Evidence for mass-loading of the Venus magnetosheath

The observed magnetic field configuration in the Venus magnetosheath contains information about the solar wind mass-loading processes occurring as a result of the extension of the neutral atmosphere into the magnetosheath. In this paper, magnetic field signatures of various mass-loading processes are discussed and experimental results from the Pioneer Venus Orbiter magnetometer experiment are examined for evidence of these signatures. The data suggest that the ?$\text{V}$X$\text{B}$ acceleration process, stochastic pickup of ionospheric ions, and $\text{J}$X$\text{B}$ force “scavenging” at the ionopause all occur at various times.     

Solar wind governing the response of Venusian atmosphere and ionosphere

The ionosphere of Venus is primarily formed by photoionization of a gaseous blanket around Venus. The impact ionization by energetic solar charged particles also plays an important role in the variability of Venusian ionospheric ion, electron density and their temperature profiles. The microscopic variations in the solar wind velocity, particle flux and orientations of frozen-in interplanetary magnetic field determine the solar wind interaction with the Venusian ionosphere. The ion and electron density profiles obtained by Pioneer Venus Orbiter and Pioneer Venus Entry Probes have been analysed in the light of simultaneous solar wind velocity and particle flux. Marked changes in height profiles of ion, electron densities and their temperatures have been found to correlate with the simultaneous changes in the solar wind velocity and particle flux. It is shown that the solar wind plays a more important role in controlling the physical properties and behavior of daytime as well as nighttime ionosphere of Venus, whereas the solar xuv sustains the primary ionization process.     

On the large-scale structure of the tail current as measured by THEMIS

The magnetic field structure and the spatial characteristics of the large-scale currents in the magnetospheric tail were studied during quiet and moderately disturbed geomagnetic conditions in 2009. The magnetic field of the currents other than the tail current was calculated in terms of a paraboloid model of the Earth’s magnetosphere, A2000, and was subtracted from measurements. It was found on the base of obtained tail current magnetic field radial distribution that the inner edge of the tail current sheet is located in the night side magnetosphere, at distances of about 10 R_E and of about 7 R_E during quiet and disturbed periods respectively. During the disturbance of February 14, 2009 (Dst_min ∼ −35 nT), the B_x and the B_z component of the tail current magnetic field near its inner edge were about 60 nT, and −60 nT that means that strong cross-tail current have been developed. The tail current parameters at different time moments during February 14, 2009 have been estimated. Solar wind conditions during this event were consistent with those during moderate magnetic storms with minimum Dst of about −100 nT. However, the magnetospheric current systems (magnetopause and cross-tail currents) were located at larger geocentric distances than typical during the 2009 extremely quiet epoch and did not provide the expected Dst magnitude. Very small disturbance on the Earth’s surface was detected consistent with an “inflated” magnetosphere.     

Model calculations of the dayside ionosphere of Venus

Model calculations of the dayside ionosphere of Venus are presented. The coupled continuity and momentum equations were solved for O₂⁺, O⁺, CO₂⁺, C⁺, N⁺, He⁺, and H⁺ density distributions, which are compared with measurements from the Pioneer Venus ion mass spectrometer. The agreement between the model results and the measurements is good for some species, such as O⁺, and rather poor for others, such as N⁺, indicating that our understanding of the dayside ion composition of Venus is incomplete. The coupled heat conduction equations for ions and electrons were solved and the calculated temperatures compared with Pioneer Venus measurements. It is shown that fluctuations in the magnetic field have a significant effect on the energy balance of the ionosphere.     

Old tail lobes effect on the solar-wind – Magnetosphere energy transport for the 27 August 2001 substorm

The magnetic flux of tail lobes Ψ is divided in two parts of comparable values Ψ₁ and Ψ₀₂, with the first that appears during substorm and the second, observed before substorm start. The first was named “new magnetic flux”, the second – “old magnetic flux”. The first, Ψ₁, is known to play a definitive role in the energy transport from the solar wind into the magnetosphere-ionosphere-atmosphere system, but the role of Ψ₀₂ in this transport is not well known. From the 27 August 2001 substorm data we study the involvement in the above transport process of the old flux Ψ₀₂. This involvement is observed in the polar cap (PC) area, which existed prior to the substorm and is called respectively “the old PC”. In this study, as distinct from earlier works, we use the balance equation of the energy stored in magnetosphere and energy consumed. Activation of the old PC magnetic flux Ψ₀₂ was found to increase the energy input by ∼85% in the event under consideration.     

