This description of the HMI science objectives is extracted from the HMI proposal of April 2002.
Fluid motions inside the Sun generate the solar magnetic field. Complex interactions between turbulent convection, rotation, large-scale flows and magnetic field produce regular patterns of solar activity changing quasi-periodically with the solar cycle. How are variations in the solar cycle related to the internal flows and surface magnetic field? How is the differential rotation produced? What is the structure of the meridional flow and how does it vary? What roles do the torsional oscillation pattern and the variations of the rotation rate in the tachocline play in the solar dynamo?
These issues are usually studied only in zonal averages by global Helioseismology but the Sun is longitudinally structured. Local helioseismology has revealed the presence of large-scale flows within the near-surface layers of the solar convection zone. These flows possess intricate patterns that change from one day to the next, accompanied by more gradually evolving patterns such as banded zonal flows and meridional circulation cells (Foldout 1.B, C). These flow structures have been described as Solar Subsurface Weather (SSW). Successive maps of these weather-like flow structures (Foldout 1.F) suggest that solar magnetism strongly modulates flow speeds and directions. Active regions tend to emerge in latitudes with stronger shear. The connections between SSW and active region development are presently unknown.
Structure and dynamics of the tachocline. Observation of the deep roots of solar activity in the tachocline is of primary importance for understanding the long-term variability of the Sun. HMI will use global and local helioseismic techniques to observe and investigate the large-scale character of the convection zone and tachocline. Topics include solar differential rotation, relations between variations of rotation and magnetic fields, longitudinal structure of zonal flows (‘torsional oscillations’), relations between the torsional pattern and active regions, subsurface shear and its variations with solar activity, and the origin of the ‘extended’ solar cycle.
Variations in differential rotation. Differential rotation (Figure C.2) is a crucial component of the solar cycle and is believed to generate the global scale toroidal magnetic field in active regions. Results from MDI and GONG have revealed intriguing 1.3-year quasi-periodic variations of the rotation rate in the tachocline, which may be a key to understanding the solar dynamo. HMI will extend this key series with better near-surface resolution.
Evolution of meridional circulation. Precise knowledge of the meridional circulation in the convection zone is crucial for understanding the long-term variability of the Sun. Helioseismology has found evidence for variation of the internal poleward flow during the solar cycle. To understand the global dynamics we must follow the evolution of the flow. HMI will generate continuous data for detailed, 3-D maps of the evolving patterns of meridional circulation providing information about how flows transport and interact with magnetic fields throughout the solar cycle.
Dynamics in the near surface shear layer. Helioseismology has revealed that significant changes in solar structure over the solar cycle occur in the near-surface shear layer. However, the physics of these variations and their role in irradiance variations are still unknown. HMI will characterize the properties of this shear layer, the interaction between surface magnetism and evolving flow patterns, and the changes in structure and dynamics as the solar cycle advances. It will assess the statistical properties of convective turbulence over the solar cycle, including the kinetic helicity and its relation to magnetic helicity – two intrinsic characteristics of dynamo action.
Observations show that magnetic flux on the Sun does not appear randomly. Once an active region emerges, there is a high probability that additional eruptions of flux will occur nearby (activity nests, active longitudes). How is magnetic flux created, concentrated, and transported to the solar surface where it emerges in the form of evolving active regions? To what extent are the appearances of active regions predictable? What roles do local flows play in their evolution?
HMI will address these questions by providing tracked sub-surface sound-speed and flow maps for individual active regions and complexes under the visible surface of the Sun combined with surface magnetograms. Current thinking suggests that flux emerging in active regions originates in the tachocline. Flux is somehow ejected from the depths in the form of loops that rise through the convection zone and emerge through the surface. Phenomenological flux transport models show that the observed photospheric distribution of the flux does not require a long-term connection to flux below the surface. Rather, field motions are described by the observed poleward flows, differential rotation, and surface diffusion acting on emerged flux of active regions. Does the active region magnetic flux really disconnect from the deeper flux ropes after emergence?
Formation and deep structure of magnetic complexes of activity. HMI will explore the nature of long-lived complexes of solar activity (‘active or preferred longitudes’), the principal sources of solar disturbances. ‘Active longitudes’ have been a puzzle of solar activity for many decades. They may continue from one cycle to the next, and may be related to variations of solar activity on the scale of 1-2 years and short-term ‘impulses’ of activity. HMI will probe beneath these features to 0.7R, the bottom of the convection zone, to search for correlated flow or thermal structures.
