Previous investigations of the authors based on the decay rates of many satellites have demonstrated the existence of a post-storm effect in the neutral atmosphere after geomagnetic storms. Its maximum appears 4–6 days after the storm onset. It generally lasts 8–10 days, but if there is also an ionospheric post-storm effect, then it is about twice as long at mid-latitudes and in the evening hours. The observed characteristics of the post-storm effect seem to indicate that it is related to the precipitation of ring current particles due to charge exchange and wave-particle interaction

Almár, Iván

Bencze, P.

Horváth, A.

Illés-Almár, Erzsébet

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Adv. SpaceRes.Vol. 7, No. 8, pp. (8)53—(8)57, 1987 0273—1177/87 $0.00 + .50Printed in Great Britain. All rightsreserved. Copyright© 1987 COSPARINVESTIGATION OF THE THERMOSPHERE-IONOSPHEREINTERACTION BY MEANS OFTHE NEUTRAL POST-STORMEFFECTE. I11és~A1már,*I. A1már,~KP. Bencze** andA. Horváth**Konkoly Observatory,H-1525 Budapest,Box 67, Hungary* ~ and GeophysicalResearchInstitute, H—9401Sopron,Box 5, HungaryABSTRACTPrevious investigations of the authors based on the decay rates of manysatellites have demonstrated the existence of a post—storm effect in theneutral atmosphere after geomagnetic storms. Its maximum appears 4—6 daysafter the storm onset. It generally lasts 8—10 days, but if tnere is alsoan ionospheric post—storm effect, then it is about twice as long at mid—lat-itudes and in the evening hours. The observed characteristics of the post—storm effect seem to indicate that it is related to the precipitation ofring current particles due to charge exchange and wave—particle interactions.INTRODUCTIONIn a former paper /1/ it has been demonstrated that the density (~) deducedfrom atmospheric drag on satellites is increased after geomagnetic stormsas compared to semiempirical models ~CIRA 72). We call this “the thermo—spheric post-storm effect”. This phenomenon was observed in the heightrange 200 - 600 km, and is similar to the well—known after—effect in themean ionospneric absorption of radio waves (MIA). The density maxima occurfour to six days after the onset of geomagnetic disturbances. In this paperwe investigate different aspects of this thermospheric post—storm effect.DATAOur data base contains about 16000 density values ~from 1965 to 1972) basedon the decay of 20 satellites having appropriate orbital eccentricities(0.05<e<O.20). Some decay rates have been partly determined by our PERLOprogram /2/, and partly taken from publications of ephemeris centres. Thetime resolution was sometimes as good as 2 days, but generally not betterthan 3—5 days. Time intervals were omitted when a time resolution at leastas good as 5—10 days could not be guaranteed.The f—curves were derived for each satellite separately, wheref = ~obs/~mode1 . All remaining long—term variations were eliminatedfrom the f—curves of each satellite. In order to remove the effect ofinadequate knowledge of satellite cross—sections, all f—values were dividedby f (the mean of all density values of a given satellite).METHODThe well-known method of superposed epochs (MSE) has been used with Dstminima as key days. Altogether those 109 events were selected during the 7years which had Dst minima exceeding -40~. Both the events and the measureddensity values were separated according to criteria described as follows:Separation of events according to geophysical parametersWe applied MSE to all events together ~Case 1) as well as to eventsseparated according to different geophysical parameters ~altogetner 40 cases:Ap -- planetary geomagnetic index; MIA -- mean ionospheric absorption;lISPS —— high speed plasma stream; SSC —— storm sudden commencement;CDR -— galactic cosmic ray intensity measured at Deep River.(8)53(8)54 E. Illés-Almár et a!.The following types of separation proved to be the most interesting:Case 2: those events when Ap maxima corresponding to Dst minima werelower than 20 (marked by —Ap, 26 events);Case 3: Ap maxima exceeding 40 (marked by +Ap, 61 events);Case 4: when Ap>40 combined with an ionospheric post-storm effect~marked by -1-Ap +MIA, 33 events);Case 5: when Ap>40 with no ionospheric post-storm effect(marked by +Ap -MIA, 20 events).Separation of density data according to the position of the perigeeThe averaged f-curves were plotted using MSE for all density data (denotedby ‘unseparated” in Fig. 1) and for those separated by the position of theperigee respectively.All f—values were separated according to the latitude of the perigee intothree groups: low latitude (IC~t < 30~), midlatitude ¼300< ~ 60~) andpolar (t(~1 >60°).Another four groups were formed according