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What Geomagnetism can Tell Us about the Solar Cycle?

Leif SvalgaardHEPL, Stanford University

Bern, 11 Nov., 2013ISSI 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)

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‘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

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Electric Current Systems in Geospace

Magnetospheric Currents

Polar CapnV2

B

BV2BV

±By

FUV

Diurnal Var.

Different Current Systems Different Magnetic Effects

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Electric Current Systems in Geospace

We can now invert the Solar Wind – Magnetosphere relationships…

Polar Cap

nV2

B

BV2BV

±By

FUV

Diurnal Var.

Different Current Systems Different Magnetic Effects

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Variometer Invented by Gauss, 1833

Nevanlinna et al.

Helsinki 1844-1912

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Magnetic Recorders

46 years ago, I used the Classic Instruments Modern Instrument

Classic Method since 1847

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Wolf’s Discovery (1852): rD = a + b RW

.

H

North X

D

Y = H sin(D)

dY = H cos(D) dD For small dD

rY

Morning

Evening

East Y

rD

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

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The Diurnal Variation of the Declination for Low, Medium, and High Solar Activity

910

-10

-8

-6

-4

-2

0

2

4

6

8Diurnal Variation of Declination at Praha

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Year

dD' 1840-1849rD

-10

-8

-6

-4

-2

0

2

4

6

8Diurnal Variation of Declination at Praha (Pruhonice)

dD' 1957-1959

1964-1965

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Year

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The Amplitude of the Diurnal Variation [from many stations] follows the Sunspot Cycle (can in fact be used to check the Sunspot Number calibration)

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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)

0

50

100

150

200

250

1880 1890 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 2010 2020

F10.7 sfu

25+Residuals

F10.7 calc = 5.42 rY - 130

Solar Activity From Diurnal Variation of Geomagnetic East Component

Nine Station Chains

232221201918171615141312

y = 5.4187x - 129.93

R2 = 0.9815

y = 0.043085x2.060402

R2 = 0.975948

0

50

100

150

200

250

300

30 35 40 45 50 55 60 65 70

rY

F10.7

rY is a Very Good Proxy for F10.7 Flux

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y = 1.1254x + 4.5545

R2 = 0.9669

30

35

40

45

50

55

60

65

70

25 30 35 40 45 50 55

Helsinki, Nurmijärvi

rY '9-station Chain'

1884-1908 1953-2008

Scaling to 9-station chain Helsinki-Nurmijärvi Diurnal Variation

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

303540455055606570

1840 1850 1860 1870 1880 1890 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 2010

Helsinki Nurmijärvi

9-station Chain

rY nT

Range of Diurnal Variation of East Component

the sunspot number (or more correctly to reconstruct the F10.7 radio flux)

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Sabine’s Discovery about Geomagnetic Disturbances

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.  Sir Edward Sabine (1788-1883)

We use the IDV-index = unsigned difference from one day to the next

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Relation to HMF Strength B

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Latest 27-day Bartels

Rotation showing B

and Kp peaks

B

V

Kp

Correlation between IMF BVn and several geomag. indices as a function of n

n = 2

n = 0

n

Lockwood (LRSP2013)

Co

rrelation

2 3 410-1-2

The IDV indices are not significantly different from having a dependence on B only. Thus, the negative part of Dst (i.e. ring current enhancement) is closest to explaining the behavior of IDV

1.0

0.5

1965-2012

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Latest Reconstruction of HMF B

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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

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The Many Stations Used for IHVin 14 ‘Boxes’ well Distributed in Longitude,

Plus Equatorial Belt

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IHV is a Measure of Power Input to the Ionosphere (Measured by POES)

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We can calculate Am [and Aa] from IHV

From IDV we get B. From IHV we get BV2. Thus we can get V

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Polar Cap Geomagnetic Observatories

Godhavn

Thule

1932-Present

1926-Present

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Svalgaard-Mansurov Effect

NP

SP

Toward

Away

Not a subtle effect…

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Sector Structure over Time

Vokhmyanin &  Ponyavin, 2013

Now

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Dominant Polarity: Rosenberg-Coleman Effect

Proves Polar Field Reversals in the Past

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How do we Know that the Poles Reversed Regularly before 1957?

Svalgaard, 1977

Wilcox & Scherrer, 1972

The predominant polarity = polar field polarity (Rosenberg-Coleman effect) annually modulated by the B-angle.

This effect combined with the Russell-McPherron 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).

“Thus, during last eight solar cycles magnetic field reversals have taken place each 11 year period”. S-M effect.Vokhmyanin & Ponyavin, 2012

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Cosmic Ray Modulation Depends on the Sign of Solar Pole Polarity

Miyahara, 2011

The shape of the modulation curve [alternating ‘peaks’ and ‘flat tops’] shows the polar field signs.

North pole

North pole

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.Svalgaard

& Wilcox, 1976

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Cross Polar Cap Hall Current

Ionospheric Hall Current across Polar Cap

1882

CHAMP

Been known a long time:

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Cross Polar Cap Potential Drop

GDH THL Space

E ~ -V×B

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Overdetermined System: 3 Eqs,

2 Unknowns

0

200

400

600

800

1000

1200

1400

1830 1840 1850 1860 1870 1880 1890 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 2010

ϕ

NeutronIon

Cosmic Ray Modulation Parameter

0

1

2

3

4

5

6

7

8

9

10

1830 1840 1850 1860 1870 1880 1890 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 2010

B

Heliospheric Magnetic Field at Earth

HMF from IDV-index HMF observed in Space

B = p (IDV)

BV2 = q (IHV)

VB = r (PCap)

Here is B back to the 1830s:

Gjøa

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R2 = 0.0019

0

1

2

3

4

5

6

1830 1840 1850 1860 1870 1880 1890 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 2010

Ceiling

Floor

Br nT

Year

Radial Component of Heliospheric Magnetic Field at Earth

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:

There seems to be both a Floor and a Ceiling and most importantly no long-term trend since the 1830s. Thus no Modern Grand Maximum.

Radial Magnetic Field (‘Open Flux’)

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Solar Activity 1835-2011

0

10

20

30

40

50

60

1840 1850 1860 1870 1880 1890 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 2010

Monthly Average Ap Index

0

2

4

6

8

10

1830 1840 1850 1860 1870 1880 1890 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 2010

B nT

Year

B (IDV)

B (obs)

Heliospheric Magnetic Field Strength B (at Earth) Inferred from IDV and Observed

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Sunspot Number

Ap Geomagnetic Index (mainly solar wind speed)

Heliospheric Magnetic Field at Earth

Activity now is similar to what it was a century ago

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Space Climate

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The Heliospheric Current Sheet

Svalgaard & Wilcox, Nature, 1976

Artist: Werner Heil

Cosmic Ray Modulation caused in part by latitudinal variations of HCS, CIRs, CMEs, and B

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The Cosmic Ray Connection

B Geomagn.B Cosmic Rays

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Back to the Future

Showing very similar conditions of the HMF B at the recent minimum and the minimum 108 years before as deduced from the Geomagnetic Record.

2008-2009 HMF B = 4.14 1901-1902 HMF B = 4.10 nTSunspot Number, Ri = 3 Sunspot Number, Rz = 4

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)

Uganda, Nov. 3rd, 2013

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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