Wind Compression Research Articles

High-latitude geomagnetic pulsation response to the passage of the front edge of the interplanetary magnetic cloud of January 10, 1997

In this paper we studied the high-latitude long period geomagnetic pulsations excited during the first hours (0430– 0830 UT ) of the magnetic cloud impact on January 10, 1997. In this study we analysed the ground-based data of the IMAGE magnetometers and the Scandinavian network of ∼30 MHz riometers. Two time intervals were selected. The first interval (0430– 0600 UT ) corresponds to the passage of the compression region on the front edge of the magnetic cloud. During this interval bursts of geomagnetic and riometer data pulsations at frequency range of 1– 3 mHz as well as a magnetic substorm were observed at polar latitudes ( Λ>72°) with the strongest amplitudes near a footprint of the open/closed field lines boundary. The waves propagated rapidly eastward at ∼6– 7 km/ s and had an azimuthal wave number m∼12. This seems to be inconsistent with the main signature of a field line resonance. The green (557.7 nm) optical emissions were observed at that time at Spitzbergen. The emissions moved rapidly eastward at about the same speed. We speculate that high-latitude geomagnetic pulsations under consideration were excited at the ionosphere altitudes, near the polar cusp footprint. The waves could be associated with quasi-periodic variations in the ionospheric conductivity produced by the particle precipitation oscillations. As the alternative hypothesis we could assume the direct penetration of solar wind compression or Alfvén waves to the polar cap. These waves can modulate the particle precipitation produced the quasi-periodic variation of ionospheric conductivity. The second time interval (0600– 0830 UT ) corresponds to an abrupt increasing of the solar wind dynamic pressure, observed about 2 h after the passage of the leading edge of the magnetic cloud. Contrary to the first interval, the strongest amplitudes of geomagnetic and cosmic noise absorption pulsations were observed in closed magnetosphere, at Λ∼66°, and showed the properties of the typical field line resonance.

Particle Acceleration in Solar Wind Compression Regions

We present the results of a theoretical investigation of the acceleration of charged particles in regions of gradual solar wind compression. The mechanism we describe is similar to diffusive shock acceleration, except that it invokes a gradual compression of the plasma over many gyroradii rather than a shock. Recent observations of energetic particles associated with corotating interaction regions (CIRs) at 1 AU suggest that the particles were accelerated within the compression region between the fast and slow solar wind rather than at the associated forward and reverse shocks, which are at larger heliocentric distances. We show that nondiffusive effects such as magnetic mirroring are important in the inner heliosphere, particularly the injection of low-energy particles (e.g., pickup ions and suprathermal solar wind ions). We integrate the trajectories of an ensemble of test particles moving in synthesized electromagnetic fields, which are similar to what is currently known about corotating interaction regions in the inner heliosphere, prior to the formation of the forward and reverse shocks. We show that compression regions associated with CIRs at 1 AU with widths ~0.03 AU can accelerate particles up to ~10 MeV and produce energy spectra which are remarkably similar to recent observations.

Semiannual variation of the geomagnetic Dst index: Evidence for a dominant nonstorm component

The geomagnetic Dst index exhibits a strong semiannual variation with amplitude of 5.3 nT (on an average baseline of −16.4 nT) for 1957–1997. If we consider the five quietest Dst days for each month during this interval, we find that while the average Dst baseline shifts from −16.4 to +4.0 nT, the amplitude of the 6‐month wave remains relatively strong at 2.5 nT. Thus ∼50% (2.5 nT/5.3 nT) of the seasonal variation of the Dst index results from modulation of its quiet time baseline. The seasonal modulation of Dst appears to consist of both a storm component, resulting from rapid variations of the ring current and other currents, and a slowly varying, nonstorm component. We estimate that the storm component accounts for only ∼30–50% of the amplitude of the 6‐month wave in Dst versus ∼50–70% for the nonstorm component. The classic equinoctial effect appears to dominate the storm component, accounting for 20–40% of the amplitude of the 6‐month wave in Dst versps ∼10% for the combined axial/Russell‐McPherron mechanisms. Candidate mechanisms for the nonstorm component of the 6‐month wave in Dst include the Malin‐Isikara effect (the seasonal displacement of ring/tail currents by solar wind compression) and a semiannual variation of magnetopause currents.

