scott m. bailey, lws workshop march 24, 2004 the observed response of the lower thermosphere to...

Scott M. Bailey, LWS Workshop March 24, 2004

The Observed Response of the Lower Thermosphere to Solar Energetic

Inputs

Scott M. Bailey, Erica M. Rodgers, Justin Yonker

Geophysical Institute

University of Alaska

Charles A. Barth, Daniel N. Baker

Laboratory for Atmospheric and Space Physics

University of Colorado

[email protected]


Abstract

The production of nitric oxide is a key response of the upper atmosphere to solar energy deposition. NO plays a strong role in the thermospheric energy balance as it emits efficiently in the infrared, it is the terminal ion in the lower ionosphere, and if transported to lower altitudes will catalytically destroy ozone. NO is primarily produced through the reaction of excited atomic nitrogen with molecular oxygen. One of the primary loss mechanisms of NO is photo-dissociation by solar ultraviolet irradiance. In order to produce the excited atomic nitrogen atom, the strong N2 molecular bond must be broken. At low latitudes, solar soft X-ray irradiance is the energy source that leads to NO. At high latitudes, auroral electrons and the energetic secondary electrons provide the source of energy that leads to the large amounts of NO observed there. Coupling between latitude regions may occur as high latitude NO is transported by winds to lower latitude.

In this talk we describe observations of NO from the Student Nitric Oxide Explorer (SNOE). SNOE obtained global observations of lower thermospheric NO from February 1998 through December 2003. We will present case studies of the observed response to large auroral storms. The SNOE observations show that auroral energy deposition produces a significant global effect on the upper atmosphere.


Nitric Oxide — Why it is important?

Present in the thermosphere - maximum density near 110 km:• Abundance — several times 10-4 mixing ratio at 130 km• Highly variable — factor of ten — 27 day and 11 year variation• Always larger in the auroral region (max at 65° geomagnetic

latitude)

Present in the mesosphere - minimum density near 75 km:• Abundance — several times 10-9 mixing ratio at 70 km• Transport downward from the lower thermosphere into the

mesosphere— Particularly during the winter polar night

Odd nitrogen reacts with odd oxygen: NO + O3 NO2 + O2

NO2 + O NO + O2

Odd nitrogen controls the composition of the lower Ionosphere:NO + O2

+ NO+ + O2

N + O2+ NO+ + O

NO+ + e N + O

Nitric Oxide is the most important cooling mechanism in the lower thermosphere:

• Heteronuclear molecule has a permanent electric dipole moment

— Infrared cooling in the 5.33 m band— Controls temperature in the critical 120 km region


Production of Nitric Oxide

NO is primarily created through the reaction of excited atomic nitrogen with molecular oxygen:

N(2D) + O2 → NO + O

A major source of excited atomic nitrogen is produced through energetic electron impact with molecular nitrogen:

e* + N2 → N(2D) + N + e*

Destruction of Nitric Oxide

NO is primarily destroyed through the reaction with ground state atomic nitrogen:

NO + N(4S) → N2 + O

NO is also destroyed through photodissociation:NO + h → N(4S) + O

Note that this reaction is doubly effective since it creates a ground state N atom which can also destroy NO.

The effective lifetime of an NO molecule is about 1 day under sunlit conditions.


So where do the energetic electrons come from?

Two sources:• Photoelectrons – solar soft X-ray irradiance, < 10 nm• Precipitating auroral electrons


SNOE obtains global observations of nitric oxide

Polar Orbit – constant local time near 11AM/PM

15 orbits per day - observations are obtained at all latitudes and 15 longitudes (same local time) each day

Spinning satellite – altitude profiles from 0 to 200 km, NO densities are obtained from 95 to 150 km

SNOE UVS observes 215, 237 nm fluorescently scattered solar radiance – NO is only measured at locations which are illuminated by the sun.


SNOE observations show that NO responds to varying solar activity as expected.

Low latitude profiles sorted by F10.7

High latitude profiles sorted by AP

Note that larger concentrations of NO are observed at the high latitudes.


SNOE observations show significantly larger amounts of NO at high latitudes.


Even if data for days of low auroral activity are considered, SNOE observations show larger amounts of NO at high latitudes.


A one-dimensional chemical model reproduces observed equatorial altitude profiles

Model Incorporates:• Solar soft X-ray irradiance, < 20 nm, measured by SNOE• Relevant photochemistry, 35 reactions• Vertical diffusion• MSIS neutral atmosphere, GLOW energetic electron transport

code


The chemical model reproduces observed equatorial NO densities for 935 days

SNOE ObservationsModel Predictions


Northern WinterSouthern Summer

Solstice


Solstice

SNOE observations for 935 days show large amounts of NO near polar regions and sporadic enhancements at all latitudes.

Geo

grap

hic

Latit

ude



Solstice


Solstice

The photochemical model can be used to calculate the global distribution of NO produced by solar soft x-rays.

Geo

grap

hic

Latit

ude


Geo

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hic

Latit

ude

Geo

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Latit

ude



Solstice


Solstice

By subtracting the calculated NO produced by solar soft x-rays from the observations, we determine the global distribution of NO produced by auroral energy.

Note that the auroral energy routinely reaches middle latitudes and occasionally reaches the equator. This excursion to lower latitudes must be caused by winds. NO transported to lower latitudes will lead to enhanced cooling as well as modification of the temperature structure and D/E region composition there.

Geo

grap

hic

Latit

ude



Solstice


Solstice

Smoothing by 30 days reveals a repeatable pattern of winter transport of NO to middle latitudes.

Note that in summer times, there is negligible NO produced by auroral energy.

Geo

grap

hic

Latit

ude



Solstice


Solstice

The photochemical model can be iterated with varying auroral inputs to obtain the auroral energy deposition required to reproduce the NO observations.

The resulting energy fluxes show strong seasonal dependence with minima at the summer solstice.

North

Day of Year


North

South

Day of Year

Day of Year



Solstice


Solstice

The ratio of hemispheric energy inputs shows that winter energy deposition is significantly larger than summer energy deposition.

The ratios may also be increasing with solar activity.

Day of Year


Conclusions:

The production of NO by solar soft X-ray irradiance is well understood.

NO produced by auroral energy is transported by winds to middle latitudes and sometimes to the equator.

Auroral energy deposition varies strongly with season and is at a minimum at summer latitudes. This is consistent with previous studies which have shown that discrete auroral events are suppressed in the summer.

scott m. bailey, lws workshop march 24, 2004 the observed response of the lower thermosphere to...

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