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OCR for page 183
1 2
INTRODUCTION
Electrical Structure of the
Middle Atmosphere
GEORGE C. REID
..
NOAA Aeronomy Laboratory
Conventional usage divides the atmosphere into lay-
ers on the basis of the average temperature profile. The
stratosphere is the region of positive vertical tempera-
ture gradient extending from the tropopause to a height
of about 50 km, and the overlying region of negative
temperature gradient is the mesosphere, extending to
about 85 km altitude, where the lowest temperatures in
the atmosphere are reached. The main heat source in
both of these regions is provided by absorption of solar-
ultraviolet radiation by ozone. At still greater heights
lies the thermosphere, in which absorption of extreme-
ultraviolet radiation causes the temperature to increase
again with height. This chapter is concerned mainly
with the electrical properties of the upper stratosphere
and the mesosphere. The troposphere and lower strato-
sphere were considered in Chapters 11 and 12 (this vol-
ume) and the thermosphere is discussed in Chapter 14.
Both the composition and the temperature play im-
portant roles in determining the electrical structure. As
noted above, the temperature in the stratosphere rises
from typical tropopause values near 200 K to values of
about 270 K at the stratopause (~50 km), above which
the temperature decreases to mesopause values that are
seasonally and latitudinally variable, occasionally drop-
ping below 140 K in the high-latitude summer.
~3
The principal atmospheric constituents are molecular
oxygen and nitrogen, just as in the lower atmosphere;
but there are a number of minor constituents that are
important from the point of view of the electrical prop-
erties. Among these are nitric oxide (NO), which dif-
fuses into the region from sources below and above;
atomic oxygen (O) and ozone (03), which are formed
locally by photodissociation of 02; and water vapor,
which can be transported from the troposphere as well
as being locally produced. The role of aerosols in the
atmosphere at heights above 30 km is uncertain and con-
troversial and is an area of active study. The occasional
presence of noctilucent clouds at the high-latitude sum-
mer mesopause and the more regular existence of a sum-
mertime polar scattering layer seen by satellites have
certainly shown that aerosols (probably ice crystals) can
exist near the top of the region, but the gap between the
mesopause and the well-known aerosol layers of the
lower stratosphere remains relatively unexplored.
In what follows, sources of ionization, the ion chemis-
try that determines the steady-state ion composition,
and the present status of our knowledge of aerosol distri-
bution are discussed. The final two sections discuss the
theory and measurement of conductivity and electric
fields in the middle atmosphere.
OCR for page 184
184
SOURCES OF IONIZATION
Figure 13.1 shows typical ion-pair production rates
(q) at middle latitudes during daytime. Throughout the
stratosphere, galactic cosmic rays provide the principal
ionization source, as in most of the lower atmosphere.
The cosmic-ray ionization rate does not vary diurnally
but does vary with geomagnetic latitude and with the
phase of the 11-year solar cycle. Heaps (1978) provided
useful relations for computing the rate of ion production
at any latitude and time. Roughly, the ion-production
rate above 30 km increases by a factor of 10 in going
from the geomagnetic equator to the polar caps at sun-
spot minimum (cosmic-ray maximum) and by a factor
of 5 at sunspot maximum. The solar-cycle modulation is
near zero at the equator, increasing to a factor of about 2
in the polar caps. The ionization rate above 30 km is
approximately proportional to the atmospheric density.
These properties are a result of (a) the shielding effect of
the geomagnetic field, which allows cosmic-ray parti-
cles to enter the atmosphere at successively higher lati-
tudes for successively lower energies, and (b) the reduc-
tion in cosmic-ray flux in the inner solar system as solar
activity intensifies.
Superimposed on these long-term global variations
are brief reductions in cosmic-ray flux known as For-
bush decreases, after their discoverer (Forbush, 1938~.
Forbush decreases occur in coincidence with geomag-
netic storms and are of brief (hours) duration. However,
as their magnitude can be as large as some tens of per-
cent, they can change global electrical parameters sig-
nificantly.
In the mesosphere the major daytime source of ioniza-
tion in undisturbed conditions is provided by the NO
90
8C
7C
60
-
,c~
I 50
4O
30 _
toys ~I ~I
a,+ UV
OR
~NO + Lyman - ~
Middle - Atmosphere ionization Sources
X=45°
1 1~111111 1 1 1
-
-
20 1 11 1 1lil1 1
10 1 10° 101 1o2
( -3 -1)
SPE /
12 Me
111111 1 1 1 11111
103 104
FIGURE 13.1 Typical ion-pair production rates in the middle atmo-
sphere.
GEORGE C. REID
molecule, whose low ionization potential of 9.25 elec-
tron volts (eV) allows it to be ionized by the intense solar
Lyman-alpha radiation. The concentration of NO in
the mesosphere is not well known and is almost certainly
variable (Solomon et al., 1982a) in response to meteoro-
logical factors. The production-rate profile in Figure
13.1 is an estimate based on reasonable values for the
NO concentration and the solar Lyman-alpha flux,
which is itself a function of solar activity (Cook et al.,
1980~.
