Energetic Neutral Atoms from the Sun: an Alternative Interpretation of a Unique Event

A&A 531, A46 (2011) Astronomy DOI: 10.1051/0004-6361/201116429 & c ESO 2011 Astrophysics

Energetic neutral atoms from the Sun: an alternative interpretation of a unique event

G. M. Simnett

School of Physics and Astronomy, University of Birmingham, B15 2TT, UK e-mail: [email protected]

Received 2 January 2011 / Accepted 9 May 2011

ABSTRACT

The high temperature of the solar corona results in virtually complete ionization of the light elements and a high degree of ionization of the heavier elements. Therefore it is not expected that many neutral atoms should be emitted from the Sun, and certainly not with high kinetic energy. A particle event associated with the first major flare of the current solar cycle, on 5 December 2006, has been interpreted as containing energetic neutral hydrogen atoms (ENA) of at least a few MeV. The ENAs were identified as such on account of their arrival direction at 1 AU, which was from the solar direction; the lack of atoms heavier than hydrogen; and the timing of their arrival, which was consistent with emission at the time of the flare X-ray burst. The observations were made from the two STEREO spacecraft which were near the Earth at the time. However, the EPAM instrument on the ACE spacecraft, which is in orbit around 6 the L1 Lagrangian point some 1.5 × 10 km away from the Earth towards the Sun, observed a pulse, or precursor, of electrons of energies of at least 50 keV but approximately one hour earlier than the pulse of ENAs at STEREO. Later ACE and STEREO detected detected a major charged particle event which is presumably associated with the 5 December flare. The relative times of the onsets of the energetic particles in both the precursor and the main solar energetic particle event at the STEREO spacecraft and ACE were consistent with corotation of the interplanetary magnetic field if the particles were the same population propagating, and probably trapped, within the field. The precursor proton intensity detected by STEREO was below the threshold of the ACE/EPAM detectors. We conclude that the interpretation of the particles seen by STEREO as energetic neutral atoms is suspect.

Key words. Sun: particle emission – Sun: ﬂares

1. Introduction However, the response to the subsequent event was normal, so we may therefore accept the precursor data at its face value. In a recent paper by Mewaldt et al. (2009) it was argued ∼ The detected electrons had a pitch angle distribution showing that a burst of protons seen 1 h following the onset of a that they were predominantly perpendicular to the local mag- GOES-class X9 flare on 5 December, 2006, at E79S06 degrees netic field, which would be consistent with a trapped distribu- on the visible solar disc, had travelled from the Sun as neutral hy- tion. How this might arise is difficult to say, and further discus- drogen atoms before entering their detector and becoming ion- sion is beyond the scope of the present paper. They were seen ized. They suggested that energetic neutral atoms (ENA) were to start arriving around 3 h and 23 min before the onset of the produced from energetic flare-accelerated protons which had un- energetic particles from the X9 flare. dergone charge exchange with heavy ions present in the corona or by radiative recombination with ambient coronal electrons. In TheprecursorseenbySTEREOstarted3hand15minbe- fore the onset of the flare particles. It would be too much of a support of this interpretation was the fact that the protons they detected were not accompanied by heavier ions, such as He nu- coincidence for the two events at ACE to be separated by essen- clei. Their observations were made from particle detectors on the tially the same time as the two events seen by STEREO if they were independent. Therefore it is worth examining the event in twin STEREO spacecraft, which were still close to the Earth on 5 December, 2006. This intriguing interpretation may possibly more detail, especially for an alternative explanation that fits all be true, but we believe it unlikely for reasons we shall discuss. the data. The Advanced Composition Explorer (ACE) spacecraft is Particles from poorly-connected flares may propagate onto interplanetary magnetic field lines which do not intersect the sunward of the Earth, in orbit around the L1 Lagrangian point. Around an hour before the supposed ENA detection at Earth. As the Sun rotates, magnetic structures populated with STEREO the EPAM instrument on ACE detected a burst of near- energetic flare particles may corotate past the Earth. This results relativistic electrons, which lasted around 15 min at an intensity in the onset at the Earth of particles accelerated in flares close above the ambient background. This burst we refer to as the pre- to the solar east limb being typically delayed by some hours cursor (following Mewaldt et al. 2009). from the onset of the flare. We believe that the main particle The electron pitch angle distribution was highly unusual, increase in the 5 December event is corotating and we shall dis- which first made us suspect that the data might not be accurate. cuss this in more detail below. A signature of a corotating event is the simultaneous increase at an observing spacecraft of par- Appendix is available in electronic form at ticles of different velocity. In other words, the fine structure in http://www.aanda.org the intensity-time profiles appears simultaneously at all energies. Article published by EDP Sciences A46, page 1 of 8 A&A 531, A46 (2011)

