DTIC ADA271457: Comparison of Ionospheric Electrical Conductances Inferred from Coincident Radar and Spacecraft Measurements and Photoionization Models

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ADA27, 1 457
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DATE 3 RE(cid:127) T 7 A. 'C2 _-AT ESE2S: VE-.
September 8,1993 REPRINT
.71TLE AND SU3TITLE -.
,,COMPARISON OF IONOSPHERIC ELECTRICAL CONDUCTANCES INFERRED
FROM COINCIDENT RADAR AND SPACECRAFT MEASUREMENTS AND PE 61102F
iPHOTOIONIZATION MODELS PR 2311
TA G5
15. AUTHCR(S)
1J. WATERMANN*, 0. DE LA BEAUJARDIERE*, F.J. RICH ;WU 02
7. PERFORMING OR'A ATCA(cid:127),O> qA,5IE;.AO ADDt(cid:127). a'5tS:S. . - .. T
!Phillips LabIGPSG ur"
.29 Randolph Road
SHanscom AFB, MA 01731-3010 ._.PL-TR-93-2189
S*Geoscience and Engineering Center, SRI International, Menlo
Park, California
Reprinted from Journal of Atmospheric and Terrestrial Physics, Vol. 55, No. 11/12,
pp 1513-1520, 1993
0SS T R ;30 UTdON .AIL < 7 A EIT
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* Approved for public release; Distribution unlimited
DTIC QTMALrrY mSpTN-- r,
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Abstract-Height-integrated electrical conductivities (conductances) inferred from coincident Sondrestrom
incoherent scatter radar and DMSP-F7 observations in the high-latitude ionosphere during solar minimum
are compared with results from photoionization models. We use radar and spacecraft measurements in
combination with atmospheric and ionospheric models to distinguish between the contributions of the two ced 2
main sources of ionization of the thermosphere, namely. solar UV,EUV radiation and auroral electron
precipitation. The model of ROsWNSON et a/. (1987, 1. yeophvs. Res. 89. 3951) of Pedersen and Haill 1n. ..................
conductances resulting from electron precipitation appears to be in accordance with radar measurements.
Published models of the conductances resulting from photoionization that use the solar zenith angle and
the solar 10.7-cm radio flux as scaling parameters are, however, in discrepancy with radar observations ..-.---------. .. . ..........
At solar zenith angles of less than 900, the solar radiation components of the Pedersen and Hall conductances )n/
are systematically overestimated by most of these models. Geophysical conditions that have some bearing
on the state of the high-latitude thermosphere (e.g. geomagnetic and substorm activity and a seasonal -ability Codes
variation of the neutral gas distribution) seem to influence the conductivity distribution but are to our
knowledge not yet sufficiently well modelled. Avail and or
Dist Special
14. SUBJECT TERMS (cid:127)5.3 '5. SER OF PAGES
Ionosphere, Conductivity, Photoionization, Sondrestrom radar, 8
DMSP 16. PC- ":ODE-6
f
17. SECURITY CLASSIFICATION 18. SECURITY CLASSIFICATION 19. SECURITY CLASSIFICATION 20. LlýTATION OF AdSTRACT
OF REPORT OF THIS PAGE OF ABS7RACT
I J
UNCLASSIFIED _LTý7CT ASSI1FIED UNCLASSIFIED
-SAR -
PL-TR-93-2189
1 t 1,4 h1 t. (cid:127) ,d , '(cid:127) tts h (cid:127)< ii N" I I .pp 151 1 . 11'929133 9t1il6.9t 'tU ,. )
Comparison of ionospheric electrical conductances inferred from
coincident radar and spacecraft measurements and
photoionization models
J. WAII-RMANN,* 0. DE LA BI:ALJARDfi.IRE* and F. J. RICHt
* Geoscience and Engineering Center. SRI International. Menlo Park, California, U.S.A.: _
"tP hillips Laboratory (AFSC0. Hanscom AFB. MA 01731. U.S.A.___
(Rec,'ied in linalfitrm 16 January 1992: accepted 31 January 1992)
Abstract--Height-integrated electrical conductivities (conductances) inferred from coincident Sondrestrom
incoherent scatter radar and DMSP-F7 observations in the high-latitude ionosphere during solar minimum-
are compared with results from photoionization models. We use radar and spacecraft measurements in
-
combination with atmospheric and ionospheric models to distinguish between the contributions of the two
main sources of ionization of the thermosphere, namely, solar UVEUV radiation and auroral electron
precipitation. The model of ROBINSON el al. (1987, J. yeophys. Res. 89, 3951) of Pedersen and Hall -
conductances resulting from electron precipitation appears to be in accordance with radar measurements.
