Author name code: howard
ADS astronomy entries on 2022-09-14
author:"Howard, Robert F."
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Title: Periodic Solar Wind Density Structures Observed with Parker
Solar Probe WISPR
Authors: Viall, N. M.; Vourlidas, A.; Howard, R.; Linton, M.; Kepko,
L.; Di Matteo, S.; Higginson, A. K.
Bibcode: 2021AAS...23812305V
Altcode:
Periodic trains of mesoscale structures in solar wind density have been
observed close to the Sun with in situ data from the Helios spacecraft,
as well as remotely in STEREO/COR2 and STEREO/HI1 white light imaging
data. While some periodic density structures may be a consequence of
the development of dynamics en route, many are remnants of the formation
and release of the solar wind, and thus provide important constraints on
solar wind models. The instrument suite on Parker Solar Probe offers an
unprecedented viewpoint of the ambient solar wind and structure therein,
shortly after its formation and release from the solar corona. Here,
we report on the first observations of periodic trains of mesoscale
structures in solar wind density observed by the Wide-field Imager
for Parker Solar PRobe (WISPR). We describe our open-source Fourier
analysis and robust spectral background estimation technique used to
identify the periodic density structures. The observation of periodic
density structures so near to the Sun allows us to begin disentangling
how much structure is created during solar wind formation, versus how
much is due to evolution as the solar wind advects outward.
Title: Low-redshift Type Ia Supernova from the LSQ/LCO Collaboration
Authors: Baltay, C.; Grossman, L.; Howard, R.; Rabinowitz, D.; Arcavi,
I.; Barbour, N.; Burke, J.; Contreras, C.; Dilday, B.; Graham, M.;
Hiramatsu, D.; Hossenzadeh, G.; Howell, D. A.; McCully, C.; McKinnon,
R.; Ment, K.; Montesi, R.; Pellegrino, C.; Valenti, S.
Bibcode: 2021PASP..133d4002B
Altcode:
This paper is the data release of a new sample of 140 type Ia
supernovae (SNe Ia) from the LaSilla-QUEST/Las Cumbres Observatory
(LCO) collaboration. The discovery of the supernovae came from the
LaSilla-QUEST variability survey, the ASASSN survey, as well as smaller
low redshift supernova surveys. All of the supernovae in this sample
were spectroscopically identified as SNe Ia using spectra from the
PESSTO survey using the 3.5 m NTT telescope at LaSilla and spectra from
the LCO 2 m Faulkes telescopes. The light-curves were obtained from a
rapid cadence photometric follow up of the supernovae with the 9 LCO 1 m
telescopes located at various observatories around the globe. Reference
images of the host galaxies were taken approximately a year after the
supernova have faded to allow precise galaxy background subtraction
from the supernova magnitudes. The supernovae in this sample were
discovered over a seven year period from 2012 October to 2019 June,
and the last galaxy reference images were taken before 2020 June.
Title: VizieR Online Data Catalog: EvryFlare. I. Cool stars's flares
in southern sky (Howard+, 2019)
Authors: Howard+; Howard, W. S.; Corbett, H.; Law, N. M.; Ratzloff,
J. K.; Glazier, A.; Fors, O.; Del Ser, D.; Haislip, J.
Bibcode: 2020yCat..18810009H
Altcode:
The Evryscope is an array of small telescopes simultaneously imaging
8150 square degrees and 18400 square degrees in total each night on the
sky at two minute cadence in g'. The Evryscope is optimized for bright,
nearby stars, with a typical dark-sky limiting magnitude of g'=16. The
Evryscope is designed to observe the entire Southern sky down to an
airmass of two and at a resolution of 13"/pixel. (2 data files).
Title: Imaging the Solar Corona From Within
Authors: Hess, P.; Howard, R.; Vourlidas, A.; Bothmer, V.; Colaninno,
R.; DeForest, C.; Gallagher, B.; Hall, J. R.; Higginson, A.; Korendyke,
C.; Kouloumvakos, A.; Lamy, P.; Liewer, P.; Linker, J.; Linton, M.;
Penteado, P.; Plunkett, S.; Poirer, N.; Raouafi, N.; Rich, N.; Rochus,
P.; Rouillard, A.; Socker, D.; Stenborg, G.; Thernisien, A.; Viall, N.
Bibcode: 2020AAS...23514907H
Altcode:
Parker Solar Probe (PSP), launched, in August 2018 is humanity's
first probe of a stellar atmosphere. It will make measurements of
the near-Sun plasma from 'within' the outer corona with gradually
reduced perihelia from its first perihelia of 35 Rs in 2018-19 to 9.8
Rs in 2025. Here we report the results from the imaging observations
of the electron and dust corona, whe PSP was 35-54 Rs from the solar
surface, taken by the Wide-field Imager for Solar Probe (WISPR). The
spacecraft was near-corotating with the solar corona throughout the
observing window, which is an unprecedented situation for any type of
coronal imaging. Our initial analysis uncovers a long-hypothesized
depletion of the primordial dust orbiting near the Sun, reveals the
plasma structure of small-scale ejections, and provides a strict test
for validating model predictions of the large-scale configuration of
the coronal plasma. Thus, WISPR imaging allows the study of near-Sun
dust dynamics as the mission progresses. The high-resolution images
of small transients, largely unresolved from 1 AU orbits, unravel
the sub-structures of small magnetic flux ropes and show that the
Sun continually releases helical magnetic fields in the background
wind. Finally, WISPR's observations of the coronal streamer evolution
confirm the large-scale topology of the solar corona but they also
reveal that, as recently predicted, streamers are composed of yet
smaller sub-streamers channeling continual density fluctuations at
all visible scales.
Title: Simulating White Light Images of Coronal Structures for WISPR/
Parker Solar Probe: Effects of the Near-Sun Elliptical Orbit
Authors: Liewer, P.; Vourlidas, A.; Thernisien, A.; Qiu, J.; Penteado,
P.; Nisticò, G.; Howard, R.; Bothmer, V.
Bibcode: 2019SoPh..294...93L
Altcode:
The three-to-five-month elliptical orbit of Parker Solar Probe
(PSP), approaching within 10 solar radii of the Sun, will allow the
Wide-field Imager for Solar Probe (WISPR) to view the corona with
unprecedented spatial resolution from multiple viewpoints. WISPR has
a wide fixed angular field of view, extending from 13.5∘
to 108∘ from the Sun and approximately 50∘
in the transverse direction, but the physical extent of the imaged
coronal region varies directly with the distance of the spacecraft from
the Sun. In a solar encounter period of approximately 10 days around
perihelion, PSP covers over 100 - 200° of heliographic longitude and
the distance from the Sun varies by a factor of two to five. In this
paper, we use synthetic white-light images to study the effects of the
rapid elliptical orbit on the images that can be anticipated for WISPR's
observations. We find that sequences of images can help identify coronal
density features that will be sampled by in-situ instruments. We also
find that the multiple viewpoints, provided by the rapid motion near
perihelion, can be used to obtain three-dimensional information on
the coronal density features.
Title: Evidence for a Circumsolar Dust Ring at about 0.4 AU
Authors: Howard, R.; Stenborg, G.; Stauffer, J. R.
Bibcode: 2018AGUFM.P53E3002H
Altcode:
In preparation for the Parker Solar Probe (PSP) and Solar Orbiter (SO)
we devised a technique to create empirical F-coronal models from STEREO
SECCHI Heliospheric Imagers. The results of a systematic study of the
resulting F-coronal models obtained from 6+ years of HI-1A data have
been published in a series of papers. The HI-1A instrument observes
the interplanetary space between 4o and 24o
elongation from the Sun, which is equivalent to about 16 to 96
Rsun. This region has not been analyzed heretofore so the properties
of the F-corona as seen from 1 AU have been determined for the first
time. In particular, we determined the ecliptic longitude dependence
of the intensity distribution along its symmetry axis (Stenborg et al,
2018). To first order, the radial intensity profiles were fit with a
single power law at each angle of the spacecraft orbital longitude. As
a test of a technique to be used in the upcoming missions, we performed
a numerical differentiation of the radial intensity profiles and found
a consistent pattern in the derivatives between about 18o
and 23o elongation that peaked at about 21o,
the pattern being observed from all along the ST-A orbit. The findings
indicate the presence of a circumsolar density enhancement that peaks
at about 23o. A straightforward integration of the excess
signal in the derivative space indicates that the intensity increase
over the background F-corona is on the order of 2%, implying a dust
density increase of about 3-4% at the center of the ring. We also
found a large-scale modulation of the inner boundary of the pattern, in
clear association with Mercury's orbit. We also found a more localized
modulation of the inner boundary attributable to the dust trail of
Comet 2P Encke, which occurs near ecliptic longitudes corresponding to
the crossing of Encke's and Mercury's orbital paths. Moreover, evidence
of dust near the S/C in a restricted range of ecliptic longitudes has
also been revealed by this technique (attributable to the dust trail
of comet P73/Schwassmann-Wachmann). We will discuss these findings. We
acknowledge the support of the NASA STEREO project.
Title: New Nova in Ophiuchus = Pnv J17422408-2053088
Authors: Waagen, E. O.; Stubbings, R.; Tyson, R.; O'Neil, J.; Domingo
Martinez, X.; Howard, R.; Fernandes Neto, J.; Blane, D.; Pearce, A.
Bibcode: 2018CBET.4540....3W
Altcode:
E. O. Waagen, American Association of Variable Star Observers
(AAVSO), reports the following visual magnitude estimates of PNV
J17422408-2053088: Aug. 9.578, 10.3 (R. Stubbings, Tetoora Road, Vic.,
Australia); 9.947, 10.2 (R. Tyson, Freeport, NY, USA); 10.091, 10.2
(J. O'Neil, Topsfield, MA, USA); 10.871, 9.8 (X. Domingo Martinez,
Alpicat, Spain); 11.188, 9.7 (R. Howard, Oakland, CA, USA); 11.882, 9.1
(Domingo Martinez); 12.205, 9.4 (Howard); 12.924, 9.0 (J. Fernandes
Neto, Florianopolis, Brazil); 13.499, 9.2 (Pearce). Waagen also
forwards the following CCD magnitudes from Pearce: Aug. 10.385,
B = 10.44; 10.386, V = 9.61 ; 10.387, R = 9.12 ; 10.388, I =
8.67. And Waagen adds that the nova is still rising, as indicated
by the following "TG" magnitudes from D. Blane in South Africa:
Aug. 13.745, 8.96; 13.807, 8.99. Waagen also notes that spectroscopy
(range 390-510 and 590-800 nm; resolution about 5400) by Williams
et al. on Aug. 9.93 UT with the 2-m Liverpool Telescope at the
Observatorio del Roque del Los Muchachos reveal this variable to be an
"Fe II"-type Galactic nova at early stages, with strong Balmer lines
with P-Cyg profiles as well as numerous other features (see website
URL http://www.astronomerstelegram.org/?read=11928).
Title: Parker Solar Probe: Exploring the Plasma Physics of the Solar
Corona and Inner Heliosphere
Authors: Velli, Marco; Bale, S.; Fox, N.; Howard, R.
Bibcode: 2018shin.confE.269V
Altcode:
The magnetic field is fundamental to solar activity and shapes the
inter-planetary environment, as demonstrated by many past and present
interplanetary and remote sensing spacecraft. Magnetic fields are also
the source for coronal heating and the very existence of the solar
wind; produced by the sun's dynamo and emerging into the corona,
magnetic fields become a conduit for waves, act to store energy,
and then propel plasma into the Heliosphere in the form of Coronal
Mass Ejections (CMEs). Magnetic fields are also at the heart of the
generation and acceleration of Solar Energetic Particle (SEPs) that
modify the space weather environment of the Earth and other planets.
Title: The Solar Orbiter Heliospheric Imager (SoloHI) for the Solar
Orbiter Mission
Authors: Howard, R.; Colaninno, R. C.; Plunkett, S. P.; Thernisien,
A. F.; Wang, D.; Rich, N.; Korendyke, C.; Socker, D. G.; Linton, M.;
McMullin, D. R.; Vourlidas, A.; Liewer, P. C.; De Jong, E.; Velli,
M.; Mikic, Z.; Bothmer, V.; Philippe, L.; Carter, M. T.
Bibcode: 2017AGUFMSH23D2681H
Altcode:
The SoloHI instrument has completed its development effort and has been
integrated onto the Solar Orbiter (SolO) spacecraft. The SolO mission,
scheduled for launch in February 2019, will undergo gravity assist
maneuvers around Venus to change both the perihelion distance as well
as the plane of the orbit to ultimately achieve a minimum perihelion
of 0.28 AU and an orbital inclination of about 35° relative to the
ecliptic plane. The remote sensing instruments will operate for three
10-day periods out of the nominal 6-month orbit. SoloHI will observe
sunlight scattered by free electrons in the corona/solar wind from 5°
to 45° elongation in visible wavelengths and will provide a coupling
between remote sensing and in situ observations. It is very similar
to the HI-1 instrument on STEREO/SECCHI except that the FOV is twice
the size at 40o. We present our efforts to prepare for the mission
including our observing plans, quick-look plans and some results of
the calibration activities. We gratefully acknowledge the support of
the NASA Solar Orbiter Collaboration project.
Title: The Wide-Field Imager for the Parker Solar Probe Mission
(WISPR)
Authors: Plunkett, S. P.; Howard, R.; Chua, D. H.; Crump, N. A.;
Dennison, H.; Korendyke, C.; Linton, M.; Rich, N.; Socker, D. G.;
Thernisien, A. F.; Wang, D.; Vourlidas, A.; Baugh, R.; Van Duyne,
J. P.; Liewer, P. C.; De Jong, E.; Boies, M. T.; Mikic, Z.; Bothmer,
V.; Rochus, P.; Halain, J. P.
Bibcode: 2017AGUFMSH23D2693P
Altcode:
The Parker Solar Probe (PSP) mission will be humanity's first visit
to the atmosphere of our nearest star, the Sun, when it is launched
in July 2018. PSP will complete 24 orbits between the Sun and Venus
with diminishing perihelia reaching as close as 7 million km (9.86
solar radii) from Sun center. In addition to a suite of in-situ probes
for the magnetic field, plasma, and energetic particles, the payload
includes the Wide Field Imager for Solar Probe (WISPR) that will
record unprecedented visible light images of the solar corona and the
inner heliosphere. WISPR is the smallest heliospheric imager to date,
and comprises two nested wide-field telescopes with large-format (2K
x 2K) APS CMOS detectors to optimize the performance over a combined
95º radial by 58º transverse field of view and to minimize the risk
of dust damage, which may be considerable close to the Sun. WISPR will
discover - in this never-before explored region of the heliosphere - the
fundamental nature of coronal structures and the source regions of the
solar wind as the PSP flies through them, and will determine whether a
dust-free zone exists near the Sun. WISPR has completed its development
effort and has been integrated onto the PSP spacecraft. In this paper,
we will present our efforts to prepare for the mission including our
observing plans and some results of the calibration activities.
Title: Properties of the Circumsolar Dust Distribution Determined
from STEREO/SECCHI and Implications for PSP and SolO
Authors: Howard, R.; Stenborg, G.
Bibcode: 2017AGUFMSH22B..07H
Altcode:
We have performed an analysis of the HI-1A instrument in the
STEREO/SECCHI suite to determine the inclination and longitude of the
ascending node of the plane of symmetry of the F-corona. The F-corona
arises from sunlight scattered by the dust in orbit about the Sun. We
find that the inclination and ascending node are not constant in the
field of view of the HI-1A (4° to 24° elongation), but are functions
of the elongation angle i.e. the distance to the Sun and are slightly
different from the parameters determined from the Helios mission. These
parameters are reflecting the gravitational influences of Jupiter,
Venus and the Sun as well as Lorentz and Poynting-Robinson forces on the
dust orbits. The center of symmetry is not the center of the Sun, but
is offset by 0.5 Rsun from the center in the direction of the average
position of Jupiter during the epoch studied: from 2007-2012. We also
observe a slight difference in the inclination when it is north or south
of the ecliptic. We suggest this may be due to remnant dust in the orbit
of the Kreutz sun-grazing comets which occur at an average rate of one
every 2-3 days. Finally, as the dust particles evaporate we expect to
see the F-coronal brightness correspondingly decrease. The detectability
of the decrease will depend on the amount of dust evaporating, but a
10% change in the density is easily detectable. If a dust free zone
surrounding the Sun exists, it will affect the F-coronal intensities
observed by PSP and SolO by an observable amount.
Title: Mapping The Territory: What Current Remote Sensing Tells Us
To Expect For PSP
Authors: DeForest, C. E.; McComas, D. J.; Vourlidas, A.; Howard, R.
Bibcode: 2017AGUFMSH21C..06D
Altcode:
Remote sensing with current coronagraphs affords the best current
estimate of plasma conditions PSP will encounter. Over the past few
years, analyses of the synoptic data sets from the STEREO/COR2 and
STEREO/HI1 imagers have yielded rough locations for critical loci such
as the Alfvén and β=1 surfaces. We now present new results from
the deepest-field coronagraph sequence made to date: the STEREO-A
deep-field campaign. Recently-developed noise reduction techniques
and the unique deep-exposure data set reveal small scale motions and
fluctuations throughout the visible corona and give new insight into
the structure of the outer corona.
Title: Solar Wind Origins, Heating and Turbulence Evolution with
Solar Probe Plus: The First Three Perihelia
Authors: Velli, M. C. M.; Panasenco, O.; Rappazzo, A. F.; Tenerani,
A.; Bale, S. D.; Fox, N. J.; Howard, R.; Kasper, J. C.; McComas, D. J.
Bibcode: 2016AGUFMSH54A..07V
Altcode:
In this presentation we will focus on some of the early science return
made possible by the Solar Probe Plus mission, and more specifically
the returns from the first three perihelia at 35.66 solar radii (Rs),
just over half the distance from the Sun of the previous closest
approaching spacecraft, Helios (62.4 Rs). The increased exploration of
the inner heliosphere will allow important new measurements on slow and
fast solar wind turbulent fluctuations, their spectra, and therefore
the origin and dynamics of the so-called Alfvénic turbulence, with
fundamental implications on both the acceleration and heating of the
wind. Will the Alfvénic turbulence cause further bursty jetting in
fast wind streams? How will the anisotropy of the particle distribution
functions eveolve and how will this impact our understanding of the
role plasma instabilities in the wind? During these first encounters,
the Solar Probe Plus spacecraft will already achieve sufficient speeds
to cross the corotation orbit at perihelion: we will therefore also
focus on the questions of the different origins of the slow and fast
solar wind, and specifically the role of the heliospheric current sheet,
the s-web, and coronal streamers and pseudo-streamers in influencing
the different plasma velocities, temperatures and fluctuation properties
in the solar wind inside 40 Rs.
Title: Estimating Coronagraph Visibility Functions - Progress Report
Authors: St Cyr, O. C.; Xie, H.; Duncan, D.; Webb, D. F.; Howard,
R.; Gurman, J. B.
Bibcode: 2015AGUFMSH31C2425S
Altcode:
Annual estimates of the coronal mass ejection (CME) rate have been
well-documented and are quasi-continuous since the mid-1970s based on
both groundbased and spacebased observations. However, coronagraphic
observations from a single viewpoint are unable to detect all CMEs
because they are limited by the properties of Thomson-scattered
photospheric radiation by coronal electrons. To overcome this limitation
and to extend the CME rate estimates beyond a single instrument,
Webb & Howard (1994) formulated the "visibility function" as an
instrument-specific calibration factor. Recently we have begun an
investigation comparing visibility functions for SOHO LASCO, STEREO
COR1/COR2, and the groundbased Mauna Loa Solar Observatory Mk3/Mk4
coronagraphs in order to extend the historical record of the CME
rate. With the launch of the twin STEREO spacecraft in late 2006,
we are able to use the combination of multiple instruments viewing
from longitudinally-separated locations to obtain new estimates of
the global CME rate. We provide a progress report on this activity.
Title: Synthetic White-light Imagery for the Wide-field Imager for
Solar Probe Plus (WISPR)
Authors: Liewer, P. C.; Thernisien, A. F.; Vourlidas, A.; Howard,
R.; DeForest, C. E.; DeJong, E.; Desai, A.
Bibcode: 2015AGUFMSH31C2426L
Altcode:
The Solar Probe Plus trajectory, approaching within 10 solar radii, will
enable the white light imager, WISPR, to fly through corona features
now only imaged remotely. The dependency of the Thomson scattering
on the imaging geometry (distance and angle from the Sun) dictates
that the outer WISPR telescope will be sensitive to the emission
from plasma close to the spacecraft, in contrast to the situation
for imaging from Earth orbit. Thus WISPR will be the first 'local'
imager providing a crucial link between the large-scale corona and
SPP's in-situ measurements. The high speed at perihelion will provide
tomographic-like views of coronal structures at ≤1° resolution. As
SPP approaches perihelion, WISPR, with a 95° radial by 58° transverse
field of view, will resolve the fine-scale structure with high spatial
resolution. To prepare for this unprecedented viewing of the structure
of the inner corona, we are creating synthetic white light images and
animations from the WISPR viewpoint using the white-light ray-tracing
package developed at NRL (available through SolarSoft). We will present
simulated observations of multi-strand models of coronal streamers and
flux ropes of various size and make comparisons with views from Earth,
Solar Orbiter and SPP. Analysis techniques for WISPR images will also
be discussed.
Title: The Mars Atmosphere and Volatile Evolution (MAVEN) Mission
Authors: Jakosky, B. M.; Lin, R. P.; Grebowsky, J. M.; Luhmann,
J. G.; Mitchell, D. F.; Beutelschies, G.; Priser, T.; Acuna, M.;
Andersson, L.; Baird, D.; Baker, D.; Bartlett, R.; Benna, M.;
Bougher, S.; Brain, D.; Carson, D.; Cauffman, S.; Chamberlin, P.;
Chaufray, J. -Y.; Cheatom, O.; Clarke, J.; Connerney, J.; Cravens,
T.; Curtis, D.; Delory, G.; Demcak, S.; DeWolfe, A.; Eparvier, F.;
Ergun, R.; Eriksson, A.; Espley, J.; Fang, X.; Folta, D.; Fox, J.;
Gomez-Rosa, C.; Habenicht, S.; Halekas, J.; Holsclaw, G.; Houghton,
M.; Howard, R.; Jarosz, M.; Jedrich, N.; Johnson, M.; Kasprzak, W.;
Kelley, M.; King, T.; Lankton, M.; Larson, D.; Leblanc, F.; Lefevre,
F.; Lillis, R.; Mahaffy, P.; Mazelle, C.; McClintock, W.; McFadden,
J.; Mitchell, D. L.; Montmessin, F.; Morrissey, J.; Peterson, W.;
Possel, W.; Sauvaud, J. -A.; Schneider, N.; Sidney, W.; Sparacino,
S.; Stewart, A. I. F.; Tolson, R.; Toublanc, D.; Waters, C.; Woods,
T.; Yelle, R.; Zurek, R.
Bibcode: 2015SSRv..195....3J
Altcode: 2015SSRv..tmp...21J
The MAVEN spacecraft launched in November 2013, arrived at Mars in
September 2014, and completed commissioning and began its one-Earth-year
primary science mission in November 2014. The orbiter's science
objectives are to explore the interactions of the Sun and the solar
wind with the Mars magnetosphere and upper atmosphere, to determine
the structure of the upper atmosphere and ionosphere and the processes
controlling it, to determine the escape rates from the upper atmosphere
to space at the present epoch, and to measure properties that allow
us to extrapolate these escape rates into the past to determine the
total loss of atmospheric gas to space through time. These results will
allow us to determine the importance of loss to space in changing the
Mars climate and atmosphere through time, thereby providing important
boundary conditions on the history of the habitability of Mars. The
MAVEN spacecraft contains eight science instruments (with nine sensors)
that measure the energy and particle input from the Sun into the Mars
upper atmosphere, the response of the upper atmosphere to that input,
and the resulting escape of gas to space. In addition, it contains an
Electra relay that will allow it to relay commands and data between
spacecraft on the surface and Earth.
Title: Tracking Prominence Eruptions to 1 AU with STEREO Heliospheric
Imaging
Authors: Wood, B. E.; Howard, R.; Linton, M.
Bibcode: 2015AGUFMSH42A..02W
Altcode:
It is rare for prominence eruptions to be observable far from the Sun
in the inner heliosphere, either in imaging or with in situ plasma
instruments. Nevertheless, we here discuss two examples of particularly
bright eruptions that are continuously trackable all the way to 1
AU by imagers on the Solar TErrestrial RElations Observatory (STEREO)
spacecraft. The two events are from 2011 June 7 and 2012 August 31. Only
these two examples of clear prominence eruptions observable this far
from the Sun could be found in the STEREO 2007-2014 image database,
consistent with the rarity of unambiguous cold prominence material
being observed in situ at 1 AU. Full 3-D reconstructions are made
of the coronal mass ejections (CMEs) that accompany the prominence
eruptions. For the 2011 June event, a time-dependent 3-D reconstruction
of the prominence structure is made using point-by-point triangulation,
which unfortunately is not possible for the August event due to a poor
viewing geometry. However, for the 2012 August event, shock normals
computed from plasma measurements at STEREO-B and Wind using the shock
jump conditions agree well with expectations from the image-based CME
reconstruction. Unlike its accompanying CME, the 2011 June prominence
exhibits little deceleration from the Sun to 1 AU, as a consequence
moving upwards within the CME. Detailed analysis of the prominence's
expansion reveals that deviation from self-similar expansion is never
large, but close to the Sun the prominence expands somewhat more
rapidly than self-similarity, with this effect decreasing with time.
Title: The first super geomagnetic storm of solar cycle 24: "The
St. Patrick day (17 March 2015)" event
Authors: Wu, C. C.; Liou, K.; Socker, D. G.; Howard, R.; Jackson,
B. V.; Yu, H. S.; Hutting, L.; Plunkett, S. P.
Bibcode: 2015AGUFMSH51A2433W
Altcode:
The first super geomagnetic storm of solar cycle 24 occurred on the
"St. Patrick's day" (17 March 2015). Notably, it was a two-step
storm. The source of the storm can be traced back to the solar event
on March 15, 2015. At ~2:10 UT on that day, SOHO/LASCO C3 recorded
a partial halo corona mass ejection (CME) which was associated with
a C9.1/1F flare (S22W25) and a series of type II/IV radio bursts. The
propagation speed of this CME is estimated to be ~668 km/s during 02:10
- 06:20 UT (Figure 1). An interplanetary (IP) shock, likely driven by
the CME, arrived at the Wind spacecraft at 03:59 UT on 17 March (Figure
2). The arrival of the IP shock at the Earth may have caused a sudden
storm commencement (SSC) at 04:45 UT on March 17. The storm intensified
(Dst dropped to -80 nT at ~10:00 UT) during the crossing of the CME
sheath. Later, the storm recovered slightly (Dst ~ -50 nT) after the
IMF turned northward. At 11:01 UT, IMF started turning southward again
due to the large magnetic cloud (MC) field itself and caused the second
storm intensification, reaching Dst = - 228 nT on March 18. We conclude
that the St. Patrick day event is a two-step storm. The first step
is associated with the sheath, whereas the second step is associated
with the MC. Here, we employ a numerical simulation using the global,
three-dimensional (3D), time-dependent, magnetohydrodynamic (MHD)
model (H3DMHD, Wu et al. 2007) to study the CME propagation from the
Sun to the Earth. The H3DMHD model has been modified so that it can be
driven by (solar wind) data at the inner boundary of the computational
domain. In this study, we use time varying, 3D solar wind velocity and
density reconstructed from STELab, Japan interplanetary scintillation
(IPS) data by the University of California, San Diego, and magnetic
field at the IPS inner boundary provided by CSSS model closed-loop
propagation (Jackson et a., 2015). The simulation result matches well
with the in situ solar wind plasma and field data at Wind, in terms of
the peak values of the IP shock and its arrival time (Figure 3). The
simulation not only helps us to identify the driver of the IP shock,
but also demonstrates that the modified H3DMHD model is capable of
realistic simulations of large solar event. In this presentation, we
will discuss the CME/storm event with detailed data from observations
(Wind and SOHO) and our numerical simulation.
Title: Requirements for an Operational Coronagraph
Authors: Howard, R.; Vourlidas, A.; Harrison, R. A.; Bisi, M. M.;
Plunkett, S. P.; Socker, D. G.; Eyles, C. J.; Webb, D. F.; DeForest,
C. E.; Davies, J. A.; Howard, T. A.; de Koning, C. A.; Gopalswamy,
N.; Davila, J. M.; Tappin, J.; Jackson, B. V.
Bibcode: 2015AGUFMSH14A..02H
Altcode:
Coronal mass ejections (CMEs) have been shown to be the major driver
of the non-recurrent space weather events and geomagnetic storms. The
utility of continuously monitoring such events has been very effectively
demonstrated by the LASCO experiment on the SOHO mission. However SOHO
is aging, having been launched 20 years ago on Dec 2, 1995. The STEREO
mission, in which two spacecraft in orbits about the sun are drifting
away from earth, has shown the utility of multiple viewpoints off the
sun-earth line. Up to now the monitoring of CMES has been performed
by scientific instruments such as LASCO and SECCHI with capabilities
beyond those required to record the parameters that are needed to
forecast the impact at earth. However, there is great interest within
the US NOAA and the UK Met Office to launch operational coronagraphs
to L1 and L5. An ad-hoc group was formed to define the requirements
of the L5 coronagraph. In this paper we present some requirements that
must be met by operational coronagraphs. The Office of Naval Research
is gratefully acknowledged.
Title: Synthetic White-light Imagery for the Wide-field Imager for
Solar Probe Plus (WISPR)
Authors: Liewer, P. C.; Su, Y.; Vourlidas, A.; Thernisien, A. F.;
Howard, R.; Hall, J. R.; DeJong, E.
Bibcode: 2014AGUFMSH21B4101L
Altcode:
The Solar Probe Plus trajectory, approaching within 10 solar radii,
will allow the white light imager, WISPR, to view the inner corona with
unprecedented spatial resolution. WISPR, with a 95° radial by 58°
transverse field of view, will image the fine-scale structure with
arcminute-scale resolution. The dependency of the Thomson scattering
on the imaging geometry (distance and angle from the Sun) dictates
that WISPR will be very sensitive to the emission from plasma close
to the spacecraft, in contrast to the situation for imaging from Earth
orbit. Thus WISPR will be the first 'local' imager providing a crucial
link between the large-scale corona and SPP's in-situ measurements. The
high speed at perihelion will provide tomographic-like views of
coronal structures. To prepare for this unprecedented viewing of the
structure of the inner corona, we are creating synthetic white light
images and animations from the WISPR viewpoint using the white-light
ray-tracing package developed at NRL (available through SolarSoft). We
will present results from multi-strand models of coronal streamers
and currents sheets as well as images of coronal mass ejections as
seen simultaneously from Earth, Solar Orbiter and SPP.
Title: When the Sun Gets in the Way: Stereo Science Observations on
the Far Side of the Sun
Authors: Vourlidas, A.; Thompson, W. T.; Gurman, J. B.; Luhmann,
J. G.; Curtis, D. W.; Schroeder, P. C.; Mewaldt, R. A.; Davis, A. J.;
Wortman, K.; Russell, C. T.; Galvin, A. B.; Popecki, M.; Kistler,
L. M.; Ellis, L.; Howard, R.; Rich, N.; Hutting, L.; Maksimovic, M.;
Bale, S. D.; Goetz, K.
Bibcode: 2014AGUFMSH53A4202V
Altcode:
With the two STEREO spacecraft on the opposite side of the Sun from
Earth, pointing the high gain antenna at Earth means that it's also
pointed very close to the Sun. This has resulted in unexpectedly
high temperatures in the antenna feed horns on both spacecraft, and
is forcing the mission operations team to take corrective action,
starting in August 2014 for STEREO Ahead, and December 2014 for STEREO
Behind. By off-pointing the antennas to use one of the lower power side
lobes instead of the main lobe, the feed horn temperatures can be kept
at a safe level while still allowing reliable communication with the
spacecraft. However, the amount of telemetry that can be brought down
will be highly reduced. Even so, significant science will still be
possible from STEREO's unique position on the solar far side. We will
discuss the science and space weather products that will be available
from each STEREO instrument, when those products will be available,
and how they will be used. Some data, including the regular space
weather beacon products, will be brought down for an average of a
few hours each day during the daily real-time passes, while the in
situ and radio beacon data will be stored on the onboard recorder to
provide a continuous 24-hour coverage for eventual downlink once the
spacecraft is back to normal operations.
Title: Observations of Near-Sun Turbulent Density Fluctuations with
the Wide Field Imager for Solar Probe Plus (WISPR)
Authors: Plunkett, S. P.; Howard, R.; Vourlidas, A.; Korendyke, C.;
Rich, N.; Thernisien, A. F.; Wang, D.; Liewer, P. C.
Bibcode: 2014AGUFMSH32A..05P
Altcode:
The trajectory of Solar Probe Plus (SPP) as it transits through
the solar corona with a perihelion of < 10 Rs will allow much
higher contrast observations of small-scale density fluctuations
with higher cadence than is possible from 1 AU. The WISPR instrument
will implement a high-cadence mode (up to 1 second) in which it will
obtain images of the corona and inner heliosphere with high spatial
resolution over a restricted field of view around specified regions of
interest. Two-dimensional power spectra of the density fluctuations
can then be constructed with variable cadences for direct comparison
to similar spectra obtained by in-situ instruments on SPP and Solar
Orbiter (SO). WISPR will provide density power spectra at or below
the spectral break between inertial and injection scales, even at the
closest perihelion approach, for different coronal structures. When
combined with tomographic information from synoptic images, the
WISPR turbulence program will be a major enhancement to the turbulence
measurements from the SPP and SO in-situ instruments resulting in a much
more robust understanding of the near-Sun turbulence. We will present
details of the planned observations and will discuss the coordinated
science objectives that can be addressed using these observations.
Title: Solar Probe Plus: A NASA Mission to Touch the Sun
Authors: Fox, N. J.; Velli, M. M. C.; Kasper, J. C.; McComas, D. J.;
Howard, R.; Bale, S. D.; Decker, R. B.
Bibcode: 2014AGUFMSH21B4096F
Altcode:
Solar Probe Plus (SPP), currently in Phase C, will be the first
mission to fly into the low solar corona, revealing how the corona is
heated and the solar wind and energetic particles are accelerated,
solving fundamental mysteries that have been top priority science
goals since such a mission was first proposed in 1958. The scale
and concept of such a mission has been revised at intervals since
that time, yet the core has always been a close encounter with the
Sun. The primary science goal of the Solar Probe Plus mission is to
determine the structure and dynamics of the Sun's coronal magnetic
field, understand how the solar corona and wind are heated and
accelerated, and determine what mechanisms accelerate and transport
energetic particles. The SPP mission will achieve this by identifying
and quantifying the basic plasma physical processes at the heart of
the Heliosphere. SPP uses an innovative mission design, significant
technology development and a risk-reducing engineering development
to meet the SPP science objectives: 1) Trace the flow of energy that
heats and accelerates the solar corona and solar wind; 2) Determine
the structure and dynamics of the plasma and magnetic fields at the
sources of the solar wind; and 3) Explore mechanisms that accelerate
and transport energetic particles. In this presentation, we present
Solar Probe Plus and examine how the mission will address the science
questions that have remained unanswered for over 5 decades.
Title: Solar Probe Plus: A NASA Mission to Touch the Sun
Authors: Fox, N. J.; Bale, S. D.; Decker, R. B.; Howard, R.; Kasper,
J. C.; McComas, D. J.; Szabo, A.; Velli, M. M.
Bibcode: 2013AGUFMSM53A2207F
Altcode:
Solar Probe Plus (SPP), currently in Phase B, will be the first mission
to fly into the low solar corona, revealing how the corona is heated
and the solar wind is accelerated, solving two fundamental mysteries
that have been top priority science goals since such a mission was
first proposed in 1958. The scale and concept of such a mission has
been revised at intervals since that time, yet the core has always been
a close encounter with the Sun. The primary science goal of the Solar
Probe Plus mission is to determine the structure and dynamics of the
Sun's coronal magnetic field, understand how the solar corona and wind
are heated and accelerated, and determine what mechanisms accelerate
and transport energetic particles. The SPP mission will achieve this
by identifying and quantifying the basic plasma physical processes at
the heart of the Heliosphere. SPP uses an innovative mission design,
significant technology development and a risk-reducing engineering
development to meet the SPP science objectives: 1) Trace the flow of
energy that heats and accelerates the solar corona and solar wind;
2) Determine the structure and dynamics of the plasma and magnetic
fields at the sources of the solar wind; and 3) Explore mechanisms
that accelerate and transport energetic particles. In this poster,
we present Solar Probe Plus and examine how the mission will address
the science questions that have remained unanswered for over 5 decades.
Title: Evidence for the return meridional flow in the convection
zone from latitude motions of sunspots
Authors: Sivaraman, K. R.; Sivaraman, H.; Gupta, S. S.; Howard, R. F.
Bibcode: 2011IAUS..273..434S
Altcode:
We have derived the latitude motions of sunspots classified into three
area categories using the measures of positions and areas of their
umbrae from the white - light images of the Sun for the period 1906 -
1987 from the Kodaikanal Observatory archives. The latitude motions
are directed equator - ward in all the three area classes. We interpret
that these equator - ward latitude motions reflect the meridional flows
at the three depths in the convection zone where the magnetic flux
loops of the spots of the three area classes are anchored. We obtain
estimates of the anchor depths through a comparison of the rotation
rates of the spots in each area class with the rotation rate profiles
from helioseismic inversions. The equator - ward flows measured by us
thus provide evidence of the return meridional flows in the convection
zone as required in the flux transport solar dynamo models. We have
done an identical analysis using a similar data set derived from the
photoheliogram collections of the Mt.Wilson Observatory for the period
1917 - 1985. There is good agreement between the results from the data
sets of the two observatories.
Title: Magnetic Fields in ICMEs At 1 AU And Flux Injection Profile
At the Sun
Authors: Kunkel, Valbona; Chen, J.; Howard, R.
Bibcode: 2011SPD....42.2309K
Altcode: 2011BAAS..43S.2309K
With the SECCHI/STEREO observations, it is now possible to observe
CME trajectories in interplanetary space. The twin spacecraft
configuration of STEREO also allows one to, for the first time,
continuously track a CME's trajectory from the Sun to 1 AU and in
cases where the ejecta is encountered by another spacecraft at 1 AU,
measure the in situ magnetic field and plasma properties of the CME
ejecta. We have examined a number of CME events whose trajectories
were continuously observed by one STEREO spacecraft and the ejecta
were intersected by the other STEREO spacecraft or ACE at L1. We have
applied the erupting flux rope model of CMEs (EFR) to these events and
calculated the best-fit solutions and the physical quantities predicted
by these solutions. For each event, it is possible to find a narrow
range of solutions that fit the observed trajectory to within 1 to
2 % of the position data. The calculated magnetic field and average
temperature and density of the resulting flux rope are compared with
the in situ data. It is found that the 1-AU quantities predicted
by the best-fit solutions for these events are in good agreement
with the in situ data and that the calculated 1 AU magnetic field
is insensitive to the form of the poloidal flux injection function,
provided the injected energy is unchanged. We discuss in detail how
the magnetic field of a CME evolves through interplanetary space,
emphasizing the quantitative relationship between the CME trajectory
and the evolution of the CME magnetic field. The discussion will focus
on a physical understanding that can be used to interpret observational
data and numerical results of simulation models of CMEs.
Title: New Results Revealed By Enhanced Extreme-Ultraviolet Images
Authors: Stenborg, Guillermo A.; Vourlidas, A.; Howard, R.
Bibcode: 2011SPD....42.1809S
Altcode: 2011BAAS..43S.1809S
Groundbreaking observations of the low solar corona at extreme
ultraviolet (EUV) wavelengths have been provided by the EIT instrument
on board SOHO for more than 15 years. At the beginning of 2007, the
EUVI instruments onboard the twin STEREO S/C opened doors and commenced
to image the EUV low corona with a better cadence and better spatial
resolution from two vantage points off the Sun-Earth line. And now,
since February 2010 the AIA instrument on board the Solar Dynamics
Observatory observes the low EUVI corona at a 10 sec cadence in 8
wavelengths. Despite the increasing quality of the EUV observations,
they have not been fully exploited. A customized wavelet-based image
cleaning and enhancing technique that exploits the multi-scale nature
of the observed solar features has been developed (Stenborg et al.,
2008) to maximize the scientific return of the EIT observations. We
have now adapted it to work with STEREO/EUVI and SDO/AIA images. Its
application has already helped unveil phenomena only theorized before,
as well as revealed phenomena that have not found a satisfactory
explanation yet. In this presentation, a brief survey of the new
products and recent discoveries will be shown.
Title: A novel technique to measure intensity fluctuations in EUV
images and to detect coronal sound waves nearby active regions
Authors: Stenborg, G.; Marsch, E.; Vourlidas, A.; Howard, R.;
Baldwin, K.
Bibcode: 2011A&A...526A..58S
Altcode:
Context. In the past years, evidence for the existence of outward-moving
(Doppler blue-shifted) plasma and slow-mode magneto-acoustic propagating
waves in various magnetic field structures (loops in particular) in
the solar corona has been found in ultraviolet images and spectra. Yet
their origin and possible connection to and importance for the mass and
energy supply to the corona and solar wind is still unclear. There has
been increasing interest in this problem thanks to the high-resolution
observations available from the extreme ultraviolet (EUV) imagers on
the Solar TErrestrial RElationships Observatory (STEREO) and the EUV
spectrometer on the Hinode mission.
