Author name code: hathaway
ADS astronomy entries on 2022-09-14
=author:"Hathaway, David" OR =author:"Hathaway, David H." OR =author:"Hathaway, D.H."
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Title: Variations in the Suns Axisymmetric Flows During Solar Cycles
23 and 24
Authors: Upton, Lisa; Hathaway, David; Mahajan, Sushant
Bibcode: 2021AGUFMSH54A..01U
Altcode:
The axisymmetric flows, differential rotation and meridional
circulation, are essential components of the solar dynamo. We have
measured these flows in the surface shear layer for each Carrington
Rotation during solar cycles 23 and 24 (1996-2021) by tracking the
motions of the magnetic network seen in magnetograms from SOHO/MDI
and SDO/HMI. We describe several improvements that have been made to
our pattern tracking algorithm, including the removal of a systematic
shift away from the disc center. Weak variations in the differential
rotation, known as the torsional oscillation, feature slower flows
on the poleward sides of the active latitudes and a high latitude
spin-up at cycle maxima. These variations were smaller during the
weaker cycle 24. Variations in the meridional flow include a slowing
of the flow in the active latitudes during cycle maxima, which was
more pronounced in Cycle 23 than in the weaker Cycle 24. Furthermore,
we find evidence of transient counter-cells at high latitudes which
appear seems unrelated to the solar activity cycle.
Title: Improved Measurements of the Sun's Meridional Flow and
Torsional Oscillation from Correlation Tracking on MDI and HMI
Magnetograms
Authors: Mahajan, Sushant S.; Hathaway, David H.; Muñoz-Jaramillo,
Andrés; Martens, Petrus C.
Bibcode: 2021ApJ...917..100M
Altcode: 2021arXiv210707731M
The Sun's axisymmetric flows, differential rotation, and meridional
flow govern the dynamics of the solar magnetic cycle, and a variety of
methods are used to measure these flows, each with its own strengths
and weaknesses. Flow measurements based on cross-correlating images of
the surface magnetic field have been made since the 1970s that require
advanced numerical techniques that are capable of detecting movements
of less than the pixel size in images of the Sun. We have identified
several systematic errors in addition to the center-to-limb effect that
influence previous measurements of these flows and propose numerical
techniques that can minimize these errors by utilizing measurements
of displacements at several time lags. Our analysis of line-of-sight
magnetograms from the Michelson Doppler Imager on the ESA/NASA Solar
and Heliospheric Observatory and the Helioseismic and Magnetic Imager
on the NASA Solar Dynamics Observatory shows long-term variations in
the meridional flow and differential rotation over two sunspot cycles
from 1996 to 2020. These improved measurements can serve as vital
inputs for solar dynamo and surface flux transport simulations.
Title: Hydrodynamic Properties of the Sun's Giant Cellular Flows
Authors: Hathaway, David H.; Upton, Lisa A.
Bibcode: 2021ApJ...908..160H
Altcode: 2020arXiv200606084H
Measurements of the large cellular flows on the Sun were made by local
correlation tracking of features (supergranules) seen in full-disk
Doppler images obtained by the Helioseismic and Magnetic Imager
(HMI) instrument on the NASA Solar Dynamics Observatory (SDO)
satellite. Several improvements made to the local correlation
tracking method allowed for more precise measurements of these
flows. Measurements were made hourly over the nearly ten years
of the mission-to-date. A four-hour time-lag between images was
determined to give the best results as a compromise between increased
feature displacement and decreased feature evolution. The hourly
measurements were averaged over the 34 days that it takes to observe all
longitudes at all latitudes to produce daily maps of the latitudinal
and longitudinal velocities. Analyses of these flow maps reveal many
interesting characteristics of these large cellular flows. While flows
at all latitudes are largely in the form of vortices with left-handed
helicity in the north and right-handed helicity in the south, there are
key distinctions between the low-latitude and high-latitude cells. The
low-latitude cells have roughly circular shapes, lifetimes of about
one month, rotate nearly rigidly, do not drift in latitude, and do not
exhibit any correlation between longitudinal and latitudinal flow. The
high-latitude cells have long extensions that spiral inward toward
the poles and can wrap nearly completely around the Sun. They have
lifetimes of several months, rotate differentially with latitude,
drift poleward at speeds approaching 2 m s-1, and have a
strong correlation between prograde and equatorward flows. Spherical
harmonic spectral analyses of maps of the divergence and curl of
the flows confirm that the flows are dominated by the curl component
with rms velocities of about 12 m s-1 at wavenumber ℓ
= 10. Fourier transforms in time over 1024 daily records of the
spherical harmonic spectra indicate two notable components—an m =
±ℓ feature representing the low-latitude component and an m =
±1 feature representing the high-latitude component. The dispersion
relation for the low-latitude component is well represented by that
derived for Rossby waves or r-modes. The high-latitude component has a
constant temporal frequency for all ℓ indicating features advected
by differential rotation at rates representative of the base of the
convection zone high latitudes. The poleward motions of these features
further suggest that the high-latitude meridional flow at the base of
the convection zone is poleward—not equatorward.
Title: Properties of the Sun's Giant Cellular Flows and their
Implications for the Sun's Activity Cycle
Authors: Hathaway, D. H.; Upton, L.
Bibcode: 2020AGUFMSH007..02H
Altcode:
No abstract at ADS
Title: The AFT Solar Cycle 25 Predictions: A Retrospective Comparison
with the Observations
Authors: Upton, L.; Hathaway, D. H.
Bibcode: 2020AGUFMSH053..03U
Altcode:
No abstract at ADS
Title: Calibration of the Sunspot and Group Numbers Using the
Waldmeier Effect
Authors: Svalgaard, Leif; Hathaway, David H.
Bibcode: 2020arXiv201101330S
Altcode:
The Waldmeier Effect is the observation that the rise time of a sunspot
cycle varies inversely with the cycle amplitude: strong cycles rise to
their maximum faster than weak cycles. The shape of the cycle and thus
the rise time does not depend on the scale factor of the sunspot number
and can thus be used to verify the constancy of the scale factor with
time as already noted by Wolfer (1902) and Waldmeier (1978). We extend
their analysis until the present using the new SILSO sunspot number
(version 2) and group number and confirm that the scale factors have
not varied significantly the past 250 years. The effect is also found
in sunspot areas, in an EUV (and F10.7) proxy (the daily range of a
geomagnetic variation), and in Cosmic Ray Modulation. The result is
that solar activity reached similar high values in every one of the
(17th?) 18th, 19th, and 20th centuries, supporting the finding that
there has been no modern Grand Maximum.
Title: An Updated Solar Cycle 25 Prediction With AFT: The Modern
Minimum
Authors: Upton, Lisa A.; Hathaway, David H.
Bibcode: 2018GeoRL..45.8091U
Altcode: 2018arXiv180804868U
Over the last decade there has been mounting evidence that the strength
of the Sun's polar magnetic fields during a solar cycle minimum is the
best predictor of the amplitude of the next solar cycle. Surface flux
transport models can be used to extend these predictions by evolving
the Sun's surface magnetic field to obtain an earlier prediction for
the strength of the polar fields, and thus the amplitude of the next
cycle. In 2016, our Advective Flux Transport (AFT) model was used to
do this, producing an early prediction for Solar Cycle 25. At that
time, AFT predicted that Cycle 25 will be similar in strength to the
Cycle 24, with an uncertainty of about 15%. AFT also predicted that
the polar fields in the southern hemisphere would weaken in late 2016
and into 2017 before recovering. That AFT prediction was based on the
magnetic field configuration at the end of January 2016. We now have
two more years of observations. We examine the accuracy of the 2016
AFT prediction and find that the new observations track well with AFT's
predictions for the last 2 years. We show that the southern relapse did
in fact occur, though the timing was off by several months. We propose
a possible cause for the southern relapse and discuss the reason for
the offset in timing. Finally, we provide an updated AFT prediction for
Solar Cycle 25 that includes solar observations through January of 2018.
Title: What to Expect for Cycle 25 - an AFT Prediction
Authors: Upton, Lisa; Hathaway, David H.
Bibcode: 2018tess.conf11501U
Altcode:
Over the course of Cycle 24, it has become evident that the best
predictor of the amplitude of a coming cycle is the strength of the
Sun's polar magnetic fields during solar cycle minimum. Surface flux
transport models allow us to make these predictions earlier by evolving
the magnetic field on the surface in order to predict the strength of
the polar fields, and thus the amplitude of the coming cycle, ahead of
time. Recently, the Advective Flux Transport (AFT) model was used to
produce a prediction of the amplitude of Solar Cycle 25 by simulating
the evolution of the polar fields leading to coming solar minimum. AFT
found that Cycle 25 will be slightly small of similar in strength to
the current cycle, with an uncertainty of about 15%. AFT also predicted
that the southern polar fields would weaken in late 2016 and into 2017
before recovering. This prediction was based on the magnetic field
configuration as of January 2016. Here I will discuss the accuracy of
that prediction based on the observations that have occurred since. I
will also present an updated Solar Cycle 25 prediction based on the
magnetic field configuration as of January 2018.
Title: AFT: Extending Solar Cycle Prediction with Data Assimilation
Authors: Upton, L.; Hathaway, D. H.
Bibcode: 2017AGUFMSH12A..01U
Altcode:
The Advective Flux Transport (AFT) model is an innovative surface flux
transport model that simulates the evolution of the radial magnetic
field on the surface of the Sun. AFT was designed to be as realistic as
possible by 1: incorporating the observed surface flows (meridional
flow, differential rotation, and an explicit evolving convective
pattern) and by 2: using data assimilation to incorporate the observed
magnetic fields directly from line-of-sight (LOS) magnetograms. AFT
has proven to be successful in simulating the evolution of the surface
magnetic fields on both short time scales (days-weeks) as well as
for long time scales (years). In particular, AFT has been shown to
accurately predict the evolution of the Sun's dipolar magnetic field
3-5 years in advance. Since the Sun's polar magnetic field strength
at solar cycle minimum is the best indicator of the amplitude of the
next cycle, this has in turn extended our ability to make solar cycle
predictions to 3-5 years before solar minimum occurs. Here, we will
discuss some of the challenges of implementing data assimilation into
AFT. We will also discuss the role of data assimilation in advancing
solar cycle predictive capability.
Title: AFT: An Updated Solar Cycle 25 Prediction
Authors: Upton, L.; Hathaway, D. H.
Bibcode: 2017AGUFMSH13A2472U
Altcode:
Over the course of Cycle 24, it has become evident that the best
predictor of the amplitude of a coming cycle is the strength of the
Sun's polar magnetic fields during solar cycle minimum. Surface flux
transport models allow us to make these predictions earlier by evolving
the magnetic field on the surface in order to predict the strength
of the polar fields, and thus the amplitude of the coming cycle,
ahead of time. Recently, the Advective Flux Transport (AFT) model was
used to produce a prediction of the amplitude of Solar Cycle 25 by
simulating the evolution of the polar fields leading to coming solar
minimum. We found that Cycle 25 will be similar in strength to the
current cycle, with an uncertainty of about 15%. We also we predicted
that the southern polar fields would weaken in late 2016 and into 2017
before recovering. This prediction was based on the magnetic field
configuration as of January 2016. Here we will discuss the accuracy of
that predicted polar field evolution. We will also present an updated
Solar Cycle 25 prediction which includes the solar observations that
have occurred since then.
Title: Addressing Systematic Errors in Correlation Tracking on
HMI Magnetograms
Authors: Mahajan, Sushant S.; Hathaway, David H.; Munoz-Jaramillo,
Andres; Martens, Petrus C.
Bibcode: 2017SPD....4820702M
Altcode:
Correlation tracking in solar magnetograms is an effective method to
measure the differential rotation and meridional flow on the solar
surface. However, since the tracking accuracy required to successfully
measure meridional flow is very high, small systematic errors have a
noticeable impact on measured meridional flow profiles. Additionally,
the uncertainties of this kind of measurements have been historically
underestimated, leading to controversy regarding flow profiles at
high latitudes extracted from measurements which are unreliable
near the solar limb.Here we present a set of systematic errors we
have identified (and potential solutions), including bias caused by
physical pixel sizes, center-to-limb systematics, and discrepancies
between measurements performed using different time intervals. We have
developed numerical techniques to get rid of these systematic errors
and in the process improve the accuracy of the measurements by an order
of magnitude.We also present a detailed analysis of uncertainties in
these measurements using synthetic magnetograms and the quantification
of an upper limit below which meridional flow measurements cannot be
trusted as a function of latitude.
Title: A Data-driven Model of the Solar Atmosphere and Heliosphere
Authors: Pogorelov, Nikolai; Hathaway, David; Kim, Tae; Liu, Yang;
Singh, Talwinder; Yalim, Mehmet Sarp
Bibcode: 2017shin.confE.123P
Altcode:
We propose a data-driven, time-dependent model of the solar atmosphere
and heliosphere suitable for nearly real time predictions of the solar
wind (SW) properties at Earth's orbit and further in the interplanetary
space. In particular, we use the SDO/HMI vector magnetograms extended to
the full Sun surface to formulate the boundary conditions appropriate
for the proposed task both physically and mathematically. Reliable
time-dependent simulations that start from the solar surface outward
have strong potential because of the wealth of observational data
currently available for space weather predictions, and also in
anticipation of new missions, such as Solar Orbiter and Parker Solar
Probe, to be launched in the near future. A real time model should not
only be numerically accurate and computationally efficient, but also
based on a solid scientific background. We describe an innovative way to
use the SDO/HMI vector and line-of-sight magnetograms, accompanied with
STEREO observations and our Advective Flux Transport model, which allows
us to specify sufficient number of mathematically admissible boundary
conditions at the solar surface. The implementation of characteristic
boundary conditions is based not only on the knowledge of how many
quantities should be specified, but also on the requirement that the
time-increments of quantities that are specified as physical boundary
conditions should be uniquely expressible in terms of the increments
of proper characteristic variables. Such boundary conditions are
implemented in a Multi-Scale Fluid-Kinetic Simulation Suite (MS-FLUKSS)
developed by the authors. MS-FLUKSS solves MHD equations with the
volumetric heating source terms. Beyond the Alfvenic surface, we
also take into account the influence of interstellar neutral atoms,
nonthermal pickup ions, and SW turbulence. In addition, we have
implemented special algorithms to track exactly the surfaces that
propagate with the SW (heliospheric current sheet, CMEs, etc). Combining
these features with AMR, we are able to analyze the CME structure
with high precision. By using high resolution observations, such as
12-minute SDO/HMI vector magnetograms, we are particularly focusing
on time-dependent phenomena in the background SW. We model CMEs by
inserting them, on analyzing multi-viewpoint observational data,
into our background solutions. We show examples of CME modeling based
on the Gibson-Low approach. Numerical results include simulations of
the solar wind and interplanetary magnetic field at Earth, Neptune,
Pluto, and Uranus, and also at STEREO, New Horizons, Ulysses, and
Voyager spacecraft.
Title: Advancing our Understanding of Active Region Evolution and
Surface Flux Transport Using Far Side Imaging from STEREO 304
Authors: Upton, L.; Ugarte-Urra, I.; Warren, H. P.; Hathaway, D. H.
Bibcode: 2016AGUFMSH42B..02U
Altcode:
The STEREO mission, combined with SDO, provides a unique opportunity
to view the solar surface continuously. These continuous observations
provide the first opportunity to track the long-term evolution of Active
Regions over multiple rotations. We present recent results in which we
illustrate how He 304 Å images can be used as a proxies for magnetic
flux measurements. We will present the long-term evolution of select
isolated Active Regions as seen in He 304 Å. These data are then
used to validate the far-side evolution of individual active regions
produced with our Advective Flux Transport model - AFT. The AFT model
is a state of the art Surface Flux Transport model, which simulates
the observed near-surface flows (including an evolving convective flow
velocity field) to model the transport of magnetic flux over the entire
Sun. Finally, we will show that when new flux emergence occurs on the
far-side of the Sun, 304 Å images can provide sufficient information
about the active region to predict its evolution. These far-side Active
Regions have a substantial impact on the coronal and interplanetary
field configuration used for space weather predictions.
Title: Effects of Far-side Evolution of Magnetic Structures on
Coronal and Interplanetary Magnetic Features
Authors: Hathaway, D. H.; Upton, L.
Bibcode: 2016AGUFMSH43A2557H
Altcode:
Models for the structure and dynamics of the corona and the
interplanetary medium are based on magnetic boundary conditions at the
Sun's surface. The degree to which this magnetic boundary condition
faithfully represents the actual conditions at the Sun's surface will
obviously influence how well the models can accurately reproduce the
conditions in the interplanetary medium. Here we show how incorporating
the evolution of far-side magnetic structures (including far-side
active regions) affects the structure of the corona and solar wind. We
compare observations of coronal structures (coronal holes) with those
produced from: 1) synoptic magnetic maps with no far-side evolution;
2) synchronic maps in which the near-side magnetic structures evolve
on the far-side via our Advective Flux Transport (AFT) code; and 3)
synchronic maps with far-side evolution via the AFT code incorporating
far-side information from STEREO EUV images. We will likewise compare
observations of solar wind magnetic structures with those produced
with these three different boundary conditions.
Title: Predicting the amplitude and hemispheric asymmetry of solar
cycle 25 with surface flux transport
Authors: Hathaway, David H.; Upton, Lisa A.
Bibcode: 2016JGRA..12110744H
Altcode: 2016arXiv161105106H
Evidence strongly indicates that the strength of the Sun's polar fields
near the time of a sunspot cycle minimum determines the strength of the
following solar activity cycle. We use our Advective Flux Transport
code, with flows well constrained by observations, to simulate the
evolution of the Sun's polar magnetic fields from early 2016 to the
end of 2019—near the expected time of cycle 24/25 minimum. We run
a series of simulations in which the uncertain conditions (convective
motion details, active region tilt, and meridional flow profile) are
varied within expected ranges. We find that the average strength of
the polar fields near the end of cycle 24 will be similar to that
measured near the end of cycle 23, indicating that cycle 25 will
be similar in strength to the current cycle. In all cases the polar
fields are asymmetric with fields in the south stronger than those
in the north. This asymmetry would be more pronounced if not for
the predicted weakening of the southern polar fields in late 2016
and through 2017. After just 4 years of simulation the variability
across our ensemble indicates an accumulated uncertainty of about
15%. This accumulated uncertainty arises from stochastic variations
in the convective motion details, the active region tilt, and changes
in the meridional flow profile. These variations limit the ultimate
predictability of the solar cycle.
Title: Unraveling the Complexity of the Evolution of the Sun's
Photospheric Magnetic Field
Authors: Hathaway, David H.
Bibcode: 2016usc..confE..87H
Altcode:
Given the emergence of tilted, bipolar active regions, surface flux
transport has been shown to reproduce much of the complex evolution
of the Sun's photospheric magnetic field. Surface flux is transported
by flows in the surface shear layer - the axisymmetric differential
rotation and meridional flow and the non-axisymmetric convective motions
(granules, supergranules, and giant cells). We have measured these
flows by correlation tracking of the magnetic elements themselves,
correlation tracking of the Doppler features (supergranules), and by
direct Doppler measurements using SDO/HMI data. These measurements fully
constrain (with no free parameters) the flows used in our surface flux
transport code - the Advective Flux Transport or AFT code. Here we show
the up-to-date evolution of these flows, their impact on the detailed
evolution of the Sun's photospheric magnetic field, and predictions
for what the polar fields will be at the next minimum in 2020.
Title: Predicting the Amplitude and Hemispheric Asymmetry of Solar
Cycle 25 with Surface Flux Transport
Authors: Hathaway, David H.; Upton, Lisa
Bibcode: 2016SPD....47.1006H
Altcode:
Evidence from 40 years of magnetic field measurements, 110 years of
polar faculae counts, and 150 years of geomagnetic field measurements,
strongly indicates that the strength of the magnetic field at the
Sun's poles near the time of a sunspot cycle minimum determines the
strength of the following solar activity cycle. The processes that
produce these polar fields are well observed and accurately modeled
as the transport of magnetic flux (which emerges in active regions)
by the horizontal flows in the Sun's near-surface shear layer,
i.e. differential rotation, poleward meridional flow, and cellular
convective motions. We use our Advective Flux Transport (AFT) code,
with flows fully constrained by observations, to simulate the evolution
of the Sun's polar magnetic fields from early 2016 to the end of 2019
- near the expected time of Cycle 24/25 minimum. We assimilate active
regions from Cycle 14 (107 years earlier) to represent the continued
development of Cycle 24. Cycle 14 was similar to Cycle 24 in size,
shape, and hemispheric asymmetry. We run a series of simulations in
which the uncertain conditions (convective motion details, active
region tilt, and meridional flow profile) are varied within expected
ranges. We find that the ensemble average of the strength of the polar
fields near the end of Cycle 24 is about the same as that measured
near the end of Cycle 23, indicating that Cycle 25 will be similar in
strength to the current cycle with an expected maximum sunspot number
(Version 2.0) of 100±15. In all cases within our ensemble the polar
fields are asymmetric with fields in the south stronger than those in
the north. After just four years of simulation the variability across
our ensemble indicates an uncertainty of about 15%. This stochastic
variability, intrinsic to the Sun itself, suggests that we may never
be able to reliably predict solar cycles more than one cycle into
the future.
Title: First flight of SMASH, the SwRI Miniature Assembly for Solar
Hard X-rays
Authors: Caspi, Amir; Laurent, Glenn Thomas; Shoffner, Michael;
Higuera Caubilla, David; Meurisse, Jeremie; Smith, Kelly; Shih,
Albert Y.; Saint-Hilaire, Pascal; DeForest, Craig; Mansour, Nagi N.;
Hathaway, David H.
Bibcode: 2016SPD....4720601C
Altcode:
The SwRI Miniature Assembly for Solar Hard X-rays (SMASH) was
successfully flown from Antarctica in January (19-30) 2016, as
a piggy-back instrument on the Gamma-Ray Imager/Polarimeter for
Solar flares (GRIPS) high altitude balloon payload. SMASH is a
technological demonstration of a new miniaturized hard X-ray (HXR)
detector for use on CubeSats and other small spacecraft, including
the proposed CubeSat Imaging X-ray Solar Spectrometer (CubIXSS).HXRs
are the observational signatures of energetic processes on the Sun,
including plasma heating and particle acceleration. One of the goals of
CubIXSS will be to address the question of how plasma is heated during
solar flares, including the relationship between thermal plasma and
non-thermal particles. SMASH demonstrated the space-borne application
of the commercial off-the-shelf Amptek X123-CdTe, a miniature cadmium
telluride photon-counting HXR spectrometer. The CdTe detector has a
physical area of 25 mm^2 and 1 mm fully-depleted thickness, with a ~100
micron Be window; with on-board thermoelectric cooling and pulse pile-up
rejection, it is sensitive to solar photons from ~5 to ~100 keV with
~0.5-1.0 keV FWHM resolution. Photons are accumulated into histogram
spectra with customizable energy binning and integration time. With
modest resource requirements (~1/8 U, ~200 g, ~2.5 W) and low cost
(~$10K), the X123-CdTe is an attractive solution for HXR measurements
from budget- and resource-limited platforms such as CubeSats. SMASH
flew two identical X123-CdTe detectors for redundancy and increased
collecting area; the supporting electronics (power, CPU) were largely
build-to-print using the Miniature X-ray Solar Spectrometer (MinXSS)
CubeSat design.We review the SMASH mission, design, and detector
performance during the 12-day Antarctic flight. We present current
progress on our data analysis of observed solar flares, and discuss
future applications of the space-qualified X123-CdTe detector, including
the CubIXSS mission concept that incorporates two such detectors.
Title: Recalculated Sunspot Cycle Characteristics Using the New
Sunspot Number Series
Authors: Hathaway, D. H.
Bibcode: 2015AGUFMSH23C2452H
Altcode:
The new sunspot number series introduces a number of changes, including
dropping the multiplicative factor of ~0.6 that was previously used
to bring the numbers into agreement with those obtained by Rudolf Wolf
with his small telescope in the mid-19th century. Using the new numbers
clearly requires changing the functional relationships between sunspot
number and other solar activity indices (e.g. sunspot area, 10.7 cm
radio flux, total solar irradiance, x-ray flares, and geomagnetic
activity). We find that, in most cases, these relationships exhibit
less scatter with the new numbers. Solar cycle prediction methods must
also be recalibrated with the new numbers. The new numbers clearly
indicate the need for a reassessment of the impact of solar variability
on terrestrial climate. There is now no appearance of a significant
secular rise in solar activity levels since the early 18th century,
suggesting, that earlier estimates of the impact of solar activity on
climate should be revised downward.
Title: Solar Cycle Prediction with the Advective Flux Transport
(AFT) Code
Authors: Hathaway, D. H.; Upton, L.
Bibcode: 2015AGUFMSH23A2434H
Altcode:
Recent observations and analyses strongly indicate that the strength of
the sun's polar fields at the end of a cycle predicts the strength of
the next solar cycle. The surface magnetic flux transport that builds
up these polar fields is now well observed and is realistically modeled
with the Advective Flux Transport (AFT) code. Given the emergence of
magnetic flux in active regions, and using the observed near surface
flows, the AFT code can reproduce, in detail, the observed magnetic
features - including both the polar fields and the large unipolar
regions - years later. The AFT code can thus be used to predict the
strength of the polar fields years before the end of a cycle and
thereby provide an earlier prediction for the strength of the next
cycle. We examine the limits of these predictions by reconstructing
the sun's magnetic field in previous cycles. We find that both the
surface flows and the active region sources change systematically
over the course of a cycle and with the strength of a cycle. However,
stochastic variations in both the flows and the active region sources
ultimately limit predictions of the solar cycle.
Title: Magnetic Flux Transport and the Long-term Evolution of Solar
Active Regions
Authors: Ugarte-Urra, Ignacio; Upton, Lisa; Warren, Harry P.; Hathaway,
David H.
Bibcode: 2015ApJ...815...90U
Altcode: 2015arXiv151104030U
With multiple vantage points around the Sun, Solar Terrestrial Relations
Observatory (STEREO) and Solar Dynamics Observatory imaging observations
provide a unique opportunity to view the solar surface continuously. We
use He ii 304 Å data from these observatories to isolate and track
ten active regions and study their long-term evolution. We find
that active regions typically follow a standard pattern of emergence
over several days followed by a slower decay that is proportional in
time to the peak intensity in the region. Since STEREO does not make
direct observations of the magnetic field, we employ a flux-luminosity
relationship to infer the total unsigned magnetic flux evolution. To
investigate this magnetic flux decay over several rotations we use
a surface flux transport model, the Advective Flux Transport model,
that simulates convective flows using a time-varying velocity field
and find that the model provides realistic predictions when information
about the active region's magnetic field strength and distribution at
peak flux is available. Finally, we illustrate how 304 Å images can
be used as a proxy for magnetic flux measurements when magnetic field
data is not accessible.
