J. R. Sharber

The loss of ions from Venus through the plasma wake

Venus, unlike Earth, is an extremely dry planet although both began with similar masses, distances from the Sun, and presumably water inventories. The high deuterium-to-hydrogen ratio in the venusian atmosphere relative to Earth’s also indicates that the atmosphere has undergone significantly different evolution over the age of the Solar System. Present-day thermal escape is low for all atmospheric species. However, hydrogen can escape by means of collisions with hot atoms from ionospheric photochemistry, and although the bulk of O and O2 are gravitationally bound, heavy ions have been observed to escape through interaction with the solar wind. Nevertheless, their relative rates of escape, spatial distribution, and composition could not be determined from these previous measurements. Here we report Venus Express measurements showing that the dominant escaping ions are O+, He+ and H+. The escaping ions leave Venus through the plasma sheet (a central portion of the plasma wake) and in a boundary layer of the induced magnetosphere. The escape rate ratios are Q(H+)/Q(O+) = 1.9; Q(He+)/Q(O+) = 0.07. The first of these implies that the escape of H+ and O+, together with the estimated escape of neutral hydrogen and oxygen, currently takes place near the stoichometric ratio corresponding to water.

The diffuse aurora: A significant source of ionization in the middle atmosphere

Energetic electrons can penetrate into the middle atmosphere causing excitation, dissociation, and ionization of neutral constituents, resulting in chemical changes. In this paper, representative electron spectra measured by the Upper Atmosphere Research Satellite particle environment monitor are used to determine the relative contributions of bremsstrahlung X rays and direct electron impact on the energy deposition and ionization production rates for altitudes between 20 and 150 km. Above 50 km most of the ionization comes from direct electron impact. However, in the stratosphere the energy contributed below 50 km is mostly due to bremsstrahlung X rays. In the diffuse aurora the ionization from the bremsstrahlung component exceeds that due to the galactic cosmic ray background to altitudes as low as 30 km during geomagnetically active periods. This paper demonstrates that a diffuse auroral source can input as much or more energy into the upper portion of the lower and middle atmosphere as previously reported for relativistic electron events. The effects of the diffuse aurora (including both the direct electron and the bremsstrahlung contributions) on atmospheric chemistry may be significant.

Evolution of the global aurora during positive IMF Bz and varying IMF By conditions

The DE 1 imaging instrumentation provides a full view of the entire auroral oval every 12 min for several hours during each orbit. We examined five examples of global evolution of the aurora that occurred during the northern hemisphere winter of 1981-1982 when the z component of the interplanetary magnetic field was positive and the y component was changing sign. Evolution of an expanded auroral emission region into a theta aurora appears to require a change in the sign of By during northward interplanetary magnetic field (IMF). Theta aurora are formed both from expanded duskside emission regions (By changes from positive to negative) and dawnside emission regions (By changes from negative to positive), however the dawnside-originating and duskside-originating evolutions are not mirror images. The persistence of a theta aurora after its formation suggests that there may be no clear relationship between the theta aurora pattern and the instantaneous configuration of the IMF.

UARS particle environment monitor observations during the November 1993 storm: Auroral morphology, spectral characterization, and energy deposition

During the major storm of November 3–11, 1993, the UARS particle environment monitor (PEM), in a 57° inclination, 585-km circular orbit, made measurements of energetic particles, X rays, and magnetic field perturbations in the midnight-dawn magnetic local time sector of the northern hemisphere and in the noon-evening sector of the southern hemisphere. X-ray images provide a global picture (∼20-min resolution) of the electron precipitation regions twice per 90-min orbit, which were used to study morphology during the main and recovery phases of the storm. Detailed examination of the precipitating electrons measured between 5 eV and 5 MeV show that in the several hundred eV to several MeV energy range the spectra may often be represented by the kappa distribution, which has a thermal character in the keV range and a power law distribution at higher energies. Selected spectral measurements were used to calculate ionization production rates as functions of altitude. The profiles peak between 95 and 120 km, each having a secondary ledge at lower altitudes produced by the bremsstrahlung X rays. We show that the ionization thus produced can be significant at altitudes as low as the upper stratosphere. Because of the length of time over which this ionization is produced globally, it is expected to have a substantial impact on atmospheric chemistry. Global power inputs to the auroral regions due to precipitating particles have been computed for each day of the storm using PEM X-ray and particle observations. On November 4, 1993, the day that includes the main and early recovery phases of the storm, the northern hemispheric power calculated by in situ measurement of the electron fluxes was 56.8 GW; that determined by using the X-ray images was 47.4 GW, These values may be compared with the northern hemisphere Joule heating rate of 290 GW on the same day computed using the PEM magnetometer and electron flux measurements.

