Search

As the twin Voyager spacecraft approach the boundary of the heliosphere, they continue returning new and unexplained measurements of the solar wind (SW) protons, energetic particles, and magnetic field that often differ markedly between the two spacecraft. Our recent studies show that time-dependent effects play a crucial role in understanding and interpreting the observational data. Since the SW is unsteady on many different time scales, its interaction with the local interstellar medium (LISM) should reflect the solar rotation and cycle, as well as merged interplanetary disturbances. Even a simplified solar cycle model allowed us to predict in 2009 the possibility of a negative radial velocity component in the SWas the heliopause is approached. Further analysis shows a nearly vanishing latitudinal velocity component, while the longitudinal component becomes comparable substantial. Here we discuss the change of the magnetic field and plasma properties across the heliopause, which is important for the identification of its spacecraft crossing. We discuss the effects of heliopause instabilities and corotating interaction regions, and demonstrate that Voyagers are unlikely to see a sharp boundary between the SW and the LISM, but rather a mixing layer of varying width.

Year 2009 was the last year of a long and pronounced solar activity minimum. In this year the solar wind in the inner heliosphere was for 90% of the time slow (< 450 km s−1) and with a weaker magnetic field strength compared to the previous solar minimum 1995-1996. We choose this year to present the results of a systematic search for small solar wind transients (STs) observed by the STEREO-Ahead (ST-A) probe. The data are from the PLASTIC and IMPACT instrument suites. By "small" we mean a duration from ∼1 to 12 hours. The parameters we search for to identify STs are (i) the total field strength, (ii) the rotation of the magnetic field vector, (iii) its smoothness, (iv) proton temperature, (v) proton beta, and (vi) Alfven Mach number. We find 45 examples. The STs have an average duration of ∼4 hours. Ensemble averages of key quantities are: (i) maximum B = 7.01 nT; (ii) proton β = 0.18; (iii) proton thermal speed = 20.8 km s−1; and (iv) Alfven Mach number = 6.13. No distinctive feature is found in the pitc...

Important results have been acquired using the measurements of VLF amplitude and phase signals from the South America VLF Network (SAVNET) stations. This network is an international project coordinated by CRAAM, Brazil in cooperation with Peru and Argentina. It started operating in April 2006, and now counts on eight stations (Atibaia, Palmas, Santa Maria and Estacao Antartica Comandante Ferraz in Brazil; Piura, Punta‐Lobos and Ica, in Peru; CASLEO, in Argentina). Researches, through the last decades, have demonstrated the versatility of the VLF technique for many scientific and technological purposes. In this work, we summarize some recent results using SAVNET data base. We have obtained daily maximum diurnal amplitude time series that exhibited behavior patterns in different time scales: 1) 1ong term variations indicating the solar activity level control of the low ionosphere; 2) characteristic periods of alternated slow and fast variations, the former being related to solar illumination conditions, and the latter that have been associated with the winter anomaly at high latitudes; 3) 27‐days period related to the solar rotation and consequently associated to the solar Lyman‐α radiation flux variations, reinforcing earlier theories about the importance of this spectral line for the D‐region formation. Finally, we conclude presenting preliminary results of simulation using LWPC, which showed very good agreement at times of observed modal amplitude minima for a given VLF propagation path.

Ulysses in‐situ measurements have given a new picture on the large‐scale properties of the fast solar wind as well as on the microscopic scales. From the large sample of data obtained during Ulysses fast latitudinal scans in 1994 and 2007 occuring both near solar minima, we will present the spectrum of the electron density fluctuations. The preliminary analysis suggests that a single Kolmogorov‐type power law spectrum does not fit the data at high latitudes in contrast to the ecliptic observations: the spectrum appears to flatten in the high‐frequency range of the inertial domain. We will discuss and compare these results and compute the probability distribution functions (for a timescale spanning from a solar rotation to two minutes), based on the use of Wavelet transforms on Ulysses electron density from the URAP experiment. We show as well an evidence of intermittence nature of the electron density fluctuations.