Title: The Sun's Photospheric Convection Spectrum
Authors: Hathaway, David H.; Teil, Thibaud; Norton, Aimee A.;
Kitiashvili, Irina
Bibcode: 2015ApJ...811..105H
Altcode: 2015arXiv150803022H
Spectra of the cellular photospheric flows are determined from
full-disk Doppler velocity observations acquired by the Helioseismic
and Magnetic Imager (HMI) instrument on the Solar Dynamics Observatory
spacecraft. Three different analysis methods are used to separately
determine spectral coefficients representing the poloidal flows, the
toroidal flows, and the radial flows. The amplitudes of these spectral
coefficients are constrained by simulated data analyzed with the same
procedures as the HMI data. We find that the total velocity spectrum
rises smoothly to a peak at a wavenumber of about 120 (wavelength of
about 35 Mm), which is typical of supergranules. The spectrum levels
off out to wavenumbers of about 400, and then rises again to a peak
at a wavenumber of about 3500 (wavelength of about 1200 km), which
is typical of granules. The velocity spectrum is dominated by the
poloidal flow component (horizontal flows with divergence but no curl)
at wavenumbers above 30. The toroidal flow component (horizontal flows
with curl but no divergence) dominates at wavenumbers less than 30. The
radial flow velocity is only about 3% of the total flow velocity at
the lowest wavenumbers, but increases in strength to become about 50%
at wavenumbers near 4000. The spectrum compares well with the spectrum
of giant cell flows at the lowest wavenumbers and with the spectrum
of granulation from a 3D radiative-hydrodynamic simulation at the
highest wavenumbers.
Title: The Solar Cycle
Authors: Hathaway, David H.
Bibcode: 2015LRSP...12....4H
Altcode: 2015arXiv150207020H
The solar cycle is reviewed. The 11-year cycle of solar activity is
characterized by the rise and fall in the numbers and surface area
of sunspots. A number of other solar activity indicators also vary
in association with the sunspots including; the 10.7 cm radio flux,
the total solar irradiance, the magnetic field, flares and coronal
mass ejections, geomagnetic activity, galactic cosmic ray fluxes,
and radioisotopes in tree rings and ice cores. Individual solar
cycles are characterized by their maxima and minima, cycle periods and
amplitudes, cycle shape, the equatorward drift of the active latitudes,
hemispheric asymmetries, and active longitudes. Cycle-to-cycle
variability includes the Maunder Minimum, the Gleissberg Cycle, and
the Gnevyshev-Ohl (even-odd) Rule. Short-term variability includes
the 154-day periodicity, quasi-biennial variations, and double-peaked
maxima. We conclude with an examination of prediction techniques for
the solar cycle and a closer look at cycles 23 and 24.
Title: On the long-term evolution of solar active regions from full
Sun observations, magnetic flux transport and hydrodynamic modeling
Authors: Ugarte-Urra, Ignacio; Upton, Lisa; Warren, Harry; Hathaway,
David H.
Bibcode: 2015TESS....120104U
Altcode:
With their multiple vantage points around the Sun, STEREO and SDO
observations provide a unique opportunity to view the solar surface
continuously. We use data from these observatories to study the
long-term evolution of solar active regions in He II 304 A. We
show that active regions follow a universal pattern of emergence
over several days followed by a decay that is proportional to the
peak intensity in the region. We find that magnetic surface flux
transport simulations are able to reproduce this evolution. Since
STEREO does not make direct observations of the magnetic field, we use
the flux-luminosity relationship to infer the total unsigned magnetic
flux from the He 304 A images. We also illustrate the use of far-side
imaging to introduce solar active regions into magnetic surface flux
transport simulations. We finally show how these models can be used to
determine the long-term coronal emission evolution in active regions
by coupling extrapolations of the magnetic flux transport simulations
field with EBTEL solutions to the hydrodynamic loop equations.
Title: Magnetic Flux Transport at the Solar Surface
Authors: Jiang, J.; Hathaway, D. H.; Cameron, R. H.; Solanki, S. K.;
Gizon, L.; Upton, L.
Bibcode: 2015sac..book..491J
Altcode:
No abstract at ADS
Title: Improving Synchronic Maps with Far-Side Active Region Emergence
Authors: Upton, L.; Hathaway, D. H.
Bibcode: 2014AGUFMSH41B4135U
Altcode:
Synchronic maps (i.e., maps of the Sun's photospheric magnetic field
over the entire surface at a single instant in time) often serve as
the inner boundary condition for global coronal magnetic field and
solar wind models. Currently, we use a surface flux transport model
to construct synchronic maps every 15 minutes with a resolution of
1024 by 512 in longitude-latitude. This model assimilates magnetic
data from SDO/HMI full-disk line-of-sight magnetograms and advects the
magnetic field with differential rotation and meridional flow profiles
taken directly from the motions of the magnetic elements. Rather than
using a diffusivity coefficient, this model explicitly incorporates
well-resolved cellular convective flows with spatial and temporal
characteristics that match observations, thus retaining the magnetic
network structure observed on the Sun. While this model accurately
transports the active regions that are observed on the near-side of the
Sun, active regions that emerge on the far-side are neglected until they
appear in the observations. Far-side active regions will obviously have
a substantial impact on the global coronal field configuration and must
be included in useful synchronic maps. We will discuss our attempts to
incorporate far-side active region emergence into our flux transport
model. We will also illustrate the impact of these improvements.
Title: Magnetic Flux Transport at the Solar Surface
Authors: Jiang, J.; Hathaway, D. H.; Cameron, R. H.; Solanki, S. K.;
Gizon, L.; Upton, L.
Bibcode: 2014SSRv..186..491J
Altcode: 2014SSRv..tmp...43J; 2014arXiv1408.3186J
After emerging to the solar surface, the Sun's magnetic field displays a
complex and intricate evolution. The evolution of the surface field is
important for several reasons. One is that the surface field, and its
dynamics, sets the boundary condition for the coronal and heliospheric
magnetic fields. Another is that the surface evolution gives us insight
into the dynamo process. In particular, it plays an essential role
in the Babcock-Leighton model of the solar dynamo. Describing this
evolution is the aim of the surface flux transport model. The model
starts from the emergence of magnetic bipoles. Thereafter, the model is
based on the induction equation and the fact that after emergence the
magnetic field is observed to evolve as if it were purely radial. The
induction equation then describes how the surface flows—differential
rotation, meridional circulation, granular, supergranular flows,
and active region inflows—determine the evolution of the field (now
taken to be purely radial). In this paper, we review the modeling of
the various processes that determine the evolution of the surface
field. We restrict our attention to their role in the surface flux
transport model. We also discuss the success of the model and some of
the results that have been obtained using this model.
Title: The Sun's Meridional Flow and Its Role in Magnetic Flux
Transport and the Sunspot Cycle
Authors: Hathaway, D. H.; Upton, L.
Bibcode: 2014AGUFMSH44A..02H
Altcode:
The Sun's meridional flow can be measured with a variety of
measurement techniques including, but not limited to: direct Doppler,
magnetic feature tracking, velocity feature tracking, time-distance
helioseismology, and ring-diagram analysis. Direct Doppler gives
information on the flow in the photosphere while the other measurement
techniques provide information about the flow at some depth or range
of depths in the Sun's convection zone. These various measurement
methods now provide a converging (but not yet fully converged)
picture of the meridional flow as a function of latitude, depth,
and time. This converging picture has a flow which is poleward from
the equator all the way to pole in the near surface layers, has an
equatorward return flow beginning at a depth of about 50 Mm, and has
another poleward branch deeper in the convection zone. The poleward
flow in the near surface layers varies systematically in strength
and latitudinal structure with the phase of the sunspot cycle and
from one cycle to the next. This near surface meridional flow is
observed to play a significant role in the poleward transport of the
magnetic flux that emerges at the surface in the form of bipolar active
regions. Variations in the strength and structure of the meridional
flow introduce variations in the strength of the Sun's polar fields,
which in turn introduce variations in the size of subsequent sunspot
cycles. The polar fields at the end of cycle 23 (2008-2009) were much
weaker than the polar fields at the end of the previous cycles. This
led to the production of the weakest sunspot cycle in 100 years -
cycle 24. Surprisingly, we find that the variations we observed in
the meridional flow during cycle 23 led to stronger polar fields than
would have been produced otherwise. This suggests that variations in
the meridional flow can be one mechanism for modulating the sizes of
sunspot cycles - helping to keep them from getting too big or too small.
Title: Revised Sunspot Numbers and the Effects on Understanding the
Sunspot Cycle
Authors: Hathaway, D. H.
Bibcode: 2014AGUFMSH13D4141H
Altcode:
While sunspot numbers provide only limited information about the
sunspot cycle, they provide that information for at least twice as many
sunspot cycles as any other direct solar observation. In particular,
sunspot numbers are available before, during, and immediately
after the Maunder Minimum (1645-1715). The instruments and methods
used to count sunspots have changed over the last 400+ years. This
leads to systematic changes in the sunspot number that can mask,
or artificially introduce, characteristics of the sunspot cycle. The
most widely used sunspot number is the International (Wolf/Zurich)
sunspot number which is now calculated at the Solar Influences Data
Center in Brussels, Belgium. These numbers extend back to 1749. The
Group sunspot number extends back to the first telescopic observations
of the Sun in 1610. There are well-known and significant differences
between these two numbers where they overlap. Recent work has helped us
to understand the sources of these differences and has led to proposed
revisions in the sunspot numbers. Independent studies now support many
of these revisions. These revised sunspot numbers suggest changes to
our understanding of the sunspot cycle itself and to our understanding
of its connection to climate change.
Title: Effects of Meridional Flow Variations on Solar Cycles 23 and 24
Authors: Upton, Lisa; Hathaway, David H.
Bibcode: 2014ApJ...792..142U
Altcode: 2014arXiv1408.0035U
The faster meridional flow that preceded the solar cycle 23/24 minimum
is thought to have led to weaker polar field strengths, producing the
extended solar minimum and the unusually weak cycle 24. To determine
the impact of meridional flow variations on the sunspot cycle, we have
simulated the Sun's surface magnetic field evolution with our newly
developed surface flux transport model. We investigate three different
cases: a constant average meridional flow, the observed time-varying
meridional flow, and a time-varying meridional flow in which the
observed variations from the average have been doubled. Comparison of
these simulations shows that the variations in the meridional flow over
cycle 23 have a significant impact (~20%) on the polar fields. However,
the variations produced polar fields that were stronger than they would
have been otherwise. We propose that the primary cause of the extended
cycle 23/24 minimum and weak cycle 24 was the weakness of cycle 23
itself—with fewer sunspots, there was insufficient flux to build a
big cycle. We also find that any polar counter-cells in the meridional
flow (equatorward flow at high latitudes) produce flux concentrations
at mid-to-high latitudes that are not consistent with observations.
Title: Observed properties of Giant Cells
Authors: Hathaway, David H.; Upton, Lisa; Colegrove, Owen
Bibcode: 2014AAS...22421809H
Altcode:
The existence of Giant Cells has been suggested by both theory
and observation for over 45 years. We have tracked the motions of
supergranules in SDO/HMI Doppler velocity data and find larger
(Giant Cell) flows that persist for months. The flows in these
cells are clockwise around centers of divergence in the north and
counter-clockwise in the south. Equatorward flows are correlated
with prograde flows - giving the transport of angular momentum toward
the equator that is needed to maintain the Sun’s rapid equatorial
rotation. The cells are most pronounced at mid- and high-latitudes where
they exhibit the rotation rates representative of those latitudes. These
are clearly large, long-lived, cellular features, with the dynamical
characteristics expected from the effects of the Sun’s rotation,
but the shapes of the cells are not well represented in numerical
models. While the Giant Cell flow velocities are small (<10 m/s),
their long lifetimes should nonetheless substantially impact the
transport of magnetic flux in the Sun’s near surface layers.
Title: Characterizing and Modeling Magnetic Flux Transport in the
Sun’s Photosphere and Determining Its Impact on the Sunspot Cycle
Authors: Upton, Lisa; Hathaway, David H.
Bibcode: 2014AAS...22410301U
Altcode:
Characterization and modeling magnetic flux transport within the surface
layers of the Sun are vital to explaining the 11 year sunspot cycle.I
have characterized the differential rotation (DR) and meridional
flow (MF) and their variations since 1996 using a cross-correlation
technique on magnetograms (maps of the magnetic field at the surface
of the Sun). The MF is faster at solar cycle minimum and slower at
maximum. Furthermore, the MF speeds that preceded the Solar Cycle 23/24
minimum were ~20% faster than the MF speeds that preceded the prior
minimum. This faster MF has been suggested to have caused weaker polar
field strengths and thus the subsequent extended solar minimum and an
unusually weak cycle 24. I have modeled surface magnetic flux transport
with a model that advects the magnetic flux emerging in sunspots using
the near-surface flows. These flows include the axisymmetric DR and MF
and the non-axisymmetric cellular convective flows (supergranules),
all of which vary in time as indicated by direct observations. At
each time step, magnetic maps of the entire Sun are created. I have
tested the predictability of this model using daily sunspot area data
as sources of new magnetic flux. I found that the evolution of the
polar fields can be reliably predicted many years in advance. The
model was then used to determine the impact of MF variations on the
sunspot cycle. One simulation included a MF that is constant, a second
included a MF that has the observed variations in time, and a third
included a MF in which the observed variations were exaggerated. The
simulations show that the variations in the MF over cycle 23 produce
polar fields that are ~20% stronger, rather than weaker. This suggests
that the cause of the weak polar fields at the end of Cycle 23 should
be attributed to the emergence of fewer active region sources, rather
that the variation in the meridional flow.
Title: The solar meridional circulation and sunspot cycle variability
Authors: Hathaway, D. H.; Upton, L.
Bibcode: 2014JGRA..119.3316H
Altcode: 2014arXiv1404.5893H
We have measured the meridional motions of the magnetic elements in
the Sun's surface layers since 1996 and find systematic and substantial
variations. In general the meridional flow speed is fast at cycle minima
and slow at cycle maxima. We find that these systematic variations are
characterized by a weakening of the meridional flow on the poleward
sides of the active (sunspot) latitudes. This can be interpreted as an
inflow toward the sunspot zones superimposed on a more general poleward
meridional flow profile. We also find variations in the meridional
flow which vary from cycle to cycle. The meridional flow was slower at
both the minimum and maximum of cycle 23 compared to similar phases
of cycles 21, 22, and 24. Models of the magnetic flux transport by a
variable meridional flow suggest that it can significantly modulate the
size and timing of the following sunspot cycle through its impact on
the Sun's polar magnetic fields. We suggest that the meridional flow
variations observed in cycle 23 contributed to the weak polar fields
at the end of the cycle which then produced a weak cycle 24 and the
extraordinary cycle 23/24 minimum.
Title: Predicting the Sun's Polar Magnetic Fields with a Surface
Flux Transport Model
Authors: Upton, Lisa; Hathaway, David H.
Bibcode: 2014ApJ...780....5U
Altcode: 2013arXiv1311.0844U; 2013arXiv1311.0844H
The Sun's polar magnetic fields are directly related to solar cycle
variability. The strength of the polar fields at the start (minimum) of
a cycle determine the subsequent amplitude of that cycle. In addition,
the polar field reversals at cycle maximum alter the propagation of
galactic cosmic rays throughout the heliosphere in fundamental ways. We
describe a surface magnetic flux transport model that advects the
magnetic flux emerging in active regions (sunspots) using detailed
observations of the near-surface flows that transport the magnetic
elements. These flows include the axisymmetric differential rotation
and meridional flow and the non-axisymmetric cellular convective flows
(supergranules), all of which vary in time in the model as indicated
by direct observations. We use this model with data assimilated from
full-disk magnetograms to produce full surface maps of the Sun's
magnetic field at 15 minute intervals from 1996 May to 2013 July
(all of sunspot cycle 23 and the rise to maximum of cycle 24). We
tested the predictability of this model using these maps as initial
conditions, but with daily sunspot area data used to give the sources
of new magnetic flux. We find that the strength of the polar fields
at cycle minimum and the polar field reversals at cycle maximum can
be reliably predicted up to 3 yr in advance. We include a prediction
for the cycle 24 polar field reversal.
Title: Giant Convection Cells Found on the Sun
Authors: Hathaway, David H.; Upton, Lisa; Colegrove, Owen
Bibcode: 2014arXiv1401.0551H
Altcode:
Heat is transported through the outermost 30% of the Sun's interior by
overturning convective motions. These motions are evident at the Sun's
surface in the form of two characteristic cellular structures - granules
and supergranules (~1000 and ~30,000 km across respectively). The
existence of much larger cells has been suggested by both theory
and observation for over 45 years. We found evidence for giant
cellular flows that persist for months by tracking the motions of
supergranules. As expected from the effects of the Sun's rotation, the
flows in these cells are clockwise around high pressure in the north,
counter-clockwise in the south and transport angular momentum toward
the equator, maintaining the Sun's rapid equatorial rotation.
Title: Giant Convection Cells Found on the Sun
Authors: Hathaway, David H.; Upton, Lisa; Colegrove, Owen
Bibcode: 2013Sci...342.1217H
Altcode:
Heat is transported through the outermost 30% of the Sun’s interior
by overturning convective motions. These motions are evident at the
Sun’s surface in the form of two characteristic cellular structures:
granules and supergranules (~1000 and ~30,000 kilometers across,
respectively). The existence of much larger cells has been suggested by
both theory and observation for more than 45 years. We found evidence
for giant cellular flows that persist for months by tracking the motions
of supergranules. As expected from the effects of the Sun’s rotation,
the flows in these cells are clockwise around high pressure in the
north and counterclockwise in the south and transport angular momentum
toward the equator, maintaining the Sun’s rapid equatorial rotation.
Title: A Curious History of Sunspot Penumbrae
Authors: Hathaway, D. H.
Bibcode: 2013SoPh..286..347H
Altcode: 2013arXiv1304.8060H
Daily records of sunspot group areas compiled by the Royal Observatory,
Greenwich, from May of 1874 through 1976 indicate a curious history for
the penumbral areas of the smaller sunspot groups. On average, the ratio
of penumbral area to umbral area in a sunspot group increases from 5
to 6 as the total sunspot group area increases from 100 to 2000 μHem
(a μHem is 10−6 the area of a solar hemisphere). This
relationship does not vary substantially with sunspot group latitude or
with the phase of the sunspot cycle. However, for the sunspot groups
with total areas < 100 μHem, this ratio changes dramatically and
systematically through this historical record. The ratio for these
smallest sunspots is near 5.5 from 1874 to 1900. After a rapid rise
to more than 7 in 1905, it drops smoothly to less than 3 by 1930 and
then rises smoothly back to more than 7 in 1961. It then returns
to near 5.5 from 1965 to 1976. The smooth variation from 1905 to
1961 shows no indication of any step-like changes that might be
attributed to changes in equipment or personnel. The overall level of
solar activity was increasing monotonically during this time period
when the penumbra-to-umbra area ratio dropped to less than half its
peak value and then returned. If this history can be confirmed by
other observations (e.g. Mt. Wilson or Kodaikanal), it may impact
our understanding of penumbra formation, our dynamo models, and our
estimates of historical changes in the solar irradiance.
Title: Meridional Flow Variations in Cycles 23 and 24: Active Latitude
Control of Sunspot Cycle Amplitudes
Authors: Hathaway, David H.; Upton, L.
Bibcode: 2013SPD....44..122H
Altcode:
We have measured the meridional motions of magnetic elements observed in
the photosphere over sunspot cycles 23 and 24 using magnetograms from
SOHO/MDI and SDO/HMI. Our measurements confirm the finding of Komm,
Howard, and Harvey (1993) that the poleward meridional flow weakens at
cycle maxima. Our high spatial and temporal resolution analyses show
that this variation is in the form of a superimposed inflow toward
the active latitudes. This inflow is weaker in cycle 24 when compared
to the inflow in 23, the stronger cycle. This systematic modulation of
the meridional flow should also modulate the amplitude of the following
sunspot cycle through its influence on the Sun’s polar fields. The
observational evidence and the theoretical consequences (similar to
those of Cameron and Schussler (2012)) will be described. Komm, Howard,
and Harvey (1993) Solar Phys. 147, 207. Cameron and Schussler (2012)
Astron. Astrophys. 548, A57.
Title: Defining the Polar Field Reversal
Authors: Upton, Lisa; Hathaway, D. H.
Bibcode: 2013SPD....4440301U
Altcode:
The polar fields on the Sun are directly related to solar cycle
variability. Recently there has been interest in studying an important
characteristic of the polar fields: the timing of the polar field
reversals. However this characteristic has been poorly defined,
mostly due to the limitations of early observations. In the past, the
reversals have been calculated by averaging the flux above some latitude
(i.e. 55° or 75°). Alternatively, the reversal could be defined by
the time in which the previous polarity is completely canceled and
replaced by the new polarity at 90°, precisely at the pole. We will
use a surface flux transport model to illustrate the differences
in the timing of the polar field reversal based on each of these
definitions and propose standardization in the definition of the polar
field reversal. The ability to predict the timing of the polar field
reversal using a surface flux transport model will also be discussed.
Title: Diary of a Wimpy Cycle
Authors: Hathaway, David H.; Upton, Lisa
Bibcode: 2013enss.confE.122H
Altcode:
The cause of the low and extended minimum in solar activity between
Sunspot Cycles 23 and 24 was the small size of Sunspot Cycle 24 itself -
small cycles start late and leave behind low minima. Cycle 24 is small
because the polar fields produced during Cycle 23 were substantially
weaker than those produced during the previous cycles and those (weak)
polar fields are the seeds for the activity of the following cycle. Here
we discuss the observed characteristics of Cycle 24 and contrast them
to the characteristics of previous cycles. We present observations
and Magnetic Flux Transport simulations with data assimilated from
SOHO/MDI and SDO/HMI that help to explain these differences and point
the way to predictions of future activity levels.
Title: Rescaling MDI Magnetic Data to Match HMI
Authors: Upton, Lisa A.; Hathaway, David H.
Bibcode: 2013enss.confE..29U
Altcode:
Comparison of Helioseismic and Magnetic Imager (HMI) and Michelson
Doppler Investigation (MDI) magnetograms reveals a systematic difference
in the field strengths as a function of center-to-limb distance and
magnetic field strength itself (Liu et al., 2012). While MDI data
exhibits an annual variation at polar latitudes, HMI does not. The
more capable HMI uses 6 samples across the spectral line, rather than
2 to obtain the magnetic field. Therefore, HMI data is expected to
more accurately represent the Sun and the MDI magnetic data should be
rescaled to match HMI. Here, the HMI magnetograms have been resampled at
the MDI resolution of 1024x1024. The magnetograms were then co-aligned
by cross-correlating blocks of pixels from each image to identify and
correct differences in orientation and magnification. The ratio of HMI
magnetic field to MDI magnetic field was obtained for each pixel. This
was repeated for 650 cotemporal HMI-MDI magnetogram. The ratios were
then averaged and plotted as a function of center-to-limb distance
and magnetic field strength. Here, we present a function f(|B|, cos
Title: Hemispheric Asymmetries of Solar Photospheric Magnetism:
Radiative, Particulate, and Heliospheric Impacts
Authors: McIntosh, Scott W.; Leamon, Robert J.; Gurman, Joseph B.;
Olive, Jean-Philippe; Cirtain, Jonathan W.; Hathaway, David H.;
Burkepile, Joan; Miesch, Mark; Markel, Robert S.; Sitongia, Leonard
Bibcode: 2013ApJ...765..146M
Altcode: 2013arXiv1302.1081M
Among many other measurable quantities, the summer of 2009 saw
a considerable low in the radiative output of the Sun that was
temporally coincident with the largest cosmic-ray flux ever measured
at 1 AU. Combining measurements and observations made by the Solar and
Heliospheric Observatory (SOHO) and Solar Dynamics Observatory (SDO)
spacecraft we begin to explore the complexities of the descending phase
of solar cycle 23, through the 2009 minimum into the ascending phase of
solar cycle 24. A hemispheric asymmetry in magnetic activity is clearly
observed and its evolution monitored and the resulting (prolonged)
magnetic imbalance must have had a considerable impact on the structure
and energetics of the heliosphere. While we cannot uniquely tie the
variance and scale of the surface magnetism to the dwindling radiative
and particulate output of the star, or the increased cosmic-ray flux
through the 2009 minimum, the timing of the decline and rapid recovery
in early 2010 would appear to inextricably link them. These observations
support a picture where the Sun's hemispheres are significantly out
of phase with each other. Studying historical sunspot records with
this picture in mind shows that the northern hemisphere has been
leading since the middle of the last century and that the hemispheric
"dominance" has changed twice in the past 130 years. The observations
presented give clear cause for concern, especially with respect to
our present understanding of the processes that produce the surface
magnetism in the (hidden) solar interior—hemispheric asymmetry is the
normal state—the strong symmetry shown in 1996 was abnormal. Further,
these observations show that the mechanism(s) which create and transport
the magnetic flux are slowly changing with time and, it appears, with
only loose coupling across the equator such that those asymmetries can
persist for a considerable time. As the current asymmetry persists and
the basal energetics of the system continue to dwindle we anticipate
new radiative and particulate lows coupled with increased cosmic-ray
fluxes heading into the next solar minimum.
Title: Erratum: "Behavior of Solar Cycles 23 and 24 Revealed by
Microwave Observations" (2012,
ApJ, 750, L42)
Authors: Gopalswamy, N.; Yashiro, S.; Mäkelä, P.; Michalek, G.;
Shibasaki, K.; Hathaway, D. H.