Active region source and evolution. By using acoustic tomography we can image sound speed perturbations that accompany magnetic flux emergence and disconnection that may occur. Vector magnetograms can give evidence on whether flux leaves the surface predominantly as ‘bubbles’, or whether it is principally the outcome of local annihilation of fields of opposing polarity. With a combination of helioseismic probing and vector field measurements HMI will provide new insight into active region flux emergence and removal.
Magnetic flux concentration in sunspots. Formation of sunspots is one of the long-standing questions of solar physics.61-63 Recent observations from MDI have revealed complicated flow patterns beneath sunspots (Figure C.3) and indicated that the highly concentrated magnetic flux in spots is accompanied by converging mass flows in the upper 3-4 Mm beneath the surface (Foldout 1.J). The evolution of these flows is not presently known. Detailed maps of subsurface flows in deeper layers, below 4 Mm, combined with surface fields and brightness for up to 9 days during disk passage will allow investigation of the relations between flow dynamics and flux concentration in spots.
Sources and mechanisms of solar irradiance variations. Magnetic features - sunspots, active regions, and network - that alter the temperature and composition of the solar atmosphere are primary sources of irradiance variability. How exactly do these features cause the irradiance variations? HMI together with the SDO Atmospheric Imaging Assembly (AIA) and Spectrometer for Irradiance (SIE), will study physical processes that govern these variations. The relation between interior processes, properties of magnetic field regions and irradiance variations, particularly the UV and EUV components that have a direct and significant effect on Earth’s atmosphere will be studied for the first time.
C.1.2.3 Sources and drivers of solar activity and disturbances
It is commonly believed that the principal driver of solar disturbances is stressed magnetic field. The stresses are released in the solar corona producing flares and coronal mass ejections (CME). The source of these stresses is believed to be in the solar interior. Flares usually occur in areas where the magnetic configuration is complex, with strong shears, high gradients, long and curved neutral lines, etc. This implies that the trigger mechanisms of flares are controlled by critical properties of magnetic field that lead eventually to MHD instabilities. But what kinds of instability actually govern, and under what conditions they are triggered are unknown. With only some theoretical ideas and models, there is no certainty of how magnetic field is stressed or twisted inside the Sun or just what the triggering process is.
Origin and dynamics of magnetic sheared structures and d-type sunspots. The spots in Figure C.5 contain two umbrae of opposite magnetic polarity within a common penumbra and were the source of powerful flares and CMEs. Such d-type sunspot regions are thought to inject magnetic flux into the solar atmosphere in a highly twisted state. It is important to determine what processes beneath the surface lead to development of these spots and allow them to become flare and CME productive. This investigation will be carried out by analysis of evolving internal mass flows and magnetic field topology of such spots.
Magnetic configuration and mechanisms of solar flares. Vector magnetic field measurements can be used to infer field topology and vertical electric current, both of which are essential to understand the flare process. Observations are required that can continuously track changes in magnetic field and electric current with sufficient spatial resolution to reveal changes of field strength and topology before and after flares. HMI will provide these unique measurements of the vector magnetic field over the whole solar disk with reasonable accuracy and at high cadence.
Emergence of magnetic flux and solar transient events. Emergence of magnetic flux is closely related to solar transient events. MDI, GONG, and BBSO data show that there can be impulsive yet long-lived changes to the fields associated with eruptive events. Emergence of magnetic flux within active regions is often associated with flares. Emerging magnetic flux regions near filaments lead to eruption of filaments. CMEs are also found to accompany emerging flux regions. Further, emergence of isolated active regions can proceed without any eruptive events. This suggests that magnetic flux emerging into the atmosphere interacts with pre-existing fields leading to loss of magnetic field stability. Observations of electric current and magnetic topology differences between newly emerging and pre-existing fields will likely lead to the understanding of why emerging flux causes solar transient events. Vector polarimetry provided by HMI will enable these quantitative studies.
Evolution of small-scale structures and magnetic carpet. The quiet Sun is covered with small regions of mixed polarity, termed ‘magnetic carpet’ (Foldout 1.G), contributing to solar activity on short timescales. As these elements emerge through the photosphere they interact with each other and with larger magnetic structures. They may provide triggers for eruptive events, and their constant interactions may be a source of coronal heating. They may also contribute to irradiance variations in the form of enhanced network emission. While HMI will certainly not see all of this flux, it will allow global scale observations of the small-scale element distribution, their interactions, and the resulting transformation of the large-scale field.