Published models of the conductances resulting from photoionization that use the solar zenith angle and .J
the solar 10.7-cm radio flux as scaling parameters are, however, in discrepancy with radar observations.
At solar zenith angles of less than 90 , the solar radiation components of the Pedersen and Hall conductances
are systematically overestimated by most of these models. Geophysical conditions that have some bearing -1
on the state of the high-latitude thermosphere (e.g. geomagnetic and substorm activity and a seasonal
variation of the neutral gas distribution) seem to influence the conductivity distribution but arc Io our
knowledge not yet sufficiently well modelled.
1. INTRODUCI'ON tribution to obtain estimates of the Pedersen and Hall
conductances. Coincident electron flux measurements
Ionospheric electrical conductivities are important from low-altitude spacecraft make it possible to dis-
parameters in ionospheric electrodynamics and tinguish between the contributions of electron pre-
magnetosphere-ionosphere coupling. Under many cipitation and photoionization to the electrical con-
circumstances (e.g. when dealing with field-aligned ductances. We have examined ten cases of coincident
current closure by Pedersen currents or Ohmic energy Sondrestrom and DMSP-F7 (Defense Meteorological
dissipation or ground magnetic field signatures of Satellite Program) measurements made during the
ionospheric currents) the knowledge ofconductances years 1984-1986 (solar minimum period). Con-
(height-integrated conductivities) is sufficient and ductance estimates from radar measurements of the
simplifies calculations considerably. electron density distribution in the dark ionosphere
In this paper we concentrate on the high-latitude were in fair agreement with those obtained from
ionosphere where solar UV and EUV radiation and spacecraft measurements of electron precipitation.
auroral electron precipitation are the two main energy This demonstrates the validity of the precipitation
sources that ionize the thermospheric gas. The ion- model suggested by RomNsON et al. (1987) that we
ization and recombination rates (and thus the elec- have chosen. The contribution of solar UV and EUV
trical conductivities) depend on the spectral dis- radiation, calculated with various photoionization
tribution of solar radiation, the path of the sun beams models (MEHTA, 1978, SENIOR. 1980. 1991; VICKREY
through the atmosphere, that is, the solar zenith angle. et al., 1981 DoE LA BEAUJARDIfRE et al., 1982; ROBN-
on the scale height and composition of the neutral SON and VONDRAK, 1984; SCHLEGEL, 1988; BREiKKE
atmosphere and possibly on the geomagnetic field, and HALL, 1988; RASMUSSEN et al., 1988). appears to
Recent papers which discuss the different parameters be in systematic discrepancy with radar measure-
that influence the daytime conductances include those ments. Our observations suggest that most of these
published by BRtEKKE and HALL (1988), DE LA BEAU- models overestimate the electrical conductances in the
JARDIERE et al. (1991) and SENIOR (1991). sunlit ionosphere.
We use Sondrestrom incoherent scatter radar In this paper we describe the method that we used
measurements of the ionospheric plasma density dis- for a quantitative comparison of radar observations
93 10 6 08 7
1513 1
1514 J. VY'ATIRMANN t-1 id.
with models, present results and discuss possible error Table I. DMSP-Ft7 pases ,elected forcolmparion ' th radkr
sources to reconcile the discrepancies. observations
Date UT EL S" , Ap
2. INSTRI NIENTS AND) DATA 30 May 1984 1203 83 54.2 120 2-
28 June 1984 1225 79 51.1 (I814
The Sondrestrom incoherent scatter radar is located 13 July 1984 1227 78 53.X 93 7o
on the west coast of Greenland at 67 geodetic and 08 October 1984 1151 75 8213 75 4-
15 January 1985 1216 NS 95S2 72 2,
74 invariant latitude (L - 13). It operates at 1290 11b6 JJaannuuaarryv 11998855 1111566 7799 9985..22 7755 2+
M Hz and measures along its line-of-sight several iono- 28 June 1985 t238 68 51.2 71 3 -
spheric parameters including plasma density and tern- 16 July 1985 1135 72 58.4 72 I1
peratures. Estimates of Debye length and electron-to- 25 September 1986 1224 69 73.8 69 3-
ion temperature ratio are used to obtain a corrected 26 September 1986 0208 63 113.5 68 3-
plasma density distribution, which is used in this
study. Up to 120 km altitude, the electron-to-ion tem-
perature ratio is assumed to be one, above 120 km the realigned the data according to the boundaries to
temperature ratios are inferred from the backscatter optimize the conditions for comparison, but still have
spectrum (at receiver gates spaced by 55 kmi). A con- assumed no significant variation of the ionospheric
tinuous temperature profile is obtained by inter- parameters between radar scan plane and spacecraft
polation and corrected densities are calculated iter- trajectory.
