Executive Summary Chapter 1 History, heritage and operation

Sodankylä Geophysical Observatory, University of Oulu, Finland

The NM network is most useful as a joint network, which has much more abilities for science and applications than a sum of individual detectors. It is a balance between national and international efforts. It is crucially important that the NMs network is as homogeneous as possible without large gaps in coverage. For example, closure of NMs in the North America would result in a dramatic drop of quality of the NM network data in international perspective.

Data from individual NMs are gathered in several international databases:

• 
  World Data Center for Cosmic Rays (WDC CR) in Japan [http://center.stelab.nagoya-u.ac.jp/WDCCR], where all hourly-resolution data from NMs are collected in an ftp format;
  
• 
  Neutron Monitor Database (NMDB) in Germany [http://www.nmdb.eu] provides a user-friendly access to many long-operating NMs;
  
• 
  Ground Level Enhancement (GLE) data base [http://gle.oulu.fi] is an international database collecting all available data on GLE events recorded by the NM network.
  

The network cannot be replaced by space-borne instruments. The primary interest of many space-borne detectors (e.g., SOHO, ACE, GOES) is in solar transients, interplanetary medium and space weather, they have low energy range and cannot give an estimate for the atmospheric penetration of cosmic rays. Special CR-dedicated missions (PAMELA, AMS-02) are focused on high-energy exotic CR and HE astrophysics. The very complex DAQ system makes it difficult for real-time analysis. Because of the low inclined orbit only a fraction of time can be used to study SEP events. These detectors have limited life-time.

The NM network data is used for several fields:

• 
  Science
  
  • 
    GCR variability (heliospheric modulation) – NMN is the primary instrument;
    
  • 
    GLE analysis (source direction, anisotropy, spectra) – NMN is the primary instrument;
    
  • 
    Solar neutron events – NMN is an important instrument ;
    
  • 
    Interplanetary transients – NMN is a useful instrument.
    
• 
  Practical use
  
  • 
    Aircraft crew dosimetry – NMN is the only instrument;
    
  • 
    Space weather: storm fore/now-casts – NMN is an important instrument;
    

Prepared by J. R. Jokipii

Figure 1. The Bartels solar-rotation-averaged counting rate. O

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.

As described by J.R. Jokipii, 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” 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 in order 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 since 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 stringent requirements.

The Sun is the only player in controlling our heliosphere and in modulating galactic cosmic rays (GCRs). As the tilt angle of the heliospheric current sheet evolves, the velocity and density of the solar wind change, and the strength and turbulence characterizing the interplanetary magnetic field (IMF) reorganize. Neutron monitors cover nearly six cycles of activity and the consequent impact on galactic cosmic ray radiation at 1 AU. Various spacecraft have been operating throughout the space age to record the Sun’s activity and its effect on the local radiation environment. These instruments often have improved resolution, albeit at lower energies, but neutron monitors provide the only source of continuous long-term monitoring while offering the possibility to inter-relate spacecraft and high-altitude balloon instruments that operate over much shorter periods of time (see Figure 1).

Due to the long-term coverage by neutron monitors, it has been possible to establish the 22-year galactic cosmic ray modulation, dominated by the 11-year solar activity, but influenced by gradient and curvature drifts in the IMF. The world wide network, which has continually grown since the first neutron monitors in 1950s, has ensured continuous coverage of the Sun’s 11 and 22 year cycles, enabling scientists to identify periods during which solar activity is unusual, e.g., the recent solar minimum of cycle 24. This unusual solar minimum has resulted in the highest intensities of galactic cosmic rays in the space age (see Figure 2) presenting increased radiation hazards to spacecraft instrumentation and astronauts (Mewaldt et al. 2010).

Of particular importance to aircraft technology and air crew, given the frequency of flights over polar routes, are the transient but intense increases in solar radiation resulting from high-energy solar energetic particle events. The highest energy solar energetic particle events, though relatively rare, occur throughout the solar cycle. The world wide network of neutron monitors offers the only real-time warning for the arrival of such events. While not enough is known of these elusive high-energy events, neutron monitors offer an important measure of their arrival times, spectral shapes, and anisotropies that help to constrain the acceleration processes and transport at play at the Sun (maybe you just need a Figure of a solar flare here).

Space Weather

The importance of this was articulated in the National Space Weather Action Plan of October 2015, where it states:

Historically, the global network of monitors was established by international scientific collaboration toward the common goal of the IGY. What and where a country could support a monitor were the main factors in how many and where stations were placed, with scientific considerations secondary—a reasonable strategy for a never-attempted exercise. However, with the potential global impact of a major space weather event, a more judicious plan for the number and location of stations is warranted. The stations should be numerous enough to cover a full range of cutoff rigidities and asymptotic directions so as to be sensitive to beamed SEPs over a wide spectral range. High latitude stations would have the lowest threshold, of course, but by themselves would not provide the spectral information necessary to assess the event’s potential radiological impact.

Medium term action: Construct a plan that has as its main priority a global network of monitors that covers the full range of geomagnetic rigidities over a mesh of asymptotic directions with some minimal angular separation determined through the analysis of archival GLEs.

Long term action: Solicit international collaboration to deploy and support these stations with potential assistance by the US.

Medium term action: Given that traditional IGY or NM-64 BF3 tubes are no longer manufactured, design around commercially available BF3 tubes, an inexpensive neutron monitor kit with a yield function as close as possible to that of the IGY or NM-64 designs. The magnitude of the yield function may be less than the traditional monitors, but should possess a similar spectral response. Keeping it small and inexpensive would facilitate wide deployment.

Implementing this plan could be accomplished as budgets permit, working toward the ultimate goal of a systematic global array of monitors with a minimum number of blind spots. The implementation of the plan must include electronic networking of the instruments, so that real-time data are immediately available to concerned parties. How these data are used can be left to the particular stakeholders and affected agencies and offices.

NMDB
Neutron Monitors routinely record ground-based cosmic ray intensities since the IGY in 1957. The improved NM64 monitor is in operation in many stations world-wide since 1964. The data of this network of Neutron Monitors has been shared among the stations, and the World Data Center for Cosmic Rays(WDC-CR) has been archiving paper records for all stations since those days.

Nowadays the data is available on CD-ROM and via an FTP server, however the standard data format has been defined for data in 1-hour resolution, and it takes anywhere from days to months until the data of a station arrives at the WDC. Since the 1990's many stations have upgraded their registration systems to measure the CR intensity at higher resolution, typically 1-minute. Most stations make this data available via their website, but due

to the lack of a standard, the stations created different data formats. The main goals of the Neutron Monitor database (http://NMDB.eu) are to provide high-resolution data from all Neutron Monitor stations in a standard format,to provide real-time data, and to make it easily accessible for everyone.

The standard data format has been implemented by storing the measurements in a SQL database. The stations are sending the data immediately after the measurement to NMDB, so that the data is available in real-time (ie less than 5 minute delay after the measurement where possible). The data is made available to everyone to an easy to use webinterface at http://nest.nmdb.eu

where data can be plotted and downloaded in ASCII format. For real-time applications a direct read-access to the database is available. The database already contains data from over 50 stations, not only real-time measurements but also historical data. To allow all stations to provide real-time data affordable registration systems have been designed during the NMDB project, the designs are freely available to all interested users.

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].

Fig.1. PAMELA’s pitch angle distribution (GCR background subtracted) in three rigidity ranges (top). Also shown is the world-wide neutron monitor pitch angle distribution (bottom) averaged between 0158 and 0220 UT.

[1] A. Bruno et al. (2015), Proc. 34th Intl. Cosmic Ray Conf., PoS(ICRC2015)085.