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Magnetosphere response to the 2005 and 2006 extreme solar events as observed by the Cluster and Double Star spacecraft
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Advances in Space Research 43 (2009) 618–623 www.elsevier.com/locate/asr
Magnetosphere response to the 2005 and 2006 extreme solar events as observed by the Cluster and Double Star spacecraft Iannis S. Dandouras a,b,*, Henri Re`me a,b, Jinbin Cao c, Philippe Escoubet d a
Universite´ de Toulouse, Centre d’Etude Spatiale des Rayonnements, F-31028 Toulouse, France b CNRS, UMR 5187, F-31028 Toulouse, France c Key Laboratory of Space Weather, CSSAR/CAS, 100080 Beijing, China d ESA/ESTEC RSSD, 2200 AG Noordwijk, The Netherlands
Received 22 February 2008; received in revised form 30 September 2008; accepted 3 October 2008
Abstract The four identical Cluster spacecraft, launched in 2000, orbit the Earth in a tetrahedral configuration and on a highly eccentric polar orbit (4–19.6 RE). This allows the crossing of critical layers that develop as a result of the interaction between the solar wind and the Earth’s magnetosphere. Since 2004 the Chinese Double Star TC-1 and TC-2 spacecraft, whose payload comprise also backup models of instruments developed by European scientists for Cluster, provided two additional points of measurement, on a larger scale: the Cluster and Double Star orbits are such that the spacecraft are almost in the same meridian, allowing conjugate studies. The Cluster and Double Star observations during the 2005 and 2006 extreme solar events are presented, showing uncommon plasma parameters values in the near-Earth solar wind and in the magnetosheath. These include solar wind velocities up to 900 km s 1 during an ICME shock arrival, accompanied by a sudden increase in the density by a factor of 5 and followed by an enrichment in He++ in the secondary front of the ICME. In the magnetosheath ion density values as high as 130 cm 3 were observed, and the plasma flow velocity there reached values even higher than the typical solar wind velocity. These resulted in unusual dayside magnetosphere compression, detection of penetrating high-energy particles in the magnetotail, and ring current development following several successive injections of energetic particles in the inner magnetosphere, which ‘‘washed out” the previously formed nose-like ion structures. Ó 2008 COSPAR. Published by Elsevier Ltd. All rights reserved. Keywords: Magnetosphere compression; Energetic particles; Ring current
1. Introduction The Cluster mission, prepared by the European Space Agency in collaboration with NASA, is based on four identical spacecraft launched in 2000. The spacecraft are on similar elliptical polar orbits with a perigee at about 4 RE and an apogee at 19.6 RE, flying in a tetrahedral configuration (Escoubet et al., 2001). This configuration allows the separation between spatial and temporal variations in the plasma populations, encountered along the spacecraft * Corresponding author. Address: Universite´ de Toulouse, Centre d’Etude Spatiale des Rayonnements, 9 Ave. Colonel Roche, BP 44346, F-31028 Toulouse Cedex 4, France. E-mail address: [email protected] (I.S. Dandouras).
orbit, and the calculation of 3D parameters. During the northern hemisphere winter the apogees of the orbits are in the near-Earth solar wind, allowing to study the magnetopause and the bow shock structure and response to solar wind conditions, whereas due to the annual orbit precession during the other half of the year the spacecraft perform measurements in the magnetotail. There they can analyze the plasma sheet dynamics during magnetospheric storms and substorms. During perigee passes the Cluster spacecraft cross the ring current region, the radiation belts and the outer plasmasphere, from south to north. On board each spacecraft 11 experiments permit a wide variety of measurements of the plasma parameters (particles and fields). Among them, the CIS (Cluster Ion Spectrometry) experiment is a comprehensive ionic plasma
0273-1177/$34.00 Ó 2008 COSPAR. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.asr.2008.10.015
I.S. Dandouras et al. / Advances in Space Research 43 (2009) 618–623
spectrometry package, capable of obtaining full threedimensional ion distributions with good time resolution (one spacecraft spin) and with mass-per-charge composition determination (Re`me et al., 2001). The CIS package consists of two different instruments, a Hot Ion Analyser (HIA) and a time-of-flight ion Composition and Distribution Function analyzer (CODIF). A fluxgate magnetometer (FGM) provides high-resolution magnetic field measurements on board the four spacecraft (Balogh et al., 2001). The data from the 11 experiments onboard Cluster are in the process of being archived at the Cluster Active Archive (CAA: http://caa.estec.esa.int/). The CAA is a depository of processed and validated high-resolution data, and is publicly accessible. Its purpose is to maximize the scientific return from the mission, and to ensure that the unique data set returned