arXiv subjects

A. A. Norton

Publications and source records attributed to A. A. Norton.

12 recordsLinked to original sources

arXiv2025

Stray Light Correction for the Helioseismic and Magnetic Imager

We report a point spread function (PSF) and deconvolution procedure to remove stray light from the Helioseismic and Magnetic Imager (HMI) data. Pre-launch calibration observations, post-launch Venus transit and lunar transit data were used to develop the PSF and evaluate how well it reproduced the observed scattering. The PSF reported differs from previous stray light removal efforts since we do not use Gaussians as the central mathematical component. Instead, we use a Lorenztian convolved with an Airy function. In 2018, the HMI team began providing full-disk, stray-light-corrected data daily. Intensity, Doppler, magnetogram, and vector magnetic field data are provided. The deconvolution uses a Richardson-Lucy algorithm and takes less than one second per full-disk image. The results, on average, show decreases in umbral continuum intensity, a doubling of the granulation intensity contrast, increases in the total field strength, most notably in plage by $\sim$1.4--2.5 the original value, and a partial correction for the convective blueshift. Local helioseismology analyses using corrected data yield results that are consistent with those from uncorrected data, with only negligible differences, except in sunspot regions. The new data are found in JSOC with names similar to the original but with the qualifying term '$\_dcon$' or '$\_dconS$' appended, denoting whether the deconvolution was applied to the filtergrams or Stokes images. The HMI team recommends using the corrected data for improved visual clarity, more accurate irradiance reconstruction, better co-alignment with high-resolution data, reduced errors in tracking algorithms, and improved magnetic field strengths.

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arXiv2024

Magnetic helicity and energy budgets of jet events from an emerging solar active region

Using photospheric vector magnetograms obtained by the Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory and a magnetic connectivity-based method, we compute the magnetic helicity and free magnetic energy budgets of a simple bipolar solar active region (AR) during its magnetic flux emergence phase which lasted $\sim$47 hrs. The AR did not produce any coronal mass ejections (CMEs) or flares with an X-ray class above C1.0 but it was the site of 60 jet events during its flux emergence phase. The helicity and free energy budgets of the AR were below established eruption-related thresholds throughout the interval we studied. However, in addition to their slowly-varying evolution, each of the time profiles of the helicity and free energy budgets showed discrete localized peaks, with eight pairs of them occurring at times of jets emanating from the AR. These jets featured larger base areas and longer durations than the other jets of the AR. We estimated, for the first time, the helicity and free magnetic energy changes associated with these eight jets which were in the ranges of $0.5-7.1 \times 10^{40}$ Mx$^2$ and $1.1-6.9 \times 10^{29}$ erg, respectively. Although these values are one to two orders of magnitude smaller than those usually associated with CMEs, the relevant percentage changes were significant and ranged from 13% to 76% for the normalized helicity and from 9% to 57% for the normalized free magnetic energy. Our study indicates that occasionally jets may have a significant imprint in the evolution of helicity and free magnetic energy budgets of emerging active regions.

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arXiv2022

Characterizing the Umbral Magnetic Knots of $δ$-Sunspots

Delta ($δ$)-spots are active regions (ARs) in which positive and negative umbrae share a penumbra. They are known to be the source of strong flares. We introduce a new quantity, the degree of $δ$ (Do$δ$), to measure the fraction of umbral flux participating in the $δ$-configuration and to isolate the dynamics of the magnetic knot, i.e. adjacent umbrae in the $δ$-configuration. Using Helioseismic and Magnetic Imager data, we analyze 19 $δ$-spots and 11 $β$-spots in detail, and 120 $δ$-spots in less detail. We find that $δ$-regions are not in a $δ$-configuration for the entire time but spend 55$\%$ of their observed time as $δ$-spots with an average, maximum Do$δ$ of 72$\%$. Compared to $β$-spots, $δ$-spots have 2.6$\times$ the maximum umbral flux, 1.9$\times$ the flux emergence rate, 2.6$\times$ the rotation, and 72$\times$ the flare energy. On average, the magnetic knots rotate 17$^{\circ}$ day$^{-1}$ while the $β$-spots rotate 2$^{\circ}$ day$^{-1}$. Approximately 72$\%$ of the magnetic knots present anti-Hale or anti-Joy tilts, contrasting starkly with only 9$\%$ of the $β$-spots. A positive correlation exists between $ϕ_{Doδ}$ and the flare energy emitted by that region. The $δ$-spots obey the hemispheric current helicity rule 64$\%$ of the time. 84$\%$ of the $δ$-spots are formed by single flux emergence events and 58$\%$ have a quadrupolar magnetic configuration. The $δ$-spot characteristics are consistent with the formation mechanism signatures as follows: 42$\%$ with the kink instability or Sigma effect, 32$\%$ with multi-segment buoyancy, 16$\%$ with collisions and two active regions that are unclassified but consistent with a rising O-ring.

