Lan K. Jian

Ion Cyclotron Waves in the Solar Wind Observed by STEREO Near 1 AU

Using high-resolution magnetic field data from the STEREO mission, we have observed strong narrow-band ion cyclotron waves (ICWs) in the solar wind near 1 AU. The waves propagate nearly parallel to the magnetic field and are below the local proton gyrofrequency in the solar wind frame. Because the twin STEREO spacecraft were far away from any planet, including the Earth, the waves do not have a planetary source. The ICWs often appear when the interplanetary magnetic field is more radial than the nominal Parker spiral. Since the wave frequency in the spacecraft frame is higher than the local proton gyrofrequency, the waves are not locally generated by ion pickup. Observations are consistent with wave generation closer to the Sun and outward transport by the super-Alfvenic solar wind. The waves are intrinsically left-hand polarized in the solar wind frame, but appear to be both left- and right-handed in the spacecraft frame, associated with outward and inward propagation, respectively. The relative amplitudes of the left-handed and right-handed waves support the closer-to-Sun generation scenario.

Behavior of current sheets at directional magnetic discontinuities in the solar wind at 0.72 AU

[1] Venus Express interplanetary magnetic field measurements have been examined for magnetic ‘‘holes,’’ accompanied by magnetic field directional changes. We examine both the thickness of the current sheet and the depth of the magnetic field depression. We find the thickness of the current sheet is not correlated with the depth of the field depression. The depth of the magnetic holes is related to directional angle change. Since total pressure should balance across these discontinuities, there must be enhanced plasma pressure within the magnetic holes. The dependence of the depth of the hole (i.e., size of the plasma pressure enhancement) on the directional changes suggests that the heating of the plasma associated with the hole formation may be provided by annihilation of the magnetic energy in the current sheet, via slow reconnection. Citation: Zhang, T. L., et al. (2008), Behavior of current sheets at directional magnetic discontinuities in the solar wind at 0.72 AU, Geophys. Res. Lett., 35, L24102, doi:10.1029/2008GL036120.

Study of the 2007 april 20 cme-comet interaction event with an mhd model

This study examines the tail disconnection event on 2007 April 20 on comet 2P/Encke, caused by a coronal mass ejection (CME) at a heliocentric distance of 0.34 AU. During their interaction, both the CME and the comet are visible with high temporal and spatial resolution by the STEREO-A spacecraft. Previously, only current sheets or shocks have been accepted as possible reasons for comet tail disconnections, so it is puzzling that the CME caused this event. The MHD simulation presented in this work reproduces the interaction process and demonstrates how the CME triggered a tail disconnection in the April 20 event. It is found that the CME disturbs the comet with a combination of a 180° sudden rotation of the interplanetary magnetic field (IMF), followed by a 90° gradual rotation. Such an interpretation applies our understanding of solar wind-comet interactions to determine the in situ IMF orientation of the CME encountering Encke.

EVOLUTION OF CORONAL MASS EJECTION MORPHOLOGY WITH INCREASING HELIOCENTRIC DISTANCE. II. IN SITU OBSERVATIONS

Interplanetary coronal mass ejections (ICMEs) are often observed to travel much faster than the ambient solar wind. If the relative speed between the two exceeds the fast magnetosonic velocity, then a shock wave will form. The Mach number and the shock standoff distance ahead of the ICME leading edge is measured to infer the vertical size of an ICME in a direction that is perpendicular to the solar wind flow. We analyze the shock standoff distance for 45 events varying between 0.5 AU and 5.5 AU in order to infer their physical dimensions. We find that the average ratio of the inferred vertical size to measured radial width, referred to as the aspect ratio, of an ICME is 2.8 ± 0.5. We also compare these results to the geometrical predictions from Paper I that forecast an aspect ratio between 3 and 6. The geometrical solution varies with heliocentric distance and appears to provide a theoretical maximum for the aspect ratio of ICMEs. The minimum aspect ratio appears to remain constant at 1 (i.e., a circular cross section) for all distances. These results suggest that possible distortions to the leading edge of ICMEs are frequent. But, these results may also indicate that the constants calculated in the empirical relationship correlating the different shock front need to be modified; or perhaps both distortions and a change in the empirical formulae are required.

