The Solar, Anomalous, and Magnetospheric Particle Explorer, SAMPEX, will carry out energetic particle studies of outstanding scientific questions in the fields of space plasma physics, solar physics, magnetospheric and middle atmospheric physics, and cosmic ray physics. SAMPEX will measure the electron and ion composition of energetic particle populations from ∼0.4 MeV/nucleon to hundreds of MeV/nucleon from a zenith‐pointing small satellite in near‐polar orbit, using a coordinate set of detectors with excellent charge and mass resolution, and with higher sensitivity than previously flown instruments. While over the magnetic poles, the instruments will study the composition of anomalous cosmic rays, solar energetic particles, and galactic cosmic rays. At lower magnetic latitudes, geomagnetic cutoff effects will allow determination of the ionization state of these particles at energies much higher than can be studied from interplanetary spacecraft. At subauroral latitudes, SAMPEX will also observe precipitating relativistic magnetospheric electrons, which undergo important interactions within the middle atmosphere.

The sensitivity of the middle atmospheric temperature and circulation to the treatment of mean-flow forcing due to breaking gravity waves at the sub-grid scale was investigated using the University of Illinois at Urbana-Champaign 40-layer General Circulation Model (GCM). The gravity-wave forcing was represented either by Rayleigh friction or by a detailed parameterization scheme with different sets of parameters. The modeled middle atmospheric temperature and circulation exhibit large sensitivity to the parameterized sub-grid gravity-wave forcing. A large warm bias of up to 50°C was found in the model's summer upper mesosphere and lower thermosphere. This warm bias was caused by the inability of the GCM to simulate the reversal of the zonal winds from easterly to westerly crossing the mesopause in the summer hemisphere. Attempts were made to slow down the easterly winds near the mesopause and to reduce the warm bias. The GCM was able to realistically simulate the semi-annual oscillation in the upper stratosphere and lower mesosphere with observational constraints on certain parameter values, but failed to simulate the quasi-biennial oscillation in any of the experiments. Budget analysis indicates that in the middle atmosphere the forces that act to maintain a steady zonal-mean zonal wind are primarily those associated with the meridional transport circulation and breaking gravity waves. Contributions from the interaction of the model-resolved eddies with the mean flow are secondary.

We have introduced additional NOy sources caused by energetic electron precipitation (EEP) during 1987 into a Chemistry‐Climate model. Comparison of two model runs with and without EEP reveals increase of reactive nitrogen by about 2 ppbv in the middle stratosphere over the tropical and middle latitudes. In the upper stratosphere over the polar winter regions the simulated NOy enhancement reaches 10 ppbv. Decreases of the ozone mixing ratio in the stratosphere by up to 5% over midlatitudes and up to 30% over southern high‐latitudes are calculated. A ∼0.5 K cooling in the middle stratosphere over the tropics and up to 2 K over southern high‐latitudes is calculated with detectable changes in the surface air temperatures. These results confirm that the magnitude of the atmospheric response to EEP events can potentially exceed the effects from solar UV fluxes. These mechanisms work in phase outside polar latitudes, but can compensate each other within polar latitudes.

Mesospheric nitric oxide as observed by the Halogen Occultation Experiment (HALOE) and calculated by the Langley Research Center two‐dimensional chemical transport model are compared on a daily and collocated basis for the period 920101 through 971231. Results show excellent agreement when energetic electron precipitation (EEP) from the outer trapping region of the magnetosphere is included. A simulation using only an upper boundary condition derived from the HALOE, but not explicitly including EEP, is deficient in NO at altitudes above 60 km. The contribution to the stratosphere of odd nitrogen formed by EEP in the mesosphere is significant and is approximately three times the contribution due to the HALOE upper boundary condition (HUBC) which approximates auroral electron precipitation, and solar EUV and solar X‐ray effects on NO formation.

