MHD heliosphere with boundary conditions from a tomographic reconstruction using interplanetary scintillation data

This talk reviews the interplanetary scintillation (IPS) technique, which allows for the determination of speed and density turbulence of the solar wind in the three-dimensional (3D) inner heliosphere. When sufficient number of scintillating sources are employed to suitably probe a given part of interplanetary space, IPS observations are powerful to study the ambient solar wind as well as to track coronal mass ejections (CMEs) in the distance range of Sun to Earth's orbit. A number of CMEs have been tracked over the entire Sun-Earth distance, by combining the white-light images from the LASCO/SOHO space mission and IPS measurements obtained from the Ooty Radio Telescope (ORT) and ISEE Multi-antenna system. Since the IPS observations at Ooty are on a grid of large number of radio sources, they provide image of disturbance associated with the CME at different distances from the Sun before its arrival at the near-Earth space. The results on the CME “speeddistance” evolution indicate a two-level deceleration: a low decline in speed at distances within or about 100 solar radii and a rapid decrease at larger distances from the Sun. However, the radial decline rates of speed, respectively within and above 100 solar radii, differ between different CMEs and suggest the involvement of input energy associated with the CME eruption in the propagation as well as the dynamics of the ambient solar wind. The typical linear size evolution of the CME with heliocentric distance suggests a pressure balance maintained between the CME driver gas and the ambient solar wind. Further, IPS estimates of density turbulence and speed have been used as inputs to the University of California, San Diego (UCSD) timedependent tomography to reconstruct the full 3D distributions of both solar wind speed and density throughout the inner heliosphere. The usefulness of IPS measurements is discussed on the possibility of forming a basic model to forecast the arrival of CMEs at 1 AU.

We report extensive interplanetary scintillation (IPS) observations of the solar wind during the period centered around 11 May 1999, when the Earth was engulfed by a region of solar wind with unusually low densities (<1 cm−3) and velocities (<350 km s−1). IPS observations with the Ooty Radio Telescope (ORT), operating at 327 MHz, probe the inner heliosphere along a large number of closely spaced lines of sight over the heliocentric distance range of 0.2 to 1.0 AU, daily, at the rate of about 150 sources in the course of 10 hours. Using these observations, the southern half of the inner heliosphere was monitored everyday between 3 and 16 May 1999. The data show that the solar wind densities were globally subdued, having started subsiding from 3 May 1999, itself at different parts of the heliosphere. This was accompanied by a noticeable decrease in the velocity of the general solar wind also. The IPS data also reveal a small subset of closely spaced sources lying to the west of the Sun that show a steep drop in inferred densities to immeasurably low amounts on and around the 11th May. The actual morphology of the subsidence event, from our observations, is shown to be that of a “void‐within‐a‐void”. The role of the polar magnetic field reversal that was taking place between Carrington rotations 1947–1954, which may have played a key role in generating the subsidence event, is also examined. To the best of our knowledge this is the first time that sustained levels of low densities and velocities have been recorded for such a long period over a large part of the inner heliosphere. Some possible signatures of high energy “strahl” electrons were also recorded in this period.

<p>Near-Earth solar wind conditions, including disturbances generated by coronal mass ejections (CMEs), are routinely forecast using 3-dimensional, numerical magnetohydrodynamic (MHD) models of the heliosphere. The resulting forecast errors are largely the result of uncertainty in the near-Sun boundary conditions, rather than heliospheric model physics or numerics. Thus ensembles of heliospheric model runs with perturbed initial conditions are used to estimate forecast uncertainty. MHD heliospheric models are relatively cheap in computational terms, requiring tens of minutes to an hour to simulate CME propagation from the Sun to Earth. Thus such ensembles can be run operationally. However, ensemble size is typically limited to ~10<sup>1</sup>-10<sup>2</sup>, which may be inadequate to sample the relevant high-dimensional parameter space. Here, we describe a simplified solar wind model that can estimate CME arrival time in approximately 0.01 seconds on a modest desktop computer and thus enables significantly larger ensembles. It is a 1-dimensional, incompressible, hydrodynamic model, which has previously been used for the steady-state solar wind, but is here used in time-dependent form. This approach is shown to adequately emulate the MHD solutions to the same boundary conditions for both steady-state solar wind and CME-like disturbances. We suggest it could serve as a “surrogate” model for the full 3-dimensional MHD models. For example, ensembles of ~10<sup>5</sup>-10<sup>6</sup> members can be used to identify regions of parameter space for more detailed investigation by the MHD models. Similarly, the simplicity of the model means it can be rewritten as an adjoint model, enabling variational data assimilation with MHD models without the need to alter their code. Model code is available as an Open Source download in the Python language.</p>

