Typical Solar Wind Research Articles

Isotopic Fractionation in Slow and Coronal Hole Associated Solar Wind

Data from the Mass and Charge Time-of-Flight spectrometers MTOF and CTOF on board SOHO have been accumulated for time periods in which coronal hole and non-coronal hole type plasma has been detected by using different methods in order to determine the abundance ratios of magnesium isotopes in these two different source regions of the solar wind. As indicators for the two different solar wind type plasma we have used several plasma parameters such as, the freeze-in temperatures, the Fe/O and the He/H ratio, which are known as reasonable indicators. For this study we have used the refractory element Mg which is an authentic whitness for matter in the solar nebula because temperatures of the Sun never have been high enough to change their isotopic composition by nuclear burning. In addition Mg has three isotopes with abundance ratios greater than about 10% and therefore they are easy to observe.

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.

Effects of the Sun on the Earth's environment

Solar disturbances are observed to have significant effects in near-Earth space. Over the past half-century of observation, a relatively clear picture has developed of how and why the typical solar wind — as well as the most extreme solar events — drive geospace responses. It is clear that magnetospheric substorms, geomagnetic storms (both recurrent and aperiodic events), and even certain atmospheric chemical changes have their origins in the solar–terrestrial coupling arena. High-speed solar wind streams and fast coronal mass ejections (CMEs) can often have strong interplanetary shock waves and southward magnetic fields which can initiate strong storm responses. We demonstrate in this review that available modern space-observing platforms and ground facilities allow us to trace drivers from the Sun to the Earth's atmosphere. This allows us to assess quantitatively the energy transport that occurs throughout the Sun–Earth system during both typical and extreme conditions. Hence, we are continuously improving our understanding of “space weather” and its effects on human society.

The solar wind as a possible source of lunar polar hydrogen deposits

We investigate the solar wind as a source for the deposits of hydrogen at the lunar poles. We create a Monte Carlo model that simulates the migration of atmospheric particles through the lunar exosphere. Making the model general enough to incorporate any physical process that might affect the particles, we develop a tool that estimates the number and form of particles that reach and stick to lunar cold traps. Each particle is allowed to follow a series of ballistic trajectories as it hops around the surface of the Moon. We trace the path of the particle until it is removed from the system by photo‐processes such as ionization or dissociation, by thermal escape, or by reaching a cold trap. Accumulating statistics on the outcomes for various input particles, we determine the amount and form of hydrogen able to migrate to the lunar cold traps over time for a typical solar wind input flux. We find that although the fraction of hydrogen delivered to the Moon that ultimately reaches the poles is small, a slow steady source like the solar wind has provided enough hydrogen over 83 Myr to account for the observed deposits. Also, we find that an enrichment in the [D/H] ratio occurs by the migration process. The amount of fractionation is dependent on the molecular form of the migrating hydrogen. Atomic deuterium/hydrogen is enriched by a factor of 4 over the delivered fraction by the migration process.

Two‐dimensional MHD simulation of the solar wind interaction with magnetic field anomalies on the surface of the Moon

Two‐dimensional magnetohydrodynamic simulations of the solar wind interaction with the magnetized regions on the surface of the Moon suggest “mini‐magnetospheres” can form around the regions on the Moon when the magnetic anomaly field strength is above 10 nT at 100 km above the surface (for a surface field strength of 290 nT) and when the solar wind ion density is below 40 cm−3, with typical observations placing anomalous magnetic field strengths around 2 nT at 100 km above the surface. The results suggest that not only can a bow shock and magnetopause form around the small anomalies, but their position and shape can change dramatically with changes in the solar wind conditions. A switch from southward to northward interplanetary magnetic field (IMF) causes the size of the mini‐magnetosphere to increase by 90% and the magnetic field at various positions inside the bow shock to increase by a factor of 10. In addition to affecting the stand‐off distance, changes in the IMF can also cause the mini‐magnetosphere to go from very round to flat and elongated. The scale size of the mini‐magnetospheres is 100 km for the range of typical solar wind conditions and the surface magnetic field strengths measured by Lunar Prospector. A stagnation point inside the shock region also exists for several solar wind conditions.

First detection of global dawn‐dusk ionospheric current intensities using Ampère's integral law on Ørsted orbits

The magnetic measurements by the Ørsted satellite in noon‐midnight orbits have enabled the derivation of the global dawn‐dusk oriented ionospheric currents from an Ampère's law closed loop line integral of the geomagnetic vector field along the satellite track. The globally integrated dawn‐to‐dusk ionospheric current is found to be proportional to the geo‐effective solar wind electric field and is around 1 million ampère for a typical solar wind electric field of 2 mV/m. Dividing the Ampère integral into semi‐orbit parts has enabled us to show that the hemispherical total current intensities depend on the respective polar cap conductivities, which relate to the daily and seasonally varying solar illumination. The more illuminated hemisphere conveys up to three times more current from dawn to dusk than does the less illuminated.