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arXiv2021

Oscillations observed in Umbra, Plage, Quiet-Sun and the Polarity Inversion Line of Active Region 11158 using HMI/SDO Data

Using data from the Helioseismic Magnetic Imager, we report on the amplitudes and phase relations of oscillations in quiet-Sun, plage, umbra and the polarity inversion line (PIL) of an active region NOAA$\#$11158. We employ Fourier, wavelet and cross correlation spectra analysis. Waves with 5-minute periods are observed in umbra, PIL and plage with common phase values of $ϕ(v,I)=\fracπ{2}$, $ϕ(v,B_{los})=-\fracπ{2}$. In addition, $ϕ(I,B_{los})=π$ in plage are observed. These phase values are consistent with slow standing or fast standing surface sausage wave modes. The line width variations, and their phase relations with intensity and magnetic oscillations, show different values within the plage and PIL regions, which may offer a way to further differentiate wave mode mechanics. Significant Doppler velocity oscillations are present along the PIL, meaning that plasma motion is perpendicular to the magnetic field lines, a signature of Alvènic waves. A time-distance diagram along a section of the PIL shows Eastward propagating Doppler oscillations converting into magnetic oscillations; the propagation speeds range between 2$-$6 km s$^{-1}$. Lastly, a 3-minute wave is observed in select regions of the umbra in the magnetogram data.

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arXiv2018

The Magnetic Response of the Solar Atmosphere to Umbral Flashes

Chromospheric observations of sunspot umbrae offer an exceptional view of magneto-acoustic shock phenomena and the impact they have on the surrounding magnetically-dominated plasma. We employ simultaneous slit-based spectro-polarimetry and spectral imaging observations of the chromospheric He I 10830Å and Ca II 8542Å lines to examine fluctuations in the umbral magnetic field caused by the steepening of magneto-acoustic waves into umbral flashes. Following the application of modern inversion routines, we find evidence to support the scenario that umbral shock events cause expansion of the embedded magnetic field lines due to the increased adiabatic pressure. The large number statistics employed allow us to calculate the adiabatic index, gamma = 1.12 +/- 0.01, for chromospheric umbral locations. Examination of the vector magnetic field fluctuations perpendicular to the solar normal revealed changes up to ~200 G at the locations of umbral flashes. Such transversal magnetic field fluctuations have not been described before. Through comparisons with non-linear force-free field extrapolations, we find that the perturbations of the transverse field components are orientated in the same direction as the quiescent field geometries. This implies that magnetic field enhancements produced by umbral flashes are directed along the motion path of the developing shock, hence producing relatively small changes, up to a maximum of ~8 degrees, in the inclination and/or azimuthal directions of the magnetic field. Importantly, this work highlights that umbral flashes are able to modify the full vector magnetic field, with the detection of the weaker transverse magnetic field components made possible by high-resolution data combined with modern inversion routines.