We present a summary of results from ten years of interplanetary scintillation (IPS) observations of stream interaction regions (SIRs) in the solar wind. Previous studies had shown that SIRs were characterized by intermediate-velocity solar wind and – in the case of compressive interactions – higher levels of scintillation. In this study we considered all cases of intermediate velocities in IPS observations from the European Incoherent SCATter (EISCAT) radar facility made at low- and mid-heliographic latitudes between 1994 and 2003. After dismissing intermediate-velocity observations which were associated with solar-wind transients (such as coronal mass ejections) we found that the remaining cases of intermediate velocities lay above coronal structures where stream interaction would be expected. An improved ballistic mapping method (compared to that used in earlier EISCAT studies of interaction regions) was used to identify the regions of raypath in IPS observations which might be expected to include interaction regions and to project these regions out to the distances of in-situ observations. The early stages of developing compression regions, consistent with their development on the leading edges of compressive stream interaction regions, were clearly detected as close to the Sun as 30 R ⊙, and further ballistic projection out to the distances of in-situ observations clearly associated these developing structures with density and velocity features characteristic of developed interaction regions in in-situ data in the cases when such data were available. The same approach was applied to study non-compressive interaction regions (shear layers) between solar-wind streams of different velocities where the stream interface lay at near-constant latitude and the results compared with those from compressive interaction regions. The results confirm that intermediate velocities seen in IPS observations above stream boundaries may arise from either detection of intermediate-velocity flow in compression regions, or from non-compressive shear layers. The variation in velocity about the mean determined from IPS measurements (representing the spread in velocity across that part of the raypath associated with the interaction region in the analysis) was comparable in compressive and non-compressive regions – a potentially interesting result which may contain important information on the geometry of developing SIRs. It is clear from these results that compressive and non-compressive interaction regions belong to the same class of stream – stream interaction, with the dominant mode determined by the latitudinal gradient of the stream interface. Finally, we discuss the results from this survey in the light of new data from the Heliospheric Imagers (HI) on the Solar TErrestrial RElations Observatory (STEREO) spacecraft and other instruments, and suggest possible directions for further work.

In this study, we simulate the Solar Wind Charge Exchange (SWCX) soft X‐ray emissions at dayside magnetosheath and cusps by using magnetohydrodynamic (MHD) and LAtmos TEst Particle (LaTeP) models. MHD models are unable to resolve the particle kinetic effects, such as the different behaviors of ions with different q/m, or distinguish the magnetospheric plasma from the solar wind plasma. We investigate these effects with the LaTeP model. As the LaTeP model does not self‐compute magnetic and electric field, the magnetic and electric field data obtained from Open Geospace General Circulation Model (OpenGGCM) and Lagrangian version of the piecewise parabolic method (PPMLR) MHD model are used as the input to LaTeP model. The soft X‐ray emissivity maps simulated from pure OpenGGCM and PPMLR MHD approaches and from LaTeP‐OpenGGCM and LaTeP‐PPMLR approaches are presented and compared. The results indicate that the LaTeP model can well resolve the kinetic effects and can be used to investigate the individual spectral characteristics. Therefore, the LaTeP model is a complementary approach for simulating the X‐ray emissions near the dayside magnetopause. We also calculate the ratio of integrated OVII/OVIII line intensities, produced by charge exchange of O7+ ions and O8+ ions, respectively. We find a relatively higher ratio at the bow shock compared to the surrounding areas, suggesting that this ratio can be an effective parameter to identify the bow shock location.

