Neutron monitor network : fundamental research and applications

Neutron monitors are standard devices located at different points on the globe. The count rates of an individual neutron monitor probe primary cosmic rays above some threshold in energy and magnetic rigidity, and coming from a limited set of directions. Since these parameters basically depend on the location of the neutron monitor on the Earth, networks of neutron monitors provide an improved capability of extracting physical information from the data, such as energy spectra and propagation directions of the primary particles. They also give us the opportunity to use neutron monitors for space weather alerts. This requires a real-time database such as NMDB.

  1. Why do we need a network of neutron monitors ?
  2. Historical development

Why do we need a network of neutron monitors ?

Neutron monitors are standard devices located at different points on the globe. The very high counting rate, in comparison with detectors in space, is the great competitive advantage of neutron monitors. This allows the stations to observe many small and short-term changes in the cosmic ray intensity (of about 0.5% magnitude), which are not accessible to detectors in space. On the other hand, unlike space borne detectors, neutron monitors cannot be saturated by intense bursts of solar energetic particles. Another advantage of neutron monitors is their long-term reliability and automatic data acquisition.

The geomagnetic field introduces two effects which are specific for each location on the Earth (see discussion of Cosmic Rays and the Earth):

  • a low rigidity cutoff (or low energy cutoff), below which particles coming from the Universe cannot reach the atmosphere above the neutron monitor,
  • a narrow cone of viewing directions, within which the primary cosmic rays must impinge on the magnetosphere in order to reach the neutron monitor (more details here).

Because of these specificities, a network of neutron monitor stations located at different geographical positions is necessary to characterise the flux of charged particles arriving at the magnetosphere, both in arrival direction and as a function of rigidity or energy. The combination of the worldwide network of neutron monitors with the Earth's atmosphere and magnetosphere can therefore be considered as a unique instrument with directional resolution and energy resolution. This is why neutron monitors have been historically conceived with a standard design. The use of all stations as a unified multidirectional detector also makes the accuracy substantially higher (< 0.1% for hourly data) than for a single instrument. The above map shows the distribution of neutron monitors around the world.

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Neutron monitor networks: research

In the following we illustrate some findings that require neutron monitors at different locations on the Earth.

Long term monitoring of cosmic ray variations

Long term studies of neutron monitor records have shown that at each station the count rate varies with the solar activity cycle. This is the phenomenon of solar modulation of galactic cosmic rays. In order to investigate the origin of the phenomenon, we cannot rely on the count rates of one neutron monitor - we need to derive the cosmic ray intensity as a function of particle energy or particle rigidity. Since each neutron monitor is sensitive to primary cosmic rays above some low-rigidity (or low-energy) cutoff, which depends on its location on the Earth, especially its latitude, we can combine measurements of stations at different latitudes, from polar to equatorial regions. This has been done to derive the long term time history of galactic cosmic rays shown in this Figure, where the cosmic ray intensity at rigidity 10 GV (kinetic energy 9 GeV) is plotted and compared with the evolution of the sunspot number during several decades.

Directional characteristics of solar cosmic rays

The high-latitude network is essential for measuring anisotropies related to transient cosmic ray events, such as solar energetic particle events and Forbush effects. If the stations are all located at comparable geomagnetic latitude, their cutoff rigidities will be similar, and any difference in their count rate profiles must be attributed to the different arrival directions of the primary cosmic rays. This is illustrated by the observations of the solar cosmic ray event of 20 January 2005 with two neutron monitors that have not too different cutoff rigidities. The initial peak is much stronger at the Terre Adélie station than at Kerguelen Island, because during this specific event the first energetic particles impinged on the Earth's magnetosphere from the south, due to an unusual orientation of the interplanetary magnetic field. If you want to know more about the directions of viewing of neutron monitors, click here.

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Neutron monitor networks and space weather alerts

So neutron monitor networks are important for extracting the maximum of scientific information from the measurements. But networks are also essential for the use of neutron monitors in space weather alerts, be they related to solar energetic particles or to Earth-directed coronal mass ejections.

