Stream Interfaces Research Articles

Solar Origin and Interplanetary Evolution of Stream Interfaces

In a Corotating Interaction Region (CIR) the stream interface is identified as a relatively sharp density drop, temperature rise, and flow shear in the solar wind, and is now generally believed to mark the boundary between solar wind which was originally slow when it left the Sun and solar wind which was originally fast. This paper summarises some important facts and open questions about the origin and nature of the boundary between fast and slow solar wind near the Sun, the evolution of stream interfaces with heliocentric distance in the inner heliosphere, and their relationship.

THERMODYNAMIC CONSTRAINTS ON NITROGENTRANSFORMATIONS AND OTHER BIOGEOCHEMICALPROCESSES AT SOIL–STREAM INTERFACES

There is much interest in biogeochemical processes that occur at the interface between soils and streams since, at the scale of landscapes, these habitats may function as control points for fluxes of nitrogen (N) and other nutrients from terrestrial to aquatic ecosystems. Here we examine whether a thermodynamic perspective can enhance our mechanistic and predictive understanding of the biogeochemical function of soil–stream interfaces, by considering how microbial communities interact with variations in supplies of electron donors and acceptors. Over a two-year period we analyzed >1400 individual samples of subsurface waters from networks of sample wells in riparian wetlands along Smith Creek, a first-order stream draining a mixed forested–agricultural landscape in southwestern Michigan, USA. We focused on areas where soil water and ground water emerged into the stream, and where we could characterize subsurface flow paths by measures of hydraulic head and/or by in situ additions of hydrologic tracers. We found strong support for the idea that the biogeochemical function of soil–stream interfaces is a predictable outcome of the interaction between microbial communities and supplies of electron donors and acceptors. Variations in key electron donors and acceptors (NO3−, N2O, NH4+, SO42−, CH4, and dissolved organic carbon [DOC]) closely followed predictions from thermodynamic theory. Transformations of N and other elements resulted from the response of microbial communities to two dominant hydrologic flow paths: (1) horizontal flow of shallow subsurface waters with high levels of electron donors (i.e., DOC, CH4, and NH4+), and (2) near-stream vertical upwelling of deep subsurface waters with high levels of energetically favorable electron acceptors (i.e., NO3−, N2O, and SO42−). Our results support the popular notion that soil–stream interfaces can possess strong potential for removing dissolved N by denitrification. Yet in contrast to prevailing ideas, we found that denitrification did not consume all NO3− that reached the soil–stream interface via subsurface flow paths. Analyses of subsurface N chemistry and natural abundances of δ15N in NO3− and NH4+ suggested a narrow near-stream region as functionally the most important location for NO3− consumption by denitrification. This region was characterized by high throughput of terrestrially derived water, by accumulation of dissolved NO3− and N2O, and by low levels of DOC. Field experiments supported our hypothesis that the sustained ability for removal of dissolved NO3− and N2O should be limited by supplies of oxidizable carbon via shallow flowpaths. In situ additions of acetate, succinate, and propionate induced rates of NO3− removal (∼1.8 g N·m−2·d−1) that were orders of magnitude greater than typically reported from riparian habitats. We propose that the immediate near-stream region may be especially important for determining the landscape-level function of many riparian wetlands. Management efforts to optimize the removal of NO3− by denitrification ought to consider promoting natural inputs of oxidizable carbon to this near-stream region.

Solar wind stream interfaces in corotating interaction regions: SWICS/Ulysses results

Motivated by the well‐known differences in charge state and even elemental composition of the slow solar wind as compared to high‐speed streams, we have analyzed observations made by the Solar Wind Ion Composition Spectrometer (SWICS)Ulysses mass spectrometer of the series of corotating interaction regions (CIRs) encountered by Ulysses in 1992/1993 at 4.5–5.4 AU from the Sun. Stream interfaces (SIs) are first identified within each CIR using the kinetic parameters (density and thermal speed) of the main components, protons and alpha particles. We then show that SIs are also the location of the expected compositional changes in the heavy ion components, such as the freezing‐in temperatures of C and O and the strength of FIP fractionation as given by the Mg/O abundance ratio, to within the time resolution with which these parameters may be obtained by SWICS. The changes in freezing‐in temperatures are so clear that they in turn may be used as a robust indicator of the SI location and even reveal that some of the SI crossings were in fact multiple. We conclude that the heavy ion composition of the solar wind changes abruptly at the SI and remains well preserved out to large heliocentric distances.

Stream interfaces and energetic ions II: Ulysses test of Pioneer results

Ulysses measurements of energetic and solar wind particles taken near 5 AU between 20 and 30 degrees south latitude during a well‐developed recurring corotating interaction region (CIR) show that the CIR's corotating energetic ion population (CEIP) associated with the trailing reverse shock starts within the CIR at the stream interface. This is consistent with an earlier result obtained by Pioneers 10 and 11 in the ecliptic plane between 4 and 6 AU. The Ulysses/Pioneer finding is noteworthy since the stream interface is not magnetically connected to the reverse shock, but lies 12–17 corotation hours from it. Thus, the finding seems to be inconsistent with the basic model that generates CEIP particles at the reverse shock and propagates them along field lines. Eliminating the inconsistency probably entails an extension of the standard model such as cross‐field diffusion or a non‐shock energization process operating near the stream interface closer to the sun.