The response of the Earth's magnetosphere to changing solar wind conditions are studied with a 3D Magnetohydrodynamic (MHD) model. One full year (155 Cluster orbits) of the Earth's magnetosphere is simulated using Grand Unified Magnetosphere Ionosphere Coupling simulation (GUMICS-4) magnetohydrodynamic code. Real solar wind measurements are given to the code as input to create the longest lasting global magnetohydrodynamics simulation to date. The applicability of the results of the simulation depends critically on the input parameters used in the model. Therefore, the validity and the variance of the OMNIWeb data is first investigated thoroughly using Cluster measurement close to the bow shock. The OMNIWeb and the Cluster data were found to correlate very well before the bow shock. The solar wind magnetic field and plasma parameters are not changed significantly from the $L_1$ Lagrange point to the foreshock, therefore the OMNIWeb data is appropriate input to the GUMICS-4. The Cluster SC3 footprints are determined by magnetic field mapping from the simulation results and the Tsyganenko (T96) model in order to compare two methods. The determined footprints are in rather good agreement with the T96. However, it was found that the footprints agree better in the northern hemisphere than the southern one during quiet conditions. If the By is not zero, the agreement of the GUMICS-4 and T96 footprint is worse in longitude in the southern hemisphere. Overall, the study implies that a 3D MHD model can increase our insight of the response of the magnetosphere to solar wind conditions.

We present here a time‐dependent three‐dimensional magnetohydrodynamic (MHD) solar wind simulation from the solar surface to the Earth's orbit driven by time‐varying line‐of‐sight solar magnetic field data. The simulation is based on the three‐dimensional (3‐D) solar‐interplanetary (SIP) adaptive mesh refinement (AMR) space‐time conservation element and solution element (CESE) MHD (SIP‐AMR‐CESE MHD) model. In this simulation, we first achieve the initial solar wind background with the time‐relaxation method by inputting a potential field obtained from the synoptic photospheric magnetic field and then generate the time‐evolving solar wind by advancing the initial 3‐D solar wind background with continuously varying photospheric magnetic field. The model updates the inner boundary conditions by using the projected normal characteristic method, inputting the high‐cadence photospheric magnetic field data corrected by solar differential rotation, and limiting the mass flux escaping from the solar photosphere. We investigate the solar wind evolution from 1 July to 11 August 2008 with the model driven by the consecutive synoptic maps from the Global Oscillation Network Group. We compare the numerical results with the previous studies on the solar wind, the solar coronal observations from the Extreme ultraviolet Imaging Telescope board on Solar and Heliospheric Observatory, and the measurements from OMNI at 1 astronomical unit (AU). Comparisons show that the present data‐driven MHD model's results have overall good agreement with the large‐scale dynamical coronal and interplanetary structures, including the sizes and distributions of the coronal holes, the positions and shapes of the streamer belts, the heliocentric distances of the Alfvénic surface, and the transitions of the solar wind speeds. However, the model fails to capture the small‐sized equatorial holes, and the modeled solar wind near 1 AU has a somewhat higher density and weaker magnetic field strength than observed. Perhaps better preprocessing of high‐cadence observed photospheric magnetic field (particularly 3‐D global measurements), combined with plasma measurements and higher resolution grids, will enable the data‐driven model to more accurately capture the time‐dependent changes of the ambient solar wind for further improvements. In addition, other measures may also be needed when the model is employed in the period of high solar activity.

This paper reports on the first combination of results from in-situ plasma measurements at Venus, using data from Venus Express, and remote sensing data from observations of interplanetary scintillation (IPS). In so doing, we demonstrate the value of combining remote sensing and in-situ techniques for the purpose of investigating interaction between solar wind, under several different conditions, and the Venusian magnetosphere. The ion mass analyser instrument (IMA) is used to investigate solar wind interaction with the Venusian magnetosphere in the presence of two different solar wind phenomena; a co-rotating interaction region (CIR) and a coronal mass ejection (CME). The CIR, detected with IPS and sampled in-situ at Venus is found to dramatically affect upstream solar wind conditions. These case studies demonstrate how combining results from these different data sources can be of considerable value when investigating such phenomena.