The suprathermal seed population for corotating interaction region ions at 1 AU deduced from composition and spectra of H+, He++, and He+ observed on Wind

We have measured H+, He++, and He+ distribution functions over the solar wind through the suprathermal energy range during two corotating interaction region (CIR) events observed by the STICS, MASS, and STEP instruments on board the Wind spacecraft at 1 AU during April and May 1995. The major properties we find are as follows: In the suprathermal energy range (∼10–500 keV/nucleon), the particle intensities peak inside the CIR itself, in the compressed and decelerated fast solar wind, in contrast to the situation at MeV energies, where the peak intensities are observed outside the CIR in the fast solar wind. The distribution functions of solar wind H+ and He++ change smoothly from the core at solar wind speeds to a power law or exponential form at higher energies, with no turnover observed at intermediate energies. CIR He+ is observed with an abundance ratio He+/He++ ∼ 16–17%, orders of magnitude higher than that in the bulk solar wind but nevertheless lower than that observed in CIRs at 4.5 AU. The H+, He++, and He+ spectra have similar slopes above speeds of ∼2.5–3 times the solar wind speed (Vsw) in the spacecraft frame. The ion speed at which the CIR He++/H+ ratio changes from typical solar wind values of 4–5% to the higher (>10%) value typical of CIRs is ∼1.5–1.7 Vsw, measured in the space‐craft reference frame. Analyzing these observations in the context of previous global observations and simple models of CIR acceleration and transport [Fisk and Lee, 1980], we conclude the following: (1) Suprathermal CIR ions at 1 AU originated close (within ∼0.5 AU) to the point of observation, not in the outer heliosphere; (2) the injection/acceleration mechanism is not especially sensitive to charge‐to‐mass ratio over the range 0.25–1.0; (3) since the particles are locally accelerated, the low‐energy ion populations we observe contain the seed population; (4) the bulk solar wind itself is not the source of the energetic ions; rather, the source is in the suprathermal tail, with an injection threshold in the spacecraft frame of ∼1.8–2.5 times the solar wind speed; and (5) in at least one of these CIRs, suprathermal particle acceleration is not shock associated and must therefore be associated with a statistical mechanism or compression in the solar wind.

Theoretical properties of electromagnetic ion cyclotron waves in the terrestrial, dayside, low‐latitude plasma depletion layer under uncompressed magnetosheath conditions

We present a numerical study of electromagnetic ion cyclotron wave (EICW) activity (growth and damping) in the terrestrial plasma depletion layer (PDL) under typical solar wind conditions. Aside from the α particles, all parameters in this study are as measured by AMPTE/IRM during 13 magnetosheath passes under low magnetic shear made in 1984–1985 and calculated with the superposed epoch analysis technique [Phan et al., 1994]. Three locations within the PDL are specified using minutes before the key time identifying the magnetopause crossing in the superposed epoch analysis (O, —5, —10 min). Our work thus characterizes average properties of EICWs along radial profiles in the PDL in the magnetic latitude and local time ranges of ±30° and 0800 to 1600 hours, respectively. In order to study the situation in the subsolar region more closely, we calculate also with PDL parameters acquired by AMPTE/IRM during the pass nearest to local noon (October 24, 1985). For α particle parameters we take reasonable estimates but also study the effect of varying the α particle temperature anisotropy. The influence of a small α particle‐proton relative drift is also included. At the magnetopause we find one peak of activity in the frequency range from 0.2 to 0.3 Ωp (the proton cyclotron frequency), which is thus below the resonance frequency of the α particles (0.5 Ωp). At locations 10 min and 5 min prior to the magnetopause crossing, the activity spectrum bifurcates, with both peaks below the a resonance and separated by an absorption band. Our explanation of this result is that as we move away from the magnetopause, the increasing plasma β enables the protons to overcome the α absorption. With a relative α particle‐proton drift of 10% of the local Alfvén speed, the absorption band is removed, but all excited frequencies are still less than 0.5 Ωp. The absorption band may also be removed by a high α particle thermal anisotropy coupled with a low α particle β along the magnetic field. On October 24, 1985, the PDL was characterized by a wider variation of β and a larger proton temperature anisotropy. Under these conditions a second EICW activity peak appears between 0.5 and 0.6 Ωp at the two locations away from the magnetopause, in addition to the α peak mentioned above, which we ascribe to the larger anisotropy of the protons. Again, a small α particle drift can remove the activity minimum. With the stagnation line flow pattern in the PDL found by AMPTE/IRM on these passes, we hypothesize that EICWs excited near noon subsequently travel along the magnetic field, bringing EICW activity to high latitudes. However, EICWs generated away from noon will be damped out while being carried to the magnetopause flanks.