Aims: Flows and waves exist
in the corona, and their signatures appear in EUV imaging observations
but are extremely difficult to analyse quantitatively because of their
weak intensity. Hence, such information is currently available mostly
from spectroscopic observations that are restricted in their spatial
and temporal coverage. To understand the nature and origin of these
fluctuations, imaging observations are essential. Here, we present
measurements of the speed of intensity fluctuations observed along
apparently open field lines with the Extreme UltraViolet Imagers (EUVI)
onboard the STEREO mission. One aim of our paper is to demonstrate that
we can make reliable kinematic measurements from these EUV images,
thereby complementing and extending the spectroscopic measurements
and opening up the full corona for such an analysis. Another aim is to
examine the assumptions that lead to flow versus wave interpretation
for these fluctuations.
Methods: We have developed a novel
image-processing method by fusing well established techniques for
the kinematic analysis of coronal mass ejections (CME) with standard
wavelet analysis. The power of our method lies with its ability
to recover weak intensity fluctuations along individual magnetic
structures at any orientation , anywhere within the full solar disk ,
and using standard synoptic observing sequences (cadence <3 min)
without the need for special observation plans.
Results: Using
information from both EUVI imagers, we obtained wave phase speeds
with values on the order of 60-90 km s-1, compatible with
those obtained by other previous measurements. Moreover, we studied the
periodicity of the observed fluctuations and established a predominance
of a 16-min period, and other periods that seem to be multiples of
an underlying 8-min period.
Conclusions: The validation of our
analysis technique opens up new possibilities for the study of coronal
flows and waves, by extending it to the full disk and to a larger
number of coronal structures than has been possible previously. It
opens up a new scientific capability for the EUV observations from
the recently launched Solar Dynamics Observatory. Here we clearly
establish the ubiquitous existence of sound waves which continuously
propagate along apparently open magnetic field lines. Movies 1
and 2 (Figs. 12 and 13) are only available in electronic form at http://www.aanda.org
Title: Return Meridional Flow in the Convection Zone from Latitudinal
Motions of Umbrae of Sunspot Groups
Authors: Sivaraman, K. R.; Sivaraman, H.; Gupta, S. S.; Howard, R. F.
Bibcode: 2010SoPh..266..247S
Altcode: 2010SoPh..tmp..157S
We have derived the velocities of meridional flows by measuring the
latitudinal motions (or drifts) of umbrae of spot groups classified
into three categories of area: 0 - 5 μ, 5 - 10 μ, and >10 μ
(μ area in millionths of the solar hemisphere). The latitudinal
drifts (or the meridional flows) in all three categories are directed
equatorward in both the northern and southern hemispheres. By sorting
the spot groups into three area classes, we are able to relate the
respective latitudinal drifts with the three depths in the convection
zone where the footpoints of the flux loops of the spot groups of each
area class are anchored. We obtain estimates of the anchor depths
through a comparison of the rotation rates of the spot groups of
each area class with the rotation-rate profiles from helioseismic
inversions. The equatorward drifts obtained provide estimates of
the meridional flows at the three depths in the convection zone and
thereby suggest the presence of return meridional flows as envisaged
in the flux-transport dynamo models, which have remained undetected so
far. The data sources for this study are measurements of positions and
areas of umbrae of sunspots from the photographic white-light images
of the Sun of the Kodaikanal Observatory archives for the period 1906 -
1987 and a very similar, but independent, data set from the Mt. Wilson
Observatory archives for the period 1917 - 1985.
Title: Examining Periodic Solar Wind Density Structures in SECCHI HI1A
Authors: Viall, Nicholeen; Vourlidas, A.; Spence, H.; Howard, R.
Bibcode: 2010AAS...21630303V
Altcode:
We present an analysis of small-scale periodic solar wind density
enhancements observed in SECCHI HI1. We discuss their possible
relationship to periodic fluctuations of the proton density observed
in-situ with the Wind SWE data. Viall et al. [2008] used 11 years
of solar wind density measurements at 1 AU and demonstrated that in
addition to turbulent fluctuations, non-turbulent periodic density
structures with length scales of tens to hundreds of megameters exist in
the solar wind. Event studies of the periodic density structures reveal
instances in which the density structures have alpha/proton abundance
ratio changes associated with the density structures. Specifically,
the alpha density varies with the same periodicity as the protons,
but in antiphase. For those events, this strongly suggests either time
varying or spatially varying coronal source plasma that created the
density structures. If such periodic density structures observed at 1
AU are generated in the corona, then they may be observable in SECCHI
HI1 data. We identify periodic density structures as they convect
with the solar wind into the field of view of SECCHI HI and follow
the train of structures as a function of time. The periodic density
structures we analyze are comparable in size to the larger structures
identified in-situ at 1 AU. This research was supported through
NASA Grant No. NNG05GK65G and an appointment to the NASA Postdoctoral
Program at the Goddard Space Flight Center, administered by Oak Ridge
Associated Universities through a contract with NASA.
Title: Tracking of Coronal White-Light Events by Texture
Authors: Goussies, N.; Stenborg, G.; Vourlidas, A.; Howard, R.
Bibcode: 2010SoPh..262..481G
Altcode: 2010SoPh..tmp....8G
The extraction of the kinematic properties of coronal mass ejections
(CMEs) from white-light coronagraph images involves a significant
degree of user interaction: defining the edge of the event, separating
the core from the front or from nearby unrelated structures, etc. To
contribute towards a less subjective and more quantitative definition,
and therefore better kinematic characterization of such events,
we have developed a novel image-processing technique based on the
concept of "texture of the event". The texture is defined by the
so-called gray-level co-occurrence matrix, and the technique consists
of a supervised segmentation algorithm to isolate a particular region
of interest based upon its similarity with a pre-specified model. Once
the event is visually defined early in its evolution, it is possible to
automatically track the event by applying the segmentation algorithm to
the corresponding time series of coronagraph images. In this paper we
describe the technique, present some examples, and show how the coronal
background, the core of the event, and even the associated shock (if
one exists) can be identified for different kind of CMEs detected by
the LASCO and SECCHI coronagraphs.
Title: ASPIICS / PROBA-3: a formation flying externally-occulted
giant coronagraph mission
Authors: Lamy, Philippe; Damé, Luc; Curdt, W.; Davila, J.; Defise,
J. M.; Fineschi, S.; Heinzel, P.; Howard, R.; Kuzin, S.; Schmutz,
W.; Tsinganos, K.; Turck-Chièze, S.; Zhukov, A.
Bibcode: 2010cosp...38.2858L
Altcode: 2010cosp.meet.2858L
Classical externally-occulted coronagraphs are presently limited in
their performances by the distance between the external occulter and
the front objective. The diffraction fringe from the occulter and
the vignetted pupil which degrades the spatial resolution prevent
useful observa-tions of the white light corona inside typically 2-2.5
Rsun. Formation flying offers an elegant solution to these limitations
and allows conceiving giant, externally-occulted coronagraphs
us-ing a two-component space system with the external occulter on
one spacecraft and the optical instrument on the other spacecraft
at distances of hundred meters. Such an instrument has just been
selected by ESA to fly (by the end of 2013) on its PROBA-3 mission,
presently in phase B, to demonstrate formation flying. It will perform
both high spatial resolution imaging of the solar corona as well as
2-dimensional spectroscopy of several emission lines (in partic-ular
the forbidden line of FeXIV at 530.285 nm) from the coronal base out
to 3 Rsun using a Fabry-Perot interferometer. The classical design of
an externally-occulted coronagraph is adapted to the formation flying
configuration allowing the detection of the very inner corona as close
as 0.05 Rsun from the solar limb. By tuning the position of the occulter
spacecraft, it may even be possible to try reaching the chromosphere
and the upper part of the spicules. ASPIICS/PROBA-3 mission, payload
and scientific objectives are detailed.
Title: Evidence for Return Meridional Flows in the Convection Zone
Authors: Sivaraman, K. R.; Sivaraman, H.; Gupta, S. S.; Howard, R. F.
Bibcode: 2010ASSP...19..357S
Altcode: 2010mcia.conf..357S
We report velocities of meridional motions derived from the latitude
drifts of spot groups measured on photographic images of the Sun
in the Kodaikanal observatory archives. They serve to measure
different meridional flows for spot groups anchored at different
depths. Comparison of spot-group rotation rates with the rotation
profile resulting from helioseismic inversions yields estimates of
the anchoring depths. The measured latitudinal drifts correspond
to meridional flows at these depths. They show evidence of return
meridional flows as required in a flux-transport dynamo.
Title: Multi-spacecraft observation of a magnetic cloud
Authors: de Lucas, Aline; Dal Lago, Alisson; Schwenn, Rainer; Clúa de
Gonzalez, Alicia L.; Marsch, Eckart; Lamy, Philippe; Damé, Luc; Curdt,
W.; Davila, J.; Defise, J. M.; Fineschi, S.; Heinzel, P.; Howard, R.;
Kuzin, S.; Schmutz, W.; Tsinganos, K.; Turck-Chièze, S.; Zhukov, A.
Bibcode: 2010cosp...38.1921D
Altcode: 2010cosp.meet.1921D
Classical externally-occulted coronagraphs are presently limited in
their performances by the distance between the external occulter and
the front objective. The diffraction fringe from the occulter and
the vignetted pupil which degrades the spatial resolution prevent
useful observa-tions of the white light corona inside typically 2-2.5
Rsun. Formation flying offers an elegant solution to these limitations
and allows conceiving giant, externally-occulted coronagraphs
us-ing a two-component space system with the external occulter on
one spacecraft and the optical instrument on the other spacecraft
at distances of hundred meters. Such an instrument has just been
selected by ESA to fly (by the end of 2013) on its PROBA-3 mission,
presently in phase B, to demonstrate formation flying. It will perform
both high spatial resolution imaging of the solar corona as well as
2-dimensional spectroscopy of several emission lines (in partic-ular
the forbidden line of FeXIV at 530.285 nm) from the coronal base out
to 3 Rsun using a Fabry-Perot interferometer. The classical design of
an externally-occulted coronagraph is adapted to the formation flying
configuration allowing the detection of the very inner corona as close
as 0.05 Rsun from the solar limb. By tuning the position of the occulter
spacecraft, it may even be possible to try reaching the chromosphere
and the upper part of the spicules. ASPIICS/PROBA-3 mission, payload
and scientific objectives are presented.
Title: Reconstructing CME and CIR Structures Using STEREO Images
Authors: Wood, Brian; Howard, R.
Bibcode: 2009SPD....40.2114W
Altcode:
In the continuing efforts to better understand the morphology of coronal
mass ejections (CMEs), the STEREO mission provides two powerful new
capabilities. The first is the use of two separate spacecraft to view
CMEs from two very different vantage points. The second is the ability
to track CMEs continuously from close to the Sun all the way to 1 AU,
thanks to its Heliospheric Imagers (HI1 and HI2). In addition to its
proficiency in tracking CMEs in the interplanetary medium (IPM), the HI2
camera for the first time offers the chance to actually view corotating
interaction regions (CIRs) propagating through the IPM. We present one
technique for reconstructing the 3D structure of both CMEs and CIRs from
extensive STEREO observations, focusing on a CME from 2008 April 26,
and a CIR from 2008 January. The CME analysis demonstrates how well a
flux rope structure reproduces the appearance of this particular April
26 event, while the CIR reconstruction explores the issue of why CIR
fronts in HI2-B images look so different from their HI2-A appearance.
Title: The SOHO/LASCO CME Catalog
Authors: Gopalswamy, N.; Yashiro, S.; Michalek, G.; Stenborg, G.;
Vourlidas, A.; Freeland, S.; Howard, R.
Bibcode: 2009EM&P..104..295G
Altcode: 2009EM&P..tmp....8G
Coronal mass ejections (CMEs) are routinely identified in the images
of the solar corona obtained by the Solar and Heliospheric Observatory
(SOHO) mission’s Large Angle and Spectrometric Coronagraph (LASCO)
since 1996. The identified CMEs are measured and their basic attributes
are cataloged in a data base known as the SOHO/LASCO CME Catalog. The
Catalog also contains digital data, movies, and plots for each CME,
so detailed scientific investigations can be performed on CMEs and
the related phenomena such as flares, radio bursts, solar energetic
particle events, and geomagnetic storms. This paper provides a brief
description of the Catalog and summarizes the statistical properties
of CMEs obtained using the Catalog. Data products relevant to space
weather research and some CME issues that can be addressed using the
Catalog are discussed. The URL of the Catalog is: <ExternalRef>
<RefSource>http://cdaw.gsfc.nasa.gov/CME_list</RefSource>
<RefTarget Address="http://cdaw.gsfc.nasa.gov/CME_list"
TargetType="URL"/> </ExternalRef>.
Title: First stereoscopic polar plume reconstructions from
STEREO/SECCHI images
Authors: Feng, L.; Inhester, B.; Solanki, S. K.; Wiegelmann, T.;
Podlipnik, B.; Howard, R.; Plunkett, S.; Wuelser, J.; Gan, W.
Bibcode: 2008AGUSMSH23A..01F
Altcode:
We present the first stereoscopic reconstruction of the
three-dimensional structures of polar plumes based on the two
simultaneously recorded images taken by the EUVI telescopes in
the SECCHI instrument package onboard the recently launched STEREO
mission. The reconstructed polar plumes were observed on April 7th,
2007 when the two spacecraft were well below the solar equatorial
plane, an appropriate time for the observation of the plumes in the
south polar coronal hole. The heliocentric separation of the two
spacecraft was 3.6 degrees at that time. We determine locations of
the footpoints of five EUV polar plumes on the solar surface as well
as their inclinations relative to the line-of-sight and to their
local radial directions. The five plumes are all within 21 degrees
of the south pole and their inclinations to the line-of-sight of
STEREO A(head) and radial directions are on average 107 degrees and
28 degrees, respectively. A simple dipole model for the south pole's
magnetic field does not provide a good correspondence with the obtained
inclinations. Of the three plumes in front of the limb only one is
associated with an EUV bright point.
Title: The Sun as the Source of Heliospheric "Space Weather": A CISM
Integrated Model Perspective and STEREO Inspiration
Authors: Luhmann, J. G.; Li, Y.; Lynch, B.; Lee, C. O.; Huttunen, E.;
Liu, Y.; Toy, V.; Odstrcil, D.; Riley, P.; Linker, J.; Mikic, Z.; Arge,
C.; Petrie, G.; Zhao, X.; Liu, Y.; Hoeksema, T.; Owens, M.; Galvin,
A.; Simunac, K.; Howard, R.; Vourlidas, A.; Jian, L. K.; Russell, C. T.
Bibcode: 2008AGUSMSH31C..01L
Altcode:
Models developed under the Center for Integrated Space weather
Modeling (CISM) represent one effort that is underway to realistically
simulate the Sun's physical controls over interplanetary conditions,
or heliospheric "space weather", in three dimensions. This capability
is critical for interpreting the latest observations from STEREO,
whose goal is to enable connections to be made between what is
observed in the heliosphere via distributed in-situ measurements
and what is observed in the corona and heliosphere via imaging from
separated 1 AU perspectives. The ways in which the CISM models are
enabling the exploitation of STEREO and other observations toward
increased understanding of the solar wind and coronal activity and
its consequences are described. In particular, the models allow the
identification of the sources of structures in the solar wind, and
analyses of how the coronal context of the observed CMEs plays a key
role in determining the ultimate terrestrial (and other planetary)
response .
Title: The Solar Terrestrial Relations Observatory (STEREO) Education
and Outreach (E/PO) Program
Authors: Peticolas, L. M.; Craig, N.; Kucera, T.; Michels, D. J.;
Gerulskis, J.; MacDowall, R. J.; Beisser, K.; Chrissotimos, C.;
Luhmann, J. G.; Galvin, A. B.; Ratta, L.; Drobnes, E.; Méndez, B. J.;
Hill, S.; Marren, K.; Howard, R.
Bibcode: 2008SSRv..136..627P
Altcode: 2007SSRv..tmp..211P
The STEREO mission’s Education and Outreach (E/PO) program began early
enough its team benefited from many lessons learned as NASA’s E/PO
profession matured. Originally made up of discrete programs, by launch
the STEREO E/PO program had developed into a quality suite containing
all the program elements now considered standard: education workshops,
teacher/student guides, national and international collaboration,
etc. The benefit of bringing so many unique programs together is the
resulting diverse portfolio, with scientists, E/PO professionals, and
their education partners all of whom can focus on excellent smaller
programs. The drawback is a less cohesive program nearly impossible
to evaluate in its entirety with the given funding. When individual
components were evaluated, we found our programs mostly made positive
impact. In this paper, we elaborate on the programs, hoping that others
will effectively use or improve upon them. When possible, we indicate
the programs’ effects on their target audiences.
Title: Stereoscopic Analysis of CME-related Coronal Activity using
STEREO/SECCHI Observations
Authors: Liewer, P. C.; Dejong, E. M.; Hall, J. R.; Braswell, S. J.;
Thompson, W. T.; Howard, R.
Bibcode: 2007AGUFMSH41B..02L
Altcode:
In May 2007, STEREO/SECCHI observed a series of coronal mass ejections
(CMEs). Here we present results from an analysis SECCHI and other
observations to study the relationship between the low-corona flaring
activity, as viewed stereoscopically by SECCHI's ultraviolet imager
EUVI, and the CMEs observed by the SECCHI coronagraphs. Activity
observed includes prominence activation, eruption and ejection and
post-flare loop arcade formation. For several May CME events, we will
show the EUVI flaring activity and the corresponding GOES X-ray flare
event that is most closely associated with the CME initiation. In some
cases, the ejecta can be seen stereoscopically (in 3D) as it crosses
the solar disk, allowing a clear identification with the CME material
observed off the disk by the SECCHI/COR1 coronagraphs. During this
period, the STEREO A and B spacecraft had reached sufficient separation
(>6 degrees) to apply stereoscopic analysis to simultaneous
EUVI images from the two spacecraft. We will report results of
3D reconstruction of flaring coronal loops and prominences using
"tiepointing" and stereoscopy (aka triangulation). This work addresses
STEREO's science objective: to understand the causes and evolution of
coronal mass ejections.
Title: Application of Stereoscopy to STEREO/SECCHI Observations
Authors: Liewer, Paulett C.; DeJong, E. M.; Hall, J. R.; Suzuki, S.;
Howard, R.; Wuesler, J.; SECCHI Team
Bibcode: 2007AAS...210.2814L
Altcode: 2007BAAS...39..325L
The two spacecraft of NASA's Solar TErrestrial RElations Observatory
(STEREO) Mission are just reaching sufficient angular separation for
stereoscopic analysis of simultaneous images. Presently, the spacecraft
are at sufficient separation ( 4 degrees) to reveal qualitative
geometric information about coronal features by viewing simultaneous
pairs in 3D using anaglyphs or special glasses. In this talk, we will
demonstrate this using anaglyphs created from SECCHI/EUVI data. For
example, we will present anaglyphs show the geometric relationship
between polar plumes and bright points in coronal holes and between
filaments/prominences and their loop systems. We will also review
results on application of stereoscopy (tiepointing and triangulation)
to synthetic EUV loop and white data CME data and discuss the range
of angular separations for which these techniques will be useful for
analysis of STEREO/SECCHI data.
Title: Early Solar Wind Observations from the Plasma and Suprathermal
Ion Composition (PLASTIC) Experiments on STEREO
Authors: Galvin, A. B.; Kistler, L. A.; Popecki, M. A.; Farrugia, C.;
Moebius, E.; Lee, M.; Ellis, L.; Simunac, K.; Singer, K.; Russell,
C.; Walker, C.; Blush, L.; Klecker, B.; Wimmer-Schweingruber, R. F.;
Thompson, B.; Bochsler, P.; Wurz, P.; Daoudi, H.; Giammanco, C.;
Karrer, R.; Opitz, A.; Koeten, M.; Luhmann, J.; Howard, R.; Wuelser,
J. P.; Acuna, M.
Bibcode: 2007AGUSMSH34A..01G
Altcode:
Two identical Plasma and Suprathermal Ion Composition (PLASTIC)
instruments are now flying on the STEREO A and B observatories, which
launched in October 2006. The STEREO observatories are drifting away
from the Earth and from each other, providing different longitudinal
perspectives. PLASTIC together with the IMPACT suite provides the
in-situ measurements for the STEREO mission, while SECCHI provides
remote imaging of the solar corona. The PLASTIC solar wind sector
measures solar wind proton bulk parameters and provides species
identification and relative abundances for the more dominant solar wind
minor (Z>2) ions. During this early part of the STEREO mission,
as we approach solar minimum conditions, there have been a series of
coronal- hole associated high speed streams and interstream sector
boundaries. In this talk we will provide initial observations of the
solar wind during selected time periods.
Title: An Attempt to Detect Coronal Mass Ejections in Lyman-α Using
SOHO Swan
Authors: Mays, M. L.; St. Cyr, O. C.; Quémerais, E.; Ferron, S.;
Bertaux, J. -L.; Yashiro, S.; Howard, R.
Bibcode: 2007SoPh..241..113M
Altcode:
In this study, the possibility that coronal mass ejections (CMEs) may
be observed in neutral Lyman-α emission was investigated. An observing
campaign was initiated for SWAN (Solar Wind ANisotropies), a Lyman-α
scanning photometer on board the Solar and Heliospheric Observatory
(SOHO) dedicated to monitoring the latitude distribution of the solar
wind from its imprints on the interstellar sky background. This was
part of SOHO Joint Observing Program (JOP) 159 and was an exploratory
investigation as it was not known how, or even if, CMEs interact with
the solar wind and interstellar neutral hydrogen at this distance
(≈60 and 120 RS). The study addresses the lack of methods
for tracking CMEs beyond the field-of-view of current coronagraphs
(30 RS). In our first method we used LASCO, white-light
coronagraphs on SOHO, and EIT, an extreme ultraviolet imaging telescope
also on SOHO, to identify CME candidates which, subject to certain
criteria, should have been observable in SWAN. The criteria included
SWAN observation time and location, CME position angle, and extrapolated
speed. None of the CME candidates that we discuss were identified in the
SWAN data. For our second method we analyzed all of the SWAN data for
184 runs of the observing campaign, and this has yielded one candidate
CME detection. The candidate CME appears as a dimming of the background
Lyman-α intensity representing ≈10% of the original intensity, moving
radially away from the Sun. Multiple candidate CMEs observed by LASCO
and EIT were found which may have caused this dimming. Here we discuss
the campaign, data analysis technique and statistics, and the results.
Title: Measurement of Kodaikanal White-Light Images: Relaxation of
Tilts of Spot Groups as Indicator of Subsurface Dynamics of Parent
Flux Loops
Authors: Sivaraman, K. R.; Gokhale, M. H.; Sivaraman, Hari; Gupta,
S. S.; Howard, Robert F.
Bibcode: 2007ApJ...657..592S
Altcode:
We reexamine the evolution of the observed tilts θ of spot groups
with life spans 2-7 days in the two latitude belts <13° and
>13°. Using an iterative procedure, we refine the linear fit
between θ and the daily tilt angle changes δθ and obtain reliable
estimates of the fit coefficients. We interpret our results in light
of the scenario implied by the theoretical model of Longcope &
Choudhuri for the subsurface dynamics of parent flux loops of bipolar
magnetic regions and arrive at the following conclusions: (1) the
parent flux tubes of spot groups possess a nonzero tilt at the onset
of rise from the depths of their origin; these ``inborn tilts'' are
~4°-11° in latitudes <13° and ~3°-15° in latitudes >13° (2)
during the rise the tilt of the omega loops of spot groups living 2-7
days get reduced to ~2°-6° in both the latitude belts, and this calls
for reexamination of the role of Coriolis force as understood so far;
(3) after emergence of the top of the loop above the surface, magnetic
tension in the legs tends to restore the tilt to the inborn tilt on
timescales of ~5 to 14 days; and (4) these timescales correspond to
field strengths in the range ~14-40 kG for the parent flux loops and
are close to the limits set by Fan et al.
Title: Demonstration of a technique for stereoscopic tracking of
CMEs for NASA's STEREO Mission
Authors: Liewer, P. C.; Dejong, E. M.; Hall, J. R.; Pournaghshband,
V. J.; Thernisien, A.; Howard, R.
Bibcode: 2006AGUFMSH51A1472L
Altcode:
We use synthetic stereoscopic coronagraph images to demonstrate
and validate a technique for determining the 3D trajectory (speed
and direction) of coronal mass ejections using stereoscopy and
triangulation. This will be used to determine the trajectory of
CMEs captured in stereo by the SECCHI coronagraphs and heliospheric
imagers on STEREO. The technique make use of a tool previously
developed for determining the 3D structure of coronal loops from
stereoscopic image pairs such as will be returned by the EUV telescope
on SECCHI/STEREO. For EUV coronal loops, the user first "seeds"
the tool by selecting the same coronal loop in the two images of
a stereoscopic pair. The tool, then, uses loop tracing algorithms
and triangulation techniques to obtain the three-dimensional (x,y,z)
coordinates of points on the loop. As long as the same emitting loop
can be identified in both EUV images, triangulation is expected to work
because the EUV images capture emission from localized regions. This is
not the case for white light images of CMEs, which capture scattered
light from an extended volume. Even though line of sight (LOS)
effects dominate the observed structure seen in white light images
of CME, we find that by "tiepointing" the bright leading edge of the
CME in both images of a stereo pair, we are able to determine the 3D
trajectory with reasonable accuracy. We will present comparisons of
the 3D trajectories determined from the synthetic white light images
with those used to create the images. The accuracy and limitations of
this approach will also be discussed.
Title: Space Weather Studies at the Planets Enabled by the STEREO
Mission
Authors: Luhmann, Janet; Thompson, W.; Schroeder, P.; Lee, C. O.;
Russell, C. T.; Galvin, A.; Howard, R.; Kaiser, M.; Odstrcil, D.;
Arge, C. N.; Riley, P.; MacNeice, P.
Bibcode: 2006DPS....38.4502L
Altcode: 2006BAAS...38..567L
Several important planetary investigations now underway as part of the
Messenger, Venus Express, Mars Express and Cassini missions depend on a
knowledge of the solar and local interplanetary conditions. For example:
Messenger will resolve the question of whether Mercury's structured
and variable sodium missions detected on the ground are a result of
solar wind and interplanetary field control, Venus and Mars Express
are monitoring the atmospheric ion escape from these terrestrial
planets to evaluate the solar EUV intensity and solar wind dynamic
pressure-related variations, and Cassini results suggest that Titan's
would-be ion torus is evidently lost by frequent intrusions of the solar
wind into the Titan orbit radius. These can be considered part of a
broader definition of "space weather" effects at the planets, together
with the responses of the planet-solar wind interactions to events
such as Coronal Mass Ejections (CMEs). None of these missions involves
regular upstream solar wind measurements to allow interpretation of
these observations, although some local on-board plasma and field
information is useful for this purpose. The twin-spacecraft STEREO
(Solar TERrEstrial Observatory) mission, launched this year, provides
new options for monitoring solar and interplanetary conditions from
perspectives other than that of the Earth. We describe the locations of
the STEREO spacecraft relative to the locations of the planets during
the upcoming two years of its prime mission, as well as the instrument
complement. The STEREO in-situ data are available on-line for general
use approximately one month after receipt on the ground. Furthermore,
models are available at the CCMC to put these measurements in the
context of conditions in the broader heliosphere.
Title: Impacts of Viewing Geometry on CME Observations in the
Heliosphere
Authors: Morrill, Jeff S.; Howard, R.; Webb, D.
Bibcode: 2006SPD....37.0807M
Altcode: 2006BAAS...38Q.231M
Optical observations of Coronal Mass Ejections (CME's) during the past
several decades have generally been confined to events observed near
the sun. Although events originating from most regions of the sun can
be observed, the assumption is often made that the CME is propagating
in the plane-of-the-sky. This assumption is generally adequate to give
lower limits of CME mass and speed. However, observations of CME's
are now being made further from the sun by SMEI and in the future by
the SECCHI instrument on STEREO which is scheduled for launch later
this year. When the CME is observed at great distances from the sun,
plane-of-the-sky assumptions are not adequate for analysis of these
events. In this presentation we will discuss some of the impacts of
geometry on the observations of intensity and velocity.
Title: Coronal Observations of CMEs. Report of Working Group A
Authors: Schwenn, R.; Raymond, J. C.; Alexander, D.; Ciaravella, A.;
Gopalswamy, N.; Howard, R.; Hudson, H.; Kaufmann, P.; Klassen, A.;
Maia, D.; Munoz-Martinez, G.; Pick, M.; Reiner, M.; Srivastava, N.;
Tripathi, D.; Vourlidas, A.; Wang, Y. -M.; Zhang, J.
Bibcode: 2006SSRv..123..127S
Altcode: 2006SSRv..tmp...58S
CMEs have been observed for over 30 years with a wide variety of
instruments. It is now possible to derive detailed and quantitative
information on CME morphology, velocity, acceleration and mass. Flares
associated with CMEs are observed in X-rays, and several different
radio signatures are also seen. Optical and UV spectra of CMEs both on
the disk and at the limb provide velocities along the line of sight
and diagnostics for temperature, density and composition. From the
vast quantity of data we attempt to synthesize the current state of
knowledge of the properties of CMEs, along with some specific observed
characteristics that illuminate the physical processes occurring during
CME eruption. These include the common three-part structures of CMEs,
which is generally attributed to compressed material at the leading
edge, a low-density magnetic bubble and dense prominence gas. Signatures
of shock waves are seen, but the location of these shocks relative
to the other structures and the occurrence rate at the heights where
Solar Energetic Particles are produced remains controversial. The
relationships among CMEs, Moreton waves, EIT waves, and EUV dimming
are also cloudy. The close connection between CMEs and flares suggests
that magnetic reconnection plays an important role in CME eruption
and evolution. We discuss the evidence for reconnection in current
sheets from white-light, X-ray, radio and UV observations. Finally, we
summarize the requirements for future instrumentation that might answer
the outstanding questions and the opportunities that new space-based
and ground-based observatories will provide in the future.
Title: Coronal Observations of CMEs
Authors: Schwenn, R.; Raymond, J. C.; Alexander, D.; Ciaravella, A.;
Gopalswamy, N.; Howard, R.; Hudson, H.; Kaufmann, P.; Klassen, A.;
Maia, D.; Munoz-Martinez, G.; Pick, M.; Reiner, M.; Srivastava, N.;
Tripathi, D.; Vourlidas, A.; Wang, Y. -M.; Zhang, J.
Bibcode: 2006cme..book..127S
Altcode:
CMEs have been observed for over 30 years with a wide variety of
instruments. It is now possible to derive detailed and quantitative
information on CME morphology, velocity, acceleration and mass. Flares
associated with CMEs are observed in X-rays, and several different
radio signatures are also seen. Optical and UV spectra of CMEs both on
the disk and at the limb provide velocities along the line of sight
and diagnostics for temperature, density and composition. From the
vast quantity of data we attempt to synthesize the current state of
knowledge of the properties of CMEs, along with some specific observed
characteristics that illuminate the physical processes occurring during
CME eruption. These include the common three-part structures of CMEs,
which is generally attributed to compressed material at the leading
edge, a low-density magnetic bubble and dense prominence gas. Signatures
of shock waves are seen, but the location of these shocks relative
to the other structures and the occurrence rate at the heights where
Solar Energetic Particles are produced remains controversial. The
relationships among CMEs, Moreton waves, EIT waves, and EUV dimming
are also cloudy. The close connection between CMEs and flares suggests
that magnetic reconnection plays an important role in CME eruption
and evolution. We discuss the evidence for reconnection in current
sheets from white-light, X-ray, radio and UV observations. Finally, we
summarize the requirements for future instrumentation that might answer
the outstanding questions and the opportunities that new space-based
and ground-based observatories will provide in the future.
Title: The SECCHI Experiment on the STEREO Mission
Authors: Howard, R.; Moses, D.; Vourlidas, A.; Davila, J.; Lemen, J.;
Harrison, R.; Eyles, C.; Defise, J. -M.; Bothmer, V.; Ravet, M. -F.;
Secchi Team
Bibcode: 2006cosp...36..870H
Altcode: 2006cosp.meet..870H
The Sun Earth Connection Coronal and Heliospheric Investigation SECCHI
on the NASA Solar Terrestrial Relations Observatory STEREO mission
is a suite of remote sensing instruments consisting of an extreme
ultraviolet EUV imager two white light coronagraphs and two telescopes
that comprise the heliospheric imager SECCHI will observe coronal mass
ejections CMEs from their birth at the sun through the corona and into
the heliosphere A complete instrument suite is being carried on each
of the two STEREO spacecraft which will provide the first sampling of
a CME from two vantage points The spacecraft will orbit the Sun one
Ahead of the Earth and the other Behind each separating from Earth at
about 22 degrees per year The varying separation means that we will
have different observational capabilities as the spacecraft separate
and therefore differing science goals The primary science objectives
all are focused on understanding the physics of the CME process -
their initiation 3D morphology propagation interaction with the
interplanetary medium and space weather effects By observing the CME
from multiple viewpoints with UV and coronagraphic telescopes and by
combining these observations with radio and in-situ observations from
the other instruments on STEREO as well as from other satellites and
ground based observatories operating at the same time answers to some
of the outstanding questions will be obtained STEREO follows the very
successful SOHO mission SOHO s success was primarily due to the highly
complementary nature of the instruments but it was
Title: Coronal and Heliospheric Science Opportunities with the SECCHI
Heliospheric Imager
Authors: Moses, J.; Socker, D.; Eyles, C.; Harrison, R.; Defise,
J.; Howard, R.; Rochus, P.; Waltham, N.; Simnett, G.; Newmark, J.;
Halain, J.; Mapson-Menard, H.; Mazy, E.; Davis, C.
Bibcode: 2005AGUSMSH51D..02M
Altcode:
The Heliospheric Imager (HI) is part of the SECCHI suite of instruments
on-board the two STEREO spacecrafts. The two HI instruments will
provide stereographic image pairs of solar coronal plasma and coronal
mass ejections (CME) over a wide field of view (85 degrees), with an
inner field limit of approximately 13 solar radii. These observations
compliment the 15 solar radii field of view of the solar corona obtained
by the other SECCHI instruments to provide unbroken coverage of the
solar corona and heliosphere from the Sun to the Earth. The as-built
characteristics of the HI will be presented along with the current data
reduction and analysis approach. The constraints that HI observations
place on models of CME propagation in the interplanetary medium will
be discussed.
Title: The Mass Properties of Coronal Mass Ejections: Evolution
& Statistics
Authors: Vourlidas, A.; Howard, R.
Bibcode: 2005AGUSMSP44A..04V
Altcode:
A defining property of a Coronal Mass Ejection (CME) is naturally its
mass. The LASCO observations of 1000s of CMEs over the last 8 years
allow us to constrain statistically the "typical" CME mass but also to
examine its evolution as a function of time during each event. Such
work could not be done in the past due to the lower cadence and
instrument sensitivity. Our analysis of the CME mass properties has
revealed some interesting and maybe unexpected results. For example,
close to half of the observed CMEs seem to blend into the background
before reaching 30 Rs. In this paper, we will discuss our findings
for the LASCO CME sample from 1996 to 2004.
Title: Of Tilt and Twist
Authors: Holder, Zachary A.; Canfield, Richard C.; McMullen, Rebecca
A.; Howard, Robert F.; Pevtsov, Alexei A.
Bibcode: 2005HiA....13..135H
Altcode:
Using Haleakala Stokes Polarimeter active-region vector magnetograms
and Mt. Wilson Observatory full-disk longitudinal magnetograms we
measure both the overall twist (using the force-free-field parameter
alpha) and tilt of 368 active regions. This dataset clearly shows two
well-known phenomena Joy's law and the hemispheric helicity rule as well
as a lesser-known twist-tilt relationship which is the point of this
work. Those regions that closely follow Joy's law show no twist-tilt
relationship as expected if the twist originates from convective
buffeting of initially untwisted and unwrithed flux tubes within the
convection zone through the Sigma effect. Those regions that strongly
depart from Joy's law show significantly larger than average twist
and a very strong twist-tilt relationship. These properties suggest
that the twist-tilt relationship in these regions is due to kinking of
flux tubes that are initially highly twisted but not strongly writhed
perhaps as a result of dynamo action.
Title: The Solar Imaging Radio Array (SIRA) Mission
Authors: Jones, D. L.; MacDowall, R.; Gopalswamy, N.; Kaiser, M.;
Reiner, M.; Demaio, L.; Weiler, K.; Kasper, J.; Bale, S.; Howard, R.
Bibcode: 2004AAS...205.1012J
Altcode: 2004BAAS...36.1351J
The Solar Imaging Radio Array will be proposed to NASA as a Medium
Explorer (MIDEX) mission by a team of investigators at GSFC, JPL, NRL,
MIT, and UC Berkeley. The main science goal of the mission is imaging
and tracking of solar radio bursts, particularly those associated with
coronal mass ejections, and understanding their evolution and influence
on Earth's magnetosphere. Related goals are mapping the 3-dimensional
morphology of the interplanetary magnetic field and improving the
prediction of geomagnetic storms. A number of topics in galactic
and extragalactic astrophysics will also be addressed by SIRA. The
mission concept is a free-flying array of about 16 small, inexpensive
satellites forming an aperture synthesis interferometer in space. By
observing from above the ionosphere, and far from terrestrial radio
interference, SIRA will cover frequencies between a few tens of kHz up
to 15 MHz. This wide spectral window is essentially unexplored with
high angular resolution. Part of this work is being carried out
at the Jet Propulsion Laboratory, California Institute of Technology,
under contract with the National Aeronautics and Space Administration.
Title: Calibration Results for the STEREO/SECCHI COR2 Coronagraphs
Authors: Vourlidas, A.; Plunkett, S.; Korendyke, C.; Gong, Q.; Socker,
D.; Howard, R.
Bibcode: 2004AGUFMSH21B0409V
Altcode:
The two SECCHI instrument suites aboard the upcoming STEREO mission
include the COR2 coronagraphs which observe the middle/outer corona;
namely, from 2.5 to 15 solar radii. As of the end of August 2004,
both COR2 instruments have been assembled and delivered to the SECCHI
project and their perfomance has been measured and analyzed. The
coronagraphs have met (and in some cases, exceeded) their performance
requirements. Here, we present in detail the results of the calibration
(photometry, polarization, stray light levels, etc) of the COR2
coronagraphs. We compare these new coronagraphs to the LASCO ones
and discuss how they will contribute to the scientific success of the
STEREO mission.
Title: Planetary Auroral Storms Trace a CME-driven Interplanetary
Shock Throughout the Solar System, from the Sun to Saturn at 9 AU
Authors: Prange, R.; Pallier, L.; Hansen, K. C.; Howard, R.; Vourlidas,
A.; Courtin, R.; Parkinson, C.
Bibcode: 2004AGUFM.P51A1419P
Altcode:
Hubble Space Telescope FUV images taken in December 2000 revealed
for the first time au auroral storm on Saturn. The Sun, the Earth,
Jupiter and Saturn were practically aligned at that time, allowing the
solar wind plasma to flow by all three planets successively within ~1
month. Observations of Jupiter coordinated with Cassini measurements
in the nearby solar wind were also executed during this period. Using
a recently developped MHD code and solar wind measurements in the
Earth vicinity, we establish that (1) the strong auroral event on
Saturn was related to the interaction of an interplanetary shock with
its magnetosphere, (2) this shock was initiated by a series of CMEs
on the Sun observed by SOHO. We follow the propagation of the shock
throughout the solar system, from the Earth where auroral storms are
recorded, to Jupiter where the auroral activity is strongly enhanced,
and to Saturn where it ultimately activates the observed unusual polar
source. This is the first report of consecutive auroral responses to a
propagating interplanetary shock. It indicates that shocks retain their
properties and their ability to trigger planetary auroral activity
thoughout the solar system, thereby unifying our understanding of
solar-planetary interactions. We discuss also the similarities and
differences observed between the planetary auroral responses.
Title: Solar Imaging Radio Array (SIRA): Imaging solar,
magnetospheric, and astrophysical sources at < 15 MHz
Authors: Howard, R.; MacDowall, R.; Gopalswamy, N.; Kaiser, M. L.;
Reiner, M. J.; Bale, S.; Jones, D.; Kasper, J.; Weiler, K.
Bibcode: 2004DPS....36.1424H
Altcode: 2004BAAS...36Q1097H
The Solar Imaging Radio Array (SIRA) is a mission to perform
aperture synthesis imaging of low frequency solar, magnetospheric, and
astrophysical radio bursts. The primary science targets are coronal mass
ejections (CMEs), which drive radio emission producing shock waves. A
space-based interferometer is required, because the frequencies of
observation (<15 MHz) do not penetrate the ionosphere. As such,
the SIRA mission serves as a lower frequency counterpart to LWA, LOFAR,
and similar ground-based radio imaging arrays. SIRA will require 12 to
16 microsatellites to establish a sufficient number of baselines with
separations on the order of kilometers. The microsat constellation
consists of microsats located quasi-randomly on a spherical shell,
initially of radius 5 km or less. The baseline microsat is 3-axis
stabilized with body-mounted solar arrays and an articulated, earth
pointing high gain antenna. A retrograde orbit at 500,000 km from Earth
was selected as the preferred orbit because it reduces the downlink
requirement while keeping the microsats sufficiently distant from
terrestrial radio interference. Also, the retrograde orbit permits
imaging of terrestrial magnetospheric radio sources from varied
perspectives. The SIRA mission serves as a pathfinder for space-based
satellite constellations and for spacecraft interferometry at shorter
wavelengths. It will be proposed to the NASA MIDEX proposal opportunity
in mid-2005.
