Evolution of polar fields and coronal holes
during the minimum between cycle 23/24:
Implications for the solar wind
Giuliana de Toma
Collaborators: S. Gibson, N. Arge, J. Burkepile, B. Emery
SORCE Meeting, Keystone, May 2010
EXTENDED MINIMUM
Aug 2009
Aug 2008
---- 3rot ave.
flat minimum
spot number
1976 1986
12.6 13.4
1996
8.6
2006
15.2
2007 2008 2009 2010*
7.6
2.9
3.1 13.7*
very quiet Sun in 2008-2009: low magnetic flux emergence
spots present less than 30% of the time
CORONA MORPHOLOGY
SOHO/LASCO C2 (fov ~1.5-6 R)
March 1996
ssn = 9.2
February 2007
ssn = 10.7
multiple white-light coronal streamers not confined to low latitudes
CORONA MORPHOLOGY
SOHO/LASCO C2 (fov ~1.5-6 R)
March 1996
ssn = 9.2
August 2008
ssn = 0
multiple white-light coronal streamers not confined to low latitudes
CORONA MORPHOLOGY
SOHO/LASCO C2 (fov ~1.5-6 R)
March 1996
August 2009
ssn = 9.2
ssn = 0
corona is flatter than in 2008 but still more complex than in 1996
CORONA MODELS: PFSS EXTRAPOLATIONS
1996
relatively flat heliospheric
current sheet (HCS) in 1996
2007
HCS still warped in 2007-2008
2008
HCS flattens in 2009
2009
Wang-Sheeley-Arge Model
POLAR MAGNETIC FIELDS
weak polar magnetic fields in 2006-2009
important for
the structure
of the corona
and heliosphere
net magnetic flux at 60-80o ~40-50% lower than in 1996
(change in |B| much smaller ~15-25%)
polar regions less unipolar in 2006-2008 than in 1996
1996
N
S
polarity
72% 20% (8%)
69% 26% (5%)
2007-2008
ratio
3.6
2.7
polarity
65% 26% (9%)
62% 32% (6%)
ratio
2.5
1.9
in collaboration with N. Arge
CORONA MODELS: PFSS EXTRAPOLATIONS
Wang-Sheeley-Arge Model
polar dipole smaller
in 2007-2008
ratio of higher moments
relatively to polar dipole
larger than in 1996
CORONAL HOLES 1996
Jun 1996
Sep 1996
Nov 1996
sin(latitude)
sin(latitude)
Mar 1996
z large polar coronal holes (N develops long &narrow extension)
z no significant low-latitude holes
CORONAL HOLES 2006-2007
Dec 2006
Apr 2007
Jun 2007
sin(latitude)
sin(latitude)
Oct 2006
z smaller polar coronal holes (without extensions)
z large and long-lived low-latitude coronal holes
CORONAL HOLES 2007-2008
Jan 2008
Mar 2008
Jun 2008
sin(latitude)
sin(latitude)
Sep 2007
z smaller polar coronal holes (without extensions)
z still large low-latitude coronal holes
CORONAL HOLES 2008-2009
Dec 2008
Jul 2009
Sep 2009
sin(latitude)
sin(latitude)
Sep 2008
z in early 2009 low-latitude coronal holes close down
z small mid-latitude holes form in the remnants of new cycle ARs
sin(latitude)
Jul-Aug 2007
sin(latitude)
CORONAL HOLE AREA
Aug 1996
Oct 2006
Apr 2007
sin(latitude)
sin(latitude)
Mar 1996
z in 1996 polar coronal holes extend below 50o (area ~7-8%)
z in 2007 polar holes are mostly confined below 60o and lowlatitude holes are large
CORONAL HOLE AREA
Oct 2008
Jul 2009
Sep 2009
sin(latitude)
sin(latitude)
Apr 2008
z in 2008-2009 polar coronal holes remains smaller than in 1996
z in 2009 smaller and short lived mid-latitude hole appear
SOLAR WIND SPEED
z mean velocity about the same but very different velocity distribution
year v (km/s)
1996
423
1986
453
2007
440
2008
450
z in 1996 the Earth was inside fast wind (> 500km/s) about 17%
of the time compared to 29% and 33% in 2007 and 2008
z high velocity tail associated with high speed streams from
low-latitude, long-lived coronal holes
SOLAR WIND SPEED
z in 2009 there is a large decrease in solar wind speed
year v (km/s)
1996
423
1986
453
2007
440
2008
450
2009
365
z in 2009 velocities fall below 600km/s
z mean solar wind speed in 2009 is ~20% lower than in 2008
SOLAR WIND SPEED
440km/s
450km/s
365km/s
solar wind in 2007-2008 is organized in recurrent high speed streams
20% decrease in 2009 in solar wind speed
Nov 27
Sep 09
Apr 09
Time
Sep 3
Oct 27
Sep 30
Jun 08
WHI
Dec 07
MgII index courtesy of M. Snow
Jun 08
Apr 09
Sep 09
Conclusions
z net polar magnetic flux about 40-50% lower
z polar regions significantly less unipolar
z polar coronal holes much smaller but large and
long-lived low-latitude holes in 2007-2008
z low-latitude holes close down by the end of 2008
z small, detached mid-latitude holes appear in
2009 in the remnants of new cycle ARs
z recurrent and periodic high-speed solar wind
streams in 2007- 2008, disappear in 2009
z mean solar wind velocity also drops in 2009