L. A. Fisk

Composition of quasi‐stationary solar wind flows from Ulysses/Solar Wind Ion Composition Spectrometer

Using improved, self-consistent analysis techniques, we determine the average solar wind charge state and elemental composition of nearly 40 ion species of He, C, N, O, Ne, Mg, Si, S, and Fe observed with the Solar Wind Ion Composition Spectrometer on Ulysses. We compare results obtained during selected time periods, including both slow solar wind and fast streams, concentrating on the quasi-stationary flows away from recurrent or intermittent disturbances such as corotating interaction regions or coronal mass ejections. In the fast streams the charge state distributions are consistent with a single freezing-in temperature for each element, whereas in the slow wind these distributions appear to be composed of contributions from a range of temperatures. The elemental composition shows the well-known first ionization potential (FIP) bias of the solar wind composition with respect to the photosphere. However, it appears that our average enrichment factor of low-FIP elements in the slow wind, not quite a factor of 3, is smaller than that in previous compilations. In fast streams the FIP bias is found to be yet smaller but still significantly above 1, clearly indicating that the FIP fractionation effect is also active beneath coronal holes from where the fast wind originates. This imposes basic requirements upon FIP fractionation models, which should reproduce the stronger and more variable low-FIP bias in the slow wind and a weaker (and perhaps conceptually different) low-FIP bias in fast streams. Taken together, these results firmly establish the fundamental difference between the two quasi-stationary solar wind types.

Motion of the footpoints of heliospheric magnetic field lines at the Sun: Implications for recurrent energetic particle events at high heliographic latitudes

The interplay between the differential rotation of the footpoints of heliospheric magnetic field lines in the photosphere and the subsequent nonradial expansion of these same field lines with the solar wind from rigidly rotating coronal holes can result in extensive excursions of heliospheric magnetic field lines with heliographic latitude. Thus magnetic field lines at high latitudes can be connected directly to corotating interactions regions (CIRs) in the solar wind at low latitudes at larger heliocentric distances. A model is presented which illustrates that this effect can account for the observations from the Ulysses spacecraft that recurrent energetic particle events, which appear to originate in low-latitude CIRs, occur at the highest latitudes observed. The model also has implications for the observation that the heliospheric magnetic field appears to have a tighter than expected spiral angle at high latitudes over the south solar pole and for the modulation of galactic cosmic rays.

Acceleration of interstellar pickup ions in the disturbed solar wind observed on Ulysses

Acceleration of interstellar pickup H+ and He+ as well as of solar wind protons and alpha particles has been observed on Ulysses during the passage of a corotating interaction region (CIR) at ∼4.5 AU. Injection efficiencies for both the high thermal speed interstellar pickup ions (H+ and He+) and the low thermal speed solar wind ions (H+ and He++) are derived using velocity distribution functions of protons, pickup He+ and alpha particles from < 1 to 60 keV/e and of ions (principally protons) above ∼60 keV. The observed spatial variations of the few keV and the few hundred keV accelerated pickup protons across the forward shock of the CIR indicate a two stage acceleration mechanism. Thermal ions are first accelerated to speeds of 3 to 4 times the solar wind speed inside the CIR, presumably by some statistical mechanism, before reaching higher energies by a shock acceleration process. Our results also indicate that (1) the injection efficiencies for pickup ions are almost 100 times higher than they are for solar wind ions, (2) pickup H+ and He+ are the two most abundant suprathermal ion species and they carry a large fraction of the particle thermal pressure, (3) the injection efficiency is highest for protons, lowest for He+, and intermediate for alpha particles, (4) both H+ and He+ have identical spectral shapes above the cutoff speed for pickup ions, and (5) the solar wind frame velocity distribution function of protons has the form F(w) = F0w−4 for 1 < w < ∼5, where w is the ion speed divided by the solar wind speed. Above w ∼ 5-10 the proton spectrum becomes steeper. These results have important implications concerning acceleration of ions by shocks and CIRs, acceleration of anomalous cosmic rays, and particle dynamics in the outer heliosphere.

Investigation of the Composition of Solar and Interstellar Matter Using Solar Wind and Pickup Ion Measurements with SWICS and SWIMS on the Ace Spacecraft

The Solar Wind Ion Composition Spectrometer (SWICS) and the Solar Wind Ions Mass Spectrometer (SWIMS) on ACE are instruments optimized for measurements of the chemical and isotopic composition of solar and interstellar matter. SWICS determines uniquely the chemical and ionic-charge composition of the solar wind, the thermal and mean speeds of all major solar wind ions from H through Fe at all solar wind speeds above 300 km s−1 (protons) and 170 km s−1 (Fe+16), and resolves H and He isotopes of both solar and interstellar sources. SWICS will measure the distribution functions of both the interstellar cloud and dust cloud pickup ions up to energies of 100 keV e−1. SWIMS will measure the chemical, isotopic and charge state composition of the solar wind for every element between He and Ni. Each of the two instruments uses electrostatic analysis followed by a time-of-flight and, as required, an energy measurement. The observations made with SWICS and SWIMS will make valuable contributions to the ISTP objectives by providing information regarding the composition and energy distribution of matter entering the magnetosphere. In addition, SWICS and SWIMS results will have an impact on many areas of solar and heliospheric physics, in particular providing important and unique information on: (i) conditions and processes in the region of the corona where the solar wind is accelerated; (ii) the location of the source regions of the solar wind in the corona; (iii) coronal heating processes; (iv) the extent and causes of variations in the composition of the solar atmosphere; (v) plasma processes in the solar wind; (vi) the acceleration of particles in the solar wind; (vii) the physics of the pickup process of interstellar He in the solar wind; and (viii) the spatial distribution and characteristics of sources of neutral matter in the inner heliosphere.

