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**Polar Neutron Monitors in the Study of Solar Cosmic Rays**

Title Polar Gateways Arctic Circle Sunrise 2008 Barrow, Alaska, January 23-29, 2008 Polar Neutron Monitors in the Study of Solar Cosmic Rays E.V. Vashenuyk, Yu.V. Balabin, B.B. Gvozdevsky Polar Geophysical Institute Apatity, Russia

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INTRODUCTION The neutron monitors (NMs) long since and down to the present time remain the basic means of relativistic solar cosmic rays study. These particles are observed in the Ground Level Enhancement (GLE) events. The rate of GLEs occurrence is ~ 1 per year. For 66 years from the first GLE registered on 28 February, 1942, only 70 events occurred up to now. The worldwide network of neutron monitors can be considered as a multidirectional cosmic ray spectrometer. The key role here is played by polar neutron monitors. Having rather narrow asymptotic cones of acceptance (viewing cones) they allow more precise determination of a direction on a source of particles and a form of pitch-angular distribution. The author’s modeling technique employing the optimization methods and modern magnetosphere models is described. It allows obtaining characteristics of high energy solar cosmic rays: rigidity (energy) spectrum, anisotropy axis and pitch angle distribution in the primary solar proton flux. With GLE modeling 14 events were analyzed and parameters of relativistic solar protons (RSP) as well as their dynamics studied. Two distinct populations of RSP: the prompt and delayed ones probably having different origins on the Sun have been revealed.

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**OUTLINE History of the GLE study with neutron monitors**

Short information about neutron monitor instrumentation. NM in Barentsburg and Apatity as part of the worldwide neutron monitor network. Neutron monitor response function and GLE modeling technique Results of relativistic solar cosmic ray events study with the GLE modeling. Neutron monitors in the study of large-scale interplanetary disturbances

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NM Apatity NM Climax Greatest in history GLEs : and

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**Neutron monitor is an instrument to register cosmic rays**

At height of 20 kms in the atmosphere the flux of primary cosmic rays, mainly protons, is transformed to secondary particles. The secondary neutrons penetrate up to the ground and are registered by the neutron monitor Neutron monitor constructed inside a marine container (Barentsburg) 2

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**Stages of creation of the Neutron Monitor in Barentsburg**

2003 B.B.Gvozdevsky 2005 2004 Y.V.Balabin 2006 3

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**Neutron monitors of the Polar Geophysical Institute**

PGI NMs Neutron monitors of the Polar Geophysical Institute Apatity (67.55N E) Barentsburg (78.08N 14.12E) Neutron monitors computer and electronics racks SERVER INTERNET 4

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**Effect of magnetosphere on cosmic rays Concept of an asymptotic cone**

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Anisotropy Solar cosmic rays anisotropy effect during the GLE on December 13, 2006 Apatity (10 sec data) Barentsburg (1 min) 6

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**Crucial role here play the polar neutron monitors**

Set of NMs The worldwide network of neutron monitors as a multidirectional cosmic ray spectrometer Crucial role here play the polar neutron monitors 7

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Asymptotic viewing cones of high latitude neutron monitors cover nearly the whole celestial sphere (by Shea and Smart, 1973) in (Duggal, 1979) One of the projects, successfully using lately a network of polar neutron monitors was the Spaceship Earth

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SPACESHIP EARTH is a network of neutron monitors strategically located to provide precise, real-time, 3-dimensional measurements of the cosmic ray angular distribution: (●) 11 Neutron Monitor Stations on 4 continents Multi-national participation: U.S.A., Russia, Australia, Canada 9 stations view equatorial plane at 40-degree intervals Thule and McMurdo provide crucial 3-dimensional perspective The project is led by Prof. J.Bieber, Univ.of Delavare, Bartol Research Institute and supported by NSF grants