to the local solar tine (LST)of the perigee: daytime (lOh< LST <14h), evening (18°< LST < 22h)night (22h<LST<20) and morning (4hKL5T~8h)RESULTSAltogether 320 curves were plotted using MSE with the Dst minima as keydays. There is a well-pronounced minimum on almost every curve around thekey day. In our opinion this phenomenon is a simple consequence of theinadequacy of the time resolution of the density curves which are notsensitive enough to follow the profile of quick geomagnetic variations; i.e.our experimental density curves cut off the peaks of sharp density maxima.The main results are summarized in Fig. 1 and Fig. 2, where the departuresof experimental densities from corresponding model values are plotted as afunction of time elapsed after each key day. In the case of a perfect modelthe scatter would be around a constant value.In Fig. 1, in column 1, MSE curves for case 1 are plotted: at the bottomdifferent geophysical parameters, above them the unseparated f-curve, thenthose belonging to different sections of the day and latitudes respectively.In the next two columns events of case 1 have been separated according tothe absence or presence of Ap storms (case 2 and 3). The thermosphericpost—storm effect is seen on every curve with a maximum around the fifthday. Comparing the unseparated curves, it is obvious that the post-stormeffect has a longer duration in case 3 than in case 2. On separated curvesit is conspicuous that the longer duration is characteristic of the mid—latitudes and of the evening hours.In Fig. 2, in column 1, the MSE curves are separated according to andLST together ~i.e. the combined latitudinal and LST effect is treated).Evening curves indicate a long—lived post—storm effect lasting about 18days at midlatitudes. Separating the events of case 3 according to thepresence (+MIA) or absence (—MIA) of an ionospheric post-storm effect(case 4 and 5) we can conclude that the long-lived thermospheric after-effect is characteristic only of +MIA cases ~column 2, case 4, plottedwith heavy line) and it is short-lived even at midlatitudes and eveningwithout a MIA effect (column 3, case 5, dashed line).DISCUSSIONThe required energy source must not be limited to the recovery phase. Theobserved conditions, i.e. the time of the increased density with its lagbehind the geomagnetic activity, the height range and the latitude regionconcerned suggest that the energy comes from the precipitation of ringcurrent (radiation belt) particles. The precipitation of particles dependson the loss processes, charge exchange and wave—particle interaction.Considering the scenario of the thermospheric post—storm effect, it appearsat low latitudes in the early recovery phase and is restricted to the inter-val from 3 days to 10 days after the onset of the disturbance. On the basisof other experimental data, charge exchange is probably responsible for theparticle precipitation at these latitudes. Energetic protons have lifetimesagainst charge exchange long enough to produce the observed effects, but donot transfer enough energy in inelastic collisions in the height range con-sidered /3/. On the other hand, soft protons have short charge exchangelifetimes /4/. Moreover, according to recent investigations, during the re—Investigationby Neutral Post-StormEffect (8)55Case 1: Case 2: Case 3:D5~, ciii D5~,-A~ D5~,+A~1.-~ ~ - polar1.-.~ ~ - - ~ midlatf/f ~ - - ~ - ~ - Low lat1.- - ~ - ~ ~ evening1.- - - night- - morning1.- - - ~ ~%~- daytime1— ~ — ~ ~ — ~ unseparated~ ~‘ ~ ~ ‘‘‘~~ atCDR1oo[~ ~ ~ ~MIA ‘~J~ ~ t4~~t~t~r-rnightA~ 20 [ ,~, ..) ~0 +10 .20 0 .10 +20 0 .10 .20 ~ daysFig. 1. Demonstration of the thermospheric post-storm effect by means ofthe method of superposed epochs (MSE). After geomagnetic storms(Dst minimum used as key day) there is a density excess whichlasts longer in midlatitudes and in the evening hours.(8)56 E. Illés-Almár et al.Case 3: Case 4: Case 5:D5~~A D +A~ oP St,+MIA St,—MIA— midlat1. - daytime—midlat1. — ~ ~ 1~t~\~ _j evening—midtat~ — — ~HI~Jf~night1.-1.- \~-~ ~ —tow (atdaytime—low lat1.- — ~‘V ~ evening—Low tat1. —nightI o.’ - -0st 50fl ~ “ “~“~ ~ ~CDR ioO[ ~ ~ ~MIA ~4ft~vl~jy~ ~ nightA~ 20[ ~ ~ jI I I0 +10 .20 0 •10 .20 0 .10 .20 ~. daysFig. 