by the Cluster mission is preserved in a stable, long-term archive for scientific analysis even beyond the end of the mission. Since 2004 the Chinese Double Star TC-1 and TC-2 spacecraft have provided two additional points of measurement, on a larger scale: the Cluster and Double Star orbits are such that the spacecraft are almost in the same meridian, allowing conjugate studies. The equatorial spacecraft (TC-1) was launched into an elliptical orbit of 1.09 13.4 RE, inclined at 28.5° to the equator. This enabled it to investigate the Earth’s magnetic tail, or the dayside outer magnetosphere during the other half of each year, and then to cut through the inner magnetosphere and get even below the inner radiation belt at perigee (Fig. 1). TC-2 was launched into a polar orbit (1.1 6.8 RE, 90° inclination). The Double Star payload comprises a combination of Chinese experiments and backup models of instruments developed by European scientists for Cluster. Among them, the HIA (Hot Ion Analyzer) instrument on board the Double Star TC-1 spacecraft is an ion spectrometer nearly identical to the HIA sensor of the CIS instrument
Fig. 1. Orbits of Double Star and Cluster during the magnetotail crossings. The Cluster orbit is given in red and the orbits of the two Double Star (DSP) satellites, i.e. the equatorial TC-1 and the polar TC-2, are in magenta. RC is the ring current region.
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on board the four Cluster spacecraft. This instrument has been specially adapted for TC-1. It measures the 3D distribution functions of the ions between 5 eV/q and 32 keV/q without mass discrimination (Re`me et al., 2005). The Double Star magnetic field data used come from the FGM (Fluxgate Magnetometer) experiment on board TC-1 (Carr et al., 2005). One of the major objectives of Cluster and Double Star is to understand the physical processes by which solar wind particles enter into the magnetosphere. During some of the 2005 and 2006 extreme solar events, presented here, the Cluster and Double Star spacecraft were favorably located to observe the solar wind parameters sudden changes, taking very uncommon values, and/or the response of the magnetosphere to these sudden changes. 2. 21 January 2005 event On January 20, 2005, an outstanding solar flare occurred and stimulated one of the largest GLE (Ground Level Enhancement) events produced at the Earth in 50 years (Belov et al., 2005). Subsequent to this flare, an ICME (Interplanetary Coronal Mass Ejection) was detected in the near-Earth space on January 21, which showed evidence of current sheet substructure near the periphery of a strongly expanding, fast magnetic cloud (Foullon et al., 2007). During the ICME arrival (17:10 UT), the Cluster spacecraft were in the solar wind, at MLT 14.5 hours and at a geocentric distance of 19.4 RE, whereas the Double Star TC-1 spacecraft was initially in the dayside magnetosphere and then in the magnetosheath, at MLT 17 hours and at a geocentric distance of 10.5 RE (Fig. 2). As shown in the
Fig. 2. Cluster spacecraft 4 orbit (in black) and Double Star TC-1 orbit (in red) for the January 21, 2005 event, projected on the Tsyganenko (1989) magnetic field model. The model is plotted for reference, to show the typical magnetospheric configuration under quite conditions. Orbit Visualization Tool plot, courtesy of the OVT team.
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Cluster data (spacecraft 4 example, Fig. 3), the arrival of the leading edge of the ICME shock resulted in a sudden strong increase of the magnetic field and strong magnetic field fluctuations, in a hardening in the spectrum of the solar wind H+ and He++ ions, in an increase of the solar wind velocity up to 900 km s 1, and in a jump by a factor of 5 in the H+ and He++ ion densities. The spacecraft entered then in the ICME sheath, and a secondary front arrival can be identified at 18:44 UT. This is characterized by a sudden increase in the He++ density. This He++ enrichment is probably indicating the arrival of the flare driver gas (Foullon et al., 2007). The Double Star TC-1 data are shown in Fig. 4. As a result of the magnetospheric compression provoked by the ICME arrival at Earth the spacecraft, initially in the dayside magnetosphere, entered in the magnetosheath at 17:11 UT, and even a short excursion in the solar wind was observed at 17:37 UT. The magnetosheath is revealed, in the energy-time ion spectrograms, by the heated plasma (broad energy range) presenting a highly anisotropic flow. A second excursion in the solar wind was observed at 18:53 UT, and then the spacecraft re-entered in the magnetosheath at 19:07 UT, traversing the bow shock at a geocentric distance of 8.5 RE and revealing thus a highly compressed dayside magnetosphere. In the magnetosheath ion density values as high as 130 cm 3 were observed, which contrast strongly with the 30– 40 cm 3 values typically observed there. The plasma flow velocity values measured in this extreme magnetosheath regime reached 630 km s 1, which is even higher than the typical solar wind velocity.
