Heliographic Latitudes Research Articles

Energetic particles during the first and third Ulysses southern high‐latitude excursions: Probing global corotating interaction region structure beyond 5 AU

We analyze energetic particle observations during the first and third ascents of Ulysses to the southern regions of the heliosphere. The first high‐latitude excursion occurred in the declining phase of solar cycle 22, while the third occurred in a later phase of solar cycle 23. In contrast to the long‐lived and well‐defined ∼26‐day recurrent intensity enhancements observed throughout the first southern excursion, observations during the third southern excursion showed a more variable structure. Transient events of solar origin were more abundant and intense. Corotating interaction regions (CIRs) were clearly present in intermittent sequences lasting only a few solar rotations. Within these sequences the relationships between solar wind structures and energetic particle events were remarkably similar to those observed in the steady recurrences of the first southern excursion. Energetic particles associated with well‐formed CIRs bounded by strong forward and reverse shock pairs were not observed until heliographic latitudes Λ ≳ 30°S (versus Λ ≳ 13°S in the first orbit). When Ulysses finally emerged into the fast (≳700 km s−1) solar wind at Λ ≥ 39°S, particle events were variable and either associated with weak reverse shocks or without solar wind signatures indicative of local CIRs (just as in solar cycle 22). The time‐intensity profiles in individual CIRs matched up well between the two southern excursions implying that essentially the same CIR structure existed in both solar cycles, albeit at a higher latitude in solar cycle 23 and only in the midst of the intermittent recurrent sequences.

Magnetically dominated structures as an important component of the solar wind turbulence

Abstract. This study focuses on the role that magnetically dominated fluctuations have within the solar wind MHD turbulence. It is well known that, as the wind expands, magnetic energy starts to dominate over kinetic energy but we lack of a statistical study apt to estimate the relevance of these fluctuations depending on wind speed, radial distance from the sun and heliographic latitude. Our results suggest that this kind of fluctuations can be interpreted as non-propagating structures, advected by the wind during its expansion. In particular, observations performed in the ecliptic revealed a clear radial dependence of these magnetic structures within fast wind, but not within slow wind. At short heliocentric distances (~0.3 AU) the turbulent population is largely dominated by Alfvénic fluctuations characterized by high values of normalized cross-helicity and a remarkable level of energy equipartition. However, as the wind expands, a new-born population, characterized by lower values of Alfvénicity and a clear imbalance in favor of magnetic energy becomes visible and clearly distinguishable from the Alfvénic population largely characterized by an outward sense of propagation. We estimate that more than 20% of all the analyzed intervals of hourly scale within fast wind are characterized by normalized cross-helicity close to zero and magnetic energy largely dominating over kinetic energy. Most of these advected magnetic structures result to be non-compressive and might represent the crossing of the border between adjacent flux tubes forming, as suggested in literature, the advected background structure of the interplanetary magnetic field. On the other hand, their features are also well fitted by the Magnetic Field Directional Turnings paradigm as proposed in literature.

Influence of solar flare's location and heliospheric current sheet on the associated shock's arrival at Earth

We study the source locations of 130 solar flare‐type II radio burst events with the associated interplanetary shocks observed by L1 spacecraft (type A events) and 217 flare‐type II events without such shocks observed at L1 (type B events) during February 1997–August 2002. In particular, we investigate the relative positions between the flare sources, the heliospheric current sheet (HCS), and the Earth. We found the following results: (1) Solar flares are usually distributed within [S30°, N30°] in heliographic latitude and [S30°, N30°] × [E10°, W30°] is the predominant source region on the solar disk that includes the majority of geoeffective solar flares. (2) The shocks with the associated flares located near the HCS would have a lower probably of reaching the Earth. For the Earth‐encountered shocks, their initial speeds are distinctly higher when their associated flares are located near the HCS. (3) The angular distance from the flare source to the Earth (defined as Ψ below) also contributes to the probability of the associated shock being observed at the Earth. The shock arrival probability decreases with the increment of Ψ and the mean initial shock speed increases with Ψ for those Earth‐encountered shocks. (4) The so‐called “same‐opposite side effect” of the HCS is confirmed to exist. That is, the shocks whose associated flares are located on the same side of the HCS as the Earth (called as “same side events”) have a greater chance of reaching the Earth than those shocks with their associated flares on the opposite side (“opposite side events”). Here for the first time, a comprehensive sample of solar transient events of both arriving and nonarriving ones (at Earth) is used to testify to the same‐opposite side effect. These results would be valuable in understanding the solar‐terrestrial relations, and helpful for space weather prediction.

