Flip-flop cycles

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.

Table 4: Stars with flip-flop cycles. References: [1] Berdyugina and Tuominen (1998 ), [2] Berdyugina et al. (2000

), [3] Berdyugina and Henry (2005 ), [4] Berdyugina and Usoskin (2003 ), [5] Berdyugina et al. (2002

), [6] Järvinen et al. (2005b ), [7] Järvinen et al. (2005a ), [8] Korhonen et al. (2002 ), [9] Fekel and Henry (2005 ).

Star	Sp	$M ∕M ⊙$	$R∕R ⊙$	$Ω ∕Ω⊙$	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]
$σ$ 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]