Based on Cluster magnetic field and particle data obtained during a 
quasi-perpendicular bow shock crossing on 24 January 2001 Lobzin et al. 
(GRL 34, 2007) have concluded that the shock is nonstationary and reforming 
on the time scale of the ion gyroperiod. The argument for shock reformation 
is mainly based on the different magnetic field profiles observed by different
spacecraft when traversing the bow shock. 

We have performed one-dimensional
full-particle simulations for parameters appropriate to the 24 January 2001
shock crossing with the physical ion to electron mass ratio (it should be
noted that the actual ion beta in the solar wind is ~ 0.6 as compared to 
2.0 given by Lobzin et al.). The simulation exhibits large amplitude small
wavelength waves in the foot, ramp, and overshoot simultaneously with 
vortices in the ion phase space of the incoming ions indicating the
excitation of the modified two stream instability between incoming ions
and electrons. These vortices lead to nonstationarity on a considerably 
smaller time scale than the ion gyroperiod. 

We have flown two closely
spaced artificial satellites from upstream through the foot and ramp 
to downstream and have recorded the magnetic field profile during 
shock traversal. The two spacecraft measure, after low-pass filtering 
the data, different magnetic profiles. This is very similar to what has
been observed after filtering the data obtained from the actual bow 
shock crossing. Based on the simulations we suggest that during the 
24 January 2001 crossing the bow shock is actually not reforming, 
but nonstationary on  two different time scales:  one time scale much
smaller than the ion gyroperiod due to mini-cycles involving ion
phase space vortices, and a nonstationarity with a time scale of
1 - 2 inverse ion gyrofrequencies  due to a periodically changing shock
potential, leading to enhanced reflection of ions, and subsequent excitation 
of waves far upstream. This nonstationarity can also be seen in the filtered 
spacecraft data. The difference of the large scale magnetic field profiles
measured aboard different Cluster spacecraft is probably due to the latter
nonstationarity, but not due to large scale reformation.