The effective recombination coefficient measured in the auroral E-region during a sudden commencement electron precipitation event

Journal ofAtmospheric andTerrestial Physics, 1975, Vol.37,pp.825-833. Perpamon Prees.Printed inNorthern Ireland

The effective recombination coefllcient measured in the aurora1E-region during a sudden commencement electron precipitation event AWEIR BREKKE* Department of Applied Physics & Information Science, University of California, San Diego, La Jolla, California, 92037, U.S.A.

(Received 3 May 1974; in revised fom 20 August 1974) Abstract-Incoherent scatter radar observations are utilized to derive the effective recombination coeffioientand the ion production rate in the aurora1E-region during a sudden commencement precipitation event. The recombination coefficient is found to be (1.1 & 0.3) x 10-r cm3 s~c-~ in good agreement with present laboratory measurements of the dissociative recombination coefficientof O$ for an electron temperature close to 500 K. The ion production rate associated with the SC is found to be at least 4.5 x lo* el cmm3set-1 at 110 km, this corresponds to an enhancement above the background ionization by more than a factor of 10. The background ionization is also larger than expected from solar EUV radiation due to ionization associated with a PCA event present. INTRODUCTION EFFECTNE recombination coefficient in the ionospheric E-region has been derived by different methods during the last couple of decades. Although good agreement exists between several authors, differences as large as a factor of 100 are not uncommon. APPELTON (1953) measured the sluggishness of the ionospheric E-region, which is the time the greatest electron density maximum in the E-region lags behind local noon, a time delay of the order of 10 min, and found recombination coefficients of the order of lo-* cm3 see-l. RATCLIFFE (1956) deduced a recombination coefficient also of the order of 1O-8 cm3 see-l from solar eclipse observations. GUSTAVSON (1964) observed the time lag between peaks in aurora1 luminosity and radio wave absorption, and deduced effective recombination coefficients close to 2 x lo+ cm3 set-l. OIEHOLT (1971) derived from simultaneous measurements of aurora1 brightness in zenith and E-layer electron densities a recombination coefficient in good agreement with Gustavson’s value, i.e. 2 x 1O-s cm3 se+. Similar data taken by KNEC~ (1956) (see OMXOLT, 1971) gives recombination coefficients in the order 3 to 5 x lo-’ cm3 see-l. Using rockets, ULWICK (1967), derived effective recombination coefficients from the primary electron flux in auroras and the intensity of the 23914 band. His values for the effective recombination coefficient ranged from 4 to 7 x lo-’ cm3 see-l in the E-region. Similarly, MCDIARMID and BUDZINSKI (1964) derived recombination coefficients between 1O-6 cm3 set-l at 85 km and 1O-7cm3 see-1 at 95 km. JESPERSEN et al. (1969) and BRYANT et al. (1970) derived effective recombination coefficients slightly less than lo--’ cm3 set-1 at 115 km and slightly larger than 2 x lo-’ cm3 set-1 at 100 km by measuring the height distributions of 24278 aurora1 intensity and the local electron density from rockets. By similar measurements BAKER (1968) got values from 3 to 6 x 10-T cm3 set-l. BARON (1972) used the time history of the electron density at different altitudes observed by the incoherent scatter radar at Chatanika, Alaska, and derived effective THE

