Download The Transport of Open Magnetic Flux on the Solar Surface and its

The Transport of Open Magnetic Flux on
the Solar Surface and its Influence on the
Solar Dynamo
L. A. Fisk
University of Michigan
Points to be Made
I want to start by reviewing the behavior of the
open magnetic flux of the Sun, the component of
the solar magnetic field that opens into the
heliosphere, as we understood it during the last
solar minimum.
I want to discuss how the current unusual solar
minimum has altered, and in some ways reenforced our understanding of the behavior of
open flux.
I then want to consider how the behavior of the
open flux may influence the solar dynamo.
This figure illustrates how
we expected the open
magnetic flux to behave at
solar minimum.
•You have a polar coronal hole,
with an axis of symmetry offset
from the solar rotation axis.
•The open flux shown in green
over-expands from the coronal
hole.
•The open flux differentially
rotates.
•The open flux is then forced to
move as shown by the red
arrows.
There was observational
support for the motions of
the open flux shown in
red.
•The motions will alter the
configuration of the
heliospheric magnetic field,
such that it is easier for lowenergy particles to be
transported in heliographic
latitude.
•Such transport was in fact
observed by Ulysses, and in
fact considered to be a major
Ulysses discovery.
Note what happens to the
open flux when it gets to
low latitudes.
•There is a single current sheet
in the heliosphere, located at
the center of the yellow band.
•The open flux has two
choices. Disconnect at the
current sheet, or reconnect with
a series of low-latitude coronal
loops, and execute a diffusive
transport around the Sun, as
shown, so there is a continuous
global transport of open
magnetic flux.
Disconnection must
occur at the current
sheet, where two open
field lines of opposite
can interact.
It must also occur
inside the Alfven point,
for a net reduction of
open flux.
•At the time of this work,
disconnection was seen to
be unlikely, because of the
absence of dropouts in
electron heat flux. The
electron heat flux data is
now ambiguous.
However, it is unlikely that
disconnection is important since it
would have to occur all on one side
of the Sun, and that should have
been noticed.
Assuming that
disconnection is not
important, and the low
latitude transport is by
diffusion due to
reconnection with loops,
there are some nice
consequences.
•You can account for the
differences in composition
between the fast and the slow
solar wind.
•You can explain why the open magnetic flux returns to same value at
each solar minimum (or was thought to do so). CMEs can temporarily
add magnetic flux to the heliosphere, but interchange reconnection, in
which the open flux reconnects with the CME loop, returns the
magnetic flux to the background level, which is being transported
about as shown in the figure.
Initially, the behavior of the heliospheric magnetic field
during the current solar minimum caused us to doubt
our understandings.
26%
24%
28%
38%
We expected
that the
magnetic field
strength
would return
to same value
as in previous
minima, due
to the lack of
disconnection.
However, the magnetic field strength dropped by 25%.
Liang Zhao, my graduate student, has a very interesting
poster here at SHINE, which I think resolves our concerns.
•Liang uses solar wind composition to identify solar wind that originates
from the streamer belt, in a band that surrounds the current sheet. It has
an identifiable composition in solar wind charge states.
•Liang finds that the band of streamer belt wind is a factor of two or more
narrower in the current minimum, compared to the previous one.
•Conversely, the solid angle occupied by non-streamer belt wind, which
is mainly coronal hole associated wind, is larger in the current minimum
compared to the previous one.
The magnetic field in the non-streamer belt wind is lower in
the current minimum, and the solid angle is larger.
Liang then demonstrates that the total magnetic flux in nonstreamer belt solar wind is in fact the same in the current
and in the previous solar minimum.
Our picture then needs
to change slightly.
The magnetic structure of the
streamer belt appears to be
impenetrable to the magnetic
field originating outside the
streamer belt, which is not
surprising since the loops in
the streamer belt have the
same orientation as the open
flux and reconnection is not
possible.
We still have to have our continuous flow pattern of
open flux, but now the reconnection with loops, and
subsequent diffusion, has to occur outside the streamer
belt.
Note that in this model,
disconnection of open
magnetic flux
originating from outside
of the streamer belt is
not possible.
There is an impenetrable
barrier that separates this
open flux from open flux of
opposite polarity.
We have every reason to assume, then, that the total
amount of open magnetic flux from outside the
streamer belt is the same in each solar minimum.
What does all have to do with the solar dynamo?
•In the solar dynamo, new magnetic flux of opposite
polarity is transported to the solar poles (by meridional
flow, diffusion) to cancel the existing polar flux.
•The new emerging flux is closed magnetic flux, but
when the flux arrives at the poles, it is observed to be
open magnetic flux.
•Presumably, the closed flux and the open flux undergo
interchange reconnection, converting the closed flux
into open flux.
However, as we have demonstrated there is a fixed
amount of open magnetic flux present, and this
constraint needs to be accounted for.
A fixed amount of open flux in the current solar minimum
In the current solar minimum, where the polar magnetic
flux is reduced, and there appear to be more low-latitude
coronal holes, the limitation on the amount of open flux
present should not matter.
One can imagine that what has varied is the transport to
the poles, and less has been gathered into the polar
regions.
However, in a cycle where the polar fields are strong, and
most of the available open magnetic flux has been
gathered into the poles, this limitation may matter.
•For example, Schrijver et al. (ApJ, 2002) did a study where they
modeled the behavior of the polar field over centuries, using only
estimates of the magnetic flux that is emitted in sunspots. The
agreement with current observations was not good, and I suspect it
would have been better if they had simply placed an upper limit on the
amount of open flux that is possible to gather at the poles.