Solar Wind Helium Abundance as a Function of Speed and Heliographic Latitude: Variation through a Solar Cycle

We present a study of the variation of the relative abundance of helium to hydrogen in the solar wind as a function of solar wind speed and heliographic latitude over the previous solar cycle. The average values of A(sub He), the ratio of helium to hydrogen number densities, are calculated in 25 speed intervals over 27-day Carrington rotations using Faraday Cup observations from the Wind spacecraft between 1995 and 2005. The higher speed and time resolution of this study compared to an earlier work with the Wind observations has led to the discovery of three new aspects of A(sub He), modulation during solar minimum from mid-1995 to mid-1997. First, we find that for solar wind speeds between 350 and 415 km/s, A(sub He), varies with a clear six-month periodicity, with a minimum value at the heliographic equatorial plane and a typical gradient of 0.01 per degree in latitude. For the slow wind this is a 30% effect. We suggest that the latitudinal gradient may be due to an additional dependence of coronal proton flux on coronal field strength or the stability of coronal loops. Second, once the gradient is subtracted, we find that A(sub He), is a remarkably linear function of solar wind speed. Finally, we identify a vanishing speed, at which A(sub He), is zero, is 259 km/s and note that this speed corresponds to the minimum solar wind speed observed at one AU. The vanishing speed may be related to previous theoretical work in which enhancements of coronal helium lead to stagnation of the escaping proton flux. During solar maximum the A(sub He), dependences on speed and latitude disappear, and we interpret this as evidence of two source regions for slow solar wind in the ecliptic plane, one being the solar minimum streamer belt and the other likely being active regions.

Interplanetary hydrogen absolute ionization rates: Retrieving the solar wind mass flux latitude and cycle dependence with SWAN/SOHO maps

We present results of the total hydrogen ionization rate obtained from the inversion of almost 10 years of full‐sky maps of the interplanetary Lyman α background measured by the SWAN instrument on SOHO. Thanks to a new estimate of the absolute calibration of the SWAN instrument and its variation during the 10 years of operation of SOHO, we are able to derive absolute values of the ionization rate as well as its latitudinal dependence. We show how the anisotropy of the ionization rate changes from solar minimum to solar maximum. At solar maximum, the so‐called ionization groove has completely disappeared and the ionizing fluxes are the same at all heliographic latitudes. We find that the hydrogen ionization cavity which surrounds the Sun increases in size with solar activity. This is evidenced by the low IP Lyman α intensities measured during and after the solar maximum. Our model calculation also shows that the increased radiation pressure is not sufficient to explain the larger cavity observed at solar maximum. We find also that ionization rates derived from in situ solar wind measurements do agree with the SWAN results at solar minimum but are significantly smaller at solar maximum. Derived in situ ionization rates do not show the solar cycle dependence we see from the SWAN data. In conclusion, we discuss possible explanations for this discrepancy.

Vector tomography for the coronal magnetic field

Aims. We study the possibility of applying tomographic techniques in order to reconstruct the 3D magnetic field configuration in the solar corona. Our simulations are based on data which can be obtained from longitudinal Zeeman-effect (or from Faraday rotation) measurements obtained during a solar rotation. Methods. The Zeeman effect provides essentially the integrated line-of-sight component of the magnetic field. The reconstruction problem relates to a family of similar problems termed vector tomography. For inversion of this type problems it is known that the curl-free part of a vector field cannot be reconstructed from the integrated along line-of-sight data. To remove the resulting ambiguity of the reconstruction we include the additional constraint, ∇· B = 0, into the inversion similar to smoothing constraints in Tikhonov regularization problems. Results. It is shown that Zeeman data obtained from vantage points in the ecliptic plane alone is sensitive only to certain magnetic field structures. For a full reconstruction it is necessary either to have also observations from viewing directions at higher heliographic latitudes, or to combine the longitudinal Zeeman-effect data with observations that provide the transverse component of the field, e.g. from the Hanle effect.

