Strong Solar Wind Research Articles

The heliospheric soft X-ray emission pattern during the ROSAT survey: Inferences on Local Bubble hot gas

Solar wind interaction with neutral gas has been shown to generate soft X-rays, through the charge-exchange of highly ionized ions with neutral atoms and molecules in the interplanetary space and in the geocorona. The resulting diffuse emission can explain the non-cosmic long term enhancements (LTE) of ROSAT soft X-ray observations, which are produced by strong solar wind flux increases. This paper focuses on the emission pattern resulting from the impact on the interstellar gas of the average, “quiet” solar wind, i.e., the non variable signal “below” the LTEs. A sky map of the heliospheric emission is calculated, based on realistic hydrogen and helium atom distributions under the influence of solar conditions such as those prevailing during the ROSAT survey. It is shown that parallax effects linked to the ROSAT observing strategy have a strong influence on the resultant emission pattern. For a stationary solar wind the modeled emission has an anisotropy factor of $\simeq$1.8, and the emission maxima do not coincide with the interstellar wind axis direction. This emission is compared with the fraction of the ROSAT 0.25 keV emission assigned to hot ($T=10^{6}$ K) gas in the Local Interstellar Bubble and with the dense gas contour maps of the Bubble drawn from NaI absorption. Assuming constancy of the global pattern, but allowing for a variable scaling factor, the maximum heliospheric contribution is derived and this allows the placement of lower limits on the truly interstellar emission in many directions. After subtraction of the heliospheric emission, there is significant interstellar 0.25 keV emission from the Local Bubble “openings” to the lower halo, as expected, but also to neighbouring interstellar “bubbles”, as well as towards a number of regions along the galactic plane. There is evidence that there is hot gas emission attributable to our local cavity and generated throughout its volume, including along the galactic plane, but significantly weaker than initially thought. This would attenuate the discrepancy by a factor of about 4–5 between the hot gas pressure initially derived without heliospheric decontamination and the pressure of the embedded local diffuse clouds.

Noble gases in chondrules and associated metal‐sulfide‐rich samples: Clues on chondrule formation and the behavior of noble gas carrier phases

Abstract— Chondrules are generally believed to have lost most or all of their trapped noble gases during their formation. We tested this assumption by measuring He, Ne, and Ar in chondrules of the carbonaceous chondrites Allende (CV3), Leoville (CV3), Renazzo (CR2), and the ordinary chondrites Semarkona (LL3.0), Bishunpur (LL3.1), and Krymka (LL3.1). Additionally, metalsulfide‐rich chondrule coatings were measured that probably formed from chondrule metal. Low primordial 20Ne concentrations are present in some chondrules, while even most of them contain small amounts of primordial 36Ar. Our preferred interpretation is that‐in contrast to CAIs‐the heating of the chondrule precursor during chondrule formation was not intense enough to expel primordial noble gases quantitatively. Those chondrules containing both primordial 20Ne and 36Ar show low presolar‐diamond‐like 36Ar/20Ne ratios. In contrast, the metal‐sulfide‐rich coatings generally show higher gas concentrations and Q‐like 36Ar/20Ne ratios. We propose that during metalsilicate fractionation in the course of chondrule formation, the Ar‐carrying phase Q became enriched in the metal‐sulfide‐rich chondrule coatings. In the silicate chondrule interior, only the most stable Ne‐carrying presolar diamonds survived the melting event leading to the low observed 36Ar/20Ne ratios. The chondrules studied here do not show evidence for substantial amounts of fractionated solar‐type noble gases from a strong solar wind irradiation of the chondrule precursor material as postulated by others for the chondrules of an enstatite chondrite.

Toward a solution to the early faint Sun paradox: A lower cosmic ray flux from a stronger solar wind

Standard solar models predict a solar luminosity that gradually increased by about 30% over the past 4.5 billion years. Under the faint Sun, Earth should have been frozen solid for most of its existence. Yet, running water is observed to have been present since very early in Earth's history. This enigma is known as the faint Sun paradox. We show here that it can be partially resolved once we consider the cooling effect that cosmic rays are suspected to have on the global climate and by considering that the younger Sun must have had a stronger solar wind such that it was more effective at stopping cosmic rays from reaching Earth. The paradox can then be completely resolved with the further contribution of modest greenhouse gas warming. When we add the cosmic ray flux modulation by a variable star formation rate in the Milky Way, we recover the long‐term glacial activity on Earth. As to the future, we find that the average global temperature will increase by typically 10°K in the coming 2 Gyr.