The spacecraft went then through a highly compressed magnetosphere, and finally entered into the ring current region where a series of injected ion populations were observed, presenting energy dispersion effects. During the January 21, 2005 event the ring current development was also monitored by the Double Star TC2 and the IMAGE spacecraft. These spacecraft are equipped with the NUADU energetic neutral atom imager (McKenna-Lawlor et al., 2005) and the HENA energetic neutral atom imager (Mitchell et al., 2000), respectively, and were both favorably placed to monitor by remote sensing the ring current development. These observations and their analysis are presented in detail in reference (McKenna-Lawlor et al., submitted for publication). 3. 14 December 2006 event On December 13, 2006, during the descending phase of the 23rd solar cycle and almost into its minimum, a significant X3 solar flare occurred, followed by a strong Earthward oriented CME (Plainaki et al., 2008). The impact of the ejecta on the Earth’s magnetosphere occurred at 14:30 UT on December 14, 2006, and a solar wind velocity of 930 km s 1 was recorded by ACE (data not shown), associated with a jump of the solar wind density to 10 cm 3 and a southward turning of the IMF. The inner magnetosphere conditions on December 14 were initially quiet, with the exception of a short negative Dst index excursion to 40 nT at 18 UT. But, at 0 UT on December 15, the Dst index intensified rapidly to
Sheath ~ 3 cm-3
~ 15 cm-3
17:10 : ICME shock arrival (hot pileup)
18:44: Secondary front arrival
Fig. 3. Cluster spacecraft 4 magnetic field and ion data for the January 21, 2005 event. From top to bottom: magnetic field, CIS-CODIF energy-time spectrograms separately for H+ and He++ (data in energy flux units: keV cm 2 s 1 sr 1 keV 1); H+ ion bulk velocity; H+ parallel temperature, H+ and He++ densities, spacecraft coordinates (GSE system) and geocentric distance in RE.
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Brief excursion into the solar wind at 17: 37 UT, R=10.13 RE
2nd excursion at 18:53 UT (R=8.84 RE)
Re-entry in the Magnetosheath at R=8.54 RE (19:07 UT).
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Injected ions in the ring current region
Fig. 4. Double Star TC-1 ion data for the January 21, 2005 event. From top to bottom: HIA energy-time spectrograms (4) for ions arriving in the 90° 180° sector with a field-of-view pointing in the sun, dusk, tail, and dawn direction, respectively, in energy flux units (keV cm 2 s 1 sr 1 keV 1); ion bulk velocity; ion density, spacecraft coordinates (GSE system) and geocentric distance in RE.
110 nT, and at 7 UT it reached a value of 147 nT (Fig. 5). The recovery phase of this intense storm started at 12 UT on December 15, 2006, and the ring current activity returned to quiet conditions on December 17, 2006. The Cluster and Double Star TC-1 orbits during this event were inside the magnetosphere, and are shown in Fig. 6. As deduced from the H+ spectrogram in the upper panel of Fig. 7, Cluster entered into the ring current region (RC), in the morning sector at 13:50 UT, while the exit, at the outbound leg of its orbit, was recorded at 16:24 UT. Strong background due to penetrating particles from the radiation belts was recorded in both the inbound and the outbound leg of its orbit, at L-shell values between 5.5 and 6.5 (‘‘red haze” in the spectrograms around 15 UT and around 16 UT). In the CODIF energy range a ‘‘banded” ion structure was observed, with its energy evolving during this pass from 4–12 keV initially to 5– 18 keV. This kind of banded structures, also called
Fig. 6. Cluster spacecraft 4 and Double Star TC-1 orbits for the December 14, 2006 event, projected on the Tsyganenko (1989) magnetic field model.
Fig. 5. Dst index values for December 2006. On December 14, 2006, the onset of an intense magnetic storm was recorded while the Cluster spacecraft were going through perigee.