An overview of Jovian electrons during the distant Ulysses Jupiter flyby

Since the 1970's interplanetary electrons in the MeV energy range, of Jovian origin, have been extensively studied from close to the Sun to beyond the Kronian orbit, near the ecliptic. The Ulysses trajectory allowed to study the propagation of these particles, in a wide range of heliographic latitudes. The location of Jupiter with respect to the structure of the heliospheric magnetic field is precisely determined and non-central. This makes Jovian electrons an ideal opportunity for studying the particle propagation parallel and perpendicular to the heliospheric magnetic field. 12 years after its first encounter in February 1992, the Ulysses mission encountered Jupiter for a second time in February 2004 at a distance of 1684 Jovian radii. The first flyby took place at a distance of closest approach of 6 Jupiter radii ( R J ) and changed the inclination of the Ulysses trajectory so that it would pass above the Sun's polar regions. During the 2004 encounter, in contrast to 1992, Ulysses did not enter the Jovian magnetosphere but remained upstream of it. In mid 2002, the MeV electron flux started increasing and displaying large short term variations. These features lasted throughout the encounter, making the electron intensities less obviously correlated with the proximity to Jupiter compared with the first Jovian encounter. In previous studies it has been shown that the diffusion coefficient κ ⊥ , ϑ perpendicular to the heliospheric magnetic field in polar direction increased in 1998 during the transition from solar minimum to solar maximum close to the ecliptic plane. Although the distant Ulysses encounter took place during the declining phase of the solar cycle the absence of an intensity variation with latitude indicate an unexpected further increase of κ ⊥ , ϑ . Thus the diffusion coefficients, and in particular perpendicular diffusion in the polar direction, are highly time-dependent. In this paper, we present the corresponding data and discuss the implication for particle propagation in the three-dimensional heliospheric magnetic field.

Solar apparent radius variability: a new statistical approach to astrolabe multisite observations

The results of the analysis of solar diameter measurements obtained by means of the astrolabe have been inconclusive so far. Temporal variations are found, but they differ in amplitude and phase. This paper presents the results of a new statistical approach to compare and analyse the astrolabe data obtained during the period 1998-2003 in Brazil, Chile, France and Turkey. We show that different time series can be considered as extracted from the same statistical population following the Gaussian law. As following on a variographic analysis, the series do not seem to be autocorrelated at small scale (40 d) and present the characteristics of a highly disorganized phenomenon. At medium scale (6 yr), a 1-yr periodicity can be seen. It could be the residual trace of a temporal latitudinal variability. At large scale (up to 30 yr, using only French and Chilean data available, respectively, from 1975 and 1990), a faint 11-yr component in the signal can be pointed out. As an 11-yr periodicity has been found in the stratospheric data, we have explored the possibility of a relationship between the measured radii and stratospheric phenomena. This would explain the discrepancy in amplitudes, the sign of the crossed correlation coefficients, and the apparent disorganized behaviour of the data. Therefore, a complementary non-determinist analysis has been done in order to distinguish a chaotic signal from a random noise. The Hurst exponent establishes the persistence of the series. It can be concluded that the solar measurements from astrolabes are not purely randomly distributed, but show the character of a deterministic series for which (i) the observations need to be far more accurate, as it is only possible if obtained from space or in real-time together with a seeing monitor; (ii) supposing that it is indeed possible to extract a solar component, data must be studied considering their heliographic latitude and (iii) data show the variability of the upper atmospheric conditions (maybe Quasi-Biennial Oscillations) and the stratospheric 11-yr periodicity, for which the modulation is caused by the solar irradiance.

