Executive Summary Chapter 1 Introduction History, heritage and operation

Figure XXII Right NM64 Sea-level Response Function to primary cosmic rays for different solar modulation spectra determined using the Yield function in the Figure (above) multiplied by primary cosmic ray spectra expected for Solar Max and Solar Min. For comparison the Compton counter, the Forbush ionization monitor, response function is also displayed. It measures the rate of charged particles that pass through the active volume (primarily muons and electrons). As shown the NM is sensitive to lower energy primary particles than that of ionization chamber. Right Neutron Monitor Count rate as shown above in Figure XX with the integrate response function shown on the left figure for solar minimum,

NEUTRON MONITOR NETWORKS

In addition to their importance in understanding the physics of both the interplanetary and interstellar media, the GCRs and SEPs are an important component of space weather. They constitute a major threat to astronauts in space when they are outside of the protective geomagnetic field. This danger can be only mitigated slightly by current technology.

SEPs 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 GCRs. 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 GCRs, preventing the full interstellar intensity from striking Earth. This modulation is most effective during periods of heightened 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, respectively, 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 inward toward the Sun.

Because of the eleven-year sunspot cycle, the reliability, stability and robustness of very long term measurements is critical for understanding changes on the Sun that go beyond particular outbursts or series of them. Of particular interest is the fact that the GCR intensity during the last (2010) solar minimum is the greatest over the period covered by the observations—by a significant factor. This would have been unknown without the long duration measurements of neutron monitors. Other measurements, both at other neutron monitors and from spacecraft exhibit 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 weak interplanetary magnetic field and low 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. Analogies quickly appeared in the literature comparing this recent GCR maximum to that inferred from the Maunder Minimum historical record, suggesting that things may be learned about the Maunder Minimum from this unusual (over the last half century) episode. Fortunately, we established a baseline of solar or space climate with which we can compare this activity in the future, i.e., the next several solar cycles—long after the current community of solar and heliospheric physicists has retired.

The observed phenomena during the last solar minimum, particularly in the intensity of GCRs, demonstrate the importance of neutron monitor data in understanding the heliosphere and space weather, or space climate when speaking about secular changes or trends occurring over several solar cycles.

As described above, it is important to note that the basic global structure of the heliosphere, including its 11-year solar activity cycle and 22-year magnetic cycle, was established by neutron monitor measurements of the galactic cosmic ray intensity. Only these high-energy particles are able to sample the entire heliosphere as they propagate from interstellar space to their collision with Earth’s atmosphere.

Neutron monitors (and earlier, electroscopes) also detected cosmic ray temporal variations on much shorter timescales. “Forbush Decreases” (Forbush, 1946?) in the cosmic ray intensity of a few to several percent were frequently observed to occur with a timescale of hours following large solar flares. The decrease recovers with a timescale of days, or longer in the case of extended recurring solar activity. Recognized to originate as the result of flare-associated transient increases in solar wind speed and magnetic turbulence “sweeping out” the cosmic rays, these variations provided a means of probing the behavior of transient disturbances in the solar wind and their eventual decay. These observations first highlighted the dynamic state of the heliosphere and the nature of the cosmic-ray response. In the case of cumulative decreases occurring with the onset of a new solar activity cycle, it appeared that the decreases do not recover. Rather, the disturbed solar wind may be viewed as characteristic of the solar wind during solar maximum activity, which reduces the cosmic-ray intensity in the heliosphere more effectively. These ideas led to the recognition that periods of multiple flares and coronal mass ejections lead to effective barriers to cosmic-ray penetration; these barriers are now known as “global merged interaction regions.” It should be noted that Forbush Decreases may exhibit a small precursor increase as the cosmic rays are swept ahead of the (shock) disturbance. Such a precursor is indeed an expected signature of diffusive shock acceleration, which is initiated as particles reflect from the approaching shock surface or from the turbulent flow downstream of the shock. Neutron monitors still provide an effective means of studying the time-dependent modulation of the bulk of galactic cosmic rays by the variable structure of the solar wind, in addition to providing a nearly 65-year record of the variable heliosphere. The recent predictions and direct measurement of the extent of the heliosphere by the Voyager spacecraft, IBEX and accompanying theoretical work provide new challenges for the theory of cosmic-ray modulation and opportunities for neutron monitor measurements to play a crucial role in advancing our understanding of the heliosphere.

