148 research outputs found
Low-Latitude Coronal Holes at the Minimum of the 23rd Solar Cycle
Low and mid-latitude coronal holes (CHs) observed on the Sun during the
current solar activity minimum (from September 21, 2006, Carrington rotation
(CR) 2048, until June 26, 2009 (CR 2084)) were analyzed using {\it SOHO}/EIT
and STEREO-A SECCHI EUVI data. From both the observations and Potential Field
Source Surface (PFSS) modeling, we find that the area occupied by CHs inside a
belt of around the solar equator is larger in the current 2007
solar minimum relative to the similar phase of the previous 1996 solar minimum.
The enhanced CH area is related to a recurrent appearance of five persistent
CHs, which survived during 7-27 solar rotations. Three of the CHs are of
positive magnetic polarity and two are negative. The most long-lived CH was
being formed during 2 days and existed for 27 rotations. This CH was associated
with fast solar wind at 1 AU of approximately 620 km s. The 3D
MHD modeling for this time period shows an open field structure above this CH.
We conclude that the global magnetic field of the Sun possessed a multi-pole
structure during this time period. Calculation of the harmonic power spectrum
of the solar magnetic field demonstrates a greater prevalence of multi-pole
components over the dipole component in the 2007 solar minimum compared to the
1996 solar minimum. The unusual large separation between the dipole and
multi-pole components is due to the very low magnitude of the dipole component,
which is three times lower than that in the previous 1996 solar minimum.Comment: 14 pages, 7 figure
May 12 1997 Cme Event: I. a Simplified Model of the Pre-Eruptive Magnetic Structure
A simple model of the coronal magnetic field prior to the CME eruption on May
12 1997 is developed. First, the magnetic field is constructed by superimposing
a large-scale background field and a localized bipolar field to model the
active region (AR) in the current-free approximation. Second, this potential
configuration is quasi-statically sheared by photospheric vortex motions
applied to two flux concentrations of the AR. Third, the resulting force-free
field is then evolved by canceling the photospheric magnetic flux with the help
of an appropriate tangential electric field applied to the central part of the
AR.
To understand the structure of the modeled configuration, we use the field
line mapping technique by generalizing it to spherical geometry. It is
demonstrated that the initial potential configuration contains a hyperbolic
flux tube (HFT) which is a union of two intersecting quasi-separatrix layers.
This HFT provides a partition of the closed magnetic flux between the AR and
the global solar magnetic field. The vortex motions applied to the AR interlock
the field lines in the coronal volume to form additionally two new HFTs pinched
into thin current layers. Reconnection in these current layers helps to
redistribute the magnetic flux and current within the AR in the
flux-cancellation phase. In this phase, a magnetic flux rope is formed together
with a bald patch separatrix surface wrapping around the rope. Other important
implications of the identified structural features of the modeled configuration
are also discussed.Comment: 25 pages, 11 figures, to appear in ApJ 200
A novel metric for coronal MHD models
[1] In the interest of quantitatively assessing the capabilities of coronal MHD models, we have developed a metric that compares the structures of the white light corona observed with SOHO LASCO C2 to model predictions. The MAS model is compared to C2 observations from two Carrington rotations during solar cycle 23, CR1913 and CR1984, which were near the minimum and maximum of solar activity, respectively, for three radial heights, 2.5 R⊙, 3.0 R⊙, and 4.5 R⊙. In addition to simulated polarization brightness images, we create a synthetic image based on the field topology along the line of sight in the model. This open-closed brightness is also compared to LASCO C2 after renormalization. In general, the model\u27s magnetic structure is a closer match to observed coronal structures than the model\u27s density structure. This is expected from the simplified energy equations used in current global corona MHD models
Magnetic Topology of Coronal Hole Linkages
In recent work, Antiochos and coworkers argued that the boundary between the open and closed field regions on the Sun can be extremely complex with narrow corridors of open ux connecting seemingly disconnected coronal holes from the main polar holes, and that these corridors may be the sources of the slow solar wind. We examine, in detail, the topology of such magnetic configurations using an analytical source surface model that allows for analysis of the eld with arbitrary resolution. Our analysis reveals three important new results: First, a coronal hole boundary can join stably to the separatrix boundary of a parasitic polarity region. Second, a single parasitic polarity region can produce multiple null points in the corona and, more important, separator lines connecting these points. Such topologies are extremely favorable for magnetic reconnection, because it can now occur over the entire length of the separators rather than being con ned to a small region around the nulls. Finally, the coronal holes are not connected by an open- eld corridor of finite width, but instead are linked by a singular line that coincides with the separatrix footprint of the parasitic polarity. We investigate how the topological features described above evolve in response to motion of the parasitic polarity region. The implications of our results for the sources of the slow solar wind and for coronal and heliospheric observations are discussed
A model for magnetically coupled sympathetic eruptions
Sympathetic eruptions on the Sun have been observed for several decades, but
the mechanisms by which one eruption can trigger another one remain poorly
understood. We present a 3D MHD simulation that suggests two possible magnetic
trigger mechanisms for sympathetic eruptions. We consider a configuration that
contains two coronal flux ropes located within a pseudo-streamer and one rope
located next to it. A sequence of eruptions is initiated by triggering the
eruption of the flux rope next to the streamer. The expansion of the rope leads
to two consecutive reconnection events, each of which triggers the eruption of
a flux rope by removing a sufficient amount of overlying flux. The simulation
qualitatively reproduces important aspects of the global sympathetic event on
2010 August 1 and provides a scenario for so-called twin filament eruptions.
The suggested mechanisms are applicable also for sympathetic eruptions
occurring in other magnetic configurations.Comment: 6 pages, 4 figures, accepted in ApJ Letter
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