In this review we summarize our current knowledge regarding the three‐dimensional structure of the quasi‐steady, large‐scale inner heliosphere. This understanding is based on the interpretation of a wide array of remote and in situ measurements, in conjunction with sophisticated numerical models. Observations by the Ulysses spacecraft, in particular, have provided an unprecedented set of measurements for more than 18 years, and observations by the STEREO spacecraft promise no less. Global MHD models of the solar corona and heliosphere have matured to the point that a wide range of measurements can now be reproduced with reasonable fidelity. In the absence of transient effects, this structure is dominated by corotating interaction regions which can be understood—to a large extent—from the consequence of solar rotation on a spatially‐variable velocity profile near the Sun, leading to parcels of plasma with different plasma and magnetic properties becoming radially aligned. This interaction is one of the pri...

This paper presents a global method for measuring solar rotation. We display its properties and advantages and show the field of application. We end by giving some results obtained using the global method on the solar cycles 9 to 23.

The rotation of the solar corona at different heliolatitudes from 1.5 to 3.0 R⊙ from Sun center has been studied at solar minimum from the reconstructed intensity time series of the O VI 1032 A and H I Lyα l216 A spectral lines and visible light polarized brightness obtained by the observations of UVCS/SOHO instrument. The time period analyzed range from mid May 1996 to mid May 1997, when, at solar minimum, some features persist for several rotations, thus allowing to analyze the UV and visible emission as time series modulated at the period of the solar rotation. The coronal differential rotation rate significantly differs from that of the photospheric plasma. The estimated equatorial synodic rotation period of the corona at 1.5 R⊙ is 27.48±0.15 days. The study of the latitudinal variation shows that the UV corona decelerates towards the photospheric rates from the equator up to the poleward boundary of the mid‐latitude streamers, reaching a peak of 28.16±0.20 days around ±30° from the equator at 1.5 R⊙, while a less evident peak is observed in the northern hemisphere, suggesting a real north‐south rotational asymmetry, the northern hemisphere the rotation looks more solid‐body‐like and slower than in the southern hemisphere. The mid‐latitude results are also confirmed by the visible light data available at 1.75 and 2.0 R⊙. The study of the radial rotation profiles shows that the corona is rotating almost rigidly with height, but we find an abrupt increase by about half a days between 2.3 and 2.5 R⊙. The larger radial and latitudinal gradients of the rotation rates are localized at the boundary between the open and closed field lines, suggesting that in these regions the differential rotation might be a source of magnetic stress and, consequently, of energy release.

We construct models of the structure and evolution of the Sun which include rotation and magnetic fields. The effects of rotation are to generate magnetic fields, redistribute magnetic angular momentum in the solar interior and to mix the elements. The effects of the magnetic fields are the corrections to the thermodynamics variables. We investigate mainly the correlation between the interior rotational properties and magnetic fields inside the Sun. We find that the magnetic fields in the solar interior are strongly affected by the rotation rate and gradient.

For parameter values typical of the solar interior, turbulent eddy viscosity is shown to be negative, with turbulence enhancing the radial shear. Both magnetic diffusivity and thermal diffusivity are severely quenched by flow shear and Alfven‐gravity waves, with values much smaller than the magnitude of the eddy viscosity. The results imply that the solar tachocline is likely to develop too strong a radial shear to be a stationary Hartmann layer, exhibiting complex time variability.

High precision photometry and spectroscopy of low-mass stars reveal a variety of properties standard stellar evolution cannot predict. Rotation, an essential ingredient of stellar evolution, is a step towards resolving the discrepancy between model predictions and observations. The first rotating stellar model, continuously tracing a low-mass star from the pre-main sequence onto the horizontal branch, is presented. The predicted luminosity functions of globular clusters and surface rotation velocities on the horizontal branch are discussed., Comment: 5 pages, 6 figures, to be published in the Proceedings of the Conference 'Unsolved Problems in Stellar Physics', Cambridge, 2-6 July 2007

We study temporal variation in the solar angular momentum, J, rotational kinetic energy, T, and the rotational contribution to the gravitational multipole moments J2k of the Sun, inferred from helioseismic data from GONG and MDI over the last 11 years, covering most of the cycle 23. We have found that the variations in J and T at high latitudes (>45°) through the convection zone are correlated positively with solar activity, while at low latitudes they are correlated negatively, except for the top 10% of solar radius, where the correlation is positive.

