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Author

R. S. Bogart

Bio: R. S. Bogart is an academic researcher from Stanford University. The author has contributed to research in topic(s): Helioseismology & Convection zone. The author has an hindex of 21, co-authored 73 publication(s) receiving 10498 citation(s).
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Journal ArticleDOI
Sebastien Couvidat1, Jesper Schou2, J. T. Hoeksema1, R. S. Bogart1  +5 moreInstitutions (2)
Abstract: NASA’s Solar Dynamics Observatory (SDO) spacecraft was launched 11 February 2010 with three instruments onboard, including the Helioseismic and Magnetic Imager (HMI). After commissioning, HMI began normal operations on 1 May 2010 and has subsequently observed the Sun’s entire visible disk almost continuously. HMI collects sequences of polarized filtergrams taken at a fixed cadence with two $4096 \times 4096$ cameras, from which are computed arcsecond-resolution maps of photospheric observables that include line-of-sight velocity and magnetic field, continuum intensity, line width, line depth, and the Stokes polarization parameters [ $I, Q, U, V$ ]. Two processing pipelines have been implemented at the SDO Joint Science Operations Center (JSOC) at Stanford University to compute these observables from calibrated Level-1 filtergrams, one that computes line-of-sight quantities every 45 seconds and the other, primarily for the vector magnetic field, that computes averages on a 720-second cadence. Corrections are made for static and temporally changing CCD characteristics, bad pixels, image alignment and distortion, polarization irregularities, filter-element uncertainty and nonuniformity, as well as Sun–spacecraft velocity. We detail the functioning of these two pipelines, explain known issues affecting the measurements of the resulting physical quantities, and describe how regular updates to the instrument calibration impact them. We also describe how the scheme for computing the observables is optimized for actual HMI observations. Initial calibration of HMI was performed on the ground using a variety of light sources and calibration sequences. During the five years of the SDO prime mission, regular calibration sequences have been taken on orbit to improve and regularly update the instrument calibration, and to monitor changes in the HMI instrument. This has resulted in several changes in the observables processing that are detailed here. The instrument more than satisfies all of the original specifications for data quality and continuity. The procedures described here still have significant room for improvement. The most significant remaining systematic errors are associated with the spacecraft orbital velocity.

131 citations


Journal ArticleDOI
Abstract: NASA's Solar Dynamics Observatory (SDO) was launched 11 February 2010 with 3 instruments on board, including the Helioseismic and Magnetic Imager (HMI). Since beginning normal operations on 1 May 2010, HMI has observed the Sun's entire visible disk almost continuously. HMI collects sequences of polarized filtergrams taken at a fixed cadence with two 4096 x 4096 cameras from which are computed arcsecond-resolution maps of photospheric observables: the line-of-sight (LoS) velocity and magnetic field, continuum intensity, line width, line depth, and the Stokes polarization parameters, I Q U V, at 6 wavelengths. Two processing pipelines implemented at the SDO Joint Science Operations Center (JSOC) at Stanford University compute observables from calibrated Level-1 filtergrams. One generates LoS quantities every 45s, and the other, primarily for the vector magnetic field, computes averages on a 720s cadence. Corrections are made for static and temporally changing CCD characteristics, bad pixels, image alignment and distortion, polarization irregularities, filter-element uncertainty and non-uniformity, as well as Sun-spacecraft velocity. This report explains issues affecting the resulting physical quantities, describes the impact of regular updates to the instrument calibration, and shows how the computations are optimized for actual HMI observations. During the 5 years of the SDO prime mission, regular calibration sequences have been used to regularly improve and update the instrument calibration and to monitor instrument changes. The instrument more than satisfies the original specifications for data quality and continuity. The procedures described here still have significant room for improvement. The most significant remaining systematic errors are associated with the spacecraft orbital velocity.

