Vector-MKdV Solitons I
Vector MKdV Equation ut + 24|u|2ux + uxxx = 0
In vector notation,
u = (
u1,...,
uN)
is a N-component variable satisfying the vector MKdV equation
ut
+ 24 u⋅u ux
+ uxxx =0
For the special case when N=2,
this vector equation is equivalent to the Hirota-MKdV equation
ut
+ 24 uuux
+ uxxx =0
in complex-variable notation
u =
u1 + i
u2.
1-Soliton Solution
Travelling waves are given by the 1-soliton solution
u(t,x)= (G/F) φ
with
G= √c, F= 2cosh(√c ξ)
where
c > 0 is the speed,
φ is the orientation unit vector,
and ξ =
x−
ct is a moving coordinate.
2-Soliton Solution
Collisions where a fast travelling wave with speed
c1 and orientation (unit vector)
φ1
overtakes a slow travelling wave with speed
c2 and orientation (unit vector)
φ2
are given by the 2-soliton solution
u(t,x)=
(G1/F) φ1 + (G2/F) φ2
with
G1 =(c1 − c2)
√c1 cosh(√c2 ξ2), G2 =(c1 − c2)
√c2 cosh(√c1 ξ1)
F= 4√c1√c2 φ1⋅φ2
+ (√c1 − √c2)2
cosh(√c1 ξ1 + √c2 ξ2)
+ (√c1 + √c2)2
cosh(√c1 ξ1 − √c2 ξ2)
where ξ
1 =
x−
c1t,
ξ
2 =
x−
c2t are moving coordinates.
Interaction Properties
Depending on their speed ratio
c1/c2
and relative orientation angle Δφ = arccos(
φ1⋅
φ2),
the fast and slow waves in a collision exhibit three different types of nonlinear interactions.
1) Merge-Split: the waves first merge together at
t=
x=0 and then split apart into their original shapes and speeds.
2) Bounce-Exchange: the waves first bounce and then exchange their shapes and
speeds at
t=
x=0.
3) Absorb-Emit: the fast wave gradually first absorbs and then emits
the slow wave.
In all three types of interactions, the waves emerging from the collision
retain the shape, speed, and orientation of the initial two waves.
The only remnant of the interaction is a forward shift in the position of the faster wave
Δx1 =
(2/√c1) ln ( (√c1 + √c2)/(√c1 − √c2) ) > 0
and a backward shift in the position of the slower wave
Δx2 =
(2/√c2) ln ( (√c1 − √c2)/(√c1 + √c2) ) < 0
where these position shifts depend only on the speeds of the waves but not on their orientations.
Overlay of 1-Soliton Solutions and Corresponding 2-Soliton Solution
Merge-Split Interaction
speed ratio: c1/c2 = 2.8
relative orientation angle: Δφ = 0.75 π
speed ratio: c1/c2 = 50
relative orientation angle: Δφ = 0.3 π
Bounce-Exchange Interaction
speed ratio: c1/c2 = 2.8
relative orientation angle: Δφ = 0.3 π
Absorb-Emit interaction
speed ratio: c1/c2 = 2.8
relative orientation angle: Δφ = 0.95 π
speed ratio: c1/c2 = 50
relative orientation angle: Δφ = 0.75 π
speed ratio: c1/c2 = 9
relative orientation angle: Δφ = π