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

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

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

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

 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

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

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

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 )

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

 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)

 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)

 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)

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

 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)

 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”

 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

 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

 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

 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

 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

 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

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)

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

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%

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)

 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

 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

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)

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

 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