Submitted Abstracts

There are 131 abstracts


HMI Measured Doppler Velocity Contamination from the SDO Orbit Velocity

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Presentation Type: Poster

Session: Session 1: Motions Inside the Sun

Abstract:

The Problem: The SDO satellite is in an inclined Geo-sync orbit which allows uninterrupted views of the Sun nearly 98% of the time. This orbit has a velocity of about 3,500 m/s with the solar line-of-sight component varying with time of day and time of year. Due to remaining calibration errors in wavelength filters the orbit velocity leaks into the line-of-sight solar velocity and magnetic field measurements. Since the same model of the filter is used in the Milne-Eddington inversions used to generate the vector magnetic field data, the orbit velocity also contaminates the vector magnetic products. These errors contribute 12h and 24h variations in most HMI data products and are known as the 24-hour problem. Early in the mission we made a patch to the calibration that corrected the disk mean velocity. The resulting LOS velocity has been used for helioseismology with no apparent problems. The velocity signal has about a 1% scale error that varies with time of day and with velocity, i.e. it is non-linear for large velocities. This causes leaks into the LOS field (which is simply the difference between velocity measured in LCP and RCP rescaled for the Zeeman splitting). This poster reviews the measurement process, shows examples of the problem, and describes recent work at resolving the issues. Since the errors are in the filter characterization it makes most sense to work first on the LOS data products since they, unlike the vector products, are directly and simply related to the filter profile without assumptions on the solar atmosphere, filling factors, etc. Therefore this poster is strictly limited to understanding how to better understand the filter profiles as they vary across the field and with time of day and time in years resulting in velocity errors of up to a percent and LOS field estimates with errors up to a few percent (of the standard LOS magnetograph method based on measuring the differences in wavelength of the line centroids in LCP and RCP light). We expect that when better filter profiles are available it will be possible to generate improved vector field data products as well.




SDO/HMI Overview of Recent Findings

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Presentation Type: Oral

Session: Session 3: Solar Magnetic Variability and the Solar Cycle

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TBD




Achieving Consistent Vector Magnetic Field Measurements from SDO/HMI

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Presentation Type: No Preference

Session: Session 4: The Evolution of Active Regions

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NASA’s Solar Dynamics Observatory (SDO) is delivering vector magnetic field observations of the full solar disk with unprecedented temporal and spatial resolution; however, the satellite is in a highly inclined geosynchronous orbit. The relative spacecraft-Sun velocity varies by ±3 km/s over a day which introduces significant orbital artifacts in the Helioseismic Magnetic Imager (HMI) data. We have recently demonstrated that the orbital artifacts contaminate all spatial and temporal scales in the data and developed a procedure for mitigating these artifacts in the Doppler data obtained from the Milne-Eddington inversions in the HMI Pipeline. Simultaneously, we have found that the orbital artifacts may be introduced by inaccurate estimates for the free-spectral ranges (FSRs) of the optical elements in HMI. We describe our approach and attempt to minimize orbital artifacts in the hmi.V_720 Dopplergram series by adjusting the FSRs for the optical elements of HMI within their measurement uncertainties of ±1%. introduces major orbital artifacts in the Helioseismic Magnetic Imager (HMI) data. We have recently demonstrated that the orbital artifacts contaminate all spatial and temporal scales in the data and developed a procedure for mitigating these artifacts in the Doppler data obtained from the Milne-Eddington inversions in the HMI Pipeline. Simultaneously, we have found that the orbital artifacts may be introduced by inaccurate estimates for the free-spectral ranges (FSRs) of the optical elements in HMI. We describe our approach and attempt to minimize orbital artifacts in the hmi.V_720 Dopplergram series by adjusting the FSRs for the optical elements in HMI within their measurement uncertainties of ±1%.




