HAO 2011 Profiles In Science: Dr. Giuliana de Toma
Giuliana de Toma is a Project Scientist II in the High Altitude Observatory of the National Center for Atmospheric Research. She received her PhD in Physics in 1994 from the University of Trieste (Italy). She came to Boulder in 1994 as a Postdoctoral Fellow within HAO, then went to the University of Colorado at Boulder, and in 2001 came back to HAO and she is part of the Long-term Solar Variability (LSV) group.
Summary of Achievements
Giuliana de Toma main scientific interests are: (1) solar cycle variability and its influence on the corona and heliosphere and
(2) analysis of coronal and chromospheric observations to study prominence dynamics, coronal prominence cavities, prominence eruptions and coronal mass ejections (CMEs).
Solar Cycle: Magnetic activity on the Sun waxes and wanes with the solar cycle which is believed to be generated by dynamo action inside the Sun. As new magnetic flux emerges into the solar photosphere, it interacts with existing magnetic structures, altering the state of the solar atmosphere and producing a rich variety of activity, including CMEs and flares. Changing coronal conditions are imprinted onto the solar wind and carried to the space environment at Earth and throughout the heliosphere. Understanding how the solar dynamo operates, how the solar atmosphere responds to the changing magnetic field, and how solar activity impacts the Earth are fundamental problems in solar physics and actively pursued at HAO. Giuliana has been active in this field for many years. She is currently working at the analysis of photospheric magnetic field observations, and in particular of polar magnetic observations, to better understand how changes in the magnetic flux distribution at the photosphere affect the structure and properties of the corona and heliosphere. Her recent work includes study of the past solar minina as well as of hemispheric asymmetries.
Coronal Holes: Coronal holes are the sources at the Sun of the fast solar wind. They are regions of one predominant magnetic polarity from where the wind flows out in the heliosphere. They are called coronal holes because have relatively low electron density and temperature so they appear dark in X-ray and EUV. Coronal holes are not easy to identify in observations because they are not the only dark regions in the corona and bright structures in the optically thin coronal lines can partially obscure their boundaries. In recent years, the availability of observations at multiple wavelengths has made their detection easier. Giuliana has developed an automatic technique to identify coronal holes. This allows us to follow changes in coronal hole area and location and provides observables to validate against coronal models. Giuliana works with HAO colleagues and collaborator Nick Arge (AFRL) to study the evolution of coronal holes, how they respond to changes in the underlying magnetic fields, and what is their impact on the solar wind at the Earth.
Prominences and Coronal Cavities: Prominences are condensations in the solar corona, which are both cooler and denser—by about 2 orders of magnitude—than the million-degree corona and are surrounded by a less dense region called cavity. They form above magnetic neutral lines, either within the strong magnetic field of active regions or in the weaker remnant magnetic fields after active regions decay. Prominences are dynamic objects, showing continuous motions down to their smaller resolvable scales. However, on large spatial scales, prominences and their associated coronal cavities are remarkably stable. For example, polar-crown prominences, are long-lived structures at high-latitude forming a circle (crown) around the Sun that can be observed for many days, or even months, before leaving the Sun in spectacular eruptions (see CMEs below). Prominences within active regions are typically more active, usually erupting over the lifetime of the active region. Evolution of quiescent prominences gives us insight on how a closed magnetic system can persist in quasi-static equilibrium for some time. Eruptions occur when this slowly evolving magnetic structure encounters a critical point and equilibrium cannot be maintained anymore. HAO has an observational program to monitor prominences and coronal cavities at Mauna Loa Solar Observatory (MLSO). Giuliana maintains a daily log of prominence/filament eruptions and coronal cavities for MLSO. She has used MLSO observations in combination with data from SOHO, STEREO, Hinode, and SDO to study the properties and evolution of prominence-cavity systems.
Coronal Eruption: Coronal mass ejections or CMEs are large ejections of mass and magnetic flux from the solar corona into interplanetary space which involve a large-scale reconfiguration of the coronal magnetic field. They are the largest manifestations of solar activity and the main sources of space weather. CMEs have a strong association with prominence-cavity eruptions and many CMEs show a 3-part structure made of a bright core (corresponding to the eruptive prominence), a dark cavity, and a bright front. In spite of the improvement in observations and the advance in CME modeling, the driver mechanisms of solar eruptions are still not fully understood. The main theoretical problem is to explain how the energy is stored in the corona before a CME and how is released. Yuhong Fan, Sarah Gibson, Joan Burkepile and Giuliana de Toma at HAO with collaborators Alysha Reinard (CU/CIRES) and Tibor Toeroek (PSI) are actively involved in a program to study the onset and acceleration phase of CMEs. This combines observational analysis of CMEs with 3D MDH models to understand the possible triggering mechanisms for CMEs as well as the effect of the overlying coronal magnetic fields on the eruption dynamic. Giuliana is responsible for the analysis of prominence eruptions and CMEs that includes measuring their trajectories and geometrical properties, and determining the evolution of other CME signatures, such as coronal dimmings. Her recent work has focused on CME events during the rising phase of cycle 24 for which multiple views are available from the two STEREO and SDO spacecraft.