There is frequently backscatter, generally from a radial distance soft X-ray monitor and was one of the largest events from so- beyond the Earth, which can result in effective trapping of the lar cycle 23. The soft X-ray maximum was around 10:30 UT particles within the interplanetary magnetic field. (Solar Geophysical Data, US Dept. Commerce, Boulder, CO). There are two criteria which are highly relevant to the in- Energetic electrons were detected at ACE as the onset of a major terpretation of unusual events: composition, including electrons; event at around 13:50 UT. However, at 10:28 UT a small pre- and the timing of the arrival at the observing spacecraft of the cursor event was detected. At STEREO energetic protons from particles. Neutral particles coming from the Sun are not affected the flare were detected around 14:45 UT and a precursor was by the interplanetary magnetic field and should be detected at detected around 11:30 UT. It is the latter increase that Mewaldt times commensurate with their speed. A reasonable hypothe- et al. (2009) have interpreted as ENA. As this was early in the sis is that energetic particles are emitted from the Sun around STEREO mission electron observations from STEREO were un- the time of the maximum of the flare soft X-ray emission, say fortunately not available. ±10 min. ACE is spinning at 5 rpm and the particle intensities are mon- The event we are discussing has three possible interpreta- itored in four sectors for some channels and eight sectors for oth- tions. (1) it is caused by ENAs; (2) it is caused by neutrons; (3) it ers (Gold et al. 1998). Figure 1 shows the onsets of both the pre- is an unusual minor particle event caused by trapped particles cursor and the main event at ACE, as measured by the deflected propagating in some kind of plasmoid which is devoid of heav- electron channels on EPAM at 38–53 keV (DE1), 53–103 keV ier energetic ions at energies that could be detected. In the latter (DE2) and 53–62 keV (E1 ). The four sectors for the electrons case it is feasible that the particles were in a corotating magnetic are plotted and the precursor is clearly anisotropic. The arrows structure which had been populated with energetic particles by in the top panel of Fig. 1 are drawn at 10:27 and 13:50 UT. It some prior (10s of minutes) solar transient event, which may eas- is clear that electrons from the precursor event are not detected ily be closely related to a solar disturbance which is part of the by EPAM above 53 keV in the roughly sunward facing detec- precursor to the major flare. We present some data from the on- tors. In the LEFS60 detector (Fig. 1c) the presursor is detected set of the subsequent major solar particle event to demonstrate mainly in sectors 4 and 5. The energetic particle intensities at that the EPAM instrument is functioning properly. The lack of ACE had been around background levels for over 2 days before contamination by solar X-rays in the prime EPAM electron data the precursor increase discussed here, so that it is unlikely that is shown in the Appendix. the precursor is associated with anything other than the major X9-class flare referred to above. The axis of the LEFS150 sensor is pointing ∼30◦ away from 2. Energetic particle observations at the ACE the direction of the Earth. However, the precursor is seen in spacecraft the 58–104 keV electron channel of LEFS150 detector (E2), shown in Fig. 1d. It is visible in only two of the four sectors, The charged particle observations we discuss were made by but the increase is over two orders of magnitude above the back- the Electron, Proton and Alpha Monitor (EPAM) instrument on ground. The LEFS150 detector has a high noise level, which ex- ACE, which has a suite of five detectors responding to electrons ceeds the threshold of the lowest energy channel. A simplistic, from ∼40–300 keV, ions from ∼50–4800 keV and nuclei above ad hoc, interpretation of the data would be to subtract a noise ∼0.5 MeV/nucleon. For a description of the EPAM instrument level of ∼30 keV from the energy channel boundaries, which and the nomenclature used to identify the data see Gold et al. would then put the energy boundaries for the E2 channel down (1998). The instrument was built in the early 1980 s as the spare from 58–104 keV to 28–74 keV, which would be consistent with unit for the HI-SCALE instrument flown on Ulysses (Lanzerotti the data from the other electron detectors. et al. 1992) and knowledge of the precise energies for the various The increase starting around 13:50 UT is interpreted as par- channels is the best available, but may be somewhat inaccurate. ticles associated with the X9 flare. The intensity fluctuations of The ACE spacecraft is orbiting the L1 point, around 1.5 × these flare particles are seen simultaneously in all energy chan- 106 km towards the Sun, while on 5 December, 2006 the nels, showing that the increases are spatial, rather than temporal, STEREO spacecraft were still both close to the Earth. Thus for as there is no velocity dispersion. This is the signature of a coro- a nominal Parker spiral magnetic field, aligned at 45◦ to the tating event. Note the slight rise from the background level for Earth-Sun line at ∼1 AU, a corotating structure would pass ACE about an hour before the main increase. around 60 min before it passed the Earth. The STEREO space- We have put a black bar in Fig. 1a between the arrows cor- craft were in transfer orbits somewhere between the Earth and responding to the onsets of the precursor and the main event. In the Moon, but uncertainty in their precise position would not af- Fig. 1b we have marked with arrows the onset time of the pre- fect this conclusion significantly, nor would minor departures of cursor and the onset of the main particle event at STEREO. We the magnetic field from the nominal Parker spiral. have copied the black bar onto Fig. 1b. Within the accuracy of The relative positions of the ACE and STEREO spacecraft the timing it is clear that the separation between the two sets provide a strong constraint on the interpretation of the precur- of arrows is the same. The onsets of the main event, which is sor event. Neutral particles or photons travelling from the Sun certainly corotating, are in no doubt, and these times serve to will do so radially. Therefore a burst of solar photons will reach establish the corotation delay between ACE and STEREO. the Earth ∼5 s later than at ACE; similarly, a burst of neutral Figure 2 shows an overview of the event as seen in the spin particles travelling at, say 0.1c, will reach the Earth about 50 s averaged intensity for all four of the deflected electron channels after reaching ACE. With these constraints in mind, we shall now (see figure caption). There is no significant velocity dispersion review the charged particle data from ACE and compare it with visible in these data after 13:50 UT, which is when the inten- that published by Mewaldt et al. (2009) from STEREO. sities rise sharply. However, the electron intensities undergo a On 5 December 2006 the Sun produced a major flare from small increase after around 12:00 UT which does have velocity an active region at E79S06 degrees on the visible solar disc. dispersion, just visible in Fig. 2. The dashed line is drawn by eye All times in this paper are on 5 December, 2006 unless oth- to aid in the identification of this phenomenon. We interpret this erwise stated. The flare was classified as X9 by the GOES as evidence that there is some continuing emission of electrons

