Twelfth International Solar Wind Conference, AIP Conference Proceedings, 1216, pp. 64-67
We investigate the quasilinear effects of the resonant wave-particle interaction under conditions of imbalanced turbulent heating in the collisionless coronal hole. We find that velocity-space transport of protons from the heated part of the distribution leads to strong wave growth in the minority (sunward) direction. In the present quasilinear analysis, the "reflected" waves grow to unphysical levels, indicating the necessity of including nonlinear processes. This mechanism is likely to be important in development of the fast solar wind, and may explain the puzzling minor ion observations of Landi & Cranmer (2009).
Twelfth International Solar Wind Conference, AIP Conference Proceedings, 1216, pp. 72-75
Resonant interactions between ions and Alfvén/ion-cyclotron (A/IC) waves may play an important role in the heating and acceleration of the fast solar wind. Although such interactions have been studied extensively for "parallel" waves, whose wave vectors k are aligned with the background magnetic field B0, much less is known about interactions between ions and oblique A/IC waves, for which the angle &theta between k and B0 is nonzero. In this paper, we present new numerical results on resonant cyclotron interactions between protons and oblique A/IC waves in collisionless low-beta plasmas such as the solar corona. We find that if some mechanism generates oblique high-frequency A/IC waves, then these waves initially modify the proton distribution function in such a way that it becomes unstable to parallel waves. Parallel waves are then amplified to the point that they dominate the wave energy at the large parallel wave numbers at which the waves resonate with the particles. Pitch-angle scattering by these waves then causes the plasma to evolve towards a state in which the proton distribution is constant along a particular set of nested "scattering surfaces" in velocity space, whose shapes have been calculated previously. As the distribution function approaches this state, the imaginary part of the frequency of parallel A/IC waves drops continuously towards zero, but oblique waves continue to undergo cyclotron damping while simultaneously causing protons to diffuse across these kinetic shells to higher energies. We conclude that oblique A/IC waves can be more effective at heating protons than parallel A/IC waves, because for oblique waves the plasma does not relax towards a state in which proton damping of oblique A/IC waves ceases.
The Astrophysical Journal, 722, pp. 710-720
We consider interactions between protons and Alfvén/ion-cyclotron (A/IC) waves in collisionless low-&beta plasmas in which the proton distribution function f is strongly modified by wave pitch-angle scattering. If the angle &theta between the wave vector and background magnetic field is zero for all the waves, then strong scattering causes f to become approximately constant on surfaces of constant &eta, where &eta ~= v2bottom + 1.5 v2/3A|vpar|4/3. Here, vbottom and vpar are the velocity components perpendicular and parallel to the background magnetic field, and vA is the Alfvén speed. If f = f(&eta), then A/IC waves with &theta = 0 are neither damped nor amplified by resonant interactions with protons. In this paper, we argue that if some mechanism generates high-frequency A/IC waves with a range of &theta values, then wave-particle interactions initially cause the proton distribution function to become so anisotropic that the plasma becomes unstable to the growth of waves with &theta = 0. The resulting amplification of &theta = 0 waves leads to an angular distribution of A/IC waves that is sharply peaked around &theta = 0 at the large wavenumbers at which A/IC waves resonate with protons. Scattering by this angular distribution of A/IC waves subsequently causes f to become approximately constant along surfaces of constant &eta, which in turn causes oblique A/IC waves to be damped by protons. We calculate the proton and electron contributions to the damping rate analytically, assuming Maxwellian electrons and f = f(&eta). Because the plasma does not relax to a state in which proton damping of oblique A/IC waves ceases, oblique A/IC waves can be significantly more effective at heating protons than A/IC waves with &theta = 0.
The Astrophysical Journal, 722, pp. 1411-1415
Sensitive SOLARC imaging spectropolarimetric observations from Haleakala reveal a diffuse coronal surface brightness in the He I 1083 nm line. A series of observations suggests that this signal originates from an "inner source" of neutral helium atoms in the solar corona. Here, we explore the possibility that this cold coronal component originates from helium ions that are neutralized by the near-Sun dust and subsequently excited to the metastable 1s2s 3S state, which then scatters photons from the solar disk. This picture suggests a deficit of coronal dust inside about 2-4 Rsun in order to account for both the flat radial brightness distribution and the small velocity line width of the observations. We find a strong correlation between the polarized He brightness and coronal white light brightness that supports the argument that electronic collisional excitation of the metastable helium triplet level is responsible for our polarization signal.
