Rona Oran

Magnetohydrodynamic Waves and Coronal Heating: Unifying Empirical and MHD Turbulence Models

We present a new global model of the solar corona, including the low corona, the transition region, and the top of the chromosphere. The realistic three-dimensional magnetic field is simulated using the data from the photospheric magnetic field measurements. The distinctive feature of the new model is incorporating MHD Alfven wave turbulence. We assume this turbulence and its nonlinear dissipation to be the only momentum and energy source for heating the coronal plasma and driving the solar wind. The difference between the turbulence dissipation efficiency in coronal holes and that in closed field regions is because the nonlinear cascade rate degrades in strongly anisotropic (imbalanced) turbulence in coronal holes (no inward propagating wave), thus resulting in colder coronal holes, from which the fast solar wind originates. The detailed presentation of the theoretical model is illustrated with the synthetic images for multi-wavelength EUV emission compared with the observations from SDO AIA and STEREO EUVI instruments for the Carrington rotation 2107.

A GLOBAL WAVE-DRIVEN MAGNETOHYDRODYNAMIC SOLAR MODEL WITH A UNIFIED TREATMENT OF OPEN AND CLOSED MAGNETIC FIELD TOPOLOGIES

We describe, analyze, and validate the recently developed Alfv´ en Wave Solar Model, a three-dimensional global model starting from the top of the chromosphere and extending into interplanetary space (out to 1–2 AU). This model solves the extended, two-temperature magnetohydrodynamics equations coupled to a wave kinetic equation for low-frequency Alfv´ en waves. In this picture, heating and acceleration of the plasma are due to wave dissipation and to wave pressure gradients, respectively. The dissipation process is described by a fully developed turbulent cascade of counterpropagating waves. We adopt a unified approach for calculating the wave dissipation in both open and closed magnetic field lines, allowing for a self-consistent treatment in any magnetic topology. Wave dissipation is the only heating mechanism assumed in the model; no geometric heating functions are invoked. Electron heat conduction and radiative cooling are also included. We demonstrate that the large-scale, steady state (in the corotating frame) properties of the solar environment are reproduced, using three adjustable parameters: the Poynting flux of chromospheric Alfv´ en waves, the perpendicular correlation length of the turbulence, and a pseudoreflection coefficient. We compare model results for Carrington rotation 2063 (2007 November–December) with remote observations in the extreme-ultraviolet and X-ray ranges from the Solar Terrestrial Relations Observatory, Solar and Heliospheric Observatory, and Hinode spacecraft and with in situ measurements by Ulysses. The results are in good agreement with observations. This is the first global simulation that is simultaneously consistent with observations of both the thermal structure of the lower corona and the wind structure beyond Earth’s orbit.

CORONAL HEATING BY SURFACE ALFVÉN WAVE DAMPING: IMPLEMENTATION IN A GLOBAL MAGNETOHYDRODYNAMICS MODEL OF THE SOLAR WIND

The heating and acceleration of the solar wind is an active area of research. Alfv´ en waves, because of their ability to accelerate and heat the plasma, are a likely candidate in both processes. Many models have explored wave dissipation mechanisms which act either in closed or open magnetic field regions. In this work, we emphasize the boundary between these regions, drawing on observations which indicate unique heating is present there. We utilize a new solar corona component of the Space Weather Modeling Framework, in which Alfvwave energy transport is self-consistently coupled to the magnetohydrodynamic equations. In this solar wind model, the wave pressure gradient accelerates and wave dissipation heats the plasma. Kolmogorov-like wave dissipation as expressed by Hollweg along open magnetic field lines was presented in van der Holst et al. Here, we introduce an additional dissipation mechanism: surface Alfv´ en wave (SAW) damping, which occurs in regions with transverse (with respect to the magnetic field) gradients in the local Alfvspeed. For solar minimum conditions, we find that SAW dissipation is weak in the polar regions (where Hollweg dissipation is strong), and strong in subpolar latitudes and the boundaries of open and closed magnetic fields (where Hollweg dissipation is weak). We show that SAW damping reproduces regions of enhanced temperature at the boundaries of open and closed magnetic fields seen in tomographic reconstructions in the low corona. Also, we argue that Ulysses data in the heliosphere show enhanced temperatures at the boundaries of fast and slow solar wind, which is reproduced by SAW dissipation. Therefore, the models temperature distribution shows best agreement with these observations when both dissipation mechanisms are considered. Lastly, we use observational constraints of shock formation in the low corona to assess the Alfv´ speed profile in the model. We find that, compared to a polytropic solar wind model, the wave-driven model with physical dissipation mechanisms presented in this work is more aligned with an empirical Alfv´ en speed profile. Therefore, a wave-driven model which includes the effects of SAW damping is a better background to simulate coronal-mass-ejection-driven shocks.

