X-RAY, gamma-RAY Emission and HIGH-ENERGY charged particles in near-Earth- space according to CORONAS-F satellite data: from maximum to minimum DURING the last solar cycle.

 

S.N. Kuznetsov, I.N. Myagkova, E.A. Muravieva, V.G. Kurt, V.I. Galkin, B.Yu. Yuskov

[E-mail: irina@srd.sinp.msu.ru]

 

Abstract.

The Russian solar observatory CORONAS-F was launched into a circular orbit with an inclination of 82.5^o, initial altitude of about 500 km and a final one of ~ 350 km, on July 31, 2001 and operated until December 12, 2005. Two of the main aims of SINP experiment on board CORONAS-F were performed: 1) simultaneous study of solar hard X-ray and gamma-ray emission with charged solar energetic particles (SEP); 2) detailed investigation of how SEP influence the near-Earth space environment. The CORONAS-F satellite orbit permits one to measure both SEP dynamics and variations of the solar particle boundary penetration as well as relativistic electrons of the outer Earth's Radiation belt during and after magnetic storms. We have found that significant enhancements of relativistic electron flux in the outer radiation belt were observed not only during strong magnetic storms near solar maximum but also after weak storms caused by high speed solar wind streams. The relativistic electrons of the outer ERB are an important source of radiation damage in the near-Earth space.

 

1. Introduction

Effects of space weather (and space climate) can range from damage to spacecraft to the disruption of power grids on Earth. The most important part of this damage is caused by solar energetic particles (SEP) - protons and electrons. Basic patterns of solar energetic particle transport by interplanetary shocks were published in e.g. Dorman and Miroshnichenko (1976). Recent experimental results and theoretical studies show that interplanetary shocks driven by Coronal Mass Ejections (CMEs) play a major role in accelerating SEPs (Cane et al (1988), Berezhko et al (2001)). Direct measurements of SEP penetration boundary variations at low altitudes are very important for in the estimation of the local space weather conditions.

Simultaneous measurements of solar flare electromagnetic emission and charged particles provide us with very useful information. Soft X-rays (SXR), hard X-rays (HXR) and g-rays produced at the Sun, their time history and spectrum in a wide energy interval provide us with the most direct information about particle injection and acceleration processes in solar flares. It is well known that the solar flare X-ray and g-ray emission is the result of charged particle interaction with the solar atmosphere - the superposition of electron bremsstrahlung continuum and g-ray line emission, e.g. Ramaty et al. (1988), Ramaty et al. (1994). The observed hard X-ray and/or g-ray spectrum shows that the charged energetic particles were accelerated up to rather high energies during the flare. Solar flare HXR and gamma-ray hardness data is very important in estimating the possible damage that may be caused by a given flare to technical systems.

Variations in Earth's Electron Radiation Belts (ERB) and the SEP cut-off during magnetic storms can also strongly affect the near-Earth environment. Previous studies have shown the relation between high speed solar winds and ERB enhancements several days later (e. g. Li et al. (1988)).

The main goals of the Russian solar observatory CORONAS-F (Complex ORbital Observations in the Near-Earth space of the Activity of the Sun) were the measurements of solar X-ray and g-ray emission and the study of how SEP events and Coronal Mass Ejections (CME) influence Earth's magnetosphere (see in Kuznetsov et al. (2002)). In this paper we present results related to these goals. Section 2 introduces the experiments and thereafter the main experimental results are given.

 

2. Experiment

CORONAS-F was launched into a circular orbit with an inclination of ~82.5^o and with an initial altitude of about 500 km on July 31, 2001. It operated until December 12, 2005 with a final altitude of about 350 km, Its orbital period was 94.8 minutes allowing it to perform ~15 number of orbits per day. The space device SONG (SOlar Neutron and Gamma-ray spectrometer) is an instrument of the SKL suite (Solar Cosmic Rays) developed by the Skobeltsyn Institute of Nuclear Physics, Moscow State University. SONG, consisting of a large CsI(Tl) crystal, detected X-ray and gamma-ray emission in a wide energy range 0.03-200 MeV (Kuznetsov et al. (2006)). Charged particles in different energy ranges (protons with energy 1-90 MeV, electrons 0.3-12 MeV) were measured by semiconductor and plastic scintillator telescopes (Kuznetsov et. al. 2002).

 

3. Observations and data analysis discussion

3.1 Soft and Hard X-ray emission

The duty cycle for the detection of solar flares on board CORONAS-F was about 40% as a result of its orbit parameters, so many major flares were lost during August 14, 2001 to September 12, 2005. However, 37 flares with gamma-emission and three solar neutron events were detected by CORONAS-F. Characteristics of the SONG flares (detection time, highest gamma-ray energy channel, HXR fluence) as well as SXR GOES characteristics are presented in Table 1.

 

Table 1. Solar gamma-ray flares detected by SONG (CORONAS-F) from August, 2001 to September, 2005.

 

3.2 Solar proton penetration boundary variations

Due to the orbit of the satellite solar energetic particles were measured by the CORONAS-F experiment only in the south and north polar caps during 15-20 minute intervals every ~45 minutes. Hence it should be highlighted that the solar relativistic electron data (1.5-3, 3-6 and 6-12 MeV) obtained by CORONAS-F from solar maximum (2001) to solar minimum (2005) are unique. Especially important are the solar extreme events measured during October and November, 2003 and January 2005. Solar event studies for November 2001, October-November 2003 and November 2004 were published in papers by Veselovsky et al. (2004), Panasyuk et al (2004), Yermolaev et al, (2005).

