Mari Paz Miralles

Prevalence of small-scale jets from the networks of the solar transition region and chromosphere

As the interface between the Sun’s photosphere and corona, the chromosphere and transition region play a key role in the formation and acceleration of the solar wind. Observations from the Interface Region Imaging Spectrograph reveal the prevalence of intermittent small-scale jets with speeds of 80 to 250 kilometers per second from the narrow bright network lanes of this interface region. These jets have lifetimes of 20 to 80 seconds and widths of ≤300 kilometers. They originate from small-scale bright regions, often preceded by footpoint brightenings and accompanied by transverse waves with amplitudes of ~20 kilometers per second. Many jets reach temperatures of at least ~105 kelvin and constitute an important element of the transition region structures. They are likely an intermittent but persistent source of mass and energy for the solar wind.

Physical Conditions in a Coronal Mass Ejection from Hinode, Stereo, and SOHO Observations

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.

Asymmetric Magnetic Reconnection in Solar Flare and Coronal Mass Ejection Current Sheets

We present two-dimensional resistive magnetohydrodynamic simulations of line-tied asymmetric magnetic reconnection in the context of solar flare and coronal mass ejection current sheets. The reconnection process is made asymmetric along the inflow direction by allowing the initial upstream magnetic field strengths and densities to differ, and along the outflow direction by placing the initial perturbation near a conducting wall boundary that represents the photosphere. When the upstream magnetic fields are asymmetric, the post-flare loop structure is distorted into a characteristic skewed candle flame shape. The simulations can thus be used to provide constraints on the reconnection asymmetry in post-flare loops. More hard X-ray emission is expected to occur at the footpoint on the weak magnetic field side because energetic particles are more likely to escape the magnetic mirror there than at the strong magnetic field footpoint. The footpoint on the weak magnetic field side is predicted to move more quickly because of the requirement in two dimensions that equal amounts of flux must be reconnected from each upstream region. The X-line drifts away from the conducting wall in all simulations with asymmetric outflow and into the strong magnetic field region during most of the simulations with asymmetric inflow. There is net plasma flow across the X-line for both the inflow and outflow directions. The reconnection exhaust directed away from the obstructing wall is significantly faster than the exhaust directed toward it. The asymmetric inflow condition allows net vorticity in the rising outflow plasmoid which would appear as rolling motions about the flux rope axis.

The role of turbulence in coronal heating and solar wind expansion.

Plasma in the Suns hot corona expands into the heliosphere as a supersonic and highly magnetized solar wind. This paper provides an overview of our current understanding of how the corona is heated and how the solar wind is accelerated. Recent models of magnetohydrodynamic turbulence have progressed to the point of successfully predicting many observed properties of this complex, multi-scale system. However, it is not clear whether the heating in open-field regions comes mainly from the dissipation of turbulent fluctuations that are launched from the solar surface, or whether the chaotic ‘magnetic carpet’ in the low corona energizes the system via magnetic reconnection. To help pin down the physics, we also review some key observational results from ultraviolet spectroscopy of the collisionless outer corona.

Comparison of Empirical Models for Polar and Equatorial Coronal Holes

We present a self-consistent empirical model for several plasma parameters of a large equatorial coronal hole observed on 1999 November 12 near solar maximum. The model was derived from observations with the Ultraviolet Coronagraph Spectrometer on the Solar and Heliospheric Observatory. In this Letter, we compare the observations of O VI λλ1032, 1037 emission lines with previous observations of a polar coronal hole observed near solar minimum. At the time of the 1999 observations, there was no evidence of large polar coronal holes. The resulting empirical model for the equatorial coronal hole describes the outflow velocities and most probable speeds for O5+, and we compared the derived ion properties with the empirical model for a solar minimum polar coronal hole. The comparison of the empirical models shows that the 1999 equatorial hole has lower O5+ outflow speeds and perpendicular temperatures than its polar counterpart from 1996 to 1997 at heights between 2 and 3 R☉. However, in situ asymptotic speeds of the wind streams coming from the 1996-1997 polar hole and from the 1999 equatorial hole are only ~15% different. Thus, the bulk of the solar wind acceleration must occur above 3 R☉ for the equatorial coronal hole. The equatorial hole also has a higher density than the polar hole at similar heights. It is not yet known whether the higher densities are responsible for the seeming inhibition of the fast ion outflow speeds and extremely large perpendicular temperatures that occur in polar coronal holes at solar minimum. We discuss the constraints and implications on various theoretical models of coronal heating and acceleration.

POST-CORONAL MASS EJECTION PLASMA OBSERVED BY HINODE

In the present work we study the evolution of an active region after the eruption of a coronal mass ejection (CME) using observations from the EIS and XRT instruments on board Hinode. The field of view includes a post-eruption arcade, a current sheet, and a coronal dimming. The goal of this paper is to provide a comprehensive set of measurements for all these aspects of the CME phenomenon made on the same CME event. The main physical properties of the plasma along the line of sight—electron density, thermal structure, plasma composition, size, and, when possible, mass—are measured and monitored with time for the first three hours following the CME event of 2008 April 9. We find that the loop arcade observed by EIS and XRT may not be related to the post-eruption arcade. Post-CME plasma is hotter than the surrounding corona, but its temperature never exceeds 3 MK. Both the electron density and thermal structure do not show significant evolution with time, while we found that the size of the loop arcade in the Hinode plane of the sky decreased with time. The plasma composition is the same in the current sheet, in the loop arcade, and in the ambient plasma, so all these plasmas are likely of coronal origin. No significant plasma flows were detected.