A46, page 2 of 8 G. M. Simnett: Solar energetic neutral atoms

EP:DE1 100004 10 (a) 38−53 keV electrons sr s) 2

10003 10 Particles/ (MeV cm

2100 10 2006/339 10 339 12 339 14 339 16 10 12 UT Date 14 16 EP:DE2 100004 10 (b) 53−103 keV electrons

sr s) 1000

2 3 10

2100 Particles/ (MeV cm 2 10

10 10 2006/339 10 339 12 339 14 339 16 10 12 UT Date 14 16 EP:E1’ 100004 10 (c) 45−62 keV electrons

1000 sr s) 3 102 Fig. 1. The onset of the precursor and the main event at ACE. The upper trace a) shows 2100 Particles/ (MeV cm Electrons / (MeV cm sr s) 10 the four sectors of the deﬂected electrons (DE1); and b) shows the same, but for chan-

10 nel DE2. The arrows in a) are drawn at 10 2006/339 10 339 12 339 14 339 16 10 12 UT Date 14 16 10:27 UT on 5 December, 2006, and 13:50 UT. EP:E2 100004 Trace c) shows the eight sectors of the LEFS60 10 channel E1;andd) shows the LEFS150 E2 (d) 58−104 keV electrons channel (see text for discussion of energy chan-

sr s) 10003 102 nel boundaries). The arrows in b) are drawn at the onset times of the precursor (11:30 UT) and the main event (14:45 UT) at STEREO 2 100 (Mewaldt et al. 2009). The dark bar in a) cov- 10Particles/ (MeV cm ers the time between the precursor and the main event onsets and is exactly reproduced in b). 10 10 2006/339 10 339 12 339 14 339 16 The data are plotted as 5 min averages and the 10 12 UT Date 14 16 nominal energy band is indicated in each plot 5 December 2006 hours UT (see text). from the Sun at a low level and from parts of the corona adjacent the magnetic field at the centre of the data accumulation time. to that from which most of the particles were emitted (the main The ordinate is the particle intensity normalised to 1 at the event) although other interpretations may be equally true. Note peak intensity. The actual intensity is S, which is given at the the good correlation of the intensity fluctuations in all four elec- top of each panel. EPAM doesn’t have coverage in the back- tron channels during the main event. This shows that the space- ward (Earth-pointing) hemisphere in the lowest channel cover- craft is sampling primarily a spatial variation of the electron in- ing 44–58 keV electrons on account of noise in the LEFS150 tensity, rather than a time-dependent increase. This is consistent detector. Figure 4 shows the pitch angle distribution of the with the data shown in Fig. 1 for the four sectors. These data also LEMS150 E2 electron channel, which nominally covers the en- show that EPAM is functioning correctly, apart from the possible ergy range 58–104 keV, from 10:20–10:44 UT. The data for the uncertainty in the numerical boundaries of the energy channels. electron pulse indicate a very anisotropic distribution, with a The magnetic field is shown in the lower panels of Fig. 2 minimum along the magnetic field direction. Note that the back- for the period covering this event. We are using spacecraft co- ground pitch angle distribution shown in the first panel from ordinates, where the reference direction is the Sun-spacecraft 10:20–10:26 UT is field-aligned. (0◦ azimuth) and the reference plane is the ecliptic. We note that For the precursor event the electron distribution is maximum the field azimuth direction is relatively constant for the period away from the magnetic field vector, which is suggestive of a 10–16 h UT on 5 December. There is nothing to suggest that trapped distribution. This is rarely seen in the EPAM data. the field should be capable of trapping or modifying the unusual For comparison, in Fig. 5 we give the electron pitch an- particle intensities we are discussing. The solar wind speed was −1 gle distribution at various times near the onset of the main around 310 km s . event. The data are plotted for successive periods of 15 min In Fig. 3 we plot the pitch angle distribution for the LEFS60 from 13:50–15:10 UT. The data show that the electron intensity E1 channel, covering electrons from 45–62 keV. The abscissa is somewhat anisotropic, with a minimum along the magnetic is the cosine of the angle the axis of the detector makes with field direction, which is approximately along the nominal Parker

