial extent of the Van Allen proton belt?
The Proton belt comes down to low altitude ~ 1.1 Earth radii (from the Geocentre) in the South Atlantic Anomaly (SAA) and goes out to approximately 4 Earth radii. The peak flux occurs at ~ 1.7 Earth rad
Considering the magnetic equator, what is the radial extent of the Van Allen electron belts?
There is an inner and out electron belt. The inner electron belt is more-or-less coincident with the proton belt, coming down to low altitude (1.1 Earth-radii) in the SAA, and going out to ~ 2.5 Earth radii, with a peak flux about 1.4 Earth-radii. There is a much more extensive outer electron belt going from approximately 3 Earth radii to 9-10 earth radii, with a peak flux at approximately 4.5 Earth radii. Between 2.5-3 Earth radii there exists the so-called slot region
Comment on the radiation environment experienced by the GPS spacecraft (approximately 17,700 km altitude
17,700 km puts he spacecraft at (17700+6378/6378) = 3.78 Earth radii more-or-less in the heart of the outer Van Allen belt. This subjects the spacecraft to very high fluxes of high energy electrons
Why does the South Atlantic Anomaly exist?
The tilt and the Earth’s magnetic dipole with respect to its rotational axis (11o), and the offset of the dipole centre from the geocentre, means that the radiation belts, which are ordered by the geomagnetic field, come closest to the Earth just off the coast of Brazil in the SAA
Explain why some degree of material shielding is helpful, but the shields should not be excessively thick.
Moderate thicknesses of material shielding can stop the lower energy particles, but the higher energy particles (in particular protons and heavy ions) cannot be stopped by any practical shield –therefore as shield thicknesses increase, to begin with, the radiation dose deposited behind the shield drops dramatically, but at a certain point additional shielding provides little extra protection for the added mass. Thick shields do also generate Bremsstrahlung (X-ray) dose
Give the approximate composition of Galactic Cosmic Rays
GCRs comprise approximately 85% protons, 14% alphas and 1% all other io
Which orbits are most at risk from solar particle event
GEO are particularly at risk, as are MEO’s if they have sufficient inclination. LEO orbits with high inclinations (such as Sun-Synchronous Orbits) also encounter SPE particles over the auroral zones. (> 65olatitude).
What are the radiation risks of interplanetary travel?
The is a continuous background GCR flux. This is a relatively low flux (2-4 particles cm-2s-1) of very high energy (typically a few GeV) particles. This is not a significant source of radiation dose, but may induce single-event effects. The main risk to interplanetary travel comes from solar particle events. These comprise very high fluxes (1000’s of particles cm-2s-1) of high energy (typically 100’s MeV) particles which can deposit significant doses (100’s of rads) in a matter of hours to a few days –easily lethal to an astronaut and damaging to spacecraft electronics. They are also a SEE threat
Explain the mechanism for total dose damage in MOSFET transistors
The MOSFET transistor relies for its operation on controlling the conductivity of a channel between a source and drain electrode by means of a potential applied to a third gate electrode. The gate electrode is isolated from the channel by means of a thin insulating oxide layer. The passage of ionising particles through this layer causes electron hole pairs to be produced, and the electrons, being mobile in SiO2 escape leaving the positively charged “holes” trapped in the oxide. Thus, exposure o ionising radiation dose causes the oxides to become increasingly positively charged. This charge applies a potential to the gate, either helping it to turn on or making it more difficult to turn it on depending upon the transistor type (nMOS or pMOS). As a result, the power consumption of a device increases and it eventually fails permanently on or off
Name two types of destructive and two types of non-destructive single-event effects
Single event upset (SEU) and single event transient (SET) error are both non-destructive SEEs –the state change is not permanent. Single event latch-up (SEL) and single event burnout (SEB) (and single event gate rupture (SEGR)) are all permanently damaging SEEs