Recent work calls into question the adequacy of using atmospheric models with an upper boundary near 60 km for interpretive and prognostic studies of stratospheric odd nitrogen (NOY = NO + NO2 + 2 × N2O5 + HNO3 + CINO3 + NO3) and ozone (O3). Studies have shown that the effects of energetic particle precipitation occurring within the mesosphere and the lower thermosphere (MLT, roughly 60–110 km) ultimately affect stratospheric NOY and, calculations suggest, O3. The magnitude of these effects— which are solar cycle‐dependent—on the stratosphere is calculated to be comparable with those due to solar flux variations associated with the 11‐year solar activity cycle. Accounting for the effects of these precipitating particles requires models with an upper boundary no lower than 100 km. Also, the polar night observations of the species required to unambiguously characterize this solar and particle‐driven coupling between atmospheric regions have not been made and are not yet planned.

Model derived sensitivities of O3 near 40 km to solar UV flux changes (11‐year and 27‐day) fall within the range 0.36–0.55 with S11y ≈ S27d. These sensitivities derived from observations have been reported to be 0.91 and 0.39–0.46, respectively. This discrepancy appears to be due to long‐term (2–10 years) fluctuations in stratospheric NO2 in the 1980s. Such fluctuations, if not taken into account in the analysis of the data, could lead to high values of the sensitivity for the 11‐year activity cycle but not for the 27‐day solar rotation period. Simulations accounting for such effects suggest a consistency between theoretical values of S11y and those derived from data if NO2 effects are included. The appropriate values are approximately 0.48, a value similar to published model calculations and results from the analysis of data from the 27‐day solar rotation period. Verification of this suggestion awaits an NO2 data base of sufficient quality and length to permit solar‐cycle trend calculations to be made.

An analysis has been carried out of the effects of energetic electron precipitation (EEP) on stratospheric NOy, NO2, and O3. Solar wind observations used together with precipitating electron fluxes observed aboard TIROS spacecraft show a close relationship between the long‐ and short‐term fluctuations in the solar wind and EEP over a period of 16 years. Daily electron energy spectra for 4.25≤E≤1050 keV and energy deposition profiles are developed for both hemispheres for L≥5 and used in two‐dimensional chemical transport simulations for the period January 15, 1979, through December 31, 1987. Results indicate that globally averaged column NOy (from 25 to 40 km) increases by ≈ 12% between 1979 and 1983–1985 with a rapid decline to 1979 levels between early 1985 and 1987. Day‐by‐day comparisons of the results with the Stratospheric Aerosol and Gas Experiment (SAGE II) column NO2 and O3 for the period October 24, 1984, and December 31, 1987, show good agreement with the inclusion of EEP in the simulations. Northern near‐hemispheric decreases of column NO2 of ≈ 35% observed by SAGE II between early 1985 and 1987 are well simulated with the inclusion of EEP. Comparisons of several simulations with one another and with SAGE II NO2 data and Solar Backscattered Ultraviolet (SBUV) (V6) O3 data suggest that SOLACE represents a solar‐ terrestrial coupling mechanism which, for solar cycle 21, is as important to stratospheric O3 as solar UV flux variations.

An analysis is carried out of the effects on middle atmospheric NOy and O3 of a coronal mass ejection (CME) event which occurred on May 12, 1997, and which is coupled with observed solar wind fluctuations. Observations of electron fluxes by instruments aboard the SAMPEX and NOAA 12 satellites indicate large enhancements of magnetospheric electron fluxes occurring with the arrival of the high‐speed solar wind. Calculations suggest that significant formation rates of NOy should occur in the mesosphere and the lower thermosphere at mid to high latitudes. Halogen Occultation Experiment (HALOE) NO observations reveal increases of more than an order of magnitude between 85 and 120 km in both hemispheres within 1–2 days after the electron flux increases. Two dimensional chemical transport simulations were carried out to assess the fate of the NOy increases. Northern hemispheric increases were lost to photochemical destruction shortly after the event ended. Southern hemispheric increases were transported in part into the stratosphere by advective descent. By October 1997, high‐latitude NOy increases of 20–40% were calculated near 25 km leading to O3 reductions of up to ≈ 20% when compared to a simulation with no electron precipitation. A solar atmospheric coupling by electrons precipitating from the outer trapping and auroral regions of the magnetosphere, and which affects middle atmospheric NO, is clearly demonstrated by the observations alone.

