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AURORAE BOREALES AND GEOMAGNETIC INCLINATIONS AS
AIDS TO ARCHAEOMAGNETIC DATING
YIANNIS LIRITZIS
Ministry of Culture, Archaeometry Division, Athens, Greece*
(Received 17 March, 1988)
Abstract. Geomagnetic virtual pole positions (VGP’s) calculated from archaeomagnetic directional data are compared with three ancient accounts of low latitude observations of the Aurorae boreales, viz. by Aristotle (384-322 BC), Seneca (55 BC-40 AD) and simultaneously by Chinese astronomers in Hangzhou, China and by European observers in Prague in 1138 AD, October 13th.
The geomagnetic latitudes of these VGP’s ranged from 65” to 70”. Inclination and declination values have also been calculated for the regions of interest, assuming dipole field symmetry.
The reduced archaeomagnetic results used, comply with those early observations of aurorae, confirming the inclination of the geomagnetic pole towards the observing sites.
A critical evaluation of the aurora1 types is given, associated to the low latitude aurora. The latter, is furthermore, discussed in relation to the ancient descriptions of the aurora phenomenon.
A critical discussion is also made on severe magnetic storms-related aurora. It has been shown that the appropriate use of relevant VGP’s, in our study, is not influenced by discernible non-dipole disturbances.
Introduction
The reconstruction of the path of the geomagnetic pole can be corroborated using ancient historic descriptions of observations of the northern lights (aurorae, northern dawn). These have excited the curiosity and wonder of men since the dawn of history. Aurorae and geomagnetic activity are related to solar activity and both are linked to magnetospheric storms generated by interplanetary disturbances. In fact, the relation- ship between the solar-terrestrial parameters have recently been discussed (Liritzis and Petropoulos, 1986, 1987; Crooker et al., 1977; Maezawa, 1978; Rostoker et al, 1967).
Aurorae are observed in an approximately circular geographic zone (the aurora1 oval) of radius of approx. 22” almost centered on the geomagnetic poles called the aurora1 zone. Historical observations of aurorae have been recorded mainly in China but in Europe and America as well. (Loomis, 1866; Rubenson, 1882; Schove, 1962, 1958, 1983; Stothers, 1979a,b; Fritz, 1881; Link, 1968; De Mairan, 1733; to mention a few).
Since 300 AD there are numerous records of aurora1 displays observed in different parts of the world, their reliability being dependent on many natural and social factors. In 584585 AD, Gregory of Tours was very precise in describing aurorae: he said “In those times there appeared towards the town of Aquilon, during the night, some brilliant rays of light which appeared to dance in the sky and the sky was so clear, in this particular region, tht if it had not been in the night, one would have been
*Present address: Geophysics dept., Edinburgh University, Edinburgh, Scotland.
Earth, Moon, and Planets 42 (1988) 151-162. 0 1988 by Kluwer Academic Publishers.
152 Y. LIRITZIS
able to seen the aurorae. . .“. In 1621 AD, Gassendi in a book describing observa- tions in southern France before 1621 coined the name aurorae bore&es. Later, Von Vlloa in 1745 and Cook in 1773 made the first observations.
Since then, the mechanism of aurora production and its physical relationship with other phenomena has been recognised and investigated by others (Feynman and Silverman, 1980; Feynman, 1983; Russell, 1975; Siscoe, 1980, Gussenhoven et al., 1981; Hakamada and Akasofu, 1981; to mention a few).
Observers in low latitude countries of the nothern hemisphere can see the northern lights only when the geomagnetic pole is inclined towards their geographic longitude resulting in a high geomagnetic inclination or occasionally due to a great magnetic storm. The former suggestion has been initiated by Keimatsu et al. (1968).
The inclination may be calculated from archaeomagnetic data for sites where the northern lights have been observed historically. Such calculations might question the validity of published inclination and declination (I, 0) data or confirm large non- dipole differences between two distant regions, when pole positions are calculated.
The three accounts of observed aurora considered here are those of: (a) Aristotle (384322 BC) in his work ‘Meteors I’, describes various types of
aurorae, saying, “Sometimes on a clear night a number of spectra can be seen taking shape in the sky, such as ‘chasms’, ‘trenches’, and blood red colours . . . the upper air condenses and takes fire and that its combustion sometimes produces the appearance of a burning fire, sometimes of ‘torches’, or stars in motion. . .“.
(b) Seneca (HAD-40 AD), who speaks of a light, “Sometimes so low in the horizon that it gives the effect of a fire in a distance. . .“, and
(c) the aurorae observed by ancient Chinese astronomers in Hangzhou, China in 1138 AD, October 13th, which is thought to refer to the same aurora as seen by European observers in Prague, Czechoslovakia (Kawai and Hirooka, 1967; Fritz, 1873).