Tail configuration during storms

The configuration of the Earth's magnetotail is controlled by the amount of energy obtained from the solar wind and the amount of energy that exits from the tail into interplanetary space or into the dissipative ionosphere. During storm time the tail structure can significantly deviate from its average pattern because of the extreme condition of the solar wind and the interplanetary magnetic field. We study the changes in the tail structure such as orientation, size, and shape of the tail during storms by reviewing Geotail observations in the distant tail. Geotail encountered the magnetosheath during all storms including those when the satellite was located near the nominal tail axis. Changes in the average orientation of the tail were consistent with that expected from the solar wind fluctuations causing geomagnetic storms, e.g., fluctuations related to a corotating interaction region. In addition to enhanced magnetic pressure due to solar wind compression, enhanced flux in the distant tail was detected during the main phase. By averaging thirteen weak-to-moderate storm cases it is found that ∼ 5 × 10 15 J is stored also in the distant tail during the storm main phase. There are evidences from Geotail and previous ISEE 3 observations that the flaring ceased earthward of X ∼ −100 R E during the main phase, whereas the location of the distant neutral line, where the flaring terminates, could be located tailward of X ∼ −200 R E region during the recovery phase of a storm.

“WCFields”: A Magnetic Rotating Stellar Wind Model from Wind Compression Theory

A stellar wind model for a magnetic rotating star is presented. We use the semianalytic wind compression model that predicts the two-dimensional geometry of outNows from rotating stars and consider the addition of a magnetic Ðeld. In the limit of weak magnetic Ðelds, in such a way that the Ðelds are unimportant in accelerating the Now, the wind compression model can be used to predict the magnetic Ðeld distribution throughout the wind, which is shown to follow the mass Nux distribution. A compression of Ðeld lines near the equator results as the Now of material from higher latitudes brings magnetic Nux toward that region. As examples, wind compression models with magnetic Ðelds (““WCField II models) are computed for both a Wolf-Rayet star and a red supergiant star. In both cases an order of magnitude enhancement of the equatorial magnetic Ðeld can result within a few stellar radii for stellar rotation rates around 20% of critical. Such enhancements could have consequences for explaining (1) nonthermal emission observed from some Wolf-Rayet winds and (2) the ring structures observed in the ejecta of SN 1987a. Subject headings : stars : magnetic Ðelds E stars : mass loss E stars : rotation E stars : Wolf-Rayet E supernovae : individual (SN 1987A)

THEORETICAL AND OBSERVATIONAL CONSEQUENCES OF STELLAR ROTATION AND MAGNETIC FIELDS FOR STELLAR WINDS

ABSTRACT My dissertation concerns the study of stellar wind structure from (1) the theoretical modeling of stellar rotation and magnetic fields and (2) the development of observational diagnostics. First, I have investigated the effects of stellar rotation for the wind structure of stars across the H-R Diagram using principles from the Wind Compressed Disk (WCD) model. Relative to a spherical wind, the effect of rotation is to increase the wind density at the equator while decreasing the density near the poles. An investigation of mild equatorial density enhancements has led to the development of the Wind Compressed Zone (WCZ) model. The WCZ model predicts that equatorial wind compressions are most likely to occur for stellar winds with low terminal speeds and/or radial velocity distributions that increase gradually from the base of the wind. In favorable cases stellar rotation can produce significant equatorial density enhancements in the winds of Wolf-Rayet stars, B~supergiants, Asymptotic Giant Branch stars, and even some novae at moderate stellar rotations of 20-30% break-up. A spectral line diagnostic based on multiline observations at Infrared wavelengths has been developed to measure the velocity law in the Wolf-Rayet winds, to determine whether they are likely susceptible to wind compression effects. The second major part of the thesis relates to the result that the WCZ model predicts the magnetic field structure in the wind, if the field strength is relatively weak. However, there are generally no good spectral diagnostics of sub-kiloGauss stellar magnetic fields. Magnetic fields at these low strengths are difficult to measure with the Zeeman effect, because the Zeeman line splitting is significantly smaller than the Doppler broadening. Thus, I have explored applications of the Hanle effect, that is sensitive to weaker magnetic fields than is the Zeeman effect, for probing the magnetic properties of stellar winds. The Hanle effect concerns the modification of resonance line polarization by the magnetic field. It has been used in studies of the Solar atmosphere, but not in other stars. Solutions for the Hanle effect in optically thin axisymmetric extended stellar envelopes have been derived. Relative to the zero field case, the Hanle effect can result in significant changes of the total line polarization. Consequences of the Hanle effect for the polarization of line profiles is also investigated, and analytic results are presented for a few special cases. A magnetic field can cause strong variations of the polarized line profile shape as compared to the case without a magnetic field. With multiline observations the Hanle effect will be a valuable diagnostic of stellar magnetic fields in the range 1-1000 Gauss.