Bibcode: 2013ApJ...763L..24G
Altcode:
No abstract at ADS
Title: Measurements of the Sun's High-latitude Meridional Circulation
Authors: Rightmire-Upton, Lisa; Hathaway, David H.; Kosak, Katie
Bibcode: 2012ApJ...761L..14R
Altcode: 2012arXiv1211.0944R
The meridional circulation at high latitudes is crucial to the
buildup and reversal of the Sun's polar magnetic fields. Here, we
characterize the axisymmetric flows by applying a magnetic feature
cross-correlation procedure to high-resolution magnetograms obtained
by the Helioseismic and Magnetic Imager (HMI) on board the Solar
Dynamics Observatory. We focus on Carrington rotations 2096-2107
(2010 April to 2011 March)—the overlap interval between HMI and the
Michelson Doppler Imager (MDI). HMI magnetograms averaged over 720 s
are first mapped into heliographic coordinates. Strips from these maps
are then cross-correlated to determine the distances in latitude and
longitude that the magnetic element pattern has moved, thus providing
meridional flow and differential rotation velocities for each rotation
of the Sun. Flow velocities were averaged for the overlap interval and
compared to results obtained from MDI data. This comparison indicates
that these HMI images are rotated counterclockwise by 0fdg075 with
respect to the Sun's rotation axis. The profiles indicate that HMI data
can be used to reliably measure these axisymmetric flow velocities to at
least within 5° of the poles. Unlike the noisier MDI measurements, no
evidence of a meridional flow counter-cell is seen in either hemisphere
with the HMI measurements: poleward flow continues all the way to the
poles. Slight north-south asymmetries are observed in the meridional
flow. These asymmetries should contribute to the observed asymmetries
in the polar fields and the timing of their reversals.
Title: Reproducing the Photospheric Magnetic Field Evolution during
the Rise of Cycle 24 with Flux Transport by Supergranules
Authors: Hathaway, D. H.; Upton, L.
Bibcode: 2012AGUFMSH13C2267H
Altcode:
We simulate the transport of magnetic flux in the Sun's
photosphere by an evolving pattern of cellular horizontal flows
(supergranules). Characteristics of the simulated flow pattern can match
observed characteristics including the velocity power spectrum, cell
lifetimes, and cell motions in longitude and latitude. Simulations using
an average, and north-south symmetric, meridional motion of the cellular
pattern produce polar magnetic fields that are too weak in the North
and too strong in the South. Simulations using cellular patterns with
meridional motions that evolve with the observed changes in strength
and north-south asymmetry will be analyzed to see if they reproduce
the polar field evolution observed during the rise of Cycle 24.
Title: Supergranules as Probes of the Sun's Meridional Circulation
Authors: Hathaway, David H.
Bibcode: 2012ApJ...760...84H
Altcode: 2012arXiv1210.3343H
Recent analysis revealed that supergranules (convection cells seen at
the Sun's surface) are advected by the zonal flows at depths equal to
the widths of the cells themselves. Here we probe the structure of
the meridional circulation by cross-correlating maps of the Doppler
velocity signal using a series of successively longer time lags between
maps. We find that the poleward meridional flow decreases in amplitude
with time lag and reverses direction to become an equatorward return
flow at time lags >24 hr. These cross-correlation results are
dominated by larger and deeper cells at longer time lags. (The smaller
cells have shorter lifetimes and do not contribute to the correlated
signal at longer time lags.) We determine the characteristic cell size
associated with each time lag by comparing the equatorial zonal flows
measured at different time lags with the zonal flows associated with
different cell sizes from a Fourier analysis. This association gives a
characteristic cell size of ~50 Mm at a 24 hr time lag. This indicates
that the poleward meridional flow returns equatorward at depths >50
Mm—just below the base of the surface shear layer. A substantial and
highly significant equatorward flow (4.6 ± 0.4 m s-1) is
found at a time lag of 28 hr corresponding to a depth of ~70 Mm. This
represents one of the first positive detections of the Sun's meridional
return flow and illustrates the power of using supergranules to probe
the Sun's internal dynamics.
Title: Behavior of Solar Cycles 23 and 24 Revealed by Microwave
Observations
Authors: Gopalswamy, N.; Yashiro, S.; Mäkelä, P.; Michalek, G.;
Shibasaki, K.; Hathaway, D. H.
Bibcode: 2012ApJ...750L..42G
Altcode: 2012arXiv1204.2816G
Using magnetic and microwave butterfly diagrams, we compare the
behavior of solar polar regions to show that (1) the polar magnetic
field and the microwave brightness temperature during solar minimum
substantially diminished during the cycle 23/24 minimum compared to
the 22/23 minimum. (2) The polar microwave brightness temperature
(Tb) seems to be a good proxy for the underlying magnetic field
strength (B). The analysis indicates a relationship, B = 0.0067Tb -
70, where B is in G and Tb in K. (3) Both the brightness temperature
and the magnetic field strength show north-south asymmetry most of
the time except for a short period during the maximum phase. (4) The
rush-to-the-pole phenomenon observed in the prominence eruption (PE)
activity seems to be complete in the northern hemisphere as of 2012
March. (5) The decline of the microwave brightness temperature in the
north polar region to the quiet-Sun levels and the sustained PE activity
poleward of 60oN suggest that solar maximum conditions have
arrived at the northern hemisphere. The southern hemisphere continues
to exhibit conditions corresponding to the rise phase of solar cycle 24.
Title: Photospheric Magnetic Flux Transport - Supergranules Rule
Authors: Hathaway, David H.; Rightmire-Upton, L.
Bibcode: 2012AAS...22011006H
Altcode:
Observations of the transport of magnetic flux in the Sun’s
photosphere show that active region magnetic flux is carried far from
its origin by a combination of flows. These flows have previously been
identified and modeled as separate axisymmetric processes: differential
rotation, meridional flow, and supergranule diffusion. Experiments with
a surface convective flow model reveal that the true nature of this
transport is advection by the non-axisymmetric cellular flows themselves
- supergranules. Magnetic elements are transported to the boundaries
of the cells and then follow the evolving boundaries. The convective
flows in supergranules have peak velocities near 500 m/s. These flows
completely overpower the superimposed 20 m/s meridional flow and 100
m/s differential rotation. The magnetic elements remain pinned at the
supergranule boundaries. Experiments with and without the superimposed
axisymmetric photospheric flows show that the axisymmetric transport of
magnetic flux is controlled by the advection of the cellular pattern by
underlying flows representative of deeper layers. The magnetic elements
follow the differential rotation and meridional flow associated with
the convection cells themselves - supergranules rule!
Title: A Statistical Test of Uniformity in Solar Cycle Indices
Authors: Hathaway, David H.
Bibcode: 2012AAS...22020601H
Altcode:
Several indices are used to characterize the solar activity cycle. Key
among these are: the International Sunspot Number, the Group Sunspot
Number, Sunspot Area, and 10.7 cm Radio Flux. A valuable aspect of
these indices is the length of the record - many decades and many
(different) 11-year cycles. However, this valuable length-of-record
attribute has an inherent problem in that it requires many different
observers and observing systems. This can lead to non-uniformity
in the datasets and subsequent erroneous conclusions about solar
cycle behavior. The sunspot numbers are obtained by counting sunspot
groups and individual sunspots on a daily basis. This suggests that
the day-to-day and month-to-month variations in these numbers should
follow Poisson Statistics and be proportional to the square-root of the
sunspot numbers themselves. Examining the historical records of these
indices indicates that this is indeed the case - even with Sunspot
Area and 10.7 cm Radio Flux. The ratios of the RMS variations to the
square-root of the indices themselves are relatively constant with
little variation over the phase of each solar cycle or from small to
large solar cycles. There are, however, important step-like changes
in these ratios associated with changes in observer and/or observer
system. Here we show how these variations can be used to construct
more uniform datasets.
Title: A Standard Law for the Equatorward Drift of the Sunspot Zones
Authors: Hathaway, David H.
Bibcode: 2012AAS...22020602H
Altcode:
The latitudinal location of the sunspot zones in each hemisphere is
determined by calculating the centroid position of sunspot areas for
each solar rotation from May 1874 to June 2012. When these centroid
positions are plotted and analyzed as functions of time from each
sunspot cycle maximum there appears to be systematic differences in
the positions and equatorward drift rates as a function of sunspot
cycle amplitude. If, instead, these centroid positions are plotted
and analyzed as functions of time from each sunspot cycle minimum then
most of the differences in the positions and equatorward drift rates
disappear. The differences that remain disappear entirely if curve
fitting is used to determine the starting times (which vary by as much
as 8 months from the times of minima). The sunspot zone latitudes and
equatorward drift measured relative to this starting time follow a
standard path for all cycles with no dependence upon cycle strength or
hemispheric dominance. Although Cycle 23 was peculiar in its length
and the strength of the polar fields it produced, it too shows no
significant variation from this standard. This standard law, and the
lack of variation with sunspot cycle characteristics, is consistent
with Dynamo Wave mechanisms but not consistent with current Flux
Transport Dynamo models for the equatorward drift of the sunspot zones.
Title: Axisymmetric Flow Properties for Magnetic Elements of
Differing Strength
Authors: Rightmire-Upton, Lisa; Hathaway, D. H.
Bibcode: 2012AAS...22020613R
Altcode:
Aspects of the structure and dynamics of the flows in the Sun’s
surface shear layer remain uncertain and yet are critically important
for understanding the observed magnetic behavior. In our previous
studies of the axisymmetric transport of magnetic elements we
found systematic changes in both the differential rotation and the
meridional flow over the course of Solar Cycle 23. Here we examine how
those flows depend upon the strength (and presumably anchoring depth)
of the magnetic elements. Line of sight magnetograms obtained by the
HMI instrument aboard SDO over the course of Carrington Rotation 2097
were mapped to heliographic coordinates and averaged over 12 minutes
to remove the 5-min oscillations. Data masks were constructed based on
the field strength of each mapped pixel to isolate magnetic elements
of differing field strength. We used Local Correlation Tracking of
the unmasked data (separated in time by 1- to 8-hours) to determine
the longitudinal and latitudinal motions of the magnetic elements. We
then calculated average flow velocities as functions of latitude and
longitude from the central meridian for 600 image pairs over the 27-day
rotation. Variations with longitude indicate and characterize systematic
errors in the flow measurements associated with changes in the signal
from disk center to limb. Removing these systematic errors reveals
changes in the axisymmetric flow properties that reflect changes in
flow properties with depth in the surface shear layer.
Title: Supergranules as Probes of Solar Convection Zone Dynamics
Authors: Hathaway, David H.
Bibcode: 2012ApJ...749L..13H
Altcode: 2012arXiv1203.2593H
Supergranules are convection cells seen at the Sun's surface as a space
filling pattern of horizontal flows. While typical supergranules have
diameters of about 35 Mm, they exhibit a broad spectrum of sizes from
~10 Mm to ~100 Mm. Here we show that supergranules of different sizes
can be used to probe the rotation rate in the Sun's outer convection
zone. We find that the equatorial rotation rate as a function of depth
as measured by global helioseismology matches the equatorial rotation
as a function of wavelength for the supergranules. This suggests that
supergranules are advected by flows at depths equal to their wavelengths
and thus can be used to probe flows at those depths. The supergranule
rotation profiles show that the surface shear layer, through which the
rotation rate increases inward, extends to depths of ~50 Mm and to
latitudes of at least 70°. Typical supergranules are well observed
at high latitudes and have a range of sizes that extend to greater
depths than those typically available for measuring subsurface flows
with local helioseismology. These characteristics indicate that probing
the solar convection zone dynamics with supergranules can complement
the results of helioseismology.
Title: Flux Transport and the Sun's Polar Magnetic Fields at Cycle
23/24 Minimum
Authors: Upton, L. A.; Hathaway, D. H.
Bibcode: 2011AGUFMSH23D..02U
Altcode:
The polar magnetic fields observed on the Sun during the Cycle
23/24 minimum were substantially weaker than those observed for
the previous three minima. In most dynamo models weak polar fields
result in weak following cycles, which are observed to start late
and leave behind a long low minimum. While this accounts for the
peculiarities observed during the Cycle 23/24 minimum, it begs an
explanation for why the polar magnetic fields were so weak. Here we
model the transport of magnetic flux from active region sources via the
observed magnetic element differential rotation and meridional flow
from 1996 to 2011 (Hathaway & Rightmire ApJ 729:80, 2011). The
active region sources are characterized by the observed sizes and
locations of the leading and following polarity magnetic flux. Both
the sources of the flux and the flows that transport them are fully
constrained by observations. Comparisons will be made between the
observed and modeled polar field reversals and the final polar field
strengths. Conclusions will be drawn concerning the need for additional
flux transport processes.
Title: Obituary: Einar A. Tandberg-Hanssen (1921-2011)
Authors: Gary, G.; Emslie, A.; Hathaway, David; Moore, Ronald
Bibcode: 2011BAAS...43..032G
Altcode:
Dr. Einar Andreas Tandberg-Hanssen was born on 6 August 1921,
in Bergen, Norway, and died on July 22, 2011, in Huntsville, AL,
USA, due to complications from ALS (Amyotrophic lateral sclerosis,
often referred to as Lou Gehrig's disease). His parents were
administrator Birger Tandberg-Hanssen (1883-1951) and secretary Antonie
"Mona" Meier (1895-1967). He married Erna Rönning (27 October
1921 - 22 November 1994), a nurse, on 22 June 1951. She was the
daughter of Captain Einar Rönning (1890-1969) and Borghild Lyshaug
(1897-1980). Einar and Erna had two daughters, Else Biesman (and
husband Allen of Rapid City, SD, USA) and Karin Brock (and husband
Mike of Gulf Shores, AL, USA). At the time of his death Einar had eight
grandchildren and eight great-grandchildren. Dr. Tandberg-Hanssen
was an internationally-known member of the solar physics community,
with over a hundred published scientific papers and several books,
including Solar Activity (1967), Solar Prominences (1974), The
Physics of Solar Flares (1988) and The Nature of Solar Prominences
(1995). Einar grew up in Langesund and Skien, Norway, where he
took the qualifying exams at Skien High School in 1941. After the war
he studied natural sciences at the University of Oslo and received his
undergraduate degree in astronomy in 1950. He worked as a research
assistant in the Institute of Theoretical Astrophysics at the University
of Oslo for three intervals in the 1950s, interspersed by fellowships
at the Institut d'Astrophysique in Paris, Caltech in Pasadena, CA, the
High Altitude Observatory in Boulder, CO, and the Cavendish Laboratory
in the UK (at the invitation of British radio-astronomer Sir Martin
Ryle). He earned a doctorate in astrophysics at the University in
Oslo in 1960 with a dissertation titled "An Investigation of the
Temperature Conditions in Prominences with a Special Study of the
Excitation of Helium." From 1959-61, Tandberg-Hanssen was a
professor at the University in Oslo. He then traveled back to the High
Altitude Observatory in Boulder, Colorado, where he was employed until
1974. He was then employed at the Space Science Laboratory at NASA's
Marshall Space Flight Center (MSFC) in Huntsville, Alabama. There,
he was a Senior Research Scientist and later Deputy Director of the
Laboratory. He served as Lab Director from 1987 until his retirement
from NASA in 1993. He promptly took a part-time post within the Center
for Space Plasma and Aeronomic Research at The University of Alabama
in Huntsville, where he worked until his death. During his tenure
at NASA, he, along with Dr. Mona Hagyard and Dr. S. T. Wu, built up
a substantial, internationally-based group of solar physicists at
MSFC and UA Huntsville. He was a lead investigator on two instruments
aboard NASA spacecraft: the S-056 X-Ray Event Analyzer on the Skylab
Apollo Telescope Mount (which provided pioneering, high-time-cadence
temperature and density information on solar X-ray-emitting regions)
and the Ultraviolet Spectrometer and Polarimeter on the Solar Maximum
Mission (which carried out sweeping new studies of EUV emission from
solar active regions and flares). Dr. Tandberg-Hanssen's books about
various aspects of solar activity, viz.Solar Activity (Blaisdell, 1967),
Solar Prominences (Reidel, 1974), The Physics of Solar Flares (with
A. G. Emslie) (Cambridge, 1988), and The Nature of Solar Prominences
(Reidel, 1995), have become international standard works within the
discipline of solar physics. In 1982, Dr. Tandberg-Hanssen
was elected to membership in the Norwegian Academy of Science
and Letters. From 1979-82 and 1982-85, respectively, he served as
vice-president and president of Commission 10 of the International
Astronomical Union (IAU). He served as president of the Federation of
Astronomical and Geophysical Data Analysis Services from 1990-1994. He
has received the NASA Exceptional Service Medal. He was also a long
time editor of the journal Solar Physics. Dr. Tandberg-Hanssen's
Solar Physics Memoir paper, entitled Solar Prominences - An Intriguing
Phenomenon http://www.springerlink.com/content/1166j74k577kv332/
was published shortly before his death. The article starts with an
autobiographical account, where the author relates how his several
study-trips abroad gradually led him to the study of solar physics
in general, and prominences particularly. Einar's residence
as a research fellow at the Institut d'Astrophysique in Paris in
the 1950s laid the foundation for a lifelong interest in France and
French culture. His great interest in and knowledge of French mediaeval
churches, as well as the Norwegian stave churches, is reflected in two
books, Letters to My Daughters (Ivy House Pub. Group, 2004), and The Joy
of Travel: More Letters to My Daughters (Pentland Press, 2007), which
serve as a review, tourist guide and history book, shaped in the form of
letters home to his two daughters, from his many travels in Norway and
France. Einar was a true gentleman and a true scholar. As evidenced
by his papers, his books, and his dealings with others, he was always
seeking not only to expand his own knowledge and understanding, but also
to find new ways of communicating his remarkable insight to others. He
is survived by his daughters, Else and Karin, and their families.
Title: What Supergranule Flow Models tell us about the Sun's Surface
Shear Layer and Magnetic Flux Transport
Authors: Hathaway, D. H.
Bibcode: 2011AGUFMSH53C..01H
Altcode:
Models of the photospheric flows due to supergranulation are generated
using an evolving spectrum of vector spherical harmonics up to spherical
harmonic wavenumber l~1500. Doppler velocity data generated from these
models are compared to direct Doppler observations from SOHO/MDI and
SDO/HMI. The models are adjusted to match the observed spatial power
spectrum as well as the wavenumber dependence of the cell lifetimes,
differential rotation velocities, meridional flow velocities, and
relative strength of radial vs. horizontal flows. The equatorial
rotation rate as a function of wavelength matches the rotation rate
as a function of depth as determined by global helioseismology. This
leads to the conclusions that the cellular structures are anchored at
depths equal to their widths, that the surface shear layer extends
to at least 70 degrees latitude, and that the poleward meridional
flow decreases in amplitude and reverses direction at the base of
the surface shear layer (~35 Mm below the surface). Using the modeled
flows to passively transport magnetic flux indicates that the observed
differential rotation and meridional flow of the magnetic elements are
directly related to the differential rotation and meridional flow of the
convective pattern itself. The magnetic elements are transported by the
evolving boundaries of the supergranule pattern (where the convective
flows converge) and are unaffected by the weaker flows associated with
the differential rotation or meridional flow of the photospheric plasma.
Title: A Standard Law for the Equatorward Drift of the Sunspot Zones
Authors: Hathaway, D. H.
Bibcode: 2011SoPh..273..221H
Altcode: 2011SoPh..tmp..341H; 2011arXiv1108.1722H
The latitudinal location of the sunspot zones in each hemisphere is
determined by calculating the centroid position of sunspot areas for
each solar rotation from May 1874 to June 2011. When these centroid
positions are plotted and analyzed as functions of time from each
sunspot cycle maximum, there appear to be systematic differences in
the positions and equatorward drift rates as a function of sunspot
cycle amplitude. If, instead, these centroid positions are plotted
and analyzed as functions of time from each sunspot cycle minimum,
then most of the differences in the positions and equatorward drift
rates disappear. The differences that remain disappear entirely if curve
fitting is used to determine the starting times (which vary by as much
as eight months from the times of minima). The sunspot zone latitudes
and equatorward drift measured relative to this starting time follow a
standard path for all cycles with no dependence upon cycle strength or
hemispheric dominance. Although Cycle 23 was peculiar in its length
and the strength of the polar fields it produced, it too shows no
significant variation from this standard. This standard law, and the
lack of variation with sunspot cycle characteristics, is consistent
with dynamo wave mechanisms but not consistent with current flux
transport dynamo models for the equatorward drift of the sunspot zones.
Title: The Sun's Meridional Circulation - not so Deep
Authors: Hathaway, David H.
Bibcode: 2011SPD....42.0203H
Altcode: 2011BAAS..43S.0203H
The Sun's global meridional circulation is evident as a slow poleward
flow at its surface. This flow is observed to carry magnetic elements
poleward - producing the Sun's polar magnetic fields as a key part
of the 11-year sunspot cycle. Flux Transport Dynamo models for the
sunspot cycle are predicated on the belief that this surface flow is
part of a circulation which sinks inward at the poles and returns to the
equator in the bottom half of the convection zone - at depths between
100 and 200 Mm. Here I use the advection of the supergranule cells by
the meridional flow to map the flow velocity in latitude and depth. My
measurements show that the equatorward return flow begins at a depth
of only 35 Mm - the base of the Sun's surface shear layer. This is the
first clear (10 sigma) detection of the meridional return flow. While
the shallow depth of the return flow indicates a false foundation
for Flux Transport Dynamo models it helps to explain the different
meridional flow rates seen for different features and provides a
mechanism for selecting the characteristic size of supergranules.
Title: The Sun's Shallow Meridional Circulation
Authors: Hathaway, David H.
Bibcode: 2011arXiv1103.1561H
Altcode:
The Sun's global meridional circulation is evident as a slow poleward
flow at its surface. This flow is observed to carry magnetic elements
poleward - producing the Sun's polar magnetic fields as a key part
of the 11-year sunspot cycle. Current theories for the sunspot cycle
assume that this surface flow is part of a circulation which sinks
inward at the poles and turns equatorward at depths below 100 Mm. Here
we use the advection of the Sun's convection cells by the meridional
flow to map the flow velocity in latitude and depth. Our measurements
show the largest cells clearly moving equatorward at depths below
35 Mm - the base of the Sun's surface shear layer. This surprisingly
shallow return flow indicates the need for substantial revisions to
solar/stellar dynamo theory.
Title: Variations in the Axisymmetric Transport of Magnetic Elements
on the Sun: 1996-2010
Authors: Hathaway, David H.; Rightmire, Lisa
Bibcode: 2011ApJ...729...80H
Altcode: 2010arXiv1010.1242H
We measure the axisymmetric transport of magnetic flux on the Sun by
cross-correlating narrow strips of data from line-of-sight magnetograms
obtained at a 96 minute cadence by the MDI instrument on the ESA/NASA
SOHO spacecraft and then averaging the flow measurements over each
synodic rotation of the Sun. Our measurements indicate that the
axisymmetric flows vary systematically over the solar cycle. The
differential rotation is weaker at maximum than at minimum. The
meridional flow is faster at minimum and slower at maximum. The
meridional flow speed on the approach to the Cycle 23/24 minimum was
substantially faster than it was at the Cycle 22/23 minimum. The
average latitudinal profile is largely a simple sinusoid that
extends to the poles and peaks at about 35° latitude. As the cycle
progresses, a pattern of inflows toward the sunspot zones develops
and moves equatorward in step with the sunspot zones. These inflows
are accompanied by the torsional oscillations. This association
is consistent with the effects of the Coriolis force acting on the
inflows. The equatorward motions associated with these inflows are
identified as the source of the decrease in net poleward flow at cycle
maxima. We also find polar countercells (equatorward flow at high
latitudes) in the south from 1996 to 2000 and in the north from 2002
to 2010. We show that these measurements of the flows are not affected
by the nonaxisymmetric diffusive motions produced by supergranulation.
Title: The Advection of Supergranules by the Sun's Axisymmetric Flows
Authors: Hathaway, David H.; Williams, Peter E.; Dela Rosa, Kevin;
Cuntz, Manfred
Bibcode: 2010ApJ...725.1082H
Altcode: 2010arXiv1008.4385H
We show that the motions of supergranules are consistent with a model
in which they are simply advected by the axisymmetric flows in the
Sun's surface shear layer. We produce a 10 day series of simulated
Doppler images at a 15 minute cadence that reproduces most spatial
and temporal characteristics seen in the SOHO/MDI Doppler data. Our
simulated data have a spectrum of cellular flows with just two
components—a granule component that peaks at spherical wavenumbers
of about 4000 and a supergranule component that peaks at wavenumbers
of about 110. We include the advection of these cellular components
by the axisymmetric flows—differential rotation and meridional
flow—whose variations with latitude and depth (wavenumber) are
consistent with observations. We mimic the evolution of the cellular
pattern by introducing random variations to the phases of the spectral
components at rates that reproduce the levels of cross-correlation as
functions of time and latitude. Our simulated data do not include any
wave-like characteristics for the supergranules yet can reproduce the
rotation characteristics previously attributed to wave-like behavior. We
find rotation rates which appear faster than the actual rotation rates
and attribute this to projection effects. We find that the measured
meridional flow does accurately represent the actual flow and that
the observations indicate poleward flow to 65°-70° latitude with
equatorward countercells in the polar regions.
Title: The Solar Cycle
Authors: Hathaway, David H.
Bibcode: 2010LRSP....7....1H
Altcode:
The Solar Cycle is reviewed. The 11-year cycle of solar activity is
characterized by the rise and fall in the numbers and surface area
of sunspots. We examine a number of other solar activity indicators
including the 10.7 cm radio flux, the total solar irradiance,
the magnetic field, flares and coronal mass ejections, geomagnetic
activity, galactic cosmic ray fluxes, and radioisotopes in tree rings
and ice cores that vary in association with the sunspots. We examine
the characteristics of individual solar cycles including their maxima
and minima, cycle periods and amplitudes, cycle shape, and the nature
of active latitudes, hemispheres, and longitudes. We examine long-term
variability including the Maunder Minimum, the Gleissberg Cycle, and
the Gnevyshev-Ohl Rule. Short-term variability includes the 154-day
periodicity, quasi-biennial variations, and double peaked maxima. We
conclude with an examination of prediction techniques for the solar
cycle.
Title: Flux Transport and the Sun's Global Magnetic Field (Invited)
Authors: Hathaway, D. H.