A46, page 3 of 8 A&A 531, A46 (2011)

S= 751.2 S= 941.0 S= 478.1 1000010000 1.0 5 1.0 5 1.0 5 38−53 keV 4

53−103 keV 0.8 0.8 0.8

103−175 keV 4 0.6 0.6 0.6 175−315 keV 4 EP / E1’ 0.4 0.4 0.4 1000 7 2 1000 6 1 3 sr s) 7

2 6 8 0.2 8 3 0.2 3 0.2 2 1 7 6 2 45−62 keV electrons 1 8 2 0.0 0.0 0.0 −1.0 −0.5 0.0 0.5 1.0 −1.0 −0.5 0.0 0.5 1.0 −1.0 −0.5 0.0 0.5 1.0 2006/339 10:29:00.00 2006/339 10:31:00.00 2006/339 10:33:00.00 2006/33910:29 10:31:00.00 − 10:31 2006/339 10:31 10:33:00.00 − 10:33 2006/339 10:33 10:35:00.00− 10:35

Particles/ (MeV cm 100 100 Fig. 3. The electron pitch angle distribution from 10:29–10:35 UT on Electrons / (MeV cm sr s) 5 December in the LEMS60 E1 channel.

S= 27.94 S= 1158. 1.0 1 4 1.0 2 1010 3 2006/339 08 339 12 339 16 339 20 3 8 12UT Date 16 20 0.8 0.8 5 December 2006 hours UT 0.6 0.6 0.4 0.4 EP / E2 2 10 B 0.2 0.2 8 0.0 0.0 14 −1.0 −0.5 0.0 0.5 1.0 −1.0 −0.5 0.0 0.5 1.0 6 nT 2006/339 10:20:00.00 2006/339 10:26:00.00

nT 10:20 − 10:26 10:26 − 10:32 2006/339 10:26:00.00 2006/339 10:32:00.00 4 S= 1469. S= 170.8 2 2006/339 08 339 12 339 16 339 20 1.0 2 1.0 3 2 UT Date 3 180 Electrons 58 − 104 keV theta 0.8 0.8 135 0.6 0.6 90 0.4 0.4 EP / E2 degrees

degrees 45 0.2 0.2 4 1 0.0 4 1 0.0 0 2006/339 08 339 12 339 16 339 20 −1.0 −0.5 0.0 0.5 1.0 −1.0 −0.5 0.0 0.5 1.0 360 UT Date phi 2006/33910:32 10:32:00.00 − 10:382006/339 10:38 10:38:00.00 − 10:44 2006/339 10:38:00.00 2006/339 10:43:50.00 270

