Pergamon Adv. Space Res. Vol. 17, No. 11, pp. (11)149-(11)155, 1996

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ENERGETIC ELECTRON PRECIPITATION DURING AURORAL EVENTS OBSERVED BY INCO~RENT SCATTER RADAR

A. Osepian,* S. ~kwood** and N. Sm~ova*~*

* Polar eeophysical Imitate, ~altari~ 15, 183023 Murmansk, Russia ** Swedish Institute of Space Physics, Box 812, S-981 28 Kiruna, Sweden *** Institute of Applied Geophysics, 107258 Moscow, Russia

ABSTRACT

Variations of electron density at the altitudes 75-120 km obtained by the EISCAT radar are examined during geomagnetic disturbances. Energy spectra and fluxes of incoming electrons dufig auroral abso~tion events are derived from the electron density pmflles using two inversion methods and a theoretical model of ion-chemise in the lower ionosphere. Typical evening and morning-side events are studied. Variations of the energy spectrum within the whole energy range (a few keV to 200-300 keV) and of electron fluxes have been determined. These can be used both to quantify the energy input from precipitating particles to the mesosphere and lower thermosphere and for testing of precipitation mechanisms. In particular it is shown that morning amoral absorption bays , which are due to p~cipitation of h&h-energy (E > 20-30 kev) electron fluxes have typical energy deposition rates of l-10 mW/kg between 80-90 km altitude.

INTRODUCI’IGN

Exertion of the spectral v~atio~ and changes of electron fluxes precipi~ting into the D and E-regions of the polar ionosphere is an extremely useful tool in understanding magnetosphere- ionosphere relations and a number of aspects of magnetospheric physics, including the energy transfer from the solar wind to the Earth’s upper atmosphere and precipitation mechanisms for different types of auroral event. Incoherent-scatter radar techniques make it is possible to measure the variations of electron density in the D and E-regions and to study variations of the electron spectrum during periods of auroral activity /1,2,3,4,5,6,7/.

In this study we use ground-based measurements of electron density profiles by incoherent scatter radar to iI.Iustrate the characteristics of typical evening and morning-side precipitation and discuss the ~ssi~ities for mo~to~ng the energy deposition in the upper mesosphere using ~~ohe~nt- scatter and riometer operations.

METHOD TO DERIVE FLUX-ENERGY SPECTRA OF PRECIPITATING ELECTRONS

A combination of two methods is used to derive flux-energy spectra from electron density profiles measured by incoherent-scatter radar. The first calculates the ionisation rates on the basis of an effective combination model followed by direct inversion using matrix algebra /8/. This method gives most information on the shape of the energy-flux spectrum. The second method /9,10/ uses an assumed spectral form (an exponential dj/dE= J exp(-E/Eo ) or power dj/dE= N Emn law ) and a seasonally and solar-zenith dependent model of D-region ion-chemistry /I11 to relate spectral hardness ( Eu and n > to the height of peak adoption of 30 MEIz radio waves, and the total absorption to the total flux. Incremental absorption profiles calculated from the electron density

(11)149

(11)150 A. Osepian et cd.

profiles are used to derive the corresponding flux-energy spectra. This method provides a convenient parameterisation of the high-energy tail in terms of spectral hardness and integrated flux. In future , when adequate models of spectral hardness have been developed for different types of events, the latter method can be used to derive particle fluxes from riometer data alone. Results of the two methods have been compared with each other and with measured electron fluxes from a rocket experiment and been found to give good agreement /Il. They can be combined to study the changes of precipitating electron fluxes and spectral variations within the energy range from a few keV to 200-300 keV. They also provide a method for quantifying the height distribution of energy dissipation in the upper mesospherc and lower thermosphere.

EXPERIMENT DETAILS

The experimental data used in this study comprises S-minute average electron density profiles measured by the EISCAT UHF incoherent-scatter radar, situated near Tromsa in northern Norway /12/ The profiles have a height resolution of 3 km and are measured along the magnetic field-line above the transmitter site. In this study we use measurements from 72-120 km altitudes.

OBSERVATIONS AND RESULTS

Electron Den&y and Radio Am

Figure 1 shows variations of electron density during two auroral events on 2502.86 at 2030-2230 UT and on 26.02.86 at 0530-0700 UT. Figure 2 shows the total radio absorption and height of maximum absorption (at 30 MHz) derived from the electron density profiles, and the spectral hardness and integrated electron fluxes derived from these parameters using the second method described above. As can be seen both in Figure 1 and in the second-lowest panels in Figure 2, the ionlsation peak during the pre-midnight event is located at much higher altitude (H=l lo-98 km) than during the morning event (96-91 km). The height of the absorbing layer also differs significantly in the evening and morning hours (second-lowest panel, Fig. 2 ). At 2120 UT the main contribution to the radio absorption occurs in the height range 85 -100 km with peak height at 90 km. At 0610 UT the main part of the absorbing layer is located at lower altitudes (80-95 km) and maximum absorption is located at 85 km. During the evening event the height of maximum absorption sinks from 97 to 87 km during the event while it rises from 83 to 88 km during the morning event. Thus the experimental data contain information on the energy range and energetic spectrum of precipitating electron fluxes and information on their changes both during each event and with local time.