(1) de Toma, G., 2010: Evolution of Coronal Holes and Implications for High Speed Solar Wind during the Minimum Between Cycle 23 and 24, Sol. Phys.M, doi:10.1007/s11207-010-9677-2.
Abstract: We analyze coronal holes present on the Sun during the extended minimum between cycles 23 and 24, study their evolution, examine the consequences for the solar wind speed near the Earth, and compare it with the previous minimum in 1996. We identify coronal holes and determine their size and location using a combination of EUV observations from SOHO/EIT and STEREO/EUVI and magnetograms. We find that the long period of low solar activity from 2006 to 2009 was characterized by weak polar magnetic fields and polar coronal holes smaller than observed during the previous minimum. We also find that large, low-latitude coronal holes were present on the Sun until 2008 and remained important sources of recurrent high-speed solar wind streams. By the end of 2008, these low-latitude coronal holes started to close down, and finally disappeared in 2009, while smaller, mid-latitude coronal holes formed in the remnants of cycle 24 active regions shifting the sources of the solar wind at the Earth to higher latitudes.
Figure 1a caption: EUV maps of the Sun in the Fe XII 19.5nm line in March-April 1996 (CR 1907) during the minimum between cycles 22 and 23 and in September-October 2008 (CR2075) during the minimum between cycles 23 and 24. The mean sunspot number for these Carrington Rotations (CR) was 9.5 and 3.1, respectively. Coronal holes appear as dark regions at this wavelength. Note that polar coronal holes are large in 1996 (total area about 15% of the Sun's surface) while in 2008 are much smaller and mostly confined above 60° in latitude. Also note that low-latitude coronal holes, that were mostly absent during the 1996 minimum, were still present on the Sun until the end of 2008, even when sunspot activity and magnetic emergence had dropped to levels below those observed in 1996.
Figure 1b caption: Distribution of solar wind speed at the Earth for 1996 (in purple), 2007 and 2008 (in blue), and 2009 (in green). The mean speeds for these years are: 423km/s, 440km/s, 449km/s and 364km/s, respectively. While the yearly averaged solar wind speed in 2007 and 2008 are comparable to the one in 1996, the velocity distribution is quite different. In 2007 and 2008 the distribution is almost a bimodal distribution with a primary peak at slightly lower velocities than in 1996 and a secondary peak near 600km/s. This high velocity tail is due to recurrent, high-speed streams originating in the large, low-latitude coronal holes present at the Sun during these years. In 2009, with the close down of the large low-latitude coronal holes, velocities drop dramatically and speeds above 600km/s become almost completely absent in the solar wind reaching the Earth.
(2) Dikpati, M. P., Gilman, G. de Toma, and R. Ulrich, 2010: Impact of Changes in the Sun's Conveyor-belt on Recent Solar Cycles, GRL, 37, CiteID L14107, doi:10.1029/2010GL044143.
Abstract: Plasma flowing poleward at the solar surface and returning equatorward near the base of the convection zone, called the meridional circulation, constitutes the Sun's conveyor-belt. Just as the Earth's great oceanic conveyor-belt carries thermal signatures that determine El Nino events, the Sun's conveyor-belt determines timing, amplitude and shape of a solar cycle in flux-transport type dynamos. In cycle 23, the Sun's surface poleward meridional flow extended all the way to the pole, while in cycle 22 it switched to equatorward near 60°. Simulations from a flux-transport dynamo model including these observed differences in meridional circulation show that the transport of dynamo-generated magnetic flux via the longer conveyor-belt, with slower return-flow in cycle 23 compared to that in cycle 22, may have caused the longer duration of cycle 23.
Figure 3: High resolution
(3) de Toma, G., S.E. Gibson, B.A. Emery, and C.N. Arge, 2010: The Minimum between Cycle 23 and 24: Is Sunspot Number the Whole Story? SOHO23 Proceedings-Understanding a Peculiar Solar Minimum, p. 217.
Abstract: During recent years we have observed a long and deep solar minimum with sunspot number in 2008 and 2009 reaching the lowest level in about a century. In spite of the lack of sunspot activity at the Sun, observations have shown that a relatively complex corona and heliosphere persisted for most of the minimum phase. The solar corona did not reach the simple "dipolar" shape often seen during solar minima, while low-latitude coronal holes, and their associated corotating high-speed solar wind streams, persisted to 2008, modulating the solar wind. We compare the current and previous minima to show how, even during very quiet times, different magnetic configurations are possible at the Sun and discuss how these different morphologies can affect the corona, heliosphere, and even the geospace.