Title: HERSCHEL Suborbital Program: 3-D Applications for the STEREO
Mission
Authors: Moses, J. D.; Newmark, J.; McMullin, D.; Antonucci, E.;
Fineschi, S.; Gardiol, D.; Zangrilli, L.; Romoli, M.; Pace, E.; Gori,
L.; Landini, F.; Gherardi, A.; da Deppo, V.; Naletto, G.; Nicolosi, P.;
Pelizzo, M.; Malvezzi, M.; Auchere, F.; Delaboudiniere, J.; Howard, R.
Bibcode: 2004AGUFMSH23A..08M
Altcode:
The HERSCHEL (HElium Resonance Scatter in the Corona and HELiosphere)
Suborbital Program is an international collaborative program between
a consortium of Italian Universities & Observatories led by
Dr. E. Antonucci (and funded by the Italian Space Agency, ASI), the
French IAS (funded in part by CNES) and the Solar Physics Branch of
NRL (by NASA SEC and the Office of Naval Research). HERSCHEL will:
investigate the slow and fast solar wind, determine the helium
distribution and abundance in the corona, and test solar wind
acceleration models; by obtaining simultaneous observations of
the electron, proton and helium solar coronae. HERSCHEL will also
establish proof-of-principle for the Ultra-Violet Coronagraph, which
is in the ESA Solar Orbiter Mission baseline. The HERSCHEL launch date
has been linked to the STEREO launch date to allow coordinated science
between the two missions. One aspect of this scientific coordination is
establishing the 3-D structure of the inner corona. HERSCHEL provides
a third viewpoint for the inner corona covered by the A&B STEREO
SECCHI COR-1. HERSCHEL is the only scheduled, space-based asset that
could provide this third viewpoint for the critical inner corona viewed
by STEREO COR-1 (although lower resolution, ground-based cononagraphs
will make a contribution). A third viewpoint dramatically increases
one's ability to establish the 3-D structure of an optically thin object
(e.g. the metric in Fig. 7 of Davila 1994, ApJ 423, 871). HERSCHEL will
provide at least a snapshot of that viewpoint, plus a wide range of
additional information on the H and He composition of the inner corona.
Title: The 3D structure of CMEs from LASCO polarization measurements
Authors: Dere, K. P.; Wang, D.; Howard, R.
Bibcode: 2004AGUFMSH22A..03D
Altcode:
The polarization of Compton scattered light from a coronal plasma
is a function of the distance of that plasma from the plane of the
sky. From an image of the polarization of the corona it is possible
to construct a 3D cube of intensity which can be visualized at any
angle to study it's three dimensional distribution. For a one month
period during July and August 2002, the LASCO C2 coronagraph performed
polarization measurements with a one hour cadence. During this period,
several CMEs were well observed. Two CMEs give the appearance of an
ejecting loop arcade. Another CME is not as easily described but may
indicate the presence of a flux tube. There are two main ambiguities in
the analysis. First, a given polarization can be produced by scattering
from in front of or behind the plane of the sky. This ambiguity can
be overcome by selecting CMEs that are completely out of the plane of
the sky. Second, in reconstructing the 3D distribution of intensity,
it is assumed that the intensity can be visualized as coming from a
single point in 3D space. The filamentary structures commonly seen in
the 3D visualizations suggest that this is not a significant problem.
Title: On the Tilt and Twist of Solar Active Regions
Authors: Holder, Zachary A.; Canfield, Richard C.; McMullen, Rebecca
A.; Nandy, Dibyendu; Howard, Robert F.; Pevtsov, Alexei A.
Bibcode: 2004ApJ...611.1149H
Altcode:
Tilt and twist are two measurable characteristics of solar active
regions that can give us information about subsurface physical
processes associated with the creation and subsequent evolution of
magnetic flux tubes inside the Sun. Using Mees Solar Observatory active
region vector magnetograms and Mount Wilson Observatory full-disk
longitudinal magnetograms, we measure the magnetic twist and tilt
angles of 368 bipolar active regions. In addition to two well-known
phenomena, Joy's law and the hemispheric helicity rule, this data set
also shows a lesser known twist-tilt relationship, which is the focus
of this study. We find that those regions that closely follow Joy's
law do not show any twist-tilt dependence. The dispersion in tilt
angles and the dispersion in twist are also found to be uncorrelated
with each other. Both of these results are predicted consequences of
convective buffeting of initially untwisted and unwrithed flux tubes
through the Σ-effect. However, we find that regions that strongly
depart from Joy's law show significantly larger than average twist
and very strong twist-tilt dependence-suggesting that the twist-tilt
relationship in these regions is due to the kinking of flux tubes that
are initially highly twisted, but not strongly writhed. This implies
that some mechanism other than the Σ-effect (e.g., the solar dynamo
itself or the process of buoyancy instability and flux tube formation)
is responsible for imparting the initial twist (at the base of the
solar convection zone) to the flux tubes that subsequently become
kink-unstable.
Title: SPECTRE: a spectro-heliograph for the transition region
Authors: Naletto, G.; Antonucci, E.; Fineschi, S.; da Deppo, V.;
Nicolosi, P.; Romoli, M.; Zangrilli, L.; Gardiol, D.; Loreggia, D.;
Malvezzi, M.; Howard, R.; Moses, D.
Bibcode: 2004ESASP.554..251N
Altcode: 2004icso.conf..251N
The SPECtro-heliograph for the Transition REgion (SPECTRE) experiment
is one of the instruments of the Solar Heliospheric Activity Research
and Prediction Program (SHARPP) suite initially foreseen aboard the
NASA mission Solar Dynamics Observa-tory (SDO) of the International
Living With a Star (ILWS) program. The scientific objective of the
SPECTRE experiment was to characterize the rapid evolution of plasma in
the transition region of the solar atmosphere, producing full-disk 1.2
arcsec-resolution images of the solar atmosphere at the very critical
63 nm OV spectral line, characterizing a solar plasma temperature of
about 250,000K. Unfortunately, NASA very recently and unexpectedly,
during the instrument Phase A study, decided not to proceed with the
realization of SHARPP. The authors of this paper think that all the
work done so far in the definition of SPECTRE should not be lost. So,
they have decided to summarize in this paper the main characteristics
of this instrument and the results of the analysis so far performed:
the hope is that in a next future this work can be used again for
realizing an instrument having similar characteristics.
Title: The Secchi Experiment on the Stereo Mission
Authors: Howard, R.; Moses, D.; Socker, D.; Cook, J.; Davila, J.;
Lemen, J.; Harrison, R.; Eyles, C.; Waltham, N.; Defise, J. -M.
Bibcode: 2004cosp...35.3893H
Altcode: 2004cosp.meet.3893H
The Sun Earth Connection Coronal and Heliospheric Investigation
(SECCHI) on the NASA Solar Terrestrial Relations Observatory (STEREO)
mission is a suite of remote sensing instruments consisting of two
white light coronagraphs, an extreme ultraviolet (EUV) imager, and
a heliospheric imager. SECCHI will observe coronal mass ejections
(CMEs) from their birth at the sun, through the corona and into the
heliosphere. A complete instrument suite is being carried on each
of the two STEREO spacecraft, which will provide the first sampling
of a CME from two vantage points. The spacecraft will orbit the Sun,
one ahead of the Earth and the other behind, separating from Earth at
about 22 degrees per year. The varying separation means that we will
have different observational capabilities as the spacecraft separate
and therefore differing science goals. The primary science objectives
all are focused on understanding the physics of the CME process -
their initiation, 3D morphology, propagation, interaction with the
interplanetary medium and space weather effects. By observing the CME
from multiple viewpoints with UV and coronagraphic telescopes and by
combining these observations with radio and in-situ observations from
the other instruments on STEREO as well as from other satellites and
ground based observatories operating at the same time, answers to some
of the outstanding questions will be obtained.
Title: Can SOHO SWAN Detect CMEs?
Authors: St. Cyr, O. C.; Malayeri, M. L.; Yashiro, S.; Quemerais,
E.; Bertaux, J.; Howard, R.
Bibcode: 2003AGUFMSH41B0462S
Altcode:
We have investigated the possibility that the Solar Wind Anisotropies
(SWAN) remote sensing instrument on SOHO may be able to detect coronal
mass ejections (CMEs) in neutral Hydrogen Lyman-α emission. We
have identified CMEs near the Sun in observations by the SOHO LASCO
white-light coronagraphs and in extreme ultraviolet emissions using SOHO
EIT. There are very few methods of tracking CMEs after they leave the
coronagraph's field-of-view, so this is an important topic to study. The
primary science goal of the SWAN investigation is the measurement of
large-scale structures in the solar wind, and these are obtained by
detecting intensity fluctuations in Lyman-α . SWAN consists of a pair
of sensors on opposite panels of SOHO. The instantaneous field-of-view
of each sensor unit is a 5° x 5° square, divided into 1° pixels. A
gimbaled periscope system allows each sensor to map the intensity
distribution of Lyman-α , and the entire sky can be scanned in less
than one day. This is the typical mode of operation for this instrument
(Bertaux et al., Solar Physics, 162, 403-439, 1995). Beginning in
May 2002 the sky-scan mode of the SWAN detectors was interrupted,
and they were held stationary for one-or-more 15-hour campaigns each
week. During those campaigns the SWAN sensors were positioned above the
East or West equator of the Sun at locations chosen to be as close to
the Sun as possible (typically 50 solar radii from Sun-center). Based
on the LASCO and EIT data, we have identified CMEs whose extrapolated
height-time measurements indicated that the events would cross the
SWAN field during the campaign period. During 12 months' observation,
there were ∼10 CMEs that met two criteria: (1) an event low in the
corona near the solar limb could be unambiguously identified in EIT; and
(2) the CME could be tracked beyond 20 R⊙ in LASCO C3. We consider
these CMEs to be particularly well-observed since the speed measured
in LASCO could be reliably extrapolated to the SWAN field-of-view. We
will report preliminary results of this novel observing campaign.
Title: The Ultraviolet and Visible-light Coronagraph of the HERSCHEL
experiment
Authors: Romoli, M.; Antonucci, E.; Fineschi, S.; Gardiol, D.;
Zangrilli, L.; Malvezzi, M. A.; Pace, E.; Gori, L.; Landini, F.;
Gherardi, A.; da Deppo, V.; Naletto, G.; Nicolosi, P.; Pelizzo, M. G.;
Moses, J. D.; Newmark, J.; Howard, R.; Auchere, F.; Delaboudinière,
J. P.
Bibcode: 2003AIPC..679..846R
Altcode:
The Herschel (HElium Resonant Scattering in the Corona and HELiosphere)
experiment, to be flown on a sounding rocket, will investigate the
helium coronal abundance and the solar wind acceleration from a
range of solar source structures by obtaining the first simultaneous
observations of the electron, proton and helium solar corona. The
HERSCHEL payload consists of the EUV Imaging Telescope (EIT), that
resembles the SOHO/EIT instrument, and the Ultraviolet and Visible
Coronagraph (UVC).UVC is an imaging coronagraph that will image
the solar corona from 1.4 to 4 solar radii in the EUV lines of HI
121.6 nm and the HeII 30.4 nm and in the visible broadband polarized
brightness. The UVC coronagraph is externally occulted with a novel
design as far as the stray light rejection is concerned. Therefore,
HERSCHEL will also establish proof-of-principle for the Ultraviolet
Coronagraph, which is in the ESA Solar Orbiter Mission baseline.The
scientific objectives of the experiment will be discussed, togetherwith
a description of the UVC coronagraph.
Title: Measurement of Kodaikanal white-light images - VI. Variation
of Rotation Rate with Age of Sunspot Groups
Authors: Sivaraman, K. R.; Sivaraman, Hari; Gupta, S. S.; Howard,
Robert F.
Bibcode: 2003SoPh..214...65S
Altcode:
We find from an analysis of the Kodaikanal sunspot group data that the
rotation rates of spot groups increase with their age when the rotation
rates are computed after sorting the spot groups life-span-wise. We
confirm these findings from an analysis of the Mt. Wilson sunspot data
set too. We show that this trend is in good agreement with the internal
rotation profiles from helioseismology (GONG) observations and is also
consistent with the concept that the footpoints of the magnetic loops
of spot groups are initially anchored in the deeper layers in the solar
interior and rise to shallower layers as the spots age, and that the
spots reflect the rotation rates at the respective depths at which their
footpoints are temporally located. We project the `first-day rotation
rates' and the `daily rotation rates' of spot groups on the rotation
profiles from the GONG observations and derive the initial anchoring
depths of the footpoints of the magnetic loops of the spot groups and
their rates of rise as the spot groups age. Our results of the rotation
rates are in antithesis to the results reported by investigators from
the Greenwich spot group data that show a deceleration in rotation
rates as the spot groups age which are also inconsistent with the
rotation profiles from helioseismology observations.
Title: The Perihelion Passage of Comet C/2002 V1 (NEAT)
Authors: Lawrence, G.; Morrill, J. S.; Andrews, M.; Hammer, D.; Jones,
G.; Schank, K.; Howard, R.
Bibcode: 2003SPD....34.0613L
Altcode: 2003BAAS...35R.819L
During mid-February the Comet C/2002 V1 (NEAT) passed through the
field-of-view of the LASCO C3 coronagraph on SOHO and yielded numerous
spectacular images. The comet passed within 0.1 AU (about 20 solar
radii) and displayed complex dust and ion tails. In addition, two CMEs
occurred and appeared to be directed toward the comet. One edge of the
second CME may have crossed the comet's tail at about the time the comet
was predicted to cross the heliospheric current sheet. Interactions
with one or both or these may have been responsible for the splitting
of the ion tail. In this presentation we will discuss the observations
and present a preliminary comparison of solar wind speeds derived from
the ion tail measurements with theoretical values.
Title: Association between CME/Flare Events and Enhanced Oxygen
Charge States
Authors: Reinard, A. A.; Dere, K.; Howard, R.
Bibcode: 2003SPD....34.0608R
Altcode: 2003BAAS...35..818R
We present results from a study of coronal mass ejection (CME) data
obtained both optically (from SOHO/LASCO and SOHO/EIT) and in situ
(from ACE). Phenomena associated with optical CMEs, such as flares
and radio bursts, are compared with heliospheric CME signatures,
such as enhanced charge states and elevated helium densities, to
determine if there is any correspondence between them. A casual
relationship has been seen between CMEs with large flares and ICMEs
with enhanced oxygen charge state ratios. This relationship will be
further investigated. An understanding of how CME observations at the
Sun relate to CME observations in the heliosphere will increase our
understanding of CME dynamics and may provide insight into CME origins.
Title: Long Lasting Type II Radio Bursts
Authors: Nunes, S.; Gopalswamy, N.; Yashiro, S.; Howard, R.
Bibcode: 2003SPD....34.0607N
Altcode: 2003BAAS...35..818N
Plasma frequencies starting in the decameter-hectametric (DH) regime and
continuing into the kilometric (km) regime correspond to approximately
to the entire Sun-Earth distance. Accordingly, we consider Type II radio
bursts observed by the WAVES experiment on the WIND spacecraft that
are observed from the DH to the km regimes and their association with
white-light coronal mass ejections (CMEs). We find that approximately
80% of these events are associated with metric Type II bursts observed
on Earth. We also consider correlations of DH/km Type II's with
sunspot numbers and other cyclical measures of solar activity, and
properties of CMEs associated with DH/km Type II bursts. This
work is supported by the Air Force Office of Scientific Research,
the National Science Foundation's SHINE Program, and NASA.
Title: The Dynamic Sun
Authors: Howard, R. F.
Bibcode: 2003SPD....34.1201H
Altcode: 2003BAAS...35..828H
The Sun presents us with an array of velocity fields, probably the
most obvious of which is granulation. A larger convective pattern,
supergranulation, is also clearly seen in the chromosphere. There
is indirect evidence for still larger organized motions within
the convective zone. This evidence comes from the effects of these
motions on the orientation and behavior of active regions at the solar
surface. As loops of magnetic flux rise through the convective zone
to emerge eventually as active regions, they are acted upon by several
forces, such as the Coriolis force, and this results in the orientations
and motions of the regions observed at the solar surface. The analysis
of orientations and motions of regions of various sizes also helps
us to understand the forces at work as buoyancy brings magnetic flux
to the surface. A picture of the emergence of active-region magnetic
flux is now in place that seems to satisfy the observations and our
understanding of the dynamo process and the convective zone.
Title: The Dynamic Sun
Authors: Howard, R. F.
Bibcode: 2003AAS...202.1601H
Altcode: 2003BAAS...35..722H
The Sun presents us with an array of velocity fields, probably the most
obvious of which is granulation. A larger convective pattern known as
"supergranulation" is also clearly seen in the chromosphere. There
is indirect evidence for still larger organized motions within
the convective zone. This evidence comes from the effects of these
motions on the orientation and behavior of active regions at the
solar surface. As loops of magnetic flux rise through the convective
zone to emerge eventually as active regions, they are acted upon
by several forces, such as the Coriolis force, and this results in
the orientations and motions of the regions observed at the solar
surface. Differential rotation is yet another large-scale velocity
pattern that is of importance for understanding the structure and the
activity cycle of the Sun - and of solar-type stars. Small systematic
variations in the differential rotation are linked in space and time
with the well-known latitude drift of activity (i.e. sunspot groups)
during a 22-year cycle. This phenomenon is clearly related to the
dynamo process that is the cause of the solar cycle of activity.
Title: In Situ and Heliospheric CME Signature Relations
Authors: Reinard, Alysha; Dere, K.; Howard, R.; Zurbuchen, T.
Bibcode: 2003IAUJD...7E..31R
Altcode:
We present results from a study of coronal mass ejection (CME) data
obtained both optically (from SOHO/LASCO and SOHO/EIT) and in situ (from
ACE). Phenomena associated with optical CMEs such as flares and radio
bursts are compared with heliospheric CME signatures such as enhanced
charge states and elevated helium densities to determine if there is
any correspondence between them. A casual relationship has been seen
between CMEs with large flares and ICMEs with enhanced oxygen charge
state ratios. This relationship will be further investigated. An
understanding of how CME observations at the Sun relate to CME
observations in the heliosphere will increase our understanding of
CME dynamics and may provide insight into CME origins.
Title: Of Twist and Tilt
Authors: Holder, Zachary A.; Canfield, Richard C.; McMullen, Rebecca
A.; Howard, Robert F.; Pevtsov, Alexei A.
Bibcode: 2003IAUJD...3E..27H
Altcode:
Using Haleakala Stokes Polarimeter active-region vector magnetograms
and Mt. Wilson Observatory full-disk longitudinal magnetograms we
measure both the overall twist (using the force-free-field parameter
alpha) and tilt of 368 active regions. This dataset clearly shows two
well-known phenomena Joy's law and the hemispheric helicity rule as well
as a lesser-known twist-tilt relationship which is the point of this
work. Those regions that closely follow Joy's law show no twist-tilt
relationship as expected if the twist originates from convective
buffeting of initially untwisted and unwrithed flux tubes within the
convection zone through the Sigma effect. Those regions that strongly
depart from Joy's law show significantly larger than average twist
and a very strong twist-tilt relationship. These properties suggest
that the twist-tilt relationship in these regions is due to kinking of
flux tubes that are initially highly twisted but not strongly writhed
perhaps as a result of dynamo action
Title: The Advanced Spectroscopic and Coronagraphic Explorer (ASCE)
Mission Concept Study
Authors: Kohl, J.; Howard, R.; Davila, J.; Noci, G.; Gardner, L.;
Socker, D.; Romoli, M.; Strachan, L.; Floyd, L.; Cranmer, S.; Raymond,
J.; van Ballegooijen, A.
Bibcode: 2002AGUFMSH52A0463K
Altcode:
The ASCE Mission is currently in a Phase A feasibility study as a
candidate for the upcoming MIDEX selection. The ASCE science payload
provides next generation spectroscopic and polarimetric instrumentation
aimed at identifying the physical processes governing solar wind
generation and coronal mass ejections (CMEs). During the current phase,
engineering design and analyses have demonstrated the feasibility of
accomplishing the original mission objectives within the MIDEX mission
constraints. The launch is planned for early 2007 and the operations
and analyses are expected to continue for 5 years. ASCE data along with
data analysis software and calibration data will be unrestricted and
available to the scientific community via an automated web site. A
Guest Investigator program is planned with an average of 15 grants
running concurrently during 2008 to 2012. Grants would be awarded in
response to proposals submitted during the first and subsequent years
of the mission.
Title: Comparison of CME ejecta at 10 solar radii and 1 AU
Authors: Reinard, A.; Howard, R.; Zurbuchen, T.
Bibcode: 2002AGUFMSH21A0493R
Altcode:
We present results from a study of CME composition profiles measured by
the ACE spacecraft at 1 AU and extrapolated back to 10 solar radii based
on the assumption of constant velocity. The results will be compared
statistically to LASCO coronagraph CME observations on the limb to
determine how CME density features are organized. We will attempt to
determine a correspondence between visible CME density features such
as leading edge, cavity, and prominence with features at 1 AU.
Title: The Twist and Tilt of Active Region Magnetic Fields
Authors: Holder, Z. A.; McMullen, R. A.; Canfield, R. C.; Howard,
R. F.; Pevtsov, A. A.
Bibcode: 2002AAS...200.0305H
Altcode: 2002BAAS...34..642H
We combine two large datasets to study the formation of the overall
twist that is present in solar active-region magnetic fields. For
purposes of discussion, we adopt a simplified model in which the
magnetic flux bundles that form active regions start as axisymmetric
toroids, without intrinsic twist, and rise in Ω -shaped loops through
the convection zone, acquiring writhe through the effect of the Coriolis
force on the large-scale flows within them, as well as buffeting by
turbulent convection. The tilt of active regions with respect to the
equator is an observable manifestation of such writhe, at photospheric
levels. Owing to magnetic helicity conservation, we expect this tilt
to be related to twist of the fields of these same regions. Using Mees
Solar Observatory active-region vector magnetograms and Mount Wilson
Observatory full-disk longitudinal magnetograms, we have measured both
the tilt (Mount Wilson) and twist (Mees) of their magnetic fields,
on active-region scales. This dataset clearly shows two well-known
phenomena, Joy's law and the hemispheric handedness rule. In this
paper we present the relationship between twist and tilt and estimate
the extent to which that relationship is due to a mutual dependence of
tilt and twist on latitude. We then compare our observational results
to the simplified model.
Title: HElium Resonance Scattering in the Corona and HELiosphere
(HERSCHEL)
Authors: Moses, J. D.; Newmark, J.; Howard, R.; Auchere, F.; Antonucci,
E.; Fineschi, S.; Romoli, M.
Bibcode: 2002AGUSMSH21B..03M
Altcode:
The proposed HERSCHEL (HElium Resonance Scattering in the Corona and
HELiosphere) program will investigate coronal heating and solar wind
acceleration from a range of solar source structures by obtaining
simultaneous observations of the electron, proton and helium solar
coronae. The HERSCHEL will establish proof-of-principle for the
Ultra-Violet Coronagraph (UVC), which is in the ESA Solar Orbiter
Mission baseline. The NRL Solar Physics Branch is joining with the
Italian UVC Consortium to address the objectives of the International
Living With a Star program with this combination of NASA suborbital
program and ESA Solar Orbiter flight opportunities. Indeed, while
the Solar Orbiter flight is still many years away, the 3 year program
being proposed here is essential in order to prove the validity of this
exciting new concept before the Solar Orbiter instrument selection is
finalized. This proposal aims to develop instrumentation that for the
first time will directly image and characterize on a global coronal
scale the two must abundant elements, hydrogen and helium. This will
directly address three outstanding questions in the Sun-Earth Connection
theme: 1) Origin of the slow solar wind, 2) Acceleration mechanisms of
the fast solar wind, and 3) Variation of Helium abundance in coronal
structures. Additionally, by establishing proof of concept for the
UVC on Solar Orbiter, this will facilitate future investigations
of CME's kinematics, and solar cycle evolution of the electron,
proton, and helium coronae. Lastly, this mission fits the goals of
the International Living With a Star (ILWS) program. This work has
been supported by the Office of Naval Research.
Title: Colliding coronal mass ejections and particle acceleration
Authors: Gopalswamy, N.; Yashiro, S.; Kaiser, M.; Reames, D.;
Howard, R.
Bibcode: 2002cosp...34E1253G
Altcode: 2002cosp.meetE1253G
Colliding Coronal Mass Ejections (CMEs) have important implications to
a number of physical processes in the near-Sun interplanetary medium:
Shock propagation, particle acceleration and solar wind composition. We
present statistical results on large solar energetic particle events,
associated CMEs and CME interaction during solar cycle 23. We show
that most of the large SEP events are preceded by CME interaction. As
an inverse study, we identified all the fast and wide front side CMEs
from the western hemisphere and examined the SEP association and
CME interaction. We found that fast and wide CMEs interacting with
preceding CMEs are more likely to be associated with SEPs. We discuss
the implications of the statistical results to the understanding of
particle acceleration by CME-driven shocks.
Title: Propagation of coronal mass ejections from Sun to 1 AU
Authors: Manoharan, P.; Gopalswamy, N.; Yashiro, S.; Howard, R.
Bibcode: 2002cosp...34E2699M
Altcode: 2002cosp.meetE2699M
We report on the study of propagation characteristics of a large number
of CMEs over the entire range of Sun-Earth distance. Using white-light
(LASCO) and interplanetary scintillation (IPS) observations, we
investigate the radial variation of the speed of CMEs. In the case
of fast CMEs (initial speed 800 kms-1 ), speed declines slowly with
distance (VcmeR-a where a0.05-0.1) within about 100 Rsun . Beyond this
distance, the speed declines as VcmeR-b where b0.5 - 1. The evolution
of size of CMEs with distance, LcmeR, suggests a pressure balance
maintained between the CME and ambient solar wind at distances greater
than 50 Rsun . We also report the detection of interaction between
fast and slow CMEs outside the LASCO field of view. The interaction
signature is seen as an unusual enhancement in the density turbulence.
Title: Properties of Narrow Coronal Mass Ejections Observed with LASCO
Authors: Yashiro, S.; Gopalswamy, N.; Howard, R.
Bibcode: 2002cosp...34E2602Y
Altcode: 2002cosp.meetE2602Y
More than 4500 coronal mass ejections (CMEs) have been observed
with SOHO LASCO coronagraph from January 1996 to December 2001. We
have measured properties of all these CMEs and published them in an
online catalog. In this paper, we describe the properties of narrow
CMEs (width < 20 deg.). We investigated 675 narrow CMEs from the
catalog and found that (1) the fraction of narrow CMEs increases from
5% to 15% towards solar maximum, (2) the average speed of the narrow
CMEs is higher than that of the wide ones, (3) the maximum speed of
narrow CMEs (1141 km/s) is much smaller than that of the wide CMEs
(2604 km/s). We also found that the wide CMEs are likely to have the
well known three-part structure, but narrow ones do not. Wide CMEs
can be explained as due to the expansion of flux tubes, but the narrow
CMEs seem to be mass flows in vertical flux tubes (streamers).
Title: Origin of coronal streamer distention
Authors: Gopalswamy, N.; Shimojo, M.; Lu, W.; Yashiro, S.; Shibasaki,
K.; Howard, R.
Bibcode: 2002cosp...34E1257G
Altcode: 2002cosp.meetE1257G
Distention of coronal streamers is considered to be one of the
pre-eruption evolution of coronal mass ejections (CMEs), although how
mass is added to the streamers is poorly understood. During our study
of eruptive prominences and their relation to CMEs, we observed a large
number of prominences associated with significant changes in the helmet
streamers overlying the prominences. We used the white light images of
the corona obtained by the Solar and Heliospheric Mission's Large Angle
and Spectrometric Coronagraph images and the microwave images from the
Nobeyama radioheliograph in Japan. We found that the streamer distention
is associated with prominence eruption with mostly horizontal motion
(parallel to the solar limb) or with eruptive prominences with most of
the mass falling back to the solar surface. We suggest that the physical
process which activates the prominences also add mass to the streamers.
Title: Variability of solar eruptions during cycle 23
Authors: Gopalswamy, N.; Nunes, S.; Yashiro, S.; Howard, R.
Bibcode: 2002cosp...34E1260G
Altcode: 2002cosp.meetE1260G
Nearly 5000 coronal mass ejections (CMEs) were observed by the Solar
and Heliospheric Observatory from the minimum to maximum of the current
solar cycle (19962001). We have measured and cataloged the properties
of all these CMEs. We have studied the variation of mean and median
speeds and the rate of CMEs (averaged over Carrington rotations)
as a function time. We compare the CME rate with those of other
energetic solar events such as interplanetary type II bursts, solar
energetic particle (SEP) events and metric type II bursts. This study
is useful in identifying the phases of the solar cycle which show rapid
variability. CMEs associated with radio bursts and SEPs belong to a
separate group characterized by high speed and large width. We discuss
the solar cycle variability of this energetic group in comparison with
the general population of CMEs.
Title: An empirical model to predict the 1-AU arrival of
interplanetary shocks
Authors: Gopalswamy, N.; Lara, A.; Manoharan, P.; Howard, R.
Bibcode: 2002cosp...34E1256G
Altcode: 2002cosp.meetE1256G
We describe an empirical model to predict the 1-AU arrival of
interplanetary shocks of solar origin. This model is an extension of
Gopalswamy et al.'s [2001] empirical CME arrival model based on an
effective acceleration acting on the CMEs as they propagate through the
interplanetary medium. We measured the properties of a large number
IP shocks, their solar sources and associated CMEs. Using in situ
observations from Wind and ACE, we obtained the physical conditions
upstream and down stream of the shock. Combining the shock data with the
known piston-shock relation, we estimate the shock arrival times. We
compare the estimated and actual arrival times of shocks to determine
the error in our shock-arrival estimates. Reference: Gopalswamy, N.,
A. Lara, S. Yashiro, M. L. Kaiser, and R. A. Howard, Predicting the
1-AU Arrival Times of Coronal Mass Ejections, J. Geophys. Res., 106,
29,207, 2001
Title: Three Dimensional Modeling of a CME event Observed in LASCO
and UVCS
Authors: Krall, J.; Chen, J.; Howard, R.; Ciaravella, A.
Bibcode: 2001AGUFMSH12B0750K
Altcode:
The dynamics of magnetic flux ropes near the sun are studied by solving
model equations [1,2] which describe a flux-rope-geometry coronal
mass ejection (CME). This model is applied to a 2000 February 11 CME
event, which was observed by both the UVCS spectrometer and the LASCO
coronagraph. The UVCS spectrometer provides line-of-sight Doppler
velocities for the CME plasma, while LASCO images provide position
and velocity information for the CME projected onto the plane of the
sky. The result is three-dimensional (3D) data that can be compared to
3D model results. However, ambiguities in the model-data correspondence
depend both on the assumed density structure within the model flux rope
and the interpretation of the data. The data (EIT, MK4, LASCO, UVCS)
and corresponding model results will be presented and discussed. [1]
Chen, J. 1996, JGR, 101, 27499 [2] Krall, J. et al., 2000, ApJ, 539,
964 Supported by ONR.
Title: The Advanced Spectroscopic and Coronagraphic Explorer Mission
Authors: Kohl, J. L.; Howard, R.; Davila, J.; Noci, G.; Esser, R.;
Ciaravella, A.; Cranmer, S.; Fineschi, S.; Gardner, L.; Raymond, J.;
Romoli, M.; Smith, P.; Socker, D.; Strachan, L.; Van Ballegooijen, A.
Bibcode: 2001AGUFMSH31B0711K
Altcode:
SOHO has provided profound insights into the physics of solar wind
acceleration and coronal mass ejections. Although significant
progress has been made, most of the dominant physical processes
controlling these phenomena are still not identified. The Advanced
Spectroscopic and Coronagraphic Explorer Mission provides next
generation spectroscopic and polarimetric instrumentation aimed at
identifying these processes. The launch is planned for March 2007 with
mission operations and data analysis continuing for 5 years. The data
will be unrestricted and available to the community. The envisioned
program includes a Guest Investigator Program with an average of 15
grants to be awarded in response to proposals submitted during the
first year of the mission. Information about the proposed scientific
goals and instrumentation will be presented.
Title: Acceleration and Deceleration of CMEs Associated with Long
Wavelength Radio Bursts
Authors: Gopalswamy, N.; Yashiro, S.; Kaiser, M. L.; Howard, R.
Bibcode: 2001AGUSM..SH31C07G
Altcode:
Type II radio bursts in the Decameter-Hectometric (DH) wavelengths
indicate powerful MHD shocks leaving the inner corona. Almost all
of these bursts are associated with massive and faster-than-average
coronal mass ejections (CMEs). A particularly interesting characteristic
of these DH CMEs is that they are predominantly decelerating in the
coronagraph field of view. In the past, it was thought that there are
mainly constant speed and accelerating CMEs. We discuss the possible
explanations for the CME deceleration in the near-Sun interplanetary
medium. Research supported by NASA, NSF and Air Force Office of
Scientific Research
Title: Sunspot Evolution
Authors: Howard, R.
Bibcode: 2000eaa..bookE2297H
Altcode:
SUNSPOTS are observed to grow from their first appearance at the solar
surface to some maximum size and then to decay until they disappear
after some hours, days, or weeks. The characteristics of this growth
and decay, the changes during the lifetime of a sunspot group in its
distribution of sunspots, surface orientation, magnetic field line
inclination, and other characteristic variations give u...
Title: MEASUREMENT OF KODAIKANAL WHITE-LIGHT IMAGES - V. Tilt-Angle
and Size Variations of Sunspot Groups
Authors: Howard, Robert F.; Sivaraman, K. R.; Gupta, S. S.
Bibcode: 2000SoPh..196..333H
Altcode:
We examine here the variations of tilt angle and polarity separation
(as defined in this paper) of multi-spot sunspot groups from the
Kodaikanal and Mount Wilson data sets covering many decades. We confirm
the tilt-angle change vs tilt-angle result found earlier from the Mount
Wilson data alone. Sunspot groups tend on average to rotate their axes
toward the average tilt angle. We point out that if we separate groups
into those with tilt angles greater than and less than the average
value, they show tilt-angle variations that vary systematically with
the growth or decay rates of the groups. This result emphasizes again
the finding that growing (presumably younger) sunspot groups rotate
their magnetic axes more rapidly than do decaying (presumably older)
groups. The tilt-angle variation as a function of tilt angle differs for
those groups whose leading spots have greater area than their following
spots and vice versa. Tilt-angle changes and polarity separation changes
show a clear relationship, which has the correct direction and magnitude
predicted by the Coriolis force, and this strongly suggests that the
Coriolis force is largely responsible for the axial tilts observed in
sunspot groups. The distribution of polarity separations shows a double
peak. These peaks are perhaps related to super- and meso-granulation
dimensions. Groups with polarity separations less than 43 Mm expand
on average, while those groups with separations more than this value
contract on average. We present evidence that the rotation of the
magnetic axes of sunspot groups is about a location closer to the
following than to the leading sunspots.
Title: Exploring Coronal Structures with SOHO
Authors: Karovska, M.; Wood, B.; Chen, J.; Cook, J.; Howard, R.
Bibcode: 2000JApA...21..403K
Altcode:
No abstract at ADS
Title: Active Region Magnetic Fields
Authors: Howard, R. F.
Bibcode: 2000JApA...21..119H
Altcode:
No abstract at ADS
Title: Space VLBI at Low Frequencies
Authors: Jones, D. L.; Allen, R.; Basart, J.; Bastian, T.; Blume, W.;
Bougeret, J. -L.; Dennison, B.; Desch, M.; Dwarakanath, K.; Erickson,
W.; Farrell, W.; Finley, D.; Gopalswamy, N.; Howard, R.; Kaiser,
M.; Kassim, N.; Kuiper, T.; MacDowall, R.; Mahoney, M.; Perley, R.;
Preston, R.; Reiner, M.; Rodriguez, P.; Stone, R.; Unwin, S.; Weiler,
K.; Woan, G.; Woo, R.
Bibcode: 2000aprs.conf..265J
Altcode: 2000astro.ph..3120J
At sufficiently low frequencies, no ground-based radio array will
be able to produce high resolution images while looking through
the ionosphere. A space-based array will be needed to explore the
objects and processes which dominate the sky at the lowest radio
frequencies. An imaging radio interferometer based on a large number
of small, inexpensive satellites would be able to track solar radio
bursts associated with coronal mass ejections out to the distance
of Earth, determine the frequency and duration of early epochs of
nonthermal activity in galaxies, and provide unique information about
the interstellar medium. This would be a ``space-space" VLBI mission,
as only baselines between satellites would be used. Angular resolution
would be limited only by interstellar and interplanetary scattering.
Title: The Astronomical Low Frequency Array: A Proposed Explorer
Mission for Radio Astronomy
Authors: Jones, D.; Allen, R.; Basart, J.; Bastian, T.; Blume, W.;
Bougeret, J. -L.; Dennison, B.; Desch, M.; Dwarakanath, K.; Erickson,
W.; Finley, D.; Gopalswamy, N.; Howard, R.; Kaiser, M.; Kassim, N.;
Kuiper, T.; MacDowall, R.; Mahoney, M.; Perley, R.; Preston, R.;
Reiner, M.; Rodriguez, P.; Stone, R.; Unwin, S.; Weiler, K.; Woan,
G.; Woo, R.
Bibcode: 2000GMS...119..339J
Altcode: 2000ralw.conf..339J
A radio interferometer array in space providing high dynamic range
images with unprecedented angular resolution over the broad frequency
range from 0.03 - 30 MHz will open new vistas in solar, terrestrial,
galactic, and extragalactic astrophysics. The ALFA interferometer
will image and track transient disturbances in the solar corona
and interplanetary medium - a new capability which is crucial
for understanding many aspects of solar-terrestrial interaction
and space weather. ALFA will also produce the first sensitive,
high-angular-resolution radio surveys of the entire sky at low
frequencies. The radio sky will look entirely different below about
30 MHz. As a result, ALFA will provide a fundamentally new view of
the universe and an extraordinarily large and varied science return.
Title: Coronal Mass Ejections and Large Scale Structure of the Corona
Authors: Maia, D.; Vourlidas, A.; Pick, M.; Howard, R.; Schwenn, R.;
Lamy, P.
Bibcode: 2000AdSpR..25.1843M
Altcode:
A comparative study of two events accompanied by both a flare and a
CME has been performed. The data analysis has been made by comparing
the observations of the LASCO/SOHO coronagraphs with those of the
Nancay radioheliograph. The observations show a clear connection
between coronal green and red line transient activity, burst radio
emission and the CME development which is due to successive loop
interactions. Signatures of these interactions are given by the radio
emission. One can identify successive sequences in the evolution
of the coronal restructuring leading to the full development of the
CME. Identification and timing of these sequences result from the radio
emission analysis. For flare-CME events , the evolution takes place
in the low corona and is extremely fast of the order, on a few minutes
Title: Measurement of Kodaikanal white-light images - IV. Axial Tilt
Angles of Sunspot Groups
Authors: Sivaraman, K. R.; Gupta, S. S.; Howard, Robert F.
Bibcode: 1999SoPh..189...69S
Altcode:
The Kodaikanal sunspot data set, covering the interval 1906-1987, is
used in conjunction with the similar Mount Wilson sunspot data set,
covering the interval 1917-1985, to examine characteristics of sunspot
group axial tilt angles. Good agreement is demonstrated between various
results derived from the two independent data sets. In particular, the
tendency for sunspot groups near the average tilt angle to be larger
than those far from the average tilt angle is confirmed. Similarly the
faster residual rotation rate for groups near the average tilt angle
is also confirmed. Other confirmations are made for the relationships
between latitude drift of sunspot groups and tilt angle, polarity
separations, and axial expansion. Evidence is presented that tilt
angles averaged over these long time intervals differ between the
north and south hemispheres by about 1.4 deg. It is suggested that
residual tilt angles show a slight systematic variation with phase in
the activity cycle.
Title: The sungrazing comets discovered with the SOHO/LASCO
coronagraphs: 1996-1998.
Authors: Biesecker, D. A.; Lamy, P.; St. Cyr, O. C.; Llebaria, A.;
Howard, R.
Bibcode: 1999DPS....31.1404B
Altcode:
An unprecedented number of Kreutz sungrazing comets have been discovered
with the LASCO coronagraphs on the SOHO spacecraft. We present here
the results of the analysis of the first 53 sungrazing comets, which
were discovered between January, 1996 and June, 1998. In this poster, we
summarize the capabilities and calibration of the LASCO coronagraphs for
comet observations. We discuss the frequency of the comet discoveries
and summarize the properties of the computed orbits. We show examples
of typical comet light curves and discuss their common features. In
particular, we show that the comets are completely disintegrated before
they reach perihelion. One particular feature of the observations is
the presence of a dust tail for only a few sungrazers while no tail
is evident for the majority of them. Analysis of the light curves is
used to investigate the properties of the nuclei (size, fragmentation,
destruction) and the dust production rates. This work was funded in
part by NASA SOHO-GI Grant NAG5-8003.
Title: The sungrazing comets discovered with the SOHO/LASCO
coronagraphs: 1996-1998.
