Midday recovery of the polar cap absorption of 19–21 March 1990: a case study

Assimilation of real-time riometer measurements into models of 30 MHz polar cap absorption

“…Leske et al (2001) observed a <1°shift of the CL towards approximately 22 MLT in 8-16 MeV/nucleon alpha particles detected on the SAMPEX satellite and a peak CL around 11 am magnetic equatorial time was observed in OGO4 satellite measurements for lower energy protons (Fanselow & Stone 1972). The poleward excursion around local noon has been cited as a cause of the ''mid-day recovery'' in absorption observed on mid-to high-latitude riometers (k = 60-65°) near the CL boundary (Hargreaves et al 1993;Ranta et al 1995;Uljev et al 1995) although Leinbach (1967) presented evidence to indicate that increased anisotropy of the energetic particle pitch angle distribution could be an alternative cause. A comparison of the error and correlation statistics using DRAP with the two CL models is given in Figure 6 for the seven riometers in the Churchill line and for all 94 SPEs.…”

Ionospheric effects of the simultaneous occurrence of a solar proton event and relativistic electron precipitation as recorded by ground-based instruments at different latitudes

“…There is some experimental evidence that pitch-angle anisotropy is a typical feature of solar protons with Ep ¡ 30 MeV but not of solar protons with Ep ¿ 30 MeV (Scholer, 1975). Furthermore, Uljev et al (1995) showed that the dominant contribution to riometer absorption at invariant latitudes ¿ 65 • is made by solar protons with Ep = 1-30 MeV, with a ux that signiÿcantly exceeded the higher energy proton ux during the SPE of May 1992 (see Fig. 1).…”

“…We took values of these parameters from Table 2 of Uljev et al (1995), since the SPEs of May 1992 and March 1990 have similar characteristics. These rates of ionization Q due to solar proton uxes, and the values of Q due to relativistic electron uxes at di erent stages of the HRE event of May 1992 as given by Gaines et al (1995), are shown in Fig.…”

“…We have calculated the ion-electron production rates, Q, due to the solar proton uxes as measured by the geostationary satellite GOES-7 for di erent phases of the SPE. The procedure is similar to that described by Uljev et al (1995), where the geomagnetic cut-o energy and the solar proton pitch angle anisotropy were taken into account.…”

“…A description of the method has been given by Uljev et al (1995). We have calculated the ion-electron production rates, Q, due to the solar proton uxes as measured by the geostationary satellite GOES-7 for di erent phases of the SPE.…”