Memorie della Societa Astronomica Italiana Supplement, 14, p. 206
The heating of the solar corona to temperatures of the order of 106 K and more is one of the outstanding problems of solar physics. Beside the high temperatures, Soho/UVCS observations have shown that heavy ions in polar corona, like O5+ and Mg9+, are heated more than protons, and that heavy ion heating is more than mass proportional; further, the perpendicular temperatures Tperpendicular are much larger than parallel temperatures Tparallel . Here we show that the heating of heavy ions can be explained by ion reflection off supercritical quasi-perpendicular collisionless shocks and the subsequent acceleration by the motional electric field E = - (1/c) V × B. The energization due to E is perpendicular to the magnetic field, and is more than mass proportional with respect to protons, because the heavy ion orbit is mostly upstream of the quasi-perpendicular shock foot.
The Astrophysical Journal, Volume 722, Issue 2, pp. 1495-1503
Some recent models for coronal heating and the origin of the solar wind postulate that the source of energy and momentum consists of Alfvén waves of solar origin dissipating via MHD turbulence. We use one of these models to predict the level of Faraday rotation fluctuations (FRFs) that should be imposed on radio signals passing through the corona. This model has the virtue of specifying the correlation length of the turbulence, knowledge of which is essential for calculating the FRFs; previous comparisons of observed FRFs with models suffered from the fact that the correlation length had to be guessed. We compare the predictions with measurements of FRFs obtained by the Helios radio experiment during occultations in 1975 through 1977, close to solar minimum. We show that only a small fraction of the FRFs are produced by density fluctuations; the bulk of the FRFs must be produced by coronal magnetic field fluctuations. The observed FRFs have periods of hours, suggesting that they are related to Alfvén waves which are observed in situ by spacecraft throughout the solar wind; other evidence also suggests that the FRFs are due to coronal Alfvén waves. We choose a model field line in an equatorial streamer which has background electron concentrations that match those inferred from the Helios occultation data. The predicted FRFs are found to agree very well with the Helios data. If the FRFs are in fact produced by Alfvén waves with the assumed correlation length, our analysis leads us to conclude that wave-turbulence models should continue to be pursued with vigor. But since we cannot prove that the FRFs are produced by Alfvén waves, we state the more conservative conclusion, still subject to the correctness of the assumed correlation length, that the corona contains long-period magnetic fluctuations with sufficient energy to heat the corona and drive the solar wind.
The Astrophysical Journal, Volume 720, Issue 1, pp. 824-847.
The physical processes that heat the solar corona and accelerate the solar wind remain unknown after many years of study. Some have suggested that the wind is driven by waves and turbulence in open magnetic flux tubes, and others have suggested that plasma is injected into the open tubes by magnetic reconnection with closed loops. In order to test the latter idea, we developed Monte Carlo simulations of the photospheric "magnetic carpet" and extrapolated the time-varying coronal field. These models were constructed for a range of different magnetic flux imbalance ratios. Completely balanced models represent quiet regions on the Sun and source regions of slow solar wind streams. Highly imbalanced models represent coronal holes and source regions of fast wind streams. The models agree with observed emergence rates, surface flux densities, and number distributions of magnetic elements. Despite having no imposed supergranular motions in the models, a realistic network of magnetic "funnels" appeared spontaneously. We computed the rate at which closed field lines open up (i.e., recycling times for open flux), and we estimated the energy flux released in reconnection events involving the opening up of closed flux tubes. For quiet regions and mixed-polarity coronal holes, these energy fluxes were found to be much lower than that which is required to accelerate the solar wind. For the most imbalanced coronal holes, the energy fluxes may be large enough to power the solar wind, but the recycling times are far longer than the time it takes the solar wind to accelerate into the low corona. Thus, it is unlikely that either the slow or fast solar wind is driven by reconnection and loop-opening processes in the magnetic carpet.
Space Science Reviews, in press
The origins of the hot solar corona and the supersonically expanding solar wind are still the subject of much debate. This paper summarizes some of the essential ingredients of realistic and self-consistent models of solar wind acceleration. It also outlines the major issues in the recent debate over what physical processes dominate the mass, momentum, and energy balance in the accelerating wind. A key obstacle in the way of producing realistic simulations of the Sun-heliosphere system is the lack of a physically motivated way of specifying the coronal heating rate. Recent models that assume the energy comes from Alfvén waves that are partially reflected, and then dissipated by magnetohydrodynamic turbulence, have been found to reproduce many of the observed features of the solar wind. This paper discusses results from these models, including detailed comparisons with measured plasma properties as a function of solar wind speed. Some suggestions are also given for future work that could answer the many remaining questions about coronal heating and solar wind acceleration.
TWELFTH INTERNATIONAL SOLAR WIND CONFERENCE. AIP Conference Proceedings, Volume 1216, pp. 432-435 (2010)
We examine the effects of including effects of both protons and electrons on the heating of the fast solar wind through two different approaches. In the first approach, we incorporate the electron temperature in an MHD turbulence transport model for the solar wind. In the second approach, we adopt more empirically based methods by analyzing the measured proton and electron temperatures to calculate the heat deposition rates. Overall, we conclude that incorporating separate proton and electron temperatures and heat conduction effects provides an improved and more complete model of the heating of the solar wind.