The Jupiter system is a source of collimated burst‐like streams of electrically charged 10‐nm dust particles. In 2004 the Ulysses spacecraft had its second flyby at Jupiter and from late 2002 to early 2005 it measured a total of 24 dust streams between 0.8 and 3.4 AU from the planet. The grains show strong coupling to the interplanetary magnetic field: their impact directions correlate with the orientation and strength of the interplanetary magnetic field vector (namely its tangential and radial components) and they occur at 26 day intervals, closely matching the solar rotation period. Ulysses measured the dust streams over a large range in jovian latitude (+75° to −35°). Enhanced dust emission was measured along the jovian equator.

Solar Forcing on closed seas and Lakes is space time dependent. The Cipher of the Dead Sea level variation since 1200 BC is solved in the context of millenium and Wolf‐Gleissberg solar cycles time scales. It is found that the pattern of Dead Sea level variation follows the pattern of major millenium solar cycles. The 70 m rise of Dead Sea around 1AD is due to the forcing of the maximum millenium major solar cycle. Although the pattern of the Dead Sea level variation is almost identical to major solar cycles pattern between 1100 and 1980 AD, there is a dating problem of the Dead Sea time series around 1100–1300 AD that time. A discrepancy that should be corrected for the solar and Dead Sea series to fit.Detailed level variations of the Dead Sea level for the past 200 years are solved in terms of the 80–120 years solar Wolf‐Gliessberg magnetic cycles. Solar induced climate changes do happen at the turning points of those cycles. Those end‐start and maximum turning points are coincident with the change in th...

A global MHD model is developed to reproduce Ulysses observations during its fast latitude transition in 1994–1995. The governing polytropic single‐fluid MHD equations are solved for a steady coronal outflow. The model includes Alfven wave momentum and energy addition into open field regions. We combine a solution for a tilted dipole magnetic field in the inner computational region (1–20 R⊙) with a three‐dimensional solution in the outer region which extends to 1 AU. The inner region solution is essentially the same as in [1], but obtained with a different numerical algorithm and rotated to match the inclination inferred for the solar dipole from observations during the Ulysses transversal. The steady solution in the outer region is constructed by a marching‐along‐radius method and accounts for solar rotation. We show that the simulated variations of plasma and magnetic field parameters and in particular the extension of slow wind belt agree fairly well with the Ulysses observations.

The distribution of the magnetic field on the solar surface from 1975 up to 2002 is analyzed using the observations taken on the WSO observatory, revealing remarkable latitudinal zonal structure in both the northern and southern hemisphere. A preliminary model combining the dipole, quadrupole, octupole and other components for the longitude distribution is discussed. The active longitude problem has been investigated. The new active regions seem to appear with the sideral period of about 25.0 ± .1 days, corresponding to the solar rotation rate at 10 degree latitude at the 0.85±.03 solar radii.

We investigate the periodicity and recurrence of FeXIV and FeX emission structures with heliospheric latitude and distance above the Sun’s surface. The data was observed by the LASCO C1 coronagraph on board the SOHO spacecraft during the solar minimum activity from April 1996 to March 1997. For the green FeXIV emission line, reliable estimates of the solar rotation period could be obtained between −60 and 60 degrees in latitude and between 1.1 and 2 solar radii. Our investigation confirms the results of (3) of an almost ridgidly rotating equatorial streamer belt with a rotation period of about 27.5±0.5 days. Even coronal emissivity structures with a shorter life time between 14 and 27 days do not seem to rotate differentially. The FeX observations on the other hand do yield some indication of a reduced rotation for coronal structures over the solar poles. However, the error of the derived rotation period estimates is considerable so that the analysis of the FeX data does not allow a definite conclusion.

Although interplanetary scintillation (IPS) is a useful means to measure the solar wind in regions where spacecraft cannot access, the IPS measurement requires a line of sight integration to relate what is observed to a location in space. We have produced a Computer Assisted Tomography (CAT) program that optimizes a three-dimensional solar wind speed distribution to fit observed interplanetary scintillation data from STE Lab. Nagoya University. The program uses solar rotation and solar wind motion to provide 3-dimensional perspective views of each point in space accessible to the IPS observations and iterates to a least-square solution fit of the observations. We plot the optimized result as a Carrington map of solar wind speed at a height of 2.5 Rs and have confirmed (1) the solar wind near a solar minimum phase has a bimodal structure near the sun, that is, a low-speed region and a high-speed region are separated by a sharp speed gradient, (2) high-speed winds get their final speed of 750–800 km/s within 0.1 AU, and subsequently the evolution of solar wind structure is small at 0.1–1AU.

