Element Abundances And The Source Of Solar Energetic Particles

The Sun can emit intense bursts of high-energy particles that can fill a large volume of the heliosphere and last several days.  The largest of these solar-energetic-particle (SEP) events are accelerated at fast shock waves driven out from the Sun by coronal mass ejections (CMEs).  These shock waves randomly sample and accelerate all the chemical elements they find as they cross the solar corona.

When these SEPs arrive near Earth, we can measure the relative abundances of all the chemical elements from the dominant H and He, through the intermediate C, O, and Fe and even up to the heavy elements like Pb and Au.  The fundamental abundance pattern of elements in the solar photosphere is modified (1) once as they are transported up into the corona before acceleration, and (2) again as they scatter and stream away from the shock along magnetic field lines after acceleration.  The first process tells us about the physical processes that form the corona; the second process actually tells us the temperature of the source plasma sampled by the shock.

The solar photosphere is nearly a cool 4000^o K so that elements with a first ionization potential (FIP) below about 10 eV (e.g. Mg, Si, Fe) are singly ionized while those with FIP above 10 eV (e.g. He, O, Ne) are neutral atoms.  Plasma waves flowing up into the corona help push the ions, but not the neutrals, so that the so-called “FIP effect” can result in an enhancement of low-FIP over the high-FIP elements by a factor of about 3.  Once the elements reach the corona they become highly ionized because the coronal temperatures are 1 to 3 million degrees Kelvin (MK).  In fact, the average charge Q of each element depends strongly upon temperature.

After acceleration, SEPs are scattered by magnetic fluctuations as they flow outward.  At a given velocity, the scattering of ions depends upon their mass-to-charge ratio A/Q, so that high-mass, low-charge ions spread rapidly, and abundance of an element is enhanced in some places and depressed in others, relative to the coronal abundance.  Thus, since Fe scatters less than O, we see Fe/O is enhanced early in events and depleted later.  At any place and time this enhancement or suppression forms a power law, i.e. the log of the enhancement has a simple linear relationship with the log of A/Q.  Since the temperature determines the Q values of the elements, we can scan through to find the best-fit temperature for the observed enhancement pattern in a SEP event.  Temperatures in coronal active regions are hotter than those found elsewhere in the corona, for example.

However, the element He is special, it has the highest value of FIP = 24.6 eV, and it is very slow to ionize as it is transported into the corona.  Some SEP events are found to have suppressed values of He that do not fit either the FIP pattern or the power-law in A/Q of all the other elements.  These events may be sampled from new channels on the fringes of active regions where the He has not yet reached equilibrium.

These new results for He and the new technique for measuring temperature are beginning to help us understand the source of SEP acceleration and the structure and physics of the corona. Measurements of SEP element abundances are a powerful new tool in this process.

References:

• Reames, D.V.: The two sources of solar energetic particles, Space Sci. Rev. 175, 53 (2013) doi: 10.1007/s11214-013-9958-9
• Reames, D.V., Temperature of the source plasma in gradual solar energetic particle events, Solar Phys., 291 911 (2016a) doi: 10.1007/s11207-016-0854-9 (arXiv: 1509.08948)
• Reames D.V., Solar Energetic Particles, Springer, Berlin, (2017) ISBN 978-3-319-50870-2, doi: 10.1007/978-3-319-50871-9

These findings are described in the article entitled The Abundance of Helium in the Source Plasma of Solar Energetic Particles, published in the journal Solar Physics. This work was led by Donald Reames from the University of Maryland.