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Before hit in the neutron monitor a cosmic ray particles (mainly protons) should pass through the magnetosphere and atmosphere of the Earth. In the atmosphere the flux of primary protons is transformed to secondary particles, including neutrons. The count rate of the neutron monitor is connected to the flux of primary cosmic rays through the Specific Yield Function (SYF). The product of the SYF by a spectrum of protons gives the response function. Characteristic response function is shown in the next Figure

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**Response function of the neutron monitor**

SYF is Specific Yield Function J(R)= a exp(R/Ro ) solar proton spectrum in exponential form ( ) Response= J(R)*SYF During the GLE 90% of response was between 1.5 and 3.7 GV for a sea level neutron monitor For all polar neutron monitors real cutoff is equal to atmospheric one: 1 GV (~450 MeV)

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The technique of deriving of the characteristics of relativistic SCR from the ground based neutron monitors data represents a rather complicated task A paper of D.F. Smart, M.A. Shea and P. Tanskanen, (1971) was a pioneer work in the GLE modeling technique. Then this technique was advanced in works: M.A. Shea, D.F. Smart, (1982), Cramp et al., (1997). Worth mentioned are papers of Bieber et al., 2003, Belov et al., 2005. Recently we developed a GLE modeling technique, allowing most precisely, from the nowadays point of view, to derive the characteristics of relativistic solar protons. It uses, in particular, the modern magnetosphere model of Tsyganenko (2002) and allows correctly account the contribution of the oblique particles into the neutron monitor response .

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GLE modeling technique of deriving the characteristics of relativistic solar protons (RSP) from the neutron monitor network data consists of a few steps: 1. Definition of asymptotic viewing cones (taking into account not only vertical but also oblique incident on a detector particles) by the particle trajectory computations in a model magnetosphere (Tsyganenko 2002) 2. Calculation of the NM responses at variable primary solar proton flux parameters. 3. Application of a least square procedure for determining primary solar proton parameters (namely, energy spectrum, anisotropy axis direction, pitch-angle distribution) outside the magnetosphere by comparison of computed ground based detector responses with observations

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**Asymptotic cones calculations**

Method: 8 direct. Asymptotic cones calculations For definition of a direction of arrival to the magnetosphere border of particles, contributing into the NM response, they calculate a trajectory of a particle with a proton mass, but negatively charged, which will start from border of the atmosphere above the given station. Calculation of a trajectory of a particle we do by integrating an equation of motion with the Rung-Cutta algorithm in a modern magnetosphere model of Tsyganenko 2002. Calculations of asymptotic cone of view for each NM station are proceeded from 1 GV (atmospheric cutoff) to 20 GV (theoretical upper limit of the spectrum of SCR) with the step in rigidity GV. To account the contribution of oblique incident particles we calculate beside a vertical, 8 trajectories of particles launched at zenith angle 20o and 8 azimuths 9

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**Neutron monitor directivity for solar cosmic rays**

Atmospheric attenuation of neutrons, produced by SCR I(θ) l = 100 g/cm2, attenuation length P is pressure Total directivity of a NM directivity dependence on zenith angle accounting the solid angle increase with θ

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**Scheme of asymptotic cones calculations:**

Method: 8 direct. Scheme of asymptotic cones calculations: To account the contribution of oblique incident particles we calculate 8 trajectories of particles launched at zenith angle 20o and 8 azimuths Asymptotic directions at magnetopause ~20° Starting directions at a launching point Calculated asymptotic directions are then used in the following modeling of a NM response 9

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**(dN/N)i is percentage increase effect at a given neutron monitor i **

Formula The response function of a i-th neutron monitor to anisotropic flux of solar protons. (dN/N)i is percentage increase effect at a given neutron monitor i J(R) = JoR-* is rigidity spectrum of RSP flux with changing slope * = + ·(R-1) where is increase per 1 GV (Cramp et al., 1997) S(R) is specific yield function (Debrunner et al., 1984), θ(R) is pitch angle (angle between the anisotropy axis given by ; parameters) F(θ(R )) ~ exp(-θ2/C) is pitch-angle distribution in a form of Gaussian (Shea&Smart, 1982) 8

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Thus, 6 parameters of anisotropic solar proton flux outside magnetosphere ; , Jo, , , C are to be determined by a solving of the nonlinear least square problem by comparison of computed responses with observations As example of such study we consider the last GLE on December 13, 2006.