Often, in association with a Forbush Decrease, neutron monitors observe an impulsive cosmic ray increase with a timescale of an hour, usually commencing about a day before the Forbush Decrease. Unlike the previous variations, these impulsive increases depend sensitively on the location of the neutron monitor. The sensitivity stems from the anisotropy of the energetic particles in the event. In contrast with the galactic cosmic rays, which have a nearly isotropic distribution, these particles accelerated at solar flares or coronal shocks are initially highly anisotropic; their detection depends on the “asymptotic direction” of the neutron monitor at the time of detection. However, combining the measurements of many neutron monitors facilitates the reconstruction of the particle distribution function as a function of time to investigate the propagation of the solar energetic particles (SEPs) to Earth and determine the time and energy dependence of the released particles. These “Ground Level Events” (GLEs) occur only a few times during each solar activity cycle. They constitute a particularly interesting class of SEP events because their energies (beyond about a GeV) are not accessible to spacecraft measurement and represent the highest energies attainable by the solar acceleration process. Tylka and Dietrich (2009) combined many neutron monitor measurements of the GLE event of 15 April 2001 to obtain the form of the energy spectral rollover beyond a rigidity of ~ 0.1 GV. These measurements by neutron monitors are crucial in establishing the origin of SEPs since the highest energies subject the proposed acceleration mechanisms (shock acceleration at a coronal shock and as a byproduct of magnetic reconnection) to the most severe requirements.

The mechanism by which energetic particles scatter and diffuse in collisionless plasma remains a key problem in astrophysics. Detailed information on transport conditions in the interplanetary medium, such as the scattering mean free path, can be obtained from modeling the time-intensity and time-anisotropy profiles of solar energetic particles (Palmer 1982; Bieber et al. 1994). Anisotropy information from neutron monitor networks is crucial for this modeling, because it permits diffusive delays in the solar wind to be distinguished from extended acceleration or release at the solar source. In turn, the analysis provides information on the particle injection profile at the Sun for comparison to solar radio and optical signatures. Neutron monitors of the four-nation Spaceship Earth network (Bieber et al. 2004) were deployed to provide optimal coverage of solar particle anisotropies, using existing monitors where possible and constructing new ones as needed.

The energy spectrum of cosmic ray variations can be deduced from a network of neutron monitors deployed over a range of geomagnetic cutoffs. For instance, Tylka and Dietrich (2009) used cutoff arrays to extend the spectrum of solar energetic particles from spacecraft energies to the neutron monitor energy range, while Oh et al. (2013) derived information on the spectrum of Galactic cosmic rays during the recent very weak solar minimum and accompanying record maximum in Galactic cosmic rays.
Because cosmic rays of neutron monitor energies are “tuned” to the various spatial dimensions of the heliosphere, they can be used to understand large disturbances in the solar wind. Pioneering observations of the anisotropy of Galactic cosmic rays (Pomerantz and Duggal 1971 and references therein) typically employed a single ground-based detector and relied on Earth’s rotation to provide a range of viewing directions. Often results were expressed as annual means, because it was necessary to average over long periods in order to extract the minuscule anisotropy signal from the many other variations present.
Now, with a network observing multiple directions simultaneously it is possible to extract the anisotropy with much better time resolution, often to one hour or better. Thus it becomes possible to study transient anisotropies in the pre-existing Galactic cosmic ray population produced by solar wind disturbances such as ICMEs. Reported effects include: bidirectional streaming, indicative of a closed magnetic field topology (Dvornikov et al. 1983; Richardson et al. 2000); gradient anisotropies, which permit determination of the cosmic-ray spatial gradient and thereby provide clues to ICME geometry (Bieber and Evenson 1998); and precursor “loss-cone” anisotropies that potentially can provide up to ~twelve hours advance warning of major geomagnetic storms (Leerungnavarat et al. 2003 and references therein).
In the last twenty years, it became clear that neutron monitors are sensitive to purely solar flare particles, in particular the secondary neutrons produced by high-energy ions with the solar atmosphere. The Sun on occasion emits relativistic neutrons with sufficient intensity to be detectable by neutron monitors (Chupp et al. 1987; Shea et al. 1991; Bieber et al. 2005). Such events provide exceptionally clear insights into acceleration/injection processes at the Sun, because the neutrons travel straight from the source to the observer unimpeded by magnetic fields. (Non-relativistic neutrons are often not observed at 1 AU because their decay time, ~15 min, is shorter than the transit time.) An optimal network for observing these events would employ low-latitude, high-altitude monitors sited to minimize atmospheric attenuation and thus increase both the probability of detection and the size of the neutron signal.

Considerable resources have been invested in space-based measurements of energetic particles. Implicit in the success of these missions is the standard baseline measurements performed by neutron monitors. Only using the entire Earth as a magnetic spectrometer, we can achieve quality data above a few GeV. It is difficult to get reasonable signals from instruments at these energies when limited by the limited size of these instruments. NASA missions have from the first have deduced the maximum science when coupled with the neutron monitor measurements. The joint data sets provide a wide vision of the spectrum and energy dependent physical processes when spanning energy ranges from several MeV to several GeV.

REFERENCES

2) Ilya Usoskin

Correlation of proton fluences from Tylka et al (2009) NM analyses versus corresponding values from GOES/HEPAD

Event-integrated integral proton spectra vs. rigidity for two GLEs. Noted are the parameters of the power law fits to the neutron monitors and of the Band-function fits to measurements above 0.137 GV (10 MeV).

Fig.1: asymptotic directions determined during the first polar pass that registered the 2012 May 17 event.

Also shown are the asymptotic directions of the NM that registered the primary GLE beam [3].