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.