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In these figures the strongest peaks that are due to the noise are marked with blue arrows. In most cases the noise does not interfere with the studied waves since it exhibits only very narrow peaks with frequencies that are higher than those of the waves.
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Only in one case Figure 14c do the frequencies of the whistler waves coincide with the frequency range of the noise. However, no noise is observed during this interval. The shock itself and the immediate upstream and downstream regions can be appreciated. In the top panel with the magnitude of the field, there are 13 wave fronts during the first 15 seconds, which means that their periods are 1. This corresponds to a frequency of 0. The following panels reveal that these waves exhibit the largest amplitudes in the B l component.
This means that downstream fluctuations begin as very elliptically, almost linearly polarized waves and eventually become completely linearly polarized. They exhibit a strong compressive component, while their transverse component is much weaker. In accordance, the MVA reveals that the 0. We used the method described in Hoppe et al. Also, proton fluxes with similar energies appear in the downstream region. After 15 hours the shock was followed by an ICME. The shock's M ms is 2. Figure 10b shows a closeup of the shock and its immediate upstream and downstream regions. It can be seen that the shock transition region is very different from the previous two.
It is very irregular and with a larger extension. They appear in two wave trains. Their amplitudes diminish with increasing distance from the shock. Their Fourier spectrum Figure 11b shows that the waves exhibit transverse and compressive components, with the first being stronger. The hodograms in Figure 11c also show that the whistlers appear with different amplitudes.
Even though the total time interval during which the whistlers were present was very short, their properties changed.
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As in the first case study, the upstream whistlers on average exhibited smaller amplitudes, were more field aligned and more planar when located further upstream of the shock, but further inspection of individual wave fronts showed that these properties vary within the wave trains and are not monotonic functions of the distance from the shock. Hence these waves are mostly transverse. Their spectrum is mostly featureless and the transverse component dominates during the entire time interval presented. The hodograms show that these waves consist of many irregular fluctuations with different amplitudes and frequencies.
These fluctuations have amplitudes 0. They can however participate in the acceleration of particles to suprathermal energies and contribute to SW modification ahead of the shock.
This is the highest time resolution at which the magnetic field profiles of the IP shocks have been observed. This allows us to study the shocks and associated waves in more detail than ever before. Such high proportion of MCs is not surprising. In their studies Richardson and Cane  and Jian et al.
This is in agreement with Jian et al. The average M ms in the sample is 1. The upstream ULF waves show no evidence of steepening. The lack of these structures upstream of the shocks in our sample could be due to their small M ms. For the three cases with enhanced upstream ULF wave activity we perform an additional analysis to determine the foreshock's extensions as observed by the spacecraft.
However, mostly transverse magnetic field fluctuations with featureless spectra in the ULF wave frequency range and with amplitudes 0. Such fluctuations may also contribute to the acceleration of ions to suprathermal energies.
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There are three possible explanations for this:. The geometry of the IP shocks is different for different points on the shock front [ Greenstadt and Mellott , ; Jian et al. Upstream of these adjacent shocks the ULF waves would be excited by backstreaming ions, thereby forming foreshock regions. Also, depending on the spacecraft's orbit relative to shock surface, part of the ULF wave foreshocks observed by the spacecraft might have been located upstream of these adjacent shocks and not upstream of the shocks that was later observed. In the case of the August 5, event we note that the properties of the upstream waves are very similar to those of the ion cyclotron waves ICWs described by Jian et al.
These waves are likely formed near the solar corona, they propagate almost parallel to the IMF and exhibit frequencies below the local proton gyrofrequency in the solar wind frame. They are intrinsically circularly LH polarized, transverse and show narrow spectra. If the observed waves are indeed the ICW waves then they were not produced by the shock and it is only a coincidence that they appear in its upstream region.
MHD simulations performed by Rouillard et al. Some portions of the shocks become more parallel and others more perpendicular. The former can eject more protons in the upstream direction and further enhance the amplitudes of the upstream compressional waves. One would expect that this mechanism acts on spatial and temporal scales that are similar to the wavelengths and periods of the upstream compressive waves and are therefore much smaller than the scales described by Rouillard et al.
Waves upstream and downstream of interplanetary shocks driven by coronal mass ejections
In the case of the terrestrial bow shock, the backstreaming suprathermal ions interact with the incoming solar wind plasma and can generate ULF waves. The regions of the ULF wave and the suprathermal ion foreshocks spatially almost coincide. The shocks observed on June 7, and April 23, exhibited the largest suprathermal proton foreshocks 0.
The enhanced suprathermal proton fluxes associated with the April 23, shock were observed as early as 11 hours before the shock.
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The protons could have been energized at the shocks themselves and then reflected back upstream if the shock's geometry permitted it at some point in the past. Alternatively, they could have leaked from the shock sheath regions. The absence of waves suggests that backstreaming ion distributions did not fulfill the conditions necessary to overcome instability threshold. An extended suprathermal proton foreshock was observed several hours before the shock. We call the upstream region permeated by these waves the HF wave foreshock.
Their properties change with the distance from the shock and within individual trains. Also, their amplitudes diminish on average with the distance from the shocks. Within individual wave trains the whistler amplitudes are largest in the middle of the trains and smallest in their edges. The wave fronts in the middle are more planar and tend to propagate more parallel with respect to the upstream IMF direction. Just upstream of the shock's ramp, between The FFT analysis shows Figure 14d that the peak frequency of the waves is 8.
They appear as LH polarized fluctuations. They must therefore be formed locally by beam instabilities. In the case of planetary shocks, the ULF waves have simple forms further upstream and they steepen and become more compressive as they are convected towards the shock fronts.
In the case of the ULF waves in our sample no steepening has been observed. It is possible that particle density gradients are not strong enough to cause the steepening [ Scholer , ].
Alternatively the IP shocks could catch up with the waves before they can steepen substantially. Their group velocities exceed those of the IP shocks so these waves may propagate away from them. The whistler waves are Landau damped, so that their amplitude on average diminishes with the distance from shock fronts. Waves that propagate more parallel to the upstream IMF experience less Landau damping than more obliquely propagating waves.
Therefore further away from the shocks the HF waves tend to be more field aligned and less compressive. Similar fluctuations were first discovered by Balikhin et al. The polarization of these waves changes from very elliptical, almost linear to completely linear. The shocks were observed during a minimum of solar activity, when most ICMEs had moderate velocities.
Miniature Shock Absorbers
Here we briefly summarize the most important results of this work:. It is the first time we observe fluctuations, similar to those reported by Balikhin et al. The majority of the shocks in their sample were probably SIR driven. In the case of upstream whistlers that are modulated in wave trains, the properties of wave fronts depend on their location in the train.
The average whistler properties also change with the distance from the shock. Closer to the shock transition the wave fronts are more compressive on average and propagate more obliquely with respect to the upstream IMF. We show that even IP shocks with small M ms , such as those in our sample, can perturb large regions in front of them. In some cases only ULF wave or suprathermal proton foreshock exists. When both are present, they can have very different extensions in the data.
The shock fronts that precede ICMEs can be very complex. Their forms vary in space and they change with time [e. The latter is especially important. The IP shocks start perturbing their upstream regions from the moment they are formed.