atively. starting from the raw densities. During our We had no radar operation specifically scheduled
experiments, the radar scanned the ionosphere in a for coincidence with DMSP-F7. Most of our data
plane pcrpendicular to the local shell of .onsktnt were :acquired during incoherent scatter World DaN
invariant latiude. Thus, each scan provides a map experiments. Table I lists the analyzed satellite passe
with date and time (UT). highest elevation (EL) of
of the electron density distribution in an ahitude vs wth sa te a nd t he ra,T ) thset e nint io gle
invariant latitude plane. During a typical scan, an the satellite seen from the radar, the solar zenith angle
(x)
integration time of 20 s corresponds to a spatial aver- at the radar site, the intensity ot the 11)7-cm flux
age over some 20 km (0.2 invariant latitude) at 120- (S,,) in units of 10 W cm ' s ', normalized to I
km altitude overhead the radar. AU and the geomagnetic activity (Kp) Note that the
The DMSP-F7 spacecraft moves on a 99 inclina- first nine events are dayside passes, two of them
tion, 830-km altitude, circular orbit that is Sun- occurred at dawn at solar zenith angles slightly exceed-
synchronous in the 1030-2230 local time plane. When ing 90 but in an already sunlit ionosphere. The last
passing through the Sondrestrom radar field-of-view, event is the only nighside example.
its trajectory crosses the contours of constant L at
an almost right angle, that is, parallel to the radar 3. METHOD
elevation scans. The on-board electron spectrometer
(RicH et al., 1985) measures one full spectrum per Teuulwýt eemn eesnadHl
(1R8IH5) me ae!a. ure on ful sectum er conductances from plasma parameters requires the
second over the energy range 30 eV-30 keV. The spec- alt
trometer is always directed upward and therefore does itude dependent evaluation of the conductivitx
not provide pitch angle discrimination, equations
We discuss cases of coincident radar and spacecraft Nc [ I.,4l, volvo
measurements that we had selected for a recent study ' - B kL',+QW + .Q(2) po
on Joule heating rates (WATERMANN and DE LA BEAU-
JARI)tIRE, 1990). During these events the satellite tra- Vo)o .x 1 ,) 1
jectory, mapped to 120 km altitude, was relatively , + Po `,+ .,,+ , p,, (I)
close to the radar scan plane (up to about 300 km
Fe[
horizontal distance). We assume, as a first approxi- Ne
mation, a two-dimensional ionosphere, with plasma an = B ~vo. + , - 1 +
parameters constant along the L shells. Close exam-
12,
ination of electron precipitation pattern and thermal
ionospheric plasma density suggests in some cases a - ±2 g Po, . . PjO, (2)
small misalignment between contours of constant
invariant latitude and poleward boundaries of the and a subsequent height integration. The parameters
diffuse and discrete aurorae. In such cases, we have Po. Po.. and pO denote the partial pressures of ionized
I
iI
Ionospheric electrical conductance comparisons 1515
atomic and molecular jxygen and nitric oxide, respec- -r(I, I,1
tively. The other symbols have their usual meaning. (a)
The plasma density N, is measured by the radar, the ,
.
gyrofrequencies are determined by the known geo- -
magnetic field, only the ion collision rates (primarily "a-
with neutrals) and electron collision rates (with ions IJ-, e
and neutrals) require model assumptions. We used , ,
I
the MSIS-83 neutral atmosphere model (HEDIN, 1983) (b)
and collision frequencies given by SCHUNK and NAGY
(1980) and integrated the conductivities from 80 to
500 km altitude. Integration up to 500 km can be
important in cases of grazing incidence sunlight
because the F-region can under such conditions 8,
contribute significantly to the height-integrated '
conductivities (RASMUSSEN et al., 1988; DE LA
BVAUJARDIERE et al.. 1991). rialr
Auroral electron precipitation is the only significant
source of E-region ionization in the dark ionosphere. I
The DMSP-F7 measurements of electron energy flux 73 75 77
and average energy were used to estimate the electrical invariant latitude (deg)
conductances after ROBINSON et al. (1987). Appli-
cation of their formula [equation (3) below] requires Fig. 1. (a) 1,p and (b) Yj, inferred from Sondrestrom radar
and DMSP-F7 electron precipitation measorements made
isotropic Maxwellian electron flux. The authors have on 26 September 1986. 0208 UT. in the dark ionosphere.
pointed out that for energies below about 500 eV the Only particle precipitation contributes to the E-region ion-
measured electron flux is considerably higher than ization and thus to Y, and YX,.