* Presentaddress: The Aurora1Observatory, P.O. Box 953, N9001 Tromse, Norway. 9

825

826

A. BREKKE

recombination coefficients between 2.5 and 6.0 x lo-’ ems se+ in the altitude region 90-140 km. Laboratory measurements indicate a dissociative recombination coefficient for NO+ ranging from 0+X-7.4 x lo-’ cm8 se+ when the electron temperature changes from 1500 to 200 K, similarly laboratory measurements for O,+ range from 1-Oto 3-O x lo-’ cm3 set-l for temperature between 700 and 200 K, BANKS and KOCEARTS(1973). According to BIONDI(1964) such coefficients should be compared with the E-region results since dissociative recombination appears to be the most important loss mechanism at these altitudes. There are obviously many difhculties to overcome when the effective recombination coefficient is measured in the ionospheric E-region, and the large scatter in the results derived indicates that the different methods applied do not always succeed in solving these difficulties. Especially it is now believed that values for the effective recombination coefficients derived during solar eclipses are not very reliable, neither can the values derived from such parameters as the total integrated aurora1 intensity in zenith and the aurora1 absorption observed by riometers be used without caution. When trying experimentally to derive effective recombination coefficients in the aurora1 E-region, one encounters a basic problem, since time variations in the ion production mechanisms are usually not well known. It is extremely difficult to correctly account for this effect, but it is believed that in the case of a sudden commencement electron precipitation event, an additional ion production mechanism is turned on and off as rapidly as possible in the ionosphere. Therefore if the effects of such an abrupt event could be unambiguously sorted out it should be the most favourable to utilize for measurements of the effective recombination coefficient in the ionosphere. Another important difficulty encountered when the effective recombination coefficient is measured in the E-region, is the presence of any wind or electric fields. Such dynamical parameters tend to sweep the plasma out of the volume where it is originally produced, and unless this drift mechanism is correctly accounted for, the values derived may be erroneous. By their nature it is difficult to actually observe simultaneously parameters involved in the production and loss mechanisms present in the E-region, and therefore assumptions must be made regarding the unknown quantities. In contrast, from incoherent scatter radar observations it is possible to simultaneously derive at least three important parameters, the electron density profile, electric fields and neutral wind in the E-region. In the following it will be shown how such observations favourably can be used to determine the effective recombination coefficient and the ion production rate in the aurora1E-region, during a sudden commencement associated electron precipitation event. DATA

The incoherent scatter radar at Chatanika, Alaska (see LEADABRAND et al., 1972, for closer description) was operating during one ot the largest magnetic storms occurring in modern time in the period 4-10 August 1972. This storm was initiated by two sudden commencements on 4 August at 0119 and 0220 UT, none of these, however, appeared to be related to any significant ionospheric response, at

The effective recombination

coefficient measured in the aurora1 E-region

827

least as seen from the radar data (BREKKEet al., 1974). A third sudden oommencement occurred on the same day at 2054 UT. This event had a much more spectacular appearance both on magnetograms worldwide and in the radar data. Recently BREKKEet al. (1974) showed how the monostatic incoherent scatter radar at Chatanika could be used to derive ionospheric electric field and currents with a time resolution down to 30 set, and in particular they used the method developed by them to investigate the dynamical behaviour of the three sudden commencements mentioned above. As this method is rather thoroughly discussed by them, it will not be repeated in this report. AGWON and S-MAN (1974) also reported on satellite measurements in the magnetosphere during the same sudden commencement occurring at 2054 UT on 4 August. It is only the results derived by the radar during this event which will be used in the following report. On Fig. 1, at the top, an enlarged picture of the sudden commencement occurring at 2054 UT is shown as it appeared in the H-component observed at College. In the middle part of Fig. 1 the electric fields derived by the radar are shown in the same time-scale, and a sharp transition occurs simultaneously in the electric field and the H-component. Finally at the bottom part of Fig. 1 the height integrated conductivities in the E-region are plotted during the same event. A strong enhancement in the height integrated Hall conductivity due to increased electron precipitation is seen during the commencement. The electron density profiles observed by the radar with 30 see time resolution between 2052 :50 and 2057:50 UT are shown in Fig. 2, and the sudden appearance of the E-layer is outstanding. The onset of this precipitation event is very abrupt, an increase in the height integrated Hall conductivity by a factor of 1.6 within 30 sec. and a similar abrupt decrease is also evident. These outstanding features in the particle precipitation will be utilized in the following. DERNATIONOB THE EFFECTIVERECOMBINATION COEFXICIENT AND THE PRODUCTION RATE DURINGTHE EVENT The rapid decrease in the conductivity at about 2057 UT may indicate that the electron flux responsible for the ionization enhancement suddenly disappeared. Based on this assumption, the electron profiles derived for this period 2052 : 502057 : 50 UT, (Fig. 2) can be used to obtain an estimate of the effective recombination coefficient and the production rate associated with the SC. A simplified continuity equation for the ionization in the E-region may be written as (RISHBETHand GA~RIOTT,1969) : div z’=Q-uN,a+v.VN, where N, is the electron density, a function of time and position, Q is the ion produotion rate, also a function of time and position, I’ is the ion drift velocity, and o( is the effective recombination coe&ient. As the vertical velocity measured in the ®ion before the sudden commencement is very small ( <6 m/set) we can neglect the vertical gradient term in Equation (1) during the period from 2050: 50 UT to 2053:50 UT. During the electron density enhancement following the SC we wil1 estimate the recombination coefficient corresponding to the altitude of the E-region