180 Fig. 4. The 58–104 keV electron pitch angle distribution at various degrees degrees 90 times during the precursor (see text for discussion of the energy channel boundaries). Notice that the electrons have a strong minimum along the 0 + 2006/3398 08 33912 12 33916 16 33920 20 ( ) magnetic field direction. The data are plotted as 6 min averages of UT Date the E2 channel. 5 December 2006 hours UT Fig. 2. Upper panel: the intensity-time history of the precursor event and the main event at ACE, seen in the four deflected electron chan- magnetically-deflected electrons do not suffer from any con- nels on EPAM. The energies of the channels are 38–53 keV (red), tamination and thus the intensity and pitch angle distributions 53–103 keV (green), 103–175 keV (blue) and 175–315 keV (cyan) and truly represent those of electrons. Furthermore, the LEFS60 and they are plotted as 5 min averages and spin averages. The dashed line LEFS150 detectors, which detect electrons, are generally reli- indicates the possible velocity dispersion seen before the main onset at able and uncontaminated, especially for the channels in the mid- around 13:50 UT. Lower panels: the magnetic field at ACE, plotted as the magnitude (B), colatitude (theta) and azimuth (phi). dle of the response function. The magnetosphere might conceivably be the source of the electrons seen by ACE. However, if this were the case, then we would expect the electrons to be field-aligned and coming from spiral towards the Sun. These data show that the EPAM instru- the anti-solar direction. Figure 4 shows that they are not field- ment is measuring the onset of the energetic electrons from the aligned. Also, a direct magnetic connection between ACE and major east limb flare in a manner which is consistent with previ- STEREO would be an extreme departure from a nominal Parker ous major flare-associated events. field. The precursor event is quite remarkable in that it has sev- eral features which are very difficult to interpret. The EPAM instrument has some detectors which have a slightly ambiguous 3. Discussion response to electrons and ions (see Gold et al. 1998; Lanzerotti et al. 1992). One might question whether the unusual behaviour The similarity between the intensity-time profile of the precursor in the EPAM electron data could be an artefact, caused, for event seen at ACE with that experienced by STEREO (Mewaldt example, by energetic solar photons. While this occurs in the et al. 2009, their Fig. 1) establishes beyond reasonable doubt LEMS30 detector, it has never been known to occur in the that the spacecraft are experiencing the same event, separated in deflected electron channels. In the Appendix we show data time simply because they are well-separated in space. The pres- from four large GOES X-class flares which occurred at vari- ence of electrons is difficult to understand if the precursor event ous times throughout the ACE mision, plus the X6.5 flare that is caused by energetic neutral hydrogen atoms. One might pos- occurred on 6 December, 2006. Thus we are confident that the tulate, however, as did Mewaldt et al., that the detected particles,

A46, page 4 of 8 G. M. Simnett: Solar energetic neutral atoms

S= 160.7 S= 341.3 S= 441.0

1 8 3 1 8 2 7 1.0 2 8 1.0 2 1.0 3 6 5 4 D 1 3 C C 4 B 4 C 0.8 7 0.8 7 0.8 B B D 5 65 0.6 A 0.6 0.6 6

0.4 0.4 0.4 D EP / E2’ E2 A A 0.2 0.2 0.2

0.0 0.0 0.0 −1.0 −0.5 0.0 0.5 1.0 −1.0 −0.5 0.0 0.5 1.0 −1.0 −0.5 0.0 0.5 1.0 2006/339 13:50:00.00 2006/339 14:05:00.00 2006/339 14:20:00.00 2006/33913:50 14:05:00.00− 14:05 2006/33914:05 14:20:00.00− 14:20 2006/33914:20 14:35:00.00− 14:35 S= 427.1 S= 777.8 S= 801.4

