2,713 research outputs found
Predicting the Sun's Polar Magnetic Fields with a Surface Flux Transport Model
The Sun's polar magnetic fields are directly related to solar cycle
variability. The strength of the polar fields at the start (minimum) of a cycle
determine the subsequent amplitude of that cycle. In addition, the polar field
reversals at cycle maximum alter the propagation of galactic cosmic rays
throughout the heliosphere in fundamental ways. We describe a surface magnetic
flux transport model that advects the magnetic flux emerging in active regions
(sunspots) using detailed observations of the near-surface flows that transport
the magnetic elements. These flows include the axisymmetric differential
rotation and meridional flow and the non-axisymmetric cellular convective flows
(supergranules) all of which vary in time in the model as indicated by direct
observations. We use this model with data assimilated from full-disk
magnetograms to produce full surface maps of the Sun's magnetic field at
15-minute intervals from 1996 May to 2013 July (all of sunspot cycle 23 and the
rise to maximum of cycle 24). We tested the predictability of this model using
these maps as initial conditions, but with daily sunspot area data used to give
the sources of new magnetic flux. We find that the strength of the polar fields
at cycle minimum and the polar field reversals at cycle maximum can be reliably
predicted up to three years in advance. We include a prediction for the cycle
24 polar field reversal.Comment: 12 pages, 9 figures, ApJ accepte
Measurements of the Sun's High Latitude Meridional Circulation
The meridional circulation at high latitudes is crucial to the build-up and
reversal of the Sun's polar magnetic fields. Here we characterize the
axisymmetric flows by applying a magnetic feature cross-correlation procedure
to high resolution magnetograms obtained by the Helioseismic and Magnetic
Imager (HMI) onboard the Solar Dynamics Observatory (SDO). We focus on
Carrington Rotations 2096-2107 (April 2010 to March 2011) - the overlap
interval between HMI and the Michelson Doppler Investigation (MDI). HMI
magnetograms averaged over 720 seconds are first mapped into heliographic
coordinates. Strips from these maps are then cross-correlated to determine the
distances in latitude and longitude that the magnetic element pattern has
moved, thus providing meridional flow and differential rotation velocities for
each rotation of the Sun. Flow velocities were averaged for the overlap
interval and compared to results obtained from MDI data. This comparison
indicates that these HMI images are rotated counter-clockwise by 0.075 degrees
with respect to the Sun's rotation axis. The profiles indicate that HMI data
can be used to reliably measure these axisymmetric flow velocities to at least
within 5 degrees of the poles. Unlike the noisier MDI measurements, no evidence
of a meridional flow counter-cell is seen in either hemisphere with the HMI
measurements: poleward flow continues all the way to the poles. Slight
North-South asymmetries are observed in the meridional flow. These asymmetries
should contribute to the observed asymmetries in the polar fields and the
timing of their reversals.Comment: 6 pages, 3 color figures, accepted for publication in The
Astrophysical Journal Lette
Investigation of advanced navigation and guidance system concepts for all-weather rotorcraft operations
Results are presented of a survey conducted of active helicopter operators to determine the extent to which they wish to operate in IMC conditions, the visibility limits under which they would operate, the revenue benefits to be gained, and the percent of aircraft cost they would pay for such increased capability. Candidate systems were examined for capability to meet the requirements of a mission model constructed to represent the modes of flight normally encountered in low visibility conditions. Recommendations are made for development of high resolution radar, simulation of the control display system for steep approaches, and for development of an obstacle sensing system for detecting wires. A cost feasibility analysis is included
Magnetic Flux Transport at the Solar Surface
After emerging to the solar surface, the Sun's magnetic field displays a
complex and intricate evolution. The evolution of the surface field is
important for several reasons. One is that the surface field, and its dynamics,
sets the boundary condition for the coronal and heliospheric magnetic fields.
Another is that the surface evolution gives us insight into the dynamo process.
In particular, it plays an essential role in the Babcock-Leighton model of the
solar dynamo. Describing this evolution is the aim of the surface flux
transport model. The model starts from the emergence of magnetic bipoles.
Thereafter, the model is based on the induction equation and the fact that
after emergence the magnetic field is observed to evolve as if it were purely
radial. The induction equation then describes how the surface flows --
differential rotation, meridional circulation, granular, supergranular flows,
and active region inflows -- determine the evolution of the field (now taken to
be purely radial). In this paper, we review the modeling of the various
processes that determine the evolution of the surface field. We restrict our
attention to their role in the surface flux transport model. We also discuss
the success of the model and some of the results that have been obtained using
this model.Comment: 39 pages, 15 figures, accepted for publication in Space Sci. Re
Reproducing the Photospheric Magnetic Field Evolution During the Rise of Cycle 24 with Flux Transport by Supergranules
We simulate the transport of magnetic flux in the Sun s photosphere by an evolving pattern of cellular horizontal flows (supergranules). Characteristics of the simulated flow pattern match observed characteristics including the velocity power spectrum, cell lifetimes, and cell pattern motion in longitude and latitude. Simulations using an average, and north-south symmetric, meridional motion of the cellular pattern produce polar magnetic fields that are too weak in the North and too strong in the South. Simulations using cellular patterns with meridional motions that evolve with the observed changes in strength and north-south asymmetry will be analyzed to see if they reproduce the polar field evolution observed during the rise of Cycle 24
Photospheric Magnetic Flux Transport - Supergranules Rule
Observations of the transport of magnetic flux in the Sun's photosphere show that active region magnetic flux is carried far from its origin by a combination of flows. These flows have previously been identified and modeled as separate axisymmetric processes: differential rotation, meridional flow, and supergranule diffusion. Experiments with a surface convective flow model reveal that the true nature of this transport is advection by the non-axisymmetric cellular flows themselves - supergranules. Magnetic elements are transported to the boundaries of the cells and then follow the evolving boundaries. The convective flows in supergranules have peak velocities near 500 m/s. These flows completely overpower the superimposed 20 m/s meridional flow and 100 m/s differential rotation. The magnetic elements remain pinned at the supergranule boundaries. Experiments with and without the superimposed axisymmetric photospheric flows show that the axisymmetric transport of magnetic flux is controlled by the advection of the cellular pattern by underlying flows representative of deeper layers. The magnetic elements follow the differential rotation and meridional flow associated with the convection cells themselves -- supergranules rule
- …