J. K. Shi

Little or no solar wind enters Venus' atmosphere at solar minimum.

Venus has no significant internal magnetic field, which allows the solar wind to interact directly with its atmosphere2,3. A field is induced in this interaction, which partially shields the atmosphere, but we have no knowledge of how effective that shield is at solar minimum. (Our current knowledge of the solar wind interaction with Venus is derived from measurements at solar maximum.) The bow shock is close to the planet, meaning that it is possible that some solar wind could be absorbed by the atmosphere and contribute to the evolution of the atmosphere. Here we report magnetic field measurements from the Venus Express spacecraft in the plasma environment surrounding Venus. The bow shock under low solar activity conditions seems to be in the position that would be expected from a complete deflection by a magnetized ionosphere. Therefore little solar wind enters the Venus ionosphere even at solar minimum.

Near-Earth substorm features from multiple satellite observations

We investigate a substorm on 3 October 2004 during which 11 satellites were located in near-Earth magnetotail (X-GSM > -10 R-E). Double Star 1 (TC-1), Cluster, and LANL-97 satellites were closely aligned in the dawn-dusk direction (<1 R-E apart) for this conjunction. After substorm expansion onset, TC-1 observed plasma sheet thinning at X approximate to -5.5 RE and later detected signature of plasma flow shear that may be associated with an auroral arc. Analysis of the dawn-dusk magnetic perturbations from GOES-10 and Polar suggests that these could be caused by a substorm current system consisting of not only the azimuthal closure of field-aligned currents (the substorm current wedge) but also the meridional closure of field-aligned currents. The temporal sequence of substorm activity (particle injection, current disruption, and dipolarization) revealed by these satellites indicates that the substorm expansion activity was initiated close to the Earth and spread later to further downstream distances. Furthermore, TC-1 and Cluster data show that there is no close relationship between some dipolarizations and Earthward plasma flows in the near-Earth region. The overall development of substorm activity is in agreement with the near-Earth initiation model for substorms. A temporal evolution of the magnetic field reconfiguration and plasma boundary motion during this substorm is constructed from these observations.

Tracing solar wind plasma entry into the magnetosphere using ion-to-electron temperature ratio

When the solar wind Mach number is low, typically such as in magnetic clouds, the physics of the bow shock leads to a downstream ion-to-electron temperature ratio that can be notably lower than usual. We utilize this property to trace solar wind plasma entry into the magnetosphere by use of Cluster measurements in the vicinity of the dusk magnetopause during the passage of a magnetic cloud at Earth on November 25, 2001. The ion-to-electron temperature ratio was indeed low in the magnetosheath (Ti/Te ∼ 3). In total, three magnetopause boundary layer intervals are encountered on that day. They all show that the low ion-to-electron temperature ratio can be preserved as the plasma enters the magnetosphere, and both with and without the observation of Kelvin-Helmholtz activity. This suggests that the ion-to-electron temperature ratio in the magnetopause boundary layer, which is usually high, is not prescribed by the heating characteristics of the plasma entry mechanism that formed these boundary layers. In the future, this property may be used to (1) further trace plasma entry into inner regions and (2) determine the preferred entry mechanisms if other theoretical, observational and simulation works can give indications on which mechanisms may alter this ratio.

Convective bursty flows in the near-Earth magnetotail inside 13 RE

With the 4-s resolution data of the magnetometer and the ion plasma analyzer on Double Star TC-1 from June to November of each year during the period of 2004-2006, we statistically analyzed the occurrence rate of both convective and field-aligned bursty flows (FABFs). In the region -13 R-E 180 km/s and V-parallel to x > 180 km/s exceeds 30 s, where V-perpendicular to x and V-parallel to x are irrespectively the X component of the velocity vector perpendicular and parallel to the local magnetic field. The statistical results show that the occurrence rate of the convective bursty flow (CBF) reached a minimum around -11.5 R-E; the occurrence rate of the FABF has a sudden decrease around -11.5 R-E, but has no clear increase inside -11.5 R-E. The spatial distribution of the all types of bursty flows shows that there had no deflection flows in the azimuthal direction related to the absence of the convective bursty flows inside 13 R-E explored by TC-1. The magnetic field analysis indicates that there exists a minimum in Bz around 12 R-E. Here, we suggest that the CBFs inside -11.5 R-E may not relate to the flows outside -12 R-E. The CBFs outside 12 R-E are likely to be the classical bursty bulk flow, originating 20-30 R-E downtail and braking outside 12 R-E. The minimum Bz may have close relation with the flow braking. The CBFs inside 12 R-E may be produced by another mechanism. The minimum Bz may be prone to the excitation of instabilities and, consequently, initiation of the substorm expansion and/or acceleration of ions resulting in CBFs inside.

