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We present some results from a statistical study of the levels of the power spectrum of the interplanetary magnetic field as measured by the Ulysses spacecraft near the ecliptic plane between 1 and 5 AU. We conclude from the studies of the probability distributions of the power spectral levels that the interplanetary medium is not totally chaotic, but can sustain high percentages (21% to 30% for the examples shown here) of discrete, low frequency modes, probably of solar origin, over a long interval of time.

Time series analysis of low-energy electron and high-energy proton cosmic ray fluxes, both measured on the Ulysses spacecraft, shows that they are quite coherent in some frequency bands in the energy range 0–6 μHz, especially near 5 μHz. Interplanetary particle fluxes with very different kinematic properties (e.g., gyrofrequencies) are modulated coherently in the heliosphere at discrete frequencies; one specific example discussed here has a period of ∼2.2 days. The analyzed data span the interval January 1, 1993, to April 20, 1994, when Ulysses traveled between 22° and 60° south heliographic latitude and was ∼4 AU from the Sun. Frequencies with high coherence appear to be associated either with solar rotation or with discrete solar modal frequencies. The frequency at 5.26 μHz is tentatively identified as a zonal harmonic mode with l = 2 and m = 0.

We report here detailed statistical studies of power spectra of 1 hour average interplanetary magnetic field (IMF) data that were acquired by the magnetometer instrument on the Ulysses spacecraft in the ecliptic plane of the heliosphere during Ulysses' cruise phase from Earth to Jupiter (October 1990 through January 1992). We have pursued these studies in order to examine in a critical manner our previously published conclusions [Thomson et al., 1995] that energetic particle fluxes in the interplanetary medium carried signatures of discrete modes. We attributed these signatures to the existence of solar acoustic and gravitational modes (p and g modes, respectively) in the heliosphere. Here we first examine the statistical distributions of g modes (less than ∼200 μHz) that are predicted by a theoretical model of the solar interior. With this background information we formulate the time series and statistical analysis procedures that must be used in order to provide information on possible modal signals in the IMF, independent of the source. We show from the statistical analyses that the modal components that are present in the IMF fluctuations can amount to ∼20% to ∼70% of the IMF fluctuation power in the frequency range of ∼10–140 μHz. We discuss these findings in the context of previously published work on searches for optical signatures of solar g modes in the solar photosphere as well as published research papers that have called into question some of the conclusions of Thomson et al. [1995]. We conclude that the interplanetary medium is not a totally chaotic medium, but can sustain high power levels of discrete, low-frequency modes, probably of solar origin, over a long interval of time.

We show that with reasonable values of anomalous resistivity, surface-wave-induced magnetic reconnection at an interface between two plasma regions (such as the magnetosheath and the magnetopause) has an intrinsic timescale that can explain observations of a delay time between the southward turning of the interplanetary magnetic field and the onset of a flow transfer event at the magnetopause.

The interaction of solar wind disturbances with the Earth's magnetosphere can produce disturbances, and at times complete disruptions, of technological systems on the Earth and in the space around the Earth. This brief report shows the changes induced in the large-scale geopotentials of the Earth (as provided from measurements across transoceanic cables) produced by a well-documented interplanetary magnetic cloud event. The study of such a well-measured event can be used to begin to make empirical space weather phenomena more quantitative. We show that geopotentials at low geomagnetic latitudes can be used to infer the time derivative of the near-equatorial magnetic disturbance index, Dst. At low geomagnetic latitudes, a peak geopotential of about 4 mV/km is found to correspond to a time rate of change of this index of about 50 nT/hr. Further, we show that in this event increases in the near-equatorial geopotential are linearly related to the energy input to the magnetosphere from the solar wind as given by the e parameter [e.g., Akasofu, 1979]. We find that an increase in the geopotential of about 4 mV/km corresponds to an energy input of about 2.8 × 1011 W for the event analyzed here.

We review the observed properties of anomalous cosmic rays and the present status of our knowledge of the processes by which they originate. We compiled a comprehensive set of ACR energy spectral data from various spacecraft throughout the heliosphere during the passes of Ulysses over the poles of the Sun and present first results of a detailed modeling effort. In several contributions, we discuss the questions of injection and possible pre-acceleration of pickup ions, summarize new observations on the ionic charge composition, and present new results on the composition of minor ions in ACRs.