Authors: Biesecker, D. A.; Lamy, P.; St. Cyr, O. C.; Llebaria, A.;
Howard, R.
Bibcode: 1999BAAS...31.1094B
Altcode:
No abstract at ADS
Title: Measurement of Kodaikanal White-Light Images - III. Rotation
Rates and Activity Cycle Variations
Authors: Gupta, S. S.; Sivaraman, K. R.; Howard, Robert F.
Bibcode: 1999SoPh..188..225G
Altcode:
The Kodaikanal sunspot data set covering the interval 1906-1987 is
analyzed for differential rotation of sunspots of different sizes. As
is known, smaller sunspots rotate faster than larger sunspots, and
this result is verified in the analysis of this data set. These results
agree well with the Mount Wilson sunspot results published earlier. The
activity cycle dependence of sunspot rotation is studied. An increase
in this rate at the minimum phase is seen, which has been reported
earlier. It is demonstrated that this cycle variation is seen for
sunspots in all size categories, which suggests that it is not a
relative increase in the number of the faster-rotating small sunspots
that causes the cycle dependence. These results are discussed as they
may relate to subsurface dynamic properties of the Sun.
Title: LASCO/SOHO Observations of Dust in the Outer Solar Corona
Authors: Kimura, H.; Mann, I.; Goldstein, B.; Korendyke, C.; Howard, R.
Bibcode: 1999DPS....31.5501K
Altcode:
The solar F-corona emission is comprised of solar radiation scattered
by dust particles and thermal radiation emitted from near-solar dust
particles. The visible brightness is mainly produced by scattering
at medium scattering angles from particles near the Sun and by
enhanced forward scattering from particles near the observer. The
infrared brightness originates from the thermal emission from hot
particles near the Sun. Studies of the F-corona are usually limited
by the influence of atmospheric stray light and by difficulties of
the separation of the K-corona, produced by sunlight scattered at
electrons. The K-corona decreases steeply with increasing elongation
and has a smaller contribution to the outer coronal brightness. This
outer corona is observed from the SOHO satellite where the lack of
atmospheric stray light and an optimized suppression of instrumental
stray light in the LASCO coronagraph allow for the detection of the
coronal brightness as far out as about 30 solar radii from the center
of the Sun. These observations yield the opportunity to study the
properties of interplanetary dust in the inner solar system. We will
present preliminary results from the analysis of the data from the LASCO
C3 coronagraph at distances from 10 to 30 solar radii from the center
of the Sun in 3 wavelength intervals between 0.4 and 1.1 micron. We
compare the data to brightness calculations in order to discuss the
distribution of dust grains in the inner solar system. The Solar
Heliospheric Observatory, SOHO, is a joint scientific space mission
developed by ESA and NASA. The Large Angle Spectrometric Coronagraph
(LASCO) was developed and is operated jointly by the Naval Research
Laboratory (USA), the Max-Planck-Institut fur Aeronomie (Germany),
the Laboratoire d'Astronomie Spatiale (France) and the University of
Birmingham (UK).
Title: LASCO/SOHO observations of dust in the outer solar corona.
Authors: Kimura, H.; Mann, I.; Goldstein, B.; Korendyke, C.; Howard, R.
Bibcode: 1999BAAS...31.1159K
Altcode:
No abstract at ADS
Title: Large-scale structure and coronal dynamics from joint radio,
SOHO/EIT and coronagraph observations
Authors: Pick, M.; Maia, D.; Vourlidas, A.; Benz, A. O.; Howard, R.;
Thompson, B. J.
Bibcode: 1999AIPC..471..649P
Altcode: 1999sowi.conf..649P
This study presents joint observations of an `halo' coronal mass
ejection from the EIT telescope and LASCO coronagraphs on SOHO, from
the Nançay Radioheliograph (NRH) and the Zurich ETH radiospectrograph
(Phoenix-2). This event includes different manifestations: a coronal
wave and a dimming region detected by EIT, a CME showing bright discrete
portions above east and west limbs. Radio signatures of all these
manifestations are found and the interpretation is briefly discussed.
Title: Radio signatures of a fast coronal mass ejection development
on November 6, 1997
Authors: Maia, D.; Vourlidas, A.; Pick, M.; Howard, R.; Schwenn, R.;
Magalhães, A.
Bibcode: 1999JGR...10412507M
Altcode:
The Oporto radiospectrograph and the Nançay radioheliograph recorded
a radio event on November 6, 1997, closely related in time with a flare
on National Oceanic and Atmospheric Administration (NOAA) active region
8100. At the beginning of the event the radio sources are located on a
rather small volume in the vicinity of the flare site. In a timescale of
only a few minutes the radio emission sites spread over a large volume
in the corona, covering a range of 100° in heliolatitude. During the
period of the radio event the Large Angle and Spectrometric Coronagraph
(LASCO) on board the Solar and Heliospheric Observatory (SOHO) observed
an extremely fast coronal mass ejection (CME), with a velocity around
2000 kms-1. This CME presents the particularity of having
a fast lateral expansion, giving it a shape reminiscent of a ``coat
hanger.'' There is a very good association between the latitudinal
extent and time development of the CME seen by LASCO and the radio
sources recorded by the radio instruments.
Title: Measurement of Kodaikanal White-Light Images - II. Rotation
Comparison and Merging with Mount Wilson Data
Authors: Howard, Robert F.; Gupta, S. S.; Sivaraman, K. R.
Bibcode: 1999SoPh..186...25H
Altcode:
Sunspot umbral positions and areas were measured for 82 years
(1906-1987) of daily, full-disk photoheliogram observations at the
Kodaikanal station of the Indian Institute of Astrophysics. The
measurement technique and reduction procedures used were nearly
identical to those used earlier for the reduction of Mount Wilson
daily full-disk photoheliograms, covering an overlapping interval of
69 years. In this paper we compare the differential rotation of the
Sun from the analysis of the Kodaikanal data with the Mount Wilson
results. In addition, we analyze the data set formed by combining the
data from the two sites for differential rotation. While doing this,
it has become apparent to us that small, subtle optical effects at
both sites produce systematic errors that have an influence on rotation
(and other) results from these data. These optical effects are analyzed
here, and corrections are made to the positional data of the sunspots
from both sites. A data set containing the combined positional data
of sunspots from both sites, corrected for these optical aberrations,
has been constructed. Results for both sunspot groups and individual
sunspots are presented. It is pointed out that optical aberrations
similar to those found in the Kodaikanal data may also exist in the
Greenwich photoheliograph data, because these two sets of solar images
were made with similar telescopes.
Title: The Advanced Solar Coronal Explorer Mission (ASCE)
Authors: Kohl, J.; Cranmer, S.; Gardner, L.; Golub, L.; Raymond, J.;
Smith, P. L.; Strachan, L.; Howard, R.; Moses, D.; Socker, D.; Wang,
D.; Fisher, R. R.; Davila, J.; St. Cyr, C.; Noci, G.; Tondello, G.
Bibcode: 1999AAS...194.6506K
Altcode: 1999BAAS...31Q.928K
The Advanced Solar Coronal Explorer (ASCE) mission was selected
for a Phase A Concept Study in the current round of proposed MIDEX
missions. It addresses three fundamental problems: 1) What physical
processes heat coronal holes and drive the fast solar wind? 2) What
physical processes heat streamers and drive the slow solar wind? and 3)
How are coronal mass ejections (CMEs) heated and accelerated, and what
role to they play in the evolution of the solar magnetic field. ASCE
has two instruments, the Spectroscopic and Polarimetric Coronagraph
(SPC) and the Extreme Ultraviolet Imager (EUVI). A deployable boom
supports a distant external occulter that allows large aperture optics
for the SPC coronagraphic channels. SPC's EUV channels will provide
spectroscopy of the extended solar corona with 30 - 200 times the
sensitivity of UVCS/SOHO and the first He II 30.4 nm spectroscopy of
the extended corona. SPC's Large Aperture Spectroscopic Coronagraph
channel will provide two orders of magnitude improvement in stray
light suppression for wide field visible spectroscopy and 2 arcsec
resolution elements for imaging and polarimetry. EUVI provides full
disk imaging with 0.9 arcsec resolution elements and extremely high
cadence. ASCE is designed to determine the thermal, kinetic, and
wave energy densities in coronal structures, determine the rates of
transformation among these forms of energy, their flow in space, and
their loss to radiation, and determine the composition and ionization
state of the corona in static and transient conditions.
Title: Measurement of Kodaikanal White-Light Images
Authors: Howard, Robert F.
Bibcode: 1999STIN...0179652H
Altcode:
Sunspot umbral positions and areas were measured for 82 years
(1906-1987) of daily, full-disk photoheliogram observations at the
Kodaikanal station of the Indian Institute of Astrophysics. The
measurement technique and reduction procedures used were nearly
identical to those used earlier for the reduction of Mount Wilson
daily full-disk photoheliograms, covering an overlapping interval of
69 years. In this paper we compare the differential rotation of the
Sun from the analysis of the Kodaikanal data with the Mount Wilson
results. In addition, we analyze the data set formed by combining the
data from the two sites for differential rotation. While doing this,
it has become apparent to us that small, subtle optical effects at
both sites produce systematic errors that have an influence on rotation
(and other) results from these data. These optical effects are analyzed
here, and corrections are made to the positional data of the sunspots
from both sites. A data set containing the combined positional data
of sunspots from both sites, corrected for these optical aberrations,
has been constructed. Results for both sunspot groups and individual
sunspots are presented. It is pointed out that optical aberrations
similar to those found in the Kodaikanal data may also exist in the
Greenwich photoheliograph data, because these two sets of solar images
were made with similar telescopes.
Title: Joint Nancay Radioheliograph and LASCO Observations of Coronal
Mass Ejections - II. The 9 July 1996 Event
Authors: Pick, M.; Maia, D.; Kerdraon, A.; Howard, R.; Brueckner,
G. E.; Michels, D. J.; Paswaters, S.; Schwenn, R.; Lamy, P.; Llebaria,
A.; Simnett, G.; Lanzerotti, L. J.; Aurass, H.
Bibcode: 1998SoPh..181..455P
Altcode:
The development of a coronal mass ejection on 9 July 1996 has been
analyzed by comparing the observations of the LASCO/SOHO coronagraphs
with those of the Nancay radioheliograph. The spatial and temporal
evolution of the associated radioburst is complex and involves a
long-duration continuum. The analysis of the time sequence of the
radio continuum reveals the existence of distinct phases associated
with distinct reconnection processes and magnetic restructuring
of the corona. Electrons are accelerated in association with these
reconnection processes. An excellent spatial association is found
between the position and extension of the radio source and the CME seen
by LASCO. Furthermore, it is shown that the topology and evolution
of the source of the radio continuum involve successive interactions
between two systems of loops. These successive interactions lead to
magnetic reconnection, then to a large scale coronal restructuring. Thus
electrons of coronal origin may have access to the interplanetary
medium in a large range of heliographic latitudes as revealed by the
Ulysses observations.
Title: Joint Nançay Radioheliograph and LASCO Observations of
Coronal Mass Ejections - I. The 1 July 1996 Event
Authors: Maia, D.; Pick, M.; Kerdraon, A.; Howard, R.; Brueckner,
G. E.; Michels, D. J.; Paswaters, S.; Schwenn, R.; Lamy, P.; Llebaria,
A.; Simnett, G.; Aurass, H.
Bibcode: 1998SoPh..181..121M
Altcode:
The development of a coronal mass ejection on 1 July 1996 has been
analyzed by comparing the observations of the LASCO/SOHO coronagraph
with those of the Nançay radioheliograph. This comparison brings new
insight and very useful diagnosis for the study of CME events. It
is shown that the initial instability took place in a small volume
located above an active region and that the occurrence of short radio
type III bursts implies a triggering process due to magnetic field
interactions. The subsequent spatial and temporal evolution of the
radio emission strongly suggests that the large scale structure becomes
unstable within the first minute of the event.
Title: Magnetic Field of the Sun as a Star: The Mount Wilson
Observatory Catalog 1970-1982
Authors: Kotov, V. A.; Scherrer, P. H.; Howard, R. F.; Haneychuk, V. I.
Bibcode: 1998ApJS..116..103K
Altcode:
Measurements of the mean magnetic field of the Sun (MMFS) seen as
a star were regularly conducted at the Mount Wilson Observatory
from 1970 October through 1982 December. A listing is presented of
all these data (2457 daily values) suitable for comparison with
similar data of other observatories and for studies of magnetic
variability and rotation of the Sun. The scatter-plot diagrams and
power spectra of the Mount Wilson data and also of the total data
1968-1991 (collected from three observatories: Crimean Astrophysical
Observatory, Mount Wilson Observatory, and Wilcox Solar Observatory)
are also presented. Time variations of the MMFS connected with solar
rotation at periods ~27-28 days and also an enigmatic 1 yr variation
are briefly discussed. The power spectrum of the 24 yr data set
shows that the most significant and phase-coherent synodic periods of
the MMFS variations are 26.92 +/- 0.02 and 27.13 +/- 0.02 days (both
are thought to be associated with rotation of the large-scale surface
magnetic field near equator of the Sun) and 28.13 +/- 0.02 days. It
is suggested that the latter period reflects ``rigid'' rotation of
the global magnetic field concentrated under the bottom of the solar
convection zone. The arguments are given in favor of reality and high
confidence level of major periodicities exhibited by MMFS variations.
Title: The November 6, 1997 event: Radio signatures of the CME
development
Authors: Maia, D.; Vourlidas, A.; Pick, M.; Howard, R.; Schwenn, R.;
Magalhaes, A.; Carneiro, J.; Agostinho, R.
Bibcode: 1998cee..workE..57M
Altcode:
The analysis of the November 6, 1997 event has taken advantage of the
new capabilities of the OPorto Radiospectrograph and of the Nanccay
Radioheliograph. The evolution of this event shows successive phases
in time scales of a fraction of a second. It is shown that the CME
observed by the LASCO/SOHO coronograph is the result of successive
interactions of multiple loop systems which occur over a latitude range
of about 100circ. These magnetic loop interactions lead to
the creation of several electron acceleration sites which are widely
separated in the corona. There is a close correspondance between the
evolution of the CME seen by LASCO and the extend of radio sources
seen by the radioheliograph. The association with particles detected
by in situ measurements in the interplanetary medium is also presented.
Title: Synoptic Solar Physics
Authors: Howard, Robert F.
Bibcode: 1998ASPC..140....3H
Altcode: 1998ssp..conf....3H
No abstract at ADS
Title: A magnetic cloud containing prominence material: January 1997
Authors: Burlaga, L.; Fitzenreiter, R.; Lepping, R.; Ogilvie, K.;
Szabo, A.; Lazarus, A.; Steinberg, J.; Gloeckler, G.; Howard, R.;
Michels, D.; Farrugia, C.; Lin, R. P.; Larson, D. E.
Bibcode: 1998JGR...103..277B
Altcode:
This work discusses the relations among (1) an interplanetary
force-free magnetic cloud containing a plug of cold high-density
material with unusual composition, (2) a coronal mass ejection (CME),
(3) an eruptive prominence, and (4) a model of prominence material
supported by a force-free magnetic flux rope in a coronal streamer. The
magnetic cloud moved past the Wind spacecraft located in the solar
wind upstream of Earth on January 10 and 11, 1997. The magnetic field
configuration in the magnetic cloud was approximately a constant-α,
force-free flux rope. The 4He++/H+
abundance in the most of the magnetic cloud was similar to that of the
streamer belt material, suggesting an association between the magnetic
cloud and a helmet streamer. A very cold region of exceptionally high
density was detected at the rear of the magnetic cloud. This dense
region had an unusual composition, including (1) a relatively high
(10%) 4He++/He+ abundance (indicating
a source near the photosphere), and (2) 4He+,
with an abundance relative to 4He++ of ~1%,
and the unusual charge states of O5+ and Fe5+
(indicating a freezing-in temperature of (1.6-4.0)×105°K,
which is unusually low, but consistent with that expected for
prominence material). Thus we suggest that the high-density region
might be prominence material. The CME was seen in the solar corona
on January 6, 1997, by the large angle and spectrometric coronagraph
(LASCO) instrument on SOHO shortly after an eruptive prominence. A
helmet streamer was observed near the latitude of the eruptive
prominence a quarter of a solar rotation before and after the eruptive
prominence. These observations are consistent with recent models,
including the conceptual model of Low and Hundhausen [1995] for a
quasi-static helmet streamer containing a force-free flux rope which
supports prominence material and the dynamical model of Wu et al. [1997]
for CMEs produced by the disruption of such a configuration.
Title: Solar origin of accelerated particles detected in the corona
and in the interplanetary medium
Authors: Pick, M.; Maia, D.; Howard, R.; Vourlidas, A.
Bibcode: 1998cee..workE..58P
Altcode:
We discuss the solar origin of accelerated particles detected in
the corona and in the interplanetary medium. This synthesis lies on
the study of several events which have been observed by the Nanccay
Radioheliograph and the LASCO/SOHO instrument. The in-situ measurements
of particles in the interplanetary medium have been made by ULYSSES,
WIND, ACE and SOHO.
Title: Development of Coronal Mass Ejections and Association with
Interplanetary Events
Authors: Pick, M.; Maia, D.; Howard, R.; Thompson, B.; Lanzerotti,
L. J. L.; Bothmer, V.; Lamy, P.
Bibcode: 1997ESASP.415..195P
Altcode: 1997cpsh.conf..195P
No abstract at ADS
Title: Solar Physics Announces CD-ROM
Authors: Svestka, Zdenek; Howard, Robert F.; Engvold, Oddbjorn
Bibcode: 1997SoPh..176..443S
Altcode:
No abstract at ADS
Title: Polar Plume Anatomy: Results of a Coordinated Observation
Authors: DeForest, C. E.; Hoeksema, J. T.; Gurman, J. B.; Thompson,
B. J.; Plunkett, S. P.; Howard, R.; Harrison, R. C.; Hasslerz, D. M.
Bibcode: 1997SoPh..175..393D
Altcode:
On 7 and 8 March 1996, the SOHO spacecraft and several other space-
and ground-based observatories cooperated in the most comprehensive
observation to date of solar polar plumes. Based on simultaneous
data from five instruments, we describe the morphology of the plumes
observed over the south pole of the Sun during the SOHO observing
campaign. Individual plumes have been characterized from the photosphere
to approximately 15 R⊙ yielding a coherent portrait of the features
for more quantitative future studies. The observed plumes arise from
small (∼ 2-5 arc sec diameter) quiescent, unipolar magnetic flux
concentrations, on chromospheric network cell boundaries. They are
denser and cooler than the surrounding coronal hole through which they
extend, and are seen clearly in both Feix and Fexii emission lines,
indicating an ionization temperature between 1.0-1.5 x 106
K. The plumes initially expand rapidly with altitude, to a diameter of
20-30 Mm about 30 Mm off the surface. Above 1.2 R⊙ plumes are observed
in white light (as `coronal rays') and extend to above 12 R⊙. They
grow superradially throughout their observed height, increasing their
subtended solid angle (relative to disk center) by a factor of ∼10
between 1.05 R⊙ and 4-5 R⊙ and by a total factor of 20-40 between
1.05 R⊙ and 12 R⊙. On spatial scales larger than 10 arc sec,
plume structure in the lower corona (R < 1.3 R⊙) is observed to
be steady-state for periods of at least 24 hours; however, on spatial
scales smaller than 10 arc sec, plume XUV intensities vary by 10-20%
(after background subtraction) on a time scale of a few minutes.
Title: A Strong Maximum Principle for Weak Solutions of Quasi-Linear
Elliptic Equations with Applications to Lorentzian and Riemannian
Geometry
Authors: Andersson, L.; Galloway, G. J.; Howard, R.
Bibcode: 1997dg.ga.....7015A
Altcode:
The strong maximum principle is proved to hold for weak (in the sense of
support functions) sub- and super-solutions to a class of quasi-linear
elliptic equations that includes the mean curvature equation for $C^0$
spacelike hypersurfaces in a Lorentzian manifold. As one application
a Lorentzian warped product splitting theorem is given.
Title: Tracking a CME from Cradle to Grave: A Multi-wavelength
Analysis of the February 6-7, 1997 Event
Authors: Gopalswamy, N.; Kundu, M. R.; Hanaoka, Y.; Kosugi, T.; Hudson,
H.; Nitta, N.; Thompson, B.; Gurman, J.; Plunkett, S.; Howard, R.;
Burkepile, J.
Bibcode: 1997SPD....28.0501G
Altcode: 1997BAAS...29..908G
The partially earth-directed coronal mass ejection (CME) event of 1997
February 6-7 originated from the southwest quadrant of the sun. The
CME accelerated from 170 km/s to about 830 km/s when it reached a
distance of 25 solar radii. The CME was an arcade eruption followed
by bright prominence core structures. The prominence core was tracked
continuously from the solar surface to the interplanetary medium by
combining data from the Nobeyama radioheliograph (microwaves), Mauna Loa
Solar Observatory (He 10830 { Angstroms}), SOHO/EIT (EUV) and SOHO/LASCO
(white light). The CME was accompanied by an arcade formation, fully
observed by the YOHKOH/SXT (soft X-rays) and SOHO/EIT (EUV). The X-ray
and EUV observations suggest that the reconnection proceeded from
the northwest end to the southeast end of a filament channel. In the
SOHO/EIT images, the the feet of the soft X-ray arcade were observed
as EUV ribbons. The CME event also caused a medium sized geomagnetic
storm: The hourly equatorial Dst values attained storm level during
18:00-19:00 UT on February 09. This means the disturbance took about
2.25 days to reach the Earth. The first signatures of an IP shock was
a pressure jump in the WIND data around 13:00 UT on Feb 09, 1997 which
lasted for about 14 hours, followed by flux rope signatures. This CME
event confirms a number of ideas about CMEs: The three part structure
(frontal bright arcade, dark cavity and prominence core), disappearing
filament, elongated arcade formation, and terrestrial effects. We make
use of the excellent data coverage from the solar surface to the Earth
to address a number of issues regarding the origin and propagation of
the geoeffective solar disturbances. We benefited from discussions at
the first SOHO-Yohkoh Coordinated Data Analysis Workshop, held March
3-7, 1997, at Goddard Space Flight Center.
Title: Observations of Coronal Features by EIT above an Active Region
by EIT and Implications for Coronal Heating
Authors: Neupert, W. M.; Newmark, J.; Thompson, B. J.; Catura, R.;
Moses, J. D.; Portier-Fozzani, F.; Delaboudiniere, J. P.; Gabriel, A.;
Artzner, G.; Clette, F.; Cugnon, P.; Maucherat, A.; Defise, J. M.;
Jamar, C.; Rochus, P.; Howard, R.; Michels, D.; Dere, K.; Freeland,
S.; Lemen, J.; Stern, R.; Gurman, J.
Bibcode: 1997SPD....28.0115N
Altcode: 1997BAAS...29..881N
The EUV Imaging Telescope (EIT) on the SOHO provides the capability
for multi-wavelength imaging of the corona in four spectral bands,
centered at 171, 195, 284, and 304 Angstroms, using multilayer telescope
technology. These bands encompass coronal temperatures from 1 MK
to 2.5 MK as well as the upper chromosphere, at about 60,000 K. In
particular, nearly simultaneous imaging in the 171 and 195 Angstrom
bands, the former including major Fe IX and Fe X emission lines, the
latter including a strong Fe XII line, provides a capability to infer
the morphology and characteristics of the corona at temperatures of
1.0 - 1.7 MK. We have examined the corona in this temperature range
over an active region observed from SOHO from May - September, 1996 and
find that low-lying loops (below a density scale height of 75,000 km,
characteristic of Fe X) vary little in brightness and temperature along
their length. For features extending to greater heights, however, both
brightness gradients and temperature gradients are observed. Preliminary
analysis of the observations when the region was on the West limb
on September 30 indicates a small positive temperature gradient of
approximately 0.5 K/km in one loop system that extended above 100,000
km. On the other hand, a nearly radial feature extending to the edge of
the EIT FOV was isothermal or had at most a slight negative temperature
gradient. Such measurements may have application to the modeling of
coronal loops and streamers and the processes of coronal heating and
solar wind acceleration.
Title: Obituary: Walter E. Mitchell, 1925-1996
Authors: Howard, Robert F.
Bibcode: 1997BAAS...29.1480H
Altcode:
No abstract at ADS
Title: Characterisation of Polar Plumes from LASCO-C2 Images in
Early 1996
Authors: Lamy, P.; Liebaria, A.; Koutchmy, S.; Reynet, P.; Molodensky,
M.; Howard, R.; Schwenn, R.; Simnett, G.
Bibcode: 1997ESASP.404..487L
Altcode: 1997cswn.conf..487L
No abstract at ADS
Title: The April 7, 1997 Event: LASCO and Nancay Radioheliograph
Joint Observations
Authors: Maia, D.; Pick, M.; Howard, R.; Brueckner, G. E.; Lamy, P.
Bibcode: 1997ESASP.404..539M
Altcode: 1997cswn.conf..539M
No abstract at ADS
Title: Electronic Densities in Coronal Holes from LASCO-C2 Images
Authors: Lamy, P.; Quemerais, E.; Llebaria, A.; Bout, M.; Howard,
R.; Schwenn, R.; Simnett, G.
Bibcode: 1997ESASP.404..491L
Altcode: 1997cswn.conf..491L
No abstract at ADS
Title: Joint Radioheliograph and LASCO Observations of Coronal
Mass Ejections
Authors: Pick, M.; Maia, D.; Howard, R.; Kerdraon, A.; Brueckner,
G. E.; Lamy, P.; Schwenn, R.; Aurass, H.
Bibcode: 1997ESASP.404..601P
Altcode: 1997cswn.conf..601P
No abstract at ADS
Title: Analysis of a High Latitude Slow CME with Travelling Ejecta
Authors: Boulade, S.; Delanné, C.; Koutchmy, S.; Lamy, P.; Llebaria,
A.; Howard, R.; Schwenn, R.; Simnett, G.
Bibcode: 1997ESASP.404..217B
Altcode: 1997cswn.conf..217B
No abstract at ADS
Title: Axial Tilt Angles of Active Regions
Authors: Howard, Robert F.
Bibcode: 1996SoPh..169..293H
Altcode:
Separate Mount Wilson plage and sunspot group data sets are analyzed in
this review to illustrate several interesting aspects of active region
axial tilt angles. (1) The distribution of tilt angles differs between
plages and sunspot groups in the sense that plages have slightly higher
tilt angles, on average, than do spot groups. (2) The distributions
of average plage total magnetic flux, or sunspot group area, with
tilt angle show a consistent effect: those groups with tilt angles
nearest the average values are larger (or have a greater total flux)
on average than those farther from the average values. Moreover,
the average tilt angles on which these size or flux distributions are
centered differ for the two types of objects, and represent closely
the actual different average tilt angles for these two features. (3)
The polarity separation distances of plages and sunspot groups show
a clear relationship to average tilt angles. In the case of each
feature, smaller polarity separations are correlated with smaller tilt
angles. (4) The dynamics of regions also show a clear relationship
with region tilt angles. The spot groups with tilt angles nearest the
average value (or perhaps 0-deg tilt angle) have on average a faster
rotation rate than those groups with extreme tilt angles.
Title: Book reviews
Authors: Howard, R. F.; van den Oord, G. H. J.; Švestka, Z.
Bibcode: 1996SoPh..169..225H
Altcode:
No abstract at ADS
Title: Tilt-Angle Variations of Active Regions
Authors: Howard, Robert F.
Bibcode: 1996SoPh..167...95H
Altcode:
An examination of the tilt angles of multi-spot sunspot groups and
plages shows that on average they tend to rotate toward the average
tilt angle in each hemisphere. This average tilt angle is about
twice as large for plages as it is for sunspot groups. The larger the
deviation from the average tilt angle, the larger, on average, is the
rotation of the magnetic axis in the direction of the average tilt
angle. The rate of rotation of the magnetic axis is about twice as
fast for sunspot groups as it is for plages. Growing plages and spot
groups rotate their axes significantly faster than do decaying plages
and spot groups. There is a latitude dependence of this effect that
follows Joy's law. The fact that these tilt angles move toward the
average tilt angle and not toward 0 deg (the east-west orientation),
combined with other results presented here, suggest that a commonly
accepted view of the origin of active region magnetic flux at the
solar surface may have to be re-examined.
Title: Measurement of solar rotation from Kodaikanal images
Authors: Gupta, S. S.; Sivaraman, K. R.; Howard, R.
Bibcode: 1996BASI...24..189G
Altcode:
No abstract at ADS
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: Observations of the South coronal hole from EIT and YOHKOH
Authors: Handy, B. N.; Catura, R.; Freeland, S.; Lemen, J.; Stern,
R.; Gurman, J. B.; Delaboudiniere, J. P.; Artzner, G.; Gabriel, A.;
Maucherat, A.; Defise, J. M.; Jamar, C.; Rochus, P.; Clette, F.;
Cugnon, P.; Howard, R.; Michels, D.; Moses, J. D.; Dere, K.; Cyr,
O. C. St.; Catura, R.; Freeland, S.; Lemen, J.; Stern, R.; Neupert,
W.; Einfalt, E.; Newmark, J.
Bibcode: 1996AAS...188.0206H
Altcode: 1996BAAS...28..821H
The Extreme ultraviolet Imaging Telescope (EIT) on board the
SOHO spacecraft is capable of studying solar transition region,
chomospheric and coronal plasmas over bandpasses optimized for He II
304 Angstroms (0.08 MK), Fe IX/X 171, 173 Angstroms (0.8 - 1.0 MK),
Fe XII 195 Angstroms (1.5 MK), and Fe XV 284 Angstroms (2.0 - 2.5
MK) with 2.5 arcsecond spatial resolution. This telescope in concert
with the Yohkoh/SXT instrument allows us to simultaneously observe
solar structures at temperatures ranging from less than 0.1MK in the
transition region to over 3MK in the solar corona. EIT has had several
opportunities to observe the South coronal hole with high spatial and
temporal resolution. We compare observations from EIT and SXT with
an eye towards correlating temporal variations over the range of
wavelengths, activity of polar crown filament systems and relating
large-scale morphology of the X-ray corona to the transition region
in He II.
Title: There's No Such Thing as the Quiet Sun: EUV Movies from SOHO
Authors: Gurman, J. B.; Delaboudiniere, J. P.; Artzner, G.; Gabriel,
A.; Maucherat, A.; Defise, J. M.; Jamar, C.; Rochus, P.; Clette, F.;
Cugnon, P.; Howard, R.; Michels, D.; Moses, J. D.; Dere, K.; Cyr,
O. C. St.; Catura, R.; Freeland, S.; Lemen, J.; Stern, R.; Neupert,
W.; Einfalt, E.; Newmark, J.
Bibcode: 1996AAS...188.3718G
Altcode: 1996BAAS...28..880G
We present unique time series of high-resolution solar images from the
normal-incidence Extreme ultraviolet Imaging Telescope (EIT) on board
the SOHO spacecraft. With a pixel scale of 2.6 arc sec and a detector
dynamic range of > 10(4) , the EIT can be used to study the dynamics
of chromospheric and coronal features in multilayer bandpasses optimized
for He II 304 Angstroms (0.08 MK), Fe IX/X 171, 173 Angstroms (0.8 -
1.0 MK), Fe XII 195 Angstroms (1.5 MK), and Fe XV 284 Angstroms (2.0 -
2.5 MK). Among the most striking features of the digital movies we will
display are: the dynamic nature of small-scale loop features in the
polar coronal holes, the constant activity of the polar crown filament
systems, the locations of the bases of polar plumes, the presence
of dark (scattering) filament material in the coronal emission line
images, and the evolution of a unique, linear, dark feature in a young
active region. The latter feature is suggestive of the ``coronal void''
observed in the electron scattering corona by Macqueen et al./ (1983).
Title: Observations of the south coronal hole from EIT and Yohkoh.
Authors: Handy, B. N.; Catura, R.; Freeland, S.; Lemen, J.; Stern,
R.; Gurman, J. B.; Delaboudinière, J. P.; Artzner, G.; Gabriel,
A.; Maucherat, A.; Defise, J. M.; Jamar, C.; Rochus, P.; Clette, F.;
Cugnon, P.; Howard, R.; Michels, D.; Moses, J. D.; Dere, K.; St. Cyr,
O. C.; Neupert, W.; Einfalt, E.; Newmark, J.
Bibcode: 1996BAAS...28Q.821H
Altcode:
No abstract at ADS
Title: Solar Active Regions As Diagnostics of Subsurface Conditions
Authors: Howard, Robert F.
Bibcode: 1996ARA&A..34...75H
Altcode:
In the past decade a number of observational and theoretical studies
have appeared that address the problem of how both the physical
conditions in subsurface layers of the Sun and the nature of the
magnetic flux tubes of active regions are reflected in the structure
and behavior of these regions at the surface. This review discusses
work in this area. Many characteristics of plages and sunspot groups
are shown to be related to the conditions encountered by the region
flux tube as it rises through the convective zone of the Sun to the
surface. Size distributions, rotation and meridional flow rates and
their covariances, and characteristics of growth and decay are among
the factors that have been shown to depend on the nature of the source
magnetic flux tube and the physical effects, such as the Coriolis
force and magnetic tension, that act deep in the convection zone.
Title: Enhancing the Spatial Resolution of LASCO/C1 Observations
Authors: Zaccheo, T. S.; Karovska, M.; Brueckner, G.; Cook, J. W.;
Howard, R.
Bibcode: 1995AAS...18712208Z
Altcode: 1995BAAS...27.1454Z
The LASCO C1 coronagraph was designed to examine the fine structure of
the solar corona in the region from 1.1R_sun to 3R_sun. Even though the
diffraction limit of the telescope is ~ 3\arcsec, the nominal resolution
is set by the CCD pixels (5.6''/). A pixel size of 1.5''/ or smaller is
needed in order to obtain diffraction limited observations (according to
the Nyquist criterium). Therefore, the resulting images are undersampled
by a factor of approximately 4. Some of this lost resolution can
be recovered by acquiring successive observations using sub-pixel
displacements of the steerable primary mirror incorporated into the
LASCO/C1 design (``Dynamic Imaging''). The spatial resolution of the
LASCO/C1 coronagraph can be enhanced by combining or co-adding multiple
observations separated by fractions of a pixel. We have identified
several methods for constructing sub-pixel estimates, evaluated the
performance of simple co-addition techniques, and developed an improved
algorithm for obtaining fractional pixel restorations. Simulations
were used to test this algorithm and to explore its limitations. In
this presentation, we describe the algorithm and the results of these
simulations. The results show that the resolution of the C1 coronagraph
can be enhanced, even in the presence of significant noise and modest
differences between successive observations.
Title: Sunspot Velocity Correlations: Are They Due to Reynolds
Stresses or to the Coriolis Force on Rising Flux Tubes?
Authors: D'Silva, Sydney; Howard, Robert F.
Bibcode: 1995SoPh..159...63D
Altcode:
Observations have consistently pointed out that the longitudinal
and latitudinal motions of sunspots are correlated. The magnitude
of the covariance was found to increase with latitude, and its sign
was found to be positive in the N-hemisphere and negative in the
S-hemisphere. This correlation was believed to be due to the underlying
turbulence where the sunspot flux tubes are anchored, and the covariance
had the right sign and magnitude needed to explain the transfer of
angular momentum toward the equator through Reynolds stresses.
Title: Characteristic Size and Diffusion of Quiet Sun Magnetic
Patterns
Authors: Komm, R. W.; Howard, R. F.; Harvey, J. W.
Bibcode: 1995SoPh..158..213K
Altcode:
We have previously studied large-scale motions using high-resolution
magnetograms taken from 1978 to 1990 with the NSO Vacuum Telescope on
Kitt Peak. Latitudinal and longitudinal motions were determined by
a two-dimensional crosscorrelation analysis of pairs of consecutive
daily observations using small magnetic features as tracers. Here we
examine the shape and amplitude of the crosscorrelation functions. We
find a characteristic length scale as indicated by the FWHM of the
crosscorrelation functions of 16.6 ± 0.2 Mm. The length scale
is constant within ±45° latitude and decreases by about 5% at
52.5° latitude; i.e., the characteristic size is almost latitude
independent. The characteristic scale is within 3% of the average value
during most times of the solar cycle, but it increases during cycle
maximum at latitudes where active regions are present. For the time
period 1978-1981 (solar cycle maximum), the length scale increases
up to 1.7 Mm or 10% at 30° latitude. In addition, we derive the
average amplitude of the crosscorrelation functions, which reflects
the diffusion of magnetic elements and their evolutionary changes
(including formation and decay). We find an average value of 0.091 ±
0.003 for the crosscorrelation amplitude at a time lag of one day, which
we interpret as being caused by the combined effect of the lifetime
of magnetic features and a diffusion process. Assuming a lifetime
of one day, we find a value of 120 km2 s−1
for the diffusion constant, while a lifetime of two days leads to 230
km2 s−1.
Title: Comparisons between Theory and Observation of Active Region
Tilts
Authors: Fisher, G. H.; Fan, Y.; Howard, R. F.
Bibcode: 1995ApJ...438..463F
Altcode:
Active regions in the Sun are generally tilted relative to the azimuthal
direction, with the leading side being closer to the equator than the
following side. This tilts is known to increase with latitude. Recently,
theoretical calculations of the dynamics of emerging, initially toroidal
active-region flux tubes have been done, showing that the observed
tilts can be explained by the Coriolis force acting on a diverging
flow field in emerging flux loops. The calculations of Fan, Fisher,
& McClymont predict that alpha proportional to Phi1/4
B0 exp -5/4 sin theta, where alpha is the tilt angle of
the active region, B0 is the magnetic field strength of the
active-region flux tube near the base of the convection zone, and phi is
the amount of magnetic flux in the tube. We compare these theoretical
predictions with the behavior of a sample of 24,701 sunspot groups
observed at Mount Wilson over a period of 68 yr, using the polarity
separation distance d as a proxy for phi. Our major findings are given.
Title: Latitude Dependence of Magnetic Field-Line Inclinations
Authors: Howard, Robert F.; Stanchfield, Donald H., II
Bibcode: 1995SoPh..156...29H
Altcode:
It is shown that leading and following magnetic field lines are
inclined toward each other by a few degrees at nearly all latitudes
in both the north and south hemispheres. The amplitudes of these
inclinations are lower by about a factor 3 for weak fields than for
strong fields. There are significant differences between the hemispheres
and from one activity cycle to the next in the leading and following
polarity field-line inclinations at latitudes poleward of the activity
latitudes. In a narrow latitude zone just south of the solar equator
the inclinations of both the leading and following fields reduce to zero
(or perhaps slightly negative values). Although one would expect such a
zone at the equator, where diffusion will mix field lines with opposite
inclinations from the two hemispheres, it is not clear why this zone
should be on one side of the equator only. The results discussed here
were obtained with Mount Wilson magnetograph data (1967-1992), and are
confirmed in many respects with National Solar Observatory/Kitt Peak
(NSO/KP) data (1976-1986).
Title: Sunspot Rotation and the Field Strengths of Subsurface
Flux Tubes
Authors: D'Silva, Sydney; Howard, Robert F.
Bibcode: 1994SoPh..151..213D
Altcode:
Observations show that bipolar magnetic regions (BMRs) have differential
rotation profiles that are faster than the local Doppler velocity
profiles by about 5%, and thep-spots in the growing sunspot groups
rotate faster than thef-spots. Also, the smaller spots rotate
faster than the larger ones. We present detailed observations of
the functional dependence of the residual rotation of sunspots on
the spot size of thep- andf-spots of growing sunspot groups. Through
numerical calculations of the dynamics of thin flux tubes we show that
flux loops emerging from the bottom of the convection zone acquire a
rotation velocity faster than the local plasma velocities, in complete
contradiction to what angular momentum conservation would demand. The
sunspot flux tubes need not be anchored to regions rotating faster than
the surface plasma velocities to exhibit the observed faster rotation;
we show that this occurs through a subtle interplay between the forces
of magnetic buoyancy and drag, coupled with the important role of the
Coriolis force acting on rising flux tubes. The dynamics of rising
flux tubes also explains the faster rotation of smaller sunspots; we
show that there is no need to evoke a radial differential rotation and
anchoring of smaller spots to faster rotating regions. The simulated
differential rotation profiles of thep- andf-legs of flux loops emerging
in the convection zone, with a latitudinal differential rotation and
velocity contours constant along cones, mimic the observed profiles
for growing sunspot groups only when the flux loops emerge radially
and obey Joy's law. (The `legs' are defined to be the vertical part
of the loops.) Also the rotation-size relation of growing sunspots
is obeyed only by radially emerging loops which obey Joy's law. This
constrains the fields at the bottom of the convection zone that are
possible for producing the BMRs we see, to lie between 60 and 160 kG,
which is in agreement with previous claims.
Title: The Covariance of Latitudinal and Longitudinal Motions of
Small Magnetic Features
Authors: Komm, R. W.; Howard, R. F.; Harvey, J. W.
