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OCR for page 166
Electrical Structure from
O to 30 Kilometers
1 (a)
INTRODUCTION
WOLFGANG GRINGEL
Universitat Tubingen
JAMES M. ROSEN and DAVID ]. HOFMANN
University of Wyoming
This chapter deals with the electrical structure of the
lower atmosphere, i.e., the troposphere and the portion
of the stratosphere below about 30 km. Here the princi-
pal observing platforms (not including surface measure-
ments) are balloons. Their limited height range, rather
than other physical considerations, is the main reason
that the electrical structure above 30 km will be dis-
cussed separately in the following chapter.
For better understanding of the electrical phenomena
taking place in the lower atmosphere and the coupling
between them, the concept of a "global circuit" will be
briefly touched on a complete discussion is presented
by Roble and Tzur (Chapter 15, this volume).
The discovery of the atmospheric conductivity raised
a question concerning the origin of the electric fields and
the electric currents that were known to exist and flow
continuously in the atmosphere. According to the classi-
cal picture of the global circuit (Dolezalek, 1972), the
total effect of all thunderstorms acting at the same time
can be regarded as the global generator, which charges
the ionosphere to several hundred kilovolts with respect
to the Earth's surface. This potential difference drives
the air-earth current downward from the ionosphere to
the ground in the nonthunderstorm areas through the
concluctive atmosphere. The value of this air-earth cur
166
rent density varies according to the ionospheric poten-
tial and the total columnar resistance between iono-
sphere and ground. Finally the local atmospheric
electric field must be consistent with this current flow-
ing through a resistive medium, i.e., the atmosphere.
In addition to the global generator there also exist ef-
fective local generators such as precipitation, convec-
tion currents (charges moved by other than electrical
forces), and blowing snow or dust. The latter create
their own local current circuits and electric fields super-
imposed on parts of the global circuit. Generators can be
regarded as local generators (Dolezalek, 1972) if the re-
sistance from the upper terminal to the ionosphere is
much greater than the resistance from that point to the
Earth's surface along the shortest possible path and with
the consequence that almost no current flows to the ion-
osphere from this generator.
In the following sections we discuss initially the
sources of ionization in the lower atmosphere together
with solar-induced and latitudinal variations. In the
next section a brief review of aerosol distributions in the
troposphere and lower stratosphere is presented. Varia-
tions following major volcanic eruptions are empha-
sized. Atmospheric conductivity, small ion concentra-
tions, and ion-mobility measurements are the subject of
the third section. Here the influence that solar activity
or aerosols have on the conductivity, and therefore on
OCR for page 167
ELECTRICAL STRUCTURE FROM 0 TO 30 KILOMETERS
parts of the columnar resistance, are discussed. In the
final section the air-earth current and electric fields in
the lower atmosphere are considered. lIere the results
are interpreted from a global viewpoint with perturba-
tions from local generators. Examples of anthropogenic
influences on the electric field near the ground are also
presented and discussed briefly.
ION PRODUCTION IN THE LOWER
ATMOSPHERE
The electric structure of the troposphere and lower
stratosphere depends strongly on the ion-pair produc-
tion rate and the physical properties of the ions pro-
duced. Cosmic rays are the primary source of ionization
in the atmosphere range under consideration. Near the
Earth's surface over the continents there is an additional
component due to ionization by radioactive materials
exhaling from the soil. This radioactive ionization com-
ponent depends on different meteorological parameters
and can exceed the cosmic-ray component by an order
of magnitude as discussed in Chapter 11. It decreases
rapidly with increasing height, and at 1 km it is already
significantly less than the contribution due to cosmic
rays (Pierce and Whitson, 1964~. The ion-production
rate by cosmic rays is shown in Figure 12.1 for different
geomagnetic latitudes during the years of solar mini
h/km
~0
_ ~
20 ~
10 _
Neher (1961,1967)
73°\ 1965
73O> - - - 1958
35°~
~~ _
__ >
q/ -3 -1
O _
0 10 20 30 40 50
FIGURE 12.1 Profiles of the ionization rate at different latitudes in
years of the minimum (1965) and maximum (1958) of the 11-yr solar
sunspot cycle (Neher, 1961, 1967~.
167
mum (1965) and solar maximum (1958) based on bal-
loon measurements by Neher (1961, 1967~. The exis-
tence of the geomagnetic field gives rise to a pronounced
latitude effect. Only at latitudes higher than about 60°
can the full energy spectrum of the cosmic rays reach the
Earth and the depth of penetration be limited only by
the increasing atmospheric density for low-energy parti-
cles. At high latitudes 100-MeV protons can penetrate to
about 30 km height, for example. Moving downward to
lower latitudes more and more particles with lower en-
ergies are deflected by the geomagnetic field and, there-
fore, are excluded. The geomagnetic equator itself can
only be reached by particles with energies greater than
about 15 GeV. The hardening of the cosmic-ray spec-
trum with decreasing latitude is indicated in Figure 12.1
by the lowering of the height at which the maximum
ionization rate occurs. Near the equator this maximum
ionization rate is observed around 10 km.
Furthermore, the ionization rate depends strongly on
solar activity in a sense that at a particular height the
ion-production rate is lower during the sunspot maxi-
mum and higher during the sunspot minimum, as illus-
trated in Figure 12.1. The mechanisms are not fully
understood, but it appears that irregularities and
enhancements of the interplanetary magnetic field tend
to exclude part of the lower-energy cosmic rays from the
inner solar system (Barouch and Burlaga, 1975~. The
effect becomes more pronounced with increasing height
and/or increasing geomagnetic latitude. At geomag-
netic latitudes around 50° the reduction of the ion-pro-
duction rate during the periods of sunspot maximum is
about 30 percent at 20 km and about 50 percent at 30
km. More recently this solar-cycle dependence was con-
firmed by measurements with open balloonborne ion-
ization chambers by Hofmann and Rosen (1979~. Ana-
lytical expressions for computing the ionization rates
dependent on latitude and solar-cycle period are given
by Heaps (1978~. Superimposed on the 11-yr solar-cycle
variation are so-called Forbush decreases (Forbush,
1954), which are somehow related to solar flares and
exhibit a temporary reduction of the incoming cosmic-
ray flux for periods of a few hours to a few days or weeks
(Duggal and Pomerantz, 1977~.
