What Geomagnetism can Tell
Us about the Solar Cycle?
Leif Svalgaard
HEPL, Stanford University
Bern, 11 Nov., 2013
ISSI Workshop: The Solar Activity Cycle: Physical
Causes and Consequences
”Wer hätte noch vor wenigen Jahren an die Möglichkeit gedacht, aus den
Sonnenfleckenbeobachtungen ein terrestrisches Phänomen zu berechnen?”
(J. R. Wolf, Bern, 1852)
1
‘Different Strokes for Different Folks’
• The key to using geomagnetism to say
something about the sun is the realization
that geomagnetic ‘indices’ can be constructed
that respond differently to different solar and
solar wind parameters, so we can
disentangle the various causes and effects
• In the last decade of research this insight has
been put to extensive use and a consensus is
emerging
2
Electric Current Systems in Geospace
Different Current Systems
Different Magnetic Effects
Diurnal
Var.
BV
BV2
FUV
B
Polar Cap
nV2
±By
Magnetospheric Currents
3
Electric Current Systems in Geospace
Different Current Systems
Different Magnetic Effects
Diurnal
Var.
BV
BV2
FUV
B
Polar Cap
nV2
±By
We can now invert the Solar Wind –
Magnetosphere relationships…
4
Variometer Invented by Gauss, 1833
Helsinki 1844-1912
Nevanlinna et al.
5
Classic Method since 1847
Magnetic
Recorders
46 years ago, I used the Classic Instruments
Modern Instrument
6
Wolf’s Discovery (1852): rD = a + b RW
.
North X
rY
Morning
H
rD
Evening
D
Y = H sin(D)
East Y
dY = H cos(D) dD For small dD
A current system in the ionosphere is created
and maintained by solar FUV radiation
The magnetic effect of this system was discovered by George Graham in 1722
7
The Diurnal Variation of the Declination for
Low, Medium, and High Solar Activity
8
6
9
10
Diurnal Variation of Declination at Praha (Pruhonice)
dD'
4
1957-1959
1964-1965
2
0
-2
-4
-6
-8
-10
Jan
Feb
Mar
Apr
Jun
Jul
Aug
Sep
Oct
Nov
Dec
Year
Diurnal Variation of Declination at Praha
8
6
May
dD'
1840-1849
rD
4
2
0
-2
-4
-6
-8
-10
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
Year
8
The Amplitude of the Diurnal Variation [from many
stations] follows the Sunspot Cycle (can in fact be
used to check the Sunspot Number calibration)
9
rY is a Very Good Proxy for F10.7 Flux
300
F10.7
Using rY from nine
‘chains’ of stations we
find that the correlation
between F10.7 and rY is
extremely good (more
than 98% of variation is
accounted for)
250
y = 5.4187x - 129.93
R2 = 0.9815
200
150
100
y = 0.043085x 2.060402
R2 = 0.975948
50
rY
0
30
35
40
45
50
55
60
65
70
Solar Activity From Diurnal Variation of Geomagnetic East Component
250
200
Nine Station Chains
F10.7 sfu
F10.7 calc = 5.42 rY - 130
150
100
12
13
14
15
16
17
18
19
20
21
22
23
1980
1990
2000
50
25+Residuals
0
1880
1890
1900
1910
1920
1930
1940
1950
1960
1970
2010
2020
10
Helsinki-Nurmijärvi Diurnal Variation
Scaling to 9-station chain
70
rY '9-station Chain'
65
60
Helsinki and its replacement station Numijärvi
scales the same way towards our composite
of nine long-running observatories and can
therefore be used to check the calibration of
the sunspot number
(or more correctly to
reconstruct the F10.7
radio flux)
y = 1.1254x + 4.5545
2
R = 0.9669
55
50
45
40
1884-1908 1953-2008
35
Helsinki, Nurmijärvi
30
25
30
35
40
45
50
55
Range of Diurnal Variation of East Component
70
65
60
55
50
45
40
35
30
rY nT
1840
9-station Chain
Helsinki
1850
1860
1870
Nurmijä rvi
1880
1890
1900
1910
1920
1930
1940
1950
1960
1970
1980
1990
2000
11
2010
Sabine’s Discovery about
Geomagnetic Disturbances
Sir Edward Sabine (1788-1883)
Edward Sabine [1843] computed the
hourly mean values for each month
and defined Disturbance as the RMS
of the differences between the actual
and mean values.
And discovered [1852] that minima
in the average rate and size of
magnetic disturbances at the widely
separated Hobarton (SH) and
Toronto (NH) observatories in 1843
corresponded to a minimum in
sunspot numbers, while maxima in
1848 corresponded to a maximum in
the decennial sunspot curve.
We use the IDV-index = unsigned difference from one day to the next
12
Relation to HMF Strength B
13
B
V
Kp
1.0
n=0
Correlation
n=2
1965-2012
0.5
-2
-1
0
n
1
Latest 27day Bartels
Rotation
showing B
and Kp
peaks
Correlation between
IMF BVn and several
geomag. indices as a
function of n
The IDV indices are not significantly
different from having a dependence
on B only. Thus, the negative part of
Lockwood Dst (i.e. ring current enhancement)
(LRSP2013) is closest to explaining the behavior
14
3
2
4 of IDV
Latest Reconstruction of HMF B
15
The IHV Index gives us BV2
Calculating the variation
(sum of unsigned differences
from one hour to the next) of
the field during the night
hours [red boxes] from
simple hourly means (the
Interhourly Variation) gives
us a quantity that correlates
with BV2 in the solar wind
16
The Many Stations Used for IHV
in 14 ‘Boxes’ well Distributed in Longitude,
Plus Equatorial Belt
17
IHV is a Measure of Power Input to the
Ionosphere (Measured by POES)
18
We can calculate Am [and Aa] from IHV
From IDV we get B. From IHV we get BV2.