NOAA 12 precipitating electron data has been examined together with ISAMS observations of nighttime NO2 during electron events in November 1991 and May 1992. Energy deposition calculations suggest the significant formation of NOy during these events near 0.1 mbar and for 60°–80°latitude. ISAMS confirms increases of NO2 from zonally averaged values of ≈1–5 ppbv prior to the events to values in excess of 30–40 ppbv with local maxima in excess of 140 ppbv. The increases occur within 2–3 days and are generally consistent with the calculated NOy formation rates due precipitating electrons. ISAMS data indicate the subsequent transport of the NO2 into the stratosphere affecting NO2 mixing ratios below 1 mbar and down to latitudes of ≈30°to 40°in both hemispheres.

A geoeffective magnetic cloud impacted the Earth early on 15 May 1997. The cloud exhibited strong initial southward interplanetary magnetic field (B(z)similar to-25 nT), which caused intense substorm activity and an intense geomagnetic storm (Dst similar to-170 nT). SAMPEX data showed that relativistic electrons (E greater than or equal to 1.0 MeV) appeared suddenly deep in the magnetosphere at L=3 to 4. These electrons were not directly "injected" from higher altitudes (i.e., from the magnetorail), nor did they come from an interplanetary source. The electron increase was preceded (for similar to 2 hrs) by remarkably strong low-frequency wave activity as seen by CANOPUS ground stations and by the GOES-8 spacecraft at geostationary orbit. POLAR/CEPPAD measurements support the result that high-energy electrons suddenly appeared deep in the magnetosphere. Thus, these new multi-point data suggest that strong magnetospheric waves can quickly and efficiently accelerate electrons to multi-MeV energies deep in the radiation belts on timescales of tens of minutes.

The SAMPEX satellite, an international collaboration with Germany, is the first in a series of small explorer missions that NASA began in 1989 to perform astrophysics and space physics investigations with small spacecraft launched on expendable launch vehicles. SAMPEX was launched from VAFB on a Scout rocket in July 1992, just 39 months after selection by NASA. Operating in an 82/spl deg/ inclination orbit with altitudes between 520 and 670 km, the 350 lb spacecraft has performed flawlessly since launch. The spacecraft bus was developed by the Small Explorer project at Goddard Space Flight Center. SAMPEX carries a payload of four scientific instruments that study particles originating at the Sun, in the so-called anomalous cosmic rays, and in the magnetosphere. The SAMPEX instruments have sensitivities >100 times larger than previous low Earth orbit spacecraft, that have led to new discoveries such as a new radiation belt of interstellar material and rare hydrogen and helium isotopes trapped in the radiation belts. SAMPEX provides routine global maps of the magnetosphere, and has given new insights into the processes by which radiation levels through the entire magnetosphere can become greatly enhanced, leading to operating spacecraft anomalies. The authors give an overview of the SAMPEX scientific goals, instrumentation, and mission development approach.

Daily odd nitrogen formation due to energetic electron precipitation has been calculated for 840101 through 841231. Electron energy spectra between 4.25 and 1050 keV were used as derived from TED and MEPED observations made aboard NOAA spacecraft. Results indicate the importance of electron precipitation to the middle atmospheric NOy budget between 70 and 111 km. The column NOy source for 42≤Z≤111 km and for invariant latitudes between 63° and 76° in both hemispheres is calculated to be ≈13 times that of the global source of NOy derived from N2O. This source is larger in the southern hemisphere and is dominated by electrons which deposit their energy above 80 km.