On the other hand, solar proton events (SPE) can
drastically increase the ion-production rate within the
stratosphere and, for high-energy solar protons, some-
times even near the ground. The duration of such SPEs is
of the order of hours, and they are normally restricted to
high-latitude regions, as discussed in more detail in the
following chapters.
OCR for page 168
168
AEROSOLS IN THE LOWER ATMOSPHERE
Within the portion of the atmosphere under consider-
ation in this chapter there are three primary regions of
interest to atmospheric electricity: the boundary layer
(approximately the first 3 to 5 km), the remaining por-
tion of the troposphere, and the stratosphere. The char-
acterizing aerosol parameters of central importance
here are concentration, size distribution, and vertical
structure. In addition, variability of the aerosol in each
of these regions must be recognized. Such variations
may influence electrical parameters and significantly
detract from the apparent repeatability of various at-
mospheric electrical measurements.
The boundary layer is generally thought of as a rela-
tively well-mixed region capped on the upper side by a
temperature inversion. Since mixing across the inver-
sion tends to be inhibited, a potential exists for the accu-
mulation of aerosols within the boundary layer if a sig-
nificant source is present. The buildup of aerosols is
frequently of sufficient magnitude to cause a notable re-
duction in visibility. Even when there is no apparent loss
in visibility as observed from the surface, the upper
boundary may still be apparent even to an airline pas-
senger at the moment when the aircraft passes through
inversion. The buildup of aerosols in the boundary layer
and the capping effect of the inversion have been ob-
served in a more quantitative sense by airborne lidar.
The ion concentration and conductivity can be greatly
affected by the aerosol buildup in the boundary layer
and a dramatic change in these quantities is often ob-
served at or near the altitude of the defining inversion.
The size distribution of aerosols near the surface of the
Earth has frequently been approximated with a power-
law function (Junge, 1963~. However, it has more re-
cently been suggested that the near-surface aerosol is ac-
tually made up of two or three size modes that when
added together approximate a power-law function over
a limited range of sizes (Willeke and Whitby, 1975~.
This interpretation of the size distribution seems to bet-
ter reflect the physical processes affecting the aerosol
concentration in the atmosphere. Some examples of typ-
ical size distributions for a variety of conditions and lo-
cations can be found in the work of Willeke and Whitby
(1975) and Patterson and Gillette (1977~.
The aerosol mixing ratio profile (and usually the con-
centration profile) typically show a relative minimum in
the upper troposphere for particle radii greater than
about 0.1 ,um. In contrast the condensation nuclei (en)
profile (corresponding to particle radii of about 0.01
,um) is more or less constant throughout the entire tro-
posphere above the boundary layer. Near the surface of
the Earth the en concentration may be relatively high
WOLFGANG GRINGEL, JAMES M. ROSEN, and DAVID J. HOFMANN
owing to local contamination (104 per cm3 or more), but
above the boundary layer the concentration ranges from
about 100 to 103 per cm3 with a global average of ap-
proximately 300 to 500 per cm3 (Rosen et al., 1978a,
1978b).
The size distribution of aerosols in this region of the
atmosphere can usually be approximated by a power-
law function between about 0.01- and 10-,um particle
diameter (Junge, 1963~. Below the minimum size of
0.01,um there are relatively few particles owing to coag-
ulation, and above 10 ,um the particle concentration
drops quickly from sedimentation effects. Thus limits of
the power-law distribution must always be specified.
The character of upper tropospheric aerosols can be
temporarily disturbed by volcanic eruptions, forest
fires, biomass burning, and large dust storms. In addi-
tion periodic annual variations of concentration have
also been observed (Hofmann et al., 1975~. Another im-
portant temporal variation of tropospherical aerosols is
associated with the so-called arctic haze events. New ev-
idence suggests that these events are characterized by
high particle loading throughout a large portion of the
troposphere. The impact of this extensive aerosol load-
ing on atmospheric electrical parameters is yet to be de-
termined.
Hogan and Mohnen (1979) reported the results of a
global survey of aerosols in the troposphere and lower
stratosphere. They found that the concentrations were
more or less symmetrically distributed about the Earth.
Measurements of this type could provide the basis for
extrapolating local or isolated observations to character-
istic worldwide values.
The morphology of stratospheric aerosols is domi-
nated by a persistent structure frequently referred to as
the 20-km sulfate layer, or Junge layer. It is now known
that the character of this layer is highly affected bY large
volcanic eruptions.
For several years prior to 1980,
stratospheric aerosols were in a quasi-steady-state con-
dition, not being under the influence of any significant
recent volcanic eruptions. During that period the size
distribution appeared to be consistent with a single
mode log-normal distribution (Pinnick et al., 1976), al-
though other types of single-mode distribution were also
employed (Russell et al., 1981) with similar results. The
composition was thought to be primarily sulfuric acid
droplets.
The appearance of the 20-km layer is not evident in
the vertical profile of all particle size ranges. The en pro-
file, for example, which is representative of particles
with sizes in the neighborhood of O. O. l-,um radius usually
shows a dramatic drop in concentration above the tro-
popause and no relative maximum at the altitude of the
stratospheric aerosol layer. This would be consistent
OCR for page 169
ELECTRICAL STRUCTURE FROM 0 TO 30 KILOMETERS
with a tropospheric en source and a vertical profile dic-
tated by diffusion and coagulation (Rosen et al., 1978a).
After a large volcanic eruption the size distribution
may be greatly disturbed and highly altitude depen-
dent. Following the eruption of E1 Chichon in April
1982, Hofmann and Rosen (1983a, 1983b) reported a
size distribution that could be approximated by a sum of
multiple log-normal distributions, each with a different
mode. The source of the smallest particle mode ~ ~ 0.01
,um) appeared to be associated with the homogeneous
condensation of highly saturated sulfuric acid vapors.
