Highly significant detection of solar neutrons

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Number: 33
1st Author: Kyoko Watanabe
2nd Author:
Published: 14 January 2006
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Solar neutrons

Solar flares can accelerate ions to high energies. These ions interact with the ambient atmosphere of the Sun, producing the γ-rays that RHESSI observes. They also produce secondary neutrons via many different kinds of nuclear interactions. Some of these solar neutrons can reach the Earth, even though they are rapidly decaying into protons and electrons. A free neutron (ie, one not bound into a nucleus) has a beta-decay half-life of 886 seconds. This is comparable to the light travel time from the Sun (499 sec), so only the fastest (relativistic) neutrons can make it to the Earth before decaying. In general the faster, more energetic neutrons arrive first, giving a crude "time-of-flight" (TOF) means of determining their energy spectrum. The decay products (protons) can be recognized by space-borne detectors, and some may also be trapped in the coronal field.

The directly arriving neutrons contain essential information about the original particle acceleration, because they are not affected by the magnetic fields. By observing the neutrons, we can directly probe the energy spectrum of the accelerated ions and also find the production time of the neutrons (which is nearly the same as the acceleration time of the ions).

Solar neutrons are observed by neutron monitors and solar neutron telescopes on the ground. Neutron monitors were originally intended for the continuous recording of the primary cosmic-ray intensity, and are now located at more than 50 stations around the world. On the other hand, a solar neutron telescope is a specialized detector designed specifically to observe solar neutrons in association with solar flares. It can measure the energies and the directions of arrival. Such telescopes are now located in seven different stations around the world. These detectors make up an international network of solar neutron observation with around-the-clock observing.

These detectors observed a large solar neutron event caused by the hugely energetic X17 flare of 2005 September 7. We report this solar neutron event and describe some of the analysis. RHESSI observed a part of this event, but, unfortunately, not its entirety.

The solar neutron event of 2005 September 7

In 2005 September, a prolific active region (NOAA 10808) produced as many as ten X-class solar flares. Among them, the first (commencing at 17:17 UT 2005 September 7) was the most energetic, with an X17 magnitude. It occurred just as the active region rotated onto the disk of the Sun, and was thus at the solar east limb.

Figure 1: The γ-ray time profile for the energy around 4.4 MeV observed by the INTEGRAL satellite on 2005 September 7. Unfortunately RHESSI was in the SAA and immediately following in eclipse on at the event onset. However, an intense emission of γ-rays was observed by INTEGRAL. In the recorded spectrum, the strong 2.2 MeV gamma-ray line due to neutron capture was not seen, but the 4.4 MeV gamma-ray line of excited 12C (carbon 12) was clearly observed (see the time profile around 4.4 MeV in Figure 1). This time profile peaks at 17:36:40 UT and reflects a convolution of the time profile of primary ion acceleration and subsequent propagation and interaction. These data give a good clue as to the time of production of the solar neutrons soon arriving at the Earth. The absence of the neutron-capture line can be explained by the limb location of the flare (we hope to describe this in a future RHESSI nugget).

Figure 2: The zenith angle of the Sun at 17:40UT on 2005 September 7. Black points represent locations of the neutron detectors. At the time of the X17 flare, the Sun was over South America; Figure 2 shows the solar position on a world map. The detectors located in Mexico and Bolivia were well positioned for observing solar neutrons, and all three of these sites recorded neutrons successfully.

Figure 3: The 5 minute counting rate observed by the Bolivia neutron monitor. Figure 3 shows data obtained by the Bolivia neutron monitor. A clear excess appeared just after 17:36:40 UT. The statistical significance was huge, about 40 sigma per 5-minute count-rate bin. The excess rates continued for about 30 minutes, thus the overall statistical significance of this detection is very high - even astounding, in a field often happy with a "few-sigma" detection. Clear excesses were also observed by the other detectors.

Analysis and Discussion

Since the largest excesses were observed by the Bolivia neutron monitor, we calculate the energy spectrum of the neutrons using these data. First, we assume that solar neutrons are produced exactly at 17:36:40 UT, when large fluxes of gamma-rays were observed by the INTEGRAL satellite. By using the time-of-flight method, the energy of the solar neutrons detected by the neutron monitor is estimated to be between 25 and 400 MeV. To calculate the original energy spectrum of the neutrons at the solar surface, we correct for attenuation in the Earth's atmosphere by using the "Shibata program." Determining the detection efficiency of the neutron monitor also requires an elaborate calculation, and we use the one of Clem and Dorman. Finally we fit the energy spectrum of source neutrons to a power law: dN/dE = 6×1027(E/100MeV)-3.8 MeV-1sr-1. We fitted only the data above 100 MeV since the data at lower energies is not as well characterized.



Figure 4: The observed and simulated time profiles of neutrons on 2005 September 7. The black solid lines are the observed two-minute counting rates from the Bolivia neutron monitor. The red points represent the simulated time profiles for solar neutrons assumed to have been produced impulsively at 17:36:40 UT with a spectral index -3.8 (a), and to have been produced with the same time profile as the 4.4-MeV gamma-rays with spectral index -3.1 (b). The high-energy cutoff of the solar neutron energy is assumed to be 500 MeV in both figures.

To explain the long duration of the emission of neutrons, we simulate the time profiles of solar neutrons by using the energy spectrum obtained above. Figure 4 (a) shows the simulated results (red points) with observed data (black solid lines) from the Bolivia neutron monitor. The shape of leading edge depends on the high-energy cutoff assumed for the neutron energies. In Figure 4 (a), this cutoff is 500 MeV, and leading edge and peak are well explained. But the long emission cannot be explained, and so something more complicated is happening.

In this flare, gamma-rays were observed continuously for about 5 minutes (Figure 1). We can use the time profile of 4.4 MeV gamma-rays as a hypothetical production time profile for the solar neutrons. Figure 4(b) shows the model result. It appears to explain the long-duration tail a little bit better than the delta-function model of Figure 4(a), but not by much. To explain the tail properly, we require that solar neutrons be produced for a long time, longer even than the time scale of the observed gamma-ray emission.


Until now, solar neutron emissions were explained by using high energy gamma-ray emission as a template for their production. This does not work for the September 7 event. To explain the strong long-duration tail of neutron emission, we need other sources of gamma-rays - for example, from pion decay via the "pi-mu-e" sequence. This is an important conclusion but it is based on a single event out of a total set of about 10 solar neutron events, This is too small a number for a proper statistical analysis, so we hope to increase the number of observed events.

Biographical note: Kyoko Watanabe is a member of the RHESSI group at UC Berkeley.

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