1. EVE non-detection of Doppler-shifted He II 304 Å H.S. Hudson 1,2, L. Fletcher 2, A. MacKinnon 2, and T. Woods 3 1 SSL, UC Berkeley, 2 University of Glasgow, 3 LASP, University of Colorado Background: It is well-known that the acceleration of non-thermal particles plays a key role in flare physics, especially as regards the extraction of energy from the magnetic field. Electrons produce bremsstrahlung X-rays and various radio emissions and can thus be detected rather directly, but high-energy ions do not have such convenient radiation signatures. They are normally detected only in situ, as SEPs, or via their  -ray emission. In the latter case, low-energy particles (below a few MeV/nucleon) are not detected because they are below the threshold energies in the excitation cross-sections. As an alternative proposal for the detection of low-energy particles, Orrall & Zirker (1976) proposed the use of charge-exchange excitation of Ly  : a proton with energy 0.1-1MeV, for example, could pick up an electron from a neutral atom and then radiate Doppler-shifted recombination radiation. This would result in broad wings of the Ly  line, especially in the red wing for downward particle beaming at disk center. The proton aurora was discovered in a related manner by Vegard (1939). Observationally, this suggestion has not found strong support, mainly because appropriate data have not been available. Fig. 1. The red-shifted 304A wing emission, as modeled by Peter et al. (1990). Generalization to He II 304Å: the analogous line for He II is 304Å. The equivalent theory for its broad wings, resulting from primary  particles, was published by Peter et al. (1990), but again searches have been difficult because of lack of proper instrumentation. Figure 1 (from Peter et al.) shows the theoretical expectation (the shaded region in the red wing). We now have excellent Sun-as-a-star coverage by the EVE instrument on SDO and can search for low-energy particles in this way, for the first time systematically. Results and explanations: We have searched for this effect in the known X-class flares and  -ray flares from the new solar cycle. We do not detect the expected broad wings. These limits are severe, by comparison with Fig. 1; we should have seen something. We have not finished our quantitative analysis of these limits but comment here on what this may mean. The theory requires several assumptions: specifically, the primary particles must penetrate into a region of low ionization and not at the same time erase the signature completely by inducing ionization themselves. Assumptions on timing and geometry are therefore necessary. It would be helpful if a more modern theoretical assessment of the problem could be undertaken now, given the great observational strides of the past twenty years. This problem is exceedingly important because of the implications for flare energetics and for the role in flares of the fast ions. Four flares: SOL2010-06-12 (M2.0) (  -ray lines) SOL2011-02-15 (X2.2) (?) SOL2011-02-24 (M3.5) (?; limb) SOL2011-03-09 (X1.5) (?) This was the entire list of suitable events prior to mid-2011 (detected above 300 keV, and thus  -ray flares; see Shih et al., 2009. We have also checked the full list of (7) X-class flares in Cycle 24 and obtain similar limits. Fig. 2 (above)) the RHESSI 100-keV and GOES light curves for the four flares, (normalized rates); see Shih et al. (2009). Fig. 3 (left) The EVE background spectra for the four events, showing the instrument’s great stability. Fig. 4 (right): time series of the chosen spectral bands. Although some continuum is present, only SOL2011-03-09 hints at an impulsive-phase component (within the dotted lines). Fig. 5 (below): The EVE difference spectra for the four events, with the spectral bands chosen for the yet-to-be-completed analysis of the upper limits; each band corresponds to a range of particle energy and geometrical assumptions. Here red shows the impulsive phase, and blue the gradual phase. Flare order is the same. References: Orrall & Zirker: ApJ 208, 618 (1976) Peter et al.: ApJ 351, 317 (1990) Shih et al.: ApJ 698, L152 (2009) Vegard: Nature 144, 1089 (1939)