This production ceased a few months after the eruption,
and consequently the mode disappeared A midsize
mode ~ ~ 0.1 Am) was also observed, which may have
resulted from coagulation and growth by condensation
of the initial particles formed by the homogeneous con-
densation. Another mode near 1-2,um diameter was also
observed that may have formed from the condensation
of sulfuric acid vapors onto the solid silicate ash particles
as well as on particles already present in the strato-
sphere. Further assessments of the aerosol injected by
the E1 Chichon eruption have been described in a collec-
tion of papers introduced by Pollack et al. (1983~.
An unexpected influence of the E1 Chichon eruption
was the enhancement of an annually appearing on layer
near 30 km altitude. Although the phenomenon was ob-
served in previous years (Rosen and Hofmann, 1983) the
concentration of on associated with these event layers
was enhanced by at least 2 orders of magnitude. Even
though the sizes of the particles were quite small
(~O.Ol-,um diameter), the concentration was large
enough to measurably affect the ambient ion concentra-
tion and conductivity (Gringel et al., 1984~. The event
particles are thought to have formed in polar regions
from highly supersaturated sulfuric acid vapors. The af-
fect of this phenomenon on the various aspects of atmo-
spheric electricity at high latitude has not been assessed
at the time of this writing. Of particular interest would
be the influence of the highly supersaturated vapors on
the ambient ion mass (and therefore mobility). Careful
measurements at appropriate locations may provide an
unexpected means of finding supporting experimental
evidence for some of the various models of ion composi-
tion (and mass) that have been proposed (Arnold, 1983;
Arnold and Buhrke, 1984~.
Figures 12.2 to 12.4 illustrate several of the character-
istics of atmospheric aerosols that have been discussed
above. The long-term influence of volcanic aerosols on
the stratosphere are illustrated in Figure 12.2, which
compares the quasi-steady-state period with the dis-
turbed conditions still observed some 18 months after
the eruption of E1 Chichon. Note also the presence of the
boundary layer near the surface (as evidenced by a
-L I """'I ' """'I " lillIll " """I " IlIlIll
1~
.
20
-
_ 50 _
~_
o: _
~_
An _
~, 100 _
in:
cat
200 _
1000
35
27 SEPT 1978
21 OCT. 1983
~""~_~`
~ (x
~ _,-"
~,_~
/, _ --
~ __, id,
c-~
_ 30
_ 25
ye
_ 20 ~
to
_ 15 as
- 10
, , ,,,,,,1 , , ,,,,,,1 , , 1111111 1 1 ,,,,,,1 , , 1111111 O
103
lo-2 10 1 10° 10 1o2
AEROSOL CONCENTRATION ( cm3)
DlA. 2 .30,Lm
FIGURE 12.2 A comparison of aerosol profiles (particle diameter
greater than 0.30 ~m) for the quasi-steady-state period (September 27,
1978) with the decay period following the April 1982 eruption of E1
Chichon.
sharp drop in concentration) and a relative minimum in
the aerosol concentration occurring in the upper tropo-
sphere.
An example of a normal and disturbed en profile is
illustrated in Figure 12.3. A significant en event layer is
evident at about 30 km altitude. Both profiles show a
relatively large drop in the on concentration just above
the surface, a relatively constant mixing ratio through-
out most of the troposphere, and a noticeable drop in
concentration near the tropopause.
An example of the influence of volcanic eruptions on
the size distributions of stratospheric aerosols is illus-
trated in Figure 12.4. As previously discussed, the size
distribution during the quasi-steady-state period could
be described quite well by a single-mode log-normal dis-
tribution. At the time of this writing some 18 months
after the eruption of E1 Chichon, the size distribution
appears still to be quite disturbed.
OCR for page 170
170
5
10 .
. _
20 _
50 _
In
`~, 100
a:
200 _
_
500 _
_ " ""'11 ' """'1 " """1 " """1 " ""I
~ - 15 J AN . 1981
- I FEB. 1983
TROPOPAUSE
_ _ -
-\~
e;~ ~
%_
ma_
T
35
_ 25
_ 20 ~
lo
_ 15~
,_
_ 5
_ --~
1000 ~ I I tl''l'1 11''''''1 1 1 l'll1~1 1 1 ''l'll1 1 1 111111 O
10 1 I00101 102 103 104
CONCENTRATION ( cm 3 )
FIGURE 12.3 A comparison of a normal on profile (January 15,
1981) with one obtained during a period of the 30-km on event.
tn3
-
E
.
' ~
At
-
~ ~ ~ ~ ~ ~ ~ I I I I 1- 1 1
1
10-1 7
En 10 _
103 _
104 _
Ad 105 _
~ loo _
He
at: 10 _
LLI
~ 10 _
1 SEPT. 1983
NONVOLCANIC
PE RIOD
\
\ \
\
\\1
'1
.01 .1
R A D I US ( '` m )
FIGURE 12.4 A comparison of the single-mode stratospheric aerosol
size distribution (solid line) typical of the quasi-steady-state period
with a recently obtained distribution (dashed line) that is believed to
be still under the influence of the April 1982 eruption of E1 Chichon.
WOLFGANG GRINGEL, JAMES M. ROSEN, and DAVID J. HOFMANN
ATMOSPHERIC CONDUCTIVITY, SMALL ION
CONCENTRATION, AND MOBILITY
30 The atmospheric conductivity depends on the exis
tence of positive and negative ions, whereas the contri
bution of free electrons can be neglected below about
45 km. There is normally a mixture of ions and the re
sulting conductivity can be expressed in terms of the
number densities and mobilities of the individual species
as
fir= (J+ + a_ = e~ni+ki+ + e~ni ki
i i
(12. 1)
with e the electronic charge and n and k the number
densities and mobilities of the particular positive and
negative ions. The ion mobility is defined by the drift
velocity v of the ions in an electric field E as
v = kE (12.2)
and depends on the mass, the collisional cross section,
and charge of the individual ions (ions are believed to be
singly charged in the lower atmosphere) as well as on the
density and polarizability of the surrounding gas. It is
inversely proportional to the air density and can be ex
pressed by the reduced mobility ho as
Kohl = k pot(h) (12.3)
where pO and To are STP pressure and temperature
(1013 mbar, 273 K) and p and T are pressure and tem
perature at height h. An empirical relationship between
reduced mobility and ion mass in nitrogen is given by
Meyerott et al., (1980) and shown later in Figure 12.13.