The Astrophysical Journal, Volume 710, Issue 1, pp. 676-688
The origins of the hot solar corona and the supersonically expanding solar wind are still the subject of debate. A key obstacle in the way of producing realistic simulations of the Sun-heliosphere system is the lack of a physically motivated way of specifying the coronal heating rate. Recent one-dimensional models have been found to reproduce many observed features of the solar wind by assuming the energy comes from Alfvén waves that are partially reflected, then dissipated by magnetohydrodynamic turbulence. However, the nonlocal physics of wave reflection has made it difficult to apply these processes to more sophisticated (three-dimensional) models. This paper presents a set of robust approximations to the solutions of the linear Alfvén wave reflection equations. A key ingredient of the turbulent heating rate is the ratio of inward-to-outward wave power, and the approximations developed here allow this to be written explicitly in terms of local plasma properties at any given location. The coronal heating also depends on the frequency spectrum of Alfvén waves in the open-field corona, which has not yet been measured directly. A model-based assumption is used here for the spectrum, but the results of future measurements can be incorporated easily. The resulting expression for the coronal heating rate is self-contained, computationally efficient, and applicable directly to global models of the corona and heliosphere. This paper tests and validates the approximations by comparing the results to exact solutions of the wave transport equations in several cases relevant to the fast and slow solar wind.
The Astrophysical Journal, Volume 722, Issue 1, pp. 625-641.
A post-coronal mass ejection (CME) current sheet (CS) is a common feature developed behind an erupting flux rope in CME models. Observationally, white light observations have recorded many occurrences of a thin ray appearing behind a CME eruption that closely resembles a post-CME CS in its spatial correspondence and morphology. UV and X-ray observations further strengthen this interpretation by the observations of high-temperature emission at locations consistent with model predictions. The next question then becomes whether the properties inside a post-CME CS predicted by a model agree with observed properties. In this work, we assume that the post-CME CS is a consequence of Petschek-like reconnection and that the observed ray-like structure is bounded by a pair of slow mode shocks developed from the reconnection site. We perform time-dependent ionization calculations and model the UV line emission. We find that such a model is consistent with SOHO/UVCS observations of the post-CME CS. The change of Fe XVIII emission in one event implies an inflow speed of ~10 km s-1 and a corresponding reconnection rate of MA ~ 0.01. We calculate the expected X-ray emission for comparison with X-ray observations by Hinode/XRT, as well as the ionic charge states as would be measured in situ at 1 AU. We find that the predicted count rate for Hinode/XRT agrees with what was observed in a post-CME CS on 2008 April 9, and the predicted ionic charge states are consistent with high ionization states commonly measured in the interplanetary CMEs. The model results depend strongly on the physical parameters in the ambient corona, namely the coronal magnetic field, the electron density, and temperature during the CME event. It is crucial to obtain these ambient coronal parameters and as many facets of the CS properties as possible by observational means so that the post-CME CS models can be scrutinized more effectively.
Advances in Space Research, Volume 46, Issue 11, pp. 1400-1408
The study concerns the streamer belt observed at high spectral resolution during the minimum of solar cycle 23 with the Ultraviolet Coronagraph Spectrometer (UVCS) onboard SOHO. On the basis of a spectroscopic analysis of the O VI doublet, the solar wind plasma parameters are inferred in the extended corona. The analysis accounts for the coronal magnetic topology, extrapolated through a 3D magneto-hydrodynamic model, in order to define the streamer boundary and to analyse the edges of coronal holes. The results of the analysis allow an accurate identification of the source regions of the slow coronal wind that are confirmed to be along the streamer boundary in the open magnetic field region.
The Astrophysical Journal, Volume 720, Issue 1, pp. 548-554
Low-frequency Alfvén-wave turbulence causes ion trajectories to become chaotic, or "stochastic," when the turbulence amplitude is sufficiently large. Stochastic orbits enable ions to absorb energy from the turbulence, increasing the perpendicular ion temperature Ti even when the fluctuation frequencies are too small for a cyclotron resonance to occur. In this paper, an analytic expression for the stochastic heating rate is used in conjunction with an observationally constrained turbulence model to obtain an analytic formula for Ti as a function of heliocentric distance r, ion mass, and ion charge in coronal holes at 2 Rsun <~ r <~ 15 Rsun. The resulting temperature profiles provide a good fit to observations of protons and O+5 ions at 2 Rsun <~ r <~ 3 Rsun from the Ultraviolet Coronagraph Spectrometer (UVCS). Stochastic heating also offers a natural explanation for several detailed features of the UVCS observations, including the preferential and anisotropic heating of minor ions, the rapid radial increase in the O+5 temperature between 1.6 Rsun and 1.9 Rsun, and the abrupt flattening of the O+5 temperature profile as r increases above 1.9 Rsun.