823

A.

I

~REKl?Z

ELECTRIC

FIELD

UNIVERSAL

TIME

1. Upper part: The H-component at College between 2040 and 2130 UT on 4 August 1972. The sudden commencement is indicated to occur at 2054 UT. Middle part: The ionospheric electrio fields derived during the period of the sudden commencement between 2040 and 2130 UT. Bottom part: The height-iterate ionospheric conducti~t~~ in the period of the sudden commencement occurring at 2054 UT. The angles given indicate the antenna azimuth reokoned positive eastward from geographic north. Fig.

maximum, so that any vertical density gradients in the eleotron density also csn be neglected during the period from 2054 to 2057 UT. Before the SC in the period 2062: 50-2053: 50 the electron profiles derived are fairly similar, so that any time variation in iV, is probabIy negligible. Purthermore, a strong PCA event ocourred over a large area, therefore, we will assume that the ionization due to this PCA was fairly uniform over an area much larger than the field of view of the radar. In such a case, the last term in equation (1) can also be

2052:50-2057:50

E x

UT

Aug.

140

jho

p.

)

55:50

)

56:2o

)

;6:5o)7:x

4

972

1 5?:5G

)

2

024024

Electron

0 density.

2

4

0

IO” electrons

2

4

02402

lK3

2

4

Fig. 2. Electron density profiles observed every 30 sec. by the radar in the period of the sudden commencement between 2052 : 50 and 2057 : 50 UT on 4 August 1977.

neglected. Taking these assumptions into account, the background production rate Qo before the SC will then be given by Q. = aNo where No is the electron density at 110 km before the SC. After the SC, when the Hall conductivity decreases rapidly between 2056:50 UT and 2057:20 UT, we assume that the ionization Q, due to electrons released by the SC suddenly disappears. Let w be the electron density at the E-region maximum associated with the SC. Then at 2057 UT, when the ion production, caused by the enhanced electron precipitation and released by the SC disappears, we have iv,=No+n

Q=Qo

and equation (1) becomes dn -xi=

-

(2aNo + ccn)n+

V . Vn

where V represents the horizontal ion drift velocity. As shown by LEINBACHet al. (1970), and BROWNet al. (1972), electron precipitation events associated with sudden commencements is limited to a latitudinal belt on the day side of the Earth, with a maximum between L = 6 and L = 7. In the zonal direction (east-west) we will therefore assume that spatial gradients in n are small and can be neglected. In the latitudinal (north-south) direction, however, we will assume that n is given by a Gaussian function such that n = no exp( -(A+)%), (4) where a is the half width between the halfvalue points of the electron precipitation peak, and AX is the distance from the peak. Since n is of the order 2 x lo5 el/cm3 at Chatanika, Ax has to be smaller than 2a for no to be less than 10’ el/cm3-an extreme value which can be used as an upper limit for no. By adopting equation

A. BREKBJS

830

(4), equation (1) may now be written dn

dt=-

V

20: No + 3 [ (

where P, is the north-south ion drift velocity. find that

1

1

+ an n,

(5)

By integration of this equation we

(6)

where At is 30 set, rcl and n8 are the electron densities st 110 km associated with the SC at time 2056:50 UT a;nd 2057:20 UT respectively, tend N=2

(

No+V&$.