Maxwellian because of secondary electrons while at
the same time such low energy electrons contribute
very little to the conductance. Therefore we use only
the 460 eV-30 keV range. In (3). E denotes the average
energy in keV, (D the electron energy flux in ergs cm - ionization model if we do not use direct measure-
s -', and Ep,, the conductances in S (Siemens) ments. Having estimated the conductance term owing
to electron precipitation. Xp. (e ) and to photo-
1
40E ionization. E.H,(uvieuv), we use the following approxi-
16=+ . (3) mate combination to obtain the total conductance
,.1 = XA(e ) +Y.2,(uv, euv). (4)
= 0.45F0 5. Assuming a Chapman-of layer, such a combination
would be correct at each individual altitude and a
In Fig. I we compare the variation of the Pedersen height integration of total conductivities would yield
(a) and Hall (b) conductances (in Siemens) over the total conductances. Because the electron pre-
invariant latitude in the dark ionosphere. The solid cipitation and photoionization models that we use
line represents 1,., obtained from DMSP electron provide only height-integrated conductivities we fol-
precipitation measurements, the dashed line Z4,1 from low the opposite approach, first height integration of
radar thermal plasma measurements. We note that the precipitation and photoionization terms indi-
the results from both methods match reasonably well, vidually and then combination following (4). These
with good agreement in Y-, and less good agreement operations do not commute; thus, we introduce an
in .,,. error. If one of the terms on the right-hand side is
The situation changes once we compare obser- small compared to the other, the result is not very
vations from the sunlit ionosphere. The method to sensitive to such an error. Therefore, we restrict our
determine F from radar measurements of N(cid:127) and quantitative dayside comparison to those invariant
models of the neutral atmosphere and collision rates latitudes where the particle precipitation was weak
remains the same as above. However, it is no longer and therefore 1(e ) small. Specifically, in all ten it
sufficient to consider only electron precipitation as an cases presented we found lp(e ) < 1.0 S and E,(e )
ionization source. We have to rely on a photo- < 1.3 S.
I-
1518 J. WAFERMANN el 1/.
_ _ _ _ _ _not merely a result of poor spatial ortemporal coinci-
dence, for the following reasons. The discrepancy
7,)93 emerged under almost perfect as well as imperfect
2 -AlO0
2.A120 coincidence between radar and satellite measure-
1-AIZ
I3-A7 ments. The solar radiation is globally constant and
3-A69 usually varies only slowly with time (except as a conse-
...... "'A" .- All quence of solar flare effects, when the ionization can
.2..A75 change substantially within a few seconds). The solar
KPASa 20A72 zenith angle varies by not more than I per 110 km
displacement. The intervals selected arc characterized
(b) by low intensity of electron precipitation, such that the
6 7 1-,f72 auroral precipitation component of the conductances
3•tL was always below 1.3 S. We therefore suggest that the
2.A2SO dependence of 1, and E on X, or alternatively on
, ,season, has not yet been correctly modelled.
BARTH et al. (1990) demonstrated that the cor-
S3-,A69 relation between the solar 10.7-cm flux and the solar
4-A75
( Lyman-u intensity is generally weak. in the short-term
. over a few solar rotations as well as in the long-term
3..-A0 over a complete solar cycle. In particular during solar
s A75 minimum, the Lyman-2 intensity can drop con-
20 0 0 A7,2 12 _, siderably. while the 10.7-cm flux does not fall below,
solar zenith angle x (deg) a threshold of about 70 units. The Lyman-o radiation
(10.2 eV) is the most intense solar UV emission. It
Fig. 4. Differences between measured and modelled iono- does not provide enough energy for any of the more
spheric conductances. (a) Yp and (b) El. owing to solar
UV/FUV radiation. The numbers to the right of the triangles efficient atmospheric ionization processes
indicate the f0.7-cm flux in units of 10 '" W m -s '. the
numbers to the left the Kp index during the time ol the DMSP N, + 15.6 eV - N _ + e
pass. The differences increase with decreasing solar zenitV
angle. 0_ + 12.1 eV - 0 + e
O+13.6eV --O+e
is evident. The three events with larger S,, suggest that. However, other terms of the Lyman series (Lyman-fl
for a constant ). the variation of 1, with S. is not and higher terms) are very efficient in 0. ionization.
