Patterns in Solar Modulation Observed by Neutron Monitors
Patterns in Solar Modulation Observed by Neutron Monitors Paul Evenson University of Delaware, Newark, DE 19716, USA Chungnam National University July 1, 2019
Abstract The basic ideas of solar modulation are all contained in the equation written down by Parker in the 1960’s but many of the details remain elusive. The flux of cosmic rays in the local region of the galaxy is thought to remain constant over thousands of years, but the flux reaching Earth is variable over all time scales that have been measured. Modulation occurs because the particles are not able to propagate freely due to magnetic fields in the plasma evolved by the sun, known as the “Solar Wind”. The solar wind dominates the region within approximately 100 AU from the sun. Because the plasma is expanding the particles that are confined by the magnetic field continuously lose energy. In order to understand the process it is necessary to consider the evolution of the particle spectrum as a whole over long periods of time. In spite of advances in spacecraft technology, neutron monitors remain the only source of information about the spectral evolution over a large range of energy and a long period of time. I will discuss what neutron monitors have recently discovered about modulation, and touch on some new approaches to increase the spectral sensitivity of neutron monitors. 2
Cosmic Ray Solar Modulation Cosmic Rays are energetic charged particles that come from sources outside the solar system -- probably from supernovae. Inside the solar system the intensity is changed (modulated) by magnetic fields originating from the Sun. Protons (>70 Me. V) seen by Voyager 1 (Cummings et al. 2016) are variable inside the solar system and constant after the spacecraft left the solar system. 3
Solar Structure – Conduction and Convection Zones It takes approximately one million years for the energy generated in the core of the sun to be conducted (by radiation) to the outer part. Near the surface, convective motion sets in. Approximately 100, 000 years of sunlight is stored in the convection zone. 4
Magnetic Dynamo Churning of the convection cells, and differential rotation of the sun generate the solar magnetic field. 5
Surface Magnetic Field • Unlike the earth, the magnetic field of the sun has not yet become nearly a dipole at the surface. • There is a dipole component, but it takes careful measurement to find it in the surface fields. • The dipole component reverses every eleven years or so. 6
Origin and Structure of the Solar Wind The hot solar corona expands, filling the solar system with plasma. This heat engine is very efficient, so the wind is “cold” and “supersonic”. The highly conductive plasma carries a magnetic field. Magnetic and electric interactions cause it to behave much like a fluid, even though the particles almost never actually collide. This is the origin of the term “wind”. 7
The Interplanetary Magnetic Field is organized into the “Parker Spiral” 8
Cosmic Ray Diffusion (a) In a uniform magnetic field a particle has a spiral orbit with a gyroradius rg = P/Bc. (b) When the field is non-uniform a particle drifts away from a field line due to the gradient and curvature of the field. (c) At a kink in the field that has a scale length >> rg, all particles will progress through the kink (but they may drift to adjacent field lines while doing so). (d) Likewise, if rg >> scale size of the kink, all particles will pass through it without being affected much. (e, f, g) When rg ≈ scale size of the kink, it depends on the gyrophase of the motion whether a particle will go through the kink (e), be reflected back (f), or effectively get stuck in the kink (g). This process is called pitch-angle scattering along the field. (h) When particles meet a kink, there is also scattering in phase angle, which leads to scattering across the field lines, leading to perpendicular diffusion. Adapted from Moraal (2010) 9
Modulation Can Be Described Without Considering Magnetic Field Details Parker (1965) showed that the spectra at Earth can be calculated by solving a Fokker-Plank equation in a spherically symmetric model of the interplanetary medium, including diffusion, convection and adiabatic deceleration. In this steady state form of his equation, the solar wind speed (V ) is independent of the distance from the sun (r). U(r, T) is the differential cosmic ray density for particles with kinetic energy T. The factor α incorporates the relativistic corrections to the kinetic energy. This works quite well reproducing primary spectra. One steady state, , approximate “force field” solution behaves as if there is a variable charge on the sun. With the assumption that the diffusion coefficient (κ) somehow depends on the level of solar activity, and the variation is slow enough, this provides a semi-quantitative description of modulation. • 10
Solar Modulation – A 22 Year Cycle 11
Electrons and Helium at 1. 2 GV The “sharp” then “flat” pattern appears in both electron and helium fluxes, but alternates. The 22 year modulation effect is related to the sign of the particle charge • 12
Charge Sign Dependent Modulation Electrons and helium at a rigidity of 1. 2 GV show a systematic differential modulation due to charge sign. • 13
Astrophysical Models Milk production at a dairy farm was low, so the farmer wrote to the local university, asking for help from academia. A multidisciplinary team of professors was assembled, headed by an astophysicist, and two weeks of intensive on-site investigation took place. The scholars then returned to the university, notebooks crammed with data, where the task of writing the report was left to the team leader. Shortly thereafter the physicist returned to the farm, saying to the farmer, “We have the solution, with only two minor assumptions".
