Review of Conditions for the Formation
and Maintenance of Filaments
http://spaceweather.com/images2002/18feb02/
Paper by Sara F. Martin, 1998
Review presented by Samuel Tun October
13, 2005
Introduction and overview
The paper reviews the
1.
the filament environment
2.
dynamic conditions related to the formation of filaments, and
3.
Additional clues from the features of filaments and their
surroundings
Typical thickness, height, and length are 5000 km, 50000 km, and
200000 km, respectively (Stix, Ch. 9).
Filaments and prominences are the same phenomenon. Here’s a
movie of an erupting one showing that.
All movies kindly provided by Dr. C. Denker, BBSO / NJIT
Conditions of the Filament
environment
Filaments and
photospheric
B-fields

filaments occur
along boundaries of
opposite polarity
line of sight
magnetic fields, or
neutral lines
Figure 1, Martin 1998


above condition applies to quiescent and active region filaments,
and of all scales (temporal and spatial)
temporal and spatial scales hold the relation big and slowly
evolving, small and quickly evolving


although the boundary containing large-scale filaments can be all
within one bipolar region, about twice as many filaments form
between ARs than within them. Filaments can form between ARs
of different ages.
the above preference points to a possible quadrupolar magnetic
configuration. Models abound, and evidence from magnetograms
seems to indicate that this is so.
Overlying arcades
seems
all filaments have
overlying arcades
Figure 2, Martin 1998



overlying coronal loops connect regions of opposite polarity
unless they have different helicities.
northern hemisphere dominated by regions of negative helicity,
while the reverse is true for the southern one
arcades are a necessary but insufficient condition for filaments
The Filament
Channel
 In medium to strong
fields, filaments form only
where there are no fibril
bridges across the neutral
line (across polarity
inversion, region of
maximum magnetic shear).
From http://www-solar.mcs.st-and.ac.uk/~duncan/prominence/


examples of fibril alignment (channel formation) preceding
filament formation are presented
some channels remain filament-less, and the mechanism for fibril
alignment is yet to be determined.
Figure 11, Martin 1998


photospheric magnetograms show field is aligned along the
filament channel. However, this is confirmed only for strong fields
and of rough spatial resolution due to instrumental limitations.
There is a permeating "background of mixed-polarity, small-scale
fields" in quiet Sun. However, filament formation in ARs occurs
between the greater plage areas, indicating that the formation of
the channels depends on the large-scale AR magnetic fields
From http://helios.obspm.fr/observat/pages/galerie/
barbs
have mass
flows in different
directions, ending
in points of
"minority polarity“
almost complete
absence of barbs in
ARs
Figure 5, Martin 1998
Dynamic conditions for the
formation of filaments
Convergence of B-fields and canceling magnetic flux, the movie.


there is evidence that
in order for filaments
to form one must
have merging areas
of opposite polarities
(flux convergence)
opposite fields
cancel, but the
mechanism is still
debated
Figure 7, Martin 1998
Changing fine structure
high-resolution
spectra show a
constant movement in
the fine structure, and
the highest resolution
images show thread
lifetimes of about 10
minutes
mass usually moves
along the observable
threads, indicating
field alignment with
threads

vector magnetograms give evidence that, in AR,
filaments form in regions of maximum magnetic
shear (low and parallel to polarity inversion line)
 some threads of large filaments appear to be
inclined to the filament axis, demonstrating that
there are fields pointing in those large angles
Field direction from
plasma flows

BBSO images were used to
show that mass flows at
different optical depths
move in different
directions in the filament
and barbs
 Detection of counter
streaming can compliment
magnetograms in
determining the filament
magnetic fields
Figure 9, Martin 1998
Discussion-pieces of prominence
puzzle in broader context
Chirality patterns in
channels, filaments,
and overlying arcades
coronal
loop
chirality is defined
such that leftskewed arcades
always lie over
dextral filaments.
Figure 10,
Martin 1998
 dextral chirality dominates in Northern hemisphere, while the
inverse is true for the South.This preference is sometimes
violated, but chirality relationships (dextral with right bearing
barbs) always hold.
Confirmation of inverse magnetic component
filaments


there exists a component of B-field perpendicular to the field of
the coronal loops above the filament. This had previously been
modeled as an x-type neutral point
Martin argues that since the footpoints of all the barbs in her
figure 5 were found to be in the minority polarity, that their
magnetic fields have a large component opposite that of the
overlying loops
Barbs to fibril relations

barbs and fibril pattern is aligned as viewed from above, but they
are not so if viewed along the filament axis
 on positive side barbs reach down into minority polarity
“plagettes”, while fibrils have upward reaching components going
into the corona...separate fields
 channel-filament relation indicates that the channel magnetic field
will dictate the filament properties
The Cavity
 Martin suggested that the cavity exists because the fields of the
filaments and the overlying arcades have different helicities
(already saw these do not interconnect)
Summary and conclusions
Filaments exist between fields of opposite polarity at sides and
ends
2 .a. all filaments have overlying arcade of coronal loops
b. arcade is skewed with respect to polarity boundary
3 .a. filament channel exists below, and fibrils aligned
b. no fibrils cross the neutral line
c. magnetic field of the channel has inverse component
4 . Opposing magnetic flux move towards each other so as to bring
the regions unto an overlap
5.
Converging magnetic fields cancel
6.
Barbs require continuous mass supply
7.
Minority polarity fields must exist nearby
8.
Chirality relations exist and must be satisfied by any model
1.
Reference (paper discussed)
Martin, S.F., 1998: Conditions for the Formation and Maintenance of Filaments, Solar Physics 182, 107-137