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At least 955 records · Page 53
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NASA NTRS1996

Ulysses at high latitudes: An overview of recent results

After its fly-by of the planet Jupiter in February 1992, the Ulysses spacecraft is now in a highly inclined heliocentric orbit that will bring it above the south polar regions of the Sun in September 1994. The high-latitude phenomena observed to date have been strongly influenced by the near-minimum solar activity conditions encountered during this phase of the mission. In late April 1993, when Ulysses was at approximately 29 deg S heliographic latitude, the recurrent high speed solar wind stream that had been observed at the location of the spacecraft for 11 consecutive solar rotation underwent a dramatic change. The wind speed in the valleys between successive peaks increased in a single step from approximately 420 km/s to aopproximately 560 km/s. This change in solar wind flow was accompanied by the disappearance at the spacecraft of the magnetic sector structure that had been observed until then. Both these finding are consistent with Ulysses having climbed beyond the latitude of the coronal streamer belt in which is embedded the heliospheric current sheet (HCS). In its subsequent poleward journey, no further evidence for an encounter with the HCS has been seen at Ulysses. Other phenomena observed include the evolution with latitude of corotating interaction region (CIRs) and their influence on the acceleration of energetic particles, and the characteristics of the solar wind flows emanating from the south polar coronal hole. In this paper, we present details of the above observations. Finally, while the polar passes of the prime mission will take place near solar minimum, an extended mission will bring Ulysses back over the poles near the maximum of the next cycle. A summary of scientific goals for Ulysses at solar maximum is given.

Marsden, R. G.↗
NASA NTRS1996

Ulysses observations of double ion beams associated with coronal mass ejections

Coronal mass ejections (CME's) are thought to result from the loss of stability within a magnetically confined coronal structure leading to its radial expansion into interplanetary space. As the CME expands into the corona current sheets will form between the expanding CME and surrounding field lines in the ambient wind. This configuration may lead to reconnection between the CME and adjacemt field lines. Such reconnection may produce double ion beams as has been observed in the terrrestrial magnetosphere. We examine all 24 distinct signatures of CME's observed by Ulysses during the in-ecliptic portion of the mission. In 5 of these 24 cases the ion spectra were not clear and thus the presence of double ion beams could not be determined. In 13 of the remaining 19 CME's double ion beams were found on the leading and/or trailing edge of the CME but not in the interior of the CME. In 3 of the CME's double ion beams were found throughout the CME while in the remaining 3 CME's double ion beams were not present near or just inside of the CME. In contrast in a control sample of 19 randomly chosen intervals, double ion beams were present at the leading and/or trailing edges of the random intervals in only 3 of the 19 cases. There appears to be no correlation between probability of occurrence of double ion beams and a magnetic cloud or non-cloud configuration of the CME and no correlation between the presence of the double ion beams at the edges of the CME and the CME being a fast or slow CME.

Hammond, C. M.↗
NASA NTRS1996

IPS observations of heliospheric density structures associated with active regions

Interplanetary scintillation (IPS) measurements of the 'disturbance factor' g, obtained with the Cambridge (UK) array can be used to explore the heliospheric density structure. We have used these data to construct synoptic (Carrington) maps, representing the large-scale enhancements of the g-factor in the inner heliosphere. These maps emphasize the stable corotating, rather than the transient heliospheric density enhancements. We have compared these maps with Carrington maps of Fe XIV observations National Solar Observatory ((NSO), Sacramento Peak) and maps based on Yohkoh Soft X-Ray Telescope (SXT) X-ray observations. Our results indicate that the regions of enhanced g tend to map to active regions rather than the current sheet. The implication is that act ve regions are the dominant source of the small-scale (approximately equal 200 km) density variations present in the quiet solar wind.

Hick, P.↗
NASA NTRS1996

Megavolt, Multigigawatt Pulsed Plasma Switch

Plasma switch proposed for use in high-voltage, high-current pulse power system. Designed not only to out-perform conventional spark-gap switch but also relatively compact and lightweight. Features inverse-pinch configuration to prevent constriction of current sheets into filaments, plus multiple-ring-electrode structure to resist high-voltage breakdown.

Lee, Ja H.↗
NASA NTRS1995

Coronal heating, nanoflares and MHD turbulence

Coronal heating is at the origin of the X-ray emission and mass loss from the sun and many other stars. While different scenarios have been proposed to explain the heating of magnetically confined and open regions of the corona, they must all rely on the transfer, storage and dissipation of the abundant energy present in photospheric motions. Here we focus on theories which rely on magnetic fields and electric currents both for the energy transfer and storage in the corona. The dissipation of this energy, whether in the form of reconnection in current sheets (nanoflare?) or the dissipation of MHD waves, depends crucially on the development of extremely small scales in the coronal magnetic field, where kinetic effects are likely to be fundamental. The question of whether coronal heating and flares may be viewed respectively as the macroscopic, low-energy average and the high-energy, temporally intermittent aspect of the same underlying driven, dissipative, turbulent system is also addressed, with emphasis placed on the main observational and theoretical stumbling blocks in the way of a confinement or disproof of such a conjecture.