In the high-latitude northern hemisphere of the heliosphere, the Heliosphere Instrument for Spectra Composition and Anisotropy at Low Energies (HI-SCALE) energetic particle detectors on Ulysses have measured quasi-recurrent ∼26-day increases of 40-65 keV electrons during 11 solar rotations from October 1995 through July 1996. They do not appear on all rotations, but when they do, they are associated with increases in 0.5-1.0 MeV protons (preceding the electron increases by several days) and sometimes with decreases in galactic cosmic rays. The northern recurrences form two series shifted half a solar rotation with respect to each other, unlike the very regular and more intense series of 21 recurrences observed by the same instrument throughout the middle-to-high-latitude southern hemisphere from mid-1993 to the beginning of 1995. Correlated energetic particle measurements from IMP 8 at Earth and Voyagers 1 and 2 at 42-62 AU establish that recurrent events during this period were intrinsically stronger in the southern heliosphere than in the north. The variability of the northern recurrences is attributed, using a generalization of the model of Fisk [1996], to temporal changes during 1966 in the near-Sum polar magnetic field configuration. These changes would affect the connection of Ulysses via magnetic field lines to the corotating interaction regions at lower latitudes >10 AU beyond the spacecraft, where the low-energy particles are accelerated and the galactic cosmic rays are modulated. The observed evolution of the northern polar coronal structure, as revealed in FeXIV (5303A) synoptic maps and confirmed by FeXII (195h) images from the Extreme Ultra-Violet Imaging Telescope on the SOHO spacecraft, is just that which is required, according to the model, to explain the evolution of the low-energy particle recurrences as observed by Ulysses/HI-SCALE in the northern heliosphere.

It is argued that surface waves with wavelengths larger than the thickness of a neutral layer that separates two different plasma regimes can induce the tearing mode instability. Magnetic reconnection is shown to occur on the tearing mode timescale.

Time-series analysis of the fluxes of interplanetary charged particles measured by the Ulysses and Voyager spacecraft reveals many periodic components. From 1 to 140 µHz, the spectral components are consistent with those estimated (but not confirmed) for gravity-mode oscillations of the Sun: from 1,000 to 4,000 µHz, the spectral lines closely match the frequencies of known solar pressure modes. These concordances imply that the solar wind and the interplanetary magnetic field transmit solar oscillations and thus might be used to probe the interior structure of the Sun.

Observations of an energetic interplanetary electron event associated with the production of Langmuir waves, both of which are identified at 4.3 AU by instruments on the Ulysses spacecraft, are presented in this paper. This electron event propagates inside a well-defined magnetic structure. The existence of this structure is firmly established by joint particle and plasma observations made by Ulysses instruments. Its local estimated radial width is of the order of 2.3 x 10(exp 7) km (0.15 AU). The electron beam is associated with a type III burst observed from Earth at high frequencies and at low frequencies from Ulysses in association with Langmuir waves detected inside the structure. The consistency of local (Ulysses) and remote (Earth) observations in terms of temporal and geometrical considerations establishes that the structure is anchored in the solar corona near the solar active region responisble for the observed type III emission and gives an accurate determination of the injection time for the observed electron beam. Propagation analysis of the electron event is presented. In order to quantify the magnetic field properties, a variance analysis has been performed and is presented in this paper. The analysis establishes that inside the structure the amount of magnetic energy involved in the fluctuations is less than 4% of the total magnetic energy; the minimal variance direction is well defined and in coincidence with the direction of the mean magnetic field. This configuration may produce conditions favorable for scatter free streaming of energetic electrons and/or Langmuir wave production. The results presented show that the magnetic field might play a role in stabilizing the coronal-origin plasma structures and then preserving them to large, approximately 4 AU, distances in the heliosphere.

The spectral and temporal characteristics of a “giant” Pc 2 (τ ∼ 7 s) hydromagnetic wave event of duration ∼110 s have been reanalyzed, as have the electromagnetic oscillations generated in the magnetosphere in an interval of about 45 min preceding and following the event. The event, which was recorded near local midnight at two mid-latitude locations, is, to the best of our knowledge, the largest such wave event reported to date. Previous analyses and interpretation of the event have centered on the bounce resonance generation of this packet of oscillations (Lanzerotti et al., 1986). The reexamination of the event, as well as other oscillations of the magnetoplasma, has caused us to identify six distinguishable spectral frequency bands which appear to encompass the Pc 2 wave packet and its evolution in time as well as to look at the development of the process as a whole. We present the details of these temporal and spectral studies and provide evidence that the Pc 2 event is probably produced by the gyroresonance swinging: amplification of the oscillatory magnetoplasma background in the frequency band ∼0.1-0.2 Hz. The analysis suggests that the oscillatory background could be produced by impulse-like disturbances. The possible source(s) of such disturbances and their nature require further study.