Constraining the common properties of active region formation using the SDO/HEAR dataset

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Presentation Type: Oral

Session: Session 4: The Evolution of Active Regions

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Observations from the Solar Dynamics Observatory (SDO) have the potential for allowing the helioseismic study of the formation of hundreds of active regions, which enable us to perform statistical analyses. We collated a uniform data set of emerging active regions (EARs) observed by the SDO/HMI instrument suitable for helioseismic analysis, where each active region can be observed up to 7 days before emergence. We call this dataset the SDO Helioseismic Emerging Active Region (SDO/HEAR) survey. We have used this dataset to to understand the nature of active region emergence. The latitudinally averaged line-of-sight magnetic field of all the EARs shows that the leading (trailing) polarity moves in a prograde (retrograde) direction with a speed of 110 ± 15 m/s (−60 ± 10 m/s) relative to the Carrington rotation rate in the first day after emergence. However, relative to the differential rotation of the surface plasma the East-West velocity is symmetric, with a mean of 90 ± 10 m/s. We have also compared the surface flows associated with the EARs at the time of emergence with surface flows from numerical simulations of flux emergence with different rise speeds. We found that the surface flows in simulations of emerging flux with a low rise speed of 70 m/s best match the observations.




A Model of the Chromosphere: Heating, Structures, and Circulation

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Presentation Type: Oral

Session: Session 2: Motions Near and Above the Solar Surface

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The problem of heating of the solar atmosphere involves two major phenomena: explaining the temperature rise from a few thousand degrees in the photosphere to a few million degrees in the corona, and accounting for the observed radiative losses from the chromosphere. Although the energy flux required for the latter is about ten times greater than that involved in the former, coronal heating has nevertheless often been perceived as the more difficult and exciting problem and is sometimes discussed to the exclusion of the more general heating of the atmosphere. We have developed over the last 5 years a model that specifically addresses the heating processes in the chromosphere. The non-thermal heating required in order to balance the radiative losses from the chromosphere is assumed to be provided by strong damping, through plasma-neutral collisions, of Alfvén waves that are driven by motions below the photosphere. The high-frequency portion of the source power spectrum is strongly damped at lower altitudes, whereas the lower-frequency perturbations are nearly undamped and can be observed in the corona and above. As a result, the waves observed above the corona constitute only a fraction of those at the photosphere and, contrary to supposition in some earlier Alfvén-wave-damping models, their power does not represent the energy input. The theory is based on classical processes and does not invoke anomalous dissipation or turbulent heating or shock heating. Calculated from parameters of a semi-empirical model for quiet-Sun conditions, the mechanism can generate sufficient heat to account for the radiative losses in the atmosphere, with most of the heat deposited at lower altitudes (the most difficult part of the chromospheric heating problem). When the magnetic field strength varies horizontally, the heating is likewise horizontally nonuniform. Since radiative loss is a strong function of temperature, the equilibrium temperature corresponding to local thermal balance between heating and radiation can be reached rapidly. Regions of stronger heating thus maintain higher temperatures and vice versa. The resulting uneven distribution of temperature drives a very slow but systematic chromospheric circulation, which distorts the magnetic field, modifying the funnel-canopy-shaped magnetic geometry (emanating from the strong field highly concentrated into small areas in the photosphere) into a wider shape, with a relatively uniform field in the upper chromosphere. The process may have some connection with the temperature minimum in the chromosphere near 600 km altitude. The remaining problem of coronal heating needs order-of-magnitude smaller heating rates but requires heating mechanisms that operate at temperatures up to the high coronal values and under nearly collisionless fully ionized plasma conditions; it is thus not addressed by our model in its present form, which relies on plasma-neutral collisions. Possible implications for the transition region, corona, and spicules will be discussed.




Active Region Formation and Subsurface Structure

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Presentation Type: Oral

Session: Session 4: The Evolution of Active Regions

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We present results from emerging magnetic flux simulations showing how several different active regions form and their very different subsurface structures. The simulations assumed an infinite sheet of uniform, untwisted, horizontal field advected into the computational domain by inflows at a depth of 20 Mm. Results from two different horizontal field strengths, 1 and 5 kG, will be presented. Convective up and down flows buckle the horizontal field into Omega and U loops. Upflows and magnetic buoyancy carry the field toward the surface, while fast downflows pin down the field. Small (granular) convective motions, near the surface, shred the emerging field into fine filaments that emerge as the observed "pepper and salt" pattern. The large (supergranular) motions, at depth, keep the overall loop structure intact, so that as the overall omega-loop emerges through the surface the opposite polarity fields counter-stream into large unipolar flux concentrations producing first pores which then coalesce into spots. These tend to be located over the supergranular downflow lanes near the bottom of the domain. The pores and spots exhibit a great variety of subsurface field structures -- from monolithic but twisted bundles to intertwined separate spaghetti sturctures. We will show movies of the surface evolution of the field and emergent continuum intensity and of the subsurface evolution of the magnetic field lines.