propely accounted for by the model. These systematic Still other UV and EUV solar emissions are neither
differences may as well be interreted as seasonal related to the 10.7-cm emission nor to the Lyman
effect. Owing to the high latitude of Sondrestrom series. Altogether we expect that the ionizing com-
(solar zenith angles near 50 occur only in summer) ponents of the solar spectrum are not well correlated
season and solar zenith angle are correlated in our with S,,. It remains to be shown whether Lyman-, can
data. There may also exist a dependence on the geo- serve as a more appropriate parameter.
magnetic activity because the most disturbed interval The geomagnetic activity influences the state of the
(Kp = 7o) is associated with the largest discrepancy. high atmosphere, for example, by heating and up-
The comparison of measured and modelled Hall welling of the neutral air, accompanied by a change
conductances [Fig. 4(b)] shows a similar trend, except in the density ratio of atomic to molecular neutral
that no systematic variation with geomagnetic activity species. Such heating can be very localized (e.g. during
(Kp dependence) is observed. The difference between substorms) and may not be well represented by the
the ROBINSON and VONDRAK (1984) model of 1,, and global Kp index. Particularly at high latitudes the AE
the radar observations is much larger than in the case index of electrojet intensity may have to be incor-
of the Pedersen conductance. porated into the conductivity model. None of the
models accounts for such an effect.
Some uncertainties in the application of the ROBIN-
5.DICUSINSON et al. (1987) precipitation ionization model
The observed discrepancies between models and remain, despite the fairly good agreement between
Sondrestrom radar measurements ot L, and El are their model and height-integrated conductances
Ionospheric electrical conductance comparisons I 19
obtained from radar measurements. The model (1988) and others. The discrepancy between radar
assumes isotropic Maxwellian electron flux. Because measurements and model calculations increases with
the DMSP electron spectrometer was always pointing decreasing solar zenith angle and. at least in the case
toward the zenith, the hypothesis of isotropic flux can of the Pedersen conductance, with increasing solar
be neither confirmed nor denied. It is known [e.g. 10.7-cm flux and possibly with increasing geomagnetic
from auroral electron fluxes observed on Spacelab- activity. Possible error sources include unsatisfactory
I, see BARROW et al. (1991)] that auroral electron modelling of the solar zenith angle dependence and
precipitation is sometimes nonisotropic. the poor correlation between certain components of
the solar UV and EUV radiation and 10.7-cm flux
intensity (BARkit et al., 1990). Variations of the state
6. CONCLUSION of the high-latitude neutral atmosphere. owing to vari-
ations in geomagnetic activity, in particular substorm
We have compared ionospheric plasma parameters activity, and to seasonal effects, are neglected in the
tn the auroral zone, inferred from nearly coincident moesThsayaocotiuettherrsBt
Sondrestrom radar and DMSP-F7 measurements aid models. This may also contribute to the errors. But
obtained the following results. our data base is still too small to identify definite Kp-
The Pedersen and Hall conductance components dependent and seasonal effects. Analysis of a larger
associatedersthenergeticalctronduc cecipitones number of cases is required to assess quantitatively
associated with energetic electron precipitation are the dependence of Pedersen and Hall conductances on
wellbre prtshe nemd odl o ROBNSO ci . wvrious geophysical conditions including monitored
(1987). It is therefore possible to use their formulae varions inothestat ofdthe neutral mosphere.
to approximately separate the electron precipitation
from the photoionization contribution to the height- Acknowledyemnents-- This work was supported b.(cid:127) NSF
integrated conductivities, in particular when the under Cooperative Agreement ATM-8822560 and Grant
ionization due to particle precipitation is low. ATM-9017725. by the Air Force Geophysics Laboratory
The ionization of the atmosphere induced by solar (now Phillips Laboratory.'Geophysics Directorate) under
UV and EUV is systematically overestimated by most Contract F19628-87-K0006, and by the Air Force Office of
Scientific Research under Task 231IG5. We thank the
available conductance models such as those presented referees for their constructive comments which helped to
by ROBINSON and VONDRAK (1984), RASMUSSEN et al. improve the paper.
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