G. de Toma

If the Sun is so quiet, why is the Earth ringing? A comparison of two solar minimum intervals

coronal holes lingered even as the sunspots disappeared. Consequently, for the months surrounding the WHI campaign, strong, long, and recurring high-speed streams in the solar wind intercepted the Earth in contrast to the weaker and more sporadic streams that occurred around the time of last cycle’s WSM campaign. In response, geospace and upper atmospheric parameters continued to ring with the periodicities of the solar wind in a manner that was absent last cycle minimum, and the flux of relativistic electrons in the Earth’s outer radiation belt was elevated to levels more than three times higher in WHI than in WSM. Such behavior could not have been predicted using sunspot numbers alone, indicating the importance of considering variation within and between solar minima in analyzing and predicting space weather responses at the Earth during solar quiet intervals, as well as in interpreting the Sun’s past behavior as preserved in geological and historical records.

THE CALM BEFORE THE STORM: THE LINK BETWEEN QUIESCENT CAVITIES AND CORONAL MASS EJECTIONS

Determining the state of the corona prior to CMEs is crucial to understanding and ultimately predicting solar eruptions. A common and compelling feature of CMEs is their three-part morphology, as seen in white-light observations of a bright expanding loop, followed by a relatively dark cavity, and finally a bright core associated with an erupting prominence/filament. This morphology is an important constraint on CME models. It is also quite common for a three-part structure of loop, cavity, and prominence core to exist quiescently in the corona, and this is equivalently an important constraint on models of CME-precursor magnetic structure. These quiescent structures exist in the low corona, primarily below approximately 1.6R� , and so are currently observable in white light duringsolar eclipses, or else by the Mauna Loa Solar Observatory Mk4 coronameter. We present the first comprehensive, quantitative analysis of white-light quiescent cavities as observed by the Mk4 coronameter. We find that such cavities are ubiquitous, as they are the coronal limb counterparts to filament channels observed on the solar disk. We consider examples that range from extremely long-lived, longitudinally extended polar-crown-filament-related cavities to smallercavitiesassociated withfilamentsnearorwithinactiveregions.Theformerareoftenvisiblefordaysandeven weeks at a time and can be identified as long-lived cavities that survive for months. We quantify cavity morphology and intensity contrast properties and consider correlations between these properties. We find multiple cases in which quiescentcavitiesdirectlyeruptintoCMEsandconsiderhowmorphologicalandintensitycontrastpropertiesofthese cases differ from the general population of cavities. Finally, we discuss the implications that these observations may have for the state of the corona just prior to a CME, and more generally for the nature of coronal MHD equilibria.

A MODEL FOR MAGNETICALLY COUPLED SYMPATHETIC ERUPTIONS

Sympathetic eruptions on the Sun have been observed for several decades, but the mechanisms by which one eruption can trigger another remain poorly understood. We present a three-dimensional MHD simulation that suggests two possible magnetic trigger mechanisms for sympathetic eruptions. We consider a configuration that contains two coronal flux ropes located within a pseudo-streamer and one rope located next to it. A sequence of eruptions is initiated by triggering the eruption of the flux rope next to the streamer. The expansion of the rope leads to two consecutive reconnection events, each of which triggers the eruption of a flux rope by removing a sufficient amount of overlying flux. The simulation qualitatively reproduces important aspects of the global sympathetic event on 2010 August 1 and provides a scenario for the so-called twin filament eruptions. The suggested mechanisms are also applicable for sympathetic eruptions occurring in other magnetic configurations.

THREE-DIMENSIONAL MORPHOLOGY OF A CORONAL PROMINENCE CAVITY

We present a three-dimensional density model of coronal prominence cavities, and a morphological fit that has been tightly constrained by a uniquely well-observed cavity. Observations were obtained as part of an International Heliophysical Year campaign by instruments from a variety of space- and ground-based observatories, spanning wavelengths from radio to soft X-ray to integrated white light. From these data it is clear that the prominence cavity is the limb manifestation of a longitudinally extended polar-crown filament channel, and that the cavity is a region of low density relative to the surrounding corona. As a first step toward quantifying density and temperature from campaign spectroscopic data, we establish the three-dimensional morphology of the cavity. This is critical for taking line-of-sight projection effects into account, since cavities are not localized in the plane of the sky and the corona is optically thin. We have augmented a global coronal streamer model to include a tunnel-like cavity with elliptical cross-section and a Gaussian variation of height along the tunnel length. We have developed a semi-automated routine that fits ellipses to cross-sections of the cavity as it rotates past the solar limb, and have applied it to Extreme Ultraviolet Imager observations from the two Solar Terrestrial Relations Observatory spacecraft. This defines the morphological parameters of our model, from which we reproduce forward-modeled cavity observables. We find that cavity morphology and orientation, in combination with the viewpoints of the observing spacecraft, explain the observed variation in cavity visibility for the east versus west limbs.