At the upper limit of the middle atmosphere, signifi-
cant amounts of ionization are produced by solar x rays,
forming the base of the E region of the ionosphere, and
by ionization of O2 in its metastable Id\ state, which is a
by-product of ozone photodissociation. While these
sources are never competitive with the NO source in
terms of ionization rates, they give rise to different pri-
mary positive-ion species (N2 and O2 as opposed to
NO + ), and hence to different chemical reaction chains.
A sporadic and intense source of ionization at high
latitudes is provided by solar-proton events (SPE) (Reid,
1974), and Figure 13.1 shows an ionization-rate profile
calculated for the peak of a major SPE in May 1959.
These events are caused by the entry into the atmo-
sphere of particles accelerated during solar flares and
traveling fairly directly from the Sun to the Earth. The
particles are mostly protons, with much smaller fluxes of
heavier nuclei and of electrons, having typical energies
of 1 to 100 MeV and considerably less atmospheric pene-
tration power than galactic cosmic rays. As a conse-
quence, their effects are largely confined to high mag-
netic latitudes ~-60°) and to altitudes well above the
lower stratosphere. Solar-proton events typically reach
their peak intensity within a few hours of a major solar
flare and then decay exponentially over the following
day or two. Their occurrence is a strong function of the
phase of the solar cycle, as illustrated in Figure 13.2,
which shows the distribution in the 1956-1973 period of
polar-cap absorption (PCA) events and of ground-level
events (Pomerantz and Duggal, 1974~. Polar-cap ab-
sorption is the name given to the intense radio-wave ab-
sorption caused by the enhanced mesospheric ionization
during an SPE, while ground-level events are the rare
events with a large enough high-energy flux to cause an
increase in cosmic-ray neutron monitors at the surface.
The frequency of the events is related to the solar-activ-
ity cycle, which peaked about 1958 and 1969, but in-
tense events can occur at any time, as evidenced by those
of February 1956 and August 1972. Figure 13.1 shows
clearly that SPEs cause major alterations in middle-
atmospheric ionization rates, and hence in the electrical
parameters.
Energetic electron precipitation from the radiation
OCR for page 185
ELEC TRI CAL S TR UC TURK OF THE MIDDLE A TMOSPHERE
An
20
10
-
us
-
o
JO o _
Aug. 1972
PCA EVENTS
1
Feb. 19 56
GROUND-LEVEL EVENTS
56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73
FIGURE 13.2 Distribution and intensity of solar energetic-particle
events, 1956-1973. The peak absorption in the upper part is a measure
of the intensity of the polar-cap absorption (PCA) events resulting
from high-latitude ionization in the mesosphere; the lower part shows
the intensity of the cosmic-ray (CR) increases recorded by neutron
monitors and caused by solar-particle-induced nuclear reactions in the
lower stratosphere.
belts also contributes to middle-atmosphere ionization
but in a manner that is highly variable in both latitude
and time. During major electron precipitation events
this can become the dominant source of ionization
above 70 km for brief periods, and ionization rates can
be as high as 105 cm~3 sect above 80 km (Reagan,
1977~. Vampola and Gorney (1983) deduced zonally av-
eraged ionization rates due to energetic electron precipi-
tation at several magnetic latitudes. Maximum ioniza-
tion rates occur between 80 and 90 km and vary
between about 0.7 cm ~ 3 see - ~ at 45° and 6 cm ~ 3 see - ~
at 65° latitude. At the higher latitudes energetic elec-
trons are competitive with solar Lyman-alpha as an ion-
ization source even in daytime. They are probably the
dominant source above 70 km at night, when the main
competitor is photoionization of NO by the weak
Lyman-alpha radiation scattered from the Earth's hy-
drogen geocorona (Strobe! et al., 1974~.
Bremsstrahlung x rays, generated by the energetic
electrons, ionize weakly at heights below 60 km
(Luhmann, 1977, Vampola and Gorney, 1983) but are
probably rarely competitive with cosmic rays as a global
ionization source.
ION CHEMISTRY IN THE MIDDLE
ATMOSPHERE
The principal primary positive ions produced in the
middle atmosphere are N2+, O2+, and NO +, all of which
IS5,
participate in a wide range of ion-molecule reactions
that lead to a rich spectrum of ambient ions. An equally
rich spectrum of negative ions is generated by reactions
that are initiated by the attachment of electrons to form
the main primary species O2- and O ~ . In this section the
current state of our knowledge of this ion chemistry and
of the steady-state ion composition that it produces are
discussed. More detailed treatments can be found in re-
view articles by Ferguson et al. (1979) and Ferguson and
Arnold (1981~.
