Abstract. The semiannual variation in geomagnetic activity is generally attributed to the Russell-McPherron effect. In that picture, enhancements of southward field B.• near the equinoxes account for the observed higher geomagnetic activity in March and September. In a contrary point of view, we argue that the bulk of the semiannual variation results from an equinoctial effect (based on the ½• angle between the solar wind flow direction and Earth's dipole axis) that makes B.• coupling less effective (by ~25% on average) at the solstices. Thus the semiannual variation is not simply due to "mountain building" (creation of B.,) at the equinoxes but results primarily from "valley digging" (loss of coupling efficiency) at the solstices. We estimate that this latter effect, which clearly reveals itself in the diurnal variation of the am index, is responsible for ~65% of the semiannual modulation. The characteristic imprint of the equinoctial hypothesis is also apparent in hourly/monthly averages of the time-differentiated Dst index and the AE index.

[1] Combined Release and Radiation Effects Satellite (CRRES) Electric Field Instrument (EFI) data are used to determine the electric field power spectral density as a function of L and Kp over the frequency range 0.2 to 15.9 mHz. The power at each frequency is fit to the function P(L, Kp) = a L b exp(cKp). Assuming a purely electrostatic field and making several other assumptions regarding the azimuthal dependence of the field fluctuations, a Kp-dependent radial diffusion coefficient D LL E is computed from the power spectra. The model average D LL E for high activity (Kp = 6) are between 1 to 2 orders of magnitude larger than that for low activity (Kp = 1), dependent upon L and first invariant.

[1] The occurrence frequency of the largest geomagnetic storms from 1868 -1998 exhibits a well-defined semiannual modulation with more than twice as many storms occurring during equinoctial months than at the solstices. To examine the cause of this seasonal imbalance, we empirically obtained a new geomagnetic index aa m that has the same seasonal and Universal Time variation as the am index. In effect, this extends the am index backward in time to 1868. By normalizing the aa m time series for C, the angle between the solar wind flow direction and Earth's dipole, we removed 75% of the amplitude of the six-month wave in monthly averages of aa m and $75% of the seasonal discrepancy in the numbers of great storms. We obtained similar percentages for the (unmodified) am index over the shorter 1959 -1998 interval. These results indicate that most, though not all, of the discrepancy in storm counts between the equinoxes and solstices is due to an equinoctial effect.

We have developed a technique to provide short‐term warnings of solar energetic proton (SEP) events that meet or exceed the Space Weather Prediction Center threshold of J (>10 MeV) = 10 pr cm−2 s−1 sr−1. The method is based on flare location, flare size, and evidence of particle acceleration/escape as parameterized by flare longitude, time‐integrated soft X‐ray intensity, and time‐integrated intensity of type III radio emission at ∼1 MHz, respectively. In this technique, warnings are issued 10 min after the maximum of ≥M2 soft X‐ray flares. For the solar cycle 23 (1995–2005) data on which it was developed, the method has a probability of detection of 63% (47/75), a false alarm rate of 42% (34/81), and a median warning time of ∼55 min for the 19 events successfully predicted by our technique for which SEP event onset times were provided by Posner (2007). These measures meet or exceed verification results for competing automated SEP warning techniques but, at the present stage of space weather forecasting, fall well short of those achieved with a human (aided by techniques such as ours) making the ultimate yes/no SEP event prediction. We give some suggestions as to how our method could be improved and provide our flare and SEP event database in the auxiliary material to facilitate quantitative comparisons with techniques developed in the future.

No abstract

We explore requirements for a solar particle event (SPE) and flare capable of producing the cosmogenic nuclide event of 775 a.d., and review solar circumstances at that time. A solar source for 775 would require a >1 GV spectrum ∼45 times stronger than that of the intense high-energy SPE of 1956 February 23. This implies a >30 MeV proton fluence (F 30 ) of ∼8 × 10 10 proton cm −2 , ∼10 times larger than that of the strongest 3 month interval of SPE activity in the modern era. This inferred F 30 value for the 775 SPE is inconsistent with the occurrence probability distribution for >30 MeV solar proton events. The best guess value for the soft X-ray classification (total energy) of an associated flare is ∼X230 (∼9 × 10 33 erg). For comparison, the flares on 2003 November 4 and 1859 September 1 had observed/inferred values of ∼X35 (∼10 33 erg) and ∼X45 (∼2 × 10 33 erg), respectively. The estimated size of the source active region for a ∼10 34 erg flare is ∼2.5 times that of the largest region yet recorded. The 775 event occurred during a period of relatively low solar activity, with a peak smoothed amplitude about half that of the second half of the 20th century. The ∼1945-1995 interval, the most active of the last ∼2000 yr, failed to witness a SPE comparable to that required for the proposed solar event in 775. These considerations challenge a recent suggestion that the 775 event is likely of solar origin.

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Svalgaard and Cliver (Astrophys. J. Lett. 661, L203, 2007) proposed that the solarwind magnetic-field strength [B] at Earth has a "floor" value of ≈ 4.6 nT in yearly averages, which is approached but not broached at solar minima. They attributed the floor to a constant baseline solar open flux. In both 2008 and 2009, the notion of such a floor was undercut by annual B averages of ≈ 4 nT. Here we present a revised view of both the level and the concept of the floor. Two independent correlations indicate that B has a floor of ≈ 2.8 nT in yearly averages. These are i) a relationship between solar polar-field strength and yearly averages of B for the last four 11-year minima (B MIN ), and ii) a precursor relationship between peak sunspot number for cycles 14 -23 and B MIN at their preceding minima. These correlations suggest that at 11-year minima, B consists of i) a floor of ≈ 2.8 nT, and ii) a component primarily due to the solar polar fields that varies from ≈ 0 nT to ≈ 3 nT. The solar polar fields provide the "seed" for the subsequent sunspot maximum. Removing the ≈ 2.8 nT floor from B MIN brings the percentage decrease in B between the 1996 and 2009 minima into agreement with the corresponding decrease in solar polar-field strength. Based on a decomposition of the solar wind (from 1972 -2009) into high-speed streams, coronal mass ejections, and slow solar wind, we suggest that the source of the floor in B is the slow solar wind. During 2009, Earth was in slow solar-wind flows ≈ 70% of the time. We propose that the floor corresponds to a baseline (non-cyclic or ground state) open solar flux of ≈ 8 × 10 13 Wb, which originates in persistent small-scale (supergranular and granular) field.