The intensities of galactic cosmic rays are modulated upon entering the heliosphere. These variations, defined as solar modulations, are classified as long-term or transient modulations based on their durations.
Studies have correlated the transient variations with the characteristics of the solar wind and the interplanetary magnetic field. Therefore, studies of transients augment our understanding of physical processes in the interplanetary medium. Processes causing transient variations may also induce geomagnetic storms. Precise measurements of cosmic ray fluxes during a transient phenomenon will have immense use in validating models of space weather prediction.
BESS (Balloon-borne Experiment with a Superconducting Spectrometer), a US-Japanese collaborative program, directly measures light elements of cosmic rays in a large energy range (∼0.1 - several hundred GeV) thus bridging the gap between the low energy space-based and high energy ground-based experiments, has a large geometrical acceptance (0.3 m2 sr), and is highly sensitive in the lowest energy regime of cosmic rays where the solar modulations occur. BESS measurements can provide study of transient variations of cosmic ray protons and helium, for same energy but different rigidities (momentum per unit charge) or the same rigidity but different energies, as separate probes. BESS is sensitive to diurnal variations, unlike space based experiments.
Proton fluxes from BESS-Polar I (flown from Williams Field, Antarctica, December 13–21, 2004), calculated for energies 0.1–100 GeV in time intervals as short as 4 hours, are analyzed for variations. Energy dependence of the observed variations is explored using smaller energy intervals. Neutron monitor data support our observations. To our knowledge, this is the first direct measurement of variations at the few–1% level by a balloon or satellite experiment at time scales of a few hours.
Proton fluxes are presented in 4-hour averages, suitable for validation of solar wind and space weather prediction models. Suggested physical interpretations of the three observed features in the proton spectra, derived by comparison with the solar wind and IMF data from space-based experiments, include presence of a corotating interaction region or a magnetic cloud or a combination of both, a turbulent interaction region, and Compton-Getting anisotropy.