6.2 Flip-flop cycles

Although active longitudes endure for a long time, the active regions they consist of evolve in size, indicating possible cyclic variations as observed in Doppler images (Berdyugina et al., 1998a , 1999a

). While one active longitude reduces its activity level, the other increases, which suggests a redistribution of the spotted area between the opposite hemispheres (see Figure 15

). When the active longitudes have about the same activity level a switch of the dominant activity from one longitude to the opposite one occurs. Such a phenomenon was first observed on FK Com (Jetsu et al., 1991 ) and was tentatively called flip-flop. Berdyugina and Tuominen (1998

) have analysed long time series of photometric data for four RS CVn stars and discovered that flip-flops are regularly repeated and, thus, indicate a new type of stellar cycle which is related to active longitudes, i.e., a flip-flop cycle. Korhonen et al. (2002

) inferred a flip-flop cycle on FK Com. In addition to RS CVn stars and FK Com, such cycles have been detected on young solar analogues (Berdyugina et al., 2002

; Järvinen et al., 2005b

; Berdyugina and Järvinen, 2005 ), examples of which are shown in Figure 11

. A list of stars exhibiting a flip-flop phenomenon is given in Table 4

Figure 15:

Doppler images of the RS CVn star II Peg in the pole-on projection illustrating a flip-flop that occurred between the end of 1997 and mid of 1998. From Berdyugina et al. (1999a

The frequency ratio between the spot cycle and flip-flop cycle appears to be different for binary components and single stars. In RS CVn-type stars exhibiting both types of cycles, flip-flops appear to occur at the frequency of the spot cycle, implying the flip-flop cycle to be twice as long as the spot cycle. In young dwarfs flip-flop cycles are 3-4 times shorter than the spot cycle.

The solar active longitudes also exhibit a flip-flop cycle. On the Sun major spot activity alternates the active longitudes in about $1 -3 yr$ , which results in flip-flop cycles of $3.8$ and $3.65 yr$ in the northern and southern hemispheres, respectively (Berdyugina and Usoskin, 2003 ). This is about 1/3 of the 11- $yr$ sunspot cycle and agrees with the results obtained for young solar analogues. The difference between flip-flop cycle lengths in the north and south is significant and produces an oscillating effect in the north-south asymmetry on a century time scale.


Star	Sp	$M/Mo.$	$R/Ro .$	$_O_/_O_ o.$	P $rot$ ,d	P $spot$ , $yr$	P $ff$ , $yr$	Reference





Binary stars

II Peg	K2 IV	0.8	3.4	3.8	6.7066		9.3	[1]
IM Peg	K2 III	1.5	13.3	1.05	24.65		13	[2]
EI Eri	G5 IV	$~$ 1.5	$>$ 3.4	13.3	1.9510		9.0	[1]
$s$ Gem	K2 III	$~$ 2	$~$ 13		19.604		14.9	[1]
HR 7275	K2 IV	$~$ 1	$>$ 8	0.90	28.263		17.5	[1]
HR 1099	K1 IV			9.2	2.841			[3]
HD 37824	K0 III	2.0	12.6	0.5	53.12		$~$ 13.5	[9]
HD 181809	K0 III-IV	1.7	6.5	0.4	59.85		$~$ 4	[9]





Single stars

Sun	G2 V	1.0	1.0	1.0	25.	11	3.7	[4]
LQ Hya	K2 V	$~$ 0.7	$~$ 0.8	16.3	1.601	15	5.2	[5]
AB Dor	K0 V	$~$ 0.8	$~$ 0.8	50.5	0.515	20	5.5	[6]
EK Dra	G1 V	$~$ 1	$~$ 1	10.0	2.606	10.5	4.5	[7]
FK Com	G5 III	$~$ 1.5	$~$ 10	10.8	2.400	13/30	6.4	[8]