Neutron Monitors and Cosmic Rays in the Heliosphere
Prepared by J. R. Jokipii
The Heliosphere is a vast spheroidal cavity in the interstellar plasma, extending out to approximately 140 AU from the Sun, created by the supersonic outward flow of the solar atmosphere (the solar wind). Galactic cosmic rays (GCR) and solar energetic particles (SEP) propagate in this medium. Because of the low ambient plasma density, the GCR and SEP do not collide with the plasma particles, but are affected only by the ambient plasma electric and magnetic fields.
In addition to their importance in understanding the physics of both the interplanetary and interstellar media, the GCR and SEP are a very important component of space weather. They constitute a major threat to astronauts in space when they are outside of the protective terrestrial magnetic field. This danger can be only mitigated slightly by current technology.
SEP are emitted sporadically by events on the Sun in discrete events, which last only hours to days, and which occur much more frequently during maximum solar activity than during minimum activity. Although their intensity at energies below some tens of MeV is quite high, the average intensity above approximately 100 MeV is dominated by GCR. The lower energy of SEP makes it possible to shield astronauts effectively against them. For this reason, I will concentrate on GCR for the rest of this document.
The heliosphere and the outflowing solar wind act to decrease (modulate) the intensity of GCR, preventing the full interstellar intensity from impacting Earth. This modulation is most effective during maximum solar activity. Figure 1 illustrates the intensity as reported for the neutron monitor at McMurdo, over the past 5 sunspot cycles. The GCR intensity maxima and minima occurring during sunspot minima and maxima are clearly visible. The alternating shapes of the GCR maxima, with a sharply peaked maximum at one solar minimum followed by a more rounded maximum at the following minimum can be understood to be a consequence of the fact that the direction of the interplanetary magnetic field changes at each sunspot minimum. The sharply peaked maxima occur when the northern interplanetary magnetic field is pointed toward the Sun.
Of particular interest is the fact that the GCR intensity during the last (2010) solar minimum is the highest over the period covered by the observations, by a significant factor. Other measurements, both at other neutron monitors and from spacecraft show the same effect. The high intensity is probably a consequence of the fact that the last solar minimum was anomalously deep and long-lasting, with an unusually small interplanetary magnetic field and solar-wind velocity. It important to determine whether this striking behavior is a harbinger of more change in the future or whether it is an anomaly.
The observed phenomena during the last solar minimum, particularly in the intensity of GCR, demonstrate the importance of neutron monitor data in understanding the heliosphere and space weather.
Figure 1. The Bartels solar-rotation-averaged counting rate. Oh etal, JGR, 118, 5431, 2013.