Bibcode: 2010AGUFMSH41D..01H
Altcode:
The Sun’s global magnetic field is produced and evolved through
the emergence of magnetic flux in active regions and its transport
across the solar surface by the axisymmetric differential rotation
and meridional flow and the non-axisymmetric convective flows of
granulation, supergranulation, and giant cell convection. Maps of
the global magnetic field serve as the inner boundary condition for
space weather. The photospheric magnetic field and its evolution
determine the coronal and solar wind structures through which CMEs
must propagate and in which solar energetic particles are accelerated
and propagate. Producing magnetic maps which best represent the actual
field configuration at any instant requires knowing the magnetic field
over the observed hemisphere as well as knowing the flows that transport
flux. From our Earth-based vantage point we only observe the front-side
hemisphere and each pole is observable for only six months of the year
at best. Models for the surface magnetic flux transport can be used
to provide updates to the magnetic field configuration in those unseen
regions. In this presentation I will describe successes and failures of
surface flux transport and present new observations on the structure,
the solar cycle variability, and the evolution of the flows involved in
magnetic flux transport. I find that supergranules play the dominant
role due to their strong flow velocities and long lifetimes. Flux is
transported by differential rotation and meridional flow only to the
extent that the supergranules participate in those two flows.
Title: Does the Current Minimum Validate (or Invalidate) Cycle
Prediction Methods?
Authors: Hathaway, D. H.
Bibcode: 2010ASPC..428..307H
Altcode: 2010arXiv1003.4208H
This deep, extended solar minimum and the slow start to Cycle 24
strongly suggest that Cycle 24 will be a small cycle. A wide array of
solar cycle prediction techniques have been applied to predicting the
amplitude of Cycle 24 with widely different results. Current conditions
and new observations indicate that some highly regarded techniques
now appear to have doubtful utility. Geomagnetic precursors have been
reliable in the past and can be tested with 12 cycles of data. Of the
three primary geomagnetic precursors only one (the minimum level of
geomagnetic activity) suggests a small cycle. The Sun's polar field
strength has also been used to successfully predict the last three
cycles. The current weak polar fields are indicative of a small
cycle. For the first time, dynamo models have been used to predict
the size of a solar cycle but with opposite predictions depending on
the model and the data assimilation. However, new measurements of the
surface meridional flow indicate that the flow was substantially faster
on the approach to Cycle 24 minimum than at Cycle 23 minimum. In both
dynamo predictions a faster meridional flow should have given a shorter
cycle 23 with stronger polar fields. This suggests that these dynamo
models are not yet ready for solar cycle prediction.
Title: Changes in the Strength and Structure of the Sun's Meridional
Flow during Cycle 23
Authors: Hathaway, David H.; Rightmire, L.
Bibcode: 2010AAS...21631902H
Altcode: 2010BAAS...41..909H
Our observations of the transport of magnetic elements across the
Sun's surface indicate that the speed of the meridional flow varies
systematically over the solar cycle - faster at minimum and slower at
maximum. The flow speed on the approach to this (Cycle 24) minimum was
substantially faster than it was at the last minimum. This increased
flow speed should have produced a short Cycle 23 with strong polar
fields in the flux transport dynamos used to predict Cycle 24 - contrary
to what we have seen. The latitudinal structure of the meridional
flow also varies systematically. The average latitudinal structure
is a simple sinusoid that extends to the poles and peaks at about 45
degrees latitude. As the cycle progresses a pattern of inflows toward
the active latitudes develops and moves equatorward in conjunction
with the activity. In addition, counter-cells occasionally form at
high latitudes. These meridional flow variations are accompanied
by changes in the differential rotation. The in-flows toward the
active latitudes are accompanied by the torsional oscillations -
slower rotation on the poleward sides and faster rotation on the
equatorward sides of the active latitudes. The counter-cells at the
poles are accompanied by slower rotation in the polar regions. These
associations are consistent with the effects of the Coriolis force
on the meridional flow. We gratefully acknowledge our funding
sources. David Hathaway was supported by a grant from NASA through
the Heliophysics Causes and Consequences of the Minimum of Cycle 23/24
Program. Lisa Rightmire was supported as a Summer Intern at Marshall
Space Flight Center by a grant from the Space Grant Consortium.
Title: Variations in the Sun’s Meridional Flow over a Solar Cycle
Authors: Hathaway, David H.; Rightmire, Lisa
Bibcode: 2010Sci...327.1350H
Altcode:
The Sun’s meridional flow is an axisymmetric flow that is generally
directed from its equator toward its poles at the surface. The structure
and strength of the meridional flow determine both the strength of the
Sun’s polar magnetic field and the intensity of sunspot cycles. We
determine the meridional flow speed of magnetic features on the Sun
using data from the Solar and Heliospheric Observatory. The average
flow is poleward at all latitudes up to 75°, which suggests that it
extends to the poles. It was faster at sunspot cycle minimum than
at maximum and substantially faster on the approach to the current
minimum than it was at the last solar minimum. This result may help
to explain why this solar activity minimum is so peculiar.
Title: The Advection of Supergranules by Large-Scale Flows
Authors: Hathaway, D. H.; Williams, P. E.; Cuntz, M.
Bibcode: 2009ASPC..416..495H
Altcode: 2010arXiv1003.4210H
We produce a 10-day series of simulated Doppler images at a 15-minute
cadence that reproduces the spatial and temporal characteristics seen
in the SoHO/MDI Doppler data. Our simulated data contains a spectrum
of cellular flows with but two necessary components—a granule
component that peaks at wavenumbers of about 4000 and a supergranule
component that peaks at wavenumbers of about 110. We include the
advection of these cellular components by a differential rotation
profile that depends on latitude and wavenumber (depth). We further
mimic the evolution of the cellular pattern by introducing random
variations to the amplitudes and phases of the spectral components at
rates that reproduce the level of cross-correlation as a function of
time and latitude. Our simulated data do not include any wave-like
characteristics for the supergranules yet can accurately reproduce
the rotation characteristics previously attributed to wave-like
characteristics.
Title: Meridional Flow Variations Over Three Solar Cycles - What
happened in Cycle 23?
Authors: Hathaway, David H.
Bibcode: 2009SPD....40.0918H
Altcode:
The meridional flow speed determines the strength of the Sun's polar
fields in both surface flux transport models and in flux transport
dynamos. The polar fields produced during cycle 23 were half as
strong as those produced in the previous two cycles. Helioseismic
measurements of the meridional flow over the rising phase of cycle 23
indicated a decrease in flow velocity. This observation was used in
flux transport dynamo models to predict a delayed start for cycle 24
and was consistent with weak polar fields and a slower equatorward
drift of the active latitudes during cycle 23. On the other hand,
the surface flux transport models require a faster meridional flow to
produce the weak polar fields. We have begun measurements of the surface
meridional flow by tracking the motions of weak (outside active regions)
magnetic field elements in magnetograms from SOHO/MDI over cycle 23 and
from NSO/Kitt Peak over cycles 21 to 23. We confirm the slowdown of the
meridional flow over the rising phase of cycle 23 but find that the
flow speed returned to its previous level during the declining phase
of cycle 23. Furthermore, this appears to be a normal feature of the
meridional flow during sunspot cycles. The flow is fast at minima and
slow at maxima. The lack of a significantly different meridional flow
during cycle 23 is very problematic for both surface flux transport
models and flux transport dynamos.
Title: Sunspots, Space Weather and Climate
Authors: Hathaway, David H.
Bibcode: 2009SPD....40.2701H
Altcode:
Four hundred years ago this year the telescope was first used for
astronomical observations. Within a year, Galileo in Italy and Harriot
in England reported seeing spots on the surface of the Sun. Yet, it
took over 230 years of observations before a Swiss amateur astronomer
noticed that the sunspots increased and decreased in number over a
period of about 11 years. Within 15 years of this discovery of the
sunspot cycle astronomers made the first observations of a flare
on the surface of the Sun. In the 150 years since that discovery we
have learned much about sunspots, the sunspot cycle, and the Sun's
explosive events - solar flares, prominence eruptions and coronal mass
ejections that usually accompany the sunspots. These events produce
what is called Space Weather. The conditions in space are dramatically
affected by these events. Space Weather can damage our satellites,
harm our astronauts, and affect our lives here on the surface of
planet Earth. Long term changes in the sunspot cycle have been linked
to changes in our climate as well. In this public lecture I will
give an introduction to sunspots, the sunspot cycle, space weather,
and the possible impact of solar variability on our climate.
Title: Solar Cycle Forecasting
Authors: Hathaway, David H.
Bibcode: 2009SSRv..144..401H
Altcode: 2008SSRv..tmp..150H
Predicting the behavior of a solar cycle after it is well underway
(2-3 years after minimum) can be done with a fair degree of skill using
auto-regression and curve fitting techniques that don’t require any
knowledge of the physics involved. Predicting the amplitude of a solar
cycle near, or before, the time of solar cycle minimum can be done
using precursors such as geomagnetic activity and polar fields that do
have some connection to the physics but the connections are uncertain
and the precursors provide less reliable forecasts. Predictions for the
amplitude of cycle 24 using these precursor techniques give drastically
different values. Recently, dynamo models have been used directly
with assimilated data to predict the amplitude of sunspot cycle 24 but
have also given significantly different predictions. While others have
questioned both the predictability of the solar cycle and the ability
of current dynamo models to provide predictions, it is clear that
cycle 24 will help to discriminate between some opposing dynamo models.
Title: Solar Cycle Forecasting
Authors: Hathaway, David H.
Bibcode: 2009odsm.book..401H
Altcode:
Predicting the behavior of a solar cycle after it is well underway
(2-3 years after minimum) can be done with a fair degree of skill
using auto-regression and curve fitting techniques that don't require
any knowledge of the physics involved. Predicting the amplitude of a
solar cycle near, or before, the time of solar cycle minimum can be done
using precursors such as geomagnetic activity and polar fields that do
have some connection to the physics but the connections are uncertain
and the precursors provide less reliable forecasts. Predictions for the
amplitude of cycle 24 using these precursor techniques give drastically
different values. Recently, dynamo models have been used directly
with assimilated data to predict the amplitude of sunspot cycle 24 but
have also given significantly different predictions. While others have
questioned both the predictability of the solar cycle and the ability
of current dynamo models to provide predictions, it is clear that
cycle 24 will help to discriminate between some opposing dynamo models.
Title: Sunspot Group Decay
Authors: Hathaway, David H.; Choudhary, Debi Prasad
Bibcode: 2008SoPh..250..269H
Altcode: 2008SoPh..tmp..126H
We examine daily records of sunspot group areas (measured in millionths
of a solar hemisphere or μHem) for the last 130 years to determine
the rate of decay of sunspot group areas. We exclude observations
of groups when they are more than 60° in longitude from the central
meridian and only include data when at least three days of observations
are available following the date of maximum area for a group's disk
passage. This leaves data for over 18 000 measurements of sunspot group
decay. We find that the decay rate increases linearly from 28 μHem
day−1 to about 140 μHem day−1 for groups with
areas increasing from 35 μHem to 1000 μHem. The decay rate tends to
level off for groups with areas larger than 1000 μHem. This behavior is
very similar to the increase in the number of sunspots per group as the
area of the group increases. Calculating the decay rate per individual
sunspot gives a decay rate of about 3.65 μHem day−1
with little dependence upon the area of the group. This suggests
that sunspots decay by a Fickian diffusion process with a diffusion
coefficient of about 10 km2 s−1. Although the
18 000 decay rate measurements are lognormally distributed, this can
be attributed to the lognormal distribution of sunspot group areas
and the linear relationship between area and decay rate for the vast
majority of groups. We find weak evidence for variations in decay
rates from one solar cycle to another and for different phases of each
sunspot cycle. However, the strongest evidence for variations is with
latitude and the variations with cycle and phase of each cycle can be
attributed to this variation. High latitude spots tend to decay faster
than low latitude spots.
Title: On the Relationship between Solar Wind Speed,
Earthward-Directed Coronal Mass Ejections, Geomagnetic Activity,
and the Sunspot Cycle Using 12-Month Moving Averages
Authors: Wilson, Robert M.; Hathaway, David H.
Bibcode: 2008STIN...0830106W
Altcode:
For 1996 .2006 (cycle 23), 12-month moving averages of the aa
geomagnetic index strongly correlate (r = 0.92) with 12-month moving
averages of solar wind speed, and 12-month moving averages of the
number of coronal mass ejections (CMEs) (halo and partial halo events)
strongly correlate (r = 0.87) with 12-month moving averages of sunspot
number. In particular, the minimum (15.8, September/October 1997) and
maximum (38.0, August 2003) values of the aa geomagnetic index occur
simultaneously with the minimum (376 km/s) and maximum (547 km/s)
solar wind speeds, both being strongly correlated with the following
recurrent component (due to high-speed streams). The large peak of aa
geomagnetic activity in cycle 23, the largest on record, spans the
interval late 2002 to mid 2004 and is associated with a decreased
number of halo and partial halo CMEs, whereas the smaller secondary
peak of early 2005 seems to be associated with a slight rebound in
the number of halo and partial halo CMEs. Based on the observed aaM
during the declining portion of cycle 23, RM for cycle 24 is predicted
to be larger than average, being about 168+/-60 (the 90% prediction
interval), whereas based on the expected aam for cycle 24 (greater
than or equal to 14.6), RM for cycle 24 should measure greater than
or equal to 118+/-30, yielding an overlap of about 128+/-20.
Title: On the Relationship Between Solar Wind Speed, Geomagnetic
Activity, and the Solar Cycle Using Annual Values
Authors: Wilson, Robert M.; Hathaway, David H.
Bibcode: 2008STIN...0822945W
Altcode:
The aa index can be decomposed into two separate components: the
leading sporadic component due to solar activity as measured by sunspot
number and the residual or recurrent component due to interplanetary
disturbances, such as coronal holes. For the interval 1964-2006, a
highly statistically important correlation (r = 0.749) is found between
annual averages of the aa index and the solar wind speed (especially
between the residual component of aa and the solar wind speed, r =
0.865). Because cyclic averages of aa (and the residual component)
have trended upward during cycles 11-23, cyclic averages of solar wind
speed are inferred to have also trended upward.
Title: Solar cycle 23
Authors: Hathaway, David H.; Suess, Steven T.
Bibcode: 2008hsac.book...21H
Altcode:
Ulysses' launch in October of 1990 was at the maximum of solar
activity cycle 22. The first passages through the polar regions of the
heliosphere came in 1994 and 1995, very near the minimum of activity
between cycles 22 and 23. The second orbit then took Ulysses through
the polar regions in 2000 and 2001, at the maximum of solar activity
for cycle 23, and its third orbit will again sample the polar regions
at near-minimum conditions (Figure 7.1). Ulysses has thus observed
heliospheric conditions through a complete solar cycle, solar cycle
23. How typical was cycle 23? In this chapter we will examine the
characteristics of this cycle, its noteworthy events, and compare it
with other cycles.
Title: Anticipating Cycle 24 Minimum and Its Consequences
Authors: Wilson, Robert M.; Hathaway, David H.
Bibcode: 2007STIN...0806637W
Altcode:
On the basis of the 12-mo moving average of monthly mean sunspot number
(R) through November 2006, cycle 23 has persisted for 126 mo, having
had a minimum of 8.0 in May 1996, a peak of 120.8 in April 2000, and
an ascent duration of 47 mo. In November 2006, the 12-mo moving average
of monthly mean sunspot number was 12.7, a value just outside the upper
observed envelope of sunspot minimum values for the most recent cycles
16-23 (range 3.4-12.3), but within the 90-percent prediction interval
(7.8 +/- 6.7). The first spotless day during the decline of cycle 23
occurred in January 2004, and the first occurrence of 10 or more and 20
or more spotless days was February 2006 and April 2007, respectively,
inferring that sunspot minimum for cycle 24 is imminent. Through May
2007, 121 spotless days have accumulated. In terms of the weighted
mean latitude (weighed by spot area) (LAT) and the highest observed
latitude spot (HLS) in November 2006, 12-mo moving averages of these
parameters measured 7.9 and 14.6 deg, respectively, these values being
the lowest values yet observed during the decline of cycle 23 and
being below corresponding mean values found for cycles 16-23. As yet,
no high-latitude new-cycle spots have been seen nor has there been
an upturn in LAT and HLS, these conditions having always preceded
new cycle minimum by several months for past cycles. Together, these
findings suggest that cycle 24 s minimum amplitude still lies well
beyond November 2006. This implies that cycle 23 s period either will
lie in the period "gap" (127-134 mo), a first for a sunspot cycle,
or it will be longer than 134 mo, thus making cycle 23 a long-period
cycle (like cycle 20) and indicating that cycle 24 s minimum will occur
after July 2007. Should cycle 23 prove to be a cycle of longer period,
a consequence might be that the maximum amplitude for cycle 24 may be
smaller than previously predicted.
Title: Solar Rossby Wave ``Hills'' Identified as Supergranules
Authors: Williams, P. E.; Hathaway, D. H.; Cuntz, M.
Bibcode: 2007ApJ...662L.135W
Altcode:
We explore the nature of ``hills'' observed on the solar surface
that had previously been attributed to Rossby waves. We investigate
the solar hills phenomenon by analyzing the output from a synthetic
model based solely on the observed solar photospheric convection
spectrum. We show that the characteristics of these hills can be
explained by the corrugation of the surface produced by the radial
flows of the convection. The hills in our simulations are dominated
by supergranules, a well-known component of solar convection. Rossby
waves have been predicted to exist within the Sun and may play an
important role in the dynamics of the solar interior, including the
Sun's differential rotation and magnetic dynamo. Our study suggests,
however, that the hills observed at the solar limb do not confirm the
existence of solar Rossby waves.
Title: The Solar Cycle
Authors: Hathaway, David H.
Bibcode: 2007AAS...210.9901H
Altcode: 2007BAAS...39R.227H
Sunspots provided the first evidence for the 11-year cycle of solar
activity and continue to provide key indicators of the level and nature
of solar activity. Solar flares, prominence eruptions, and coronal mass
ejections increase in frequency as the number of sunspots increases
during the rising phase of the solar cycle. The total irradiance
of the Sun and its irradiance in ultraviolet light and x-rays also
increase as the sunspot number increases. On the other hand, the flux
of galactic cosmic rays reaching Earth decreases as the sunspot number
increases. These changes in the heliospheric environment produce
significant effects on our environment. Our technological assets,
in space, in the air, and on the ground, can be adversely affected
by solar activity. Satellite drag, single-event upsets in electronic
components, radio communication outages, power outages, and terrestrial
climate can all be influenced by solar activity. In this Parker Lecture
I will describe many of the significant characteristics of the solar
cycle, their roots in solar magnetism, the mechanisms of the Sun’s
magnetic dynamo, and predictions for the amplitude and timing of next
solar cycle.
Title: Curious Behavior of Sunspot Umbrae in the First Half of the
20th Century
Authors: Hathaway, David H.; Wilson, R. M.; Campbell, A.
Bibcode: 2007AAS...210.9203H
Altcode: 2007BAAS...39..209H
We examined the behavior of the areas of sunspot umbrae and penumbrae
as reported daily by the Royal Observatory, Greenwich (RGO) from May
1874 to December 1976. We calculated the ratio of the umbral area
to the penumbral area (corrected for foreshortening as observed on
the solar disc) for each sunspot group and for each day. We found: 1)
that this ratio is about 0.2 on average, 2) that larger sunspot groups
have slightly smaller ratios, 3) that there is a weak dependence
on the phase of the solar cycle, 4) that there is no dependence on
the latitude of the sunspot groups, and curiously 5) that for the
smaller sunspot groups this ratio increased dramatically from about
1910 to 1930 and then returned to “normal” from 1930 to 1950. We
examined other sunspot records to determine whether this behavior was
an artifact of the RGO data and find evidence to indicate that the
behavior was real. For the smaller sunspots (constituting the vast
majority in both number and total area), the proportional size of the
sunspot umbrae slowly increased by more than 50% and then returned to
“normal” over this 40-year period.
Title: An Examination of Selected Geomagnetic Indices in Relation
to the Sunspot Cycle
Authors: Wilson, Robert M.; Hathaway, David H.
Bibcode: 2006STIN...0721477W
Altcode:
Previous studies have shown geomagnetic indices to be useful for
providing early estimates for the size of the following sunspot
cycle several years in advance. Examined this study are various
precursor methods for predicting the minimum and maximum amplitude
of the following sunspot cycle, these precursors based on the aa and
Ap geomagnetic indices and the number of disturbed days (NDD), days
when the daily Ap index equaled or exceeded 25. Also examined is the
yearly peak of the daily Ap index (Apmax), the number of days when
Ap greater than or equal to 100, cyclic averages of sunspot number R,
aa, Ap, NDD, and the number of sudden storm commencements (NSSC), as
well the cyclic sums of NDD and NSSC. The analysis yields 90-percent
prediction intervals for both the minimum and maximum amplitudes
for cycle 24, the next sunspot cycle. In terms of yearly averages,
the best regressions give Rmin = 9.8+/-2.9 and Rmax = 153.8+/-24.7,
equivalent to Rm = 8.8+/-2.8 and RM = 159+/-5.5, based on the 12-mo
moving average (or smoothed monthly mean sunspot number). Hence, cycle
24 is expected to be above average in size, similar to cycles 21 and
22, producing more than 300 sudden storm commencements and more than
560 disturbed days, of which about 25 will be Ap greater than or equal
to 100. On the basis of annual averages, the sunspot minimum year for
cycle 24 will be either 2006 or 2007.
Title: Geomagnetic activity indicates large amplitude for sunspot
cycle 24
Authors: Hathaway, D. H.; Wilson, R. M.
Bibcode: 2006AGUFMSH21A0330H
Altcode:
The level of geomagnetic activity near the time of solar activity
minimum has been shown to be a reliable indicator for the amplitude of
the following solar activity maximum. The geomagnetic activity index
aa can be split into two components: one associated with solar flares,
prominence eruptions, and coronal mass ejections which follows the solar
activity cycle and a second component associated with recurrent high
speed solar wind streams which is out of phase with the solar activity
cycle. This second component often peaks before solar activity minimum
and has been one of the most reliable indicators for the amplitude of
the following maximum. The size of the recent maximum in this second
component indicates that solar activity cycle 24 will be much higher
than average similar in size to cycles 21 and 22 with a peak smoothed
sunspot number of 160±25.
Title: Supergranules -- The True Nature of Solar Rossby Hills?
Authors: Williams, Peter; Cuntz, Manfred; Hathaway, David
Bibcode: 2006APS..TSF.P1002W
Altcode:
Supergranulation is a well established component of solar convection
and visible on the solar surface as cellular structures. The convective
upflow within a supergranule cell overshoots the equilibrium solar
surface creating a corrugated surface. The hills associated with these
upflows have been detected as they pass over the solar limb. Their
discovery was initially attributed to Rossby waves, arising from r-mode
oscillations in the Sun where the Coriolis force acts as a restoring
force on internal gravity waves. We analyze these hills by producing an
artificial height map derived from the radial component of supergranule
Doppler velocity data constructed from the spectral components of a
synthetic photospheric convection spectrum. We are able to show that
the observed signals leading to the detection of these solar hills can
be modeled by applying the same methods that lead to the Rossby wave
`discovery', prompting the conclusion that the corrugation has its
origins in supergranulation.
Title: Geomagnetic activity indicates large amplitude for sunspot
cycle 24
Authors: Hathaway, David H.; Wilson, Robert M.
Bibcode: 2006GeoRL..3318101H
Altcode:
The level of geomagnetic activity near the time of solar activity
minimum has been shown to be a reliable indicator for the amplitude of
the following solar activity maximum. The geomagnetic activity index
aa can be split into two components: one associated with solar flares,
prominence eruptions, and coronal mass ejections which follows the solar
activity cycle and a second component associated with recurrent high
speed solar wind streams which is out of phase with the solar activity
cycle. This second component often peaks before solar activity minimum
and has been one of the most reliable indicators for the amplitude of
the following maximum. The size of the recent maximum in this second
component indicates that solar activity cycle 24 will be much higher
than average - similar in size to cycles 21 and 22 with a peak smoothed
sunspot number of 160 +/- 25.
Title: On the Relationship Between Spotless Days and the Sunspot
Cycle: A Supplement
Authors: Wilson, Robert M.; Hathaway, David H.
Bibcode: 2006STIN...0718264W
Altcode:
This study provides supplemental material to an earlier study concerning
the relationship between spotless days and the sunspot cycle. Our
previous study, Technical Publication (TP)-2005-213608 determined the
timing and size of sunspot minimum and maximum for the new sunspot
cycle, relative to the occurrence of the first spotless day during
the declining phase of the old sunspot cycle and the last spotless
day during the rising portion of the new cycle. Because the number
of spotless days (NSD) rapidly increases as the cycle nears sunspot
minimum and rapidly decreases thereafter, the size and timing of sunspot
minimum and maximum might be more accurately determined using a higher
threshold for comparison, rather than using the first and last spotless
day occurrences. It is this aspect that is investigated more thoroughly
in this TP.
Title: Rossby 'Hills' Identified as Supergranule Manifestations
Authors: Williams, Peter E.; Hathaway, D. H.; Cuntz, M.
Bibcode: 2006SPD....37.3002W
Altcode: 2006BAAS...38S.256W
Rossby waves have been well established as oceanographic and atmospheric
features on Earth in which the Coriolis force acts as a restoring force
on internal gravity waves. Rossby waves have also been predicted to
exist as "r-mode oscillations" on rotating stars and the Sun. Recently,
reports have claimed that such phenomena exist as low amplitude,
long wavelength features - "hills" - on the surface of the Sun by
analyzing spatial and temporal signatures of the solar limb from the MDI
instrument on SOHO. We have used simulated data to conduct a similar
analysis of the limb and discovered that the reported signatures can
be obtained by considering only the supergranule convection pattern.
Title: Supergranule Superrotation Identified as a Projection Effect
Authors: Hathaway, D. H.; Williams, P. E.; Cuntz, M.
Bibcode: 2006ApJ...644..598H
Altcode:
Previous measurements of the rotation rate of the supergranule
Doppler velocity pattern revealed surprising characteristics: (1) the
pattern rotates faster than the plasma at the surface, and, at each
latitude, it rotates faster than the plasma at any level below the
surface (superrotation), (2) larger cells rotate more rapidly than
smaller cells, and (3) faster rotation rates are found when using
cross-correlation techniques with larger time lags between Doppler
images. We simulate the supergranulation velocity pattern using a
spectrum for the cellular flows that matches the observed spectrum,
but we keep the pattern unchanged and rotating rigidly. Our simulation
shows that the superrotation and its dependence on cell size can be
largely reproduced by projection effects on the line-of-sight Doppler
velocity signal. The remaining variation in rotation rate with cell
size can be attributed to cells smaller than supergranules extending
through shallower layers that have slower rotation rates.