Testing the Hill model of transpolar potential saturation

The Hill model predicts that the transpolar potential (ΦPC) saturates for strong solar wind electric fields (ESW) and saturates at higher values for larger solar wind ram pressures (PSW) and/or lower ionospheric Pedersen conductances (ΣP). Three months of Defense Meteorological Satellite Program (DMSP) measurements of polar cap potentials have been compared with predictions of the Hill model. Data from representative days that span a wide range of interplanetary and solar‐cycle conditions were selected for presentation. In the comparison we augmented the Hill model by adding a constant term to the magnetospheric potential and expressing ΣP in terms of the 10.7‐cm solar radio flux (F10.7). Temporal variations in observed ΦPC are in good agreement with the predictions of the augmented Hill model. Comparison of two events with significantly different average PSW but similar F10.7 and ESW found that DMSP observed higher potentials when PSW was larger. DMSP also observed higher potentials when ΣP was lower, when comparing two events with significantly different F10.7 (solar minimum versus solar maximum) but similar PSW and ESW. Both comparisons agree with the predictions of the Hill model.

Probability distribution invariance of 1‐minute auroral‐zone geomagnetic field fluctuations

A statistical model of short time‐scale geomagnetic fluctuations is developed and used to evaluate how geomagnetic dynamics are influenced by different solar wind controlling parameters. The functional form of the probability distribution function (PDF) that describes extreme‐value (greater than 4σ) minute‐to‐minute changes in the ground magnetic field (Δx) at magnetometer station Sodankylä (geomagnetic latitude and longitude of [63.87,107.61]) is shown to be nearly independent of the variables solar wind (SW) forcing, local time (LT), and day of year (DOY). Instead of modifying the intrinsic dynamics, as characterized by the functional form of the PDF of Δx, these variables are shown either to amplify or reduce the absolute level of variability of the fluctuations: The primary difference in the PDF tail of Δx during weak and strong solar wind forcing is the standard deviation, σ; the functional form of the PDF = f[Δx/σ(DOY,LT,SW)] is nearly invariant. In a statistical interpretation, we conclude that differences in solar‐generated conductivity, seasonal effects, strength of solar wind forcing and variability, and position of the magnetometer ground station in local time do not change the structure of the extreme‐value dynamics, as characterized by the probability distribution of Δx, but they serve to amplify the intrinsic variability.

Effects of the solar wind electric field and ionospheric conductance on the cross polar cap potential: Results of global MHD modeling

The behavior of the cross polar cap potential, ΦPC, under strong solar wind conditions is studied using global MHD simulations. Simulations using two typical values of the ionospheric Pedersen conductance in agreement with others show that the cross polar cap potential is reduced compared to the corresponding potential in the solar wind due to the stagnation of the magnetosheath flow and the existence of parallel potentials. However, it is the ionospheric conductance that affects the value of ΦPC the most: the transpolar potential saturates only for high enough ionospheric conductance. A mechanism in which the ionospheric conductance changes the properties of the magnetosheath flow is proposed. This mechanism assumes mapping of the electrostatic potential in the ideal MHD system and yields a self‐consistent response of the reconnection and transpolar potentials to changes in the ionospheric conductance.

Energetic [formula omitted] intensity ratio under solar maximum and solar minimum conditions: Ulysses observations

We study the solar cycle variability of the heliospheric energetic proton-to-helium abundance ratios. We use 0.5–1.0 MeV nucleon −1 proton and helium intensities observed by the Ulysses spacecraft at both high and low heliographic latitudes. Ulysses observations show that during solar maximum the 0.5–1.0 MeV nucleon −1 H He intensity ratios are, on average, higher than during solar minimum. Under solar minimum conditions the interaction between slow and fast solar wind streams is strong, producing long-lasting and stable corotating interaction regions (CIRs) which are efficient accelerators of pickup He +. During solar maximum, transient events of solar origin (characterized by high H He ratios) are able to globally fill the heliosphere. In addition, the absence of large and stable coronal holes results in a lack of recurrent strong corotating solar wind interactions, and consequently a less efficient acceleration of pickup He +. Even when solar wind stream interaction regions (SIRS) are observed, the H He intensity ratio during solar maximum rarely decreases to the low (∼6) values typically observed during solar minimum CIR events. The latest data collected by Ulysses during its solar maximum descent from northern polar latitudes (mid-2002) show nearly-recurrent CIR events, which occasionally decrease the H/He ratios to these low (∼6) values. The still frequent occurrence of SEP events, however, produces increases of the H/He intensity ratios to high (∼30) values. Although SEP events still dominate the particle population in the inner heliosphere, the low H He ratios observed in these specific CIR events suggest an efficient acceleration of pickup He + rather than the acceleration of remnant SEP material.