Time-dependent ionization is important for diagnostics of coronal streamers and pseudostreamers. We describe time-dependent ionization calculations for a three-dimensional magnetohydrodynamic (MHD) model of the solar corona and inner heliosphere. We analyze how non-equilibrium ionization (NEI) influences emission from a pseudostreamer during the Whole Sun Month interval (Carrington rotation CR1913, 1996 August 22 to September 18). We use a time-dependent code to calculate NEI states, based on the plasma temperature, density, velocity, and magnetic field in the MHD model, to obtain the synthetic emissivities and predict the intensities of the Lyα, O vi, Mg x, and Si xii emission lines observed by the SOHO/Ultraviolet Coronagraph Spectrometer (UVCS). At low coronal heights, the predicted intensity profiles of both Lyα and O vi lines match UVCS observations well, but the Mg x and Si xii emission are predicted to be too bright. At larger heights, the O vi and Mg x lines are predicted to be brighter for NEI than equilibrium ionization around this pseudostreamer, and Si xii is predicted to be fainter for NEI cases. The differences of predicted UVCS intensities between NEI and equilibrium ionization are around a factor of 2, but neither matches the observed intensity distributions along the full length of the UVCS slit. Variations in elemental abundances in closed field regions due to the gravitational settling and the FIP effect may significantly contribute to the predicted uncertainty. The assumption of Maxwellian electron distributions and errors in the magnetic field on the solar surface may also have notable effects on the mismatch between observations and model predictions.

A polytropic model for the solar wind

Abstract. Observations from the second Ulysses fast latitude scan show that the global structure of solar wind near solar maximum is much more complex than at solar minimum. Soon after solar maximum, Ulysses observed a polar coronal hole (high speed) plasma with magnetic polarity of the new solar cycle in the Northern Hemisphere. We analyze the solar wind structure at and near solar maximum using interplanetary scintillation (IPS) measurements. To do this, we have developed a new tomographic technique, which improves our ability to examine the complex structure of the solar wind at solar maximum. Our IPS results show that in 1999 and 2000 the total area with speed greater than 700 km s-1 is significantly reduced first in the Northern Hemisphere and then in the Southern Hemisphere. For year 2001, we find that the formation of large areas of fast solar wind around the north pole precedes the formation of large polar coronal holes around the southern pole by several months. The IPS observations show a high level agreement to the Ulysses observation, particularly in coronal holes.Key words. Interplanetary physics (solar wind plasma) – Radio science (remote sensing)

The recently refurbished Ooty Radio Telescope in southern India was used in a two-month campaign of interplanetary scintillation (IPS) observations in collaboration with the Cambridge IPS array in England during April–May 1992. The unique feature of this campaign was that, for the first time, scintillation enhancements were predicted in real time by observing solar events on 7–8 May, 1992 and then detected at both Ooty and Cambridge. Also, for the first time, high spatial resolution (∼ 100 sources sr−1) solar wind all-sky velocity maps were obtained at Ooty. Good consistency is found between the IPS observations from both observatories andin-situ shocks detected at Earth by IMP-8.Yohkoh soft X-ray images were used to infer the generation of a coronal mass ejection on 7 May, 1992.