Transient Coronal Holes as Seen in the He I 1083 nm MLSO Observations

Observations from Yohkoh SXT and SOHO EIT have shown that dimming regions often appear on the solar disk near the location of a coronal mass ejection (CME). We now can see brightenings in He I 1083 nm observations made at the Mauna Loa Solar Observatory (MLSO) that form at the same time and are cospatial with the EUV intensity dimmings observed from space. The He I 1083 nm brightenings are induced by a decrease of the overlying coronal radiation. The EUV and X-ray dimmings and He I 1083 nm brightenings can thus be interpreted as different manifestations of the decreased coronal density caused by the ejection of coronal material during the eruption, i.e., as transient coronal holes. In this paper we present examples of transient coronal holes that form during the CME onset as seen in He I 1083 nm data and compare them with simultaneous observations in the Fe XII 19.5 nm line. We find that there is good agreement in both shape and size of the transient coronal holes at these two wavelengths. The 3 minute cadence of the He I 1083 nm observations taken at MLSO is used to determine the appearance and evolution of transient coronal holes with high temporal accuracy. Additional data in the H? line and in broadband visible light are used to investigate the relation of transient coronal holes to the flare, filament eruption, and CME. The cases presented here illustrate how the higher time cadence of the MLSO observations can complement space data to establish the chronology of the various manifestations of solar activity associated with a CME.

Observations of Coronal Mass Ejections with the Coronal Multichannel Polarimeter

The Coronal Multichannel Polarimeter (CoMP) measures not only the polarization of coronal emission, but also the full radiance profiles of coronal emission lines. For the first time, CoMP observations provide high-cadence image sequences of the coronal line intensity, Doppler shift, and line width simultaneously over a large field of view. By studying the Doppler shift and line width we may explore more of the physical processes of the initiation and propagation of coronal mass ejections (CMEs). Here we identify a list of CMEs observed by CoMP and present the first results of these observations. Our preliminary analysis shows that CMEs are usually associated with greatly increased Doppler shift and enhanced line width. These new observations provide not only valuable information to constrain CME models and probe various processes during the initial propagation of CMEs in the low corona, but also offer a possible cost-effective and low-risk means of space-weather monitoring.

Scientific objectives and capabilities of the Coronal Solar Magnetism Observatory

Magnetic influences increase in importance in the solar atmosphere from the photosphere out into the corona, yet our ability to routinely measure magnetic fields in the outer solar atmosphere is lacking. We describe the scientific objectives and capabilities of the COronal Solar Magnetism Observatory (COSMO), a proposed synoptic facility designed to measure magnetic fields and plasma properties in the large-scale solar atmosphere. COSMO comprises a suite of three instruments chosen to enable the study of the solar atmosphere as a coupled system: (1) a coronagraph with a 1.5 m aperture to measure the magnetic field, temperature, density, and dynamics of the corona; (2) an instrument for diagnostics of chromospheric and prominence magnetic fields and plasma properties; and (3) a white light K-coronagraph to measure the density structure and dynamics of the corona and coronal mass ejections. COSMO will provide a unique combination of magnetic field, density, temperature, and velocity observations in the corona and chromosphere that have the potential to transform our understanding of fundamental physical processes in the solar atmosphere and their role in the origins of solar variability and space weather.

Analysis of sunspot area over two solar cycles

We examine changes in sunspots and faculae and their effect on total solar irradiance during solar cycles 22 and 23 using photometric images from the San Fernando Observatory. We find important differences in the very large spots between the two cycles, both in their number and time of appearance. In particular, there is a noticeable lack of very large spots in cycle 23 with areas larger than 700 millionths of a solar hemisphere which corresponds to a decrease of about 40% relative to cycle 22. We do not find large differences in the frequencies of small to medium spots between the two cycles. There is a decrease in the number of pores and very small spots during the maximum phase of cycle 23 which is largely compensated by an increase during other phases of the solar cycle. The decrease of the very large spots, in spite of the fact that they represent only a few percent of all spots in a cycle, is primarily responsible for the observed changes in total sunspot area and total sunspot deficit during cycle 23 maximum. The cumulative effect of the decrease in the very small spots is an order of magnitude smaller than the decrease caused by the lack of large spots. These data demonstrate that the main difference between cycles 22 and 23 was in the frequency of very large spots and not in the very small spots, as previously concluded. Analysis of the USAF/NOAA and Debrecen sunspot areas confirms these findings.

TEMPORAL STABILITY OF SUNSPOT UMBRAL INTENSITIES: 1986-2012

We examine the relative intensity of sunspot umbrae during the period from 1986 to 2012 using photometric images from the San Fernando Observatory. We confirm the presence of a relationship between the mean umbral core intensity and the mean sunspot area, as found in previous studies, and do not find a notable change in this relationship between cycles 22 and 23. We looked for a possible time variation in the sunspot umbral contrast during the 27 yr covering cycles 22, 23, and the rise of cycle 24, and we did not find a significant change. These findings do not indicate that sunspots have become less dark during cycles 23 and 24.