‘‘nose-like” structures due to the shape they can sometimes take in the energy-time ion spectrograms, are characterized by a deeper inward penetration of particles coming from the tail at a given energy (typically a few keV), and then
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RC
Dst = 2 nT
Dst = -40nT Nose-like structure: 4-12 keV → 5-18 keV
>20 keV 10-14 keV
Ring current population involving several successive injections
Fig. 7. Combined Cluster sc 4 (CODIF H+ and O+ data) and Double Star TC-1 (HIA ion data) for the December 14, 2006 perigee pass.
spreading to both higher and lower energies at larger Lshell values (Vallat et al., 2007; Dandouras et al., in press). A similar strong inbound/outbound asymmetry is also observed in the profile of the radiation belts penetrating particles, and it is apparently related to the highly tilted magnetic dipole (Fig. 6). It should be noted that the Cluster perigee pass occurred just when the ejecta cloud hit the magnetosphere, but before the ring current intensification as registered by the Dst index. The Double Star TC-1 spacecraft went through perigee at 11:40 UT (Fig. 7). At its inbound leg the HIA instrument recorded an entry into in the ring current at 03:30 UT, in the 23 MLT sector, i.e. in the opposite MLT sector with respect to Cluster. The high-energy resolution of the instrument allows the analysis in two energy bands of the observed, during the inbound leg, nose-like structure, one band at 10–14 keV and one above 20 keV, its upper limit being beyond the instrument energy domain. At its outbound leg, and after the exit from the heavy radiation belts background zone, i.e. after 14 UT, the HIA instrument recorded a much more active ring current ion population, with several successive injections, the bulk of the energy being apparently above the instrument energy domain (E > 32 keV) and HIA detecting only the lower part of their energy spectrum. Starting from 19:39 UT, and when the TC-1 spacecraft was at a geocentric distance of 8.7 RE in the pre-midnight magnetotail, the instrument recorded a background due to penetrating SEPs (Solar Energetic Particles), which appears as a ‘‘green haze” in the energy-time ion spectrogram.
4. Conclusions During some of the 2005 and 2006 extreme solar events, the Cluster and Double Star spacecraft were favorably positioned to provide coordinated measurements and monitor the solar wind parameters sudden changes, taking very uncommon values, and/or the response of the magnetosphere to these changes. In particular: Solar wind velocities up to 900 km s 1 were measured during an ICME shock arrival, accompanied by a sudden increase in the density by a factor of 5 (21 January 2005 event). During the secondary front of this ICME an enrichment in He++ was observed, probably indicating the arrival of the flare driver gas. The ICME resulted in a very strong magnetospheric compression. In the magnetosheath ion density values as high as 130 cm 3 were observed, and the plasma flow velocity values measured in this extreme magnetosheath regime reached 630 km s 1, which is even higher than the typical solar wind velocity. Ring current development was monitored (14 December 2006 event). A ‘‘nose-like” ion structure, previously formed in the ring current region and simultaneously detected by the Cluster and Double Star spacecraft in opposite MLT sectors, was ‘‘washed out” by several successive injections of energetic particles. These injections resulted in a much harder ring current energy spectrum.
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During this event the arrival of penetrating SEPs (Solar Energetic Particles) was recorded inside the pre-midnight magnetotail, about 5 hours after the impact of the CME ejecta on the Earth’s magnetosphere.
Acknowledgment The authors acknowledge the use of ACE data in this study. The Dst index was provided by the World Data Center for Geomagnetism, Kyoto. References Balogh, A., Carr, C.M., Acuna, M.H., et al. The Cluster magnetic field investigation: overview of in-flight performance and initial results. Ann. Geophys. 19, 1207, 2001. Belov, A.V., Eroshenko, E.A., Mavromichalaki, H., et al. Ground level enhancement of the solar cosmic rays on January 20, 2005, Proc. 29th Internat. Cosmic Ray Conf. (Pune), 1, 189–192, 2005. Carr, C., Brown, P., Zhang, T.L., et al. The Double Star magnetic field investigation: instrument design, performance and highlights of the first year’s observations. Ann. Geophys. 23, 2713–2732, 2005. Dandouras, I., Cao, J., Vallat, C. Energetic ion dynamics of the inner magnetosphere revealed in coordinated Cluster-Double Star observations. J. Geophys. Res., in press, doi:10.1029/2007JA012757. Escoubet, C.P., Fehringer, M., Goldstein, M. The Cluster mission. Ann. Geophys. 19, 1197–1200, 2001.
