193 research outputs found
On the evolution of outward and inward Alfvénic fluctuations in the polar wind
No full textPlasma and magnetic field measurements by Ulysses are used to investigate the radial evolution of hourly-scale Alfvénic fluctuations in the polar wind. The data span from 1.4 to 4.3 AU in heliocentric distance. Different radial regimes at different distances emerge. Inside about 2.5 AU the large outward traveling fluctuations decrease faster, in terms of energy per unit mass, than the small inward ones. This is in agreement with previous low-latitude observations inside 1 AU within the trailing edge of fast streams. As a result of this different gradient the ratio of inward to outward fluctuation energy rises to about 0.5 near 2.5 AU. Beyond this distance the radial gradient of the inward fluctuations becomes increasingly steeper, while that of the outward ones does not vary appreciably. A state is quickly reached where both populations decline at almost the same rate. These results on the behavior of outward and inward Alfvénic fluctuations are new and represent a constraint for models of turbulence evolution in steadily expanding flows like the polar wind. Finally, an extrapolation to regions near the Sun would suggest that Alfvénic fluctuations at hourly scale should not play a relevant role in solar wind heating and acceleration. Obviously, this last conclusion may be invalidated by non-WKB effects and by compressive and dissipative processess close to the Sun
Modeling the cyclic modulation of photospheric lines
No full textWe have studied the behavior of three photospheric lines (Fe I 537.9, C I 538.0 and Ti II 538.1 nm), which have been monitored on the Sun for more than twenty years, either as full-disk or as center-disk measurements (Gray & Livingston 1997; Livingston & Wallace 2003). The aim is to detect a possible photospheric variation with the cycle. We try to reconstruct the cyclic variations of full-disk line depths as due to active region (AR) modulation through a spectral synthesis with FAL semi-empirical models (Fontenla et al. 1999) weighted by AR coverage factors. The sensitivity of these lines to thermodynamic variations and to AR presence is analyzed. We show that the AR modulation alone cannot explain all the observational results, either in amplitude or in phase. The "residual", i.e. the difference between observed behavior of these three lines at full-disk and that predicted by models for the AR modulation, results in a signal that is correlated with the measured center-disk line variations, and should be free from magnetic effects. Both the full-disk and the center-disk data show several periodicities; furthermore there are two periodicities shared by the three lines, one close to the 11 yr magnetic cycle and the other of 2.8 yr
The solar wind alpha-particle content as a clue for the origin of slow flows
No full textSome aspects of the variation of the alpha-particle abundance in the solar wind are studied using Ulysses data. We focus on the first mid-latitude phase of the mission, when the spacecraft intermittently enters the fast high-latitude wind, and on the region around the first perihelion passage, when low-latitude wind in conditions of low solar activity is observed. In particular, we look for the existence of a relationship between the alpha-particle abundance and the solar wind velocity. No evidence of correlation is found for the midlatitude wind, while at low latitudes a positive correlation emerges. This difference may be indicative of a different relevance, in the two regions, of the processes that can be at the origin of low-speed solar wind. At mid latitudes the low velocity of the wind would be mainly due to the topology of the solar magnetic field, with a large expansion factor. Near the equator this effect would become weaker and a dominant role would be played by source region features related to the alpha-particle content
Line depth variations along the solar cycle: a biennial periodicity?
No full textWe study the behaviour of three photospheric lines (Fe I 537.9, C I 538.0 and Ti II 538.1 nm), monitored on the Sun since 1978. The aim is to detect photospheric variations with the cycle. We reconstruct the cyclic variations of full-disk line depths as due to the active regions (ARs). We show that ARs alone cannot account all the observational results. The differences between observed behaviour and the AR contribution correlate with the measured center-disk line variations, and a common periodicity of ̃ 2.7 yr is present
Cross-helicity and residual energy in solar wind turbulence: radial evolution and latitudinal dependence in the region from 1 to 5 AU
No full textSolar wind plasma and magnetic field measurements by Ulysses have been used to study MHD turbulence in different heliospheric regions. Four intervals of six solar rotations have been analyzed. Two of them are on the ecliptic around 2 and 5 AU, respectively, one is at midlatitude near 5 AU, and the last one is at high latitude around 3 AU. Conditions on the ecliptic are those typical of high solar activity periods. The midlatitude interval is characterized by very strong gradients in the wind speed, due to an intermittent appearance of the wind coming from the polar coronal hole. In the high-latitude interval, fully inside the polar wind, the speed is steadily high. We investigated at three different scales the level of correlation between velocity and magnetic field fluctuations, as given by the normalized cross-helicity, and the sharing of the fluctuation energy between its kinetic and magnetic component, as measured by the normalized residual energy. The observations on the ecliptic, while confirming previous findings based on Voyagers data, clearly indicate that the normalized crosshelicity is well different from zero also at distances as large as 5 AU. The midlatitude turbulence, when compared to that at low and high heliographic latitudes, appears much more evolved, with a remarkably lower normalized cross-helicity. This unambiguously highlights that processes at velocity gradients are an important factor in the turbulence evolution
Alfvenic fluctuations in "newborn" polar solar wind
No full textThe 3-D structure of the solar wind is strongly dependent: upon the Sun's activity cycle. At low solar activity a bimodal structure is dominant, with a fast and uniform flow at the high latitudes, and slow and variable flows at low latitudes. Around solar maximum, in sharp contrast, variable flows are observed at all latitudes. This last kind of pattern, however, is a relatively short-lived feature, and quite soon after solar maximum the polar wind tends to regain its role. The plasma parameter distributions for these newborn polar flows appear very similar to those typically observed in polar wind at low solar activity. The point addressed here is about polar wind fluctuations. As is well known, the low-solar-activity polar wind is characterized by a strong flow of Alfvenic fluctuations. Does this hold for the new polar flows too? An answer to this question is given here through a comparative statistical analysis on parameters such as total energy, cross helicity, and residual energy, that are of general use to describe the Alfvenic character of fluctuations. Our results indicate that the main features of the Alfvenic fluctuations observed in low-solar-activity polar wind have been quickly recovered in the new polar flows developed shortly after solar maximum
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