StefanR wrote:Space Weather Prediction Center Topic Paper:
Satellites and Space Weather
http://www.swpc.noaa.gov/info/Satellites.html
Quote:
Types of Spacecraft Anomalies
Spacecraft anomalies are grouped into broad categories based upon the effect upon the spacecraft. A list of potential effects follows:
Surface charging*
Deep dielectric or bulk charging*
Single Event Upset (SEU) * a) Galactic cosmic rays and b) Solar proton events*
Heh... There goes that pesky "no charge separation in space" notion and "charges will instantly neutralize," right out the window. Maybe they didn't NOTICE the implication(s).
(Space Weather Prediction Center Topic Paper: Satellites and Space Weather)
http://www.swpc.noaa.gov/info/Satellites.html
Surface Charging
Surface charging to a high voltage does not usually cause immediate problems for a spacecraft.
But, they apparently admit it HAPPENS, in general, without the charges "immediately neutralizing." IE, the craft apparently becomes a charged body in space capable of undergoing electrical interactions (with the environment, or within itself).
However, electrical discharges resulting from differential charging can damage surface material and create electromagnetic interference that can result in damage to electronic devices. Variations in low energy plasma parameters around the spacecraft, along with the photoelectric effect from sunlight, cause most surface charging. Due to the low energy of the plasma, this type of charging does not penetrate directly into interior components. Surface charging can be largely mitigated through proper materials selection and grounding techniques.
Surface charging occurs predominantly during geomagnetic storms. It is usually more severe in the spacecraft local times of midnight to dawn but can occur at any time. Night to day, and day to night transitions are especially problematic during storms since the photoelectric effect is abruptly present or absent, which can trip discharges. Additionally, thruster firings can change the local plasma environment and trigger discharges.
The common measure for geomagnetic storms, and hence the occurrence of surface charging, is the K index. This index is a 3 hourly measure ranging from 0-9 (0=quiet, 9=severely disturbed.). It is derived from ground-based magnetometer data and is used as a surrogate for actual plasma measurements at satellite altitudes. In general, surface charging effects begin at the K=4 to K=5 level. Charging is probable at K>=6. Geomagnetic substorms can be somewhat localized in space so the use of the planetary K index (Kp) may mask the severity of effect upon a specific spacecraft. Some claim better correlation is achieved using data from a ground-based magnetometer at the "footpoint" of the magnetic field line that passes through the affected spacecraft. The Los Alamos National Laboratory (LANL) also has sensors on several geosynchronous (GEO) spacecraft that directly measure the appropriate particle energy ranges to determine if surface charging is probable.
Deep Dielectric or Bulk Charging
This phenomenon is a problem primarily for high altitude spacecraft. At times, when Earth is immersed in a high-speed solar wind stream, the Van Allen belts become populated with high fluxes of relativistic (>~1 MeV) electrons. These electrons easily penetrate spacecraft shielding and can build up charge where they come to rest in dielectrics such as coax cable, circuit boards, electrically floating radiation shields, etc. If the electron flux is high for extended periods, abrupt discharges (tiny "lightening strokes") deep in the spacecraft can occur.
High fluxes of these electrons vary with the 11 year solar cycle and are most prevalent late in the cycle and at solar minimum. Occasionally, high-energy electron events recur with a 27-day periodicity - the rotation period of the Sun. Discharges appear to correlate well with long periods of high fluxes. At these times, charge buildup exceeds the natural charge leakage rate of the dielectric. The charge builds and discharge occurs after the breakdown voltage is reached.
Apparently not an "immediate neutralization" process. Only once the discharge threshold is reached. Gee, I wonder if the Earth works this way?
*Cough*
In the past, some energetic electron enhancements at GEO have approached two weeks in duration. It was at the end of one of these long duration enhancements in 1994 that two Canadian satellites experienced debilitating upsets.
SWPC operates electron flux sensors on the GOES GEO spacecraft. These instruments measure electron fluxes of >0.6 and >2 MeV. (See GOES electron data ) Historically, deep dielectric discharges begin to occur when the
>2 MeV fluxes exceed 1000 particles/cm/sec/ster. In general, fluxes become elevated for all GEO spacecraft at the same time. However, there is a diurnal variation where fluxes peak by approximately an order of magnitude for spacecraft at local noon.
Again, it seems like things "charge up," take their sweet time, and eventually "discharge" (like little lightning bolts) inside satellites, etc.
Cheers,
~Michael Gmirkin