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Foullon, C., Owen, C.J., Dasso, S., et al. Multi-spacecraft study of the January 21st 2005 ICME: evidence of current sheet substructure near the periphery of a strongly expanding, fast, magnetic cloud. Solar Phys. 244 (1–2), 139–165, 2007. McKenna-Lawlor, S., Li, L., Barabash, S., et al. The NUADU experiment on TC-2 and the first Energetic Neutral Atom (ENA) images recorded by this instrument. Ann. Geophys., 1432-0576/ag/2005-23-2825, 2005. McKenna-Lawlor, S., Li, L., Dandouras, et al. Observations in energetic neutral atoms of the development and early decay of a ring current event on 21–22 January and modeling of its associated magnetospheric magnetic field, 2005, J. Geophys. Res., submitted for publication. Mitchell, D.G., Jaskulek, S.E., Schlemm, C.H., et al. High energy neutral atom HENA imager for the IMAGE mission. Space Sci. Rev. 91, 67– 112, 2000. Plainaki, C., Mavromichalaki, H., Belov, A., et al. Application of the NM-BANGLE model to GLE70, in Proc. 30th Inter. Cosmic Ray Conference. In: Caballero, R., D’Olivo, J.C., Medina-Tanco, G., et al. (Eds.), Universidad Nacional Auto´noma de Me´xico, Me´xico City, vol. 1, 281–284, 2008. Re`me, H., Aoustin, C., Bosqued, J.M., et al. First multispacecraft ion measurements in and near the Earth’s magnetosphere with the identical Cluster ion spectrometry (CIS) experiment. Ann. Geophys. 19, 1303, 2001. Re`me, H., Dandouras, I., Aoustin, C., et al. The HIA instrument onboard the Tan Ce 1 Double Star near-Equatorial Spacecraft and its first results. Ann. Geophys. 23, 1432-0576/ag/2005-23-2757, 2005. Tsyganenko, N.A. A magnetospheric magnetic field model with a warped tail current sheet. Planet. Space Sci. 37, 5–20, 1989. Vallat, C., Ganushkina, N., Dandouras, I., et al. Ion multi-nose structures observed by Cluster in the inner magnetosphere. Ann. Geophys. 25, 171–190, 2007.
Advances in Space Research 43 (2009) 618–623 www.elsevier.com/locate/asr
Magnetosphere response to the 2005 and 2006 extreme solar events as observed by the Cluster and Double Star spacecraft Iannis S. Dandouras a,b,*, Henri Re`me a,b, Jinbin Cao c, Philippe Escoubet d a
Universite´ de Toulouse, Centre d’Etude Spatiale des Rayonnements, F-31028 Toulouse, France b CNRS, UMR 5187, F-31028 Toulouse, France c Key Laboratory of Space Weather, CSSAR/CAS, 100080 Beijing, China d ESA/ESTEC RSSD, 2200 AG Noordwijk, The Netherlands
Received 22 February 2008; received in revised form 30 September 2008; accepted 3 October 2008
Abstract The four identical Cluster spacecraft, launched in 2000, orbit the Earth in a tetrahedral configuration and on a highly eccentric polar orbit (4–19.6 RE). This allows the crossing of critical layers that develop as a result of the interaction between the solar wind and the Earth’s magnetosphere. Since 2004 the Chinese Double Star TC-1 and TC-2 spacecraft, whose payload comprise also backup models of instruments developed by European scientists for Cluster, provided two additional points of measurement, on a larger scale: the Cluster and Double Star orbits are such that the spacecraft are almost in the same meridian, allowing conjugate studies. The Cluster and Double Star observations during the 2005 and 2006 extreme solar events are presented, showing uncommon plasma parameters values in the near-Earth solar wind and in the magnetosheath. These include solar wind velocities up to 900 km s 1 during an ICME shock arrival, accompanied by a sudden increase in the density by a factor of 5 and followed by an enrichment in He++ in the secondary front of the ICME. In the magnetosheath ion density values as high as 130 cm 3 were observed, and the plasma flow velocity there reached values even higher than the typical solar wind velocity. These resulted in unusual dayside magnetosphere compression, detection of penetrating high-energy particles in the magnetotail, and ring current development following several successive injections of energetic particles in the inner magnetosphere, which ‘‘washed out” the previously formed nose-like ion structures. Ó 2008 COSPAR. Published by Elsevier Ltd. All rights reserved. Keywords: Magnetosphere compression; Energetic particles; Ring current
1. Introduction The Cluster mission, prepared by the European Space Agency in collaboration with NASA, is based on four identical spacecraft launched in 2000. The spacecraft are on similar elliptical polar orbits with a perigee at about 4 RE and an apogee at 19.6 RE, flying in a tetrahedral configuration (Escoubet et al., 2001). This configuration allows the separation between spatial and temporal variations in the plasma populations, encountered along the spacecraft * Corresponding author. Address: Universite´ de Toulouse, Centre d’Etude Spatiale des Rayonnements, 9 Ave. Colonel Roche, BP 44346, F-31028 Toulouse Cedex 4, France. E-mail address: [email protected] (I.S. Dandouras).