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**The Sun on December 13, 2006 White light 30 nm emission**

Active region AR10930 Ground level effect of a solar flare Х3.4/2В S06 W UT

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GLE 70 Increase profiles at some NM stations: Oulu, Apatity, Moscow, Barentsburg, Fort Smith GLE 70 The asymptotic cones (1-20 GV), for the above NM stations and Th-Thule, McM-McMurdo, SA-SANAE, Ma-Mawson, No-Norilsk, Ti-Tixie, CS-Cape Shmidt, In-Inuvik, Pe-Pewanuk. The derived anisotropy axis and pitch angle grid lines for solar proton flux at UT are shown. The cross is the IMF direction (ACE data). 9

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**Observed and modeled responses at a number neutron monitor stations**

Fitting Observed and modeled responses at a number neutron monitor stations ─── increase profiles at neutron monitors ●●● modeling responses

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**Dynamics of pitch angle distributions (PAD) derived from neutron monitors data**

5 to Sun 1 3 3 1 2 Numbers mark the moments of time 4 PAD demonstrates an initial highly collimated beam of particles (prompt component) followed by a delayed quasi-isotropic population (delayed component) 5 6

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**■ GOES-11 TOM intensities ● balloons, 10 UT**

Dynamics of energetic spectra of relativistic solar protons Direct solar proton data ■ GOES-11 TOM intensities ● balloons, 10 UT Spectra derived from NM data 03:05 03:30 04:00

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**Prompt and delayed components of relativistic solar protons (RSP)**

The modeling analysis of 14 large GLEs occurred in the period on the data of the worldwide neutron monitors carried out by us revealed two distinct RSP populations (components): Prompt Component (PC): the early collimated impulse-like intensity increase with exponential energy spectrum, Delayed component (DC): the late quasi-isotropic gradual increase with a softer energy spectrum of the power law form. The exponential spectrum may be an evidence of the acceleration by electric fields arising in the reconnecting current sheets in the corona. The possible source of delayed component particles can be stochastic acceleration at the MHD turbulence in expanding flare plasma. E.V. Vashenyuk, Yu.V. Balabin, L.I. Miroshnichenko J. Perez-Peraza , A. Gallegos-Cruz, ASR, V.38 (3), 411; (2006); 30 icrc, Merida, Mexico, paper 0658 (2007)

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Two components of relativistic SCR were revealed also in the Greatest in history GLEs: and On a general background of "ordinary" GLE, differing in amplitude from ~1 to 600 %, two giant superevents: February 23, 1956 (GLE 05), and January 20, 2006 (GLE 69) are allocated. The amplitude of increase on neutron monitors in these events reached ~5000 %. As our modeling analysis revealed, two mentioned above particle populations (components), prompt (PC) with high anisotropy and exponential energy spectrum and delayed one (DC) with moderate anisotropy and power-law spectrum, exist in both cases. The prompt component was a cause of a giant pulse-like increase at a limited number of NM stations, and the DC caused a gradual increase with moderate amplitude at the most NM stations over the globe.

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**The GLE 20.01.2005 Flare X7.1 N14 W61 (GOES-12)**

Greatest Increase effects were observed at South Pole (5000%) and McMurdo (3000 %). Numbers mark time intervals when the prompt (1) and delayed (2) components dominated

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Prompt and delayed RSP components given rise to the exponential and power law spectra in the GLE SYF- specific yield function Debrunner et al., 1984 c a b d Increase profiles at the McMurdo and Mawson neutron monitors (a), rigidity spectra derived at the moments 07:00 (1) and 08:00 (2) UT (b), SYF and spectra 1 and 2 (c); differential responses (d) of the McMurdo neutron monitor to the exponential spectrum at the moment 1 (blue shading) and to the power-law spectrum at the moment 2 (red shading).

