The effect of Joule heating on the density of nitric oxide in the thermosphere was studied using observations from the Student Nitric Oxide Explorer (SNOE) satellite and model calculations from the Thermospheric Ionosphere Electrodynamics General Circulation Model for a Joule heating event that occurred on 25 September 1998. Model results and SNOE observations from fifteen orbits were compared in the latitude range 82°S to 82°N over the altitude range 100–150 km. Joule heating which occurred in the 55°‐ to 60°‐latitude region produced a meridional wind blowing equatorward and a gravity wave propagating equatorward, which caused an increase in the temperature of the thermosphere in the 20°‐ to 75°‐latitude region. When the heated thermosphere was illuminated by solar radiation, the density of nitric oxide increased over this entire latitude region because of a temperature‐sensitive reaction between ground state nitrogen atoms and molecular oxygen. In the 24 hours following the Joule heating event, the increased nitric oxide diffused downward from the 150‐km region to the 110‐km level of the thermosphere.

The nitric oxide (NO) density in the lower thermosphere has been calculated by a photochemical model for NOx and compared with measured NO densities from Student Nitric Oxide Explorer (SNOE). At higher latitudes the most important contributor for NO density increases is energetic electron precipitation. The electron energy is estimated in two ways, from auroral ultraviolet (UV) and X‐ray measurements obtained from Ultraviolet Imager (UVI) and Polar Ionospheric X‐ray Imaging Experiment (PIXIE) on board the Polar satellite and from ground magnetometer measurements. For the time intervals when the Polar satellite was not above the northern hemisphere, a parameterization of the electron energy flux from ground magnetic measurements was used. This parameterization was based on data from the SuperMAG database compared to UVI/PIXIE derived electron energy fluxes. The negative perturbation in the northward ground magnetic component is found to be linearly related to the precipitating electron energy flux. The 4‐day period studied is from 30 April (day 120) until 4 May 1998, where the onset of a geomagnetic storm occurred 2 May (day 122). The results of the comparisons show an overall larger modeled nitric oxide density at auroral latitudes than what was measured by SNOE. The largest discrepancies were for the day of the storm onset, when the background atmosphere was more distorted by Joule heating. The next day the agreement between the model and the observations was far better, which might be due to less amount of Joule heating this day.

We compare the ionospheric electron precipitation morphology and power from a global MHD simulation (GUMICS-4) with direct measurements of auroral energy flux during a pair of substorms on 28-29 March 1998. The electron precipitation power is computed directly from global images of auroral light observed by the Polar satellite ultraviolet imager (UVI). Independent of the Polar UVI measurements, the electron precipitation energy is determined from SNOE satellite observations on the thermospheric nitric oxide (NO) density. We find that the GUMICS-4 simulation reproduces the spatial variation of the global aurora rather reliably in the sense that the onset of the substorm is shown in GUMICS-4 simulation as enhanced precipitation in the right location at the right time. The total integrated precipitation power in the GUMICS-4 simulation is in quantitative agreement with the observations during quiet times, i.e., before the two substorm intensifications. We find that during active times the GUMICS-4 integrated precipitation is a factor of 5 lower than the observations indicate. However, we also find factor of 2-3 differences in the precipitation power among the three different UVI processing methods tested here. The findings of this paper are used to complete an earlier objective, in which the total ionospheric power deposition in the simulation is forecasted from a mathematical expression, which is a function of solar wind density, velocity and magnetic field. We find that during this event, the correlation coefficient between the outcome of the forecasting expression and the simulation results is 0.83. During the event, the simulation result on the total ionospheric power deposition agrees with observations (correlation coefficient 0.8) and the AE index (0.85).

