Upload
others
View
0
Download
0
Embed Size (px)
Citation preview
Comparison between the polar coronal holes during the Cycle22/23 and Cycle
23/24 minima using magnetic, microwave, and EUV butterfly diagrams
N. Gopalswamy, S. Yashiro, P. Mäkelä, K. Shibasaki & D. Hathaway
Nobeyama Radio Images show many solar features
ACTIVE REGIONFILAMENT
CORONAL HOLE
PROMINENCE
In this paper we are interested in the polar coronal holes
Coronal holes are bright in microwaves
96102117 GHzMWO 961020SXT 961020
Dark in X‐rays Enhanced unipolar B Bright in Microwaves
Coronal holes are bright in the frequency range 15‐80 GHz (polar & low‐latitude)Quiet‐sun emission at 17 GHz comes from the chromosphereMicrowave enhancement is due to different physical conditions in the Chromosphere beneath coronal holes (hotter than quiet sun)( See Gopalswamy et al., 1999 for a review)
Here we study the solar‐cycle variation of the 17 GHz brightness in polar coronal holes
From Gopalswamy, 2008 JASTP
CH Brightness Temperature and underlying photospheric field strength
Brightness Temperature (x 104 K)
Peak Pho
tosphe
ric Field Strength (G
)
71 Coronal Holes1/1996 – 4/1999
Gopalswamy et al., 2000
Low‐latitudecoronal holes
Tb – B Relationship
Motivation
• Magnetic connection of the solar wind speed to deeper layers of the solar atmosphere has been proposed in terms of expansion factor (Wang & Sheeley, 1990), photospheric field strength (Fisk et al., 1999) and a combination of the two (Fujiki et al. 2005)
• Coronal holes have distinct signatures at the photospheric (enhanced unipolar field) and chromospheric layers (enhanced microwave brightness temperature)
• Microwave brightness temperature (Tb) in low‐latitude coronal holes is related to the photospheric field strength B (Gopalswamy, Salem & Shibasaki, 2000)
• Because of the extended observations available from Nobeyama, we have the opportunity to connect conditions in the photosphere, chromosphere, corona, and the solar wind
• We are interested in the implications of the Tb – B relationship to the current (23/24) subdued minimum?
Data
• Photospheric magnetograms (Kitt Peak, SOLIS, SOHO/MDI)
• 17 GHz microwave brightness temperature images (from the Nobeyama radioheliograph)
• SOHO/EIT images
• Ulysses solar wind speed from SWOOPS (Solar Wind Observations Over the Poles of the Sun)
Nobeyama Synoptic Charts
CORONAL HOLE
CORONAL HOLE
ACTIVE REGIONFILAMENT
Constructed from daily best images taken around noon timeTake a 13° strip near the central meridian and assemble them over a Carrington Rotation periodOne can see different solar features including the prominent polar coronal “holes”We use these synoptic charts to construct the microwave butterfly diagram
24?
2322
23
CH
22/23 minimum 23/24 minimum
Microwave butterfly Diagram (Nobeyama)
(Negative Image: Back ‐> Bright)Two bright features: Active region belt (corona) and polar coronal holes (chromosphere)Polar brightness is lower during cycle 23/24 minimum compared to the 22/23 minimum
YEAR
LATITU
DE
30S
0
30N
90N
90S
Magnetic Butterfly Diagram (Hathaway)
Mostly Kitt Peak Data; MDI data for a few rotationsClose similarity between microwave and magnetic butterfly diagrams
EUV bright features are dim in microwaves
Magnetic & Microwave butterfly Diagrams
Microwave contour levels at [1.033, 1.067, 1.10, 1.133, 1.167, 1.20, 1.233] x 104 KThere is a good spatial correspondence between the enhanced B and Tb
Blue: Negative; Red: positive
Temporal Relationship in the Polar Region
Averaged over latitudes poleward of 60 degrees
Smoothed over 13 Carrington Rot.
All the symptoms of the subduedSolar minimum 23/24 are evident.Radio
Magnetic Field
Cycle 22/23 & 23/24 minimaMostly Kitt Peak Data and some MDI datathat goes into Hathaway magnetic butterfly
During 22/23 minimum, the field strengthand Microwave brightness are higherthan the corresponding values during the23/24 minimum
23/24
22/23 22/23
23/24
Cycle 22/23 & 23/24 minima: N
22/23
23/24
•The difference between 22/23 and 23/24 minima is not substantial in terms of Microwave brightness (150 K)•The 22/23 field strength is higher by a factor of ~ 8.5/ 5 = 1.7•The correlation is poorer than that in the southern hemisphere
North‐South Asymmetry∆T = 450 K ∆T = 150 K
∆T ∆T
23/24
22/2322/23
23/24
Microwave Brightness and SW speed
• Compare the 17 GHz polar brightness temperature with solar wind speed measured by Ulysses in the polar region (poleward of 60 degrees)
• North and south polar passes corresponding to 22/23 and 23/24 minima
• Higher 17 GHz Tb higher solar wind speed
Solar Wind Speed At Ulysses (|L|>60o)
23/24
22/23
23/24
22/23
Wind speed difference similar to Brightness temperature differenceThe southern and northern polar wind speeds are at different timesHigher Tb higher solar wind speed
Max 23
Decl 23
S W Speed At Ulysses (|L|>60o) related to polar B (similar to 17 GHz Tb)
Summary
• The microwave enhancement in polar coronal holes during 23/24 minimum is diminished with respect to the 22/23 minimum
• The decrease is consistent with the reduced polar field strength during the 23/24 minimum as measured by KPNO
• There is a clear north‐south asymmetry in the polar microwave brightness difference between the two minima
• During the 23/24 minimum, B and Tb varied over a very narrow range of values in the southern hemisphere
• The reduced microwave brightness is consistent with the reduced solar wind speed measured by Ulysses/SWOOPS at high latitudes