Increase in the carbon dioxide infrared emissions during solar proton events

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  • 151

    ISSN 0016-7932, Geomagnetism and Aeronomy, 2006, Vol. 46, No. 2, pp. 151158. Pleiades Publishing, Inc., 2006.Original Russian Text V.P. Ogibalov, S.N. Khvorostovskii, G.M. Shved, 2006, published in Geomagnetizm i Aeronomiya, 2006, Vol. 46, No. 2, pp. 159167.

    1. INTRODUCTION

    As a result of a decrease in the molecular collisionfrequency with altitude, not only collisional vibrationaltransitions but also optical transitions contribute to theformation of the

    CO

    2

    vibrational state population in themesosphere and lower thermosphere (MLT) [Lpez-Puertas and Taylor, 2001]. Therefore, the population ofthese states in the MLT region becomes non-Boltz-mann; i.e., the local thermodynamic equilibrium (LTE)is disturbed for vibrational states. The disturbance ofLTE for the states excited due to the deformation vibra-tion (

    2

    ) of

    CO

    2

    molecule begins from approximately70 km [Khvorostovskaya et al., 2002]. The same distur-bance but for the states excited due to the stretchingasymmetrical vibration (

    3

    ) begins from approximately60 km at night and even from lower altitudes (fromapproximately 50 km) in daytime due to the excitationof vibrational states during solar radiation absorption in

    CO

    2

    infrared bands [Shved et al., 1998].Optical transitions, when the vibrational quantum

    number corresponding to the

    2

    vibration changes byunity, generate atmospheric emission in the 15

    m

    CO

    2

    band. The same transitions but for the

    3

    vibration pro-duce emission in the 4.3

    m band. Nonequilibrium(nonthermal) MLT emissions in the above bands werefirst measured in 19731974 on rockets that registeredemission from the zenith [Stair et al., 1974, 1975]. Sub-sequently, the nighttime and daytime nonequilibriumMLT emissions in these and other infrared vibrationalrotational

    CO

    2

    bands were intensely studied duringboth rocket-borne and satellite observations and using

    numerical simulation [

    L

    pez

    -Puertas and Taylor,2001]. Some researchers even tried to measure the 4.3and 15

    m emissions in order to determine the

    CO

    2

    mixture ratio in the MLT region [Kaufmann et al.,2002; Mertens et al., 2003; Kostsov and Timofeev,2003].

    Nighttime rocket-borne measurements of the4.3

    m emission at a latitude of

    65

    N [Stair et al.,1975] indicated that the brightness of this emission inthe MLT region increases by an order of magnitude andmore during auroras. Some researchers tried to simu-late this enhancement [Kumer, 1977, 1978]. Theenhancement is explained by excitation of nitrogenvibrational states when

    N

    2

    molecules are bombarded byenergetic electrons precipitating during auroras andvibrational excitation is subsequently transferred to

    CO

    2

    molecules in the course of

    N

    2

    CO

    2

    collisions. Thisprocess of vibrational energy exchange is very rapidbecause the vibrational quantum energy (

    3

    ) is close tothe energy of vibrational quantum of

    N

    2

    molecule. Inthe 1990s, some researchers became interested insprites, i.e., optical emission bursts in the mesospherethat occur above thunderclouds simultaneously withlightning strokes [Pasko et al., 1997]. Lightning strokesare accompanied by the generation of strong short-lived(~10 ms) quasielectrostatic fields at altitudes of 5095 km. Atmospheric electrons are accelerated in thesefields and excite states of air molecules. As in the caseof auroras, excitation of

    N

    2

    (1)

    states should result in anenhancement of emission in the 4.3

    m

    CO

    2

    band in theMLT region. The models of this enhancement were,

    Increase in the Carbon Dioxide Infrared Emissions during Solar Proton Events

    V. P. Ogibalov, S. N. Khvorostovskii, and G. M. Shved

    V. A. Fok Research Institute of Physics, St. Petersburg State University, St. Petersburg, Russia

    e-mail: shved@pobox.spbu.ru

    Received November 10, 2004

    Abstract

    Increase in the nighttime high-latitude nonthermal emissions in the mesosphere and lower thermo-sphere in the 4.3 and 15

    m

    CO

    2

    bands during solar proton events has been estimated for the first time. Theestimations have been performed for protons with energies not lower than 1 MeV precipitating into the atmo-sphere. A strong increase in the 4.3

    m emission can be anticipated during the above events; however, a sub-stantial increase in the 15

    m emission is improbable. The 4.3

    m emission can increase only above approxi-mately 80 km regardless of the energy of precipitating protons. The excitation of CO

    2

    vibrational states, tran-sitions from which generate the 4.3

    m emission, is caused by the vibrational excitation of

    N

    2

    molecules dueto collisions with secondary electrons, produced during solar proton events, and the following transfer of thisexcitation to

    CO

    2

    (00

    0

    1)

    molecules during

    N

    2

    CO

    2

    collisions.PACS numbers: N94.20D

    DOI:

    10.1134/S0016793206020034

  • 152

    GEOMAGNETISM AND AERONOMY

    Vol. 46

    No. 2

    2006

    OGIBALOV et al.

    correspondingly, proposed [Picard et al., 1997; Milikhet al., 1998].

