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| − | '''2008 GEM Bow Shock Session''' | + | Leader: ''Nick Omidi <omidi [at] solanasci.com>'' |
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| − | '''Russell et al. :''' Addressed the structure and evolution of weak collisionless shocks. In regards to the evolution of the shock, STEREO A-B observations show examples of shock steepening by coalescence of two weaker shocks where the trailing shock overtakes the leading shock. Regarding shock structure, the original theory of weak shocks suggested that their structure was due to dispersive waves whose damping provide the necessary dissipation at the shock. According to this theory these waves are present upstream of the shock layer for oblique cases and downstream of the shock for perpendicular cases. STEREO observations, however, show that waves are present both upstream and downstream of the shock.
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| − | '''Krauss-Varban et al. :''' Addressed the question of whether observed energetic ion fluxes at CME-driven shocks can be understood and quantified for the purpose of space weather forecasts. Previous attempts at ordering fluxes with Mach number and shock normal angle were not successful. To address this problem 2.5-D hybrid simulations were performed to compare fluxes of energetic ions to observations. The results show that in the case of quasi-parallel shocks both fluxes and the power law index agree well with observations without the presence of seed population of ions. In the case of the oblique shocks, the simulated fluxes are 5 orders of magnitude below the observed levels and lead to a soft power law. Addition of 1% seed population with a kappa distribution function and running for much longer times enhance the fluxes by 2 orders of magnitude. This enhancement is due to generation of fast waves in the upstream and their convection back into the shock causing the undulation of the shock surface. To account for the missing 3 orders of magnitude in the fluxes the effects of particle mirroring in the sunward converging field lines was included by reflecting the energetic ions back into the simulation box leading to lager fluxes and harder spectrum. However, additional mechanisms are still needed and under consideration for future studies.
| + | The Foreshock, Bowshock, Magnetosheath Focus Group has ended and published a final report in [http://gem.epss.ucla.edu/mediawiki/GEMstone/GEMstone_Vol19_No2.pdf GEMstone Volume 19 No.2 - December 2009]. |
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| − | '''Zhang et al. :''' Reported on THEMIS observations of a weak interplanetary shock interacting with the bow shock and its magnetospheric consequences. The interaction results in the earthward motion of the magnetopause at the speed of ~ 40 km/s. In addition, ground stations observed compressions over a wide range of MLTs and latitudes. It was also found that the transmitted IP shock took the form of a discontinuity associated with enhancement of B and N and a decrease in T. The field rotation associated with this structure is similar to that of the IP shock prior to its encounter with the shock. However, the fast shock expected to form based on fluid theory was not present.
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| − | '''Eastwood et al. :''' Reported on THEMIS observations of an HFA and comparisons to global hybrid simulations. During this event, THEMIS A was upstream of the bow shock while B,C,D and E were in the downstream region. THEMIS E observed the signatures of the HFA which consisted of a series of fast and slow magnetosonic waves and the remnants of the discontinuity whose interaction with the bow shock resulted in the formation of the HFA. In addition, THEMIS ground based observatories tracked the progress of the associated magnetic impulse event across the magnetosphere. An interesting aspect of these observations was that while the HFA was formed on the dusk side the associated magnetic impulse was observed on the dawn side. To compare these observations with hybrid simulations two runs using different upstream conditions guessed at based on observations were performed. In the first run, an HFA was formed on the dawn side but not on the dusk side. The lack of an HFA on the dusk side is due to the quasi-perpendicular nature of the simulated bow shock. In second run, the IMF was mostly radial leading to quasi-parallel geometry at the dayside bow shock. The interaction of the backstreaming ions with the incoming discontinuity resulted in the formation of a large structure in the foreshock which was subsequently convected back towards the shock. While the results show some similarities to the THEMIS observations no true HFA were formed. These results indicate the sensitivity of the solutions to the upstream conditions and the difficulties in deciphering the “true” upstream conditions from the observations due to the turbulent nature of this region during the event. Further work is planned for future.
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| − | '''Omidi et al. :''' Reported on the properties the ion foreshock boundary using global hybrid simulations. This new boundary was first detected in a global hybrid run with radial IMF geometry which led to a new model for foreshock cavities by Sibeck et al. (2008). According to this model, foreshock cavities correspond to spacecraft going from solar wind through the foreshock boundary into the foreshock and back into the solar wind. This accounts for the isolated nature of foreshock cavities. The results of simulations show that the ion foreshock boundary is weak at low Mach numbers and is enhanced with increasing Mach number. By performing a series of local hybrid simulations it was found that this Mach number dependence is in turn tied to the properties of the backstreaming ions where both beam velocity and density lead to the strengthening of the ion foreshock boundary. The results also show that while for small IMF cone angles the ion foreshock boundary forms on both sides of the foreshock. At larger cone angles (e.g. 20o) an asymmetry forms where the boundary is present only on one side of the foreshock.
