PART I — THE SOLAR-TERRESTRIAL ENVIRONMENT 



The HF radio communicator therefore 

 requires long-range and short-term 

 forecasts of the specific frequencies 

 that will effectively propagate 

 throughout the day. This is known 

 as frequency management and means, 

 in short, the determination of the 

 frequency that can be used from a 

 particular transmitter to a particular 

 receiver at a particular time. Propa- 

 gation of the HF signal to a distant 

 receiver employs single or multiple 

 "reflections" from the ionosphere and 

 the earth. Since the state of the iono- 

 sphere is dynamic and highly respon- 

 sive to solar activity, the number of 

 usable frequencies depends on (a) the 

 intensity of ionizing solar ultraviolet 

 and X-ray emissions and (b) the de- 

 gree of disturbance of the magneto- 

 spheric-ionospheric environment. 

 These are in addition to such factors 

 as time of day, latitude, and equip- 

 ment characteristics. 



The HF communicator also requires 

 forecasts and real-time advisories of 

 short-wave fadeouts caused by X-ray 

 emissions related to solar flares. If he 

 gets these, he can insure that alternate 

 means of communication (satellite or 

 microwave methods) are available for 

 use in sending the highest-priority 

 messages. He can also differentiate 

 between communication outages 

 caused by propagation and those 

 caused by equipment malfunction. If 

 he knows that an outage is due to 

 a short-wave fadeout, the communi- 

 cator can simply wait for his circuit 

 to return to normal to continue 

 low-priority traffic rather than take 

 time-consuming action to switch fre- 

 quencies. 



Other solar-terrestrial disturbances 

 which disrupt communications, such 

 as "polar cap absorption" events, 

 geomagnetic-ionospheric storms, and 

 auroral and geomagnetic substorm 

 events, must also be forecast to allow 

 the communicator to prepare to use 

 alternate means of communication. 



Finally, the communicator needs an 

 accurate and complete history of ion- 



ospheric disturbances to post-analyze 

 his system's performance. Outages 

 that have been attributed to poor 

 propagation when no disturbances 

 were observed can then be identified 

 as being due to mechanical or pro- 

 cedural problems. 



High-Altitude Density — Space 

 vehicles which spend all or part of 

 their orbits in the region from 100 to 

 1,000 kilometers above ground are 

 subject to significant drag from the 

 neutral atmosphere. The density of 

 this region and the resulting satellite 

 drag are dynamic parameters. Their 

 variations reflect heating of the high 

 atmosphere produced by solar ultra- 

 violet variations and corpuscular pre- 

 cipitations, mostly at polar latitudes. 



Satellite drag perturbs the orbital 

 parameters of the vehicles and, in 

 turn, complicates cataloguing, track- 

 ing, and control. Density variations 

 can sometimes alter the orbit enough 

 to carry the vehicle out of an area of 

 scientific interest or otherwise de- 

 grade its mission. If mission con- 

 trollers are to be able to compensate 

 adequately for orbital changes, they 

 need the following: 



1. A dynamic, accurate model of 

 the global distribution of at- 

 mospheric density throughout 

 the region of interest; 



2. Accurate observations of such 

 parameters of the model as 

 ultraviolet flux, solar-wind en- 

 ergy, and density; and 



3. Accurate forecasts of these pa- 

 rameters. 



Space Radiation — Man in space 

 faces radiation hazards from galactic 

 cosmic rays, trapped radiation, and 

 storms of particles (mostly protons) 

 from solar flares. 



Cosmic radiation is so penetrating 

 that there is no practical means of 

 shielding against it. Astronauts sim- 

 ply must live with it. Its intensity is 



low enough that it does not pose a 

 serious hazard. 



The trapped-radiation environment 

 of near-earth space, however, is so in- 

 tense that prolonged exposure would 

 be fatal. Consequently, mission plan- 

 ners avoid that region by orbiting 

 below it or arranging to pass through 

 it quickly. 



Solar-flare radiation poses a threat 

 for a lightly shielded astronaut. The 

 threat is not especially significant, 

 however, because (a) major events 

 occur rarely, (b) the astronaut can be 

 shielded effectively from most of the 

 radiation (in effect, the Apollo com- 

 mand module is a "storm cellar"), 

 and (c) the astronaut can return to 

 the safety of a shielded vehicle before 

 significant doses have time to build 

 up. 



Despite the rather low critical na- 

 ture of this hazard, certain space- 

 environment support is essential to 

 protect man effectively from the haz- 

 ards of solar-flare radiation. Mission 

 planners need forecasts of the likeli- 

 hood of a particle event to insure that 

 they have enough options available 

 in case an event occurs. Observations 

 of the flare radiation are needed to 

 alert the astronauts. Techniques are 

 required to project the course and 

 intensity of an observed event so that 

 the radiation threat can be accurately 

 assessed. 



Today there is some concern over 

 the radiation hazard to passengers 

 and crew of supersonic transports, 

 especially for polar flights. Though 

 not completely resolved, it appears 

 that the threat is minimal, since solar 

 cosmic-ray events sufficiently intense 

 to cause undesirably high radiation 

 doses are exceedingly rare and prob- 

 ably occur less than once every ten 

 years. But forecasts, observations, 

 and alerts will be needed to insure 

 full protection. Warning systems are 

 being developed, but warnings are 

 unlikely to reach aircraft already in 

 polar regions unless communication 



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