The Voyager 2 measurements of energetic (∼1 MeV/nucleon) H3 molecules in Jupiter's magnetosphere provided strong evidence that the planet's ionosphere is an important plasma source in the Jovian environment (Hamilton et al., 1980). During the Ulysses spacecraft encounter with Jupiter in February 1992 the Heliosphere Instrument for Spectra, Composition, and Anisotropy at Low Energies (HI-SCALE) on this satellite also detected significant fluxes of energetic H3 molecules. These molecules were measured throughout the dayside inbound pass (near the plasma sheet) as well as during the outbound pass at high Jovian latitudes on the duskside. The spectral shapes of the energetic 4He and the H3 are found to be similar over the range ∼0.6-1.6 MeV/nucleon, with a power law spectral slope of n∼3. The abundance ratio R (≡H3/4He) is found to be ∼0.1 over this energy range for the inbound pass and to vary from ∼0.2 at ∼0.6 MeV/nucleon to ∼0.1 at ∼1.6 MeV/nucleon at duskside high latitudes. These observations, especially those from high latitudes, confirm that the energetic molecules must be accelerated out of the Jovian topside ionosphere into the magnetosphere.

Magnetic impulse events were selected by a computer algorithm procedure from magnetic records obtained at the near cusp latitude conjugate stations Iqaluit, Northwest Territories, Canada, and South Pole Station, Antarctica. The algorithm was constructed to select large (≳ 50 nT in the vertical component of the magnetic field), short lived (6 to 12 min) events. These events were found to be highly localized in the 06 to 18 LT sector at the two stations. A strong minimum in occurrence was found during hour 11 LT. The field changes associated with these events can be interpreted as due to an approximately half-cycle, odd-mode, Alfven wave along a near-magnetopause flux tube. From the vertical magnetic deflections of the impulse events the directions of field-aligned currents into the conjugate ionospheres were inferred. In the morning LT sector, field-aligned currents were directed into the ionospheres, while in the afternoon LT sector, field-aligned currents were directed out of the ionospheres. These findings are comparable with the statistical results for quasi-stationary field-aligned currents and suggest that at the times of these events, Iqaluit and South Pole are at a higher effective magnetic latitude. The average deflection in the vertical component for the events was measured to be ∼ 95 nT. From this the magnitude of the average field-aligned currents was calculated to be J∥ ∼ 2 × 10−7 A/m².

Simultaneous measurements of magnetic field variations at the nominally conjugate sites of South Pole station, Antarctica, and Iqaluit, Canada, have been used to study latitude differences in the location of corresponding westward electrojets in the nightside polar cap. Substorm-related auroral events were selected from 1986 data with the criterion that the magnitude of the negative bay exceed 100 nT in at least one hemisphere and that the electrojet, oriented mainly in the magnetic east–west direction, pass overhead of this station. The resulting data base comprised 60 events distributed seasonally as follows: austral summer (17); fall (6); winter (18); spring (19). The interhemispheric latitude difference in the locations of the electrojets was obtained by examining the ratio of the vertical and horizontal component magnetic variations. Seasonal and diurnal variations of the latitude of westward electrojets were evident during solstice periods. In particular, it is noted that the substorm westward electrojet flows at higher latitudes in the winter hemisphere than in the summer hemisphere, after 2000 MLT. The latitude difference between the two hemispheres maximizes at ∼4° near local midnight. The latitude difference becomes smaller away from midnight and reverses for local times prior to ∼2000 MLT. The observed seasonal and diurnal variations are generally consistent with predictions of the effect of the magnetic dipole tilt on the location of last closed field lines. However, because on the nightside the field lines originating from South Pole station and Iqaluit reach deep into the magnetotail, they are likely to be influenced by the plasma sheet. Thus the implications of an asymmetric distribution of the plasma sheet with respect to the neutral sheet have also been considered. Such a postulated distribution can account for the seasonal variations while conserving magnetic flux and maintaining pressure balance across the tail.