Magnetic flux ropes in the Venus ionosphere: In situ observations of force-free structures?

Force-free magnetic structures with cylindrical geometry appear under a variety of conditions in nature. Filamentary helical magnetic structures are observed to be associated with prominences and flares in the solar atmosphere, and can arise in superconductors and laboratory plasmas. Another example of cylindrical quasi-force-free configurations appears to exist in the Venus ionosphere. Magnetic flux ropes with diameters of ～20 – 30 km have been observed by the Pioneer Venus Orbiter to be a nearly ubiquitous feature of the dayside Venus ionosphere. Models of flux ropes suggest that many of these structures tend to be quasi-force-free, i.e., $\text{J}$×$\text{B}$～0, while others are correlated with pressure variations in the ambient thermal plasma, $\text{J}$×$\text{B}$=-?(nkT).     

Electron densities and temperatures in the Venus ionosphere: Effects of solar EUV,solar wind pressure and magnetic field

The Venus ionosphere is influenced by variations in both solar EUV flux and solar wind conditions. On the dayside the location of the topside of the ionosphere, the ionopause, is controlled by solar wind dynamic pressure. Within the dayside ionosphere, however, electron density is affected mainly by solar EUV variations, and is relatively unaffected by solar wind variations and associated magnetic fields induced within the ionosphere. The existence of a substantial nightside ionosphere of Venus is thought to be due to the rapid nightward transport of dayside ionospheric plasma across the terminator. Typical solar wind conditions do not strongly affect this transport and consequently have little direct influence on nightside ionospheric conditions, except on occasions of extremely high solar wind dynamic pressure. However, both nightside electron density and temperature are affected by the presence of magnetic field, as in the case of ionospheric holes.     

The magnetic fields of Mercury,Venus and Mars

Just as clearly as Mariner 10 established that Mercury has an intrinsic magnetic field, the Pioneer Venus orbiter has established that Venus has no significant intrinsic field. This is perhaps the opposite of what might be expected. Mercury, a small planet might be expected to cool rapidly and its internal dynamo to cease, while Venus, which is almost as large as the Earth, should not have lost much heat. On the contrary the source of energy of the Mercury dynamo appears to be extant whereas that of Venus appears to be extinct.The existence of a Martian magnetic field is controversial. No unambiguous signature of a Martian magnetic field has been reported. If the field on the nightside of Mars is of planetary rather than solar origin the Russian Mars spacecraft observations indicate the Martian dipole lies near the planetary equator rather than its pole.     

Effects of large-scale magnetic fields in the Venus ionosphere

Theoretical models of the ionosphere of Venus have been constructed in the past without due consideration of the fact that the ionosphere is sometimes magnetized. This paper examines some differences between the magnetized and unmagnetized dayside Venus ionosphere using the Pioneer Venus Orbiter Langmuir probe and magnetometer data. Particular attention is given to the evaluation of the altitude profiles of the thermal electron heating and comparison of the magnitude of the magnetic force(¯vׯB) ׯB with other forces in the ionosphere. Several examples illustrate how heating profiles are different in the magnetized ionosphere with effective heating below ～200 km altitude reduced by orders of magnitude compared to the field-free ionosphere. The force associated with the magnetic field is comparable to other forces in the magnetized ionosphere. The measured plasma density, electron temperature and magnetic field thus suggest that large-scale magnetic fields should be included in future ionosphere models.     

Studies of the configuration of the Venus ionospheric magnetic field

The dayside ionospheric magnetic field of Venus has been modelled from two different points of view. The Cloutier et al. electrodynamic model makes specific predictions about the behavior of the global magnetic field configuration that can be compared with those expected from the alternate diffusion/convection model. Although the diffusion/convection model is currently only one-dimensional, it is found that it is consistent with the observations in several areas where the 3-dimensional electrodynamic model is not.     

