On the probability of solar cosmic ray fluency during SEP event On the probability of solar cosmic ray fluency during SEP event in dependence of the level of solar activityin dependence of the level of solar activity

L.I. Dormana,b, L.A. Pustil’nik a

(a) Israel Cosmic Ray & Space Weather Center and Emilio Segre’ Observatory, affiliated to Tel Aviv University, Technion and Israel Space Agency, Qazrin 12900, Israel;(b) Cosmic Ray Department of IZMIRAN, Russian Academy of Science, Troitsk 142092, Moscow Region, Russia; Supported by INTAS 5777 and NMDB (FP-7)

Observational data on cosmic ray fluency in the space near the Earth accumulated by ground and space CR observatories during last decades present unique possibility to estimate statistical properties real of the space radiation and its sensitivity to solar activity. This information on fluency distribution is necessary for estimation of radiation hazard both in the space (satellites, interplanetary missions, space stations) and in atmosphere (aircraft level).

For this aims we study the probability of difference fluency levels of solar cosmic ray in different phases of solar activity. We estimated probabilities of fluencies on the base of long time observation of proton flux (both direct measurements on the space detectors and on the base of recalculation from ground stations). Restored amplitude-frequency spectrum (distribution of events with given interval of amplitudes of the fluency or for fluencies more than given) allows to us estimate probability of dangerous states and extrapolate this distribution to rare events of extremely high amplitudes. These result allow to estimate radiation dangerous level both for long term missions in far space and for short time expeditions in near space for different states of solar activity

Main source of data for this analysis is data from the Catalog of Shea and Smart for 31 year observations’ fluency in space (1990) and for detailed analysis – data from satellites GOES during the last 11 years (1994–2004).

UN/ESA/NASA/JAXA Workshop on the International Heliophysical Year 2007 and Basic Space Science "First Results from the International Heliophysical Year 2007"

sted by the Solar-Terrestrial Influences Laboratory of the Bulgarian Academy of Sciences,on behalf of the Government of Bulgaria

2-6 June 2008, Sozopol, Bulgaria

Dependence of frequency (event/year) of events with fluency exceed Dependence of frequency (event/year) of events with fluency exceed critical level P(F>Fcritical level P(F>Fc c ) on amplitude of the level F) on amplitude of the level Fcc

• Three year averaged statistics of on the basis of Catalog of Shea and Smart (1990) show universal character of dependence of frequency of events on their fluency’ amplitudes for different energies – power spectrum for high energy events with universal exponent: 1.2.

• These universality we interpret as manifestation of “avalanche” self-organization nature of solar flare process (connected energy accumulation in random network with percolation of primary energy through convective zone in randomly high fragmented magnetic structures in corona with the next energy release in turbulent current sheets on the boundaries of this structures.

(a) Histograms of the dependencies of fluency frequency (event/year) from the lg (F) for particles with energies >10 MeV and >30 MeV, averagedfor three solar cycles; curves are according to fitting approximation (lower) with parameters and according to formulas lower for particles with energy >10 MeV, and for particles with energy >30 MeV; (b) Contour map form of 2-D dependence of frequency P of observed events (events/year): abscissa axis – the level of solar activity (yearly sunspot numbers), axis ordinate – logarithm of fluency of solar protons with energy >10 MeV, step in is of requency levels is 0.2 events/year; (c) the same as in Fig. 1b, but for solar protons with energy >30 MeV.

Daily fluency (in particle/(cmDaily fluency (in particle/(cm22 ster)) dynamics during 1994–2004 ster)) dynamics during 1994–2004 years for particle energies >1, >10, >100 MeV from GOES data.years for particle energies >1, >10, >100 MeV from GOES data.

• Integrated input to accumulated proton fluency during total 1994–2004 period from rare but strong events (lg (daily fluency) > 9 or daily fluency > 109 particle/(cm2 ster)) is much more than the input from the permanent and numerous weak events (noise like) with daily fluencies 104.5–107 particle/(cm2 ster). It is correct both for fluencies, which included low energy particles (E > 1 MeV) and for fluencies of high energy particles with energies >10 MeV and >100 MeV.