In the troposphere and lower stratosphere the atmo
spheric conductivity is maintained by the so-called
small ions with reduced mobilities around 1.5 cm2/V
see, whereas the mobilities of large ions (charged aerosol
particles or large molecular clusters) are too small to
contribute directly to the conductivity (e.g., Israel,
1973b). As will be discussed later, large ions as well as
attachment by aerosol particles can reduce the conduc
tivity significantly. The production and annihilation of
small ions is shown schematically in Figure 12.5. The
molecular ion and remaining electron created by the io
nizing process form charged molecular clusters (the
small ions) after several reactions. These small ions are
annihilated by mutual recombination and neutraliza
tion, or they become almost immobile by attachment to
aerosol particles or large ions. Under steady-state condi
tions and assuming equal densities of positive and nega
tive small ions, the fundamental balance equation be
comes in its most simplified form
dn/dt= 0 - q - an2- hnZ, (12.4)
OCR for page 171
ELECTRICAL STRUCTURE FROM 0 TO 30 KILOMETERS
MOLECULAR SMALL
ION ION ~
(a ~HMENT ~ ( ) PARTICLE
COSMIC RAY /
~ / RECOMBINATION
GAS ~ ) HYDRATION AND DESTRUCTION GAS MOLECULES
MOLECULE in\ OF HYDRATION SHELL i'
ELECTRON \ ~
~/AT T A C H M E N T go) A E RO ISCOLLE
MOLECULAR SMALL
ION ION
where q is the ionization rate, ax the recombination coef
ficient, ~ the attachment coefficient, and Z the aerosol
concentration. An expression for (r) that is relevant to
stratospheric conditions has been developed by
Zikmunda and Mohnen (1972) and is approximately
$(r) = 2.35 X 10-4 r]-457, where r is the radius of the
aerosol particle in micrometers. Recombination coeffi
cients were reported by Smith and Church (1977) and
experimentally derived from nearly simultaneous bal
loon measurements of the ionization rate, the small ion
density, and the conductivity by Rosen and Hofmann
(1981a, 1981b) and Gringel et al. (1983) .
The atmospheric conductivity within balloon alti
tudes is measured mostly with the Gerdien condenser in/km
technique (e.g., Israel, 1973a). In contrast to ion
density measurements, conductivity measurements are
largely independent of the airflow rate through the Ger
clien tube. This seems to be the main reason for the rela
tively good agreement among different conductivity
profiles as reported by Meyerott et al. (1980~. A direct
comparison of three different Gerdien-type sondes has
shown in fact an agreement of the resulting conductivity
profiles within 10 percent to 32 km height (Rosen et al.,
1982~.
Figure 12.6 shows a mean profile (21 soundings) of
the positive polar conductivity for quiet solar conditions
and a geomagnetic latitude around 50° (Gringel, 1978~.
This profile can be analytically expressed by the relation
3
cr + (TO - 14 mho/m) = expti Lo aizi), (12.5)
with z the altitude (in kilometers) and an = 6.363 X
10-~, al = 3.6008 X 10-~, as = -8.605 X 10-4, and
as = 1.0331 X 10-~.
The scattering of the mean values as shown for kilo-
meter intervals is considerably below 15 km, indicating
a highly variable aerosol density throughout the tropos-
phere. The noticeable increase of the conductivity
171
FIGURE 12.5 Schematic representation of
the production and annihilation of atmo-
spheric small ions.
around 13 km can also be attributed to a sharp decrease
of aerosol particle concentrations, especially condensa-
tion nuclei, above the tropopause.
For comparison, Figure 12.6 shows the mean of three
conductivity measurements conducted within 1 week
following solar flares. One profile was obtained on Au-
gust 8, 1972, during a period of intense solar events. The
other two flights were conducted 3 and 7 days, respec-
tively, after a solar flare associated with an intense type-
IV radio burst on April 11, 1978. For all three flights the
30
20
10
~-- , , , A4~_
o
1 10 100 1000
polar conductivity At
· active sun
X quiet sun
FIGURE 12.6 Profiles of the positive polar conductivity versus alti-
tude (a+ = a ) during quiet and active solar conditions. Mean values
and standard deviations are shown for altitude intervals in kilometers
(Gringel, 1978).
OCR for page 172
172
ionization rate was appreciably reduced by the flare-
related Forbush decrease, resulting in a remarkable
conductivity reduction throughout the lower strato-
sphere. For the August 8, 1972, flight the conductivity
reduction reached deep into the troposphere (see also
Figure 12.8 below).
The ratio between the values of positive and negative
polar conductivity exhibit a noticeable altitude depen-
dence as shown in Figure 12.7 (Gringel, 1978~. In the
troposphere the negative conductivity values exceed the
positive ones by about 12 percent. Between 15 and 20
km both polarities are about equal, whereas above 20 to
25 km the negative conductivity becomes 5 to 10 percent
smaller than the positive on the average. Assuming
equal densities of positive and negative small ions, the
same can be concluded for the corresponding average
small-ion mobilities.
As shown by Roble and Tzur (Chapter 15, this vol-
ume) the vertical columnar resistance is an important
parameter within the scope of the global atmospheric
electrical circuit. This columnar resistance Rc is related
to the atmospheric conductivity by
he
Rc= ~ (CJ+ + ~ Ah (12.6)
J
hi
and denotes the resistance of a vertical air column with a
1-m2 base between the ground and the equalization
in/km
]4 ~
12 _
JO F
8 F
6
l
I A+/A
1___ 1 1 1 1 1 1 ~
0,6 0,8 1,0
FIGURE 12.7 The ratio of positive to negative polar conductivity as
deduced from 10 balloon flights (Gringel, 1978~.