C.1.2.4 Links between the internal processes and dynamics of the corona and heliosphere
The highly structured solar atmosphere is predominately governed by magnetic field emerged from in the solar interior. Magnetic fields and the consequent coronal structures occur on many spatial and temporal scales. Intrinsic connectivity between multi-scale patterns increases coronal structure complexity leading to variability. For example, CMEs apparently interact with to the global-scale magnetic field, but many CMEs, especially fast CMEs, are associated with flares, which are believed to be local phenomena. Model-based reconstruction of 3-D magnetic structure is one way to estimate the field from observations. Models using vector field data in active regions provide the best match to the observations. More realistic MHD coronal models based on HMI high-cadence vector-field maps as boundary conditions will greatly enhance our understanding of how the corona responds to evolving, non-potential active regions.
Complexity and energetics of the solar corona. Observations from
Large-scale coronal field estimates. Models computed from line-of-sight photospheric magnetic maps have been used to reproduce coronal forms that show multi-scale closed field structures as well as the source of open field that starts from coronal holes but spreads to fill interplanetary space. Modeled coronal field demonstrates two types of closed field regions: helmet streamers that form the heliospheric current sheet and a region sandwiched between the like-polarity open field regions. There is evidence that most CMEs are associated with helmet streamers and with newly opened flux. HMI will provide uniform magnetic coverage at a high cadence, and together with simultaneous AIA, WCI and STEREO coronal images will enable the development of coronal field models and study of the relationship between pre-existing patterns, newly opening fields, long distance connectivity, and CMEs.
Coronal magnetic structure and solar wind. MHD simulation and current-free coronal field modeling based on magnetograms are two ways to study solar wind properties and their relations with coronal magnetic field structure (Figure C.6). These methods have proven effective and promising, showing potential in applications of real-time space weather forecasting. It has been demonstrated that modeling of the solar wind can be significantly improved with increased cadence of the input magnetic data. By providing full-disk vector field data at high cadence, HMI will enable these models to describe the distribution of the solar wind, coronal holes and open field regions, and how magnetic fields in active regions connect with interplanetary magnetic field lines.
C.1.2.5 Precursors of solar disturbances for space-weather forecasts
Variations in the solar spectral irradiance and total irradiance may have profound effects on life through their potential but poorly understood role in climate changes. The variation from cycle to cycle of the number, strength, and timing of the strongest eruptive events is unpredictable at present. We are far from answering simple questions like ‘will the next cycle be larger than the current one?' 'When will the next large eruption occur?' Or even 'when will there be several successive quiet days?' As we learn more about the fundamental processes through studies of internal motions, magnetic flux transport and evolution, relations between active regions, UV irradiance, and solar shape variations we will be vigilant for opportunities to develop prediction tools. Nevertheless, there are several near term practical possibilities to improve the situation with HMI observations.
Far-side imaging and activity index. A procedure for solar far-side imaging was developed using data from MDI (Figure C.7), and has led to the routine mapping of the Sun’s far-side. Acoustic travel-time perturbations are correlated with strong magnetic fields, providing a view of active regions well before they become visible as rotate onto the disk at the east limb. Synoptic images, which are now able to cover the entire far hemisphere of the Sun, will provide the ability to forecast the appearance of large active regions up to 2 weeks in advance and allow the detection of regions which emerge just a few days before rotating into view. HMI's full coverage to the limb will allow lower-noise far-side estimates.
Predicting emergence of active regions by helioseismic imaging. Rising magnetic flux tubes in the solar convection zone may produce detectable seismic signatures (Fig. C.4), which would provide warning of their impending emergence. Helioseismic images of the base of the convection zone will employ a similar range of p-modes as those used to construct images of the far side. A goal is to detect and monitor seismic signatures of persistent or recurring solar activity near the tachocline. Success here could lead to long-term forecasts of solar activity.
Determination of magnetic cloud Bs events. Potentially valuable information for geomagnetic forecasts - predictions of magnetic cloud Bs (southward field) events - can be obtained from the vector field measurements. Long intervals of large southward interplanetary magnetic field, Bs events, and high solar wind speed are believed to be the primary cause of intense geomagnetic disturbances with the Bs component the more important quantity. It has been shown that orientation in ‘clouds’ remains basically unchanged while propagating from the solar surface to Earth’s orbit. This provides a plausible chain of related phenomena that should allow prediction to be made from solar observations of the geo-effectiveness of CMEs directed toward Earth. Estimates of embedded Bs will be significantly improved by incorporating frequently updated vector field maps into coronal field projections with the potential addition of coronagraphic observations from AIA, WCI, and STEREO.