Title: The Supergranule Super-Rotation Illusion
Authors: Hathaway, David H.; Williams, P.; Cuntz, M.
Bibcode: 2006SPD....37.3001H
Altcode: 2006BAAS...38..256H
Peculiar aspects of the rotation rate of the supergranules have been
noted for over 20 years now. This has culminated in recent reports
suggesting that the supergranules have wave-like characteristics and
propagate prograde at a rate that exceeds that of the plasma anywhere
below the surface. We have simulated supergranules that rotate at a rate
that is independent of position or size and find that they appear to
rotate at a more rapid rate. This super-rotation of the supergranules
is seen in both cross-correlation and Fourier analyses of the Doppler
velocity pattern. The amplitude of the rotation excess as a function of
size matches that seen in the Fourier analyses of MDI data. The source
of this rotation excess is identified with the effect of projecting
velocity signals into the line-of-sight. We conclude that supergranules
are merely advected by the flow in the near-surface shear layer and that
their apparent super-rotation does not indicate wave-like properties.
Title: An Examination of Sunspot Number Rates of Growth and Decay
in Relation to the Sunspot Cycle
Authors: Wilson, Robert M.; Hathaway, David H.
Bibcode: 2006STIN...0709843W
Altcode:
On the basis of annual sunspot number averages, sunspot number
rates of growth and decay are examined relative to both minimum and
maximum amplitudes and the time of their occurrences using cycles 12
through present, the most reliably determined sunspot cycles. Indeed,
strong correlations are found for predicting the minimum and maximum
amplitudes and the time of their occurrences years in advance. As
applied to predicting sunspot minimum for cycle 24, the next cycle,
its minimum appears likely to occur in 2006, especially if it is a
robust cycle similar in nature to cycles 17-23.
Title: On the Relation Between Sunspot Area and Sunspot Number
Authors: Wilson, Robert M.; Hathaway, David H.
Bibcode: 2006STIN...0620186W
Altcode:
Often, the relation between monthly or yearly averages of total sunspot
area, A, and sunspot number, R, has been described using the formula A =
16.7 R. Such a simple relation, however, is erroneous. The yearly ratio
of A/R has varied between 5.3 in 1964 to 19.7 in 1926, having a mean
of 13.1 with a standard deviation of 3.5. For 1875-1976 (corresponding
to the Royal Greenwich Observatory timeframe), the yearly ratio of A/R
has a mean of 14.1 with a standard deviation of 3.2, and it is found to
differ significantly from the mean for 1977-2004 (corresponding to the
United States Air Force/National Oceanic and Atmospheric Administration
Solar Optical Observing Network timeframe), which equals 9.8 with a
standard deviation of 2.1. Scatterplots of yearly values of A versus
R are highly correlated for both timeframes and they suggest that a
value of R = 100 implies A=1,538 +/- 174 during the first timeframe,
but only A=1,076 +/- 123 for the second timeframe. Comparison of the
yearly ratios adjusted for same day coverage against yearly ratios
using Rome Observatory measures for the interval 1958-1998 indicates
that sunspot areas during the second timeframe are inherently too low.
Title: Obituary: Jason G. Porter, 1954-2005
Authors: Hathaway, David H.
Bibcode: 2005BAAS...37.1555H
Altcode:
Jason Porter, a solar astronomer at NASA's Marshall Space Flight
Center (MSFC), died on 23 July 2005 from complications associated
with his 18-year battle with a form of non-Hodgkin's lymphoma. He was
born on 28 June 1954. Jason was Texas born and bred. He received
his Bachelor's degree from Texas A&M in 1976 and then went to the
University of Colorado for his graduate work. He received his PhD from
the Department of Astrophysical, Planetary, and Atmospheric Sciences in
1984. His thesis, "Ultraviolet Spectral Diagnostics of Solar Flares and
Heating Events," was written under the guidance of Katharine Gebbie and
Juri Toomre. The ideas behind his thesis and much of his later work were
formulated while he was a Graduate Research Assistant at Goddard Space
Flight Center (GSFC) working on analysis of data from the Ultraviolet
Spectrometer and Polarimeter, a major instrument on the Solar Maximum
Mission (SMM). While at Goddard, he met his wife-to-be, Linda Zimmerman,
who was working as a computer system administrator at the SMM Operations
Center. They married and moved to Huntsville, Alabama in 1984 where
Jason had an appointment as an NAS/NRC Resident Research Associate in
the Solar Physics Branch of MSFC and Linda was a system administrator
for the Space Science Laboratory. After a short stint at the University
of Alabama in Huntsville, Jason joined NASA as a Senior Scientist in
the Space Science Laboratory in 1987, a position he still held at the
time of his death. Jason's early work brought forth the idea that
"microflares" make a significant contribution to the heating of the
solar corona, an idea which he continued to champion throughout his
career. He also searched for coronal emission from white dwarf stars
using the ROSAT and Chandra Space Observatories, and served as the
NASA Project Scientist for a lunar based ultraviolet telescope. More
recently he was leading a team of engineers and scientists, from MSFC,
GSFC, and the National Solar Observatory on the development of a
solar ultraviolet magnetograph instrument (SUMI) capable of measuring
vector magnetic fields in the upper chromosphere and transition region
where the magnetic reconnection that powers solar flares and CMEs is
believed to occur. He continued to provide inspiring leadership to the
development of SUMI up until the last month of his life. Jason
was admired by his colleagues on both a professional and personal
level. He also had a rich life outside of his professional work. He
loved the outdoors - hiking, camping, and fishing in particular. He
loved music. Bluegrass was one of his favorites. He played the steel
guitar, the Dobro, and the trombone, and spent many evenings playing
in a local bluegrass band. He also loved finely crafted lagers and
ales and would occasionally bring some strange brew to liven up an
evening of poker. Jason and Linda have two sons, Graham (13) and Allen
(11). All who knew him well will miss him dearly.
Title: How Large-scale Flows May Influence Solar Activity?
Authors: Hathaway, D. H.
Bibcode: 2005ASPC..346...19H
Altcode:
Large-scale flows within the solar convection zone are the primary
drivers of the Sun's magnetic activity cycle and play important roles
in shaping the Sun's magnetic field. Differential rotation amplifies
the magnetic field through its shearing action and converts poloidal
field into toroidal field. Poleward meridional flow near the surface
carries magnetic flux that reverses the magnetic poles at about the
time of solar maximum. The deeper, equatorward meridional flow can
carry magnetic flux back toward the lower latitudes where it erupts
through the surface to form tilted active regions that convert toroidal
fields into oppositely directed poloidal fields. These axisymmetric
flows are themselves driven by large-scale convective motions. The
effects of the Sun's rotation on convection produce velocity
correlations that can maintain both the differential rotation and the
meridional circulation. These convective motions can also influence
solar activity directly by shaping the magnetic field pattern. While
considerable theoretical advances have been made toward understanding
these large-scale flows, outstanding problems in matching theory to
observations still remain.
Title: A Comparison of Rome Observatory Sunspot Area and Sunspot
Number Determinations With International Measures, 1958-1998
Authors: Wilson, Robert M.; Hathaway, David H.
Bibcode: 2005STIN...0622159W
Altcode:
Two changes in recording the sunspot record have occurred in recent
years. First, in 1976, the longer-than-100-yr daily photographic record
of the Royal Greenwich Observatory (RGO), used for determination of
numbers and positions of sunspot groups and sunspot areas ended, and
second, at the end of 1980, after more than 130 years, Zurich s Swiss
Federal Observatory stopped providing daily sunspot numbers. To extend
the sunspot record beyond 1976, use of United States Air Force/National
Oceanic and Atmospheric Administration (USAF/NOAA) sunspot drawing
observations from the Solar Optical Observing Network began in 1977,
and the combined record of sunspot activity from RGO/USAF/NOAA was made
accessible at http://science.nasa.gov/ssl/PAD/SOLAR/greenwch.htm. Also,
in 1981, the task of providing daily sunspot numbers was taken up by
the Royal Observatory of Belgium s Solar Influences and Data analysis
Center, and the combined Zurich/International sunspot number database
was made available at http://sidc.oma.be/index.php3. In this study,
Rome Observatory 1958-1998 photographic records of sunspot areas,
numbers of groups, and derived sunspot numbers are compared against
same-day international values to determine relative behaviors and to
evaluate whether any potential changes might have been introduced in
the overall sunspot record, due to the aforementioned changes.
Title: Determining the Sun's Deep Meridional Flow Speed Using Active
Latitude Drift Rates Since 1874
Authors: Hathaway, D. H.; Wilson, R. M.
Bibcode: 2005AGUSMSP32A..02H
Altcode:
Dynamo models that incorporate a deep meridional return flow indicate
that this flow regulates both the period and the amplitude of the
sunspot cycle (Dikpati & Charbonneau 1999, ApJ, 518, 508 and
Charbonneau & Dikpati 2000, ApJ, 543, 1027). We recently examined
the equatorward drift of the active latitudes (as given by the centroid
of the sunspot areas in each hemisphere) and found evidence supporting
this view (Hathaway et al. 2003, ApJ, 589, 665 and Hathaway et al. 2004,
ApJ, 602, 543). In those studies we fit the equatorward drift in each
hemisphere for each sunspot cycle with a simple parabola - giving
us a drift rate and its deceleration for each hemisphere/cycle. Here
we analyze the same data (the Royal Greenwich Observatory/USAF/NOAA
daily active region summaries) to determine the drift rates in each
hemisphere on a yearly basis (rotation-by-rotation measurements smoothed
to remove high frequencies) and fit them with a simple model for the
meridional flow that provides the meridional flow speed as a function
of latitude and time from 1874 to 2005. These flow speeds can be used
to test dynamo models -- some of which have predictive capabilities.
Title: Coronal Heating, Spicules, and SolarB
Authors: Moore, R. L.; Falconer, D. A.; Porter, J. G.; Hathaway,
D. H.; Yamauchi, Y.; Rabin, D. M.
Bibcode: 2004ASPC..325..283M
Altcode:
We summarize certain observations of coronal luminosity, network
magnetic flux, spicules, and macrospicules. These observations together
imply that in quiet regions that are not influenced by active regions
the coronal heating comes from magnetic activity in the edges of the
network flux, possibly from explosions of sheared core fields around
granule-sized inclusions of opposite-polarity flux. This scenario can
be tested by SolarB.
Title: What the Sunspot Record Tells Us About Space Climate
Authors: Hathaway, David H.; Wilson, Robert M.
Bibcode: 2004SoPh..224....5H
Altcode: 2005SoPh..224....5H
The records concerning the number, sizes, and positions of sunspots
provide a direct means of characterizing solar activity over nearly 400
years. Sunspot numbers are strongly correlated with modern measures of
solar activity including: 10.7-cm radio flux, total irradiance, X-ray
flares, sunspot area, the baseline level of geomagnetic activity, and
the flux of galactic cosmic rays. The Group Sunspot Number provides
information on 27 sunspot cycles, far more than any of the modern
measures of solar activity, and enough to provide important details
about long-term variations in solar activity or "Space Climate." The
sunspot record shows: 1) sunspot cycles have periods of 131± 14
months with a normal distribution; 2) sunspot cycles are asymmetric
with a fast rise and slow decline; 3) the rise time from minimum to
maximum decreases with cycle amplitude; 4) large amplitude cycles are
preceded by short period cycles; 5) large amplitude cycles are preceded
by high minima; 6) although the two hemispheres remain linked in phase,
there are significant asymmetries in the activity in each hemisphere;
7) the rate at which the active latitudes drift toward the equator is
anti-correlated with the cycle period; 8) the rate at which the active
latitudes drift toward the equator is positively correlated with the
amplitude of the cycle after the next; 9) there has been a significant
secular increase in the amplitudes of the sunspot cycles since the end
of the Maunder Minimum (1715); and 10) there is weak evidence for a
quasi-periodic variation in the sunspot cycle amplitudes with a period
of about 90 years. These characteristics indicate that the next solar
cycle should have a maximum smoothed sunspot number of about 145 ±
30 in 2010 while the following cycle should have a maximum of about
70 ± 30 in 2023.
Title: Supergranule Diffusion and Active Region Decay
Authors: Hathaway, D. H.; Choudhary, D. P.
Bibcode: 2004AAS...204.3712H
Altcode: 2004BAAS...36..711H
Models of the Sun's magnetic dynamo include turbulent diffusion to
parameterize the effects of convective motions on the evolution of
the Sun's magnetic field. Supergranules are known to dominate the
evolution of the surface magnetic field structure as evidenced by
the structure of both the active and quiet magnetic network. However,
estimates for the diffusivity attributed to supergranules differ by
an order of magnitude - from about 100 km2/s to more than
1000 km2/s. We examine this question of the diffusivity
using three different approaches. 1) We study the decay of more than
30,000 active regions by determining the rate of change in the sunspot
area of each active region from day-to-day. 2) We study the decay
of a single isolated active region near the time of solar minimum by
examining the magnetic field evolution over five solar rotations from
SOHO/MDI magnetograms obtained at 96-minute intervals. 3) We study
the characteristics of supergranules that influence the estimates of
their diffusive properties - flow speeds and lifetimes as functions
of size ∼V from SOHO/MDI Dopplergrams.
Title: Erratum: ``Evidence that a Deep Meridional Flow Sets the
Sunspot Cycle Period'' (ApJ, 589,
665 [2003])
Authors: Hathaway, David H.; Nandy, Dibyendu; Wilson, Robert M.;
Reichmann, Edwin J.
Bibcode: 2004ApJ...602..543H
Altcode:
An error was made in entering the data used in Figure 6. This changes
the results concerning the length of the time lag between the variations
in the meridional flow speed and those in the cycle amplitude. The
final paragraph on page 667 should read: ``Finally, we study
the relationship between the drift velocities and the amplitudes
of the hemisphere/cycles. In Figure 5 we compare the drift velocity
at the maximum of the cycle to the amplitude of that cycle for that
hemisphere. There is a positive (0.5) and significant (95%) correlation
between the two. However, an even stronger relationship is found between
the drift velocity and the amplitude of the N+2 cycle. The correlation
is stronger (0.7) and more significant (99%), as shown in Figure 6. This
relationship is suggestive of a ``memory'' in the solar cycle, again
a property of dynamo models that use meridional circulation. Indeed,
the two-cycle lag is precisely the relationship found by Charbonneau
& Dikpati (ApJ, 589, 665
[2003]). This behavior is, however, more difficult to interpret,
and we elaborate on this in the next section. In either case, these
correlations only explain part of the variance in cycle amplitude (25%
for the current cycle and 50% for the N+2 cycle). Obviously, other
mechanisms, such as variations in the gradient in the rotation rate,
also contribute to the cycle amplitude variations. Our investigation of
possible connections between drift rates and the amplitudes of the N+1
and N+3 cycles gives no significant correlations at these alternative
time lags.'' The revised Figure 6 and its caption are given below
Title: Solar Coronal Heating and the Magnetic Flux Content of
the Network
Authors: Falconer, D. A.; Moore, R. L.; Porter, J. G.; Hathaway, D. H.
Bibcode: 2003ApJ...593..549F
Altcode:
We investigate the heating of the quiet corona by measuring the increase
of coronal luminosity with the amount of magnetic flux in the underlying
network at solar minimum when there were no active regions on the face
of the Sun. The coronal luminosity is measured from Fe IX/X-Fe XII pairs
of coronal images from SOHO/EIT, under the assumption that practically
all of the coronal luminosity in our quiet regions comes from plasma in
the temperature range 0.9×106K<=T<=1.3×106
K. The network magnetic flux content is measured from SOHO/MDI
magnetograms. We find that the luminosity of the corona in our quiet
regions increases roughly in proportion to the square root of the
magnetic flux content of the network and roughly in proportion to the
length of the perimeter of the network magnetic flux clumps. From (1)
this result, (2) other observations of many fine-scale explosive events
at the edges of network flux clumps, and (3) a demonstration that it is
energetically feasible for the heating of the corona in quiet regions
to be driven by explosions of granule-sized sheared-core magnetic
bipoles embedded in the edges of network flux clumps, we infer that in
quiet regions that are not influenced by active regions the corona is
mainly heated by such magnetic activity in the edges of the network
flux clumps. Our observational results together with our feasibility
analysis allow us to predict that (1) at the edges of the network flux
clumps there are many transient sheared-core bipoles of the size and
lifetime of granules and having transverse field strengths greater
than ~100 G, (2) ~30 of these bipoles are present per supergranule,
and (3) most spicules are produced by explosions of these bipoles.
Title: Evidence That a Deep Meridional Flow Sets the Sunspot Cycle
Period
Authors: Hathaway, David H.; Nandy, Dibyendu; Wilson, Robert M.;
Reichmann, Edwin J.
Bibcode: 2003ApJ...589..665H
Altcode:
Sunspots appear on the Sun in two bands on either side of the equator
that drift toward lower latitudes as each sunspot cycle progresses. We
examine the drift of the centroid of the sunspot area toward the
equator in each hemisphere from 1874 to 2002 and find that the drift
rate slows as the centroid approaches the equator. We compare the
drift rate at sunspot cycle maximum with the period of each cycle
for each hemisphere and find a highly significant anticorrelation:
hemispheres with faster drift rates have shorter periods. These
observations are consistent with a meridional counterflow deep within
the Sun as the primary driver of the migration toward the equator and
the period associated with the sunspot cycle. We also find that the
drift rate at maximum is significantly correlated with the amplitude of
the following cycle, a prediction of dynamo models that employ a deep
meridional flow toward the equator. Our results indicate an amplitude
of about 1.2 m s-1 for the meridional flow velocity at the
base of the solar convection zone.
Title: Evidence that a Deep Meridional Flow Sets the Sunspot Cycle
Period
Authors: Hathaway, D. H.; Nandy, D.; Wilson, R. M.; Reichmann, E. J.
Bibcode: 2003SPD....34.2604H
Altcode: 2003BAAS...35..854H
Sunspots appear on the Sun in two bands on either side of the equator
that drift toward lower latitudes as each sunspot cycle progresses. We
examine the equatorward drift of the centroid of the sunspot area in
each hemisphere from 1874 to 2002 and find that the drift rate slows
as the centroid approaches the equator. We compare the drift rate at
sunspot cycle maximum to the cycle-period for each hemisphere and find
a highly significant anti-correlation: hemispheres with faster drift
rates have shorter periods. These observations are consistent with
an equatorward meridional counterflow, deep within the Sun, as the
primary driver of the equatorward migration and the period associated
with the sunspot cycle. We also find that the drift rate at maximum is
significantly correlated with the amplitude of the following cycle, a
prediction of dynamo models that employ a deep equatorward meridional
flow. Our results indicate an amplitude of about 1.2 m/s for the
meridional flow velocity at the base of the solar convection zone.
Title: Coronal Heating and the Magnetic Flux Content of the Network
Authors: Moore, R. L.; Falconer, D. A.; Porter, J. G.; Hathaway, D. H.
Bibcode: 2003SPD....34.1010M
Altcode: 2003BAAS...35..826M
We investigate the heating of the quiet corona by measuring the
increase of coronal luminosity with the amount of magnetic flux in
the underlying network at solar minimum when there were no active
regions on the face of the Sun. The coronal luminosity is measured
from Fe IX/X-Fe XII pairs of coronal images from SOHO/EIT. The network
magnetic flux content is measured from SOHO/MDI magnetograms. We find
that the luminosity of the corona in our quiet regions increases roughly
in proportion to the square root of the magnetic flux content of the
network and roughly in proportion to the length of the perimeter of
the network magnetic flux clumps. From (1) this result, (2) other
observations of many fine-scale explosive events at the edges of
network flux clumps, and (3) a demonstration that it is energetically
feasible for the heating of the corona in quiet regions to be driven by
explosions of granule-sized sheared-core magnetic bipoles embedded in
the edges of network flux clumps, we infer that in quiet regions that
are not influenced by active regions the corona is mainly heated by
such magnetic activity in the edges of the network flux clumps. Our
observational results together with our feasibility analysis allow
us to predict that (1) at the edges of the network flux clumps there
are many transient sheared-core bipoles of the size and lifetime of
granules and having transverse field strengths > 100 G, (2) 30 of
these bipoles are present per supergranule, and (3) most spicules are
produced by explosions of these bipoles. This work was supported
by NASA's Office of Space Science through its Solar and Heliospheric
Physics Supporting Research and Technology Program and its Sun-Earth
Connection Guest Investigator Program.
Title: Beyond Solar-B: MTRAP, the Magnetic TRAnsition Region Probe
Authors: Davis, J. M.; Moore, R. L.; Hathaway, D. H.; Science
Definition CommitteeHigh-Resolution Solar Magnetography Beyond
Solar-B Team
Bibcode: 2003SPD....34.2014D
Altcode: 2003BAAS...35..846D
The next generation of solar missions will reveal and measure
fine-scale solar magnetic fields and their effects in the solar
atmosphere at heights, small scales, sensitivities, and fields
of view well beyond the reach of Solar-B. The necessity for, and
potential of, such observations for understanding solar magnetic
fields, their generation in and below the photosphere, and their
control of the solar atmosphere and heliosphere, were the focus of
a science definition workshop, "High-Resolution Solar Magnetography
from Space: Beyond Solar-B," held in Huntsville Alabama in April
2001. Forty internationally prominent scientists active in solar
research involving fine-scale solar magnetism participated in this
Workshop and reached consensus that the key science objective to be
pursued beyond Solar-B is a physical understanding of the fine-scale
magnetic structure and activity in the magnetic transition region,
defined as the region between the photosphere and corona where neither
the plasma nor the magnetic field strongly dominates the other. The
observational objective requires high cadence (< 10s) vector
magnetic field maps, and spatially resolved spectra from the IR,
visible, vacuum UV, to the EUV at high resolution (< 50km) over
a large FOV ( 140,000 km). A polarimetric resolution of one part in
ten thousand is required to measure transverse magnetic fields of <
30G. The latest SEC Roadmap includes a mission identified as MTRAP to
meet these requirements. Enabling technology development requirements
include large, lightweight, reflecting optics, large format sensors
(16K x 16K pixels) with high QE at 150 nm, and extendable spacecraft
structures. The Science Organizing Committee of the Beyond Solar-B
Workshop recommends that: 1. Science and Technology Definition Teams
should be established in FY04 to finalize the science requirements
and to define technology development efforts needed to ensure the
practicality of MTRAP's observational goals. 2. The necessary technology
development funding should be included in Code S budgets for FY06 and
beyond to prepare MTRAP for a new start no later than the nominal end
of the Solar-B mission, around 2010.
Title: Large scale flows through the solar cycle
Authors: Hathaway, David H.
Bibcode: 2003ESASP.517...87H
Altcode: 2003soho...12...87H
Large scale flows within the solar convection zone are thought to be
the primary drivers of the Sun's magnetic activity cycle. Differential
rotation amplifies the magnetic field and converts poloidal fields
into toroidal fields. Poleward meridional flow near the surface
carries magnetic flux that reverses the polar magnetic polarities. A
deeper, equatorward meridional flow may carry magnetic flux toward the
equator. These axisymmetric flows are themselves driven by large scale
convective motions. Given these intimate connections between the large
scale flows and solar activity, it would be surprising if there were not
solar cycle variations in the flow characteristics. Some variations,
namely the torsional oscillations, are well established. Other
variations, namely changes in the meridional flow and in the shear at
the base of the convection zone, are more controversial. In this paper
I describe the observed characteristics of the solar cycle and the
large scale flows and discuss the nature of the solar cycle variations.
Title: Group Sunspot Numbers: Sunspot Cycle Characteristics
Authors: Hathaway, David H.; Wilson, Robert M.; Reichmann, Edwin J.
Bibcode: 2002SoPh..211..357H
Altcode:
We examine the `Group' sunspot numbers constructed by Hoyt and Schatten
to determine their utility in characterizing the solar activity
cycle. We compare smoothed monthly Group sunspot numbers to Zürich
(International) sunspot numbers, 10.7-cm radio flux, and total sunspot
area. We find that the Zürich numbers follow the 10.7-cm radio flux
and total sunspot area measurements only slightly better than the Group
numbers. We examine several significant characteristics of the sunspot
cycle using both Group numbers and Zürich numbers. We find that the
`Waldmeier Effect' - the anti-correlation between cycle amplitude
and the elapsed time between minimum and maximum of a cycle - is much
more apparent in the Zürich numbers. The `Amplitude-Period Effect'
- the anti-correlation between cycle amplitude and the length of the
previous cycle from minimum to minimum - is also much more apparent in
the Zürich numbers. The `Amplitude-Minimum Effect' - the correlation
between cycle amplitude and the activity level at the previous (onset)
minimum is equally apparent in both the Zürich numbers and the Group
numbers. The `Even-Odd Effect' - in which odd-numbered cycles are larger
than their even-numbered precursors - is somewhat stronger in the Group
numbers but with a tighter relationship in the Zürich numbers. The
`Secular Trend' - the increase in cycle amplitudes since the Maunder
Minimum - is much stronger in Group numbers. After removing this trend
we find little evidence for multi-cycle periodicities like the 80-year
Gleissberg cycle or the two- and three-cycle periodicities. We also find
little evidence for a correlation between the amplitude of a cycle and
its period or for a bimodal distribution of cycle periods. We conclude
that the Group numbers are most useful for extending the sunspot cycle
data further back in time and thereby adding more cycles and improving
the statistics. However, the Zürich numbers are slightly more useful
for characterizing the on-going levels of solar activity.
Title: Coronal Heating and the Increase of Coronal Luminosity with
Magnetic Flux
Authors: Moore, R. L.; Falconer, D. A.; Porter, J. G.; Hathaway, D. H.
