W. B. Hanson

A proposed production model of rapid subauroral ion drifts and their relationship to substorm evolution

Multisatellite data are used to examine the temporal relationship between Subauroral Ion Drifts (SAID) and the phases of an auroral substorm. Utilizing images of auroral luminosities taken by the Dynamics Explorer 1 (DE 1) spacecraft and observations of particle injection at geosynchronous orbit, we identify the time of expansive phase onset and estimate the time at which recovery begins. Noting the times at which SAID are observed simultaneously by the DE 2 spacecraft, we find that SAID typically occur well after substorm onset (more than 30 min), during the substorm recovery phase. Substantial westward ion drifts and field-aligned currents are observed well equatorward of the auroral oval during the expansion phase of a substorm, but the drifts lack the narrow spike signature associated with SAID. Prior to substorm onset and after substorm recovery, field-aligned currents are absent equatorward of the auroral oval and the ionosphere is very nearly corotating. A phenomenological model of SAID production is proposed that qualitatively agrees with the observed ionospheric signatures and substorm temporal relationship. In this model, substorm-generated, subauroral field-aligned currents close via Pedersen currents with the outward flowing, region 1 currents at higher latitudes. These Pedersen currents flow in the region of low conductivity equatorward of the auroral oval and are associated with relatively large, poleward directed electric fields. The frictional heating of the ions caused by collisions with the corotating neutral atmosphere substantially increases the rate of ion-atom interchange between O+ and N2. Subsequent fast recombination of NO+ with electrons further reduces the subauroral F region conductivities with a corresponding increase in the electric field and the frictional heating. This heating leads to thermal expansion, substantial field-aligned plasma flow, and very large depletions in the F peak concentration, thus additionally reducing the height-integrated Pedersen conductivity.

Composition and Structure of the Martian Atmosphere: Preliminary Results from Viking 1

Results from the aeroshell-mounted neutral mass spectrometer on Viking I indicate that the upper atmosphere of Mars is composed mainly of CO2 with trace quantities of N2, Ar, O, O2, and CO. The mixing ratios by volume relative to CO2 for N2, Ar, and O2 are about 0.06, 0.015, and 0.003, respectively, at an altitude near 135 kilometers. Molecular oxygen (O2+) is a major component of the ionosphere according to results from the retarding potential analyzer. The atmosphere between 140 and 200 kilometers has an average temperature of about 180� � 20�K. Atmospheric pressure at the landing site for Viking 1 was 7.3 millibars at an air temperature of 241�K. The descent data are consistent with the view that CO2 should be the major constituent of the lower martian atmosphere.

Electric field observations of equatorial bubbles

We present here results from the double floating probe experiment carried on the San Marco D satellite, with emphasis on the observation of large incremental changes in the convective electric field vector at the boundary of equatorial plasma bubbles. This study concentrates on isolated bubble structures in the upper ionospheric F region and divides these observed bubble encounters into two types, type I (live bubbles) and type II (dead bubbles). Type I bubbles show varying degrees of plasma density depletion and upward velocities ranging from 100 to 1000 m/s. Type II bubbles show plasma density depletion but no appreciable upward convection. Both types of events are often surrounded by a halo of plasma turbulence extending considerably outside the regions plasma depletion. Most type I events show some evidence for local continuity in the eastward (y) electric current, where the y component of the observed electric field (Ey) shows hyperbolic correlation with the plasma density (n), as dictated by horizontal current continuity. This model stresses the importance of including magnetic field aligned currents in deriving the electric potential equation from the divergence equation ▽ · j = 0. All of the type I (live) events examined exhibit a striking and systematic lack of conservation of the vertical component (x) of the electric field vector (Ex) on crossing these structures. This lack of conservation of Ex is of the order of 1.5 mV/m from west to east, directly implying that type I bubbles are not steady state plasma structures. A straightforward interpretation of this jump phenomenon in Ex leads to the conclusion that the walls of most of the type I bubbles are collapsing inward at the rate of some 50 m/s. Since the average east-west dimension of the bubble structures we have examined here is of the order of 40 km, we conclude that the average lifetime of the strong upward convection phase is about 15 min. This suggests that after 15 min or so these type I events may be pinched off from the low densities of the bottomside F region and the bubbles perhaps become type II events which continue to drift eastward with the general background zonal plasma flow during the equatorial night. It is argued that the collapse motions may be driven by an asymmetry between the upwind (west) and downwind (east) E region drag on the F region eastward dynamo motions. Such an asymmetry appears to be of the proper magnitude and direction to produce the observed (∼1.5 mV/m) jump in the tangential (vertical) component of E on crossing these events.

A comment on plasma 'pile-up' in the F-region

Abstract At ionospheric heights the geomagnetic field is virtually incompressible. In consequence, an electromagnetic drift can only compress the F-region plasma by moving it in a direction in which the field becomes stronger. This paper examines the rate of compression at mid-latitudes for three different assumptions about the ion motion.

Satellite measurements through the center of a substorm surge

Measurements have been made of electric and magnetic fields, plasma drifts, and electron precipitation within a surge at the westward, leading edge of the auroral “bulge” at the peak of the substorm expansion phase. The trajectory of the DE 2 satellite over the auroral emissions is determined from nearly simultaneous observations with the imager on the DE 1 satellite at a higher altitude. The electric field and plasma drift measurements have enabled us to deduce the basic configuration of the ionospheric electric potential, or plasma convection, around the surge. The electric potential shows that the bulge is associated with a protrusion of the dawn convection cell into the dusk cell, poleward of the “Harang discontinuity”. This protrusion contains a westward electric field that strongly enhances the westward electrojet current by the creation of a “Cowling channel”. This westward electric field, and the associated Cowling current, appear to terminate within the surge, which contains an intense, upward field-aligned current. The magnetic field measurements show that the region containing this field-aligned current is shaped more like a cylinder rather than a long sheet. The total current is found to exceed one-half million amperes.