The origin of the 1/f low-frequency spectrum of fluctuations observed in the solar wind is discussed. It is suggested that this spectrum arises due to sampling random perturbations produced at the Sun. The perturbation process is characterized by a power-law distribution. It is pointed out that, due to solar rotation, the sampling is equivalent to scanning across different size scales on the Sun. The limits of the spectrum found from the observations are used to estimate the characteristic scales on the Sun, the scales related to the origin of the solar wind.

We have investigated energetic ions of non-corotating nature in the high latitude heliosphere. Major particle events were observed by Ulysses up to latitudes of 60 °S. All were associated with passage of coronal mass ejections (CMEs) over the spacecraft. The relationship of these events with solar activity was investigated using Yohkoh soft X-ray images.

The large-scale temperature structure of the low corona is investigated using synoptic temperature maps, derived from the intensity ratio of the green (Fe XIV) and red (Fe X) coronal lines as observed at the National Solar Observatory/Sacramento Peak. This intensity ratio is sensitive to coronal plasma with temperatures in the range of 1-2 MK. The synoptic maps indicate an association between high coronal temperature and the large-scale magnetic field. A comparison with WSO 'source surface' synoptic maps shows that especially when the heliospheric current sheet is stable over several rotations, the large-scale high-temperature features follow the current sheet remarkably well. For recent Carrington rotations temperature maps have been constructed for various heights between 1.15 and 1.45 solar radii. For these maps the correspondence with the current sheet (calculated at 2.5 solar radii) improves with height. Deviations between temperature structure and magnetic structure appears to be largest when the magnetic structure changes rapidly from rotation to rotation.

We discuss the latitude variation of the coronal hole differential rotation investigating persistent structures in high speed streams as observed from SWICS Ulysses during its first passage of the southern polar hole in 1993–1994. We find a slower rotation rate near the ecliptic than what is inferred from averaged photospheric features, e.g. from sunspots. At intermediate latitudes we find a rate similar to the equatorial rotation rate indicating a quasi-rigid rotation of the polar coronal hole. At latitudes >65° no persistent structures to determine the polar rotation have been observed. For the passage of the southern heliosphere in 1993/94 we find a latitudinal dependence of the sidereal rotation rate of the coronal hole which can be approximated by ωSW=[13.13+1.94 sin2(Θ)]°/day, where Θ denotes the solar latitude.

We present here observations of energetic ions during the following phases of the Ulysses prime mission: the first south polar pass, the low-latitude pass and part of the first north polar pass. Peaks are observed in the energetic ion intensity which recur either once per solar rotation during the ascent to high southern latitudes, or twice per rotation during the low latitude pass. The intensity of the peaks also rises with each major solar event, decaying slowly thereafter over a period of several rotations. The peaks are observed up to ∼70° during the ascent to high southern latitudes, but not seen again until around 45° during the descent, this asymmetry most likely being caused by a decrease in the number of solar events.

A simplified numerical model of Corotating Interaction Regions created by the solar wind together with the sun rotation and tilt of the current sheet is advanced. Forward and reverse shocks are identified and the frozen-in fields are computed.

Helioseismic studies have revealed that the only sharp change in the Sun’s internal rotation occurs near the interface between the convective zone and the radiative interior. This region is generally regarded as the source of the solar activity cycle. Other helioseismic clues to the properties of the interface concern the magnetic field and the temporal stability of rotation there.

A spatially variable coronal expansion, when coupled with solar rotation, leads to the formation of high speed solar wind streams which evolve considerably with increasing heliocentric distance. Initially the streams steepen for simple kinematic reasons, but this steepening is resisted by pressure forces, leading eventually to the formation of forward‐reverse shock pairs in the distant heliosphere. The basic physical processes responsible for stream steepening and evolution are explored and model calculations are compared with actual spacecraft observations of the process. Solar wind stream evolution is relatively well understood both observationally and theoretically. Tools developed in achieving this understanding should be applicable to other astrophysical systems where a spatially or temporally variable outflow is associated with a rotating object.