Positive ions
The first measurements of positive-ion composition in
the mesosphere were made by a rocketborne mass spec-
trometer in 1963 (Narcisi and Bailey, 1965~. The domi-
nant species below the mesopause were found to be pro-
ton hydrates, i.e., members of the family H+(H2O)n,
with a sharp transition at about the mesopause to such
simple species as O2+, NO +, and several metallic species,
probably of meteoric origin. Many subsequent measure-
ments have verified these results and have shown that
the size spectrum of the proton hydrates is very tempera-
ture sensitive. At the cold high-latitude summer meso-
pause, as many as 20 water molecules have been seen
clustered in individual ions (Bjorn and Arnold, 1981~.
The currently proposed positive-ion reaction scheme
leading from the primary ions to the proton hydrates is
illustrated in Figure 13.3. Since N2+ is rapidly converted
into O2+ by charge exchange with O2; the two primary
ions of concern are O2+ and NO +. The chain that con-
verts O2+ into the proton hydrates was identified by
Fehsenfeld and Ferguson (1969) and Good et al. (1970)
and is fairly straightforward. Clustering of O2+ to O2
forms 04, which rapidly undergoes a switching reac-
tion in which the O2 molecule forming the cluster
switches with an H2O molecule to form O2+(H2O).
When they are energetically allowed, such switching re-
actions are usually fast, occurring at virtually every col-
lision between the two species. Subsequent collisions
with water molecules lead rapidly to the proton hy-
drates.
The failure of this mechanism above the mesopause is
probably due to a combination of factors: the decreasing
water-vapor concentration, the increasing electron con-
centration leading to shorter ion lifetimes against re-
combination, and the increasing concentration of
atomic oxygen. The latter attacks the O4+ clusters
through the reaction
O4+ + O - O2+ + O3 (13.1)
The chain of reactions leading from NO + to the pro-
ton hydrates is less certain but probably involves several
OCR for page 186
86
FIGURE 13.3 Schematic diagram of the
principal positive-ion reactions in the meso-
sphere. The details of the NO+ hydration
scheme, enclosed by the broken lines, are not
yet established.
. _
NO+(N ~ r
l _
NO 002'
T ~
1
NO+(H20XNA
| - -a I L - . ~ I
I NAHUM I
NO (H,O)~No)
steps of clustering and switching as shown in Figure
13.3. This mechanism, first proposed by Ferguson
(1974), yields steady-state ion distributions that are rea-
sonably close to those observed when appropriate reac-
tion rates are used in model calculations (Reid, 1977~.
Most of the critical reaction rates are still unmeasured at
mesospheric temperatures, however.
In the stratosphere, the picture is rather more compli-
cated. Mass spectrometry has recently been developed
for use at the high ambient gas pressures of the strato-
sphere, and measurements of positive-ion composition
have been made from rockets and balloons (Arnold et
al., 1978~. These experiments showed the existence of
the proton hydrates, as in the mesosphere, and also that
below about 40 km the proton hydrates are replaced as
the dominant species by ions with a core of mass 42 emu.
This has been tentatively identified as protonated aceto-
nitrile, H + (CH3CN) (Arnold et al., 1978) an identifi-
cation that is reasonable in view of the high proton affin-
ity of CH3CN and its recent discovery in the troposphere
(Becker and Ionescu, 1982~ .
It should be emphasized that our knowledge of strato-
spheric ion composition is very sketchy. Almost nothing
is known of the composition at heights below 30 km or at
locations other than continental middle latitudes.
Negative ions
Our knowledge of negative-ion composition in the
middle atmosphere is in an unsatisfactory state. Labora
GEORGE C. REID
ionization
1 · 1
. I _
1 N2+
-
02+ 1 -
!~ -~T
~1
~3
~ N~(H30)3 1 j ~
~1
| H3O (H20) ~
- ;~1
.
~ t
3
tory measurements of the negative-ion reactions
thought to be the most important ones in the atmosphere
have led to the reaction scheme shown in Figure 13.4
(Ferguson et al., 1979~. In this scheme, direct attach-
ment of electrons takes place only to O2 and 03; associa-
tive detachment reactions occurring chiefly with atomic
oxygen quickly destroy most of the resulting O2 and O~
ions in regions where O is present. The ions that escape
destruction in this way, however, go on to form a wide
variety of species whose electron affinity increases as we
progress down the chain. In the absence of annihilation
by positive ions, the dominant terminal species in the
chain would be the nitrate ion, NO3, with the high elec-
tron affinity of 3.9 eV (Ferguson et al., 1972~.
Mass-spectrometer measurements of negative-ion
composition are much more difficult to make than the
corresponding positive-ion measurements, largely ow-
ing to the problem of contamination by electrons. As a
result, few measurements have been made in the meso-
sphere, and these have given somewhat conflicting
results (Narcisi et al., 1971; Arnold et al., 1971, 1982~.
The predicted dominance of such species as NO3 and
CO3 at heights below 80 km appears to be borne out,
but many unidentified light ions have been seen in the
mesosphere. Above 80 km, there appears to be a layer of
heavy (> 100 emu) ions (Arnold et al., 1982) that may
be a result of attachment to neutral species of meteoric
origin, perhaps forming the very stable silicon species
SiO3 (Viggiano et al., 1982~.
