The Helioseismic and Magnetic Imager (HMI) instrument and investigation as a part of the NASA Solar Dynamics Observatory (SDO) is designed to study convection-zone dynamics and the solar dynamo, the origin and evolution of sunspots, active regions, and complexes of activity, the sources and drivers of solar magnetic activity and disturbances, links between the internal processes and dynamics of the corona and heliosphere, and precursors of solar disturbances for space-weather forecasts. A brief overview of the instrument, investigation objectives, and standard data products is presented.

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The Helioseismic and Magnetic Imager (HMI) Investigation for the Solar Dynamics Observatory (SDO)

Monthly Notices is one of the three largest general primary astronomical research publications. It is an international journal, published by the Royal Astronomical Society. This article 1 describes its publication policy and practice.

Monthly Notices of the Royal Astronomical Society

We propose a new model for the initiation of a solar coronal mass ejection (CME). The model agrees with two properties of CMEs and eruptive flares that have proved to be very difficult to explain with previous models: (1) very low-lying magnetic field lines, down to the photospheric neutral line, can open toward infinity during an eruption; and (2) the eruption is driven solely by magnetic free energy stored in a closed, sheared arcade. Consequently, the magnetic energy of the closed state is well above that of the posteruption open state. The key new feature of our model is that CMEs occur in multipolar topologies in which reconnection between a sheared arcade and neighboring flux systems triggers the eruption. In this "magnetic breakout" model, reconnection removes the unsheared field above the low-lying, sheared core flux near the neutral line, thereby allowing this core flux to burst open. We present numerical simulations that demonstrate our model can account for the energy requirements for CMEs. We discuss the implication of the model for CME/flare prediction.

https://iopscience.iop.org/article/10.1086/306563/pdf

A Model for Solar Coronal Mass Ejections

The ideal helical kink instability of a force-free coronal magnetic flux rope, anchored in the photosphere, is studied as a model for solar eruptions. Using the flux rope model of Titov and D?moulin as the initial condition in MHD simulations, both the development of helical shape and the rise profile of a confined (or failed) filament eruption (on 2002 May 27) are reproduced in very good agreement with the observations. By modifying the model such that the magnetic field decreases more rapidly with height above the flux rope, a full (or ejective) eruption of the rope is obtained in very good agreement with the developing helical shape and the exponential-to-linear rise profile of a fast coronal mass ejection (CME) on 2001 May 15. This confirms that the helical kink instability of a twisted magnetic flux rope can be the mechanism of the initiation and the initial driver of solar eruptions. The agreement of the simulations with properties that are characteristic of many eruptions suggests that they are often triggered by the kink instability. The decrease of the overlying field with height is a main factor in deciding whether the instability leads to a confined event or to a CME.

https://iopscience.iop.org/article/10.1086/462412/pdf

Confined and Ejective Eruptions of Kink-unstable Flux Ropes

Solar Probe Plus (SPP) will be the first spacecraft to fly into the low solar corona. SPP’s main science goal is to determine the structure and dynamics of the Sun’s coronal magnetic field, understand how the solar corona and wind are heated and accelerated, and determine what processes accelerate energetic particles. Understanding these fundamental phenomena has been a top-priority science goal for over five decades, dating back to the 1958 Simpson Committee Report. The scale and concept of such a mission has been revised at intervals since that time, yet the core has always been a close encounter with the Sun. The mission design and the technology and engineering developments enable SPP to meet its science objectives to: (1) Trace the flow of energy that heats and accelerates the solar corona and solar wind; (2) Determine the structure and dynamics of the plasma and magnetic fields at the sources of the solar wind; and (3) Explore mechanisms that accelerate and transport energetic particles. The SPP mission was confirmed in March 2014 and is under development as a part of NASA’s Living with a Star (LWS) Program. SPP is scheduled for launch in mid-2018, and will perform 24 orbits over a 7-year nominal mission duration. Seven Venus gravity assists gradually reduce SPP’s perihelion from 35 solar radii (
 $R_{S}$
 ) for the first orbit to ${<}10~R_{S}$
 for the final three orbits. In this paper we present the science, mission concept and the baseline vehicle for SPP, and examine how the mission will address the key science questions

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The Solar Probe Plus Mission: Humanity’s First Visit to Our Star

Global Coronal Modeling and Space Weather Prediction

Helmet streamers on the sun were observed to be the site of coronal mass ejections, dynamic events that eject coronal plasma and magnetic fields into the solar wind. A two dimensional (azimuthally symmetric) helmet streamer configuration was developed by computing solutions of the time dependent magnetohydrodynamic (MHD) equations, for a specified magnetic flux distribution on the sun. The helmet streamer is not symmetric about the equator. The evolution of the configuration, when differential rotation is applied, was investigated. It was found that after many rotations the configuration does not reach a steady state, but disrupts recurrently with the ejection of a plasmoid. These results suggest that differential rotation may be one of the mechanisms by which mass ejections are initiated.