Solar energetic particle alerts

Enhanced fluxes of energetic particles from the Sun (SEP=solar energetic particles) are a major nuisance for spacecraft equipments and other technology, for radio communications in polar regions, and also for human spaceflight (more details here). With growing dependence on space based technology it becomes increasingly necessary to develop tools to forecast such events. Solar cosmic ray protons and possibly neutrons are, besides high energy electrons, the fastest particles that will reach the Earth during a given event. They are not numerous, and therefore are not by themselves a major danger. But their arrival signals that some time later the bulk of the much more numerous protons and ions of lower energy must be expected. And since solar cosmic rays are always produced in large events, where the number of protons and ions at lower energies is among the highest, neutron monitor networks can be used to develop real time warning systems, SEP alerts. There are two fundamental requirements for such an alert: predict events reliably, and avoid false alarms.

The importance of real time neutron monitor data for this purpose is one of the justifications for the NMDB project. Within this project we develop such a warning system, using data from at least three neutron monitor stations at high latitudes – since those are the most sensitive because of the low geomagnetic cutoff – and combining them with soft X-ray data from satellites in order to check if a flare is ongoing. When at a given neutron monitor the count rate exceeds the running average by some amount during several successive one-minute measurements, a ‘station alert’ is set. A Ground Level Enhancement is considered to have started when at least three stations are in the station alert mode, and when one X-ray channel shows that a flare has started.

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Cosmic rays as an early warning of geo-effective coronal mass ejections

But it is not only fast particles that play a role in space weather. Coronal mass ejections (CMEs) propagating through interplanetary space may generate a geomagnetic storm when they hurt the Earth's magnetosphere. The disturbance of the Earth's magnetic field induces electric currents that can interfere with technical equipment on the ground, especially in the polar regions of the Earth, and also with space borne electronics. Neutron monitor measurements can provide early warning of the arrival of Earth-directed CMEs, because these disturbances modify the propagation of galactic cosmic rays in the Heliosphere.

When a fast coronal mass ejection travels through interplanetary space (ICME), driving a shock wave in front of it, it affects the propagation of galactic cosmic rays and their arrival directions at Earth (more information here). As a shock wave may reflect charged particles, cosmic rays are depleted behind the shock. Since cosmic rays propagate much faster than the ICME, their monitoring can be used to inform about the incoming disturbed region well in advance of its arrival at Earth. Precursor signatures of ICMEs were indeed identified in the data of neutron monitors before the beginning of strong magnetic storms and large Forbush effects. Detailed researches of these effects showed that precursor signatures may be a decrease or an increase of the cosmic ray count rate.

Precursory decreases apparently result when a neutron monitor station is magnetically connected to the cosmic ray-depleted region downstream of the shock (which means behind the shock). But for the same reason - reflection at the shock - one expects an enhanced cosmic ray flux in front of the shock (upstream region). If the Earth is connected to this region, neutron monitors will detect an enhanced cosmic ray intensity before the ICME shock arrives. The shock effect is most prominent over the distance corresponding to one circular orbit of the cosmic ray particle in the magnetic field (the Larmor radius) in front of the shock. For protons of 10 GV rigidity on the quiet background of the mean interplanetary magnetic field intensity before the shock arrival (about 5 nT) the Larmor radius is bout 0.04 AU (1 astronomical unit = average Sun-Earth distance). A shock at 500 km/s needs about 4 hours to travel this distance before arriving at Earth. So, these anomalies are most often observed in the last hours before shock arrival. The neutron monitor network can identify these signatures and therefore emit a warning of the imminient onset of a geomagnetic storm.

