R. G. Marsden

The Ulysses south polar pass: Energetic ion observations

We present here a preliminary analysis of observations of energetic ions during the first polar pass of the Ulysses spacecraft, concentrating mainly on the region where the spacecraft was continually immersed in high speed flow from the polar coronal hole. During the ascent to high latitudes, a single recurrent peak was observed once per solar rotation. From 70°S during ascent to 43°S during descent no major peaks were observed. Thereafter, two peaks per solar rotation were observed.

Energy spectra of 50-keV to 20-MeV protons accelerated at corotating interaction regions at Ulysses

We analyze here the energy spectra of 50-keV to 20-MeV protons accelerated at corotating shocks observed at Ulysses in the southern heliosphere during 1992 and 1993. In general, our results are inconsistent with the predictions of two models based on diffusive shock acceleration theory (Fisk and Lee (1980) and Jones and Ellison (1991)). The main results are the following: (1) The relationships between the spectral indices and the shock compression ratio show significant departures from those predicted by the two models; the observed spectra are substantially harder than predicted; (2) the spectral indices at the reverse shocks depend strongly on the heliographic latitude of Ulysses; the hardest spectra are associated with strong quasi-perpendicular shocks observed between 208S and 308S; (3) the spectral indices at the reverse shocks are anticorrelated with the upstream plasma velocity V9 in the de Hoffman-Teller frame of the shock; and (4) the spectral indices at the forward shocks show no dependence on either the shock parameters or the heliographic latitude. The latitudinal dependence of the reverse shock spectra is probably related to the tilt (;258) of the heliospheric current sheet with respect to the solar equatorial plane during 1992 and 1993. The negative correlation between the spectral indices at the reverse shocks and V9 u indicates that the gradient drift mechanism plays a crucial role in accelerating particles at corotating interaction regions (CIRs). The differences in our observations at the forward and reverse shocks may be due to the presence of a more energetic seed population and an enhancement in the level of magnetic field fluctuations upstream of the trailing edges of the CIRs.

Ulysses high‐latitude observations of ions accelerated by co‐rotating interaction regions

We present observations of energetic ions (E∼1 MeV) from the Ulysses spacecraft during its first pass from the ecliptic plane to the southern high-latitude regions of the heliosphere. At latitudes less than ∼13°S Ulysses was completely immersed in the heliomagnetic streamer belt, and observed a ∼1 MeV proton intensity which showed little evidence of a periodic structure. Between ∼13°S and ∼29°S Ulysses observed one dominant recurrent co-rotating interaction region, its reverse shocks being mainly responsible for accelerating the ∼1 MeV protons. At ∼29°S the spacecraft left this region and entered the solar wind flow from the polar coronal hole. From ∼29° up to ∼45°S, reverse shocks from this and other interaction regions were still being observed. Accelerated energetic ions, with proton-to-alpha ratio signatures consistent with having been accelerated by the reverse shocks of these co-rotating interacting regions, were still being observed up to latitudes of ∼50°S.

Properties of a large-scale interplanetary loop structure as deduced from low-energy proton anisotropy and magnetic field measurements

We present correlated particle and magnetic field measurements by the ISEE 3 spacecraft for the loop structure behind the interplanetary traveling shock event of November 12, 1978. Following the passage of the turbulent shock region, we observe strong bidirectional streaming of low-energy protons for approximately 6 hours, corresponding to a loop thickness of about 0.07 AU. This region is also characterized by a low relative variance of the magnetic field, a depressed proton intensity, and a reduction in the magnetic power spectral density. Using quasi-linear theory applied to a slab model, we derive a value of 3 AU for the mean free path during the passage of the closed loop. We infer from this observation that the proton regime associated with the loop structure is experiencing scatter-free transport and that either the length of the loop is approximately 3 AU between the sun and the earth or else the protons are being reflected at both ends of a smaller loop.

The Ulysses south polar pass: Transient fluxes of energetic ions

Using Ulysses low energy telescope (LET) measurements of protons and alpha particles with energies of ∼1–5 MeV/n, we present first observations of transient energetic particle events in the high latitude heliosphere. Three transient particle events with gradual onsets and time durations of several days have been identified in the LET data at heliographic latitudes as high as ∼60°S, at radial distances from the Sun between 3.3–3.6 AU. No transient increases were observed polewards of 60°S. All three of the events were associated with passage of a coronal mass ejection (CME) over Ulysses, and two were associated with the newly identified type of forward/reverse shock pairs in the solar wind caused by over-expanding CMEs. The largest of the three transient events has also been observed at Earths orbit, i.e. in the ecliptic. The observations show that energetic particles can be convected into the high latitude heliosphere through CMEs.

