Solar energetic electrons related to the 28 October 2003 flare

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 110,, doi: /2004ja010910, 2005 Solar energetic electrons related to the 28 October 2003 flare A. Klassen, 1 S. Krucker, 2 H. Kunow, 1 R. Müller-Mellin, 1 R. Wimmer-Schweingruber, 1 G. Mann, 3 and A. Posner 4 Received 19 November 2004; revised 20 May 2005; accepted 2 June 2005; published 8 September [1] We investigate the solar origin of near-relativistic electrons and protons during the X17.2/4B flare as observed by the Comprehensive Suprathermal and Energetic Particle Analyser (COSTEP) and Three-Dimensional Plasma (3DP) analyzer experiments on board the SOHO and Wind spacecraft. These observations are combined with groundand space-based spectral radio data obtained by the Potsdam spectrograph and the Wind/Waves instrument. Additionally, we use measurements of relativistic protons (ground-level event (GLE)) by neutron monitors (Kiel and Moscow). Timing and electron energy spectrum analysis suggest that there are three separate stages of electron injection into interplanetary space: (1) An injection of radio type III producing electrons is observed first; (2) an impulsive injection with an almost symmetric time profile with a short duration (18 min) is released 11 min later, followed by (3) a gradual, long (>1 hour) lasting injection, with an onset 25 min after the first type III burst. While the first escaping type III producing electrons are more likely related to the reconnection processes during the impulsive flare phase, the association of the two delayed electron injections with solar events is not well understood. Citation: Klassen, A., S. Krucker, H. Kunow, R. Müller-Mellin, R. Wimmer-Schweingruber, G. Mann, and A. Posner (2005), Solar energetic electrons related to the 28 October 2003 flare, J. Geophys. Res., 110,, doi: /2004ja Introduction [2] The time interval from mid-october to early November 2003 was one of the most powerful periods of solar activity in solar cycle 23. Three active regions (AR 10484, 10486, and generated twelve X class flares. Some flares were associated with strong high-energy proton and electron events and fast-moving coronal mass ejections (CMEs), hitting the Earth s magnetosphere and producing strong geomagnetic storms and well visible aurorae around the Earth, even down to low geomagnetic latitudes. [3] Impulsive electron events are often flare related, and about 90% of all events are accompanied by radio type III bursts [e.g., Lin, 1985]. Type III bursts are radiation from plasma oscillations excited by beams of fast electrons escaping from the Sun into interplanetary space. From the drift rate of the type III bursts it can be derived that the energies of the type III producing electrons are usually below 30 kev. This electron population is observed by in situ electron observations near 1 AU [e.g., Lin, 1985]. However, recent studies have shown that the near-relativistic 1 Institut für Experimentelle und Angewandte Physik, Universität Kiel, Kiel, Germany. 2 Space Sciences Laboratory, University of California, Berkeley, California, USA. 3 Astrophysikalisches Institut Potsdam, Potsdam, Germany. 4 Southwest Research Institute, San Antonio, Texas, USA. Copyright 2005 by the American Geophysical Union /05/2004JA electron events at energies above 30 kev are released up to 30 min later in comparison to type III radio emission [Krucker et al., 1999; Klassen et al., 2002; Haggerty and Roelof, 2002; L. Wang et al., Evidence for two different injections in three solar impulsive events, submitted to Geophysical Research Letters, 2005]. [4] The origin of these delayed electron events is still debated in three main directions. (1) Delayed events are accelerated by coronal shocks (observed as type II radio bursts) and/or by large-scale coronal waves (Moreton/EIT waves) in conjunction with CMEs [Krucker et al., 1999; Klassen et al., 2002; Haggerty and Roelof, 2002]. (2) It is suggested that reconfiguration (reconnection) of the low corona behind the coronal shock/cme plays a main role in the acceleration of delayed particle events. In this case, the flare itself and the shock play a minor role [Laitinen et al., 2000; Klein et al., 2001; Cane, 2003; Maia and Pick, 2004]. (3) Cane [2003] argues that the delay is due to particle propagation effects across magnetic field lines, and all events therefore originate from a single electron population. [5] In this paper, we investigate the solar origin and release of highly energetic electron events on 28 October 2003, which were associated with a X17.2/4B X-ray/optical flare at S16E08, AR (Solar-Geophysical Data no. 716, part 2, 2004) and a very fast 2459 km s 1 halo CME (Yashiro and Michalek, CME catalogue, nasa.gov/cme_list/index.html). We focus on the timing of energetic electrons in comparison with associated phenomena in the corona and discuss the energy spectra of different electron populations during this event. We will show that 1of6