The Astrophysical Journal Volume 720, Issue 1, pp. 130-143
We report on the study of a fast coronal mass ejection (CME)-driven shock associated with the solar eruption of 2002 March 22. This event was observed in the intermediate corona both in white light and the extreme ultraviolet (EUV) by the LASCO and UVCS instruments on board the Solar and Heliospheric Observatory, as well as in metric and decametric wavelengths through space- and ground-based radio observatories. Clear signatures of shock transit are (1) strong type II emission lanes observed after the CME initiation, (2) strong O VI ??1032, 1037 line profile broadenings (up to ~2 × 107 K) associated with the shock transit across the UVCS slit field of view, and (3) a density enhancement located in LASCO images above the CME front. Since the UVCS slit was centered at 4.1 R sun, in correspondence with the flank of the expanding CME, this observation represents the highest UV detection of a shock obtained so far with the UVCS instrument. White-light and EUV data have been combined in order to estimate not only the shock compression ratio and the plasma temperature, but also the strength of the involved coronal magnetic fields, by applying the Rankine-Hugoniot equations for the general case of oblique shocks. Results show that, for a compression ratio X = 2.06 as derived from LASCO data, the coronal plasma is heated across the shock from an initial temperature of 2.3 × 105 K up to 1.9 × 106 K, while at the same time the magnetic field undergoes a compression from a pre-shock value of ~0.02 G up to a post-shock field of ~0.04 G. Magnetic and kinetic energy density increases at the shock are comparable (in agreement with the idea of equipartition of energy), and both are more than two times larger than the thermal energy density increase. This is the first time that a complete characterization of pre- and post-shock plasma physical parameters has been derived in the solar corona.
The Astrophysical Journal, Volume 718, Issue 1, pp. 251-265
Over the last few years coronagraphic and spectroscopic observations have demonstrated that small-scale eruptions, such as "jets," "narrow coronal mass ejections (CMEs)," "mini CMEs," "streamer puffs," "streamer detachments," and others, occur ubiquitously on the Sun. Nevertheless, the origin of small-scale eruptive events and how these are interrelated with larger scale CMEs have been poorly investigated so far. In this work, we study a series of small-scale side eruptions that occurred during and after a large-scale CME. Observations show that a CME can be associated not only with a single reconnection process, leading to the large-scale phenomenon, but also with many other side reconnections occurring at different locations and times around the main flux rope, possibly induced by the CME expansion in the surrounding corona. White light and EUV observations of a slow CME acquired by the SOHO/LASCO and SOHO/UVCS instruments are analyzed here to characterize the locations of side reconnections induced by the CME. The magnetic reconnection rate M has been estimated from the UVCS data from the ratio between the inflows and outflows observed around the reconnection region, and from the LASCO data from the observed aperture angles between the slow mode shocks (SMSs) associated with the reconnection. It turns out that M ~= 0.05 at the heliocentric distance of 1.8 Rsun, while between ~2.5 and 5.5 Rsun, M values progressively decrease with time/altitude from M ~ 1 down to M ~ 0.3. Such large values of M are theoretically acceptable only if flux pile-up reconnection is envisaged. The observed occurrence of multiple reconnections associated with a CME is verified by numerical simulations of an eruption occurring within multiple helmet streamers. The simulations confirm that small side reconnections are a consequence of CME expansion against the surrounding coronal streamers. The simulated and observed evolution of aperture angles between the SMSs are in good agreement as well. These results demonstrate the effect of the global coronal magnetic field in the occurrence of small-scale eruptions due to lateral reconnection in a preceding CME event.
SOHO-23: Understanding a Peculiar Solar Minimum, ASP Conference Series, vol. 428, eds., Cranmer, S.R., Hoeksema, J. T., Kohl, J. L.,p. 177.
Analyzing the data from the UVCS and other instruments on board SOHO, we studied the plasma features of the current sheet observed in various temperatures. The results suggest thick current sheet with small interior structures, which implies the development of plasma instabilities in the sheet. Direct measurements yielded the sheet thickness d between 5.0E4 and 4.6E5 km, increasing with both time and altitude. The possibility that this large apparent d results from projection effects is examined and rejected. We derive theoretical scaling laws relating d to other observables that corroborate this conclusion and thus help confine both d and other parameters to a reasonable range. The possible impact of our results on the existing models of particle acceleration in reconnecting current sheets is also briefly discussed.