The Variability of the Aurora1 Oval
Aurora1 reports can be used only cautiously, to corroborate the archaeomagnetic data (1) for the reconstruction of the geomagnetic pole positions in the past and (2) for archaeomagnetic dating, that is the aim of the present attempt.
As the aurora1 oval is formed around the pole, any shift of the geomagnetic pole, ideally, would result in a change in observational latitudes of aurorae. However, this is not always the case; a change in the pattern of observed occurrence of aurorae can be due to several factors including, a change in the Earth’s dipole field or a change in the solar wind driving the auroras.
At our present stage of knowledge it is not possible to identify unequivocally the solar wind parameters that might change to affect either the annual frequency or latitudinal distribution of aurora1 observations. The problem has been approached either through direct studies of auroras or through the closely related geomagnetic activity; the latter, in turn, being related to some function of the solar wind velocity
AURORAE BOREALES AND GEOMAGNETIC INCLINATIONS 153
u, and the southward component of the interplanetary field, Bz (Liritzis and Petropou- los, 1987).
Two effects of change in the solar wind that could cause a change in aurora1 occurrence frequency at a particular latitude are changes in the number of geomagnet- ically disturbed days per year and changes in the position of aurorae for the same level of geomagnetic activity.
Aurora1 accounts and distributions in the Medieval and current epoch in Europe has been studied extensively (Feldstein and Starkov, 1968; Feynman and Silverman, 1980; see also references in the introduction). For the current epoch these studies have shown that the southward extension of the aurora1 oval edges is accompanied by increasing geomagnetic activity (R-index, geomagnetic storms) and the position and dynamics of the aurora1 oval was changed noticeably in a period as short as four years.
For the last 200 years such a change in aurora1 occurrence patterns was due to the solar wind or some underlying variability of the sun (see, e.g., Kamide et al., 1977; Svalgaard, 1977). For example, during the International Geophysical Year (IGY) 1957/58, when solar activity was maximum, aurorae was observed in low latitudes and could be seen in Japan ( = 30 - 40” N) down to places of geomagnetic latitude 35” N (1957, July 5, 1120-1200 UT) (Japanese contribution IGY, Science Council of Japan, Tokyo, p. 44). One should, however, note that the low latitude aurora is quite different from the mid-high-latitude aurora in the colour and display; the former being reddish and diffuse and of veil form while the latter tend to be very active and of long duration in time.
From all the above, we are aware that only simultaneous accounts in the Occident and the Orient would be reasonably utilized to infer the position of geomagnetic pole axis because of the variability of aurora1 oval position with individual magnetic storms.
Since the loss of Prof. Keimatsu, in 1976, this work has only recently been revived (Fukushima et al., 1985).
In our work we calculate the pole positions from archaeodirectional data and consequently the inclinations for three sites, and incorporate observing positions of auroras to corroborate for such computations.
Results and Discussion
Archaeomagnetic measurements have been made for various countries (see e.g. Creer et al., 1983) though not precisely at the geographic sites where the northern lights have been observed. Thus, it is necessary to make some small corrections to published archaeomagnetic directions to allow for the particular differences in latitude and longitude.
Directions of magnetization may be represented by their corresponding virtual pole positions, calculated assuming a geocentric dipolar (but not axial) geomagnetic field.
With this model the inclination and declination for a particular geographic site is directly related to the angular distance from the geomagnetic pole, and if the location
154 Y. LIRITZIS
of the site is in normal geographic latitude & and longitude, 4,, then the pole’s latitude, ;1, and longitude, 4P, often called virtual pole, can be determined (Irving, 1964).
The reverse procedure to determine the Inclination and Declination (Ired, Dred) at another geographic site from the location of the virtual pole is also made here. Alternatively, the quoted Z,C value reduced to a given latitude is used on the assumption of an axial dipole field, which predicts D = 0 everywhere.
These calculations are given to a first approximation and correction of the present geomagnetic field directions in an area of no more than 750 x 750 km2 shows differences of less than 1” in both Z and D at a central location (Tarling, 1983).
Clearly large errors (of a few degrees) will arise for extensions over wider areas, as the correction assumes that for any such region the geomagnetic pole corresponds to a geocentric dipole. This is in fact the basic assumption of palaeomagnetism applied to geological formations.
The non-dipolar components can cause, of course, large errors in the determina- tion of the actual pole. However, our computation corroborated by ancient aurora1 accounts offer further ways to judge the reliability of some archaeomagnetic data (see Tables I, II, III).
The calculations pertaining to each case are as follows:
ARISTOTLE'SOBSERVATIONS
Three pairs of data (Z, D) were used (Table 1). On average the pole position is A, = 69.5” + 3”; 4p = 7.5” + 10”.
The Zrd is similar to Z$ within the experimental error for I. Figure 1 illustrates the possible extension of the aurora1 oval of 26” to 30” radius around the calculated geomagnetic pole position corresponding, probably, to a diffused type of aurora.