Velli, M.↗
NASA NTRS1995

Evidence of active region imprints on the solar wind structure

A common descriptive framework for discussing the solar wind structure in the inner heliosphere uses the global magnetic field as a reference: low density, high velocity solar wind emanates from open magnetic fields, with high density, low speed solar wind flowing outward near the current sheet. In this picture, active regions, underlying closed magnetic field structures in the streamer belt, leave little or no imprint on the solar wind. We present evidence from interplanetary scintillation measurements of the 'disturbance factor' g that active regions play a role in modulating the solar wind and possibly contribute to the solar wind mass output. Hence we find that the traditional view of the solar wind, though useful in understanding many features of solar wind structure, is oversimplified and possibly neglects important aspects of solar wind dynamics

Hick, P.↗
NASA NTRS1995

The solar/interplanetary event of 14 April 1994 observed by Yohkoh/SXT

The polar crown event of April 14 1994 is one of the largest scale eruptive events observed by the Yohkoh/SXT. Associated with the formation of an arcade of soft X-ray loops at the Sun was the detection of an interplanetary forward/reverse shock event by the Ulysses spacecraft some 4-7 days later. The relationship between the coronal and interplanetary signatures of these events is important if we are to address fully the initialization and consequent acceleration of interplanetary phenomena, such as CMEs and counter-streaming electrons, originating at the Sun. From detailed analysis of the energetics of the arcade formed during the eruption of April 14 1994, we find peak temperatures and emission measures of approximately 5MK and approximately 10(exp 48)cm(exp -3) respectively. The total thermal content of the arcade loop structure observed in soft X-rays is calculated to be some 5 x 10(exp 29) ergs. The development of these parameters as the event proceeds and their relationship to the dynamics of the eruption are investigated. Although spanning a longitudinal range of some 150 degrees the April 14 event displayed the typical helmet streamer structure normally associated with coronal mass ejections These helmet streamers are thought to be related to the global solar magnetic field through the heliospheric current sheet (HCS). The arcade formation, together with the eruption of material into interplanetary space, signifies a large-scale reconfiguration of the coronal magnetic field. We examine the effects of the formation of such a coronal arcade structure on the HCS and discuss the dynamics involved with the passage of a large scale disturbance through the interplanetary magnetic field.

Alexander, D.↗
NASA NTRS1995

Large-scale structure of the solar corona and inner heliosphere

The large-scale structure of the solar corona influences solar activity particularly coronal mass ejections (CMEs). The helmet streamers that are observed to dominate the structure of the inner corona are formed by the interaction of the solar wind with coronal magnetic fields. We have simulated this interaction in three dimensions using the magnetohydrodynamic (MHD) equations. In order to create a realistic model, we use the magnetic field that is observed at the Sun's surface (deduced from daily Wilcox Solar Observatory magnetograms) as input, in combination with specified density and temperature profiles at the surface. A self-consistent 3D solar-wind solution is developed by integrating the MHD equations in time to steady state. Such solutions can reproduce the observed structures that are seen in coronagraph images and eclipse photographs of the corona. This model allows us to accurately determine the position of the heliospheric current sheet. We will compare the results obtained from our model with Ulysses observations during the period May-June 1993, and with an eclipse photograph of the corona on November 3, 1994.

Mikic, Z.↗
NASA NTRS1995

Coronal roots of solar wind streams: 3-D MHD modeling

Weak (discontinuous) solutions of the 3-D MHD equations look like a promising tool to model the transonic solar wind with structural elements: current sheets, coronal plumes etc. Using the observational information about various coronal emissions one can include these structural elements into the 3-D MHD solar wind model by embedding the discontinuities of given type. Such 3-D MHD structured solar wind is calculated self-consistently: variants are examined via numerical experiments. In particular, the behavior of coronal plumes in the transonic solar wind flow, is modeled. The input information for numerical modeling (for example, the magnetic field map at the very base of the solar corona) can be adjusted so that fast stream arises over the center of the coronal hole, over the coronal hole boundaries and, even, over the region with closed magnetic topology. 3-D MHD equations have the analytical solution which can serve as a model of supersonic trans-alfvenic solar wind in the (5-20) solar radii heliocentric distance interval. The transverse, nonradial total (gas + magnetic field) pressure balance in the flow is the corner-stone of this solution. The solution describes the filamentation (ray-like structure of the solar corona) and streaming (formation of high-speed streams with velocities up to 800 km/sec) as a consequence of the magnetic field spatial inhomogeneous structure and trans-alfvenic character of the flow. The magnetic field works in the model as a 'controller' for the solar wind streaming and filamentation.