SOHO-23: Understanding a Peculiar Solar Minimum, ASP Conference Series, vol. 428, eds., Cranmer, S.R., Hoeksema, J. T., Kohl, J. L., p. 181.
We investigated the evolutionary features of the magnetic configuration that includes a current-carrying flux rope, which is used to model the filament, after the loss of equilibrium in the system takes place in a catastrophic fashion. The flux rope is thrust upward rapidly and various types of disturbance around it and even on the boundary surface are consequently invoked. A slow mode shock is produced in front of the flux rope when its speed exceeds the local slow magnetoacoustic wave speed, and a fast mode shock appears as the speed exceeds the fast wave speed. Both shocks expand toward the flank, the fast one eventually reaches the boundary surface and its reflection forms two echoes from the footpoints, but the slow mode shock decays somewhere in the lower corona and cannot approach to the boundary. The interaction of the fast shock with the boundary leads to a disturbance of which the propagation accounts for the Moreton wave when observed in H\u03b1. Our results also suggest that the disturbance in the corona caused by the slow shock and the velocity vortices might account for the EIT wave whose speed is about 40% that of the Moreton wave. Implication of these results to the observed correlation of the type II radio burst to the fast and the slow mode shocks will also be discussed.
SOHO-23: Understanding a Peculiar Solar Minimum, ASP Conference Series, vol. 428, eds., Cranmer, S.R., Hoeksema, J. T., Kohl, J. L., p. 187.
We compute coronal outflow velocity maps at a fixed height in the corona for more than 10 years of Solar Cycle 23. The velocities are estimated from the UVCS/SOHO synoptic observations of the O VI 103.2 nm and 103.7 nm intensities. We chose a height as close as possible to the traditional coronal source surface height of 2.5 solar radii (from Sun center) to perform the velocity computations. Variations of the velocities over the solar cycle show the response of the corona to changes in both the magnetic field geometry and plasma conditions in the regions where the wind is generated. In addition to providing a description of the coronal regions of interest, the maps can be used to show the connections between different coronal structures and the fast and slow speed wind streams that exist further out in the heliosphere.
SOHO-23: Understanding a Peculiar Solar Minimum, ASP Conference Series, vol. 428, eds., Cranmer, S.R., Hoeksema, J. T., Kohl, J. L., p. 191.
The well maintained UVCS/SOHO radiometric calibration to determine the variations in the ultraviolet intensities in polar coronal holes between the Solar Cycle 22/23 and Cycle 23/24 minima. The radiometric calibration has been carefully monitored and updated during the mission by observing an ensemble of B-stars, which as a group, are believed to have a stable mean irradiance. These observations along with data from the freshly calibrated Ultraviolet Coronal Spectrometer on the Spartan 201 satellite in 1998, have been used to determine the changes in the calibration. The Spartan 201 inter-calibration, together with the original laboratory calibration, was used to establish the in-flight absolute radiometric calibration scale. This paper summarizes the in- flight radiometric calibration of UVCS/SOHO and observed variations in polar coronal hole intensities as a function of heliographic height above the poles of the Sun.
SOHO-23: Understanding a Peculiar Solar Minimum, ASP Conference Series, vol. 428, eds., Cranmer, S.R., Hoeksema, J. T., Kohl, J. L., p. 197.
In the present work we use a deep-exposure spectrum taken by the SUMER spectrometer in a polar coronal hole in 1996 to measure the ion temperatures of a large number of ions at many different heights above the limb between 0.03 and 0.17 solar radii. We find that the measured ion temperatures are almost always larger than the electron temperatures and exhibit a non-monotonic dependence on the charge-to-mass ratio. We use these measurements to provide empirical constraints to a theoretical model of ion heating and acceleration based on gradually replenished ion-cyclotron waves. We compare the wave power required to heat the ions to the observed levels to a prediction based on a model of anisotropic magnetohydrodynamic turbulence. We find that the empirical heating model and the turbulent cascade model agree with one another, and explain the measured ion temperatures, for charge-to-mass ratios smaller than about 0.25. However, ions with charge-to-mass ratios exceeding 0.25 disagree with the model the wave power they require to be heated to the measured ion temperatures shows an increase with charge-to-mass ratio (i.e., with increasing frequency) that cannot be explained by a traditional cascade model. We discuss possible additional processes that might be responsible for the inferred surplus of wave power.
SOHO-23: Understanding a Peculiar Solar Minimum, ASP Conference Series, vol. 428, eds., Cranmer, S.R., Hoeksema, J. T., Kohl, J. L., p. 201.