Obviously north of Greece is within this region and northern lights could have been seen by Aristotle. The Z and D for north of Greece at that time are calculated as 63.3 f 3” and - 10” + lo”, respectively (Note 1975’s value of 57” and -2” E for north Greece).
SENECA'S OBSERVATION
Seven pairs of data (Z, D) were used for the 90 BC to 100 AD (Table II). On average the calculated pole position is &, = 71.6” f 3, bp = 5 + 5”. The ZFd is similar to Z,” within the errors.
The No. 2 of Table II is a unique (I, D) pair of Thellier’s data whereas no. 1 is the average of the (Z, D) measurements.
Figure 1 illustrates the possible extension of the aurora1 oval around the calculated geomagnetic pole position. Obviously Rome (A, = 40.6” N) is within this oval for Seneca to have seen this light. The Z and D for Rome at that time is calculated to 61.6 f 3 and 7 f 13, respectively (Note Zand D for 1975 are approx., 56” and 2” W, respectively).
It is worth noting that the 1, and ZFd for N. Greece (approx. 40.5” N) and Rome (approx. 40.6”) are similar within the errors, as expected. The apparent differences in
AURORAE BOREALES AND GEOMAGNETIC INCLINATIONS 155
156 Y. LIRITZIS
TABL
E III
Old
Chi
nese
ast
rono
mic
al
text
s,
I138
Oct
. 13
, Han
gcho
u,
Loya
ng
regi
on,
Chi
na
and
in P
ragu
e
Auth
or-s
ite
DO
4
A D
Ed
s ZC
I
Dat
e AD
1. r
ef.
[3]
(Chi
na,
Loya
ng)
2. r
ef.
[7]
i Arka
nsas
Sout
hwes
t Am
eric
a
3. r
ef.
IS]
(Jap
an)
4. r
ef.
[5]
p. 1
52
vario
us
auth
ors
(take
n fro
m
curv
es)
5. r
ef.
[IO]
(Icel
and)
6. r
ef.
8 (U
krai
ne)
7. r
ef.
[4]
(Bul
garia
)
8. r
ef.
[2]
(Brit
ain)
9. r
ef.
[I]
(Fra
nce)
Aver
ages
fo
r H
angc
hou:
W
ithou
t (2
.5):
With
out
(2,
5, 8
, 9)”
55f2
15
Wk2
34
.42
58.1
18
W
35.2
2
55 *
5 O
&8
34 L
3
70 *
5
17 *
5 47
*
3
62.5
k 5
-
42.8
63av
22
av.
51.5
61 k
3 15
+6
46av
.
112E
137.
05
135
* 3
30+4
23.6
E
0 7av.
E
67 f
2
80.6
78av
.
64_+
3
65 f
4
-78
65.5
5 3
73.6
73 +
3
71.6
&4
68 *
3 65
.4 +
4
90 f
2
186.
2E
192a
v.
125_
f3
64 +
3
140+
5 ~
N 1
22E
71.5
54
62 k
5
65 +
5
(to
Prag
ue)
- -
37
58 *
4
32 &
6
58 &
4
110_
+80
60.5
_+ 4
82
.6
60.5
+ 4
10
7 63
-2o+
10
-20+
5 45
+ 5
- -3+5
-4&5
-122
13
-12+
13
-2
o+
11
54
1140
+ 1
0
1100
-120
0
llO~l
175
57.4
10
50-l
17.5
1140
f 10
1140
+ 10
58.8
+ 5
11
40+5
72
+5
55
110~
1200
68
(to
Pr
ague
)
54.7
Fi
Prag
ue)
105&
l 15
0
50.5
+ 3
64
k 3
(to
Pr
ague
)
55
55
56.3
115&
1200
105&
1200
10
5G12
00
105G
1200
“Due
to
the
drif
t of
the
non
dip
ole
field
as
wel
l as
the
loca
l de
velo
pmen
t of
mag
netic
an
omal
ies
the
aver
age
redu
ced
Z, D
val
ues
are
cons
ider
ed
as m
ost
relia
ble
whe
n de
duce
d fro
m
regi
ons
near
est
to t
hose
of
inte
rest
.
158 Y. LIRITZIS
-n -- 30 02 02 30 30 4 4
Fig. 1. Reconstructrion of aurora1 oval from calculated pole positions for Aristotle’s and Seneca’s times. H = Hangchou, P = Prague, G = Greece, R = Rome. G maximum extension of aurora1 pole latitude
and longitude. Aurora1 oval radius ~28” (26”-30”).
longitude reflected in $P, opd are overmasked by the attached errors. It would thus be interesting to search for records in Greece for evidence of aurora at Seneca’s lifespan (Stothers, 1979a).
In the two above times the aurorae might have been intensified and enhanced by great magnetic storms as well.