Bibcode: 1994SoPh..151...15K
Altcode:
We study the covariance of longitudinal and latitudinal motions of small
magnetic features after subtracting long-term averages of differential
rotation and meridional flow. The covariance is generally interpreted
as Reynolds stress and linked to the equatorward transport of angular
momentum. Using high-resolution magnetograms taken daily with the NSO
Vacuum Telescope on Kitt Peak, we determine large-scale motions by
a two-dimensional crosscorrelation analysis of pairs of consecutive
daily observations from which active regions are excluded, i.e., we
analyze the motions of small magnetic features. In the present work,
we focus on 107 day pairs obtained during the year 1988 and on 472
day pairs taken in selected intervals from 1978 to 1990. We find
that all covariance values are very small (below 250 m2
s−2), which is about one to two orders of magnitude smaller
than the values from sunspot measurements derived by other authors. At
active region latitudes, the masking process increases the noise,
which increases the chance that the covariances at these latitudes
are not significantly different from zero. We find that the results
depend strongly on the temporal averaging involved. Daily unaveraged
crosscorrelations lead to no apparent correlation between the residual
velocities, while in the monthly averages of the 1988 data, we find a
covariance of −37 ± 15 m2 s−2 at 45° with
a linear correlation of −0.59, which is significantly different from
zero and has the right sign for an equatorial transport of angular
momentum. When we average over longer time periods, the covariance
values decrease again. The annual averages of the 1978-1990 data
show both no significant covariances and the smallest errors. These
small covariances imply that the motions of small magnetic features
do not reflect the transport of angular momentum via the mechanism of
Reynolds stress.
Title: A Possible Coriolis-Force Contribution to the Tilt-Angle
Rotation of Sunspot Groups
Authors: Howard, Robert F.
Bibcode: 1994SoPh..149...23H
Altcode:
The rotation of the magnetic axes of sunspot groups is studied as a
function of the expansion and contraction of the groups along their
magnetic axes. In general, except for the extreme values of tilt-angle
change, slow rates of rotation of the magnetic axes are associated with
low values of expansion or contraction, and faster rotation of the
magnetic axes is associated with rapid expansion or contraction. The
direction of rotation of the magnetic axes is related to expansion
or contraction in the sense that would be predicted by the Coriolis
force. A comparison of the effect at high and low latitudes shows
a difference that further supports the Coriolis force hypothesis,
and an examination of the amplitude of the effect also suggests that
the Coriolis force may be a factor in the tilt-angle rotation of
spot groups.
Title: Active Regions on the Sun
Authors: Howard, Robert F.
Bibcode: 1994ASPC...68....1H
Altcode: 1994sare.conf....1H
No abstract at ADS
Title: Polarity Separation in Active Regions
Authors: Howard, R. F.
Bibcode: 1994smf..conf...49H
Altcode:
No abstract at ADS
Title: Solar non-rotational motions
Authors: Komm, R. W.; Howard, R. F.; Harvey, J. W.
Bibcode: 1994smf..conf...68K
Altcode:
No abstract at ADS
Title: Average east-west inclinations of surface magnetic field lines
Authors: Howard, R. F.
Bibcode: 1994ASIC..433..297H
Altcode:
No abstract at ADS
Title: Limits on the Magnetic Field Strength at the Base of the
Solar Convection Zone
Authors: D'Silva, Sydney; Howard, Robert F.
Bibcode: 1993SoPh..148....1D
Altcode:
Howard (1993) finds a relationship between the tilt angles of
BMRs (Bipolar Magnetic Regions) and the separation between their
leading and following polarities; the tilt angle increases with
polarity separation. Here we present a more detailed analysis of
this relationship and show that this effect constrains the strength
of the magnetic field at the bottom of the convection zone to a value
between 40 and 150 kG, which confirms the constraints put by D'Silva and
Choudhuri (1993) based on Joy's law (the tilt-latitude relationship),
through an entirely different approach.
Title: Meridional Flow of Small Photospheric Magnetic Features
Authors: Komm, R. W.; Howard, R. F.; Harvey, J. W.
Bibcode: 1993SoPh..147..207K
Altcode:
We study the meridional flow of small magnetic features, using
high-resolution magnetograms taken from 1978 to 1990 with the NSO
Vacuum Telescope on Kitt Peak. Latitudinal motions are determined by a
two-dimensional crosscorrelation analysis of 514 pairs of consecutive
daily observations from which active regions are excluded. We find a
meridional flow of the order of 10 m s−1, which is poleward
in each hemisphere, increases in amplitude from 0 at the equator,
reaches a maximum at mid-latitude, and slowly decreases poleward. The
average observed meridional flow is fit adequately by an expansion
of the formM (θ) = 12.9(±0.6) sin(2θ) + 1.4(±0.6) sin(4θ), in m
s−1 whereθ is the latitude and which reaches a maximum of
13.2 m s−1 at 39°. We also find a solar-cycle dependence
of the meridional flow. The flow remains poleward during the cycle, but
the amplitude changes from smaller-than-average during cycle maximum to
larger-than-average during cycle minimum for latitudes between about
15° and 45°. The difference in amplitude between the flows at cycle
minimum and maximum depends on latitude and is about 25% of the grand
average value. The change of the flow amplitude from cycle maximum to
minimum occurs rapidly, in about one year, for the 15-45° latitude
range. At the highest latitude range analyzed, centered at 52.5°,
the flow is more poleward-than-average during minimumand maximum,
and less at other times. These data show no equatorward migration of
the meridional flow pattern during the solar cycle and no significant
hemispheric asymmetry. Our results agree with the meridional flow and
its temporal variation derived from Doppler data. They also agree on
average with the meridional flow derived from the poleward migration
of the weak large-scale magnetic field patterns but differ in the
solar-cycle dependence. Our results, however, disagree with the
meridional flow derived from sunspots or plages.
Title: Some Factors Affecting the Growth and Decay of Plages
Authors: Howard, Robert F.
Bibcode: 1993SoPh..147....1H
Altcode:
The Mount Wilson coarse array magnetograph data set is analyzed to
examine the dependence of growth and decay rates on the tilt angles
of the magnetic axes of the regions. It is found that there is a
relationship between these quantities which is similar to that found
earlier for sunspot groups. Regions near the average tilt angle show
larger average (absolute) growth and decay rates. Thepercentage growth
and decay rates show minima (in absolute values) at the average tilt
angles because the average areas of regions are largest near this
angle. This result is similar to that derived earlier for sunspot
groups. As in the case of spot groups, this suggests that, for decay,
the effect results from the fact that the average tilt angle may
represent the simplest subsurface configuration of the flux loop or
loops that make up the region. In the case of region growth, it was
suggested that the more complicated loop configuration should result
in increased magnetic tension in the flux loop, and thus in a slower
ascent of the loop to the surface, and thus a slower growth rate. In
order to examine this further, the growth and decay rates of plage
regions were examined as functions of the magnetic complexity of the
regions. In the case of decay, the result was as expected from the
model suggested above - that is, the more complex regions decayed
more slowly. But for growing regions the effect is the opposite to
that expected (more complex regions grow faster, even in terms of
percentage growth), so the explanation of the tilt angle effect for
growing regions proposed earlier may not be valid.
Title: Measurement of Kodiakanal White-Light Images - Part One
Authors: Sivaraman, K. R.; Gupta, S. S.; Howard, Robert F.
Bibcode: 1993SoPh..146...27S
Altcode:
A program of digitization of the daily white-light solar images from
the Kodaikanal station of the Indian Institute of Astrophysics is in
progress. A similar set of white-light data from the Mount Wilson
Observatory was digitized some years ago. In both cases, areas
and positions of individual sunspot umbrae are measured. In this
preliminary report, comparisons of these measurements from the two
sites are made. It is shown that both area and position measurements
are in quite good agreement. The agreement is sufficiently good that
it is possible to measure motions and area changes of sunspots from
one site to the next, involving time differences from about 12 hours to
about 36 hours. This enables us to trace the motions of many more small
sunspots than could be done from one site alone. Very small systematic
differences in rotation rate between the two sites of about 0.4% are
found. A portion of this discrepancy is apparently due to the difference
in plate scales between the two sites. Another contributing factor in
the difference is the latitude visibility of sunspots. In addition it is
suggested that a small, systematic difference in the measured radii at
the two sites may contribute a small amount to this discrepancy, but it
has not been possible to confirm this hypothesis. It is concluded that
in general, when dealing with high precision rotation results of this
sort, one must be extremely careful about subtle systematic effects.
Title: The East-West Inclination Angles of Weak Magnetic Fields
Authors: Howard, R. F.
Bibcode: 1993BAAS...25.1182H
Altcode:
No abstract at ADS
Title: The Covariance of Latitudinal and Longitudinal Motions of
Small Magnetic Features
Authors: Komm, R. W.; Howard, R. F.; Harvey, J. W.
Bibcode: 1993BAAS...25.1220K
Altcode:
No abstract at ADS
Title: How Growth and Decay of Sunspot Groups Depend on Axial
Tilt Angles
Authors: Howard, Robert F.
Bibcode: 1993SoPh..145...95H
Altcode:
Digitized Mount Wilson sunspot data covering the interval from 1917
to 1985 are analyzed to examine the average growth and decay rates
of sunspot groups as a function of the tilt angles of the magnetic
axes of the groups. It is found that in absolute terms, both growth
and decay rates of groups peak at the average tilt angle of the groups
(about +5°). In percentage terms these rates are a minimum near these
tilt angles because average group areas are largest at the average tilt
angle. The clear peaks at the average tilt angle (rather than at 0°)
may be related to the structure or geometry of the subsurface flux loops
that form the regions. One suggestion to explain this effect is that
this is the angle that represents no twist of these subsurface flux
loops. This implies, however, that these loops do not get twisted,
on average, during their ascent to the surface by Coriolis forces,
as has been suggested in the past. The average percentage growth rates
for groups with negative tilt angles show high average values and large
dispersions for certain tilt angle intervals, suggesting slower growth
rates, for some unknown reason, for many small spot groups in certain
tilt angle ranges.
Title: Rotation Rates of Small Magnetic Features from Two-Dimensional
and One-Dimensional Cross-Correlation Analyses
Authors: Komm, R. W.; Howard, R. F.; Harvey, J. W.
Bibcode: 1993SoPh..145....1K
Altcode:
We present results of an analysis of 628 high-resolution magnetograms
taken daily with the NSO Vacuum Telescope on Kitt Peak from 1975 to
1991. Motions in longitude on the solar surface are determined by a
two-dimensional cross-correlation analysis of consecutive day pairs. We
find that the measured rotation rate of small magnetic features, i.e.,
excluding active regions, is in excellent agreement with the results
of the previous one-dimensional analysis of the same data (Komm,
Howard, and Harvey, 1993). The polynomial fits show magnetic torsional
oscillations, i.e., a more rigid rotation during cycle maximum and
a more differential rotation during cycle minimum, but with smaller
amplitudes than the one-dimensional analysis. The full width at half
maximum of the cross-correlations is almost constant over latitude
which shows that the active regions are effectively excluded. The
agreement between the one- and two-dimensional cross-correlation
analyses shows that the two different techniques are consistent and that
the large-scale motions can be divided into rotational and meridional
components that are not affected by each other.
Title: Axial Tilt Angles of Active Regions and Their Polarity
Separations
Authors: Howard, Robert F.
Bibcode: 1993SoPh..145..105H
Altcode:
Sunspot group and magnetic (plage) data are examined to search for
a relationship between the tilt angles of active regions and the
separations of their leading and following portions. A relationship
is found in the sense that larger positive tilt angles are associated
with larger polarity separations. This is the direction predicted by
recent theoretical work (D'Silva and Choudhuri, 1992). The explanation
for this appears to be that smaller surface polarity separations lead
to larger magnetic tension forces, which diminish the effect of the
Coriolis force that acts to twist rising flux tubes.
Title: The Development of Sunspot Groups
Authors: Howard, R. F.
Bibcode: 1993ASPC...46..492H
Altcode: 1993IAUCo.141..492H; 1993mvfs.conf..492H
No abstract at ADS
Title: Editorial
Authors: de Jager, Cornelis; Howard, Robert F.; švestka, Zdeněk
Bibcode: 1993SoPh..143D...7D
Altcode:
No abstract at ADS
Title: Torsional Oscillations and Internal Rotation
Authors: Komm, R. W.; Harvey, J. W.; Howard, R. F.
Bibcode: 1993ASPC...42..269K
Altcode: 1993gong.conf..269K
No abstract at ADS
Title: Torsional Oscillation Patterns in Photospheric Magnetic
Features
Authors: Komm, R. W.; Howard, R. F.; Harvey, J. W.
Bibcode: 1993SoPh..143...19K
Altcode:
We analyzed 689 high-resolution magnetograms taken daily with the NSO
Vacuum Telescope on Kitt Peak from 1975 to 1991. Motions in longitude on
the solar surface are determined by a one-dimensional crosscorrelation
analysis of consecutive day pairs. The main sidereal rotation rate
of small magnetic features is best fit byω = 2.913(±0.004) −
0.405(±0.027) sin2φ − 0.422(±0.030) sin4φ,
in µrad s−1, whereφ is the latitude. Small features and
the large-scale field pattern show the same general cycle dependence;
both show a torsional oscillation pattern. Alternating bands of
faster and slower rotation travel from higher latitudes toward the
equator during the solar cycle in such a way that the faster bands
reach the equator at cycle minimum. For the magnetic field pattern,
the slower bands coincide with larger widths of the crosscorrelations
(corresponding to larger features) and also with zones of enhanced
magnetic flux. Active regions thus rotate slower than small magnetic
features. This magnetic torsional oscillation resembles the pattern
derived from Doppler measurements, but its velocities are larger by a
factor of more than 1.5, it lies closer to the equator, and it leads
the Doppler pattern by about two years. These differences could be
due to different depths at which the different torsional oscillation
indicators are rooted.
Title: The Rotation of Active Regions with Differing Magnetic
Polarity Separations
Authors: Howard, Robert F.
Bibcode: 1992SoPh..142..233H
Altcode:
The separation of the leading and following portions of plages and
(multi-spot) sunspot groups is examined as a parameter in the analysis
of plage and spot group rotation. The magnetic complexity of plages
affects their average properties in such a study because it tends to
make the polarity separations of the plages less than they really are
(by the definition of polarity separation used here). Correcting
for this effect, one finds a clear and very significant
dependence of the total magnetic flux of a region on its polarity
separation. Extrapolating this relationship to zero total flux leads
to an X intercept of about 25 Mm in polarity separation. The average
residual rotation rates of regions depend upon the polarity separation
in the sense that larger separations correspond to slower rotation rates
(except for small values of separation, which are affected by region
complexity). In the case of sunspots, the result that smaller individual
spots rotate faster than larger spots is confirmed and quantified. It is
shown also that smaller spot groups rotate faster than larger groups,
but this is a much weaker effect than that for individual spots. It
is suggested that the principal effect is for spots, and that this
individual spot effect is responsible for much or all of the group
effect, including that attributed in the past to group age. Although
larger spot groups have larger polarity separations, it is shown that
the rotation rate-polarity separation effect is the opposite in groups
than one finds in plages: groups with larger polarity separations rotate
faster than those with smaller separations. This anomalous effect
may be related to the evolution of plages and spot groups, or it may
be related to connections with subsurface toroidal flux tubes. It is
suggested that the polarity separation is a parameter of solar active
regions that may shed some light on their origin and evolution.
Title: Solar Meridional Flow Detected in Small Magnetic Features
Authors: Komm, R. W.; Howard, R. F.; Harvey, J. W.
Bibcode: 1992AAS...181.8102K
Altcode: 1992BAAS...24.1252K
We present results of an analysis of 514 high-resolution magnetograms
taken daily with the NSO Vacuum Telescope on Kitt Peak from 1978 to
1990. Motions in latitude on the solar surface are determined by a
two-dimensional crosscorrelation analysis of consecutive day pairs
after excluding large active regions. We find a meridional flow of the
order of 10 ms(-1) , which is poleward in each hemisphere, increases
in amplitude from 0 at the equator, reaches a maximum at mid-latitude,
and slowly decreases poleward. The average meridional flow is fit by an
expansion of derivatives of even Legendre polynomials $M(theta ) = 8.88
(+/- 0.45) {{partial P_2}/{partial theta }} - 0.66 (+/-0.26) {{partial
P_4}/{partial theta }} in ms^{-1}\ where \theta is the latitude, which
reaches a maximum of 13.2 ms^{-1}\ at 39 deg. We also find a solar
cycle dependence of the meridional flow. The flow remains poleward
during the cycle, but the amplitude (at latitudes poleward of 20 deg)
changes from smaller-than-average during maximum to larger-than-average
during minimum. The meridional flow fit of the maximum activity years
1980--1982 peaks at 10.9 ms^{-1}, while the fit of the minimum years
1984--1986 reaches a maximum velocity of 14.5 ms^{-1}$; the difference
is about 27% of the average value.
Title: Some Characteristics of the Development and Decay of Active
Region Magnetic Flux
Authors: Howard, Robert F.
Bibcode: 1992SoPh..142...47H
Altcode:
Mount Wilson synoptic data of both plages and sunspots are examined
in an effort to determine in some detail the manner of the appearance
and disappearance of the magnetic flux of active regions at the solar
surface. Separating regions into leading and following portions by
magnetic polarity in the case of the plages and by position in the case
of sunspots (for which there is no magnetic information available in
this data set), various characteristics of these features are studied,
namely their rotation, their relative longitudinal motions, and the
east-west inclinations of their magnetic fields. The evidence, taken
together, suggests that the magnetic flux loops which comprise a region
rise to the surface at the time of its formation, and (at least some
of them) sink back below the surface at the time of the decay of the
region. It is likely that not all the magnetic flux that arises sinks
again below the surface.
Title: Solar corona synoptic observations from SOHO with an Extreme
Ultraviolet Imaging Telescope.
Authors: Delaboudinière, J. P.; Gabriel, A. H.; Artzner, G. E.;
Dere, K.; Howard, R.; Michels, D.; Catura, R.; Lemen, J.; Stern, R.;
Gurman, J.; Neupert, W.; Cugnon, P.; Koeckelenbergh, A.; van Dessel,
E. L.; Jamar, C.; Maucherat, A.
Bibcode: 1992ESASP.348...21D
Altcode: 1992cscl.work...21D
The major scientific objective of the EUV Imaging Telescope (EIT)
is to study the evolution of coronal structure over a wide range
of spatial and temporal scales and temperatures. A second strategic
objective is to provide full disk synoptic maps of the global corona
to aid in unifying SOHO (Solar and Heliospheric Observatory)/Cluster
investigations. EIT will also provide images to support the planning
of detailed spectroscopic investigations by the CDS (Coronal Diagnostic
Spectrometer) and SUMER spectrometers in SOHO. EIT observations will be
made in four narrow spectral bands, centered at 171 A (Fe 9), 195 A(Fe
12), 284 A (Fe 15), and 304 A (He 2) representing restricted temperature
domains within a wide temperature range from 40,000 to 3,000,000
K. The results will be images of the solar atmosphere from the upper
chromosphere and transition region to the active region corona. These
maps, made at appropriate time intervals, will be used to study the fine
structures in the solar corona and to relate their dynamic properties
to the underlying chromosphere and photosphere. Dynamic events in the
inner corona will be related to white light transients in the outer
corona, and observations of the internal structure of coronal holes
will be used to investigate origins of the solar wind.
Title: Crosscorrelation Analysis of Small Photospheric Magnetic
Features
Authors: Komm, Rudolf W.; Howard, Robert F.; Harvey, John W.
Bibcode: 1992AAS...180.5110K
Altcode: 1992BAAS...24..815K
We present results of an analysis of high-resolution magnetograms
taken daily with the NSO Vacuum Telescope on Kitt Peak from 1975 to
1991. Motions in longitude on the solar surface are determined by a
one-dimensional crosscorrelation analysis of consecutive day pairs. The
mean sidereal rotation rate of small magnetic features is best fit by
$omega = 2.913 (+/- 0.004) -0.405 (+/- 0.027) sin(2phi ) -0.422 (+/-
0.030) sin(4phi ) in \mu rad s^{-1} where \phi$ is the latitude. The
small features show a torsional oscillation pattern; alternating bands
of faster and slower rotation travel from higher latitudes toward the
equator during the solar cycle in such a way that the faster bands
reach the equator at cycle minimum. The magnetic torsional oscillation
resembles the pattern derived from Doppler measurements, but is
different in three respects. Its velocities are larger by a factor of
more than 1.5, it lies closer to the equator, and leads the Doppler
pattern by about two years. Motions in longitude and also in latitude
are determined by a two-dimensional crosscorrelation analysis. The mean
sidereal rotation rate of the two-dimensional analysis is in excellent
agreement with the one-dimensional rate which assures the robustness of
the two-dimensional analysis. In latitude, we find meridional motions
of the order of 10 m/s which are poleward in each hemisphere.
Title: Rotation and Magnetic Polarity Separation in Active Regions
Authors: Howard, Robert F.
Bibcode: 1992AAS...180.5111H
Altcode: 1992BAAS...24..815H
The separations of the leading and following components of plages and,
separately, of sunspot groups are used as parameters in the study of
the rotation of these features. These quantities are determined by
calculating the magnetic flux-weighted positions of fields of leading
or following polarity magnetic flux in the case of plages and by
calculating the area weighted positions of the spots west or east of the
area-weighted longitude centroid of the sunspot groups. (For the sunspot
groups there is no magnetic information available as a part of this data
set.) After correcting for the effects of magnetically complex regions,
it is found that there is a significant correlation between polarity
separation and rotation rate; plages with larger polarity separation
rotate slower than those with smaller polarity separation. In the
case of individual spots, it is known that smaller spots rotate faster
than larger spots and for groups the same effect is found, but with a
very low amplitude. It is suggested that the group effect is strongly,
and perhaps totally, influenced by the spot effect. When the rotation
rates of spot groups of varying polarity separations are examined,
the opposite effect is seen: Groups with larger polarity separations
rotate faster than those with smaller polarity separations. It is
suggested that this discrepancy, which is somewhat analogous to other
recently-discovered differences in dynamic behavior between plages
and sunspot groups, may result from the fact that the magnetic fields
of sunspots and plages are anchored in toroidal magnetic flux tubes
that are located at different distances beneath the solar surface. The
variation with rotation rate in each case might then be a depth effect
and represent an indication of the vertical angular velocity gradient.
Title: The East-West Inclination of Magnetic Field Lines in Sunspots
Authors: Howard, Robert F.
Bibcode: 1992SoPh..137..205H
Altcode:
Digitized Mount Wilson sunspot data covering the interval from 1917 to
1985 are analyzed to examine the average areas of individual sunspot
umbrae over small zones of central meridian distance. Assuming that
systematic, east-west differences in these quantities are due to the
inclination of the magnetic fields of the spots, one can calculate
average east-west inclination angles for all spots and for subsets of
the full data set. It is found from such an analysis that on average
spot fields are inclined such as to trail the rotation by a few
deg. Leading and following spots may show a tendency to be inclined
slightly away from each other, in contrast to the results of an earlier
study of plage magnetic fields. Growing spots tend to be inclined much
more to the east than decaying spots. This is in the opposite sense
to the analogous result derived from plage magnetic fields.
Title: The Growth and Decay of Sunspot Groups
Authors: Howard, Robert F.
Bibcode: 1992SoPh..137...51H
Altcode:
Digitized Mount Wilson sunspot data from 1917 to 1985 are analyzed to
examine the growth and decay rates of sunspot group umbral areas. These
rates are distributed roughly symmetrically about a median rate of
decay of a few μhemisphere day-1. Percentage area change
rates average 502% day-1 for growing groups and -45%
day-1 for decaying groups. These values are significantly
higher than the comparable rates for plage magnetic fields because
spot groups have shorter lifetimes than do plages. The distribution
of percentage decay rates also differs from that of plage magnetic
fields. Small spot groups grow at faster rates on average than they
decay, and large spot groups decay on average at faster rates than they
grow. Near solar minimum there is a marked decrease in daily percentage
spot area growth rates. This decrease is not related to group area,
nor is it due to latitude effects. Sunspot groups with rotation
rates close to the average (for each latitude) have markedly slower
average rates of daily group growth and decay than do those groups
with rotation rates faster or slower than the average. Similarly,
sunspot groups with latitude drift rates near zero have markedly
slower average rates of daily group growth and decay than do groups
with significant latitude drifts in either direction. Both of these
findings are similar to results for plage magnetic fields. These
various correlations are discussed in the light of our views of
the connection of the magnetic fields of spot groups to subsurface
magnetic flux tubes. It is suggested that a factor in the rates of
growth or decay of spot groups and plages may be the inclination angle
to the vertical of the magnetic fields of the spots or plages. Larger
inclination angles may result in faster growth and decay rates.
Title: Rotation of Leading; Following Portions of Plages; Sunspot
Groups
Authors: Howard, Robert F.
Bibcode: 1992ASPC...27..297H
Altcode: 1992socy.work..297H
No abstract at ADS
Title: Editorial
Authors: de Jager, Cornelis; Švestka, Zdeněk; Howard, Robert F.
Bibcode: 1992SoPh..137D...5D
Altcode:
No abstract at ADS
Title: Rotation Rate Determined from Small Photospheric Magnetic
Features
Authors: Komm, R. W.; Howard, R. F.; Harvey, J. W.; Forgach, S.
Bibcode: 1992ASPC...27..325K
Altcode: 1992socy.work..325K
No abstract at ADS
Title: The East-West Inclinations of Magnetic Fields in the Solar
Photosphere
Authors: Howard, R. F.
Bibcode: 1992ASPC...26..243H
Altcode: 1992csss....7..243H
No abstract at ADS
Title: On the Large-Scale Distribution of Solar Magnetic Fields
Authors: Howard, Robert F.
Bibcode: 1992ASPC...27...44H
Altcode: 1992socy.work...44H
No abstract at ADS
Title: Axial Tilt Angles of Sunspot Groups
Authors: Howard, Robert F.
Bibcode: 1991SoPh..136..251H
Altcode:
Digitized Mount Wilson sunspot data from 1917 to 1985 are analyzed
to examine tilt angles determined from the area-weighted positions
of leading and following sunspots. These spot group tilt angles are
examined in relation to other group characteristics to give information
which may relate to the formation and evolution of sunspot groups and
the magnetic connection of groups to subsurface magnetic flux tubes. The
average tilt angle of all 24816 (multiple-spot) group observations in
this study is found to be + 4.2 ± 0.2 deg, where the positive sign
signifies that the leading spots lie equatorward of the following
spots. Sunspot group areas are significantly larger on average for
groups nearer the average tilt angle, which is similar to a result
found earlier for active region plages. Average tilt angles are found
to be larger at higher latitudes, confirming earlier results. There is
a strong negative correlation between average daily latitudinal motion
(plus to poles) and group tilt angle. That is, for groups within about
40 deg of the average tilt angle, smaller tilt angles are associated
with more positive (poleward) daily drift. Groups nearest the average
tilt angle rotate the fastest, on average, the amplitude differences
being between about +0.1 and − 0.1 deg day−1 for groups
near and far from the average tilt angle, respectively. Groups with
tilt angles near the average show a negative daily separation change
between leading and following spots of close to 4 Mm day−1
on average. Groups on either side of the average tilt angle show spot
separations that are on average more positive. A similar effect is
not seen for the daily variations of group areas. These results are
discussed in relation to analogous recent results for active region
magnetic fields. More evidence is found for a qualitative difference
between the magnetic fields of sunspots and of plages, relating,
perhaps, to a difference in subsurface connection of the field lines
or to different physical mechanisms that may play a role for fields
of different field strengths.
Title: Cycle latitude effects for sunspot groups
Authors: Howard, Robert F.
Bibcode: 1991SoPh..135..327H
Altcode:
Digitized Mount Wilson sunspot data from 1917 to 1985 are analyzed
to examine meridional motion and rotation properties as a function
of latitude and distance (ξ) from the average latitude of activity
(ξ0) in each hemisphere. Latitude dependence similar to
previous results is found, but only for spot groups whose areas are
decreasing from one day to the next. A previous study of active region
magnetic fields, using this technique of motions as a function of the
average latitude of activity, had shown meridional motions on average
toward ξ0. In this analysis of spot data some evidence is
seen for motion away from ξ0, with some slight evidence for
faster rotation equatorward of ξ0 and slower motion poleward
of ξ0, similar to the torsional oscillation phenomenon. For
reasons that are not clear, both of these effects are significantly
more pronounced for sunspot groups whose areas are decreasing.
Title: Sunspot group areas and the latitude distance from the average
latitude of activity
Authors: Howard, Robert F.
Bibcode: 1991SoPh..135..339H
Altcode:
Digitized Mount Wilson sunspot data from 1917 to 1985 are analyzed
to examine group areas as a function of latitude distance (ξ) from
the central latitude of activity in each hemisphere. On average these
group areas are larger for the smallest values of ¦ξ¦. The effect is
similar to that seen for the magnetic fields of active regions (Howard,
1991). It is concluded that this is fundamentally a ξ dependence, and
not a latitude dependence. The suggestion is made that the cause of this
effect is the influence of large-scale convective motions on the rising
flux tubes that make up the active regions. The smaller flux tubes
(spot groups) are more easily displaced in latitude during their ascent
to the surface by this velocity field than are the larger flux tubes.
Title: The Magnetic Fields of Active Regions - Part Eight
Authors: Howard, Robert F.
Bibcode: 1991SoPh..135...43H
Altcode:
The Mount Wilson coarse array magnetograph data set is analyzed
to determine characteristics of magnetic regions as a function of
distance from the average latitude, ξ0, of regions in each
hemisphere, a quantity which varies during the activity cycle. Regions
with normal polarity axis orientations are distributed asymmetrically
about ξ0 with the median latitude about 1 deg equatorward of
ξ0. Reversed polarity orientation regions show a somewhat
broader and more symmetric distribution. Average sizes for regions at
ξ = 0 (ξ0) are nearly twice as large as those located at
10 deg latitude in either direction. Regions poleward of ξ0
tend to show a net magnetic field biased toward the following polarity,
and regions equatorward of ξ0 are biased toward the leading
polarity, both by around 10%. Neither region growth rates nor decay
rates are related to ξ. The average polarity axis tilt angles of
regions are lower for regions near the equator than for those nearer the
poles. It is most likely that this is basically an effect of latitude
rather than ξ. Meridional motions of young regions are shown to be
toward ξ0. Older regions do not show this behavior. This may
be a magnetic effect rather than being due to large-scale circulatory
motion, as has been suggested in the past. East-west inclination angles
of active region magnetic fields show a slight tendency to trail the
rotation direction (eastward inclination) by a few deg for regions
with ξ0> 0 and lead the rotation (westward inclination)
by a few deg for regions with ξ0 > 0. This effect may
be related to the torsional oscillations. These various results are
discussed in terms of a hypothetical subsurface magnetic flux tube
which gives rise to the surface activity.
Title: The Magnetic Fields of Active Regions - Part Seven
Authors: Howard, Robert F.
Bibcode: 1991SoPh..134..233H
Altcode:
Magnetogram data are analyzed to study east-west magnetic flux
differences interpreted as the component of magnetic field line
inclination at the photospheric level in a plane parallel to the solar
equator. This component is determined by comparing average east-west
pairs of flux values at equal distances from the central meridian. The
average inclination of a whole region is such as to trail the rotation
(incline toward the east) by about 1.9 deg. Leading and following
polarities tilt toward each other by about 16 deg. Growing regions
are strongly inclined to the west (to lead the rotation) with large
differences between leading and following portions. Decaying regions
are slightly inclined to the east with more normal differences between
leading and following portions. These results concerning growing and
decaying regions are seen with greater amplitude for reversed polarity
regions. As the activity cycle progresses, the average inclination of
the field lines of the following portions of regions varies from about
10 to about 3 deg (leading the rotation), and the average difference in
inclination of the leading and following portions of regions decreases
monotonically during the cycle from nearly 20 to about 11 deg. A
slight difference is seen between the average east-west inclination
angles of regions that are rotating faster than average and those that
are rotating slower than average in the sense that slower regions
are slightly inclined toward the east and faster regions toward the
west. Some of these results may be related to the location or nature
of the subsurface flux tubes to which the active regions fields are
connected and also, perhaps, to the nature of this connection.
Title: The Magnetic Fields of Active Regions - Part Six
Authors: Howard, Robert F.
Bibcode: 1991SoPh..132..257H
Altcode:
Magnetogram data are analyzed to study daily variations of tilt angles
of the magnetic axes of active regions defined by magnetic fields
measured with the Mount Wilson magnetograph. The period covered by
this daily data set is 1967 through April 9, 1990. It is found that
on average regions with positive tilt angles (leading portions of the
regions equatorward of following portions) show average negative daily
tilt angle changes (decreases in tilt angle) and regions with negative
tilt angles show average positive daily tilt angle changes. Generally
the larger the tilt angle of either sign, the larger is the average
daily tilt angle change. Although at times some young regions are
observed to rotate their magnetic axes rapidly as they are formed,
or shortly thereafter, age does not seem to be an important factor
in distinguishing (among those regions that have large tilt angles)
between those that rotate their tilt angles rapidly and those that do
not. Other characteristics were also investigated without success to see
if they provided such a distinguishing factor. These were: net magnetic
flux, total magnetic flux, and magnetic polarity separation. One
characteristic that does provide such a distinction is cycle phase:
large daily tilt angle changes are clustered around solar maximum. A
clear correlation is found between polarity axis rotation and latitude:
polarity axis rotation rates are larger at higher latitudes up to about
20 deg. Another parameter that is correlated with large tilt angle
change is rotation rate (about the rotation axis of the Sun). Regions
with large tilt angle rotation rates tend to rotate by 1-2 deg day
−1 faster than do those regions that show slow tilt
angle twist. These results may be related to characteristics of the
subsurface connection of the magnetic flux tubes that form the regions.
Title: The Magnetic Fields of Active Regions - Part Five
Authors: Howard, Robert F.
Bibcode: 1991SoPh..132...49H
Altcode:
Daily magnetogram data are analyzed to study the orientation angles of
the magnetic axes of active regions defined by magnetic fields measured
with the Mount Wilson magnetograph. The period covered by this daily
data set is 1967 through April 9, 1990. The well-known tilt of the
magnetic axes of active regions is seen clearly in these data. The
dependence of tilt angle on latitude is different from that found by
previous investigators, and it is suggested that this is due to the
fact that this study uses all active regions, not spot groups or young
active regions, and that there may be systematic variations in time of
the tilt angles of regions. The variation of tilt angle with latitude
is shown not to be a variation with cycle phase. Regions with smaller
absolute tilt angles are larger than those with larger absolute tilt
angles. Regions tilted in the normal orientation, with leading fields
equatorward of following fields (positive tilt angles), are larger on
average than those regions with negative tilt angles. Although there is
no obvious relationship between tilt angle and daily region area change,
it is found that regions with large tilt angles show on average rapid
separation of the magnetic poles of the regions. This is not an effect
of differential rotation shear. Normally oriented regions with small
positive tilt angles rotate slower on average than those with small
negative tilt angles. Some, but not all, of these results suggest that
regions rise from subsurface flux ropes.
Title: The Magnetic Fields of Active Regions - Part Four
Authors: Howard, Robert F.
Bibcode: 1991SoPh..131..259H
Altcode:
Daily magnetogram data are analyzed to examine the meridional component
of motions of active regions defined by magnetic fields measured with
the Mount Wilson magnetograph. The period covered by this daily data
set is 1967 through April 9, 1990. Meridional motions of all the
active regions observed in this interval show a different latitude
dependence than is shown by most other solar surface tracers: the
higher the latitude, the greater is the relative equatorward drift
velocity. However, the subsets of the data containing no increase or
decrease of region size or showing decrease in size only lead to no
latitude dependence of meridional motion, which implies that there is
a very large effect due to the growth of regions. In this study only
regions showing no growth or decay were used in the analysis. There
is only a very slight variation of latitude dependence of meridional
motions with cycle phase. At the decline from maximum there is a weak
relationship found, at other phases there is no correlation. Meridional
motions are correlated with rotation rate residual velocities. Slower
rotation rates are found for poleward moving regions, and faster
rotation rates are found for equatorward moving regions - a result
similar to that for sunspots. Covariances of these quantities are
comparable in magnitude to those found for sunspot groups.
Title: The Magnetic Fields of Active Regions - Part Three
Authors: Howard, Robert F.
Bibcode: 1991SoPh..131..239H
Altcode:
The Mount Wilson daily magnetogram data set is analyzed to examine
the daily growth and decay of active region magnetic fields. Rates
of magnetic flux growth and decay, both absolute and in terms of
percentage, are studied for various subsets of the data. The daily
percentage flux growth of regions is about twice as great as the
percentage flux decay. Relationships are found between daily flux
change and region size, hemisphere, cycle phase, leading and following
polarities, normal and reversed polarity orientation, rotation rate,
meridional drift rate and region complexity. In general, growth and
decay of flux is faster in more active situations: at cycle maximum or
for the more active hemisphere. Leading polarity fields grow and decay
more rapidly than following polarity fields. Regions with reversed
magnetic polarity orientations have much larger rates of flux growth
and decay than do normally oriented regions, but this is an effect
of region size. The smallest rates of flux growth and decay are seen
for regions rotating nearest the average rate and with the smallest
meridional drift rates. These results are discussed in terms of our
current understanding of how regions form and decay.
Title: Large-scale velocity fields.
Authors: Howard, Robert F.; Kichatinov, L. L.; Bogart, Richard S.;
Ribes, Elizabeth
Bibcode: 1991sia..book..748H
Altcode:
Results in the area of solar rotation over the past few years are
reviewed. Considerable effort has gone into rotation determinations
from tracers - especially sunspots - in recent years. The present
status of our knowledge of solar supergranulation, mesogranulation and
giant-scale convection is briefly reviewed. Theoretical suggestions
are discussed which may serve to reconcile theory and observations
of giant-scale solar convection. Global meridional circulation in
the photosphere is suggested by models of the solar rotation. The
solar rotation shows a latitude and cycle dependence. The effect
is clearly seen in the magnetic tracers (sunspot data) as well as
the spectroscopic observations, although the latter are affected by
systematic errors which cannot easily be removed from a real solar
signal. This variability can be understood in terms of a time-dependent
convective toroidal roll pattern. The connection of the rolls with
rigidly rotating layers favors a solar dynamo located below the
convective zone, possibly in the radiative interior which rotates like
a solid body. Such a new picture of the rotation and convection has
to be confirmed. Undoubtedly helioseismology will further add to our
knowledge of the solar rotation in the near future.
Title: Solar Surface Velocity Fields Determined from Small Magnetic
Features
Authors: Howard, R. F.; Harvey, J. W.; Forgach, S.
Bibcode: 1990SoPh..130..295H
Altcode:
We describe a method for the analysis of magnetic data taken daily
at the Vacuum Telescope at Kitt Peak. In this technique, accurate
position differences of very small magnetic features on the solar
surface outside active regions are determined from one day to the next
by a cross-correlation analysis. In order to minimize systematic errors,
a number of corrections are applied to the data for effects originating
in the instrument and in the Earth's atmosphere. The resulting maps
of solar latitude vs central meridian distance are cross-correlated
from one day to the next to determine daily motions in longitude and
latitude. Some examples of rotation and meridional motion results are
presented. For the months of May 1988 and October-November 1987, we find
rotation coefficients A = 2.894 ± 0.011, B = - 0.428 ± 0.070, and C =
-0.370 ± 0.077 in μrad s−1 from the expansion ω = A +
B sin2φ + C sin4φ, where φ is the latitude. The
differential rotation curve for this interval is essentially flat within
20 deg of the equator in these intervals. For the same intervals we
find a poleward meridional motion a = 16.0 ± 2.8 m sec -1
from the relation v = a sinφ, where v is the line-of-sight velocity.
Title: The Magnetic Fields of Active Regions - Part Two
Authors: Howard, Robert F.
Bibcode: 1990SoPh..126..299H
Altcode:
The Mount Wilson coarse array data set is used to define active regions
in the interval 1967 to August, 1988. From the positions of these
active regions on consecutive days, rotation rates are derived. The
differential rotation of the active regions is calculated and compared
with previous magnetic field and plage rates. The agreement is good
except for the variation with time. The active region rates are
slower by a few percent than the magnetic field or facular rates. The
differential rotation rate of active regions with reversed magnetic
polarity orientations is calculated. These regions show little or no
evidence for differential rotation, although uncertainties in this
determination are large. A correlation is found between rotation rate
and region size in the sense that larger regions rotate more slowly. A
correlation between rotation rate and cycle phase is suggested which
is in agreement with earlier sunspot results. Leading and following
portions of active regions, unlike leading and following spots, show
little or no difference in their rotation rates. The regions with
polarity orientations nearest the normal configuration tend to show
rotation rates that are nearest the average values. Most of these
results generally support the conclusion that old, weaker magnetic
fields have evolved different subsurface connections from the time
they were a part of sunspots or plages. It seems possible that they
are connected at a shallower layer than are sunspot or plage fields.
Title: Sunspot Motions from a Study of Kodaikanal and Mount Wilson
Observations
Authors: Howard, Robert F.; Sivaraman, K. R.; Gupta, S. S.; Gilman,
Pamela I.
Bibcode: 1990IAUS..142..107H
Altcode:
Results are presented of a study of the daily motions of individual
sunspots and of sunspot groups on the basis of Kodaikanal and Mount
Wilson white-light observations. A comparison of the two data sets show
a good agreement between them in spot areas and motions. Preliminary
rotation and latitude drift reduction from the combined data set
confirm earlier results from the Mount Wilson data alone.
Title: The Magnetic Fields of Active Regions - Part One
Authors: Howard, Robert F.