orbit, and the calculation of 3D parameters. During the northern hemisphere winter the apogees of the orbits are in the near-Earth solar wind, allowing to study the magnetopause and the bow shock structure and response to solar wind conditions, whereas due to the annual orbit precession during the other half of the year the spacecraft perform measurements in the magnetotail. There they can analyze the plasma sheet dynamics during magnetospheric storms and substorms. During perigee passes the Cluster spacecraft cross the ring current region, the radiation belts and the outer plasmasphere, from south to north. On board each spacecraft 11 experiments permit a wide variety of measurements of the plasma parameters (particles and fields). Among them, the CIS (Cluster Ion Spectrometry) experiment is a comprehensive ionic plasma
0273-1177/$34.00 Ó 2008 COSPAR. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.asr.2008.10.015
I.S. Dandouras et al. / Advances in Space Research 43 (2009) 618–623
spectrometry package, capable of obtaining full threedimensional ion distributions with good time resolution (one spacecraft spin) and with mass-per-charge composition determination (Re`me et al., 2001). The CIS package consists of two different instruments, a Hot Ion Analyser (HIA) and a time-of-flight ion Composition and Distribution Function analyzer (CODIF). A fluxgate magnetometer (FGM) provides high-resolution magnetic field measurements on board the four spacecraft (Balogh et al., 2001). The data from the 11 experiments onboard Cluster are in the process of being archived at the Cluster Active Archive (CAA: http://caa.estec.esa.int/). The CAA is a depository of processed and validated high-resolution data, and is publicly accessible. Its purpose is to maximize the scientific return from the mission, and to ensure that the unique data set returned by the Cluster mission is preserved in a stable, long-term archive for scientific analysis even beyond the end of the mission. Since 2004 the Chinese Double Star TC-1 and TC-2 spacecraft have provided two additional points of measurement, on a larger scale: the Cluster and Double Star orbits are such that the spacecraft are almost in the same meridian, allowing conjugate studies. The equatorial spacecraft (TC-1) was launched into an elliptical orbit of 1.09 13.4 RE, inclined at 28.5° to the equator. This enabled it to investigate the Earth’s magnetic tail, or the dayside outer magnetosphere during the other half of each year, and then to cut through the inner magnetosphere and get even below the inner radiation belt at perigee (Fig. 1). TC-2 was launched into a polar orbit (1.1 6.8 RE, 90° inclination). The Double Star payload comprises a combination of Chinese experiments and backup models of instruments developed by European scientists for Cluster. Among them, the HIA (Hot Ion Analyzer) instrument on board the Double Star TC-1 spacecraft is an ion spectrometer nearly identical to the HIA sensor of the CIS instrument
Fig. 1. Orbits of Double Star and Cluster during the magnetotail crossings. The Cluster orbit is given in red and the orbits of the two Double Star (DSP) satellites, i.e. the equatorial TC-1 and the polar TC-2, are in magenta. RC is the ring current region.
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on board the four Cluster spacecraft. This instrument has been specially adapted for TC-1. It measures the 3D distribution functions of the ions between 5 eV/q and 32 keV/q without mass discrimination (Re`me et al., 2005). The Double Star magnetic field data used come from the FGM (Fluxgate Magnetometer) experiment on board TC-1 (Carr et al., 2005). One of the major objectives of Cluster and Double Star is to understand the physical processes by which solar wind particles enter into the magnetosphere. During some of the 2005 and 2006 extreme solar events, presented here, the Cluster and Double Star spacecraft were favorably located to observe the solar wind parameters sudden changes, taking very uncommon values, and/or the response of the magnetosphere to these sudden changes. 2. 21 January 2005 event On January 20, 2005, an outstanding solar flare occurred and stimulated one of the largest GLE (Ground Level Enhancement) events produced at the Earth in 50 years (Belov et al., 2005). Subsequent to this flare, an ICME (Interplanetary Coronal Mass Ejection) was detected in the near-Earth space on January 21, which showed evidence of current sheet substructure near the periphery of a strongly expanding, fast magnetic cloud (Foullon et al., 2007). During the ICME arrival (17:10 UT), the Cluster spacecraft were in the solar wind, at MLT 14.5 hours and at a geocentric distance of 19.4 RE, whereas the Double Star TC-1 spacecraft was initially in the dayside magnetosphere and then in the magnetosheath, at MLT 17 hours and at a geocentric distance of 10.5 RE (Fig. 2). As shown in the
Fig. 2. Cluster spacecraft 4 orbit (in black) and Double Star TC-1 orbit (in red) for the January 21, 2005 event, projected on the Tsyganenko (1989) magnetic field model. The model is plotted for reference, to show the typical magnetospheric configuration under quite conditions. Orbit Visualization Tool plot, courtesy of the OVT team.