This paper presents values of particle spectral parameters and estimates of plasma densities, temperatures, and beta parameters, obtained with the Low Energy Charged Particle instrument during the Voyager 2 encounter with Neptune on August 24-25, 1989. In addition, trapped electron intensities are compared with the whistler mode stably trapped limits. The results revealed a very good inbound-outbound symmetry for both the proton and the electron profiles, suggesting that there was little dynamical activity in Neptune's magnetosphere during the Voyager encounter. The similarities and differences observed between Neptune and Uranus in the values of plasma density, pressure, and beta are discussed.

The heliosphere exists because of the presence of the solar wind, the expanding hot upper atmosphere (the corona) of the Sun, which excludes the local interstellar medium (LISM) from the vicinity of the Sun and planets. The size and boundaries of the heliosphere are determined through the interaction between the solar wind and the LISM. The internal properties, structure, and dynamics of the heliospheric medium are defined by spatial and temporal variability of the regions of origin of the solar wind in the solar corona. The variability of the solar wind leads to evolving, dynamic phenomena throughout the heliosphere on all spatial and temporal scales. The most important timescale is imposed by the approximately 11-year solar activity cycle and the approximately 22-year solar magnetic cycle (the Hale cycle). Regions of the origin and of the properties of the solar wind undergo considerable change on this 11-year timescale, the dominant parameter in the description of the global heliosphere. Due to the rotation of the Sun and the interaction of this rotation with the generation of the internal and external magnetic fields of the Sun, the 11-year periodicity is most significant in the heliosphere in the solar meridian, and thus as a function of heliolatitude.

During the Galileo probe's descent through Jupiter's atmosphere, under the ionosphere, the lightning and radio emission detector measured radio frequency signals at levels significantly above the probe's electromagnetic noise. The signal strengths at 3 and 15 kilohertz were relatively large at the beginning of the descent, decreased with depth to a pressure level of about 5 bars, and then increased slowly until the end of the mission. The 15-kilohertz signals show arrival direction anisotropies. Measurements of radio frequency wave forms show that the probe passed through an atmospheric region that did not support lightning within at least 100 kilometers and more likely a few thousand kilometers of the descent trajectory. The apparent opacity of the jovian atmosphere increases sharply at pressures greater than about 4 bars.

Several years ago we reported evidence for distinct modal structure in particle flux data from Ulysses and other interplanetary spacecraft, based on detailed time series analyses (1). Furthermore, we concluded that numerous discrete frequencies could be identified with known solar p-mode frequencies and with several calculated solar g-mode frequencies. This paper continues to explore the nature of the modal structure of the interplanetary medium by investigating the statistics of the coherence in electron fluxes measured at two separate locations in the near-ecliptic heliosphere, at 1 AU and at 5 AU, on the ACE and the Ulysses spacecraft, respectively. We demonstrate that there are highly significant coincident frequency peaks at the two locations in data taken at different times, and that, for data acquired at the same time at the two locations, there are high levels of coherence at multiple frequency pairs across a wide spectral band.

The occurrence of the high fluence solar events in October 1989 led to a reexamination of some of the assumptions made on the appropriate energy spectral representation of the large solar-flare proton events used for engineering designs of solar arrays for geosynchronous communication spacecraft. This paper discusses the solar event characteristics and some aspects of the design of solar proton fluence models. It is estimated that, for a design of a spacecraft with about 12-15-yr life expectancy, it will be necessary to take into account the launch data in the particular phase of the solar cycle (i.e., whether two solar maxima or one) that is likely to be encountered. 19 refs.

Anomalous cosmic-ray (ACR) oxygen (O) fluxes in the energy range 0.5-5.0 MeV nucleon-1 were measured in the ecliptic plane at 1 and 5 AU in 1997 and 1998. Using measurements from essentially identical instruments on the Advanced Composition Explorer (ACE) and Ulysses spacecraft, we were able to monitor the decrease with time of the ACR O fluxes in the inner heliosphere during the onset of solar cycle 23. The data obtained from three time intervals that are relatively undisturbed by solar activity during this period show that the e-folding time is approximately 140+/-40 days for the disappearance of 2-5 MeV nucleon-1 ACR oxygen from the ecliptic plane at 5 AU.