UARS observations of Birkeland currents and Joule heating rates for the November 4, 1993, storm

Magnetic field and particle observations from the Upper Atmosphere Research Satellite particle environment monitor (UARS/PEM) are used to estimate field-aligned currents, electron precipitation energy flux, ionospheric conductivities, and Joule heating rates during the main phase of the November 4, 1993, geomagnetic storm. From 0300 to 1200 UT on November 4 the auroral oval expanded equatorward of 65° magnetic latitude (MLAT), and UARS encountered the polar cap on seven consecutive passes during the storm main phase. These passes provide data appropriate to determine field-aligned currents and estimate ionospheric Joule heating. For this storm, UARS sampled the midnight to dawn sector in the northern hemisphere and the noon to dusk sector in the southern hemisphere. The maximum net currents on the day side and nightside are comparable and reach 1 A/m for several hours. The average Joule heating rates are comparable at midnight, early morning, and noon, where they are 9.2, 6.6, and 7.7 GW/h, respectively, but have a strong peak in the late afternoon, where they are 25.6 GW/h. In contrast, the electron precipitation energy deposition is highest near midnight at 5.6 GW/h but drops to less than half this level to 2.4 GW/h and 1.9 GW/h in the early morning and at dusk, respectively, but is very small near noon, only 0.24 GW/h. The Joule to particle energy deposition rate ratio thus varies by roughly an order of magnitude with local time, being over 40 near noon, about 20 at dusk, 3 near dawn, and 2 at midnight. The hemispherical Joule and electron precipitation heating rates, HJ and Helec, are estimated to have been 290 GW and 50 GW, respectively, giving HJ/Helec = 4.5 and HJ + Helec = 340 GW. Differences between these averages and assimilative mapping of ionospheric dynamics (AMIE) results, HJ = 200 GW and Helec = 80 GW, reflect time variability during the storm and are largely resolved when AMIE results only at the times of UARS passes are considered.

Observations of the UARS Particle Environment Monitor and computation of ionization rates in the middle and upper atmosphere during a geomagnetic storm

During the geomagnetic storm of 8–9 November 1991, the Upper Atmosphere Research Satellite (UARS), orbiting at 585 km, passed through the expanded auroral zone during portions of several successive orbits. The instruments of the UARS Particle Environment Monitor (PEM) observed particle precipitation, auroral x-ray emissions, and large-scale disturbances in the geomagnetic field during this time. In this paper we present observations made by the PEM instruments during this storm and compute ionization and energy deposition rates as functions of altitude in the middle and upper atmosphere by incident electrons and positive ions in the storm interval. The suite of PEM instruments provides a systematic measurement of energetic particles and their associated x-rays over an energy range not fully covered by previous satellite missions. In this energy range the ionization and dissociation rate in the atmosphere may be inferred from the upper stratosphere to the thermosphere. The impact on the chemistry of the region may be investigated.