WOLFGANG GRINGEL. NAMES M. ROSEN and DAVID T. HOFMANN
24
he ~
:2 _
8 ~ ~ ~ , ~ ,,,'l ,~, ', ',,,
o
5 lol6 1017
Qm2
An+ ~_( ~ - 691 )
+~+ :{ A-782
RC (hi) ~ +` 5:
FIGURE 12.8 Mean columnar resistance [RC(h~] between the
lower-altitude hi and 60 km. The profiles Rc+ (hi) are calculated from
positive polar conductivity profiles only for quiet solar conditions (A + )
and after solar flares measured on August 8, 1972 (A-691) and on April
14, 1978 (A-782.
layer. From Eq. (12.6) Rc is determined mainly by the
low conductivity values near the ground. Figure 12.8
shows the columnar resistance RC(h~) between the lower
height hi and 60 km as function of hi under quiet solar
conditions as calculated from the conductivity profile in
Figure 12.6 and using the ratio between positive and
negative conductivity shown in Figure 12.7. The mean
value from the ground to 60 km was found to be 1.3 X
10~7 Q m2 at Weissenau (South Germany, 450 m above
sea level) during fair-weather conditions. The varia-
tions are on the average about 30 percent and are caused
by a changing ionization from radioactive materials
near the ground and by varying aerosol concentrations
in the lower troposphere. The first 2 km of the atmo-
sphere contribute about 50 percent and the first 13 km
about 95 percent to the total columnar resistance.
The right side of Figure 12.8 shows three profiles of
the positive polar columnar resistance R+ as a function
of the lower height hi for quiet solar conditions and two
profiles following solar flares. These three profiles were
calculated using only positive conductivity values and
therefore show about twice the value of the actual Rc
obtained from both polarities. Whereas the Forbush de-
crease observed on April 14, 1978, influenced mainly
the part of the columnar resistance above the tropos-
phere, the enhancement reached deep into the tropos-
phere on August 8, 1972, at a geomagnetic latitude of
48°. The part of RC above 13 km was found to be higher
by 13 and 28 percent, respectively, when compared
with RC obtained under quiet solar conditions. As pro-
posed by Markson (1978) this could cause a reduction of
the current flowing from the top of the thunderclouds to
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Representative terms from entire chapter:
atmospheric conductivity
ELECTRICAL STRUCTURE FROM 0 TO 30 KILOMETERS
the ionosphere, resulting in a lower value for the ionos-
pheric potential reported for periods of high solar activ-
ity by Fischer and Muhleisen (1972~.
Layers of an increased aerosol density can signifi-
cantly reduce the atmospheric conductivity owing to at-
tachment of small ions on these aerosol particles. Figure
12.9, as an extreme, shows a conductivity profile
through a Sahara dust layer between 1.7 and 3.7 km
height and 2200 km west of the West African coast
(Gringel and Muhleisen, 1978~. The dust concentration
Z* responsible for the conductivity decrease is also
shown. The authors report a mean mass concentration
of 1200 g~ 3 throughout the layer. Owing to the low alti-
tude of the layer the total columnar resistance was in-
creased about 30 to 50 percent within these large-scale
areas of Sahara dust transport across the North Atlantic.
It is estimated that about the same increase of Rc occurs
40:
km I
3OI
20
n
km
Ascent at Pos. 16,5°N, 37 W to
Launching 26.11.73 14.42 Gem
h
10
km
5
o
.
.
.
.
.
~i/ Z~'cm3
~i- 46 -
-*/ ,' - 127
6dA ~' -140
~,, ~ 44
Sahara dust
layer
. . ., , I A
50
1~14 Q~1 -
1 10
FIGURE 12.9 Polar conductivity as a function of altitude and the
concentration of mineral dust particles Z* derived from the conductiv-
ity decrease ^X in the main Sahara dust transport layer at 2200 km
distance from West Africa (Gringel and Muhleisen, 1978~.
173
,~
Aster (1968)
I ~uti (1966)
h I
_~
0 5000
Ah
10
a)
~5)
~ . .
1 0000 cm~3
~ Kroening (1960)
~ C)
Paltridga .
it, /'
__' 10 _
~~i' ( 1966) :~
OCR for page 174
174
authors as reported by Riekert (1971~. The profiles show
large variations among each other and differ by almost a
factor of 5 at the ion density maximum near 15 km. The
small-scale fluctuations shown in the profiles of Kroen
ing (1960) and Paltridge (1965) have been attributed to
the attachment of small ions on particles. However, it is
not clear that the known stratospheric aerosol size and
concentration can quantitatively account for the re
quired amount of ion depletion, except under rare cir- ~ 0.2 _
cumstances. O
If, on the other hand, the airflow through the Ger-
ELECTRICAL STRUCTURE FROM O TO 30 KILOMETERS
entire altitude range. Following these measurements a
balloonborne ion-mobility spectrometer was developed
and for the first time was flown successfully at Laramie,
Wyoming, on June 6, 1983 (Gringel, in preparation).
The spectrometer consists of a voltage-stepped Gerdien
condenser with divided collector electrode, force venti-
lated by a lobe pump. The maximum electric-field
strength is kept well below 1 V/cm Torr in order to avoid
a breakup of the weakly bonded small ions. As prelimi-
nary results, the fractional abundances of positive and
negative small ions with respect to the reduced mobility
are shown in Figures 12.13 and 12.14 for altitudes of 26
and 24.5 km, respectively. These results indicate that
most of the positive ~ ~ 78 percent) and negative ~ ~ 90
percent) ions have reduced mobilities between ~ 1 and
~2.2 cm2/V sec. The mass-mobility relation by Mey-
erott et at. (1980) indicates that most of the ions of both
polarities have masses between 55 and 400 emu.
Whereas only 20 percent of the negative ions show re-
duced mobilities larger than ~ 1.7 cm2/V see, approxi-
mately 43 percent of the positive ions exceed this value
and have masses smaller than about 100 emu. The re
1 ' '
x 1
x
xl
IX
W - 204
MAY 15, 1979
30
ye
~ 20 _
c,
J
10 _
x 1
ko+= ( 1.35 + 0.08 ) cm2 V Is 1
x l x
Ix
x1x x
1 ,`
1 x
lxx
I x
l xx
x
I x
xx
x xx
xx
Xl
IX
x
- -4 1
1
~ 1
1
1
1
rot
~ 1
O
o
1111
1.0
kO.t,(cm2 V Is I )
2.0
FIGURE 12.13 Reduced mobilities of positive small ions versus alti-
tude calculated from simultaneous measurements of small-ion density
and conductivity (Gringel et al., 1983).