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| − | '''Blanco-Cano et al. :''' Reported on the properties of foreshock ULF waves in global hybrid simulations and their dependence on the IMF cone angle and comparisons with CLUSTER observations. At intermediate cone angles the simulations show the presence of sinusoidal waves which propagate at small angles to the magnetic field and shocklets which propagate at larger wave normal angles. These waves are present in different parts of the foreshock. On the other hand, during small cone angles the two types of waves generated by the backstreaming ions correspond to parallel propagating sinusoidal waves and highly oblique fast magnetosonic waves. The properties of the excited waves were compared to linear theory which showed very good agreement. In contrast to intermediate cone angles, both types of waves are generated in the same regions of the foreshock and interact strongly during their nonlinear evolution. As a result, structures associated with large (~ 50 %) drops in solar wind density and magnetic field are formed which have been named foreshock cavitons. The size of the foreshock cavitons is of the order of 1 RE and they are convected back across the shock and into the magnetosheath leading to a highly turbulent plasma. A search through the CLUSTER data was conducted where a number of foreshock cavitons were observed. Comparisons between properties of the simulated and observed foreshock cavitons show very good agreement.
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| − | '''Scholer et al. :''' Reported on Cluster observations in the quasi-parallel foreshock region of the Earth bow shock. They investigated the association of the intensity of the diffuse ion population with the intensity of the compressional and tangential magnetic field wave intensity. They found that although the intensity of the diffuse ion population increases with decreasing distance to the Earth bow shock the wave intensity stays constant. This effect cannot be explained by existing theories and there is some evidence that this is effect is associated with filed-aligned ion beams originating at the perpendicular region of the Earth’s bow shock.
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| − | '''Desai et al.:''' Reported on abrupt enhancements in the intensities of ions in the energy range of a few 10s of keV to 100s of keV upstream of the Earth’s Bow Shock – upstream ion events – are characterized by short durations (~1-2 hours), steeply falling spectra, large (>100:1) field-aligned sunward anisotropies, and positive correlations with the solar wind speed and geomagnetic activity indices. Despite the wealth of information available, it is still not clear whether these ions are accelerated at the bow shock or somewhere inside the Earth’s magnetosphere. Furthermore, such events are also often observed simultaneously at two or more spacecraft, indicating that a large source region perhaps covering the entire size of the bow shock fills large spatial structures in the upstream region. In this paper we use simultaneous measurements of >40 keV upstream ions observed at ACE, Wind, and STEREO-A between 2007, day 1 through 2007, day 181 to calculate the occurrence probability of upstream events as a function of lateral and radial separation between L1 and STEREO-A. During the end of this ~6-month period, Wind (or ACE) and STEREO-A were separated by ~1750 RE in the radial direction and laterally in YGSE by ~3800 RE. Despite this large separation, STEREO-A continued to observe upstream events right up until the end of our survey period. More surprisingly, we found that the occurrence probability for measuring simultaneous upstream events at Wind or ACE at L1 and STEREO-A was ~20-30%, i.e., far greater than that expected from accidental coincidences. We discuss the implications of these results for the size of the source region, the conditions under which upstream events occur, and the size and nature of the spatial structures in which these ions populate and propagate in the upstream region.
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| − | '''Kucharek et al.:''' Reported on intensity variations of suprathermal ions at interplanetary discontinuities such as shocks, shocks associated with CIR’s, and compression regions. Observation from ACE/SPEICA and STEREO PLASTIC in the energy range of 250 – 800keV were used to investigate the enhancements of He+, He2+, 3He2+ at CIRs. Numerical (test particle) simulations have been used to explain why 3He2+ is less enhanced the pickup helium and solar wind alpha particles. Reasonable agreement has been achieved and there is evidence that the enhancements of the various species are controlled by the local turbulence at the shock ramp as well as local shock parameters. Future observations as well as simulations are planned to investigate not only the reflection and accelerations properties of the different interplanetary discontinuities for the various species but also the impact of these structures on the (far) downstream region of the Earth’s bow shock.
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