Comparison between the polar coronal holes during the Cycle22/23 and Cycle 23/24 minima using magnetic, microwave, and EUV butterfly diagrams

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 PROMINENCE FILAMENT ACTIVE REGION CORONAL HOLE In this paper we are interested in the polar coronal holes

Coronal holes are 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 chromosphere Microwave 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 SXT 961020 MWO 961020 17 GHz 961021 Dark in X rays Enhanced unipolar B Bright in Microwaves From Gopalswamy, 2008 JASTP

CH Brightness Temperature and underlying photospheric field strength Peak Photospheric Field Strength (G) 71 Coronal Holes 1/1996 4/1999 Tb B Relationship Brightness Temperature (x 10 4 K) Low latitude coronal holes Gopalswamy et al., 2000

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 Constructed from daily best images taken around noon time Take a 13 strip near the central meridian and assemble them over a Carrington Rotation period One can see different solar features including the prominent polar coronal holes We use these synoptic charts to construct the microwave butterfly diagram CORONAL HOLE ACTIVE REGION FILAMENT CORONAL HOLE

Microwave butterfly Diagram (Nobeyama) 22 23 23 24? CH 22/23 minimum 23/24 minimum (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

Magnetic Butterfly Diagram (Hathaway) 90N 30N LATITUDE 0 30S 90S YEAR Mostly Kitt Peak Data; MDI data for a few rotations Close similarity between microwave and magnetic butterfly diagrams

EUV bright features are dim in microwaves

Magnetic & Microwave butterfly Diagrams Blue: Negative; Red: positive Microwave contour levels at [1.033, 1.067, 1.10, 1.133, 1.167, 1.20, 1.233] x 10 4 K There is a good spatial correspondence between the enhanced B and Tb

Temporal Relationship in the Polar Region Averaged over latitudes poleward of 60 degrees Smoothed over 13 Carrington Rot. Radio All the symptoms of the subdued Solar minimum 23/24 are evident. Magnetic Field

Cycle 22/23 & 23/24 minima Mostly Kitt Peak Data and some MDI data that goes into Hathaway magnetic butterfly 23/24 During 22/23 minimum, the field strength and Microwave brightness are higher than the corresponding values during the 23/24 minimum 22/23 22/23 23/24

Cycle 22/23 & 23/24 minima: N 22/23 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 23/24

North South Asymmetry T = 450 K T = 150 K 22/23 22/23 23/24 T 23/24 T

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 >60 o ) 22/23 22/23 23/24 23/24 Max 23 Decl 23 Wind speed difference similar to Brightness temperature difference The southern and northern polar wind speeds are at different times Higher Tb higher solar wind speed

S W Speed At Ulysses ( L >60 o ) 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