Inhibition of Wind-Compressed Disk Formation by Nonradial Line Forces in Rotating Hot-Star Winds

We investigate the effects of nonradial line forces on the formation of a wind-compressed (WCD) around a rapidly rotating B star. Such nonradial forces can arise both from asymmetries in the line resonances in the rotating wind and from rotational distortion of the stellar surface. They characteristically include a latitudinal force component directed away from the equator and an azimuthal force component acting against the sense of rotation. Here we present results from radiation-hydrodynamical simulations showing that these nonradial forces can lead to an effective suppression of the equatorward flow needed to form a WCD as well as a modest (~20%) spin-down of the wind rotation. Furthermore, contrary to previous expectations that the wind mass flux should be enhanced by the reduced effective gravity near the equator, we show here that gravity darkening effects can actually lead to a reduced mass loss, and thus lower density, in the wind from the equatorial region. Overall, the results here thus imply a flow configuration that is markedly different from that derived in previous models of winds from rotating early-type stars. In particular, a major conclusion is that equatorial wind compression effects should be effectively suppressed in any radiatively driven stellar wind for which, as in the usual CAK formalism, the driving includes a significant component from optically thick lines. This presents a serious challenge to the WCD paradigm as an explanation for disk formation around Be and other rapidly rotating hot stars thought to have CAK-type, line-driven winds.

Traversal of comet SL‐9 through the Jovian magnetosphere and impact with Jupiter: radio upper limits

Continuous radio observations below 1 MHz of Jupiter from the Ulysses spacecraft are used to establish an upper limit to the radiated power at low frequencies associated with the traversal through the magnetosphere and impact of Comet SL‐9 with the planet. Although Jovian emissions were observed throughout the impact interval, no systematic intensity changes were observed before, during, or after the fragment impact times as a whole. Examined individually, a large intensity increase, probably associated with a solar wind compression at Jupiter, was observed at the time of the P impact. Intense solar type III bursts, which can sometimes be confused with Jovian emissions, occurred often, which serves as a caution to other (groundbased) radio observers. We derive an upper limit for the interfragment dust density of 10−3/m3 in the context of a dust‐magnetosphere interaction model proposed earlier.

The effect of oblateness and gravity darkening on the radiation driving in winds from rapidly rotating B stars

We calculate the radiative driving force for winds around rapidly rotating oblate B stars, and we estimate the impact these forces should have on the production of a wind compressed disk. The effects of limb darkening, gravity darkening, oblateness, and an arbitrary wind velocity field are included in the computation of vector 'oblate finite disk' (OFD) factors, which depend on both radius and colatitude in the wind. The impact of limb darkening alone, with or without rotation, can increase the mass loss by as much as 10% over values computed using the standard uniformly bright spherical finite disk factor. For rapidly rotating stars, limb darkening makes 'sub-stellar' gravity darkening the dominant effect in the radial and latitudinal OFD factors, and lessens the impact of gravity darkening at other visible latitudes (nearer to the oblate limb). Thus, the radial radiative driving is generally stronger over the poles and weaker over the equator, following the gravity darkening at these latitudes. The nonradial radiative driving is considerably smaller in magnitude than the radial component, but is directed both away from the equatorial plane and in a retrograde azimuthal direction, acting to decrease the effective stellar rotation velocity. These forces thus weaken the equatorward wind compression compared to wind models computed with nonrotating finite disk factors.