The properties of the low altitude magnetic belt in the Venus ionosphere

When the solar wind dynamic pressure is high, the Venus ionosphere usually contains a belt of steady magnetic field at the very lowest altitudes to which Pioneer Venus probes. The current layer that flows on the high altitude side of this low altitude belt is centered at an altitude which ranges from 170 to 190 km with a most probable altitude of 182 km. This altitude is independent of solar zenith angle and hence the current system is flowing horizontally rather than vertically as proposed by Cloutier and co-workers. The lower edge of the magnetic belt was probed only on the lowest altitude passes of Pioneer Venus. This boundary is even more stable in location. The belt has decayed to 90% of its maximum strength usually by 162 km and to 50% of its maximum strength by 155 km. We interpret these data to indicate that the observed magnetic structure of the Venus ionosphere is a product of temporal evolution rather than of spacecraft motion through a spatially varying static structure.     

Laboratory experiments and space phenomena

Two types of convection were observed in the laboratory model of the magnetosphere: viscous convection and convection due to field lines common to both the magnetosphere and artificial solar wind. With a southward field component in the solar wind, convection from the Sun is observed in the polar cap, while with a large northward component, convection is directed toward the Sun. Merging of the field lines occurs in the cleft. With the southward component, a visor appears in front of the magnetosphere boundary. The decay of the visor into small magnetic structure is observed. The formation of an induced magnetosphere with a magnetic tail is shown in the experiments of the simulated conditions near non-magnetic bodies with a plasma shell (Venus, comets). A combined induced-intrinsic magnetosphere also was investigated.     

Low-altitude field-aligned currents used to diagnose plasma distribution and magnetic topology in the outer magnetosphere

Field-aligned currents (FAC) can be used to determine changes in the total plasma content (TPC) of convecting flux tubes. The observed steady-state FAC system is combined with the observed equipotential pattern to determine contours of TPC as mapped to the ionosphere. Criteria for a qualitative mapping of the FAC, equipotentials and TCP contours along magnetic field lines to the equatorial plane are set up and the result is shown in figure 3. Some interesting features are: (1) There is a considerable distortion, which is most obvious near midnight, due to the existence of FAC; (2) There is a dusk to dawn component of convection across the tail; (3) The reversal of this component in the pre-midnight quadrant produces the Harang discontinuity. A discussion of time-dependent flows suggests that both plasma depletion associated with FAC and neutral lines may be necessary in a substorm expansion. Between substorm expansions, convection is faster than FAC-produced collapse of flux tubes. Finally it is pointed out that the current and electric field are probably not parallel in the tail, requiring a rethinking of tail models.     

Evolution of solar wind structures from 0.72 to 1 AU

Understanding the evolution of solar wind structures in the inner heliosphere as they approach the Earth is important to space weather prediction. From the in situ solar wind plasma and magnetic field measurements of Pioneer Venus Orbiter (PVO) at 0.72 AU (1979–1988), and of Wind/Advanced Composition Explorer (ACE) missions at 1 AU (1995–2004), we identify and characterize two major solar wind structures, stream interaction regions (SIRs) and interplanetary coronal mass ejections (ICMEs). The average percentage of SIRs occurring with shocks increases significantly from 3% to 24% as they evolve from 0.72 to 1 AU. The average occurrence rate, radial extent, and bulk velocity variation of SIRs do not change from 0.72 to 1 AU, while peak pressure and magnetic field strength both decrease with the radial evolution of SIRs. Within the 0.28 AU distance from the orbit of Venus to that of Earth, the average fraction of ICMEs with shocks increases from 49% to 66%, and the typical radial extent of ICMEs expands by about a fraction of 1.4, with peak pressure and magnetic field strength decreasing significantly. The mean occurrence rate and expansion velocity of ICMEs do not change from 0.72 to 1 AU.     

磁场重联的二维粒子模拟研究

利用二维全粒子模拟方法研究了无碰撞等离子体中的磁场重联过程，得到了不同区域的离子和电子速度分布．计算结果表明，电子和离子在扩散区中的不同动力学特性产生的Hall电流使磁场的y分量By呈现四极形分布．离子和电子的速度分布偏离了初态时的Maxwell分布，呈现非局域的多重分布．同时由于磁场重联而产生的电场使电子在X点附近得到加速和加热，因而在电子的能谱分布中形成-高能尾。     