Previous studies have shown the connection between electron precipitation and the excess amounts of nitric oxide at auroral latitudes. In this study the electron energy deposition derived from thermospheric nitric oxide (NO) measurements is compared with the electron energy deposition derived from X‐ray bremsstrahlung measurements. The electron energy deposition is derived from nitric oxide densities by use of a photochemical model for nitric oxide and is referred to as the modeled energy deposition. The comparisons are made for the beginning of five geomagnetic storms in 1998: 21 March, 2 May, 14 June, 26 June, and 16 July. By using these quite different methods to derive the total electron energy deposition (4–100 keV), the results show that we have a generally good understanding of the physics and chemistry of the energy transfer from electron precipitation in the lower thermosphere. The comparisons also show some discrepancies. The modeled energy deposition is typically larger than the energy deposition derived from X‐ray bremsstrahlung in the beginning of the storm period, whereas later on in the storm the energy deposition derived from X‐ray measurements is largest. The cases where the modeled energy deposition is largest is probably due to production of NO occurring before the bremsstrahlung measurements. The systematic underestimate of the calculated energy deposition could be due to uncertainties in the reaction rates or in the characteristic electron energy used in the photochemical model. The effects from horizontal neutral wind on the NO gas from the production on the nightside to the observations on the dayside can also be a source of discrepancy.

Global observations of nitric oxide obtained from the Student Nitric Oxide Explorer (SNOE) have been used to determine the density of nitric oxide in the lower thermosphere. A thermospheric model which takes into account all of the photochemical processes that produce and destroy nitric oxide has been used to deduce the flux of precipitating auroral electrons that produce nitric oxide. The results of this analysis allow us to infer the daily variability of precipitating electron energy deposition. The most important result of this paper is that there is a clear‐cut minimum in the electron flux at the time of the summer solstice in both hemispheres. This seasonal dependence of precipitating auroral electrons supports the idea that auroral arcs are suppressed in the solar‐illuminated hemisphere due to high ionospheric conductivity. The strength of the seasonal variations suggests that the bulk of the energy deposition may be due to discrete auroral events.

Nitric oxide (NO) densities at heights between 96 and 150 km in the Earth's upper atmosphere are directly compared with the energy deposition from precipitating energetic electrons. The comparisons are done for the beginning of a geomagnetic storm event on 2 May 1998. The electron energy is derived from X‐ray bremsstrahlung observations from the Polar Ionospheric X‐ray Imaging Experiment (PIXIE) on board the Polar spacecraft. Measurements of the NO density are performed by the Student Nitric Oxide Explorer (SNOE) on the dayside by measuring airglow spectral features of the NO γ‐band. Since a significant part of the electron precipitation takes place during the night, and considering the long lifetime of NO, we have accumulated the X‐ray data in geographical boxes. This enables us to follow the development of the total energy deposition over a specific area during the night and morning hours. In agreement with theoretical predictions we find an increase in NO at higher latitudes due to electron precipitation. At 106 km altitude, which is found to be the average altitude of the peak values of both NO intensity and precipitating electron energy, ∼83% of the NO density is produced by electron precipitation. At this altitude we find that the electron precipitation results in the production of ∼8 NO molecules per keV deposited energy. The comparison of the data also shows effects of a horizontal neutral wind. Above 100 km the peak in NO density is displaced equatorward of the peak in electron energy deposition.

A time‐dependent thermospheric model has been used to calculate the nitric oxide density in the lower thermosphere for a 935‐day period, 11 March 1998 to 30 September 2000. This model uses daily values of the observed solar soft X‐ray irradiance (2–7 nm) as an energy input parameter. The model does not include an energy input from auroral electron precipitation. The results of the model calculation of nitric oxide density at 110 km were compared with observations of nitric oxide density made with the Student Nitric Oxide Explorer (SNOE) for the 935‐day period. At the equator the model calculations and the observations agree very well with a linear correlation coefficient of 0.876. The correlation coefficient remains high for the altitude region 107–117 km, the region where solar soft X‐rays (2–7 nm) are the major source of nitric oxide production. The comparison of the model calculations with the observations as a function of latitude show that there is excess nitric oxide poleward of 30°N and S latitude particularly during the fall‐winter season. We believe that the source of this excess nitric oxide is the nitric oxide that is produced in the auroral region (65°–75°N and S geomagnetic latitude) by precipitating auroral electrons. We believe that aurorally produced nitric oxide is transported equatorward by horizontal winds. At midlatitudes the excess nitric oxide decays to about half of its initial value in one day. At times of large geomagnetic storms we believe that aurorally produced nitric oxide is transported all the way to the equator by horizontal winds. The excellent correlation of the model calculations and the SNOE observations of nitric oxide at 110 km between 30°S and 30°N support the hypothesis that solar soft X‐rays are the source of the variability of nitric oxide in the thermosphere at low latitudes.

