Accurate Polar and small scale observations during the solar cycle

1
Accurate Polar and small scale observations
during the solar cycle

• Elena E. Benevolenskaya
• Yang Liu
• J. Todd Hoeksema
• Stanford University

HMI/AIA meeting, Monterey, 13-17 February, 2006
2
Figure 1. a) Estimated values of the magnetic
flux of the radial field component in the
latitude zones from 78o to 88o in Northern (blue
line) and Southern Hemispheres (red line) b) The
relative positive polarity parts of magnetic flux
in Northern (blue line) and Southern hemispheres
(red line). c) Total signed magnetic flux. The
polar magnetic field reversal was in CR1975 2
(April 2001) in the North and in CR1981 2
(September 2001) in South.
3
The estimated values of the total unsigned
magnetic flux Fr F F-
for polar caps 78o 88o are presented in Figure
1 (a) for low-resolution synoptic maps (with step
1o in both latitude and longitude). There is a
N-S asymmetry in the distributions of the total
polar magnetic flux for low-resolution maps Fr
1.5-1.8 x 1022 Mx and Fr 2.0-2.5 x 1022 Mx for
the North and South polar caps, correspondingly,
before CR2007 (September 2003). After that, the
total magnetic flux displays a small increasing
in North and decreasing in South. The positive
F /Fr part of the magnetic flux is plotted in
Figures 4 (b). Total signed flux is present in
Figure 1c. The time of reversals can be easily
determined at F /Fr 0.5. This was in CR1974
(March 2001) in the North and in CR1980 (August
2001) in South. This is close to the periods
obtained by Durrant and Wilson (2003) CR1975 in
North and CR1981 in South using the Kitt Peak
synoptic maps.
4
a)
c)
b)
d)
Figure 2
5
Figure 2. Magnetic maps averaged over 60 min for
periods a) When SOHO/MDI was rotated, P_angle
-180.0o. b) When SOHO/MDI was not rotated,
P_angle 0.0o. Noise level as s - distribution
for 1 min images in 60 min series c) for
P_angle -180.0o and d) for P_angle 0.0o.
Figure 3. Noise level as s - distribution for
images averaged over 5 min (a, c) and 1 min image
(b, d) for two time sets in 1 hour series. Top
panels SOHO/MDI was rotated. Bottom panels
SOHO/MDI was not rotated.
Figure 4. Variations of the values of total
unsigned flux as function of number averaged
images for polar caps a) for North and b) for
South
Figure 5. Synoptic magnetic maps for CR2027
(25February 25 March, 2005) obtained without
and with averaged a) one image, b) 60 images.
Latitude in sin (latitude). Resolution is 1o in
both latitude and longitude.
Figure 6. Noise level (s distribution) versus
latitude for CR 1993 and CR 2020 for 5 min and 1
hour averaged magnetograms. a) s9.3G for
87oN-88oN and s10.7G for 80oS-81oS (P_angle
0.0o) b) s2.7G for 87oN-88oN and s4.8G for
80oS-81oS (P_angle 0.0o) c) s9.7G for
87oN-88oN and s12.3G for 80oS-81oS (P_angle
-180.0o) d) s3.9G for 87oN-88oN and s4.8G for
80oS-81oS (P_angle -180.0o)
6
N-S sigma distributions
Figure 3
7
Figure 4
8
The values of the total polar magnetic flux are
depended on the number of averaged images
(Figure1). The noise level of the MDI
magnetograms was estimated by Ortiz et.al (2002).
They obtained that the 20 min averaged MDI
magnetograms have a reasonably low noise level.
The averaged magnetic noise level for 5 min
integrated magnetograms is about 9G. But 1-s
noise level for 1 min longitudinal magnetograms
is 20 G. They, also found the increase of the
noise in the direction SW limb, which we can see
in Figure 2 (c,d). The predicted noise level
decreases as 1/vN, where N is number of
consequent images. If the noise level for 1 min
image is about 20G, it is 9G for 5 min, 2.8G for
50 min and 2.6 G for 60 min averaged
images. There is some discrepancy in the values
of the total magnetic flux which estimated from
synoptic maps obtained from 15 averaged images
per day and the consecutive 1 min images
averaged over 60 min (see Figures 1 (a) and 4 (a)
). For example, for CR 2033 (8 August 4
September, 2005) the total magnetic flux was
2.2560e022 Mx for 15 images taken during day
and it is higher for any consequent averaged
images up to the N60. It is connected with the
reduction of the supergranulation noise which
more hopefully suppressed in the first case.
9
Figure 5
10
Figure 6
11
Influence of the Shutter noise on the estimation
of the polar magnetic flux. The shutter noise of
the SOHO/MDI instrument induces a small random
offset into the magnetic measurements. It was
determined by Yang Liu, Xuepu Zhao and J.Todd
Hoeksema (2004). The correction is applied to
each magnetogram and this offset is
subtracted. We have estimated the magnetic flux
of north polar cap (78oN-88oN) during the CR2033
with offset correction and no offset
correction. There is a small difference in the
relative positive magnetic flux and magnetic
strength. With correction we obtain (F/ Fr)
0.3616 , and the averaged radial component of the
magnetic field is B r - 8.13G. No correction
(F/ Fr) 0.3676 and averaged radial component
of the magnetic field is B r -7.62G. There is
a small differences for total magnetic flux. For
example, the total unsigned magnetic flux in
CR2021 is 1.7524 x 1022 Mx without correction
and it is 1.7912 x 1022 Mx with correction.
12
Conclutions The estimated values of the total
magnetic flux depends on the number of averaged
magnetograms in the synoptic map, it is decreased
with the number of the averaged magnetograms due
to noise reduction. Shutter noise contributes a
small portion in the estimation of the total
unsigned magnetic flux of polar caps for the
individual synoptic map. The individual
magnetograms reveal the non-uniform
s-distribution over solar disk. The noise
increases in the SW direction on normal
magnetograms, and in the NE direction, when
SOHO/MDI is rotated by 180o.
References E. E. Benevolenskaya, 2004, AA, 428,
L5 C. J. Durrant, and P. R. Wilson, 2003, Solar
Phys., 214, 23 Y. Liu, Xuepu Zhao, and J. T.
Hoeksema, 2004, Solar Phys., 219, 39 Ortiz, S. K.
Solanki, V. Domingo, M. Fligge, and B. Sanahuja,
2002, AA, 388, 1036