In the present work we analize multiwavelength observations from Hinode, SOHO and STEREO of the early phases of a Coronal Mass Ejection (CME). We use Hinode/EIS and SOHO/UVCS high resolution spectra to measure the physical properties of the CME ejecta as a function of time at 1.1 and 1.9 solar radii. Hinode/XRT images are used in combination with EIS spectra to constrain the high temperature plasma properties of the ejecta. SECCHI/EUVI, SECCHI/COR1 and SOHO/LASCO images are used to measure the CME velocity and acceleration. The combination of measurements of plane of the sky velocities and line of sight speeds from EIS and UVCS Doppler shifts allows us to determine the absolute velocities of the CME plasma at 1.1 and 3.0 solar radii. Plasma properties, dynamical status, thermal structure and brightness distributions are used to constrain the energy contents of the CME plasma and to compare it to theoretical predictions from models of CME plasma heating and acceleration.
SOHO-23: Understanding a Peculiar Solar Minimum, ASP Conference Series, vol. 428, eds., Cranmer, S.R., Hoeksema, J. T., Kohl, J. L., p. 209.
This presentation will review our growing understanding of the physics behind coronal heating (in open-field regions) and the acceleration of the solar wind. Many new insights have come from the last solar cycles worth of observations and theoretical work. Measurements of the plasma properties in the extended corona, where the primary solar wind acceleration occurs, have been key to discriminating between competing theories. This talk will describe how UVCS/SOHO measurements of coronal holes and streamers over the last 14 years have provided clues about the detailed kinetic processes that energize both fast and slow wind regions. We will also present a brief survey of current ideas involving the coronal source regions of fast and slow wind streams, and how these change over the solar cycle. These source regions are discussed in the context of recent theoretical models (based on MHD turbulence) that have begun to successfully predict both the heating and acceleration in fast and slow wind regions with essentially no free parameters. Some new results regarding these models -- including the realization that the turbulent heating rate ends up scaling with the mean coronal magnetic flux density -- will also be presented.
SOHO-23: Understanding a Peculiar Solar Minimum, ASP Conference Series, vol. 428, eds., Cranmer, S.R., Hoeksema, J. T., Kohl, J. L., p. 223.
The Whole Heliosphere Interval (WHI) is an internationally coordinated observing and modeling effort to characterize the 3-dimensional interconnected solar-heliospheric-planetary system - a.k.a. the heliophysical system. The heart of the WHI campaign is the study of the interconnected global 3-D heliophysical domain, from the interior of the Sun, to the Earth, outer planets, and into interstellar space. WHI observing campaigns began with the 3-D solar structure from solar Carrington Rotation 2068, which ran from March 20 - April 16, 2008. Observations and models of the outer heliosphere and planetary impacts extend beyond those dates as necessary for example, the solar wind transit time to the outer planets can take months. WHI occurred during this extended solar minimum, which optimizes our ability to characterize the 3-D heliosphere and trace the structure to the outer limits of the heliosphere. Highlights include the 3-D reconstruction of the solar wind and complex geospace response during this solar minimum, contrasts with the past solar minimum, and the effect of transient activity on the quiet heliosphere. Nearly 200 scientists are participating in WHI data and modeling efforts, ensuring that the WHI integrated observations and models will give us a new view of the heliophysical system. A summary of some of the key results so far from the WHI, especially from the IAU JD16 meeting in August 2009, will be given.
SOHO-23: Understanding a Peculiar Solar Minimum, ASP Conference Series, vol. 428, eds., Cranmer, S.R., Hoeksema, J. T., Kohl, J. L., p. 237.
We have measured the physical properties of polar coronal holes from the minimum activity phase of solar cycle 23 (1996-1997) to the present peculiar minimum of solar cycle 24 (2007-2009) using the UVCS instrument on SOHO. Observations in H I Lyman alpha (121.6 nm) and O VI (103.2, 103.7 nm) provide spectroscopic diagnostics of proton and O5+ bulk outflow velocities and velocity distributions as a function of heliocentric distance above the poles of the Sun. These observations have allowed us to follow the changes in the physical properties of the polar coronal holes during solar cycle 23 and its approach to the current minimum. Recent ground- and space-based observations have reported a variety of unusual phenomena associated with the current minimum. We present the comparison of observed oxygen line intensities, line ratios, and profiles for polar coronal holes at both minima and during solar cycle 23 and show how this new minimum manifests itself in the ultraviolet corona. The comparison of the physical properties of these two minima as seen by UVCS in the extended corona, now possible for the first time, may provide crucial empirical constraints on models of extended coronal heating and acceleration for the fast solar wind.
SOHO-23: Understanding a Peculiar Solar Minimum, ASP Conference Series, vol. 428, eds., Cranmer, S.R., Hoeksema, J. T., Kohl, J. L., p. 321.