Bibcode: 1989SoPh..123..271H
Altcode:
The Mount Wilson daily magnetogram data set is used in its coarse
format to determine various statistical properties of magnetic
regions. The method of defining magnetic regions is described, and
also the criteria for a `return' of a magnetic region from one day
to the next are given. Region sizes, polarity separations, total and
net magnetic fluxes, magnetic complexities, and polarity orientations
are defined. A relationship is found between polarity orientation and
region size in the sense that regions with less magnetic flux tend to
show greater deviation on average from the usual polarity orientation.
Title: Large-Scale Motions of Small-Scale Magnetic Fields
Authors: Harvey, J.; Howard, R.; Forgach, S.
Bibcode: 1989BAAS...21..854H
Altcode:
No abstract at ADS
Title: Editorial
Authors: de Jager, Cornelis; Švestka, Zdeněk; Howard, Robert F.
Bibcode: 1989SoPh..121D...9D
Altcode:
No abstract at ADS
Title: Distribution of Sunspot Umbral Areas: 1917--1982
Authors: Bogdan, T. J.; Gilman, Peter A.; Lerche, I.; Howard, Robert
Bibcode: 1988ApJ...327..451B
Altcode:
Over 24,000 measurements of individual sunspot umbral areas taken from
the Mount Wilson white-light plate collection covering the period
1917-1982 are used to determine the relative size distribution
of sunspot umbras. In the range 1.5-141 millionths of a solar
hemisphere, the sunspot umbral areas are found to be distributed
lognormally. Moreover, the same distribution is obtained for all phases
of the solar cycle (maximum, minimum, ascending, descending), as well
as for various individual cycles, between 1917 and 1982. Both the mean
and the geometric logarithmic standard deviation of this distribution
appear to be intrinsically constant over the entire data set; only
the number of spots exhibits the familiar solar cycle variations. If
the observed lognormal umbral size distribution is not a particular
attribute of the sunspot umbras but is instead of a more fundamental
property of emerging magnetic flux, then the data would predict a
maximum in the size spectrum of photospheric magnetic structures
for flux tubes with radii in the range 500-800 km. The absence of
solar cycle variations in the relative distribution of umbral areas
and especially the lognormal character of this distribution may both
argue for the fragmentation of magnetic elements in the solar envelope.
Title: Models of motions in the Sun
Authors: Howard, Robert F.
Bibcode: 1987Natur.328..667H
Altcode:
No abstract at ADS
Title: Distribution of Sunspot Umbral Areas: 1917-1982
Authors: Bogdan, T. J.; Gilman, P. A.; Lerche, I.; Howard, R.
Bibcode: 1987BAAS...19..924B
Altcode:
No abstract at ADS
Title: Observations of Surface Velocity Fields
Authors: Howard, Robert F.
Bibcode: 1987ASSL..137...23H
Altcode: 1987isav.symp...23H
This brief review discusses several recent papers on the motions of
individual sunspots and sunspot groups. Measurements of spot areas
and positions from Mount Wilson white-light, full-disk plates were
carried out for an interval covering most of this century. Rotation and
meridional motions were derived from these measures. In addition motions
of individual spots within sunspot groups were analyzed. A number of
results have emerged which may shed some light on the problem of the
linkage of sunspot magnetic flux tubes to subsurface layers and to
each other.
Title: Coronal Activity at Meter-Decameter Wavelengths: Clark Lake
Observations
Authors: Schmahl, E. J.; Kundu, M. R.; Szabo, A.; Gergely, T. E.;
Howard, R.
Bibcode: 1986BAAS...18R.900S
Altcode:
No abstract at ADS
Title: Meridional Motions of Sunspots and Sunspot Groups
Authors: Howard, R.; Gilman, P. A.
Bibcode: 1986ApJ...307..389H
Altcode:
Mount Wilson white-light plate data for north-south sunspot motions
are studied, taking both sunspot groups and individual spots into
consideration. The average results as a function of latitude show a
midlatitude northward flow and an amplitude of a few hundredths of a
degree per day in each hemisphere. For sunspot groups, a dependence on
latitude is seen that tends generally toward more poleward motions at
higher latitudes. A previously reported, systematic variation of the
latitude dependence of the meridional motion of sunspot groups with
phase in the solar cycle is not confirmed.
Title: The Sun. (Book Reviews: Physics of the Sun)
Authors: Howard, Robert F.
Bibcode: 1986Sci...233..483S
Altcode:
No abstract at ADS
Title: Comet 1983 XX (Solwind 6)
Authors: Howard, R.; Koomen, M.; Michels, D.; Sheeley, N.; Marsden,
B. G.
Bibcode: 1986IAUC.4229....1H
Altcode:
R. Howard, M. Koomen, D. Michels and N. Sheeley, Naval Research
Laboratory, report identification of another comet in the SOLWIND
coronagraph data. The apparent heliocentric separation F (solar radii)
and position angle O (counted from the solar north pole) have been
converted to R.A. and Decl. by the undersigned. On the sixth data
frame the coma was behind the occulting disk. 1983 UT F O R.A. (1950.0)
Decl. Sept. 24.863 8.1 230.5 11 54.4 - 0 50 24.870 7.8 230.4 11 54.8
- 0 49 24.878 7.6 231.2 11 55.0 - 0 47 24.885 7.3 230.6 11 55.3 - 0
47 24.892 7.2 230.5 11 55.5 - 0 47 25.062 < 2.5 231.8 (12 00.9 -
0 32) Preliminary examination of the raw data suggests that SOLWIND
6 was brighter than SOLWIND 2-5 (but certainly not SOLWIND 1). The
tail was still quite bright on Sept. 25.08 and present but fainter on
Sept. 25.14. Careful scrutiny of subsequent data reveals no trace of
the comet reappearing from behind the occulted area. The line of sight
was clearly very nearly in the comet's orbital plane. Computations
by the undersigned suggest that the comet was very probably a member
of the Kreutz group, a representative set of orbital elements being
as follows: T = 1983 Sept. 25.19 ET, Peri. = 78.39, Node = 357.94,
Incl. = 143.95 (equinox 1950.0), q = 0.0076 AU.
Title: Rotation and Expansion within Sunspot Groups
Authors: Gilman, P. A.; Howard, R.
Bibcode: 1986ApJ...303..480G
Altcode:
By superposing data for many sunspot groups measured on the Mount
Wilson white-light plate collection, the authors demonstrate that
differential rotation, about equal to the ambient rate, occurs between
sunspots within the group. It is also shown that the relative motions
of leader and follower sunspots can be characterized primarily as a
simple expansion of the group along its major axis, with very little
rotation of the pattern about group center.
Title: Probable Sungrazing Comets
Authors: Sheeley, N. R., Jr.; Howard, R.; Koomen, M.; Michels, D.;
Marsden, B. G.
Bibcode: 1985IAUC.4129....1S
Altcode:
N. R. Sheeley, Jr., Naval Research Laboratory, reports observations
of two more probable sungrazing comets (cf. IAUC 3640, 3718, 3719)
in the coronagraphic data from the P78-1 SOLWIND satellite. R. Howard,
M. Koomen and D. Michels were also involved with these observations of
what are tentatively called SOLWIND 4 and 5, and the Central Bureau
in fact received the provisional data below some months before the
deliberate destruction of the satellite on Sept. 13. Improved positions
are anticipated for all five objects. The apparent heliocentric
separation F (solar radii) and position angle O have been converted
to R.A. and Decl. by the undersigned. SOLWIND 4 UT F O R.A. (1950.0)
Decl. 1981 Nov. 3.999 10.5 205.0 14 29.5 -17 42 4.038 9.7 205.0 14
30.0 -17 31 4.105 8.4 203.0 14 31.2 -17 15 4.171 7.2 200.6 14 32.3
-17 01 4.238 5.8 197.1 14 33.5 -16 43 4.304 4.6 195.5 14 34.3 -16
26 4.371 3.3? 188.5? 14 35.4 -16 08 SOLWIND 5 UT F O R.A. (1950.0)
Decl. 1984 July 28.302 6.7 243.4 8 22.4 +18 15 28.309 6.6 243.8 8 22.5
+18 16 28.316 6.4 244.2 8 22.7 +18 18 28.324 6.4 244.9 8 22.7 +18 19
28.331 5.9 246.0 8 23.2 +18 24 28.368 4.8 247.6 8 24.4 +18 32 28.375
4.6 248.3 8 24.6 +18 34 28.383 4.4 249.2 8 24.8 +18 36 28.390 4.2 250.2
8 25.0 +18 38 28.397 3.9 250.8 8 25.3 +18 41 28.435 2.7 258.5 8 26.6
+18 52 28.443 2.7 259.0 8 26.6 +18 52 Computations by the undersigned
suggest that SOLWIND 5 is a member of the Kreutz group with T = 1984
July 28.48 ET, Peri. = 62.28, Node = 337.30, Incl. = 139.14 (equinox
1950.0), q = 0.0044 AU. Of all the comets SOLWIND 4 is least likely
to belong to the Kreutz group, and the following very hypothetical
elements have been derived: T = 1981 Nov. 4.6 ET, Peri. = 97.2, Node =
25.6, Incl. = 113.7, q = 0.008 AU.
Title: Eight Decades of Solar Research at Mount-Wilson
Authors: Howard, R.
Bibcode: 1985SoPh..100..171H
Altcode:
The Mount Wilson solar program has figured prominently in the field
of solar physics throughout this century. This review describes the
development of the instrumentation and the progress of the research
at Mount Wilson from 1904 to 1984.
Title: Rotation rates of leader and follower sunspots
Authors: Gilman, P. A.; Howard, R.
Bibcode: 1985ApJ...295..233G
Altcode:
The rotation rates of leader and follower sunspots found on Mount Wilson
white light plates have been measured for the years 1917-1983. It is
found that at all latitudes, leader spots rotate faster than follower
spots by about 0.1 deg per day, or 14 m/s. It is also found that, when
examined separately, leaders and followers show the same variations in
rotation with cycle phase as do all spots taken together, as reported
earlier in Gilman and Howard (1984). Leaders and followers show similar
variations in rotation rate even on an annual basis. Thus, while leaders
and followers in each group diverge in longitude from each other at an
average rate of about 0.1 deg per day, each is separately speeding up
or slowing down its rotation according to the phase in the cycle, and
by a similar amount. Leaders and followers also give about the same
covariance of longitude and latitude motions, indicating that whole
sunspot groups participate in tracing the apparent angular momentum
transport toward the equator, as previously reported for all spots in
Gilman and Howard (1984).
Title: Torsional Oscillations of the Sun
Authors: Snodgrass, H. B.; Howard, R.
Bibcode: 1985Sci...228..945S
Altcode:
The sun's differential rotation has a cyclic pattern of change that is
tightly correlated with the sunspot, or magnetic activity, cycle. This
pattern can be described as a torsional oscillation, in which the solar
rotation is periodically sped up or slowed down in certain zones of
latitude while elsewhere the rotation remains essentially steady. The
zones of anomalous rotation move on the sun in wavelike fashion,
keeping pace with and flanking the zones of magnetic activity. It is
uncertain whether this torsional oscillation is a globally coherent
ringing of the sun or whether it is a local pattern caused by and
causing local changes in the magnetic fields. In either case, it may
be an important link in the connection between the rotation and the
cycle that is widely believed to exist but is not yet understood.
Title: Solar Meridional Flow During 1982 - 1984
Authors: Zirker, J. B.; Howard, R. F.
Bibcode: 1985BAAS...17..634Z
Altcode:
No abstract at ADS
Title: Torsional Oscillations of Low Mode
Authors: Snodgrass, H. B.; Howard, R.
Bibcode: 1985SoPh...95..221S
Altcode:
Standing wave torsional oscillations of wavenumber 1/2 and 1
hemisphere−1 are studied using an improved fit to Mount
Wilson magnetograph data. These oscillations are seen to be in phase
with each other and with the magnetic activity cycle, and seem best
represented as a flexing of the differential rotation curve. Superposing
them gives a differential rotation which at solar minimum is slightly
flattened at the equator but considerably (∼ 5%) steepened at the
poles, and also tends to produce a travelling wave with wavenumber
1 hemisphere−1 that moves from pole to equator as the
cycle progresses.
Title: Solar Rotation
Authors: Howard, R. F.
Bibcode: 1985IAUTA..19..100H
Altcode: 1985IAUT...19..100H
No abstract at ADS
Title: Synoptic charts of solar magnetic fields, January - December
1985.
Authors: Howard, R.
Bibcode: 1985QBSA...27....1H
Altcode:
No abstract at ADS
Title: Evaluation of a magneto-optical filter and a Fabry-Perot
interferometer for the measurement of solar velocity fields from
space.
Authors: Rhodes, E. J., Jr.; Cacciani, A.; Blamont, J.; Tomczyk, S.;
Ulrich, R. K.; Howard, R. F.
Bibcode: 1984sses.nasa..125R
Altcode: 1984sss..conf..125R
A program was developed to evaluate the performance of three
different devices as possible space-borne solar velocity field
imagers. Two of these three devices, a magneto-optical filter and
a molecular adherence Fabry-Perot interferometer were installed in
a newly-constructed observing system located at the 60-foot tower
telescope at the Mt. Wilson Observatory. Time series of solar
filtergrams and Dopplergrams lasting up to 10 hours per day were
obtained with the filter while shorter runs were obtained with the
Fabry-Perot. Two-dimensional k h-omega power spectra which
show clearly the well-known p-mode ridges were computed from the time
series obtained with the magneto-optical filter. These power spectra
were compared with similar power spectra obtained recently with the
13.7-m McMath spectrograph at Kitt Peak.
Title: Observations of Solar Velocity Fields With Large-Format CCD
Cameras at the Mount Wilson Observatory
Authors: Rhodes, E. J., Jr.; Cacciani, A.; Tomczyk, S.; Ulrich, R. K.;
Dumont, P.; Howard, R. F.
Bibcode: 1984BAAS...16..979R
Altcode:
No abstract at ADS
Title: Limits on photospheric Doppler signatures for solar giant cells
Authors: Snodgrass, H. B.; Howard, R.
Bibcode: 1984ApJ...284..848S
Altcode:
Mount Wilson solar-velocity data taken since improvement of the
spectrograph in May 1982 are analyzed to search for photospheric traces
of persistent velocity patterns that are anticipated in recent model
predictions. The method involves time-averaged autocorrelations and
cross correlations of the residuals that remain after least-squares fits
for differential rotation, limb shift, and meridional circulation are
extracted from the daily-magnetogram velocity arrays. It is argued that,
owing to the supergranular motions in the photosphere, the sensitivity
in applying the present method to the new Mount Wilson data is close to
the ultimate sensitivity possible for detection of this phenomenon. The
following limits are currently established through this analysis: (1)
there is no sharply peaked power spectrum with amplitude above about 1
m/s per wavenumber, and (2) there is no broad-band power spectrum for
which the total integrated power is greater than about 10 sq m/sq sec.
Title: The Summer 1984 Solar Oscillation Program of the Mount Wilson
60-foot Solar Telescope
Authors: Tomczyk, S.; Rhodes, E. J., Jr.; Cacciani, A.; Ulrich, R. K.;
Howard, R. F.
Bibcode: 1984BAAS...16..978T
Altcode:
No abstract at ADS
Title: Variations in solar rotation with the sunspot cycle
Authors: Gilman, P. A.; Howard, R.
Bibcode: 1984ApJ...283..385G
Altcode:
The positions of sunspots as photographed in white light at Mt. Wilson
from 1921-82 were analyzed to detect any systematic variations,
particularly in relation to the solar cycle. The study analyzed 5 deg sq
bins of sunspots for both hemispheres. The residual rotation rates were
calculated for individual and grouped sunspots. Peaks in the sunspot
rotation rate were detected near the solar maximum and minimum and
in high solar latitudes 3 yr before the end of the cycle. The highest
sunspot rotation peaks were 0.5 deg/day. The ubiquity of the rotation
rate changes over the whole solar disk implied periodic angular momentum
exchanges between the photosphere and deeper layers of the convective
zone. Finally, the interpretations of the differences observed between
variations in sunspot and Doppler rotation are discussed.
Title: Rotation of the sun measured from Mount Wilson white-light
images
Authors: Howard, R.; Gilman, P. I.; Gilman, P. A.
Bibcode: 1984ApJ...283..373H
Altcode:
The instrumentation, data and data reduction procedures used in white
light observations of sunspot rotation rates are described. The study
covered 62 yr of rotation observations. The data were all gathered using
the same Mt. Wilson telescope, which has had three different main lenses
in the interval 1981-82. Details of the exposure calibration and lens
operation procedures are provided. The data were treated in terms of
eight evenly space determinations of the solar limb and account was
taken of all sunspots within 60 deg of the central meridian. Spot
movements were traced in terms of groups of contiguous individual
spots. Large spots rotated slower than small spots, a condition
attributed to greater viscous drag in the larger flux tubes in the
photosphere. The data tend to confirm theories that the photospheric
gas revolves at a different rate than the sunspots.
Title: On the Correlation of Longitudinal and Latitudinal Motions
of Sunspots
Authors: Gilman, P. A.; Howard, R.
Bibcode: 1984SoPh...93..171G
Altcode:
Using new measurements of positions of individual sunspots and sunspot
groups obtained from 62 years of the Mt. Wilson white-light plate
collection, we have recomputed the correlation between longitude and
latitude motion. Our results for groups are similar to those of Ward
(1965a) computed from the Greenwich record, but for individual spots
the covariance is reduced by a factor of about 3 from the Ward values,
though still of the same sign and still statistically significant. We
conclude that there is a real correlation between longitude and latitude
movement of individual spots, implying angular momentum transport
toward the equator as inferred by Ward. The two thirds reduction in
the covariance for individual spots as opposed to groups is probably
due to certain properties of spot groups, as first pointed out in an
unpublished manuscript by Leighton.
Title: Magneto-Optical Filter Observations of Solar Oscillations at
the Mt. Wilson Observatory
Authors: Rhodes, E. J., Jr.; Cacciani, A.; Tomczyk, S.; Ulrich, R. K.;
Dumont, P.; Howard, R. F.
Bibcode: 1984BAAS...16..451R
Altcode:
No abstract at ADS
Title: A statistical study of active regions 1967 1981
Authors: Tang, F.; Howard, R.; Adkins, J. M.
Bibcode: 1984SoPh...91...75T
Altcode:
We have studied 15 years of active region data based on the Mount
Wilson daily magnetograms in the interval 1967-1981. The analysis
revealed the following: (1) The integral number of regions decreases
exponentially with increasing region sizes, or N(A) = 4788 exp(-A/175)
for the 15 years of data, where A is the area in square degrees and N(A)
is the number of active regions with area ≥A. (2) The average area
of active regions varies with the phase of the solar cycle. There are
more larger regions during maximum than during minimum. (3) Regions
in the north are 10% larger on average than those in the south during
this interval. This coincides with a similar asymmetry in the total
magnetic flux between the hemispheres. (4) Regions of all sizes and
magnetic complexities show the same characteristic latitude variation
with phase in the solar cycle. The largest regions, however, show a
narrower latitude range.
Title: The Rotation of the Sun from Mount Wilson Sunspot Measurements
Authors: Howard, R. F.; Gilman, P. A.
Bibcode: 1984KodOB...4....1H
Altcode:
The authors have completed the measurement and reduction of 62 years
of white-light solar images taken at the Mount Wilson Observatory. The
data have been analyzed for differential rotation and time variations
of this quantity. This is a brief review of the work.
Title: A compact dopplergraph/magnetograph suitable for space-based
measurements of solar oscillations and magnetic fields
Authors: Rhodes, E. J., Jr.; Cacciani, A.; Tomczyk, S.; Ulrich, R. K.;
Blamont, J.; Howard, R. F.; Dumont, P.; Smith, E. J.
Bibcode: 1984AdSpR...4h.103R
Altcode: 1984AdSpR...4..103R
A compact Dopplergraph/magnetograph placed in a continuous solar-viewing
orbit will allow us to make major advancements in our understanding
of solar internal structure and dynamics. An international program
is currently being conducted at JPL and Mt. Wilson to develop such an
instrument. By combining a unique magneto-optical resonance filter with
CID and CCD cameras we have been able to obtain full- and partial-disk
Dopplergrams and magnetograms. Time series of the velocity images are
converted into k-ω power spectra which show clear- the solar nonradial
p-mode oscillations. Magnetograms suitable for studying the long-term
evolution of solar active regions have also been obtained with this
instrument. A flight instrument based on this concept is being studied
for possible inclusion in the SOHO mission.
Title: Evolution of solar magnetic flux.
Authors: Boris, J. P.; DeVore, C. R.; Golub, L.; Howard, R. F.; Low,
B. C.; Sheeley, N. R., Jr.; Simon, G. W.; Tsinganos, K. C.
Bibcode: 1984NASRP1120....3B
Altcode:
Contents: Introduction. Appearance of magnetic flux: models for flux
emergence, unexplained observations. Dynamics of surface magnetic
flux: magnetic flux transport, magnetic flux structure. Disappearance
of magnetic flux: theoretical considerations, observations of flux
disappearance. Summary.
Title: Solar Rotation
Authors: Howard, Robert
Bibcode: 1984ARA&A..22..131H
Altcode:
Contents: (1) Introduction. (2) Surface rotation: Early
measurements. Modern measurements. The rotation rate. Differential
rotation. Torsional oscillations. Variation of rotation rate with
height in the atmosphere. (3) Rotation from tracers: Sunspots. Rotation
of photospheric magnetic fields. (4) Chromosphere and corona. (5)
Rotation of the solar interior.
Title: Synoptic charts of solar magnetic fields, January - December
1984.
Authors: Howard, R.
Bibcode: 1984QBSA...26....1H
Altcode:
No abstract at ADS
Title: Magnetic field - plasma interaction on the Sun. Proceedings of
a colloquium held at Kodaikanal on January 23, 1984 in commemoration
of the 75th anniversary of the discovery of the Evershed effect.
Authors: Sivaraman, K. R.; Raju, P. K.; Bhattacharyya, J. C.;
Howard, R.
Bibcode: 1984mafi.book.....S
Altcode:
No abstract at ADS
Title: Radio and Visible Light Observations of a Coronal Arcade
Transient
Authors: Gergely, T. E.; Kundu, M. R.; Erksine, F. T., III; Sawyer, C.;
Wagner, W. J.; Illing, R.; House, L. L.; McCabe, M. K.; Stewart, R. T.;
Nelson, G. J.; Koomen, M. J.; Michels, D.; Howard, R.; Sheeley, N.
Bibcode: 1984SoPh...90..161G
Altcode:
We discuss simultaneous visible-light and radio observations of
a coronal transient that occurred on 9 April, 1980. Visible-light
observations of the transient and the associated erupting prominence
were available from the Coronagraph/Polarimeter carried aboard SMM,
the P78-1 coronagraph, and from the Haleakala Observatory. Radio
observations of the related type III-II-IV bursts were available from
the Clark Lake and Culgoora Observatories. The transient was extremely
complex; we suggest that an entire coronal arcade rather than just a
single loop participated in the event. Type III burst sources observed
at the beginning of the event were located along a nearby streamer,
which was not disrupted, but was displaced by the outmoving loops. The
type II burst showed large tangential motion, but unlike such sources
usually do, it had no related herringbone structure. A moving type
IV burst source can be associated with the most dense feature of the
white-light transient.
Title: Recalibration of Mount-Wilson Doppler Measurements
Authors: Snodgrass, H. B.; Howard, R.; Webster, L.
Bibcode: 1984SoPh...90..199S
Altcode:
A new calibration of the spectrograph at the Mount Wilson 150-foot
Tower Telescope demonstrates that all reported solar Doppler rates
to date measured at λ5250.2 with this instrument are too high by a
factor of 0.55%.
Title: Active Regions in the Solar Cycle
Authors: Tang, F.; Howard, R.
Bibcode: 1984stp..conf...61T
Altcode:
No abstract at ADS
Title: The QSO 1156+295 : a multifrequency study of recent activity.
Authors: Wills, B. J.; Pollock, J. T.; Aller, H. D.; Aller, M. F.;
Balonek, T. J.; Barvainis, R. E.; Binzel, R. P.; Chaffee, F. H.;
Dent, W. A.; Douglas, J. N.; Fanti, C.; Garrett, D. B.; Gregorini,
L.; Henry, R. B. C.; Hill, R. E.; Howard, R.; Jeske, N.; Kepler,
S. O.; Leacock, R. J.; Mantovani, F.; O'Dea, C. P.; Padrielli, L.;
Perley, P.; Pica, A. J.; Puschell, J. J.; Sanduleak, N.; Shields,
G. A.; Smith, A. G.; Thuan, T. X.; Wade, C. M.; Wasilewski, A. J.;
Webb, J. R.; Wills, D.; Wisniewski, W. Z.
Bibcode: 1983ApJ...274...62W
Altcode:
Photometric observations are presented for outbursts of the
quasi-stellar object QSO 1156+295 in 1981 and 1982. High time resolution
photometry is included showing variations on time scales of weeks
to about half an hour. Data from early plate material show that the
object may have been quite bright at the beginning of this century
but was quite faint and probably inactive between about 1950 and the
beginning of recent activity in 1977-1979. Some results of optical
linear polarization are discussed. In examining spectrophotometric
results, broadband spectra show very little change in spectral shape
during changes in continuum brightness. High resolution spectra (0.1-A
FWHM) show no evidence for the Mg II 2798-A absorption sometimes
seen in optically violent variables and other QSOs. The radio flux
density variations at several frequencies are documented and discussed
and compared with the optical light curve. Models for 1156+295 are
discussed including variability time scales, and the observed energy
output is compared with a magnetic accretion disk model proposed by
Shields and Wheeler (1976). Results are summarized and the different
sizes of active and inactive regions of the object are compared.
Title: A search for solar radial-velocity variations.
Authors: Bruning, D. H.; Howard, R.
Bibcode: 1983PASP...95..587B
Altcode:
No abstract at ADS
Title: A Statistical Study of Active Regions 1967-1981
Authors: Tang, F.; Howard, R.; Adkins, J. M.
Bibcode: 1983BAAS...15..971T
Altcode:
No abstract at ADS
Title: Limits on Giant Cell Signatures in the Photosphere
Authors: Snodgrass, H. B.; Howard, R.
Bibcode: 1983BAAS...15..953S
Altcode:
No abstract at ADS
Title: The Mount Wilson Magnetograph (Report from a Solar Institute)
Authors: Howard, R.; Boyden, J. E.; Bruning, D. H.; Clark, M. K.;
Crist, H. W.; Labonte, B. J.
Bibcode: 1983SoPh...87..195H
Altcode:
Alterations to the Mount Wilson Observatory solar magnetograph were
made during 1981. The present state of the instrument, including
the spectrograph, is described. The magnetic and Doppler velocity
signals and the setup procedure for the magnetogram observation are
discussed. The advantages of the new system are described.
Title: Solar Rotation Results at Mount-Wilson - Part Four - Results
Authors: Howard, R.; Adkins, J. M.; Boyden, J. E.; Cragg, T. A.;
Gregory, T. S.; Labonte, B. J.; Padilla, S. P.; Webster, L.
Bibcode: 1983SoPh...83..321H
Altcode:
We publish here rotation results from Doppler velocity measurements
made at Mount Wilson over a period of more than 14 years. Altogether
data from 188 rotations are presented. These results are displayed in
various tables and figures. Measurements of scattered light along with
its effect on the measured rotation rate are shown.
Title: Major Mass Ejection Rate From Three Space Coronagraphs
Authors: Sawyer, C.; Howard, R.; Sheeley, N.; Koomen, M.; Michels, D.
Bibcode: 1983BAAS...15..706S
Altcode: 1983BAAS...15..683M
No abstract at ADS
Title: Variations in Sunspot Rotation and the Activity Cycle
Authors: Howard, R.; Gilman, P. A.
Bibcode: 1983BAAS...15..698H
Altcode:
No abstract at ADS
Title: Large-Scale Doppler Shifts in the Solar Photosphere
Authors: Snodgrass, H. B.; Howard, R.
Bibcode: 1983BAAS...15..719S
Altcode:
No abstract at ADS
Title: A Wavelength Calibration Device for the Mount Wilson
Magnetograph
Authors: Bruning, D. H.; Howard, R.
Bibcode: 1983BAAS...15Q.701B
Altcode:
No abstract at ADS
Title: Solar Astronomy. (Book Reviews: The Sun, Our Star)
Authors: Howard, Robert
Bibcode: 1983Sci...219.1419N
Altcode:
No abstract at ADS
Title: Solar Astronomy. (Book Reviews: The Sun, Our Star)
Authors: Howard, Robert
Bibcode: 1983Sci...219.1419H
Altcode:
No abstract at ADS
Title: Torsional Oscillations of the Sun
Authors: Howard, Robert
Bibcode: 1983SoPh...82..437H
Altcode: 1983IAUCo..66..437H
A series of digitized synoptic observations of solar magnetic and
velocity fields has been carried out at the Mount Wilson Observatory
since 1967. In recent studies (Howard and LaBonte, 1980; LaBonte and
Howard, 1981), the existence of slow, large-scale torsional (toroidal)
oscillations of the Sun has been demonstrated. Two modes have been
identified. The first is a travelling wave, symmetric about the equator,
with wave number 2 per hemisphere. The pattern-alternately slower and
faster than the average rotation-starts at the poles and drifts to
the equator in an interval of 22 years. At any one latitude on the
Sun, the period of the oscillation is 11 years, and the amplitude
is 3 m s-1. The magnetic flux emergence that is seen as
the solar cycle occurs on average at the latitude of one shear zone
of this oscillation. The amplitude of the shear is quite constant
from the polar latitudes to the equator. The other mode of torsional
oscillation, superposed on the first mode, is a wave number 1 per
hemisphere pattern consisting of faster than average rotation at high
latitudes around solar maximum and faster than average rotation at low
latitudes near solar minimum. The amplitude of the effect is about 5 m
s-1. For the first mode, the close relationship in latitude
between the activity-related magnetic flux eruption and the torsional
shear zone suggests strongly that there is a close connection between
these motions and the cycle mechanism. It has been suggested (Yoshimura,
1981; Schüssler, 1981) that the effect is caused by a subsurface
Lorentz force wave resulting from the dynamo action of magnetic flux
ropes. But, this seems unlikely because of the high latitudes at which
the shear wave is seen to originate and the constancy of the magnitude
of the shear throughout the life time of the wave.
Title: A New System for Observing Solar Oscillations at the MT.WILSON
Observatory - Part One - System Design and Installation
Authors: Rhodes, E. J., Jr.; Howard, R. F.; Ulrich, R. K.; Smith, E. J.
Bibcode: 1983SoPh...82..245R
Altcode: 1983IAUCo..66..245R
In this paper we describe a new observing system which is currently
nearing completation at the Mount Wilson Observatory. This system has
been designed to obtain daily measurements of solar photospheric and
subphotospheric rotational velocities from the frequency splitting
of non-radial solar p-mode oscillations of moderate to high degree
(i.e. l > 150). The completed system will combine a 244 × 248
pixel CID camera with a high-speed floating point array processor,
a 32-bit minicomputer, and a large-capacity disc storage system. We
are integrating these components into the spectrograph of the 60-foot
solar tower telescope at Mount Wilson in order to provide a facility
which will be dedicated to the acquisition of oscillation data.
Title: The ISPM Unified Radio and Plasma wave experiment.
Authors: Stone, R. G.; Caldwell, J.; de Conchy, Y.; Deschanciaux, C.;
Ebbett, R.; Epstein, G.; Goetz, K.; Harvey, C. C.; Hoang, S.; Howard,
R.; Hulin, R.; Huntzinger, G.; Kellogg, P.; Klein, B.; Knoll, R.;
Lokerson, D.; Manning, R.; Mengué, J. P.; Meyer, A.; Monge, N.;
Monson, S.; Nicol, G.; Phan, V.; Steinberg, J. L.; Tilloles, P.;
Torres, E.; Wouters, F.
Bibcode: 1983ESASP1050..185S
Altcode:
The scientific objectives of the ISPM Unified Radio and Plasma (URAP)
wave experiment are twofold: the determination of the direction
and polarisation of distant radio sources for remote sensing of the
heliosphere, and the detailed study of local wave phenomena which
determine the transport coefficients of the ambient plasma. After
a brief discussion of these scientific objectives, a comprehensive
description of the experiment is presented.
Title: The observed relationships between some solar rotation
parameters and the activity cycle
Authors: Howard, R.; Labonte, B. J.
Bibcode: 1983IAUS..102..101H
Altcode:
Several parameters of the solar rotation show variations which appear
to relate to the phase of the solar-activity cycle. The latitude
gradient of the differential rotation, as seen in the coefficients
of the sin2 and sin4 terms in the latitude expansion, shows marked
variations with the cycle. One of these variations may be described
as a one-cycle-per-hemisphere torsional oscillation with a period of
11 years, where the high latitudes rotate faster at solar-activity
maximum and slower at minimum, and the low latitudes rotate faster at
solar-activity minimum and slower at maximum. Another variation is a
periodic oscillation of the fractional difference in the low-latitude
rotation between north and south hemispheres. The possibility of a
variation in the absolute rotational velocity of the sun in phase with
the solar cycle remains an open question. The two-cycle-per-hemisphere
torsional waves in the solar rotation also represent an aspect of the
rotation which varies with the cycle. It is shown that the amplitude
of the fast flowing zone rises a year before the rise to activity
maximum. The fast zone seems to be physically the more significant of
the two zones.
Title: Synoptic charts of solar magnetic fields, January - December
1983.
Authors: Howard, R.
Bibcode: 1983QBSA...25....1H
Altcode:
No abstract at ADS
Title: Solar Rotation Measurements at Mount-Wilson - Part Three -
Meridional Flow and Limbshift
Authors: Labonte, B. J.; Howard, R.
Bibcode: 1982SoPh...80..361L
Altcode:
The `ears' velocity pattern described in Paper I (Howard et al., 1980)
had no physical explanation. A reanalysis shows that the large scale
solar velocity patterns are better described by a nonmonotonic limbshift
and a meridional flow. The results of the new analysis imply that the
study of solar velocity pattern at the level of a few ms−1
required that magnetic regions be treated separately from nonmagnetic
regions.
Title: Are the high-latitude torsional oscillations of the sun real?
Authors: Labonte, B. J.; Howard, R.
Bibcode: 1982SoPh...80..373L
Altcode:
A numerical test is made to determine if the high-latitude torsional
wave is generated from the low-latitude torsional pattern as a result of
our reduction procedures. The results indicate that the high-latitude
motions are not an artifact of the analysis, but are true solar
features. We demonstrate also that the one-wave-per-hemisphere torsional
oscillation does not result from the reduction procedure. These results
place the observations in conflict with the predictions of α - (ω)
models of the solar cycle.
Title: Associations of Nuclear Gamma Rays with Other Flares Emissions
Authors: Cliver, E.; Share, G.; Chupp, E.; Matz, S.; Howard, R.;
Koomen, M.; McGuire, R.; von Rosenvinge, T.
Bibcode: 1982BAAS...14..874C
Altcode:
No abstract at ADS
Title: The Magnetic Flux in the Quiet Sun Network
Authors: Labonte, B. J.; Howard, R.
Bibcode: 1982SoPh...80...15L
Altcode:
The Ca II K line emission from the quiet Sun network does not vary with
the 11-year cycle (White and Livinston, 1981). We confirm this result
from direct magnetic measurements. This effect is not simply explained
by present empirical models of the evolution of surface magnetic fields.
Title: Evidence for a Poleward Meridional Flow on the Sun
Authors: Topka, K.; Moore, R.; Labonte, B. J.; Howard, R.
Bibcode: 1982SoPh...79..231T
Altcode:
We define for observational study two subsets of all polar zone
filaments, which we call polemost filaments and polar filament
bands. The behavior of the mean latitude of both the polemost filaments
and the polar filament bands is examined and compared with the evolution
of the polar magnetic field over an activity cycle as recently distilled
by Howard and LaBonte (1981) from the past 13 years of Mt. Wilson
full-disk magnetograms. The magnetic data reveal that the polar
magnetic fields are built up and maintained by the episodic arrival of
discrete f-polarity regions that originate in active region latitudes
and subsequently drift to the poles. After leaving the active-region
latitudes, these unipolar f-polarity regions do not spread equatorward
even though there is less net flux equatorward; this indicates that
the f-polarity regions are carried poleward by a meridional flow,
rather than by diffusion. The polar zone filaments are an independent
tracer which confirms both the episodic polar field formation and the
meridional flow. We find: The mean latitude of the polemost filaments
tracks the boundary of the polar field cap and undergoes an equatorward
dip during each arrival of additional polar field.
Title: On the Dissolution of Sunspot Groups
Authors: Wallenhorst, S. G.; Howard, R.
Bibcode: 1982SoPh...76..203W
Altcode:
The behavior of magnetic fluxes from active regions is investigated for
times near sunspot disappearance. It is found that the magnetic fluxes
decrease on or near the date the spot vanishes. We investigate this
effect, and conclude that it is actually due to changes in the field,
rather than through dissipation of the active region fields. This
is important in considerations of the large-scale behavior of solar
magnetic fields.
Title: Solar Observations Pertaining to the Search for Extra-Solar
Planetary Systems
Authors: Howard, R.; Bruning, D. H.
Bibcode: 1982BAAS...14..626H
Altcode:
No abstract at ADS
Title: Magnetic fields on the sun.
Authors: Howard, R.
Bibcode: 1982SAOSR.392A.155H
Altcode: 1982csss....2..155H
Synoptic observations of solar magnetic fields are discussed. Seen in
long-term averages, the magnetic fields of the Sun show distinctive
behavior. The active-region latitudes are characterized by magnetic
fields of preceding polarity. The flow of following polarity fields
to make up the polar fields is episodic, not continuous. This field
motion is a directed poleward flow and is not due to diffusion. The
total magnetic flux on the solar surface, which is related linearly
to the calcium emission in integrated sunlight, varies from activity
minimum to maximum by a factor of 2 or 3. Nearly all this flux is seen
at active-region latitudes-only about 1% is at the poles. The total
flux of the Sun disappears from the surface at a very rapid rate and
is replaced by new flux. All the field and flux patterns that we see
originate in active-region latitudes. The polar magnetic fields of the
Sun were observed to change polarity recently. The variations of the
full-disk solar flux are shown to lead to the proper rotation rate of
the Sun, but the phase of the variations is constant for only a year
or two at most.
Title: Torsional Waves on the Sun and the Activity Cycle
Authors: Labonte, B. J.; Howard, R.
Bibcode: 1982SoPh...75..161L
Altcode:
Some properties of the recently-discovered torsional oscillations of
the Sun are presented. The detailed relation of this velocity feature
to magnetic activity gives evidence that these motions represent a
fundamental oscillation within the Sun that is responsible for the
solar activity cycle and that they are not a natural consequence
of an α-ω dynamo. A new torsional oscillation with wave number 1
hemisphere−1 is demonstrated to exist on the Sun.
Title: Solar magnetic fields, January - December 1982.
Authors: Howard, R.
Bibcode: 1982QBSA...24....1H
Altcode:
No abstract at ADS
Title: Surface Magnetic Fields during the Solar Activity Cycle
Authors: Howard, R.; Labonte, B. J.
Bibcode: 1981SoPh...74..131H
Altcode:
We examine magnetic field measurements from Mount Wilson that cover the
solar surface over a 13 1/2 year interval, from 1967 to mid-1980. Seen
in long-term averages, the sunspot latitudes are characterized by
fields of preceding polarity, while the polar fields are built up by
a few discrete flows of following polarity fields. These drift speeds
average about 10 m s-1 in latitude - slower early in the
cycle and faster later in the cycle - and result from a large-scale
poleward displacement of field lines, not diffusion. Weak field
plots show essentially the same pattern as the stronger fields, and
both data indicate that the large-scale field patterns result only
from fields emerging at active region latitudes. The total magnetic
flux over the solar surface varies only by a factor of about 3 from
minimum to a very strong maximum (1979). Magnetic flux is highly
concentrated toward the solar equator; only about 1% of the flux is
at the poles. Magnetic flux appears at the solar surface at a rate
which is sufficient to create all the flux that is seen at the solar
surface within a period of only 10 days. Flux can spread relatively
rapidly over the solar surface from outbreaks of activity. This is
presumably caused by diffusion. In general, magnetic field lines at
the photospheric level are nearly radial.
Title: Measurement of Solar Radius Changes
Authors: Labonte, B. J.; Howard, R.
Bibcode: 1981Sci...214..907L
Altcode:
Photoelectric solar radius measurements since 1974 at Mount Wilson show
no change in the solar radius, with a limit of about 0.1 arc second
(1 standard deviation), over the interval. The limit is set by residual
systematic effects.
Title: An improved search for large-scale convection cells in the
solar atmosphere
Authors: Labonte, B. J.; Howard, R.; Gilman, P. A.
Bibcode: 1981ApJ...250..796L
Altcode:
A reanalysis of Mount Wilson solar velocity observations was made to
search for giant cellular patterns. The reanalysis avoids several errors
made in a previous search. No cells are detected with sensitivity of
3 to 12 m/s depending upon wavenumber. The observed amplitudes do not
conflict with recent model predictions.
Title: Book-Review - Phenomena in Stars and Stellar Systems
Authors: Bonnet, R. M.; Dupree, A. K.; Howard, R.
Bibcode: 1981Sci...214..902B
Altcode:
No abstract at ADS
Title: The Solar-Stellar Connection. (Book Reviews: Solar Phenomena
in Stars and Stellar Systems)
Authors: Howard, Robert
Bibcode: 1981Sci...214Q.902H
Altcode: 1981Sci...214Q.902B
No abstract at ADS
Title: Comparative Magnetospherology - Part 12 - Evidences to Support
the Two-Hemisphere Model on Rotational Reversing of the Heliodipole
in Sunspot Maximum Phase
Authors: Saito, Takao; Howard, R.
