Comment on "Are periodic solar wind number density structures formed in the solar corona?" by N. M. Viall et al., 2009, Geophys. Res. Lett., 36, L23102, doi:10.1029/2009GL041191

Location of formation of periodic solar wind number density structures is discussed. Observation of proton and alpha anticorrelation in these structures [Viall et al., 2009] indicates that taking into account that bulk velocity of aplha-particles is higher than that of proton the place of formation for these structures should be located at distance less 0.002 AU from place of observation.Comment: 6 pages, submitted in GR

Cluster four spacecraft measurements of small traveling compression regions in the near-tail

Cluster observations taken during a substorm on September 19, 2001 have revealed the presence of small traveling compression regions (TCRs) in the near tail. These measurements are used to determine directly the speed and direction of TCR propagation and the amplitude of the underlying bulge in the plasma sheet. The time-of-flight speeds derived from the arrival times of the magnetic perturbations at the different Cluster s/c yielded a mean speed of 413 km/s. For 2 of the TCRs s/c 1, 2 and 4 were located sufficiently close to the plasma sheet that they were immersed in the central plasma sheet plasma as the TCR swept over s/c 3. In this manner the Cluster measurements directly demonstrated that these small TCRs are caused by moving bulges in the plasma sheet-lobe interface. In summary, our analysis of the Cluster measurements has directly demonstrated the existence of moving bulges in the north-south thickness of the plasma sheet, most probably due to the formation of flux ropes, and their role in producing traveling compression regions

Cluster electric current density measurements within a magnetic flux rope in the plasma sheet

[1] On August 22, 2001 all 4 Cluster spacecraft nearly simultaneously penetrated a magnetic flux rope in the tail. The flux rope encounter took place in the central plasma sheet, beta(i) similar to1-2, near the leading edge of a bursty bulk flow. The "time-of-flight'' of the flux rope across the 4 spacecraft yielded V-x similar to 700 km/s and a diameter of similar to1 R-e. The speed at which the flux rope moved over the spacecraft is in close agreement with the Cluster plasma measurements. The magnetic field profiles measured at each spacecraft were first modeled separately using the Lepping-Burlaga force-free flux rope model. The results indicated that the center of the flux rope passed northward ( above) s/c 3, but southward (below) of s/c 1, 2 and 4. The peak electric currents along the central axis of the flux rope predicted by these single-s/c models were similar to15-19 nA/m(2). The 4-spacecraft Cluster magnetic field measurements provide a second means to determine the electric current density without any assumption regarding flux rope structure. The current profile determined using the curlometer technique was qualitatively similar to those determined by modeling the individual spacecraft magnetic field observations and yielded a peak current density of 17 nA/m(2) near the central axis of the rope. However, the curlometer results also showed that the flux rope was not force-free with the component of the current density perpendicular to the magnetic field exceeding the parallel component over the forward half of the rope, perhaps due to the pressure gradients generated by the collision of the BBF with the inner magnetosphere. Hence, while the single-spacecraft models are very successful in fitting flux rope magnetic field and current variations, they do not provide a stringent test of the force-free condition

Cluster observations of "crater" flux transfer events at the dayside high-latitude magnetopause

On 11 January 2002 Cluster detected two "crater"-like flux transfer event (FTE) signatures. The four spacecraft were in quasi-linear formation, spread over similar to 2 R-E in the magnetopause normal direction, and sample a range of distances from it. The observations occur near a southward turning of the IMF, but no solar wind pressure pulses are detected. Analysis reveals: (1) C3, closest to the magnetopause, made two transient excursions into the magnetosheath which bracket two "crater"-FTE signatures detected at the other three spacecraft; (2) magnetic field deflections observed at the other spacecraft do not match the magnetosheath field direction at C3; (3) these FTE signatures involve encounters with reconnected field lines and associated boundary layers lying just inside the magnetopause, including a "separatrix layer" of accelerated magnetosheath electrons and an injection of magnetosheath ions. Under the observed conditions, reconnected flux tubes created by a transient and localized patch of reconnection located nearer to the subsolar point, will move northward and duskward over Cluster, consistent with observations inside the magnetosphere. The FTE signatures arise from this transient inward motion of reconnection-associated boundary layers over the spacecraft. We postulate that the transient relocation of C3 into the magnetosheath is due to a region of eroded magnetic flux, lying in the wake of the recoiling FTE, which itself is driven duskward at some fraction of the magnetosheath flow speed. The FTEs pass northward of C3, but the eroded wake, which we term the "traveling magnetopause erosion region" (TMER), is located equatorward of the FTEs and moves duskward over C3