175
NEGATIVE IONS: 30- 25 mb
_ 2
Ko ~ 1.47cm /Vs
0.4
it
m 0 3 ~
as
At
o
~0.2
fir
0.1
o
0.1
L AR AMIE, WY
JUINE 7. 1983
~r;:
1.0
REDUCED MOBILITY ( cm 2/Vs )
10
FIGURE 12.14 Reduced mobility spectrum of negative ions around
24.5 km (Gringel, in preparation).
duced average mobility values are 1.5 cm2/V see for the
negative ions and 1.8 cm2/V see for the positive ions with
an accuracy range of + 10 to - 20 percent. It cannot be
decided at this time whether the reduced mobilities
measured in 1979 (see Figure 12.13) and 1983 are really
different from present values.
The negative ion-mobility spectrum shown in Figure
12.14 is quite consistent with ion mass spectrometer
measurements reported recently by Viggiano et al.
(1983~. These authors found that the main mass
peaks of negative ions between 125 and 489 emu be-
long to the main ion families NO3 (HNO3)n and
HSO4H2SO4)m(HNO3)n. The heavy HSO4 core ion
family was found during their flights (September/Octo-
ber 1981) mainly at altitudes above 30 km. However the
major volcanic eruption of E1 Chichon in early April
1982 has changed the H2SO4 content of the stratosphere
dramatically, as reported among others by Hofmann
and Rosen (1983a, 1983b). Their aerosol measurements
as well as the negative-ion mobility spectra indicate the
presence of the heavy HSO 4 core ion family well below
30 km.
In contrast to negative-ion mass spectrometer mea-
surements, as of this writing the presence of positive ions
having masses greater than 140 emu in the lower strato
176
sphere (e.g., Arnold et al., 1981) have not been de
tected. This is not consistent with ion-mobility measure- h
meets and clearly needs further investigation.
AIR-EARTH CURRENT AND ELECTRIC 10
FIELDS IN THE LOWER ATMOSPHERE
The atmospheric electric circuit is characterized by a
difference in voltage (on the order of 300 kV) between
the highly conductive ionosphere (commonly referred to
as the equalization layer) and the Earth's surface, which
is also a relatively good conductor. This voltage Vat is
thought to be maintained principally by thunderstorms
acting as the generators and the atmospheric conductiv
ity acting to discharge the ionosphere through continu
ous flow of current. The value of this air-earth current
density in fair-weather areas depends on the voltage Vat
and the columnar resistance Rc and is, according to
Ohm's law,
iv= V~/Rc. (12.7)
The value of the atmospheric electric field E(h) de
pends on the air-earth current density and the electrical
conductivity of the air according to the following Ohm's
law relationship:
Ah) = E(h) [~+ (h) + ~ Chid.
Under steady-state conditions it is expected for rea-
sons of continuity that the air-earth current density is
constant with altitude if large-scale horizontally homo-
geneous conditions exist and if no charged clouds or
other disturbances alter the so-called fair-weather con-
ditions.
Figure 12.15 shows the atmospheric electric field and
both polar conductivities as measured simultaneously
over the North Atlantic by Gringel et al. (1978~. The
field and conductivity profiles show clearly the inverse
pattern to each other, which is expected for a constant
air-earth current density through the atmosphere. Even
a thin cloud layer at 6 km did not disturb this inverse
pattern. The mean vertical air-earth current density for
this particular balloon flight was calculated to be iv =
(2.35 + O.1S) pA/m2, a typical value for our oceanic
measurements. Another profile Of iv obtained at Lara-
mie, Wyoming (Rosen et al., 1982) to 31 km height is
shown in Figure 12.16. Again the profiles of both polar
current densities show the expected constancy with alti-
tude. At the same time of this flight at Laramie, the
ionospheric potential Vat was determined over Weisse-
nau, Germany, by integration of the measured electric-
field strength (Fischer and Muhleisen, 1975) and found
to be 330 kV, a value that should be the same as over
Laramie according to the classical picture of the global
WOLFGANG GRINGEL, JAMES M. ROSEN, and DAVID J. HOFMANN
km
5
o
~ Lot
_1 ~
_ -A. ~ )-
_
_ , . . ~ A_ t~ ,_-'
~1''
r I A;J
Gl l
1 2 5 10 20 50
O 5
_ E
do;
- 1 2 JV.
,_
i 1 1 1ol2 AL 2
1^
_ OX
_ ~
Xk
x? x.ExA.
x, a. ExA_
x8x
_ ALL
X~
Xt
_ I
~ _=, 1
., 1. 1 _
E/Vm'
100 A/1~44~1m'
FIGURE 12.15 Polar air-earth current densities iv+ and iv- versus
altitude calculated from simultaneously measured electric-field
strength E and both polaI: conductivities ()\ = a) over the North Atlan-
tic (Gringel et al., 1978~.
electrical circuit. In addition, the columnar resistance
was obtained from an atmospheric conductivity profile
and found to be RC = 0.65 X 1017 Q m2. This implies that
the total conduction current density, calculated from VI
and Rc, is S.1 pA/m2. The good agreement with the
mean value calculated from the conductivity and elec-
tric-field profiles proves that the Earth surface and the
ionosphere can be regarded as good conductors where
charges are distributed worldwide within short times.
The fact that iv over Laramie shows about twice the
value as over the Atlantic is explained by the high alti-
tude of Laramie (2150 m above sea level) resulting in the
low RC value given above. These 2 km normally contrib-
ute about SO percent to the total columnar resistance of
around 1.3 X 10~7 Q m2 between sea level and the iono-
sphere.
Direct measurements for the air-earth current density
in the free atmosphere have been carried out also with
long-wire antenna sondes described by Kasemir (19603.