This Figure shows an example: it is a map of variations of cosmiic ray intensities as a function of the asymptotic arrival direction (vertical axis) and time (fractional days, horizontal axis). Red circles signify a decrease of intensity and yellow circles - an increase. The size of the circle is proportional to the amplitude of the cosmic ray variation. The vertical line indicates the time when the shock arrived at Earth. Starting at that time the cosmic ray intensity is depressed at all neutron monitors, as shown by the ubiquitous red circles. But the picture shows clearly that intensity decreases of cosmic rays appeared well before within a narrow longitude zone 135°-180°, which corresponds to the direction of the interplanetary magnetic field. This peculiarity became especially well pronounced from September 7 ~23:00 UT (24 hours prior to the shock arrival at Earth!). The depression of the magnetic field-aligned cosmic ray intensity signalled that the interplanetary magnetic field line through the Earth was then connected to a region which prevented the arrival of cosmic rays - the region behind the shock of the Earthward travelling ICME. This way the real time observation of cosmic rays with the worldwide network of neutron monitors could serve to warn about the incoming ICME hours before its arrival.

The exploitation of this tool for operational services is a project for the future. The above picture was obtained using about 45 neutron monitors. With this number of stations all longitudes of arrival are observed at each instant. While rotating with the Earth, each station scans a complete circle of longitudes during a day, and, the more stations at different locations are used, the more complete is the picture we obtain. If we have only one station at each instant, only one longitude direction will be scanned. The European and near-European stations alone will not be sufficient either: the same example as above is pictured in these Figures by the data from only European (left)and European+Russian stations (right), hence from a narrow range of longitudes. Aspects of the precursor are still visible, but only sporadically, that is not sufficient for a reliable warning system.

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Historical development

Historically, the neutron monitor network started from J.A. Simpson who invented his instrument for the registration of cosmic ray generated atmospheric neutrons in 1948. At many stations of the present-day world wide network the continuous monitoring of this neutron component started in July 1957, with the official beginning of the International Geophysical Year (IGY).

The early period: International Geophysical Year and IGY neutron monitors

In 1957 – 1958 research was conducted under the IGY plan, and just after, in 1959, the IGC program (International Geophysical Collaboration) followed, as a prolongation of the IGY. On 15th September 1957 the World Data Center (WDC-B2) was created in IZMIRAN, Moscow (NIZMIR). In this place all observations from around the world were to be collected by exchanging data. At the same time, data obtained from the Soviet stations and the European-Asian regions were forwarded to data centres in the USA (WDC-A) and Japan (WDC-C). This data exchange established the mutual understanding of, and contacts between scientists of all countries.

The renewal of the network: NM64 neutron monitors

In the 1960s, the international scientific activity on the investigation of cosmic rays continued expanding, notably within the International Year of the quiescent Sun. In 1964 a new type of neutron monitor (NM64) was created by Hatton and Carmichael, with larger counters in order to provide better statistical accuracy. The old stations were re-equipped and the new super monitors were installed in new stations. The evolution of the number of stations equipped with IGY and NM64 neutron monitors and the evolution of the count rates can be seen in the Figure.

Towards a real time database

For the first time neutron monitor data (from the Moscow station) were posted on the Internet for real time consulting in 1997; this led, de facto, to a new era of real time collecting, processing and data presentation.

At present the world-wide network consists of about 50 operational neutron monitors with different specific energy characteristics and responses to primary cosmic rays. All neutron monitors operate continuously with 1- or 5- minute intervals of data collection. The majority of the stations (about 30) present their data on the Internet in real time. Since January 2008, the high-resolution Neutron Monitor Database (NMDB), an e-Infrastructures project supported by the European Commission in the Seventh Framework Programme (in the Capacities section), is being developed. This effort is focussed on the development of a real-time database for high resolution neutron monitor measurements, to include data from as many neutron monitors as possible. The main objective is the development of a digital repository with cosmic-ray data that will be available via internet to a broad range of user communities, via direct access to the database through standardised web interfaces.

Future developments can be envisaged whereby cosmic-ray related data from other scientific measurements (e.g. networks of muon telescopes) could also be pooled in this new cosmic-ray database.

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