Winds from rotating Wolf-Rayet stars: the wind-compressed zone model

Theoretical models for Wolf-Rayet winds that include the effects of rotation and magnetic fields are discussed. These are of two types: magnetic rotator models and wind compression models. The magnetic rotator models are examples of equatorial expulsion models, typically requiring large rotation rates to produce significant equatorial density enhancements. Wind- compression models are introduced here as an application of the two-dimensional Wind-Compressed Disk (WCD) model that has recently been developed for Be stars. An equatorial disk forms because of the supersonic confluence of the flow from the upper and lower hemispheres of the star. In the application to WR stars, we suggest that disk formation is not required, and instead a less extreme example that we call a wind-compressed zone (WCZ) may suffice. Because the winds of WR stars have a geometrically extended acceleration region, or a “slow” velocity law, there can be compressions by an order of magnitude, even with moderate stellar rotation rates of about 16% critical. Magnetic flux conservation is then used to estimate the enhancement of the field strength that occurs because of the equatorial wind-compression. We consider an application of the model to explain the occurrence of polarization and dust formation among only some WR stars. Finally, we consider the WR + compact companion system WR 147, and suggest that the enhanced accretion in a WCZ could help explain the large X-ray emission.

A millimeter-wave line study of L1287 - A case of induced star formation by stellar wind compression?

view Abstract Citations (61) References (45) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS A Millimeter-Wave Line Study of L1287: A Case of Induced Star Formation by Stellar Wind Compression? Yang, Ji ; Umemoto, Tomofumi ; Iwata, Takahiro ; Fukui, Yasuo Abstract Results are presented of both low- and high-resolution molecular-line observations made toward the dark cloud L1287. The cloud appears filamentary in (C-13)O emission and extends over 10 pc along the Galactic plane. At least four separate cores are aligned along the filament. The most intense emission of (C-13)O comes from the L1287 main core. The total mass of the cloud is estimated to be about 240 solar masses. An energetic molecular outflow has been discovered in (C-12)O emission toward the peak of the main cloud core where a cold-type IRAS source, which is suggested to be the driving source of the outflow, is located. The central part of the main (C-13)O core has been mapped in HCN and HCO(+) (J = 1-0) lines. These lines are intense and optically thick, as suggested by the strong self-absorption. Both emission components have sizes of 0.1 pc. The lower limit of the mass of the HCN core is estimated to be 13 solar masses. Bipolar wing emission is also revealed in the HCO(+) spectra. The main (C-13)O core exhibits an unusually high ratio of the luminosity of the central object to the associated (C-13)O core mass in comparison with the typical isolated dark clouds in the solar neighborhood. The stellar wind from the neighboring Be star Kappa Cas is suggested to have played a role. Publication: The Astrophysical Journal Pub Date: May 1991 DOI: 10.1086/170032 Bibcode: 1991ApJ...373..137Y Keywords: Infrared Sources (Astronomy); Interstellar Chemistry; Interstellar Gas; Molecular Clouds; Star Formation; Stellar Winds; Carbon Monoxide; Carbon 13; Infrared Astronomy Satellite; Interstellar Extinction; Line Spectra; Millimeter Waves; Astrophysics; INFRARED: SOURCES; INTERSTELLAR: MOLECULES; NEBULAE: INDIVIDUAL ALPHANUMERIC: L1287 (LYNDS 1287); STARS: FORMATION full text sources ADS | data products SIMBAD (3)

Modeling Jupiter’s magnetospheric currents using Pioneer data: Evidence for a low‐latitude cusp