    In addition to the above causes, energetic electronsare also originated in the MLT region during solar pro-ton events (SPEs) (see, e.g., [Bazilevskaya et al.,2003]). In the present study the

    CO

    2

    vibrational statepopulations, excited as a result of high-energy electronprecipitation into the atmosphere during these events,are calculated for the first time. The populations are cal-culated for nighttime conditions; i.e., in the case whenthe states are not excited during solar radiation absorp-tion. The obtained populations are used to simulate ver-tical profiles of emission intensities in the 4.3 and15

    m

    CO

    2

    bands for two versions of emission registra-tion: from the zenith and planetary limb. The estima-tions are performed for the proton energy distributionsregistered during SPEs of July 16, 1959, August 4,1972, and July 13, 1982, and presented in [Solomonet al., 1983].

    2. MODEL DESCRIPTIONThe vertical profile of the atmospheric temperature,

    typical of the high-latitude MLT region in the secondhalf of the summer, was selected based on the MSISE-90 model of the atmosphere [Hedin, 1991]. The modelof the composition of the atmosphere was taken from[Shved et al., 1998].

    The vibrational state systems and optical and colli-sional vibrational transitions used in the calculationswere described in detail by Shved et al. [1998]. Thesame work presents the rate constants of the collisionalprocesses except for the rate constant of the

    CO

    2

    (01

    1

    0)

    state quenching during

    CO

    2

    O

    collisions, which wastaken from [Khvorostovskaya et al., 2002]. The intensi-ties of the

    CO

    2

    vibrationalrotational lines are takenfrom the HITRAN-2000 database for the spectroscopicparameters of atmospheric molecules. The appliedmodel of the Lorentz width of these lines is presentedin [Shved et al., 1998]. Additional excitation of

    CO

    2

    vibrational states in the vicinity of the mesopause dueto the transfer of the vibrational excitation energy ofOH molecule to these states [Shved et al., 1998] wasignored in these estimates of the molecule state popula-tion since the rate of this excitation is very uncertain.

    The energy of energetic protons precipitating duringSPEs dissipates in the atmosphere and is transformed incascade into the energy of the secondary particleshydrogen atoms, protons, and electronsduring colli-sions [Eather, 1967]. Produced energetic electronsshow a specific energy distribution [Khvorostovskiiand Zelenkova, 1997b; Khvorostovskii, 2001a], andcollisions with these electrons represent the mainmechanism of additional excitation of air moleculevibrational states during SPEs. We took into accountdifferent ways that result in excitation of

    2

    and

    3

    vibrations of

    CO

    2

    molecule. First of all, we took intoaccount direct excitation of the

    01

    1

    0, 2

    2

    , and

    00

    0

    1

    states during collision of

    CO

    2

    (00

    0

    0)

    molecule withelectron, where

    2

    2

    denotes the set of the

    10

    0

    0, 02

    2

    0

    ,

    and

    02

    0

    0

    states that have close energies and are interre-lated via rapid intramolecular vibrational energyexchange during collisions [Shved et al., 1998;Ogibalov et al., 1998]. As in the case when the 4.3

    memission was generated during auroras and lightningstrokes (see Introduction), we took into account themost effective mechanism of the

    CO

    2

    (00

    0

    1)

    state exci-tation by electrons, which represents the sequence ofthe processes

    N

    2

    (0) +

    e

    N

    2

    (

    v

    ) +

    e,

    (1)

    CO

    2

    (00

    0

    0) + N

    2

    (1) CO

    2

    (00

    0

    1) + N

    2

    (0). (2)

    In so doing, we assumed that

    N

    2

    (

    v

    )

    molecules with thevibrational quantum number

    v

    > 1 almost instanta-neously redistribute their excitation over

    N

    2

    (1)

    mole-cules since the rate of the vibrational energy exchangebetween nitrogen molecules during their collision ishigh. We also took into account additional pumping ofthe nitrogen molecule vibrational states as

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