During the solar minimum STEREO observations show that the three-dimensional structure of the solar corona can be described well by a tilted bi-polar magnetic configuration. The slow solar wind is modeled using three-fluid model that includes heavy ions, such as He II and O VI. The model is initialized with photospheric magnetic field and potential extrapolation. The resulting steady state non-potential streamer configuration calculated with this model is compared to STEREO observations of the streamer density structure. SOHO/UVCS observations are used to compare the O VI emission to model results. We will discuss the unique properties of the solar wind produced in this configuration.
The Astrophysical Journal Letters, Volume 713, Issue 1, pp. L69-L73
UV spectra of the bright sungrazing comet C-2003K7 detected at 2.37 R sun above the Sun surface by the Ultraviolet Coronagraph Spectrometer (UVCS) during the daily synoptic scan show bright lines of H I Ly&alpha, Si III &lambda 1206, and C III &lambda 977. The derived outgassing rate is an order of magnitude larger than those of the other sungrazers observed by UVCS. Analysis of the spectra suggests that the comet broke apart into smaller pieces before it reached the UVCS slit. The observations provide lower and upper limits to the values of the Si III/C III ratio, in the range 8-22. The ratio indicates a larger abundance of silicates in the cometary dust as compared to organic refractory materials.
The Astrophysical Journal, Volume 711, Issue 1, pp. 75-98
In the present work, we analyze multiwavelength observations from Hinode, Solar and Heliospheric Observatory (SOHO), and STEREO of the early phases of a coronal mass ejection (CME). We use Hinode/EIS and SOHO/UVCS high-resolution spectra to measure the physical properties of the CME ejecta as a function of time at 1.1 and 1.9 solar radii. Hinode/XRT images are used in combination with EIS spectra to constrain the high temperature plasma properties of the ejecta. SECCHI/EUVI, SECCHI/COR 1, SOHO/EIT, and SOHO/LASCO images are used to measure the CME trajectory, velocity, and acceleration. The combination of measurements of plane of the sky velocities from two different directions allows us to determine the total velocity of the CME plasma up to 5 solar radii. Plasma properties, dynamical status, thermal structure, and brightness distributions are used to constrain the energy content of the CME plasma and to determine the heating rate. We find that the heating is larger than the kinetic energy, and compare it to theoretical predictions from models of CME plasma heating and acceleration.
TWELFTH INTERNATIONAL SOLAR WIND CONFERENCE. AIP Conference Proceedings, Volume 1216, pp. 432-435 (2010)
The intensity of the H I Ly&alpha line measured by the UltraViolet Coronagraph Spectrometer (UVCS) onboard the Solar and Heliospheric Observatory (SOHO) is used to investigate the density turbulence within the coronal mass ejection (CME) occurred on 2006 December 24, in the South polar coronal hole. In order to compare the spectral index inside the CME with those found in the undisturbed coronal plasma, we examined the CME data by applying the wavelet technique. This temporal analysis reveals, during the whole observation time, the existence of large-scale density fluctuations of periods from tens of minutes to a few hours. However, during the CME, the power spectrum becomes less steep with a spectral slope about 5/3, typical of the turbulent regime, whilst prior to the CME and in the recovery phase the spectral slope is about 3. The Kolmogorov-like spectrum observed within the CME is evidence for the nearly incompressible turbulent character of the CME plasma. This spectrum is significantly different from that of the high-speed flow from coronal holes and the low-speed wind originating above closed-field coronal streamers. This result is particularly important to advance in the understanding of where the main source of CME flux injection resides.
TWELFTH INTERNATIONAL SOLAR WIND CONFERENCE. AIP Conference Proceedings, Volume 1216, pp. 420-423
On June 2, 2003, SOHO/LASCO coronagraph observed two CMEs at the West limb of the Sun, at 00.30 and 08:54 UT, respectively, which appeared to originate from the same source region. Both CMEs show the typical three-part structure. These events have been also observed by SOHO/UVCS, allowing us to infer their physical parameters. We also looked for interplanetary signatures of the CMEs in ACE `in situ' observations but we did not find evidence of the ejected flux rope; however, the solar wind appeared significantly distorted, probably as a consequence of the influence of both CMEs on their surrounding interplanetary plasma.
TWELFTH INTERNATIONAL SOLAR WIND CONFERENCE. AIP Conference Proceedings, Volume 1216, pp. 387-390
The present study concerns a comparison between the slow solar wind plasma parameters obtained in the extended corona by the UV spectroscopic data from the Ultraviolet Coronagraph Spectrometer (UVCS) onboard SOHO during the minimum of solar activity (1996) and the results of a time-dependent 2.5 D three-fluid MHD model of coronal streamer belt. The aim of the study is to improve the knowledge of the slow solar wind acceleration mechanism and the origin of its variability.