2 two electron populations observed in interplanetary space were released at different times and were separate populations relative to those which generate type III bursts. 2. Instrumentation [6] The ground-based radio observations were obtained with the radio spectrometer ( MHz) of the Potsdam Astrophysical Institute [Mann et al., 1992]. The presented radio spectra in Figure 1 are background subtracted. The bright horizontal stripes appearing at a constant frequency are caused by terrestrial disturbances. [7] The space-based radio observations were obtained with the WAVES instrument on board the Wind spacecraft, which includes several radio receivers that cover the frequency range from 4.09 khz to MHz [Bougeret et al., 1995]. [8] The solar energetic electron events were observed with the Comprehensive Suprathermal and Energetic Particle Analyser (COSTEP) [Müller-Mellin et al., 1995] aboard the Solar and Heliospheric Observatory (SOHO) and with the 3-D plasma instrument on board the Wind spacecraft [Lin et al., 1995]. [9] White light observations of the corona were provided by the Large Angle Spectroscopic Coronagraph (LASCO/ SOHO) [Brueckner et al., 1995]. LASCO consists of three coronagraphs C1, C2, C3 which image the corona of the Sun from 1.1 to 30 solar radii. [10] The soft X-ray flux recordings have been obtained by the GOES instruments. The relativistic protons were observed with ground-based neutron monitors (Kiel and Moscow) with cutoff rigidities of 2.29 and 2.46 GV, respectively. We used 1 min data corrected for pressure, only. A description of this type of neutron monitor is given by Carmichael [1968]. 3. Observation 3.1. Determination of the Solar Release Time (SRT) [11] To compare the electron and proton injection time at the Sun we use the solar release time (SRT). The SRT of particles t SRT is estimated from the observed risetime (UT) of the particle flux at about 1 AU (t AU )as t SRT ¼ t AU L=v; where L is the length of the path along the magnetic field spiral from the release site on the Sun up to the spacecraft, v denotes the particle velocity. The interplanetary magnetic field (IMF) spiral can be approximated by a Parker spiral whose length depends only on the solar wind speed. The observed solar wind velocity of 650 km/s on 28 October, 2003 (from SOHO/CELIAS/MTOF Proton Monitor) implies an IMF line length from the Sun to Earth (SOHO) of 1.06 (1.04) AU. We also assume that the particles would propagate scatter free along the IMF spiral with a pitch angle 0. [12] The observed time of type II, III, IV bursts, CME and soft X-ray emissions were also expressed in terms of the solar release time according to t SRT ¼ t AU 500 s; where 500 s is the time required for electromagnetic emission to travel from the Sun to the Earth. The approximated time delay between the light propagation time along the Sun-Earth line and the MeV electrons travel time along the Parker spiral is about min X-Ray and Radio Observations [13] Figure 1 presents the time history of energetic electrons in comparison with the soft X-ray, radio emission, and with the neutron monitor count rates. [14] The initial rise in soft X-ray (SXR) emission at SRT is associated with a series of type III bursts in the range below 400 MHz. The impulsive flare phase starts with a steep rise of X rays at 1052 SRT and peaks at about 1102 SRT. This later stage was accompanied by a strong decimeter to meter radio emission (type IV burst), interplanetary type III bursts and a type II burst (see Table 1). [15] In the meter radio domain, the impulsive flare phase was accompanied by narrow-band radio signatures at SRT (range MHz), followed by a type IV burst 1054 UT ( MHz), a group of strong type III bursts at 1054 SRT (range MHz), and a type II burst at 1055 SRT with continuation at least until 1130 UT at 1 MHz. The type II burst is identified as slowly drifting radio lanes in the range MHz and is schematically shown by the red curve in Figure 1, drawn along the fundamental emission lane. It was partially overlapped by other types of radio emissions and therefore does not show a distinct two lane structure. [16] At frequencies below 14 MHz a group of type III bursts is detected at 1054: SRT. The arrival of type III emission at the plasma line (at 1140 SRT) was accompanied by local generation of Langmuir waves. That is a hint that the Wind spacecraft indeed intercepted the electron beams producing these type IIIs. The onset time of the type III bursts is indicated by the dotted line (I) in Figure 1. [17] During the flare, a strong continuum emission (type IV burst) with repeated maxima occurred in the whole radio range MHz. It is accepted that radio type III bursts are produced by electrons escaping the solar corona into interplanetary space. On the other hand, appearance of radio type IV bursts indicates the emission from electrons confined in closed magnetic fields in the corona. That points to a long-lasting electron acceleration in the corona [Trottet, 1986; Klein et al., 2005]. The type II burst is a radio signature of a shock wave traveling across the corona and is presumed to be an efficient accelerator of energetic electrons [Reames, 1999, and references therein] Relativistic Protons (Neutron Monitors) [18] The neutron monitor count rates are shown in the third panel of Figure 1 and represent the arrival of relativistic protons of 1.4 GeV at Earth (so-called ground-level event (GLE)). Their injection into interplanetary space occurs at about 1107 SRT after the SXR maximum and during the injection of the impulsive electron event (interval II, Figure 1). Both stations Kiel and Moscow at cutoff rigidities of 2.29 GV and 2.46 GV, respectively, are shown with a time resolution of 1 min. 2of6