Bibcode: 2002AAS...200.8808M
Altcode: 2002BAAS...34R.790M
We present the observed scaling of coronal luminosity with magnetic
flux in a set of quiet regions. Comparison of this with the observed
scaling found for active regions by Fisher et al (1998, ApJ, 508,
985) suggests an underlying difference between coronal heating in
active regions and quiet regions. From SOHO/EIT coronal images and
SOHO/MDI magnetograms of 4 similar large quiet regions, we measure
LCorona and Φ Total in random subregions
ranging in area from about 4 supergranules [(70,000 km)2]
to about 100 supergranules [(0.5 RSun)2], where
LCorona is the luminosity of the corona in a subregion and
Φ Total is the flux content of the magnetic network in the
subregion. This sampling of our quiet regions yields a correlation plot
of Log(LCorona) vs Log(Φ Total) appropriate for
comparison with the corresponding plot from Fisher et al for active
regions. For our quiet regions, the mean values of LCorona
and Φ Total both increase linearly with area (simply
because each set of subregions of the same area has very nearly
the same mean coronal luminosity per unit area and mean magnetic
flux per unit area), and in each constant-area set the values of
LCorona and Φ Total "scatter" about their
means for that area. This results in the linear least-squares fit to
the Log(LCorona) vs Log(Φ Total) plot having
a slope somewhat less than 1. If active regions mimicked our quiet
regions in that all large sets of same-area active regions had the same
mean coronal luminosity per unit area and same mean magnetic flux per
unit area, then the least-squares fit to their Log(LCorona)
vs Log(Φ Total) plot would also have a slope of less than
1. Instead, the slope for active regions is 1.2. Given the observed
factor of 3 scatter about the least-squares linear fit, this slope
is consistent with Φ Total on average increasing linearly
with area (A) as in quiet regions, but LCorona on average
increasing as the volume (A1.5) of the active region instead
of as the area. This possiblity is reasonable if the heating in active
regions is a burning down of previously-stored coronal magnetic energy
rather than a steady dissipation of energy flux from below as expected
in quiet regions. This work is supported by NASA, OSS, through its
S&HP SR&T and SEC GI programs.
Title: Supergranule Rotation Rates and Lifetimes
Authors: Hathaway, D. H.
Bibcode: 2002AAS...200.0415H
Altcode: 2002BAAS...34R.645H
Previous measurements of the rotation rate of the supergranule
Doppler pattern have revealed three interesting characteristics. 1) The
supergranule pattern rotates faster than the plasma at the surface and,
at each latitude, it rotates faster than the plasma at any level below
the surface. 2) Larger cells rotate more rapidly than smaller cells. 3)
Faster rotation rates are found when using larger time lags between
Doppler images. These last two characteristics are consistent with the
idea that large cells live longer and extend deeper into the Sun where
the rotation rate is faster. A re-examination of the rotation rates
and lifetimes of the Doppler patterns seen with the MDI instrument
on SOHO confirms these characteristics. However, a simulation of the
data using a spectrum for the cellular flows that matches the observed
spectrum shows that these characteristics can be largely reproduced by
cellular patterns that rotate at the same rate without any dependence
upon cell size. The rotation rate, and its dependence on latitude,
is nonetheless still faster than the surface or internal rotation
rate. The difference in rotation rates as functions of cell size and
time lag between observations is attributed to projection effects on
the line-of-sight Doppler signal. This data simulation is also used
to determine characteristic lifetimes for the cellular patterns as a
function of cell size. These lifetime determinations are also affected
by projection effects on the line-of-sight Doppler signal.
Title: Characterizing Photospheric Flows
Authors: Hathaway, David
Bibcode: 2002smra.progE...8H
Altcode:
No abstract at ADS
Title: Torsional oscillation & meridional flows
Authors: Hathaway, David
Bibcode: 2002ocnd.confE..14H
Altcode:
No abstract at ADS
Title: Radial Flows in Supergranules
Authors: Hathaway, D. H.; Beck, J. G.; Han, S.; Raymond, J.
Bibcode: 2002SoPh..205...25H
Altcode:
We determine the radial component of the supergranular flow velocity
by examining the center-to-limb variation of the Doppler velocity
signal. We acquire individual Doppler images obtained with the MDI
instrument on the SOHO spacecraft and process them to remove the
p-mode oscillation signal, the axisymmetric flows, the convective
blueshift signal, and instrumental artifacts. The remaining Doppler
signal contains only non-axisymmetric flow structures. The Doppler
signal from the horizontal flows in these structures varies like sin
ρ, where ρ is the heliocentric angle from disk center. The Doppler
signal from radial flows varies like cos ρ. We fit the center-to-limb
variation of the mean squared velocity signal to a straight line in
sin2 ρ over the central portion of the disk. The intercept
of this line at disk center gives the amplitude of the radial component
of the flow. The slope of the line gives the amplitude of the horizontal
component. We find that the radial flows for typical supergranules have
speeds about 10% that of their associated horizontal flows or about 30
m s−1. The ratio of the radial to horizontal flow speed
increases from 9% to about 18% as the size of the structure decreases
from > 60 Mm to ∼ 5 Mm. We use data simulations to check these
results and find a ratio that increases from 5% to only about 12%
over the same range of sizes. These smaller ratios are attributed to
an underestimation of the horizontal flow speeds due to the fact that
the transverse component of the horizontal flow is not detected by
Doppler measurements.
Title: Latitudinal Transport of Angular Momentum by Cellular Flows
Observed with MDI
Authors: Hathaway, D. H.; Gilman, P. A.; Beck, J. G.
Bibcode: 2001AGUSM..SP21C09H
Altcode:
We have analyzed Doppler velocity images from the MDI instrument on
SOHO to determine the latitudinal transport of angular momentum by the
cellular photospheric flows. Doppler velocity images from 60-days in
May to July of 1996 were processed to remove the p-mode oscillations,
the convective blue shift, the axisymmetric flows, and any instrumental
artifacts. The remaining cellular flows were examined for evidence of
latitudinal angular momentum transport. Small cells show no evidence
of any such transport. Cells the size of supergranules (30,000 km in
diameter) show strong evidence for a poleward transport of angular
momentum. This would be expected if supergranules are influenced by
the Coriolis force, and if the cells are elongated in an east-west
direction. We find good evidence for just such an east- west elongation
of the supergranules. This elongation may be the result of differential
rotation shearing the cellular structures. Data simulations of this
effect support the conclusion that elongated supergranules transport
angular momentum from the equator toward the poles. Cells somewhat
larger than supergranules do not show evidence for this poleward
transport. Further analysis of the data is planned to determine if
the direction of angular momentum transport reverses for even larger
cellular structures. The Sun's rapidly rotating equator must be
maintained by such transport somewhere within the convection zone.
Title: Sun-Earth Day, 2001
Authors: Adams, M.; Mortfield, P.; Hathaway, D. H.
Bibcode: 2001AAS...198.1508A
Altcode: 2001BAAS...33..809A
In order to promote awareness of the Sun-Earth connection, NASA's
Marshall Space Flight Center, in collaboration with the Stanford
SOLAR Center, sponsored a one-day Sun-Earth Day event on April 27,
2001. Although "celebrated" on only one day, teachers and students
from across the nation, prepared for over a month in advance. Workshops
were held in March to train teachers. Students performed experiments,
results of which were shared through video clips and an internet web
cast. Our poster includes highlights from student experiments (grades
2 - 12), lessons learned from the teacher workshops, and plans for
Sun-Earth Day 2002.
Title: Coronal Heating and the Magnetic Flux Content of the Network
Authors: Falconer, D. A.; Moore, R. L.; Porter, J. G.; Hathaway, D. H.
Bibcode: 2001AGUSM..SH31D06F
Altcode:
Previously, from analysis of SOHO/EIT coronal images in combination
with Kitt Peak magnetograms (Falconer et al 1998, ApJ, 501, 386-396),
we found that the quiet corona is the sum of two components: the
large-scale corona and the coronal network. The large-scale corona
consists of all coronal-temperature ( million-degree) structures larger
than the width of a chromospheric network lane (> 10,000 km). The
coronal network (1) consists of all coronal-temperature structures
of the scale of the network lanes and smaller (< 10,000 km), (2)
is rooted in and loosely traces the photospheric magnetic network, (3)
has its brightest features seated on polarity dividing lines (neutral
lines) in the network magnetic flux, and (4) produces only about 5%
of the total coronal emission in quiet regions. The heating of the
coronal network is apparently magnetic in origin. Here, from analysis
of EIT coronal images of quiet regions in combination with magnetograms
of the same quiet regions from SOHO/MDI and from Kitt Peak, we examine
the other 95% of the quiet corona and its relation to the underlying
magnetic network. We find: (1) Dividing the large-scale corona into
its bright and dim halves divides the area into bright "continents" and
dark "oceans" having spans of 2-4 supergranules. (2) These patterns are
also present in the photospheric magnetograms: the network is stronger
under the bright half and weaker under the dim half. (3) The radiation
from the large-scale corona increases roughly as the cube root of the
magnetic flux content of the underlying magnetic network. In contrast,
Fisher et al (1998, ApJ, 508, 985-998) found that the coronal radiation
from an active region increases roughly linearly with the magnetic
flux content of the active region. We assume, as is widely held, that
nearly all of the large-scale corona is magnetically rooted in the
network. Our results, together with the result of Fisher et al (1998),
suggest that either the coronal heating in quiet regions has a large
non-magnetic component, or, if the heating is predominantly produced via
the magnetic field, the mechanism is significantly different than in
active regions. This work is funded by NASA's Office of Space Science
through the Solar Physics Supporting Research and Technology Program
and the Sun-Earth Connection Guest Investigator Program.
Title: The Photospheric Convection Spectrum
Authors: Hathaway, D. H.; Beck, J. G.; Bogart, R. S.; Bachmann, K. T.;
Khatri, G.; Betitto, J. M.; Han, S.; Raymond, J.
Bibcode: 2000SPD....31.0504H
Altcode: 2000BAAS...32..836H
Spectra of the cellular photospheric flows are determined
from observations acquired by the MDI instrument on the SOHO
spacecraft. Spherical harmonic spectra are obtained from the full-disk
observations. Fourier spectra are obtained from the high-resolution
observations. The p-mode oscillation signal and instrumental artifacts
are reduced by temporal filtering of the Doppler data. The resulting
spectra give power (kinetic energy) per wavenumber for effective
spherical harmonic degrees from 1 to over 3000. Significant power is
found at all wavenumbers, including the small wavenumbers representative
of giant cells. The time evolution of the spectral coefficients
indicates that these small wavenumber components rotate at the solar
rotation rate and thus represent a component of the photospheric
cellular flows. The spectra show distinct peaks representing granules
and supergranules but no distinct features at wavenumbers representative
of mesogranules or giant cells. The observed cellular patterns and
spectra are well represented by a model that only includes only two
distinct modes --- granules and supergranules. The radial component of
the supergranular flow is determined by examining the center-to-limb
variation of the Doppler velocity signal. Doppler velocity images
are constructed from sections of the spectrum representing cells of
different sizes. The center-to-limb variation of the mean squared signal
in each of these images is fit over the central portion of the disk out
to where foreshortening begins to affect the signal. The results of this
analysis suggest that the radial flows for typical supergranules have
speeds about 9% that of their associated horizontal flows or about 30
m/s. The ratio of the radial to horizontal flow speed increases from
9% to about 13% as the size of the cells decreases from >30 Mm to
<5 Mm. Data simulations are used to confirm these conclusions.
Title: Large-Scale Coronal Heating from `Cool' Activity in the Solar
Magnetic Network
Authors: Falconer, D. A.; Moore, R. L.; Porter, J. G.; Hathaway, D. H.
Bibcode: 2000SPD....31.0208F
Altcode: 2000BAAS...32..812F
In either Fe IX/X images of Fe XII images from SOHO/EIT, the quiet solar
corona shows structure on scales ranging from sub-supergranular (i.e.,
bright points and coronal network) to multi-supergranular (large-scale
corona). In Falconer et al 1998 (Ap.J., 501, 386) we suppressed the
large-scale corona and found that the network-scale coronal features are
predominantly rooted in the magnetic network lanes at the boundaries
of the supergranules. Here we investigate the relationship between
the large-scale corona and the network as seen in three different
EIT filters (He II, Fe IX/X, and Fe XII), and in the magnetic field
from SOHO/MDI. We find that, underlying the brighter regions of the
large-scale corona (either Fe IX/X or Fe XII), the coronal network
(Fe IX/X, and Fe XII), the transition region network (He II), and
the magnetic flux content of the network are all enhanced relative to
that underlying the dimmer regions of the large-scale corona. We find
that the transition region network radiates more than the large-scale
corona, which radiates more than the coronal network. From our results
we infer that quiet-sun regions (supergranular or larger in size)
with enhanced magnetic flux produce enhanced network activity. The
small fraction of the network activity manifested as coronal network
also increases with increasing magnetic flux. The network activity,
predominately the transition region network activity, (or something
else also correlated with the magnetic field) drives the heating of
the large-scale corona. If the large-scale corona is being heated by
the transition region activity, the heating must be done primarily by
some nonthermal process (nonjet, possibly waves or currents), because
the transition region is cool relative to the corona. This work was
funded by the Solar Physics Branch of NASA's office of Space Science
through the SR&T Program and the SEC Guest Investigator Program.
Title: The Photospheric Convection Spectrum
Authors: Hathaway, D. H.; Beck, J. G.; Bogart, R. S.; Bachmann, K. T.;
Khatri, G.; Petitto, J. M.; Han, S.; Raymond, J.
Bibcode: 2000SoPh..193..299H
Altcode:
Spectra of the cellular photospheric flows are determined
from observations acquired by the MDI instrument on the SOHO
spacecraft. Spherical harmonic spectra are obtained from the full-disk
observations. Fourier spectra are obtained from the high-resolution
observations. The p-mode oscillation signal and instrumental artifacts
are reduced by temporal filtering of the Doppler data. The resulting
spectra give power (kinetic energy) per wave number for effective
spherical harmonic degrees from 1 to over 3000. Significant power is
found at all wavenumbers, including the small wavenumbers representative
of giant cells. The time evolution of the spectral coefficients
indicates that these small wavenumber components rotate at the solar
rotation rate and thus represent a component of the photospheric
cellular flows. The spectra show distinct peaks representing granules
and supergranules but no distinct features at wavenumbers representative
of mesogranules or giant cells. The observed cellular patterns and
spectra are well represented by a model that includes two distinct
modes - granules and supergranules.
Title: Multi-Wavelength Analysis of the March 26, 1991 Solar Flare and
Relation of Spatial and Temporal Characteristics of Hα Emission to
the Dynamics of the Magnetic Field and Charged Particle Acceleration
Authors: Kurt, V. G.; Akimov, V. V.; Hagyard, M. J.; Hathaway, D. H.
Bibcode: 2000ASPC..206..426K
Altcode: 2000hesp.conf..426K
No abstract at ADS
Title: A synthesis of solar cycle prediction techniques
Authors: Hathaway, David H.; Wilson, Robert M.; Reichmann, Edwin J.
Bibcode: 1999JGR...10422375H
Altcode:
A number of techniques currently in use for predicting solar activity
on a solar cycle timescale are tested with historical data. Some
techniques, e.g., regression and curve fitting, work well as solar
activity approaches maximum and provide a month-by-month description
of future activity, while others, e.g., geomagnetic precursors, work
well near solar minimum but only provide an estimate of the amplitude
of the cycle. A synthesis of different techniques is shown to provide
a more accurate and useful forecast of solar cycle activity levels. A
combination of two uncorrelated geomagnetic precursor techniques
provides a more accurate prediction for the amplitude of a solar
activity cycle at a time well before activity minimum. This combined
precursor method gives a smoothed sunspot number maximum of 154±21 at
the 95% level of confidence for the next cycle maximum. A mathematical
function dependent on the time of cycle initiation and the cycle
amplitude is used to describe the level of solar activity month by month
for the next cycle. As the time of cycle maximum approaches a better
estimate of the cycle activity is obtained by including the fit between
previous activity levels and this function. This combined solar cycle
activity forecast gives, as of January 1999, a smoothed sunspot maximum
of 146±20 at the 95% level of confidence for the next cycle maximum.
Title: Large-Scale Coronal Heating from the Solar Magnetic Network
Authors: Falconer, D. A.; Moore, R. L.; Porter, J. G.; Hathaway, D. H.
Bibcode: 1999AAS...194.2301F
Altcode: 1999BAAS...31..860F
In Fe XII images from SOHO/EIT, the quiet solar corona shows structure
on scales ranging from sub-supergranular (i.e., bright points and
coronal network) to multi-supergranular. In Falconer et al 1998 (Ap.J.,
501, 386) we suppressed the large-scale background and found that
the network-scale features are predominantly rooted in the magnetic
network lanes at the boundaries of the supergranules. The emission
of the coronal network and bright points contribute only about 5% of
the entire quiet solar coronal Fe XII emission. Here we investigate
the large-scale corona, the supergranular and larger-scale structure
that we had previously treated as a background, and that emits 95%
of the total Fe XII emission. We compare the dim and bright halves
of the large-scale corona and find that the bright half is 1.5 times
brighter than the dim half, has an order of magnitude greater area
of bright point coverage, has three times brighter coronal network,
and has about 1.5 times more magnetic flux than the dim half. These
results suggest that the brightness of the large-scale corona is
more closely related to the large-scale total magnetic flux than to
bright point activity. We conclude that in the quiet sun: (1) Magnetic
flux is modulated (concentrated/diluted) on size scales larger than
supergranules. (2) The large-scale enhanced magnetic flux gives an
enhanced, more active, magnetic network and an increased incidence
of network bright point formation. (3) The heating of the large-scale
corona is dominated by more widespread, but weaker, network activity
than that which heats the bright points. This work was funded by the
Solar Physics Branch of NASA's office of Space Science through the
SR&T Program and the SEC Guest Investigator Program.
Title: Comment on ``The predicted size of cycle 23 based on the
inferred three-cycle quasiperiodicity of the planetary index Ap''
by H. S. Ahluwalia
Authors: Wilson, Robert M.; Hathaway, David H.
Bibcode: 1999JGR...104.2555W
Altcode:
No abstract at ADS
Title: The Photospheric Convection Spectrum
Authors: Hathaway, David H.
Bibcode: 1999soho....9E...6H
Altcode:
Spectra of the photospheric cellular flows are determined from SOHO/MDI
observations. Spherical harmonic decompositions are obtained from
the full-disc observations. Fourier decompositions are obtained from
the high-resolution observations. The p-mode oscillation signals and
instrumental artifacts are reduced by temporal filtering of the Doppler
data. The resulting spectra give power (kinetic energy) per wavenumber
for effective spherical harmonic degrees from 1 to about 3000. The
spectra show distinct peaks representing granules and supergranules
but no distinct features at wavenumbers representative of mesogranules
or giant cells.
Title: Large-scale Coronal Heating, Clustering of Coronal Bright
Points, and Concentration of Magnetic Flux
Authors: Falconer, D. A.; Moore, R. L.; Porter, J. G.; Hathaway, D. H.
Bibcode: 1999SSRv...87..181F
Altcode:
By combining quiet-region Fe XII coronal images from SOHO/EIT with
magnetograms from NSO/Kitt Peak and from SOHO/MDI, we show that the
population of network coronal bright points and the magnetic flux
content of the network are both markedly greater under the bright
half of the large-scale quiet corona than under the dim half. These
results (1) support the view that the heating of the entire corona
in quiet regions and coronal holes is driven by fine-scale magnetic
activity (microflares, explosive events, spicules) seated low in the
magnetic network, and (2) suggest that this large-scale modulation
of the magnetic flux and coronal heating is a signature of giant
convection cells.
Title: On the Correlation Between Maximum Amplitude and Smoothed
Monthly Mean Sunspot Number during the Rise of the Cycle (from T =
0-48 months Past Sunspot Minimum)
Authors: Wilson, Robert M.; Hathaway, David H.; Reichmann, Edwin J.
Bibcode: 1998nasa.reptX....W
Altcode:
During the rise from sunspot minimum to maximum, the observed value of
smoothed monthly mean sunspot number at maximum RM is found to correlate
with increasing strength against the current value of smoothed monthly
mean sunspot number R(t), where t is the elapsed time in months from
minimum. On the basis of the modern era sunspot cycles (i.e., cycles
10-22), the inferred linear correlation is found to be statistically
important (i.e., at the 95-percent level of confidence) from about 11 mo
past minimum and statistically very important (i.e.. at the 99-percent
level of confidence) from about 15 mo past minimum; ignoring cycle 19,
the largest cycle of the modern era, the inferred linear correlation
is found to be statistically important from cycle onset. On the basis
of R(t), estimates of RM can be gauged usually to within about +/-
30 percent during the first 2 yr and to within about +/- 20 percent
(or better) after the first 2 yr of a cycle's onset. For cycle 23,
because controversy exists regarding the placement of its minimum
(i.e., its onset), being either May 1996 or perhaps August 1996 (or
shortly thereafter), estimates of its RM are divergent, being lower
(more like a mean size cycle) when using the earlier epoch of minimum
and higher (above average in size) when using the later-occurring
minimum. For smoothed monthly mean sunspot number through October 1997
(t = 17 or 14 mo, respectively), having a provisional value of 32.0. the
earlier minimum date projects an RM of 110.3 +/- 33.1, while the later
minimum date projects one of 137.2 +/- 41.2. The projection is slowly
decreasing in size using the earlier onset date, while it is slowly
increasing in size using the later onset date.
Title: Estimating the size and timing of maximum amplitude for cycle
23 from its early cycle behavior
Authors: Wilson, Robert M.; Hathaway, David H.; Reichmann, Edwin J.
Bibcode: 1998JGR...10317411W
Altcode:
On the basis of the lowest observed smoothed monthly mean sunspot
number, cycle 23 appears to have conventionally begun in May 1996, in
conjunction with the first appearance of a new cycle, high-latitude
spot group. Such behavior, however, is considered rather unusual,
since, previously (based on the data-available cycles 12-22), the first
appearance of a new cycle, high-latitude spot group has always preceded
conventional onset by at least 3 months. Furthermore, accepting May
1996 as the official start for cycle 23 poses a dilemma regarding
its projected size and timing of maximum amplitude. Specifically,
from the maximum-minimum and amplitude-period relationships we infer
that cycle 23 should be above average in size and a fast riser, with
maximum amplitude occurring before May 2000 (being in agreement with
projections for cycle 23 based on precursor information), yet from its
initial languid rate of rise (during the first 6 months of the cycle)
we infer that it should be below average in size and a slow riser,
with maximum amplitude occurring after May 2000. The dilemma vanishes,
however, when we use a slightly later-occurring onset. For example,
using August 1996, a date associated with a local secondary minimum
prior to the rapid rise that began shortly thereafter (in early 1997),
we infer that the cycle 23 rate of rise is above that for the mean of
cycles 1-22, the mean of cycles 10-22 (the modern era cycles), the mean
of the modern era ``fast risers,'' and the largest of the modern era
``slow risers'' (i.e., cycle 20), thereby suggesting that cycle 23 will
be both fast rising and above average in size, peaking before August
2000. Additionally, presuming cycle 23 to be a well-behaved fast-rising
cycle (regardless of whichever onset date is used), we also infer that
its maximum amplitude likely will measure about 144.0+/-28.8 (from the
general behavior found for the bulk of modern era fast risers; i.e.,
5 of 7 have had their maximum amplitude to lie within 20% of the mean
curve for modern era fast risers). It is apparent, then, that sunspot
number growth during 1998 will prove crucial for correctly establishing
the size and shape of cycle 23.
Title: Network Coronal Bright Points: Coronal Heating Concentrations
Found in the Solar Magnetic Network
Authors: Falconer, D. A.; Moore, R. L.; Porter, J. G.; Hathaway, D. H.
Bibcode: 1998ApJ...501..386F
Altcode:
We examine the magnetic origins of coronal heating in quiet
regions by combining SOHO/EIT Fe XII coronal images and Kitt Peak
magnetograms. Spatial filtering of the coronal images shows a network
of enhanced structures on the scale of the magnetic network in quiet
regions. Superposition of the filtered coronal images on maps of the
magnetic network extracted from the magnetograms shows that the coronal
network does indeed trace and stem from the magnetic network. Network
coronal bright points, the brightest features in the network lanes,
are found to have a highly significant coincidence with polarity
dividing lines (neutral lines) in the network and are often at the feet
of enhanced coronal structures that stem from the network and reach
out over the cell interiors. These results indicate that, similar to
the close linkage of neutral-line core fields with coronal heating in
active regions (shown in previous work), low-lying core fields encasing
neutral lines in the magnetic network often drive noticeable coronal
heating both within themselves (the network coronal bright points)
and on more extended field lines rooted around them. This behavior
favors the possibility that active core fields in the network are the
main drivers of the heating of the bulk of the quiet corona, on scales
much larger than the network lanes and cells.
Title: An estimate for the size of cycle 23 based on near minimum
conditions
Authors: Wilson, Robert M.; Hathaway, David H.; Reichmann, Edwin J.
Bibcode: 1998JGR...103.6595W
Altcode:
The first occurrence of a high-latitude, new cycle spot group for cycle
23 was in May 1996, in conjunction with a minimum in the smoothed
monthly mean sunspot number. Since then, new cycle spot groups have
become more predominant, and the smoothed monthly mean sunspot number
has slowly risen. Such behavior indicates that new cycle 23 probably
had its minimum annual average sunspot number, R(min), equal to 8.7,
in 1996. Because this value is larger than the average for R(min),
cycle 23 is expected to have a maximum amplitude, R(max), that,
likewise, will be larger than average, suggesting further that it
probably will be both fast rising (i.e., peaking before May 2000) and
of shorter than average length (i.e., ending before May 2007). Another
parameter well correlated with R(max) is the minimum amplitude of the aa
geomagnetic index, aa(min), which usually occurs either in the year of
R(min) occurrence or, more often, in the following year. For 1996 the
annual average of aa measured 18.6. Presuming this value to be aa(min)
for cycle 23, we calculate cycle 23's R(max) to be about 171.0+/-17.6
(i.e., the 90% prediction interval), based on the stronger (r=0.98)
bivariate fit of R(max) versus both R(min) and aa(min). Comparison of
this estimate with others, using various combinations of parameters,
yields an overlap in the prediction intervals for R(max) of about
168+/-15, a range that is within the consensus recently reported by
Joselyn et al. [1997] (=160+/-30). Thus this study supports the view
that cycle 23 will have an R(max) that will be larger than average
but smaller than was seen for cycle 19, the largest cycle on record
with R(max)=190.2.
Title: On the correlation between maximum amplitude and smoothed
monthly mean sunspot number during the rise of the cycle (from t=0-48
months past sunspot minimum)
Authors: Wilson, Robert M.; Hathaway, David H.; Reichmann, Edwin J.
Bibcode: 1998cbma.book.....W
Altcode:
No abstract at ADS
Title: The Solar Dynamo
Authors: Hathaway, David H.