In the stratosphere, the first mass-spectrometer mea
OCR for page 187
ELECTRICAL STRUCTURE OF THE MIDDLE ATMOSPHERE
ionization
02,1~tO 1O3 O
°2 ~ ~103O2 O
__ I On ~
I NO NO2 CH4
I I O I NO2 ~· l OH- I
1 1 103
L NO3 ~
1
~1o 1 1
~1 ~
~ COZ CO3- ~
O
NO
NOR
O3
CH4
FIGURE 13.4 Schematic diagram of the principal negative-ion re-
actions in the mesosphere. The chain leading to the terminal species
HCO3 is probably of minor importance.
surements (Arnold and Henschen, 1978) showed the
dominance of heavy ions. Laboratory measurements by
Viggiano et al. (1980) showed that in the presence of
sulfuric acid the species HSO4 would become an impor-
tant core ion. Sulfuric acid is known to be the major
component of the stratospheric aerosol layer and is a by-
pro~luct of volcanic activity, as discussed in Chapter 12
(this volume).
There thus appear to be three fairly distinct negative-
ion strata in the middle atmosphere. The central region
between about 55 and 80 km is formed mainly by ion-
molecule reactions involving the commoner minor con-
stituents, following initial attachment to O2. This cen-
tral layer has layers of heavier ions both above and
below the upper one probably a result of reactions in-
volving meteoric species and the lower one built by clus-
tering around lISO4 . At present this is a very sketchy
and incomplete picture, and many more observations
are needed to clarify it.
~7
got
an~-
~ 6c
-
-
Q) 50
40
an
1 1 1 1
QUIET
Proton Hydrates
H+(H~O)n
O2 SPE
2 Intermediates
,
~1 1
3 )
20 40 60 80 100 20 40 60 80 100
Percentage Composition
FIGURE 13.5 Model calculations of the steady-state positive-ion
composition of the middle atmosphere, omitting the reactions leading
to nonproton hydrates in the stratosphere. The left-hand panel repre-
sents quiet conditions, and the right-hand panel is for the ease of an
intense solar proton event.
Model Calculations
1 co2
.If the rates of production of the various ion species
HCO3-and the rates of the important chemical reactions are
known, it is possible to calculate the steady-state ion
composition. Many such calculations have been made,
and examples are shown in Figures 13.5 and 13.6.
Figure 13.5 illustrates the positive-ion composition
calculated for an intense solar-particle event (right-
hand panel) and for undisturbed daytime conditions
90
80
70
60
-
C5)
_ 50
40
30:
20'
1 1 ~1
<~CO3
NO3
QUIET l SPE ~
20 40 60 80 1 00 20 40 60 80 1 00
Percentage Composition
NO3
FIGURE 13.6 Model calculations of the steady-state negative-ion
composition of the middle atmosphere, omitting reactions involving
meteoric species at the higher altitudes and reactions involving sulfur
species in the stratosphere. The left-hand panel represents quiet condi-
tions, and the right-hand panel is for the case of an intense solar proton
event.
OCR for page 188
188
(left-hand panel). The stratospheric ion chemistry lead-
ing to the formation of nonproton hydrates has been
omitted. In both cases the proton hydrates are dominant
in the lower mesosphere with a fairly abrupt transition
to NO + at greater heights in the quiet case and to mostly
O2+ in the SPE case. A thin layer containing mostly the
intermediate clusters of O2+ or NO + separates the two
regions. The tendency toward horizontal layering of the
principal proton-hydrate species is caused by the shift in
equilibrium toward lighter species as the temperature
increases.
Figure 13.6 shows the result of similar calculations of
negative-ion composition, again omitting both the up-
per meteoric layer above 80 km and the stratospheric
region of heavy HSO4- derived ions. In the undisturbed
case NO3- is dominant in most of the middle atmo-
sphere, but the great enhancement in the rate of ion-ion
recombination during the SPE inhibits the formation of
NO3 and leads to an increase in the fraction of CO3-.
The initial ions O ~ and O2- are the main components at
the top of the mesosphere.
The ultimate loss of ions in the middle atmosphere
takes place mainly through recombination with elec-
trons or with ions of the opposite charge. In the case of
negative ions, photodetachment by sunlight provides
another loss mechanism about which little quantitative
information is available for the main atmospheric spe-
cies. Loss to aerosol particles is presumably also a signifi-
cant sink, especially in the vicinity of the stratospheric
aerosol layer and possibly of the polar aerosol layers near
the summer mesopause. Neither photodetachment nor
aerosol attachment were included in the calculations
represented by Figure 13.6.
MESOSPHERIC AEROSOLS
Stratospheric aerosols were discussed in Chapter 12
(this volume). The aerosol content of the mesosphere
and the effect of these aerosols on the electrical proper-
ties are much more speculative. Noctilucent clouds pro-
vide direct evidence that particulate material does exist
at mesospheric heights, at least on some occasions.
