An overview of the cycle variations in the solar corona

Louise Harra

lkh@mssl.ucl.ac.uk

UCL Department of Space and Climate PhysicsMullard Space Science Laboratory

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The solar wind is weaker…

McComas et al., 2008

Differences between previous minimum and this one

In this minimum the solar wind • Lower wind speed by 3%• Less dense (17%)• Cooler (14%)• Lower mass flux (20%)• Lower dynamic pressure (22%)• Lower thermal pressure (25%)• More fast/slow stream interaction regions (due to more

mid-low latitude CHs

The heliosphere is likely to be smaller now. Both polar and ecliptic observations show similar variations – the changes are global not just polar.

Solar cycle evolution of the solar wind

The fast wind regions in ’08 show a marked increase at lower latitudesIt also showed decrease at high latitudes. This difference is likely to be due to weaker polar fields during the ‘08 minimum. Tokumaru et al., 2010

Coronal magnetic fields ：1996(CR1910) and 2008(CR2070)

Tokumaru et al., 2009

1996 2008

Solar minimum low in heliospheric magnetic field

The peak B varies with cycle, but the minimum has stayed the same over the past 130 yrs.This past minimum it reached a new low – possibly due to lower CME rate?

Courtesy R. Forsyth

Weakening of polar magnetic fieldsPolar magnetic fields formed through the transport of trailing magnetic flux from active regions to the poles. Simulations carried out between 1965 to present day and it was found that increasing the surface meridional flow by 15% can create current day behaviour.Wang, Robbrecht and Sheeley, 2009

Low latitude Coronal holes

• The area occupied by CH is larger than in 2007 min.

• This area is related to recurrence of 5 persistent CHs.

• The Sun had a multi-pole structure during this time period – dominantly due to the weak polar fields.

Abramenko et al., 2010

Coronal holes forming after active region decay

Karachik, Pevstov and Abramenko, 2010

The CHs develop both in the leading and lagging polarities of AR.

• The CHs have their magnetic field closed on the N polar CH.• The change from E-W to N-S may indicate a transformation from

toroidal to poloidal field (Babcock-Leighton model?).• This was observed at solar minimum or the onset of the cycle

24.• The poles were showing a flux imbalance – the linkage from the

CH to the N pole reduces this imbalance.

Early signs of solar cycle 25??

• Wilson et al. (1988) found that each cycle of solar activity begins at polar latitudes.

• New results have shown that flux emergence in coronal holes is already showing the appearance of cycle 25! Savcheva et al., 2009

Mid-range Periodicities in activity• Solar flare index is ~ proportional to the total energy of a flare = it (i,

intensity in Hα , t is duration in minutes).• Well established periodicities are 27 days and 11 years.• Mid-range periodicities have been found in Hα, HXR peak rate, 10.7 cm

radio peak flux, energetic electrons all show 153 day periodicity.• Additional 323 and 540 day periodicity has been found in daily sunspot

number and area.• Quasi-period of 157 days found in flares > M5.• Recent re-evaluation of cycles 21-23 by Kilcik et al (2010) found

prominent periods of 152 days for cycle 21, 73 days for cycle 22 and 62 days for cycle 23.

North-south asymmetry in cycle 23.

Either one or other hemisphere has dominant activity at different parts of the cycle.

The switch between dominant hemispheres is very quick, and the persistent longitudes last for 1-1.5 years.

Rotation parameters for sunspots suggest that the northern hemisphere rotated faster than the southern during the past 3 cycles (Zhang et al., 2011)

Zharkov and Zharkova, 2011

X-flares can occur at any stage of the cycle.

Hathaway, 2010

When do large flares occur in the cycle?

A tendency exists that the more powerful a flare the later it takes place in the cycle.

The most powerful flares take place in the decay phase of the cycle.(Tan, 2011).

Influence of the lower atmosphere

Detection of a signal of visible irradiance has been found even for C-class flares.

An estimate of 70% of the total energy of flares was determined to be from WL continuum. The lower solar atmosphere behaviour is important in understanding flare energetics.(Kretzschmar, 2011)

From 1976-2010, the flares and sunspot number correlated at 94.8%

The baseline level of geomagnetic activity also increases when sunspot number increases.

Hathway, 2010

CME occurrence rate

The occurrence rate tracks solar cycle but with a peak delay of 6-12 mnths.CME rate increases from 0.5/day to ~6/day.

Cycle behaviour of magnetic clouds

Fluxropes are complicated, but many reflect the magnetic field of their source regions.

The poloidal field of MCs exhibits a polarity reversal during the 22yr Hale cycle.

This provides evidence that CMEs preferentially remove the like polarity of the solar dipole field and support CME participation in the solar cycle.

Minima in geomagnetic activity occur just after those in sunspot number.Geomagnetic activity tends to stay high during the decay phase – maybe due to low latitude CHs.