Pisanko, Yu. V.↗
NASA NTRS1995

Large-scale variations of solar wind elemental composition and charge states with heliospheric latitude

The Solar Wind Ion Composition Spectrometer (SWICS) onboard Ulysses allows determination of the elemental composition of the solar wind and the charge states of all major solar wind ion species. Ulysses left the ecliptic plane in early 1992, crossed the Sun's south polar region in late 1994 and made a fast approach back towards the ecliptic in the first half of 1995. Data from this period were investigated for long-term variations in the solar wind composition. At midlatitudes Ulysses encountered periodically the fast solar wind stream emerging from the south coronal hole. As a consequence, dramatic variations in the charge-states arise, between high charge-states dominating in the current sheet solar wind and low charge states in the coronal hole stream. However, the initial analysis indicates that from midlatitudes onwards, with Ulysses permanently immersed in the coronal hole stream, the charge state and elemental abundance ratios of the major solar wind ion species stayed essentially constant. This implies that the temperature profile in the coronal hole at solar wind source altitudes exhibit no variation with solar latitude. It confirms that the south coronal hole is essentially unstructured down to scale lengths of several degrees in solar latitude.

Woch, J.↗
NASA NTRS1995

The observed North-South Asymmetry of IMF spiral

We appraise the finding, reported in the literature, that a small but finite north-south asymmetry (NSA) exists in the interplanetary magnetic field (IMF) spiral at Earth's orbit. We have analyzed the data available on the Omnitape for the 1963 to 1993 period. The coverage is very uneven, ranging from less than 40% to greater than 80%. The magnitude of NSA fluctuates considerably during the period of our analysis. This is true even if one considers the period 1967 to 1982 when the coverage is greater than 50%. The values of NSA derived from 27-day averages of the hourly data points range from greater than +50 deg to less than -40 deg. If one arranges the data according to the magnetic polarity epochs of the solar polar field, the epoch averages gives the magnitude of NSA less than approximately 2 deg. This is also true, if one considers the average magnitude of NSA for the 1965 to 1993 period, when the coverage is greater than 25%. A genuine, persistent, NSA of IMF spiral is likely to affect the cosmic ray modulation, on either side of the current sheet, by introducing a corresponding change in the radial diffusion coefficient of energetic particle transport in the heliosphere. The annual mean values of the observed NSA of IMF spiral are compared with the observed off-ecliptic contributions to cosmic ray modulation.

Ahluwalia, H. S.↗
NASA NTRS1995

Search for fine scale structures in high latitude solar wind

About 25 years ago, E. Parker suggested that, as a consequence of the inhomogeneous structure of the corona, the solar wind might consist of adjacent structures with different physical conditions. Since that suggestion was made, the solar wind plasma characteristics have been measured in situ through many experiments, but little has been done to check whether the solar wind shows any evidence for fine scale structures, and, in the affirmative, how far from the Sun these structures persist. A previous work on this subject, by Thieme, Marsch and Schwenn (1990), based on Helios data, lead these authors to claim that the solar wind, between 0.3 and 1 AU, is inhomogeneous on a scale consistent with the hypothesis that the plume-interplume plasmas, at those distances, still retain their identity. In this work we present preliminary results from an investigation of the solar wind fine structure from Ulysses high latitude observations. To this end, we have analyzed data over several months, during 1994, at times well after Ulysses's last encounter with the Heliospheric Current Sheet, when the spacecraft was at latitudes above 50 degrees. These data refer to high speed wind coming from southern polar coronal holes and are best suited for plume-interplume identification. We have performed a power spectra analysis of typical plasma parameters, to test whether the wind plasma consist of two distinct plasma populations. We also examined data to check whether there is any evidence for an horizontal pressure balance over the hypothesized distinct structures. Our results are discussed and compared with previous findings.

Livi, S.↗
NASA NTRS1995

Numerical simulation of corotating interaction regions

We report on a numerical simulation of Corotating Interaction Regions (CIRs) in the solar wind. We employ an MHD calculation with a tilted dipole model: slow wind is assumed around the tilted current sheet. As a simplifying assumption the solar wind is kept radial, meridional and azimuthal deflections are neglected. Despite of this simplification the model reproduces several observed features of CIR-s. CIR-s turn out most prominent at mid-latitudes. Toward increasing heliographic latitudes, reverse shocks become dominant, forward shocks tend to become weaker then disappear. The frozen in magnetic field is calculated and applied for the simulation of the transport of cosmic rays in the heliosphere. Some consequences on recurrent cosmic ray phenomena and on the acceleration of energetic particles will be discussed.