1

From (6) and (7) it is readily seen that unless V, and AZ are zero Q has to be determined by an iteration procedure. We will adopt from the density profiles in Fig. 2 the following values No = 1+4 x lo6 el/cms (at 2053:20 UT), n1 = 2.3 x 10bel/cms (at 2056:60 UT) and na = O-4 x lo5 el/cma (at 2057:20 UT). From Fig. 1 one can see that the eastward electric field is of the order of 6 mv/m around 2067 UT. This field corresponds to a northward ion drift of about 100 m,Jsec. We therefore infer 100 mjsec as a r-onable value for V, in (7). Inserting these values in equation (6) the effective recomb~ation coefficient will be a -+ l-6 x lo-’ cm* sea-l

(3) for Ax = 0, i.e. if Chatanika were at the center of the precipitation maximum. For a precipitation pattern having a halfwidth of the order of 1000 km (a = X000 km) tc will vary between 25 x 10m7cm3 set-l and 7.6 x 10q cma set-1 when Ax varies between -2a and +2a, for a -+ 750 km cc will vary between 2.7 x lo--’ cm3 set-1 and 4.8 x lo-* ornaset-1 for similar variations in Ax. For halfwidths less than 500 km, however, IAz[ has to be less than a in order to have physical acceptable values for the effective recombination coefficient. Halfwidths less than 500 km, however, are most unlikely since the electron precipitation associated with ttn SC usually covers about 15 degrees in latitude, i.e. about 1600 km. (BROWN et aZ., 1972). Therefore it is believed that a = 750 km probably is the most representative vslue, and the recomb~&tion coe~~ient given by equation (8) will be adopted. It is also likely that College is fairly close to the maximum p~~ipi~tion according to the results obtained by LEINBACH et ccl. (1970). By using a chain of riometers distributed at different latitudes but approximately at the same longitude they found that the electron precipitation usually associated with SC peaks close to L = 65 and therefore probably within 200 km of the College latitude. Assuming a halfwidth of 750 km and further that College WM within 200 km from the peak the derived recombination coefficient had to be between 1.75 x lop7 cm3 see-1 and 1.47 x 1O-7 cm9 se+. Between 2056: 20 UT and 2056: 50 UT the height integrated Hdl conductivity does not change appreciably, such that in this period we will assume that dn/& = 0 at 110 km, and the production rate associsted with the SC can be found as g = a(2iVo + n)n

(9)

The ef%otive recombination coefficientmeacjuredin the auroraI E-region

831

where the drift term has been neglected and equation (1) has been used. Adopting the value n = 2*3 x l@ el/cms at 110 km at 2056:50 UT from Fig. 2, the ion production rate associated with the SC will be Q = 1.9 x lo4 el/oms-sec.

(10)

This is, however, a lower limit for q during the SC, as it must have been larger at UT. Assuming dn/dt is zero at this time, a production rate of 4-5 x lo* el/ cm3-set is derived. In any case, the ion production rate was increased by about a factor of 10 above the background level which c&n be found from equation (2) to be 2055: 20

QO

=

3.3

x lo8 el/cms-sec.