8 3 8 1.0 1.01 2 1.0 1 1 7 8 7 7 3 4 5 2 3 6 2 6 C4 5 C 0.8 0.8 4 6 0.8 B Electrons 60 − 100 keV B5 C Fig. 5. The ∼60–100 keV electron pitch angle 0.6 0.6 0.6 B D distribution at various times around the onset of D 0.4 0.4 0.4 the main event. Notice that the electrons have a EP / E2’ E2 A D A weak anisotropy, with a minimum along the (+) A 0.2 0.2 0.2 magnetic field direction. The data are plotted as 15 min averages of the E2 and E2 channels. 0.0 0.0 0.0 −1.0 −0.5 0.0 0.5 1.0 −1.0 −0.5 0.0 0.5 1.0 −1.0 −0.5 0.0 0.5 1.0 The DE2 channel has similar pitch angle cov- 2006/339 14:35:00.00 2006/339 14:50:00.00 2006/339 15:05:00.00 erage to the E2 channel and for simplicity is 2006/33914:35 14:50:00.00− 14:50 2006/339 14:50 15:05:00.00− 15:05 2006/339 15:05 15:20:00.00− 15:20 not shown. be they protons or electrons, are the result of neutron decay. EPAM resemble narrow beams of particles, which are detected This phenomenon has previously been discussed at length by in some sectors but not others, and have a sharp upper energy Dröge et al. (1996). They show that the electrons from energetic cut-off for electrons. There is no evidence for the precursor in neutron decay have an energy spectrum which peaks around DE2. In the LEFS150 detector the precursor is clearly seen in E2 300 keV (see Dröge et al. 1996, their Fig. 2). This is inconsistent (Fig. 2), which nominally responds to 58–104 keV. However, we with the spectrum of the detected electrons at ACE, which do not speculate that due to the increased noise level in this detector, exceed 100 keV at the sensitivity level of the detector and there- which makes the lowest channel unusable, the channel energy fore must have an energy spectrum that is steeply falling above, boundaries may be lower than nominal. say, 50 keV. Mewaldt et al., for different reasons, also rejected By the time the corotating structure reaches STEREO, the the neutron decay hypothesis. protons trapped within it are predominantly coming from the We have shown above that the precursor detected by EPAM sunward direction, which Mewaldt et al. (2009) find extremely is a very unusual event, just as that detected by STEREO is difficult to explain other than by neutral particles. Unusual as also unusual. At STEREO the main particle increase, presum- this may seem, we can only point out that the event detected by ably from the X9 flare at E79S06 on the visible solar disc, EPAM is also difficult to explain. But a provenance in neutrons started around 14:45 UT (Mewaldt et al. 2009). The precursor, or neutral atoms is ruled out. We should emphasise that the mag- which Mewaldt et al. attribute to neutral atoms, started around netic field, as shown in Fig. 2, does not suggest that there could 11:30 UT. The timing of the events at the various spacecraft sup- be any significant local particle acceleration taking place. ports our hypothesis that they are the same. The corotation delay The composition of the precursor is unusual, in that Mewaldt between ACE and Earth is about 1 h, with ACE leading. This is et al. (2009) do not detect any energetic ions heavier than pro- consistent with the ACE-STEREO delay, both for the precursor tons. EPAM can also measure the composition of energetic par- and the main event. ticles. Near the onset of the main event, around 15:00 UT the Finally, we might suppose that any neutral atoms detected ratio of 0.52–1.73 MeV protons to 0.39–6.98 MeV helium nu- by STEREO could also be detected by EPAM. When would this clei measured by EPAM was around 6000. (NB This number is be? For illustrative purposes we will perform the calculation for uncorrected for the instrument response.) Near the maximum of atoms of kinetic energy 4.7 MeV, which have a velocity of 0.1 c. the particle event on 8 December this ratio had dropped to ∼90. ACE is ∼1.5 × 106 km closer to the Sun, and thus should en- Thus near the onset of the main event the energetic nuclei are counter the hypothetical neutral atoms 1.5 × 106/(0.13 × 105)s, deficient in helium, which is also the case at STEREO. or 50 s, before STEREO. Throughout the period between the end of the precursor and the onset of the main particle event the ions 4. Conclusions at EPAM were at background levels. However, the sensitivity of EPAM is at the level of ∼5 × 10−2 particles/(MeV cm2 ssr)at The precursor energetic particle increase seen on 5 December around 1 MeV, so it might not be sensitive enough to detect the 2006 is a very interesting and unusual event. The data we have particles reported by Mewaldt et al. (2009). analysed from ACE/EPAM are unique in our experience, and We are now faced with the conclusion that the precursor one might be tempted to reach an intriguing and plausible ex- at EPAM is neither from neutral atoms as it includes near- planation, as was done by Mewaldt et al. (2009). We have con- relativistic electrons, nor from neutrons, as the electron spectrum sidered three possible scenarios to account for it, namely ENAs, is contrary to that expected from neutron decay. Therefore we are solar neutrons produced at the Sun following an X9 flare, and left with the third of our options given in the introduction. The an unusual particle event propagating though the interplane- precursor is very unusual in that the particle distributions seen by tary medium from the Sun to beyond the Earth. The neutral

A46, page 5 of 8 A&A 531, A46 (2011) atom hypothesis can be eliminated as it is accompanied by near- with small pitch angles have been lost. The proton intensity, if relativistic electrons and if it were to relate to the ENAs sug- present, appears to be below the threshold of the EPAM detec- gested by Mewaldt et al., it comes about an hour too early. Also, tors. the neutron hypothesis can be eliminated as neutron decay would If the rather unusual electron event detected by ACE/EPAM be accompanied by electrons of much higher energy than seen by is entirely separate from the proton event detected by STEREO, EPAM. then we need two, previously unknown, interpretations of these The major energetic particle event, which had an onset at data. If they are linked by an unusual corotating interplanetary ACE at around 13:50 UT, has an intensity-time profile similar event then only one new interpretation is needed (Occam’s ra- to that seen by STEREO to start at 14:45 UT. The fluctuations zor). The lack of heavy nuclei in the STEREO data suggests that in particle intensity for different energy (velocity) particles are some type of resonant acceleration process may be needed, al- observed simultaneously (see Figs. 1 and 2, and Mewaldt et al. though we would have no objection to other mechanisms. We 2009, their Fig. 1), proving that the spacecraft are sampling spa- conclude that the interpretation of Mewaldt et al. (2009)that tial, rather than temporal, effects. Therefore the main particle they have detected energetic neutral hydrogen atoms from the event is corotating with the solar rotation speed, through the Sun, is suspect, as we would then have to find separate solution interplanetary medium. We have shown that the nominal coro- to the electron event at EPAM. tation delay between ACE and the Earth is around one hour, with ACE leading. This nominal time difference is almost ex- Acknowledgements. The author wishes to thank the referee for helpful com- actly what is seen for both the precursor and the main event. ments. The most plausible explanation for the precursor is that it contains energetic protons (detected by STEREO) and near References relativistic electrons (detected by ACE) emitted from the Sun before, but close to the time of the X9 flare. It is possible that these Dröge, W., Ruffolo, D., & Klecker, B. 1996, ApJ, 464, L87 Gold, R. E., Krimigis, S. M., Hawkins, S. E., III, et al. 1998, Space Sci. Rev., 86, particles are accelerated in the interplanetary medium within 541 an unusual magnetic structure, but we shall not discuss this Lanzerotti, L. J., Gold, R. E., Anderson, K. A., et al. 1992, A&A, 92, 349 speculation further. The magnetic structure which is corotating Mewaldt, R. A., Leske, R. A., Stone, E. C., et al. 2009, ApJ, 693, L11 with the Sun appears to contain trapped electrons; any electrons Simnett, G. M. 2005, J. Geophys. Res., 110, A09S01