Determination of the abundance of aluminum in the solar wind with SOHO/CELIAS/MTOF

The Al/Mg abundance ratio provides an excellent test case for investigating possible fractionation processes among low First lonization Potential (FIP) elements in the solar wind. Al and Mg are refractory elements; their abundance ratio has been well determined in solar system materials and inferences for the abundance ratio in the solar atmosphere are reliable. Al and Mg are at neighboring masses and have similar charge state properties in the solar corona; hence mass fractionation effects in the solar wind acceleration process and instrumental mass fractionation are minimal. From first observations during two relatively short periods, one recorded in coronal hole associated solar wind, the other in typical interstream solar wind, it is concluded that the solar wind ratio in both regimes is consistent with the solar system ratio. The Al/Mg ratio in interstream solar wind is 0.081±0.012, and in the sample of coronal hole associated solar wind it amounts to 0.076±0.011. A comparison of these results with the solar system ratio of 0.079±0.005 gives no indication for fractionation occurring among low FIP elements in the solar wind.

Interaction of Mercury with the Solar Wind

We present the structure of the hermean magnetosphere obtained by a global three-dimensional MHD simulation. The magnetic field of Mercury is strong enough to form a permanent magnetosphere under typical solar wind conditions. Mercury does not have a substantial atmosphere or ionosphere which makes the magnetosphere unique in the solar system. We study in detail the hermean magnetosphere for the typical solar wind parameters at perihelion and the nominal Parker spiral interplanetary magnetic field (IMF). Although the magnetosphere of Mercury is qualitatively similar to Earth's magnetosphere, its much smaller size results in many quantitative differences. For example, the magnetic field lines of Mercury are closed only for latitudes less than ≈50°, while for Earth the similar latitude will be ≈75°. We find that a direct interaction of the solar wind with the surface of the planet and the formation of a “bald” subsolar spot become possible if the solar wind speed is increased by a factor of 2.5 of if density is increased by a factor of 9 as compared with the nominal values. We present the results of our simulation for this case a well, although our conclusion is that direct interaction of Mercury with the solar wind is a rare phenomenon.

Coupling between high and low latitudes as observed with LASCO in the solar corona and in interplanetary space

Abstract From Skylab, Helios and Ulysses observations we had learnt already how closely the 3D heliosphere is related to the underlying coronal structure. Around solar activity minimum, large polar coronal holes dominate the major part of the heliosphere, through the high-speed solar wind streams emanating from them. A different type of solar wind is restricted to a narrow near-equatorial belt (about 30 degrees in latitude). The magnetic topology is dominated by strong multipole components and multiple current sheets in the upper corona, and by a large-scale dipole field further outside, respectively. New observations by LASCO on SOHO cover both regions and may reveal clues to the processes responsible for the generation of the different types of solar wind.

Heliospheric 3D structure and CME propagation as seen from SOHO: Recent lessons for space weather predictions

Observations from Skylab, Helios, Ulysses, and SOHO have demonstrated how closely the 3D heliosphere is related to the underlying coronal structure. Around solar activity minimum, the large polar coronal holes dominate the major part of the heliosphere, through the high-speed solar wind streams emanating from them. A distinctively different type of solar wind is restricted to a narrow near-equatorial belt (about 30 0 in latitude). Its magnetic topology is dominated by strong multipole components and multiple current sheets. Owing to the new telescopes on SOHO, various effects of CME disturbances propagating through the heliosphere can now be observed in much greater detail: optically from the photosphere out to 32 R s, and later on by in situ spacecraft. It appears that the prediction reliability of space weather at Earth's orbit can be raised substantially in the near future.

Here comes Solar Probe!

