30TH INTERNATIONAL COSMIC RAY CONFERENCE

Effects of the January 2005 GLE/SEP events on minor atmospheric components

M. STORINI1, A. DAMIANI 2.1Istituto di Fisica dello Spazio Interplanetario - INAF, Via del Fosso del Cavaliere, 100 - 00133 Roma, Italy2 ICES - International Center for Earth Sciences c/o CNR Istituto di Acustica ”O.M. Corbino”, Via delFosso del Cavaliere, 100 - 00133 Roma, Italystorini@ifsi-roma.inaf.it

Abstract: It is known from long ago that solar energetic charged particles, driven by the geomagneticfield, are able to produce ionization at different altitudes of the terrestrial atmosphere. Moreover, they caninitiate catalytic cycles for the ozone depletion, involving NOx (N, NO, NO2) and HOx (H, OH, HO2)components. Nevertheless, only in recent years it was possible to compare chemical models involvingatmospheric minor components with satellite data. In this work we looked for effects of the GLE/SEPevents occurred during January 2005 on the OH and HNO3 species of the atmosphere. Results show thatthere is a response on the minor atmospheric components, which is different in the winter and summerterrestrial hemispheres.

Introduction

It is widely known that minor atmospheric con-stituents are strongly dependent on the geoma-gnetic and solar activity. In particular, several in-duced effects by cosmic rays (CRs) can be singledout at diverse altitudes. For example, in the tro-posphere the CR flux seems to be connected to thecloudiness formation (see [8] for an early work and[9] for a recent analisys); in the lower stratosphereCRs are an important source of nitric oxide (NO)molecules, by the ionization and dissociation ofthe molecular nitrogen (N2). At Polar Regions andduring the winter time, when the lack of sun lightprevents the formation of NO via the reaction ofnitrogen protoxide (N2O) with atomic oxygen, theCR flux can become the main source of nitric oxide[6]. Besides, in stratosphere and mesosphere, so-lar energetic particle (SEP) events are able to trig-ger catalytic cycles of O3 destruction [1]; O3 isthe most important atmospheric component for theEarth’s lives.

During January 2005, after a low/moderate solaractivity interval, a big injection of solar energeticparticles in the terrestrial atmosphere occurred (∼16-22 January). Two Ground-Level Enhancementswere identified (GLE68/SEP: 17 January, start∼

10 UT and GLE69/SEP: 20 January, start∼ 07UT) in the world-wide network of cosmic ray de-tectors. This paper focuses on the January 2005GLE/SEP’s effects on the polar atmospheric che-mistry.

Used Data

The employed atmospheric data come from theMicro-wave Limb Sounder (MLS) instrument onthe AURA satellite. The NASA EOS (Earth Ob-serving System) MLS is one of the four instru-ment of AURA launched on 15 July 2004 to sun-synchronous near polar orbit.

The MLS is a limb emission instrument that scansthe Earth’s limb viewing the microwave emissionat different spectral regions. The measured che-mical components are: O3, H2O, BrO, ClO, HCl,HOCl, OH, HO2, HCN, CO, HNO3, N2O, andSO2 mixing ratios.

In this work we used EOS MLS Version 1.5 Level2 Data (http://mls.jpl.nasa.gov/data/), from whichO3, HNO3, and OH values were taken. Theproton flux was retrieved from GOES 11 files(http://www.ngdc.noaa.gov/stp/GOES/).

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SEPEFFECTS ON THE ATMOSPHERIC CHEMISTRY

January 2005 GLE/SEP events

SEP events are able to produce ionization at dif-ferent altitudes of the terrestrial environment andcan initiate catalytic cycles for the ozone deple-tion, involving NOx (NO, NO2) and HOx (OH,HO2) components. Such effects generally referto the Polar Cap regions (geomagnetic latitudesabove 60◦). Nevertheless, [3] showed that January2005 GLE/SEP events produced also two weak andvery short (< 12 h) ozone depletions at the externalboundary of the Southern Polar Cap.

Moreover, Damiani et al. [2], analyzing the in-terhemispheric differences, emphasized that: atthe Southern high latitudes (day state) the O3 de-crease is weak while in the Northern (night state)ones it is strong and long lasting, particularly inthe mesosphere. For this reason, Figure 1 (upperpanel) exemplifies only the daily ozone profiles inthe Northern Hemisphere (location:∼ 75◦-82◦),before and after the GLE/SEP events of January2005. The elevate values of the mesospheric O3

mixing ratio on the day before the event (blue pro-file) is related to the existence of a third ozonemaximum during the winter time [5]. In fact, atelevate latitudes of the winter hemisphere the lowSun light leads to a minor efficiency of the UV fluxon the water vapour photolysis; therefore, the re-duced production of HOx components in ”night”condition facilitates the increase of the ozone con-centration. Mesospheric ozone depletion (morethan 1 ppmv) is evident on 18 January (the day af-ter the first GLE/SEP) and lasts for some days. Inthe stratosphere (above 1 hPa) the ozone decreaseis less evident from the daily profiles; an analy-sis of NOx component should be performed. TheMLS instrument cannot measure atmospheric NOx

