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Digital archive of theses discussed at the University of Pisa


Thesis etd-12022010-193039

Thesis type
Tesi di dottorato di ricerca
Thesis title
Kelvin Helmholtz instability and associated phenomena in the Magnetopause: Observations-Simulations
Academic discipline
Course of study
relatore Prof. Pegoraro, Francesco
tutor Prof. Califano, Francesco
  • collisionless shock
  • Kelvin Helmholtz instability
  • magnetopause
Graduation session start date
Release date
Many questions about the role of Kelvin-Helmholtz (KH) instability at the transition regions between
the magnetosphere and the magnetosheath are still open. For example it is not clear if this instability could interest the inner boundary layer of the magnetopause as suggested by some models and observations.
In the first part of the work we have used density, temperature and magnetic field observations obtained from the THEMIS spacecrafts to investigate the KH instability in the inner regions of the magnetopause.
In particular we have discussed the interpretation of particular event observed
from two of the five THEMIS spacecrafts near the dusk flank of the magnetopause at low latitudes.
During this event, THD and THE probes located in the magnetosphere were almost aligned in the antisolar direction.
Both spacecrafts measured a sequence of magnetic and velocity field oscillations in a time period of 5 minutes with an amplitude up to 20 nT and up to 200 km/s respectively.
The magnetic field fluctuations as well as the ion moments are mostly consistent with tailward propagating KH vortices showing bipolar signatures of the normal component of the ion velocity and of the oscillating magnetic field.
However, the typical mixing layer between the shocked solar wind plasma and the magnetospheric plasma is not evident. In fact, the ion energy-time spectrogram
did not indicate the intrusion of the cold magnetosheath ions and the ion
density and temperature always oscillated around the magnetospheric values during the whole time period. Therefore, the vortex structures look like magnetospheric vortices. During the same time period another probe (THC), was located in the magnetosheath. From the observations of THC, THE and THD we have estimated the typical plasma conditions in each side of the magnetopause.
We suggest that the KH instability grows at the inner magnetopause boundary instead of the usual magnetosheath-magnetopause interface. To investigate this possibility we have carried out two sets of simulations using a two-dimensional two-fluid code.
Two different settings of initial conditions were used corresponding to the study of the KH instability (i) at the magnetosheath-magnetosphere and (ii) at the magnetopause-magnetosphere interface.
The numerical results have confirmed that the signatures of the KH instability are not consistent with the first set of initial condition (i) while being in good agreement with the second set (ii).
Our results support previous predictions and observations of KH instability recorded by a single spacecraft at the inner boundary layer of the magnetopause.
At the same time, this study identify some important characteristics of signatures on plasma quantities that could appear whenever a probe passes through a KH vortex.

In the second part of the work we have studied the KH instability in the transition to supermagnetosonic regimes.
The shocked solar wind in the magnetosheath becomes more and more supersonic towards the tail and consequently it is expected to become supermagnetosonic. On the basis of observational data, we performed several numerical simulations in which for a fixed density jump we varied the intensity of the sheared velocity field in
order to study the transition from low to high magnetosonic Mach numbers.
We recall that assuming a compressible plasma, in presence of uniform density configurations, increasing the velocity field compressible effects play a very important role by gradually stabilizing the KH instability when the flow becomes supermagnetosonic.
We point out that in the magnetospheric case the density is not uniform. In particular the density and temperature jumps between the magnetosphere
and the magnetosheath are so large that the solar wind becomes supermagnetosonic on the magnetosheath side. However, due to the density drop the solar wind remains submagnetosonic on the magnetospheric side and as a consequence, the KH instability is not suppressed.
The most important point concerning the transition towards magnetosonic Mach numbers of the order of unity is the role of the vortex acting now as an obstacle and possibly leading to the formation of shock structures extending
far from the transition region.
From the simulations we deduce that very strong solar wind velocities are not necessary to generate shocks in the magnetosheath. We have
also observed the possible formation of shocks in the magnetosphere and varying the angle between the magnetic field and the velocity of shear we have investigated the dispersive structure of shocks in two-fluid approximation.