2. The thermospheric post-storm effect is long—lived whenmidlatitude plus evening cases are combined and there is anafter—effect in mean ionospheric absorption (MIA) as well.Investigation by Neutral Post-Storm Effect (8)57covery phase, 0+ decays rather rapidly ~-l day) in the bulk of the energydensity (>20 key), but not at lower energies /5/. Since low energy 0+ ionshave long charge exchange lifetimes and show the greatest magnetic activitydependence, they might be considered as candidates for the energy source.Precipitating particles can directly and indirectly affect the upperatmosphere at low latitudes. The direct effect can be the result of precipi-tating neutrals, produced by charge exchange between the ring current ionsand the geocorona. The neutrals directed towards the Earth impact atlatitudes determined by the geometry of the pitch angle distribution; i.e.within 300 of the dip equator /6/. The 0 atoms of low energy (<10 key)which have lifetimes long enough to produce the post-storm effect arethermalized by ionizing (stripping) and excite the ambient atoms in theheight range 200-600 km. This process results in the heating of the upperatmosphere /7/. The indirect effect can be attributed to a process sug-gested by Moritz /8/ and confirmed by Mizera and Blake /9/ and by Scholeret al. /10/. According to Moritz /8/, the energetic neutral atoms formedin a charge exchange of ring current ions with neutral hydrogen in the geo-corona and moving freely in the upper atmosphere are reionized. These par-ticles can be neutralized by charge exchange and untrapped and temporarilytrapped at lower altitudes in the low latitude upper atmosphere (doublecharge exchange). The flux of these particles depends on the flux of theincident energetic neutrals. Our hypothesis seems to be supported by theobservation of increased intensity of H Lyman alpha emission /11/, /12/.The H Lyman alpha emission measured during the recovery phase of a geomag-netic storm at low latitudes indicates that the radiation could be attrib-uted to the precipitation of protons in the energy range 10-300 keyobserved by Moritz /8/ and lvlizera and Blake /9/. Since the asymmetric ringcurrent is concentrated in the evening hours, the circumstance that thethermospheric post—storm effect is more developed in this time interval mayalso hint at the ring current origin of this effect.At midlatitudes the thermospheric post—storm effect has been found morelong—lived than at low latitudes —- in the presence of MIA in particular.Other observations show that plasma waves (periodically structured Pd typemicropulsations) are enhanced inside the plasmasphere in the late recoveryphase /13/, /14/, /15/. Thus, at midlatitudes, wave-particle interactionscan also contribute to the observed post—storm effect, when the plasma wavesare amplified by ion cyclotron instability. Theoretical investigationsindicate that the stable trapping limit for a given flux of energetic par-ticles is decreased as the ambient cold plasma density increases /16/. Suchconditions occure, where, during the recovery phase, the plasmapause movesoutward into the ring current due to the filling of the plasnasphere. Theintersection of the ring current with the increasing cold plasma density ofthe plasmasphere produces the ion cyclotron instability. Concerning the lo-cal time dependence of the thermospheric post—storm effect at midlatitudes,our results seem to show that such intersections are more effective at thebulge of the plasmasphere located in the evening sector. It should again benoted that the asymmetric ring current is concentrated also in the eveninghours during the recovery phase. The observation that the thermosphericpost—storm effect is not a regular phenomenon may also be a proof of theworking of the wave—particle interaction. Thus, it has been found that theunmodelled thermospheric density increase can be due to the long-liveddissipation of the energy of the geomagnetic storm into the thermosphere.REFERENCES1. E. Illés—Almár, P. Bencze and F. MSrz, Nabl. ISZ 23, 333 (1984).2. A. Horváth and E. Illés—Almár, Nabl. ISZ 9, 277 (1970).3. R.H. Eather and K.M. Burrows, Aust. J. Phys., 19, 309 (1966).4. P.H. Smith, N.K. Bewtra and R.A.Hoffman, J.Geophys.Res. 86,3470, (1981).5. S.14. Krimigis, G. Gloeckler, R.W.McEntire, T.A. Potemra, F.L. Scarf andE.G. Shelley, Geophys. Res. Letters 12, 329 (1985).6. G.W. Prôlls, Planet. Space Sci. 21, 1681 (1973).7. M. Torr end D.G. Torr, Geophys. Res. Letters 6, 700 (1979).8. J. Moritz, Z. Geophys. 38, 701 (1972).9. P.F. ~4izera and J.B. Blake, J. Geophys. Res. 78, 1058 ~l973).10. M. Scholer, D. Hovestadt and G. Morfill, J. Geophys. Res. 80, 80 ~l975).11. G.E. Thomas, J. Geophys. Res. 85, 6055 (1980).12. S. Cazes and C. Emerich, J. Geophys. Res. 85, 6049 (1980).13. R.C. Wentworth, J. Geophys. Res. 69, 2291 (1964).14. R.R. Heacock and M. Kivinen, J. Geophys. Res. 77, 6746 (1972)15. W.W.L. Taylor and L.R. Lyons, ~ 81, 6177 ~l976).16. J.i4.Cornwall, F.V.Coroniti and R.M.Thorne,J.Geophys.Res.75,4699(l97O).

Investigation of the thermosphere-ionosphere interaction by means of the neutral post-storm effect,

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