Improved dynamic geomagnetic rigidity cutoff modeling: Testing predictive accuracy

Abstract

[1] In the polar atmosphere, significant chemical and ionization changes occur during solar proton events (SPEs). The access of solar protons to this region is limited by the dynamically changing geomagnetic field. In this study, we have used riometer absorption observations to investigate the accuracy of a model to predict K p -dependent geomagnetic rigidity cutoffs, and hence the changing proton fluxes. The imaging riometer at Halley, Antarctica is ideally situated for such a study, as the rigidity cutoff sweeps back and forth across the instrument's field of view, providing a severe test of the rigidity cutoff model. Using observations from this riometer during five solar proton events, we have confirmed the basic accuracy of this rigidity model. However, we find that the model can be improved by setting a lower K p limit (i.e., K p = 5 instead of 6) at which the rigidity modeling saturates. We also find that, for L > 4.5, the apparent L-shell of the beam moves equatorward. In addition, the Sodankylä Ion and Neutral Chemistry (SIC) model is used to determine an empirical relationship between integral proton precipitation fluxes and nighttime ionosphere riometer absorption, in order to allow consideration of wintertime SPEs. We find that during the nighttime, the proton flux energy threshold is lowered to include protons with energies of >5 MeV in comparison with >10 MeV for the daytime empirical relationships. In addition, we provide an indication of the southern and northern geographic regions inside which SPEs play a role in modifying the neutral chemistry of the stratosphere and mesosphere.Citation: Clilverd, M. A., C. J. Rodger, T. Moffat-Griffin, and P. T. Verronen (2007), Improved dynamic geomagnetic rigidity cutoff modeling: Testing predictive accuracy,

Latitudinal extent of the January 2005 solar proton event in the Northern Hemisphere from satellite observations of hydroxyl

“…However, it was suggested that the need for K p restriction indicated that the stretching of the geomagnectic field was overestimated by the model, and that the latitudinal limit of the forcing was more poleward than predicted before. Clilverd et al (2007) tested the predictive accuracy of the Rodger model by comparing its results with Halley riometer observations during several SPE. They concluded that the K p saturation in the model leads to even better agreement with the data if enforced at 5.5 but only near the equatorial boundary of the proton forcing.…”

“…This is a useful addition to the knowledge about cutoff behaviour, because the Halley riometer is not adequately placed that a good idea of the equatorward boundary location could be obtained during big storms. Both Clilverd et al (2007) and this study use the Rodger cutoff estimation method, with the K p saturated at 6. So, the suggested over-stretching of the geomagnetic field is taken into account in these two studies.…”

Probing the high latitude ionosphere from ground-based observations: The state of current knowledge and capabilities during IPY (2007–2009)

“…Potemra (1972) demonstrated that the riometer absorption is related to the flux of protons 410 MeV and Kavanagh et al (2004b) applied this to an imaging riometer and demonstrated that solar radio emission, though a problem at these times, is not significant enough to disrupt the reliability of the relationship. Clilverd et al (2007) and Rodger et al (2006) have used an imaging riometer close to the cutoff boundary to test cut-off models and found good agreement. The implementation of an ion chemistry model allowed the authors to estimate the relationship between PCA and proton flux during nighttime conditions.…”

Atmosphere‐ionosphere conductivity enhancements during a hard solar energetic particle event

“…SIC‐calculated ion densities are currently estimated to be accurate to within a few tens of percent ( personal communication, P. Verronen ). The SIC‐calculated ionization levels have been studied many times during SEP events have been tested experimentally using incoherent scatter radar [ Verronen et al , 2002], riometers [ Rodger et al , 2006; Clilverd et al , 2007], and subionospheric VLF propagation [ Clilverd et al , 2005].…”