• Dependence of frequency – number of days N(F) with given daily fluency F has power character

Value k changed from k = 0.6 for particles with energy >1 MeV, k = 0.4 for particles with energy >10 MeV and k = 0.2 for particles with energy >100 MeV.

Distribution of total particle number during 1994–2004 years for particle energies E > 1, >10, and >100 MeV (left ordinate axis) in intervals of fluencies for proton energies E > 1 MeV (abscissa axis) and distribution of days number with given fluency interval integrated in total interval 1994–2004(right ordinate axis).

k

F

FN)F(N

00

Does Fluency Activity on Solar Activity depend on particles energy?Does Fluency Activity on Solar Activity depend on particles energy?

•Variations of sunspot number W (right Y-axis) and number of days per year N (left Y-axis) with given fluency interval for different energy channels during 1994–2004: (a)for events with daily fluencies F(E > 100 MeV) > 104 particle/(cm2 ster) – correlation coefficient 0.55; (b) the same for events with daily fluencies F(E > 10MeV) > 104.5 particle/(cm2 ster) – correlation coefficient 0.86; (c) the same for events with daily fluencies F(E > 1 MeV) > 106.5 particle/(cm2 ster) correlation coefficient 0.58; (d) the same for events with daily fluencies F(E > 1 MeV) > 107 particle/(cm2 ster)– correlation coefficient 0.73.

The asymmetryThe asymmetry in the frequency–fluency in the frequency–fluencydistributions for minimum and maximum solar distributions for minimum and maximum solar

activityactivity

(a) Number of events per year (left scale) with fluency F(E > 100 MeV) > 107 particle/(cm2 ster) and F(E > 100 MeV) > 106 particle/(cm2 ster), and the level of solar activity (yearly sunspot numbers W, right scale) in dependence of time in interval 1994–2004; (b) the asymmetry in the frequency–fluency distributions for minimum and maximum solar activity. Ordinate axis – lg(number of days with fluency F(E > 1 MeV)), abscissa axis – lg(fluency F(E > 1 MeV)) in interval of fluencies 104.5–9 particle/(cm2 ster).

(b) Asymmetry exists in reality but its level decreases with decrease of the level of fluency.

Two populations of fluency eventsTwo populations of fluency events

Two different populations of events with different sources of particles for them:

• First group provides input to low energy particles with a wide interval of fluency (lg (F(>1 MeV) = 4.5–8.5)), which does not give any input to medium- and high-energy fluencies. Possible sources of these particles are small solar flare events, CME, and interplanetary shock waves without high energy particle generation.

• Second group of the SEP is the source of higher amplitude SEP fluencies, which shows evident correlation between fluencies in different energetic channels: between high energy and medium energy, and between high energy and low energy.

ConclusionsConclusions• On the basis of 11-year observations on GOES 7–11 for 1994–2004 of daily fluencies for particles with energies E > 1, >10,

and >100 MeV it was shown that main input to the integral fluency during 1994–2004 was caused by few rare events with the highest fluencies. Several rare giant events gave a bigger input to integral fluency for 11-year cycle than thousands of lower fluency events.

• Fluencies for different energy channels demonstrate different asymmetry relative to minimum and maximum states of solar activity. In spite of general correlation of frequency of high fluency SEP events with sunspot number (correlation coefficients from 0.55 to 0.86), there are strong deviations between variation of sunspot numbers and frequencies of fluency (in agreement with Shea and Smart (1990)).

• Energy spectrum of particles appears in observed fluencies and its distribution in 2-D diagrams (Fluency (low energies)– Fluency (high energies)). It shows existence of two different populations of events: first population consists of events in low energy particles without any reaction in high energy particle fluencies; second population consists of events with simultaneous proportional reaction in all energetic channels from >1 to >100MeV (regression coefficients about 0.9 and correlation coefficients 0.7–0.8). We suppose that the origin of the second population is SEP generation in great solar flares with wide energy spectrum; the origin of the first low energy population may be caused by weak solar flares, CME and interplanetary shock waves without high energy particle generation.

• Our preliminary estimation of the radiation doses in space from the greatest SEP events with fluencies F(E > 1 MeV) > 109 particle/(cm2 ster) shows that this group of events is the source of real radiation hazard (tens–hundreds rem during event).

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