TWELFTH INTERNATIONAL SOLAR WIND CONFERENCE. AIP Conference Proceedings, Volume 1216, pp. 363-366
We investigated the proton enhancement and O6+/H depletion in the vicinity of the heliospheric current sheet (HCS) using data from STEREO/PLASTIC and STEREO/IMPACT. Three HCS crossing events were studied. For the first two events, the proton enhancement and O6+/H depletion are found to lie at one edge of the HCS. The proton density has a steep slope both at the HCS and at the other boundary of the enhancement. In the third event the proton enhancement and O6+/H depletion surround the HCS and last for 8 hours while the density profile is very different from the other two events. Velocity shear is observed at the HCS for the first two events but not for the third. The enhancement of hydrogen and depletion of oxygen at the streamer belt in the solar corona have been reported using UVCS observation. A potential connection with our observations is based on the similar features observed at 1 AU. How the plasma flows out of the streamer belt, and why there are different features in HCS encounters remain open questions for future study.
TWELFTH INTERNATIONAL SOLAR WIND CONFERENCE. AIP Conference Proceedings, Volume 1216, pp. 52-55
In addition to the high temperatures, SOHO/UVCS observations have shown that heavy ions in the polar corona are heated more than protons, and that heavy ion heating is more than mass proportional; further, the perpendicular temperatures are much larger than parallel temperatures. Here, we propose that the more than mass proportional heating of heavy ions in coronal holes is due to the ion reflection at supercritical quasi-perpendicular shocks and to the ion acceleration by the motional electric field in the shock frame. We also discuss the formation of collisionless shock in the polar corona.
Solar Physics, (arXiv: 1001.3843)
We present a detailed model of stray-light suppression in the spectrometer channels of the Ultraviolet Coronagraph Spectrometer (UVCS) on the SOHO spacecraft. The control of diffracted and scattered stray light from the bright solar disk is one of the most important tasks of a coronagraph. We compute the fractions of light that diffract past the UVCS external occulter and non-specularly pass into the spectrometer slit. The diffracted component of the stray light depends on the finite aperture of the primary mirror and on its figure. The amount of non-specular scattering depends mainly on the micro-roughness of the mirror. For reasonable choices of these quantities, the modeled stray-light fraction agrees well with measurements of stray light made both in the laboratory and during the UVCS mission. The models were constructed for the bright H I Ly&alpha emission line, but they are applicable to other spectral lines as well.
The Astrophysical Journal, Volume 708, Issue 2, pp. 1135-1144
We study the evolution and physical parameters of three consecutive coronal mass ejections (CMEs) that occurred at the west limb of the Sun on 2003 June 2 at 00:30, 08:54, 16:08 UT, respectively. The Large Angle and Spectrometric Coronagraph Experiment (LASCO) CME catalog shows that the CMEs entered the C2 field of view with position angles within a 5° interval. This suggests a common origin for the ejections, to be identified with the magnetic system associated with the active region that lies below the CMEs. The close proximity in time and source location of the events prompted us to analyze LASCO white light data and Ultraviolet Coronagraph Spectrometer (UVCS) spectra with the aim of identifying similarities and differences among the three CMEs. It turns out that two of them display the typical three-part structure, while no conclusion can be drawn about the morphology of the third ejection. The CMEs plasma is "cool," i.e., electron temperatures in the CMEs front are of the order of 2 × 105 K, with no significant variation between different events. However, ejection speeds vary by a factor of ~1.5 between consecutive events and electron densities (more precisely emission measures) by a factor of ~6 between the first CME and the second and third CMEs. In the aftermath of all events, we found evidence of current sheets (CSs) both in LASCO and UVCS. We give here the CS physical parameters (electron temperature, density, and kinetic temperature) and follow, in one of the events, their temporal evolution over a 6 hr time interval. A discussion of our results, in the framework of previous findings, concludes the paper.
TWELFTH INTERNATIONAL SOLAR WIND CONFERENCE. AIP Conference Proceedings, Volume 1216, pp. 56-59
We have recently presented a mechanism for preferential acceleration and heating of coronal hole minor ions [1, 2]. The energization is due to the effect of multiple cyclotron resonances in the presence of sunward and anti-sunward dispersive ion cyclotron waves, providing a second-order Fermi interaction. The mechanism is preferential because coronal hole protons do not experience such multiple resonances. The detailed model results depend on many parameters, including poorly-known quantities such as the wave intensities, spectral shapes and radial profiles. In this paper, we show that reasonable choices for these quantities can yield excellent agreement with the observations. We find that only a small fraction of the extrapolated wave power is needed to provide the observed heating, and there is an indication that the resonant wave levels are increasing with radial position between r = 2 Rs and 4 Rs.