Particle acceleration at corotating interaction regions in the three-dimensional heliosphere

We have investigated the relationship between the energetic (∼1 MeV) proton intensity (J) and the magnetic compression ratio (C) measured at the trailing edges of corotating interaction regions observed at Ulysses. In general, our results show that the proton intensity was well correlated with the compression ratio, provided that the seed intensity remained constant, consistent with predictions of the Fermi model. Specifically, our results indicate that particles were accelerated to above ∼1 MeV in energy at or near the trailing edges of the compression regions observed in the midlatitude southern heliosphere, irrespective of whether the bounding reverse shocks were present or not. On the basis of this, we conclude that shock acceleration is probably not the only mechanism by which particles are accelerated to above ∼1 MeV in energy at compression or interaction regions (CIRs). On the basis of magnetic field measurements obtained near the trailing edges of several midlatitude CIRs, we propose that particles could have been accelerated via the Fermi mechanism by being scattered back and forth across the trailing edges of the compression regions by large-amplitude Alfven waves. Our results also show that the proton intensity was well correlated with the compression ratio during low solar activity periods but was essentially independent of C during periods of high solar activity. We suggest that the correlation between J and C was not observed during solar active periods because of significant variations in the seed intensity that result from sporadic contributions from transient solar events. In contrast, the correlation was observable during quiescent periods probably because contributions from transients had decreased dramatically, which allowed the CIRs to accelerate particles out of a seed population whose intensity remained relatively unperturbed.

Energetic proton observations at 1 and 5 AU: 2. Rising phase of the solar cycle 23

The increasing level of solar activity at the end of 1997 coincides with the observation of solar energetic particle (SEP) events by the Wind and Ulysses spacecraft. The proximity of Ulysses to the ecliptic plane during this period allows us to study the effects of the solar activity at two different heliolongitudinal and heliocentric distances but at similar heliolatitudes. We identify the main transient proton enhancements seen by Wind and Ulysses from October 1997 to the end of December 1998. We compare their characteristics and suggest an interpretation in terms of their heliospheric location, the activity occurring at the Sun, and the conditions for particle propagation between the Sun and both spacecraft. Whereas at 1 AU different SEP events can be associated with individual coronal mass ejections (CMEs), at 5 AU long-lasting SEP events are observed while a sequence of CMEs takes place at the Sun. Continuous injection of particles from traveling CME-driven shocks, the effects of particle propagation along interplanetary magnetic field lines, and the corotation of these field lines across the Ulysses location are the main factors responsible for these long-lasting periods of high particle intensity observed by Ulysses.

Observational signatures of the influence of the interplanetary shocks on the associated low‐energy particle events

We have analyzed the temporal evolution of the upstream particle fluxes and anisotropies in a set of particle events associated with interplanetary shocks, detected by ISEE 3 during the period 1978-1980 in the 147-238 and 620-1000 keV energy ranges. In particular, we have studied those features that can perhaps be used as signatures of the history of the influence of the shock on the particle event. We have paid special attention to the evolution of the flux anisotropy that can carry relevant information on the contribution of shock accelerated particles to the total observed flux. Our analysis shows that during most events, long-lasting (between 5 and 36 hours) large anisotropies are observed upstream of the shock, supporting the hypothesis of continuous injection of shock accelerated particles in the interplanetary medium while the shock is propagating outward from the Sun. The evolution of the anisotropy throughout each event shows a dependence on the longitude of the parent solar source. We have interpreted the observations considering that shock accelerated particles contribute significantly to the total observed flux from the time when the shock intersects the magnetic field line that connects with the observer. The heliocentric distance of this initial intersection point has been derived from the flux anisotropy observations for each event. The values obtained show a distribution with respect to the longitude of the solar source that can be reproduced with a simple model, which assumes a spherical shock with an angular extension ≈ 100° and an archimedian spiral structure for the interplanetary magnetic field. The results of this work provide a further insight into both the importance of shock acceleration, as well as the influence of the large-scale shock structure in low-energy particle events.

ENERGETIC PARTICLE OBSERVATIONS AND PROPAGATION IN THE THREE-DIMENSIONAL HELIOSPHERE DURING THE 2006 DECEMBER EVENTS

We report observations of solar energetic particles obtained by the HI-SCALE and COSPIN/LET instruments onboard Ulysses during the period of isolated but intense solar activity in 2006 December, in the declining phase of the solar activity cycle. We present measurements of particle intensities and also discuss observations of particle anisotropies and composition in selected energy ranges. Active Region 10930 produced a series of major solar flares with the strongest one (X9.0) recorded on December 5 after it rotated into view on the solar east limb. Located over the South Pole of the Sun, at >72°S heliographic latitude and 2.8 AU radial distance, Ulysses provided unique measurements for assessing the nature of particle propagation to high latitudes under near-minimum solar activity conditions, in a relatively undisturbed heliosphere. The observations seem to exclude the possibility that magnetic field lines originating at low latitudes reached Ulysses, suggesting either that the energetic particles observed as large solar energetic particle (SEP) events over the South Pole of the Sun in 2006 December were released when propagating coronal waves reached high-latitude field lines connected to Ulysses, or underwent perpendicular diffusion. We also discuss comparisons with energetic particle data acquired by the STEREO and Advanced Composition Explorer in the ecliptic plane near 1 AU during this period.

Stochastic Fermi acceleration of ions in the pre-shock region of comet P/Halley

Energetic cometary ion fluxes measured between 2.5106 km and 106 km from the cometary nucleus along the inbound trajectory of s/c VEGA-1 are used to derive the temperatures of the ion distributions in the solar wind frame. The increase of the temperature is modelled by the temperature change derived from a Fokker-Planck type equation with a source term and a stochastic acceleration term. The temperature increase predicted by theory is about 3 keV, higher than the observed one (≃ 1.4 keV). The difference may be due to the approximations applied. The second order Fermi mechanism is thus capable of producing the temperature increase observed.