Calculated distributions of hydrogen and helium ions in the low-latitude ionosphere

Abstract The simultaneous time-dependent continuity equations for O+, H+ and He+ in the low latitude F-region are solved. Account is taken of E × B drift, a meridional neutral wind and ion-ion and ion-neutral drag. The calculated profiles of O+ and H+ concentrations at 1630 LT are in fair agreement with the observations of Hanson and his co-workers. The He+ field-aligned velocity is almost matched to the O+ field-aligned velocity and, above the chemical equilibrium region and around the He+ peak, the He+ concentration is determined largely by production and transport. There is disagreement between the theoretical vertical He+ profile and the profile observed by Hanson et al. Satisfactory agreement is obtained with Taylors satellite results at fixed height for O+ and H+. It is found that the He+ concentration is greater in the winter hemisphere than in the summer hemisphere, even if the neutral helium distribution is symmetrical about the Equator. The He+ results are consistent with Taylors results.

Auroral ionospheric signatures of the plasma sheet boundary layer in the evening sector

We report on particles and fields observed during Defense Meteorological Satellite Program (DMSP) F9 and DE 2 crossings of the polar cap/auroral oval boundary in the evening MLT sector. Season-dependent, latitudinally narrow regions of rapid, eastward plasma flows were encountered by DMSP near the poleward boundary of auroral electron precipitation. Ten DE 2 orbits exhibiting electric field spikes that drive these plasma flows were chosen for detailed analysis. The boundary region is characterized by pairs of oppositely-directed, field-aligned current sheets. The more poleward of the two current sheets is directed into the ionosphere. Within this downward current sheet, precipitating electrons either had average energies of a few hundred eV or were below polar rain flux levels. Near the transition to upward currents, DE 2 generally detected intense fluxes of accelerated electrons and weak fluxes of ions, both with average energies between 5 and 12 keV. In two instances, precipitating ions with energies >5 keV spanned both current sheets. Comparisons with satellite measurements at higher altitudes suggest that the particles and fields originated in the magnetotail inside the distant reconnection region and propagated to Earth through the plasma sheet boundary layer. Auroral electrons are accelerated by parallel electric fields produced by the different pitch angle distributions of protons and electrons in this layer interacting with the near-Earth magnetic mirror. Electric field spikes driving rapid plasma flows along the poleward boundaries of intense, keV electron precipitation represent ionospheric responses to the field-aligned currents and conductivity gradients. The generation of field-aligned currents in the boundary layer may be understood qualitatively as resulting from the different rates of earthward drift for electrons and protons in the magnetotails current sheet.

Downdrafting plasma flow in equatorial bubbles

The electric field experiment carried aboard the San Marco D equatorial ionospheric satellite regularly measured updrafting in plasma depletion channels or “equatorial bubbles” which form on the bottomside of the nightside equatorial F region. We report here observations of downdrafting vertical plasma velocities inside such depletion regions in the nightside equatorial ionosphere. Both updrafting and downdrafting motions can be expected on the basis of a generalized gradient drift/collisional Rayleigh-Taylor instability process in the ionospheric F region. Although the gravitation can only drive upward plasma flow in plasma depletion regions, both background westward zonal electric fields and upward vertical neutral winds can cause an occurrence of downdrafting (i.e., a downward motion of the plasma within the bubble) if those parameters are strong enough. We show that as the background zonal electric field becomes westward (often after ∼2100 LT) in the equatorial ionosphere, the plasma interior to an existing bubble at altitudes of ∼400 km and less at the magnetic equator may assume a downdrafting motion, while at higher altitudes in the same bubble channel, the plasma flow remains upward. Such a simultaneous occurrence of the updrafting and downdrafting plasma flow in a single bubble channel may lead to the pinching off of the upper part of the depletion region from the lower altitude regions, causing the decay of a bubble or the formation of a “dead” bubble.

Entry Science Experiments for Viking 1975

Abstract A review is given of our present knowledge of the Martian atmosphere with special emphasis on the results obtained by the Mariner 4, 6, and 7 fly-bys. The Viking Project offers the first opportunity for in situ measurements which should resolve many questions left open by previous work. A description is given of the neutral gas mass spectrometer and retarding potential analyzer experiments to be performed as the lander enters the upper atmosphere and the experiments planned for determining atmospheric structure as the lander approaches the surface of the planet.

Small‐scale electrodynamics of the cusp with northward interplanetary magnetic field

We report on possible, low-altitude field signatures of merging occurring at high-latitudes during a period of strong, northward directed interplanetary magnetic field. Our study of the small-scale electrodynamics in a vicinity of the poleward cusp boundary is based on multisensor measurements from a single DE 2 satellite pass over northern hemisphere cusp/cleft regions. Observed sunward convection over the polar cap and presence of the reverse ion dispersion feature are in agreement with expectations for merging occurring at the magnetopause, on the field lines mapping to the poleward boundary of the cusp. Large electric and magnetic field spikes detected at the poleward edge of the magnetosheathlike particle precipitation are interpreted as field signatures of the low-altitude footprint of such merging line locations. A train of phase-shifted, almost linearly polarized electric and magnetic field fluctuations was detected just equatorward of the large electromagnetic spike. These may be due to either ion cyclotron waves excited by penetrating magnetosheath ions or transient oscillations in the frame of convecting plasma, brought about by the sudden change in the flow at the magnetospheric end of the field line.