Bibcode: 1998fsam.conf..113H
Altcode:
The solar dynamo is the process by which the Sun's magnetic field is
generated through the interaction of the field with convection and
rotation. In this, it is kin to planetary dynamos and other stellar
dynamos. Although the precise mechanism by which the Sun generates
its field remains poorly understood despite decades of theoretical
and observational work, recent advances suggest that solutions to
this solar dynamo problem may be forthcoming. Two basic processes
are involved in dynamo activity. When the fluid stresses dominate
the magnetic stresses (high plasma beta = 8(pi)rho/B(sup 2)), shear
flows can stretch magnetic field lines in the direction of the shear
(the "alpha effect") and helical flows can lift and twist field lines
into orthogonal planes (the "alpha effect"). These two processes can
be active anywhere in the solar convection zone but with different
results depending upon their relative strengths and signs. Little
is known about how and where these processes occur. Other processes,
such as magnetic diffusion and the effects of the fine scale structure
of the solar magnetic field, pose additional problems.
Title: Synoptic Datasets and Solar Activity Predictions
Authors: Hathaway, David H.
Bibcode: 1998ASPC..140...47H
Altcode: 1998ssp..conf...47H
No abstract at ADS
Title: Mesogranulation as a Distinct Scale of Convection in the Sun
Authors: Bachmann, K. T.; Khatri, G.; Petitto, J. M.; Hathaway, D. H.
Bibcode: 1997AAS...191.7404B
Altcode: 1997BAAS...29.1324B
We present evidence for the existence of mesogranulation as a scale
of convection distinct from granulation and supergranulation through
analysis of full-disk Doppler velocity images of the Sun collected
by the Michelson Doppler Imager (MDI) aboard the NASA/ESA Solar and
Heliospheric Observatory (SOHO). Our analysis procedures isolate
nearly steady flows in the solar photosphere and yield power spectra
of convection for spherical harmonic degrees up to l = 1000. Each
spectrum exhibits an obvious supergranulation peak at l 130 and a
broad secondary peak at l 600 with a distinct break in the spectrum
between these peaks at l 300. We believe that this secondary peak
is a signature of mesogranulation with typical cell diameters of
about 7 Mm. Our standard analysis procedure is to first remove the
p-mode oscillation signal by averaging individual Dopplergrams over
17-minute intervals. Next, by fitting to standard functional forms
we remove Doppler signals due to the motion of the spacecraft, the
convective blueshift, solar rotation including differential rotation,
and the meridional circulation in order to produce Dopplergrams
dominated by convective motions. By mapping these processed images
onto heliographic coordinates and projecting onto spherical harmonics,
we produce a power spectrum of solar convection for each 17-minute
period. We construct synthetic images and pass them through the same
analysis procedure in order to determine the actual solar convection
spectrum that reproduces the analyzed results. We find that a small
but increasing percentage of high-degree convective power is lost in
the analysis as we approach the limit of resolution of the detector
but that the broad, mesogranulation peak at l 600 must be included in
the convection spectrum of the synthetic images.
Title: 3-D Magnetic Field Configuration Late in a Large Two-Ribbon
Flare
Authors: Moore, R. L.; Schmieder, B.; Hathaway, D. H.; Tarbell, T. D.
Bibcode: 1997SoPh..176..153M
Altcode:
We present Hα and coronal X-ray images of the large two-ribbon flare
of 25-26 June, 1992 during its long-lasting gradual decay phase. From
these observations we deduce that the 3-D magnetic field configuration
late in this flare was similar to that at and before the onset of
such large eruptive bipolar flares: the sheared core field running
under and out of the flare arcade was S-shaped, and at least one
elbow of the S looped into the low corona. From previous observations
of filament-eruption flares, we infer that such core-field coronal
elbows, though rarely observed, are probably a common feature of the
3-D magnetic field configuration late in large two-ribbon flares. The
rare circumstance that apparently resulted in a coronal elbow of the
core field being visible in Hα in our flare was the occurrence of a
series of subflares low in the core field under the late-phase arcade
of the large flare; these subflares probably produced flaring arches
in the northern coronal elbow, thereby rendering this elbow visible
in Hα. The observed late-phase 3-D field configuration presented
here, together with the recent sheared-core bipolar magnetic field
model of Antiochos, Dahlburg, and Klimchuk (1994) and recent Yohkoh
SXT observations of the coronal magnetic field configuration at
and before the onset of large eruptive bipolar flares, supports the
seminal 3-D model for eruptive two-ribbon flares proposed by Hirayama
(1974), with three modifications: (1) the preflare magnetic field is
closed over the filament-holding core field; (2) the preflare core
field has the shape of an S (or backward S) with coronal elbows; (3)
a lower part of the core field does not erupt and open, but remains
closed throughout flare, and can have prominent coronal elbows. In
this picture, the rest of the core field, the upper part, does erupt
and open along with the preflare arcade envelope field in which it
rides; the flare arcade is formed by reconnection that begins in the
middle of the core field at the start of the eruption and progresses
from reconnecting closed core field early in the flare to reconnecting
`opened' envelope field late in the flare.
Title: Evaluation of Two Fractal Methods for Magnetogram Image
Analysis
Authors: Stark, B.; Adams, M.; Hathaway, D. H.; Hagyard, M. J.
Bibcode: 1997SoPh..174..297S
Altcode:
Fractal and multifractal techniques have been applied to various types
of solar data to study the fractal properties of sunspots as well as
the distribution of photospheric magnetic fields and the role of random
motions on the solar surface in this distribution. Other research
includes the investigation of changes in the fractal dimension as
an indicator for solar flares. Here we evaluate the efficacy of two
methods for determining the fractal dimension of an image data set:
the Differential Box Counting scheme and a new method, the Jaenisch
scheme. To determine the sensitivity of the techniques to changes in
image complexity, various types of constructed images are analyzed. In
addition, we apply this method to solar magnetogram data from Marshall
Space Flight Center's vector magnetograph.
Title: A Study of Magnetic Complexity Using HURST`S Rescaled Range
Analysis
Authors: Adams, M.; Hathaway, D. H.; Stark, B. A.; Musielak, Z. E.
Bibcode: 1997SoPh..174..341A
Altcode:
A fractal analysis using the classical Hurst method has been
applied to artificial data, simulated sunspot magnetic field data,
and to data acquired with NASA/Marshall Space Flight Center's
vector magnetograph. The main goals of this study are to quantify
the complexity of an active region and to determine if significant
changes in complexity are associated with flare activity. We tested the
analysis using three basic types of two-dimensional synthetic data: (1)
data composed of gaussians with various types of superimposed features,
(2) random data, and (3) synthetic sunspots created from a basic, simple
configuration on which are placed increasingly smaller structures. Our
results confirm that the Hurst method of analysis is sensitive to the
presence of large-scale structures within a two-dimensional image. When
the large-scale structure has been removed, the value of the Hurst
exponent is inversely proportional to increasing complexity in the
image. The Hurst exponent of magnetograph data with the large-scale
structure of the sunspot removed, shows a tantalizing variation in
the shear parameter five minutes prior to a flare.
Title: The Solar-B Mission
Authors: Antiochos, Spiro; Acton, Loren; Canfield, Richard; Davila,
Joseph; Davis, John; Dere, Kenneth; Doschek, George; Golub, Leon;
Harvey, John; Hathaway, David; Hudson, Hugh; Moore, Ronald; Lites,
Bruce; Rust, David; Strong, Keith; Title, Alan
Bibcode: 1997STIN...9721329A
Altcode:
Solar-B, the next ISAS mission (with major NASA participation), is
designed to address the fundamental question of how magnetic fields
interact with plasma to produce solar variability. The mission has
a number of unique capabilities that will enable it to answer the
outstanding questions of solar magnetism. First, by escaping atmospheric
seeing, it will deliver continuous observations of the solar surface
with unprecedented spatial resolution. Second, Solar-B will deliver the
first accurate measurements of all three components of the photospheric
magnetic field. Solar-B will measure both the magnetic energy driving
the photosphere and simultaneously its effects in the corona. Solar-B
offers unique programmatic opportunities to NASA. It will continue an
effective collaboration with our most reliable international partner. It
will deliver images and data that will have strong public outreach
potential. Finally, the science of Solar-B is clearly related to the
themes of origins and plasma astrophysics, and contributes directly
to the national space weather and global change programs.
Title: 3-D Magnetic Field Configuration Late in a Large Two-Ribbon
Flare
Authors: Moore, R. L.; Schmieder, B.; Hathaway, D. H.; Tarbell, T. D.
Bibcode: 1997SPD....28.0157M
Altcode: 1997BAAS...29R.889M
We present H-alpha and coronal X-ray images of the large two-ribbon
flare of 25/26 June 1992 during its long-lasting gradual decay
phase. From these observations we deduce that the 3-D magnetic field
configuration late in this flare was similar to that at and before the
onset of such large eruptive bipolar flares: the sheared core field
running under and out of the flare arcade was S-shaped, and at least one
elbow of the S looped into the low corona. From previous observations
of filament-eruption flares, we infer that such core-field coronal
elbows, though rarely observed, are probably a common feature of the
3-D magnetic field configuration late in large two-ribbon flares. The
rare circumstance that apparently resulted in a coronal elbow of the
core field being visible in H-alpha in our flare was the occurrence
of a series of subflares low in the core field under the late-phase
arcade of the large flare; these subflares probably produced flaring
arches in the northern coronal elbow, thereby rendering this elbow
visible in H-alpha. The observed late-phase 3-D field configuration
presented here, together with the recent sheared-core bipolar magnetic
field model of Antiochos, Dahlburg, and Klimchuk (1994) and recent
Yohkoh SXT observations of the coronal magnetic field configuration
at and before the onset of large eruptive bipolar flares, supports the
seminal 3-D model for eruptive two-ribbon flares proposed by Hirayama
(1974), with three modifications: (1) the preflare magnetic field is
closed over the filament-holding core field; (2) the preflare core
field has the shape of an S (or backward S) with coronal elbows; (3)
a lower part of the core field does not erupt and open, but remains
closed throughout flare, and can have prominent coronal elbows. In
this picture, the rest of the core field, the upper part, does erupt
and open along with the preflare arcade envelope field in which it
rides; the flare arcade is formed by reconnection that begins in the
middle of the core field at the start of the eruption and progresses
from reconnecting closed core field early in the flare to reconnecting
"opened" envelope field late in the flare.
Title: Multi-Wavelength Analysis of the March 26, 1991, Solar Flare
Authors: Hagyard, M. J.; Stark, B. A.; Hathaway, D. H.; Kurt, V. G.;
Akimov, V. V.
Bibcode: 1997SPD....28.0155H
Altcode: 1997BAAS...29R.889H
A large 4B/X5 solar flare erupted on the Sun on March 26, 1991, at 20:26
UT. The event was characterized by double peaks in emission in radio,
soft X-rays, and high energy (>100 Mev) gamma-ray wavelengths. The
Marshall Space Flight Center vector magnetograph obtained magnetic
field data prior to and during this flare, and H-alpha images of the
flare were video taped from the H-alpha telescope coaligned with the
magnetograph. An examination of a time series of vector magnetograms
showed that emerging flux near a large filament was the probable
trigger of the flare; the H-alpha images showed the eruption of this
filament which did not reform following the flare. We analyzed the
H-alpha video images for the H-alpha emission time profile so as to
determine the H-alpha counterparts to the two energetic events that
produced the double peaks in the high energy data. We also analyzed the
magnetic field data in those areas to ascertain the field morphology
associated with the two energetic events.
Title: Gauging the nearness and size of cycle minimum.
Authors: Wilson, Robert M.; Hathaway, David H.; Reichmann, Edwin J.
Bibcode: 1997msfc.rept.....W
Altcode:
Because the first occurrence of a new cycle, high-latitude (≥25°)
spot has always preceded conventional onset of the new cycle by at
least 3 months (for the data-available interval of cycles 12 - 22),
conventional onset for cycle 23 is not expected until about August 1996
or later, based on the first occurrence of a new cycle 23, high-latitude
spot during the decline of old cycle 22 in May 1996. Although much
excitement for an earlier-occurring minimum (about March 1996) for
cycle 23 was voiced earlier this year, the present study shows that
this exuberance is unfounded. The decline of cycle 22 continues to
favor cycle 23 minimum sometime during the latter portion of 1996 to
the early portion of 1997.
Title: On the behavior of the sunspot cycle near minimum
Authors: Wilson, Robert M.; Hathaway, David H.; Reichmann, Edwin J.
Bibcode: 1996JGR...10119967W
Altcode:
The decline of cycle 22 is shown to be consistent with the notion
that it will have a period <11 years. On the basis of the modern
era of sunspot cycles, the average length of short-period cycles
has been 123+/-3 months, suggesting that onset for cycle 23 will be
about December 1996 (+/-3 months). As yet, no high-latitude (25° or
more) new cycle spots have been reported. Because the occurrence of a
high-latitude new cycle spot group has always preceded conventional
cycle onset by at least 3 months, one infers that its occurrence is
most imminent.
Title: On determining the rise, size, and duration classes of a
sunspot cycle.
Authors: Wilson, Robert M.; Hathaway, David H.; Reichmann, Edwin J.
Bibcode: 1996msfc.reptR....W
Altcode:
The authors investigate the interrelationships of ascent duration,
maximum amplitude, period, and cycle numberedness (even-odd) for cycles
1 to 21. Based on the inferred correlations and the early behavior of
cycle 22, they characterize cycle 22 in terms of its probable rise,
size, and duration classes. Application to cycle 23 is also given.
Title: On the importance of cycle minimum in sunspot cycle prediction.
Authors: Wilson, Robert M.; Hathaway, David H.; Reichmann, Edwin J.
Bibcode: 1996msfc.reptQ....W
Altcode:
Several features of the sunspot cycle that relate to cycle minimum and
to the prediction of sunspot maximum are examined, in particular with
application to cycle 23.
Title: GONG Observations of Solar Surface Flows
Authors: Hathaway, D. H.; Gilman, P. A.; Harvey, J. W.; Hill, F.;
Howard, R. F.; Jones, H. P.; Kasher, J. C.; Leibacher, J. W.; Pintar,
J. A.; Simon, G. W.
Bibcode: 1996Sci...272.1306H
Altcode:
Doppler velocity observations obtained by the Global Oscillation Network
Group (GONG) instruments directly measure the nearly steady flows in
the solar photosphere. The sun's differential rotation is accurately
determined from single observations. The rotation profile with respect
to latitude agrees well with previous measures, but it also shows a
slight north-south asymmetry. Rotation profiles averaged over 27-day
rotations of the sun reveal the torsional oscillation signal-weak,
jetlike features, with amplitudes of 5 meters per second, that are
associated with the sunspot latitude activity belts. A meridional
circulation with a poleward flow of about 20 meters per second is
also evident. Several characteristics of the surface flows suggest
the presence of large convection cells.
Title: GONG Observations of Solar Surface Flows
Authors: Hathaway, D. H.; Gilman, P. A.; Jones, H. P.; Kasher, J.;
Simon, G. W.; GONG Nearly Steady Flows Team; GONG Magnetic Fields Team
Bibcode: 1996AAS...188.5304H
Altcode: 1996BAAS...28..903H
Doppler velocity observations obtained by the GONG instruments directly
measure the nearly steady flows in the solar photosphere. The Sun's
differential rotation profile is accurately determined from single
observations. This profile is well represented by a fourth order
polynomial which includes a rapidly rotating equator and a slight
north-south asymmetry. Rotation profiles averaged over 27 day rotations
of the Sun are sufficient to reveal the torsional oscillation signal -
weak, 5 m/s, jet-like features associated with the sunspot latitude
activity belts. A meridional circulation with poleward flow of about 20
m/s is also found from single observations and its spatial structure
is well determined. Several of the observed characteristics of the
surface flows suggest the presence of large convection cells. The
convection spectrum is measured and found to have peak power for cells
with wavelengths of about 50,000 km but the spectrum extends to much
larger wavelengths. Day-to-day variations in the observed structure of
the differential rotation and meridional circulation profiles indicate
the presence of large-scale, nonaxisymmetric velocity signals which may
be of solar origin. Studies correlating the convective flow patterns on
consecutive days also indicate the presence of large cellular patterns
that rotate at the Sun's rotation rate.
Title: The Solar Acoustic Spectrum and Eigenmode Parameters
Authors: Hill, F.; Stark, P. B.; Stebbins, R. T.; Anderson, E. R.;
Antia, H. M.; Brown, T. M.; Duvall, T. L., Jr.; Haber, D. A.;
Harvey, J. W.; Hathaway, D. H.; Howe, R.; Hubbard, R. P.; Jones,
H. P.; Kennedy, J. R.; Korzennik, S. G.; Kosovichev, A. G.; Leibacher,
J. W.; Libbrecht, K. G.; Pintar, J. A.; Rhodes, E. J., Jr.; Schou, J.;
Thompson, M. J.; Tomczyk, S.; Toner, C. G.; Toussaint, R.; Williams,
W. E.
Bibcode: 1996Sci...272.1292H
Altcode:
The Global Oscillation Network Group (GONG) project estimates
the frequencies, amplitudes, and linewidths of more than 250,000
acoustic resonances of the sun from data sets lasting 36 days. The
frequency resolution of a single data set is 0.321 microhertz. For
frequencies averaged over the azimuthal order m, the median formal
error is 0.044 microhertz, and the associated median fractional error
is 1.6 x 10-5. For a 3-year data set, the fractional error
is expected to be 3 x 10-6. The GONG m-averaged frequency
measurements differ from other helioseismic data sets by 0.03 to 0.08
microhertz. The differences arise from a combination of systematic
errors, random errors, and possible changes in solar structure.
Title: Doppler Measurements of the Sun's Meridional Flow
Authors: Hathaway, David H.
Bibcode: 1996ApJ...460.1027H
Altcode:
Doppler velocity data obtained with the Global Oscillation Network
Group (GONG) instruments in Tucson from 1992 August through 1995 April
were analyzed to determine the structure and evolution of the Sun's
meridional flow. Individual measurements of the flow were derived
from line-of-sight velocity images averaged over 17 minutes to remove
the p-mode oscillation signal. Typical flow velocities are poleward
at ∼20 m s-1, but the results suggest that episodes may
occur with much stronger flows. Such variations may help to explain
some of the many disparate reports on the strength and structure of
the Sun's meridional flow.
Title: Prelude to Cycle 23: The Case for a Fast-Rising, Large
Amplitude Cycle
Authors: Wilson, Robert M.; Hathaway, David H.; Reichmann, Edwin J.
Bibcode: 1996nasa.reptR....W
Altcode:
For the common data-available interval of cycles 12 to 22, we show
that annual averages of sunspot number for minimum years (R(min)) and
maximum years (R(max)) and of the minimum value of the aa geomagnetic
index in the vicinity of sunspot minimum (aa(min)) are consistent
with the notion that each has embedded within its respective record a
long-term, linear, secular increase. Extrapolating each of these fits
to cycle 23, we infer that it will have R(min) = 12.7 +/- 5.7, R(max)
= 176.7 +/- 61.8, and aa(min) = 21.0 +/- 5.0 (at the 95-percent level
of confidence), suggesting that cycle 23 will have R(min) greater
than 7.0, R(max) greater than 114.9, and aa(min) greater than 16.0
(at the 97.5-percent level of confidence). Such values imply that
cycle 23 will be larger than average in size and, consequently (by the
Waidmeier effect), will be a fast riser. We also infer from the R(max)
and aa(min) records the existence of an even- odd cycle effect, one in
which the odd-following cycle is numerically larger in value than the
even-leading cycle. For cycle 23, the even-odd cycle effect suggests
that R(max) greater than 157.6 and aa(min) greater than 19.0, values
that were recorded for cycle 22, the even-leading cycle of the current
even-odd cycle pair (cycles 22 and 23). For 1995, the annual average
of the aa index measured about 22, while for sunspot number, it was
about 18. Because aa(min) usually lags R(min) by 1 year (true for 8 of
11 cycles) and 1996 seems destined to be the year of R(min) for cycle
23, it may be that aa(min) will occur in 1997, although it could occur
in 1996 in conjunction with R(min) (true for 3 of 11 cycles). Because
of this ambiguity in determining aa(min), no formal prediction based
on the correlation of R(max) against aa(min), having r = 0.90, or of
R(max) against the combined effects of R(min) and aa(min)-the bivariate
technique-having r = 0.99, is possible until 1997, at the earliest.
Title: Observing large-scale solar surface flows with GONG:
Investigation of a key element in solar activity buildup
Authors: Beck, John G.; Simon, George W.; Hathaway, David H.
Bibcode: 1996msfc.rept.....B
Altcode:
The Global Oscillation Network Group (GONG) solar telescope network has
begun regular operations, and will provide continuous Doppler images
of large-scale nearly-steady motions at the solar surface, primarily
those due to supergranulation. Not only the Sun's well-known magnetic
network, but also flux diffusion, dispersal, and concentration at the
surface appear to be controlled by supergranulation. Through such
magnetoconvective interactions, magnetic stresses develop, leading
to solar activity. We show a Doppler movie made from a 45.5 hr time
series obtained 1995 May 9-10 using data from three of the six GONG
sites (Learmonth, Tenerife, Tucson), to demonstrate the capability of
this system.
Title: Observing Large-Scale Solar Surface Flows with GONG:
Investigation of a Key Element in Solar Activity Buildup
Authors: Beck, John C.; Hathaway, David H.; Simon, George W.
Bibcode: 1996ASPC...95..196B
Altcode: 1996sdit.conf..196B
No abstract at ADS
Title: Evolution of the Fractal Dimension in a Flaring Active Region
Authors: Adams, M.; Hathaway, D. H.; Musielak, Z. E.
Bibcode: 1995SPD....26.1016A
Altcode: 1995BAAS...27..980A
No abstract at ADS
Title: Klein-Gordon Equations for Acoustic Waves and Their
Applications in Helioseismology
Authors: Neergaard, L. F.; Musielak, Z. E.; Hathaway, D. H.
Bibcode: 1995SPD....26..401N
Altcode: 1995BAAS...27..954N
No abstract at ADS
Title: Temporal Variations of the Sun's Meridional Flow
Authors: Hathaway, D. H.
Bibcode: 1995SPD....26..103H
Altcode: 1995BAAS...27..949H
No abstract at ADS
Title: Journey to the heart of the Sun.
Authors: Hathaway, D. H.
Bibcode: 1995Ast....23...38H
Altcode:
Energy generated in the Sun's core takes a million years to reach its
surface. The author sketches the internal structure of the Sun.
Title: Nearly Steady Flows in GONG Prototype Data
Authors: Hathaway, D. H.
Bibcode: 1995ASPC...76..204H
Altcode: 1995gong.conf..204H
No abstract at ADS
Title: The Shape of the Sunspot Cycle
Authors: Hathaway, David H.; Wilson, Robert M.; Reichmann, Edwin J.
Bibcode: 1994SoPh..151..177H
Altcode:
The temporal behavior of a sunspot cycle, as described by the
International sunspot numbers, can be represented by a simple
function with four parameters: starting time, amplitude, rise time,
and asymmetry. Of these, the parameter that governs the asymmetry
between the rise to maximum and the fall to minimum is found to
vary little from cycle to cycle and can be fixed at a single value
for all cycles. A close relationship is found between rise time
and amplitude which allows for a representation of each cycle by a
function containing only two parameters: the starting time and the
amplitude. These parameters are determined for the previous 22 sunspot
cycles and examined for any predictable behavior. A weak correlation is
found between the amplitude of a cycle and the length of the previous
cycle. This allows for an estimate of the amplitude accurate to within
about 30% right at the start of the cycle. As the cycle progresses,
the amplitude can be better determined to within 20% at 30 months and
to within 10% at 42 months into the cycle, thereby providing a good
prediction both for the timing and size of sunspot maximum and for
the behavior of the remaining 7-12 years of the cycle.
Title: Producing the solar dynamo
Authors: Hathaway, D. H.
Bibcode: 1994EOSTr..75..548H
Altcode:
This article is part of a series that investigates topics in space
physics and aeronomy. The solar dynamo is the process by which the
Sun's magnetic field is generated through the interaction of the field
with convection and rotation. In this, it is kin to planetary dynamos
and other stellar dynamos. Although the precise mechanism by which
the Sun generates its field remains poorly understood despite decades
of theoretical and observational work, recent advances suggest that
solutions to this solar dynamo problem may be forthcoming.
Title: The solar dynamo
Authors: Hathaway, David H.
Bibcode: 1994STIN...9611025H
Altcode:
The solar dynamo is the process by which the Sun's magnetic field is
generated through the interaction of the field with convection and
rotation. In this, it is kin to planetary dynamos and other stellar
dynamos. Although the precise mechanism by which the Sun generates its
field remains poorly understood in spite of decades of theoretical and
observational work, recent advances suggest that solutions to this solar
dynamo problem may be forthcoming. The two basic processes involved
in dynamo activity are demonstrated and the Sun's activity effects
are presented in this document, along with a historical perspective
regarding solar dynamos and the efforts to understand and measure them.
Title: Nearly steady flows in GONG prototype data
Authors: Hathaway, David H.
Bibcode: 1993STIN...9530863H
Altcode:
Doppler velocity images obtained with the GONG prototype
instrument were analyzed to measure the nearly steady photospheric
flows. The data consists of 88 images each of velocity, intensity,
and modulation obtained at 20:00 UT on 88 days from July 1992
to February 1994. Each velocity image was temporally filtered to
remove the p-mode oscillations, masked to exclude active regions,
and then analyzed using spherical harmonics and orthogonal functions
as described by Hathaway (1992). The spectral coefficients show very
consistent results for the entire time interval with some evidence
of year-to-year variations. The rotation profile agrees well with
previous results and exhibits a north-south asymmetry that reverses
sign during the 20 month interval. The residual rotation velocities
exhibit structures with amplitudes of approximately 5 m/s that may
be related to torsional oscillations. The meridional circulation is
directed from the equator toward the poles with a peak velocity in
the photosphere of approximately 50 m/s. The higher order components
are very weak but indicate a divergent flow from the mid-latitudes
(opposite that found for the June 1989 data). The convective limb
shift is well fit by a 3rd order polynomial. The convection spectrum
has a prominent peak at spherical harmonic degrees of l approximately
150 with very little signal in the low degree modes. Analysis of this
signal shows that there is no evidence for giant cell convection at
the level of approximately 10 m/s for all modes up to l = 32.
Title: The Shape of the Solar Sunspot Cycle
Authors: Hathaway, D. H.; Wilson, R. M.; Reichmann, E. J.
Bibcode: 1993BAAS...25R1216H
Altcode:
No abstract at ADS
Title: Measurement of p-Mode Energy Propagation in the Quiet Solar
Photosphere
Authors: Fontenla, J. M.; Rabin, D.; Hathaway, D. H.; Moore, R. L.