Despite recent advances, fundamental questions remain about the nature of the solar corona and the solar wind: 1) What heats the corona and accelerates the solar wind? 2) Where do the different types of solar wind originate? 3) Where and how are energetic particles produced and transported near the Sun? 4) What role do plasma turbulence and waves play in the corona and solar wind production? 5) What is the nature of the magnetic field and photospheric structures near the solar poles? Flying a trajectory perpendicular to the Earth-Sun line during its perihelion passage, Solar Probe will use in-situ and imaging instruments to provide the first three dimensional viewing of the corona, direct observations of solar polar regions, and local sampling of the solar environment. These primary observations are complemented by context-setting measurements and Earth-based observations. Solar Probe is currently scheduled for launch in February 2007 as the third in the new Outer Planets/Solar Probe mission line of NASA and will arrive at the Sun in 2010 under solar maximum conditions with a second closest approach near solar minimum in 2015.

New observations of heavy‐ion‐rich solar particle events from ACE

Following launch of the Advanced Composition Explorer in August 1997, the Solar Isotope Spectrometer measured the composition of nine solar energetic particle events. We have used isotopic measurements of Ne to determine the degree of charge‐to‐mass‐dependent fractionation and infer the charge states of C‐Ni in the four most heavy‐ion‐rich of the nine events. The results indicate a source temperature of ∼4×106 K; this and the measured abundances suggest that these four events are more characteristic of impulsive events than gradual. Although the ³He/4He ratios are not enhanced to the level commonly ascribed to impulsive events, there are sizable enhancements over typical solar wind values measured in three of the events.

Solar Wind Models from the Sun to 1 AU: Constraints by in situ and Remote Sensing Measurements

There are three major types of solar wind: The steady fast wind originating on open magnetic field lines in coronal holes, the unsteady slow wind coming probably from the temporarily open streamer belt and the transient wind in the form of large coronal mass ejections. The majority of the models is concerned with the fast wind, which is, at least during solar minimum, the normal mode of the wind and most easily modeled by multi-fluid equations involving waves. The in-situ constraints imposed on the models, mainly by the Helios (in ecliptic) and Ulysses (high-latitude) interplanetary measurements, are extensively discussed with respect to fluid and kinetic properties of the wind. The recent SOHO observations have brought a wealth of new information about the boundary conditions for the wind in the inner solar corona and about the plasma conditions prevailing in the transition region and chromospheric sources of the wind plasma. These results are presented, and then some key questions and scientific issues are identified.

Solar wind acceleration by large‐amplitude nonlinear waves: Parametric study

We investigate the parametric dependence of the solar wind acceleration by large‐amplitude nonlinear (LAN) magnetohydrodynamic waves. For this purpose we model numerically the self‐consistent problem of the solar wind with waves by solving time‐dependent, nonlinear, resistive 2.5‐dimensional (three‐dimensional with azimuthal symmetry) MHD equations driven by Alfvén waves. We find that when the Alfvén wave amplitude is above a parameter‐dependent threshold, LAN waves are generated in the model coronal hole. For typical coronal parameters the solar wind speed and density fluctuate considerably on a timescale of ∼10–40 min and with an amplitude of up to several hundred kmilometers per second near the Sun (r ≲ 10 RS) in agreement with recent interplanetary scintillation observations. The solar wind speed is inversely dependent on the driving frequency in the range 0.35–3 mHz. The amplitude of the velocity fluctuations increases with the amplitude of the magnetic field and the driving Alfvén waves at the base of the corona and decreases with the coronal temperature. We found that for the same typical solar wind and Alfvén wave parameters and an isothermal initial atmosphere, the WKB model predicts 30% higher flow velocities far from the Sun (32 RS) than our self‐consistent wave model with high‐frequency Alfvén waves (ƒ = 2.78 mHz), conforming to the WKB approximation. However, our model predicts significantly higher average flow speed near the Sun. When low‐frequency non‐WKB waves drive the wind, our model predicts 25% higher solar wind speed than the WKB model far from the Sun. This result of our model is in agreement with linear studies of solar wind acceleration by Alfvén waves that take into account Alfvén wave reflection.

Characteristics of magnetic fluctuations within coronal mass ejections: The January 1997 event

We determine the geometry of the fluctuations of the magnetic field at frequencies just above the proton gyrofrequency for the January 10, 1997 CME and the magnetic cloud within. The transverse magnetic fluctuations represent a greater fraction of the magnetic energy than is the case in the typical undisturbed solar wind. The break in the power spectrum that is associated with the the onset of magnetic dissipation falls within the frequency range of interest. The fluctuation geometry is markedly different above and below the spectral break frequency. The inertial range geometry remains unchanged in the cloud with only ∼30% of the energy associated with field‐aligned wave vectors. The dissipation range wave vectors are highly oblique to the mean magnetic field B with up to 96% of the energy associated with oblique wave vectors.

