The definitions below are meant to assist user's of the RHESSI SAA & Eclipse Data page.

Sunlight Start Time:  The time at which RHESSI exits from eclipse and the spacecraft maintains a direct line of sight to the Sun.

Sunlight Stop Time:  The time at which RHESSI enters into eclipse and sunlight is no longer visible.

Eclipse:  The period of time (approximately 35 minutes every 1.5 hours) during which RHESSI can not observe the Sun due to occultation by the Earth.

Eclipse Start Time:  Same as "Sunlight Stop Time".

Eclipse Stop Time:  Same as "Sunlight Start Time".

Atmospheric Absorption Zone:  Points near the Earth's surface where the attenuation effects of the atmosphere must be taken into account.  The start to stop time lasts for approximately 23 seconds and occurs roughly at the same time as the sunlight start and stop times.  

South Atlantic Anomaly (SAA):  Spacecraft flying in orbits inclined between 35 and 60 degrees against Earth's equator and having an altitude of a few hundred miles periodically pass through a zone of reduced natural shielding and thus spend a few minutes during each passage exposed to higher particle fluxes than outside it. This occurs because the magnetosphere is not quite spherical but shows a "pothole", or dip, which is the result of the eccentric displacement of the center of the magnetic field from the geographical center of the Earth (by 280 miles) as well as the displacement between the magnetic and geographic poles of Earth. This effect is known as the the South Atlantic Anomaly (SAA). The design of the RHESSI spacecraft, which flies at an inclination of 38 degrees, must take the effects of the SAA into account.  See also: http://www.ll.mit.edu/ST/sbv/saa.html

Northern and Southern Magnetic Zones:  Established zones around the north and south magnetic poles where compass behavior is deemed to be "erratic" and "unusable". These zones are defined where H (the horizontal intensity) is between 3000 nT - 6000 nT (erratic zone) and H is less than 3000 nT (unusable zone).  See also:  http://www.ngdc.noaa.gov/seg/potfld/faqgeom.shtml#q5b

This page last updated: June 27, 2008