26-day Analysis of Energetic Ion Observations at High and Low Heliolatitudes: Ulysses and ACE

We present observations of energetic (0.34–8 MeV) ions from the Ulysses spacecraft during its second ascent to southern high latitude regions of the heliosphere. We cover the period from January 1999 until mid-2000 as Ulysses moved from 5.2 AU and 18° S to 3.5 AU and 55° S. In contrast to the long-lived and well-defined ∼26-day recurrences that were observed throughout Ulysses‘ first southern pass, energetic ion fluxes during the first portion of the Ulysses’ second polar orbit are highly irregular. Although corotating interaction regions (CIRs) are clearly present in solar wind and magnetic field data throughout the first half of 1999, their effects on energetic ion intensities are quite different from what they were in 1992–1993. No dominant strictly recurrent ion flux increases are observed in association with the arrival of these CIRs. Correspondingly, there is no stable structure of large polar coronal holes during the same period. Isolated transient solar energetic particle (SEP) events are observed at low and high latitudes. We compare energetic ion observations from the ACE and Ulysses spacecraft during the first half of 1999 to determine the influence of these SEP events in the observed recurrent CIR structure. Such SEP events occurred only occasionally during 1992–1993, but when they occurred, they obscured the recurrences in a manner similar to that observed in 1999–2000. We therefore conclude that the basic differences in the behavior of energetic ion events between the first and second southern passes are due to the short life of the corotating structure and the higher frequency of SEP events occurring in 1999–2000. This revised version was published online in August 2006 with corrections to the Cover Date.     

Origins of Anisotropic 40–300 keV Electron Events Observed at Low and High Latitudes

Using a survey of anisotropic electron events in the energy range of ～40–300 keV observed by HI-SCALE on Ulysses, we have selected several time intervals during 1999 when Ulysses traveled from about 20° S at 5.2 AU (January 1999) to 42° S at 4.2 AU (January 2000). We compare these events with observations at ～1 AU using the nearly identical instrument, EPAM on ACE. In order to study the solar origins of these electrons using the imaging Nançay Radioheliograph, we further restricted the list of events to those in which interplanetary magnetic field lines with origins on the visible solar disk, intersected Ulysses. We find that not all the anisotropic electron events are observed by both spacecraft and there exists a strong dependence on the spacecraft's magnetic connection back to the Sun. We have identified the solar origin for five electron events using radio observations, and correlate these with interplanetary type-III radio emissions using the WIND/WAVES experiment.     

Oxygen and Iron Ions at High Heliolatitudes

The fluxes of O and Fe ions at high heliolatitudes measured by the HiScale instrument on Ulysses reflect the dynamical processes that affect the charged particle populations in the heliosphere. Both the O and Fe ions show more latitude dependence in the first (solar minimum) orbit to high southern heliolatitudes than during the second (solar maximum) orbit. The ion fluxes are larger during the solar minimum orbit; the flux levels are influenced by the occurrence of corotating interaction regions. The Fe/O abundance ratios are found to be similar at 1 AU and at high heliolatitudes. This revised version was published online in August 2006 with corrections to the Cover Date.     

Simultaneous Observations of Solar Energetic Particle Events by imp 8 and the Ulysses Cospin High Energy Telescope at High Solar Latitudes

With Ulysses approaching the south solar polar latitudes during a period of high solar activity, it is for the first time possible to study the distribution of solar energetic particles (SEPs) in solar latitude as well as in radius and longitude. From July 1997 to August 2000, Ulysses moved from near the solar equator at ∼5 AU to ∼67° S latitude at ∼3 AU. Using observations of >∼30 MeV protons from Ulysses and IMP-8 at Earth we find good correlation between large SEP increases observed at IMP and Ulysses, almost regardless of the relative locations of the spacecraft. The observations show that within a few days after injection of SEPs, the flux in the inner heliosphere is often almost uniform, depending only weakly on the position of the observer. No clear effect of the increasing solar latitude of Ulysses is evident. Since the typical latitudinal extent of CMEs, which most likely accelerate the SEPs, is only ∼30°, this suggests that the enhanced cross-field propagation for cosmic rays and CIR-accelerated particles deduced from Ulysses’ high latitude studies near solar minimum is also true for SEPs near solar maximum. This revised version was published online in August 2006 with corrections to the Cover Date.     