3 Figure 1. Relation between electron intensities, neutron monitor count rates, radio, and soft X-ray emissions for the event on 28 October 2003: (top to bottom) electron intensities in the range MeV as observed by Wind/3DP and SOHO/COSTEP instruments, neutron monitor count rates, dynamic radio spectrum (range MHz, Potsdam and Wind), and X rays (1 8 Å and Å, GOES). The times shown are solar release times (SRT); that is, the time of flight is subtracted, and thus the effect of electron velocity dispersion is removed. In the insert, uncorrected time profiles show a distinct velocity dispersion. The red curve shows schematically the type II burst. Injection time of type III bursts is indicated by the dotted vertical line. Impulsive electron event (0.182 MeV), the injection time interval, is shown by dashed vertical lines. The relativistic protons (GLE) which triggered the neutron monitors were released in the same time interval. Injection interval of the gradual event in the range MeV is shown by solid vertical lines. A distinct delay is observed between type III bursts and the injection of two electron populations. The interval marked in orange indicates the period during which the COSTEP detector was affected by the strong hard X-ray emission. 3of6

4 Table 1. Event Timing Event Injection Time a (Onset Interval), SRT Maximum/End Time, a SRT Onset Time, b UT Range c Impulsive soft X rays First radio signature : MHz Type III bursts 1054: : MHz Type IV burst 1054 > MHz Type II burst (270) 1 MHz Impulsive electrons 1105 ( ) MeV Gradual electrons 1119 ( ) MeV Relativistic protons (GLE) 1107 ( ) MeV a Determined release time at the Sun (see section 3.1). b Times as observed at 1 AU. c Type II frequency in parentheses means the harmonic lane. The detected neutrons are secondary cosmic rays generated by cascades in the Earth s atmosphere. The primary cosmic rays initiating the cascades are predominantly protons, in our case of solar origin Electrons [19] Energetic electrons were observed by the SOHO/ COSTEP [Müller-Mellin et al., 1995] and the Wind/Three- Dimensional Plasma (3DP) analyzer [Lin et al., 1995] instruments. There are two distinct electron components (Figure 1, first and second panels). The first component, a highly anisotropic burst with a short duration of 18 min and an almost symmetric time profile, is detected below MeV. It shows an impulsive time profile (henceforth impulsive event) with a clear velocity dispersion in the peak flux (Figure 1, insert). The velocity dispersion at the onset shows an instrumental effect: If a high-energy electron loses only a fraction of its energy in the detector (i.e., it leaves the detector before it lost all its energy), a count at a too low energy is recorded resulting in too early onset times at low energies. The electron time profiles corrected for the time of flight presented in Figure 1 show this effect clearly, best seen for the lowest two energy channels that show an artificially too early onset. Since we do not see many electrons at high energies (soft spectrum, no counts above 300 kev), therefore the highest channel 182 kev gives the true onset. [20] The second component, a weakly anisotropic electron enhancement and detected at energies below 10.4 MeV, shows a long-lasting gradual character (henceforth gradual event) and was observed by both the Wind/3DP and COSTEP instruments. The terms impulsive and gradual are used here to describe the onset profile of the electron event, only. It does not indicate a short or long duration of related soft X-ray burst. The impulsive event electrons (182 kev) were injected at about 1105 SRT (interval II), 11 min after the start of the radio type III bursts (indicated by I). The later gradual event electrons ( MeV, interval III) were injected still later, about 14 min after the onset of the impulsive event. The vertical dashed and solid lines in Figure 1 indicate the injection times of the impulsive and gradual electron events, respectively. The injection time for the impulsive electrons was obtained using the steep rise of electron intensities as observed at 1 AU and then correcting for the propagation time in each energy channel, see chapter 3.1. [21] For the gradual event it was not so simple to determine the injection time because first the intensity increase was slow, and secondly, the COSTEP data during the impulsive flare phase were contaminated by the strong hard X-ray flux and pileup effect at SRT as marked in orange in Figure 1. Therefore these data were not used. To estimate the onset times in the COSTEP energy range, we used a linear backward extrapolation of the time profiles after 1132 SRT. The injection times (SRT) of electron populations and associated events are presented in Table 1. For comparison the onset times (UT) as observed at 1 AU are also presented. [22] A very fast 2459 km s 1 halo CME was observed by LASCO/SOHO coronagraph in association with this major flare. The CME first appeared at 1122 SRT when the CME was at about 5.8 R in the projection of the sky plane. A plot of the height-time measurements of the CME leading edge is shown in Figure 2. The speed 2459 km s 1 of the CME was obtained from the online SOHO/LASCO CME catalogue ( in which CME kinematics are estimated and compiled from LASCO C2 and C3 images. [23] By comparison of the CME evolution and the particle injection times we found that the CME was between 2.5 and 3.5 R and between 5 and 6 R at the time of the impulsive and the gradual electron event injections, Figure 2. Height-time measurements at the CME front as observed by LASCO/SOHO on 28 October The vertical lines are the same as in Figure 1 and indicate the injection times of different electron populations. 4of6

5 Figure 3. Spectra of initial impulsive (diamonds) and later gradual (pluses) events during the 28 October 2003 event. Horizontal bars mark the energy bins. respectively. Furthermore, the projected CME liftoff time (1054 SRT) match the type III bursts onset time Electron Events Spectra [24] Figure 3 shows the electron energy spectra measured between 8.9 and 310 kev for the impulsive event and between 27 and 510 kev for the later gradual event during their maxima. The spectra were obtained at the peak of each energy channel (peak flux spectrum) and at the center of each energy bin (e.g., P. H. Oakley et al., Spectra and frequency distribution of solar impulsive electron events, submitted to Astrophysical Journal, 2005, hereinafter referred to as Oakley et al., submitted manuscript, 2005). [25] The spectrum of the impulsive event can be fitted with two power law indices. For the low-energy electrons (40 kev) the spectral index is g 1.9. However, at the higher energies above 66 kev, the spectrum is extremely soft (g 6.2). The gradual event on the other hand is extremely hard with g = The evident spectral difference between the impulsive and the gradual events together with their delayed injection times suggest that these events belong to different electron populations. Unfortunately, it was not possible to extend the spectrum above 510 kev, because the COSTEP data at the event maximum were unusable because of the saturation and pileup effects. 4. Summary of Observational Results [26] The timing and spectral analysis of different kinds of electron signatures escaping the Sun during the flare in conjunction with a halo CME on 28 October 2003, reveals three stages of particle injection into interplanetary space, which are delayed with respect to each other. [27] 1. Electrons producing radio type III bursts (range MHz) occurred during the impulsive flare phase and strong decimeter to meter radio emission (Figure 1). [28] 2. Impulsive electron event (0.182 MeV) and relativistic protons (1.4 GeV) were injected about simultaneously after the maximum of soft X rays and coincident with a strong decimeter to meter radio emission (type IV burst) and a coronal shock wave (type II burst). These particles were injected about 11 min later than the electrons producing radio type III bursts. [29] 3. The relativistic electrons of the gradual event ( MeV) were released after the X-ray maximum and with a delay of about 25 min with respect to the onset of the impulsive flare phase and type III radio bursts. This gradual event was also delayed with respect to the impulsive electron event by about 14 min. [30] Above 66 kev the spectra of the impulsive and the gradual events are significantly different. The first show a very soft spectrum (g 6.2), the second a hard spectrum (g = ). That suggests, the impulsive and the gradual events belong to different electron populations. Furthermore, both the impulsive and the gradual events were released during a strong type IV burst and while a coronal shock wave (type II burst) was propagating through the corona. 5. Discussion [31] The in situ electron observations of the flare on 28 October 2003 discussed in this paper show two electron events, both delayed relative to the electrons causing the type III radio bursts: (1) An impulsive event with a short duration of 18 min with an almost symmetric time profile that is delayed relative to the type III emission by 11 min; and (2) a gradual event with a slow increase and a long duration of several hours that is delayed by 25 min. Obviously the type III producing electrons originate from the middle corona (start frequency above 80 MHz). They move along magnetic field lines well connected to the spacecraft. Indeed the arrival of the electrons was associated with local generation of Langmuir waves. From the timing it is possible that the electrons were released during the impulsive flare phase and originated from the flare site. However, it is more likely that the type III electrons were accelerated westward of the flare site because it was an east side flare (E08) connected