Figure 3 caption: Modeled coronal magnetic field at 5 solar radii for Carrington Rotations 1911, 2068, and 2081 in June 1996, March 2008, and March 2009, respectively, computed using the Wang-Sheeley-Arge (WSA) model. In spite of the fact that magnetic activity in 2008 was as low or lower than in 1996, the heliospheric current sheet was still significantly warped (middle panel). Only in early 2009 did the structure of the heliosphere start to resemble the one observed in 1996 (top and middle panels). This change in morphology corresponded to a significant decrease in the solar wind speed at the Earth.
(4) Judge, P.G., J. Burkepile, G. de Toma, and M. Druckmueller, 2010: Historic Eclipses and the Recent Solar Minimum Corona, SOHO23 Proceedings—Understanding a Peculiar Solar Minimum, p. 171.
We have studied the corona as seen at the eclipses of 1878, 1900, 1901, and others. These eclipses occurred during extended sunspot minimum conditions. We compare these data with those of the recent solar minimum corona, using data from the eclipses of July 22, 2009 and August 1, 2008. An attempt to characterize the global solar magnetic fields is made. We speculate on the origin of the non-dipolar structure seen in the 2008 and 2009 eclipse images.
Figure 5: High resolution
(5) de Toma, G., S.E. Gibson, B.A. Emery, and J. Kozyra. 2010: Solar Cycle 23: An Unusual Solar Minimum? In the Twelfth International Solar Wind Conference, AIP Conference Proceedings, p. 667, doi: 10.1063/1.3395955.
Abstract: We are currently observing the minimum phase of Cycle 23. Magnetic activity during the years 2006-2009 has been very weak with sunspot numbers reaching the lowest values in about 100 years. This long and extended minimum is characterized by weak polar magnetic fields, small polar coronal holes, and a relatively complex coronal morphology. This magnetic configuration at the Sun is remarkably different from the one observed during the previous two solar minima. We review observations made at the Sun and in the solar wind during the recent solar minima and discuss the implications of the observed differences for the heliosphere and geospace.
Figure 5 caption: White-light images of the solar corona from LASCO/C2 for the previous solar minimum on February 20 1996 (left panel), and the recent minimum (last three panels) on February 15 2007, July 25 2008, and May 22 2009. All images correspond to very quiet times: the observed sunspot number for the four days was 8, 0, 8, and 0, respectively. During the 1996 minimum, the shape of the corona was dipolar and coronal streamers were confined to a narrow region around the heliographic equator while in 2007, 2008, and 2009 coronal streamers extended to relatively high heliolatitudes.
Figure 6: High resolution
(6) de Toma, G. and C.N. Arge. 2010: The Sun's Magnetic Field During The Past Two Minima in the Twelfth International Solar Wind Conference, AIP Conference Proceedings, p.679, doi: 10.1063/1.3395958.
Abstract: The past three years have been characterized by very weak sunspot and CME activity making this a long and deep solar minimum. In spite of the lack of magnetic activity at the Sun, white light and EUV images have shown that a relatively complex corona with multiple streamers and low latitude coronal holes persisted during most of this extended minimum. At the same time, solar wind observations indicated that the heliospheric current sheet was more warped during the declining and minimum phase of this cycle. This morphology of the corona and heliosphere differs from the one observed during the previous solar minimum when coronal streamers were confined to low heliolatitudes and the heliospheric current sheet was nearly flat. Interestingly, the polar magnetic fields observed at the solar photosphere during the present minimum are about 40% weaker than during the previous minimum. We use a potential field model to test if the weaker polar fields (and associated weaker dipole moment) can explain the differences observed in the corona and heliosphere during the past two minima.
Figure 6 caption: Net polar magnetic flux as measured at the Sun's photosphere between 60° and 80° latitude by SOHO/MDI (squares) and at NSO (diamonds) from 1995 to 2010. The polar magnetic flux has been at about the same level since 2004 and is significantly lower than during the previous minimum in 1996.
Figure 7: High resolution
(7) de Toma, G., R. Casini, T.E. Berger, B.C. Low, A. de Wijn, J. Burkepile, K.S. Balasubramaniam. 2009: Observations of Large-Scale Dynamic Bubbles in Prominences in The Second Hinode Science Meeting: Beyond Discovery-Toward Understanding, ASP Conference Series, 415, 163.
Abstract: Solar prominences are very dynamic objects, showing continuous motions down to their smallest resolvable spatial and temporal scales. However, as macroscopic magnetic structures, they are remarkably stable during their quiescent phase. We present recent ground-based and Hinode observations of large-scale bubble-like, dynamic sub-structures that form within and rise through quiescent prominences without disrupting them. We investigate the similarities and differences of the Hinode and ground-based observations and discuss their implications for models of prominences.
Figure 7 caption: The figure on the right side shows the Halpha images of a prominence taken with Hinode/SOT on August 16 2007 showing the rise, break-up, and subsequent reformation of a large bubble inside the prominence (top three rows). Note that the bubble shape changed as it moved upward. The trajectory of the bubble during its fast ascent is shown in the bottom left, and a lower-resolution image of the bubble taken at MLSO in the bottom right.