The goal of the proposed work was to understand the latitude structure of nitric oxide in the mesosphere and lower thermosphere. The problem was portrayed by a clear difference between predictions of the nitric oxide distribution from chemical/dynamical models and data from observations made by the Solar Mesosphere Explorer (SMEE) in the early to mid eighties. The data exhibits a flat latitude structure of NO, the models tend to produce at equatorial maximum. The first task was to use the UARS-HALOE data to confirm the SME observations. The purpose of this first phase was to verify the UARS-NO structure is consistent with the SME data. The next task was to determine the cause of the discrepancy between modeled and observed nitric oxide latitude structure. The result from the final phase indicated that the latitude structure in the Photo-Electron (PE) production rate was the most important

Fuller-Rowell, T.J.

English

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Title: Understanding the latitude structure of nitric oxide in the mesosphere
and lower thermosphere
Agency: NASA PI: T.J. Fuller-Rowell Award: NAG5-2753
The goal of the proposed work was to understand the latitude structure of nitric oxide in the
mesosphere and lower thermosphere. The problem was portrayed by a clear difference
between predictions of the nitric oxide distribution from chemical/dynamical models and
data from observations made by the Solar Mesosphere Explorer (SME) in the early to mid
eighties. The data exhibits a flat latitude structure of NO, the models tend to produce at
equatorial maximum.
The first task was to use the UARS-HALOE data to confirm the SME observations. Data
from the UARS-HALOE instrument was successfully extracted from the database for the
years 1992 -1995 inclusive. These data were converted from mixing ratios to absolute
number densities, and sorted for periods surrounding the equinoxes (one month either side)
for both sunrise and sunset periods. Maps of NO density as a function of latitude and
height, from 70 to 130 km were created for each equinox for both sunrise and sunset.
The purpose of this first phase was to verify the UARS NO structure is consistent with the
SME data. One of the fundamental differences between the UARS and SME observations
is the sampling. SME, in polar orbit, recorded the latitude structure in the afternoon sector
each overpass. UARS, however, in its inclined orbit only builds the latitude picture at
sunrise and sunset over a period of about a month. The structure is therefore modulated by
any change that has occurred over that period in the production sources, such as EUV flux.
The data must therefore be sorted as a function of solar activity, and, if the data is over a
short period, it must also be sorted by geomagnetic activity. Sufficient data now exists that
the equinox structure can be obtained by sorting only by solar activity.
Another difference in the UARS-HALOE data is that all longitudes are sampled. A picture
of the latitude/longitude distribution can be obtained at sunrise and sunset. This
presentation of the data clearly shows the response to geomagnetic or auroral activity. In
the longitude sector of the magnetic pole, in both north and south hemispheres, the
signature of auroral production penetrates well into mid latitudes in the geographic frame.
It is therefore important when averaging in longitude to use the geomagnetic coordinate
system. This was done for the figures presented in this report.
A useful consequence of the sampling in latitude and longitude is that it is possible to
search for a signature of the South Atlantic Anomaly (SAA). This region has been known
for decades to contain significantly higher fluxes of energetic proton and electron
precipitation. What has not been quantified is the response to this source. The UARS NO
data was analyzed at various altitudes in the mesosphere and lower thermosphere to
determine if any signature of the SAA was present; none could be found.
The important conclusion from this first phase of the data analysis was that the HALOE
sunset data showed close agreement with the 3.00 pm SME data This is illustrated in
Figure 1, which shows both the latitude structure and the absolute magnitudes are in
excellent agreement. Small differences were also seen in the sunrise and sunset structure.
https://ntrs.nasa.gov/search.jsp?R=19980018101 2020-06-16T01:06:10+00:00Z
Thenexttaskwasto determinethecauseof thediscrepancybetweenmodeledandobserved
nitric oxidelatitudestructure.Progressin modelinghasinvolved,firstly, the
implementationof arevisedsolarflux modelfor softX-ray flux. This newEUV model
incorporated atarecordedby theSOLRADsatellite.Thedatawascompletelyrecalibrated
andrevealedsoftX-ray fluxessignificantlylargerthanpreviousempiricalsolarflux
models.It hasbeenshownby manyauthorsthatthesoftX-raysareresponsiblefor a large
partof theNO productionatmid andlow latitudes.
Thisnewsolarflux modelwasincorporatedinto the2Dzonally-averagedupper
mesosphereandthermospheremodelto investigatetheimpacton thesimulatedlatitude
structureof NO. Theresultsshowedaslightlyweakerlatitudestructurethanwasapparent
usingtheoriginalHintereggerspectrum,improvingtheagreementbetweenthemodeland
SMEandUARSdatasomewhat,but themodelstill showedasignificantequatorial
maximum.
Toperformmorecomprehensivemodelsensitivitystudiesaone-dimensionalphoto-
chemicaldiurnalmodelwasused.The 1Dmodelwasrun for aseriesof latitudesto build a
pictureof thelatitudestructure.Themodelwasusedto investigatethe impactof three
changesin theoddnitrogenchemistry.Theseinclude:useof thenewJPLratesfor the
reactionN(4S)+O2_ NO+O;theeffectof increasedphotolysisof NO by S-Rbands;and
theeffectof usingfastercollisionaldeactivationratefor reactionN(+S)+O2_ NO+O.
Figure2 illustratestheeffectof eachof theseprocesses.Thecombinationhasclearlylead
