Solar flares produce increases in coherent-scatter power from the mesosphere due to the increase in free electrons produced by X-ray photoionization. Thirteen such power enhancements were observed at Urbana. When such an enhancement occurs at an altitude containing a turbulence layer with constant strength, the relative enhancement of electon density is estimated from the enhancement in power. Such estimates of enchanced electron density are compared with estimates of the X-ray photoionization at that altitude, deduced from geostationary satellite measurements. It is found that possible types ion-chemical reaction scheme may be distinguished, and the nonflare ion-pair production function may be estimated. The type of ion-chemical scheme and the nonflare ion-production function are shown to depend on the solar zenith angle

Bowhill, S. A.

Parker, J.

English

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310 " .. !.,  18 7- 10479
5.1.1 AN ANALYSIS AT MESOSPHERIC COHER_T-SCATTER POWER
_qHANCEMENTS DURING SOLAR FLARE EVENTS
ABSTRACT
J. Parker and S. A. Buwhill
Department of Electrical and Computer Engineering .....) \w/_'__/J-
University of Illinois _ _\_!Urbana, IL 61801 '
/
Solar flares produce increases in coherent-scatter power from the
mesoaphere due to the increase in free electrons produced by X-ray photo-
ionization. Thirteen such power enhancements have been observed at Urbana.
When such an enhancement occurs at an altitude containing a turbulent layer
with constant strength, we may estimate the relative enhancement of electron
density from the enhancement in power. Such estimates of enhanced electron
density are compared with estimates of the X-ray photoion/zat/on at that
altitude, deduced from geostationary satellite measurements. It is found that
possible types ion-chemical reaction scheme may be distinguished, and the non-
flare ion-pair production function may be estimated. The type of ion-chemical
scheme and the nonflare ion-production function are shown to depend on the
solar zenith angle.
INTRODUCTION
It has, of course, been known for some time that solar flares produce an
increase in ionization in the D region. This flare-time enhancement in
ionization results in an increase in coherent scattered power which has bee_
assT-,ed to account for an exceptionally full set of good mesospheric velocity
measurements during the event of April 11, 1978, 0800 CST at Urbana (MILLER et
al., 1978), and also measured directly for the event of January 5, 1981, at
1218 AST at Arecibo (ROTTGER, 1983).
However, the processes linking the solar X-ray enhancement to the co-
herent-scatter power increase involve many unknowns. Photoionization by X-rays
may be considered as the driving function of a set of ion-chemical reactions
which finally determines the electron-density profile; this profile must then
be advected by turbulence to produce the scattered power. Many details of
these processes can only be deduced indirectly.
This paper describes how models of these processes may be constructed
which account for some of the features of the power enhancements observed
during solar flares, Early work along these lines may be found in PARKER and
BOWHILL (1984).
COHER_T SCATTER DEPENDENCE ON ELECTRON DENSITY
The radar scattering cross section is proportioned to the mean-square
fluctuation of the retraetive index _. At VHF:
2 2
n = i - Ne2/¢ m_
o
where N is the electron concentration, e and m the charge and mass of the
electron, e the permitivity of free space, and _ the angular frequency of the
O
impinging wave. The right-hand term is small compared to unity, eo fluctua-
tions in _ and N are proportional. Further, if the electron concentration
increases in a scattering volume by a constant factor, the mean-square fluctua-
https://ntrs.nasa.gov/search.jsp?R=19870001046 2020-03-23T14:38:44+00:00Z
311
tion in electron density A_
<AN2> = (N,) 2 _ N 2
due to turbulence will be
where N' is the vertical gradient _f the electron density N. Thus, the scat-
tered power P is proportional to N- within tlme-scales where the turbulence
is characterized by constant mean-squared statistics. (This assumes no time-
lag due to turbulent advection; we shall demonstrate below that this holds at
least some of the time.
Finally, when we divide the flare-time scattered power P at a given
altitude by the pre-flare power P we obtain
0
P/Po = (No + AN)2/No 2
which implies
AN/N o = _TF ° - i
D-REGION FLARE EFFECTS OBSERVED AT URBANA
Table 1 displays features of 13 solar flare events which produced enhanced
scatter, the enhancement shown for the altitude range 60-75 kin. The sizes of
the peak X-ray fluxes from two detectors on the GOES II satellite are shown for
comparison. No correlation of X-ray event size and coherent-scatter power
enhancement is evident. This is probably due to the wide variability of tur-
bulant strength and the nonflare ion-production rate from one flare event to
