M. K. Reuss

The elemental composition in energetic particle events at high heliospheric latitudes

The relative abundances of low energy ions (0.6-2.0 MeV/n) in solar energetic particle (SEP) and corotating interaction region (CIR) events have been measured by the EPAC experiment aboard Ulysses since launch in October 1990 until the present time. We give an overview of the abundances of heavy ions (He, C, Ne, Fe) relative to oxygen during energetic particle events lasting longer than 5 days during the in-and out-of-ecliptic phase of the mission. While the period Oct. 1990 to Aug. 1992 was dominated by high solar activity the Ulysses out of ecliptic passage at solar latitudes up to 45° went parallel to the declining phase of solar activity. Thus a very clear structure of corotating interaction regions was observed. While the in-ecliptic composition is in general agreement with measurements made near the Earth, the development of the CIR-composition shows two phases: From Aug. 1992 to May 1993 the C/O-ratio is 0.55-0.70, afterwards it increases to 0.8-0.9. This increase is correlated to the disappearance of the current sheet at 30° solar latitude reported by Smith et al.(1993).

Cosmic ray modulation by expanding, high-latitude streams

As ‘Ulysses’ moved to latitudes above 45°, the regular, recurring series of mid-latitude high-speed streams is no longer observed out to 4 AU. However, the associated, near-relativistic particle modulation persists to at least 70°. We explain this remote shadowing as due to the spread in latitude of the fast streams beyond the ‘Ulysses’ trajectory and show that an estimate of the particle diffusion coefficient within the stream, based on magnetometer data, yields sufficient modulation to make the model plausible.

Energetic particle observations in high heliographic latitudes

We report on observations obtained with the EPAC instrument on board of ULYSSES during the period after the Jupiter passage when the spacecraft started to climb towards higher heliographic latitudes. Observations on the encounters with shocks related to corotating interaction regions (CIR) and coronal mass ejections (CME) will be discussed, spanning a period until the end of 1993. We also report on findings related to the modulation of galactic cosmic rays.

Energetic particle anisotropies and remote magnetic connection at high solar latitudes

Abstract A very stable pattern of energetic particle events recurring with the solar rotation period dominates the Ulysses high latitude energetic particle observations during the current solar minimum. Most of these events can be associated with reverse shocks of corotating interaction regions (CIRs) but at latitudes above 50° there are either no shocks observable at all or the events are retarded by 2–4 days with respect to the shock passage. This has been explained either by cross-field diffusion from low latitude CIRs or by remote magnetic connection to a CIR expanding in latitude with distance. Using data from the Ulysses EPAC instrument we show that anisotropies of 0.7 MeV/N Helium ions are in favour of the remote magnetic connection theory.

A ‘Ulysses’ cosmic-ray latitude gradient observation and modulation models

The ‘EPAC’ instrument on Ulysses is sensitive to relativistic cosmic rays when far from Jupiter and in the absence of energetic solar particles. Measurement of the latitude gradient of these particles, after correction for time variations, has been made for the 1993–1994 south polar pass. The average magnitude of the gradient is about the same or smaller than predicted by a model which includes full gradient drift. However, the latitude dependence of the solar plasma output into interplanetary space (including fast-stream and magnetic turbulence effects) seems to be important in determining the magnitude of modulation.

Observations of energetic particles with EPAC on Ulysses in polar latitudes of the heliosphere

SummaryMeasurements with the EPAC energetic-charged-particle instrument aboard Ulysses show between −15° and −65° ions and, to some extent, also electrons apparently accelerated by shocks associated with a cororating interaction region (CIR) operating at low latitudes. Particles could have reached Ulysses along magnetic-field lines which connect to the shocks in the more distant heliosphere. Such connections evidently do not exist above −65°. Between the recurrent streams we find the underlying composition to be similar to that of the anomalous component of cosmic rays (ACR). One channel (ELH), sensitive also to high-energy protons (E>210 MeV), shows that, superimposed to the large-scale heliospheric modulation of galactic ions, a 26-day variation of the flux is observed. Such modulation is also observed for the ACR, in phase with the galactic particle modulation, but anticorrelated to the CIR-related low-energy particles. An estimate of the latitudinal and radial gradients of the galactic cosmic rays at 1 GV gives +0.4%/degree and −11%/AU, respectively.

A particle event at 5 AU and 20o southern latitude from measurements with the EPAC instrument on Ulysses

In November 1992, the Ulysses spacecraft observed a multiple solar particle event and a CME event at 5.2 AU and a heliographic latitude of 20° S which were superimposed to the recurrent corotating interacting region. Distinct particle flux increases caused by these events were observed in all energy channels of the EPAC experiment. The experimental findings are discussed.

Ion measurements in the Jovian magnetosphere by the Ulysses EPAC experiment: A comparison between dawn and dusk sectors

We report on results from the EPAC experiment on board the Ulysses spacecraft during the Jupiter Flyby in February 1992. We compare ion anisotropies (E = 0.24−0.77 MeV/N) received on the inbound pass in the noon sector at low magnetic latitudes with those from the outbound pass in the dusk sector at high magnetic latitudes. The results can be summarized as follows: (1) The classification of an inner and middle magnetosphere is observed inbound at low magnetic latitudes as well as outbound at high magnetic latitudes. (2) Three-dimensional anisotropies of ions in the noon sector were found in the corotation direction with radial components, whereas in the dusk sector field aligned anisotropies as well as anisotropies in corotation and anti-corotation were observed. (3) The steady increase of HeO with increasing latitude in the inner magnetosphere suggests the ionosphere as an internal He-source. (4) We find a difference in the HeO-ratio between the day- and the dusk side of the magnetosphere.