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Transcript
High-latitude activity and its relationship to the mid-latitude solar
activity.
Elena E. Benevolenskaya & J. Todd Hoeksema
Stanford University
Abstract. The high-latitude activity at photosphere and corona, and their relation to the mid-latitude activity in cycle 23 using the Extreme Ultraviolet (EUV)
coronal observation have been explored using the MDI magnetic synoptic maps available on the SOHO web page with a new calibration and EIT synoptic
maps. The EIT synoptic maps of EUV images in three lines Fe and in one line He II (171A, 195A, 284A and 304A) are obtained for period June 1996 May 2006 (CR1911-CR2042) from the full disk EIT images. They are represented by values of the line intensity centered on the central meridian and can
be directly compared with magnetic synoptic maps (MDI maps). It was found that the solar cycle dependence of the EUV polar corona occurs because of
the large-scale topology of the solar corona and its relationship with the mid-latitude magnetic flux. It is seen more pronounced on the rising phase of the
solar cycle due to the connectivity of the coronal structures extended from the mid-latitude to the high-latitude. But, after the solar cycle maximum the EUV
polar corona shows a less dependence of the mid-latitude corona. In the polar regions the absent of the correlation of the unsigned magnetic flux and EUV
corona occurs not only due to the effect of projection. But it tells about the numerous emerging bi-polar and unipolar regions inside the polar region which
does not contribute to the brightness of the EUV corona. Really, during the solar minimum when the polar magnetic field reaches its maximum and number
of unipolar magnetic regions of strong magnetic field increases, but we observe dimming and coronal holes instead of the bright EUV corona.
Mid-latitude and high-latitude magnetic fields.
Magnetic elements
Polar magnetic field, its reversals
Zonal Structure of the Solar cycle 23
Figure 6. Synoptic frames of magnetic field. Left image: Δt=0 hours, middle image: Δt=14 hours, right image:Δt=25 hours
The hourly averaged (50 images inside 1 hour) MDI images were transformed
to the Carrington coordinate system. As a result, the magnetic frames centered
on the central meridian from -40o to 40o in longitude stepped 0.1o and in sin
latitude with resolution equals 0.001 of sin latitude are obtained. The area of
the magnetic frames is about 1.735 · 1011 km2,and area of each pixel equals
8.45 · 105 km2.
Example for two magnetic element in Mid latitude
Figure 3. Polar magnetic field (at 55o) from Wilcox Solar Observatory. North
hemisphere Is marked by the blue color, South hemisphere is marked by the red color.
Start of reversal at 55o in South
End of reversal at 55o in South
Δt~ 2 years
Time of polar magnetic field reversals
L. Svalgaard And E. W. Cliver, ApJ, 2007
Figure 7. Left panels: Displacement of the magnetic
element in longitude and in Latitude. Right panel: Area of
magnetic element with B|| >10 G.
two sets of
activity waves
Figure 8. Left panels: Displacement of the magnetic element in
longitude and in Latitude. Right panel: Area of magnetic element with
B|| <-10 G.
Dynamics of the small-scale magnetic field which forms the streamers or
‘surges’ is very complicated. Near the pole we did not observe continually
latitudinal motion for individual magnetic elements and it may be related to slow down
of the meridional circulation (Raouafi, Harvey, 2007).
Examples for magnetic elements in High latitude (about 80o)
Durrant and Wilson (2003) found time of reversals using The Kitt Peak data:
CR1975± 2 in North and CR1981± 1 in South. MDI data confirmed this results.
Figure1. A) Sunspot area as a function of time from June
1996 to May 2006 (Carrington rotations from 1911 to 2042);
Axisymmetrical distributions as a function of latitude and
time for: B) EUV flux in Fe XII and C) He II lines ;
D) Unsigned magnetic flux [0 20G], MDI (old calibration).
E) B||- component of the magnetic field, in the blue-red
color map is [-1G 1G], MDI (old calibration);
What contributes these uncertainties?
Line of sight component? Radial field approximation?
Space scale of the averaged magnetic field?
Figure 9. Left panels: Displacement of the magnetic shortlived element in longitude and in Latitude. Right panel: Area
of magnetic element with |B||| >10 G.
Figure 10. Left panels: Displacement of the long-lived magnetic element in longitude and in
Latitude. Right panel: Area of magnetic element with |B||| >10 G.
Conclusions
•
Figure 2. Left image: YOHKOH Soft X-ray image, right panel:
topolgy of the Magnetic field during the rising phase of the Sun.
Foot-points of the giants loops Forms the high-latitude activity
waves.
Figure 5. The total MDI unsigned magnetic flux of the radial field component in the
latitude zones from 78o to 88o in Northern (solid line) and Southern Hemispheres (dash
lines); b) The relative positive polarity parts of magnetic flux in Northern (solid line) and
Southern (dash line) hemispheres ; c) The total signed magnetic flux. The polar magnetic
field reversal was in CR1975± 2 (March 2001) in the North and in CR1980± 2 (September
2001) in South.
•
•
The zonal or axisymmetrical structure of the solar cycle reveals the transport of the
magnetic flux from the mid to high latitude.
Migration of the zonal neutral line defines the reversal of the magnetic field during the
solar cycle.
The transport of the magnetic energy is a complex process related to the surface,
subsurface and coronal processes.