Physical properties of cooling plasma in quiescent active region loops

In the present work, we use SOHO/SUMER, SOHO/UVCS, SOHO/EIT, SOHO/LASCO, STEREO/EUVI, and Hinode/EIS coordinated observations of an active region (AR 10989) at the west limb taken on 2008 April 8 to study the cooling of coronal loops. The cooling plasma is identified using the intensities of SUMER spectral lines emitted at temperatures in the 4.15 ≤ log T ≤ 5.45 range. EIS and SUMER spectral observations are used to measure the physical properties of the loops. We found that before cooling took place these loops were filled with coronal hole-like plasma, with temperatures in the 5.6 ≤ log T ≤ 5.9 range. SUMER spectra also allowed us to determine the plasma temperature, density, emission measure, element abundances, and dynamic status during the cooling process. The ability of EUVI to observe the emitting region from a different direction allowed us to measure the volume of the emitting region and estimate its emission measure. Comparison with values measured from line intensities provided us with an estimate of the filling factor. UVCS observations of the coronal emission above the active region showed no streamer structure associated with AR 10989 at position angles between 242°and 253 EIT, LASCO, and EUVI-A narrowband images and UVCS spectral observations were used to discriminate between different scenarios and monitor the behavior of the active region in time. The present study provides the first detailed measurements of the physical properties of cooling loops, a very important benchmark for theoretical models of loop cooling and condensation.

Low-latitude coronal holes during solar maximum

Abstract The Ultraviolet Coronagraph Spectrometer (UVCS) on SOHO has been used to observe large low-latitude coronal holes during solar maximum that produced fast solar wind streams. UVCS observations show that large low-latitude coronal holes at solar maximum, coronal holes of at least 10° in longitude, have plasma properties that seem to bridge the gap between solar minimum polar coronal holes and streamers. The ion kinetic perpendicular temperatures in equatorial coronal holes are about 2 times larger than those in a solar minimum equatorial streamer, and about a factor of 2 smaller than those in polar coronal holes above 2 R ⊙ . The outflow speeds for the large equatorial coronal holes observed by UVCS are 3–4 times lower than those in polar coronal holes between 2 and 3 R ⊙ . The values for high- and mid-latitude coronal holes are in between those. In all these cases, the in situ data corresponding to these coronal holes showed high-speed wind streams with asymptotic speeds of 600–750 km s −1 . These wind speeds approach those observed over polar coronal holes at solar minimum, but the outflow speeds in these coronal holes between 2 and 3 R ⊙ are different. In contrast to the polar coronal holes, the bulk of the solar wind acceleration must occur above 3 R ⊙ for large low-latitude coronal holes at solar maximum. These observations provide detailed empirical constraints for theoretical models and may be key to understanding how the various types of solar wind plasma are heated and accelerated.

The Sun, the Solar Wind, and the Heliosphere

We describe the aims and contents of the book entitled “The Sun, the Solar Wind, and the Heliosphere”. This is a volume in the IAGA Special Book Series dedicated to the science covered by IAGA Division IV, Solar Wind and Interplanetary Field. The book features review articles on topics from the interior of the Sun to the outermost regions of the heliosphere. In addition, we highlight some of the results presented during the Division IV symposia at the 11th Scientific Assembly of IAGA in Sopron, Hungary, which was planned simultaneously with this book.

PROPERTIES OF A POLAR CORONAL HOLE DURING THE SOLAR MINIMUM IN 2007

We report measurements of a polar coronal hole during the recent solar minimum using the Extreme Ultraviolet Imaging Spectrometer on Hinode. Five observations are analyzed that span the polar coronal hole from the central meridian to the boundary with the quiet-Sun corona. We study the observations above the solar limb in the height range of 1.03-1.20 R ☉. The electron temperature T e and emission measure (EM) are found using a geometric mean emission measure method. The EM derived from the elements Fe, Si, S, and Al are compared in order to measure relative coronal-to-photospheric abundance enhancement factors. We also studied the ion temperature T i and the non-thermal velocity v nt using the line profiles. All these measurements are compared to polar coronal hole observations from the previous (1996-1997) solar minimum and to model predictions for relative abundances. There are many similarities in the physical properties of the polar coronal holes between the two minima at these low heights. We find that the electron density, T e, and T i are comparable in both minima. T e shows a comparable gradient with height. Both minima show a decreasing T i with increasing charge-to-mass ratio q/M. A previously observed upturn of T i for ions above q/M>0.25 was not found here. We also compared relative coronal-to-photospheric elemental abundance enhancement factors for a number of elements. These ratios were ~1 for both the low first ionization potential (FIP) elements Si and Al and the marginally high FIP element S relative to the low FIP element Fe, as is expected based on earlier observations and models for a polar coronal hole. These results are consistent with no FIP effect in a polar coronal hole.