Stream interfaces and energetic ions closer than expected: Analyses of Pioneers 10 and 11 observations

An empirical study of corotating interaction regions (CIRs) observed between 3.9 AU and 5.9 AU on Pioneers 10 and 11 shows that the main corotation energetic ion population (CEIP), which is associated with the trailing reverse shock, terminates within the CIR at a definite, structural boundary, which we show here is the stream interface. This new result has significant implications for solar wind and energetic particle modeling. In particular it implies either that the reverse shock forms closer to the stream interface than models suggest or that the theories that treat the generation and transport of these energetic ions, such as preshock Fermi acceleration and cross‐field diffusion must be combined or extended. We test these scenarios by comparing the CEIP intensity profiles on the two sides of the stream interface. We find that while each automatically accounts for one or two aspects of the results none of them alone can account for all of our empirical results.

Three views of two giant streams: Aligned observations at 1 AU, 4.6 AU, and 5.9 AU

A close radial alignment of the IMP and Pioneers 10 and 11 spacecraft in 1974 allows a nearly unambiguous, empirical study of the radial evolution of the interaction regions of two contrasting weak and strong, giant streams. The study confirms the main aspects of the standard model of corotating interaction regions: an expanding and strengthening pair of forward‐reverse shocks sandwich a stream interface. It adds the following concepts: stream group speed — the speed at the stream interface tends to remain constant with distance; corotating stream complexes — interaction regions can include features like noncompressive density enhancements and streamer belts; secondary interfaces — a possible precursor to the reverse shock; and emerging stream interfaces — one emerged between 1 AU and 4.6 AU. The study uses the conservation specific entropy to correlate features between spacecraft.

Solar wind stream interfaces

Measurements with Los Alamos Scientific Laboratory instrumentation aboard Imp 6, 7, and 8 reveal that approximately one third of all high‐speed solar wind streams observed at 1 AU contain a sharp boundary (of thickness less than ∼4 × 104 km) near their leading edge, called a stream interface, which separates plasma of distinctly different properties and origins. Identified as discontinuities across which the density drops abruptly, the proton temperature increases abruptly, and the speed rises, stream interfaces are remarkably similar in character from one stream to the next. A superposed epoch analysis of plasma data has been performed for 23 discontinuous stream interfaces observed during the interval March 1971 through August 1974. Among the results of this analysis are the following: (1) a stream interface separates what was originally thick (i.e., dense) slow gas from what was originally thin (i.e., rare) fast gas; (2) the interface is the site of a discontinuous shear in the solar wind flow in a frame of reference corotating with the sun; (3) stream interfaces occur at speeds less than 450 km s−1 and close to or at the maximum of the pressure ridge at the leading edges of high‐speed streams; (4) a discontinuous rise by ∼40% in electron temperature occurs at the interface; and (5) discontinuous changes (usually rises) in alpha particle abundance and flow speed relative to the protons occur at the interface. Stream interfaces do not generally recur on successive solar rotations, even though the streams in which they are embedded often do. At distances beyond several astronomical units, stream interfaces should be bounded by forward‐reverse shock pairs; three of four reverse shocks observed at 1 AU during 1971–1974 were preceded within ∼1 day by stream interfaces. Although stream interfaces often occur in close proximity to reversals in direction of the interplanetary magnetic field, the field reversals generally precede the interfaces by 1½ hours to 1½ days. Approximately 40% of stream interfaces at 1 AU produce si− in the geomagnetic field. It has been suggested previously that stream interfaces are the result of the nonlinear evolution of high‐speed streams produced by a smoothly varying temperature elevation in the solar envelope. Our measurements appear to contradict this thesis: many streams do not appear to be smoothly varying close to the sun, and a temperature elevation in the solar envelope is not required to explain the abrupt jumps in plasma properties which occur at stream interfaces. Our observations suggest that many streams close to the sun are bounded on all sides by large radial velocity shears separating rapidly expanding plasma from more slowly expanding plasma. The shear at the leading edge of such streams becomes the stream interface observed at 1 AU; however, momentum transfer across the interface reduces the magnitude of the speed jump across the shear with increasing distance from the sun. The large abrupt increases in electron and proton temperature observed at interfaces are primarily a consequence of the fact that interfaces separate what was originally thick (i.e., dense) slow gas from what was originally thin (i.e., rare) fast gas.

A model for the origin of solar wind stream interfaces

The basic variations in solar wind properties that have been observed at 'stream interfaces' near 1 AU are explained by a gas dynamic model in which a radially propagating stream, produced by a temperature variation in the solar envelope, steepens nonlinearly while moving through interplanetary space. The region thus identified with the stream interface separates the ambient solar wind from the fresh hot material originally in the stream. However, the interface regions given by the present model are thicker than most stream interfaces observed in the solar wind, a fact suggesting that some additional physical process may be important in determining that thickness. Variations in the density, speed, or Alfven pressure alone appear not to produce streams with such an interface.

Interplanetary stream interfaces

At l AU there is a distinct boundary (the stream interface) at the leading edge of a stream in the solar wind, characterized by an abrupt drop in density, a similar increase in temperature and a small increase in speed. It is suggested that stream interfaces form in the interplanetary medium as a consequence of the non-linear evolution of streams generated by an increase in temperature in the solar envelope. This evolution eventually leads to the formation of a reverse shock behind the interface and a forward shock ahead of it. Two instances in which both a stream interface and a reverse shock had developed at l AU are presented. Examples of flare generated shocks which passed through a stream and were observed near a stream interface are also presented. It is shown that stream interfaces are definitely not the same structures as piston boundaries. It is noted that slow shocks, like stream interfaces, always occur ahead of streams and may develop in the interplanetary medium.