Cosmic ray variations due to the Earth’s magnetic field

Fluctuations of neutron monitor count rates occur on various time scales and for various reasons. An observed fluctuation need not imply that the cosmic ray intensity itself changed. We illustrate this here with a recent observation, where a temporary enhancement was observed by some neutron monitors. The enhancement can be explained by the simultaneously observed variation of the Earth’s magnetic field, which for a few hours allows particles of lower energy than usual to penetrate.

Fluctuations of the count rate of cosmic rays can be of many origins. Some are due to an actual variation of the flux of cosmic rays, be it galactic cosmic rays whose flux is disturbed by the magnetic field of the Sun, be it bursts of solar relativistic protons. Other variations are introduced by the propagation of the incoming cosmic rays in the Earth’s magnetic field. For instance there is a daily fluctuation of the galactic cosmic ray count rate, due to the fact that a neutron monitor on the Earth rotates with respect to the magnetosphere, whose shape and position are principally determined by the Sun-Earth direction.

CR evolution 1-4 Mar 2011

FFig. 1: Variation of some neutron monitor count rates (Lomnicky Stit and two monitors at Jungfraujoch; green and orange), Oulu (red) between 1st and 4th March 2011. Plotted using the Nest interface (see also NMDB online access tools).

Here we illustrate another phenomenon created by a fluctuation of the Earth’s magnetic field : on 1 March 2011 some neutron monitors at low latitudes, such as Jungfraujoch, Lomnicky Stit, Athens, Rome … observed an enhancement of their count rates that lasted for a few hours. The plot (Fig. 1) shows the enhancement of count rates near 12:00 Universal Time at the Jungfraujoch (Switzerland) and Lomnicky Stit (Slovakia) neutron monitors. The event was also observed by the new Plateau de Bure neutron monitor in the French Alps. For comparison, the monitor at Oulu (Finland) whose count rates are plotted by the fat red line in Fig. 1 shows no enhancement at this time.

What is the reason for this particular count rate enhancement, and why is it seen by neutron monitors at mid-latitude, but not by those closer to the magnetic poles, such as Oulu ? If newly accelerated cosmic rays were coming from the Sun, one would expect to see them much more clearly with the polar neutron monitors, because there the magnetic field of the Earth is a less efficient shield than at lower latitudes. And indeed there was no solar flare or coronal mass ejection that could have accounted for solar high-energy particles on 1st March.

Kp index

Fig. 2: Evolution of the Kp index measuring perturbations of the Earth’s magnetic field. The Kp index measures the fluctuation of the horizontal magnetic field compared to a quiet day. Measurements by different observatories are averaged. Kp ranges from 0 to 9 units, where an increase by 1 unit corresponds to doubling the magnetic field perturbation. Kp indices greater than or equal to 5 indicate storm-level geomagnetic activity. Obtained from the NOAA Space Weather Prediction Center.

However, the measurement of the magnetic field from ground-based observatories does show a perturbation at this time. In Fig. 2 the time history of the Kp index is shown in the first days of March, with a clear disturbance indicated by the red bars near noon on 1st March. This is the time when neutron monitor stations detected the enhanced count rate ! So the explanation of the enhancement must be sought in the influence of the Earth’s magnetic field on the count rate: The fluctuation of the magnetic field changes the minimum energy (or minimum magnetic rigidity) that a cosmic ray must have to traverse the magnetic field – the low-energy cutoff. Strong magnetic fields orthogonal to the cosmic ray trajectory are a more efficient shield than weaker fields. This minimum energy is reduced at times of magnetic field perturbations. Since the flux of cosmic ray particles decreases with increasing energy, a decrease of the cutoff means that each second more particles traverse the magnetosphere and reach the atmosphere above the neutron monitors.

But why does this not lead to an enhancement of count rates at the Oulu neutron monitor ? The answer is that at high latitudes, such as Oulu, the magnetic field is always only a weak shield. There the main obstacle for a cosmic ray is the Earth’s atmosphere, which is not affected by the magnetic field perturbation. Stations at mid-latitudes have cutoff energies near the maximum sensitivity of the neutron monitor and are therefore particularly sensitive to such changes in the Earth’s magnetic field. The weakened magnetic field of the Earth therefore explains the enhanced neutron monitor count rate at the mid-latitude stations.

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