部分开放多极磁场中的日冕磁绳灾变

采用球坐标下二维三分量理想MHD模型，研究部分开放多极背景磁场中日冕磁绳的灾变现象．背景磁场由含3个闭合双极场的冕流和带赤道中性电流片的开放场构成，磁绳位于中心双极场的下方，其特性由环向磁通和轴向磁通表征．对给定的环向磁通，存在轴向磁通的一个临界值；对给定的轴向磁通，也存在环向磁通的一个临界值．在该临界值以下，磁绳附着于太阳表面，系统处于平衡状态；该临界值一旦被超越，磁绳将脱离太阳表面向上喷发，说明部分开放多极背景磁场中的日冕磁绳系统存在灾变现象．本文算例表明，灾变点对应的磁能阈值超过对应部分开放场(中心双极场开放，两侧的双极场仍维持闭合)能量约15％，其超过部分可为日冕物质抛射一类太阳爆发提供能源．     

Cometary probe of the Venera-Halley mission

Venera-Halley mission is to be launched to Venus in Dec. 1984. It will fly by Venus in June 1985. Separation of the cometary probe and Venera descend module will take place at that time. The gravitational swing-by at Venus will provide the encounter with the Halley comet in March 1986. The remote sensing of the inner coma (TV-imagery, spectrometry in the region from 1200 A to 12 μm, polarimetry) and of the nucleus, direct measurements of dust fluxes, dust composition, plasma and magnetic field are planned in the framework of multinational cooperation.     

On the source region of flux transfer events

We have examined the region of occurrence of flux transfer events for three distinct orientations of the interplanetary magnetic field: nearly horizontal in the solar magnetospheric equator, diagonally southward at 45° to the magnetospheric equator and nearly due south. For horizontal IMF conditions the FTE's occur in a horizontal band about ± 6 R_E wide. For diagonally southward IMF conditions, the FTE's occur in a diagonal swath about ± 6 R_E wide passing through the subsolar point. For duskward but nearly due southward IMF conditions, our observations reveal FTE's throughout the northern morning quadrant. These observations are consistent with a near equatorial source for flux transfer events and hence with component merging and not anti-parallel merging. These observations also help understand the energetic ion anisotropies seen in these events.     

Electromagnetic waves observed by Venus Express at periapsis: Detection and analysis techniques

The magnetometer on Venus Express was designed to be able to obtain 128 Hz samples of the magnetic field from two sensors in a gradiometer configuration. This mode is used around periapsis to determine whether the signals reported at low altitudes near 100 Hz, had the properties of electromagnetic waves generated by electric discharges in the Venus atmosphere. The lack of a magnetic cleanliness program and the shortness of the magnetometer boom make this a challenging measurement. Fortunately the signals are sufficiently strong that they can be easily resolved with rather straightforward analysis techniques.     

Temporal and spatial variations observed in the ionospheric composition of Venus: Implications for empirical modelling

In situ measurements of the thermal ion composition of the ionosphere of Venus have been obtained for a period of two Venus years from the Bennett rf ion mass spectrometer on the Pioneer Venus Orbiter. Ion measurements within an altitude interval of 160 to 300 kilometers, corresponding to an overall latitude interval of about ?4° to 34°N, are assembled from the interval December 1978 to March 1980. This time interval corresponds to two revolutions of Venus about the Sun, designated as two “diurnal cycles”. The distributions of several ion species in this data base have been sorted to identify temporal and spatial variations, and to determine the feasibility of an analytical representation of the experimental results. The first results from the sorting of several prominent ions including O⁺, O₂⁺, and H⁺ and several minor ions including CO₂⁺, C⁺, and H₂⁺ reveal significant diurnal variations, with superimposed modulation associated with solar activity and solar wind variations. The diurnal variation consists of strong day to night contrast in the ion concentrations, with differences of one to two orders of magnitude, depending upon ion mass and altitude. The concentrations of O₂⁺, O⁺, CO₂⁺ and C⁺ peak throughout the dayside decreasing sharply at the terminators to nightside levels, lower by one to two orders of magnitude relative to the dayside. The diurnal variations of the light ions H⁺ and H₂⁺ peak during the night, exhibiting asymmetric nightside bulges favoring the pre-dawn sector, near 0400 solar hour angle. Superimposed upon the diurnal distributions are modulation signatures which correlate well with modulation in the F_10.7 index, indicating a strong influence of solar variability on the ion production and distribution. The influence of solar wind perturbations upon the ion distributions are also indicated, by a significant increase in the scatter of the observations with increasing altitude as higher altitudes, approaching 300 kilometers, are sampled. Together, these temporal and spatial variations make the task of modelling the ionosphere of Venus both very interesting and challenging.