Solar modulation of galactic cosmic rays in the three-dimensional heliosphere according to meteorite data

Cosmogenic radionuclides with distinctive half-lives from chondritic falls were used as natural detectors of galactic cosmic rays (GCR). A unique series of uniform data was obtained for variations in the integral gradients of GCR with a rigidity of R > 0.5 GV in 1955–2000 on heliocentric distances of 1.5–3.3 AU and heliographic latitudes between 23° S and 16° N. Correlation analysis was performed for the variations in GCR gradients and variations in solar activity (number of sunspots, SS, and intensity of the green coronal line, GCL), the intensity of the interplanetary magnetic field (IMF), and the inclination of the heliospheric current sheet (HCS). Distribution and variations of GCR were analyzed in 11-year solar cycles and during a change in 22-year magnetic cycles. The detected dependencies of GCR gradients on the intensity of IMF and HCS inclination provided insight into the differences in the processes of structural transformation of IMF during changes between various phases of solar and magnetic cycles. The investigated relationships lead to the conclusion that a change of secular solar cycles occurred during solar cycle 20; moreover, there is probably still an increase in the 600-year solar cycle, which can be among the major reasons for the observed global warming.

The Latitudinal Excursion of Coronal Magnetic Field Lines in Response to Differential Rotation: MHD Simulations

Solar energetic particles, which are believed to originate from corotating interacting regions (CIRS) at low heliographic latitude, were observed by the Ulysses spacecraft even as it passed over the Sun's poles. One interpretation of this result is that high-latitude field lines intercepted by Ulysses connect to low-latitude CIRs at much larger heliocentric distances. The Fisk model explains the latitudinal excursion of magnetic field lines in the solar corona and heliosphere as the inevitable consequence of the interaction of a in a differentially rotating photosphere with rigidly rotating coronal holes. We use a time-dependent three-dimensional magnetohydrodynamic (MHD) algorithm to follow the evolution of a simple model of the solar corona in response to the differential rotation of the photospheric magnetic flux. We examine the changes of the coronal-hole boundaries, the redistribution of the line-of-sight magnetic field, and the precession of field lines in the corona. Our results confirm the basic idea of the Fisk model, that differential rotation leads to changes in the heliographic latitude of magnetic field lines. However, the latitudinal excursion of magnetic field lines in this simple tilted dipole model is too small to explain the Ulysses observations. Although coronal holes in our model rotate more rigidly than do photospheric features (in general agreement with observations), they do not rotate strictly rigidly as assumed by Fisk. This basic difference between our model and Fisk's will be explored in the future by considering more realistic magnetic flux distributions, as observed during Ulysses polar excursions.

Cyclic variations in the differential rotation of the solar corona

The rotation of the solar corona is analyzed using the original database on the brightness of the FeXIV 530.3 nm coronal green line covering six recent activity cycles. The rate of the differential rotation of the corona depends on the cycle phase. In decay phases, there are only small differences in the rotation, which are similar to that of a rigid body. The differences are more significant (though less pronounced than in the photosphere) during rise phases, just before maxima, and sometimes at maxima. The total rate of the coronal rotation is represented as a superposition of two, i.e., fast and slow modes. The synodic period of the fast mode is approximately 27 days at the equator and varies slightly with time. This mode displays weak differences in rotation and is most pronounced in the middle of decay phases. The slow mode is manifested only at high latitudes during the rise phases of activity, and displays a mean period of 31 days. The relative contribution of each mode to the total rotational rate is determined as a function of time and heliographic latitude. These results indicate that the structure of the velocity field in the convective zone must also vary with time. This conclusion can be verified by helioseismology measurements in the near future.

Time-dependent 3D modulation of Jovian electrons

We report on the modelling of the time-dependent transport of a few MeV electrons in the heliosphere. These low-energetic electrons, observed with the Kiel Electron Telescope (KET) on board the Ulysses spacecraft, are for the first time simulated over a complete solar activity cycle on the basis of a time-dependent three-dimensional modulation model. It is demonstrated that the solar activity has an effect especially at high heliographic latitudes being observed by Ulysses. The findings of the Ulysses mission lasting almost 15 years now provide a substantial data base for this study. Both galactic cosmic rays and the Jovian electron source are considered for the simulation of the electron fluxes within the heliosphere. Originating from these simulations two models for the time-dependent solar wind expansion are suggested, formulated and tested. Both characterise the varying velocity field of the solar wind over a solar cycle. A time-dependent anisotropic diffusion tensor was also tested in order to figure out from KET observations the variation of the release rate of low-energetic electrons from the Jovian magnetosphere.