but it is able to retrieve nitric acid, a good proxyfor NOy (reservoir nitrogen). During January 2005SEPs, together with the stratospheric ozone deple-tion, an HNO3 increase appears between 10 and2 hPa (∼ 30-40 km), persisting till the end of themonth. Figure 1 (lower panel) shows its tempo-ral evolution (location:∼ 75◦-82◦ N) by using de-rived HNO3 (mixing ratio) contours, from 15 to 31January 2005. This increase of nitric acid can orig-inate from two different processes.

The first process involves the most importantHNO3 production phenomenon:

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Figure 1: Daily ozone profile of averaged val-ues (volume mixing ratio) during several days ofJanuary 2005 (upper panel) and contours of aver-aged HNO3 (volume mixing ratio) values duringthe second part of January 2005 (lower panel). Se-lected location:∼ 75◦-82◦ N.

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30TH INTERNATIONAL COSMIC RAY CONFERENCE

NO2 + OH→ HNO3 + M.

Therefore, the HNO3 increase can be the result ofthe OH and NO2 raise during SEPs; this reactionis fast enough to produce the amount of nitric acidsimilar to that observed during October-November2003 GLE/SEP events [4].

The second process is related to the ion chemistry[7] and may be important since it is an active pro-cess also during night time (i.e. winter hemisphereat elevate latitudes). Through the reaction of watercluster ions with NO3 the nitric acid is able to raise.Nevertheless, the elevate photolysis at the summerhemisphere prevents to single out the HNO3 vari-ability for the January GLE/SEPs.

During January 2005 GLE/SEP events it was pos-sible also to highlight the increase of the OHmolecules (proxy for HOx) associated with the so-lar protons transit in the atmosphere. Figure 2shows the OH temporal evolution for∼ 75◦-82◦

N (upper panel) and S (lower panel) for 15-24January 2005. The sudden and intense OH en-hancement evidences the goodness of expectationsfrom available models. The OH concentrationraises several hundreds of% at Northern latitudes,whereas it remains almost constant at South (only aweak modulation is present) where the intense so-lar illumination increases the OH background val-ues and hides the induced effects by SEPs. To no-tice that the response of odd hydrogen species toSEPs is very fast, almost contemporary, either insummer or winter hemisphere.

Summary and conclusions

GLE/SEP induced effects on some minor compo-nents of the terrestrial atmosphere have been il-lustrated for the GLE/SEP events occurred duringJanuary 2005. Results for a Polar Cap location (∼75◦-82◦) can be summarized as follows:

• A relevant increase of the HNO3 from 19 to28 January 2005 was identified from∼ 2.5hPa to 5.5 hPa at the Northern hemisphere(Figure 1, lower panel).

• The OH increase during the GLE/SEPevents presented a fairly good reproductionin the Northern hemisphere of the protonflux profile (Figure 2, upper panel).

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Figure 2: Solar proton flux (particle energy E>10 MeV) from GOES 11 data and contours of OHvalues (volume mixing ratio) from MLS/EOS datafor ∼ 75◦-82◦ N (upper panel) and∼ 75◦-82◦ S(lower panel).

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SEPEFFECTS ON THE ATMOSPHERIC CHEMISTRY

Moreover, in the upper mesosphere (∼ 60-80 km)the OH variability seems to be triggered by a cut-off value in the proton flux (∼ 10 pfu for particleenergy E> 10 MeV). In fact, the SEP flux arrivingon the terrestrial environment on 16 January initi-ated the OH modulation, which is ended when theproton flux went below∼ 10 pfu. This behaviour isless evident in the Southern hemisphere (Figure 2,upper panel). Moreover, in the lower mesospherethe scenario is more complex.

In conclusion, it is stressed that the high OH valuesunder solar illumination make difficult to single outthe OH rise induced by SEPs, since the backgroundconcentration of H-species depends on the H2Oconcentration (more elevated in the summer meso-sphere). On the contrary, the reduced solar illu-mination during the winter time facilitates to high-light the atmospheric chemistry changes. More-over, in the winter hemisphere the OH changes arelong lasting (if compared with the summer ones,because of the OH short life under solar light).Very probably, the different OH concentration andlife during the extreme seasons is the main raisonof the elevated and long lasting O3 depletion in theNorthern mesosphere and the short and feeble O3

decrease at the Southern one.

Acknowledgements

Work faced for COST 724 Action, supported bythe Italian PNRA and partly performed for aPhD thesis under development at Siena Univer-sity/Dept. of Earth Science.

References

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