The Cassini Ultraviolet Imaging Spectrograph (UVIS) is part of the remote sensing payload of the Cassini orbiter spacecraft. UVIS has two spectrographic channels that provide images and spectra covering the ranges from 56 to 118 nm and 110 to 190 nm. A third optical path with a solar blind CsI photocathode is used for high signal-to-noise-ratio stellar occultations by rings and atmospheres. A separate Hydrogen Deuterium Absorption Cell measures the relative abundance of deuterium and hydrogen from their Lyman-α emission. The UVIS science objectives include investigation of the chemistry, aerosols, clouds, and energy balance of the Titan and Saturn atmospheres; neutrals in the Saturn magnetosphere; the deuterium-to-hydrogen (D/H) ratio for Titan and Saturn; icy satellite surface properties; and the structure and evolution of Saturn’s rings.

Nitric oxide density in the lower thermosphere (97–150 km) has been measured from the polar‐orbiting Student Nitric Oxide Explorer (SNOE) satellite as a function of latitude, longitude, and altitude for the 2 1/2 year period from 11 March 1998 until 30 September 2000. The observations show that the maximum density occurs near 106–110 km and that the density is highly variable. The nitric oxide density at low latitudes correlates well with the solar soft X‐ray irradiance (2–7 nm), indicating that it is the solar X‐rays that produce thermospheric nitric oxide at low and midlatitudes. Nitric oxide is produced at auroral latitudes (60°–70° geomagnetic) by the precipitation of electrons (1–10 keV) into the thermosphere. During high geomagnetic activity, increased nitric oxide may be present at midlatitudes as the result of meridional winds that carry the nitric oxide equatorward.

[1] Three years (March 1998 through March 2001) of nitric oxide ( NO) observations in the Northern Hemispheric thermosphere (90-170 km) as made by the Student Nitric Oxide Explorer (SNOE) spacecraft are compared in a broad statistical analysis with the daily-averaged northern auroral bremsstrahlung x-ray observations, which are taken to be a good proxy for the population of precipitating energetic electrons. The latter are made by the Polar Ionospheric X-ray Imaging Experiment (PIXIE) on board the NASA GGS Polar spacecraft. A modest correlation between these two data sets is found, indicating that about 20-40% of the variation in the number density of thermospheric nitric oxide at high latitudes is caused by variations in the precipitation of energetic particles from the magnetosphere into the auroral regions of the ionosphere. Time delays between the two data sets are examined in this study, as well as the altitude profile of the nitric oxide observations. Differences in the response and recovery times in the two data sets are also carefully considered along with hemispheric asymmetries as a function of season, leading to stronger correlations which are then discussed in terms of the properties of the data sets used.

A model of NO abundance in the lower thermosphere is described. The model includes time dependence, an energetic electron flux calculation that includes transport, neutral and ion photochemistry, and vertical diffusion. We show that a steady state calculation is inadequate for calculating NO abundance. We examine the relationship between observed NO abundance and the integrated energy input to the lower thermosphere over the previous day. It is shown that the relationship between the energy input and the NO abundance varies with the local time of the NO measurement and with the length of daylight. These dependencies arise due to the role of photodissociation as a loss mechanism for NO. This model is designed for analysis of NO observations and will be used in the analysis of observations by the SNOE spacecraft [Barth et al., 1999].