ICMEs at High Latitudes and in the Outer Heliosphere

Interplanetary coronal mass ejections (ICMEs) propagate into the outer heliosphere, where they can have a significant effect on the structure, evolution, and morphology of the solar wind, particularly during times of high solar activity. They are known to play an important role in cosmic ray modulation and the acceleration of energetic particles. ICMEs are also believed to be associated with the large global transient events that swept through the heliosphere during the declining phases of solar cycles 21 and 22. But until recently, little was known about the actual behavior of ICMEs at large heliographic latitudes and large distances from the Sun. Over the past decade, the Ulysses spacecraft has provided in situ observations of ICMEs at moderate heliographic distances over a broad range of heliographic latitudes. More recently, observations of alpha particle enhancements, proton temperature depressions, and magnetic clouds at the Voyager and Pioneer spacecraft have begun to provide comparable information regarding the behavior of ICMEs at extremely large heliocentric distances. At the same time, advances in modeling have provided new insights into the dynamics and evolution of ICMEs and their effects on cosmic rays and energetic particles.

Axially symmetric large-scale transport of the magnetic flux during solar activity cycle

Transport of the solar background large-scale magnetic regions is followed between individual consecutive magnetic synoptic charts derived from observing data of Kitt Peak NSO. During many solar rotations the horizontal magnetic flux displacement was described by large-scale horizontal transport velocities, inferred in many points over the whole solar photosphere. Large-scale transport velocities contains from both axially symmetric and non-axially symmetric components. The first one describes zonal and meridional global transport studied in time interval during three last solar activity cycles. Cycle dependent global velocities are found as values varying in heliographic latitude and in the phase of the solar cycle.

Quasi-Periodic Oscillations of Microwave Emission in Solar Active Regions

Abstract In this paper we discuss QPOs registered at microwaves using the Nobeyama Radioheliograph (NoRH). Our main conclusions concerning the QPO, found from NoRH observations, include: In addition to widely known 3-min oscillations and comparatively less investigated shorter fluctuations, we have registered over sunspots at microwave range also a diversity of more long-term periodic processes having typical periods of tens and hundreds of minutes. In some sunspots these periods are dominating. In a single active region, different plasma structures can simultaneously oscillate within a long-term range with different periods. Most of the different plasma structures of the solar atmosphere registered at the Nobeyama radio maps at a wavelength of 1.76 cm at all heliographic latitudes show the presence of the long-term QPO. In observations of the QPO at microwaves, we have dealt mostly with nonstationary processes. We have thus used wavelet analysis to estimate the quality of proper plasma resonators responsible for QPOs. In our interpretation, three types of plasma oscillators were proposed to be of importance: resonators that coincide with the emitting region; resonators that are outside, but close to the radio emitter; and resonators of the global solar nature. We conclude that NoRH has opened a new era in the study of different quasi-periodic oscillations and waves in the plasma structures of the solar atmosphere.

Ulysses observations of Jovian relativistic electrons in the interplanetary space near Jupiter: Determination of perpendicular particle transport coefficients and their energy dependence

Intensities of ∼1–10 MeV relativistic electrons in several energy channels of the high-energy telescope (HET) on Ulysses increase dramatically during its flybys of Jupiter in 1992 and 2004. Perpendicular diffusion coefficients of these particles are derived by fitting the spatial profile of Jovian electron intensity to a diffusion-convection model of particle transport. It is found that the latitudinal diffusion coefficient during the 2004 Jupiter flyby has to be enhanced from its value during the 1992 Jupiter flyby and it is also enhanced relative to the radial perpendicular diffusion. Such an enhancement of latitudinal particle transport was implied previously through the observations of Jovian electrons, cosmic rays and solar energetic particles at high heliographic latitudes, and now this requirement extends further to low latitude region of the heliosphere. Energy dependence of the perpendicular diffusion coefficient is obtained quite precisely through the variation in the slope of energy spectrum of Jovian electrons. The perpendicular diffusion coefficient increases with energy, which can put a tight constraint on models of the particle transport coefficient. The newest nonlinear guiding center (NLGC) theory of perpendicular diffusion is consistent with this observation, but only when it is combined with a parallel diffusion coefficient from the quasilinear theory in a slab magnetic turbulence without dynamic damping.

