HF absorption near the polar cap edge during PCA events

Sometimes during PCA events the hf (3 to 30 MHz) absorption observed at a riometer station near the polar cap edge decreases, although it remains constant or even increases at a higher-latitude station. The absorption stays depressed for a few hours and then returns to or near the predecrease level. The effect often occurs in the local forenoon hours, is called the midday recovery, and has been attributed to an increase in geomagnetic cutoff energy alpha f solar protons or to development of an anisotropic pitch angle distribution of solar protons. Two recoveries are discussed in relation to satellite proton observations along meridians near those of the ground station. (WDM)

[1]  A. J. Zmuda,et al.  Bombardment of the polar-cap ionosphere by solar cosmic rays , 1972 .

[2]  T. Potemra.  The Empirical Connection of Riometer Absorption to Solar Protons During PCA Events , 1972 .

[3]  A. J. Zmuda,et al.  VLF Phase Disturbances, HF Absorption, and Solar Protons in the PCA Events of 1967 , 1970 .

[4]  T. Potemra,et al.  VLF PHASE DISTURBANCES, HF ABSORPTION, AND SOLAR PROTONS IN THE EVENTS OF AUGUST 28 AND SEPTEMBER 2, 1966. , 1969 .

[5]  H. Sauer.  Nonconjugate aspects of recent polar cap absorption events , 1968 .

[6]  J. Jiménez,et al.  Arrival of low‐energy cosmic rays via the magnetospheric tail , 1968 .

[7]  H. Leinbach.  Midday recoveries of polar cap absorption , 1967 .

[8]  R. Parthasarathy,et al.  Observations of the september 2, 1966 PCA event: Midday recovery in absorption , 1967 .

[9]  Donald J. Williams,et al.  The February 5, 1965, solar proton event: 1. Time history and spectrums observed at 1100 kilometers , 1967 .

[10]  H. E. Taylor.  Latitude local-time dependence of low-energy cosmic-ray cutoffs in a realistic geomagnetic field. , 1967 .

[11]  H. Leinbach,et al.  The influence of geomagnetic activity on polar cap absorption , 1965 .

[12]  F. Michel,et al.  PHYSICAL SIGNIFICANCE OF INHOMOGENEITIES IN POLAR CAP ABSORPTION EVENTS , 1965 .

[13]  J. Aarons,et al.  Effects of the solar flares of 7 July 1958 , 1959 .

[14]  H. Sauer,et al.  The influence of the geomagnetic tail on low‐energy cosmic‐ray cutoffs , 1967 .