The connection of solar wind velocity with an intensity of energetic electrons trapped by geomagnetic field

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THE CONNECTION OF SOLAR WIND VELOCITY WITH AN INTENSITY OF ENERGETIC ELECTRONS TRAPPED BY GEOMAGNETIC FIELD E. V. Gorchalcovand Yu. V. Mineev Scobeltsyn Institute of Nuclear Physics, Moscow State University, Moscow 119899, Russia

ABSTRACT According to data from a number of satellites (“Prognoz”, “Intercosmos” and “Cosmos” series) a quantitative connection is established between values of electron fluxes with energy >30 keV on magnetopause and (with energies 0.03-15 Mev) in the outer radiation belt, and values of the solar wind velocity and K-index. Temporal relationships of this effect are also determined. In particular the delay of an appearance of energetic electrons in the magnetosphere relative to the maximum of the solar wind is determined. The explanation of these effects is discussed in the frame work of the theory of radial diffusion. A radial diffusion coefficient is estimated on the basis of values of time delays for energetic electrons peak fluxes on the magnetopause and in the outer radiation belt relative to the maximum of the solar wind velocity. INTRODUCTION Many of experimental data about energetic electron fluxes in the near Earth cosmic space obtained with a help of satellites “Molnia”, “Prognoz”, “Cosmos”, “Intercosmos” series are not taken into account to mathematical models. Among these data are: a) an appearance of the maximum in differential energetic electron spectra at energies Ee -1 MeV on L=3.5-5.0 in relatively quiet geomagnetic conditions (Dst <30 nT) /l/; b) a presence of minimum in differential energetic spectra of electrons on L=1.2-1.4 at energies Ee =0.6-1.0 MeV out of dependence on geomagnetic conditions /2/; c) connection of relativistic electron fluxes in the magnetosheath on the magnetopause and in the outer radiation belt of the Earth with the velocity of the solar wind, Dst variation, components of interplanetary magnetic field /3,4/. For the “a” block of the data the effect of variations is highly essential. For the “b” block of the data the effect of geomagnetic forces is traced in more complicated form. The effect of the solar wind on energetic electron fluxes is especially high in the magnetosheath on the magnetopause and also in the outer magnetosphere and on outer shells of the outer radiation belt of the Earth. EXPERIMENTAL DATA Since more precise spectrometers for electrons on satellites have been used it became clear that electron fluxes and electron spectra in the magnetosheath and in the outer radiation belt of the Earth depend strongly on the solar and geomagnetic activity, primary on D,t variations, the B, component of the interplanetary magnetic field and the solar wind velocity. (2)1??

(2)134

E.V. GodmkovandYu v. hain&v

1) The evolution of differential energetic electron spectra with energies 0.3-2.0 MeV are obtained /I/ from the data by the satellite “Intercosmos-19” for the outer radiation belt of the Earth on the altitudes 500-1000 km. At a magneticelly quiet period the spectra have maximum at the energies ~1 MeV on L=3.5-5.0. The spectra of the inner radiation belt of the Earth have minima at L=l.2-1.3 for Ee -0.6-l .O MeV. Similar spectra were obtained on the satellite “Cosmos-1686” on altitudes 350-500 km in the years 19851987 are shown on figure 1. _ N (e/c$sters

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Fig. 1, The evalution of differential energetical electrons in inner belt.

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Fig.2. Dependence of time for to

2) The discovered relationship between fluxes of relativistic electrons on the outer radiation belt of the Earth and the solar wind velocity is of interest. On the satellites “Prognoz-4-7”, “Raduga”, “Intercosmos-19”, “Cosmos-900” dependencies of the maximum of fluxes appearance in the outer radiation belt (L=7-4 and less) on the maximum of the solar wind velocity have been found. For the satellites “Prognoz-6-7” data /4/ and “Raduga”, analytical dependencies were obtained for electron flux density on the geosynchronous orbit from solar wind velocity V(t) J(t+z)=-19.8+5.73.10 -2 V(t)

(1)

where -c=2.5 days, t-time. Annalogous correlation may be obtained for magnetopause electrons also but the delay time is ‘I:=1.5 days. Correlation J(t+t)=a+bV(t) reflects many of experimental data about dependencies of the maximal fluxes of energetic electrons Ee >0.3 MeV in the outer radiation belt and magnetopause from the solar wind velocity. On the basis of the theory of radial diffusion, expected times of difhtsion for energetic electrons inside the magnetosphere for L=6.6 may be calculated with the formulae: xr, -0.01 L 2/ D,+k DoL-8 where k-coeff-icient -100. Do-5.10 -I4 c-l

(2)

Solar Wind Velocity

For estimation of D, values it is possible index of geomagnetic activity.

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(3)

VZ k, CK, +300 km/c, where k, 69; CK, is a sum of the K,, index values for 12 hours. According to the work /5/ good accord exists between expected times for electrons diffision from the boundary of the magnetoshere for I,&.6 and from the magnetopause zu and observed delay times z (fig.2). 3) High-velocity flow of the solar wind and the fluxes of energetic electrons in the Earth’s magnetosphere. The results of the “Cosmos-900” satellite were made by a global Cherenkov detector with the large geometric factor (~0.9 m2sr). The increase of the intensity was observed ina wide range of magnetic shells L=3 to 10. All the events have a f’eature in common a high velocity of solar wind (>700 km s-l). Table lists the paramctcrs of all events. TABLE

4) The correlation of the maximal amplitudes for energetic electrons bursts J, on the magnetopause with the solar wind velocity were obtained. The coefficient of correlation between these parameters r (ln Jm V)=O.65fl.O9.It should be detected that the observed dynamics for fluxes of energetic electrons in the magnetosheath and the outer radiation belt corresponds to the ideas about an acceleration of electrons near the magnetopause with following difhrsion them into the magnetosphere. This allows to make a prognoz for increasing of electron fluxes at least for period of 3 days extreme L shells. CONCLUSION Existing designed models do not take into account the experimental data here: 1) During the last two cycles of solar activity the specificial features were observed in the differential energetic spectra of electrons. These features are resistant character and should be taken into account in designed models.

6. v.Gxcbkov andYuv. MO,,,W

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2) Variations of relativistic electron fluxes in the outer radiation belt of the Earth are determined mainly by the force of magnetic storm and by the solar wind velocity, but not by the solar activity cycle. 3) The obtained electron fluxes dependencies, including the maximal fluxes dependencies, form a value of the maximal velocity of the solar wind may be used for short-term prognoses of radiation safety. REFERENCES l.P.V.Vakulov, L.M.Kovrigina, Yu.V.Mineev, L.V.Tverskaya. Geomagnetism and aeronomija. l5, 1028-1032 (1975) 2.Yu.V.Mineev, Mathem. Model for near Earth Cosmos. Moscow, INP MSU, 68-72 (1989). 3.I.N.Senchuro, P.I.Shavrin. Kosmich. issled., l4, (2) 314-316 (1976) 4.I.P.Bezrodnych, E.G.Berezko. Izvestija AN SSSR, ser. phys., $3, 2165-2167 (1984). S.I.P.Bezrodnych, E.G.Berezko, Yu.V.Mineev, et. al. 20 th ICRC, 4,453-456

(1987).

6.K.G.Afanasiev, Ye.V.Gorchakov, et. al. Adv. Space Res., 11, (9) 283-286 (1991).