These silvery-blue translucent clouds appear sporadi-
cally at high latitudes during the summer months, when
their great height allows them to be seen by scattered
sunlight long after the Sun has set at the surface. Their
phenomenology has been reviewed by Fogle and
Haurwitz (1966~. Optical measurements suggest that
the particle concentrations are 1 to 50 cm-3 in these
clouds, with an individual particle radius of the order of
0.1 ,um. Satellite measurements of backscattered sun-
light reveal the existence near the mesopause of a denser
semipermanent particle layer over the summer polar
GEORGE C. REID
cap (Donahue et al., 1972; Thomas et al., 1982), which
is probably related to the noctilucent cloud layer.
There is general agreement that the particles forming
these layers are ice crystals formed in the extremely low
temperatures of the summer mesopause region by con-
densation from the low background concentration of
water vapor (Hesstvedt, 1961~. Several model calcula-
tions have shown that it is reasonable to expect ice crys-
tals to form under these conditions (e.g., Charlson,
1965; Reid, 1975; Turco et al., 1982) provided that suit-
able nucleation centers exist. Mass-spectrometer results
suggest that nucleation does occur on positive ions
(Goldberg and Witt, 1977; Bjorn and Arnold, 1981),
and the particles themselves could then act as surfaces
for ion capture. There is evidence for abnormally low
electron concentrations in the high-latitude summer
mesopause region, perhaps indicating enhanced elec-
tron loss through attachment to particles (e. g., Pedersen
et al., 1970~.
Below the mesopause, the evidence is much less di-
rect. Volz and Goody (1962) found evidence from twi-
light measurements of low concentrations of dust parti-
cles throughout the mesosphere. Hunten et al. (1980)
calculated the flux of particles produced by condensa-
tion of meteor ablation products. Depending on the
model conditions, this calculation predicted meso-
spheric concentrations of 102 to 103 cm ~ 3 and individual
particle radii of a few nanometers. The influence of such
a particle distribution on the ion and electron concen-
trations would be small. Chesworth and Hale (1974)
proposed the existence of mesospheric particle concen-
"rations of 103 to 104 cm - 3 to explain certain discrepan-
cies in the electrical parameters. The evidence for such
large concentrations was indirect, and further work is
needed in this area. In particular, the relationship, if
any, between the meteor-ablation particles of Hunten et
al. (1980) and the particles suggested by Chesworth and
Hale (1974) should be studied.
CONDUCTIVITY IN THE MIDDLE
ATMOSPHERE
The current density, j, and the electric field, E, are
related by the familiar Ohm's law expression
E.
1 = (J,
(13.2)
where (' is the conductivity. In the lower atmosphere
and most of the middle atmosphere, (, is a scalar, and the
electric field and the current lie in the same direction.
Above about 70 km, however, collisions between elec-
trons and air molecules become infrequent enough that
the bending of the electron path by the Earth's magnetic
field becomes appreciable between collisions, and mo
OCR for page 189
ELECTRICAL STR UCTURE OF THE MIDDLE ATMOSPHERE
lion perpendicular to the field becomes less easy than
motion along the field. In these circumstances, ~ be-
comes a tensor, and an applied electric field with a com-
ponent perpendicular to the magnetic field drives a cur-
rent in a different direction. This anisotropy of the
conductivity becomes a dominant influence on the elec-
trical properties of the thermosphere (see Chapter 14,
this volume).
Conductivity can be measured directly by rocket-
borne probes, and a substantial number of such conduc-
tivity values are reported in the literature, especially us-
ing the so-called blunt-probe technique (Hale et al.,
1968~. The problem of shock-wave effects associated
with a supersonic rocket are usually avoided in these ex-
periments by deploying the probe with a parachute at
the top of the trajectory and making the measurements
during the subsonic descent.
In the presence of a mixture of ions, the conductivity
can be expressed in terms of the mobilities of the individ-
ual species as
a= eEn:+k2+ + eEn'-k7~ + eneke, (13.3)
where n and k are the concentrations and mobilities of
the positive ions, negative ions, and electrons. tEqua-
tion (13.3) is identical to Eq. (12.1) in Chapter 12, this
volume, except that in the middle atmosphere electrons
come into consideration. ~ An experimentally derived re-
lationship between reduced mobility and ion mass in ni-
trogen is shown in Figure 13.7 (Meyerott et al., 1980~.
1000
100
Jo _
1 1 1 1 1 1 1
2
Reduced Mobility (10-4m2volt-1s-1)
FIGURE 13.7 Reduced mobility as a function of ion mass.
189
The reduced mobility, ho, is the mobility in a standard
atmosphere and is related to the actual mobility, k, by
k = ko(2-73)( p )' (13.4)
where T is absolute temperature and p is pressure in mil-
libars.
The ion mobility can be measured by the Gerdien
condenser technique (e.g., Conley, 1974), and a num-
ber of measurements using rockets have been reported.
As discussed by Meyerott et al. (1980), there is no gen-
eral agreement among the various measurements, al-
though the data of Conley (1974) and Widdel et al.
