Solar wind spatial structure: The meaning of latitude gradients in observations averaged over solar longitude

This paper explores theoretical aspects of corotating solar wind dynamics on a global scale by means of numerical simulations executed with a nonlinear, inviscid, adiabatic, single-fluid, three-dimensional C3-D) hydrodynamic formulation. The study begins with a simple, hypothetical 3-D stream structure defined on a source surface located at 35 RS and carefully documents its evolution to 1 AU under the influence of solar rotation. By manipulating the structure of this prototype configuration at the source surface, it is possible to'elucidate the factors most strongly affecting stream evolution: 1) the intrinsic correlations among density, temperature, and velocity existing near the source; 2) the amplitude of the stream; 3) the longitudinal breadth of the stream; 4) the latitudinal breadth of the stream; and 5) the heliographic latitude of the centroid of the stream. The action of these factors is best understood in terms of momentum arguments-of relative simplicity and general application (to the extent that waves, conduction, and kinetic effects may be ignored).Corotating structure is viewed as a spiral standing wave (in the rotating frame) in which there is an ongoing competition between the kinematic tendency of the stream to steepen (as high-speed material overtakes slow) and the dynamical reaction of the gas to resist compression (through acceleration and tangential deflection of material by pressure gradients in the interaction region). Longitudinal gradients in the radial velocity distribution determine how fast material is brought into the interaction region, but the detailed momentum balance as a function of position within the stream dictates what happens when the material collides. Reasonable specifications of the five factors mentioned above can so affect this kinematic-dynamic balance that even a high-amplitude stream (e.g., peak-to-trough velocity differences at 1 AU o 480 km/s) may be prevented from shocking inside 1 AU, where the nonradial-flow broadening mechanism operates most efficiently. The nonlinear 3-D capabilities of the model allow quantitative study of the global development of this induced tangential flow in some detail. The nonradial motions lead to the net latitudinal transport of small amounts (a few percent) of mass, energy, and momentum. The effects of the latitudinal transport upon the evolution 2 within an east-west plane are minimal, and the latitudinal spreading of stream material in interplanetary spade, even in the presence of steep meridional gradients (up to 30 km/s/deg), is limited to a few degrees.Thus, for corotating structures, with their favorable spiral geometry, the 2-D approximation adequately describes the dynamical interaction. However, certain important research topics can only be approached in the full 3-D formulation. For example, the systematic pattern of the meridional flow changes across stream-fronts contain information on the 3-D structure of the stream and thus offers promise as a practical diagnostic tool. Also, since the magnetic field is tied ...

Section: Computational Resultsmentioning

confidence: 99%

A three‐dimensional model of corotating streams in the solar wind 2. Hydrodynamic streams

Pizzo

1980

“…However, the properties of the solar wind are supposed to be strongly related to the solar magnetic features and could as a consequence present a symmetry around an instantaneous magnetic axis. Such an axis cotfid be significantly tilted from the solar rotation axis [Hundhausen, 1978] -2(1 -A sin 2 •) where/1o represents the ionization rate at 1 AU from the sun in the solar equatorial plane, r is the radial distance in AU units, and A is a parameter (0 < A < 1) describing the degree of anisotropy. The/1o and the A are the two parameters which we propose to derive from a comparison with the data.…”

Section: -5 the Most Intricate Problem Is To Disentangle The Role Omentioning

confidence: 99%

Solar wind decrease at high heliographic latitudes detected from Prognoz interplanetary lyman alpha mapping

Lallement¹,

Bertaux²,

Kurt³

1985

105

New evidence for a latitudinal decrease of the solar wind mass flux is presented from observations of the interplanetary Lyman alpha emission collected in 1976 and 1977 with satellites Prognoz 5 and 6. The flow of interstellar hydrogen atoms in the solar system is ionized by EUV solar radiation and charge exchange with solar wind protons which accounts for about 80% of the total ionization rate. The resulting gradual decrease of the neutral H density from the upwind region down to the downwind region observed from Ly α intensity measurements allowed the determination of the absolute value of the total ionization rate β for one H atom at 1 AU against ionization. Collected in 1976 and 1977 at five places in the solar system, the measurements are first compared to a model which assumes isotropy of the EUV and solar wind. Strong departures are obvious toward high‐latitude regions, especially when the observer is in the downwind region where the solar wind ionization has had more time to act (cumulative effect). A model was constructed which included a decrease of the ionization rate with heliographic latitude. The adjustment of data allowed for the measurement of the absolute value of the total ionization rate and implies a 50% latitude decrease of the ionization rate due to charge exchange with the solar wind, from βsw = (3.9±0.5) × 10−8; s−1 at the equator to βsw = (2.0±0.5) × 10−8 s−1 at the pole. The corresponding absolute value of the solar wind proton flux is (2.4‐3.6) × 108 cm−2 s−1 at the equator and twice less at the pole if a constant velocity is assumed for the solar wind. Even if the solar wind velocity increases from 400 to 800 km s−1, which would decrease the charge exchange cross section by 25%, there is still a decrease by about 30% of the solar wind mass flux from equator to pole. The Lyman alpha data from Mariner 10 (Kumar and Broadfoot, 1978) had already shown a similar trend in 1974, showing a persistence of the solar wind anisotropy for 3–4 years during a solar minimum. The large‐scale properties of the solar wind mass flux can therefore be monitored at all latitudes by remote sensing of Ly α interplanetary emission, since the solar wind is carving the flow of interstellar H and its anisotropies are ''printed'' on the interplanetary H distribution. With uncalibrated Ly α measurements, an absolute value of the solar wind flux can be determined at all latitudes averaged over a typical 1‐year period.

“…The solar wind structure has often been studied by taking averages over heliographic longitude or heliomagnetic longitude [-Newkirk and Fisk, 1985, and references therein]. However, unless the solar wind spatial structure is simple and sym-metrical with respect to a rotational axis or a dipole axis, it is difficult to infer the solar wind structure from this longitudinal average [Hundhausen, 1978]. On the other hand, mapping the solar wind speed structure in the longitude and latitude coordinates can provide us with two-dimensional information without such ambiguities.…”

mentioning

confidence: 99%

Solar cycle evolution of solar wind speed structure between 1973 and 1985 observed with the interplanetary scintillation method

Kojima¹,

Kakinuma²

1987

132

The solar cycle evolution of solar wind speed structure was studied for the years from 1973 to 1985 on a basis of interplanetary scintillation observations using a new method for mapping solar wind speed to the source surface. The major minimum‐speed regions are distributed along a neutral line through the whole period of a solar cycle: when solar activity is low, they are distributed on the wavy neutral line along the solar equator; in the active phase they also tend to be distributed along the neutral line, which has a large latitudinal amplitude. The minimum‐speed regions tend to be distributed not only along the neutral line but also at low magnetic intensity regions and/or coronal bright regions which do not correspond to the neutral line. As the polar high‐speed regions extend equatorward around the minimum phase, the latitudinal gradient of speed increases at the boundaries of the low‐speed region, and the width of the low‐speed region decreases. One or two years before the minimum of solar activity, two localized minimum‐speed regions appear on the neutral line, and their locations are longitudinally separated by 180°.