Solar cycle variations of the energetic H/He intensity ratio at high heliolatitudes and in the ecliptic plane

Abstract. We study the variability of the heliospheric energetic proton-to-helium abundance ratios during different phases of the solar cycle. We use energetic particle, solar wind, and magnetic field data from the Ulysses, ACE and IMP-8 spacecraft to compare the H/He intensity ratio at high heliographic latitudes and in the ecliptic plane. During the first out-of-ecliptic excursion of Ulysses (1992–1996), the HI-SCALE instrument measured corotating energetic particle intensity enhancements characterized by low values (< 10) of the 0.5–1.0 MeV nucleon-1 H/He intensity ratio. During the second out-of-ecliptic excursion of Ulysses (1999–2002), the more frequent occurrence of solar energetic particle events resulted in almost continuously high (< 20) values of the H/He ratio, even at the highest heliolatitudes reached by Ulysses. Comparison with in-ecliptic measurements from an identical instrument on the ACE spacecraft showed similar H/He values at ACE and Ulysses, suggesting a remarkable uniformity of energetic particle intensities in the solar maximum heliosphere at high heliolatitudes and in the ecliptic plane. In-ecliptic observations of the H/He intensity ratio from the IMP-8 spacecraft show variations between solar maximum and solar minimum similar to those observed by Ulysses at high heliographic latitudes. We suggest that the variation of the H/He intensity ratio throughout the solar cycle is due to the different level of transient solar activity, as well as the different structure and duration that corotating solar wind structures have under solar maximum and solar minimum conditions. During solar minimum, the interactions between the two different types of solar wind streams (slow vs. fast) are strong and long-lasting, allowing for a continuous and efficient acceleration of interstellar pickup He +. During solar maximum, transient events of solar origin (characterized by high values of the H/He ratio) are able to globally fill the heliosphere. In addition, during solar maximum, the lack of strong recurrent high-speed solar wind streams, together with the dynamic character of the Sun, lead to weak and short-lived solar wind stream interactions. This results in a less efficient acceleration of pickup He +, and thus a higher value of the H/He intensity ratio.Key words. Interplanetary physics (energetic particles, interplanetary shocks; solar wind plasma)

Halo-coronal mass ejections near the 23rd solar minimum: lift-off, inner heliosphere, and in situ (1 AU) signatures

Abstract. The extreme ultraviolet (EUV) signatures of a solar lift-off, decametric and kilometric radio burst emissions and energetic particle (EP) inner heliospheric signatures of an interplanetary shock, and in situ identification of its driver through solar wind observations are discussed for 12 isolated halo coronal mass ejections (H-CMEs) occurring between December 1996 and 1997. For the aforementioned twelve and the one event added in the discussion, it is found that ten passed several necessary conditions for being a "Sun-Earth connection". It is found that low corona EUV and Ha chromospheric signatures indicate filament eruption as the cause of H-CME. These signatures indicate that the 12 events can be divided into two major subsets, 7 related to active regions (ARs) and 5 unrelated or related to decayed AR. In the case of events related to AR, there is indication of a faster lift-off, while a more gradual lift-off appears to characterize the second set. Inner heliospheric signatures – the presence of long lasting enhanced energetic particle flux and/or kilometric type II radio bursts – of a driven shock were identified in half of the 12 events. The in situ (1 AU) analyses using five different solar wind ejecta signatures and comparisons with the bidirectional flow of suprathermal particles and Forbush decreases result in indications of a strong solar wind ejecta signatures for 11 out of 12 cases. From the discussion of these results, combined with work by other authors for overlapping events, we conclude that good Sun-Earth connection candidates originate most likely from solar filament eruptions with at least one of its extremities located closer to the central meridian than ~ 30° E or ~ 35° W with a larger extension in latitudinal location possible. In seven of the twelve cases it appears that the encountered ejecta was driving a shock at 1 AU. Support for this interpretation is found on the approximately equal velocity of the shock and the ejecta leading-edge. These shocks were weak to moderate in strength, and a comparison of their transit time with their local speed indicated a deceleration. In contradistinction with this result on shocks, the transit time versus the local speed of the ejecta appeared either to indicate that the ejecta as a whole traveled at constant speed or underwent a small amount of acceleration. This is a result that stands for cases with and without fast stream observations at their rear end. Seven out of twelve ejecta candidate intervals were themselves interplanetary magnetic cloud (IMC) or contained a previously identified IMC. As a by-product of this study, we noticed two good ejecta candidates at 1 AU for which observation of a H-CME or CME appears to be missing.Key words. Radio science (remote sensing); Solar physics, astrophysics and astronomy (flares and mass ejections); Space plasma physics (nonlinear phenomena)

