Analysis of Interball 2 and ground-based measurements of the increase in SCR intensity during the solar flares on November 4 and 6, 1997

V. E. Timofeev, I. I. Sosin, V. G. Grigor'ev, and S. A. Starodubtsev

Institute of Space Physics and Aeronomy, Yakutsk, Sakha Republic, Russia

Received October 15, 1999

Contents

Abstract

Introduction

The spacecraft Interball 2 was launched almost simultaneously with the onset of the new 23rd solar cycle. However, it did not detect a considerable increase in the solar cosmic ray intensity until November, 1997. The only exception was a small burst in the intensity of charged particles with energies > 7 MeV on April 7, 1997, which was likely to be connected with acceleration of the charged particles in the interplanetary space due to coronal mass ejection (according to Solar-Geophys. Data, 1997).

This paper presents the observed characteristics of the first intense proton events of the new 23rd solar cycle, which were induced by the flares on November 4, 1997 (magnitude  B2, coordinates 14^o S, 32^o W) and on November 6, 1997 (magnitude  B2, coordinates 18^o S, 63^o W). They were detected at the near-Earth orbit and on the Earth's surface.

A Brief Description of the Experiment

The Interball 2 spacecraft was launched on August 29, 1996, into an elliptic orbit with an inclination of 62^o, a period of 5 h 46.9 min, perigee of 770 km, and apogee of 19,184 km. The satellite-borne instrument 10K-80 consisted of electronic and detection units with wide-angle scintillation detector in the form of a spherical layer containing a semiconductor detector 100  m m in width and 1 cm² in area. The geometric factor of the telescope was 4 cm² s^-1 sr^-1. The device detected protons in 5 differential channels within the energy range from 27 MeV to 300 MeV and one integral energy channel with E_p > 7 MeV.

A more detailed description of the detection unit and results of its calibration with a monochromatic beam of protons from the isochromatic cyclotron U-240 installed at the Nuclear Research Institute of Ukranian Academy of Sciences were given by Grigorov et al. [1991] and Komarov et al. [1980].

The ground-based cosmic ray stations Irkutsk, Moscow, Yakutsk, Apatity, and Tixie are equipped by conventional neutron supermonitors with median energy E 10 GeV.

Data Analysis and Discussion

The event on November 4, 1997, was observed only at the spacecrafts. The event was associated with the active region NOAA 8100 (S21L352) where the flare X2/2B (14^oS, 32^oW) took place. The emission maximum was observed at 0558 UT. A coronal mass ejection was detected between 0552 and 0608 UT at the SOHO/EIT spacecraft [Lario et al., 1998]. At 0640 UT, in ~40 min after the flare, all channels of the 10K-80 device detected the onset of increase in the solar cosmic ray (SCR) intensity (see Figure 1). The arrows in Figure 1 indicate the moments of solar flares and their coordinates, the solid vertical lines show the moments of arrival of the shock wave and magnetic cloud boundaries [Lario et al., 1998].

Unfortunately, the arrival moments of the first particles and their energy dependence cannot be determined more accurately because Interball 2 was within the Earth's radiation belts. Nevertheless, since we know the coordinates of the burst (generation) of the SCRs that are typically accelerated immediately in the region of the optical burst [Reinhard and Wibberen, 1974] and the coordinates of the SCR injection into the interplanetary space (the base of the Sun-Earth field line) that have been determined taking into account the solar wind velocity ~350 km s ^-1 (Solar-Geophys. Data, 1997) and the Sun rotation velocity DL = 37^o, we may assume that a fast coronal transport of SCR from the generation point to the injection point took place. The time behavior of the SCR intensity increase in the undisturbed interplanetary magnetic field [Krimigis, 1965] can be satisfactorily approximated by the solution of the diffusion relation under the assumption of pulsed injection of particles and under the condition that the diffusion coefficient k depends on the heliodistance r as k = k₀ r^b [Mason et al., 1999], where b asymptotically approaches unity with increasing energy. The free path length l increases with energy from 5.6 10¹¹ cm to 9 10¹¹ cm.

The event is characterized by a rather rigid differential energy spectrum g = -1.89 (see Figure 2).

The event on November 6, 1997 is the greatest in the last 5 years. It was observed in the band of soft X rays by a number of spacecrafts and manifested itself in the ground-based measurements of the SCR neutron component as GLE. The flare X9.4/2B took place in the same active region NOAA 8100, its coordinates were 18^oS, 63^oW, and the H_a intensity maximum was at 1155 UT on November 6. The motion of the coronal mass ejection from the western limb of the Sun was observed at the same time [Lario et al., 1998]. In ~30 min, against the background of the SCR intensity decrease from the previous event, all the channels of 10K-80 detected the onset of a sharp increase with the amplitude exceeding the previous one by nearly a factor of 3 for 27- to 300-MeV protons (see Figure 1). In the integral channel detecting the particles with the energies higher than 7 MeV, the increase amplitude on November 6 was smaller than that of the November 4 event, which was probably due to a longer diffusion process for the particles with lower energies. The differential energy spectrum of this event was even more rigid, g = -1.58 (see Figure 2).

Figure 3 illustrates the results of detection of this event by a number of Russian cosmic ray stations. The data are presented in order of decreasing R that is geomagnetic cutoff threshold of a station. As we can see from Figure 3, particles with the magnetic rigidity to R < 4 GV were observed in this event. The estimation of the integral energy spectrum index (with consideration of the device coefficients needed to recalculate the data for the regions beyond the Earth's magnetosphere) is g = -2.59, which is rather rigid for a GLE. Since in this event the SCR particle injection took place during the travel of the magnetic cloud formed by the coronal mass ejection on November 4 in the interplanetary space, the particle flux appeared to be considerably anisotropic. For instance, the peak of the SCR increase was observed in Apatity 5 min earlier than that in Tixie. The front edge of the intensity increase in Apatity was about 10 min ahead that in Tixie.

According to the data from Interball 2, the intensity decay phase is longer than in the previous case, which points to the effect of the composite inhomogeneous interplanetary magnetic field caused by the coronal mass ejection on November 4, 1997, whose shock wave arrived at the Earth in 64 hours, at 2248 UT on November 6, 1997. The strong anisotropy, as well as the contribution into the measured particle flux from the previous event, hinders the use of the diffusion model for determining the parameters of the charged particle propagation in the interplanetary space in this event.

Summary

The analysis of the increase in SCR intensity in the events on November 4 and 6, 1997, has shown that they have a large amplitude and a rather rigid energy spectrum ( g = -1.89 and g =-1.58, respectively). The interval between the events of increase in the SCR intensity was almost equal to the time of propagation of the flare-induced disturbances from the Sun to the Earth (2-3 days). In the November 4 event the SCR propagated in the undisturbed solar wind, whereas on November 6 they propagated in the presence of a propagating shock wave generated in the previous event. The November 6 event was detected by the ground-based neutron monitors. An anisotropy in the energy range up to relativistic energies and spectrum steepening were observed. The obtained results were confirmed by the measurements from the Ulysses and Wind spacecrafts [Lario et al., 1998; Mason et al., 1999; Mobius et al., 1999].

Acknowledgments

References

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