(1976) suggest a single reduced mobility of about 2.7 X
10-4 m2 V~~ sect for the entire middle atmosphere.
This corresponds to an ion mass of about 30 emu accord-
ing to Figure 13.7, which is clearly in disagreement with
both the mass-spectrometer measurements and the
model studies discussed above. Meyerott et al. (1980)
suggested that the Gerdien condenser measurements
were affected by the breakup of cluster ions by both
shock-wave and instrumental electric-field effects.
Much heavier ions have been seen in some flights. Rose
and Widdel (1972), for example, reported a group of
positive ions above 60 km whose altitude dependence
gives a reduced mobility of about 3.7 X 10-5 m2 V-i
sec~ i, corresponding to an ion mass of several thousand
emu. The origin of these ions is unknown.
Ion concentrations are also measured by the Gerdien
condenser technique, and here again there are puzzling
differences between observations and predictions.
Above about 60 km, ionization of nitric oxide by solar
Lyman-alpha radiation becomes an important source,
and a considerable amount of variability in ion concen-
tration is expected (even in quiet conditions) owing to
the variability in NO concentration (Solomon et al.,
1982b). At lower altitudes, however, the only signifi-
cant source is cosmic-ray ionization, and the ion-pro-
duction rate due to cosmic rays can be calculated with a
fair degree of. certainty. The loss rate through ion-ion
recombination is also well known (Smith and Church,
1977; Smith and Adams, 1982), and the steady-state ion
concentration can thus be calculated with correspond-
ing accuracy. Figure 13.8 shows a typical set of results.
The points are taken from experimental data reported
by Widdel et al. (1976), and the solid line is the positive-
ion concentration calculated from the same model used
to produce the ion-composition profiles shown in Figure
13.6. The overall shape of the altitude profile shows rea-
sonable agreement, but the measured values are gener-
ally lower than the calculated values by about a factor of
3. A similar discrepancy was found by Meyerott et al.
OCR for page 190
190
90
so
Al I 1 1 11111| I I I Ill!| i~IT
70
60L
ME 50 _
~ 40 _
I
30 _
20 _
10 _
1 1 1 1111
1o1 1o2
1: 1
1975 ~
/
1 1 1 111111 1 1 1 11111
103 104
lon Concentration (cm-3)
FIGURE 13.8 Results of measurements of ion concentration (Wid-
del et al., 1976) and of model calculations for quiet conditions.
(1980), who suggested that it might be due to the lack of
the experimental technique to measure low-mobility
ions. Further measurements are clearly needed.
Another puzzling feature shown in Figure 13.8 is the
abrupt increase in concentration occurring at about 65
km in the 1968 data and 45 km in the 1975 data. A simi-
lar abrupt increase in conductivity at about 65 km is
seen in blunt-probe measurements and has been attrib-
utec] to the transition from a cosmic-ray ionization
source below to a Lyman-alpha source above (Mitchell
and Hale, 1973~. The model results show such a transi-
tion, but it is much less abrupt and smaller in magnitude
than the one observed. The same explanation is cer-
tainly ruled out for the 45-km transition, since solar Ly-
man-alpha radiation is almost totally extinguished be-
low 50 km.
Figure 13.9 shows the theoretical profiles of positive-
ion, negative-ion, and electron contributions to the day-
time conductivity, using the same model as before. The
dotted curve shows an average of several blunt-probe
measurements of positive conductivity in quiet condi-
tions (Mitchell and Hale, 1973) and is taken as represen-
tative of the current experimental situation. Clearly the
overall shape of the theoretical positive-conductivity
profile matches the observations reasonably well par-
ticularly below 60 km where cosmic rays are the princi-
pal source of ionization. At higher levels, the expected
GEORGE C. REID
variability of ion concentration should lead to a corre-
sponding variability in positive conductivity.
The role of electrons is noteworthy. The theoretical
profiles show that electrons make a negligible contribu-
tion to the total conductivity below 50 km but com-
pletely dominate the conductivity above 60 km, where
they give rise to an extremely steep upward gradient in
conductivity [the"equalizing" layer (Dolezalek, 1972~.
The electron mobility is so large that the electron contri-
bution to the conductivity becomes equal to the ion con-
tribution at a level where the electron concentration is
less than 1 cm ~ 3. In this region the model predictions of
the electron-ion balance are not trustworthy. In partic-
ular, the model used here does not include negative-ion
photodetachment as a source of electrons, as photode-
tachment cross sections of the principal atmospheric
negative ions are not known. Even small amounts of
photodetachment, however, will have an important ef-
fect on electron concentrations in the region of the stra-
topause, and hence on the model conductivity profiles.