The Jovian magnetospheric field measured by Pioneer 10 and 11 can be well modeled by a combination of current systems composing an azimuthally symmetric current disc, a dusk‐dawn current sheet in both the dayside and the nightside magnetosphere, and an image dipole to represent the effects of currents on the magnetopause. The inclusion of a dusk‐dawn current sheet in the dayside magnetosphere allows observations obtained both inbound and outbound to be simultaneously fit by an azimuthally symmetric current disc (i.e., without the need for local time dependent current densities). Similar disc current intensities are found to describe both Pioneer 10 and Pioneer 11 encounters. During the Pioneer 10 inbound passage the magnetopause was rapidly pushed inside the spacecraft position by a solar wind compression event. The changes that occurred in the magnetospheric field at this time can be described by relatively simple changes in our model parameters. When the field lines of our model are extended beyond the region of fitting, both the Pioneer 10 and the Pioneer 11 encounters give similar profiles with magnetopause distances that are consistent with the actual encounters. The most striking feature of the models is that they suggest that the Jovian cusp is at much lower latitudes than is the case with the earth’s magnetosphere.

The dynamics of the Venus ionosphere: 1. A simulation of the solar wind compression of the upper dayside ionosphere

The effects of the solar wind compression of the dayside Venus ionosphere are simulated numerically. The initial ionosphere is assumed to be in pressure equilibrium with the (shocked) solar wind at the upper boundary of the ionosphere. Composition, densities, and temperatures of ions and electrons in the ionosphere are chosen in accordance with Pioneer Venus data. A spherically symmetric Lagrangian hydrodynamic code using a two‐fluid model of the ionosphere consisting of O+ and electrons is employed to simulate the effects on the ionosphere of rapid changes in solar wind pressure. Sudden increases in solar wind pressure are found to generate shock waves in the ions that propagate from the ionopause downward into the ionosphere with velocities as high as 5 km/s. The effect of shock waves on ionospheric density profiles is dramatic with distinct ‘ledges’ developing in the ionosphere at the shock front. Comparison of density profiles from our simulation with select Pioneer Venus ion mass spectrometer data suggest possible agreement between shock produced ionospheric ledges resulting from rapid solar wind compression and observed ionospheric ledges.

Solar cycle variation in geomagnetic external spherical harmonic coefficients.

The first degree external spherical harmonic coefficients are obtained for each year of a 12-year period centered at 1958.5, using annual mean values of X, Y, and Z components from 54 magnetic observatories. Values of the coefficient g01e of the zonal first degree harmonic clearly show a solar cycle variation. The peak-to-peak amplitude of the variation is approximately 29 nT, which is consistent with values obtained earlier by first filtering data to retain only those variations having periods near the solar cycle and then subjecting the filtered data to a spherical harmonic analysis. The variation in g01e is found to correlate extremely well with the annual mean Dst, index and with the annual number of days having Ap≥60. Based on statistics of the mean square successive difference, an explanation is presented why the obtained solar cycle variation in g01e, which is very much smaller in magnitude than the standard deviations calculated by the conventional method, is statistically meaningful. The determined absolute (not relative) values of g01e are in agreement, within several nanotesla, with the expectation from a theoretical model of solar wind compression of the magnetosphere and an analysis of the Dst index.

Pitch angle and radial diffusion of MeV protons in the outer magnetosphere

The latitude dependence of the field line-averaged pitch angle diffusion coefficient of 1-MeV protons in the magnetosphere has been calculated under the following assumptions: Scattering is due to gyroresonant interaction with a broad band spectrum of ion cyclotron waves, and the magnetospheric magnetic field model includes the internal dipole and the external solar wind compression as well as night side current sheet effects. In addition, calculations were performed in a model which simulates geomagnetic storm time ring currents. It could be shown that the power levels of normal quiet time magnetospheric wave activity are not sufficient to cause strong pitch angle scattering but that this situation may change during geomagnetic storms, as has indeed been observed. By using the field line-averaged pitch angle diffusion coefficient the associated radial diffusion coefficent kappa/sub perpendicular/ (either through gyroresonant interaction or through shell splitting) was calculated. It was shown that kappa/sub perpendicular/ was too small in the pseudotrapping region to compete with direct particle access and that it yielded values comparable with previous estimates due to other processes in the high-latitude closed drift shell region. In contrast to these other estimates, which involved sudden impluses and sudden commencements to provide the magnetic fluctuations, themore » wave-particle interaction picture developed here implies a steady radial diffusion at all times, temporal enhancements occurring during geomagnetically disturbed times. (AIP)« less