to aflatteningof the latitudevariationof theNO densityattheheightof thepeaknear110
km. Thelowerpanelshowstheequivalentheightlatitudeassembledfrom theseriesof 1D
simulationsat aseriesof latitudes.This workwaspresentedattheAGU fall meeting1995
(Fuller-Rowellet al. 1995). Notethatthe1Dmodeldoesnot includeauroralprecipitation
sothatthehigh latitudeincre_,seis notproducedor expected.
At first sightit appearstheproblemcanberesolvedby adjustingoddnitrogenreaction
rates.Whenthisknowledgewasincorporatedintoathree-dimensional,chemical/
dynamicalmodeltheresultscouldnotbeduplicated.Thethreedimensionalmodelclearly
differsfrom the 1Dmodelin anumberof ways. Theneutralatmosphereandtemperatures
arecalculatedself-consistentlyratherthanrelyingonMSISempiricalmodel;horizontal
advectionis includedto transporthe long-livedchemicalspecies;the3Dmodelusesavery
simpletreatmentfor estimatingthephoto-electronionizationrates;andthe3Dmodel
includesauroralproductionwith theappropriateoffsetbetweenthegeomagneticand
geographicoordinatesystems.The final phaseof theprojectwasto determinewhichof
thesedifferenceswasthecauseof thedifferencebetweenthe3Dmodelandthedata/1D
model. Thetheoryfor thedependenceof PEionizationonNO structurewaspresentedat
theBirminghamCOSPAR(Fuller-Rowellet al., 1996).
Theresultsfrom thef'malphaseindicatedthatthe latitudestructurein thephoto-electron
(PE)productionratewasthemostimportant. In theearlyversionsof the3Dcodethe
assumptionwasmadethattheratiobetweensolarphotonionizationandsecondaryPE
ionizationwassimplyafunctionof opticaldepth;this translatesto apressurelevel
dependenceonly. The 1Ddimensionalcodewasnotsorestrictedby computerresources
sousedamoresophisticatedPEproductionalgorithm.Examinationof the latitude
structureof theratioof photon/PEionizationratesin the1Dmodelrevealedalatitude,or
zenithangle,dependencein additionto thepressurelevelvariation. Thetheoryis clear:
photonionizationrates,neartheNO peakheightaround110kin, arisefrom photonswith
wavelengthsfrom softX-raysto 102.nm. SoftX-rays,producingthePEspectrum,tend
to havesmalleropticaldepthscomparedto thelongerwavelengthphotonsource.This
leadsto aflatterlatitudestructurein thePEionizationsourceandintroducesalatitude
dependencein theratioof photon/PEionizationrates.This latitudedependencein thePE
ionizationrateswasparameterizedfromthe 1Dphoto-electrontransportcodeasa function
of solarzenithangle,solarcycle,andintensityof X-ray flux, andwasincorporatedinto the
3Dmodel. Usingthisparameterizedphoto-electronionizationratemodelthe3D
simulationswereabletoproduceamuchflatterlatitudestructurefor nitricoxide,in much
betteragreementwith theSMEandUARS-HALOEdata.
Althoughit now appearsthatchemistryandthelatitudestructureof the primary ionization
sources can resolve much of the NO problem, the other primary influence on the latitude
structure is transport by the wind field. This aspect was pursued by identifying empirical
zonally-averaged wind models to incorporate into the model. Three wind models were
identified. The first, and most obvious is the CIRA wind model assembled from an
amalgamation of thermospheric and middle atmosphere observation. The second is a
model assembled from many years of ground-based wind observations from meteor traces
and partial reflection techniques, by Portnyagin and Soloveva. The model covers altitudes
from 70-110 km so must be merged either with the CIRA model at higher altitudes of with
the modeled structure. The third wind model identified as a potential source for this study
is that constructed from the UARS WINDII data by Charles McLandress. This model has
its attractions since it is consistent, in time, with the observing periods used to comtruct the
NO distribution. The altitude range covered by the model is limited to the 70-110 km by
the lack of suitable emission features in the WINDII observations. Although altering the
wind field did introduce additional structure in the latitude distribution of NO, it did not
significantly improve the agreement between model and data at equinox.
The UARS-HALOE NO data was also sorted as a function of season and solar activity. A
subset of the data is shown in Figure 3. The sunrise and sunset data has been sorted in
three levels of solar activity and three seasons (Nothern summer, Northern winter, and
equinox). The data shows a clear pattern, with a steady increase in mid and low latitude
NO with the rise of the solar cycle. There is also a distinct difference between the
height/structure in the illunfinated summer high latitude compared with winter. Simulations
with the 3D model shows reasonable agreement with the overall structure as a function of
season, latitude, and solar cycle (Figure 4).
A review of modeling the distribution of nitric oxide including the results of this study will
be presented at the Nagoya COSPAR meeting, and will be submitted for publication.
Figure Captions
Figure 1. Comparison of the latitude structure from SME at the 3pm local time sector
(upper panel) with HALOE observations at sunset (lower panel). Both are from data one
month either side of equinox at moderate solar activity.
Figure 2. Top panel shows the effect on the latitude structure of NO at 106 km of changing
chemical reaction rates (see text). The lower panel shows the height/latitude structure with
all three reactions taken into consideration.
Figure 3. Summary of seasonal, latitude, and solar cycle dependence of NO density from
HALOE sunset observations.
Figure 4. Model simuations of seasonal, latitude, and solar cycle dependence of NO
density at sunset for comparison with Fig. 3.
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Understanding the Latitude Structure of Nitric Oxide in the Mesosphere and Lower Thermosphere

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