HANGZHOU AND PRAGUE, OCTOBER I ~TH, 1138
Nine pairs of data (JO) were used for the time span AD 10%&1200. On average the pole position is Lp = 71.6” f 4, 4P = 110” + 80. The Icelandic, European and North American values are apparently higher than the other parts of the world near China.
AURORAE BOREALES AND GEOMAGNETIC lNCLlNATlONS 159
180
..* or 30” E
~~&lTt)i\ EQUAL-RRER : 75737152
Fig. 2. As per Figure 1 but for Hangchou and Prague. Aurora1 oval radius =30” (27”-33”). The dashed circle is the most probable averaged oval.
Due to the non-dipole field, spatial and temporal variations and its westward drift, the Chinese, Japanese and possibly Ukrainian results would be favoured, giving on average & = 65.4” and #, = 107” (dashed oval, Figure 2). Naturally the reduced directional values from regions near to the location of interest will be more reliable. Thus, the I,” of the Asian data give for Hangzhou, Zs = 56 f 2, and the European give for Prague Zs = 67 + 4. F’ igure 2 illustrates the aurora1 oval around the calcu- lated geomagnetic pole position and Hangzhou and Prague are within this aurora1 extension for ancient Chinese astronomers in Loyang and European observers in Prague to be able to have seen it.
In comparison with the & from the two earlier accounts the lower values found above are in perfect accord with the lower latitude of Hangzhou (approx. 30” N) with
160 Y. LIRITZIS
respect to Rome and N. Greece (approx. 40.5 N”). Moreover, the aurora1 oval radius is of the expected figure; which holds for an inclined geomagnetic pole to China in combination with a diffused type of aurorae and a highly increased geomagnetic activity (and solar activity). The aurora1 oval drawn by the late Kawai et al. (1967) for these two localities, that shows a perfect agreement with the deviation to be expected from their proposed hypotrochoid motion, being of a radius of around 58” contradicts with ours (Figure 2). Their oval certainly is exaggerated, which also implies that their model should be reconsidered.
There is a possibility that the aurora1 oval drawn by them cannot be rejected if it was during a severe magnetic storm, such as the low latitude aurorae on 11.2.1958 seen at 23” N geomagnetic latitude in Japan.
Furthermore to the above calculated figures for Z and D the aurora1 form could also be predicted. According to current knowledge (see e.g. Stormer, 1955; Kamide et al., 1977; Gussenhoven et al., 1981) the aurora1 form rays (narrow almost vertical columns of luminosity, field aligned) have a considerably greater vertical extent than normal aurora in the height distributions that varies from IO&600 km. The longer extents e.g 600 km (approx. 3” in latitude) are observed in lower latitude aurora near the sunspot maximum and are greater aurorae associated with atmospheric heating.
The diffused aurorae of the veil form are also extensive to a diffused surface > 10” in extent.
Conclusions
Taking into consideration these aurora1 forms, the aurora1 oval extends to at most a 35” radius around the pole. Thus, both the aurora1 type and a possibly coexisting great magnetic storm produce aurora1 extensions. We do not know at present which one was more pronounced; though for ancient times, we favour the former as such a phenomenon could have happened more frequently for this to have attracted the wonder and curiosity of the philosophers. Thus, such am-oral extensions, plus an inclined geomagnetic pole to a 1, = 65-70” for the considered, here, sites, both make it possible for the northern lights to be observed as far south as 35”40”.
This work, proves also that ancient D and Z values in the location of the aurora1 observation, are not influenced by discernible non-dipole disturbances. Therefore, the relevant VGP position obtained in this way (rather than getting a worldwide average, so as to average regional disturbances) stands as correct at least in the present study.
Overall, the appropriate use of such archaeo-astronomical accounts, as illustrated here, has a benefit in aiding archaeomagnetic dating as the corrected or reduced Z and D are reliable guides with which future archaeomagnetic data from these areas may be worth comparing.
This study, finally, initiates archaeologists to revisit old texts of ancient civilisa- tions that are situated within the aurora1 zones of Figures 1, 2 and try to locate such aurora1 accounts that are otherwise predicted from these figures.
AURORAE BOREALES AND GEOMAGNETIC INCLINATIONS 161
Acknowledgements
I wish to thank Prof. K. Creer for reading the manuscript and for very constructive comments, and to Prof. Silverman for valuable comments and helpful correspon- dence. I am grateful for UNESCO for assistance under grant No. OBL NO 9-0514- 8901 DTP 3812., and The British Council for contribution towards travel costs.
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I62 Y. LIRlTZIS
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‘Archaeometric Sampling in Greece, Field Report, Dept. of Physics, Edinburgh Univ. (1977). [lo] Brynjolfson, A.: 1957, Phil. Mug. Supp. Adv. Phys. 1, 247.
