The Heliospheric Plasma Sheet Observed in situ by Three Spacecraft over Four Solar Rotations

Abstract

In this paper we present in situ observations of the heliospheric plasma sheet (HPS) from STEREO-A, Wind, and STEREO-B over four solar rotations in the declining phase of Solar Cycle 23, covering late March through late June 2007. During this time period the three spacecraft were located in the ecliptic plane, and were gradually separating in heliographic longitude from about 3 degrees to 14 degrees. Crossings of the HPS were identified using the following criteria: reversal of the interplanetary magnetic field sector, enhanced proton density, and local minima in both the proton specific entropy argument and in the alpha particle-to-proton number density ratio (N a /N p ). Two interplanetary coronal mass ejections (ICMEs) were observed during the third solar rotation of our study period, which disrupted the HPS from its quasi-stationary state. We find differences in the in situ proton parameters at the HPS between the three spacecraft despite temporal separations of less than one day. We attribute these differences to both small separations in heliographic latitude and radial evolution of the solar wind leading to the development of compression The Sun 360 Guest Editors: Bernhard Fleck, Bernd Heber, and Angelos Vourlidas regions associated with stream interaction regions (SIRs). We also observed a modest enhancement in the density of iron ions at the HPS.

In situ properties of small and large flux ropes in the solar wind

“…The HPS thickness at 1 AU was found to be below 0.01 AU by Winterhalter et al [], while larger values, in the interval [0.01,0.03] AU, were measured by Bavassano et al [] and Zhou et al []. These larger thickness values were confirmed by Foullon et al [] and Simunac et al [] with the crossing of the same HPS by three spacecraft during five solar rotations. They found that the HPS thickness does not significantly evolve on the time scale of a day but does evolve at the scale of a solar rotation, with a thickness evolving between 0.02 and 0.06 AU.…”

Properties of the HPS‐ICME‐CIR Interaction Event of 9–10 September 2011

“…The presence of the magnetic strength, total pressure, and proton temperature increase supports the existence of this shock at this position. At the onset of the HPS, Wind experience a decline in entropy (Figure ), followed by a sharp rise during the ICME forward shock passage, before stabilizing between the shock and the second HPS boundary, due to stable density and temperature. This entropy behavior, where the increase occurs within the HPS, is rather different from the cases documented by Simunac et al (), in which an increase is reported at the second HPS boundary. We attribute this difference to the existence of the ICME forward shock within the HPS. The total pressure increased at the onset of the first HPS boundary until the ICME shock.…”

“…For further clarification, we designate the green‐shaded region as region A in Figure . This is the HPS, which has two boundaries (two vertical blue lines) identified by three main criteria: an enhancement in proton density, plasma beta enhancement, and the inversion of the interplanetary magnetic field sector (see, e.g., Crooker et al, ; Liu et al, ; Simunac et al, ; Suess et al, ; Winterhalter et al, ). For the purposes of comparison and to compensate for missing ACE data, Wind data are shown in Figure .…”

A statistical analysis of heliospheric plasma sheets, heliospheric current sheets, and sector boundaries observed in situ by STEREO

“…We greatly expand in this paper the study of Simunac et al [2012] by including all events observed by STEREO between 4 March 2007 and 29 February 2008. In addition, we use suprathermal electron and magnetic field data to select only those HCS events which can be considered to be a global structure.…”