Latitude Survey

Cosmic rays are energetic particles (including electrons, protons, and heavier elements) produced in the galaxy by processes that are not fully understood. The propagation of these particles is influenced by magnetic fields in the solar wind and in the magnetosphere of the Earth. By recording variations and fluctuations in the cosmic ray intensity, we are able to investigate changes in these magnetic fields.

Neutron monitors measure the “debris” left after the primary cosmic rays enter the atmosphere of the earth. Much of the information regarding the composition of the cosmic rays is lost in this process, but the large size of the neutron monitor permits an extremely accurate determination of the intensity of cosmic rays striking the atmosphere directly above the detector. The paths taken by these cosmic rays are influenced by the magnetic field of the earth, so that the monitor is “looking” toward a particular direction in space. This direction is in general very much removed from the local vertical. Our network of neutron monitors measures the anisotropy of cosmic rays.

Near the equator, the magnetic field of the earth is more effective in keeping cosmic rays from reaching the atmosphere than it is near the poles; so only higher energy particles are admitted. A parameter called the “Apparent Cutoff Rigidity” can be calculated to summarize this process. (Rigidity, or momentum per unit charge, is closely related to the particle energy but is more useful in the actual calculations.)

In a latitude survey, a neutron monitor is transported through a range of cutoff rigidities as rapidly as possible. (Speed is important because the cosmic ray intensity changes with time.) The variation in counting rate of the neutron monitor as a function of effective cutoff rigidity permits a measurement of the cosmic ray spectrum (integrated over particle type) at energies where direct spacecraft measurement is difficult or impossible due to the large detector sizes required. Observed deviations from perfect ordering of the data by the calculated cutoff    rigidity may be used to produce refined models of the magnetic field of the earth.

(LEFT) Insulated container used for 3NM64 neutron monitor latitude surveys on the Polar Star and Polar Sea. (RIGHT) 3NM64 installed inside the container. For this and several other voyages a small calibration neutron monitor (Krüger et al. 2008) was installed near the door.  Image Credit: Nuntiyakul et al. 2014