The Ionosphere of Titan: Ideal Diurnal and Nocturnal Cases

We have solved a stationary Boltzmann transport equation to describe the ionosphere of Titan in two simple cases. The first one deals with the satellite being outside the Kronian magnetosphere on the dayside of Saturn, which happens under strong solar wind conditions. In that case, the main energy source of ionization is the solar photons. We show the effect of the photoionization and the secondary ion production for a solar zenith angle of 45°. The electron production peaks at 25 electrons s−1 cm−3 around 1000 km. We estimate the electron density from a comprehensive chemical code. This electron density is then compared with the one computed from a simple recombination model. Finally, we determine the intensity of nitrogen emissions, which are compared to the Voyager 1 measurements.In the second case, the satellite is inside Saturn's magnetosphere. We show the effect of the ionization due to electron precipitation at night, above the polar regions. The input electron flux is measured by the Voyager probes, gathered from several instruments on board. A simple Kappa distribution is given to model a mean electron flux precipitating on Titan. We show that the electron production ranges between 1 to 5 electrons s−1 cm−3 between about 550 and 650 km. The electron production due to the photoionization above the pole is evaluated and compared to the effect of the kronian electron precipitation.

The cusp/magnetosheath interface on May 29, 1996: Interball‐1 and Polar observations

On May 29, 1996, under steady strong northward IMF and high solar wind dynamic pressure conditions both Polar and Interball cross field lines that pass through the northern cusp and apparently close to the post‐cusp reconnection site. The magnetopause current observed by Interball consists of two quite distinct layers, an inner broad current that is quite turbulent and another current that is quite abrupt and quiet. Polar also crosses current layers, similar to the Interball inner one. These observations support a model in which cusp field lines experience essentially stochastic behavior but on average provide topological connection between the cusp and magnetosheath.

Hemispherical and latitudinal differences in the response of geomagnetic activity to recurrent solar wind streams

Abstract The data-adaptive technique of Singular Spectrum Analysis isolates significant signal components from background noise in a time series. This methodology is adopted for analysis of indices of geomagnetic activity, interplanetary magnetic field (IMF) and solar wind parameters during an interval in 1974 marked by strong recurrent solar wind streams to bring out the latitudinal dependence and differences in the geomagnetic response between the northern and the southern hemispheres. The geoeffectiveness of the azimuthal component of IMF in the observed 28-day periodicity at low latitudes can be attributed to the seasonally dependent IMF polarity influence through the Russell–McPherron mechanism. In respect of the 14-day oscillation, the solar wind velocity, the variability and density which characterize the recurrent streams appear significant in their efficacy at subauroral and auroral latitudes as also in generating North–South asymmetry in the geomagnetic response. A clear difference in the response to recurrent solar wind streams, between the two hemispheres, is brought out with the southern (northern) hemisphere showing greater activity with ∼14-d (∼28-d) periodicity. The observed responses can be interpreted in terms of the geomagnetic changes associated with the sector structure and sector boundary of IMF through the Russell–McPherron (R–M) effect.