The CORONAS-F experiment (due to its low polar orbit) has demonstrated that for the estimation of possible SEP damage both the intensity of energetic solar particles and the data concerning the boundaries of solar particle penetration in the Earth's magnetosphere are very important. High energy solar particle penetration in the polar caps during the main phase of magnetic storms is one of the important sources of radiation danger in the near-Earth space, especially for low-altitude satellites. The size of the proton penetration area depends on proton energy and on geomagnetic conditions. Some earlier CORONAS-F results of similar studies were published in Panasyuk et al (2004), Kuznetsov et al. (2002), Yermolaev et al, (2005).

Fig.1. SEP penetration boundary variations during geomagnetic storms in May 2005. Dst-variation is shown by solid line.

 

As an example, in Figure 1 we present and discuss the variations observed in the SEP penetration boundary (or SEP cut-off rigidity variations) measured by CORONAS-F (protons with energies 1-5 MeV) during magnetic storms during the time period 19 to 21 August 2002. It is clearly seen that the proton flux on the penetration boundary does decrease abruptly. Therefore it is possible to apply different criteria to the analysis of the penetration boundary position. As was done in Kuznetsov et al. (2002) and Yermolaev et al, (2005), in this work we use the traditional 'Skobeltsyn Institute of Nuclear Physics Lomonosov Moscow State University' criterion - 'twice below the maximum of the SEP flux'. The values of penetration boundary obtained during the morning magnetic local time (MLT) are marked as plusses, during evening MLT as solid diamonds.

For comparison, time variation of the Dst index is also shown in Figure 1. In regard to Dst the main results are: (a) CORONAS-F data dynamics is in good agreement with Dst both for the morning and evening sectors of MLT, but for the evening sector the SEP penetration was deeper; (b) the penetration boundary for the evening sector was shifted closer to Earth (to L~3) not only during the strong magnetic storm near noon of August 21 (Dst= -115 nT) but also during the significantly weaker storm which took place 1.5 days earlier (Dst= -70 nT) and even during the very small storm of August, 19 (Dst= -55 nT). This observed effect - the penetration of SEPs during even rather weak magnetic storms - is very important for space weather forecasting studies.

 

3.3 Outer electron radiation belt variations

Earth's electron radiation belt (ERB) dynamics is one of the most important physical processes occurring during magnetic storms. The upper panels in figures 2a and 2b) show the relativistic electron dynamics in the outer radiation belt during April-June 2004 (a) and March-May 2005 (b). The electron flux is shown by grey-scale color. White color indicates the absence of data mostly connected with telemetry problems. During both time periods there were no significant solar flares and SEPs according to CORONAS-F (1.5-3 MeV) measurements. The X and Y axes show the day and the L-shell value respectively. Middle panel figures show the Dst index, and the bottom ones the solar plasma wind speed.

It is seen that the most significant enhancements of relativistic electrons were observed some days after the occurrence of small magnetic storms connected with incoming high speed solar wind streams. During the total operation time of CORONAS-F we have found more than ten such cases. The observed time-delay between solar wind speed enhancements and relativistic electrons in ERB supports previous work and is very useful for predictions studies.

Such electrons are sometimes named "killer electrons" as they are very dangerous to electronic devices, in particular the microcircuits that are used in space. Relativistic electrons of the outer ERB produce volumetric ionization in microcircuits of spacecrafts and breakdown their normal operation. Therefore, the measurement of relativistic electron dynamics has both practical and scientific interest (e.g. Myagkova (2005)) and references therein). We suggest that the high enhancements of relativistic electrons in the outer ERB founded in CORONAS-F experiment is necessary to take into account for space weather and space climate studies.

a)

b)

Fig. 2. Relativistic electron flux variation in the outer ERB in April-June 2004 (a) and March-May 2005 (b) according to CORONAS-F data. Middle panel shows the Dst index, bottom ones the solar plasma wind speed

 

 

4. Summary

 

In this paper we have presented observational results regarding X-ray, γ-ray and particle (protons and electrons) observations obtained by the CORONAS-F spacecraft. The main results are listed here:

Measurements of solar flare HXR and gamma-ray emission permits one to estimate the hardness of the flare emission so as to better be able to predict the possible damage that can be caused by the flares.

The monitoring of SEP penetration boundaries in the Earth's magnetosphere is useful also during magnetic storms, since solar energetic particles can penetrate deep into the Earth's magnetosphere during the main phase of even rather weak magnetic storms. In particular due to measurements of the SEP penetration boundaries one has the opportunity to estimate the radiation damage for different space missions.

The CORONAS-F observations provide evidence that the observed significant variations of relativistic electrons in the outer ERB at low altitudes (from 350 to 500 km) during and after geomagnetic storms can be connected both with strong magnetic storms caused by CMEs and with small magnetic storms connected with incoming high speed solar wind streams.

Results are important for space weather effect studies, especially how they are a function of solar activity.

.

Acknowledgements

This work has been partly supported by grant N 05-02-17487 of Russian Foundation for Basic Research

 

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