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A46, page 6 of 8 G. M. Simnett: Solar energetic neutral atoms

DRV:P4+6.00000 Appendix A: Response of EPAM to solar X-rays 105 P4 193−315 keV ions 104 sr s)

It is well known that energetic particle instruments using solid 2 state detectors experience a response to solar X-rays if they 103 are pointed at the Sun. Therefore it is customary to ignore the

2 data from such detectors at times of intense solar X-ray activity. 2 10 The LEMS30 detector on the EPAM instrument has two sec- Particles/ (MeV cm 101 2001/ 92 21:30 92 21:40 92 21:50 92 22:00 92 22:10 92 22:20 92 22:3 tors out of four which scan the Sun as the spacecraft spins. The 21:30 21:40 21:50UT22:00 Date 22:10 22:20 22:30 magnetically-deﬂected electrons enter the instrument through EP:DE1 106 the same aperture as the ions detected by LEMS30, but a magnet 38−53 keV electrons Particles / (MeV cm sr s)

sweeps them into a well-shielded detector which has negligible sr s) response to solar X-rays. In this appendix we demonstrate the 2 integrity of the deﬂected electron data by showing the responses 105 of both the electron detector and the LEMS30 ion detector at times of intense solar X-ray activity. Particles/ (MeV cm 104 2001/ 92 21:30 92 21:40 92 21:50 92 22:00 92 22:10 92 22:20 92 22:3 21:30 21:40 21:50UT 22:00 Date 22:10 22:20 22:30 Table A.1. Large X-ray ﬂares which contaminate sunward-facing de- 2 April 2001 −− UT tectors. Fig. A.2. As Fig. A.1 except for 2 April 2001.

Date X-ray ﬂare Time of Hα Flare EP:P4 1.24 GOES class size maximum location 1.2•10 4 1.0•10 P4 193−315 keV ions

4 November 1997 X2.1 05:58 2B S13W33 sr s) 2 April 2001 X20 21:49 N17W78 2 8.0•100.83 6.0•103

15 April 2001 X14.4 13:50 2B S20W85 4 28 October 2003 X17.2 11:10 4B S16E08 4.0•100.43

3 6 December 2006 X6.5 18:47 3B S06E63 Particles/ (MeV cm 2.0•10 00 13:30 13:40 13:50 14:00 14:10 14:20 14:30

2 2001/105 13:30 105 13:40 105 13:50 105 14:00 105 14:10 105 14:20 105 14:3 UT Date EP:DE1 The ﬂares we have chosen are given in Table A.1.Theyare 2.5•105 all GOES X-ray class X-ﬂares, and three of them are among 2.0•10205 38−53 keV electrons sr s) the largest of the solar cycle 23. Figure A.1 shows the intensity- 2 time history from 05:45–06:45 UT on 4 November 1997 for 1.5•105