Study of Plasma Heating in Solar Eruptive Events

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Presentation Type: Oral

Session: Session 5: Studies of Solar Eruptive Events (SEEs)

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The temperature of plasma is usually heated to over 10 MK by magnetic reconnection in Solar Eruptive Events. However, the details of the process are not known. With an improved way of DEM calculation, we are able to constrain the high-temperature DEMs using SDO/AIA data alone and study the heating process from the beginning to the end of SEEs. The results are also compared with other observations from RHESSI and GOES.




Unexpectedly Strong Lorentz-Force Impulse Observed During a Solar Eruption

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Presentation Type: Oral

Session: Session 5: Studies of Solar Eruptive Events (SEEs)

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For fast coronal mass ejections (CMEs), the acceleration phase takes place in the low corona; the momentum process is presumably dominated by the Lorentz force. Using ultra-high-cadence vector magnetic data from the Helioseismic and Magnetic Imager (HMI) and numerical simulations, we show that the observed fast-evolving photospheric field can be used to characterize the impulse of the Lorentz force during a CME. While the peak Lorentz force concurs with the maximum ejecta acceleration, the observed total force impulse surprisingly exceeds the CME momentum by over an order of magnitude. We conjecture that most of the Lorentz force impulse is "trapped" in the thin layer of the photosphere above the HMI line-formation height and is counter-balanced by gravity. This implies a consequent upward plasma motion which we coin "gentle photospheric upwelling". The unexpected effect dominates the momentum processes, but is negligible for the energy budget, suggesting a complex coupling between different layers of the solar atmosphere during CMEs.




The Asymmetric Polar Field Reversal of Solar Cycle 24

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Presentation Type: No Preference

Session: Session 3: Solar Magnetic Variability and the Solar Cycle

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As each solar cycle progresses, remnant magnetic flux from active regions (ARs) migrates poleward to cancel the old-cycle polar field. We describe this polarity reversal process during Cycle 24 using over six years of magnetic field measurements from the Helioseismic and Magnetic Imager. The total flux associated with ARs reached maximum in the north in 2011, more than two years earlier than the south; the maximum is significantly weaker than Cycle 23. The process of polar field reversal is relatively slow, north–south asymmetric, and episodic. We estimate that the global axial dipole changed sign in 2013 October; the northern and southern polar fields (mean above 60 deg latitude) reversed in 2012 November and 2014 March, respectively, about 16 months apart. Notably, the poleward surges of flux in each hemisphere alternated in polarity, giving rise to multiple reversals in the north. The southern polar field, on the other hand, kept increasing in strength. As of 2016 July it has become almost 80% higher than the 2010 level, suggesting that Cycle 25 is at least as strong as Cycle 24. We discuss the heliospheric consequences of such north-south asymmetric evolution, and the prospect of utilizing HMI vector field measurement at the polar region.




Prediction of Cycle 25 based on Polar Fields

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Presentation Type: Oral

Session: Session 3: Solar Magnetic Variability and the Solar Cycle

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WSO: The pole-most aperture measures the lineof-sight field between about 55° and the poles. Each 10 days the usable daily polar field measurements in a centered 30-day window are averaged. A 20nHz low pass filter eliminates yearly geometric projection effects. SDO-HMI: Line-of-sight magnetic observations (Blos above 60° lat.) at 720s cadence are converted to radial field (Br), under the assumption that the actual field vector is radial. Twice-per-day values are calculated as the mean weighted by de-projected image pixel areas for each latitudinal bin within ±45-deg longitude. These raw (12-hour) data have been averaged into the same windows as WSO’s and reduced to the WSO scale taking saturation (1.8) and projection (COS(72°)) into account. We have argued that the ‘poloidal’ field in the years leading up to solar minimum is a good proxy for the size of the next cycle (SNmax≈ DM [WSO scale μT]). The successful prediction of Cycle 24 seems to bear that out, as well as the observed corroboration from previous cycles. As a measure of the poloidal field we used the average ‘Dipole Moment’, i.e. the difference, DM, between the fields at the North pole and the South pole. The 20nHz filtered WSO DM matches well the HMI DM on the WSO scale using the same 30-day window as WSO. So, we can extend WSO using HMI into the future as needed. Preliminarily, the polar fields now are as strong as before the last minimum and may increase further, so Cycle 25 may be at least a repeat of Cycle 24, not any smaller and possible a bit stronger.