PCA Events Research Articles

On the relationship between polar-glow aurora and solar cosmic ray fluxes

Calculations of the λ3914 emission rate due to solar cosmic ray bombardment, based on exponential rigidity spectra for solar protons and alpha particles, are compared with spectrographic observations from the polar regions during PCA events. While limited data on the time variations of polar-glow aurora agree with predictions from observations of solar cosmic ray fluxes, the observed emission rates exceed calculated values by about an order of magnitude. The bearing of these results on the PCA problem is discussed.

Solar cosmic ray events during late August 1957

Reanalysis of radio wave observations of PCA events and balloon measurements of energetic particles leads to the conclusion that two instead of three solar cosmic ray events occurred during late August 1957. This revision involves different flare assignments, and the balloon data show that the delay time between flare beginning and the first arrival of particles at the earth is much less than was reported previously on the basis of radio wave measurements. An importance 3 flare on August 28, 1957, began at 2010 UT. This flare is the most likely source of the solar cosmic rays that began to arrive at the earth after 2140 UT on August 28. Evidence found for an event beginning on August 29, is uncertain. The second cosmic ray event that can be identified during this period began at 1340 UT on August 31. The associated flare is probably the one that began at 1257 UT on this date.

A study of the relationship between absorption-time profiles of polar-cap-absorption events and Forbush decreases of cosmic-ray intensity

Forbush decrease data recorded by neutron monitors during PCA events support the interpretation of the absorption-time profiles of PCA events as due to solar particles under the influence of magnetic fields carried by solar plasma. All PCA events showing evidence of strong trapping of solar particles are accompanied by strong Forbush decreases, the magnitudes of which are a function of the delay time from flare to SC storm. Those PCA events showing little evidence of trapping are accompanied by only very weak decreases. The large western excess of these latter events is explained in terms of sun-earth magnetic linkage set up by earlier flare activity. The relative merits of two models of the interplanetary fields are examined. (auth)

A study of solar activity associated with polar-cap absorption

Systematic examination of all unusual solar radio and optical emission before the onset of 43 polar-cap absorption events shows which type of solar activity most frequently precedes PCA. This provides an objective basis for assignment of the solar event most probably associated with each of the individual PCA events. Relations among various characteristics of solar and PCA events are examined. These data present little evidence for association of PCA with solar flares that occur on the far side of the sun, nor do they indicate any significant dependence of occurrence or of delay time of PCA on position of the associated flare. Delay from flare to absorption varies from values of several minutes, appropriate to the energy of the solar particles, up to several hours. A definite relation is found between delay time and phase of the solar cycle, with long delay times near the time of maximum solar activity.