The Solar Wind's Fractionation: First Observation of a Mass-Dependent Effect|Mass-independent fractionation|Towards Understanding Mass-Dependent Fractionation of Solar Wind Isotopic Compositions

 The Solar Wind's Fractionation: First Observation of a Mass-Dependent Effect

About this Research

The sun is made up largely of hydrogen (by mass, 90%), a little amount of helium (10%), and all the other naturally occurring elements in the periodic table (around 1%). Although every element in the periodic table is present in the solar wind, the proportions are varied. For instance, the solar wind has a helium abundance that is two times lower than that of the sun. Fractionation is the term for the difference in an element's abundance between the sun and solar wind. It has long been understood that the fractionation of various elements does not occur at random but rather follows a pattern that may be seen. In other words, the elements that are more willing to give up an electron have a greater abundance in the solar wind than those that are less willing to do so.

For example, the energy required to ionize neon is three times greater than that required to ionize iron, and as a result, neon has around one-third less iron in the solar wind than is present in the sun. It has been the subject of research for more than three decades and is referred to as the First Ionization Potential (or FIP) effect.

The fact that the high-speed solar wind typically has abundances closer to solar values than the low-speed solar wind (i.e., is less fractionated) provides another hint about the issue of solar wind fractionation. Due to this, it has been hypothesized that whatever process is in charge of fractionation needs time to work, and that it may have less time to do so in the coronal hole regions of the sun, where the fast wind originates, than in the so-called interstream regions, where the slow wind originates.

Researchers have long looked for alternative abundance trends, particularly those that depend on atomic mass, that might provide insight into solar wind fractionation. Telescopic views of huge, heavy-ion-poor coronal loops have revealed mass-dependent effects, but no mass-dependent effects have ever been detected by solar wind-measuring instruments. Fortunately, this is no longer true. We have examined fractionation as a function of solar wind speed using newly re-analyzed data from the SWICS sensor on board the Advanced Composition Explorer, or ACE, spacecraft. The amount that fractionation varies with solar wind speed is found to be dependent on the mass of an element. The change in fractionation rate with speed for a particular element is plotted in the figure below as a function of mass.

We compare the other elements using the fractionation versus speed for magnesium as the reference. We observe that the difference in fractionation vs. speed increases as the mass difference between a specific element and magnesium increases.

Since this approach only considers relative masses, more research is necessary to identify whether the heavier elements or the lighter ones exhibit a bigger fractionation change with speed. Nevertheless, the fact that there is such a distinct reliance on mass indicates that the elemental mass does in fact affect the solar wind fractionation process. The physics governing the fractionation of elements heavier than hydrogen between the Sun and the solar wind should become clearer with an understanding of this effect. In the end, it might answer the more fundamental query of what processes are driving the solar wind's acceleration in the first place.