Modeling coronal evolution

A sequence of apparently coupled eruptions was observed on 2010 August 1-2 by SDO and STEREO. The eruptions were closely synchronized with one another, even though some of them occurred at widely separated locations. In an attempt to identify a plausible reason for such synchronization, we study the large-scale structure of the background magnetic configuration. The coronal field was computed from the photospheric magnetic field observed at the appropriate time period by using the potential field source-surface model. 
We investigate the resulting field structure by analyzing the so-called squashing factor calculated at the photospheric and source-surface boundaries, as well as at different coronal cross-sections. Using this information as a guide, we determine the underlying structural skeleton of the configuration, including separatrix and quasi-separatrix surfaces. Our analysis reveals, in particular, several pseudo-streamers in the regions where the eruptions occurred. Of special interest to us are the magnetic null points and separators associated with the pseudo-streamers. We propose that magnetic reconnection triggered along these separators by the first eruption likely played a key role in establishing the assumed link between the sequential eruptions. The present work substantiates our recent simplified magnetohydrodynamic model of sympathetic eruptions and provides a guide for further deeper study of these phenomena. Several important implications of our results for the S-web model of the slow solar wind are also addressed.

2010 August 1-2 sympathetic eruptions: I. Magnetic topology of the source-surface background field

Ensemble modeling is a method of prediction based on the use of a representative sample of possible future states. Global models of the solar corona and inner heliosphere are now maturing to the point of becoming predictive tools, thus, it is both meaningful and necessary to quantitatively assess their uncertainty and limitations. In this study, we apply simple ensemble modeling techniques in a first step towards these goals. We focus on one relatively quiescent time period, Carrington rotation 2062, which occurred during the late declining phase of solar cycle 23 and assess the sensitivity of the model results to variations in boundary conditions, models, and free parameter values. We present variance maps, "whisker" plots, and Taylor diagrams to estimate the accuracy of the solutions, which demonstrate that the ensemble mean solution outperforms any of the individual realizations. Our results provide a baseline against which future model improvements can be compared.

Ensemble modeling of the ambient solar wind

The Solar Probe Plus (SPP) mission scheduled for launch in 2018, will orbit between the Sun and Venus with
diminishing perihelia reaching as close as 7 million km (9.86 solar radii) from Sun center. In addition to a suite of in-situ
probes for the magnetic field, plasma, and energetic particles, SPP will be equipped with an imager. The Wide-field
Imager for the Solar PRobe+ (WISPR), with a 95° radial by 58° transverse field of view, will image the fine-scale
coronal structure of the corona, derive the 3D structure of the large-scale corona, and determine whether a dust-free zone
exists near the Sun. Given the tight mass constrains of the mission, WISPR incorporates an efficient design of two widefield
telescopes and their associated focal plane arrays based on novel large-format (2kx2k) APS CMOS detectors into
the smallest heliospheric imaging package to date. The flexible control electronics allow WISPR to collect individual
images at cadences up to 1 second at perihelion or sum several of them to increase the signal-to-noise during the
outbound part of the orbit. The use of two telescopes minimizes the risk of dust damage which may be considerable
close to the Sun. The dependency of the Thomson scattering emission of the corona on the imaging geometry dictates
that WISPR will be very sensitive to the emission from plasma close to the spacecraft in contrast to the situation for
imaging from Earth orbit. WISPR will be the first ‘local’ imager providing a crucial link between the large scale corona
and the in-situ measurements.

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Zoran Mikic

Papers

Global Coronal Modeling and Space Weather Prediction

Modeling coronal evolution

2010 August 1-2 sympathetic eruptions: I. Magnetic topology of the source-surface background field

Ensemble modeling of the ambient solar wind

Seeing the corona with the solar probe plus mission: the wide-field imager for solar probe+ (WISPR)