On the Estimation of Solar Energetic Particle Injection Timing from Onset Times near Earth
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Abstract:
We examine the accuracy of a common technique for estimating the start time of solar energetic particle injection based on a linear fit to the observed onset time versus 1/(particle velocity). This is based on a concept that the first arriving particles move directly along the magnetic field with no scattering. We check this by performing numerical simulations of the transport of solar protons between 2 and 2000 MeV from the Sun to the Earth, for several assumptions regarding interplanetary scattering and the duration of particle injection, and by analyzing the results using the inverse velocity fit. We find that, in most cases, the onset times align close to a straight line as a function of inverse velocity. Despite this, the estimated injection time can be in error by several minutes. Also, the estimated path length can deviate greatly from the actual path length along the interplanetary magnetic field. The major difference between the estimated and actual path lengths implies that the first arriving particles cannot be viewed as moving directly along the interplanetary magnetic field.Keywords:
Solar energetic particles
Path length
Interplanetary medium
Particle (ecology)
Abstract. We present a new reconstruction of the interplanetary magnetic field (IMF, B) for 1846–2012 with a full analysis of errors, based on the homogeneously constructed IDV(1d) composite of geomagnetic activity presented in Part 1 (Lockwood et al., 2013a). Analysis of the dependence of the commonly used geomagnetic indices on solar wind parameters is presented which helps explain why annual means of interdiurnal range data, such as the new composite, depend only on the IMF with only a very weak influence of the solar wind flow speed. The best results are obtained using a polynomial (rather than a linear) fit of the form B = χ · (IDV(1d) − β)α with best-fit coefficients χ = 3.469, β = 1.393 nT, and α = 0.420. The results are contrasted with the reconstruction of the IMF since 1835 by Svalgaard and Cliver (2010).
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Based on cosmic ray data obtained by neutron monitors at the Earth's surface, and data on near-relativistic electrons measured by the WIND satellite, as well as on solar X-ray and radio burst data, the solar energetic particle (SEP) event of 2005 January 20 is studied. The results show that this event is a mixed event where the flare is dominant in the acceleration of the SEPs, the interplanetary shock accelerates mainly solar protons with energies below 130 MeV, while the relativistic protons are only accelerated by the solar flare. The interplanetary shock had an obvious acceleration effect on relativistic electrons with energies greater than 2 MeV. It was found that the solar release time for the relativistic protons was about 06:41 UT, while that for the near-relativistic electrons was about 06:39 UT. The latter turned out to be about 2 min later than the onset time of the interplanetary type III burst.
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Correlations of the Dst index with various interplanetary parameters are stud ied by using the solar wind and interplanetary magnetic field data from the ACE spacecraft. Among all parameters investigated, Ey, the down-dusk component of the interplanetary electric field is found to be best correlated with the Dst index. During strong and moderate geomagnetic storms, it is seen that 2 to 3 hours before the storm main phase starts, Ey suddenly begins to increase and that the increase of Ey, is almost synchronous with the decrease of the Dst, with a time a head about 2 to 3 hours. Such an abruptly changing characteristic curve of Ey is much easier to be identified than those corresponding curves of V , V2Bz .. VB2 and e. Weak disturbances in the magnetospheric background field, fluctuations of both the IMF Bz and the interplanetary Ey all strongly influence the development of storms, causing multiple-step main phase decrease and enhancement of the storm intensity. The physical mechanism responsible for the close correlation of Dst with the interplanetary electric field is discussed. A viewpoint that the present study of multiple-step main phase storms needs to be improved is proposed.
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Ionospheric dynamo region
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An overview is given of various phenomena observed by numerous spacecraft in the interplanetary medium. These phenomena are related to transient solar events such as flares and coronal holes. The effects of such transient solar events are extensive. At times, a solar event can affect the interplanetary medium out to distances as far as 17.2 AU and over a wide range of azimuthal angles. Also some phenomena, such as high frequency fluctuations (precursors of shocks) in the interplanetary medium, appear beyond 1 AU. Thus, transient phenomena are significantly modified by their passage through this medium. The conclusion is reached that transient events originating on the sun and passing through the interplanetary medium represent complex and significant physical problems.
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Solar flare
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Van Allen and Krimigis find that solar electrons diffuse through the interplanetary medium at a rate that is similar to that of protons of the same velocity, despite the disparity by a factor of 2 × 10³ in their respective magnetic rigidities, and they suggest that this fact serves to extend knowledge of the structure of the interplanetary magnetic field to much smaller scale than has been measured by Coleman. This suggestion is developed quantitatively in the framework of Roelof's theory. It is found that the power spectrum of the interplanetary magnetic field varies inversely as the square of the frequency ƒ in the range 2.7 × 10−4 < ƒ < 0.5 cps.
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