Bibcode: 1993ApJ...405..787F
Altcode:
We have measured and analyzed the p-mode oscillations in the profile
of the Mg I 4571 A line in a quiet region near disk center. The
oscillations are found to be mostly standing waves, in agreement with
previous work. However, a small propagating component is measured, and
we determine the direction, magnitude, and vertical variation of the
energy propagation. The work integral indicates an upward energy flow of
about 2 x 10 exp 7 ergs/sq cm/s at a height of 50 km above the base of
the photosphere for waves with frequencies of 2-16 mHz. This energy flow
decreases exponentially with height and drops below 10 exp 5 ergs/sq
cm/s in the uppermost photosphere. The energy flow leaving the upper
photosphere is at least an order of magnitude too small to constitute a
significant source of heating for the chromosphere. However, the p-mode
damping in the lower photosphere approaches levels large enough to
account for the measured p-mode line widths. The relative amplitudes
and phases of the thermodynamic quantities indicate that the p-mode
are neither adiabatic nor isothermal in the photosphere.
Title: Doppler Measurement of the Solar Meridional Circulation
Authors: Hathaway, D. H.
Bibcode: 1993ASPC...42..265H
Altcode: 1993gong.conf..265H
No abstract at ADS
Title: Artificial Data for Testing Helioseismology Algorithms
Authors: Bogart, R. S.; Hill, F.; Toussaint, R.; Hathaway, D. H.;
Duvall, T. L., Jr.
Bibcode: 1993ASPC...42..429B
Altcode: 1993gong.conf..429B
No abstract at ADS
Title: Doppler Measurement of the Solar Meridional Circulation
Authors: Hathaway, D. H.
Bibcode: 1992AAS...180.0601H
Altcode: 1992BAAS...24Q.736H
No abstract at ADS
Title: Spherical Harmonic Analysis of Steady Photospheric Flows -
Part Two
Authors: Hathaway, David H.
Bibcode: 1992SoPh..137...15H
Altcode:
The use of the spherical harmonic functions to analyse the nearly
steady flows in the solar photosphere is extended to situations in
which B0, the latitude at disk center, is nonzero and
spurious velocities are present. The procedures for extracting the
rotation profile and meridional circulation are altered to account for
the seasonal tilt of the Sun's rotation axis toward and away from the
observer. A more robust and accurate method for separating the limb
shift and meridional circulation signals is described. The analysis
procedures include the ability to mask out areas containing spurious
velocities (velocity-like signals that do not represent true flow
velocities in the photosphere). The procedures are shown to work well
in extracting the various flow components from realistic artificial
data with a broad, continuous spectrum for the supergranulation. The
presence of this supergranulation signal introduces errors of a few m
s -1 in the measurements of the rotation profile, meridional
circulation, and limb shift from a single Doppler image. While averaging
the results of 24 hourly measurements has little effect in reducing
these errors, an average of 27 daily measurements reduces the errors
to well under 1 m s -1.
Title: Rotation Rate of the Supergranulation Pattern
Authors: Hathaway, D. H.; Rhodes, E. J., Jr.; Korzennik, S.;
Cacciani, A.
Bibcode: 1991BAAS...23.1051H
Altcode:
No abstract at ADS
Title: How Deeply Might Sunspots and Supergranules Be Anchored within
the Sun?
Authors: Rhodes, E. J., Jr.; Korzennik, S. G.; Hathaway, D. H.;
Cacciani, A.
Bibcode: 1991BAAS...23.1033R
Altcode:
No abstract at ADS
Title: The Supergranulation Spectrum
Authors: Hathaway, David H.; Rhodes, Edward J.; Cacciani, Alessandro;
Korzennik, Sylvain G.
Bibcode: 1991LNP...388..163H
Altcode: 1991ctsm.conf..163H
Full-disk Dopplergram observations obtained at the 60-foot
tower of the Mount Wilson Observatory with the Cacciani sodium
magneto-optical filter were analyzed to determine the spectrum of
the solar supergranulation. Individual Dopplergrams were averaged
together using a weighted average over 20-minute intervals to remove
the p-mode oscillations. The Doppler signals due to the motion of the
observer, the solar rotation, differential rotation, and limb shift
were then removed from the data to produce Dopplergrams dominated
by the supergranular flows. These data were mapped to heliographic
coordinates and projected onto the spherical harmonics. The resulting
spectrum exhibits a peak at spherical harmonic degree } 100, which
corresponds to typical cell diameters of about 40 Mm. Synthetic data
were constructed and passed through the same analysis procedures to
determine the actual spectrum required to reproduce the results. A
good fit was obtained with a kinetic energy spectrum which peaks at }
100 and decreases exponentially out to } 500 with an e-folding range
of } 90. A power law fit to the spectrum over this range in yields an
exponent of about -2.75. No corrections for seeing were included in
the analysis. Although the image was sampled at 8 arcsec resolution,
the effects of seeing may alter the actual slope of the spectrum and
make it somewhat flatter. The spectrum does not exhibit any evidence
for a distinct mesogranulation component out to } 500 (corresponding
to cell diameters of about 8 Mm).
Title: Solar Rotation and the Sunspot Cycle
Authors: Hathaway, David H.; Wilson, Robert M.
Bibcode: 1990ApJ...357..271H
Altcode:
Reexamination of the published sunspot rotation rates from Mount Wilson
for the period from 1921 to 1982 suggests that the sun rotates more
rapidly when there are fewer sunspots. This behavior is seen over the
course of each cycle with the most rapid rotation usually observed at
sunspot minimum. It is also seen in hemispheric differences with the
southern hemisphere, having fewer spots, rotating more rapidly than
the northern hemisphere. Furthermore, the rotation rate averaged over
each cycle also shows that the sun rotates more rapidly during cycles
with fewer sunspots and less sunspots area. This inverse correlation
between sunspot area and rotation rate suggests that during the Maunder
minimum the sun may have rotated slightly faster than is observed today.
Title: Analysis of a 116 Year Record of Sunspot Positions and Sizes
Authors: Hathaway, D. H.; Harvey, K. L.
Bibcode: 1990BAAS...22..873H
Altcode:
No abstract at ADS
Title: Measurement of Dissipation or Pumping of P-Modes in the
Solar Photosphere
Authors: Fontenla, J. M.; Hathaway, D. H.; Rabin, D.; Moore, R.
Bibcode: 1990BAAS...22..856F
Altcode:
No abstract at ADS
Title: Evaluation of Magnetic Shear in Off-Disk Center Active Regions
Authors: Venkatakrishnan, P.; Hagyard, M. J.; Hathaway, D. H.
Bibcode: 1989SoPh..122..215V
Altcode:
We analyze the changes that projection effects produce in the evaluation
of magnetic shear in off-disk center active regions by comparing angular
shear calculated in image plane and heliographic coordinates. We
describe the procedure for properly evaluating magnetic shear by
transforming the observed vector magnetic field into the heliographic
system and then apply this procedure to evaluate magnetic shear along
the magnetic neutral line in an active region that was observed on 1984
April 24 at a longitude offset of -45°. In particular, we show that
the number of `critically sheared' pixels along an east-west directed
segment of the neutral line in the leader sunspot group changes from 16
in the image plane magnetogram to 14 in the heliographic magnetogram. We
also show that the critical shear as calculated in the image plane
served as a good predictor for the location of flaring activity since
the flare ribbons of the great flare of April 24 bracketed the inversion
line where the critical shear was located. These results indicate that
for this particular region, projection effects did not significantly
affect the evaluation of critical shear.
Title: Spectrum Lifetime, and Rotation Rate of Supergranules
Authors: Hathaway, D. H.; Rhodes, E. J., Jr.; Cacciani, A.;
Korzennik, S.
Bibcode: 1989BAAS...21..829H
Altcode:
No abstract at ADS
Title: Solar Rotation and the Sunspot Cycle
Authors: Wilson, R. M.; Hathaway, D. H.
Bibcode: 1989BAAS...21..843W
Altcode:
No abstract at ADS
Title: Evaluation of Magnetic Shear in Off-Disk Center Active Regions
Authors: Hagyard, M. J.; Hathaway, D. H.; Venkatakrishnan, P.
Bibcode: 1989BAAS...21..838H
Altcode:
No abstract at ADS
Title: The GONG data reduction and analysis system.
Authors: Pintar, J. A.; Andersen, B.; Anderson, E. R.; Armet, D. B.;
Brown, T. M.; Hathaway, D. H.; Hill, F.; Jones, H. P.; GONG Data Team
Bibcode: 1988ESASP.286..217P
Altcode:
Each of the six GONG observing stations will produce three, 16-bit,
256×256 images of the Sun every 60 seconds of sunlight. These
data will be transferred from the observing sites to the GONG Data
Management and Analysis Center (DMAC), in Tucson, on high-density tapes
at a combined rate of over 1 gigabyte per day. The contemporaneous
processing of these data will produce several standard data products
and will require a sustained throughput in excess of 7 megaflops. Peak
rates may exceed 50 megaflops. Archives will accumulate at the rate
of approximately 1 terabyte per year, reaching nearly 3 terabytes in
three years of observing. Researchers will access the data products
with a machine-independent GONG Reduction and Analysis Software
Package (GRASP). Based on the Image Reduction and Analysis Facility
(IRAF), this package will include database facilities and helioseismic
analysis tools. Users may access the data as visitors in Tucson, or
may access DMAC remotely through networks, or may process subsets of
the data at their local institutions using GRASP or other systems of
their choice. Elements of the system will reach the prototype stage
by the end of 1988. Full operation is expected in 1992 when data
acquisition begins.
Title: Simulating Photospheric Doppler Velocity Fields
Authors: Hathaway, David H.
Bibcode: 1988SoPh..117..329H
Altcode:
A method is described for constructing artificial data that
realistically simulate photospheric velocity fields. The velocity
fields include rotation, differential rotation, meridional circulation,
giant cell convection, supergranulation, convective limb shift, p-mode
oscillations, and observer motion. Data constructed by this method can
be used for testing algorithms designed to extract and analyze these
velocity fields in real Doppler velocity data.
Title: Elimination of Projection Effects from Vector Magnetograms -
the Pre-Flare Configuration of Active Region AR:4474
Authors: Venkatakrishnan, P.; Hagyard, M. J.; Hathaway, D. H.
Bibcode: 1988SoPh..115..125V
Altcode:
We demonstrate a simple method of transforming vector magnetograms
to heliographic coordinates. The merits of this transformation are
illustrated using a vector magnetogram obtained with the MSFC vector
magnetograph 80 minutes prior to a white light flare in active region AR
4474 on 25 April, 1984. The original magnetogram shows strong magnetic
shear along the neutral line at both the flare site and a non-flaring
site. The transformation of the magnetogram to heliographic coordinates
shows that the elimination of projection effects results in a much
shorter length of the sheared region at the non-flaring site than
what is inferred from the image plane vector magnetogram. The length
of the sheared region at the flare site is relatively less affected
by the transformation.
Title: Temporal Filters for Analyzing Steady Photospheric Flows
Authors: Hathaway, D. H.
Bibcode: 1988BAAS...20..683H
Altcode:
No abstract at ADS
Title: Temporal Filters for Isolating Steady Photospheric Flows
Authors: Hathaway, David H.
Bibcode: 1988SoPh..117....1H
Altcode:
A variety of temporal filters are tested on artificial data with 60
and 75 s sampling intervals to determine their accuracy in separating
the nearly-steady photospheric flows from the p-mode oscillations
in Doppler velocity data. Longer temporal averages are better
at reducing the residual signal due to p-modes but they introduce
additional errors from the rotation of the supergranule pattern across
the solar disk. Unweighted filters (boxcar averages) leave residual
r.m.s. errors of about 6 m s−1 from the p-modes after 60
min of averaging. Weighted filters, with nearly Gaussian shapes, leave
similar residual errors after only 20 min of averaging and introduce
smaller errors from the rotation of the supergranule pattern. The best
filters found are weighted filters that use data separated by 150
or 120 s so that the p-modes are sampled at opposite phases. These
filters achieve an optimum error level after about 20 min, with the
r.m.s. errors due to the p-mode oscillations and the rotation of the
supergranules both at a level of only 1.5 m s−1.
Title: Asymmetry of the heliosphere
Authors: Suess, S. T.; Hathaway, D. H.; Dessler, A. J.
Bibcode: 1987GeoRL..14..977S
Altcode:
The outflowing solar wind interacts with the local interstellar
medium to form the heliospheric cavity within which the solar wind is
supersonic. Because the interstellar medium is moving with respect
to the Sun, and because the solar wind has a latitude dependence,
the heliosphere is asymmetric. The flow of the interstellar medium
past the heliosphere produces an asymmetry because of the Bernoulli
effect, which draws the heliosphere out in a direction orthogonal to
the upstream-downstream axis, and because of a viscous interaction,
which draws out the heliosphere downstream. We consider a variety of
cases and find the effects to be significant with, typically, the
upstream direction having a heliospheric dimension that is 2/3 the
downstream dimension. Suggestions have been put forth to the effect
that a spacecraft penetration of the heliospheric shock wave may be
imminent. Because one of the most distant spacecraft is moving roughly
in the upstream direction relative to the interstellar flow, and the
other is moving in the downstream direction, the distance to their
encounters with the heliospheric shock may differ by as much as 40 AU.
Title: Spherical Harmonic Analysis of Steady Photospheric
Flows-Effects due to Nonzero B-angles, Limited Spatial Resolution
and Limited Spatial Coverage
Authors: Hathaway, D. H.
Bibcode: 1987BAAS...19R.935H
Altcode:
No abstract at ADS
Title: Spherical Harmonic Analysis of Steady Photospheric Flows
Authors: Hathaway, David H.
Bibcode: 1987SoPh..108....1H
Altcode:
Steady photospheric flows can be represented by a spectrum of
spherical harmonic modes. A technique is described in which full
disc doppler velocity measurements are analysed using the spherical
harmonic functions to determine the characteristics of this spectrum
and the nature of these flows. Synthetic data is constructed for
testing this technique. This data contains limb shift, rotation,
differential rotation, meridional circulation, supergranules, giant
cells and various levels of noise.
Title: Thermal convection in a rotating shear flow
Authors: Hathaway, David H.; Somerville, Richard C. J.
Bibcode: 1987GApFD..38...43H
Altcode:
The dynamics of thermal convection in a rotating shear flow was
studied using a three-dimensional time-dependent numerical model
of the flow. The model assumes a sheared zonal wind in the form of
an atmospheric jet stream and considers convective motions imbedded
in this flow. It was found that rotation plays a vital role in the
dynamics. Without rotation, the convective motions extract energy
and momentum from the mean zonal flow. With rotation, the convective
motions feed the energy and momentum into the mean flow. The results
show how the small-scale convection might influence the large-scale
dynamics of rotating stars and planets.
Title: A Spherical Harmonic Decomposition Technique for Analysing
Steady Photospheric Flows
Authors: Hathaway, David H.
Bibcode: 1987ASSL..137..115H
Altcode: 1987isav.symp..115H
Steady flows in the photosphere, including differential rotation,
meridional circulation and convection, can be represented by a spectrum
of modes. A technique is described in which the spherical harmonic
functions are used to determine the characteristics of this spectrum and
the nature of these flows. Some information about the spectrum is lost
because only one hemisphere is seen, only the line of sight velocity is
measured and the measurements contain noise. This produces an apparent
mixing between spectral modes. By analysing synthetic data it is found
that, in spite of this mixing, differential rotation can be accurately
measured, meridional circulations with small amplitudes can be extracted
from the data and giant cells might be separated from supergranules.
Title: Convective forcing of global circulations on the Jovian planets
Authors: Hathaway, David H.
Bibcode: 1986joat.conf..144H
Altcode:
Examples of convection in rotating layers are presented to illustrate
how convection can drive global circulations on the Jovian planets. For
rapid rotation the convective motions become largely two-dimensional
and produce Reynold stresses which drive large scale flows. The initial
tendency is to produce a prograde equatorial jet and a meridional
circulation which is directed toward the poles in the surface
layers. Fully nonlinear numerical simulations for the slowly rotating
solar convection zone show that the meridional circulation does not
reach the poles. Instead a multicellular meridional circulation is
produced which has a downward flowing branch in the mid-latitudes. For
more rapidly rotating objects such as Jupiter and Saturn this meridional
circulation may consist of a larger number of cells. Axisymmetric
convective models then show that prograde jets form at the downflow
latitudes. A nonlinear numerical simulation of convection in a prograde
jet is presented to illustrate the interactions which occur between
convection and these jets. Without rotation the convection removes
energy and momentum from the jet. With rotation the convection feeds
energy and momentum into the jet.
Title: Magnetic reversals of Jupiter and Saturn
Authors: Hathaway, D. H.; Dessler, A. J.
Bibcode: 1986Icar...67...88H
Altcode:
We propose that the origin and behavior of the internal magnetic fields
of Jupiter and Saturn should be much like the Sun's. Jupiter and Saturn
are predominantly gaseous, they have significant internal heat sources,
and their surface angular rotation rates vary with latitude. There
is also empirical evidence showing that the rotation rates of their
magnetic fields vary with latitude. This differential rotation is
instrumental in producing solar-type dynamos, which are characterized
by quasi-periodic field reversals. When we apply the theory and scaling
parameters for the reversal period of the magnetic field of the Sun
to Jupiter and Saturn, we derive an estimate for the time interval
between magnetic reversals to be on the order of centuries. This time
scale is consistent with observed changes in Jupiter's magnetic field
over the last 2 decades.
Title: Spherical Harmonic Analysis of Steady Photospheric Flows
Authors: Hathaway, D. H.
Bibcode: 1986BAAS...18..702H
Altcode:
No abstract at ADS
Title: Nonlinear interactions between convection, rotation and flows
with vertical shear
Authors: Hathaway, D. H.; Somerville, R. C. J.
Bibcode: 1986JFM...164...91H
Altcode:
The effects of a mean flow with vertical shear on the convective
motions in a rotating layer are examined using a three-dimensional
and time-dependent numerical model. In the absence of rotation, the
convective motions are shown to be dominated by the shear flow when
the Richardson number becomes greater than about -1.0. Both heat and
momentum are carried down their respective gradients. For rotating
cases with vertical rotation vectors, the Coriolis force turns the
flow induced by the convection to produce a more complicated shear that
changes direction with height. For rotating cases with tilted rotation
vectors, the results depend on the direction of the shear. When the
imposed flow is in the opposite direction, the convection motions
are less energetic and are even suppressed entirely when the shear
is strong. When the imposed flow is in the same direction, as that
produced by the rotation, the convective motions are enhanced and a
countergradient flux of momentum can be produced.
Title: Convective forcing of global circulations on the Jovian
planets.
Authors: Hathaway, D. H.
Bibcode: 1986NASCP2441..144H
Altcode:
Two examples of convection in rotating layers are presented to
illustrate how convection can drive global circulations on the Jovian
planets. The first is from an analytical model that the author developed
with the hope of getting a handle on how to parameterize small scale
convection in a global circulation model. The second example is a
numerical simulation of convection in a zonal flow which illustrates the
interactions between the convection, rotation and a zonal shear flow.
Title: Jupiter and Saturn's Magnetic Differential Rotation and
Expected Periods for Magnetic Field Reversals
Authors: Hathaway, D. H.; Dessler, A. J.
Bibcode: 1985BAAS...17..925H
Altcode:
No abstract at ADS
Title: Numerical Simulation in Three Space Dimensions of
Time-Dependent Thermal Convection in a Rotating Fluid
Authors: Hathaway, D. H.; Somerville, R. C.
Bibcode: 1985LApM...22..309H
Altcode:
No abstract at ADS
Title: A Numerical Model of Convection in Jupiter's North Temperate
Belt
Authors: Hathaway, D. H.; Somerville, R. C. J.
Bibcode: 1984BAAS...16..640H
Altcode:
No abstract at ADS
Title: A Hydrodynamical Model for Sunspot Blocking
Authors: Hathaway, D. H.
Bibcode: 1984BAAS...16Q.412H
Altcode:
No abstract at ADS
Title: A convective model for turbulent mixing in rotating convection
zones
Authors: Hathaway, D. H.
Bibcode: 1984ApJ...276..316H
Altcode:
The effects of rotation are included in an analytical model for the
convective motions in a plane-parallel layer of an ideal fluid. The
turbulent stress tensor, formed by taking products and averages of
the various velocity components, is calculated for an arbitrary eddy
size and shape. Heuristic formulae presented for determining the size
and shape of the dominant eddy then give a fully specified stress
tensor. Applications for this stress tensor in problems of stellar
internal dynamics, heat flow, scalar diffusion, and dynamo theory
are suggested. The resultant stresses tend to produce differential
rotation profiles with rapidly rotating equators and interiors. The
dynamo activity associated with these convective motions tends to
occur near the lower boundary of the convection zone.
Title: A Hydrodynamical Model for Sunspot Blocking
Authors: Hathaway, D. H.
Bibcode: 1983BAAS...15R.950H
Altcode:
No abstract at ADS
Title: A Spherical Harmonic Decomposition of Photospheric Velocity
Fields
Authors: Hathaway, D. H.
Bibcode: 1982BAAS...14Q.939H
Altcode:
No abstract at ADS
Title: Nonlinear Simulations of Solar Rotation Effects in
Supergranules
Authors: Hathaway, D. H.
Bibcode: 1982SoPh...77..341H
Altcode:
Nonlinear calculations for the three-dimensional and time dependent
convective flow in a plane parallel layer of fluid are carried out with
parameter values appropriate for supergranules on the Sun. A rotation
vector is used which is tilted from the vertical to represent various
latitudes. For the incompressible fluid used in this model the solar
rotation produces turning motions sufficient to completely twist a
fluid column in about one day. It is suggested that this effect will
be greatly enhanced in a compressible fluid. The tilted rotation
vector produces anisotropies and systematic Reynolds stresses which
drive mean flows. The resulting flows produce a rotation rate which
increases inward and a meridional circulation with poleward flow along
the outer surface.
Title: Global Circulations Driven by Small Scale Convection:
3-Dimensional Simulations of Locally Driven Flows
Authors: Hathaway, D. H.; Somerville, R. C. J.
Bibcode: 1981BAAS...13..907H
Altcode:
No abstract at ADS
Title: Nonlinear Simulations of Rotational Effects in Supergranules
Authors: Hathaway, D. H.; Toomre, J.
Bibcode: 1980BAAS...12Q.894H
Altcode:
No abstract at ADS
Title: Axisymmetric Convection Driven by Latitudinal Temperature
Gradients in Rotating Spherical Shells.
Authors: Hathaway, D. H.; Gilman, P. A.; Miller, J.; Toomre, J.
Bibcode: 1980BAAS...12..686H
Altcode:
No abstract at ADS
Title: Convective instability when the temperature gradient and
rotation vector are oblique to gravity. II. Real fluids with effects
of diffusion
Authors: Hathaway, D. H.; Toomre, J.; Gilman, P. A.
Bibcode: 1980GApFD..15....7H
Altcode:
The linear stability analysis of Hathaway, Gilman and Toomre (1979)
(hereafter referred to as Paper I) is repeated for Boussinesq fluids
with viscous and thermal diffusion. As in Paper I the fluid is confined
between plane parallel boundaries and the rotation vector is oblique
to gravity. This tilted rotation vector introduces a preference
for roll-like disturbances whose axes are oriented north-south;
the preference is particularly strong in the equatorial region. The
presence of a latitudinal temperature gradient produces a thermal
wind shear which favors axisymmetric convective rolls if the gradient
exceeds some critical value. For vanishingly small diffusivities the
value of this transition temperature gradient approaches the inviscid
value found in Paper I. For larger diffusivities larger gradients are
required particularly in the high latitudes. These results are largely
independent of the Prandtl number. Diffusion tends to stabilize the
large wavenumber rolls with the result that a unique wavenumber can
be found at which the growth rate is maximized. These preferred rolls
have widths comparable to the depth of the layer and tend to be broader
near the equator. The axisymmetric rolls are similar in many respects
to the cloud bands on Jupiter provided they extend to a depth of about
15,000 km.
Title: Convective Instability in Rotating Layers with Thermal Winds
and Application to Jupiter
Authors: Hathaway, D. H.; Gilman, P. A.; Toomre, J.
Bibcode: 1979BAAS...11Q.618H
Altcode:
No abstract at ADS
Title: Convection in rotating layers with thermal winds and
application to Jupiter
Authors: Hathaway, D. H.
Bibcode: 1979STIN...8017943H
Altcode:
A linear stabilty analysis is carried out for fluid layers under
uniform rotation which possess both vertical and horizontal temperature
gradients. In order to represent various latitudes with these plane
parallel layers, a rotation vector is used which is generally oblique to
gravity. The preferred convective modes are assessed as a function of
latitude for both ideal and real fluid. It is found that the diffusive
effects of viscosity and thermal conductivity are relatively unimportant
in determining the preferred orientation of these connective rolls. It
is proposed that the axisymmetric cloud hands observed on the planet
Jupiter may be produced by these east-west convective instabilities. A
simple radiative-convective model for Jupiter is used to estimate the
parameter values relevant in the stability analysis.
Title: Convective Instability when the Temperature Gradient and
Rotation Vector are Oblique to Gravity. I. Fluids without Diffusion
Authors: Hathaway, D. H.; Toomre, J.; Gilman, P. A.
Bibcode: 1979GApFD..13..289H
Altcode:
No abstract at ADS
Title: Convection in Rotating Layers with Thermal Winds and
Application to Jupiter.
Authors: Hathaway, D. H.
Bibcode: 1979PhDT.........9H
Altcode:
A linear stability analysis is carried out for fluid layers under
uniform rotation which possess both vertical and horizontal temperature
gradients. In order to represent various latitudes with these
plane parallel layers, a rotation vector is used which is generally
oblique to gravity. The configuration is used to assess the preferred
convective modes as a function of latitude. Two major effects arise:
(1) the tilted rotation vector introduces a preference for roll
like disturbances with north-south orientations, the preference is
particularly strong in the equatorial regions, and, (2) the presence of
a north-south temperature gradient produces a thermal wind shear which
favors axisymmetric convective rolls oriented parallel to the flow in
an east-west direction. The axisymmetric cloud bands observed on the
planet Jupiter is suggested to be produced by the east-west convective
instabilities. A simple radiative-convective model for Jupiter is used
to estimate the parameter values relevant in the stability analysis.