a : evening event b : morning event

72

2030 2100 2130 2200 2230 0530 0600 0630

time (UT) time (UT)

Fig. 1. Contour plots of electron density variation during a) an evening-side substomi and b) a morning-side slowly-varying absorption event. The figures indicate electron density in units of 10” m-3

Energetic Electron F’rwipition (11)151

a : evening event b : morning event

2100 2200

time (UT)

0600 0630 0700

time (UT)

Fig. 2. Radio-wave absorption at 30 MHz (lowest panel), heights of maximum absorption (solid line) and peak electron density (dashed line - second-lowest panel), spectral hardness (second-top panel) and total flux of electrons with energies >lO keV (a) or >30 keV (b) (top panel, units cm-2s-1).

Selected energy spectra deduced by a combination of the two methods described above are given in Figute 3. They show in more detail how the spectra change during the events. In the evening sector (Figs. la, 2a and 3a) the largest changes of electron density, the height of the ionisation peak and absorption peak and accordingly of the spectral hardness occur during the first part of the event (2030-2135 UT). According to the rna~~rne~r records, a subset onset occurred at 2110 UT. First of all it should be noted that at 2105 UT a hole in the electron density was observed at altitudes below 98 km. As a result the value of amoral absorption fell to 0.1 dB and the energy spectrum at this moment was very soft and &h~a~~~sed by a very low in~~ty of precipitating particles at energies E>lO keV (Fig. 3a, dj/dE=2.5*107 *exp(-E / 2.4). The phenomenon of ” auroral fading” before the substorm onset has been remarked on by /13/. In this case the signatures of fading are seen in the EISCAT data as a temporary reduction of electron density over all altitudes below 100 km and in the spectrum as a fast decrease of precipitating electrons especially at energies E>20 keV.

A. Osepian et al. (11)152

108,

a: evening event

OOUT 05UT

w+-2110UT

0 20 40 60 80 loo

E(keV)

b:moming event

- 0545UT e OSSOUT - 0555UT

40 60

E (keV)

80

Fig. 3. Flux-energy spectra of precipitating electrons derived from the electron density profiles for (a) the evening event and (b) the morning event.

One can see that the moment of substorm onset is char~te~sed by an overall increase in flux with Es10 keV but an especially fast one at E>20 keV, i.e. the spectrum hardens (dj/dE= l.96*106 *exp(-E/10)). A hardening of the spectrum at substonn onset (due to acceleration of the electrons to provide an increasing field-aligned current) is known to occur for the low-energy part of the spectrum (E<lO-10 keV, e.g./14/). However this occurs on time scales less than the integration time used here and cannot account for the effects at higher energies. These can rather be attributed to direct precipitation from the plasma sheet of freshly injected electrons. At the time of the following intensification at 2135 UT one can again see an increase of electrons at E>lO keV (dj/dE= 8.3*106*exp(-E/11) while the changes of softer electrons are small . As the absorption peaks the spectrum hardens, and the total electron flux with E> 10 keV also increases. This may be due to a new injection of accelerated electrons. The decay of auroral activity is finally character&d by softening of the spectra together with decreasing of flux at E>lO keV.

In the morning sector (Figs lb, 2b and 3b) before the growth of activity one can see a steep reduction in the precipitation of electrons at energies between 20-30 keV. The low-energy part of the precipitating flux changes very little (Eo = 4-5 keV) during the whole active period. The largest changes occurred for electrons with energies 20-30 keV. At the beginning of the event the spectra of electrons with E>30 keV were rather hard (Eo=26-18 keV). A steep decrease of electron density at high altitudes , especially at h = 91-95 km , observed at 0545 UT (Fig.1 b), is a result of a sharp reduction in the precipitation of electrons at energies between 15-30 keV (Fig. 3b). During the first part of the event the hole in the flux at E =20-30 keV is gradually tilled and the energy spectrum is softens somewhat (see Fig. 2b and Fig. 3b ). The spectrum does not change between 0600 UT and 0645 UT (Eo= 14-16 keV). The decay of the event is characterised by a further softening of the spectrum ( Eo=l2-10 keV). It is apparent that the sources of soft ( Ec25-30 keV) and hard (E>30 keV) electrons are different. The harder spectrum in the morning sector (as compared with the evening ) and the gradual softening of the spectrum in the precipitating flux of electrons with E>25-30 keV can be explained by gradient-curvature drift of electrons from a source of injection of fresh electrons near local midnight during a substorm. The precipitation of electrons at such energies from the magnetosphere in the morning sector is usually explained by pitch-angle diffusion of resonant electrons ( the energy range of resonant electrons is several tens keV- several hundred keV /15,16,17/ e.g. by whistler-mode waves.

