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Centennial Variations of Near-Earth IMF and Solar Wind Speed 09:50, Friday November 21 auditorium Reine Elisabeth Session: 14 Space Climate Time allowed 20 min

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Mike Lockwood, L. Barnard, C.J. Scott, & M.J. Owens (Department of Meteorology, University of Reading, & Space Science and Technology Department, STFC/Rutherford Appleton Laboratory ) Centennial Variations of Near- Earth IMF & Solar Wind Speed 11 th European Space Weather Week, Liege, 18th November 2014 Session: 14 Space Climate

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Responses of indices 4 long index data series B and V sw reconstructions Solar change on timescales of days to millennia Origin of the B & V sw 2 signals Conclusions

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Responses of indices 4 long index data series B and V sw reconstructions Solar change on timescales of days to millennia Origin of the B & V sw 2 signals Conclusions

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Correlations with B V SW n as a function of n (B = IMF; V SW = solar wind speed) Two main classes of geomagnetic index: (1). Depend on B (2). Depend on BV SW 2 (Lockwood et al, 2013) Responses of the different geomagnetic activity indices

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Responses of indices 4 long index data series B and V sw reconstructions Solar change on timescales of days to millennia Origin of the B & V sw 2 signals Conclusions

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Latitude dependence of geomagnetic activity on B, V SW and BV SW 2 Finch et al. (2008) Data for The V SW 2 correlation comes only from the (nightside) auroral oval N. hemi. S. Hemi not significant at 2 level Modulus of geomagnetic latitude, | | (deg) auroral oval

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Origin of the B dependence (Lockwood, 2013) Southward IMF v. IMF for averaging timescale T T = 1 hour T = 1 day p.d.f. of the ratio geomagnetic indices give B to within uncertainty given by (c )

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Dependence of substorm current wedge on V SW 2 (Lockwood, 2013) Growth phase: adding open magnetospheric flux to tail causes far tail (where magnetopause || V SW ) to flare as tail lobe flux F TL rises) magnetic pressure in far tail lobe, B TL 2 /(2 o ) balances static SW pressure, hence B TL and cross-tail current density, J CT indep. of F TL

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In mid-tail, tail flaring angle means that solar wind dynamic pressure ( V SW 2 ) confines tail radius somewhat This means the rise in F TL in growth phase causes rise in mid-tail B TL (by an amount that depends on V SW 2 ) Dependence of substorm current wedge on V SW 2 (Lockwood, 2013)

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In mid-tail, tail flaring angle means that solar wind dynamic pressure ( V SW 2 ) confines tail radius somewhat This means the rise in F TL in growth phase causes rise in mid-tail B TL causes mid tail J CT rise (that depends on V SW 2 ) Dependence of substorm current wedge on V SW 2 (Lockwood, 2013)

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Hence at onset of substorm expansion phase, the J CT that is deflected into auroral ionosphere in substorm depends on V SW 2 current in westward electroject of substorm current wedge depends on V SW 2 Dependence of substorm current wedge on V SW 2 (Lockwood, 2013)

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Responses of indices 4 long index data series B and V sw reconstructions Solar change on timescales of days to millennia Origin of the B & V sw 2 signals Conclusions

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IDV(1d) Based on day-to-day difference in daily means (Bartels’ u) from three intercalibrated stations: Helsinki ( ) Niemegk ( ) Eskdalemuir (1911- present day) Stations selected because response function to IMF and solar wind in modern data is the same for all 3 stations (Lockwood et al, 2013) Geomagnetic activity indices used a). 1-day Interdiurnal Variation index, IDV(1d)

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IDV Based on day-to-day difference in near- midnight values Uses all non-auroral stations available (1 in 1880 rising to >50 for the space age – so inhomogeneous in construction) Before 1872 it is not IDV at all, but a proxy for Bartel’s u index from diurnal variation (Svalgaard & Cliver, 2010 ) Geomagnetic activity indices used b). Interdiurnal Variation index, IDV “u”

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n of peak correlation (top) and peak correlation (bottom) for IDV (triangles) and IDV(1d) (circles) Helsinki in green, Eskdalemuir in red and Niemegk in blue Optimum n for IDV(1d) near zero for these 3 stations Optimum n for IDV varies with even outside auroral oval and so response will changes as distribution of IDV(1d) and IDV – station latitude available station changes