The Common Spectrum for Accelerated Ions in the Quiet-Time Solar Wind

Suprathermal tails, formed in part from accelerated interstellar pickup ions, are observed ubiquitously in the solar wind. In quiet-time conditions, the tails are found to have a common spectral shape, a distribution function in velocity space that is a power law with a spectral index of -5. It is shown that such a spectral index is to be expected if the tails are formed by stochastic acceleration due to compressional turbulence in the solar wind, in which the particles are accelerated by, and do an equal amount of work on, the turbulence. The spectrum is formed by a cascade in energy, analogous to turbulent cascades.

On the Coronal Magnetic Field: Consequences of Large-Scale Motions

A model has been introduced for the magnetic field in the heliosphere in which the field lines execute large excursions in heliographic latitude. The excursions result from the interplay between the differential rotation of the photosphere and the nonradial expansion of the solar wind near the Sun. The model accounts for the observed ease with which low-rigidity particles propagate in latitude in the solar wind. In this paper the consequences of this model for the behavior of the coronal magnetic field are explored. It is pointed out that the model provides an explanation for the time evolution and apparent rigid rotation of polar coronal holes and the differences between fast and slow solar wind.

Iron charge distribution as an identifier of interplanetary coronal mass ejections

We present solar wind Fe charge state data measured on the Advanced Composition Explorer (ACE) from early 1998 to the middle of 2000. Average Fe charge states in the solar wind are typically around 9 to 11. However, deviations from these average charge states occur, including intervals with a large fraction of Fe 16 which are consistently associated with interplanetary coronal mass ejections (ICMEs). By studying the Fe charge state distribution we are able to extract coronal electron temperatures often exceeding 2 10 6 kelvins. We also discuss the temporal trends of these events, indicating the more frequent appearance of periods with high Fe charge states as solar activity increases.

Elemental fractionation in the slow solar wind

Slow solar wind, found at low heliographic latitudes, and fast solar wind, associated with high-latitude coronal holes, are fundamentally different: slow solar wind is more variable and is biased in elements with low first ionization potentials (FIPs), whereas fast solar wind is steady and has at most limited FIP bias. It has been recently argued that these differences are consequences of a continuous reorganization of the Suns magnetic field described by a new heliospheric magnetic field model. This continuous reorganization requires a sustained reconnection process at low latitudes between open magnetic field lines and large coronal loops. Arguably, the slow solar wind originates from material stored on large coronal loops which is released sporadically because of reconnection. The fast solar wind is released on continuously open magnetic field lines. In this paper a theory for FIP fractionation is developed which depends on the wave heating of minor ions which extends below the transition region. The wave heating may be natural on the closed field configurations of large coronal loops but not in the open configurations associated with fields emanating from coronal holes. Therefore, the theory naturally leads to a differentiation in FIP fractionation between fast and slow solar wind.

An empirical study of the electron temperature and heavy ion velocities in the south polar coronal hole

The solar wind ions flowing outward through the solar corona generally have their ionic fractions ‘freeze-in’ within 5 solar radii. The altitude where the freeze-in occurs depends on the competition between two time scales: the time over which the wind flows through a density scale height, and the time over which the ions achieve ionization equilibrium. Therefore, electron temperature, electron density, and the velocity of the ions are the three main physical quantities which determine the freeze-in process, and thus the solar wind ionic charge states. These physical quantities are determined by the heating and acceleration of the solar wind, as well as the geometry of the expansion. In this work, we present a parametric study of the electron temperature profile and velocities of the heavy ions in the inner solar corona. We use the ionic charge composition data observed by the SWICS experiment on Ulysses during the south polar pass to derive empirically the electron temperature profile in the south polar coronal hole. We find that the electron temperature profile in the solar inner corona is well constrained by the solar wind charge composition data. The data also indicate that the electron temperature profile must have a maximum within 2 solar radii. We also find that the velocities of heavy ions in their freeze-in regions are small (<100 km s-1) and different elements must flow at different velocities in the inner corona.

The solar wind and suprathermal ion composition investigation on the wind spacecraft

The Solar Wind and Suprathermal Ion Composition Experiment (SMS) on WIND is designed to determine uniquely the elemental, isotopic, and ionic-charge composition of the solar wind, the temperatures and mean speeds of all major solar-wind ions, from H through Fe, at solar wind speeds ranging from 175 kms−1 (protons) to 1280 kms−1 (Fe+8), and the composition, charge states as well as the 3-dimensional distribution functions of suprathermal ions, including interstellar pick-up He+, of energies up to 230 keV/e. The experiment consists of three instruments with a common Data Processing Unit. Each of the three instruments uses electrostatic analysis followed by a time-of-flight and, as required, an energy measurement. The observations made by SMS will make valuable contributions to the ISTP objectives by providing information regarding the composition and energy distribution of matter entering the magnetosphere. In addition SMS results will have an impact on many areas of solar and heliospheric physics, in particular providing important and unique information on: (i) conditions and processes in the region of the corona where the solar wind is accelerated; (ii) the location of the source regions of the solar wind in the corona; (iii) coronal heating processes; (iv) the extent and causes of variations in the composition of the solar atmosphere; (v) plasma processes in the solar wind; (vi) the acceleration of particles in the solar wind; and (vii) the physics of the pick-up process of interstellar He as well as lunar particles in the solar wind, and the isotopic composition of interstellar helium.