North-south distribution of solar flares during cycle 23

In this paper, we investigate the spatial distribution of solar flares in the northern and southern hemispheres of the Sun that occurred during the period 1996 to 2003. This period of investigation includes the ascending phase, the maximum and part of the descending phase of solar cycle 23. It is revealed that the flare activity during this cycle is low compared to the previous solar cycle, indicating the violation of Gnevyshev-Ohl rule. The distribution of flares with respect to heliographic latitudes shows a significant asymmetry between northern and southern hemisphere which is maximum during the minimum phase of the solar cycle. The present study indicates that the activity dominates the northern hemisphere in general during the rising phase of the cycle (1997–2000). The dominance of northern hemisphere shifted towards the southern hemisphere after the solar maximum in 2000 and remained there in the successive years. Although the annual variations in the asymmetry time series during cycle 23 are quite different from cycle 22, they are comparable to cycle 21.

A Flux Tube Solar Dynamo Model Based on the Competing Role of Buoyancy and Downflows

A magnetic flux tube can be considered both as a separate body and as a confined field. As a field, it is affected by both differential rotation (Ω-effect) and cyclonic convection (α-effect). As a body, the tube experiences not only a buoyant force, but also a dynamic pressure due to downflows above the tube. These two competing dynamic effects are incorporated into the α-Ω dynamo equations through the total magnetic turbulent diffusivity, leading to a flux tube dynamo operating in the convection zone. We analyze and solve the extended dynamo equations in the linear approximation by adopting the observed solar internal rotation and assuming a downflow effect derived from numerical simulations of a solar convection zone. The model reproduces the 22 yr cycle period; the extended butterfly diagram with the confinement of strong activity to low heliographic latitudes |Φ| ≤ 35°; the evidence that at low latitudes the radial field is in an approximately π phase lag compared to the toroidal field at the same latitude; the evidence that the poleward branch is in a π/2 phase lag with respect to the equatorward branch; and the evidence that most of the magnetic flux is present in an intermittent form, concentrated into strong flux tubes.

Solar diameter measurements at São Paulo Observatory

In this work we describe solar radius measurements made from 1972 with the Sao Paulo astrolabe. We find values of 959.52 ± 0.03 arcsec for the visual data and 959.61 ± 0.05 arcsec for the CCD data using a modified data acquisition system. We compare our results with other astrolabe measurements and with SOHO-MDI. Observations of the solar radius show contradictory results when we try to correlate changes in the diameter with the solar cycle. Our series is not correlated with the solar cycle but it shows a period of 13.4 ± 2.2 yr. We have also compared our time series with other long-term measurements of solar radius along solar cycle 21 and analysed our measurements as a function of heliographic latitude. We do not find a significant solar oblateness.

Injection and propagation of solar protons to high heliospheric latitudes: Ulysses Ket observations

The Ulysses spacecraft at high heliographic latitudes has detected eight large solar energetic particle events during the recent solar maximum in 2000 and 2001 years. These events are easily identified with famous episodes of the solar activity and the corresponding SEP events near the Earth. Analyzing the absolute intensities of ∼40–100 MeV proton time profiles from the Ulysses COSPIN/KET and the GOES detector near Earth we find that for these eight events the intensities vary only within a factor of 2–3 from event to event at high latitudes during the first 30 h. In contrast the intensities at Earth differ from one event to another by several orders of magnitude. Thus, the time history during these 30 h has only a weak dependence on the relative position of Ulysses to the possible flare location. This implies: (1) a nearly isotropic injection of protons with comparable intensities to high heliolatitudes, and (2) a similar propagation process during the first 30 h of the events. We attribute the fact that the decay phases of these events are nearly the same at Ulysses and in the ecliptic rather to cross-field diffusion than to the presence of a shock.