(11)

In comparison, BANES and KOCIWXTS(1973) found by theoretical ~~o~atio~ an ion production rate close to 2 x 10s eljcm*-set at 110 km due to solar radiation for a solar zenith angle close to 50”. The excess ionization found here may be due to the PCA event present. Also, it is believed that the true solar fluxes are higher than those used by BARES and KOCKARTS(1973) (BANKS,personal communication). Of course, by neglecting any gradients in the electron density as usually is done, the effective recombination coefficient can be derived for several altitudes and not only for the maximum density in the E-region. Such calculations have been done in the altitude region loo-120 km where the density enhancement is most pronounced. The v&es derived for the effective recombination coefficient are shown versus altitude in l?ig. 3. Also included are the range within which the effective recomb~ation coefficient were found to vary at 110 km for a p~cipi~tion region with a half~dth of 780 km. As an average value of the effective recombiuation coefficient in the altitude region 100-120 km one gets from these results a = (1.11 Ifr:O-34) X lo+ cm8 set-l.

(12)

Compared with the values derived by BAROX (1972) in a similar manner from time variations of the electron densities at different altitudes during aurora1 particle precipitation events, the value derived in this report is about a factor of 2 smaller.

EFFECTIVE

REC.

COEFF.

[cm3 SW-‘I

Fig. 3. The ef%&ive reoombination coeffi&nt a, versus altitude derived from the radar data by neglecting the gradient term in equation (1). The horizontal bar indicates the range within which a will vary depending on the latitu~al extent of the electron precipitation. For a more detailed explanation, see the text.

832

A. BEE-

BARONet al. (1972) however, neglected the last term in equation (1) due to lack of simultaneous ion velocity observations. As ion velocities of the order 100-1000 m/set are not uncommon in the aurora1 zone during aurora1 particle precipitation events, it is evident that the presence of such high velocities will leave the last term in equation (1) relatively important, and the derived recombination coefficients will be in error. By neglecting the drift term in (5) (i.e. V, = 0) the effective recombination coefficient will be given by the formula 1 a=xqG

ln

1 + 2No/n3li2 1 + 2No/n,>

(13)

from which it is evident that the derived value of a depends crucially on a proper estimation of the background electron density No. It is not simple to derive such estimations in active aurora1 displays because effects from several different electron precipitation events may be present at the same time, and isolated events are difficult to find contrary to the sudden commencement event presented above. These may be some of the reasons for the discrepancy between the values derived by BARON(1972) and the one obtained here in this report. CoNCLTJsIoN

The abrupt changes in the electron density proties measured during a sudden commencement make them very favourable for a study of the ion-production rate associated with the electron precipitation released by the event, and also for measuring the effective recombination coefficient in the E-region ionosphere. The analysis given here shows that the ion production rate probably was increased by more than an order of magnitude above the background level, to at least 4.5 x lo4 el/cm3-set and the effective recombination coefficient at 110 km was found to be (1.1 f 0.3) x lo-’ cm3 see-l, in good agreement with laboratory measurements of the dissociative recombination coefficient for the 02+ ions at electron temperatures between 400 and 700 K. Good agreement is also found between the derived values and laboratory measurements of the dissociative reoombination coefficient for NO+ at electron temperatures close to 1000 K. As such high temperatures are rather unlikely in the lower E-region, one can infer that the dissociative recombination of the O,+ ions probably was the most important loss process during this sudden commencement event. REFERENCES

AGGSON T. L. and SKILLMANT. L. A~PELTONE.V.

1974

BAKERK. D.

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1973 1972

National de la Recherche Scientiflque. Aeronomy, pp. 253-256. Academic Press, New York. Auroral Ionospheric iWea.surements,pp. 43-63. DNA Project 617 Radar. Stanford Research Institute, California.

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co&Tic&&,measured in the &u.romlE-region

833

1964 1974

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1972

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1964 1971

Cala.J. Phys. 413, 2048. The Optical Aurora, p. 26, 180-182. Springer,Nsw York. Solar Eclipses and the Ionosphere, pp, 1-13. (Edited by W. J. C. BEYNON and G. M. BROWN). Pergamon Press, Oxford. I~t~od~t~n to Ionospheric Physics, p. 126. Academic Press, New York, Awora mzd Airglow (Edited by B. 5I. McCOR~C). Reinhold, New York.

RATCLJ~FEJ. A.

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1967