Elemental composition of the January 6, 1997, CME

Using solar wind particle data from the CELIAS/MTOF sensor on the SOHO mission, we studied the abundance of the elements O, Ne, Mg, Si, S, Ca, and Fe for the time period around the January 6, 1997, coronal mass ejection event (CME). In the interstream and coronal hole regions before and after this event we found elemental abundances consistent with the expected abundance patterns of the respective flow regimes. However, during the passage of the CME and during the passage of the erupted filament, which followed the CME, we found that the elemental composition differed markedly from the interstream and coronal hole regions before and after this event. During the passage of the CME and the passage of the erupted filament we found a mass‐dependent element fractionation, with a monotonic increase toward heavier elements. We observed Si/O and Fe/O abundance ratios of about one half during these time periods, which is significantly higher than for typical solar wind.

Diffusive Shock Acceleration Of Electrons At An Interplanetary Shock Observed On 21 Feb 1994

For the diffusive shock acceleration process to work, we need both upstream and downstream scattering centers. For ions of energies > ∼ several tens of keV, the necessary scattering centers are identified as magneto-hydrodynamic (Alfven) waves, whose frequency determined by the cyclotron resonance condition, (1) becomes ∼ MeV can resonate with and be scattered by Alfven waves of the frequency range similar to the above waves. Electrons of surpathermal energies (< ∼ several tens of keV), on the other hand, can resonate only with waves of much higher frequencies (1–10 Hz), which are no longer on the Alfven wave branch but on the whistler wave branch. Difficulty arises from the matching condition on the helicities of waves: To resonate with electrons streaming away from a shock front, waves should have the right-hand helicity (RHH). Since whistler waves propagating away from the shock purely parallel to the background magnetic field have a right-hand polarization but a left-hand helicity (LHH), we should take into account of obliquely propagating whistler waves which consist of both LHH and RHH. However, since the RHH component has amplitude much less than that of LHH component, the wave-particle cyclotron interaction between upstream going electrons and oblique whistler waves can occur with much reduced efficiency. Because of this reason, Levinson (1992) obtained a high threshold Mach number for a MHD shock to be able to excite upstream whistler waves self-consistently by accelerated electrons; (2) which is prohibitively high for heliospheric shocks (β e is the ratio between the electron thermal pressure and magnetic pressure, which is the order of 0.1-1 for the typical solar wind condition). Therefore, for electron DSA processes to be possible at heliospheric shocks, either the following two conditions should be satisfied: 1. Injection of ∼ MeV electrons by other processes.

Foreshock Langmuir waves for unusually constant solar wind conditions: Data and implications for foreshock structure

Plasma wave data are compared with ISEE 1's position in the electron foreshock for an interval with unusually constant (but otherwise typical) solar wind magnetic field and plasma characteristics. For this period, temporal variations in the wave characteristics can be confidently separated from sweeping of the spatially varying foreshock back and forth across the spacecraft. The spacecraft's location, particularly the coordinate Dƒ downstream from the foreshock boundary (often termed DIFF), is calculated by using three shock models and the observed solar wind magnetometer and plasma data. Scatterplots of the wave field versus Dƒ are used to constrain viable shock models, to investigate the observed scatter in the wave fields at constant Dƒ, and to test the theoretical predictions of linear instability theory. The scatterplots confirm the abrupt onset of the foreshock waves near the upstream boundary, the narrow width in Dƒ of the region with high fields, and the relatively slow falloff of the fields at large Dƒ, as seen in earlier studies, but with much smaller statistical scatter. The plots also show an offset of the high‐field region from the foreshock boundary. It is shown that an adaptive, time‐varying shock model with no free parameters, determined by the observed solar wind data and published shock crossings, is viable but that two alternative models are not. Foreshock wave studies can therefore remotely constrain the bow shock's location. The observed scatter in wave field at constant Dƒ is shown to be real and to correspond to real temporal variations, not to unresolved changes in Dƒ. By comparing the wave data with a linear instability theory based on a published model for the electron beam it is found that the theory can account qualitatively and semiquantitatively for the abrupt onset of the waves near Dƒ = 0, for the narrow width and offset of the high‐field region, and for the decrease in wave intensity with increasing Dƒ. Quantitative differences between observations and theory remain, including large overprediction of the wave fields and the slower than predicted falloff at large Dƒ of the wave fields. These differences, as well as the unresolved issue of the electron beam speed in the high‐field region of the foreshock, are discussed. The intrinsic temporal variability of the wave fields, as well as their overprediction based on homogeneous plasma theory, are indicative of stochastic growth physics, which causes wave growth to be random and varying in sign, rather than secular.