NUMERICAL SIMULATIONS OF CORONAL MASS EJECTION ON 2011 MARCH 7: ONE-TEMPERATURE AND TWO-TEMPERATURE MODEL COMPARISON

During Carrington rotation (CR) 2107, a fast coronal mass ejection (CME; >2000 km s −1 ) occurred in active region NOAA 11164. This event is also associated with a solar energetic particle event. In this study, we present simulations of this CME with one-temperature (1T) and two-temperature (2T: coupled thermodynamics of the electron and proton populations) models. Both the 1T and 2T models start from the chromosphere with heat conduction and radiative cooling. The background solar wind is driven by Alfv´ en-wave pressure and heated by Alfv´ en-wave dissipation in which we have incorporated the balanced turbulence at the top of the closed field lines. The magnetic field of the inner boundary is set up using a synoptic map from Solar Dynamics Observatory/ Helioseismic and Magnetic Imager. The Titov–D´ emoulin flux-rope model is used to initiate the CME event. We compare the propagation of fast CMEs and the thermodynamics of CME-driven shocks in both the 1T and 2T CME simulations. Also, the synthesized white light images are compared with the Solar and Heliospheric Observatory/ Large Angle and Spectrometric Coronagraph observations. Because there is no distinction between electron and proton temperatures, heat conduction in the 1T model creates an unphysical temperature precursor in front of the CME-driven shock and makes the shock parameters (e.g., shock Mach number, compression ratio) incorrect. Our results demonstrate the importance of the electron heat conduction in conjunction with proton shock heating in order to produce the physically correct CME structures and CME-driven shocks.

A Steady-state Picture of Solar Wind Acceleration and Charge State Composition Derived from a Global Wave-driven MHD Model

The higher charge states found in slow (<400 km s−1) solar wind streams compared to fast streams have supported the hypothesis that the slow wind originates in closed coronal loops and is released intermittently through reconnection. Here we examine whether a highly ionized slow wind can also form along steady and open magnetic field lines. We model the steady-state solar atmosphere using the Alfven Wave Solar Model (AWSoM), a global MHD model driven by Alfven waves, and apply an ionization code to calculate the charge state evolution along modeled open field lines. This constitutes the first charge state calculation covering all latitudes in a realistic magnetic field. The ratios and are compared to in situ Ulysses observations and are found to be higher in the slow wind, as observed; however, they are underpredicted in both wind types. The modeled ion fractions of S, Si, and Fe are used to calculate line-of-sight intensities, which are compared to Extreme-ultraviolet Imaging Spectrometer (EIS) observations above a coronal hole. The agreement is partial and suggests that all ionization rates are underpredicted. Assuming the presence of suprathermal electrons improved the agreement with both EIS and Ulysses observations; importantly, the trend of higher ionization in the slow wind was maintained. The results suggest that there can be a sub-class of slow wind that is steady and highly ionized. Further analysis shows that it originates from coronal hole boundaries (CHBs), where the modeled electron density and temperature are higher than inside the hole, leading to faster ionization. This property of CHBs is global and observationally supported by EUV tomography.

On the Origin of Mid-latitude Fast Wind: Challenging the Two-state Solar Wind Paradigm

The bimodal paradigm of solar wind describes a slow solar wind situated near the heliospheric current sheet while a fast wind overexpands from the poles to fill in the remainder of the heliosphere. In this paper, we challenge this paradigm and focus here on mid-latitude wind using three fast-latitude passes completed by the Ulysses spacecraft. Based on its composition and dynamic properties, we discuss how this wind differs from both the fast, polar coronal hole wind and the low latitude, streamer-associated slow solar wind. Using a detailed analysis of ionic and elemental abundances, as well as solar wind dynamic properties, we conclude that there is a third quasi-stationary solar wind state, called the boundary wind. This boundary wind is characterized by a charge-state distribution that is similar to slow wind, but with an elemental composition that is coronal hole like. Based on these data, we present arguments for the location of the origin of this wind. We conclude that the boundary wind is a subset of the fast wind emanating from regions close to the boundaries of coronal holes and is accelerated by a similar process.

CHARGE STATE EVOLUTION IN THE SOLAR WIND. III. MODEL COMPARISON WITH OBSERVATIONS

We test three theoretical models of the fast solar wind with a set of remote sensing observations and in-situ measurements taken during the minimum of solar cycle 23. First, the model electron density and temperature are compared to SOHO/SUMER spectroscopic measurements. Second, the model electron density, temperature, and wind speed are used to predict the charge state evolution of the wind plasma from the source regions to the freeze-in point. Frozen-in charge states are compared with Ulysses/SWICS measurements at 1 AU, while charge states close to the Sun are combined with the CHIANTI spectral code to calculate the intensities of selected spectral lines, to be compared with SOHO/SUMER observations in the north polar coronal hole. We find that none of the theoretical models are able to completely reproduce all observations; namely, all of them underestimate the charge state distribution of the solar wind everywhere, although the levels of disagreement vary from model to model. We discuss possible causes of the disagreement, namely, uncertainties in the calculation of the charge state evolution and of line intensities, in the atomic data, and in the assumptions on the wind plasma conditions. Last, we discuss the scenario where the wind is accelerated from a region located in the solar corona rather than in the chromosphere as assumed in the three theoretical models, and find that a wind originating from the corona is in much closer agreement with observations.

Mass loading of the solar wind by a sungrazing comet

Collisionless mass loading was suggested by Biermann et al. (1967) for describing interactions between the solar wind and cometary atmospheres. Recent observations have led to an increased interest in coronal mass loading due to sungrazing comets and collisional debris of sunward migrating interplanetary dust particles. In a previous paper, we presented a 3-D MHD model of the solar corona based on the Block-Adaptive-Tree-Solarwind-Roe-Upwind-Scheme code which includes the interaction of dust with the solar wind. We have shown the impact on the solar wind from abrupt mass loading in the coronal region. We apply the model to a sungrazing cometary source, using ejected dust dynamics to generate tail-shaped mass-loading regions. Results help predict the effects on the solar wind acceleration and composition due to sungrazing comets, such as Comet C/2011 W3 (Lovejoy). We show how these effects may be detected by the upcoming Solar Probe Plus Mission.