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Checks with other historic data Greenwich data unreliable before 1880 because of temperature correction St Petersburg data (SPE) support IDV(1d) and not IDV before 1870 IDV(1d) and IDV – early years

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IHV Based on hour-to-hour variation around midnight Uses all stations available (so not homogeneous in construction) Spot values give different values to hourly means and must be avoided (Svalgaard and Cliver, 2007 ) Geomagnetic activity indices used c). Inter-hour Variation index, IHV

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aa C Based on 3-hr range values. Mean of series from southern England and Australia, each using 3 stations Corrections made for station inter-calibration problems (particularly Abinger-to-Hartland move in 1957) Before 1868 uses correlated range A k (D) data from Helsinki (Lockwood et al., 2014) Geomagnetic activity indices used a). Corrected aa index, aa C A k (D) HLS

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RI aac Sargent’s 27-day recurrence index. Based on correlations between 27-day intervals and the next Uses aa C Extended back before 1868 using correlated recurrence index from the range A k (H) values from Helsinki (Lockwood et al, 2014) Geomagnetic activity indices used N.B.) 27-day recurrence index from aa C, RI aac A k (H) HLS

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Extensions to aa C and its recurrence index using Helsinki (HLS) range Ak(D) and Ak(H) indices (blue and mauve lines). Checked using values from St Petersburg (SPE) IDV(1d) and IDV – early years

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Dependence of different geomagnetic activity indices on IMF B (V SW ) n all indices depend on B but depend on (V SW ) n with different n We use pairings with different n to reconstruct B and V SW n = 1.7±0.8 r = n = 1.6±0.8 r = n = −0.1±1.1 r = best fit aa C best fit IHV best fit IDV(1d) n = −0.1±1.1 r = best fit IDV interplanetary data (annual means)

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Responses of indices 4 long index data series B and V sw reconstructions Solar change on timescales of days to millennia Origin of the B & V sw 2 signals Conclusions

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Are the differences significant? dependence on exponent n of :- Correlation coefficients Significance of difference from peak Probability that pairs share the same n 6% - 10%

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Geomagnetic Reconstructions of near-Earth IMF, B, solar wind speed, V SW, and the Open Solar Flux (OSF), F S Sunspot number, R near-Earth IMF, B near-Earth solar wind speed,V SW open solar flux (OSF) (from Lockwood et al., 2014)

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10,000 polynomial fits that minimise r.m.s. difference of observed and fitted values For each, annual mean data points shifted by errors such that the 10,000 fits give a Gaussian distributions of known 2 uncertainties Uncertainties include effect of data gaps (shown) and the difference between the mean IMF B and the mean of its southward component B z (the latter being what determines geomagnetic activity) Fitting procedure & uncertainty 2 upper limit median of 10,000 fits 2 lower limit

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an example of a reconstructed year Distributions of the B & V SW values from the 10,000 fits for the 4 pairings that minimise combined normalised r.m.s. differences of both B and V SW For each, the blue line is the overall p.d.f., the product of the 4 p.d.f.s The grey band is bounded by the 2 points of the overall p.d.f. Combining Uncertainties

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Geomagnetic Reconstructions of near-Earth IMF, B, solar wind speed, V SW, and the Open Solar Flux (OSF), F S Sunspot number, R near-Earth IMF, B near-Earth solar wind speed,V SW open solar flux (OSF) (from Lockwood et al., 2014)

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Responses of indices 4 long index data series B and V sw reconstructions Solar change on timescales of days to millennia Origin of the B & V sw 2 signals Conclusions

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Correlations between geomagnetic indices and interplanetary parameters depend on index type and locations of stations Role of southward IMF (through magnetic reconnection at the magnetopause) causes all indexes to depend on IMF B on annual timescales with a known uncertainty distribution V SW 2 dependence arises from dynamic pressure effect on the mid tail and influences all indices that respond strongly to the substorm current wedge We have reliable, homogeneous indices for 1844 onward We can reconstruct B and V SW (and open solar flux: not covered in this talk) with low 2 uncertainties from 1844 onward Conclusions

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