Whereas Ogawa et al. (1977) reported a constant air-
earth current throughout the troposphere and lower
stratosphere, the measurements by Cobb (1977) at the
South Pole indicated a slight decrease of the iv values
ELECTRICAL STRUCTURE FROM 0 TO 30 KILOMETERS
above the tropopause. The reasons for this current de
crease are unknown.
At globally representative stations the air-earth cur-
rent density shows a diurnal variation versus universal
time with a minimum at around 0300 GMT and a maxi-
mum near 1800 GMT, reflecting the diurnal variations
of the ionospheric potential. Figure 12.17shows this di-
urnal variation measured near the surface in the North
Atlantic (Gringel et at., 1978) and near the ground at
the South Pole (Cobb, 1977) during fair weather. The
larger variation over the North Atlantic is probably due
to the relatively short observation time of only 15 days.
The agreement of the two curves also strongly supports
the concept of an universally controlled global circuit.
Direct-current density measurements near the ground
at a continental station have been reported by Burke
and Few (1978~. They observed a typical sunrise effect
that is characterized by a gradual increase of the atmo-
spheric conduction current within an hour after sunrise,
reaching a peak about 2 hours after sunrise. The current
density then gradually decreased but usually remained
at a higher value than was observed before sunrise. This
sunrise effect is thought to be caused by mechanical
transport of positive charges following the onset of con
35
30 _
25 _
ye
- 20 _
ca
~ _
10
5 _
O 1 1,,1 1 1 1 1 1 1 111
0.5 1 2 5
'"'1 1 1 1 1 1 1 1111
W - 186
x E x>+
o Ex x
x
o'
1 x
Ol
x
J
IX
b
olx
olx
Ix
o
IX
Ix
1 o
Xl
x1°
o
1 o
x
iv = ( 5 1+ 0.3) pA /ml2
1
_O
iv+ (pA m ')
FIGURE 12.16 Polar air-earth current densities iv+ and iv- mea-
sured on August 4, 1978, at Laramie, Wyoming. The mean total air-
earth current density is j' = (5.1 + 0.3) pA/m2.
140
Yo
120 ~
L . ,_
JV/]Y
100
i,""'
177
80
60
LO
North Atlantic Jv ~ 2,9 pA/m2
26.10.73- 23.11.73
- South Pole: Jv. 2,5 pA/m2
Nov. 72 - March 74
oh 6h 12h
18 GMT 24h
FIGURE 12.17 Mean diurnal variations of the air-earth current
density in relative units over the North Atlantic (Gringel et al., 1978)
and at the South Pole (Cobb, 1977~.
vection. During fog they observed low values of iv that
can be attributed to an enhancement of the columnar
resistance, caused by the attachment of small ions to the
fog droplets. Beneath low clouds without precipitation
they usually found negative current readings, which are
interpreted by Burke and Few (1978) as charge-separa-
tion processes occurring in most of the low clouds,
whether or not the clouds finally produced local precipi-
tation or developed into thunderclouds.
The electric field in the lower atmosphere is vertical
and directed downward during fair-weather conditions
and large-scale atmospheric homogeneity. In the litera-
ture of atmospheric electricity, that direction is defined
as the direction a positive charge moves in the electric
field (e.g., Chalmers, 1967~. At globally representative
stations, such as ocean or polar stations, the vertical co-
lumnar resistance remains nearly constant during fair
weather (Dolezalek, 1972~. Here the electric-field
strength near the ground shows a diurnal variation with
universal time similar to that shown for the air-earth
current density in Figure 12.17, both reflecting varia-
tions of the ionospheric potential. Over the continents
consideration must be given to a varying ionization rate
in the first few hundred meters caused by the exhalation
of radioactive materials from the Earth. This ionization
rate depends strongly on different meteorological pa-
rameters, such as convection, and therefore the colum-
nar resistance can no longer be regarded as constant. As
shown by Israel (1973b) the global diurnal variation of
the electric field is normally masked by local variations
at these stations. If local generators, such as precipita-
tion, convection currents, and blowing snow or dust,
178
FIGURE 12.18 Typical variations of the
vertical electric field near the ground and
their relative amplitude distribution during
fair weather (0), haze God, and fog ~-~ (from
Fischer, 1977~.
-
>
to
-
IL
-
J
0 1 2
TIME ( hr)
_O ac
o
7C
60
~1
_ SO
0-
~40
AS
-
J
~0
to
10
o
L
-200 0 +200
also become active, the description becomes increas-
ingly complicated and the vertical electric-field strength
Ez can vary considerably.
Figure 12.18 shows typical variations of Ez at a conti-
nental mid-latitude station during fair weather and also
during haze and fog (Fischer, 1977~. The relative occur-
rence of the field amplitudes is also shown. During fair
weather the variation is small with a mean value of Ez
about 120 V/m with no negative fields. During haze and
fog the variations become much larger and even nega-
tive values of Ez occur indicating the presence of space
charges around the station. The higher positive Ez val-
ues are mainly caused by a drastic reduction of the at-
mospheric conductivity due to the attachment of small
ions to haze or fog droplets.
The highest values of Ez are measured during rain or
snow showers and thunderstorms as shown in Figure
12.19 (Fischer, 1977~. For both cases the amplitude dis-
tribution shows a typical U pattern with mainly high
positive or negative field values. The highest field values
can reach 5000 V/m at the ground. This seems to be an
upper limit because corona discharges build up a space-
charge layer with the appropriate sign so as to reduce
the original field values. Examples of anthropogenic in-
fluences are shown in Figures 12.20 and 12.21. Figure
WOLFGANG GRINGEL, JAMES M. ROSEN, and DAVID J. HOFMANN
O -TYP .- TYP
400
500
200
+100
o
-100
eon
70
60:
SOL
40h
Sol
2O
1O
O -
- - TYP
400 ~
~'1~-
5 0 1 2 5
TIME ( hr ) 0 , 2 5
TIME ( hr )
n FAIR WEATHER
1 ~
1 1
1 1
ll ll
1 1
r1 r.,R wE..~E~
- L 20 _ ~'
so
60
~0
~0
so
1 1 1 1 1 1 1 1 1 1 1 1 1 1
- 200 0 ~ 200 - 200 0 + 200
ELECTRIC FIELD ( v/m )
12.20 shows the undisturbed and disturbed electric-
field values near the ground on the upwind and down-
wind side of a high-voltage power line. Figure 12.21
shows the undisturbed and disturbed fields values near a
large city in Germany (Fischer, 1977~. Whereas the sta-
tion at the south shows almost a fair-weather field pat-
tern, the values at the northern station exhibit large var-
iations, and even negative values of Ez occur. The
reasons for the large variations at the disturbed stations
are in both cases drifting pollution and/or space charges.