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arXiv2017

Magnetic Flux Emergence and Decay Rates for Preceder and Follower Sunspots Observed with HMI

We quantify the emergence and decay rates of preceder (p) and follower (f) sunspots within ten active regions from 2010-2014 using Space-weather Helioseismic Magnetic Imager Active Region Patch data. The sunspots are small- to mid-sized regions and contain a signed flux within a single polarity sunspot of $(1.1-6.5)\times 10^{21}$ Mx. The net unsigned flux within the regions, including plage, ranges from $(5.1-20)\times 10^{21}$ Mx. Rates are calculated with and without intensity contours to differentiate between sunspot formation and flux emergence. Signed flux emergence rates, calculated with intensity contours, for the p (f) spots average $6.8$ (4.9) $\times 10^{19}$ Mx h$^{-1}$, while decay rates are $-1.9 (-3.4)\times 10^{19}$ Mx h$^{-1}$. The mean, signed flux emergence rate of the regions, including plage, is $7.1 \times 10^{19}$ Mx h$^{-1}$ for a mean peak flux of $5.9 \times 10^{21}$ Mx. Using a synthesis of these results and others reported previously, there is a clear trend for larger flux regions to emerge faster than smaller ones. Observed emergence rates ($dϕ/dt$, Mx h$^{-1}$) scale with total signed peak flux, $\tildeϕ_{max}$, as a power law with an exponent of 0.36, i.e., $dϕ/dt = A \tildeϕ_{max}^{0.36}$. The observed rates may assist in constraining the boundary and initial conditions in simulations which already demonstrate increased rates for flux tubes with higher buoyancy and twist, or in the presence of a strong upflow. Overall, the observed emergence rates are smaller than those in simulations, which may indicate a slower rise of the flux in the interior than captured in simulations.

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arXiv2014

The Sun's interior structure and dynamics, and the solar cycle

The Sun's internal structure and dynamics can be studied with helioseismology, which uses the Sun's natural acoustic oscillations to build up a profile of the solar interior. We discuss how solar acoustic oscillations are affected by the Sun's magnetic field. Careful observations of these effects can be inverted to determine the variations in the structure and dynamics of the Sun's interior as the solar cycle progresses. Observed variations in the structure and dynamics can then be used to inform models of the solar dynamo, which are crucial to our understanding of how the Sun's magnetic field is generated and maintained.

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arXiv2010

Solar-Cycle Characteristics Examined in Separate Hemispheres: Phase, Gnevyshev Gap, and Length of Minimum

Research results from solar-dynamo models show the northern and southern hemispheres may evolve separately throughout the solar cycle. The observed phase lag between the hemispheres provides information regarding the strength of hemispheric coupling. Using hemispheric sunspot-area and sunspot-number data from Cycles 12 - 23, we determine how out of phase the separate hemispheres are during the rising, maximum, and declining period of each solar cycle. Hemispheric phase differences range from 0 - 11, 0 - 14, and 2 - 19 months for the rising, maximum, and declining periods, respectively. The phases appear randomly distributed between zero months (in phase) and half of the rise (or decline) time of the solar cycle. An analysis of the Gnevyshev gap is conducted to determine if the double-peak is caused by the averaging of two hemispheres that are out of phase. We confirm previous findings that the Gnevyshev gap is a phenomenon that occurs in the separate hemispheres and is not due to a superposition of sunspot indices from hemispheres slightly out of phase. Cross hemispheric coupling could be strongest at solar minimum, when there are large quantities of magnetic flux at the Equator. We search for a correlation between the hemispheric phase difference near the end of the solar cycle and the length of solar-cycle minimum, but found none. Because magnetic flux diffusion across the Equator is a mechanism by which the hemispheres couple, we measured the magnetic flux crossing the Equator by examining magnetograms for Solar Cycles 21 - 23. We find, on average, a surplus of northern hemisphere magnetic flux crossing during the mid-declining phase of each solar cycle. However, we find no correlation between magnitude of magnetic flux crossing the Equator, length of solar minima, and phase lag between the hemispheres.