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Cluster data (spacecraft 4 example, Fig. 3), the arrival of the leading edge of the ICME shock resulted in a sudden strong increase of the magnetic field and strong magnetic field fluctuations, in a hardening in the spectrum of the solar wind H+ and He++ ions, in an increase of the solar wind velocity up to 900 km s 1, and in a jump by a factor of 5 in the H+ and He++ ion densities. The spacecraft entered then in the ICME sheath, and a secondary front arrival can be identified at 18:44 UT. This is characterized by a sudden increase in the He++ density. This He++ enrichment is probably indicating the arrival of the flare driver gas (Foullon et al., 2007). The Double Star TC-1 data are shown in Fig. 4. As a result of the magnetospheric compression provoked by the ICME arrival at Earth the spacecraft, initially in the dayside magnetosphere, entered in the magnetosheath at 17:11 UT, and even a short excursion in the solar wind was observed at 17:37 UT. The magnetosheath is revealed, in the energy-time ion spectrograms, by the heated plasma (broad energy range) presenting a highly anisotropic flow. A second excursion in the solar wind was observed at 18:53 UT, and then the spacecraft re-entered in the magnetosheath at 19:07 UT, traversing the bow shock at a geocentric distance of 8.5 RE and revealing thus a highly compressed dayside magnetosphere. In the magnetosheath ion density values as high as 130 cm 3 were observed, which contrast strongly with the 30– 40 cm 3 values typically observed there. The plasma flow velocity values measured in this extreme magnetosheath regime reached 630 km s 1, which is even higher than the typical solar wind velocity.
The spacecraft went then through a highly compressed magnetosphere, and finally entered into the ring current region where a series of injected ion populations were observed, presenting energy dispersion effects. During the January 21, 2005 event the ring current development was also monitored by the Double Star TC2 and the IMAGE spacecraft. These spacecraft are equipped with the NUADU energetic neutral atom imager (McKenna-Lawlor et al., 2005) and the HENA energetic neutral atom imager (Mitchell et al., 2000), respectively, and were both favorably placed to monitor by remote sensing the ring current development. These observations and their analysis are presented in detail in reference (McKenna-Lawlor et al., submitted for publication). 3. 14 December 2006 event On December 13, 2006, during the descending phase of the 23rd solar cycle and almost into its minimum, a significant X3 solar flare occurred, followed by a strong Earthward oriented CME (Plainaki et al., 2008). The impact of the ejecta on the Earth’s magnetosphere occurred at 14:30 UT on December 14, 2006, and a solar wind velocity of 930 km s 1 was recorded by ACE (data not shown), associated with a jump of the solar wind density to 10 cm 3 and a southward turning of the IMF. The inner magnetosphere conditions on December 14 were initially quiet, with the exception of a short negative Dst index excursion to 40 nT at 18 UT. But, at 0 UT on December 15, the Dst index intensified rapidly to
Sheath ~ 3 cm-3
~ 15 cm-3
17:10 : ICME shock arrival (hot pileup)
18:44: Secondary front arrival
Fig. 3. Cluster spacecraft 4 magnetic field and ion data for the January 21, 2005 event. From top to bottom: magnetic field, CIS-CODIF energy-time spectrograms separately for H+ and He++ (data in energy flux units: keV cm 2 s 1 sr 1 keV 1); H+ ion bulk velocity; H+ parallel temperature, H+ and He++ densities, spacecraft coordinates (GSE system) and geocentric distance in RE.
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Brief excursion into the solar wind at 17: 37 UT, R=10.13 RE
2nd excursion at 18:53 UT (R=8.84 RE)
Re-entry in the Magnetosheath at R=8.54 RE (19:07 UT).
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Injected ions in the ring current region
Fig. 4. Double Star TC-1 ion data for the January 21, 2005 event. From top to bottom: HIA energy-time spectrograms (4) for ions arriving in the 90° 180° sector with a field-of-view pointing in the sun, dusk, tail, and dawn direction, respectively, in energy flux units (keV cm 2 s 1 sr 1 keV 1); ion bulk velocity; ion density, spacecraft coordinates (GSE system) and geocentric distance in RE.