Modulation Near Solar Maximum at High Solar Latitudes Observations From the Ulysses Cospin High Energy Telescope

The three-dimensional structure of the solar maximum modulation of cosmic rays in the heliosphere can be studied for the first time by comparing observations from Ulysses at high solar latitudes to those from in-ecliptic spacecraft, such as IMP-8. Observations through mid-2000 show that changes in modulation remain well correlated at Earth and Ulysses up to latitudes of ∼60° south. The observed changes seem to be best correlated with changes in the inclination of the heliospheric current sheet. The spectral index of the proton spectra at energies <100 MeV in the ecliptic and at high latitudes remain roughly consistent with the T ⁺¹ spectrum expected from modulation models, while the spectral index of the helium spectrum at both locations has changed smoothly from the flat or even negative index spectra characteristic of anomalous component fluxes toward the T ⁺¹ galactic spectrum with increasing modulation. Intensities near the equator and at high latitude remain nearly equal, and latitudinal gradients for nucleonic cosmic rays thus remain small (<1% deg⁻¹) at solar maximum. In the most recent data fluxes of protons and helium with energies less than ∼100 MeV nucl⁻¹ measured by Ulysses are smaller than those measured at IMP-8, suggesting that the gradients may have switched to become negative toward the poles even before a clear reversal of polarity for the solar magnetic dipole has been completed. This revised version was published online in August 2006 with corrections to the Cover Date.     

Remote Sensing of H from Ulysses and Galileo

We model interplanetary H Lyman-α (Lα) observations from Galileo UVS (Ultraviolet Spectrometer) and EUVS (Extreme Ultraviolet Spectrometer) (Hord et al., 1992) and the Ulysses interstellar neutral gas (GAS) instrument (Witte et al., 1992). EUVS measurements near solar maximum (max) in 1990–1992 have a peaked brightness maximum upwind due to a rather isotropic solar wind charge-exchange ionization pattern (A=0–0.25). GAS measurements from solar minimum (min) in 1997 have a plateau in the upwind direction that we model using Ulysses SWOOPS (solar wind plasma experiment) solar min data on solar wind density and velocity at different heliographic latitudes. The isotropic ionization pattern deduced from EUVS at solar max may be consistent with recent SWOOPS results (McComas et al., 2000b, c) that high speed solar wind is absent at high latitudes during solar max. Galileo and Ulysses Lα data favor higher H temperatures (15 000–18 000 K) than previous models. This revised version was published online in August 2006 with corrections to the Cover Date.     

Source Region of High and Low Speed Wind during the Spartan 201-05 Flight

The large-scale coronal magnetic fields of the Sun are believed to play an important role in organizing the coronal plasma and channeling the high and low speed solar wind along the open magnetic field lines of the polar coronal holes and the rapidly diverging field lines close to the current sheet regions, as has been observed by the instruments aboard the Ulysses spacecraft from March 1992 to March 1997. We have performed a study of this phenomena within the framework of a semi-empirical model of the coronal expansion and solar wind using Spartan, SOHO, and Ulysses observations during the quiescent phase of the solar cycle. Key to this understanding is the demonstration that the white light coronagraph data can be used to trace out the topology of the coronal magnetic field and then using the Ulysses data to fix the strength of the surface magnetic field of the Sun. As a consequence, it is possible to utilize this semi-empirical model with remote sensing observation of the shape and density of the solar corona and in situ data of magnetic field and mass flux to predict values of the solar wind at all latitudes through out the solar system. We have applied this technique to the observations of Spartan 201-05 on 1–2 November, 1998, SOHO and Ulysses during the rising phase of this solar cycle and speculate on what solar wind velocities Ulysses will observe during its polar passes over the south and the north poles during September of 2000 and 2001. In order to do this the model has been generalized to include multiple streamer belts and co-located current sheets. The model shows some interesting new results. This revised version was published online in August 2006 with corrections to the Cover Date.     

Latitudinal Variation of the Underlying Heliospheric Magnetic Field Direction: Comparison of the Ulysses First and Second Orbits

We discuss the underlying direction of the heliospheric magnetic field measured by Ulysses in the latitude range 6° S-65° S by examining distributions of the magnetic field azimuthal angle with respect to the simple Parker spiral model. During the first Ulysses traversal of this latitude range in 1992–1994, while solar activity was declining, the shape of the distributions obtained at high latitudes in the fast solar wind differed from that at lower latitudes. In the present data set, obtained during rising solar activity, both field polarities are present at all latitudes and the peaks of the distributions agree with the predicted spiral direction to first approximation. However, compared to the first orbit, a significantly greater percentage of the observed field vectors have large deviations from the spiral direction. This revised version was published online in August 2006 with corrections to the Cover Date.     

CME-Driven Solar Wind Disturbances at High Heliographic Latitudes

We have identified 20 coronal mass ejections, or CMEs, in the solar wind in the Ulysses data obtained between S30° and S75° during the second polar orbit. Unlike CME-driven disturbances observed at high latitudes during Ulysses’ first polar orbit, these disturbances had plasma and magnetic field characteristics similar to those observed in the ecliptic plane near 1 AU when one allows for evolution with heliocentric distance. Here we provide a brief overview of CME observations at high latitudes both close to and far from the Sun, with emphasis on the recent Ulysses measurements on the rising portion of solar cycle 23. This revised version was published online in August 2006 with corrections to the Cover Date.     