Bibcode: 1981lupl.symp...91S
Altcode:
No abstract at ADS
Title: Probable Sungrazing Comet
Authors: Howard, R.; Koomen, N.; Michels, D. J.; Sheeley, N.; Marsden,
B. G.
Bibcode: 1981IAUC.3640....1H
Altcode:
Images of a probable comet have been found at the Naval Research
Laboratory, Washington, on coronagraph exposures obtained from
the satellite P78-1 in 1979. The object was found by R. Howard as
a result of instrumentation developed and operated by N. Koomen and
D. J. Michels. The following heliocentric separations rho (in units of
the instantaneous solar radius) and position angles theta (measured
counterclockwise from the sun's north pole) have been derived by
N. Sheeley and converted to R.A. and Decl. by the undersigned. The head
of the object was somewhat brighter than Venus, and a tail was directed
roughly away from the center of the sun. 1979 UT rho theta R.A. (1950)
Decl. Aug. 30.789 5.96 234.2 10 26.47 + 8 45.1 30.796 5.67 233.8 10
26.80 + 8 45.5 30.802 5.27 234.3 10 27.22 + 8 47.8 30.809 5.16 234.6
10 27.36 + 8 48.6 30.816 5.09 235.4 10 27.43 + 8 49.8 30.856 3.65 235.7
10 29.07 + 8 54.8 30.867 3.11 236.1 10 29.67 + 8 56.9 30.885 2.56 239.2
10 30.28 + 9 00.7 At the last observation the object's head was at the
edge of the coronagraph's occulting disk. On the next exposure, taken at
Aug. 30.989 UT, the tail is still present, and during the next several
hours cometary material evidently diffused around to p.a. ~ 360o. The
comet's orbit cannot be unequivocally determined, but computations by
the undersigned suggest that a retrograde solution is to be preferred,
for this better explains the previous failure to detect the comet in a
twilit sky. Other possible coronagraphic or hitherto unreported visual
detections of the object would of course be very useful. Retrograde
orbit solutions show some resemblance to the orbits of the members
of the Kreutz sungrazing comet group; there would seem to be a good
chance that the comet hit the sun (for the head was not detected after
perihelion). The following possible orbital solution has been selected
solely because of its general resemblance to the Kreutz-type orbits:
T = 1979 Aug. 30.92 ET Peri. = 83.42 Node = 9.81 1950.0 q = 0.001 AU
(assumed) Incl. = 142.42
Title: Solar measurements at Mount Wilson. II. Systematic instrumental
effects and the absolute rotation rate.
Authors: Labonte, B. J.; Howard, R.
Bibcode: 1981SoPh...73....3L
Altcode:
Possible sources of systematic error in solar Doppler rotational
velocities are examined. Scattered light is shown to affect the
Mount Wilson solar rotation results, but this effect is not enough
to bring the spectroscopic results in coincidence with the sunspot
rotation. Interference fringes at the spectrograph focus at Mount
Wilson have in two intervals affected the rotation results. It has been
possible to correlate this error with temperature and thus correct for
it. A misalignment between the entrance and exit slits is a possible
source of error, but for the Mount Wilson slit configuration the
amplitude of this effect is negligibly small. Rapid scanning of the
solar image also produces no measurable effect.
Title: Transient Brightenings of Interconnecting Loops - Part Two -
Dynamics of the Brightened Loops
Authors: Svestka, Z.; Howard, R.
Bibcode: 1981SoPh...71..349S
Altcode:
We discuss three different kinds of dynamic events related to
interconnecting loops observed in soft X-rays aboard Skylab: (1)
A newly born transequatorial loop that was either emerging from
subphotospheric layers or gradually filled in with hot plasma. (2)
Large-scale twists of interconnecting loops which never relax, and
often only form, after the loop brightenings. (3) Three events where
the loop that later interconnected two active regions had been visible
long before one of the interconnecting regions was born. Several impacts
this observation might have upon our understanding of the process of
flux emergence are suggested.
Title: On the outburst of flare activity of 26 November, 1973
Authors: Howard, R.; Svestka, Z.
Bibcode: 1981SoPh...71...49H
Altcode:
We draw attention of flare build-up observers to a strong 30 hour-long
outburst of homologous flare activity and unusual growth and brightening
of coronal loops, seen on Skylab. We suggest that these events might
have been closely associated with newly emerging magnetic flux, in
spite of the fact that the flux effects in Hα and EUV were first seen
only late after the activity had started, and the flux emerged at the
opposite end of the coronal loops from where the flares occurred.
Title: Horizontal Motions on the Solar Surface
Authors: Howard, R.; Labonte, B.
Bibcode: 1981siwn.conf...93H
Altcode:
No abstract at ADS
Title: M. Wilson Doppler Velocity Measurements
Authors: Howard, R.
Bibcode: 1981siwn.conf...97H
Altcode:
No abstract at ADS
Title: New Instrumentation for Solar Oscillation Measurements at
MT. Wilson
Authors: Rhodes, E.; Howard, R.; Ulrich, R.; Smith, E.
Bibcode: 1981siwn.conf..102R
Altcode:
No abstract at ADS
Title: Mount Wilson Solar Diameter Measurements
Authors: Labonte, B.; Howard, R.
Bibcode: 1981siwn.conf..362L
Altcode:
No abstract at ADS
Title: Global velocity fields of the sun and the activity cycle
Authors: Howard, R.
Bibcode: 1981AmSci..69...28H
Altcode:
Some features of the solar activity cycle are discussed from the
vantage point of ground-based observations at Mt. Wilson. Solar rotation
and large-scale surface velocities are examined in relation to solar
activity. It is proposed that torsional oscillations take place at
a layer where the differential rotation, i.e. the latitude gradient,
does not exist or has a very low amplitude.
Title: A 0.4K bolometer receiver for millimeter astronomy.
Authors: Ade, P. A. R.; Davis, J.; Howard, R.; Nolt, I.; Payne, J. M.;
Predko, S.; Radostitz, J. V.
Bibcode: 1981imw..conf..W35A
Altcode:
No abstract at ADS
Title: Solar magnetic fields, January - December 1981.
Authors: Howard, R.
Bibcode: 1981QBSA...23....1H
Altcode:
No abstract at ADS
Title: Measurement of the MT.WILSON Solar White-Light Plate Collection
Authors: Howard, R.
Bibcode: 1981phss.conf...59H
Altcode:
No abstract at ADS
Title: On the Dissolution of Sunspot Groups
Authors: Wallenhorst, S. G.; Howard, R.
Bibcode: 1981phss.conf...55W
Altcode:
No abstract at ADS
Title: Book-Review - the Ancient Sun
Authors: Pepin, R. O.; Eddy, J. A.; Merrill, R. B.; Howard, R.
Bibcode: 1981S&T....62..252P
Altcode:
No abstract at ADS
Title: Currents on the Sun
Authors: Howard, R.; LaBonte, B.
Bibcode: 1980S&T....60..485H
Altcode:
No abstract at ADS
Title: Large-Scale Solar Magnetic Fields Over the Last Cycle
Authors: Howard, Robert; Labonte, Barry J.
Bibcode: 1980BAAS...12..893H
Altcode:
No abstract at ADS
Title: Polar Crown Filaments and the Polar Magnetic Field
Authors: Topka, K.; Moore, R. L.; Labonte, B. J.; Howard, R.
Bibcode: 1980BAAS...12..893T
Altcode:
No abstract at ADS
Title: Observational Search for Variations in the Solar Convection
Authors: Labonte, B. J.; Howard, R.
Bibcode: 1980BAAS...12..914L
Altcode:
No abstract at ADS
Title: The sun is observed to be a torsional oscillator with a period
of 11 years
Authors: Howard, R.; Labonte, B. J.
Bibcode: 1980ApJ...239L..33H
Altcode:
Twelve years of full-disk Mount Wilson velocity data have been analyzed
to study horizontal east-west motions. A torsional wave pattern with
alternating latitude zones of slow and fast rotation is found, after
subtracting a differentially rotating frame. Amplitudes of the flow
pattern average about 3 m/s. It requires about 22 years for zones
to drift from the poles, where they originate, to the equator, where
they disappear. The pattern is symmetric about the equator. The zones
representing the next solar cycle (No. 22) are seen now at high solar
latitudes. Solar active regions are formed in a latitude strip centered
on the boundary of fast- and slow-velocity zones. This pattern evidently
represents a deep-seated circulation pattern and is the first evidence
of the association of mass motions with large-scale characteristics
of the solar activity cycle.
Title: A search for large-scale convection cells in the solar
atmosphere
Authors: Howard, R.; Labonte, B. J.
Bibcode: 1980ApJ...239..738H
Altcode:
Mount Wilson magnetograph velocity observations are used to search
for east-west motions resulting from hypothetical cellular patterns
extending over one or two hemispheres in the latitude direction. No
such solar patterns were found. Upper limits established by this
analysis depend on the cell lifetime and the pattern stability, but
in all cases they are no more than about 10 m/s.
Title: Solar Rotation Measurements at Mount-Wilson - Part One -
Analysis and Instrumental Effects
Authors: Howard, R.; Boyden, J. E.; Labonte, B. J.
Bibcode: 1980SoPh...66..167H
Altcode:
We examine the background velocity fields of the Sun as observed at
Mount Wilson. The method of velocity reduction of the full-disk Mount
Wilson data is outlined. We describe a number of tests that have been
carried out in order to find an instrumental origin for short-term
rotation variations and a large-scale background line-shift - the
ears. No instrumental cause can be found for this ear effect, although
such a cause cannot yet be ruled out.
Title: The Dynamics of Brightened Interconnecting Loops
Authors: Howard, R.; Svestka, Z.
Bibcode: 1980BAAS...12..519H
Altcode:
No abstract at ADS
Title: Torsional Oscillations of the Sun and Magnetic Flux Eruption
Authors: Labonte, B. J.; Howard, R.
Bibcode: 1980BAAS...12..473L
Altcode:
No abstract at ADS
Title: Solar Currents and the Magnetic Cycle
Authors: Howard, R.; Labonte, B.; Dicke, R. H.; Wilcox, J.
Bibcode: 1980SciN..117..245H
Altcode:
No abstract at ADS
Title: Search for giant cells in the solar convection zone
Authors: Labonte, B. J.; Howard, R.
Bibcode: 1980IAUS...91...21L
Altcode:
No abstract at ADS
Title: Synoptic charts of solar magnetic fields. Mount Wilson
Observatory.
Authors: Howard, R.
Bibcode: 1980QBSA...21....1H
Altcode:
No abstract at ADS
Title: Synoptic charts of solar magnetic fields. Mount Wilson
Observatory.
Authors: Howard, R.
Bibcode: 1980QBSA..203..275H
Altcode:
No abstract at ADS
Title: Unraveling Solar Magnetism
Authors: Wilcox, J. M.; Scherrer, P. H.; Hoeksema, J. T.; Howard,
R.; Labonte, B.
Bibcode: 1980SciN..117..374W
Altcode:
No abstract at ADS
Title: Transient brightenings of interconnecting loops. Morphology
of the sudden brightenings.
Authors: Svestka, Z.; Howard, R.
Bibcode: 1979SoPh...63..297S
Altcode:
We study sudden brightenings of coronal loops that interconnect
active regions. Such brightenings often occur within one or two
days after the birth of a new interconnecting loop, as well as
in some old interconnections. The brightenings of young loops are
obviously associated with the emergence of new magnetic flux near their
footpoints, whereas some enhancements of old loops may be triggered by
slowly moving disturbances propagating from other centers of activity. A
few loop brightenings are associated with flares, but the loop does
not brighten in consequence of energy supply from the flare. Both the
flare and the loop brightening are independent consequences of one
common agent, presumably newly emerging flux.
Title: The birthplaces of active regions and X-ray bright points.
Authors: Howard, R.; Fritzova-Svestkova, L.; Svestka, Z.
Bibcode: 1979SoPh...63..105H
Altcode:
A comparison of soft X-ray pictures of the Sun (S-054 experiment of
Skylab) with K-line spectroheliograms (Mount Wilson) shows that the
X-ray bright points tend to emerge randomly throughout the Ca network
pattern. However, all those bright points that developed into active
regions emerged at the boundaries of network cells. This suggests that
the magnetic flux of active regions comes from greater depths in the
convection zone than the shallow flux that gives rise to the random
emergence of bright points.
Title: Effects of the Sun. (Book Reviews: The New Solar
Physics. Papers from an AAAS symposium, Denver, Feb. 1977)
Authors: Howard, Robert
Bibcode: 1979Sci...204..607H
Altcode: 1979Sci...204..607E
No abstract at ADS
Title: Evidence for large-scale velocity features on the sun.
Authors: Howard, R.
Bibcode: 1979ApJ...228L..45H
Altcode:
Giant velocity features on the solar surface are seen in line-of-sight
velocity data from the Mount Wilson magnetograph. Velocity amplitudes
around 40 m/s are seen in features with dimensions about 15 deg in
latitude and 30-60 deg in longitude. These features are associated
with solar activity. The largest such feature accompanied the largest
complex of activity in this interval, lived for about 16 rotations,
and predated the first magnetic fields in the area by about two
rotations. A general pattern of upward motion at the equator is seen,
and motions away from the earth at higher latitudes could represent
a meridional flow toward the poles of the order of 20 m/s.
Title: Synoptic charts of solar magnetic fields. Mount Wilson
Observatory.
Authors: Howard, R.
Bibcode: 1979QBSA..202..213H
Altcode:
No abstract at ADS
Title: Physics of the Sun - Synoptic Observations at MT.WILSON
Rotation of the Sun - Large-Scale Velocity Fields - Active Regions
Regions - Solar Axis Elements - Big Bear Solar Observatory -
Instruments - Blue Continuum in Flares - Thermal X-Ray Plasma in
Solar Flares
Authors: Howard, R.; Goeden, R.; Eaton, S.; Labonte, B.; Patterson,
A.; Zirin, H.; Tanaka, H.; Moore, R.
Bibcode: 1979haob.rept..716H
Altcode:
No abstract at ADS
Title: Differential rotation and global-scale velocity fields.
Authors: Howard, R.; Yoshimura, H.
Bibcode: 1979psa..conf...28H
Altcode:
No abstract at ADS
Title: The rotation of the sun.
Authors: Howard, R.
Bibcode: 1978RvGSP..16..721H
Altcode: 1978RvGeo..16..721H
Observations of solar rotation are reviewed. There are two basic methods
of determining the rotation of the sun. One is to measure the daily or
monthly positions of tracers on the surface or in the corona, and the
other is to measure line-of-sight velocities with the Doppler effect in
spectrum lines. The results of a number of investigations involving both
methods are compared. The experimental and interpretational problems
associated with observational determinations of solar rotation are
reviewed and compared. The theoretical situation in this field is
reviewed, including the significance of different rotation rates for
surface magnetic features and the solar plasma. The rotation of the
solar interior is discussed in terms of both model calculations and
recent p mode oscillation rotation rates, which reflect the interior
rotation.
Title: Magnetic field rotation at high solar latitudes.
Authors: Howard, R.
Bibcode: 1978SoPh...59..243H
Altcode:
Measurements of the rotation rate of polar magnetic features during
1974-76 lead to a significantly slower rotation rate than that found
earlier for polar faculae in 1951-54. Similarly, the rotation rate of
these features is slower than the Doppler-determined rate at polar
latitudes or the rotation rate of polar filaments. It is suggested
that the strong latitude rotation gradient in the subsurface magnetic
flux tubes which is implied by these results may presage a very active
solar maximum for cycle 21.
Title: Erratum: "Development of a complex of activity in the solar
corona" [Sol. Phys., Vol. 54, p. 65 - 105 (1977)].
Authors: Howard, R.; Svestka, Z.
Bibcode: 1978SoPh...56..471H
Altcode:
No abstract at ADS
Title: Sunspots (sunspot relative-numbers and sunspot-areas); Synoptic
charts of solar magnetic fields (Mount Wilson Observatory); Eruptions
chromosphériques brillantes; Intensité de la couronne solaire;
Solar radio emission.
Authors: Waldmeier, M.; Howard, R.; Simon, P.; Enome, S.
Bibcode: 1978QBSA..197....1W
Altcode:
No abstract at ADS
Title: Synoptic Charts of Solar Magnetic Fields
Authors: Howard, R.
Bibcode: 1978QBSA..204..333H
Altcode:
No abstract at ADS
Title: Synoptic Charts of Solar Magnetic Fields
Authors: Howard, R.
Bibcode: 1978QBSA..203..275H
Altcode:
No abstract at ADS
Title: Synoptic charts of solar magnetic fields, Mount Wilson
Observatory. 1978 January - March.
Authors: Howard, R.
Bibcode: 1978QBSA..201..159H
Altcode:
No abstract at ADS
Title: Sunspots (sunspot relative-numbers and sunspot-areas); Synoptic
charts of solar magnetic fields (Mount Wilson Observatory); Eruptions
chromosphériques brillantes; Intensité de la couronne solaire;
Solar radio emission.
Authors: Waldmeier, M.; Howard, R.; Simon, P.; Enome, S.
Bibcode: 1978QBSA..199...71W
Altcode:
No abstract at ADS
Title: Open magnetic fields in active regions.
Authors: Svestka, Z.; Solodyna, C. V.; Howard, R.; Levine, R. H.
Bibcode: 1977SoPh...55..359S
Altcode:
Soft X-ray observations confirm that some of the dark gaps seen between
interconnecting loops and inner cores of active regions may be loci of
open fields, as it has been predicted by global potential extrapolation
of photospheric magnetic fields. It seems that the field lines may
open only in a later state of the active region development.
Title: Development of a complex of activity in the solar corona.
Authors: Howard, R.; Svestka, Z.
Bibcode: 1977SoPh...54...65H
Altcode:
Skylab observations of the Sun in soft X-rays gave us the first
possibility to study the development of a complex of activity in the
solar corona during its whole lifetime of seven solar rotations. The
basic components of the activity complex were permanently interconnected
(including across the equator) through sets of magnetic field lines,
which suggests similar connections also below the photosphere. However,
the visibility of individual loops in these connections was greatly
variable and typically shorter than one day. Each brightening of
a coronal loop in X-rays seems to be related to a variation in the
photospheric magnetic field near its footpoint. Only loops (rarely
visible) connecting active regions with remnants of old fields can be
seen in about the same shape for many days. The interconnecting X-ray
loops do not connect sunspots.
Title: Studies of solar magnetic fields. V: The true average field
strengths near the poles.
Authors: Howard, R.
Bibcode: 1977SoPh...52..243H
Altcode:
An estimate of the average magnetic field strength at the poles of the
Sun from Mount Wilson measurements is made by comparing low latitude
magnetic measurements in the same regions made near the center of
the disk and near the limb. There is still some uncertainty because
the orientation angle of the field lines in the meridional plane is
unknown, but the most likely possibility is that the true average
field strengths are about twice the measured values (0-2 G), with an
absolute upper limit on the underestimation of the field strengths of
about a factor 5. The measurements refer to latitudes below about 80°.
Title: Transequatorial loops interconnecting McMath regions 12472
and 12474.
Authors: Svestka, Z.; Krieger, A. S.; Chase, R. C.; Howard, R.
Bibcode: 1977SoPh...52...69S
Altcode:
We discuss the life-story of a transequatorial loop system which
interconnected the newly born active region McMath 12474 with the old
region 12472. The loop system was probably born through reconnection
accomplished 1.5 to 5 days after the birth of 12474 and the loops were
observed in soft X-rays for at least 1.5 days. Transient `sharpenings'
of the interconnection and a striking brightening of the whole loop
system for about 6 hr appear to be caused by magnetic field variations
in the region 12474. A flare might have been related to the brightening,
but only in an indirect way: the same emerging flux could have triggered
the flare and at the same time strengthened the magnetic field at the
foot-points of the loops. Electron temperature in the loop system,
equal to 2.1 × 106 K in its quiet phase, increased to
3.1 × 106 K during the brightening. Electron density in
the loop system was ≤ 1.3 × 109 cm−3 and
it could be estimated to ∼7 × 108 cm−3
prior to the brightening. During the brightening the loops became
twisted. There was no obvious effect whatsoever of the activity in
12474 upon the in erconnected old region. The final decay of the loop
system reflected the decay of magnetic field in the region 12474.
Title: The mean magnetic field of the Sun: Method of observation
and relation to the interplanetary magnetic field
Authors: Scherrer, Philip H.; Wilcox, John M.; Kotov, Valeri; Severny,
A. B.; Howard, Robert
Bibcode: 1977SoPh...52D...6S
Altcode:
The mean solar magnetic field as measured in integrated light has
been observed since 1968. Since 1970 it has been observed both at
Hale Observatories and at the Crimean Astrophysical Observatory. The
observing procedures at both observatories and their implications for
mean field measurements are discussed. A comparison of the two sets
of daily observations shows that similar results are obtained at both
observatories. A comparison of the mean field with the interplanetary
magnetic polarity shows that the IMF sector structure has the same
pattern as the mean field polarity.
Title: The mean magnetic field of the sun: method of observation
and relation to the interplanetary magnetic field.
Authors: Scherrer, P. H.; Wilcox, J. M.; Kotov, V.; Severnyi, A. B.;
Howard, R.
Bibcode: 1977SoPh...52....3S
Altcode:
The mean solar magnetic field as measured in integrated light has
been observed since 1968. Since 1970 it has been observed both at
Hale Observatories and at the Crimean Astrophysical Observatory. The
observing procedures at both observatories and their implications for
mean field measurements are discussed. A comparison of the two sets
of daily observations shows that similar results are obtained at both
observatories. A comparison of the mean field with the interplanetary
magnetic polarity shows that the IMF sector structure has the same
pattern as the mean field polarity.
Title: Open Magnetic Fields in Active Regions.
Authors: Svestka, Z.; Solodyna, C. V.; Howard, R.; Levine, R. H.
Bibcode: 1977BAAS....9Q.344S
Altcode:
No abstract at ADS
Title: Solar Cycle, Solar Rotation and Large-Scale Circulation
Authors: Howard, R.
Bibcode: 1977ASSL...69....7H
Altcode: 1977igss.conf....7H
Solar (Activity) Cycle Hale Cycle Long-Term Activity Variations Dynamos
Differential Rotation Rotation of the Convection Zone Carrington
Rotation Oblateness (Solar) Meridional Flow Giant Cells or Large-Scale
Circulation
Title: Sunspots (sunspot relative-numbers and sunspot-areas); Synoptic
charts of solar magnetic fields (Mount Wilson Observatory); Eruptions
chromosphériques brillantes; Intensité de la couronne solaire;
Solar radio emission.
Authors: Waldmeier, M.; Howard, R.; Simon, P.; Tanaka, H.
Bibcode: 1977QBSA..193..153W
Altcode:
No abstract at ADS
Title: Solar Rotation and Large~Sca1e Velocity Fields
Authors: Howard, Robert F.
Bibcode: 1977lsms.proc...58H
Altcode:
No abstract at ADS
Title: Sunspots (sunspot relative-numbers and sunspot-areas); Synoptic
charts of solar magnetic fields (Mount Wilson Observatory); Eruptions
chromosphériques brillantes; Intensité de la couronne solaire;
Solar radio emission.
Authors: Waldmeier, M.; Howard, R.; Simon, P.; Tanaka, H.
Bibcode: 1977QBSA..195..233W
Altcode:
No abstract at ADS
Title: Large-scale solar magnetic fields.
Authors: Howard, R.
Bibcode: 1977ARA&A..15..153H
Altcode:
Topics discussed in this review of large-scale solar magnetic fields
include large-scale magnetic surface features, the solar activity
cycle and the large-scale patterns, and magnetic fields in the
corona. Features considered include the decay of active regions, the
background field pattern, the polar fields, giant regular structures,
expansion of the field in surface harmonics, and the average inclination
of magnetic-field lines in the photosphere. Also considered are the
appearance of the background fields through the cycle, the variation
of the fields through the cycle, variations in magnetic flux, the
overall patterns and its variations, and the magnetic field of the
sun as a star. The expansion of the surface fields as well as X-ray
coronal loops are examined, and goals of future research are indicated.
Title: A possible variation of the solar rotation with the activity
cycle.
Authors: Howard, R.
Bibcode: 1976ApJ...210L.159H
Altcode:
Daily spectroscopic observation of the rotation of the sun indicates
that several slow changes have taken place since 1967. The equatorial
rotation rate of the photospheric gas has gradually increased until,
in 1976, it is close to the sunspot rate determined by Newton and Nunn
(1951). The latitude gradient at middle latitudes decreased starting
in 1974, and the latitude gradient at high latitudes has increased in
the same interval. An increase in the number of low-latitude active
regions may be responsible for accelerating the photospheric gas.
Title: The Mount Wilson solar magnetograph: scanning and data system.
Authors: Howard, R.
Bibcode: 1976SoPh...48..411H
Altcode:
A description is given of a newly-installed computer-operated image
scanning and data system for the 150-foot Tower Telescope at the Mount
Wilson Observatory. This new system provides improved flexibility,
accuracy, and reliability in the magnetograph observations.
Title: Studies of solar magnetic fields. IV: The effects of angular
resolution.
Authors: Howard, R.
Bibcode: 1976SoPh...47..575H
Altcode:
In order to provide a smooth transition to a smaller aperture for the
Mount Wilson daily magnetograms, a 2-step change was made, with two
daily observations made using two different apertures covering an
interval of several months. A comparison of these observations has
made possible a check on the zero-level and calibration errors of
the Mount Wilson magnetograph in recent years, and it has shown that
an interval of low measured total magnetic flux resulted at least in
part from an increase in the mixing of magnetic elements of the two
polarities on a scale comparable with the aperture size.
Title: Solar activity (Activité solaire).
Authors: Newkirk, G.; Dunn, R. B.; Mehltretter, P.; MacQueen, R.;
Bonnet, R. M.; White, O. R.; Fokker, A. D.; Zwaan, C.; Bruzek, A.;
Durrant, C.; Grossmann-Doerth, U.; Mehltretter, J. P.; Svestka, Z.;
de Feiter, L. D.; Tandberg-Hanssen, E.; Howard, R.; Stix, M.; Pneuman,
G. W.; Hundhausen, A. J.; Sawyer, C.; Simon, P.
Bibcode: 1976IAUTA..16b..13N
Altcode:
No abstract at ADS
Title: Differential Rotation and Global-Scale Velocity Fields
Authors: Howard, R.; Yoshimura, H.
Bibcode: 1976IAUS...71...19H
Altcode:
No abstract at ADS
Title: Phenomenological understanding of the solar cycle.
Authors: Howard, R.
Bibcode: 1976pspe.proc...34H
Altcode: 1976pspe.conf...34H
The Babcock-Leighton model of a solar dynamo is reviewed, and a general
discussion of magnetic fields on the solar surface is provided. The
model assumes that dipole-like fields, in the form of flux tubes, run
from one polar region to the other at some distance below the solar
surface, and that these flux tubes become twisted as well as stretched
by differential rotation. Aspects of the solar cycle explained by
the model are considered to include the frequency of active region
formation, the latitude variation of the activity, the observed Hale
polarity law, and the tilts of the axes of active regions. The model
does not explain certain regularities in the distribution of activity
of the solar surface, for example, instances when the solar activity
is confined to one side of the sun. The model also does not explain
the regularity on a large scale shown by the interplanetary magnetic
field sector pattern, or the existence of large scale irregularities.
Title: Sunspots (sunspot relative-numbers and sunspot-areas);
Synoptic charts of solar magnetic fields (Mount Wilson Observatory);
Eruptions chromoshériques brillantes; Intensité de la couronne
solaire; Solar radio emission.
Authors: Waldmeier, M.; Howard, R.; Olivieri, G.; Bernot, M.;
Tanaka, H.
Bibcode: 1976QBSA..189....1W
Altcode:
No abstract at ADS
Title: Sunspots (sunspot relative-numbers and sunspot areas);
Synoptic charts of solar magnetic fields (Mount Wilson Observatory);
Eruptions chromosphériques brillants; Intensité de la couronne
solaire; Solar radio emission.
Authors: Waldmeier, M.; Howard, R.; Olivieri, G.; Bernot, M.;
Tanaka, H.
Bibcode: 1976QBSA..191...73W
Altcode:
No abstract at ADS
Title: The Sun's Magnetic Sector Structure
Authors: Svalgaard, L.; Wilcox, J. M.; Scherrer, P. H.; Howard, R.
Bibcode: 1975SoPh...45...83S
Altcode:
The synoptic appearance of solar magnetic sectors is studied using 454
sector boundaries observed at Earth during 1959-1973. The sectors are
clearly visible in the photospheric magnetic field. Sector boundaries
can be clearly identified as north-south running demarcation lines
between regions of persistent magnetic polarity imbalances. These
regions extend up to about 35 ° of latitude on both sides of the
equator. They generally do not extend into the polar caps. The polar
cap boundary can be identified as an east-west demarcation line marking
the poleward limit of the sectors. The typical flux imbalance for a
magnetic sector is about 4 × 1021 Mx.
Title: The sun's magnetic sector structure
Authors: Svalgaard, L.; Wilcox, J. M.; Scherrer, P. H.; Howard, R.
Bibcode: 1975suiprrept.1959S
Altcode:
The synoptic appearance of solar magnetic sectors is studied using 454
sector boundaries observed at earth during 1959-1973. The sectors are
clearly visible in the photospheric magnetic field. Sector boundaries
can be clearly identified as north-south running demarcation lines
between regions of persistent magnetic polarity imbalances. These
regions extend up to about 35 deg of latitude on both sides of the
equator. They generally do not extend into the polar caps. The polar
cap boundary can be identified as an east-west demarcation line marking
the poleward limit of the sectors. The typical flux imbalance for a
magnetic sector is about 4 x 10 to the 21st power Maxwells.
Title: The rotation of the sun.
Authors: Howard, R.
Bibcode: 1975SciAm.232d.106H
Altcode: 1975SciAm.232..106H
The nonuniform rotation of the sun is discussed. Timing of tracers
(such as sunspots) and differential Doppler shifting of spectrographs
indicate that the material near the sun's equator rotates faster than
the material near its poles. Moreover, rotation of markers, all of which
are associated with magnetic fields, is faster than rotation of gas as
determined by photospheric spectra; this means that the solar magnetic
field rotates faster than the mass of the sun. A rapidly rotating core,
as evinced by an observed oblateness greater than that calculated
from the sun's observed rotation rate, could explain the differential
rotations of mass and field. A transportation of angular momentum from
high solar latitudes to low solar latitudes may account for nonuniform
mass rotation; several mechanisms for this transfer are considered.
Title: Tabulation of the Harmonic Coefficients of the Solar Magnetic
Fields
Authors: Altschuler, Martin D.; Trotter, Dorothy E.; Newkirk, Gordon,
Jr.; Howard, Robert
Bibcode: 1975SoPh...41..225A
Altcode:
Tables of spherical harmonic coefficients for the global
photospheric magnetic field between 1959 and 1974 are now available
on microfilm. (These are the same coefficients which were used to
construct the maps of the coronal magnetic atlas.)
Title: Sunspots (sunspot relative-number and sunspot-areas); Synoptic
charts of solar magnetic fields (Mount Wilson Observatory); Éruptions
chromosphériques brillantes; Intensité de la couronne solaire;
Solar radio emission.
Authors: Waldmeier, M.; Howard, R.; Olivieri, G.; Bernot, M.;
Tanaka, H.
Bibcode: 1975QBSA..185..175W
Altcode:
No abstract at ADS
Title: Sunspots (sunspot relative-numbers and sunspot-areas); Synoptic
charts of solar magnetic fields (Mount Wilson Observatory); Eruptions
chromosphériques brillantes; Intensité de la couronne solaire;
Solar radio emission.
Authors: Waldmeier, M.; Howard, R.; Olivieri, G.; Bernot, M.;
Tanaka, H.
Bibcode: 1975QBSA..187..255W
Altcode:
No abstract at ADS
Title: Studies of solar magnetic fields. III: The east-west
orientation of field lines.
Authors: Howard, R.
Bibcode: 1974A&A....36..275H
Altcode:
No abstract at ADS
Title: The Large-Scale Solar Magnetic Field
Authors: Altschuler, M. D.; Trotter, D. E.; Newkirk, G., Jr.;
Howard, R.
Bibcode: 1974SoPh...39....3A
Altcode:
The large-scale photospheric magnetic field, measured by
the Mt. Wilson magnetograph, has been analyzed in terms of
surface harmonics (Pnm)(θ)cosmφ and
Pnm(θ)sinmφ) for the years 1959 through
1972. Our results are as follows. The single harmonic which most
often characterized the general solar magnetic field throughout the
period of observation corresponds to a dipole lying in the plane
of the equator (2 sectors, n = m = 1). This 2-sector harmonic was
particularly dominant during the active years of solar cycles 19
and 20. The north-south dipole harmonic (n = 1, m = 0) was prominent
only during quiet years and was relatively insignificant during the
active years. (The derived north-south dipole includes magnetic fields
from the entire solar surface and does not necessarily correlate with
either the dipole-like appearance of the polar regions of the Sun or
with the weak polar magnetic fields.) The 4-sector structure (n = m =
2) was prominent, and often dominant, at various times throughout the
cycle. A 6-sector structure (n = m = 3) occasionally became dominant
for very brief periods during the active years. Contributions to the
general solar magnetic field from harmonics of principal index 4 ⩽
n ⩽ 9 were generally relatively small throughout this entire solar
cycle with one outstanding exception. For a period of several months
prior to the large August 1972 flares, the global photospheric field
was dominated by an n = 5 harmonic; this harmonic returned to a low
value shortly after the August 1972 flare events. Rapid changes in
the global harmonics, in particular, relative and absolute changes in
the contributions of harmonics of different principal index n to the
global field, imply that the global solar field is not very deep or
that very strong fluid flows connect the photosphere with deeper layers.
Title: Studies of Solar Magnetic Fields. I: The Average Field
Strengths
Authors: Howard, R.
Bibcode: 1974SoPh...38..283H
Altcode:
The telescope, spectrograph, and magnetograph at the 150-ft
Tower Telescope are described, and a chronology of changes in the
instrumentation is given. The average magnetic field strengths over the
last seven years are discussed. The changes in polarity at the poles of
the Sun are described. The characteristics of these polarity reversals
at both poles are similar. A reversal is not seen in the sunspot
latitudes (≲ 40°) but is observed to start in the 40-50° zone and
proceed slowly poleward, reaching the pole within 12 to 18 months. At
the time of the polarity reversal at the pole, field strengths over
a large portion of the disk show similar behavior. Rapid changes
of solar magnetic fields over large portions of the solar disk are
discussed. Two possible models are suggested to explain the frequent
`monopole' appearance of the solar fields. The poleward drift of the
magnetic field reversals in each hemisphere was not closely in phase
with the polar filament migrations or the variations in mean latitude
of high-latitude coronal activity. The behavior of the low-latitude
field strengths with phase in the cycle follows earlier correlations
of activity with predominantly negative magnetic fields.
Title: Studies of Solar Magnetic Fields. II: The Magnetic Fluxes
Authors: Howard, R.
Bibcode: 1974SoPh...38...59H
Altcode:
Magnetic flux data from the Mount Wilson magnetograph are examined over
the interval 1967-1973. The total flux in the north is greater than that
in the south by about 7% over this interval, reflecting a higher level
of activity in the northern hemisphere. Close to 95% of the total flux
is confined to latitudes equatorward of 40°, which means that close
to 95% of the flux cancels with flux of opposite polarity before it
can migrate poleward of 40°. It is pointed out that a consequence of
this flux distribution is that ephemeral regions must make a negligible
contribution to the long-term largescale magnetic flux distribution. A
broad peak in the total flux may be seen centered about one year after
activity maximum in the north below 40°. In the south there is a
very sharp increase in flux about the same time. In the north, several
poleward migrations of flux may be seen. Two of these may correspond
with the two poleward prominence migrations seen by Waldmeier. In both
the north and the south there is a poleward migration of negative
flux about the time of activity maximum. Poleward flux drift rates
are about 20 m s−1.
Title: Solar Sector Boundary Configuration from Comparison of
Synoptic Charts of the Photospheric Magnetic Field with the Observed
Interplanetary Field
Authors: Scherrer, P. H.; Wilcox, J. M.; Howard, R.
Bibcode: 1974BAAS....6Q.293S
Altcode:
No abstract at ADS
Title: Magnetic Flux Measurements in the Photosphere
Authors: Howard, Robert
Bibcode: 1974BAAS....6Q.289H
Altcode:
No abstract at ADS
Title: Studies of Solar Magnetic Fields. III: The East-West
Orientation of Field Lines
Authors: Howard, R.
Bibcode: 1974SoPh...39..275H
Altcode:
Solar magnetic flux data accumulated from the magnetograph of the Mount
Wilson Observatory are used to infer average east-west field inclination
angles for the interval 1967-1973. In all latitude zones the total flux
(∣F+∣ + ∣F−1∣) measurements indicate
that the field is inclined so as to trail the rotation by a small
amount. Averaged over the whole disk, this angle is . No clear pattern
may be seen in the variations of this quantity with time in any latitude
zone. The individual polarities show some systematic behavior. In the
north, the negative (preceding) fields are inclined so as to trail the
rotation at all latitudes. The positive fields are inclined toward the
rotation by a smaller amount. In the south, a similar situation exists
for the fields below 40° latitude, but poleward of 40° the following
polarity fields are strongly inclined to trail the rotation. In the
north, there has been a gradual decrease of the inclination angles of
both polarities during the seven-year interval. At the higher latitudes
the sign of the east-west inclinations actually changed during the
interval. From an examination of magnetograms it is clear that there are
no systematic east-west inclinations of field lines outside sunspots
greater than about 30° from the vertical. Cross correlations of the
east-west inclination data indicate that equatorward of 40° variations
in time of the orientation of fields of the two polarities tend to be
parallel, and poleward of 40° these variations are such that the two
polarities incline toward or away from each other.
Title: Sunspots (sunspot relative-numbers and sunspot-areas); Synoptic
charts of solar magnetic fields (Mount Wilson Observatory); Eruptions
chromosphériques brillantes; Intensité de la couronne solaire;
Solar radio emission.
Authors: Waldmeier, M.; Howard, R.; Olivieri, G.; Bernot, M.;
Tanaka, H.
Bibcode: 1974QBSA..184..133W
Altcode:
No abstract at ADS
Title: Review of Publications- Solar Magnetic Fields, I.A.U. Symposium
No. 43
Authors: Howard, Robert
Bibcode: 1973JRASC..67..263H
Altcode:
No abstract at ADS
Title: Solar Rotation and Solar Activity
Authors: Howard, Robert
Bibcode: 1973BAAS....5T.273H
Altcode:
No abstract at ADS
Title: The Mean Solar Magnetic Field Observed at the Mt. Wilson
Solar Observatory
Authors: Scherrer, P. H.; Wilcox, J. M.; Howard, R. F.
Bibcode: 1973BAAS....5R.279S
Altcode:
No abstract at ADS
Title: On the Random Nature of the Eruption of Magnetic Flux at the
Solar Surface
Authors: Howard, Robert; Edberg, Stephen J.
Bibcode: 1973SoPh...28...73H
Altcode:
No abstract at ADS
Title: Sunspots (sunspot relative numbers and sunspot-areas); Synoptic
charts of solar magnetic fields (Mount Wilson Observatory); Eruption
chromosphériques brillantes; Intensité de la couronne solaire;
Solar radio emission.
Authors: Waldmeier, M.; Howard, R.; Olivieri, G.; Bernot, M.;
Tanaka, H.
Bibcode: 1973QBSA..179..241W
Altcode:
No abstract at ADS
Title: Large-Scale Photospheric Magnetic Field: The Diffusion of
Active Region Fields
Authors: Schatten, Kenneth H.; Leighton, Robert B.; Howard, Robert;
Wilcox, John M.
Bibcode: 1972SoPh...26..283S
Altcode:
The large-scale photospheric magnetic field has been computed by
allowing observed active region fields to diffuse and to be sheared
by differential rotation in accordance with the Leighton (1969)
magnetokinematic model of the solar cycle. The differential rotation
of the computed field patterns as determined by autocorrelation curves
is similar to that of the observed photospheric field, and poleward of
20° latitude both are significantly different from the differential
rotation of the long-lived sunspots (Newton and Nunn, 1951) used as
an input into the computations.
Title: Recent Solar Research
Authors: Howard, Robert
Bibcode: 1972Sci...177.1157H
Altcode:
No abstract at ADS
Title: Polar magnetic fields of the Sun: 1960 1971
Authors: Howard, Robert
Bibcode: 1972SoPh...25....5H
Altcode:
Observations of the magnetic fields in the polar regions of the Sun are
presented for the period 1960-1971. At the start of this interval the
fields at the two poles were consistently of opposite sign and averaged
around 1 G. Early in 1961 the field in the south decreased suddenly and
the field in the north decreased in strength slowly over the next few
years. By the mid-1960's the fields at both poles were quite weak and
irregular. Throughout the period of these observations the fields at
both poles often showed a remarkable tendency to vary in unison. About
the middle of 1971 the north polar field became significantly positive,
first at lower latitudes, then above 70 °. An autocorrelation analysis
of the polar fields in the north shows a weak rotation peak, indicating
significant `features' in these regions. A comparison of field strengths
in the east and west quadrants in the north suggests that even at the
extreme polar latitudes the following polarity fields are inclined
slightly toward the rotation and the preceding polarity field lines
are inclined slightly to trail the rotation.