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arXiv2008

Evidence for polar jets as precursors of polar plume formation

Observations from the Hinode/XRT telescope and STEREO/SECCHI/EUVI are utilized to study polar coronal jets and plumes. The study focuses on the temporal evolution of both structures and their relationship. The data sample, spanning April 7-8 2007, shows that over 90% of the 28 observed jet events are associated with polar plumes. EUV images (STEREO/SECCHI) show plume haze rising from the location of approximately 70% of the polar X-ray (Hinode/XRT) and EUV jets, with the plume haze appearing minutes to hours after the jet was observed. The remaining jets occurred in areas where plume material previously existed causing a brightness enhancement of the latter after the jet event. Short-lived, jet-like events and small transient bright points are seen (one at a time) at different locations within the base of pre-existing long-lived plumes. X-ray images also show instances (at least two events) of collimated-thin jets rapidly evolving into significantly wider plume-like structures that are followed by the delayed appearance of plume haze in the EUV. These observations provide evidence that X-ray jets are precursors of polar plumes, and in some cases cause brightenings of plumes. Possible mechanisms to explain the observed jet and plume relationship are discussed.

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arXiv2007

SOLIS Vector Spectromagnetograph: status and science

The Vector Spectromagnetograph (VSM) instrument has been recording photospheric and chromospheric magnetograms daily since August 2003. Full-disk photospheric vector magnetograms are observed at least weekly and, since November 2006, area-scans of active regions daily. Quick-look vector magnetic images, plus X3D and FITS formated files, are now publicly available daily. In the near future, Milne-Eddington inversion parameter data will also be available and a typical observing day will include three full-disk photospheric vector magnetograms. Besides full-disk observations, the VSM is capable of high temporal cadence area-scans of both the photosphere and chromosphere. Carrington rotation and daily synoptic maps are also available from the photospheric magnetograms and coronal hole estimate images.

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arXiv2006

Spectral Line Selection for HMI: A Comparison of Fe I 6173 and Ni I 6768

We present a study of two spectral lines, Fe I 6173 Angstroms and Ni I 6768 Angstroms, that were candidates to be used in the Helioseismic and Magnetic Imager (HMI) for observing Doppler velocity and the vector magnetic field. The line profiles were studied using the Mt. Wilson Observatory, the Advanced Stokes Polarimeter and the Kitt Peak McMath telescope and one meter Fourier transform spectrometer atlas. Both Fe I and Ni I profiles have clean continua and no blends that threaten instrument performance. The Fe I line is 2% deeper, 15% narrower and has a 6% smaller equivalent width than the Ni I line. The potential of each spectral line to recover pre-assigned solar conditions is tested using a least-squares minimization technique to fit Milne-Eddington models to tens of thousands of line profiles that have been sampled at five spectral positions across the line. Overall, the Fe I line has a better performance than the Ni I line for vector magnetic field retrieval. We selected the Fe I spectral line for use in HMI due to its better performance for magnetic diagnostics while not sacrificing velocity information.

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arXiv1999

Characteristics of Magnetohydrodynamic Oscillations Observed with Michelson Doppler Imager

We report on the spatial distribution of magnetogram oscillatory power and phase angles between velocity and magnetogram signals as observed with the Michelson Doppler Imager. The dataset is 151.25 arcsec times 151.25 arcsec containing sunspot from Dec 2, 1997 with a temporal sampling interval of 60 seconds and spatial sampling of 0.605 arcsec. Simultaneously observed continuum intensity and surface velocity accompany the magnetic information. We focus on three frequency regimes: 0.5-1.0, 3.0-3.5 and 5.5-6.0 mHz corresponding roughly to timescales of magnetic evolution, p-modes and the 3 minute resonant sunspot oscillation. Significant low frequency magnetogram power is found in lower flux pixels, 100-300 Gauss, in a striking ring with filamentary structure surrounding sunspot. Five minute magnetogram power peaks in extended regions of flux 600-800 Gauss. The 3 minute oscillation is observed in sunspot umbra in pixels whose flux measures 1300-1500 Gauss. Phase angles of approximately -90 degrees between velocity and magnetic flux in the 3.0-3.5 and 5.5-6.0 mHz regimes are found in regions of significant cross amplitude.

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