Fine Structure in the Corona at High Latitudes at Solar Maximum

Microstreams and pressure balance structures in fast solar wind were more easily detected at Ulysses at 2.2 AU over the poles than at Helios at 0.3 AU. This is because solar rotation leads to dynamic interactions between different speed regimes at a rate that depends on latitude for the same size features. Dynamic interactions make structures more difficult to detect with increasing distance from the Sun. At solar maximum, Ulysses will sample high latitude solar wind coming from streamers, providing information on fine structure at the tops of streamers and on the source of slow solar wind. Examples are given here of the detectability of various sized structures at Ulysses when it is over the polar regions of the Sun. This revised version was published online in August 2006 with corrections to the Cover Date.     

Ulysses' Second Orbit: Remarkably Different Solar Wind

By the time of the 34th ESLAB symposium, dedicated to the memory of John Simpson, Ulysses had nearly reached its peak southerly latitude in its second polar orbit. The global solar wind structure observed thus far in Ulysses' second orbit is remarkably different from that observed over its first orbit. In particular, Ulysses observed highly irregular solar wind with less periodic stream interaction regions, much more frequent coronal mass ejections, and only a single, short interval of fast solar wind. Ulysses also observed the slowest solar wind seen thus far in its ten-year journey (∼270 km s⁻¹). The complicated solar wind structure undoubtedly arises from the more complex coronal structure found around solar activity maximum, when the large polar coronal holes have disappeared and coronal streamers, small-scale coronal holes, and frequent CMEs are found at all heliolatitudes. This revised version was published online in August 2006 with corrections to the Cover Date.     

The Evolution of the Anomalous Cosmic Ray Oxygen Spectra From 1995 to 1998: Ulysses Observations

Moraal and Steenberg (1999), showed that the peak energy in the anomalous cosmic ray spectra is independent of the radial distance up to a few AU away from the termination shock but dependent on the solar wind speed, the radius of the termination shock and the scattering strength. In this paper we will discuss the variation of the cosmic ray oxygen energy spectrum as measured by the Ulysses EPAC and the COSPIN/LET on board Ulysses. We found that the peak energy decreased from ∼5 MeV nucl⁻¹, when Ulysses was at high northern heliographic latitudes embedded in the fast solar wind to ∼3.5 MeV n⁻¹, in the streamer belt. The shift towards lower energy might also be caused by changing modulation although Voyager measurements indicate no variation of the ACR Oxygen spectrum at ∼60 AU. This revised version was published online in August 2006 with corrections to the Cover Date.     

Comparison of ≳40 keV Electron Events at High and Low Heliolatitudes: Ulysses/HI-SCALE and ACE/EPAM

We have performed a joint survey of anisotropic ≳40 keV electron events from August 1997 to September 2000 using the matched detectors on the Ulysses (ULS)/HI-SCALE and the ACE/EPAM instruments. A computer algorithm selected events with strong, statistically significant pitch-angle anisotropies. Electron pitch-angle distributions at ACE (∼1 AU) are often ‘beams’ that are strongly collimated along the local interplanetary magnetic field (IMF). These flare-associated impulsive injections can display rapid rise times (∼15 min) and slower decays, or more irregular intensity histories. At ULS, the electron intensities are lower and the time histories smoother, but strong anisotropies are still observable, indicating direct, nearly field-aligned propagation outward from the Sun. We focus on four event periods, selected from the survey, during times when the angle between the footpoints of the IMF lines intersecting ACE and ULS is small. These events span three full years and cover a wide range of distances and heliographic latitudes. We found one reasonably good association between impulsive electron events at ACE and ULS, and two events with small field-aligned gradients. This revised version was published online in August 2006 with corrections to the Cover Date.     

Langmuir Wave Activity: Comparing the Ulysses Solar Minimum and Solar Maximum Orbits

We examine the occurrence and intensity of Langmuir wave activity (electrostatic waves at the electron plasma frequency) during the solar minimum and solar maximum orbits of Ulysses. At high latitudes during the solar minimum orbit, occurrences of Langmuir waves in magnetic holes were frequent; in the second orbit, they were less common. This difference, in comparison with observations from the first Ulysses fast heliolatitude scan, suggests that Langmuir wave activity in magnetic holes is enhanced in solar wind from polar coronal holes. This revised version was published online in August 2006 with corrections to the Cover Date.     