Title: Atlas of Magnetic Fields in the Solar Corona
Authors: Newkirk, Gordon; Trotter, Dorothy E.; Altschuler, Martin D.;
Howard, Robert
Bibcode: 1972SoPh...24..370N
Altcode:
No abstract at ADS
Title: The Velocity Fields in Active Regions
Authors: Howard, Robert
Bibcode: 1972SoPh...24..123H
Altcode:
From line-shift observations in two spectrum lines it is determined
that the downward motions observed in plages may represent a real
downward transport of material, not an apparent downward flow due to
brightness or ionization differences in a multistream velocity model.
Title: Observations of Short Period Oscillations in Two Dimensions
Authors: Harvey, J.; Howard, Robert
Bibcode: 1972SoPh...23..300H
Altcode:
Observations of the photospheric velocity field at the disk center
with a cadence of five frames per second strongly support the idea that
short period oscillations arise from a combination of image motion and
horizontal gradients of the line of sight velocity field. Any genuine
solar short period oscillations are effectively masked by these false
short period oscillations.
Title: On the Filamentary Nature of Solar Magnetic Fields
Authors: Howard, Robert; Stenflo, J. O.
Bibcode: 1972SoPh...22..402H
Altcode:
A method is presented for obtaining information about the unresolved
filamentary structure of solar magnetic fields. A comparison is made
of pairs of Mount Wilson magnetograph recordings made in the two
spectral lines FeI 5250 Å and FeI 5233 Å obtained on 26 different
days. Due to line weakenings and saturation in the magnetic filaments,
the apparent field strengths measured in the 5250 Å line are too
low, while the 5233 Å line is expected to give essentially correct
results. From a comparison between the apparent field strengths and
fluxes and their center to limb variations, we draw the following
tentative conclusions: (a) More than 90 % of the total flux seen with
a 17 by 17 arc sec magnetograph aperture is channeled through narrow
filaments with very high field strengths in plages and at the boundaries
of supergranular cells. (b) An upper limit for the interfilamentary
field strength integrated over the same aperture seems to be about
3 G. (c) The field lines in a filament are confined in a very small
region in the photosphere but spread out very rapidly higher up in
the atmosphere. (d) All earlier Mount Wilson magnetograph data should
be multiplied by a factor that is about 1.8 at the center of the disk
and decreased toward the limb in order to give the correct value of
the longitudinal magnetic field averaged over the scanning aperture.
Title: The Mean Photospheric Magnetic Field from Solar Magnetograms:
Comparisons with the Interplanetary Magnetic Field
Authors: Scherrer, Philip H.; Wilcox, John M.; Howard, Robert
Bibcode: 1972SoPh...22..418S
Altcode:
Large-scale averages of daily solar magnetograms have been compared by
cross-correlation with the interplanetary magnetic sector pattern during
a 2 1/2 yr interval. A significant correlation was found at a lag of
about 4 1/2 days, with the amplitude of the correlation depending on
the area included in the magnetogram averages. The highest correlation
was found when an area of one quarter of the solar disk was used,
which is consistent with the idea that the photospheric features which
are to be associated with the interplanetary sector pattern are large
scale features.
Title: Sunspots (sunspot relative numbers and sunspot areas); Synoptic
charts of solar magnetic fields (Mount Wilson Observatory); Eruptions
chromosphériques brillantes; Intensité de la couronne solaire;
Solar radio emission.
Authors: Waldmeier, M.; Howard, R.; Michard, R.; Olivieri, G.;
Bernot, M.
Bibcode: 1972QBSA..177..149W
Altcode:
No abstract at ADS
Title: Sunspots (sunspot relative numbers and sunspot areas); Synoptic
charts of solar magnetic fields (Mount Wilson Observatory); Eruptions
chromosphériques brillantes; Intensité de la couronne solaire;
Solar radio emission.
Authors: Waldmeier, M.; Howard, R.; Michard, R.; Olivieri, G.;
Bernot, M.
Bibcode: 1972QBSA..175...73W
Altcode:
No abstract at ADS
Title: On the Filamentary Nature of Solar Magnetic Fields
Authors: Howard, R.; Stenflo, J. O.
Bibcode: 1972lfpm.conf..251H
Altcode:
No abstract at ADS
Title: Photospheric Magnetic Fields
Authors: Howard, Robert
Bibcode: 1972NASSP.308....3H
Altcode: 1972sowi.conf....3H
No abstract at ADS
Title: Sunspots (sunspot relative numbers and sunspotareas); Synoptic
charts of solar magnetic fields (Mount Wilson Observatory); Eruptions
chromosphériques brillantes; Intensité de la couronne solaire;
Solar radio emission.
Authors: Waldmeier, M.; Howard, R.; Michard, R.; Olivieri, G.;
Bernot, M.
Bibcode: 1972QBSA..173....1W
Altcode:
No abstract at ADS
Title: Large-Scale Negative Polarity Magnetic Fields on the Sun and
Particle-Emitting Flares - Comments
Authors: Scherrer, P. H.; Howard, Robert; Wilcox, John
Bibcode: 1972NASSP.308...39S
Altcode: 1972sowi.conf...39S
No abstract at ADS
Title: General Magnetic Field of the Sun
Authors: Sugiura, M.; Heppner, J. P.; Boldt, E.; Babcock, H. W.;
Howard, R. F.
Bibcode: 1972AIPH....5..304S
Altcode:
No abstract at ADS
Title: Solar Magnetic Fields - Large Scale
Authors: Howard, Robert
Bibcode: 1971PASP...83..550H
Altcode:
Key words: sun - magnetic fields
Title: Photospheric magnetic field rotation: Rigid and differential.
Authors: Tanenbaum, A. S.; Wilcox, J. M.; Schatten, K. H.; Howard, R.
Bibcode: 1971BAAS....3R.264T
Altcode:
No abstract at ADS
Title: The Large-Scale Velocity Fields of the Solar Atmosphere
Authors: Howard, Robert
Bibcode: 1971SoPh...16...21H
Altcode:
Magnetograph velocity data are studied for evidence of large-scale
velocity fields. It is established that there exist on the surface of
the sun regions of more or less coherent downward motion with dimensions
of the order of a solar radius. Velocity amplitudes in these regions
are in the range 50-75 m/sec. Downward-moving large-scale features are
observed to live for at least several days in general and to rotate at
least approximately with the solar rotation rate. Horizontal east-west
motions appears to have lifetimes of at least many months. The extent
in longitude of these horizontal features is about 25°. There is no
evidence for meridional motions from these data, with an upper limit
to the line-of-sight velocity of about 30 m/sec. Active regions,
as reported previously, are areas of generally downward motion. Some
features in the autocorrelation of the rotational velocity of the sun
remain unexplained.
Title: Solar magnetic fields. Symposium no. 43, held at the College
de France Paris, France, August 31 to September 4, 1970.
Authors: Howard, Robert
Bibcode: 1971IAUS...43.....H
Altcode:
No abstract at ADS
Title: Time Evolution of the Large-Scale Solar Magnetic Fields
Authors: Altschuler, M. D.; Newkirk, G., Jr.; Trotter, D. E.;
Howard, R.
Bibcode: 1971IAUS...43..588A
Altcode:
No abstract at ADS
Title: Five-Minute Oscillations in the Solar Magnetic Field
Authors: Tanenbaum, A. S.; Wilcox, J. M.; Howard, R.
Bibcode: 1971IAUS...43..348T
Altcode:
No abstract at ADS
Title: Opposite Polarities in the Development of Some Regularities
in the Distribution of Large-Scale Magnetic Fields
Authors: Ambroz, P.; Bumba, V.; Howard, R.; Sýkora, J.
Bibcode: 1971IAUS...43..696A
Altcode:
No abstract at ADS
Title: Solar magnetic fields
Authors: Howard, Robert
Bibcode: 1971smf..conf.....H
Altcode: 1971QB539.S64......
No abstract at ADS
Title: Sunspots (sunspot relative-numbers and sunspot areas); Synoptic
charts of solar magnetic fields (Mount Wilson Observatory); Eruptions
chromosphériques brillantes; Intensité de la couronne solaire;
Solar radio emission.
Authors: Waldmeier, M.; Howard, R.; Michard, R.; Bastiaans, J. G.;
Fokker, A. D.
Bibcode: 1971QBSA..169....1W
Altcode:
No abstract at ADS
Title: Sunspots (sunspot relative numbers, and sunspot areas);
Synoptic charts of solar magnetic fields (Mount Wilson Observatory);
Eruptions chromosphériques brillantes; Intensité de la couronne
solaire; Solar radio emission.
Authors: Waldmeier, M.; Howard, R.; Michard, R.; Olivieri, G.;
Bernot, M.
Bibcode: 1971QBSA..171..241W
Altcode:
No abstract at ADS
Title: Photospheric Magnetic Field Rotation: Rigid and Differential
Authors: Wilcox, John M.; Schatten, Kenneth H.; Tanenbaum, Andrew S.;
Howard, Robert
Bibcode: 1970SoPh...14..255W
Altcode:
An autocorrelation of the direction of the large-scale photospheric
magnetic field observed during 1959-1967 has yielded evidence that
the field structure at some heliographic latitudes can display both
differential rotation and rigid rotation properties.
Title: Differential Rotation of the Photospheric Magnetic Field
Authors: Wilcox, John M.; Howard, Robert
Bibcode: 1970SoPh...13..251W
Altcode:
The differential rotation of the large-scale photospheric magnetic field
has been investigated with an autocorrelation technique using synoptic
charts of the photospheric field during the interval 1959-66. Near
the equator the rotation period of the field is nearly the same
as the rotation rate of long-lived sunspots studied by Newton and
Nunn. Away from the equatorial zone the field has a significantly
shorter rotation period than the spots. Over the entire range of
latitudes investigated the average rotation period of the photospheric
magnetic field was about 1 1/4 days less than the average rotation
period of the material observed with Doppler shifts by Livingston and
by Howard and Harvey. Near the equator the photospheric field results
agree with the results obtained from recurrent sunspots, while above
15° the photospheric field rotation rates agree with the rotation
rates of the K corona and the filaments.
Title: Spectroscopic Determinations of Solar Rotation
Authors: Howard, Robert; Harvey, J.
Bibcode: 1970SoPh...12...23H
Altcode:
Spectral line shift data obtained from full-disk magnetograms recorded
at Mt. Wilson are analyzed for differential rotation. The method of
analysis is discussed and the results from the data for 1966 through
1968 are presented. The average equatorial velocity over this period is
found to be 1.93 km/sec or 13.76 deg/day (sidereal). This corresponds
to a sidereal period of 26.16 days. The average results areω =
2.78 × 10-6 - 3.51 × 10-7 sin2B -
4.43 × 10-7 sin4B rad/sec, whereB is the solar
latitude. This indicates a smaller decrease of angular velocity with
latitude than found by earlier investigators. Variations from day to
day are caused by large-scale short-lived velocity fields on the solar
surface. There also appear to be secular variations.
Title: Sunspots; Synoptic charts of solar magnetic fields (Mount
Wilson Observatory); Eruptions chromosphériques brillantes;
Intensité de la couronne solaire; Solar radio emission.
Authors: Waldmeier, M.; Howard, R.; Michard, R.; Olivieri, G.; Bernot,
M.; Bastiaans, J. G.; Fokker, A. D.
Bibcode: 1970QBSA..165....1W
Altcode:
No abstract at ADS
Title: On the Spectrum of Granular and Intergranular Regions
Authors: Howard, R.; Bhatnagar, A.
Bibcode: 1969SoPh...10..245H
Altcode:
A very high quality wiggly-line spectrogram was analyzed by making
high-resolution spectral scans of numerous small solar features. An
attempt from the line profiles to detect a magnetic field difference
between the granular and intergranular regions, resulted in a field
increase of 20 ± 15 G in the darker regions of the granular field. Line
width increases apparently due to small-scale turbulent velocities are
seen in the darker regions. It is postulated that in general darker
regions show increased turbulent velocities. Conspicuous asymmetries in
line profiles are seen in dark intergranular regions. It is suggested
that these are the result of velocity gradients in the downward flow
of material. An ionized Cr line showed a conspicuous increase in
equivalent width in the darker regions of the granular field, thus
indicating a decrease in electron pressure in these areas.
Title: Solar Velocity Fields: 5-Min Oscillations and Supergranulation
Authors: Tanenbaum, Andrew S.; Wilcox, John M.; Franzier, Edward N.;
Howard, Robert
Bibcode: 1969SoPh....9..328T
Altcode:
One dimensional magnetograph scans have been used to study the 5-min
photospheric velocity oscillations and the supergranulation. The
oscillations in wing brightness lead the oscillations in velocity by
less than 90° in the photosphere, and about 90° in the chromosphere,
suggesting that they are traveling waves at lower levels and standing
waves at higher levels. Downward flows have been observed to be
coincident with the chromospheric network confirming the hypothesis
that material is flowing downward at supergranular boundaries.
Title: The Difference between the Spectra of Granular and
Intergranular Regions
Authors: Howard, R.; Bhatnagar, A.
Bibcode: 1969BAAS....1T.279H
Altcode:
No abstract at ADS
Title: Polar Magnetic Fields of the Sun from 1960-68
Authors: Howard, Robert
Bibcode: 1969BAAS....1Q.280H
Altcode:
No abstract at ADS
Title: Solar Research at the Mount Wilson and Palomar Observatories
Authors: Howard, Robert
Bibcode: 1969SoPh....7..153H
Altcode:
No abstract at ADS
Title: Solar Activity and Recurrences in Magnetic-Field Distribution
Authors: Bumba, V.; Howard, R.
Bibcode: 1969SoPh....7...28B
Altcode:
A study of the Mount Wilson magnetic-field synoptic chart material
divided into latitude zones for the interval 1959-67, and a comparison
of the data with sunspot groups have provided a better understanding
of the structure of the background-field pattern and its relation to
activity. The interaction of old and new fields within the pattern
seems to result in long-lived sections of alternating polarity in both
hemispheres. We postulate subsurface sources with rotation periods of
about 27 days which produce active regions over a longitude zone of
some tens of degrees. There is a tendency for the background-field
features with strong fields to resist to some extent the shearing
effects of differential rotation. A prediction is made concerning the
nature of the interplanetary magnetic field above the ecliptic.
Title: Some regularities in the distribution of large-scale magnetic
fields on the Sun
Authors: Bumba, V.; Howard, R.; Kopecký, M.; Kuklin, G. V.
Bibcode: 1969BAICz..20...18B
Altcode:
No abstract at ADS
Title: On the solar sources of recurrent geophysical effects
Authors: Bumba, V.; Howard, R.
Bibcode: 1969BAICz..20...61B
Altcode:
No abstract at ADS
Title: Reply to K. R. Sivaraman
Authors: Howard, Robert
Bibcode: 1969SoPh....6..154H
Altcode:
No abstract at ADS
Title: On long-term forecasts of solar activity
Authors: Bumba, V.; Howard, R.
Bibcode: 1969sfsr.conf..387B
Altcode:
No abstract at ADS
Title: Atlas of solar magnetic fields.
Authors: Howard, R.; Bumba, V.; Smith, S. F.
Bibcode: 1969asmf.book.....H
Altcode:
No abstract at ADS
Title: A large scale pattern in the solar magnetic field
Authors: Wilcox, J. M.; Howard, R.
Bibcode: 1969sfsr.conf..327W
Altcode:
No abstract at ADS
Title: On forecasts of interplanetary and geophysical conditions
Authors: Howard, R.; Bumba, V.
Bibcode: 1969sfsr.conf..397H
Altcode:
No abstract at ADS
Title: A Large-Scale Pattern in the Solar Magnetic Field
Authors: Wilcox, John M.; Howard, Robert
Bibcode: 1968SoPh....5..564W
Altcode:
A clearly evident large-scale pattern in the interplanetary magnetic
field during 1964 is used to search for a similar large-scale pattern in
the solar magnetic field. It is found that such a pattern did exist in
the photospheric field observations on both sides of the equator over
a range of at least 40°N to 35°S. The pattern is basically similar
at all these latitudes, and differs from that to be expected from
solar differential rotation in three important respects. It is found
that the solar magnetic pattern changed at all latitudes investigated
within an interval of a few solar rotations.
Title: A new method of magnetograph observation of the photospheric
brightness, velocity, and magnetic fields
Authors: Howard, Robert; Tanenbaum, Andrew S.; Wilcox, John M.
Bibcode: 1968SoPh....4..286H
Altcode:
Several improvements have been made to the Mount Wilson Observatory
solar magnetograph, including changes to the guider, the Doppler
compensator, and the data-handling system. The improved magnetograph has
been used for a new type of solar observation consisting of several
hundred scans back and forth along a straight line of length 3/4
R0 perpendicular to central meridian. The data reduction,
which is done entirely with a computer, eliminates those effects which
have their origin in the earth-sun geometry. The spatial and temporal
properties of the 5-min oscillations are discussed.
Title: Persistent Solar Magnetic Pattern Extending Over Equatorial
Latitudes
Authors: Wilcox, John M.; Howard, Robert
Bibcode: 1968PhRvL..20.1252W
Altcode:
During an interval of one year near the minimum of the 11-yr sunspot
cycle, an interpolated pattern of the interplanetary magnetic field
has been compared with the photospheric magnetic field. This reveals
a persistent pattern in the solar field extending over a wide range
of heliographic latitude on both sides of the equator.
Title: Some Observations bearing on the Problem of the Short-Period
Oscillations
Authors: Howard, Robert; Livingston, William C.
Bibcode: 1968SoPh....3..434H
Altcode:
Observations of solar velocity fields made simultaneously at Mount
Wilson and at Kitt Peak with the same size aperture (5 arc-sec)
and same position on the disk (± 1 arc-sec) are presented. The
object is to clarify whether the short-period oscillations (SPO's)
previously reported (Howard, 1967), could be caused by local seeing
conditions. The time of onset and general character of the SPO's are
found to be well correlated for the two sites, a condition that favors
a solar origin. However, because correlation in complete detail did
not prove possible, some doubt must remain regarding the source of
the SPO's.
Title: Magnetic Classification of Active Regions
Authors: Smith, Sara F.; Howard, Robert
Bibcode: 1968IAUS...35...33S
Altcode:
No abstract at ADS
Title: The interplanetary sector structure and solar radio emission
Authors: Krüger, A.; Bumba, V.; Howard, R.; Kleczek, J.
Bibcode: 1968BAICz..19..180K
Altcode:
No abstract at ADS
Title: Patterns of Active Region Magnetic Field Development
Authors: Bumba, V.; Howard, R.; Martres, M. J.; Soru-Iscovici, I.
Bibcode: 1968IAUS...35...13B
Altcode:
No abstract at ADS
Title: Large-Scale Structure of the Photospheric Magnetic Field.
Authors: Wilcox, John M.; Howard, Robert
Bibcode: 1968AJS....73Q..84W
Altcode:
The large-scale configuration of the interplanetary magnetic field
(sector pattern) observed by IMP-1 at the start of 1964 was very
similar to that observed by IMP-2 and Mariner 4 near the end of
1964. The pattern of recurring geomagnetic disturbance and some cosmic
ray observations also suggest that the interplanetary field pattern
observed by IMP-1 was quasi-stationary during 1964. We therefore assume
that the IMP-I sector pattern pattern persisted in the interplanetai~y
field during 1964, and investigate cross correlations of the direction
of this interplanetary field pattern with the directions of the
photospheric magnetic field observed with the solar magnetograph
at Mt. Wilson Observatory. At 200N heliographic latitude a peak in
the cross correlation is observed at a lag of approximately ~2' days
from the appearance of a magnetic feature at central meridian to its
observation by spacecraft near the earth. Similar cross correlations
for other heliographic latitudes were investigated from 400N to 3505,
in increments of 5~. Similar peaks in the cross correlations are found
at these other latitudes, and there is a smoothly varying change in
the position of the peak corresponding to a smaller value of the lag
as the distance from 200N increases in either northward or southward
direction. This suggests the possibility that a sectorlike pattern
existed on the sun at latitudes from 400N to 3505 and the pattern at
these latitudes rotated almost as a solid body. A slow evolution of
the pattern with time can be observed, but the magnitude is much less
than that associated with differential rotation.
Title: A New Method of Magnetograph Observations of the Photospheric
Brightness, Velocity, and Magnetic Fields.
Authors: Tanenbaum, Andrew S.; Wilcox, John M.; Howard, Robert
Bibcode: 1968AJS....73Q..80T
Altcode:
A new type of observation has been made with the Mt. Wilson
solar magnetograph consisting of several hundred scans back and
forth across a straight line of length ~43 R0 perpendicular to the
central meridian. Both spatial and temporal characteristics of the
5-min oscillations are discussed. Considering only quiet regions,
oscillations of amplitude greater than 50,100,150, and 200 m/sec occupy
96+1, 83+5, 61+10, and 39+10% of the sun's surface. The evolution of
various features in the magnetic field, velocity, and brightness can be
followed for several hours. Regions of enhanced magnetic field within
a plage correspond in detail to brightenings in the wings of the line
Fe I X5250.
Title: Budapest Symposium on Solar Active Regions
Authors: Howard, Robert
Bibcode: 1967S&T....34..296H
Altcode:
No abstract at ADS
Title: Velocity Fields in the Solar Atmosphere
Authors: Howard, Robert
Bibcode: 1967SoPh....2....3H
Altcode:
Observations of velocity fields in the solar atmosphere made with the
Mount Wilson solar magnetograph are analyzed. These observations, which
were made with very high velocity sensitivity, cover nearly 250 hours
and were made with apertures of several sizes and at various parts of
the solar disk, and in strong and weak magnetic fields. The amplitudes
of the 300-sec oscillations are about 25% weaker in regions where the
magnetic field is greater than 80 gauss than where the field is less
than 10 gauss. No difference in the frequencies of the oscillations
could be found between strong-field and field-free regions. It is
suggested that the oscillations occur only where the field is absent and
the lower amplitude in a strong field represents the fraction of the
magnetograph aperture occupied by a magnetic field. The element sizes
for the 300-sec oscillations are probably at least 5-10 arc seconds.
Title: Magnetic Fields and the Solar Cycle
Authors: Howard, Robert
Bibcode: 1967ASPL...10...25H
Altcode: 1967ASPL..454.....H
No abstract at ADS
Title: The Sun as a Magnetic Star
Authors: Bumba, V.; Howard, Robert; Smith, Sara F.
Bibcode: 1967mrs..conf..131B
Altcode:
No abstract at ADS
Title: Magnetic Field of the Sun (observational)
Authors: Howard, Robert
Bibcode: 1967ARA&A...5....1H
Altcode:
No abstract at ADS
Title: Atlas of solar magnetic fields
Authors: Howard, Robert; Bumba, V.; Smith, Sara F.
Bibcode: 1967asmf.book.....H
Altcode: 1967QB539.H65......
No abstract at ADS
Title: The Structure of the Solar Chromosphere. II. Spectroheliograms
in λ 10830 Å and Their Interpretation
Authors: Zirin, Harold; Howard, Robert
Bibcode: 1966ApJ...146..367Z
Altcode:
Spectroheliograms made in the X 10830 A line of He 1 show a network
pattern of absorption which coincides with the Ca and Ha network. The
absorption in 10830 is limited to the edges of the network cells, to
plages and filaments. It is concluded that the edges of the network
cells are the only areas in the chromosphere where temperature and
density are sufficiently high to excite the 223 level.
Title: On the measurement of small-scale magnetic fields on the Sun
Authors: Howard, R.
Bibcode: 1966Obs....86..160H
Altcode:
No abstract at ADS
Title: A Note on the Identification of "m" Regions
Authors: Bumba, V.; Howard, Robert
Bibcode: 1966ApJ...143..592B
Altcode:
No abstract at ADS
Title: Solar Magnetic Fields
Authors: Bumba, V.; Howard, Robert
Bibcode: 1965Sci...149.1331B
Altcode:
No abstract at ADS
Title: On the Development of Solar Flares Within the Calcium Network.
Authors: Bumba, V.; Howard, Robert
Bibcode: 1965ApJ...142..796B
Altcode:
No abstract at ADS
Title: Large-Scale Distribution of Solar Magnetic Fields.
Authors: Bumba, V.; Howard, Robert
Bibcode: 1965ApJ...141.1502B
Altcode:
Solar magnetograms covering a period of years were used to study
the distribution of weak magnetic fields on the solar surface. The
semiregular pattern of these background fields is the result of the
expansion, weakening, and stretching by differential rotation of
magnetic fields of old active regions and their interaction with
neighboring fields, and of the continuing development of magnetic
fields of new regions within the pattern. The net result is a slowly
changing pattern of background fields which occupies at times during
the solar cycle more than 50 per cent of the surface area of the Sun,
as seen with the 23" resolution of the magnetograph. The interaction
of nearby magnetic fields takes the form of the apparent attraction of
features of the same polarity and the apparent repulsion of features of
opposite polarity. Sometimes weak magnetic features covering a large
area apparently disappear over a period of a few rotations. It is not
clear from the observations what the mechanism of this disappearance
could be. It is clear from the period near minimum that active regions
are concentrated in complexes of activity whose location and development
are clearly defined. The largest of these complexes, consisting of
many active regions, result in the formation of Unipolar Magnetic
Regions (UMR). The polar fields are discussed from the standpoint of
the poleward drift of the UMR's.
Title: A Study of the Development of Active Regions on the Sun.
Authors: Bumba, V.; Howard, Robert
Bibcode: 1965ApJ...141.1492B
Altcode:
The early development of a large number of active regions was
studied. Magnetic, photospheric, and chromospheric observations from
Mount Wilson were used. It was found that the supergranular pattern of
the solar atmosphere plays a very fundamental role in the development of
active regions. New regions form in or immediately adjacent to expanding
weak old regions that are seen as an enhanced calcium network. The
first appearance of the new calcium plage (also of sunspots) takes
place in the space between several supergranules, and the subsequent
brightening occurs between supergranules, usually filling in several
of them. In many cases it appeared that during the first day or two of
the development of the group, the magnetic fields did not show zero
net flux. Usually the direction of the development of the plage was
from following to leading The increase of magnetic flux (the initial
growth of the plage) takes place only during the first few days in the
life of a region. During this period the boundary of the filamentary
structure in Ha increases at the rate of about 200 m/sec. This evidently
represents the rapid ordering of magnetic fields around the plage.
Title: Solar Astronomy Neglected
Authors: Howard, Robert; Leighton, Robert; Zirin, Harold; Whitford,
A. E.
Bibcode: 1965Sci...147.1087H
Altcode:
No abstract at ADS
Title: Association of Green-Line Coronal Features with Photospheric
Magnetic Fields
Authors: Bumba, V.; Howard, R.; Kleczek, J.
Bibcode: 1965PASP...77...55B
Altcode:
No abstract at ADS
Title: Introductory report
Authors: Howard, R.
Bibcode: 1965IAUS...22..129H
Altcode:
No abstract at ADS
Title: Some Characteristic Properties of Solar Magnetic Fields.
Authors: Smith, Sara F.; Ramsey, Harry E.; Howard, Robert
Bibcode: 1965AJ.....70R.330S
Altcode:
A two-year project in reducing and analyzing the Mt. Wilson magnetograms
has led to increased knowledge about the properties of solar magnetic
fields. It is now recognized that all solar magnetic fields are
basically bipolar units or products of bipolar units. These bipolar
units (or sometimes multipolar units) are the magnetic fields of active
regions. As the active regions age, their magnetic fields expand and
weaken (as measured with the 23 sec of arc resolution of the magneto-
graph), and gradually merge into a large slowly changing pattern of
weak background fields. Some chromospheric phenomena such as flares and
filaments exhibit a generally consistent pattern with respect to the
photo spheric magnetic field configuration. Both flares and filaments
in strong magnetic regions appear to be related to the neutral line
between opposite polarities. Filaments, both inside and outside of
active regions, are located, almost without exception, along neutral
lines of the longitudinal magnetic field. Occasionally a complicated
configuration of polarities will result in an active region filament
apparently crossing photospheric isogauss lines. We believe this to be a
height effect; the neutral line may exist above the photosphere. Of 21
flares of importance 1 or greater observed on 15 days, all were found
to lie adjacent to a neutral line, as closely as could be determined
within the accuracy of the isogauss maps and the method of overlaying
flare positions on these maps of photo- spheric magnetic fields. In
most situations where the flare consisted of more than one segment,
these segments lay on both sides of the neutral line and occasionally
extended across the neutral line. In some cases, the flare segments
extended into areas of weak magnetic field but maintained their general
orientation with respect to the neutral line. This work was supported
by the Advanced Research Projects Agency through Contract Nonr3933
(00) with the Lockheed Solar Physics Laboratory.
Title: Photospheric Magnetic Fields and Chromospheric Features.
Authors: Howard, Robert; Harvey, J. W.
Bibcode: 1964ApJ...139.1328H
Altcode:
Fine-scan magnetograms and large-scale Ha filtergrams of an active
region were made simultaneously. From the on-band pictures we could
identify bright and dark fine mottles (<1600 km), coarse dark mottles
( 5000 km), bright and dark fibrils, and filaments. Small dark mottles
have lifetimes of about 10 min, and large dark mottles have lifetimes of
about 15 min. The lifetimes of bright fine mottles are much longer than
those of the dark mottles. There are two clear-cut distinctions between
dark fibrils and filaments. The fibrils show increased contrast when
seen on the blue wing of Ha, while on the same filtergrams the contrast
of the filaments decreases. The fibrils seem to lie perpendicular to
isogauss lines of the longitudinal field measured in the photosphere,
and the filaments in general lie parallel to these isogauss lines and
over the null line of the field. It is evident that the filaments lie at
higher layers than do the fibrils, and are different in nature. A ring
of fibrils is found to occupy the position of the 15-G contour line
(also the outline of the calcium plage). The calcium network pattern
can be seen on the bluewing Ha filtergrams as regions of small plages
surrounded by fibrils. We suggest that these fibrils are associated
with spicules. In Ha movies it is evident that the portion of the
chromosphere outside the 15-G contour lines is undergoing some type
of random seething motion. Most of this (seen on-band) is actually
a change in size and shape of the mottles. An important 1- flare
occurred during the observations. No changes in the isogauss maps
could be detected before and after the flare, but some slight changes
in some chromospheric structures were noted.
Title: Magnetic Observations Relating to Solar Flares
Authors: Howard, R.
Bibcode: 1964NASSP..50...89H
Altcode: 1964psf..conf...89H
No abstract at ADS
Title: The development of sunspot groups and the supergranular pattern
Authors: Bumba, V.; Howard, R.
Bibcode: 1964susp.conf..220B
Altcode:
No abstract at ADS
Title: Large-Scale Patterns of the Solar Magnetic Field.
Authors: Bumba, V.; Howard, Robert; Smith, Sara F.
Bibcode: 1964AJ.....69Q.535B
Altcode:
Reduction of 421 years of daily magnetograms obtained with the
magnetograph built by Dr. H. W. Babcock is in progress. The angular
resolution from these observations is 23 sec of arc. Isogauss drawings
were made from the magnetograms, and synoptic charts were drawn for
each solar rotation. It is apparent that the large-scale pattern of the
solar magnetic field is for the most part the result of the spreading
out and stretching by differential rotation of portions of the magnetic
fields of old active regions. These large-scale features persist for
many months, while the small-scale patterns which are connected with
active regions can change in a matter of days. The main direction of
motion of the migrating fields is eastward and poleward. The following
polarity in each hemisphere usually predominates in the poleward drift
of fields. The polar magnetic field measurements record il~is quantized
migration of fields (Undoubtedly, as has already been pointed out,
this drift of following polarities was responsible for the reversal
in polarity observed in the polar fields during the last maximum.) It
appears that if there is a fixed component of a general solar field
it is not apparent in these observations and would have to be less
than a few tenths of a gauss. Judging from the distribution of the
large-scale magnetic fields, one would observe the sun at a distance
during some parts of the 22-yr cycle as a magnetic variable star (if
it were possible to observe variations of a few Gauss) with irregular
fluctuations and reversals in intervals of the order of a few da~~s. A
portion of this work was made possible by the Advanced Research Projects
Agency by means of a contract with the Lockheed Solar Observatory.
Title: Large-Scale Patterns of Solar Magnetic Fields
Authors: Bumba, V.; Howard, R. F.; Smith, S. F.
Bibcode: 1964Ast....69..535B
Altcode:
No abstract at ADS
Title: On the Relation of Major Solar Flares with Changes in Sunspot
Areas.
Authors: Howard, Robert
Bibcode: 1963ApJ...138.1312H
Altcode:
No abstract at ADS
Title: Vertical Structures in the Chromosphere.
Authors: Cragg, T.; Howard, R.; Zirin, H.
Bibcode: 1963ApJ...138..303C
Altcode:
No abstract at ADS
Title: Solar Magnetic Fields and the Great Flare of July 16, 1959.
Authors: Howard, Robert; Severny, A.
Bibcode: 1963ApJ...137.1242H
Altcode:
Ten magnetic maps of the active region on several days at the time of
the 3+ flare of July 16, 1 were obtained at the Crimean Astrophysical
Observatory. At some time during a 15-hour interval wil which the
flare occurred, the higher magnetic fields near and inside the sunspots
decreased by near factor of 3. The resulting loss of magnetic energy
amounted to about 1032 ergs, which is the estim energy emitted in the
form of cosmic rays from flares. Magnetic observations made of the
active re at Mount Wilson during the course of the flare could not have
shown the changes in the stronger fi seen in the Crimean observations.
Title: Preliminary Solar Magnetograph Observations with Small
Apertures.
Authors: Howard, Robert
Bibcode: 1962ApJ...136..211H
Altcode:
Some observations made with the solar magnetograph in conjunction
with apertures the size of large granules are analyzed to obtain
autocorrelation functions and power spectra. These observations
include east-west traces recording magnetic-field strength or
velocity and stationary observations for time-correlation studies
recording velocities. From the former observations it was found that
the root-meansquare magnetic fluctuations were 8 2 + 44 gauss. The
autocorrelation function for the magnetic traces was similar in form to
that obtained by Rogerson (1955) for intensity fluctuations on calcium
spectroheliograms The time-correlation studies indicate that there is
oscillatory vertical motion in the solar photosphere with a period of
about 296 seconds.
Title: Solar Magnetic Fields
Authors: Howard, Robert
Bibcode: 1962ASPL....8..359H
Altcode: 1962ASPL..396.....H
No abstract at ADS
Title: Magnetic Fields Associated with the Solar Flare of July
16, 1959.
Authors: Howard, Robert; Babcock, Horace W.
Bibcode: 1960ApJ...132..218H
Altcode:
Fourteen "fine-scan" magnetograms at intervals of 15 minutes were
obtained during the progress of a large solar flare on July 16,
1959. The pattern of magnetic fields associated with the flare
showed no changes other than minor effects attributable to seeing and
imperfections in scanning.
Title: Magnetic Field Associated with a Great Solar Flare
Authors: Howard, Robert; Cragg, Thomas; Babcock, Horace W.
Bibcode: 1959Natur.184..351H
Altcode:
AN unusually large solar flare, of intensity 3+, was observed at
Mount Wilson on July 16, 1959. The flare was in an active region
centered on the spot group at approximately 18° N., 29° W. It
showed a predominantly S-shaped or double spiral configuration, with
marked variations of relative intensity in its various parts. Visual
observations of the spectrum showed that the flare commenced abruptly
between 21.19 and 21.24 U.T.; maximum was between 22.01 and 22.13
U.T. Lines of Ca II, Na, He and H were observed to be in emission
for more than 1 hr.; the width of the Hα emission was greater than
6 A. Emission persisted in the lines of Ca II and of H until after
observations were terminated at 01.00 U.T. on July 17.
Title: Observations of Solar Magnitic Fields.
Authors: Howard, Robert
Bibcode: 1959ApJ...130..193H
Altcode:
The magnetograph at the 150-foot solar tower at Mount Wilson Observatory
was used to trace a number of active and quiet regions on the surface
of the sun with an angular resolution of about 10 seconds of arc. It
was found that magnetic features exist near sunspots with fields
exceeding 75 gauss. Changes in the magnetic features, as seen from
magnetic contour maps made on different days, were frequent. The flux
measured near spots was in most cases sufficient to balance the flux
from the spots. There is good evidence for a tilting of the lines
of force of the photospheric magnetic field in the direction of a
sunspot. Magnetic contour maps show that magnetic features bear a
close resemblance to calciumplage regions. In all cases the plages
were outlined very nearly by a 10-gauss contour line. Where filaments
(prominences) were present in regions removed from plages, in all
cases it was found that they occurred in areas where the field was
less than 10 gauss. Moreover, the magnetic features on either side
of the filaments were always of different polarities. It is suggested
that the magnetic field in the solar photosphere and chromosphere is
in the form of more or less vertical columns.
Title: Note on Hydromagnetic Waves Passing Through an Atmosphere
with a Density Gradient.
Authors: Weymann, Ray; Howard, Robert
Bibcode: 1958ApJ...128..142W
Altcode:
No abstract at ADS
Title: Excitation Temperatures and Turbulent Velocities in Sunspots.
Authors: Howard, Robert
Bibcode: 1958ApJ...127..108H
Altcode:
High-dispersion spectra of several small and medium-sized sunspots
were obtained with the 16-inch coronagraph of the Sacramento Peak
Observatory. Equivalent widths and half-widths were measured. A curve of
growth was constructed for each spot. Excitation temperatures for iron
derived from the curve- of-growth analysis averaged 41000, assuming an
excitation temperature of 49000 for the photosphere. There is evidence
for a higher excitation temperature for smaller spots. Turbulent
velocities derived from the curves of growth averaged 2.9 km/sec
compared to 1.7 km/sec for the photosphere, while a line-profile
analysis gave an average of 3.7 km/sec compared to 1.5 km/sec for
the photosphere.
Title: Excitation Temperatures and Turbulent Velocities in Sunspots.
Authors: Howard, Robert
Bibcode: 1957PhDT.........2H
Altcode:
No abstract at ADS
Title: Inhomogeneous Stellar Models. V. a. Solar Model with Convective
Envelope and Inhomogeneous Interior.
Authors: Schwarzschild, M.; Howard, R.; Härm, R.
Bibcode: 1957ApJ...125..233S
Altcode:
A model for the sun has computed in which account has been taken both
of the deep hydrogen convection zone and of the internal inhomogeneity
in composition caused by the transmutation of hydrogen during the last
five billion years. The model is found to permit good agreement with
the analysis of the solar photosphere as regards chemical composition
and as regards the depth of the hydrogen convection zone. Furthermore,
the model indicates that the sun must have become brighter by nearly
2 mag. during the five billion years.
Title: An analysis of the spectra of sunspots.
Authors: Howard, Robert
Bibcode: 1957AJ.....62R.143H
Altcode:
Spectra of three sunspots were obtained on several of the first days
of their growth with the i6-inch coronograph of the Sacramento
Peak Observatory. The equivalent widths of fifty lines were
measured. These lines were selected to obtain the minimum magnetic
intensification. Curves of growth were drawn and 43000 was derived as
the average excitation temperature for iron. This value is an upper
limit because of the effects of scattered photospheric light. The
turbulent velocity derived from the curve of growth was 2.5 km/sec, from
line profiles 1.8 ~ 0.2 km/sec. Both these values should be considered
as lower limits because of the effects of scattered photospheric light,
and both these values are higher than the turbulent velocities for
the photosphere. Princeton University Observatory Princeton, N. J.
Title: On the Colors of Subdwarfs.
Authors: Schwarzschild, M.; Searle, L.; Howard, R.
Bibcode: 1955ApJ...122..353S
Altcode:
No abstract at ADS
Title: Axial Rotation in the Brighter Stars of Draper Types B2-B5.
Authors: Slettebak, Arne; Howard, Robert F.
Bibcode: 1955ApJ...121..102S
Altcode:
Rotational velocities (v sin i) of the stars of Draper types B2-B5,
brighter than 5.51 mag. and north of declination - 20 , were determined
by comparing observed profiles of the He 1 4471 line with sets of
profiles computed by the graphical method of Shajn and Struve. Spectral
types and luminosity classes on the MK system of classification were
provided by Dr. W. W. Morgan. The mean true rotational velocity of the
main-sequence stars of MK types B2-B5 was found to be 201 kin/sec,
a value somewhat larger than the corresponding one for the stars. A
subdivision of the main-sequence stars mto MK types and BSB7 shows
that the BSB7 stars appear to have the greatest axial rotation, with
decreasing rotational velocities for both earlier and later types. The
stars of intermediate luminosity have smaller axial rotation than the
main-sequence stars. This was also found to be the case for the stars,
but the opposite situation was obtained for the stars. A number of
spectroscopic binaries included in this study are discussed from the
point of view of possible synchronism between axial rotation and orbital
revolution. The luminosity effect of the forbidden line of He I at X
4470 is briefly discussed. It is suggested that the observed relation
of axial rotation to position on the H-R diagram may be interpreted
in terms of the evolutionary sequences recently computed by Sandage
and Schwarzschild.
Title: Charta Prima, Praesidi, Concilio, et Sodalibus Regalis
Societatis Londini, a Rege Carolo Secundo Concessa, a. D. Mdclxii
Authors: Secundo, Carolo; Howard
Bibcode: 1781RSPT...71R...1S
Altcode:
No abstract at ADS