Above the ground the vertical electric field Ez drops rap-
idly with increasing altitude owing to the increasing at-
mospheric conductivity. Figure 12.22 shows the de-
crease of the vertical electric field with altitude during
fair weather, during cloudiness without precipitation,
and during haze and fog as measured again over Weisse-
nau, Germany (Fischer, 1977~. The positive sign of Ez
means that the field vector is pointed downward again.
The variations of different profiles, shown by the ha-
chured areas, are mainly caused by conductivity varia-
tions, especially in the lower troposphere, rather than
by variations of the ionospheric potential itself. The
scatter is greatly increased during periods of cloudiness
and haze or fog, whereas the mean profile of the same 20
balloon flights (indicated by the thick curve in Figure
ELECTRICAL STRUCTURE FROM 0 TO 30 KILOMETERS
\7- TYP R-TYP
J
-
-
40
IJJ
C)
3
20
It'
3000 _
2000 _
+ 1000 _
O U t.
- 1000 _
2000 -1 1
0 1
TIME (hours)
LL
-
~- 200 0 +200
0 1 2
TIME (hours)
50
40
30
20
10
O
1 1 1 1 1 1 1
-200 0 +200
ELECTRIC FIELD ~ v/m )
FIGURE 12.19 Same as Figure 12.18 for rain or snow showers ~ v
and for thunderstorms ~ ~ ~ (from Fischer, 1977).
]79
300
200
100
o
can
lo:
can
~200
300
100
o
_
1 -
7h 8h
gh lob ,, h
;
I
~.v
_
7h oh 9 h 1oh Oh
FIGURE 12.20 Influence of a high-voltage power line (220 kV) on
the electric field near the ground (black station) compared with the
electric field at the undisturbed station (Fischer, 1977).
12.22) still shows a pattern typical for fair-weather con-
ditions. During rain or snow and especially in thunder-
clouds, the scatter in the field values becomes much
larger, including regions with large negative field val-
ues. Temporal variations are fast, and the horizontal
components of the electric-field strength can reach the
same order of magnitude as the vertical components as
measured by Winn et al. (1978~. These large variations
of the electric-field strength in shower and thunder-
clouds are caused by regions of high-space-charge den-
sity of both signs in these clouds.
Above the tropopause the vertical electric-field
strength continues to decrease nearly exponentially as
the atmospheric conductivity increases and normally
drops to around 300 mV/m at 30 km at mid-latitudes.
Holzworth and Mozer (1979) showed that solar proton
events can cause large reductions of the stratospheric
electric-field values at high latitudes by more than an
180
FIGURE 12.21 Disturbed electric field
near the ground in the neighborhood of a big
city (black station). The electric-field pattern
at the south is not disturbed and shows typical
fair-weather values (Fischer, 1977~.
Lid
order of magnitude. The authors could explain their be-
havior with the greatly enhanced ionization by solar
protons, which in turn enhances the stratospheric con-
ductivity and thereby reduces the local electric fields.
CONCLUSION
Galactic cosmic rays are the primary source of ioniza-
tion in the lower atmosphere. They control the bulk at-
mospheric conductivity parameter, which in turn is lin-
early related to the small-ion concentration and
small-ion mobility. Although the existence of a solar-
induced modulation of the ionization rate and of the at-
mospheric conductivity is evident, the basic physical
mechanisms are not fully understood. More measure
WOLFGANG GRINGEL, JAMES M. ROSEN, and DAVID J. HOFMANN
300
O .
11 h 12 h 13 h 14h 15h
200 ~If,
0 1 2 3 4 5km
h 12h 13h 14h 15h
AUGSBURG
meets of both parameters (if possible simultaneously)
are desirable in order to establish cyclic and transient
solar modulation effects. The accuracy of direct mea-
surements of small-ion concentrations and mobilities
have shown improvements, but the results among dif-
ferent researchers are still contradictory. Almost noth-
ing is known about the mobility distribution of atmo-
spheric ions throughout the troposphere and lower
stratosphere. The existence of heavy ions with masses of
several hundred emu, as inferred from mobility mea-
surements, has only just recently been established by
mass spectrometer measurements. Problems with the
sampling procedure employed can not be overlooked.
Simultaneous measurements of ion masses, ion concen-
trations, and ion mobilities, together with conductivity
ELECTRIC FIELD ( v/m )
FIGURE 12.22 The atmospheric electric field versus altitude during fair weather (O), cloudiness without precipitation (a), and during haze and
fog (c=, - ). The black curves show typical measurements, the white curves show mean profiles and the hachured areas show the scattering of the
values (Fischer, 1977).
ELECTRICAL STRUCTURE FROM 0 TO 30 KILOMETERS
and aerosol measurements, are clearly needed to gain a
deeper insight into the physics and chemistry of atmo-
spheric ions. Volcanic eruptions can result in a rather
dramatic increase of the aerosol content of the lower at-
mosphere. The extent to which these aerosols directly
affect the atmospheric conductivity, small-ion concen-
tration, and mobility should be investigated to a greater
extent. On the other hand, the sulfuric acid content of
the lower atmosphere is also drastically enhanced fol-
lowing volcanic eruptions and might considerably influ-
ence the ion composition. Air-earth current-density
measurements seem to be consistent with the classical
picture of the global circuit with some exceptions. The
question of whether there are other global generators in
the lower atmosphere in addition to thunderstorms
could probably be unraveled by ground-based and bal-
loonborne current measurements at different locations.
The electric-field strength in the lower atmosphere,
which is closely related to the air-earth current and the
atmospheric conductivity, can undergo considerable
fluctuations near the ground owing to conductivity vari-
ations and the influence of a local generator.
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