another. This suggests the need for a more sophisticated analysis.
MODELS RE_ATING SOLAR X-RAYS AND ELECTRON DENSITY
Given the GOES II X-ray measurements, we may calculate the X-ray ion-pair
production rate qx at a given altitude as follows. First, we must estimate
the X-ray spectrum from the two data points provided by the GOES detectors at
each time. This may be done by assuming a power law spectral form:
@ = AX B
(see for example ROWE et al., 1970) and solving for A and B based on the wave-
length response characteristics of the X-ray detectors (published in DONNELLY,
1977).
This form of the X-ray intensity @ (k) is used to calculate the desired
ion-palr production function qx according to Chapman theory. Constants
necessary for this calculation are the average air absorption cross sections
o=(l) and the ionization efficlencies for X-rays n (l)from BANKS and KOf_IERTS
(_973), and the scale height H and average air density M from the US STANDARD
ATMOSPHERE (1976).
The relationship between q (= q + o) and N is perticularly unclear
O iX
in the lower D region. If we assume over_ll charge neutrallty, an unchanged
ratio of negative ions and positive ions, and unchanged proportions of the re-
combining species, we may derive the followlng (MITRA, 1974):
q = _N 2
where the constant of proportionality_ is called the effective recombination
coezficient (call this Case A). MITRA (1974) also proposes (Case B)
q = BN
312
Table i. X-ray flare events producing measurable coherent-scatter radar
power enhancements at Urbana between April 1978 and December 1983.
61.5-75 km
pk flux x 10-5w/m2 pk CS power
yr mo da cst SID Ha 0.1-0.8r4n 0.05-0.4nm dB above P
o
78 04 ii 0820 3+ NA 20 3.5 7.7
80 05 21 1510 3 3B 14 3.2 29.8
80 05 28 1354 2+ 2B 12 2.0 3.5
80 11 13 1324 2+ 1N 4.2 0.7 11.2
80 11 14 1239 2+ 1B 2.4 0.4 13.8
81 01 27 0947 2 1B 4.6 1.3 6.1
81 05 05 0809 3 3B 12 3.3 2.8
81 08 03 1425 2+ 1N 7.1 1.8 10.1
81 i0 14 IIII 2+ 1B 30 11 7.4
82 03 31 1626 2+ NA 7.5 1.7 4.9
82 06 02 0953 3+ NA 10 2.9 17.4
83 08 13 1215 1 2B 5.2 0.9 3.7
83 08 21 1159 1 2B 2.2 0.3 6.8
as a possible relationship, given other conditions. Both relationships require
chemical equilibrium, which may or may not hold during a flare. If not, the
relationship is far more complicated, but a model by THOMAS et al. (1973) pre-
dicts delays in the lower D region between peak q and peak N of up to 14 min-
utes, increasin 8 with decreasing altitude (Case D).
DISTINGUISHING CHEMICAL SCHEME TYPES AND ESTIMATING NONFLARE ION-PAIR FRO-
DU CT ION
To distinguish between these three possibilities (and possibly others) at
a given altitude we may assume each possiblity in turn, compare qx at each
time with AN/NO from the coherent-scatter data, and find the best fit. To
simplify this comparison, note that Case A implies P=q, so that we may
estimate qx/qo as
qx/qo = P/Po - 1
For Case B, q and N are proportional, so that
qx/qo = _- i
If we make two plots, one for each of these estimates of qx/qo against the
same values at q from the satellite data, the result woul_ be a line of
unit slope only _or that case which is correct. If the points of the initial
313
part of the flare (ascendin 8) do not lie alon 8 the same path as those of the
decay phase of the flare, we may have Case D, or possibly a time la& due to
turbulent advection.
The result of this technique is shown in _igure i, for the flare of Nov-
ember 14. 1980, end the scattered power from 70.5 kin. The plot of upper left
shows a close fit to a line, but not of unit slope. The plot of upper right
shows a fairly close fit to a line of slope 1. We may conclude q = BN, and
more: note that the intercept of this llne with the log (qx/qo) = 0 line
implies _th,_t when q_ = qo" log (qx) = 0.3. Thus we may estimateqn =
0.5 cm-_s -_. The l%wer plot shows the log scattered power anti,he estimate
of qx based on the satellite data. The coincidence of the peaks of these
curves validates the chemical equilibrium hypothesis, and demonstrates that
there is no lag due to turbulent advection.
COMBINED RESULTS FOR SEVERAL FLARES
Figure 2 shows the type of power enhancement found for each altitude for
each of seven flare e_ents, arranged in order of decreasin 8 solar zenith engle.
The symbol E represents extremely large enhencements (some as much as 30 dB)
which cannot be accounted for by this model. Als0, note that the altitudes
showing no response seem to proliferate downward with increasing solar zenith
angle. This is not what Chapman theory predicts for ionization, and so pos-
sibly indicates a variability in existence of turbulent layers. Some events,
particularly at high zenith angles, show delays (Case D), with delay increasin S
with decreasing altitude. Note, however, that a delay may be either a chemical
or turbulent mixin 8 effect.
I
ff
-I
_,_
-:3
3-
I
-I
, 1 !
EBT LOB(OXIBH)
LRBSUME5 @=RN+'_
I I I
/
! I
EBT LnB(_X/_Z)
.BB_UMEB B-BN
I I I
LOB(BX) LOIB(SX)