magnetically to the spacecraft not well enough. [32] The large difference in the spectra between the impulsive and gradual events support the idea that the two delayed events are injected not only at different times, but also that different drivers or/and mechanisms are responsible for the acceleration. Determination of the solar processes responsible for the different electron events of 28 October is very difficult because a mixture of signatures in the X-ray and radio domains is observed (impulsive flare phase, type II and type IV bursts in conjunction with a CME) all indicating acceleration of electrons. [33] The first electron population associated with type III bursts can be explained by the reconnection process during the impulsive flare phase in the low corona. The second population, the impulsive electron event, occurs during the maximum of radio emission in the decimeter to meter range and could therefore be related to reconfiguration of magnetic field structures in the low corona. Strong maxima of type IV emission indicate a repeated acceleration in the low corona occurring near and/or far from the flare site behind the associated CME [Maia and Pick, 2004; Klein et al., 2005]. On the other hand, the impulsive event also occurred during a radio type II burst indicating a shock was present in 5of6

6 the corona at that time. Therefore shock acceleration is another possibility for the origin of the impulsive event [e.g., Klassen et al., 2002; Simnett et al., 2002]. As suggested by the neutron monitor observations, relativistic protons (1.4 GeV) are injected at about the same time as the impulsive electron event having a similar peak time and duration as the impulsive event. Therefore it seems likely that both were accelerated simultaneously by the same process and at the same place. [34] The third population, the gradual event, is also difficult to explain. The long delay of about 25 min relative to the flare impulsive phase together with the gradual intensity profile rather excludes that the gradual event is related to the impulsive character of reconnection process in the corona. The gradual event is more likely related to reconfiguration-reconnection behind the shock/cme leading edge or to the shock itself. Since very hard electron spectra, as seen for the gradual event, tend to be observed for events that also show radio type II bursts [Klassen et al., 2002; Oakley et al., submitted manuscript, 2005], the gradual event could be related to the shock acceleration process in the corona and interplanetary space in association with CME. [35] An alternative scenario is the so-called reservoir effect [e.g., Roelof et al., 1992]. The mildly relativistic electrons are ejected into a magnetic structure created by CMEs propagating outward from the earlier flares (e.g., CMEs on 26 October). The electrons propagate out past 1 AU and scatter back toward the Earth/Sun. They stream along a field configuration other than the standard spiral structure and fill gradually the reservoir. In this case the observed long-lasting electron acceleration in the low corona (type IV burst) could play an important role in filling the reservoir with relativistic electrons. This concept can more easily explain the very slow rise of the electron flux and the weak anisotropy of the event. 6. Conclusion [36] We found at least three stages of electron injection into interplanetary space during the flare/cme on 28 October 2003, which could be associated with different drivers and acceleration processes in the solar corona. The timing and spectral analysis together with the flare location in the east suggest that the two in situ observed electron events are different populations than the population that generate radio type III bursts. Our study confirms the existence of multiple populations of energetic electrons during a single solar flare in conjunction with a CME, but the association between the different populations and solar events is still not well understood. [37] Acknowledgments. We are thankful to the IZMIRAN neutron monitor team for data access. The CME catalog is generated and maintained by NASA and the Catholic University of America in cooperation with the Naval Research Laboratory. SOHO is a project of international cooperation between ESA and NASA. This work was supported by DLR grant 50 OC [38] Arthur Richmond thanks Dennis Haggerty and Karl-Ludwig Klein for their assistance in evaluating this paper. References Bougeret, J.-L., et al. (1995), Waves: The radio and plasma wave investigation on the Wind spacecraft, Space Sci. Rev., 71, Brueckner, G. E., et al. (1995), The large angle spectroscopic coronagraph (LASCO), Sol. Phys., 162, Cane, H. V. (2003), Near-relativistic solar electrons and type III radio bursts, Astrophys. J., 598, Carmichael, H. (1968), Cosmic ray measurements, Ann. IQSY, 1, Haggerty, D. K., and E. C. Roelof (2002), Impulsive near-relativistic solar electron events: Delayed injection with respect to solar electromagnetic emission, Astrophys. J., 579, Klassen, A., V. Bothmer, G. Mann, M. J. Reiner, S. 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