Landau damping in space plasmas with generalized (r,q) distribution function

Space plasmas possessing non-Maxwellian particle distribution functions with an enhanced high-energy tail and shoulder in the profile of distribution function take an important role to the wave particle interaction. In the present paper Landau damping of electron plasma (Langmuir) waves and ion-acoustic waves in a hot, isotropic, unmagnetized plasma is studied with the generalized (r,q) distribution function. The results show that for the Langmuir oscillations Landau damping becomes severe as the spectral index r or q reduces. However, for the ion-acoustic waves Landau damping is more sensitive to the ion temperature than the spectral indices.

Nonlinear waves in a low-β plasma with cylindrical symmetry

The “Sagdeev potential” is derived from the magnetohydrodynamic equations in a cylindrical coordinate system, and nonlinear electrostatic density waves propagating along the magnetic field in a low-β plasma with cylindrical symmetry are studied. The results show the existence not only of periodic density waves, solitons with a density hump and solitons with a density dip, but also of density shock waves.

Theoretical study on the magnetospheric distribution of up-flowing ions

Abstract Based on the dynamic equation, the static distribution function of the ions along the field line is found and features of density distributions of up-flowing ions along the field line are studied. The results show that, (1) the ion densities away from the Earth reduce more quickly in near geo-space, (2) the larger the ion mass is, the more quickly the ion density reduces along the field line, (3) when Kp=3∼5, the region that the up-flowing ions occupy is larger than that when Kp=0. Some results are consistent with the observation.

The research of control system for Cesium atomic fountain clock

The control system for Cesium atomic fountain clock is described in detail in this paper. It consists of five parts, automatic digital frequency locking system of extended cavity diode laser (ECDL), the time serial control system, the data acquisition system, the microwave frequency synthesizer control system and the servo control system. With automatic digital frequency locking system, the frequency of ECDL can be locked to any saturated absorption curve automatically and remains locked throughout a series of abrupt disturbance. And moreover, the laser system can relock itself in few seconds when the frequency of the laser is out of range. The time serial control system generate thirteen time serial signals synchronously, including Transistor-Transistor Logic (TTL) signals and arbitrary waveform signals. Moreover, the result shows that these signals are synchronized within 10 μs. The time of flight (TOF) signals are obtained combined with data acquisition system. The resulting Ramsey resonance signal is obtained by sweeping frequency of the synthesizer. The frequency of the 9.2 GHz synthesizer is square-wave modulated by the servo control system so that the frequency of the quartz crystal oscillator is locked to the atomic resonance of the fountain.

Laser Power Stabilization for the Detection of the Populations of the Atomic Double Levels in Cs Fountain Clock

In this paper we present experimental results on the laser power stabilization of the detection system in Cs fountain clock. In order to stabilize the laser power, we have constructed a feedback loop by using an acousto-optic modulator, a photodetector, and a servo controller. The control-loop can make the relative intensity noise of the laser beam reduce from 1.33×10-4 Hz-1/2 to 7.16×10-6 Hz-1/2 at 2 Hz. However, if the beam used in the control-loop is obtained by using PBS, the two detection beams in fountain clock cannot be stabilized at the same time. Therefore, we propose a new scheme by using a piece of uncoated optical glass plate as a splitter to obtain the branch beam used in the control-loop. Experimental results demonstrate that the power stabilities of the two detection beams are both at 10-5 level (based on rms value) for 1.5 hours at the same time.

Study the phase transient of the RF Mach-Zehnder interferometric switch

The traditional switch exhibits phase discontinuity and transients in few hundreds microradian which will induce the larger microwave leakage shift in the atomic fountain clock. The RF Mach-Zehnder interferometric switch is developed in order to decrease the phase variations without inducing phase discontinuity. To detect the phase variations, the phase transient measurement system is also built and described in detail in this paper. The result shows the peak to peak phase variations of RF Mach-Zehnder interferometric is 10 microradians within the measurement resolution of a few microradians after five days averaging.