Thus we can get V
19
Polar Cap Geomagnetic Observatories
1926-Present
Godhavn
1932-Present
Thule
20
Svalgaard-Mansurov Effect
NP
Toward
SP
Away
Not a subtle effect…
21
Vokhmyanin & Ponyavin, 2013
Sector
Structure
over Time
Now
22
Dominant Polarity: Rosenberg-Coleman Effect
Proves Polar Field Reversals in the Past
23
How do we Know that the Poles
Reversed Regularly before 1957?
Wilcox & Scherrer, 1972
Svalgaard, 1977
“Thus, during last eight solar cycles
magnetic field reversals have taken
place each 11 year period”. S-M effect.
Vokhmyanin & Ponyavin, 2012
The predominant polarity = polar field polarity
(Rosenberg-Coleman effect) annually
modulated by the B-angle.
This effect combined with the RussellMcPherron effect [geomagnetic activity
enhanced by the Southward Component
of the HMF] predicts a 22-year cycle in
geomagnetic activity synchronized with
polar field reversals, as observed (now for
1840s-Present).
24
Cosmic Ray Modulation Depends
on the Sign of Solar Pole Polarity
The shape of the
modulation curve
[alternating ‘peaks’
and ‘flat tops’] shows
the polar field signs.
North pole
North pole
Miyahara, 2011
Svalgaard
& Wilcox,
1976
Ice cores contain a long
record of 10Be atoms
produced by cosmic
rays. The record can be
inverted to yield the
cosmic ray intensity.
The technique is not yet
good enough to show
peaks and flats, but
might with time be
refined to allow this.
25
Cross Polar Cap
Hall Current
Ionospheric Hall Current across Polar Cap
CHAMP
Been known a long time:
1882
26
Cross Polar Cap Potential Drop
GDH
THL
Space
E ~ -V×B
27
Overdetermined
System: 3 Eqs,
2 Unknowns
B
= p (IDV)
BV2 = q (IHV)
Gjøa
VB = r (PCap)
Here is B back to the 1830s:
Heliospheric Magnetic Field at Earth
10
9
B
8
7
6
5
4
3
HMF from IDV-index
2
HMF observed in Space
1
0
1830
1840
1860
1870
1880
1890
1900
1910
1920
1930
1940
Cosmic Ray Modulation Parameter
1400
ϕ
1200
1850
1950
1960
1970
1980
1990
2000
2010
28
Radial Magnetic Field (‘Open Flux’)
Since we can also estimate solar wind speed from geomagnetic indices
[Svalgaard & Cliver, JGR 2007] we can calculate the radial magnetic flux
from the total B using the Parker Spiral formula:
Radial Component of Heliospheric Magnetic Field at Earth
6
Br nT
5
Ceiling
4
R2 = 0.0019
3
2
Floor
1
Year
0
1830
1840
1850
1860
1870
1880
1890
1900
1910
1920
1930
1940
1950
1960
1970
1980
1990
2000
2010
There seems to be both a Floor and a Ceiling and most importantly no longterm trend since the 1830s. Thus no Modern Grand Maximum.
29
Solar Activity 1835-2011
Sunspot Number
Monthly Average Ap Index
60
Ap Geomagnetic Index (mainly solar wind speed)
50
40
30
20
10
B nT
0
1840
Heliospheric Magnetic Field Strength B (at Earth) Inferred from IDV and Observed
1850
1860
1870
1880
1890
1900
1910
1920
1930
1940
1950
1960
1970
1980
1990
2000
2010
10
B (IDV)
8
6
13
23
4
0
1830
B (obs)
Heliospheric Magnetic Field at Earth
2
1840
1850
1860
1870
1880
1890
1900
1910
1920
1930
1940
1950
1960
1970
1980
Activity now is similar to what it was a century ago
1990
2000
2010
Year
30
Space
Climate
31
The Heliospheric Current Sheet
Artist: Werner Heil
Cosmic Ray
Modulation
caused in
part by
latitudinal
variations of
HCS, CIRs,
CMEs, and B
Svalgaard & Wilcox, Nature, 1976
32
The Cosmic Ray Connection
B Cosmic Rays
B Geomagn.
33
Back to the Future
2008-2009 HMF B = 4.14 1901-1902 HMF B = 4.10 nT
Sunspot Number, Ri = 3 Sunspot Number, Rz = 4
Showing very similar conditions of the
HMF B at the recent minimum and the
minimum 108 years before as deduced
from the Geomagnetic Record.
Uganda,
Nov. 3rd, 2013
The first known report of the
red flash, produced by
spicules requiring the
presence of widespread
solar magnetic fields, comes
from Stannyan observing the
eclipse of 1706 at Bern,
Switzerland. The second
observation, at the 1715
eclipse in England, was
made by, among others,
Edmund Halley. These first
observations of the red flash
imply that a significant level
of solar magnetism must
have existed even when
very few spots were
observed, during the latter
part of the Maunder
Minimum (Foukal & Eddy,
2007)
34
Conclusions
•
•
•
•
•
We can determine B, V, and n back to 1830s
Polar field reversals occurred that far back
No Modern Grand Maximum
FUV radiation varies with Sunspot Number
Solar Cycle Variations can be tracked with
Geomagnetism
• Caveat: The Earth’s main field is decreasing
and we don’t know how that affects the
geomagnetic inferences
35