5 EPAM channnels P4 and DE1, which are designed to measure 1.0•10Particles / (MeV cm sr s) x 10 10

5.0•104

193–315 keV ions and 38–53 keV electrons. This ﬂare was Particles/ (MeV cm

GOES class X2.1 and it was preceded by a relatively quiet pe- 00 2001/10513:30 13:30 105 13:40 13:40 105 13:50 13:50 10514:00 14:00 105 14:10 14:10 105 14:20 14:20 105 14:30 14:3 riod, with a low ambient background of charged particles. The UT Date 15 April 2001 −− UT data are plotted showing the four sectors independently. The detected ions and electrons enter the instrument though the same Fig. A.3. As Fig. A.1 except for 15 April 2001. aperture, which for two of the sectors (blue and green) sweep past the Sun. It is clear that the P4 channel is responding to solar X-rays in these two sectors, but not in the other two sectors, plotted as red and cyan. The colour code for the electrons is identical. About 20 min later a beam of electrons is detected, which is the normal timing given the longitude of the flare at W33. DRV:P4+6.00000 800 The second event we have chosen was GOES class X20, P4 193−315 keV ions 600 and Fig. A.2 shows the P4 and DE1 channels for the period sr s) 2 21:30–22:30 UT on 2 April, 2001. April 2001 was an active 400 month and the pre-flare background intensity was enhanced over quiet times. Note that the intensities in Fig. A.2 are plotted log- 200 2 Particles/ (MeV cm arithmically. The P4 channel responds to the flare X-rays in the 0 blue and green sectors. The electron channel does not repond to 1997/3085:50 05:50 3086:00 06:00 308 6:1006:10 3086:20 06:20 308 6:30 06:30 308 6:40 06:40 UT Date the solar X-rays and ∼20 min later the electrons start arriving at EP:DE1 3•104 ACE as an anisotropic beam. This behaviour is normal for parti-

38−53 keV electrons cles associated with a ﬂare near the solar west limb. sr s) 2 Particles / (MeV cm sr s) 2•104 Figure A.3 shows the same channels for the time period 13:30–14:30 UT on 15 April 2001. The detector response to the

1•104 ﬂare is the same as before. Figure A.4 shows the same channels

Particles/ (MeV cm for the period 10:50–11:50 UT on 28 October 2003. The ﬂare

0 was GOES class 17.2 and we have chosen this event partly be- 1997/3085:50 05:50 3086:00 06:00 308 6:1006:10 308 6:20 06:20 308 6:30 06:30 308 6:40 06:40 UT Date cause there is a very small electron increase aound the onset of 4 November 1997 −− UT the solar X-ray event. However, this is a short-lived beam of solar Fig. A.1. Contamination in the LEMS30 detector from solar X-rays on electrons which must have left the Sun some 15–20 min before 4 November 1997. The upper panel is the P4 channel, which is designed the ﬂare onset. We know that the response is not due to X-rays to respond to ions with energies between 193 and 315 keV. The lower as the peak sector (red) is not a sector that views the Sun, and the panel are the magnetically-deﬂected electrons, which enter the EPAM sector with the lowest intensity (cyan) is one of the sectors where instrument through the same aperture as the LEMS30 ions. A46, page 7 of 8 A&A 531, A46 (2011)

EP:P4 EP:P3 106 4.5•105 P4 193−315 keV ions 4.0•1045 5 P3 115−193 keV ions 10 5 sr s) sr s)

2 3.5•10 2

5 3.0•103 104 2.5•105

2.0•1025 103 2 Particles/ (MeV cm Particles/ (MeV cm 1.5•105 2 102 1.0•1015 2003/30110:50 10:50 301 11:00 11:00 301 11:10 11:10 301 11:20 11:20 301 11:30 11:30 30111:40 11:40 30111:50 11:5 2006/34018:30 18:30 340 18:40 18:40 340 18:50 18:50 34019:00 19:00 340 19:10 19:10 340 19:20 19:20 340 19:30 19:3 UT Date UT Date EP:DE1 EP:DE1 5 107 1.1•10 38−53 keV electrons 1.0•101.05 38−53 keV electrons sr s) sr s) 2 2 6

Particles / (MeV cm sr s) 10 9.0•100.94 Particles / (MeV cm sr s) x 10

8.0•100.84 105 7.0•100.74 Particles/ (MeV cm Particles/ (MeV cm

4 104 6.0•100.6 2003/30110:50 10:50 301 11:00 11:00 301 11:10 11:10 301 11:20 11:20 301 11:30 11:30 301 11:40 11:40 30111:50 11:5 2006/34018:30 18:30 340 18:40 18:40 340 18:50 18:50 340 19:00 19:00 340 19:10 19:10 340 19:20 19:20 34019:30 19:3 UT Date UT Date 28 October 2003 −− UT 6 December 2006 −− UT Fig. A.4. As Fig. A.1 except for 28 October, 2003. Fig. A.5. As Fig. A.1 except for 2 April 2001, and the upper panel shows the P3 channel of LEMS30, designed to detect ions from 115–193 keV. the instrument aperture sweeps past the Sun. The 28 October 2003 event has been analysed in depth by Simnett (2005). previous events, with the two sunward sectors of LEMS30 show- The final event we present is that occurring on 6 December, ing an increase above the (enhanced) background but with no 2006. We have chosen this event as (a) it is a GOES X6.5 flare; increase in either the non-sunward sectors or the electrons. and (b) it is the day after the event which is the subject of this The conclusion we draw from this analysis is that the de- paper. Figure A.5 shows the data from 18:30–19:30 UT. The flected electron channels in the EPAM instrument are not con- response to this event is essentially identical to that shown in the taminated by solar X-rays.

A46, page 8 of 8