Annual variation in near‐Earth solar wind speed: Evidence for persistent north‐south asymmetry related to solar magnetic polarity

We study the annual variation in solar wind speed at Earth's orbit and in geomagnetic activity since mid‐1960's. The two parameters depict a very similar annual variation during the whole period. Annual variation has maximum amplitude around sunspot minima. The phase of annual variation reverses soon after solar maxima, following the Sun's polarity reversal and indicating a new type of 22‐year periodicity. Stronger solar wind is found at or close to the Earth's highest northern (southern, resp.) heliographic latitudes during solar minima with a negative (positive) magnetic polarity. This implies an asymmetric SW speed distribution across heliographic equator such that the minimum speed region during solar minimum times is displaced away from heliographic equator towards the northern magnetic hemisphere. This may result e.g. from a systematically larger extension of polar coronal holes from the Sun's magnetic south pole toward solar equator. We exclude the earlier explanations proposed for annual variation, such as accumulation of small comets within 1 AU, or internal solar variation.

Loss of Water on the Young Venus: The Effect of a Strong Primitive Solar Wind

An enhanced primitive solar wind, such as may have prevailed during the first few 100 million years of the solar system history, is shown to have had the potential to stimulate strong thermal atmospheric escape from the young Venus. Due to heating by solar wind bombardment of an extended dense planetary corona, typically 10 times more extensive than the solid planet, an escape flux of pure atomic hydrogen as large as 3 × 10 14cm −2sec −1is found to be possible, provided the solar wind was ≈10 3–10 4more intense than now. Even if escape was diffusion-limited, an enhanced primitive solar UV flux (a factor of ≈5 above present level), absorbed by ≈0.3 mbar of thermospheric water vapor, was able to supply the flow at the required rate. For these high escape rates, oxygen was massively dragged off along with hydrogen, and water molecules could be lost at a rate of ≈6 × 10 13molecules cm −2sec −1. Because, at this rate, a terrestrial-type ocean was completely lost in ≈10 million years, short compared to typical accretion and outgassing times, water was lost “as soon” as it was outgassed. This mechanism could explain the present lack of oxygen in the Venus atmosphere. Because it is expected to affect all sunlike stars in the early phase of planet formation, abiotic oxygen atmospheres could be rare in the universe.

Search for the “unknown” quantity in the solar wind: A personal account

In 1964 when S. Chapman and I concluded that there is some “unknown” quantity in the solar wind controlling the development of the main phase of geomagnetic storms. This statement was based on our study of the variety of the development of geomagnetic storms and on the resulting conclusion that the Chapman‐Ferraro's solar wind flows around the Earth and confines the Earth in a cavity (the magnetosphere) but does not transfer the energy to the magnetosphere. It was thought to be an outrageous statement, since it had been firmly believed in the 1960s that a strong solar wind was all that was needed to cause geomagnetic storms. Our search for the unknown quantity in the solar wind eventually led us to a study of auroral/magnetospheric substorms, which, in turn, led us to the conclusion that the unknown quantity is the north–south component of the IMF, or more accurately, a specific combination of three parameters: the solar wind speed V, the IMF magnitude B, and its polar angle θ. This paper describes a personal account of the history of this search for the unknown quantity. One important lesson I learned during this research is that a scientific field is often in trouble when the majority of researchers agree on one particular theory or one particular interpretation of observations. In general, one particular theory in a scientific field, however popular it may be, is eventually bound to fail. Thus, when the majority of researchers believe in that particular theory for a very long time, it means that the advance in that field is temporarily stalled.

Electromagnetic induction heating of meteorite parent bodies by the primordial solar wind

We consider the electromagnetic induction heating of meteorite parent bodies in the strong solar wind in the T Tauri phase of the Sun including the effect of the Alfvén wing generated by the interaction between a steady uniformly flowing magnetized plasma and a conductor. The characteristics of the electric current induced in the conductor are determined by the shortest of the three characteristic times: time for the Alfvén wave to propagate from one boundary of the conductor to the other (Alfvén travel time), time for the flow to cross the conductor, and time for the conductor to diffuse the magnetic field. The limit for which the Alfvén travel time is the shortest is applied to the interaction between a meteorite parent body and the solar wind in the T Tauri phase. In this limit the increase of the temperature of meteorite parent bodies is independent of the magnetic field, and inversely proportional to the electrical conductivity of meteorite parent bodies. Numerical calculations are carried out in the case where the electrical conductivity depends on the temperature. The result shows that the region of the high heating rate is confined to the surface. Since the conductivity has a strong positive dependence on the temperature, negative feedback on the heating process tends to suppress the temperature increase. The result also indicates that the lower the surface temperature, the higher the heating rate, since the conductivity is kept low.