Identification of auroral oval boundaries from in situ magnetic field measurements

We have conducted an investigation into the relationship between magnetic field fluctuations below 100 Hz observed with the UARS and Freja magnetic field experiments and concluded that increases in this “ac” activity serve as an excellent indicator of the boundaries of large-scale field-aligned current systems. Magnetic field fluctuations in these regions generally correspond to increased electron fluxes at energies below roughly 2 keV. Using a single equatorward boundary crossing as a reference point, the statistical field-aligned current pattern of Iijima and Potemra [1978] for disturbed magnetic conditions can be extrapolated to provide an estimate of the global position of the auroral oval. Real-time in situ magnetic field measurements from high-latitude spacecraft can be used to provide a quick, simple, locator of the equatorward boundaries of the large-scale field-aligned currents, accurate to within ± 2° in latitude. A proof-of-concept implementation of this technique has been performed using magnetic field data acquired by the Freja spacecraft. Development of the automated detection algorithm, essentially a measurement of the standard deviation of the low-frequency magnetic field measurements, was aided by using UARS magnetic field data. The UARS data were binned by MLT and magnetic latitude for varying Kp conditions and demonstrated observable ac boundaries at all local times and all Kp levels. In a separate statistical study consisting of 96 UARS high-latitude observations, we determined the differences between the observed and estimated equatorward boundaries of the large-scale field-aligned current system for each high-latitude pass. This difference was within 2 deg of latitude in 53 of 96 cases, with 84 of 96 cases within 5 deg.

Nitric oxide variations in the mesosphere and lower thermosphere during the November 1993 storm period

The variability of nitric oxide has not been fully quantified. Particle precipitation is known to enhance nitric oxide; however, a detailed understanding of this process is lacking, and its importance has not been quantified. In this paper, we present nitric oxide measurements between 80 and 200 km altitudes from the Halogen Occultation Experiment (HALOE) on UARS during the November 1993 space weather special study period, near solar minimum conditions. The nitric oxide mass mixing ratio routinely varies by 3 orders of magnitude above 80 km altitudes. The variation of NO with magnetic activity is discussed, using two measures of magnetic activity level. The first is the well-known Kp index, and the second is derived from the particle environment monitor (PEM) on UARS. The Kp index varied between 0 and 7 during the study period. A detailed comparison of NO enhancements with ionization rates derived from the PEM data indicates that the particle precipitation can account for much of the NO. In the context of the November 1993 storm period, this study is interesting because it emphasizes the potential importance of space weather for the entire atmosphere and not only regions above 100 km altitude.

Validation of UARS particle environment monitor electron energy deposition

One of the primary goals of the particle environment monitor (PEM) on the Upper Atmosphere Research Satellite is the determination of the solar and magnetospheric particle energy inputs to the upper and middle atmosphere. This is accomplished by measuring the incident particle spectra and computing energy deposition and ionization rates as functions of altitude. The particle spectral measurements are made by detectors covering the energy range 5 eV to 5 MeV for electrons and 5 eV to 150 MeV for protons. An X ray camera provides global-scale images and energy spectra of ∼3 to 100 keV bremsstrahlung X rays produced by electrons incident on the atmosphere. Thus PEM measures particles which lose their energy in an atmospheric region extending from the thermosphere down to ∼50 km, with the altitude of associated bremsstrahlung X rays reaching ∼25-km levels. PEM validation documented in this report has involved an assessment of the data quality of each PEM component instrument, intercalibration of individual instruments, and validation of the computations of the rates of electron energy deposition into the atmosphere. The energy deposition rate calculations are validated against existing energy deposition codes over the complete energy range of PEM measurements.

Variations of the thermospheric nitric oxide mass mixing ratio as a function of Kp, altitude, and magnetic local time

In this letter, data from the Halogen Occultation Experiment (HALOE) and the Particle Environment Monitor on the Upper Atmospheric Research Satellite are presented. Five years of data from each instrument have been used to create high-latitude maps of Nitric Oxide (NO) mass mixing ratio and electron impact and bremsstrahlung ionization rates as a function of altitude and Kp. We show that enhancements in NO at 100 km are coupled to the auroral oval and to increases in the local ionization rate, with both the peak in the ionization rate and NO mass mixing ratio occuring in the dawn magnetic local time sector between 60° and 70° magnetic latitude. Below this altitude, the NO is shown to be enhanced at lower latitudes and sunward of the dawn-dusk meridian, most likely due to solar illumination. Above 100 km, the NO density enhancement stays in approximately the same MLT sector, while the peak ionization rate expands in MLT towards the night side. This is attributed to a build up of NO created by particle precipitation during the night, which is corotated towards the dawn region.