The conductivity is greatly reduced at night at all
heights above 50 km. The decrease is partly due to the
absence of solar ionizing radiation and partly to changes
in neutral chemistry, notably the conversion of atomic
oxygen to ozone in the mesosphere. Figure 13.10 shows
model profiles of daytime and nighttime conductivity
for quiet mid-latitude conditions, in which the night-
time sources of ionization are mostly solar Lyman-alpha
radiation scattered from the geocorona and the zonally
averaged flux of energetic electrons at 45° magnetic lati-
tude given by Vampola and Gorney (1983~. The night
90
80
7n
~ ~ I 1 1 1 11111
_ _
_
E 60 _
c)
_ 50 ~
40 _
30 _
1 1 1 1
· ~~ Elections
.-~ A!
-a-. ,?Y Quiet: x = 45°
/
1 1 111111 1 ~1 k
Positive
ions
20 I,,, ,,,1 , , ,,,, ,,1 , ,,,, ,,,1 , ,,,, 1 1,1 , ,,,,, 1
-l2 1o-ll 10-10 10_9 10-8 10-7
Conductivity (MHO M-1)
FIGURE 13.9 Middle-atmosphere daytime conductivity. The dot-
ted curve shows direct measurements (Mitchell and Hale, 1973) of pos-
itive conductivity, and the other curves show the result of model calcu-
lations of the contributions of the electrons, positive ions, and negative
ions.
OCR for page 191
ELECTRICAL STRUCTURE OF THE MIDDLE ATMOSPHERE
90
80
70
-
Cal
._
a)
I
50
40
30
1 1 1 1 1 1 1 // '
sight
/Day
/ 1 1 1 1 1 1 1
10 - 12 10 - 10
10-8 10ff
Conductivity (MHO mat)
10-4 10-2
FIGURE 13.10 Model calculations of the daytime and nighttime
conductivity of the middle atmosphere during quiet conditions.
time equalizing layer lies nearly 10 km higher than the
daytime layer, and the transition between the two
should take place rapidly during twilight.
During a solar-proton event, the conductivity profile
of the middle atmosphere is greatly changed. Individual
events are so variable in ionization rate, however, that a
representative SPE-conductivity profile would not be
meaningful. Figure 13.11 is simply intended as an illus-
tration of the conductivity profile calculated from the
above mode! for a fairly intense SPE during daytime.
The largest fractional increase from the quiet-time con-
ductivity occurs between 50 and 60 km, and the greatly
enhanced ionization in this region tends to eliminate, or
90
70
60
50 t
- F I i,lll trill 1l,ll ',lil ilill l,,l, ,~ ,:
40 _
30
V _
20 1 ~1, ,,,~1' ',~1' t''1~ ''t1' t''1' ''lit '''1' 1:
10-12 10-11 10-10 10-9 10-8 10-7 10-6 10-5 10-4 10-3
Conductivity (MHO M-1)
FIGURE 13.11 Model calculations of the daytime conductivity of
the middle atmosphere during quiet conditions and during an intense
solar-proton event.
191
at least substantially reduce, the steep gradient immedi-
ately above.
To summarize, neither direct measurements nor
modeling studies of the bulk electrical parameters of the
middle atmosphere are in a satisfactory condition. The
measurements are difficult to make and tend to be beset
by problems of interpretation. Models are based on defi-
cient knowledge of several of the important mechanisms
and cannot yet give more than estimates of the altitude
profiles of such important parameters as the ion concen-
tration and the conductivity. Much remains to be done
to place our knowledge on a secure footing.
ELECTRIC FIELDS IN THE MIDDLE
ATMOSPHERE
The normal electric field in the middle atmosphere is
a superposition of fields mapped upward from thunder-
storm generators in the lower atmosphere and fields
mapped downward from magnetospheric and ionos-
pheric dynamo generators. The possible existence of lo-
cal electric-field generators within the middle atmo-
sphere is a controversial topic and will be discussed
briefly later.
The mapping of the fair-weather field in the vertical
direction has been studied by a number of authors (e. g.,
Mozer and Serlin, 1969; Park and Dejnakarintra, 1973),
and a full three-dimensional mode} that includes a real-
istic thunderstorm distribution and surface topography
has been constructed by Hays and Roble (1979~. The
principal component of the middIe-atmosphere electric
field provided by this tropospheric source is vertically
directed and arises from the necessity for continuity of
the vertical current. The vertical electric field is thus
roughly inversely proportional to the conductivity, and
its order of magnitude varies from 10- ~ V/m at balloon
altitudes to 10-6 V/m at the base of the thermosphere.
The horizontal component of the electric field arises
from the nonuniform distribution of thunderstorm gen-
erators over the Earth and is largely removed by the
short-circuiting effect of the equalizing layer in the
mesosphere, where the conductivity increases sharply
(see Figure 13.10~. The attenuation is not complete,
however, and the model of Hays and Roble (1979) pre-
dicts horizontal electric fields of magnitude up to a few
tenths of a millivolt per meter in the lower thermosphere
arising from the fair-weather source.
The corresponding problem of mapping the iono-
spheric and magnetospheric electric fields downward
through the middle atmosphere to the ground has also
been examined by several authors (e.g., Mozer and
Serlin, 1969; Volland, 1972; Chin, 1974; Park, 1976)
using simple one-dimensional models and by Roble and
OCR for page 192
192
Hays (1979) with a full three-dimensional model. The
generation of these fields and their mapping into the
global electrical circuit will be discussed in Chapters 14
and 15 (this volume).