Venus Express observations of an atypically distant bow shock during the passage of an interplanetary coronal mass ejection

[1] On 10-11 September 2006 the Venus Express magnetometer detected a very strong Interplanetary Coronal Mass Ejection (ICME) event with an average field about 2 times higher than that of a typical ICME at 0.72 AU. While the effective obstacle to the solar wind is compressed to a smaller dimension during this ICME event, the bow shock is located far upstream of its nominal location. The observed shocks are weak and appear very dynamic. The location of the shock crossing can be found all along the Venus Express trajectory, which has an apocenter of 12 R v . We attribute the atypical distant bow shock location as an effect of the extremely low Mach number during the ICME.

An unusual current sheet in an ICME: Possible association with C/2006 P1 (McNaught)

[1] On December 15, 2006, a strong ICME crossed the twin STEREO spacecraft while they were still close together in near-Earth space. In the midst of this traversal, a strong current sheet was observed in the magnetic records, whose properties are inconsistent with our present understanding of the magnetic structure of an interplanetary coronal mass ejection (ICME). A possible cause of this current sheet is the extended dust trail of C/2006 P 1 (McNaught), whose orbital plane the ICME traversed close to the Sun. If this association is correct, then charged dust can produce twists in the interplanetary magnetic field that can persist for an AU or longer. Further, as a result of this electromagnetic interaction, small dust particles can be carried out of the solar system.

How unprecedented a solar minimum was it

The end of the last solar cycle was at least 3 years late, and to date, the new solar cycle has seen mainly weaker activity since the onset of the rising phase toward the new solar maximum. The newspapers now even report when auroras are seen in Norway. This paper is an update of our review paper written during the deepest part of the last solar minimum [1]. We update the records of solar activity and its consequent effects on the interplanetary fields and solar wind density. The arrival of solar minimum allows us to use two techniques that predict sunspot maximum from readings obtained at solar minimum. It is clear that the Sun is still behaving strangely compared to the last few solar minima even though we are well beyond the minimum phase of the cycle 23–24 transition.

Proton enhancement and decreased O6+/H at the heliospheric current sheet: implications for the origin of slow solar wind

We investigated the proton enhancement and O6+/H depletion in the vicinity of the heliospheric current sheet (HCS) using data from STEREO/PLASTIC and STEREO/IMPACT. Three HCS crossing events were studied. For the first two events, the proton enhancement and O6+/H depletion are found to lie at one edge of the HCS. The proton density has a steep slope both at the HCS and at the other boundary of the enhancement. In the third event the proton enhancement and O6+/H depletion surround the HCS and last for 8 hours while the density profile is very different from the other two events. Velocity shear is observed at the HCS for the first two events but not for the third. The enhancement of hydrogen and depletion of oxygen at the streamer belt in the solar corona have been reported using UVCS observation. A potential connection with our observations is based on the similar features observed at 1 AU. How the plasma flows out of the streamer belt, and why there are different features in HCS encounters remain open que...

Mirror Mode Structures in the Solar Wind: STEREO Observations

Mirror mode structures occur in the solar wind either as an isolated magnetic field depression or as trains of magnetic holes (or peaks). Some trains have long durations and have been named mirror mode storms [1]. In this work we investigate mirror mode structures at 1 AU using STEREO A and B high resolution data. Magnetic field data were scanned to search for magnetic holes and peaks in a relatively steady ambient solar wind. We found several examples of mirror mode structures present in the ambient solar wind and also associated with SIRs. In order to study mirror mode origin, we present a case study with mirror mode structures present in the leading edge of a SIR during almost 8 hours corresponding to mirror mode storms. We analyze mirror mode shape and duration as well as plasma and magnetic field conditions that occur in the region surrounding mirror mode storms.

Study of Interplanetary Shocks Using Multi‐Spacecraft Observations

We investigate the characteristics of interplanetary (IP) shock waves associated with a stream interaction region (SIR) observed during April 21–24, 2007 by STEREO‐A/B, WIND and ACE spacecraft. During the years 2007–2008 STEREO‐A observed 43 and STEREO‐B crossed 41 shocks. As IP shocks propagate, they encounter solar wind with different characteristics (density, speed) and different orientations of the ambient magnetic field. Hence, it is expected that shock profiles will vary strongly through the space. We use magnetic field and plasma data to study shock structure, strength and orientation. In this example of a SIR we find that the characteristics of the shocks change dramatically from one region to another, the shock structure can be quasi‐perpendicular as observed in one spacecraft and quasi‐parallel when crossed at other point. Low frequency waves with different characteristics appear upstream and downstream of forward and reverse shocks. In this example the region upstream of the forward quasi‐perpe...