120 citations


Journal ArticleDOI
Abstract: We report observations of white-light ejecta in the low corona, for two X-class flares on 2013 May 13, using data from the Helioseismic and Magnetic Imager (HMI) of the Solar Dynamics Observatory. At least two distinct kinds of sources appeared (chromospheric and coronal), in the early and later phases of flare development, in addition to the white-light footpoint sources commonly observed in the lower atmosphere. The gradual emissions have a clear identification with the classical loop-prominence system, but are brighter than expected and possibly seen here in the continuum rather than line emission. We find the HMI flux exceeds the radio/X-ray interpolation of the bremsstrahlung produced in the flare soft X-ray sources by at least one order of magnitude. This implies the participation of cooler sources that can produce free-bound continua and possibly line emission detectable by HMI. One of the early sources dynamically resembles {sup c}oronal rain{sup ,} appearing at a maximum apparent height and moving toward the photosphere at an apparent constant projected speed of 134 ± 8 km s{sup –1}. Not much literature exists on the detection of optical continuum sources above the limb of the Sun by non-coronagraphic instruments and these observations have potential implications for our basicmore » understanding of flare development, since visible observations can in principle provide high spatial and temporal resolution.« less

34 citations



Journal ArticleDOI
Abstract: We report observations of white-light ejecta in the low corona, for two X-class flares on the 2013 May 13, using data from the Helioseismic and Magnetic Imager (HMI) of the Solar Dynamics Observatory. At least two distinct kinds of sources appeared (chromospheric and coronal), in the early and later phases of flare development, in addition to the white-light footpoint sources commonly observed in the lower atmosphere. The gradual emissions have a clear identification with the classical loop-prominence system, but are brighter than expected and possibly seen here in the continuum rather than line emission. We find the HMI flux exceeds the radio/X-ray interpolation of the bremsstrahlung produced in the flare soft X-ray sources by at least one order of magnitude. This implies the participation of cooler sources that can produce free-bound continua and possibly line emission detectable by HMI. One of the early sources dynamically resembles "coronal rain", appearing at a maximum apparent height and moving toward the photosphere at an apparent constant projected speed of 134 $\pm$ 8 $\mathrm{km s^{-1}}$. Not much literature exists on the detection of optical continuum sources above the limb of the Sun by non-coronagraphic instruments, and these observations have potential implications for our basic understanding of flare development, since visible observations can in principle provide high spatial and temporal resolution.

17 citations


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Journal ArticleDOI
Abstract: Forecasting of solar flares remains a challenge due to the limited understanding of the underlying physical processes of these events, in particular magnetic reconnection. Studies have indicated that changes to the photospheric magnetic fields associated with magnetic reconnection – particularly in relation to the field helicity – occur during solar flare events. This study utilized data from the Solar Dynamics Observatory (SDO) Helioseismic and Magnetic Imager (HMI) and SpaceWeather HMI Active Region Patches (SHARPs) to analyze full vector-field component data of the photospheric magnetic field during solar flare events within a near decade long HMI dataset. Analysis of the data was used to identify and compare the trends of differing flare classes for varying time intervals leading up to an event, as well as the trends of flares that occur with and without a precursor flare, in order to discern signatures of the physical mechanisms involved. The data suggests that active regions that produce flares with precursors are continuing to evolve and appreciably more complex than those that produce a flare without a precursor. Additionally, precursor flares were found to enhance the shear across an active region, helping set up the conditions necessary for a larger solar flare to occur. Ultimately none of the SHARP parameters showed a distinct signature of magnetic reconnection.

Journal ArticleDOI
Naoto Nishizuka1, Yuki Kubo1, Komei Sugiura2, Mitsue Den1  +1 moreInstitutions (2)
Abstract: We developed an operational solar flare prediction model using deep neural networks, named Deep Flare Net (DeFN). DeFN can issue probabilistic forecasts of solar flares in two categories, such as >=M-class and =C-class and =M-class flares and TSS = 0.63 for >=C-class flares. For comparison, we evaluated the operationally forecast results from January 2019 to June 2020. We found that operational DeFN forecasts achieved TSS = 0.70 (0.84) for >=C-class flares with the probability threshold of 50 (40)%, although there were very few M-class flares during this period and we should continue monitoring the results for a longer time. Here, we adopted a chronological split to divide the database into two for training and testing. The chronological split appears suitable for evaluating operational models. Furthermore, we proposed the use of time-series cross-validation. The procedure achieved TSS = 0.70 for >=M-class flares and 0.59 for >=C-class flares using the datasets obtained from 2010 to 2017.



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Performance
Metrics

Author's H-index: 21

No. of papers from the Author in previous years
YearPapers
20162
201310
20124
20117
20101
20091