Kota, J.↗
NASA NTRS1995

Geomagnetic field as a sensor of the large-scale structure of the interplanetary medium

Application of various geomagnetic data to restoration of the heliospheric current sheet configuration in the past is discussed. Technique for such a restoration is proposed based on analysis of information on the interplanetary magnetic field polarity inferred from diurnal variation of the geomagnetic field in polar regions outer solar corona pictures resulting from solar eclipse observations are concerned with the heliospheric sheet configuration. So, days of past solar eclipses must serve as a kind of reference points in the course of the heliospheric sheet reconstruction.

Gulyaev, R. A.↗
NASA NTRS1996

Preliminary Results of Observations of Comets De Vico and Hyakutake by the Ulysses Comet Watch Network

Cometary interactions with the solar wind allow us to use comets as probes of the inner regions of the heliosphere. During their close passage to the Sun, comets are exposed to different environments depending on their latitude. Until recently, characterizing these environments has been difficult because most spacecraft studying the sun have been confined to studying its mid-latitudes. A valuable source of information about the differing regimes of the solar wind is the joint ESA/NASA ULYSSES mission, which is the first spacecraft to explore the polar regions of the heliosphere. In 1995, ULYSSES' orbit covered a range of solar latitudes from -80 degrees to +80 degrees - an interval referred to as the 'fast latitude scan.' The Ulysses Comet Watch incorporates in-situ measurements during these periods by the ULYSSES spacecraft with images contributed by a world-wide network of observers (both amateur and professional). Bright comets whose paths come within 20 degrees solar latitude of the spacecraft are considered especially good targets for correlation between spacecraft data and plasma tail activity. Ulysses findings of interest to cometary plasma research are: Verification of global differences in solar wind properties (speed and density) at different solar latitudes. At polar latitudes - ranging from roughly +/-30 degrees to +/-80 degrees - the solar wind speed is about 750 kilometers/sec, and has a proton density (1 AU) around 3 cm(exp -3). Changes in properties are small and the heliospheric current sheet (HCS) is not seen. In the equatorial latitudes (roughly +30 to -30 degrees), the average solar wind speed is about 450 kilometers/sec, with an average proton density (at 1 AU) around 9 cm(exp -3). The HCS is seen and changes in properties can be large. An object, spacecraft or comet, at a given latitude, can be entirely in the polar, entirely in the equatorial, or can experience both - sort of a transition region.

Petersen, C. C.↗
NASA NTRS1996

Interball substorm observations: Christmas for space scientists

Observational results from the Interball Tail Probe spacecraft are presented. One of the main objectives of the Interball project is to study the dynamic processes in the magnetosphere. Three events observed by the spacecraft's instruments are investigated: a pseudobreakup during which earthward streaming ions were observed in the vicinity of a thin current sheet; a substorm in which the magnetic signatures in the lobe and on the ground were preceeded by northward re-orientation of the interplanetary magnetic field Bz component; and a magnetic storm at the beginning of which extreme deformation of the magnetotail was observed.

Sandahl, Ingrid↗
NASA NTRS1996

Inferences on Coronal Magnetic Fields from SOHO UVCS Observations

The characteristics of the magnetic field ubiquitously permeating the coronal plasma are still largely unknown. In this paper we analyze some aspects of coronal physics, related to the magnetic field behavior, which forthcoming SOHO UVCS observations can help better understand. To this end, three coronal structures will be examined: streamers, coronal mass ejections (CME's) and coronal holes. As to streamers and CME's, we show, via simulations of the Ly-alpha and white light emission from these objects, calculated on the basis of recent theoretical models, how new data from SOHO can help advancing our knowledge of the streamer/CME magnetic configuration. Our discussion highlights also those observational signatures which might offer clues on reconnection processes in streamers' current sheets. Coronal holes (CH's) are discussed in the last section of the paper. Little is known about CH flux tube geometry, which is closely related to the behavior of the solar wind at small heliocentric distances. Indirect evidence for the flux tube spreading factors, within a few solar radii, is here examined.

Poletto, G.↗
NASA NTRS1997

Physics of Boundaries and their Interactions in Space Plasmas

This report describes the work done by SciberNet, Inc. during the month of August. We have resolved the issues associated with the implementation of the dipole field in our large scale hybrid simulations of the magnetopause. We have setup several runs and will spend the next several months analyzing the data. The results will be presented at the Fall AGU. We are also continuing our analysis of the 3-D simulations of thin current sheets at the magnetopause, paying special attention to the conditions under which Kelvin-Helmholtz would lead to sizable perturbations of the magnetopause. In a related study, we are in the process of developing a new kinetic linear code that would for the first time enable us to examine the linear properties of the Kelvin-Helmholtz instability in the fully kinetic regime. Finally, we are continuing our code development to include inflow-outflow boundary conditions in our 2-D and 3-D hybrid codes. We are also comparing the different methods of code parallelization in order to extend the limits of our calculations.

Omidi, Nojan↗