A critical review on the viability of a space propulsion based on the solar wind momentum flux

In this paper three major aspects of the magnetic sail propulsion concept are critically reviewed. Two of them concern spacecraft technology in relation to large superconducting devices onboard. The third is concerned with the real physics of the interaction between the solar wind and the magnetic spacecraft. It is pointed out that technological difficulties, although considerable, do not appear insurmountable. In contrast, much work has to be done to understand the important aspects of the interaction mode by which the spacecraft would be thrusted. In any case, the strong solar wind fluctuations cannot be ignored and may render this potential propulsion mode impracticable in the solar system.

Elsässer variable analysis of fluctuations in the ion foreshock and undisturbed solar wind

MHD fluctuations in the solar wind have been investigated previously by use of Elsässer variables. In this paper, we present a comparison of the spectra of Elsässer variables in the undisturbed solar wind at 1 AU and in the ion foreshock in front of the Earth. Both observations take place under relatively strong solar wind flow speed conditions (≈ 600 km s−1). In the undisturbed solar wind we find that outward propagating Alfvén waves dominate, as reported by other observers. In the ion foreshock the situation is more complex, with neither outward nor inward propagation dominating over the entire range investigated (1‐10 mHz). Measurements of the Poynting vectors associated with the fluctuations are consistent with the Elsässer variable analysis. These results generally support interpretations of the Elsässer variables which have been made based strictly on solar wind data and provide additional insight into the nature of the ion foreshock turbulence.

Alpha particle heating in hot diamagnetic cavities

The heating of solar wind alpha particles in hot diamagnetic cavities (HDCs) is examined both through data analysis and computer simulation. The Lockheed Plasma Composition Experiment on ISEE 1 shows more than an order of magnitude increase in the alpha temperature between the solar wind and the HDCs, analogous to the strong solar wind proton heating evidenced in the same transition. A one‐dimensional electromagnetic hybrid computer simulation is used to study alpha heating by ion/ion instabilities, where the initial condition is a cool field‐aligned proton beam streaming relative to the solar wind protons and alpha particles. Low beam densities excite the proton/proton right‐hand resonant instability which pitch angle scatters the beam without significantly heating the alphas. At larger beam densities, the proton/proton nonresonant instability saturates by strong trapping of all three ion components; after saturation the large amplitude magnetic fluctuations lead to stochastic scattering and heating of the ions such that the final temperatures of the alphas are typically greater than the final proton temperatures. These results provide further support to the hypothesis of Thomsen et al. (1988) that the nonresonant instability is the primary source of ion heating in hot diamagnetic cavities.

The radiation belt origin of Uranus' Nightside radio emission

From the location of the source field lines of the smooth nightside component of Uranus kilometric radiation (UKR) (Kaiser et al., this issue) the most likely free energy source is the outer radiation belts. As the terminator sweeps over the magnetic north polar region, precipitation of electrons produced by solar heating of the upper atmosphere and consequent submergence of the electron mirror points deeper in the atmosphere will create a backscattered electron distribution with an enhanced population at large pitch angles. This distribution will propagate to the conjugate magnetic south pole of Uranus, where the converging magnetic field lines will produce a mirroring electron distribution moving upward from the south pole, which will drive a cyclotron maser instability. The upper frequency limit to the UKR emissions corresponds closely to the electron gyrofrequency of the mirror point of the weaker northern pole and is consistent with a backscattered electron population originating from precipitation near the northern pole and hence mirroring at higher altitudes at the conjugate point in the southern magnetic hemisphere. The rise in emission intensity is shown to scale directly with the terminator's speed across the northern pole; the smooth emission result from sunrise over the northern pole and collision‐induced precipitation. Strong radio emissions are limited to the continually dark south pole, since only there is the electron plasma frequency to gyrofrequency ratio sufficiently small. The clocklike radio emission is shown to be a direct consequence of the terminator's control of the emission process. Hence solar‐driven upper atmospheric tides underlie the Uranian radio clock. Since the radiation belt population is most likely only weakly dependent on solar wind conditions, strong solar wind control of smooth UKR is not expected.