An upper limit of 16% (at 95% c.l.) is derived for the photon fraction in cosmic rays with energies greater than 10 19 eV, based on observations of the depth of shower maximum performed with the hybrid detector of the Pierre Auger Observatory. This is the first such limit on photons obtained by observing the fluorescence light profile of air showers. This upper limit confirms and improves on previous results from the Haverah Park and AGASA surface arrays. Additional data recorded with the Auger surface detectors for a subset of the event sample support the conclusion that a photon origin of the observed events is not favored.
Abstract. Interplanetary shock waves, propagating in the heliosphere faster than earlier-emitted coronal ejecta, penetrate them and modify their parameters during this interaction. Using two and one half dimensional MHD simulations, we show how a magnetic cloud (flux rope) propagating with a speed 3 times higher than the ambient solar wind is affected by an even faster traveling shock wave overtaking the cloud. The magnetic field increases inside the cloud during the interaction as it is compressed in the radial direction and becomes very oblate. The cloud is also accelerated and moves faster, as a whole, while both shocks (driven by the cloud and the faster interplanetary shock) merge upstream of the cloud. This interaction may be a rather common phenomenon due to the frequency of coronal mass ejections and occurrence of shock waves during periods of high solar activity.
[1] In late October and early November 2003 the ACE spacecraft at 1 AU detected two shock-associated interplanetary coronal mass ejections (ICMEs). In the sheath region formed in front of both ICMEs, some of the highest speeds ever directly measured in the solar wind were observed. We analyze in detail the energetic particle signatures measured at 1 AU by the EPAM experiment on board ACE during the passage and in the vicinity of these ICMEs. Solar energetic particles (SEPs) are utilized as diagnostic tracers of the large-scale structure and topology of the interplanetary magnetic field (IMF) embedded within both ICME events. In order to explain the bidirectional particle flows observed within both ICMEs, we have examined two candidate scenarios for these ICMEs in terms of open and closed magnetic field configurations. In the context of an open field configuration, the enhanced magnetic field regions associated with the CME-driven shocks mirror the energetic particles and hence the observed bidirectional flows. In the context of a closed field configuration, bidirectional flows result from particle circulation and reflection in a looped field configuration. Furthermore, we use the ACE/EPAM observations to reassess the leading and trailing boundaries of the ICMEs with respect to those previously proposed based upon ACE/SWEPAM solar wind plasma, suprathermal electron measurements, and ACE/MAG magnetic field data.
We present here magnetic force-free solutions for spherical, oblate, and prolate clouds and show their magnetic field configurations. It is shown that spheroidal models can fit observed clouds as well as the cylindrical model. The spherical model is free of the limitation of the cylindrical model that allows only reduced increase of the magnetic field to 2 x of the boundary value following from properties of the Bessel functions. For the tested cases, the cloud diameters following from the fit are generally larger for the spherical model than for the cylindrical one. An analysis of 14 cases shows that the fit using the spherical model is of a comparable accuracy in comparison with the cylindrical model. Generally, no exact determination of the cloud boundaries has been given up to now. We try to estimate cloud boundaries from the plasma data as an independent check, and compare them with cloud boundaries following from models of magnetic clouds. The bom•daries given by the spheroidal models are near irregular temperature increases, and we suggest taking these increases as a possible indicator of the cloud physical boundaries.
11,46711,468 VANDAS ET AL.: SPHEROIDAL MODELS OF MAGNETIC CLOUDS
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