A new catalogue of 314 solar energetic particle (SEP) events extending over a large time span from 1984 to 2013 has been compiled. The properties as well as the associations of these SEP events with their parent solar sources have been thoroughly examined. The properties of the events include the proton peak integral flux and the fluence for energies above 10, 30, 60 and 100 MeV. The associated solar events were parametrized by solar flare (SF) and coronal mass ejection (CME) characteristics, as well as related radio emissions. In particular, for SFs: the soft X-ray (SXR) peak flux, the SXR fluence, the heliographic location, the rise time and the duration were exploited; for CMEs the plane-of-sky velocity as well as the angular width were utilized. For radio emissions, type III, II and IV radio bursts were identified. Furthermore, we utilized element abundances of Fe and O. We found evidence that most of the SEP events in our catalogue do not conform to a simple two-class paradigm, with the 73% of them exhibiting both type III and type II radio bursts, and that a continuum of event properties is present. Although, the so-called hybrid or mixed events are found to be present in our catalogue, it was not possible to attribute each SEP event to a mixed/hybrid sub-category. Moreover, it appears that the start of the type III burst most often precedes the maximum of the SF and thus falls within the impulsive phase of the associated SF. At the same time, type III bursts take place within %5.22 min, on average, in advance from the time of maximum of the derivative of the SXR flux (Neupert effect). We further performed a statistical analysis and a mapping of the logarithm of the proton peak flux at E > 10 MeV, on different pairs of the parent solar source characteristics. This revealed correlations in 3-D space and demonstrated that the gradual SEP events that stem from the central part of the visible solar disk constitute a significant radiation risk. The velocity of the associated CMEs, as well as the SXR peak flux and fluence, are all fairly significantly correlated to both the proton peak flux and the fluence of the SEP events in our catalogue. The strongest correlation to SEP characteristics is manifested by the CME velocity.

The electron density profile of the nightside high latitude region has been determined from a geocentric distance 1.5 RE to 3 RE by the use of the Viking Langmuir probe. Inside this region, density depletions are observed. Most of them coincide with acceleration structure crossings. Generation of Auroral Kilometric Radiation (AKR) is observed in the strongest depletions between 1.5 and 2.5 RE. A threshold on the ratio plasma to electron gyrofrequency for AKR generation to occur is estimated at 0.14. This is in good agreement with the cyclotron maser instability theory for AKR generation.

The present paper is a summary of studies carried out from Viking measurements on the propagation and the generauon of the auroral kilometric radiation (AKR hereafter). Advantage is taken of the spin modulation of the AKR observed as Viking was rotating in the cartwheel mode. This, together with the study of the cutoff of the various spectral components, confirms that low-amplitude Z and O modes are generated at the same time as a larger-amplitude X mode. Hence Z, O and X mode AKR is all generated by the same sources. The spectrum of the dominant polarization, the X mode, usually contains several spectral peaks. An AKR source crossing is characterized by a minimum in the frequency of the lowest-frequency peak (fpeak) and by a maximum of its amplitude. About 50 AKR source crossings are used to demonstrate that fpeak approaches fce, the electron gyrofrequency: (fpeak-fce)/fce •-0.025 in AKR sources. Similarly, the lo•v-frequency cutoff of the AKR is found, on average, to coincide with fce. The density inside AKR sources is determined from four sets of independent measurements, namely (1) the upper frequency cutoff of the hiss, (2) the relaxation sounder, (3) the Langmuir probe, and (4) particle measurements. It is shown that an AKR source coincides with a strong depletion in the density of the cold/cool electrons that becomes comparable to or less than the density of energetic electrons (E > 1 keV). The total density inside AKR sources is of the order of 1 cm -3, typically a factor 5 to 10 below that of the surrounding regions. AKR sources are found to coincide also with an acceleration region characterized by a potential drop of > 1 kV, both below and above the spacecraft. Evidence for this comes from the observation of electrons accelerated above the spacecraft and ions accelerated below it. In addition to a strong depletion in the density of the cool electrons, particle measurements on Viking give evidence of several possible free energy sources that could drive unstable the AKR, namely (1) a loss cone, (2) a hole for parallel velocities smaller than that of the observed downgoing electron beam, and (3) a trapped electron component for a pitch angle a •-90% The trapped electron component, bounded at low perpendicular energies (a few keVs) by an enhanced loss cone, is observed inside, and only inside, AKR sources. It is therefore concluded that the corresponding o•fio•v.L > 0, for small parallel velocities, is the free energy source that drives unstable the cyclotron maser. 11,657 11,658 Roux ET AL.: AURORAL KILOMETRIC RADIATION SOURCES

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When the Viking satellite crossed the auroral acceleration region, where the electron density is supposed to be less than a few particles per cubic centimeter, a strong spin modulation of the signal of a positively biased Langrnuir probe was observed. This spin modulation is better correlated with the angle of the antenna to the geomagnetic field than with the angle to the Sun. A simple model based on the photoelectron orbits in a spherically symmetric electric field, in the presence of a uniform magnetic field, gives the right phase and amplitude of the extrema. In addition, it shows that the perturbation could be negligible when the antenna is perpendicular to the magnetic field, provided that the photoelectron sources are far enough from the magnetic field lines connected to the probe (about 2 Larmor radii). As a consequence, we deduce important information for the design of experiments devoted to very low density measurements, and we show that the magnetic field may be a crucial parameter for the study of the exchange of photoelectrons between parts of a satellite. The most important result of this study is to facilitate the probe density measurements inside the acceleration structures. It is shown that in these regions the cold plasma density is depleted, with a density of thermal electrons below 5 cm -3 at altitudes between 6500 km and 8500 kin. When auroral kilometric radiation (AKR) is generatedinside such structures, the density of thermal electrons can be below I cm -3 . The edges of the structures are sharply defined, and the gradient scale can be as low as 1.4 kin. This is the first time that density measurements of thermal electrons have been realized inside clearly identified AKR sources, with appropriate resolution. techniques fail. In the region in which we are interested, the geocentric distance range of Viking is between 1.5 RE and 3 RE. At 2 RE the Viking velocity is about 7 km/s; hence the spatial resolution is about 140 m.One direct application of density measurements is the characterization of transient or static plasma structures. Above 3000 km altitude, many structures have been observed to be associated with density modifications. One can mention, for instance, the acceleration structures [Ternerin et al., 1981; Mozer and Ternerin, 1983], the sources of auroral kilometric radiation (AKR hereafter) [Benson and Calvert, 1979; Benson and Akasofu, 1984; de Feraudy et al., 1987; Pottelette et al., 1988; Bahnsen et al., 1989; Hilgers et al., 1989, 1991; Louarn et al., 1990; Ungstrup et al., 1990; Pertaut et al., 1990], the auroral plasma cavity [Calvert, 1981; Persoon et al., 1988], and the double layers and solitary waves [Koskinen et al., 1987; BostrSm et al., 1988].These structures were discovered independently and were named for the physical phenomenon used to identify them; however, some of them are strongly related to each other. This kind of correlation, sketched by Persoon et al. [1988]and Hilgers et al. [1989, 1991], will be presented in detail in a forthcoming paper. Density measureme...