Astrophysical Models The two minor assumptions: 1) Cows are spherical 2) Cows emit milk isotropically
The Heliosphere Is Not Spherically Symmetric and Particles Do Not Diffuse Isotropically • Large Scale: Drifts • Small Scale: Helicity 16
. Drifts Lack of axial symmetry in a magnetic field allows opposite charges to propagate differently. The Parker Spiral Field is not axially symmetric so large scale “gradient and curvature” drifts of particles in opposite directions can occur. Coupled with energy changes in the expanding solar wind this can result in different modulation levels for different charge signs. • 17
Heliospheric Current Sheet The dipole component of the magnetic field in the solar wind only begins to dominate when the field lines can no longer close. In the magnetosphere of the Earth there are only a few lines like this, forming the “tail”. In the heliosphere, a relatively thin current carrying region separates the regions of opposite polarity. This “current sheet” can have a rather complicated shape and can greatly affect the overall transport of particles in the heliosphere. As an extreme case of curvature drift this transport is charge sign dependent. 18
. Helicity Using interplanetary magnetic field data acquired at 1 AU, Bieber, Evenson and Matthaeus (GRL 14, 864 -867, 1987) showed that the distribution of magnetic helicity in the solar wind is asymmetric about the current sheet. The explanation is complicated, but asymmetric helicity and the opposite direction of the magnetic field cause systematically different diffusion coefficients for oppositely charged particles. Simply speaking, the helical trajectory of a particle can either have the same “twist” as the magnetic field or the opposite twist. The twist of the trajectory depends on the product of the charge sign and the field polarity. Coupled with energy changes in the expanding solar wind this can result in different modulation levels for different charge signs • 19
Ground Based Detectors Low energy cosmic rays are plentiful, and small detectors on spacecraft and balloons are excellent ways to detect them. Spacecraft can make useful measurements up to about 1015 electron volts. Above about 109 electron volts the measurements require long exposures, and above 1015 electron volts the exposures are impossibly long. Only ground based detectors can be made large enough to overcome these limits. The cosmic ray particles themselves interact in the atmosphere long before they reach the ground, but beginning at about 109 electron volts they produce secondary particles that reach the ground and carry information about the primary particle that produced them. The following slides show the development of a shower of secondary particles from a primary of about 1017 electron volts.
electrons/positrons muons photons neutrons
Neutron Monitors • Older type “BP 28” – proportional counter filled with BF 3: n + 10 B → α + 7 Li • Modern type – counter filled with 3 He: n + 3 He → p + 3 H • Both types are called “NM 64” Neutron Monitor in Nain, Labrador Construction completed November 2000 • 29
Neutron Monitors Primarily Observe the Spectrum Using Geomagnetic Cutoff Example: Latitude Survey in 2009 on the Swedish Icebreaker Oden 30
2009 Latitude Survey 31
Current Status Recent work by Nuntiyakul et al. (2014) and Mangeard et al. (2018) indicates that the interplay of drift, helicity and current sheet propagation is rather complicated and energy dependent.
Air Showers in a Self-Triggered Neutron Monitor Alejandro Sáiz, David Ruffolo, Warit Mitthumsiri, Chanoknan Banglieng, Pierre-Simon Mangeard, Waraporn Nuntiyakul, Paul Evenson 33
What this is about … • I have always been interested in detecting air showers with a neutron monitor, but it has been mostly a curiosity • I have never thought seriously about what science may be possible • Our recent work has focused on very tiny showers, but larger ones have proved to be easily detectable • So … I would like to get some help to see whethere might be some scientific value in looking into the larger showers with more detail • Comments and suggestions are therefore greatly appreciated!! 34
Neutron Monitors • High energy cosmic rays are rare. Observing them at high time resolution requires a large detector. • Ground based instruments remain the state-of-the-art method for studying these elusive particles. • Neutron monitors and muon detectors on the surface record the byproducts of nuclear interactions primary cosmic rays with Earth's atmosphere. 35
Solar Modulation We look for tiny air showers resulting from particles that are influenced by solar activity. 36
Neutron Monitor Principle • An incoming hadron interacts with a nucleus of lead to produce several low energy neutrons. • These neutrons thermalize in polyethylene or other material containing a lot of hydrogen. • Thermal neutrons cause fission reaction in a 10 B (7 Li + 4 He) or 3 He (3 H + p) gas proportional counter. • The large amount of energy released in the fission process dominates that of all penetrating charged particles. There is essentially no background. 37
Simulated Interaction In a Neutron Monitor 38
Monitor Counting Rate • Mostly we just use the counting rate of the monitor 39
Pierre-Simon’s Recent Project Just Accepted by Ap. J! 40
Measuring Particle Spectra • Variable geomagnetic cutoff can be used to investigate spectra – but what about the region near and above the highest available cutoff. 41
Interaction in the Lead Shows Some Dependence on Energy of Incoming Particle 42
For Several Years We Have Looked at the Time Distribution of Neutrons in Individual Detectors 43
Recently We Have Expanded to Correlations Among Detectors For adjacent detectors the neutrons can propagate, but for larger separations the correlations must result from distinct secondary particles from the same primary. It is more or less obvious that these must (on average) be higher energy primaries 44
To Make A Long Story Short … • We are now working on getting time histories of these multiple events to go beyond simple two-fold correlations. We know how to proceed with the small events – or at least think we do. • But we also (surprisingly) trigger nicely on much bigger events, and have no real idea how interesting these might be. • Do we try to publish this as a new result or just continue with our program as planned? 45
Trigger Where All Detectors Fire 46
We are working on improved electronics to get better resolution near the main interaction – but that is years away. Is it worth proposing? 47
Possible Applications • The main objective – study of modulation near 18 Ge. V with the monitor in Thailand. • Addition of energy resolution to other monitors, specifically South Pole • Study of air showers using the muon information from Syowa (next slide) • Coincidences with the Ice. Cube detector at South Pole 48
Combination Neutron Monitor and Directional Muon Detector Now Operating at Syowa Station Antarctica 49
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