(CM+-3_,B_-I ) (CfH'-'4,5_- i
eI i l _:3 e e3 -2 Z 3II m e I I- H -I B I
.LOB C5 PON. RBOVE PRE-FLRRE RVB.
LO_ HX (£M+-3tS_-I)
I¢@IIIII¢IIII f I
NOVEMBER IH, IHBB POHER (_ PLOT) RLT5 BVID FROM 7D.E TO 7U.E KM,
Figure I. Flare-time scattered power enhancement and estimated
electron production rate qx due to flare X-rays at 70.5 km
for the November 14, 1980 event.
314
KM
88.5 I
85.5 !
i
82.5
79.5
76.5
73.5
70.5
67.5
64.5
61.5
CN
CN
CR
CR
CN
-- CN A CR
-- E A CN
- - CN -- A CN
-- -- CN B?
-- CN CN E
D4 B E
D4 B -- B D--2 E
D4 D4 D6 D4 D-2 E/D8
D6 -- D6 D5 _ CR
D6 _ CR D8 -- AB?
CR -- A D8 -- A
CN AB? _ D8 -- A
CN AB? AB? DI3 -- AB?
AB?
CN
CN
CN
CN
CN
CN
CN
E
E
E
E
E
A/D-2?
A
AB?
AB?
A
o o o o o
n I H , ! I u
x x x x x x x
Figure 2. Types of response at each mesospheric altitude for each
flare: A) q = _N 2. B) q = 8N. AB?) Probably A or B, but plots
are ambiguous. D#) Delayed effect, with peak power following q
by # minutes. E) Extreme response, power increase greater than
cases A or B. CN) Coincident flare-time power response, but
unable to fit to A or B model due to comparable nonflare
fluctuations in power at that altitude. CR) Coincident response
well above nonflare base power, but unable to fit to A or B
models due to large fluctuations on the order of the one minute
sample time. --) no detectable response.
Figure 3 shows estimates of qo made by this technique. When the
interpretation was unclear between Case A and Case B, but equilibrium seems to
hold, a dashed horizontal line indicates the range of possible q_ estimates.
Note that the estimates for qo fall into two clusters, correspoUnding to small
and large zenith angles. Also, the values of qo for large zenith angle are
smaller, as expected. Also, q generally increases with altitude for a given
O
flare eZfect, also as expected. We may conclude that the estlmate of qo has
some degree of reliability, although its absolute accuracy is not estimated
hers.
DISCUSSION AND CONCLUSION
By using models for the uncertain links between the causal X-ray emissions
and the coherent-scatter power enhancements, we have gained insights into fea-
tures which are not otherwise clear. Often one model fits the data better than
others, and so we are able to choose between some features of ion-chemical
schemes. When no existing model fits the data, as with the extreme power
enhancements shown in Figure 3, it is unclear which part of the analysis should
be modified: an extreme enhancement may be due to an unknown ion-chemical
scheme, or may be due instead to some unknown condition which allows P to
increase faster than N 2. This suggests further research.
315
]
v
t,i
r_
D
90
80
7O
6O
5O
I I I /j,,/ I
/,,,"
/.- Rotnosiri Theoreticol
secx =105
Jan. 27, 1981
sec x=225 _ / =/-- Oct. 14, 1981
.'--7/'--" S ............
,.........y .... °
.-e sec x = 1.15
secx = 2,26
I I I I I
i_ I I0 ° I0 = i0 z IOs
qo (c_3 s'l)
Figure 3. Estimates of q_ made by finding intercept of log (qx) vs
log (qx/qo) plots with line log (_)_ 0 for each altitude and
flare event which shows type A (q =aN _) or type B (q =BN)
response. Horizontal dashed lines represent the span between the
type A estimate at q and type B estimate at q at altitudes
• O . O
where the cholce between type A and type B _s ambiguous. The
estimate of qo by RATNASIRI and SECHRIST (1975) is reproduced for
comparison.
Only altitudes with steady turbulent layers produce useful data, so
altitude gaps are inevitable. However, the coherent-scatter radar technique
compares favorably with earlier methods of observing changes in N during solar
flare, such as partial reflection and wave interaction. The temporal and
spatial resolution are excellent for the coherent-scatter technique, and the
relative changes in N may be measured accurately.
ACKNOWLEDGMENT
The work described in this paper was supported in part by the National
Aeronautics and Space Administration under grant NSG 7506.
REFERENCES
Banks, P. M. and G. Kockarts (1973), Aeronomy_ Part A, Academic Press, New
York.
Donnelly, R. F. (1977), Solar X-ray measurements from SHS-1, SMS-2, and GOES-1
information for data users, NOAA Technical Memorandum ERL SEL-48.
Miller, K. L., S. A. Bowhill, K. P. Gibbs and I. D. Countryman (1978), First
measurements of mesospheric vertical velocities by VHF radar at
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Mitra, A. P. (1974), Ionospheric effects of solar flares, Reidel Publishing
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Parker, J. W. and S. A. Bowhill (1984), Observations of solar-flare ionization
in the mesosphere using coherent-scatter radar, Adv. Space Res., 4,
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Rottser . J. (1983)m Origin of refractive index fluctuations in the mesosphere
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An analysis at mesospheric coherent-scatter power enhancements during solar flare events

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