110 nT, and at 7 UT it reached a value of 147 nT (Fig. 5). The recovery phase of this intense storm started at 12 UT on December 15, 2006, and the ring current activity returned to quiet conditions on December 17, 2006. The Cluster and Double Star TC-1 orbits during this event were inside the magnetosphere, and are shown in Fig. 6. As deduced from the H+ spectrogram in the upper panel of Fig. 7, Cluster entered into the ring current region (RC), in the morning sector at 13:50 UT, while the exit, at the outbound leg of its orbit, was recorded at 16:24 UT. Strong background due to penetrating particles from the radiation belts was recorded in both the inbound and the outbound leg of its orbit, at L-shell values between 5.5 and 6.5 (‘‘red haze” in the spectrograms around 15 UT and around 16 UT). In the CODIF energy range a ‘‘banded” ion structure was observed, with its energy evolving during this pass from 4–12 keV initially to 5– 18 keV. This kind of banded structures, also called
Fig. 6. Cluster spacecraft 4 and Double Star TC-1 orbits for the December 14, 2006 event, projected on the Tsyganenko (1989) magnetic field model.
Fig. 5. Dst index values for December 2006. On December 14, 2006, the onset of an intense magnetic storm was recorded while the Cluster spacecraft were going through perigee.
‘‘nose-like” structures due to the shape they can sometimes take in the energy-time ion spectrograms, are characterized by a deeper inward penetration of particles coming from the tail at a given energy (typically a few keV), and then
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RC
Dst = 2 nT
Dst = -40nT Nose-like structure: 4-12 keV → 5-18 keV
>20 keV 10-14 keV
Ring current population involving several successive injections
Fig. 7. Combined Cluster sc 4 (CODIF H+ and O+ data) and Double Star TC-1 (HIA ion data) for the December 14, 2006 perigee pass.
spreading to both higher and lower energies at larger Lshell values (Vallat et al., 2007; Dandouras et al., in press). A similar strong inbound/outbound asymmetry is also observed in the profile of the radiation belts penetrating particles, and it is apparently related to the highly tilted magnetic dipole (Fig. 6). It should be noted that the Cluster perigee pass occurred just when the ejecta cloud hit the magnetosphere, but before the ring current intensification as registered by the Dst index. The Double Star TC-1 spacecraft went through perigee at 11:40 UT (Fig. 7). At its inbound leg the HIA instrument recorded an entry into in the ring current at 03:30 UT, in the 23 MLT sector, i.e. in the opposite MLT sector with respect to Cluster. The high-energy resolution of the instrument allows the analysis in two energy bands of the observed, during the inbound leg, nose-like structure, one band at 10–14 keV and one above 20 keV, its upper limit being beyond the instrument energy domain. At its outbound leg, and after the exit from the heavy radiation belts background zone, i.e. after 14 UT, the HIA instrument recorded a much more active ring current ion population, with several successive injections, the bulk of the energy being apparently above the instrument energy domain (E > 32 keV) and HIA detecting only the lower part of their energy spectrum. Starting from 19:39 UT, and when the TC-1 spacecraft was at a geocentric distance of 8.7 RE in the pre-midnight magnetotail, the instrument recorded a background due to penetrating SEPs (Solar Energetic Particles), which appears as a ‘‘green haze” in the energy-time ion spectrogram.
4. Conclusions During some of the 2005 and 2006 extreme solar events, the Cluster and Double Star spacecraft were favorably positioned to provide coordinated measurements and monitor the solar wind parameters sudden changes, taking very uncommon values, and/or the response of the magnetosphere to these changes. In particular: Solar wind velocities up to 900 km s 1 were measured during an ICME shock arrival, accompanied by a sudden increase in the density by a factor of 5 (21 January 2005 event). During the secondary front of this ICME an enrichment in He++ was observed, probably indicating the arrival of the flare driver gas. The ICME resulted in a very strong magnetospheric compression. In the magnetosheath ion density values as high as 130 cm 3 were observed, and the plasma flow velocity values measured in this extreme magnetosheath regime reached 630 km s 1, which is even higher than the typical solar wind velocity. Ring current development was monitored (14 December 2006 event). A ‘‘nose-like” ion structure, previously formed in the ring current region and simultaneously detected by the Cluster and Double Star spacecraft in opposite MLT sectors, was ‘‘washed out” by several successive injections of energetic particles. These injections resulted in a much harder ring current energy spectrum.
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During this event the arrival of penetrating SEPs (Solar Energetic Particles) was recorded inside the pre-midnight magnetotail, about 5 hours after the impact of the CME ejecta on the Earth’s magnetosphere.
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