Solar wind features a latitudinal structure that evolves during the solar activity cycle. The only in-situ measurements of the solar wind speed and density available so far were performed by Ulysses at the turn of the 20th and 21st centuries. They showed that when the solar activity is low, the wind is fast and rare, with an equatorial band filled with a slower but denser outflow. During epochs of high activity, the band of slow wind expands to all latitudes. However, details of possible north/south asymmetries and regular structure evolution at shorter time scales could not be established because the measurements were performed in-situ along a highly elliptical orbit with a period of the order of half of the solar cycle.Complementary methods of monitoring the solar wind latitudinal profiles include remote-sensing observations such as interplanetary scintillations (IPS) and hydrogen Lyman-α backscatter glow observations. Existing analyses of IPS and helioglow observations returned somewhat conflicting conclusions. Helioglow maps observed by SOHO/SWAN suggested that the solar wind flux temporarily features flux maxima at mid-latitudes. Analysis of IPS observations for equivalent times did not reveal such maxima in the solar wind speed.Insight from Ulysses resulted in a hypothesis that the energy flux of solar wind is latitudinally invariant, which cannot be verified without additional observations. A confirmation of this invariance would be an important milestone in the understanding of the solar wind emission mechanism and provide a handy tool supporting retrieval of the solar wind structure from observations of either IPS or the helioglow.GLObal solar Wind Structure (GLOWS) is a Lyman-α photometer onboard Interstellar Mapping and Acceleration Probe (IMAP), dedicated to helioglow observations aimed at retrieval of the solar wind structure. We present how GLOWS observations can be interpreted to resolve the helioglow/IPS solar wind structure dilemma and to verify the hypothesis of latitudinal invariance of the solar wind energy flux.

Interplanetary scintillation (IPS) observations made with the 327‐MHz multistation system of the Solar‐Terrestrial Environment Laboratory (STEL) are analyzed to investigate the global distribution of solar wind speed and density fluctuations (ΔNe) and their evolution during 1997–2009. This study aims at elucidating the evolution of ΔNe distribution during the cycle 23 and subsequent extended minimum, which is useful for improving understanding of the heliospheric response to the peculiar solar activity. The computer‐assisted tomography (CAT) method is used in the present study to deconvolve the line‐of‐sight integration of STEL IPS observations. This CAT method enables retrieval of the quasi‐stationary large‐scale structure of the background solar wind. The results show that the high (low)‐latitude region is dominated by reduced (enhanced) ΔNe plasma, being closely associated with the fast (slow) solar wind. The solar wind speed data show a distinct change with solar activity, and an excellent positive (negative) correlation is revealed between the fast (slow) wind area and the polar field strength of the Sun. In contrast, the ΔNe data do not show such a solar cycle variation, but instead reveal a significant increase in the fractional area of low‐ΔNe region in 2004 preceded by a constant value with a small amount of fluctuation. This change is observed for all latitudes, distinctly after 2007 for low latitudes. Our finding is consistent with the long‐term variation of the solar wind density revealed from in situ measurements at the Earth orbit, if ΔNe ∝ Ne (where Ne is the solar wind electron density), and also consistent with the coronal hole distribution during the last solar cycle. It is found that ΔNe is inversely correlated with the solar wind speed V. We obtain the best fit power law function ΔNe ∝ V−0.36±0.14 for V > 350 km/s, which is basically consistent with our earlier result. This fact suggests that the fractional density fluctuations ΔNe/Ne are greater in the fast wind than in the slow wind. There is no systematic variation in the power law index or slope during 1997–2009 except for a null slope in 2000, which may be ascribed to an insufficient resolution of the CAT analysis. Thus, the inverse relation between ΔNe and V is regarded as a general rule for solar wind turbulence. The important point to note is that a marked drop in ΔNe occurs for the slow speed wind, V < 350 km/s, particularly for 2004 and 2009. This fact may be attributed to different source conditions of the very‐low‐speed solar wind.

Three major cusp models are systematically compared with low-resolution (large-scale) and high-resolution (fine-scale) low-altitude observations. Those models are (a) global magnetohydrodynamics (MHD) models (including MHD+drift models), (b) turbulent/diffusive entry models, and (c) direct flowing entry models. Although low-resolution data are mostly consistent with MHD models, high-resolution data mostly contradicts them. The data instead supports the other models in which the cusp is considered as a local `extra' open region. This is a good lesson to us: past supportive `tests' of MHD cusp models might have essential flaws in the methodology, and high-resolution data is necessary even for large-scale modelings.