The dramatic increase in middle-atmosphere conduc-
tivity during a major solar-proton event causes large
changes in the local electric fields at high latitudes.
Holzworth and Mozer (1979) carried out balloon mea-
surements of the stratospheric electric field over north-
ern Canada during the intense event of August 1972 and
reported a decrease in the vertical field by more than an
order of magnitude. The decrease closely paralleled the
increase in solar-proton flux and could be explained
qualitatively by conservation of the fair-weather cur-
rent in the presence of the greatly enhanced conductiv-
ity. The upward mapping of the thunderstorm-gener-
ated fields of the lower atmosphere is sensitive to
changes in middle-atmosphere conductivity, since the
middle atmosphere represents a low-resistance load to
the generator even in quiet conditions. The downward
mapping of electric fields Generated in the ionosphere
~. .
GEORGE C. REID
must involve either a local mesospheric generation
mechanism or a dramatic local decrease in conductiv
ity, in which case the strong electric field would be
needed to maintain current continuity. The latter possi
bility appears to be ruled out by simultaneous conduc
tivitv measurements made on the same flight (Maynard
et al., 1981~. These measurements show that the con
ductivity is indeed low in the region of the strong electric
fields but is still large enough to provide a vertical cur
rent density about 200 times larger than the fair
weather value.
The fact that the strong fields are usually seen near
the 60- to 65-km height region, where the equalizing
layer exists, is a possible clue. In this region the domi
nant negatively charged current carriers change from
the slow-moving negative ions below to the highly mo
bile electrons above, and a fairly sharp change in elec
tric field must result from the need to conserve vertical
current alone. However, as mentioned above, the fields
measured are much larger than those associated with
this upward mapping process, and there is no obvious
anct magnetosphere, however, is much less sensitive, reason why strong fields should be generated in this
since the conductivity of the middle atmosphere re- neighborhood. The observations challenge our picture
mains much less than that of the lower ionosphere even
during a major solar-proton event (see Figure 13.11~.
The changes in the global electric circuit arising from
the August 1972 SPE have been examined in detail by
Reagan et al. (1983) and Tzur and Roble (1983), all of
whom pointed out the importance in estimating the
changes in middle-atmosphere electrical parameters of
including the current carried by the precipitating pro
tons themselves in the polar-cap region. Changes in the
global circuit, however, arose mainly from the Forbush
decrease in galactic cosmic-ray flux that accompanied
the event rather than from the solar-proton flux.
Measurements of the electric field in the mesosphere
have been carried out with rocketborne techniques and
have yielded conflicting and unexpected results. Most
startling of these is the measurement of strong electric
fields in the lower mesosphere (Tyutin, 1976; Hale and
Croskey, 1979; Maynard et al., 1981) with intensities
that can be orders of magnitude larger than those re
quired to maintain continuity of the fair-weather cur
rent. The reality of these fields has been questioned
(Kelley et al., 1983), and the possibility that they are
instrumental artifacts has not been entirely laid to rest.
No satisfactory explanation of their existence, either as a
genuine atmospheric phenomenon or as an instrumental
effect, has yet been proposed. However, they remain an
intriguing feature of the middle atmosphere. The anom
alous fields, if they are real, cannot be mapped from
above or below, since they are present only in relatively
well-defined height ranges. Any plausible explanation
lo
~ O
of the middle atmosphere as a passive element in the
global electrical circuit and suggest that there may be
field-generating mechanisms that we do not yet under-
stand. Even if the electric fields do turn out to be instru-
mental artifacts, their explanation will contribute to our
understanding of the limitations of in situ electric-field
measurements in the terrestrial environment.
CONCLUSION
In this brief review we have summarized our present
understanding of the electrical structure of the atmo-
sphere in the 30- to 100-km height range. The sources of
ionization in this region are reasonably well known, and
their variations in time and space are at least qualita-
tively understood. The complexities of the ion chemistry
that connects the ionization sources to the ambient ion
composition still require a great deal of unraveling. We
are still quite ignorant of many aspects, including pho-
todetachment of negative ions and the role of reactive
neutral species with extremely low concentrations.
These uncertainties lead to corresponding uncertainties
in ion concentration and mobility and in such bulk elec-
trical parameters as the conductivity. Direct experimen-
tal measurements have led to considerable progress, but
they are beset by difficulties of interpretation and by
inconsistencies among themselves. Finally, the recent
observations of large mesospheric electric fields have
raised first-order questions about our understanding of
atmospheric field-generating mechanisms or of the in
OCR for page 193
ELECTRICAL STR UCTURE OF THE MIDDLE ATMOSPHERE
strumental techniques used in these measurements. Sig
nificant challenges for the future exist in almost every
aspect of middle-atmospheric electricity.
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\
Representative terms from entire chapter:
electric fields