Energetic Ions Observed at Low to High Latitudes in the Southern Heliosphere During Declining and Rising Solar Activity

We present and compare observations of energetic protons during the two first transits of the Ulysses spacecraft from low to high latitudes in the southern heliosphere. Protons in the energy range 1.8–3.8 MeV from the COSPIN experiment are studied for global trends and in relation to some ambient structures in the solar wind (corotating interaction region, forward/reverse shock). The global trends show the large dependence on the heliospheric condition and solar activity, including indications of a larger ambient particle population during the rising phase of solar activity and more efficient solar wind particle accelerators during the declining phase. More enhancements in the proton flux intensity are time associated with forward shocks than reverse contrary to first pass. Recurrent structures are found even during the second transit. Some latitude dependent periodicities are observed that could relate to the differential solar rotation. This revised version was published online in August 2006 with corrections to the Cover Date.     

Solar Origin of Anisotropic Electron Events observed by ULYSSES and ACE

We investigate the solar origin of energetic collimated electron events observed in situ by the EPAM and/or HISCALE particle experiments aboard the ACE and ULYSSES spacecraft, respectively. This study has been performed for two months, September and October 1999 when Ulysses was magnetically connected to the solar disk at heliolongitudes visible at the Earth. The two spacecraft were separated by about 35° in latitude and up to 60° in longitude. In this paper, results are described for one period only, October 24–November 1, 1999. It is found that not all the anisotropic events are observed by both spacecrafts; there exists a strong dependence on the spacecraft's magnetic connection back to the Sun. This revised version was published online in August 2006 with corrections to the Cover Date.     

Magnetic Field Observations of Transient Events at Ulysses, 1996–2000

During the years 1996–2000 solar activity has been gradually rising and is now close to maximum. At the same time the Ulysses spacecraft has performed a north to south traverse of the low latitude regions of the heliosphere and is now once again travelling through high southerly latitudes. We show some examples and report on the occurence rates of transient solar wind disturbances which have been identified by their magnetic field signatures. ‘Magnetic clouds’ remain more common at low (compared to high) latitudes despite the rise in solar activity. However, more events were observed at high latitudes than at solar minimum. This revised version was published online in August 2006 with corrections to the Cover Date.     

Associating the Solar Wind Measured by Ulysses with its Source at the sun

Radio occultation, ultraviolet, and white-light measurements have expanded our knowledge of the morphology of density and velocity in polar coronal holes, and made it possible to carry out the first systematic comparisons between the Ulysses solar wind measurements and quantitative white-light observations of the solar corona. This paper summarizes the rationale and salient features of this new approach which has been used to relate the solar wind observed by Ulysses in 1993–1995 to the inner corona. The statistical characteristics (average, standard deviation, and autocorrelation function) of the Ulysses density measurements of the fast wind are found to be mirrored in those of polarized brightness measurements of path-integrated density made by the High Altitude Observatory (HAO) Mauna Loa K-coronagraph at 1.15 R _⊙. These results reinforce the conclusions from comparisons between measurements of the outer and inner corona. They show that the polar coronal hole extends radially into the solar wind, and that sources of the fast wind are not limited to coronal holes. This revised version was published online in August 2006 with corrections to the Cover Date.     

Cosmic Ray Modulation over the Poles at Solar Maximum: Observations

Our knowledge of how galactic and anomalous cosmic rays are modulated in the inner heliosphere has been dramatically enlarged as a result of measurements from several missions launched in the past ten years. Among them, Ulysses explored the polar regions of the inner heliosphere during the last solar minimum period and is now revisiting southern polar latitudes under solar maximum conditions. This gives us for the first time the possibility to compare modulation of cosmic rays at high heliographic latitudes during such different time periods. We present data from different instruments on board the Ulysses spacecraft together with 1 AU measurements in the ecliptic. In this paper we focus on measurements that have direct implications for our understanding of modulation of cosmic rays in the inner heliosphere. This revised version was published online in August 2006 with corrections to the Cover Date.     

Three-Dimensional Solar Wind Modeling Using Remote-Sensing Data

We have developed a computer-assisted tomography (CAT) technique that iteratively modifies a kinematic solar wind model to least-squares fit heliospheric remote sensing observations (interplanetary scintillation and Thomson-scattering observations). These remote sensing data cover a large range of solar elongations, and access high-latitude regions over the solar poles. The technique can be applied to a time-independent solar wind model, assuming strict co-rotation, or, when sufficient remote sensing observations are available, to a time-dependent model. For the time-dependent case the technique depends primarily on outward motion of structures in the solar wind to provide the perspective views required for a tomographic reconstruction. We show results of corotating tomographic reconstructions primarily using IPS velocity observations from the Solar-Terrestrial Environment Laboratory (STELab, Nagoya, Japan), and include comparisons with in situ velocity data out of the ecliptic (Ulysses) and in the ecliptic (ACE). This revised version was published online in August 2006 with corrections to the Cover Date.