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**Two relativistic solar proton components in the GLE 23 February, 1956**

(a) Increase profiles at the Leeds and Ottawa neutron monitors; (b) energy spectra derived at the moments 04:00 (1) and 06:00 UT (2), (c) SYF and spectra (1 and 2) and differential responses of the Leeds neutron monitor to the exponential spectrum (1,blue) and to the power-law spectrum (2,red). By numbers are marked, respectively, the moments when the prompt component (1) or delayed one (2) were dominating. One can see comparable responses of both neutron monitors to the power-law spectrum at moment (2).

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**Results of modeling analysis of 14 major GLEs showing existence of 2 RSP components**

Spectrum of prompt component: J=J0exp(E/E0), E (GeV); J0, J1 (m2 s st GeV) -1 Spectrum of delayed component J=J1E- γ E.V. Vashenyuk, Yu.V. Balabin, L.I. Miroshnichenko J. Perez-Peraza , A. Gallegos-Cruz3, 30 icrc, paper 0588

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**Polar neutron monitors in the study of large-scale structure of IMF**

Sun flare Polar neutron monitors in the study of large-scale structure of IMF Coronal Mass Ejection on October 26, 2003 12

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**The scheme of Forbush-effect**

A series of Forbush decreases in the events of October-November, 2003 GLE effect The scheme of Forbush-effect CME 13

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Bi-directional flux GLE The large scale IMF structure in a form of giant loop constructed with the help of ground based neutron monitor data. It explains, how high-energy protons (HEP) from eastern solar flare could come to the Earth from antisunward direction Miroshnichenko L.I., Klein K.-L., Trottet G., Lantos P., Vashenyuk E.V., Balabin Yu.V., Gvozdevsky B.B., J. Geophys. Res Vol A09S08

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N-S asymmetry During the GLEs occurred in the steady effect of South-North anisotropy of solar cosmic ray was observed. The scheme of North-South asymmetry of SCR as observed by a pair of antipodal stations Barentsburg and McMurdo SCR anisotropy 15

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**N/S anisotropy of solar cosmic rays**

North-South anisotropy of relativistic solar cosmic rays sometimes registered by polar neutron monitors may be an indicator of large scale north-south asymmetry in the IMF that cannot be discovered by the spacecraft magnetometer measuring the IMF in one point of space. It should be noted that because of very large gyro-radius of relativistic protons in IMF (≥ 106 km) the noted effect could not be related with some reconnection process of the IMF field lines with geomagnetic ones. Steady South-North relativistic SCR anisotropy was observed during GLEs in October-November, The similar effect was observed also in the SCR of moderate energies (units, tens and hundreds MeV) as measured on a spacecraft CORONAS-F (Veselovsky et al., 2004), and in the GLE 70, 20 January, In the recent GLE 70, the N/S anisotropy was absent

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Results Polar neutron monitor network is an effective tool for the relativistic solar cosmic ray study. The modeling technique employing the optimization methods and modern magnetosphere models allows obtaining characteristics of high energy solar cosmic rays: rigidity (energy) spectrum, anisotropy axis and pitch angle distribution of the primary solar proton flux. There is good agreement of these characteristics with direct measurements in adjacent energy intervals on balloons and spacecrafts. The presence of the prompt and delayed components (PC and DC) of relativistic solar protons in all studied GLEs (14) as well as in superevents and has been shown. Moreover, the huge increases in both superevents on a limited number NM stations were caused by the prompt component having an exponential energetic spectrum. The polar neutron monitors are effective tool in revealing the large scale structure of interplanetary magnetic field during disturbed periods

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Aknowledgments We express our gratitude to the organizers of this conference given to us opportunity to communicate through huge distances over the globe. We are grateful to all colleagues presented the data of ground based NM observations used in this work. Our special sympathy is to the Bartol neutron monitor research team maintaining neutron monitor network under ( NSF Grant No ATM )

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