Energetic Eleam Precipitation (11)153

a : evfming event b : morning event 1000

28 100

2 s to

s 0 1 it .a

G .1

1 .01

.OOl

2030 2100 2130 2200 2230 053b 0600 0630

time (UT) time (UT)

Fig. 4. Energy deposition at altitudes 75 - 100 km for the two events.

Enemv Denosition in the A~osoher~

Figures 4a and 4b show the energy deposited at different heights by the ~~cipitation. Clearly, the much harder spectral characteristics in the morning event are reflected in the much higher energy deposition below 90 km altitude in that event. The evening event, on the other hand, deposits much more energy above 90 km altitude. In the evening sector during the substorm the spectra am not only rather soft, but the hardness is also rather variable. In the morning sector the spectra are overall much harder but on the whole not as variable as on the evening side. The result is much more variable energy deposition rates at 90 km and below for the evening event.

DISCUSSION AND CONTUSIONS

Clearly, the flux-energy spectra during the two events are very different although they give very similar signatures on riometer records . and morning periods is , however ,

The difference in spectral hardness between the evening much larger than the variations within each period. If we plot

the energy deposition at selected altitudes as a function of absorption we find a rather good relationship between the two, taking each event separately (Fig. 5). This means that it may indeed

a : evening event b : morning event

1 I 0 0.5 1 1.5

absorption (dB)

Fig 5. Relations~p between absotption and energy de~sition for the two events.

+ QOkm

* 1OOkm

(I I)154 A. Osepian et at.

lo5 +

I I

0 1 2

absorption (dB)

oooo November, evening

0000 November, morning

x*x* 25.02.86 , evening

++++ 25.02.86 , morning

Figure 6. Relationship between absorption and total flux for the two events studied here and for two other events at similar local times.

be realistic to use riometers to monitor the energy input to the upper mesosphere, particularly for morning-side events which are those which most affect the relevant altitudes. It will, however, be important to continue to study a large number of events with incoherent-scatter radar to verify the relationships in Figure 5, and to identify possible other types of events with different hardness characteristics.

If we plot the relationship between the value of total precipitation flux J(>E keV (elcm-2,s’)) and the value of radio absorption we find that they also show a regular relationship and compare well with results obtained previously /lo/ for the evening and morning absorption bays on 17-18 November 1983 (Fig. 6). This is encouraging both in the context of being able to use riometer data to determine energy deposition and for the possibility to study the mechanism of precipitation. The relationship between absorption and the precipitating flux at energies exceeding rni~m~ resonant energy will in p~nciple enable the diffusion regime and electron pitch-angle diffusion coefficients to be estimated during morning amoral activity.

Acknowkdgemcnts The EISCAT Scientific Association is supported by the Centre National de la Recherche S~ientifique of France, Suomen Akatemia of Finland, Max Planck Gesellschaft of Germany, Norges Almenvitenskaplige Forskningtid of Norway, Naturvetenskapliga Forskningsr4det of Sweden and the Science and Engineering Research Council of the United Kingdom. The work of S.K. is supported by the Natmvetenskapliga Fo&nings&det of Sweden.

REFERENCES

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2. A. Brekke, C. Hall, Aurora1 ionospheric conductances during disturbed conditions, Ann. Geo~~y~. Z(3), 269-280 (1989).

3. T. Devlin, J. K. Hargreaves, P. N. Collis, EISCAT observations of the ionospheric D-region during auroral radio absorption events, 1. Armos. Terr. Phys, 48, 795-806 (1986).

Energehc Elear~n Precipilation (11)155

4. S. Kirkwood, L. Eliasson, Energetic particle precipitation in the substorm growth phase measured by EISCAT and Viking, J. Geo~hy~. Res. $& ~25-6037 (1990).

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15. G.T. Davidson, Fitch angle diffusion in momingside aurorae. 1. The role of the loss cone in the formation of impulsive bursts of precipitation, J. Geophys. Res. !& 44134427 (1986).

16. G. T. Davidson, P.C. Filbert, R.W. Nightingale, W.L. Imhof, J. B. Reagan, EC. Whipple, Observations of intense trappaed electron fluxes at synchronous altitudes, J. Geophys. Res. f&j, 77- 95 (1988).

17. M. Schulz, G. Davidson, Limiting energy spectrum of a saturated radiation belt, J. Geophys. Res. s, 59-76 (1988).